JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
PAGES 303^325
2007
doi:10.1093/petrology/egl062
The Graveyard Point Intrusion: an Example
of Extreme Differentiation of Snake River
Plain Basalt in a Shallow Crustal Pluton
CRAIG M. WHITE*
GEOSCIENCES DEPARTMENT, BOISE STATE UNIVERSITY, BOISE, ID 83725 USA
RECEIVED AUGUST 29, 2005; ACCEPTED SEPTEMBER 28, 2006;
ADVANCE ACCESS PUBLICATION NOVEMBER 8, 2006
The Graveyard Point intrusion is the only known example of a well-exposed differentiated mafic pluton associated with the late
Miocene^Pleistocene magmatism of the western Snake River Plain
(SRP). It is exposed in a 6 km by 4 km area adjacent to the Oregon^
Idaho border, and exposures range in thickness from 20 to 160 m.The
thicker parts of the intrusion are strongly differentiated and contain a
25^60 m thick section of well-laminated cumulus-textured gabbros
that grade upward into pegmatoidal ferrogabbro. Evolved liquids
formed sheets of Fe-rich siliceous granophyre. At least two injections
of magma are indicated by abrupt discontinuities in the rock and
mineral compositions, and by the lack of mass balance between the
bulk intrusion and its chilled borders. The laminated gabbros are
interpreted to have formed from a tongue of augite and plagioclase
crystals that were carried in with the second pulse of magma.
Following the final emplacement of the intrusion, in situ differentiation proceeded through a two-stage process: the ferrogabbros are
explained as interstitial liquids forced out of the crystal mush by compaction, and the siliceous granophyres are interpreted to be residual
liquids that migrated out of the partly crystallized ferrogabbros in
response to the exsolution of volatiles. Because the geochemical trend
inferred for the mafic to intermediate composition liquids in the
Graveyard Point intrusion is similar to the trend for many western
Snake River Plain lavas, the pluton may be a good model for shallow
sub-volcanic magma chambers elsewhere in the SRP. However,
some western SRP lavas contain anomalously high concentrations
of P2O5 , which are best explained by mixing within the active crustal mush column or with partial melts of previously formed
differentiated mafic intrusions.
I N T RO D U C T I O N
granophyre
Recent papers by Marsh (2000, 2004) emphasized that
magmas evolve in response to combined physical and
chemical processes that occur over a wide range of
scales in spatially integrated intrusive complexes referred
to as ‘magmatic mush columns’. However, in most young
volcanic provinces the intrusive parts of this system are
hidden and their petrological features can be only
inferred from petrographic, chemical and isotopic studies
of lavas and tephra. The Snake River Plain (SRP) of
Idaho and eastern Oregon is one such young province
where extensive complexes of crustal level, possibly interconnected, mafic intrusions have been indicated by geophysical evidence, but remain largely unknown because
they have not been exposed by erosion (Mabey, 1982;
Sparlin et al., 1982; Peng & Humphreys, 1998). The SRP
contains a compositionally diverse suite of mainly tholeiitic lavas, many of which appear to have evolved by
fractional crystallization over a range of crustal depths
(e.g. Leeman & Vitaliano, 1976; Leeman, 1982; Reid,
1995; Geist et al., 2002). For this reason, the concept of a
magmatic mush column may be particularly applicable
to this province, and knowing the nature of the plutonic
system becomes a critical part of understanding the magmatic system. To that end, this paper presents results of
the first detailed study of a western SRP intrusive complex and suggests a physical model for the evolution of
magmas in the upper part of the mush column beneath
this well-known volcanic province.
*Corresponding author. Telephone: 208-426-3633.
E-mail: [email protected]
ß The Author 2006. Published by Oxford University Press. All
rights reserved. For Permissions, please email: journals.permissions@
oxfordjournals.org
KEY WORDS:
Snake River Plain; mafic intrusions; tholeiitic; sill;
JOURNAL OF PETROLOGY
VOLUME 48
GEOLOGIC A L S ET T I NG
The western SRP is a NW-trending intracontinental rift
basin about 70 km wide and 300 km long (Fig. 1a). The
boundary faults are parallel to major structural trends in
the Pacific Northwest, but perpendicular to the assumed
NE-trending track of the Yellowstone hotspot (Pierce &
Morgan, 1992; Smith & Braile, 1994). Although there is no
consensus on the origin of the western SRP, many workers
have suggested that the 17^14 Ma episode of voluminous
magmatism attributed to the Yellowstone hotspot either
directly or indirectly initiated the formation of the western
SRP (e.g. Geist & Richards, 1993; Glen & Ponce, 2002;
Shervais et al., 2002). The rifting and subsidence that
appears to have begun about 10 Ma is commonly attributed to regional extension that occurred throughout the
northern Basin and Range Province (e.g. Hooper et al.,
2002). Basaltic magmatism in the western SRP began
with the onset of extension and has produced a diverse
suite of tholeiitic lavas, many of which are strongly
enriched in iron (FeO 414%) (Bonnichsen & Godchaux,
2002). More than 2 km of sediments fill the deepest parts
of the western SRP basin in southwestern Idaho (Wood,
1994), but the basin shallows rapidly to the west and dies
out in eastern Oregon where it abuts the older, north^
south-trending Oregon^Idaho graben (Cummings et al.,
2000). Erosion has exposed the Graveyard Point intrusion
because it was emplaced near the western margin of the
plain where the least amount of subsidence has occurred.
G E O L O G Y O F T H E I N T RU S I O N
Age and field relations
Mafic intrusive rocks forming the Graveyard Point complex are exposed in a series of discontinuous outcrops
within an area of c. 6 km by 4 km. The magmas were
emplaced into middle Miocene fluvial and lacustrine
sediments and silicic pyroclastic rocks originally mapped
by Kittleman et al. (1967) as part of the Sucker Creek
Formation. Ferns (1989) cited a K^Ar whole-rock date of
67 04 Ma for diabase from the lower part of the main
intrusion, a value that is only slightly younger than
40
Ar^39Ar ages of basalts of similar composition erupted
along the southern margin of the western SRP in Idaho
(White et al., 2002). Mafic rocks of the Graveyard Point
complex occur in three types of exposures: (1) 1^10 m
wide dikes of olivine diabase; (2) 20^30 m thick sills,
also composed of olivine diabase; (3) 100^160 m thick,
irregularly shaped but generally sheet-like intrusions
composed of olivine diabase, gabbroic cumulates, ferrogabbro and granophyre. Chilled borders in all of the floored
intrusions are chemically and petrographically similar to
one another and to the fine-grained diabase in the dikes.
The field relations are consistent with a structural
model in which all exposures of the sill-like bodies are
NUMBER 2
FEBRUARY 2007
part of a single, wedge-shaped intrusion that was offset
by normal faults.
Igneous stratigraphy
The thickest and most complete sections through the
intrusion are located near its eastern end where contacts
at both the roof and floor are exposed at several localities.
Measured stratigraphic columns were constructed at three
sites in this area, and their locations are shown in Fig. 1b.
Specimens for thin sectioning were collected at vertical
intervals of 2^5 m at each locality. Section A^A0 contains
an exposure of 135 m thickness in which the lower contact
is visible but an unknown thickness of the upper part of
the intrusion has been removed by erosion. Section B^B0
contains a complete section of 150 m thickness through
the intrusion, although the upper contact is exposed
about 100 m south of the rest of the measured section.
Both of these sections contain several texturally and
petrographically distinctive sub-horizontal lithological
layers that are described in detail below. The intrusion
thins to the south and is only 24 m thick at section C^C0,
where it consists entirely of olivine diabase. The stratigraphic columns for sections A^A0, B^B0 and C^C0 are
shown in Fig. 2. Rock exposures between sections A^A0
and B^B0 are nearly continuous and the distinctive lithological units in this part of the intrusion can be correlated
with confidence; however, the interval between these sections and section C^C0 is not exposed and the pinching
out of these units shown in Fig. 2 is speculative. The
modal abundances of plagioclase, pyroxene, olivine and
interstitial granophyre in specimens collected at section
B^B0 are shown vs stratigraphic height in Fig. 3. Modal
analyses of representative specimens from the major lithological units within the intrusion are given with the
bulk-rock chemical analyses in Table 1.
