JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
PAGES 2287±2312
2003
DOI: 10.1093/petrology/egg078
Textural and Thermal History of Partial
Melting in Tonalitic Wallrock at the Margin
of a Basalt Dike, Wallowa Mountains, Oregon
H. L. PETCOVIC* AND A. L. GRUNDER
DEPARTMENT OF GEOSCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331, USA
RECEIVED SEPTEMBER 15, 2002; ACCEPTED JUNE 18, 2003
Columbia River Basalt Group dikes invade biotite---hornblende
tonalite to granodiorite rocks of the Wallowa Mountains. Most
dikes are strongly quenched against wallrock, but rare dike
segments have preserved zones of partial melt in adjacent wallrock and provide an opportunity to examine shallow crustal
melting. At Maxwell Lake, the 4 m thick wallrock partial
melt zone contains as much as 47 vol. % melt (glass plus quench
crystals) around mineral reaction sites and along quartz--feldspar boundaries. Bulk compositional data indicate that
melting took place under closed conditions (excepting volatiles).
With progressive melting, hornblende, biotite, and orthoclase
were consumed but plagioclase, quartz, and magnetite persisted
in the restite. Clinopyroxene, orthopyroxene, plagioclase, and
Fe---Ti oxides were produced during dehydration-melting reactions involving hornblende and biotite. Reacting phases became
more heterogeneous with progressive melting; crystallizing
phases were relatively homogeneous. Progressive melting produced an early clear glass, followed by light (high-K) and dark
(high-Ca) brown glass domains overprinted by devitrification.
Melts were metaluminous and granitic to granodioritic. Thermal
modeling of the partial melt zone suggests that melting took
place over a period of about 4 years. Thus, rare dikes with
melted margins represent long-lived portions of the Columbia
River Basalt dike system and may have sustained large flows.
Columbia River Basalt dike; crustal melting;
dehydration-melting; tonalite---granodiorite; thermal model
KEY WORDS:
INTRODUCTION
sampled. The Wallowa Mountains of northeastern
Oregon, however, provide a natural laboratory in
which to examine shallow crustal melting. In this
area, hundreds of Columbia River Basalt Group
(CRBG) feeder dikes cut granitoid rocks of the Wallowa
Batholith. The batholith is a biotite- and hornblendebearing tonalite to granodiorite. Although most dikes
were strongly quenched against their wallrock, a few
dikes have developed partially melted contact zones,
commonly with up to 50 vol. % quenched melt in
the wallrock at their margins. This melt is represented
by devitrified silicic glass plus plagioclase, pyroxene,
and magnetite quench crystals. We have examined the
partially melted zone in tonalite at the margin of a
CRBG (Grande Ronde) dike where quenched melt is
preserved over a distance of 4 m from the dike margin
and reaches 47 vol. % near the dike---wallrock contact.
Recent work has shown that dehydration-melting
plays a crucial role in the generation of silicic melts in
the crust. Dehydration-melting, also called fluid- or
vapor-absent melting, is the incongruent reaction of a
hydrous mineral assemblage to form melt plus residual
minerals. Previous studies of crustal dehydration-melting considered protoliths with either amphibole or
mica. Melting of protoliths with both hydrous phases
was examined by Rutter & Wyllie (1988) and Skjerlie
& Johnston (1996) at high pressure (10 kbar). This
study differs from previous work in that both hornblende and biotite are present in the Wallowa parent
rock and melting was shallow.
Although there is broad consensus that basalt injection
can be fundamental in crustal melting, there are few
places where stages of the interaction can be directly
Related studies of partial melting
*Corresponding author. Telephone: 541-737-1201. Fax: 541-7371200. E-mail: [email protected]
Journal of Petrology 44(12) # Oxford University Press 2003; all rights
reserved
Dehydration-melting of mafic to intermediate composition amphibolites at pressures 510 kbar (1000 MPa)
JOURNAL OF PETROLOGY
VOLUME 44
have produced trondhjemitic, to tonalitic, to granodioritic melt coexisting with a restite of clinopyroxene ‡
orthopyroxene ‡ plagioclase quartz Fe---Ti oxides
(Beard & Lofgren, 1991; Rapp et al., 1991; Rushmer,
1991; Wolf & Wyllie, 1994; Pati~
no Douce & Beard,
1995). Biotite dehydration-melting reactions at
510 kbar have produced granitic to granodioritic melt
and a restite of orthopyroxene ‡ plagioclase ‡ Fe---Ti
oxides (generally ilmenite and/or magnetite) alkali
feldspar quartz from protoliths as diverse as biotite
tonalites (Pati~
no Douce & Beard, 1995; Singh &
Johannes, 1996a, 1996b), a biotite-bearing metagreywacke (Vielzeuf & Montel, 1994), pelites (Le Breton &
Thompson, 1988; Vielzeuf & Holloway, 1988), and a
high-F tonalitic gneiss (also containing 2 wt % hornblende; Skjerlie & Johnston, 1992, 1993). At pressures
around 10 kbar and greater, garnet was a crucial phase
in the restite for nearly all experimental protoliths.
Partial melting experiments have been performed
on protoliths containing both biotite and amphibole
at 10 kbar. A garnet---biotite---hornblende tonalite
yielded up to 40 vol. % melt (melt composition
not given) with a restite of orthopyroxene ‡
clinopyroxene ‡ rutile ‡ garnet (Rutter & Wyllie,
1988). A biotite---hornblende---epidote gneiss produced
about 35 vol. % peraluminous, granodioritic to granitic
melt and a restite of orthopyroxene ‡ clinopyroxene ‡
plagioclase ‡ garnet ‡ ferro-pargasitic amphibole
(Skjerlie & Johnston, 1996).
Whereas work on piston-cylinder experiments has
rarely reported coexisting, compositionally distinct
melts, the occurrence of multiple melts is common in
natural examples and rock core experiments. Shallow
(51 kbar) partial melting of Sierra Nevada biotite
granite at the contact with a trachyandesite plug produced both brown and clear glass coexisting with relict
plagioclase ‡ sanidine ‡ quartz, with magnetite,
rutile, and Mg-cordierite replacing biotite (Al-Rawi
& Carmichael, 1967; Kaczor et al., 1988; Tommasini
& Davies, 1997). Brown and clear glass coexisting with
a restite of plagioclase ‡ quartz ‡orthoclase ‡ Fe---Ti
oxides ‡ minor orthopyroxene were also noted by
Green (1994) in partially melted biotite granodiorite
xenoliths hosted in andesitic to dacitic dikes. Philpotts
& Asher (1993) noted abundant disequilibrium melting textures, such as sieve-textured feldspar, in partially melted biotite gneiss at the contact of a basalt
dike at paleodepth of about 10 km. Knesel & Davidson
(1996) melted 2 cm cubes of biotite alkali granite under
atmospheric conditions, yielding coexisting clear and
brown melts plus a restite of quartz ‡ plagioclase ‡
Fe---Ti oxides alkali feldspar. Multiple composition
melts attributed to muscovite and biotite (‡ quartz ‡
plagioclase) dehydration-melting reactions in pelite at
7 kbar were also noted by Rushmer (2001).
NUMBER 12
DECEMBER 2003
Fig. 1. Simplified geological map showing the location of the
Maxwell Lake dike, modified after Taubeneck (1995).
THE NATURAL LABORATORY
The Wallowa Batholith and CRBG dikes
The Wallowa Mountains are largely composed of the
Wallowa Batholith, a series of Late Jurassic plutons
(140---160 Ma; Armstrong et al., 1977) intruded into
island arc terranes accreted to the western margin of
North America (Fig. 1). Dikes exposed in the batholith
are part of the Chief Joseph dike swarm, which fed the
Columbia River flood basalts (Taubeneck, 1970).
Imnaha Basalt (17.3---17.0 Ma; Baksi, 1989) is preserved as erosional remnants on some peaks of the
Wallowa Mountains and as dikes. Most dikes in the
batholith are of Grande Ronde Basalt (16.9---15.6 Ma;
Baksi, 1989). Paleodepth at the time of Grande Ronde
dike emplacement was as great as 2.5 km, as estimated
from a thickness of about 1 km of Imnaha flows unconformably overlying about 1.5 km of relief in the
Wallowa Mountains.
Individual basalt dikes within the Wallowa Mountains can extend several kilometers along strike, are a
few centimeters to 50 m thick (average 7---10 m), and
are steeply dipping (average 70 ). Overall, dikes strike
N10 W, although strikes may vary from N55 W to
N30 E within sub-swarms where dikes occur in zones
of concentration of 7---12 dikes per km2 (Taubeneck &
Duncan, 1997; Petcovic et al., 2001). Dikes have one
or more of the following morphologies: dikes with
2288
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
quenched margins and no interaction with the wallrock, dikes with partially melted wallrock at their
margins, dikes that have eroded their wallrock, and
dikes containing whole to disaggregated crustal
xenoliths that constitute locally as much as 30% of
the dike (Grunder & Taubeneck, 1997). The majority
of Wallowa dikes have an aphanitic quench zone
(a few centimeters to 20 cm thick) at their margins.
Rarely, these dikes may have localized zones of
partial melting that extend into wallrock for up to
about 50 cm from the dike---wallrock contact. Typically, localized melting zones occur where dikes have
eroded their own quenched margins so that coarsegrained basalt is directly in contact with the wallrock.
Partial melting is most common in narrow wallrock
screens or in wallrock trapped between two crosscutting dikes.
Only a handful of dikes in the Wallowas exhibit
extensive partial melting (i.e. zones of partial melting
that are meters thick and continuous for tens to hundreds of meters). These dikes consistently lack an aphanitic quench zone at the dike margin. In these dikes,
melted margins are typically one-quarter to one-third
of the width of the dike, and in cases where dikes are
not vertical, the hanging wall has a thicker melted
margin (Grunder & Taubeneck, 1997).
The Maxwell Lake Dike
This study focuses on a single, well-exposed Grande
Ronde basalt dike with well-developed partially
melted wallrock margins (Fig. 2). The dike strikes
N20 E, dips steeply to the west (averaging about
75 ), and is from 2.6 to 7.8 m thick. It extends as en
echelon segments for at least 1 km along strike. Paleodepth at the time of dike emplacement was at most
2 km, as reconstructed from regional geology.
The partial melt margin along the hanging wall
(western margin) of the dike is generally 2---2.5 m
thick but reaches nearly 5 m thick at the southern end
of the outcrop (Fig. 2). The footwall partial melt margin is about 1.5 m thick. The thickness of the melt zone
in the hanging wall is typically 1.7---1.5 times as thick
as that of the footwall and is 0.3---0.5 times as thick as
the dike.
