Lithos 206–207 (2014) 321–337
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Lithos
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The Cryogenian intra-continental rifting of Rodinia: Evidence from the
Laurentian margin in eastern North America
Elizabeth McClellan a,⁎, Esteban Gazel b,1
a
b
Department of Geology, Radford University, P.O. Box 6939, Radford, VA 24142, United States
Department of Geosciences, Virginia Tech, 5041 Derring Hall (0420), Blacksburg, VA 24061, United States
a r t i c l e
i n f o
Article history:
Received 22 March 2014
Accepted 4 August 2014
Available online 14 August 2014
Keywords:
Wilson cycles
Rodinia
Continental rifting
Laurentian margin
a b s t r a c t
The geologic history of the eastern North American (Laurentian) margin encompasses two complete Wilson
cycles that brought about the assembly and subsequent disaggregation of two supercontinents, Rodinia and
Pangea. In the southern and central Appalachian region, basement rocks were affected by two episodes of crustal
extension separated by N100 m.y.; a Cryogenian phase spanning the interval 765–700 Ma and an Ediacaran
event at ~ 565 Ma. During the Cryogenian phase, the Mesoproterozoic continental crust was intruded by
numerous A-type felsic plutons and extensional mafic dikes. At ~ 760–750 Ma a bimodal volcanic sequence
erupted onto the uplifted and eroded basement. This sequence, known as the Mount Rogers Formation (MRF),
comprises a bimodal basalt–rhyolite lower section and an upper section of dominantly peralkaline rhyolitic
sheets. Here, we provide new geochemical evidence from the well-preserved volcanic rocks of the Cryogenian
lower MRF, with the goal of elucidating the process that induced the initial stage of the break-up of Rodinia
and how this affected the evolution of the eastern Laurentian margin. The geochemical compositions of
the Cryogenian lavas are remarkably similar to modern continental intra-plate settings (e.g., East African Rift,
Yellowstone–Snake River Plain). Geochemical, geophysical and tectonic evidence suggests that the common
denominator controlling the melting processes in these settings is deep mantle plume activity. Thus, evidence
from the MRF suggests that the initial phase of extension of the Laurentian margin at ~760–750 Ma was possibly
triggered by mantle plume activity. It is possible that lithospheric weakness caused by a mantle plume that
impacted Rodinia triggered the regional extension and produced the intra-continental rifting that preceded
the breakup of the Laurentian margin.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Global cycles of continental collision and breakup have been
recognized since the proposal of the plate tectonics model in the 60s
(Wilson, 1965, 1966). Although this model is widely accepted today, it
is still not clear what processes are responsible for those cycles, especially
the breakup of supercontinents. The geologic history of the eastern North
American (Laurentian) margin encompasses two complete Wilson
cycles that brought about the assembly and subsequent disaggregation
of two supercontinents, Rodinia and Pangea. The lithotectonic record
involves several orogenic events, including the composite Grenville
orogeny (~ 1.2–0.9 Ga) during the formation of Rodinia and three
major collisional events (Taconic ~ 490–440 Ma, Acadian ~ 420–360
and Alleghanian ~320–260 Ma) that built the Appalachian orogen and
culminated in the formation of Pangea (e.g., Hatcher, 2010). The record
also holds evidence of the breakup of each supercontinent, and therefore can provide key information concerning the plate tectonics model
⁎ Corresponding author. Tel.: +1 540 831 5430.
E-mail addresses: [email protected] (E. McClellan), [email protected] (E. Gazel).
1
Tel.: +1 540 231 2296.
http://dx.doi.org/10.1016/j.lithos.2014.08.006
0024-4937/© 2014 Elsevier B.V. All rights reserved.
and continental evolution. During the breakup of Pangea, the rift-todrift transition along the eastern North American margin began with
rifting in southeastern North America at ca. 230 Ma, followed by rapid
emplacement of mafic magmas as flood basalts and dikes at 200 Ma,
and initial seafloor spreading at 190–170 Ma (e.g., Labails et al., 2010;
Schettino and Turco, 2009; Schlische et al., 2003). In contrast to the
b50 million-year duration of events leading to the initial opening of
the Central Atlantic, fragmentation of the eastern Laurentian margin
of Rodinia was a protracted event that spanned ~ 200 million years,
from early intracontinental rifting to the onset of seafloor spreading.
Magmatic activity associated with final breakup of Rodinia and opening
of the Iapetus Ocean is broadly referred to as the Central Iapetus magmatic event (Ernst and Buchan, 1997). Basaltic rocks were emplaced
during three pulses, including the 615 Ma Long Range dikes in
Newfoundland and southeastern Labrador, the 590 Ma Grenville dike
swarm, and the 565–555 Ma Catoctin volcanics and related intrusions
(Ernst and Bleeker, 2010; Puffer, 2002). The Central Iapetus magmatic
event was preceded by Cryogenian (ca. 765–680 Ma) intracontinental
rifting and magmatism (e.g., Ernst and Bleeker, 2010). In this study
we present new field and geochemical evidence for the triggers and
processes related to this early stage of rifting of the Laurentian margin.
322
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
by Neoproterozoic to early Cambrian volcanic and sedimentary deposits.
Crystallization ages of basement gneisses and granitoids of 1.3–1.0 Ga
correspond to the timing of Grenville collisional orogenic events
(McLelland et al., 1996) related to assembly of Rodinia (Carrigan et al.,
2003; Tollo et al., 2004a, 2010). The basement rocks of the Blue Ridge
ai
ns
In the Blue Ridge province of the southern and central Appalachian
region, rocks that record the Proterozoic history occur primarily in two
allochthonous, fault-bounded inliers, the French Broad and Shenandoah
massifs (Fig. 1). These two inliers together extend N 700 km along strike
and expose Mesoproterozoic basement rocks unconformably overlain
40° N
hi
an
M
ou
nt
PA
Ap
p
al
ac
400 km
MD
765-565 Ma
Anorogenic
Magmatic
Events
Catoctin rhyolite,
564 ± 9 Ma
VA
Neopro terozoi c
Clastic sedimen tary strata;
minor interbeded volcanic s
Battle Mtn. felsite,
ca. 702 Ma
Anorogenic granitoid plutons
if
ss
do
ah
Ma
Charlottesville
Rockfish River, 680 ± 9 Ma
en
Polly Wright Cove, 706 ± 4 Ma
Mobley Mountain, 653 ± 19 Ma
Sh
Suck Mountain, 680 ± 4 / 727 ± 20 Ma
Roanoke
Fr
MRF rhyolites
~760-750 Ma
Area of Figure 2A
VA
TN
White Oak, 724 ± 3 Ma
Rivanna, 735 ± 4 Ma
an
Mesoproterozoi c
Grenville basement
Hitt Mountain, 706 ± 2 Ma
Catoctin basalt,
570 ± 36 Ma
Anorogenic mafic and felsic
volcanic s (includes Mount Rogers
& Catoctin Fms.)
Area intruded by
Bakersville dike swarm
Cobbler Mtn.,
722 ± 3 Ma
Laurel Mills,
729 ± 1 Ma
Amissville, 745 ± 9 Ma
Battle Mtn. granite,
705 ± 2 Ma
Arrington Mtn., 730 ± 4 Ma
Robertson River Igneous Suite
WV
40° N
Bristol
Beech
745 ± 6 Ma
Bakersville dikes
~734-728 Ma
fa
s
ie
ul t
VA
Striped Rock
ca. 748 Ma
NC
Lansing
739 ± 4 Ma
GMF rhyolite
742 ± 2 Ma
Winston-Salem
sif
as
Brown Mtn.