Chilled borders of fine-grained, intergranular microgabbro are well developed next to the upper and lower contacts wherever they are exposed. Plagioclase (An81) and
olivine (Fo79) are the only phenocrysts in these rocks;
they make up about 15^20% of the mode and commonly
occur together in glomeroporphyritic clots up to 4 mm
across (Fig. 4a). In all sections, the lower chilled border
grades upward into fine- to medium-grained, dense, black,
subophitic- to ophitic-textured diabase in which olivine
and plagioclase are partly or completely enclosed by optically continuous anhedral crystals of augite. Section C^C0
is composed entirely of this rock type, but in the thicker
parts of the intrusion (sections A^A0 and B^B0 ) a noticeable change in the petrographic character of the rocks
occurs at about 10 m above the floor. At this height, augite
appears for the first time as discrete subhedral crystals
and the rocks become distinctly granular in texture, with
augite and olivine typically occurring in crystals up to
3 mm across and plagioclase crystals generally being
slightly smaller. This petrographic boundary also marks a
304
WHITE
THE GRAVEYARD POINT INTRUSION
Fig. 1. (a) Map showing the location of the Graveyard Point intrusion and the general outline of the western Snake River Plain in southwestern
Idaho and eastern Oregon. Inset illustrates the track of the Yellowstone hotspot from southeastern Oregon to its present location beneath
Yellowstone National Park (Y). (b) Simplified geological map of the eastern half of the Graveyard Point intrusion [modified from Ferns
(1989)], showing the locations of the three measured sections. Circles indicate the locations of samples from the chilled borders of the intrusion;
the numbers refer to analyses given in Table 1.
shift in the chemical compositions of the rocks, which is
discussed below. At 30 m above the lower contact, the
gabbros take on a moderately to strongly sub-horizontal
igneous lamination caused by the shape-preferred orientations of augite and plagioclase (Fig. 4b). These rocks are
coarser grained than the granular gabbros, with crystals
typically being 5^8 mm in length. Augite and plagioclase
are tabular in shape and form compact frameworks of
touching subhedral to euhedral grains typical of cumulates
(Irvine, 1982). Imbricate textures (tiling) are common,
suggesting that the planar fabrics were caused at least in
part by laminar flow (e.g. Nicolas, 1992; Shelley, 1993).
Near the middle of the intrusion, about 55 m above the
floor, the rocks change from laminated gabbro to patchy
textured coarse-grained ferrogabbro having no preferred
orientation of crystals. Some zones within this unit contain
crystals as much as 4 cm in length. In thin sections the
ferrogabbros are composed of a porous framework of
plagioclase, monoclinic pyroxene, olivine, apatite and
magnetite surrounding interstitial pools of fine-grained
granophyre and granophyre-lined vesicles (Fig. 4c).
The oxides commonly occur as skeletal crystals. Within
this unit are one or more lens-shaped layers in which the
granophyre forms as much as 50% of the total rock.
These exceptionally granophyre-rich zones, which have
the bulk composition of ferrodiorite, are easily recognized
in the field because they weather to a reddish brown
color, contain abundant vesicles, and have a distinctive
brecciated appearance (Fig. 5a). The granophyre in these
rocks is extremely fine-grained and contains swallowtailed or hollow-centered crystals indicative of rapid
crystallization (Lofgren, 1980); the coarse-grained parts
consist of clusters of minerals that are petrographically
similar to those in the surrounding ferrogabbro.
Overlying the ferrogabbro and ferrodiorite is a sequence
of much less differentiated, poikilitic-textured olivine
gabbros, which typically consist of small (1mm)
subhedral crystals of plagioclase surrounded by much
larger (1cm) anhedral poikilitic crystals of augite or,
less commonly, olivine (Fig. 4d). This rock type is not
present in section A^A0 , where the upper part of the intrusion has been removed by erosion. The poikilitic textures
305
JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
FEBRUARY 2007
Fig. 2. Stratigraphic columns through the Graveyard Point intrusion showing the vertical distribution of rock types at measured sections A^A0 ,
B^B0 and C^C0 (Fig. 1). Correlations of rock units between sections A^A0 and B^B0 can be made with confidence, but units cannot be traced in
the field between sections B^B0 and C^C0. The vertical exaggeration is 7.
of these rocks and their position in the upper part of the
intrusion suggest that they may form an upper border
series that crystallized in situ near the roof. Where the
upper contact of the intrusion is preserved, the poikilitictextured gabbro is overlain by a few meters of dark gray,
fine-grained, olivine diabase similar to the rocks adjacent
to the floor.
Distinct veins, sheets and pods of granophyre intrude
all the other rock types except the olivine diabase in
the lower 10 m of the intrusion. Granophyre is present
as millimeter- to centimeter-wide veins in the laminated
gabbro (Fig. 4b) and forms cross-cutting and conformable
sheets as much as 30 cm thick in the upper one-third of the
intrusion (Fig. 5b). At one locality, poikilitic gabbro is cut
by an irregularly shaped, diapir-like, granophyre body
about 5 m across. Medium- to coarse-grained granophyres
are similar to the ferrogabbros in appearance and
composition, and generally contain less than 52% SiO2.
Fine-grained granophyres have higher silica contents,
up to 68% in one dike, and contain small crystals
of sodic plagioclase, ferroaugite, magnetite and apatite,
pigeonite, fayalitic olivine, in a fine-grained matrix
of intergrown quartz and alkali feldspar. Although discrete
granophyre bodies are most abundant in the interior of
the intrusion, a 1m thick sheet of fine-grained granophyre
also occurs for about 100 m along the roof of the intrusion,
where it is in contact with hornfelsed country rock.
G E O C H E M I S T RY
Analyses of bulk-rocks and small veins
About 100 samples from all parts of the intrusion and nine
specimens from the wall-rocks adjacent to the roof
and floor were analyzed for major elements and 14 trace
elements by X-ray fluorescence spectrometry (XRF)
using the Rigaku 3370 system at the Washington State
University Geoanalytical Laboratory (the WSU laboratory
has since replaced this instrument). Sample preparation
methods, operating conditions and statistics for these
analyses have been described by Johnson et al. (1999).
Twenty-six samples were analyzed for additional trace
elements by inductively coupled plasma mass spectrometry
(ICP-MS) using the HP-4500þ instrument at the WSU
laboratory. Technical notes for the ICP-MS analyses
are available on the WSU Geoanalytical Laboratory
web site. Representative chemical analyses of bulkrock specimens are given in Table 1, along with data on
analytical precision. In addition, the granophyre in several
small (51mm wide) veins in thin sections of samples
from the laminated gabbro and ferrogabbro was analyzed
for eight major oxides with the Cameca Camebax
electron microprobe at Washington State University.
These analyses were made using the Kevex spectrometer,
with a beam width of 100^300 mm and an accelerating
voltage of 20 kV.
306
WHITE
THE GRAVEYARD POINT INTRUSION
Fig. 3. Distribution of modal olivine, Ca-rich pyroxene, plagioclase, and interstitial granophyre in samples from section B^B0. Line A marks the
first appearance of pyroxene as discrete subhedral crystals; line B indicates the base of the laminated gabbros.
Analyses of four samples from the chilled borders
collected at various locations around the intrusion are
given in Table 1 (analyses 1, 18, 19, 20). The compositions
cluster at the boundary between olivine tholeiite and
quartz tholeiite, according to the normative classification
system of Yoder & Tilley (1962), and plot within the
field of low-K, high-Ti transitional basalts (LKTB) on the
MgO^TiO2^K2O diagram of Hart et al. (1984). This composition is intermediate between mid-ocean ridge basalt
(MORB)-like high-alumina olivine tholeiites (HAOT)
from the northwestern margin of the Basin and Range
province and the more evolved tholeiites typical of the
younger (53 Ma) basalts in the western Snake River Plain
(SROT). The intrusion’s chilled margin is, however,
very similar in composition to the earliest erupted
basalts (6^8 Ma) in the western Snake River Plain
(White et al., 2002).