The dike margins are divided into four mappable
zones based on outcrop-scale textural characteristics
(Fig. 2). These are: the unmelted country rock, the
mafics-out zone, the mottled zone, and the mush
zone. These textural zones are 10 cm to 2 m wide with
gradational transitions from one zone to the next.
Zones parallel the dike and, like the partial melt zone
overall, are proportionally wider in the hanging wall.
The unmelted wallrock is a hypidiomorphic
granular hornblende biotite granodiorite by IUGS
classification. Based on the compositional classification
of Barker (1979) it is a tonalite; we refer to it as a
tonalite owing to the paucity of orthoclase relative to
plagioclase feldspar (Table 1; Fig. 3). The mafics-out
zone is characterized by the absence of biotite and
hornblende. Instead, fine-grained pyroxene and Fe--Ti oxides occupy former biotite and hornblende
mineral sites. Glass may be present as thin seams surrounding quartz and feldspar grains. The mottled zone
is characterized by a blue---gray mottled texture made
up of residual quartz and feldspar grains that lack
distinct margins, mafic mineral reaction domains,
and brown glass seams that surround grains. The
mush zone is a discontinuous, 10---50 cm wide zone
paralleling the dike margin. This zone contains sparse
amorphous grains of quartz and feldspar in a finegrained, blue---gray groundmass. The presence of the
mush zone appears to correlate with thicker parts of the
dike. A dense network of blue---gray veins and a cataclastic texture occur in the partially melted tonalite at
the southern end of the outcrop.
PROGRESSIVE STAGES OF MELTING
Quenched partial melt zones in the margins of the
Maxwell Lake dike have captured a continuum of
textural reactions, most of which can only be viewed
in thin section. We have grouped samples collected
from the hanging wall partial melt zones into five
progressive stages based on interpretations of the melt
reaction textures. Samples representing Stage 1 were
collected from the unmelted wallrock zone (Fig. 2) at a
distance of 44 m from the dike---wallrock contact.
Samples for Stages 2 and 3 were collected from the
mafics-out zone ( 2---4 m from the contact), and samples representing Stages 4 and 5 were collected from
the mottled zone (0.5---2 m from the contact) (Fig. 2).
These stages of reaction range from unmelted wallrock
(Stage 1) to nearly 47 vol. % quenched melt (Stage 5).
Stage 1
The unmelted tonalite is medium to coarse grained
with an inhomogeneous distribution of biotite and
hornblende (Fig. 4). Biotite rims are commonly altered
to chlorite whereas hornblende grains may contain
small quartz inclusions and/or clinopyroxene cores.
Magnetite and trace phases, including (in decreasing
order of abundance) apatite, zircon, and titanite, are
associated with biotite and hornblende and also may
be enclosed within grains.
Stage 2
Stage 2 contains trace amounts (51 vol. %) of glass,
thus representing the onset of melting (Table 1, Fig. 3).
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JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
DECEMBER 2003
Fig. 2. Outcrop map of the Maxwell Lake dike and its partially melted margins, showing sample labels and locations.
Hornblende contains sub-microscopic dusty reaction
products whereas biotite contains dusty opaque reaction minerals along cleavage planes and up to 0.5 mm
inward from crystal edges. No glass is present on
quartz---feldspar boundaries, but thin (510 mm) seams
of clear glass are present along fractures within quartz
crystals. These seams locally grade into rare small pools
of yellow---brown glass at the margins of biotite and
hornblende crystals. The sample of Stage 2 has a cataclastic overprint as evidenced by abundant veins,
minutely fractured quartz and feldspar crystals, and
offset twinning in plagioclase.
Stage 3
Stage 3 is most conspicuously characterized by the
appearance of continuous glass seams and the absence
of both hornblende and biotite (Table 1, Fig. 3). Dusty
brown reaction products rimmed by optically aligned
pyroxene grains occur in hornblende reaction sites
(Fig. 5a). Sites of biotite consumption are occupied
by a fine-grained intergrowth of glass, dusty magnetite
and lesser ilmenite, orthopyroxene, and plagioclase
feldspar with opaque oxides aligned in parallel bands
(Fig. 6a). When in contact with glass, plagioclase
generally has a spongy texture and poorly developed
2290
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
Table 1: Modal percentages of phases in each stage of melting
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
1390
418
1092
954
1142
42.9
19.3
49.0
19.6
52.3
8.2
59.4
2.5
43.3
3.0
Orthoclase
8.0
0
0
14.7
14.0
5.7
15.3
9.3
1.7
Hornblende
0
0
0
0
Fe---Ti oxide
1.0
1.0
0
1.0
Hornblende site
Pyroxene
0
0
0
0.1
8.8
Biotite site
Pyroxene
0
0
Plagioclase
0
0
Total counts:
Primary/relict
Plagioclase
Quartz
Biotite
Glass
Quench crystals
Fe---Ti oxide
0
0
Browny
0
51
Clearz
0
51
Plagioclase
0
0
Pyroxene
0
0
Fe---Ti oxide
0
0
12.5
6.5
0.6
7.4
1.3
2.1
7.1
1.5
2.4
2.9
1.1
1.5
12.3
0.1
14.3
3.9
30.1
1.1
0.5
1.0
0.3
0.3
0.3
5.3
1.9
0.9
0.2
Numbers are given as vol. % of whole rock, and modal data were collected by point counts on thin sections. Stage 1 is
represented by two samples. All other stages are represented by a single sample (for sample names and locations, see Fig. 2).
Point counts were performed on multiple thin sections (one thin section for Stage 2). Error is 1% for most counts and may
be higher in Stage 2 because of the low count number. Error is probably higher in Stages 2 and 4 as a result of the cataclastic
texture of these samples. All stages contain trace (50.5 vol. %) zircon, fluorapatite, and titanite.
Plagioclase data include plagioclase plus spongy rims containing trapped melt. Visual estimates are 4 vol. % trapped melt in
Stage 3, 5 vol. % trapped melt in Stage 4, and 7 vol. % trapped melt in Stage 5.
yBrown glass data include both dark and light brown domains.
zClear data are for granular domains in Stages 3---5. Description of clear glass from Stage 2 is given in text.
fritted margins. Individual cells in spongy plagioclase
are rounded and filled with brown glass. We estimate
4 vol. % glass is trapped within spongy plagioclase.
About 12 vol. % devitrified brown glass is localized
around sites where biotite and hornblende have been
consumed and along quartz---plagioclase grain boundaries (Fig. 7a). Seams are as thin as a few tens of
microns and as thick as 1 mm. The brown glass occurs
as two textural domains that grade into one another
(Fig. 7b). The dominant light brown glass domain is
characterized by radiating sheaves of microfibrous
crystals that we interpret as fine spherulitic devitrification. Opaque oxides occur as sparse needles. The dark
brown domain remains largely glassy and is characterized by dusty appearance under the microscope. Light
brown domains surround dark brown domains, and
both domains are irregularly distributed around biotite and hornblende reaction sites and along seams
between quartz and feldspar (Fig. 7a). Microlites of
acicular to hopper plagioclase, acicular pyroxene, and
equant magnetite are associated with the brown glass
domains (Fig. 7b). These quench crystals make up
typically 2 vol. % of the bulk mode, but up to
20 vol. % of the quenched groundmass. Minor clear,
granular domains have sharp boundaries with the
brown glass domains (Fig. 7b). These clear granular
domains are somewhat crystalline (i.e. not completely
isotropic) and almost always associated with embayed
quartz crystals.
Stage 4
Stage 4 is characterized by the absence of orthoclase;
the modal proportion of quartz is 53 vol. %, and the
sample contains about 18 vol. % glass and about
1 vol. % quench crystals (Table 1, Fig. 3). Small
(5100 mm long), optically aligned clinopyroxene and
orthopyroxene crystals occupy sites where hornblende
has been consumed. Magnetite and lesser ilmenite are
concentrated towards the center of the hornblende
reaction sites. A fine-grained intergrowth of glass,
aligned magnetite and lesser ilmenite, orthopyroxene,
and plagioclase occupies sites of biotite consumption.
The three devitrified glass domains described for
Stage 3 are also present in Stage 4. The sample of
Stage 4 has a cataclastic overprint as evidenced by
brecciated crystal fragments, microscopic fractures in
crystals, and offset twinning in plagioclase.
2291
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
DECEMBER 2003
Fig. 3. Modal data as a function of melt fraction. Dashed lines
indicate estimates of when phases react out. The modal abundance
of melt is shown by the stippled fields. Melt is represented by glass
and devitrified glass (both grouped as glass here), quench crystals
(shown separately), and reaction melt trapped in spongy plagioclase
(shown separately). An estimate of the actual amount of plagioclase
was made by subtracting the estimated proportion of glass in spongy
plagioclase from the modal plagioclase data. Ox, iron---titanium
oxide (magnetite and ilmenite); Plag or Pl, plagioclase feldspar;
Qtz, quartz; Or, orthoclase feldspar; Hbl, hornblende; Bio, biotite;
Pyx, pyroxene; xtls, crystals.
Fig. 5. Photomicrographs of hornblende reaction sites in Stages 3
and 5. (a) In Stage 3, a hornblende reaction site, in plane-polarized
light, has a dusty core with a coarser fringe of aligned pyroxene. Seam
of finely devitrified brown glass (BG, lower right) and spongy reaction in plagioclase should be noted. (b) In Stage 5, hornblende site in
crossed nicols is a network of optically aligned orthopyroxene (Opx)
and minor magnetite. Dark interstitial material is brown glass.
Fig. 4. Photomicrograph of unmelted tonalitic wallrock (Stage 1) in
crossed nicols. Plag, plagioclase; Qtz, quartz; Or, orthoclase; Hbl,
hornblende; Bio, biotite; Mag, magnetite; Ap, apatite; Zr, zircon; Ti,
titanite.
Stage 5
Stage 5 contains about 31 vol. % glass and 9 vol. %
quench crystals (Table 1, Fig. 3), thus representing the
maximum degree of partial melting at the Maxwell
Lake dike. Optically aligned orthopyroxene with
minor magnetite occurs in hornblende consumption
sites (Fig. 5b). Aligned magnetite and lesser ilmenite
intergrown with orthopyroxene and plagioclase occurs
in biotite reaction sites (Fig. 6b). Although still very
fine, crystals are coarser than in Stage 3. Relict quartz
crystals are rounded and embayed, and may be surrounded by haloes of acicular clinopyroxene. Plagioclase crystals in contact with glass have well-developed
spongy texture, fritted margins, and a thin (525 mm
wide) optically distinct rim. Spongy plagioclase may
contain up to 7 vol. % glass; 30% of the plagioclase
has a spongy texture (Fig. 3).