765 ± 7 Ma
Crossnore
ca. 754 Ma
Asheville
Stewartsville, 666 ± 10 Ma
White Oak Creek, 745 ± 9 Ma
Dillons Mill, 695 ± 16 Ma
ch
n
re
ad
M
0
o
Br
50
100 km
80° W
F
Fig. 1. Neoproterozoic anorogenic magmatic events along the Laurentian margin of eastern North America during the breakup of Rodinia, as represented by plutonic and volcanic rocks
presently exposed in the French Broad and Shenandoah massifs of the Appalachian Blue Ridge. See Table 1 for references to isotopic ages.
Modified from Burton and Southworth (2010) and Tollo et al. (2012).
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
province were subsequently affected by two episodes of crustal extension separated by N100 m.y.: a Cryogenian phase spanning the interval
765–700 Ma and an Ediacaran event at ~565 Ma (Aleinikoff et al., 1995;
Badger and Sinha, 1988). The latter event produced extensive flood
basalts, with intercalated rhyolites, of the Catoctin Formation (Badger
and Sinha, 1988; Reed, 1955), which record fragmentation of Rodinia
(Badger et al., 2010) and opening of the Iapetus Ocean (Aleinikoff
et al., 1995; Wehr and Glover, 1985).
During the earlier Cryogenian phase, however, the Mesoproterozoic
continental crust was intruded by numerous A-type felsic plutons and
extensional mafic dikes (e.g., Goldberg et al., 1986; Tollo et al., 2004b)
(Fig. 1, Table 1). At ~760–750 Ma a bimodal volcanic sequence erupted
onto the uplifted and eroded basement. This sequence, known as the
Mount Rogers Formation (Rankin, 1970, 1993), comprises a bimodal
basalt–rhyolite lower section, and an upper section of dominantly
peralkaline rhyolitic sheets (Fig. 2). The contact of the Mount Rogers
Formation with underlying 1.3–1.0 billion year old basement granitoid
rocks (Tollo et al., 2012) is interpreted as an unconformity (Rankin,
1993), although presently is a fault (Bailey and Rose, 1998) or a sheared
unconformity (McClellan et al., 2012a).
The Mount Rogers Formation is overlain by the Konnarock Formation
(Fig. 2), a sequence of sedimentary rocks interpreted as Cryogenian
glaciogenic deposits (Blondeau and Lowe, 1972; Miller, 1989; Rankin,
1993; Schwab, 1976). Although the two units are mostly separated
structurally by Paleozoic faults, Rankin (1993) recognized several areas
of stratigraphic contact. Merschat and Southworth (2011) mapped
volcaniclastic deposits and rhyolite of the Mount Rogers Formation
grading into and interbedded with laminated mudstone of the
323
Konnarock Formation, suggesting essentially contemporaneous volcanic activity and glaciation. Rhyolites, with associated basalts and clastic
sedimentary deposits, also occur in two volcanic centers that are structurally removed from the main body of the Mount Rogers Formation
(Rankin, 1993) (Fig. 2). The Razor Ridge volcanic center crops out to
the east and is at least in part dissected by faults, whereas the Pond
Mountain volcanic center occurs solely in the Catface thrust sheet,
west of and structurally below the Stone Mountain thrust sheet
(Bailey and Rose, 1998; Merschat and Southworth, 2011; Rubin and
Tollo, 2012; Tollo et al., 2012).
The driving mechanisms that trigger supercontinental breakup are
not well understood, and there is ongoing debate concerning the
relative roles of active rifting, related to upwelling of mantle material,
vs. passive rifting, related to crustal extension (e.g., Murphy and
Nance, 2013). For example, active processes related to mantle plumes
or a superplume are commonly invoked as causing rifting and fragmentation of Rodinia (e.g., Li et al., 2008; Wang et al., 2009), whereas a twostage process involving first passive, then active rifting has been
suggested for the breakup of Pangea (Hynes, 1990). We provide new
geochemical evidence from the well-preserved volcanic rocks of the
Cryogenian lower Mount Rogers Formation, with the goal of elucidating
the process that induced the initial stage of the breakup of Rodinia along
what would become the Eastern Laurentian margin.
2. Stratigraphy of the Cryogenian Mount Rogers Formation
The Mount Rogers Formation is divided into an upper section
dominated by rhyolitic volcanic rocks, and a lower sequence comprising
Table 1
Summary of the Neoproterozoic igneous rocks of the French Broad and Shenandoah massifs, southern and central Blue Ridge, as shown in Fig. 1.
Formation/pluton
Latitudea
Cryogenian plutons and volcanic rocks
Brown Mountain
35.930556
metagranite
Beech metagranite
36.203499
Crossnore metagranite
36.021794
Grandfather Mountain Fm. 36.190833
Bakersville dike swarm
Bakersville dike swarm
Lansing
Mount Rogers Fm.
Striped Rock
Dillons Mill
White Oak Creek
Stewartsville
Suck Mountain
Suck Mountain
Mobley Mountain
Polly Wright Cove
Rockfish River
Longitudea
Rock type
Isotopic age (Ma) Technique
Reference
−81.740833
Alkali granite
765 ± 7
U–Pb TIMS
Fetter and Goldberg (1995)
745 ± 6
754 ± 5
742 ± 2
U–Pb TIMS
U–Pb TIMS
U–Pb TIMS
Su et al. (1994)
Su et al. (1994)
Fetter and Goldberg (1995)
734
728
739
758
±
±
±
±
26
16
4
12
Rb–Sr
U–Pb TIMS
U–Pb TIMS
U–Pb TIMS
Goldberg et al. (1986)
Ownby et al. (2004)
Su et al. (1994)
Aleinikoff et al. (1995)
748
695
745
666
680
727
653
706
±
±
±
±
±
±
±
±
11
16
9
10
4
20
19
4
U–Pb TIMS
U–Pb SIMS
U–Pb SIMS
U–Pb SIMS
SHRIMP
U–Pb SIMS
U–Pb SIMS
U–Pb TIMS
Essex (1992)
Fokin (2003)
Fokin (2003)
Fokin (2003)
Tollo et al. (2004b)
Fokin (2003)
Fokin (2003)
Tollo et al. (2004b)
680 ± 9
U–Pb SIMS
Fokin (2003)
Tollo and Aleinikoff (1996)
Tollo and Aleinikoff (1996)
Tollo and Aleinikoff (1996)
Tollo and Aleinikoff (1996)
Tollo and Aleinikoff (1996)
Tollo and Aleinikoff (1996)
Fokin (2003)
Tollo and Aleinikoff (1996)
Tollo and Aleinikoff (1996)
Southworth et al. (2009)
−82.081969
−81.929558
−81.746111
Alkali granite
Alkalic to peralkaline granite
(Meta)rhyolite (with basalt and clastic
sediments)
Multiple locations Multiple locations Metadiabase dikes
36.075936
−82.087929
Mafic orthogneiss
36.501838
−81.513168
Alkali granite
36.637292
−81.607360
Whitetop Rhyolite (metarhyolite,
interpreted as lava)
36.659868
−81.160786
A-type granite
37.030032
−80.052387
Alkali-feldspar granite (metaluminous)
37.645151
−79.465202
Biotite granite (peralkaline)
37.225244
−79.804840
Biotite granite (metaluminous)
37.424440
−79.533890
Biotite granite (peraluminous)
37.428016
−79.549770
Biotite granite (peraluminous)
37.643056
−79.097923
2-feldspar biotite granite (metaluminous)
37.800560
−78.882500
Metaluminous biotite granite to
leucogranite
37.814440
−78.775280
Granodiorite
Robertson River (RR) igneous suite
RR — Rivanna
38.111111
RR — Hitt Mountain
38.246667
RR — White Oak
38.413056
RR — Arrington Mountain 38.462778
RR — Battle Mountain
38.648056
RR — Battle Mountain
38.576111
RR — Amissville
38.756845
RR — Laurel Mills
38.638056
RR — Cobbler Mountain
38.903333
RR — Quartz trachyte
38.799785
−78.464722
−78.368056
−78.238889
−78.246667
−78.067500
−78.058333
−78.036067
−78.096667
−77.931389
−78.168777
Biotite granite (metaluminous)
Alkali fsp syenite (metaluminous)
Alkali fsp granite (metaluminous)
Alkali fsp granite (metaluminous)
Felsite (peralkaline)
Alkali fsp granite (peralkaline)
Alkali fsp granite (peralkaline)
Granite (metaluminous)
Alkali fsp quartz syenite (metaluminous)
Quartz trachyte
735 ± 4
706 ± 2
724 ± 3
730 ± 4
ca. 702
705 ± 2
745 ± 9
729 ± 1
722 ± 3
714 ± 5
719 ± 6
U–Pb TIMS
U–Pb TIMS
U–Pb TIMS
U–Pb TIMS
U–Pb TIMS
U–Pb TIMS
U–Pb SIMS
U–Pb TIMS
U–Pb TIMS
SHRIMP
Ediacaran volcanic rocks
Catoctin Fm.