When all of the analyses of bulk-rock specimens and
granophyre veins are plotted on an AFM diagram
(Fig. 6a), they form a well-defined tholeiitic trend of iron
enrichment followed by iron depletion and enrichment in
alkalis. Concentrations of FeOT peak at around 19% in
the ferrogabbros, well below the maximum iron enrichment of the Skaergaard liquid (McBirney, 1996), but
similar to the maximum values found in volcanic glasses
(e.g. Brooks et al., 1991) and close to the limit of iron enrichment predicted by the fractionation models of Toplis &
Carroll (1996). Silica contents range between 45 and 51%
for all analyzed rock specimens with the exception of those
collected from the granophyre-enriched, mixed-textured
ferrodiorites and the discrete granophyre intrusions
(Table
1).
Analyses
recalculated
to
CMAS
end-members are plotted on the 1 atmosphere Ol^Di^Q
pseudoternary diagram in Fig. 6b. The chilled borders
307
JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
FEBRUARY 2007
Table 1: Chemical analyses and modes of bulk rocks from the Graveyard Point intrusion
Specimens collected from measured section B–B0
Analysis:
1
2
3
4
5
6
Strat ht. (m):1
01
44
69
121
171
256
303
Rock unit:
chilled
olivine
olivine
granular
granular
granular
laminated
diabase
diabase
gabbro3
gabbro
gabbro
gabbro
GP-303
GP-340
GP-339
GP-338
GP-337
GP-335
GP-3
SiO2
4908
4825
4922
4887
4946
4851
4974
Al2O3
1579
1577
1696
1366
1511
1537
1345
TiO2
185
176
183
262
217
243
238
FeOT
1085
1129
985
1254
1178
1165
1106
MnO
019
018
016
022
020
020
019
MgO
753
799
667
547
554
507
643
CaO
1116
1099
1161
1099
1144
1129
1258
Na2O
239
235
241
256
260
258
245
border
Sample no.:
2
7
Oxide wt% (XRF)
K2O
037
048
048
066
060
056
047
P2O5
028
027
028
041
035
036
032
Total
9948
9933
9947
9800
9925
9802
9907
Trace elements ppm (XRF)
Ni
90
100
83
24
29
26
28
Cr
267
262
265
241
267
218
327
Sc
41
43
40
56
50
48
64
V
311
298
294
402
331
368
433
Ba
311
260
264
431
332
352
276
Rb
5
8
7
9
8
7
7
Sr
240
232
245
229
238
244
217
Zr
111
109
111
158
136
135
115
Y
32
29
31
45
37
38
35
Nb
11
12
11
17
13
15
12
Trace elements ppm (ICP-MS)
La
132
119
132
189
158
163
139
Sm
49
46
51
72
60
63
56
Eu
17
16
17
24
21
22
20
Yb
30
28
29
42
34
36
33
Th
062
051
057
080
066
070
067
Hf
28
25
28
43
32
33
32
Ta
063
057
064
13
076
081
069
Modes (based on point counts)
plagioclase
150
505
602
479
542
572
490
augite
—
268
199
324
262
256
361
olivine
32
177
131
85
74
64
40
opaques
—
28
34
62
39
31
38
apatite
—
—
02
06
08
06
08
granophyre
—
—
—
—
56
23
20
other4
818
22
32
44
19
48
43
continued
308
WHITE
THE GRAVEYARD POINT INTRUSION
Table 1: Continued
Specimens collected from measured section B–B0
Analysis:
8
9
10
11
12
13
Strat ht. (m):
335
457
567
672
811
930
14
1019
Rock unit:
laminated
laminated
ferro-
ferro-
ferro-
poikilitic
ferro-
gabbro
gabbro
gabbro
gabbro
diorite
gabbro
diorite
Sample no.:
GP-333
GP-331
GP-329
GP-327
GP-325
GP-323
GP-321
Oxides wt% (XRF)
SiO2
5088
4934
5023
4981
5113
4892
5520
Al2O3
1324
1219
1197
1187
1052
1589
1166
TiO2
235
244
273
355
285
249
226
FeOT
1079
1316
1331
1539
1761
1168
1435
MnO
020
022
023
025
029
019
025
MgO
629
651
579
386
181
526
124
CaO
1227
1135
1126
919
682
1166
586
Na2O
259
250
267
302
321
267
377
K2O
057
060
066
106
169
054
203
P2O5
034
038
043
065
121
032
088
Total
9952
9869
9928
9865
9715
9961
9749
Trace elements ppm (XRF)
Ni
28
29
16
1
0
28
0
Cr
223
94
45
51
1
213
0
Sc
64
60
64
51
36
45
41
V
455
430
462
407
73
363
42
Ba
322
331
356
555
983
415
1144
Rb
8
8
8
15
27
7
35
Sr
214
206
210
212
206
253
214
Zr
131
144
154
245
411
125
478
Y
39
43
45
67
109
33
119
Nb
15
16
16
27
43
14
48
Trace elements ppm (ICP-MS)
La
142
173
181
282
548
131
591
Sm
56
65
71
102
192
50
199
Eu
19
22
24
33
53
19
54
Yb
35
40
42
61
104
40
113
Th
060
073
074
123
240
056
275
Hf
30
35
38
59
106
27
119
Ta
075
087
098
139
236
075
259
Modes (based on point counts)
plagioclase
471
413
401
376
284
528
191
augite
458
405
382
195
147
272
138
olivine
02
109
—
—
16
39
—
opaques
38
30
68
46
44
61
21
apatite
05
04
03
02
15
05
10
granophyre
09
—
74
272
343
55
476
other
17
39
72
109
150
39
164
continued
309
JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
FEBRUARY 2007
Table 1: Continued
Specimens from measured section B–B0
Additional specimens of the chilled border
Analysis:
15
16
17
18
19
20
Strat ht. (m):
1152
1212
1289
Elev (m):
977
987
975
Rock unit:
ferro-
poikilitic
poikilitic
Latitude:
43831490
43831080
43831710
gabbro
gabbro
gabbro
Longitude:
11783770
11782890
11783650
GP-319
GP-317
GP-315
Sample no.:
GP-2324
GP-2125
GP-2966
SiO2
4774
5001
4941
4931
4910
4905
Al2O3
1263
1618
1597
1555
1598
1565
TiO2
363
191
192
181
162
184
FeOT
1486
1058
1064
1123
1064
1091
MnO
023
018
018
018
018
019
MgO
486
591
636
756
745
818
CaO
1095
1174
1204
1116
1156
1048
Na2O
278
272
249
244
237
225
K2O
076
054
044
049
048
052
P2O5
051
027
027
028
025
028
Total
9895
10003
9972
10001
9963
9935
Sample no.:
Oxides wt% (XRF)
Trace elements ppm (XRF)
Ni
12
42
41
98
129
99
Cr
41
232
315
276
281
268
Sc
59
46
51
39
42
44
V
604
325
330
305
298
300
Ba
403
281
323
447
412
272
Rb
11
8
8
9
10
8
Sr
222
248
243
254
257
218
Zr
184
120
106
112
101
111
Y
49
32
30
32
29
31
Nb
22
12
12
13
11
11
Trace elements ppm (ICP-MS)
La
205
123
114
132
119
132
Sm
81
46
46
49
45
48
Eu
26
17
18
17
16
17
Yb
45
28
27
30
28
29
Th
090
055
046
059
057
057
Hf
47
27
25
26
24
26
Ta
119
060
060
062
056
062
Modes (based on point counts)
plagioclase
449
550
539
99
165
146
augite
398
312
328
—
—
—
olivine
—
70
65
39
50
51
opaques
30
37
24
—
—
—
apatite
04
01
03
—
—
—
granophyre
45
02
0
—
—
—
other
21
20
38
862
785
803
continued
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Table 1: Continued
Other ferrogabbros and ferrodiorites
Granophyres from dikes and sheets
Laboratory precision
Analysis:
21
22
23
24
25
Elev. (m):
1008
1028
1009
999
978
26
Latitude:
43831560
43831540
43831770
43831730
43831720
Longitude:
11783460
11783560
11783620
11783580
11783610
Sample no.:
GP-13
GP-289
GP-2357
GP-2428
GP-2939
BCR-P10
SD%
Rel.%
Oxides wt% (XRF)
SiO2
4556
5048
5663
6247
6683
5540
005
008
Al2O3
1019
1003
1150
1174
1256
1356
002
016
TiO2
525
287
250
124
089
2256
00048
022
FeOT
1912
1916
1402
1084
681
1273
0014
011
MnO
026
032
023
018
013
0185
00007
038
MgO
478
212
148
051
055
344
0028
081
CaO
955
696
539
420
257
698
0008
011
Na2O
262
325
368
392
419
334
0014
042
K2O
079
165
229
291
362
173
0000
000
P2O5
048
135
098
032
019
0381
00018
047
Total
9860
9819
9870
9833
9834
100002
Trace elements ppm (XRF)
Ni
15
0
0
1
8
0
0
0
Cr
139
0
4
2
4
181
137
76
Sc
62
37
33
23
17
348
27
77
V
1049
40
53
0
2
391
66
17
Ba
422
907
1189
1539
1677
6755
1687
25
Rb
11
26
39
50
65
466
097
21
Sr
189
200
189
177
121
3256
08
026
Zr
190
398
478
659
748
1743
106
061
Y
50
113
114
137
135
375
053
14
Nb
22
45
46
62
55
138
081
59
Trace elements ppm (ICP-MS)
La
n.d.