As in Stages 3 and 4, glass is localized around reacted
mafic sites and as seams up to 2 mm thick on quartz--plagioclase grain boundaries. Stage 5 glass occurs as
dominant, microfibrous light brown domains, glassy
dark brown domains, and minor clear granular
domains. Acicular to hopper quench crystals of plagioclase, pyroxene, and magnetite are localized in the
brown glass domains, typically making up 25 vol. %,
but up to 50 vol. %, of the groundmass.
2292
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
Fig. 6. Photomicrographs of biotite reaction sites in Stages 3 and 5.
(a) In Stage 3 (plane-polarized light), the biotite reaction site is
occupied by aligned, dusty opaque oxides (Mag ‡ Ilm). Light
domains are an intergrowth of plagioclase, orthopyroxene, and
glass (Plag ‡ Opx). (b) Stage 5 biotite reaction site in planepolarized light. Magnetite and lesser ilmenite are slightly more
coarse-grained than in Stage 3.
COMPOSITIONAL
CHARACTERISTICS OF
PROGRESSIVE MELTING
Analytical methods
Fig. 7. Photomicrographs of glass domains in Stage 3. (a) Domains
of light brown glass (LBG) and dark brown glass (DBG) distributed
along quartz---plagioclase grain boundaries. (b) Light brown glass,
dark brown glass, and clear granular (CG) domains adjacent to a
plagioclase grain and hornblende reaction site. Dark brown glass
remains largely isotropic whereas light brown glass exhibits feathery
devitrification. Sparse quench crystals of plagioclase (Q plag) and
pyroxene (Q pyx) occur in brown glass domains.
because of its susceptibility to migration. Additional
details have been provided by Petcovic (2000).
Whole-rock bulk analyses were performed using the
Rigaku 3370 X-ray fluorescence (XRF) spectrometer
at Washington State University's GeoAnalytical
Laboratory. Analyses of crystals and glass were performed using the CAMECA SX-50 Electron Microprobe at Oregon State University. Analyses of
hornblende, pyroxene, feldspar, and biotite used a
beam current of 30 nA, an accelerating voltage of
15 kV, and a 3---5 mm diameter beam. Glass was analyzed using the same conditions and a broad (20 mm)
beam. Sodium was counted first in glass and crystals
Bulk composition
Most analyses of major and trace elements for the five
stages are identical within analytical error (Table 2).
The samples from Stage 2 and, to a lesser degree
Stage 4, are slightly altered, which is reflected in low
Na2O values. Nevertheless, minor loss of Na2O during
melting is possibly indicated. Some variations greater
than analytical error, such as higher La, Ce, and Th in
Stages 3, 4, and 5 compared with Stage 1, probably
reflect differences in the proportion of trace phases
2293
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
Table 2: Bulk-rock major and trace element
data
Table 3: Selected biotite and hornblende
analyses
Stage 1
Stage 2
Stage 3
Stage 41
Stage 5
59.32
0.70
62.61
0.50
60.02
0.59
61.05
0.64
61.15
0.73
No. of analyses:
17.97
5.35
17.09
4.23
17.32
5.24
18.03
4.77
17.00
5.17
wt %
0.10
3.84
0.07
2.59
0.12
3.93
0.08
3.25
0.10
3.89
6.45
4.12
7.28
1.84
7.52
3.21
7.06
2.49
6.07
3.48
P2O5
1.35
0.18
1.18
0.13
1.23
0.18
1.58
0.16
1.58
0.19
Total
99.37
97.53
99.35
99.09
99.36
Hornbende
wt %
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
ppm
Ni
39
23
48
26
33
Cr
65
28
61
37
45
Sc
17
15
13
14
18
V
124
99
117
126
147
Ba
504
371
332
495
492
Rb
25
21
22
30
32
Sr
626
586
453
524
541
Zr
125
84
123
171
117
Y
20
11
22
14
17
Nb
5
5
4
4
5
Ga
18
17
19
17
17
Cu
34
22
2
26
15
Zn
44
44
67
48
66
Pb
4
1
4
0
4
La
8
0
21
16
22
Ce
21
28
42
43
41
Th
1
3
8
3
3
DECEMBER 2003
Biotite
Stage 1
Stage 2
Stage 1
Stage 2
69
23
43
26
SiO2 (0.13/0.12)
TiO2 (0.02/0.03)
49.28
0.80
49.52
0.62
36.54
3.21
43.16
3.97
Al2O3 (0.05/0.07)
FeO (0.09)
5.77
13.45
7.03
14.40
16.45
14.18
13.19
Fe2O3 (calc)
6.37
13.52
7.25
FeO (calc)
MnO (0.03)
6.88
0.45
7.13
0.39
MgO (0.08/0.07)
CaO (0.06/0.01)
14.55
11.67
14.48
11.54
Na2O (0.03/0.02)
K2O (0.01/0.05)
0.69
0.34
0.74
0.13
H2O (calc)
2.00
0.18
b.d.
0.04
0.08
b.d.
0.03
100.42
F (0.02)
Cl (0.01)
2.04
0.14
b.d.
13.36
11.82
0.50
b.d.
0.12
8.98
3.74
0.28
0.11
0.73
7.32
3.80
0.67
0.08
0.30
99.36
0.15
97.18
Mg no.
66
66
59
62
Range Mg no.
63---74
61---67
58---60
24---74
O ˆ F, Cl
Total
99.12
All Fe as FeO.
Fe2O3, FeO, and H2O calculated by charge balance using the
Cameca SX Formula-1 program. b.d., concentration below
detection limit. Numbers in parentheses indicate analytical
precision determined from microprobe counting statistics;
first number refers to precision for hornblende, second number for biotite.
1
Data for Stage 4 are averages of two analyses. All others are
a single analysis.
2
Unexplained total of 1133 ppm.
All Fe as FeO.
Error is 10% for all except Ba, Zr, Y, Nb (15%) and Al, Si
(0.5 wt %).
between samples. Overall, however, the process of
partial melting of the Wallowa tonalite was a closed
system, excepting volatile components, based on the
near identity of bulk-rock analyses of unmelted and
partially melted rock.
Phase compositions
Hornblende and biotite
Hornblende (Table 3, Fig. 8a) is present only in
Stages 1 and 2 (Fig. 3). Relative to unmelted rock,
Stage 2 hornblende is compositionally more heterogeneous, and has lost virtually all Cl, half of the F,
and much of the K (Fig. 8). Whereas Stage 1 hornblende has a normal trend of increasing FeO with
decreasing MgO, Stage 2 hornblende has a scattered
increase in FeO with increasing MgO (Fig. 8b).
Like hornblende, biotite is present only in Stages 1
and 2 (Fig. 3, Table 3). Stage 1 biotite is tightly clustered in composition, relative to Stage 2 biotite, which
has generally lower FeO and higher TiO2 and highly
variable MgO concentrations (Fig. 9). Stage 2 biotite
has as great as 10-fold enrichment in Na2O with relatively modest loss in K2O (Fig. 9b). Fluorine concentrations are also enriched in most Stage 2 biotite and
correspond to high TiO2 compositions (Fig. 9c).
Chlorine concentration is broadly the same in Stage 1
and in Stage 2 (Cl of 0.5---0.15 wt %) but Stage 2
2294
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
Fig. 8. Hornblende compositions in Stages 1 and 2. (a) Amphibole classification after Deer et al. (1992). (b) FeO and MgO. (c) K2O and
Na2O. (d) Cl and F.
biotite with low F (50.3 wt %) also has low Cl
(50.05 wt %; Fig. 9c).
Feldspar
Plagioclase and orthoclase together make up nearly
50 vol. % of the bulk unmelted wallrock (Fig. 3).
Orthoclase is absent from the restite assemblage by
Stage 4 (Table 4, Fig. 3), indicating that it is consumed
during the early stages of melting. Stage 2 orthoclase is
less potassic than Stage 1 orthoclase (Fig. 10a) and
contains up to three times more Ba (0.54---1.69 wt %
in Stage 2 vs 0.08---0.55 wt % in Stage 1).
Plagioclase makes up the bulk of the mode in all
stages (Fig. 3). In the partially melted wallrock, it
occurs as a residual phase (Stages 2---5), as interstitial
material in biotite reaction sites (Stages 3---5), and as
quench crystals associated with glass seams (Stages
3---5) (Table 4). Changes in residual plagioclase composition are not systematic with increased melting
(Fig. 10). Stage 5 plagioclase is less different from
Stage 1 than are the intervening stages. On the
whole, residual plagioclase becomes richer in K2O,
CaO, FeO, and MgO, and poorer in Na2O with continued melting (Fig. 10). Optically distinct plagioclase
rims in Stage 5 are also compositionally distinct,
having higher concentrations of CaO, FeO, and
MgO than Stage 5 relict plagioclase (Fig. 10b).
Plagioclase from reacted biotite sites in Stages 3---5
(Fig. 6) is predominantly labradorite (Table 4). Plagioclase in biotite reaction sites is generally more calcic
than residual plagioclase and contains 51 wt % K2O,
but is compositionally variable. MgO and FeO concentrations are slightly higher than in residual plagioclase (up to 0.34 wt % MgO and 1.54 wt % FeO in
Stage 5), and concentrations of these oxides increase
with continued melting.
Andesine to labradorite quench crystals were analyzed in Stages 3---5 (Table 4). The compositional
range of quench plagioclase is similar to that of relict
plagioclase, but quench crystals may be slightly less
potassic. FeO and MgO concentrations in quench plagioclase are slightly higher in more advanced stages
of melting. Stage 5 quench crystals contain up to
0.13 wt % MgO and 1.31 wt % FeO.
2295
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
DECEMBER 2003
reaction sites, as interstitial material in biotite reaction
sites, and as acicular quench crystals associated with
brown glass seams. Both orthopyroxene and clinopyroxene occur in hornblende reaction sites as optically aligned microlites (Fig. 5). In Stage 3, augite,
pigeonite, and lesser enstatitic orthopyroxene occupy
decomposed hornblende sites (Table 5). However, by
Stage 5, nearly all of this pyroxene is enstatitic orthopyroxene (Table 5). Early clinopyroxene and orthopyroxene crystals have heterogeneous compositions,
but pyroxene compositions become more homogeneous
with continued reaction (Fig. 11). Concentrations of
Na2O in augite decrease from 1.3 wt % in Stage 3 to
50.4 wt % in Stage 4. Stage 3 augite contains up to
8 wt % Al2O3 whereas Stage 4 augite contains 55 wt %.