Catoctin Fm.
−78.642447
−77.149104
Basalt flows (greenstone)
Metarhyolite (interpreted as lava)
570 ± 36
564 ± 9
Rb–Sr (cpx) Badger and Sinha (1988)
U–Pb TIMS Aleinikoff et al. (1995)
a
38.033128
40.061012
Location approximated if not specified in reference.
324
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
A)
Cc
Zk
o
36
37 ‘
30 “
Cc
nt
u
Fa
Zmwt
Yg
ut
St one
Zmw
n t ai
Yg
n
Zmb
Zmr r
Zmwt
Zmu
ou
ou
nM
Zk
t
le
ul t
Yg
M
Ir o
ul
Fa
ai n
Tr
o
Da
Zmr r
t
Fa
ul
Fa
Zmwt
Zmbr
Zm br
lt
e
Ca t f a c
Zmp
Zmf
Zmb
Zml
Zmf
R
C
Zml
0
2
4
6
8 km
Yg
o
o
81 37 ‘ 30 “
B)
81 30 ‘
Chilhowee Group (Cambrian)
Wilburn Rhyolite
Zmwt
Whitetop Rhyolite
Zml
s
Zmb
Metabasalt (greenstone)
Zmf
Fees Rhyolite
Yg
Lower MRF
Zmbr Buzzard Rock Rhyolite
750
Zmw
Fees
Whitetop
755
Pond Mountain volcanic center (undiff.)
Neoproterozoic)
Zmp
Buzzard Rock
Mount RogersForma
Razor Ridge volcanic center (undiff.)
Upper MRF
Zmrr
Age in millions of years
Wilburn
Zk
760
Cc
745
Map Leyend
Granitoid gneiss (Mesoproterozoic)
Fig. 2. A. Geological map of Mount Rogers Formation, modified from Rankin (1993). Contacts in the lower Mount Rogers Formation from this study. Placement of the Stone Mountain fault
in North Carolina from Bailey and Rose (1998). B. Uranium-lead zircon CA-TIMS ages of rhyolites from the Mount Rogers Formation (Tollo et al., 2012). Bars represent preferred ages within
error: Fees — 753.1 ± 2.7 Ma; Buzzard Rock — 755.0 ± 6.6 Ma; Whitetop — 753.3 ± 2.0 Ma; Wilburn Ridge — 749.7 ± 3.1 Ma.
bimodal volcanics intermixed with clastic sedimentary rocks (Fig. 2).
The upper Mount Rogers Formation rhyolites were mapped and
described in detail by Rankin (1993) and have been the subject of later
studies (Novak and Rankin, 2004; Tollo et al., 2012). Rankin (1993)
also formalized the Fees Rhyolite Member of the lower Mount Rogers
Formation; however, the remainder of the lower section, described as
“complexly folded greywacke, phyllite, conglomerate, arkose, greenstone, and rhyolite” (Rankin, 1993, p. 7), remained undifferentiated.
Stratigraphy of the lower Mount Rogers, based on our recent detailed
mapping (McClellan et al., 2011, 2012a, 2012b) is described herein.
Volcanic and clastic sedimentary rocks of the Mount Rogers
Formation were metamorphosed to lower greenschist facies during
Paleozoic collisional tectonism (Rankin, 1993). Despite the metamorphic overprint and locally intense foliation development, identification
of protoliths is generally straightforward. Therefore, the prefix ‘meta-’
will be omitted in the descriptions below. Thickness estimates are
included, but may be imprecise due to complex deformation throughout the unit and subsequent erosion.
2.1. Upper Mount Rogers Formation
Rankin (1993) divided the upper Mount Rogers Formation into
three rhyolite units (Fig. 2), with an estimated total thickness of approximately 1850 m. Aleinikoff et al. (1995) obtained a U–Pb zircon age of
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
758 ± 12 Ma for the Whitetop Rhyolite. More recently, Tollo et al.
(2012) reported CA (chemical abrasion)-TIMS zircon ages for all three
units, ranging from ~ 750 to 755 Ma (Fig. 2). The Wilburn Rhyolite
is an extraordinary well-preserved ash-flow sheet (ignimbrite).
Geochemical interpretations suggest that the magma chamber was
compositionally zoned from metaluminous to peralkaline, and this relationship was inverted during eruption (Novak and Rankin, 2004). Fewer
published geochemical analyses exist for the other units; however,
these show that the Whitetop Rhyolite is geochemically similar to the
Wilburn, whereas the Buzzard Rock Rhyolite is lower in silica and less
chemically evolved (e.g., higher in FeOt, TiO2, Sc, V) than the other
units (Tollo et al., 2012). Major and trace-element data indicate that
the Mount Rogers rhyolites were formed in a within-plate setting and
are interpreted as part of an A-type suite of plutonic and volcanic
rocks (Fig. 1) related to intracontinental rifting (Novak and Rankin,
2004; Tollo et al., 2004b, 2012). Tollo et al. (2012) suggest the
Yellowstone system as a modern analog for the volcanic rocks.
2.2. Lower Mount Rogers Formation
In the lower Mount Rogers Formation, definition of the original
stratigraphic order is complicated by episodic eruption and local erosion
of underlying volcanic strata coupled with abrupt facies changes in the
clastic sedimentary deposits, all of which was modified by Paleozoic
folding (McClellan et al., 2011). In the following descriptions, the units
are arranged in their interpreted order, from oldest to youngest.