n.d.
631
756
699
2626
049
186
Sm
n.d.
n.d.
206
218
208
703
015
207
Eu
n.d.
n.d.
52
58
51
214
004
192
Yb
n.d.
n.d.
116
146
134
336
003
094
Th
n.d.
n.d.
334
408
376
513
049
950
Hf
n.d.
n.d.
135
174
170
467
007
147
Ta
n.d.
n.d.
255
337
288
082
002
270
Modes (based on point counts)
plagioclase
443
359
n.d.
n.d.
n.d.
augite
260
131
n.d.
n.d.
n.d.
olivine
66
25
n.d.
n.d.
n.d.
opaques
104
40
n.d.
n.d.
n.d.
apatite
03
24
n.d.
n.d.
n.d.
granophyre
33
245
n.d.
n.d.
n.d.
other
89
175
n.d.
n.d.
n.d.
1
Vertical height above exposed base of intrusion in section B–B0 , Fig. 1b.
2
Lower chilled border in contact with tuffaceous siltstone (100 m south of measured section B–B0 ).
3
Base of inferred second magma pulse.
4
Mainly products of deuteric alteration or secondary minerals in cavities; groundmass in modes of chilled
5
Lower chilled border in contact with porcellanitized tuffaceous siltstone.
6
Chilled border in contact with partially fused tuffaceous sandstone in roof pendant.
20 cm wide granophyre dike cutting poikilitic gabbro.
40 cm thick conformable sheet within poikilitic gabbro.
35 cm thick conformable sheet within poikilitic gabbro.
SU internal standard from same site as USGS standard BCR-1: n ¼ 10 for XRF, n ¼ 50 for ICP-MS.
n.d., not determined.
7
A
8
A
9
A
10
311
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JOURNAL OF PETROLOGY
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FEBRUARY 2007
Fig. 4. Photomicrographs of representative specimens from the Graveyard Point intrusion; all were taken in plane-polarized light and are at the
same scale, shown by the 5 mm bar in (a). (See text for detailed descriptions.) (a) Lower chilled border with small glomerocrysts of plagioclase
and olivine; (b) laminated gabbro (arrow points to a granophyre vein that cuts across the plane of lamination); (c) ferrogabbro containing
plagioclase, augite, skeletal Fe^Ti oxide, and pools of interstitial granophyre; (d) poikilitic gabbro consisting mainly of large, optically continuous augite enclosing small crystals of plagioclase.
Fig. 5. (a) Outcrop of mixed-textured rock composed of medium- to coarse-grained ferrogabbro containing veins and clots of fine grained
siliceous granophyre (G). Scale bar represents 10 cm. (b) Area of intersecting sheets of siliceous granophyre cutting poikilitic gabbro in the
upper part of the intrusion. Hammer is 36 cm long
and ophitic-textured olivine diabases plot within the
field of olivine (þplagioclase), the granular-textured
augite-bearing gabbros in the interior of the intrusion
(including the laminated rocks) plot along the olivine^
Ca-rich pyroxene boundary or within the field of Ca-rich
pyroxene (þplagioclase), and the granophyres plot along
the pigeonite^Ca-rich pyroxene boundary (þplagioclase).
Samples of diabase from the lower part of the intrusion
plot closer to the Ol corner than the chilled border
samples, which is consistent with the small spike in modal
olivine at about 5 m above the floor of the intrusion (Fig. 3)
and may indicate that these rocks contain excess olivine.
312
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Fig. 6. (a) AMF plot of bulk-rocks and granophyre veins from the Graveyard Point intrusion shown with the experimentally determined liquid
trend for the Skaergaard intrusion (McBirney, 1975). (b) Di^Ol^Q plot of all analyzed rocks and veins from the intrusion using the plagioclase
projection of Grove & Baker (1984).
The laminated gabbros plot closer to the Di corner than
any of the other specimens, which is consistent with their
high content of modal augite (Fig. 3) and suggests that they
may contain excess pyroxene.
Chemical profiles for MgO, Ni, Sc and Zr are shown
for measured section B^B0 in Fig. 7. A notable feature
in these profiles is the chemical discontinuity at about
10 m above the base of the intrusion, coincident with the
boundary between the ophitic-textured olivine diabase
and the coarser-grained, granular-textured olivine gabbro.
Rocks immediately above this boundary contain less
MgO, substantially less Ni, and somewhat more Sc and
Zr than the rocks below it. The behavior of other oxides
and trace elements across this discontinuity can be seen
by comparing analyses 3 and 4 in Table 1. XRF analyses of
seven specimens from the lower part of section A^A0 show
a similar discontinuity about 13 m above the lower contact.
Concentrations of MgO and Ni abruptly increase
again near the top of the section at the contact between
the poikilitic gabbros and the olivine diabase adjacent
to the roof. It is clear from these profiles that the average
composition of the intrusion at this section does not
correspond to the compositions of the chilled borders.
The lack of mass balance within this part of the intrusion
combined with the abruptness of the lower and upper
chemical discontinuities can be explained by the addition
of more evolved magma into a sheet initially formed
by a pulse of more mafic magma. The lack of internal
chilled borders indicates that the second influx of magma
must have occurred while the initial sheet was still hot or
partly molten.
The strongly incompatible trace elements Zr, Ba, Rb,
Nb, La, Th, Hf and Ta maintain nearly constant ratios
with one another throughout the suite of rock types
within the intrusion. Plots utilizing these elements result
in straight-line trends that project to the origin (Fig. 8),
while the volcanic and sedimentary rocks adjacent to
the intrusion generally plot well away from these trends.
Although small degrees of contamination by the shallow
crust cannot be ruled out, these relations are consistent
with the granophyres and other evolved rocks within the
intrusion having formed by fractional crystallization
of a magma or magmas similar in composition to the
chilled border. None of the granophyres, including the
1m thick sheet adjacent to the roof, appear to be partial
melts of the surrounding country rock.
Mineral compositions
The compositions of olivine, pyroxene and plagioclase
in selected specimens from the intrusion were determined
using a Cameca Camebax electron microprobe at
Washington State University using a 20 kV current and
a 4 mm beam width. Additional microprobe analyses
of plagioclase were obtained at the University of Oregon
using a Cameca SX50. Typically, the compositions
of about 8^10 crystals each of plagioclase, pyroxene and
olivine (if present) were determined for a specimen,
and between one and three points were analyzed in the
interior of each crystal. Representative mineral analyses
313
JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
FEBRUARY 2007
Fig. 7. Chemical stratigraphy through the Graveyard Point intrusion at section B^B0. Line A from Fig. 3 marks a pronounced chemical discontinuity. Line B indicates the base of the laminated gabbros.
are included in Electronic Appendix 1 at http://
petrology.oxfordjournals.org.