Orthopyroxene intergrown with plagioclase and
opaque oxides occurs in biotite reaction sites in Stages
3---5 (Fig. 6). Enstatitic orthopyroxene in biotite reaction sites is slightly more Mg- and Al-rich and Ca- and
Na-poor than enstatitic orthopyroxene in hornblende
reaction sites (Table 5).
Enstatitic orthopyroxene quench crystals are associated with brown glass seams, and sparse augite
quench crystals also occur in Stage 5 (Table 5).
Quench enstatitic orthopyroxene is similar in composition to enstatitic orthopyroxene in decomposed
hornblende sites, but is Al-poor in comparison with
enstatitic orthopyroxene in decomposed biotite sites
(typically 0.8---1.8 wt % Al2O3 vs 43 wt % for biotite).
Quench augite is also Al-poor in comparison with
augite in reacted hornblende sites from Stages 3 and 4.
Magnetite and ilmenite
Fig. 9. Biotite compositions in Stages 1 and 2. (a) FeO and MgO.
(b) Na2O and K2O. (c) TiO2 and F. Cl concentration in all samples
with 50.3 wt % F is 50.05 wt %. Sample marked with has high Cl
(0.28 wt %). Others have Cl between 0.05 and 0.15 wt %.
Pyroxene
Excepting rare cores in hornblende, pyroxene is not
present in the wallrock assemblage until Stage 3,
indicating that it is produced during partial melting
reactions (Fig. 3). Texturally, pyroxene occurs in
Stages 3---5 as microlites associated with hornblende
Magnetite is ubiquitous in all stages, occurring as a
primary phase in the unmelted wallrock, as a residual
phase in Stages 2---5, texturally associated with hornblende and biotite reaction sites in Stages 2---5 (Figs 5
and 6), and as quench crystals associated with brown
seams of glass. Residual magnetite, as well as magnetite associated with decomposed hornblende and
biotite sites, may contain up to 15 wt % TiO2. Sparse
ilmenite is present as scattered grains in Stages 3 and 5,
in Stage 4 decomposed hornblende sites, and in reacted
biotite sites from Stages 3---5. The small size of Fe---Ti
oxides made microprobe analysis difficult and often
yielded low analytical totals (Table 6, and see Petcovic,
2000). Only a few magnetite---ilmenite pairs in Stages 3
and 4 yielded analyses suitable for geothermometry
(see thermal modeling section, below).
Glass
Glass analyzed from Stages 3---5 yields a different composition for each textural domain (Table 7, Fig. 12).
2296
Orthoclase
Plagioclase
Plagioclase in biotite sites
Stage 31
Stage 41
Quench plagioclase
Stage 51
Stage 41
Stage 2
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
5
57
19
31
14
38
SiO2 (0.14)
Al2O3 (0.09/0.07)
64.03
18.07
63.01
18.89
58.18
25.52
57.42
26.33
55.32
26.92
56.59
26.64
53.00
28.35
51.56
29.24
53.80
27.75
56.18
26.23
57.32
25.23
54.81
27.12
FeO (0.03/0.02)
MgO (0.01)
0.12
b.d.
b.d.
0.21
0.23
0.23
b.d.
b.d.
8.00
b.d.
8.55
1.25
0.22
1.00
0.08
1.04
0.09
1.31
0.12
10.61
b.d.
9.51
1.29
0.16
b.d.
0.45
b.d.
7.74
0.73
0.05
CaO (0.05/0.01)
BaO (0.01/0.02)
b.d.
0.37
0.98
0.11
Na2O (0.06/0.03)
K2O (0.02/0.07)
0.43
15.56
b.d.
5.77
b.d.
4.24
b.d.
5.42
10.22
0.13
10.47
b.d.
5.54
10.43
0.15
9.52
b.d.
7.01
10.74
0.59
12.93
1.58
1.92
12.20
0.10
1.38
0.61
0.63
0.40
4.40
0.55
0.42
4.74
0.62
0.55
3.53
1.16
Total
98.69
98.05
100.39
98.91
98.95
98.86
98.90
98.39
98.87
98.97
98.99
98.73
99.07
Stage 3
1
Stage 5
16
wt %
59.34
26.10
2297
b.d.
3.22
b.d.
5.40
b.d.
4.88
0.33
An
0
2
38
41
43
56
48
55
67
53
48
56
53
Ab
4
19
61
51
53
40
50
41
30
44
49
36
45
Or
96
79
1
8
4
4
2
3
3
4
3
8
Range An, Or2
94---97
74---80
1
Average of two analyses.
2
Range in An for plagioclase,
32---50
35---49
34---56
40---64
38---58
55
65---69
48---57
49---63
2
48---59
Or for orthoclase.
All Fe as FeO.
b.d., concentration below detection limit. Numbers in parentheses indicate analytical precision determined from microprobe counting statistics; first number refers to
precision for plagioclase feldspar, second number for orthoclase feldspar.
MELTING AT BASALT DIKE MARGIN
Stage 1
14
No. of analyses:
PETCOVIC AND GRUNDER
Table 4: Representative feldspar analyses
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
DECEMBER 2003
successfully analyzed. These seams proved to generally
be too thin and/or altered for successful analysis.
DISCUSSION
Partial melting at the margin of the Maxwell Lake
CRBG dike in the Wallowa Mountains provides a
macrocosm of textural and compositional information
about progressive melting of continental crust at a scale
between experimental charges and granitoid intrusions. We first discuss the nature of the melting reactions, including the nature of melts produced. We then
compare this example of natural melting with other
natural examples and experimental work on similar
protoliths. Finally, the conditions of melting are evaluated, followed by development of a thermal model
that predicts the timescale over which melting
took place.
Nature of the melting reactions
Closed-system melting
Fig. 10. Feldspar compositions in Stages 1---5. (a) Ternary feldspar
compositions of primary and residual feldspar. (b) CaO and FeO in
plagioclase. Stage 5 plagioclase rims have 0.5---1.5 wt % MgO, compared with relict plagioclase with typically 50.5 wt %.
The light brown, microfibrous glass domains are metaluminous to mildly peraluminous (generally 51%
normative corundum) and granitic with up to 8 wt %
K2O in Stage 3 and 6.5 wt % K2O in Stage 5
(Table 7). The dark brown glass domains are metaluminous and tonalitic in composition. The two
domains are best distinguished by the amount of K2O
relative to CaO (Fig. 12a). With respect to Na2O,
TiO2, MgO, FeO, and Al2O3, the brown glass domains
largely overlap at each stage. With increased degree of
melting (from Stage 3 to Stage 5), K2O and SiO2
concentrations decrease and FeO, MgO, and TiO2
concentrations increase slightly in both brown glass
domains (Fig. 12).
The clear granular domains of Stages 3---5 are essentially composed of SiO2 (Table 7, Fig. 12b). The clear
to yellow---brown glass domains in Stage 2 were not
Within the range of samples we have examined, partial
melting appears to have taken place under closed conditions. Nearly all major elements (as well as most trace
elements) were conserved from Stage 1 to Stage 3 to
Stage 5 (Table 2), indicating that these samples represent a chemically closed system. Stages 2 and 4 are
weighted less in this discussion because of the cataclastic overprint and slight bulk compositional differences
attributed to alteration. We do not consider water and
other volatile components here. Compositional nearidentity during progressive melting also suggests that
melts did not separate from their restite. Indeed, we
observed no textural evidence in thin section or in
outcrop that suggests large-scale melt flow.
The melting reactions
During the progress of partial melting observed over
4 m of wallrock adjacent to the Maxwell Lake dike,
hornblende, biotite, and orthoclase were entirely consumed. The composition of these phases became highly
variable as they broke down. Plagioclase, quartz, and
magnetite were partially consumed yet persisted in the
restite with as much as 31 vol. % glass. With progressive melting, relict plagioclase developed a spongy
residuum by the reaction of andesine to produce
labradorite plus an albitic melt partially trapped
within the plagioclase.
During progressive melting, orthopyroxene, clinopyroxene, secondary plagioclase, magnetite, sparse
ilmenite, and melt (now represented by devitrified
glass ‡ quench crystals) were produced. The compositions of new minerals were initially variable, yet
2298
Clinopyroxene in
Pigeonite in
Orthopyroxene in
Orthopyroxene in
Quench
Quench
hornblende sites
hornblende sites
hornblende sites
biotite sites
orthopyroxene
clinopyroxene
Stage 3
Stage 41
No. of analyses: 34
Stage 3
Stage 4
Stage 5
Stage 3
Stage 4
Stage 5
Stage 4
5
14
1
17
32
51
1
Stage 51
Stage 31
Stage 41
Stage 5
Stage 5
6
3
wt %
52.53
0.41
52.33
0.64
53.86
0.30
51.94
0.37
52.78
0.41
54.57
0.19
53.44
0.47
53.71
0.26
50.77
0.49
52.96
0.56
53.17
0.26
51.91
0.34
53.27
0.45
52.37
0.46
Al2O3 (0.02)
FeO (0.10/0.08)
2.49
10.23
3.77
13.38
2.86
15.82
2.42
17.31
1.43
14.51
1.11
17.30
1.52
17.70
1.20
15.48
3.67
18.43
3.30
14.42
0.83
18.57
1.75
18.22
1.72
16.10
1.47
10.18
MgO (0.10/0.08)
MnO (0.03)
15.64
0.41
19.58
0.35
23.23
0.46
22.67
0.47
22.22
0.37
25.15
0.62
25.03
0.46
26.20
0.40
22.49
0.44
28.03
0.39
23.92
0.54
24.29
0.37
25.92
0.46
15.67
0.37
CaO (0.03/0.07)
Na2O (0.02)
18.03
0.46
10.53
0.15
2.54
0.40
2.74
0.19
7.53
0.12
2.00
1.97
1.74
1.43
0.18
0.47
1.34
1.13
1.67
100.22
100.74
99.47
98.12
99.37
Total
b.d.
b.d.
101.00
100.66
b.d.
99.03
97.90
b.d.
b.d.
b.d.
b.d.