2.2.1. Fees Rhyolite
The Fees Rhyolite Member occurs at or near the base of the lower
Mount Rogers Formation. First described by Rankin (1993), the rhyolite
was also the subject of a study by James (1999). Our recent mapping has
extended the known outcrop area of the unit (Fig. 2), and we estimate a
maximum thickness of approximately 300 m. Single zircon CA-TIMS
analysis of the Fees yielded an age of 753.1 ± 2.7 Ma (Tollo et al.,
2012) (Fig. 2). The dominant rock type is a porphyritic rhyolite with
prominent phenocrysts of perthitic alkali feldspar and quartz and lesser
plagioclase (Fig. 3A, B), which distinguishes this unit from similar rhyolites in the upper Mount Rogers Formation (Rankin, 1993, and this
study). Locally, fiamme (elongate lenses, likely representing devitrified
flattened pumice clasts) are present in outcrop (Fig. 3A). Lithics within
the Fees Rhyolite include those of volcanic and plutonic origin. The
presence of ignimbrite textures, lithic clasts, and fiamme suggests that
the Fees Rhyolite is dominantly pyroclastic in origin. Within its outcrop
belt, the Fees Rhyolite is associated with coarse rhyolite porphyry
distinguished by abundant alkali feldspar and quartz phenocrysts. Feldspars are commonly up to 1 cm or more in cross section. This lithology is
similar to felsic rocks described by Rankin (1993) in the Pond Mountain
volcanic center (Fig. 2), and may represent a hypabyssal intrusive.
2.2.2. Basalt (greenstone)
Although previous workers recognized the occurrence of metabasalt
in the lower Mount Rogers Formation, its relative abundance was not
fully appreciated. Our mapping shows that mafic rocks dominate the
lower Mount Rogers Formation in the western part of its outcrop area
(Fig. 2). The rocks range from relatively undeformed basalt to greenstone or foliated greenschist, all having the typical chlorite and epidote
assemblages characteristic of the low-grade metamorphic overprint
(Rankin, 1993, and this study). Several different varieties occur
(Fig. 3C, D), including vesicular/amygdaloidal basalt, homogeneous
aphyric greenstone or greenschist, and plagioclase-phyric basalt
porphyry. The latter rock may represent a hypabyssal intrusive, but
relationships in outcrop show interlayering between vesicular flows
and the porphyry, suggesting an extrusive origin. In the study area
thickness estimates are complicated by folding, but we estimate a
minimum thickness of 1300 m. Lindsey (2010) calculated a thickness
of approximately 1600 m for lower Mount Rogers Formation basalts
325
exposed in the Trout Dale thrust sheet north of the present study area
(Fig. 2).
2.2.3. Clastic sedimentary rocks
The volcanic rocks are overlain by a sequence of coarse-grained
clastic rocks, comprising polymict conglomerate that grades laterally
into pebbly feldspathic to lithic wacke. Ranging from grain- to matrixsupported, the conglomerate contains cobble-sized to boulder-sized
clasts (Fig. 3E, F). Framework grain composition is dominated by
rhyolite clasts, many of which resemble Fees Rhyolite, and lithic clasts
of basement rocks, with lesser basalt, all of which appear to be locally
derived from the underlying rocks. Original bedding is occasionally
observed and is defined by layers of pebbly sandstone within the
conglomerate. The conglomerate grades laterally into coarse arkosic
sandstones that contain granules and pebbles of vein quartz, basement
granitoids, and rhyolite. Although the original geometry of the Mount
Rogers clastic deposits has been highly modified by later tectonic deformation, major changes in lateral and vertical facies and generally
wedge-shaped geometry are still evident (Jessee et al., 2012). The clastic
sedimentary rocks are interpreted as alluvial fan deposits, which likely
represent a progradational sequence formed during synsedimentary
faulting associated with crustal extension (McClellan et al., 2012b).
3. Data and methods
We present new major and trace-element data from 36 rhyolites
and basaltic lavas from the lower Mount Rogers Formation. Representative analyses are in Table 2; all analyses are provided in Supplementary
Table S2. During the course of detailed mapping we collected samples
from road cuts, streams, and mountain slope outcrops in Grayson
County, VA (GPS locations in Table 2). We also sampled the Pond
Mountain and Razor Ridge rhyolites during reconnaissance surveys. In
the lab, weathered portions of the samples were removed using a
water-lubricated saw. Samples were then fragmented into smaller
pieces and any remaining weathered material was manually removed.
Major and trace element data were collected at the certified
Activation Laboratories (http://www.actlabs.com/) in Ancaster,
Canada and reported in Table S2 (Supplementary materials). These
data were collected with a combined INAA/ICP-MS research-grade
package. For INAA, the methods used follow those of Hoffman (1992).
ICP-MS methods involved a lithium metaborate/tetraborate fusion of
the whole rock, then dissolved and analyzed by Perkin Elmer Sciex
ELAN 9000 ICP-MS. Three blanks and five controls (three before sample
group and two after) were analyzed per group of samples. Duplicates
were fused and analyzed every 15 samples. The instrument was
recalibrated every 40 samples. The accuracy for standards BIR-1a, W-2
and JR-1 run with samples was better than 10% relative to the certified
values for the trace elements and better than 1% for major elements
(with the exception of Na2O and P2O5, that were better than 7%). For
the duplicates precision was better than 1% and 5% RSD for major
and trace elements respectively. The blanks run with the samples
were below the detection limit for all the elements reported. All the
standards, duplicates and blanks are reported in Table S3 in the
Supplementary materials.
4. Results
New major (wt.%) and trace elements (ppm) are reported in Table 2
and Supplementary Table S2. Geochemical data from upper Mount
Rogers Formation rhyolites (Novak and Rankin, 2004; Tollo et al.,
2012) are included in the plots for comparison. The samples are bimodal
in composition, with one population of basaltic magmas that includes
lava flows and feeder dikes, and another of rhyolites (Fig. 4A). The
basaltic samples belong to the tholeiitic series (Fig. 4B). The few samples
that plot as trachybasalts (Fig. 4A) could be the result of mobility of
alkalis (Na2O, K2O) by secondary processes rather than belonging to
326
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
A)
B)
Afs
Qz
C)
D)
Pl
E)
F)
Fig. 3. Lithologies of the lower Mount Rogers Formation; all lithologies metamorphosed to lower greenschist facies. Mineral abbreviations from Whitney and Evans (2010). Petrographic
descriptions can be found in Table S1 of the Supplementary material. A. Outcrop of Fees Rhyolite with prominent aligned fiamme. B. Photomicrograph of Fees Rhyolite, characterized
by phenocrysts of embayed quartz (Qz) and perthitic alkali feldspar (Afs). Scale bar = 2 mm. C. Plagioclase-phyric basalt, described as “turkey-track” texture by Rankin (1993). Pl —
plagioclase. D. Basalt with vesicular texture. E. Polymict conglomerate dominated by clasts of pink granitoids (light-colored clasts) and rhyolite (dark-colored clasts). F. Clast-supported
conglomerate with boulder-sized clasts of rhyolite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
an alkaline series, as less mobile FeO(t) (Fig. 5A), and TiO2 correlate
negatively with MgO as expected in tholeiitic magmas (e.g., Zimmer
et al., 2010).
In the basaltic samples, major elements correlate negatively with
increasing MgO, with the exception of CaO and Al2O3 that correlate
positively, indicating fractionation of olivine and plagioclase as expected
in tholeiitic suites (Fig. 5A). Trace elements mostly show a negative
correlation (e.g., Hf in Fig. 5A) or do not vary with MgO (e.g., Sc in
Fig. 5A). The relatively constant values of Sc, ~ 30–40 ppm, and the
lack of correlation with MgO (Fig. 5A), suggest that the fractionation
of clinopyroxene was not significant in the evolution of these samples.
On multi-element diagrams (Fig. 6), samples display enrichment in
highly to moderately incompatible elements, with all samples having
steep rare earth element (REE) patterns (La/Yb, 5–10) and high-field
strength (HFSE, Th, Nb, Ta, Zr, Ti, Y, Hf) enrichments.