Variations in the compositions of the major silicate
phases with height through section B^B0 are shown in
Fig. 9. A notable feature in this diagram is the abrupt
shift in the compositions of olivine and plagioclase that
takes place at the same stratigraphic height as the bulkrock chemical discontinuity (line A in Fig. 9). Pyroxene
compositions are not displaced at this boundary; however,
this mineral occurs only as interstitial grains or poikilitic
plates in the lower 10 m of the section and was probably
not a primocryst. The compositions of plagioclase and
pyroxene in the laminated gabbros are noteworthy for
their relatively constant average values throughout the
unit and for the small compositional variations among
crystals in a single thin section. The uniform compositions
of the minerals in these rocks supports the interpretation
that variations in the bulk-rock chemistry of samples from
this unit are probably due to differences in the amounts
and proportions of excess crystals.
Isotopic ratios
Sr isotopic ratios were determined for 12 samples from the
Graveyard Point intrusion and four specimens of wall-rock
by thermal ionization mass spectrometry at Miami
University (Ohio) (Table 2). Measured analyses were corrected to 87Sr/86Sr ¼ 070800 for the E&A SrCO3
standard, and initial ratios were calculated for an age of
314
WHITE
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Fig. 8. Variation of Rb vs Zr (ppm) for all analyzed bulk-rock
samples from the intrusion compared with analyzed samples of silicic
tuffs and sediments from the surrounding country rocks. Symbols for
samples from the intrusion are the same as in Fig. 6; country rocks are
indicated by open stars.
67 Ma. The 12 analyzed rocks from the intrusion yielded
initial 87Sr/86Sr ratios between 07059 and 07065, with
specimens of the Ni-rich olivine diabase having slightly
lower values than those from the interior of the intrusion
(Fig. 10). On the other hand, there are no systematic
differences between the Sr isotopic ratios of the granophyre
sheets and those of the more mafic gabbro and ferrogabbro.
The silicic pyroclastic rocks and tuffaceous sediments that
surround the intrusion display a greater range of Sr ratios
(07048^07067), which increase with increasing silica
content. Taken together, the Sr isotopic analyses support
the interpretation that the granophyric liquids formed
by differentiation of the more mafic magmas after
the final emplacement of the intrusion and without any
significant contribution from the wall-rocks.
DISCUSSION
Emplacement of the intrusion
Several types of field evidence indicate that the Graveyard
Point intrusion was emplaced at a shallow depth. These
include the presence of well-developed, fine-grained
chilled borders, the abundance of round vesicles in some
parts of the upper chilled border, and the miarolitic
texture of evolved rocks in the interior of the intrusion.
Moreover, XRD studies have shown that tridymite is
present in fused siltstone within a few centimeters of
the contacts (Ferns, 1989). This occurrence allows the
maximum pressure at the site of emplacement to be
estimated using the pressure^temperature values for
the thermodynamically and experimentally determined
b-quartz^tridymite transition (Tuttle & Bowen, 1958;
Berman, 1988). An estimate of the maximum temperature
of the first magma injected into the intrusion can be made
using the MELTS (Ghiorso & Sack, 1995) and MIXFRAC
(Nielsen, 1988) computer fractionation models. These
programs yield temperatures of 11858C and 12058C,
respectively, for the first appearance of olivine þ plagioclase
in a liquid with the composition of the chilled border.
A temperature of 10968C is obtained for the chilled
border magma using the plagioclase þ liquid thermometer
of Putirka (2005), although this value is substantially lower
than temperatures estimated for SRP basalts in previous
studies (Leeman & Vitaliano, 1976; Honjo & Leeman,
1987). If a temperature of 11808C is assumed for the
magma emplaced by the first pulse, then a maximum pressure of 175 MPa can be calculated from the relation
P (MPa) ¼ 0561 T (8C) ^ 487 6 for the quartz^tridymite
transition (Hirschmann et al., 1997). This value corresponds
to a maximum depth of around 65 km, given an average
density of 2700 kg/m3 for the overlying rock. Putirka
(2005)
also
formulated
an
expression
for
calculating pressure based on plagioclase þ liquid
equilibria, which, when applied to the mineral and
bulk-rock analyses for the chilled border, yields a value of
129 MPa (equivalent to 48 km). Because this would be
the pressure at which plagioclase equilibrated with the
liquid prior to injection, a shallower depth can be inferred
for the intrusion.
The simplest explanation for the lack of mass balance
between the composition of the chilled border and the
bulk composition of the intrusion is that it was formed by
more than one injection of magma. The abrupt shift in
rock and mineral compositions at about 10 m above the
base of the intrusion is inferred here to mark the lower
contact of the second magma pulse. The initial influx of
tholeiitic magma contained small phenocrysts of olivine
and plagioclase, as indicated by thin sections of the chilled
border (Fig. 4a). The second pulse was more voluminous
and more chemically evolved, and carried with it a substantial proportion of medium- to coarse-grained crystals
of augite and plagioclase, and a smaller amount of olivine.
The depth at which this magma originated can only be
roughly estimated because its composition is not known
with any certainty; however, experiments utilizing basalts
from the Snake River Plain indicate that Ca-rich pyroxene
is not likely to be a near-liquidus phase at pressures less
than 500 MPa (Thompson, 1972; Leeman & Vitaliano,
1976). The well-defined layer of laminated gabbro beginning about 20 m above the presumed base of the second
magma pulse (30 m above the floor of the intrusion) probably represents a crystal-rich tongue formed by flow differentiation during the ascent and lateral injection of the
magma (e.g. Simkin, 1967; Upton & Wadsworth, 1967;
Komar, 1972; Richardson, 1979; Husch, 1990; Mangan
et al., 1993). The imbricate textures in these rocks and
the wedge-shaped geometry of the unit support this
315
JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
FEBRUARY 2007
Fig. 9. Variations in the compositions of olivine, pyroxene and plagioclase in rocks sampled from measured section B^B0. Most of the points are
averages of microprobe analyses of 8^10 crystals in a single thin section; error bars are based on 1 SD. Lines A and B are drawn at the textural
and chemical discontinuities described for Figs. 3 and 7.
interpretation. Because the laminated gabbro contains
variable proportions of excess augite and plagioclase, chemical profiles through it do not show the well-developed
D-shaped enrichments for MgO that have been noted
in sills containing similarly formed concentrations of
olivine and orthopyroxene (e.g. Gibb & Henderson, 1992;
Marsh, 1996).
It is not entirely clear when the magma that formed
the poikilitic gabbro in the upper part of the intrusion in
section B^B0 was emplaced. The Sr isotopic ratios of these
rocks are more like those of samples from the interior of the
intrusion than they are to analyses of the olivine diabase
(Fig. 10); however, evidence discussed below indicates that
the poikilitic gabbros are more probably related to the
initial influx of magma. Although these rocks are less
mafic than the chilled border and the basal olivine diabase,
they contain more MgO and Ni and have lower contents
of excluded elements than any of the other rocks in the
intrusion, including the laminated cumulates (Table 1).
Moreover, the small primocrysts of plagioclase in the
poikilitic gabbros are more calcic than plagioclase in the
laminated rocks (Fig. 9). These relations are the reverse
of what is generally observed in intrusions where an
upper border series and a lower cumulate series are
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Table 2: Sr isotopic ratios
Sample no.
Lithologic unit
Rb (ppm)
Sr (ppm)
87
Sr/86Src
87
Sr/86Sri1
Specimens from the Graveyard Point intrusion
GP-303
lower chilled border
5
240
0705912
44
070591
GP-205
olivine diabase
4
232
0705926
38
070592
GP-340
olivine diabase
8
232
0706001
32
070599
GP-334
olivine diabase
14
219
0706111
25
070609
GP-292
ophitic gabbro
7
247
0706418
24
070641
GP-318
ophitic gabbro
6
258
0706248
26
070624
GP-315
ophitic gabbro
8
243
0706549
22
070654
GP-337
granular gabbro
8
238
0706161
33
070615
GP-327
ferrogabbro
15
212
0706186
27
070617
GP-321
ferrodiorite
35
214
0706176
44
070613
GP-235
granophyre dike
39
189
0706320
22
070627
GP-242
granophyre dike
50
177
0706245
34
070617
Specimens of rocks bordering the intrusion
GP-234
crystal lithic tuff
45
436
0704822
44
070479
GP-301
lithic tuff
54
142
0705022
28
070492
GP-302
vitric tuff
167
95
0707200
30
070673
GP-304
vitric tuff
119
110
0705995
29
070571
1
Age corrected to 67 Ma.