18.68
0.26
100.15
98.70
98.06
99.63
99.46
En
46
56
68
66
62
69
69
72
66
77
68
69
72
45
Fs
17
22
26
28
23
27
27
24
31
22
30
29
25
16
Wo
38
22
5
6
15
4
4
3
3
1
3
2
3
Range En
40---52
43---44
64---69
57---67
Range Wo
25---43
39---41
5---11
5---16
1
Average
66---73
65---72
64---75
76---78
66---69
68---69
65---76
39
42---46
36---40
of two analyses.
All Fe as FeO.
b.d., concentration below detection limit. Numbers in parentheses indicate analytical precision determined from microprobe counting statistics; first number refers to
precision for orthopyroxene, second number for clinopyroxene.
MELTING AT BASALT DIKE MARGIN
2299
SiO2 (0.10/0.09)
TiO2 (0.02)
PETCOVIC AND GRUNDER
Table 5: Representative pyroxene analyses
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
DECEMBER 2003
phases, allows us to determine a general, initial meltproducing reaction:
hornblende ‡ biotite ‡ quartz ‡ orthoclase
‡ Ab from plagioclase ˆ clinopyroxene
‡ orthopyroxene ‡ plagioclase ‡ magnetite
‡ melt …glass ‡ quench crystals†:
…1†
This reaction was terminal for both amphibole and
biotite and yielded 18 vol. % melt (12 vol. % glass ‡
2 vol. % quench crystals ‡ 4 vol. % glass trapped in
spongy plagioclase; Table 1). Between Stage 3 and
Stage 5 (Fig. 3), wallrock closer to the dike margin
experienced higher temperature conditions, suggesting
that additional melt was produced from the following
reaction:
plagioclase ‡ quartz ‡ orthoclase ‡ clinopyroxene
‡ magnetite ˆ melt …glass ‡ quench crystals†: …2†
Fig. 11. Pyroxene compositions in Stages 3---5. (a) Ternary composition of pyroxenes from hornblende reaction sites. Data for Ca-poor
pyroxenes of Stages 3---5 are virtually overlapping. Augite is common
only in Stage 3. Stage 4 augites fall into the center of the distribution.
(b) Al2O3 and Na2O in pyroxene from hornblende reaction sites.
became more homogeneous with continued melting.
Melt developed within and around decomposed biotite
and hornblende sites, and also along seams up to 2 mm
thick between relict quartz and plagioclase crystals.
In Stage 3, aligned clinopyroxene and orthopyroxene
microlites, minor magnetite and rare ilmenite, and
glass occupy decomposed hornblende sites, suggesting
that dehydration-melting reactions involving hornblende produced these phases. By Stage 5, all of the
clinopyroxene in hornblende decomposition sites
has reacted to produce enstatitic orthopyroxene. In
Stages 3---5, biotite dehydration-melting reactions produced aligned magnetite and lesser ilmenite in an
intergrown matrix of orthopyroxene, plagioclase, and
glass. Because partial melting took place in a closed
system, components released from phases that break
down must have been accommodated either in the
restite or in the melt.
Consideration of the difference between modal proportion of phases between Stage 1 and Stage 3 (Fig. 3),
along with consideration of the composition of these
This second reaction was terminal for orthoclase and
clinopyroxene, and yielded an additional 29 vol. %
melt for a total of nearly 47 vol. % melt (31 vol. %
glass ‡ 9 vol. % quench crystals ‡ 7 vol. % glass
trapped in spongy plagioclase; Table 1).
Although we have established general meltproducing reactions, other work on partial melting in
crystalline rocks (e.g. Wolf & Wyllie, 1991; Philpotts &
Asher, 1993; Hammouda et al., 1996; Knesel &
Davidson, 1996) has suggested that dehydration-melting reactions are locally controlled by stoichiometry of
melt reactions and kinetics rather than by the overall
bulk assemblage. For example, melt in reaction (1)
could have been produced by local reactions such as
hornblende ‡ quartz, biotite ‡ plagioclase, biotite ‡
hornblende ‡ orthoclase, or quartz ‡ plagioclase. This
process results in a disequilibrium assemblage of
heterogeneous local melts, residual minerals and secondary minerals in different stages of reaction. Abundant disequilibrium textures (e.g. spongy plagioclase)
and initially variable phase compositions suggest that
partial melting reactions in the Wallowa tonalite were
also controlled by local assemblages.
Composition of the melts produced
We analyzed three distinct glass domains in the partially melted wallrock: a dominant, light brown,
high-K glass exhibiting feathery devitrification; a less
abundant, dark brown, high-Ca glass; and a sparse,
clear, high-Si glass. The distribution of both brown
glasses is highly irregular, occurring in seams between
quartz and plagioclase, adjacent to hornblende and
biotite reaction sites, and within these reaction sites.
The compositions of each glass domain largely overlap
for each stage, except for high K2O in the light brown
2300
Table 6: Selected magnetite and ilmenite analyses used in oxide geothermometry
Stage 4
Magnetite
Analysis no.:
A.C.3
Ilmenite
F.4
Magnetite
A.K.3
A.K.4
A.A.4
B.C.1
Ilmenite
.16
.6
.3
b.d.
b.d.
10.68
b.d.
.13
G.1
b.d.
0.49
53.22
wt %
SiO2 (0.01)
TiO2 (0.07/0.21)
0.18
10.49
0.12
10.24
b.d.
7.56
0.11
47.53
b.d.
46.68
0.11
41.35
Al2O3 (0.02)
Cr2O3 (0.02/0.01)
5.62
7.14
1.53
0.14
0.26
0.27
0.15
0.16
b.d.
0.20
b.d.
b.d.
0.71
77.43
b.d.
41.30
b.d.
43.45
0.34
48.94
Fe2O3 (calc)
71.27
40.87
69.18
39.74
47.87
9.28
32.95
10.55
33.95
18.22
32.55
FeO (calc)
MnO (0.02/0.03)
34.49
0.47
33.41
0.43
34.36
0.32
0.58
5.22
0.57
4.17
0.29
2.50
MgO (0.02/0.03)
Total
3.76
96.16
4.29
95.64
1.13
93.66
Xmag
0.66
0.65
0.76
95.93
96.24
95.67
0.90
Xilm
Equilib. match
B.C.1
T ( C)
969
A.K.4
808
A.A.4
745
A.C.3
804
0.89
A.C.3
830
15.48
13.90
2.43
0.15
2.09
0.13
2.25
0.22
0.80
77.15
0.67
73.60
0.97
73.87
43.97
35.30
37.61
40.83
0.22
37.59
0.19
41.83
0.20
40.03
0.21
2.32
98.03
1.95
97.85
1.97
97.72
2.04
97.29
0.54
0.67
0.53
0.57
0.43
1.10
73.18
35.96
0.81
A.K.3
861
2.35
b.d.
1.42
31.99
0.00
31.99
b.d.
3.68
93.31
1.00
G.1
1088
G.1
988
G.1
1098
G.1
1066
All Fe as FeO.
b.d., concentration below detection limit. Low totals are due to the small size of the oxides. Numbers in parentheses indicate analytical precision determined from
microprobe counting statistics; first number refers to magnetite, second number to ilmenite. Magnetite and ilmenite recalculations by the method of Stormer (1983).
Potential equilibrium pairs for geothermometry identified by Mg---Mn partitioning after Bacon & Hirschmann (1988). Geothermometry temperatures calculated after
Ghiorso & Sack (1991).
MELTING AT BASALT DIKE MARGIN
2301
b.d.
0.22
V2O3 (0.01)
FeO (0.22/0.15)
14.93
2.19
PETCOVIC AND GRUNDER
Stage 3
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
DECEMBER 2003
Table 7: Representative glass analyses
Dark brown
No. of analyses:
Light brown
Clear granular
Stage 3
Stage 4
Stage 5
Stage 3
Stage 4
Stage 5
Stage 3
15
7
8
38
36
50
4
74.18
0.21
76.19
0.78
78.90
0.58
75.99
0.49
72.73
0.74
76.26
0.87
11.49
0.49
11.40
0.89
11.46
0.56
11.62
0.44
10.46
0.97
11.02
0.95
b.d.
3.51
0.14
3.08
b.d.
3.36
b.d.
0.53
0.14
0.68
0.05
0.73
b.d.
0.68
3.15
0.31
2.88
1.50
3.51
0.33
2.17
6.21
2.64
4.40
2.81
5.35
0.20
0.06
b.d.
93.05
wt %
SiO2 (0.11)
TiO2 (0.02)
Al2O3 (0.04)
FeO (0.04)
MgO (0.01)
CaO (0.02)
Na2O (0.04)
K2O (0.03)
P2O5 (0.03)
0.07
0.16
97.15
0.16
0.08
97.65
0.14
0.27
93.57
0.14
0.19
99.03
0.15
0.35
Total
F (0.02)
93.25
0.18
98.45
0.14
Cl (0.01)
0.02
b.d.
0.02
b.d.
0.04
b.d.
90.44
b.d.
1.43
b.d.
0.15
0.02
CIPW norm (wt %)
Q
47.1
47.3
50.7
39.6
1.0
38.4
87.7
0
38.7
0.6
C
0
0
Or
1 .9
26.7
8.8
24.4
0
0
1.9
29.7
36.7
18.4
26.0
22.4
31.6
23.7
0.4
1.7
16.3
0 .6
13.8
0.4
14.6
0.3
2.1
1.1
2.8
0
0
1.7
0.2
Wo
0
0
0.2
0
0
0
0.2
Hy
0 .3
0
0 .4
0 .2
0.1
0.9
0.9
1.4
0.4
1.7
0
Il
0.6
1.5
0.2
0.8
0.6
0
Ab
An
Di
Ap
0.4
1.1
0.4
0
0.1
All Fe as FeO.
b.d., concentration below detection limit. Number in parentheses indicates analytical precision determined from microprobe
counting statistics. Concentrations of MnO and S were below detection limits for all samples. CIPW norm calculated with all
Fe as FeO. CIPW norm abbreviations: Q, quartz; C, corundum, Or, orthoclase; Ab, albite; An, anorthite; Di, diopside; Wo,
wollastonite; Hy, hypersthene; Il, ilmenite; Ap, apatite.
glass (4---8 wt %) and high CaO in the dark brown
glass (3---5 wt %). It is possible that the high-K glass
was produced by dehydration-melting reactions
dominated by biotite, whereas the high-Ca glass was
produced by reactions dominated by hornblende.
Compositionally, high-K glass data in Fig. 12 largely
overlap with the biotite dehydration-melting field in
Fig. 13, and high-Ca glass data overlap with the
compositional field for amphibole-derived melts.