In the rhyolites, negative correlation between major elements
and SiO2 is common, controlled mostly by Na–K feldspar and Fe–Ti
oxides (Fig. 5B). Steep slopes in P2O5 and TiO2 can be explained by
crystallization of apatite and Fe–Ti oxides respectively (Fig. 5B). On
chondrite- and primitive mantle-normalized plots (Fig. 6) the samples
also present enrichments in highly to moderately incompatible
elements and steep REE patterns (La/Yb, 5–15). The rhyolite patterns
show slight depletions in Nb and Ta and significant depletions in Sr, P,
Eu, and Ti, that suggest the fractionation of feldspar together with
apatite and Fe–Ti oxides during the evolution of these rocks (Fig. 6).
5. Discussion
5.1. Geochemistry and magmatic processes
Although the Cryogenian volcanics from the Mt. Rogers location
underwent low-grade metamorphism, they are exceptionally preserved
considering the complex ~760 m.y. history of these rocks. Most of the
major and trace elements are also well preserved (Figs. 5, 6), especially
the REEs, the HFSE and other elements that are immobile during fluid–
rock interactions (Pearce, 2008). Thus, we feel confident in interpreting
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
327
Table 2
Major- and trace-element chemistry of representative samples from this study (full analyses in Supplementary Table S2). Major oxides reported as wt.% and trace elements as ppm.
Elements with − symbol were below detection limit. LMRF — lower Mount Rogers Formation.
Sample
P-11-167B
GC-11-289B
GC-11-271
TD-12-498A
GC-11-257
PM-1
RR-1
LMRF metabasalt Metabasaltic dike in basement LMRF Fees Rhyolite LMRF Fees Rhyolite LMRF Fees Rhyolite Pond Mountain Rhyolite Razor Ridge Rhyolite
Latitude
36.624126
36.610755
36.638161
36.612289
36.607977
36.71362
Longitude −81.523064
36.581268
−81.444566
−81.481656
−81.443754
−81.483952
−81.641113
−81.33965
Major elements
47.85
SiO2
14.18
Al2O3
13.58
Fe2O3
MnO
0.19
MgO
4.79
CaO
7.27
3.85
Na2O
0.63
K2O
2.69
TiO2
0.28
P2O5
LOI
3.39
Total
98.69
49.14
14.89
12.53
0.17
6.82
7.37
3.00
0.86
2.10
0.23
3.06
100.20
76.39
10.82
2.48
0.03
0.16
0.21
2.83
4.77
0.14
0.01
0.48
98.33
73.39
12.06
3.39
0.06
0.08
0.55
3.54
4.82
0.25
0.01
0.55
98.72
75.19
11.52
2.54
0.03
0.15
0.10
1.95
5.57
0.25
0.04
0.91
98.25
76.43
11.89
3.13
0.04
0.26
0.22
1.19
5.06
0.28
0.05
1.60
100.20
69.51
13.36
4.71
0.05
0.32
0.99
2.53
5.51
0.52
0.11
1.56
99.16
Trace elements
Ba
204
Sr
191
Ga
21
Ni
29
Co
43.8
Cr
29.3
Cu
159
Ge
1.5
Hf
4.5
Nb
16.5
Pb
–
Rb
12
Sb
0.30
Sc
34.00
Ta
1.02
Th
2.29
U
0.55
V
301
Y
33
Zn
102
Zr
182
La
20.0
Ce
45.1
Pr
5.78
Nd
25.5
Sm
6.31
Eu
2.01
Gd
6.21
Tb
1.03
Dy
6.14
Ho
1.23
Er
3.44
Tl
0.09
Tm
0.49
Yb
3.00
Lu
0.45
273
235
18
64
46.1
108
40
1.7
3.3
12.8
–
31
0.20
30.50
0.75
1.40
0.36
262
27
87
137
14.4
32.1
4.21
18.7
4.72
1.84
4.67
0.81
4.79
0.99
2.68
0.15
0.40
2.46
0.39
136
18
32
2
1.5
–
5
1.3
23.2
85.1
21
154
0.20
0.67
5.75
22.60
3.97
8
135
112
709
59.4
130.0
16.30
63.8
18.60
0.22
19.60
3.76
25.10
5.19
14.70
0.64
2.22
14.60
2.37
60
19
29
3
–
11.2
6
2.1
15.2
52.2
–
113
1.20
0.87
3.00
13.70
1.79
7
89
133
764
136.0
278.0
32.30
121.0
21.60
0.45
21.00
3.13
17.20
3.15
8.86
0.50
1.35
8.62
1.32
97
14
30
2
1.4
3.1
5
1.3
17.0
56.9
–
149
0.20
1.18
3.12
12.20
2.08
8
69
88
683
86.5
194.0
20.10
75.4
14.50
0.26
11.80
2.04
12.60
2.62
8.04
0.60
1.25
8.61
1.39
716
39
24
3
2.3
–
4
1.6
9.6
28.8
6
166
0.40
3.84
1.67
16.40
1.69
12
96
86
358
246.0
167.0
51.70
185.0
31.50
3.54
22.30
3.53
19.30
3.79
9.86
0.71
1.45
8.76
1.32
1257
106
25
2
–
10.4
12
2.1
14.0
30.4
–
139
–
6.26
1.83
7.99
1.81
17
73
80
692
101.0
183.0
26.00
99.4
17.50
2.47
15.90
2.39
14.10
2.61
7.49
0.84
1.11
6.69
0.97
the geochemical evidence from our samples to elucidate the magmatic
processes that produced them. As both basalts and rhyolites have
well-preserved REE and HFSE we use them to evaluate if there is a
cogenetic relation between the two. Based on the REEs, it is clear that
these samples belong to two different trends (Fig. 7A).
The basaltic samples belong to the tholeiitic series (Fig. 4B),
controlled by crystallization of olivine and plagioclase. These suggest
that the parental magma was relatively dry in composition as observed
today in MORB and some intra-plate settings (Zimmer et al., 2010). Both
the basalts and the rhyolites have steep REE patterns in chondritenormalized plots (Fig. 6A and B). In the basaltic samples, the variation
in heavy REE between the different samples is probably related to
melting depth, with the most depleted (Sm/Yb N 2) being produced in
the garnet stability field (Green and Ringwood, 1967; Hofmann and
Feigenson, 1983; Johnson, 1998) (Fig. 7A).
When normalized to a primitive mantle reference (Fig. 6B) it is
clear that the magmas that produced the Cryogenian basalts show
incompatible element patterns controlled by upwelling asthenosphere,
as they are enriched in LREE and HFSE (e.g. positive Nb, Ta, Ti), typical of
intraplate magmas (Hofmann, 1997; Jackson et al., 2008; Pearce, 2008).
The asthenospheric signature of the mafic magmas is confirmed by endmembers having Nb/U (Fig. 7B) values between 40 and 60, characteristic of melts derived from upwelling asthenosphere in MORB and OIB
settings (Hofmann et al., 1986).
The rhyolite patterns have HFSE depletions (Fig. 6D), as would be
expected if there was crustal interaction. It is possible that the rhyolites
328
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
A)
10
PhonoTephrite
Na2O+K2O
8
6
Trachyandesite
Tephrite or
trachyBasanite
andesite
Trachybasalt
Trachydacite
Dacite
LMRF basalts
4
Basalt
2
Rhyolite
Andesite
andesite
LMRF Fees rhyolites
UMRF Wilburn Ridge rhyolites
Picro-
other UMRF rhyolites
basalt
Pond Mountain rhyolites
Razor Ridge rhyolites
0
40
45
50
55
60
65
70
75
80
SiO2
B)
FeO*
Tholeiitic
5.3. A deep source for the breakup of Rodinia
Calc-Alkaline
Mt. Rogers Cryogenian Basalts
basaltic flows
basaltic dikes
Alk
Mesoproterozoic basement that was intruded by the Neoproterozoic
Brown Mountain alkali granite. To constrain the age of the GMF
sequence, Fetter and Goldberg (1995) obtained U–Pb zircon ages on
the underlying Brown Mountain Granite (765 ± 7 Ma) and the
stratigraphically highest rhyolite in the GMF (742 ± 2 Ma).