Fig. 10. Sr-isotope compositions of bulk-rock specimens from the
Graveyard Point intrusion and the surrounding country rocks
(open stars) vs wt% SiO2. Other symbols are the same as in Fig. 6.
coeval (e.g. Naslund, 1984). It is more likely that small
crystals of plagioclase and olivine carried in with the first
pulse of magma settled out of the upper part of the intrusion and accumulated in the lower part, resulting in formation of the olivine diabase. Major-element based leastsquares calculations (Bryan et al., 1969) support this
hypothesis: analyses of the poikilitic gabbro can be closely
approximated by subtracting 8^12% olivine þ plagioclase
in roughly equal amounts from the chilled border composition, and the olivine diabase can be duplicated by adding
4^11% of these minerals to the analysis of the chilled
border. The sums of the squares of the residuals range
from 002 to 021 when the compositions of olivine
and plagioclase in the chilled border are used in the calculations. The redistribution of crystals in the amounts and
proportions indicated by the least-squares calculations
is plausible given that the average phenocryst content in
samples of the chilled border is around 5% olivine and
13% plagioclase. If this interpretation is correct, then
almost all of the olivine phenocrysts and about half of the
plagioclase crystals would have been removed from the
upper^middle part of the intrusion and concentrated in
the lower 10 m. However, given that the zone of poikilitic
gabbro in section B^B0 is substantially thicker than the
layer of olivine diabase near the floor, it is surprising
that the diabase samples do not contain an even greater
proportion of accumulated crystals. This discrepancy
may be due in part to insufficient sampling of the lower
diabase. Alternatively, residual liquid left over after the
separation of the olivine and plagioclase phenocrysts
may have migrated along a sloping roof and become concentrated in a cupola, in which case the poikilitic gabbro
would not be as voluminous elsewhere in the intrusion as
it appears to be from section B^B0.
317
JOURNAL OF PETROLOGY
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FEBRUARY 2007
Considering the discussion above, the following
sequence of events is proposed for the emplacement of the
Graveyard Point intrusion.
(1) Basaltic magma containing about 18% small phenocrysts of olivine and plagioclase was injected into
a sequence of silicic volcanic rocks and tuffaceous
sediments, forming an irregularly shaped but generally sheet-like intrusion that was as much as 60 m
thick in the area of measured section B^B0 but
thinned to about 25 m at section C^C0.
(2) Following the formation of the chilled borders,
the phenocrysts of olivine and, to a lesser extent,
plagioclase that were carried in with the magma
settled and accumulated in the lower 10 m of the
intrusion, producing a layer of MgO- and Ni-enriched
olivine diabase. Crystal-depleted liquids in the
upper part of the intrusion may have become concentrated in one or more cupolas in the roof where
they solidified in situ, forming the distinctive poikilitic
textured gabbros.
(3) A second injection of more evolved basaltic magma
was emplaced into the thickest part of the intrusion
after the initial pulse had mostly, but probably not
completely, crystallized. It did not invade the thinner
distal part of the original intrusion (section C^C0 ),
perhaps because those areas were already solid. The
second pulse carried in a large proportion of augite
and plagioclase phenocrysts and inflated the intrusion
to at least 150 m thick in the area of sections A^A0
and B^B0. The phenocrysts were concentrated by flow
differentiation during the ascent and emplacement
of the magma, and formed a wedge-shaped layer
of crystal mush between 60 and 25 m thick.
Differentiation processes
The chemical composition of magma in the leading edge
of the first pulse is preserved in the chilled border, and,
as discussed above, the primary process causing this
magma to differentiate after emplacement appears to
have been the redistribution of olivine and plagioclase
phenocrysts. The composition of the second magma is not
as well constrained because it did not form distinctive
chilled borders; however, the granular-textured gabbro
just above the chemical discontinuity in section B^B0 may
provide a reasonable approximation (sample GP-338,
Table 1, column 4). If this is assumed to be true, then the
magma injected in the second pulse was a quartz tholeiite
with SiO2 between 49 and 50% and MgO between
5 and 6%. The lack of internal chilled borders implies
a short time interval between the emplacement of the two
magmas; for this reason, a simple scenario in which
the second magma evolved from the first during the
time between injections is probably unrealistic. It is more
likely that they were derived from different parts of
Fig. 11. Incompatible element diagram for representative specimens
of laminated gabbro, ferrogabbro, ferrodiorite and granophyre.
Analyses are normalized to GP-338 (Table 1), which is inferred to
approximate the bulk composition of the second magma emplaced
into the intrusion.
a complex system of interconnected reservoirs that had
undergone differing amounts of fractional crystallization
of the same or a similar parent magma; in other words,
from different regions and probably from different depths
within the magmatic mush column.
There is no evidence to indicate that any magmas with
compositions more evolved than quartz tholeiite were
added from outside the intrusion, and the geochemical
and isotopic data preclude significant amounts of wallrock contamination. Therefore, the ferrogabbros, mixedtextured ferrodiorites and silicic granophyres all must
have evolved within the intrusion itself. Using the analysis
of sample GP-338 as a proxy for the bulk composition
of the second magma, enrichments of the most strongly
excluded trace elements (Zr and Th) indicate that
20^40% fractional crystallization would be needed to
generate the range of compositions in the ferrogabbros,
and about 65^75% crystallization is necessary to form the
silicic granophyres. The behavior of the incompatible
minor and trace elements in these rocks is shown
on the normalized trace-element diagram in Fig. 11,
along with a specimen of the laminated gabbro.
Element concentrations were normalized to the analysis of
sample GP-338. The typical ferrogabbro is enriched in
nearly all of the 22 elements on the diagram relative to its
inferred parent magma, with the notable exception of Sr,
which is enriched in the laminated gabbro. Although not
shown in Fig. 11, Sc and Cr are also depleted in the ferrogabbros and enriched in the laminated rocks relative to
GP-338 (Table 1). The trace-element relations are consistent with fractionation of plagioclase þ augite olivine,
the minerals that appear to have been concentrated in the
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WHITE
THE GRAVEYARD POINT INTRUSION
laminated gabbro. The mixed-textured ferrodiorite and the
silicic granophyre are progressively more enriched in most
of the incompatible elements but also produce increasingly
strong negative anomalies for Sr and Ti (Fig. 11). Although
the ferrodiorite is enriched in P relative to the ferrogabbro,
the granophyre is strongly depleted in this element,
even compared with the inferred composition of the
parent magma. The variations in Sr, Ti and P in these
rocks are consistent with a fractionation scheme that
initially included plagioclase and progressively added
Ti^Fe oxides and apatite. This does not mean to say that
these minerals settled out of a compositionally evolving
liquid; it is more likely that liquids separated from
frameworks of crystals in which these minerals were
progressively crystallizing.
The physical processes that result in the segregation of
chemically evolved liquids have been the focus of many of
the more recent studies of small to moderate-sized mafic
intrusions and thick lava flows. Philpotts et al. (1996) and
Marsh (2002) emphasized a number of important differences between the processes that form low-Si, generally
coarse-grained gabbroic segregations (e.g. ‘gabbroic
pegmatites’ of Wager & Deer, 1938; ‘dolerite pegmatites’
of Walker, 1949) and those that form Si-rich,
commonly fine-grained granophyres (e.g. Hotz, 1953).