Additionally, the high-K glass is more abundant than
the high-Ca glass.
Texturally, we find that an overprint of devitrification has obscured whether the light and dark brown
glasses were initially separate melts originating from
biotite and hornblende breakdown reactions, respectively. Light brown glass, which exhibits spherulitic
devitrification, surrounds cores of dark brown, largely
isotropic glass. This textural relationship occurs in melt
seams and adjacent to biotite and hornblende breakdown sites. We are currently unable to determine
whether the dark and light brown glass domains are
relicts of coexisting melts, or whether they are the
product of Ca---K migration during devitrification.
However, it is likely that locally controlled melting
reactions initially produced distinct melts of variable
composition, that these melts homogenized to some
degree during continued melting, and that they experienced devitrification after quenching.
2302
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
Fig. 12. Glass compositions in Stages 3---5. (a) Normative An (anorthite, CaAl2Si2O8)---Ab (albite, NaAlSi3O8)---Or (orthoclase, KAlSi3O8).
Fields after Barker (1979). (b) Normative Q (quartz, SiO2)---Ab---Or. Cotectic line for haplogranite system at 2 kbar and Xmelt(H2O) ˆ 0.7
(water undersaturated) after Holtz et al. (1992). Filled star marks position of the granite minimum. (c) TiO2 and FeO . The behavior of MgO
largely mimics that of FeO .
The clear granular domains observed in Stages 3---5
are nearly always associated with embayed to resorbed
quartz grains. Texturally, clear domains are somewhat
crystalline with undulose to patchy extinction. They
are composed essentially of SiO2, suggesting that they
represent fully (re)crystallized polymorphs of quartz.
It is unclear whether these domains were ever melted.
Clear glass localized on quartz fractures in Stage 2 is
probably a product of initial melting reactions involving quartz and feldspar.
To reconstitute the bulk melt composition from the
glass domains and quench crystals, we employed three
methods (Table 8). (1) A composite melt analysis was
achieved by averaging electron microprobe point analyses from a grid of evenly spaced points in a region
encompassing glass domains and quench crystals.
(2) Using a melt modal reconstitution, the melt composition was calculated by summing the compositions
of the devitrified glass domains and plagioclase, pyroxene, and magnetite quench crystals, each weighted
by their modal abundance. (3) Using a bulk modal
reconstitution, the difference between modally weighted composition of restite phases and whole-rock bulk
composition yielded the composition of the melt.
On the whole, we think the grid analysis average is
the best approximation of the real bulk composition,
because it is not subject to errors in modal estimates.
There is poor agreement among the three compositions
reconstituted for Stage 3 and good agreement for
Stage 5 (Table 8). The bulk modal reconstitution was
difficult to complete because of the variability in
mineral phase compositions, and due to the difficulty
in evaluating the mode of extremely fine mineral reaction products in decomposed biotite and hornblende
sites. For example, the differences in Stage 3 are probably due to difficulties in modal estimates, in particular
overestimation of the abundance of Fe---Ti oxides leading to high values of FeO in the reconstructed melt.
The reconstructed melt in Stage 3 is metaluminous to
mildly peraluminous (0.3 wt % normative corundum)
and granitic (Fig. 13a). Stage 5 reconstructed melt is
metaluminous and granodioritic to granitic (Fig. 13a).
2303
JOURNAL OF PETROLOGY
VOLUME 44
NUMBER 12
DECEMBER 2003
Fig. 13. Comparison between reconstructed melt compositions in Stages 3 and 5 and other natural and experimental melts. Field for
amphibole-bearing protoliths includes data from Beard & Lofgren (1991), Rapp et al. (1991), Rushmer (1991), and Pati~
no Douce & Beard
(1995). Field for biotite-bearing protoliths includes data from Kaczor et al. (1988), Vielzeuf & Holloway (1988), Kitchen (1989), Skjerlie &
Johnston (1992, 1993), Philpotts & Asher (1993), Pati~
no Douce & Beard (1995), and Singh & Johannes (1996a, 1996b). Individual data
points are shown for Skjerlie & Johnston (1996). (a) Normative An---Ab---Or of melts. (b) Normative Q---Ab---Or in melts. (c) TiO2 and FeO
in melts.
With increased melting, concentrations of SiO2 and
K2O decrease whereas concentrations of most other
components (notably Al2O3, CaO and FeO) increase
(Fig. 13).
The natural vs the experimental laboratory
The Maxwell dike locality bears out the conclusion
that progressive partial melting in crystalline rocks
produces a heterogeneous assemblage of compositionally variable melts, based on other examples of melting
(e.g. B
usch et al., 1974; Kaczor et al., 1988; Wolf &
Wyllie, 1991; Philpotts & Asher, 1993; Hammouda
et al., 1996; Knesel & Davidson, 1996; Rushmer,
2001). In contrast, dehydration-melting experiments
of powdered starting materials do not produce such
melt heterogeneity. Despite the inherent disequilibrium nature of melting in the natural examples, we
observe that the overall modal and compositional
record of progressive melting is similar to that of equilibrium experiments. In the following comparisons, we
bear in mind that the Wallowa tonalite differs from
most other melting studies of crustal rocks in that it
contains subequal proportions of amphibole and
mica, and that melting took place at51 kbar. However,
melting of granitoid protoliths appears to be insensitive
to pressures as great as 8 kbar (Singh & Johannes,
1996a), which suggests that the results of melting in
the Wallowa example are applicable to much of the
upper crust.
Comparison of mineral compositions
Plagioclase formed part of the parent rock in all granitoid natural and experimental protoliths, and residual
plagioclase was a component of restite assemblages
under nearly all pressure---temperature conditions up
to 10 kbar. In these studies, as in the Wallowa rocks,
2304
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
Table 8: Reconstructed bulk melt compositions
Stage 3
Stage 5
matrix
melt
bulk
bulk
matrix
melt
bulk
average1
mode2
mode3
mode4
average1
mode2
mode3
71.78
0.28
68.50
0.50
65.59
0.54
66.93
0.60
68.01
0.97
70.73
0.40
11.89
1.95
10.65
5.99
18.63
0.54
13.47
2.73
12.42
5.53
14.45
2.44
0.00
1.08
0.09
2.45
0.13
0.16
0.05
1.40
0.07
1.64
0.09
0.09
1.39
2.17
5.31
1.68
11.39
ÿ4.56
9.62
3.22
2.20
3.82
1.69
5.08
1.02
3.74
3.07
3.81
2.84
2.82
3.40
3.57
3.21
0.07
0.61
0.45
0.34
96.08
0.11
104.81
100.41
F
0.09
96.18
0.18
95.86
0.20
0.17
98.01
0.10
Cl
0.02
0.01
0.02
0.03
wt %
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
81.08
ÿ1.80
25.58
ÿ14.26
0.02
4.54
0.29
99.76
CIPW norm (wt %)
33.1
0.3
29.3
0
Ab
31.4
18.3
22.6
18.6
An
6.3
7.8
Di
0
Hy
Il
5.9
0.5
16.4
0.9
Ap
0.2
0.2
Q
C
Or
27.7
0
25.0
27.0
28.3
0
0
22.1
8 .6
22.5
26.0
0
20.1
23.9
19.0
30.2
35.2
1 .5
11.7
1.7
11.2
1.5
13.4
5.9
0
1 .0
6.8
1.2
12.0
1.8
1.1
0.8
1 .0
0.8
0.4
0.7
0
All
1
Fe as FeO. CIPW norm abbreviations same as for Table 7.
Average of 79 analyses in a 212 mm 387 mm grid for Stage 3, and average of 256 analyses in four 800 mm 800 mm
(approximately) grids for Stage 5.
2
Modal melt reconstitution based on proportions of melt domains and quench crystals as follows: dark brown glass : light
brown glass : clear granular glass : quench plagioclase : quench orthopyroxene : quench clinopyroxene : quench magnetite,
3.5:8.8:0.2:0.5:1.0:0:0.3 for Stage 3 and 4.2:25.9:1.1:5.3:1.3:0.6:0.9 for Stage 5. Numbers are given in vol. % of the bulk
rock.
3
Melt calculated by difference of bulk and restite given mode in Table 1 and average mineral compositions. Details have been
given by Petcovic (2000). F and Cl data not available for bulk analyses.
4
Melt calculated by difference of bulk and restite with mode given in Table 1 except 0.5% primary magnetite, 10% pyroxene
in hornblende reaction sites, 1.1% Fe---Ti oxides in biotite reaction sites, and 19% melt. Numbers are given in vol. % of bulk
rock.
plagioclase was consumed only at large extents of melting. Dissolution features, such as fritted margins and
spongy textures observed in reacted Wallowa plagioclase, were also observed by B
usch et al. (1974), Kaczor
et al. (1988), Green (1994), and Philpotts and Asher
(1993), who attributed fritted margins to melting along
cleavage planes. By Stage 5, Or and Ab components of
residual Wallowa plagioclase had increased, and relict
rims were high in An, FeO, and MgO. High-An rims in
relict plagioclase were also observed by B
usch et al.
(1974). Vielzeuf & Montel (1994) reported an increase
in the Or component of residual plagioclase, and many
studies (B
usch et al., 1974; Kaczor et al., 1988; Beard &
Lofgren, 1991; Philpotts & Asher, 1993; Singh &
Johannes, 1996b) observed an increase in An in residual plagioclase relative to starting plagioclase.
Amphibole dehydration-melting reactions produced
clinopyroxene ‡ lesser orthopyroxene in amphibole
reaction sites under nearly all pressure---temperature
conditions up to 10 kbar (e.g. Beard & Lofgren, 1991;
2305
JOURNAL OF PETROLOGY
VOLUME 44
Rapp et al., 1991; Rushmer, 1991; Pati~
no Douce &
Beard, 1995). In the Wallowa rocks, dehydrationmelting reactions involving hornblende initially produced aligned augite ‡ pigeonite ‡ lesser enstatitic
orthopyroxene ‡ sparse magnetite. Beard & Lofgren
(1991) observed enstatitic orthopyroxene, augite, and
sparse pigeonite as amphibole reaction products, but
their augite contained 55 wt % Al2O3. In contrast,
Wallowa augite contained up to 8 wt % Al2O3.
Studies involving biotite dehydration-melting reactions document orthopyroxene ‡ Fe---Ti oxides alkali feldspar (rarely ‡ amphibole) in biotite reaction
sites (e.g. B
usch et al., 1974; Kaczor et al., 1988;
Vielzeuf & Montel, 1994; Pati~
no-Douce & Beard,
1995; Singh & Johannes, 1996a; Rushmer, 2001).