Other than the Brown Mountain Granite, Cryogenian plutons in the
French Broad massif range in age from ~ 748 to 739 Ma (Fig. 1,
Table 1) and were intruded by a suite of diabase dikes and gabbros of
the Bakersville dike swarm between ~ 734 and 728 Ma (Goldberg
et al., 1986; Ownby et al., 2004). Along strike in the Shenandoah massif,
anorogenic granitoid plutons appear to represent two age groups.
Plutons N700 Ma are concentrated in the Robertson River Igneous
Suite (Tollo and Aleinikoff, 1996), whereas a small group of b700 Ma
plutons is restricted to an area between latitudes 37°N and 38°N
(Figs. 1, 8). With the exception of this latter group of younger plutons,
magmatism in the French Broad and Shenandoah massifs generally
appears to have proceeded from older to younger along strike to the
northeast. In this area, there is no record of igneous activity subsequent
to the youngest of the latter group of plutons (Mobley Mountain,
~ 653 Ma, Fokin, 2003) until outpouring of the Catoctin flood basalts
(Figs. 1, 8) at ~570–565 Ma (Aleinikoff et al., 1995; Badger and Sinha,
1988), an event that corresponded to the final breakup of Rodinia and
opening of the Iapetus Ocean (Aleinikoff et al., 1995; Badger et al.,
2010; Wehr and Glover, 1985). Therefore, a period of 100–150 m.y.
separated Cryogenian anorogenic magmatism and crustal extension,
and the Ediacaran volcanism that marked successful rifting of the
margin.
MgO
Fig. 4. Geochemical classification based on major element compositions of the Cryogenian
Mount Rogers Formation volcanic rocks, indicating bimodal volcanism represented by
basalts and rhyolites. Samples normalized to 100% in a water-free basis. Abbreviations:
LMRF — lower Mount Rogers Formation. UMRF — upper Mount Rogers Formation.
were produced when asthenosphere-derived melts interacted with the
so-called “hot zone”, a region at the boundary where basaltic melts
pond and interact with the lower crust, as the samples have HREE
depletions indicative of a garnet-bearing residue common in the lower
crust (Annen et al., 2006; Hildreth and Moorbath, 1988). This interpretation is consistent with the Nb/Ta–Zr/Sm systematics shown in Fig. 7C
(Foley et al., 2002), that suggests a mixing between mantle derived
melts and melts produced by lower crustal lithologies like amphibolite
and rutile-free eclogite.
5.2. Temporal and spatial variations of the rifting of Rodinia at the
Laurentian-eastern North America Margin
Remnants of Cryogenian anorogenic magmatism that preceded
breakup of Rodinia and opening of the Iapetus Ocean are exposed over
a distance of ~ 700 km in the French Broad and Shenandoah massifs
(Fig. 1) and show a general pattern of younging from southwest to
northeast (Figs. 1, 8, Table 1). Volcanic rocks of this age are dominantly
preserved in the Mount Rogers and Grandfather Mountain Formations
of the French Broad massif (Fig. 1). The Grandfather Mountain
Formation (GMF) comprises a thick sequence of clastic alluvial fan
deposits interbedded with basalt and rhyolite (Schwab, 1977). Neton
(1992) recognized five coarsening upward basin fill sequences indicating discrete episodes of rifting. The GMF unconformably overlies
The Cryogenian episode of crustal extension has been considered to
represent an initial stage of rifting or a “failed rift” that did not lead to
continental separation and opening of an ocean basin (Aleinikoff et al.,
1995; Rankin, 1993). Attempts to explain the mechanism of this phase
of rifting, and the gap in time between it and the successful opening of
Iapetus at ~565 Ma, include an origin as a failed arm of a triple junction
(Rankin, 1976, 1993), a plume track forming a zone of crustal weakness
that was exploited during the later extension (Fokin, 2003), and a rifttransform model that created fault-bounded blocks having independent
extensional histories (Burton and Southworth, 2010).
The use of HFSE and the HREE has been recognized as a particularly
useful tool for the determination of the tectonic processes that triggered
magmatic production, not only because different tectonic environments
today produce magmas with particular HFSE signatures but also
because these elements are less mobile during secondary processes
such as metamorphism and weathering (Condie, 2005; Fitton et al.,
1997; Pearce, 2008). Herein, we use different HFSE (e.g., Nb, Ti, Zr, Y,
Yb) proxies to provide evidence of the tectonic environment that produced the Cryogenian Mount Rogers Formation volcanic rocks (Fig. 9).
Basaltic magmas from the Mount Rogers Formation were produced in
an intra-plate setting (Fig. 9A). Origin of the magmas by melting of an
enriched mantle, combined with interaction with lithospheric mantle
that was metasomatized by fluids, is suggested by the near vertical
increase in Th/Yb at near constant Nb/Yb (Fig. 9B, Pearce, 2008). The
Y–Nb systematics of the rhyolites coupled with Rb (a fluid mobile
element) also confirms an intraplate tectonic setting for the Cryogenian
event (Fig. 9C, Pearce et al., 1984). The Zr/Y–Nb/Y systematics of the
basalts (Fig. 9D) suggests the requirement of a component common in
modern ocean island basalts derived from recycled oceanic crust, and
a deep depleted component common in large igneous provinces. Both
of these components are associated with mantle plume activity
throughout Earth history (Condie, 2005). This association is confirmed
by the dominantly positive ΔNb (ΔNb = − 1.92log(Zr/Y) + 1.740 +
log(Nb/Y), Fitton et al., 1997) values of the lavas (Fig. 9D) that can be
explained by a mantle plume source in the production of the Cryogenian
mafic lavas (Fitton et al., 1997; Jackson et al., 2008).
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
A)
18
329
18
17
17
16
16
14
ol +
Al2O3
FeOT
15
pla
g.
13
15
g.
14
l+
o
12
basalts (LMRF)
13
11
10
pla
2
3
4
5
6
7
8
12
9
8
2
3
4
5
6
7
8
9
50
45
6
40
Hf
Sc 35
4
ol +
ol + plag.
pla
g.
30
2
25
0
2
3
4
5
6
7
8
20
9
2
3
4
5
MgO
6
0.7
5
0.6
es
0.4
es
FeO
9
id
ox
io
xid
2
0.3
0.2
1
0
70
8
Ti
0.5
Fe
-T
3
7
Fe
4
TiO2
B)
6
MgO
0.1
0.0
72
74
76
78
70
80
72
74
76
78
80
0.20
16
Cryogenian rhyolites
15
Fees (LMRF)
Wilburn Ridge (UMRF)
other UMRF
Pond Mountain
Razor Ridge
0.15
Na
-K
13
feld
P2O5
Al2O3
14
sp
ar
0.10
12
0.05
11
10
70
72
74
76
78
80
SiO2
0.00
70
72
74
76
78
80
SiO2
Fig. 5. Variations of major element compositions from basaltic and rhyolite samples from the Cryogenian Mount Rogers Formation, and inferred crystallization phases. Additional data for
the upper Mount Rogers Formation from Novak and Rankin (2004) and Tollo et al. (2012). Abbreviations as in Fig. 4.