Following their lead, these rock types are discussed
separately below, beginning with the ferrogabbros. Where
they are present in the Graveyard Point intrusion, the
ferrogabbros are similar in texture and composition to
the coarse-grained, iron-rich (but not silica-rich) gabbroic
pegmatites or pegmatitic segregation veins commonly
found in diabase sills, solidified lava lakes and thick
basalt flows. With few exceptions, studies of these bodies
have attributed their origin to the upward migration and
segregation of liquids formed within the lower crystal
mush after 20^30% crystallization (e.g. Puffer & Horter,
1993; Philpotts et al., 1996; Mitchell et al., 1997). Because
these liquids are commonly iron-rich and therefore dense,
various mechanisms have been suggested that would
enhance their buoyancy and facilitate their movement
upward through the crystal pile. Shirley (1986, 1987)
emphasized the role of crystal compaction to redistribute
interstitial liquids in the Palisades Sill, and compactiondriven upward movement of evolved low-silica melts has
been documented in thick basalt flows (Philpotts et al.,
1996; Philpotts & Philpotts, 2005). Larsen & Brooks (1994)
suggested that gabbroic pegmatites in the Skaergaard
intrusion formed by the convective rise of interstitial
liquids whose densities had been reduced by dissolved
water. Puffer & Horter (1993) attributed the pegmatitic
segregation veins in flood basalts to the transport of interstitial liquids in segregation vesicles, a process described
by Helz et al. (1989) for the transfer of evolved melts in the
Kilauea Iki lava lake. This model is based in part on the
process described as ‘gas filter pressing’ by Anderson et al.
(1984) and as ‘vapor-differentiation’ by Goff (1996), by
which interstitial melts are forced into vesicles
formed when magmatic gases exsolve in response to crystallization. The evolved liquid is transported upward in the
buoyant mixture of vapor- and melt-filled bubbles.
Although gas vesicles are common in the evolved
mafic rocks of the Graveyard Point intrusion, they
are not present in the underlying laminated gabbros.
This suggests that exsolution of the gas phase took place
after the ferrogabbroic liquids became segregated.
Moreover, most of the ferrogabbros are depleted in Ba,
Rb and Zr relative to values predicted by major-oxide
based mass-balance calculations, whereas these elements
should be preferentially enriched in melts that were transported upward in volatile-rich segregation vesicles (Puffer
& Horter, 1993). If a gas transport mechanism did not play
an important role at this stage, then it is likely that
the evolved mafic liquids migrated upward in response to
compaction of the crystals within the laminated unit.
Although iron-rich, the buoyancy of these liquids would
have been enhanced by increasing amounts of dissolved
magmatic gases as crystallization proceeded within the
crystal mush. In addition, field relations indicate that
slabs of partly or wholly crystallized rock became detached
from the lower part of the poikilitic gabbro that had
crystallized next to the roof of the intrusion, causing the
stratigraphic complexity observed in section B^B0 (Fig. 2).
Downward movement of this slab of largely crystalline
crust would have augmented compaction in the crystal
mush and caused the interstitial liquids to be drawn into
the tear in a manner described by Marsh et al. (1991) and
Carman (1994).
The silicic granophyres are mainly concentrated in the
upper third of the intrusion, where they occur as dikes,
sheets and pods within the poikilitic gabbro. As noted
above, the lowTiO2 and P2O5 contents of the silicic granophyres complement the high values of these oxides in many
of the ferrogabbros. This relationship can be explained by
the movement of late-stage, interstitial liquids out of
a framework of crystals in which apatite and Fe^Ti oxides
were retained. Segregation of these liquids was probably
driven by the vapor-differentiation process described
above. The abundance of irregularly shaped miarolitic
cavities in the mixed-textured ferrodiorite and the
presence of granophyre-lined vesicles in the ferrogabbro
(Fig. 12) are compelling evidence in support of this process.
The brecciated appearance of the mixed-textured
ferrodiorites (Fig. 5a) suggests that vesiculation may
have been very rapid and was perhaps initiated by
decompression triggered by fracturing of the overlying
country rocks. An abrupt drop in vapor pressure may also
explain the quench textures of crystals in the interstitial
granophyres in many of the ferrodiorites.
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JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
FEBRUARY 2007
Fig. 12. Photomicrograph (plane-polarized light) of a granophyre-lined vesicle in ferrogabbro from the interior of the Graveyard Point intrusion. The center of the vesicle is filled with limonite (lim). Small crystals of feldspar in the surrounding granophyre display quench textures and
have compositions around Ab75Or10An15. Coarse-grained crystals are plagioclase (pl), augite (aug) and opaque oxides. Scale bar represents
1mm.
A two-stage process is therefore suggested in which ironrich mafic liquids were formed by localized fractionation
within the tongue of crystals emplaced with the second
magma pulse and were expelled by crystal compaction.
Further in situ crystallization of this liquid produced
a second generation of more evolved, silica-enriched,
liquids, which in turn separated from their own rigid, but
porous, crystal framework in response to the exsolution
and migration of bubbles of magmatic vapor. The combined process is generally similar to that described by
Philpotts et al. (1996) for the differentiation of the thick
Holyoke lava flow, although compelling evidence does
not exist at the Graveyard Point intrusion to support their
suggestion that the late-stage silica-rich granophyres form
from immiscible liquids.
Liquid trend
Although cumulus processes in small intrusions are
generally inefficient compared with those in larger bodies,
some of the rocks in the Graveyard Point intrusion clearly
contain excess amounts pyroxene and/or plagioclase,
and therefore analyses of these specimens cannot be
considered to represent any liquid. On the other hand,
analyses of the small sheets and dikes of granophyre must
represent late-stage liquid compositions, and many of
the ferrodiorites and ferrogabbros are probably close to
the compositions of liquids that existed at different places
and times within the intrusion. Analyses of these rocks are
plotted on Fenner diagrams in Fig. 13 to illustrate the trend
of liquids inferred to have evolved within the intrusion
after the second magma was emplaced. The laminated
and granular gabbros from the lower middle part of the
intrusion have been excluded from these plots, and they
also do not include any data for the mafic rocks related
to the first influx of magma. The chemical trend for the
late-stage liquids is compared with model liquid trends
produced by the fractional crystallization simulations
of the experimentally constrained MIXFRAC program of
Nielsen (1988). The analysis of the first sample above the
chemical discontinuity in section B^B0 (GP-338, Table 1,
column 4) was used as the composition of the parent
magma in the computer model and is indicated by a star
in Fig. 13. The calculations were made with the oxygen
fugacity fixed at the fayalite^magnetite^quartz (FMQ)
buffer. The low-pressure trend produced by MIXFRAC
broadly mimics the sample compositions and inflection
points on the Fenner diagrams, but does not reach the
level of iron enrichment observed in the rocks. The
poor fit for FeOt may be due to the inability of computer
fractionation models to accurately predict the saturation
temperature of Fe^Ti oxides in iron-rich magmas
[see the discussion by Toplis & Carroll (1996)]. In general,
320
WHITE
THE GRAVEYARD POINT INTRUSION
Fig. 14. AMF trends for analyzed rocks from the Graveyard Point
intrusion (GPI) and tholeiitic lavas from the western Snake River
Plain (WSRP). Analyses of western SRP lavas are from Bonnichsen
& Godchaux (2002), Shervais et al. (2002) and White et al. (2002). The
continuous line divides the fields for tholeiitic (TH) and calc-alkaline
(CA) rocks (after Irvine & Baragar, 1971). The lower third of the diagram is not shown in this figure.
Fig. 13. Variation of major and minor oxides (wt%) plotted against
MgO (wt%) for ferrogabbros, ferrodiorites and granophyres from the
Graveyard Point intrusion. [See text for discussion of the low-pressure
model liquid trend derived from MIXFRAC (Nielsen, 1988).]
however, the model is consistent with the liquid
trend inferred from the bulk-rock analyses, and supports
the interpretation that, following the emplacement of the
second pulse of magma, the remaining liquid evolved
within the intrusion by some process of crystallizationdriven differentiation.
Comparison with Snake River Plain lavas
The late Miocene to Pleistocene basalts of the western SRP
erupted from as many as 400 different vents associated
with scoria cones, small shields and phreatomagmatic
centers. All but the very youngest of these lavas plot in
the field of tholeiitic rocks on AMF diagrams (Fig. 14) and
display trends of increasing FeOT and TiO2, and slightly
decreasing SiO2, as MgO decreases. To date, only one
tholeiitic lava flow has been identified in the western SRP
that appears to have differentiated to the point where SiO2
enrichment had begun (Bonnichsen & Godchaux, 2002).