Reactions involving biotite breakdown in the Wallowa
wallrock produced orthopyroxene ‡ plagioclase ‡
high-Ti magnetite ‡ sparse ilmenite. Similar to what
we observed from Stage 3 to Stage 5, Pati~
no Douce &
Beard (1995) and Singh & Johannes (1996b) observed
an increase in En component in orthopyroxene with
rising temperature. In contrast to other studies involving biotite breakdown, alkali feldspar was not
observed. It is possible that the high water content of
initial dehydration melts and low total pressure destabilized alkali feldspar as a reaction product [compare
experimental work by Naney (1983) and Johnson &
Rutherford (1989)]. On the other hand, hornblende
dehydration-melting reactions may have contributed
Ca to form plagioclase in biotite reaction sites.
Comparison of melts
In crystalline rocks such as the Wallowa tonalite, in situ
melt of multiple compositions (preserved as glass or
granophyre) has been reported as seams around crystals, along fractures within crystals, and localized
around decomposed mafic sites (e.g. B
usch et al.,
1974; Kaczor et al., 1988; Philpotts & Asher, 1993;
Green, 1994; Knesel & Davidson, 1996; Tommasini
& Davies, 1997; Rushmer, 2001). In the partially
melted granite studied by Kaczor et al. (1988), clear
glass (high SiO2, alkalis, and Rb) localized around
quartz was a product of biotite breakdown and early
quartz---feldspar melting. Brown glass (high CaO,
Al2O3, MgO, FeO, and TiO2), localized around oxides
and spongy feldspar, became more abundant at higher
extents of melting. Philpotts & Asher (1993) reported
two distinct melt compositions: a high-K glass located
between quartz and orthoclase, and a low-K glass
between quartz and andesine.
In their granite cube experiment, Knesel &
Davidson (1996) observed clear (trachytic) glass locally
grading into brown (mugearitic) glass that surrounded
Fe---Ti oxides replacing biotite. Rushmer (2001)
NUMBER 12
DECEMBER 2003
observed voluminous, granitic glass derived from muscovite breakdown along grain boundaries and cracks
in quartz grains, with minor, biotite-derived glass
(higher FeO and TiO2) localized around spinel in
biotite reaction sites. Locally, melt compositions
mixed along grain boundaries, producing an intermediate-composition glass. Previous workers have
pointed out that progressive disequilibrium melting of
this type produces melts that were initially enriched in
many incompatible elements, particularly Rb as well
as 87 Sr/86 Sr derived from the breakdown of biotite--hornblende, whereas successive melts were enriched in
the restite component (particularly Sr) released with
plagioclase (e.g. Kaczor et al., 1988; Hammouda et al.,
1996; Knesel & Davidson, 1996; Tommasini & Davies,
1997; Davies & Tommasini, 2000).
Similar to other studies, melt in the Wallowa tonalite
occurs on quartz---plagioclase boundaries, trapped
within spongy plagioclase, and around decomposed
hornblende and biotite sites. The heterogeneity in
quenched melt from the Wallowa samples may, in
part, be attributable to devitrification textures; however, comparison with other studies suggests that local
reactions also played a role. Dehydration-melting reactions dominated by either biotite or hornblende could
account for the high-K and high-Ca brown glasses,
respectively. Sparse clear (Stage 2) glass, which we
have not analyzed, probably represents an early product of melting on quartz---feldspar boundaries, similar
to the clear glass observed by Kaczor et al. (1988).
The Stage 3 bulk (reconstructed) melt was produced
by dehydration-melting reactions involving biotite,
hornblende, plagioclase, quartz, and orthoclase.
Although biotite and hornblende are modally subequal in the unmelted Wallowa tonalite, the Stage 3
bulk melt lies within the field for melts produced from
biotite-bearing protoliths (Fig. 13). Biotite dehydrationmelting reactions, however, may produce 2---3 times
more melt than amphibole dehydration-melting
reactions (Pati~
no Douce & Beard, 1995). The composition of Stage 5 bulk melt (Fig. 13) reflects continued
melting and the consumption of clinopyroxene and
plagioclase.
Overall, the bulk Wallowa melts are metaluminous
to barely peraluminous, in contrast to peraluminous
(1---5% normative corundum) melts produced from
biotite-bearing protoliths. Melts from amphibolebearing protoliths are trondhjemitic to granodioritic,
metaluminous to peraluminous (0---7% normative corundum), silicic, and low in mafic oxides; Wallowa
melts differ in that they are more K-rich and Al-poor
(Fig. 13). Our bulk melt composition reflects the involvement of both biotite and hornblende in dehydrationmelting reactions. With increased degree of melting
(Stage 3 to Stage 5), Wallowa melts become slightly
2306
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
Fig. 14. Results of thermal model predicting wallrock heating due to dike flow followed by dike and wallrock cooling. Parameters include:
initial wallrock temperature 55 C, dike temperature 1140 C, dike thickness 8 m, a ˆ 4.11 10 ÿ7 . Additional details are given in the
Appendix. (a) Part A of the thermal model. Temperature plotted against distance from the dike center as a function of time simulating the
period when the dike was active. Square symbols and vertical error bars indicate average and range (respectively) of results for Fe---Ti oxide
thermometry (Table 6). Range of temperatures for hornblende and biotite breakdown are from comparison with partial melting experiments
(see text). Horizontal error bars reflect typical thickness for each stage of melting (1 m). (b) Part B of the thermal model. Temperature
plotted against distance from the dike center as a function of time once basalt flow has ceased. Dashed line is initial isotherm, equivalent to 3.6
years of heating in Part A. (c) Enlargement of framed area shown in (b).
more mafic, more aluminous, less potassic, and less
silicic, as is consistent with other partial melting studies. Overall, Stage 3 Wallowa bulk melt is similar to
average A-type granite (as compiled by Skjerlie &
Johnston, 1992) and some high-silica rhyolites (see
Streck, 2002). Stage 5 Wallowa bulk melt is similar to
metaluminous rhyodacites thought to be mainly of
a crustal-melt origin (see Feeley & Grunder, 1991;
Johnson & Grunder, 2002).
Thermal history and implications of diking
Thermal conditions of melting
Temperatures recorded by minerals in the melt zone
represent an integrated thermal effect from heating
during diking followed by cooling when basalt flow
ceased. The maximum thermal gradient at the edge
of the Maxwell Lake dike was about 1090 C, assuming
a geothermal gradient of 25 C/km, a depth of 2 km to
the dike, and a basalt magma temperature of 1140 C
[liquidus temperature from Murase & McBirney
(1973)]. The wallrock could have been hotter than
estimated if it had been subjected to heating by previous dike intrusion. Mineral thermometry and comparison with experimental phase equilibria have been
used to estimate the temperature conditions represented by each progressive melting stage (Fig. 14a).
The small grain size and heterogeneity of Fe---Ti
oxides compromise their use in geothermometry.
Nevertheless, potential equilibrium pairs of magnetite
and ilmenite were identified in Stages 3 and 4 on the
basis of the Mg---Mn partitioning criteria of Bacon &
Hirschmann (1988) (Table 6). Stage 3 magnetite--ilmenite pairs give a range of values with an average
temperature of about 830 C (Fig. 14a). Stage 4 pairs
2307
JOURNAL OF PETROLOGY
VOLUME 44
suggest an average temperature of 1060 C (Fig. 14a).
Stage 5 contained no equilibrium pairs. Pyroxene
pairs did not produce equilibrium tie lines (after
Lindsley, 1983), and were therefore not successful for
thermometry.
Comparison with experimental phase equilibria
from intermediate composition protoliths can help constrain the temperature at which melt-forming reactions
took place. From a series of low-pressure experiments
designed to locate the solidus of a synthetic biotite
tonalite, Singh & Johannes (1996a, 1996b) concluded
that the onset of melting in biotite-bearing rocks may
be as low as 700 C. However, 1---3 kbar melting experiments on a meta-greywacke indicated a biotite breakdown range of 800---860 C (Vielzeuf & Montel,
1994), and 3 kbar experiments on a synthetic biotite
tonalite indicated a range of 850---930 C (Pati~
no Douce
& Beard, 1995). We have chosen an initial biotitebreakdown temperature of 800 C, which is therefore a
minimum estimate for Stage 2 where the dehydrationmelting of biotite produced thin glass seams (Fig. 14a).
Under pressure conditions of 5 kbar, hornblende in
most intermediate composition amphibolites reacted
out between about 850 and 925 C (Beard & Lofgren,
1991; Pati~
no Douce & Beard, 1995). Because Stage 3
lacks both biotite and hornblende, temperature at this
location probably reached at least 925 C (Fig. 14a).
Thermal model of wallrock melting
We have constructed a one-dimensional thermal model
to estimate how long basalt flowed in the dike to provide sufficient thermal flux to the wallrock to propagate partial melt zones and related temperatures as far
as 4 m from the dike---wallrock contact (additional
details are given in the Appendix). In Part A of the
model (Fig. 14a), the dike was held at a constant
temperature of 1140 C to simulate magma flow while
the dike was active. The final conditions of this simulation provided the initial conditions for Part B (Fig. 14b
and c), in which magma flow has ceased and the dike
and wallrock were allowed to cool. Both parts of the
model assume all heat flow was via conduction. Additionally, we assume that the latent heat of crystallization of the basalt was equal to the latent heat of melting
of the wallrock, and that both were released linearly
over the entire temperature interval. Latent heat of
crystallization of the wallrock was neglected.
Results for Part A of the model (for an intermediate
thermal diffusivity value of 4.11 10 ÿ7 m2 /s; Fig. 14a)
suggest that tonalite located within the first few centimeters of the dike---wallrock contact began to undergo
partial melting within a few hours of dike intrusion.
This timescale is similar to that of experimental studies,
such as the water-saturated melting experiments of
NUMBER 12
DECEMBER 2003
B
usch et al. (1974) and the granite cube experiments
by Knesel & Davidson (1996), where small volumes of
rock underwent melting over timescales of hours to
days. Within about 1 year of dike injection, all Stage 5
wallrock (0---1 m from the dike---wallrock contact) had
reached temperatures in excess of 925 C. Stage 4 wallrock (1---2 m from the contact) had begun to melt after
about 1 year, whereas Stage 3 wallrock (2---3 m from
the contact) underwent melting after nearly 3 years.