When the geochemical compositions of the Cryogenian lavas are
compared with samples from two modern continental intra-plate
settings in multi-element spider diagrams, the East African Rift and
the Yellowstone–Snake River Plain system, both the basaltic and rhyolitic samples share geochemical similarities with modern lavas from
those two intra-plate locations (Fig. 6). The rhyolites are closer to the
Yellowstone samples as they also share Nb–Ta depletions suggestive
of a higher degree of crustal interaction than the ones from the East
African Rift, but the overall patterns are comparable to both locations.
The similarity between modern magmas produced in the Yellowstone–
Snake River Plain system and the Cryogenian lavas is also notable in
the trace-element diagrams in Fig. 9.
Geochemical, geophysical and tectonic evidence suggests that the
common denominator controlling the melting processes in the East
African Rift and the Yellowstone–Snake River Plane system is deep
mantle plume activity (e.g., Dodson et al., 1997; Furman et al., 2006;
330
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
A) 1000
B) 1000
Mount Rogers Fm. Basalts
Rock/Primitive Mantle
Rock/Chondrites
Mount Rogers Fm. Basalts
100
10
East African Rift
100
10
1
East African Rift
Yellowstone-Snake River Plain
Yellowstone-Snake River Plain
1
.1
Ce
La
Nd
Pr
Sm
Pm
Gd
Eu
Dy
Tb
Er
Ho
Yb
Tm
U
Lu
C) 1000
Th
Ta
Nb
La
K
P
Sr
Zr
Nd
Eu Dy Yb
Sm Ti
Y
Lu
D) 1000
Mount Rogers Fm. Rhyolites
Rock/Primitive Mantle
Mount Rogers Fm. Rhyolites
Rock/Chondrites
Pr
Ce
100
10
East African Rift
100
10
East African Rift
1
YellowstoneSnake River Plain
YellowstoneSnake River Plain
1
.1
Ce
La
Nd
Pr
Sm
Pm
Gd
Eu
Dy
Tb
Er
Ho
Yb
Tm
U
Lu
Th
Ta
Nb
La
K
Pr
Ce
P
Sr
Zr
Nd
Eu Dy Yb
Sm Ti
Y
Lu
Fig. 6. Incompatible element compositions of samples from the Cryogenian Mount Rogers Formation. A and B are REE patterns normalized to chondrites (Sun and McDonough, 1989) and
C and D are multi-element diagrams normalized to primitive mantle (McDonough and Sun, 1995). The shaded areas are samples from East African Rift and Yellowstone from the GEOROC
database for comparison (http://georoc.mpch-mainz.gwdg.de/georoc/).
Nyblade et al., 2000; Scarsi and Craig, 1996; Schutt and Dueker, 2008).
Applying a uniformitarianism approach to the new geochemical
evidence presented here, we suggest that the initial phase of extension
of the eastern Laurentian margin at ~ 760 was triggered by mantle
plume activity. Lithospheric weakness caused by such a mantle plume,
as suggested by numerical modeling in other plume–lithosphere interaction scenarios (Sobolev et al., 2011), may have triggered the later
regional extension and intra-continental rifting to form the rifted margin
(Fig. 10).
Li et al. (2003) assembled evidence that points to two major phases
of bimodal anorogenic magmatism affecting Rodinian continental
blocks, the first spanning ~840–790 Ma (South China, Australia, India),
and the second spanning ~780–720 Ma (South China, Australia, India,
Laurentia, and southwestern Africa). Large-scale lithospheric doming
and unroofing, consistent with plume activity, accompanied or preceded
these events. Recognizing that the widespread nature and duration of
the magmatic events required a large, sustained heat source, they
proposed the existence of a mantle superplume underneath Rodinia,
which eventually led to its breakup. The superplume is interpreted to
have risen first in the northern polar and western regions of Rodinia
(Li et al., 2003, 2008) with widespread magmatism and rifting affecting
South China, Australia and India ~825 Ma. After a global hiatus in plumerelated magmatism, and rotation of Rodinia away from the polar region,
rifting and magmatism related to a second plume breakout spread
across the rifting supercontinent to Laurentia and its conjugate
blocks (Li et al., 2008). The Rodinian plume may have taken the form
of a “superswell” (e.g., Davies, 1999), a broad mantle upwelling that
produced clusters of normal-sized plumes (Ernst and Buchan, 2003).
Identifying evidence for plumes in the geologic past is complicated by
subsequent erosion and, commonly, deformation due to later tectonic
events, including continental collision. Such is the case along the eastern
margin of Laurentia. Plume-related features may be preserved in the
geologic record, however, and include broad domal uplifts, dike swarms,
flood basalts, and intracontinental rifting and breakup (Ernst and
Buchan, 2003). Significant domal uplift that precedes rifting is commonly
associated with plumes (e.g., Campbell, 2005, 2007; Ernst and Buchan,
2003; Şengör and Natal'in, 2001), commonly tens of millions of years
before magmatic activity (Rainbird and Ernst, 2001), and may lead to
a widespread sedimentary hiatus between basement rocks and
plume-related volcanics (Li et al., 2003). In the region of this study, a
significant unconformity exists between Mesoproterozoic basement
rocks and volcanic and sedimentary deposits of the Mount Rogers and
Grandfather Mountain Formations in the French Broad massif. To the
north in the Shenandoah massif, Neoproterozoic sedimentary rocks
occur in the Mechum River and Swift Run Formations. Although the
exact ages of these units are uncertain, a maximum depositional age
for the Mechum River Formation is indicated by clasts of ~730 Ma granite of the Robertson River suite contained in conglomerates (Bailey
et al., 2007). The younger Swift Run clastic rocks interfinger with basalts
of the Catoctin Formation (Bailey et al., 2002; Jonas and Stose, 1939;
Southworth and Brezinski, 1996); therefore both units are younger
than the Mount Rogers and Grandfather Mountain Formations. Detrital
zircon data for Neoproterozoic and Paleozoic sedimentary rocks of
the southern and central Appalachians show a predominance of
Mesoproterozoic “Grenville” ages, along with peaks at ~ 780 Ma
and younger, but a near total absence of ages between ~ 900 and
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
A)
331
40.5
20
40.0
15
39.5
10
39.0
Basalts
Ediacaran
5
garnet
0
1.0
1.5
2.0
2.5
3.0
Sm/Yb
B) 2.5
Latitude Today
La/Yb
Cryogenian
Rhyolites
Melt fraction/
source enrichment
38.5
38.0
37.5
37.0
36.5
Lower Mt. Rogers Fm.
2.0
La/Nb
36.0
1.5
35.5
500
550
600
650
700
750
800
Age in millions of years
lithosphere
Nb/U<20
1.0
0.5
10
Mixing
20
Asthenosphere
(MORB-OIB, Nb/U=30-50)
30
40
50
60
Nb/U
C)
30
25
PM
DM: Depleted mantle
Nb/Ta
20
15%
PM
15
DM
5%
10
Lower Mt. Rogers Fm.
Amphibolite
(batch)
Amphibolite
1%
5
Rhyolites
0
1
10
100
1000
Zr/Sm
Fig. 7. Assessment of melting conditions using non-fluid mobile REE and HFSE ratios
of Cryogenian lavas from Mt. Rogers Formation. A. Melt fraction/source enrichment and
garnet in the source evaluation. In both ratios Sm as a denominator was used to simplify
mixing as straight lines. Assuming that the basalts and rhyolites had a similar source, it
is possible that the rhyolites were produced by different melt fractions and mixing between the extreme end-members. The effect of melt fraction is not as obvious in the basalts, but there is evidence of higher garnet component in the residue as indicated by
the increase in Sm/Yb (e.g., Green and Ringwood, 1967; Hofmann and Feigenson, 1983;
Johnson, 1998). The array of data from the basalt samples can also be explained by mixing.