Maximum concentrations for FeOT and TiO2 in the
western SRP volcanic suite are about 175% and 45%,
respectively. Similar values are recorded for many of
the ferrogabbros in the Graveyard Point intrusion,
although a few specimens contain as much as 20% FeOT
and 55% TiO2, possibly owing to small amounts of
excess magnetite.
Ratios of CaO/Al2O3 in both suites initially increase
slightly with differentiation and then decline (Fig. 15).
Although this trend is not particularly strong in the
volcanic suite, it does suggest that small amounts of
Ca-rich pyroxene were removed from these magmas. This
interpretation is in apparent conflict with the observation
that pyroxene-phyric lavas have never been reported
from the western SRP, although the Graveyard Point
intrusion provides compelling evidence that pyroxene can
be an abundant crystallizing phase at some depth within
the magmatic mush column.
The greatest differences between the compositions
of the SRP lavas and those of the intrusion occur in
the behaviors of the incompatible minor oxides and trace
elements. Although abundances of these elements are
about the same in the least differentiated rocks of each
suite, at moderate degrees of differentiation (Mg numbers
40^50) their concentrations are much greater in the lavas
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JOURNAL OF PETROLOGY
VOLUME 48
NUMBER 2
FEBRUARY 2007
Fig. 15. Selected oxide and trace-element ratios plotted against Mg-number (Mg-) for analyzed rocks from the Graveyard Point intrusion
(open circles) and lavas from the western SRP (crosses). The average K2O/P2O5 ratio in rocks from Layered Series Upper Zone b of the
Skaergaard intrusion (Skg UZb; McBirney, 1996) is shown for comparison. (See text for discussion.)
than they are in the plutonic rocks. The enrichment of
P2O5 in the volcanic suite is particularly noteworthy,
as it exceeds that of either K2O or TiO2. Among the
incompatible trace elements, Y and the heavy REE are
only slightly enriched in the ferrobasalts compared with
the strong increases in the concentrations of Zr, Sr, Ba,
La and Nb (Fig. 15). The behaviors of the excluded
elements in the lavas are best explained by a process of
fractional crystallization combined with open-system
mixing with a source rich in excluded elements and particularly enriched in phosphorus. This source was probably
not within the granitic rocks of the Idaho Batholith
because most of them have high K/P ratios. A similar
argument can be used to rule out the Miocene age SRP
rhyolites erupted just before the beginning of western SRP
basalt magmatism. An older cratonic source is unlikely
because Sr-isotopic analyses for western SRP lavas indicate
that 87Sr/86Sr ratios are lower for the high P2O5 ferrobasalts than they are for less evolved basalts in the same
region (White et al., 2002).
A similar trend of P2O5 enrichment and decreasing
K2O/P2O5 was noted by Geist et al. (2002) for basaltic
lavas sampled in drill core at the Idaho National
Laboratory in the eastern Snake River Plain. They
proposed that these magmas evolved by combined
fractional crystallization and assimilation (AFC) of highP2O5 ferrogabbro contained within a differentiated mafic
intrusion emplaced at mid-crustal levels below the eastern
SRP. A similar model is proposed here to explain why
the trace element trends in the western SRP lavas are so
different from those in the Graveyard Point intrusion.
The high incompatible element abundances and variable
but generally low K2O/P2O5 ratios in the lavas are consistent with mixing of a crystallizing basaltic magma with
partial melts of ferrogabbro and granophyre in layers or
veins within a differentiated mafic intrusion. Partial
melts of ferrogabbros containing a few percent cumulus
apatite would be enriched in phosphorus; however, if
some apatite were retained in the melting residuum, the
bulk distribution coefficients for Yand Yb would be greater
than those for elements such as Zr and La, which are
excluded from all likely minerals in the residuum including
apatite. There is good evidence suggesting that mafic
intrusions may be common beneath the western SRP: fast
seismic velocities at mid-crustal levels beneath the western
plain and high gravity anomalies along the axis of the
plain have both been attributed to the presence of mafic
intrusions in the mid-crust (Mabey, 1982; Wood &
Clemens, 2002). The Graveyard Point intrusion demonstrates that even relatively small western SRP sills can
322
WHITE
THE GRAVEYARD POINT INTRUSION
contain ferrogabbros with relatively high abundances
of P2O5 (415 wt%) and sheets of granophyre enriched in
excluded trace elements (Zr 4700 ppm; Ba 41500 ppm).
More extreme enrichment of P2O5 might be expected
in larger intrusions, where differentiation is likely to be
more efficient (for example, Upper Zone b in the Layered
Series of the Skaergaard intrusion).
S U M M A RY A N D C O N C L U S I O N S
Variations in the textures and compositions of rocks and
minerals composing the Graveyard Point intrusion can
be explained by multiple intrusions of magma followed
by shallow-level in situ crystallization, compaction and
liquid migration. The initial injection of magma consisted
of olivine tholeiite, which is preserved in chilled borders
next to both the roof and floor of the intrusion.
Redistribution of existing phenocrysts of olivine and
plagioclase produced a lower zone of olivine diabase
enriched in MgO relative to the chilled border, and an
upper zone of poikilitic-textured gabbro that is depleted
in MgO. Injection of a second magma caused some parts
of the intrusion to inflate to as much as 150 m in thickness.
In contrast to the pattern of re-injection observed in many
differentiated sills, the second pulse was more evolved than
the resident magma, although it also entrained large
amounts of plagioclase and augite phenocrysts. Rock
textures and chemical profiles suggest that these phenocrysts were concentrated by flow differentiation into a
wedge-shaped horizon of distinctive laminated gabbro as
much as 60 m thick. Following emplacement of the second
magma, interstitial ferrogabbroic liquids separated from
the crystal mush, which compacted, and accumulated
beneath the earlier formed poikilitic-textured gabbro of
the roof zone. Crystallization of the ferrogabbroic magma
in turn produced granophyric liquids, which migrated out
of interstitial spaces, probably in response to oversaturation and exsolution of dissolved volatiles. Upward movement of this low-density mixture of evolved granophyric
liquid and exsolved vapor inflated and fractured the
uppermost zones of the partially crystallized ferrogabbro,
and sheets of granophyric liquid intruded upward into the
poikilitic gabbro.
The complex origin inferred for the Graveyard
Point intrusion supports the recent observation by
Gibb & Henderson (2006) that ‘with few exceptions’,
detailed investigations of large mafic sills indicate a history
of multiple intrusions. The substantial differences in the
chemical compositions and phenocryst contents of the
two pulses of magma (but not in their excluded trace
element ratios) suggest that they evolved in different
parts, and probably at different depths, within an
interconnected magmatic mush column similar to that
described by Marsh (2004). The extraordinary degree of
differentiation observed in this relatively small intrusion
is attributed to a combination of factors including the
emplacement of a large proportion of crystals with the
second magma pulse and a relatively high volatile content
in the second-stage liquid.
The Graveyard Point intrusion provides the only
documented example of the extended differentiation of
a western SRP magma in a shallow pluton. Comparison
of the major element chemical trends inferred for the
Graveyard Point liquid with those produced by western
SRP lavas shows that they are generally similar. This
supports the interpretations of many researchers that
SRP tholeiites have evolved by fractional crystallization
at relatively low pressures (e.g. Leeman & Vitaliano,
1976). In contrast, the trends for P2O5 and some trace
element ratios such as La/Yb are substantially different
for the lavas compared with the intrusive suite. These
differences are attributed to the interaction of rising SRP
magmas with apatite-bearing mafic cumulates and evolved
granophyric liquids at various levels within the crust.
S U P P L E M E N TA RY DATA
Supplementary data for this paper may be obtained at
Journal of Petrology online.
AC K N O W L E D G E M E N T S
I am grateful to Gregg Beukelman and Doug Brown for
their help in the field, and to Bill Hart for generously providing the Sr isotopic analyses. Discussions on the outcrop
with Bill Bonnichsen, Bill Hart and Gregor Markl were
very helpful. I thank Bruce Marsh, Bill Leeman and
Wendy Bohrson for their insightful reviews and helpful
suggestions, which greatly improved this paper. This
research was supported by a grant from the National
Science Foundation (EAR-9219127).
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