Results suggest that about 4 years were required for
rocks at 4 m from the dike---wallrock contact to reach
800 C. This, therefore, is the maximum time that the
dike was active; wallrock beyond this distance shows
no evidence of biotite breakdown or onset of partial
melting.
Results of modeling the cooling history of both the
dike and wallrock are shown in Fig. 14b. Because the
heat pulse continued to propagate during cooling,
wallrock further than about 3 m from the contact
experienced additional heating. However, at 4 m
from the contact (the critical transition between
unmelted and partially melted wallrock), there was
only about 10 C of additional heating (Fig. 14c)
between 0.05 and 1 year following cessation of dike
flow. Isotherms were steep enough so that wallrock at
distances 44 m never experienced temperatures in
excess of 800 C and therefore experienced no partial
melting.
We found that both parts of the model were sensitive
to thermal properties of the dike and wallrock
[expressed as the thermal diffusivity value (a)]. For
example, a minimum thermal diffusivity value (1.22 10 ÿ6 m2 /s) yielded model results indicating that wallrock at 4 m from the contact began to break down after
1.2 years. A maximum thermal diffusivity value (1.38 10 ÿ7 m2 /s) suggested that breakdown began after
about 10.6 years. The model was rather insensitive to
dike thickness. Part A was independent of dike thickness, whereas both dike and wallrock cooled more
slowly with a thicker dike in Part B, yet the shape of the
isotherms remained largely unaffected. For example, a
dike of 16 m thickness was 200 C warmer after 20 years
of cooling than the results shown in Fig. 14b.
The 4 year heating interval implied by this model is a
maximum time if the wallrock were preheated by prior
dike activity. Preheating the wallrock to 200 C at the
time of dike injection allowed temperatures to reach
800 C at 4 m from the contact in 2.6 years, one year
faster than with wallrock at 55 C. On the other hand,
it is more likely that the 4 year timescale is a minimum
if intermittent flow in the dike or cooling by means
other than conduction occurred. Long pauses in flow
of the dike could potentially cause complex overprints
of melting and crystallization reactions. Although we
cannot preclude some fluctuations, the regular textural
2308
PETCOVIC AND GRUNDER
MELTING AT BASALT DIKE MARGIN
progression observed in the melt zone is consistent with
continuous or pulsating flow. Cooling of dike and wallrock by means other than conduction, such as cooling
by circulating groundwater [as suggested by Delaney
(1987)], may account for the rapid cooling history
implied by the presence of glass and quench crystals
in the partial melt zones. If such cooling took place, the
calculated heating times based on conductive cooling
are minima.
Implications of the thermal model for CRBG volcanism
Dikes with substantial wallrock melting are rare in the
Wallowas; in mapping of four sub-swarms we have
found only two dikes with well-developed wallrock
partial melt zones. In both cases where there is substantial partial melt in wallrock adjacent to a dike, the
dike is not prominently quenched. Instead, coarsegrained basalt extends to the dike---wallrock contact.
The thermal model of the Maxwell Lake dike suggests
that magma flowed through this dike for at least several years. We believe that, in general, the higher
thermal flux experienced by dikes with partial melt at
their margins indicates that they had a prolonged history of activity and therefore were more likely to have
fed major CRBG flows. Dikes with quenched margins
and no interaction with the wallrock, on the other
hand, probably represent conduits used for short periods of time before solidifying. We take this to be analogous to Hawaiian-style eruptions where early dike-fed
fissure eruptions become localized to yield long-lived
central vent eruptions that feed flows.
CONCLUSIONS
In rare instances, wallrock adjacent to Columbia River
Basalt Group dikes in the Wallowa Mountains has
undergone partial melting, providing a unique opportunity to examine crustal melting in a natural setting.
The unmelted wallrock is a hornblende---biotite granodiorite to tonalite, a lithology that is rarely examined in
the experimental partial melting literature but is probably common in natural settings. Samples collected
from the margin of a Grande Ronde dike at Maxwell
Lake represent progressive stages of closed-system partial melting over a distance of about 4 m from unmelted
tonalite (Stage 1) to about 47 vol. % quenched melt
(Stage 5). Partial melt reactions took place at a paleodepth of about 2---2.5 km.
With the onset of melting, a trace amount of a clear
melt was produced, now preserved along fractures in
quartz. Dehydration-melting reactions involving both
biotite and hornblende, plus plagioclase, orthoclase
and quartz, produced melt (preserved as variably devitrified glass and quench crystals) localized around
decomposed mafic sites, on quartz---feldspar boundaries and in spongy plagioclase. Comparison with
other natural examples and experimental work indicates that a dominant high-K (light brown) glass
resulted from biotite dehydration-melting, leaving
aligned magnetite and ilmenite intergrown with
plagioclase and orthopyroxene in the former biotite
sites. A less abundant high-Ca (dark brown) glass was
produced during dehydration-melting of hornblende
leaving a dusty intergrowth of clinopyroxene, lesser
orthopyroxene, and sparse magnetite in former hornblende sites. Approximately 18 vol. % melt were produced in these early stages of melting. Up to 29 vol. %
additional melt were produced by the reaction of
orthoclase, clinopyroxene, quartz, plagioclase and
magnetite. This reaction was terminal for both orthoclase and clinopyroxene, leaving optically aligned
orthopyroxenes in former hornblende sites. With progressive melting, phases being consumed became strikingly more heterogeneous in composition, whereas
reaction products were relatively homogeneous. The
progress of disequilibrium melting reactions as well as
the composition of reaction products are broadly similar to those observed in other natural and experimental
case studies.
The bulk composition of the reconstructed early
melts was granitic and metaluminous to barely peraluminous and closely approximates the composition of
many A-type granites. With progressive reaction, the
melt became more granodioritic and metaluminous
and closely mimics the composition of rhyodacitic volcanic rocks thought to be the products of crustal melting. In general, the Wallowa bulk melt, produced
by simultaneous dehydration-melting of biotite and
hornblende, was intermediate in composition between
granitic peraluminous liquids produced from biotite
dehydration melting and the tonalitic liquids produced
from amphibole dehydration melting.
Thermal modeling suggests that for a dike of 8 m
thickness carrying magma at 1140 C, about 4 years
were required to initiate breakdown reactions in wallrock at a distance of 4 m from the dike---wallrock contact. Depending on the choices of thermal properties of
the dike and host rock, 1---10 years were required to
initiate the melting reactions at 4 m. If flow of magma
in the dike was intermittent, or cooling was enhanced
by groundwater circulation, then the dike could have
been active longer. We think that dikes with substantial partially melted margins represent long-lived portions of the Columbia River Basalt feeder system and
may have sustained large flows, possibly analogous to
Hawaiian-style central vent eruptions. In contrast, the
majority of dikes, which are quenched against the wallrock, represent dike propagation and fissure eruption
events that were short-lived.
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ACKNOWLEDGEMENTS
We would especially like to thank Bill Taubeneck for
his assistance with the petrographic analysis and his
guidance on the field component of this research. We
would also like to thank Roger Nielsen of the Oregon
State University Electron Microprobe Laboratory for
his advice on and assistance with data collection.
George Bergantz and Roy Haggerty provided valuable
ideas and input for the thermal modeling. Discussions
with Peter Reiners, John Dilles, and Joe Dufek helped
to clarify this work. Reviews by Tracy Rushmer and
Mike Williams substantially improved the manuscript.
Thanks go to Mike Winkler, Brandon Browne, Jesse
Dickinson, Lang Farmer, and George Bergantz for
participation in field work. This research was supported in part by the Geological Society of America
grant number 6514-99 awarded to H.L.P.
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APPENDIX: THERMAL MODEL
Symbols used in model
Symbol
Units
Value(s) used in model
4
1.38 10 ÿ7 , 4.11 10 ÿ7 , 1.22 10 ÿ6
a
distance from dike center (x ˆ 0) to dike---wallrock contact (x ˆ a)
m
a
modified thermal diffusivity
m2 /s
Cp
specific heat capacity
J/kg K
1000, 2000, 3000
r
densityy
kg/m3
2400
erfc
complementary error function
fs
solid fraction in magma
none
k
conductivity
J/m s K
1, 2, 3
L
latent heat of crystallization/melting
J/kg
30 000
t
time
s
y
dimensionless temperature y ˆ …TÿTw†=…TdÿTw†
none
Td
initial dike temperature
C
1140
Tw
initial wallrock temperature
C
55
wr
wallrock value
x
distance normal to dike-wallrock contact from dike center
Values used to calculate a for model results shown in Fig. 14 are in bold.
yIntermediate value used for both dike and wallrock.
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VOLUME 44
Part A
To simulate heating of wallrock while magma is flowing through the dike, we model the system as heat flow
via conduction in a semi-infinite solid (e.g. Carslaw &
Jaeger, 1959). The general heat equation, given an
initial condition of T(x 4 a, 0) ˆ Tw and boundary
conditions of dike margin constant at Td [T(x ˆ a, t) ˆ
Td], and T must be Tw at infinity [T(x ! 1, t) ˆ 0] is
@T
@2T
dfs
rwr Cpwr
ˆ kwr 2 ‡ rwr Lwr
:
@t
@x
dt
…A1†
Because the solid fraction is a function of temperature,
the heat equation is rewritten as
rwr Cpwr
@T
@2T
dfs @T
ˆ kwr 2 ‡ rwr Lwr
:
@t
@x
dT @t
…A2†
Introducing dimensionless temperature yields
2
dfs
@y
Lwr
ˆ kwr @ y :
rwr Cpwr ‡
…Td ÿ Tw† dy @t
@x2
NUMBER 12
DECEMBER 2003
Assuming that fs is linear in y (i.e. that jdfs=dyj ˆ 1),
the solution is
xÿa
y…x4a, t† ˆ erfc p
…A4†
2 at
where
aˆ
kwr
:
rwr ‰Cpwr ‡ Lwr =…Td ÿ Tw†Š
This equation was solved and plotted using the
program Mathematica 4.2.
Part B
To simulate cooling of the dike and wallrock, we model
the system as an infinite solid. At t ˆ tp, dike injection is
stopped, and dike and wallrock are allowed to cool.
The general solution to equation (A4) at t ˆ tp is (after
Carslaw & Jaeger, 1959)
"
#
Z 1
1
ÿ…x ÿ x0 †2
0
y…x, t† ˆ p
dx0,
yp…x †exp
4a…t ÿ tp†
2 pa…t ÿ tp† ÿ1
t4tp:
…A3†
…A5†
…A6†
This equation was solved by numerical integration
and plotted using Mathematica 4.2.
2312