B. La/Nb and Nb/U ratios of the Cryogenian basalts from Mt. Rogers suggest mixing (black
lines) between lithospheric and asthenospheric mantle components (Hofmann, 1997;
Hofmann et al., 1986). C. Possible source compositions for the Cryogenian volcanic
samples presented in this study from Foley et al. (2002). Note that according to this
diagram the basaltic samples are consistent with peridotite sources and the rhyolites are
better explained by lower crust sources as rutile-free eclogite source and/or amphibolite
with mixing toward mantle components, thus suggesting the interaction between
mantle-derived melts and lower crust lithologies as suggested in other scenarios
(e.g., Annen et al., 2006; Hildreth and Moorbath, 1988).
Fig. 8. Spatial variation of plutonic and volcanic events in the central and southern
Appalachian Blue Ridge (French Broad and Shenandoah massifs) related to the breakup
of the eastern North American margin of Rodinia, in present-day coordinates. Stage 1,
Cryogenian intracontinental rifting, is represented by plutons and volcanic rocks that
generally young from south to north (squares), and a regional cluster of slightly younger
plutons (diamonds). Stage 2, final rift to drift transition, is represented by Ediacaran
volcanic rocks of the Catoctin Formation.
780 Ma (e.g., Bream et al., 2004; Chakraborty et al., 2012; HolmDenoma et al., 2012; Holm-Denoma et al., in press). These data suggest
considerable uplift and erosion of Mesoproterozoic basement rocks
prior to extrusion of the Mount Rogers/Grandfather Mountain sequence, consistent with a pre-rift doming event.
In the French Broad massif, the ~ 730 Ma Bakersville mafic dike
swarm intrudes Mesoproterozoic basement over an area at least
200 km long and 25 km wide. Where most densely concentrated
(Fig. 1), dikes comprise up to 20–50% of the total rock mass, although
5–15% is more typical (Goldberg et al., 1986). Similar dikes are abundant
in the Mount Rogers area and continue to the north end of the exposed
French Broad massif (Burton and Southworth, 2010). The linear N to
NNE trend of the dikes does not appear to form the radiating pattern
typical of plume-related dike swarms, although the present orientation
may be partially due to realignment during Paleozoic deformation.
Mafic dikes are also abundant in the northern part of the Shenandoah
massif; however, the chemical similarity and proximity to basalts of
the Catoctin Formation suggest that these are younger feeder dikes to
the Catoctin basalt flows (Burton and Southworth, 2010), related to
the ~565 Ma rifting and opening of the Iapetus ocean basin. Therefore,
preserved Cryogenian rocks in the Shenandoah massif are dominated
by granitoid plutons and rare felsic volcanics.
To the east of the study area, the Fries fault (Fig. 1) juxtaposes
the Ashe Metamorphic Suite (Abbot and Raymond, 1984) against
Mesoproterozoic basement. Amphibolite of probable volcanic origin comprises up to 50% of the Ashe (Rankin et al., 1973), and Bartholomew
et al. (1983) suggested that the Bakersville dikes might have been
feeders to the Ashe volcanics. On the other hand, Abbot and Raymond
(1984) interpreted the Ashe as having been deposited on oceanic
crust. Uncertainties concerning the age and depositional setting of the
Ashe and its structural relationship with the French Broad massif
therefore preclude a demonstrable correlation with the Cryogenian
332
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
B)
A)
10
Ti/100
Basalts
Rhyolites
Modern Cont. Intraplate
1
RB
O
.1
Island-arc
O
RB
Y-SRPB
Ocean-floor
C
M
Int
AFC
EM
D
ra-
pla
t
e
East African
A
Lithosphere interaction/
Th/Yb
Yellowstone-SRP
O
IB
Yellowstone
-Snake River
Plain
Calc-alkalic
.01
Zr
.1
Y*3
C)
S
CE
R
OU
E
10
Enriched
Compoent
.1
Ocean Ridge
Volcanic Arc
OIB
Recycled
Componet
PL
Nb/Y
Yellowstone
-Snake River
Plain
Rb
S
UM
1
East African
100
100
10
Intra-Plate
Syn-collisional
10
Nb/Yb
D)
1000
1
ES
Deep Depleted
Componet
E
U
SO
M
U
ne Shallow Depleted
PL
Mantle
b
N
N
MORB NO
Δ
Li
1
RC
ARC
.01
10
100
1000
Y+Nb
1
Zr/Y
10
Fig. 9. Tectonic discrimination diagrams for the Cryogenian volcanic rocks compared to modern intra-plate magmas from East-African Rift and Yellowstone–Snake River Plain (SRP)
(http://georoc.mpch-mainz.gwdg.de/georoc/). A. Zr–Ti–Y diagram from Pearce and Cann (1973), for basaltic rocks with CaO + MgO 12–20 wt.%. Notice that the Cryogenian basalts
from Mount Rogers plot in the intra-plate area together with samples from the East African Rift and some overlap with Yellowstone-SRP basalts. B. Rb–Y + Nb diagram from Pearce
et al. (1984) for granitic rocks but also widely used for rhyolites. The Cryogenian rhyolites clearly plot in the intra-plate setting within the area defined by rhyolites from the
Yellowstone–Snake River Plain (SRP). D. Plume vs non-plume components from Condie (2005) for the Cryogenian basaltic samples consistent with the involvement of plume sources
with positive ΔNb (mixing between a deep depleted component and OIB recycled component, Fitton et al., 1997).
rift event. The true extent of Cryogenian mafic dikes and flows, as well
as related felsic plutonism and volcanism, is unknown, as the rocks
are overridden by Paleozoic faults on the east and buried by late
Neoproterozoic to Paleozoic sediments to the west and south. The
present exposure of the French Broad and Shenandoah massifs for
greater than 700 km along strike (Fig. 1), however, is consistent with
a plume head of ‘normal’ 1000-km radius (Ernst and Buchan, 2003)
particularly if we accept that some of the evidence is buried, eroded,
or perhaps exists on the rifted conjugate margin.
5.4. Conjugate margins and tectonic reconstructions
At present we can only speculate as to the identity of the conjugate
margin that subsequently rifted from eastern Laurentia. The configuration of assembled Rodinia remains controversial (e.g., Evans, 2009; Li
et al., 2008), as does the paleogeographic positions of cratonic blocks
during Neoproterozoic supercontinental breakup (Meert et al., 2013).
Of the many competing models of Rodinia at ~ 1 Ga, most show the
Amazonian, Rio de la Plata, or Kalahari cratonic blocks (Fig. 11)
positioned close to the eastern or southern Laurentian margin (this
and discussion below is in present day coordinates).
5.4.1. Possible South American elements
Paleomagnetic, isotopic, and geologic evidence support the conventional juxtaposition of the Rondonia–Sunsas orogenic belt in southwestern Amazonia with the Grenville province of eastern Laurentia
(e.g., Chew et al., 2010; Dalziel, 1991; Hoffman, 1991; Li et al., 2008;
Pisarevsky et al., 2003; Sadowski and Bettencourt, 1996; Weil et al.,
1998). Tohver et al. (2002) suggested that Amazonia first collided
with the Llano segment of the Laurentian Grenville belt (present-day
Texas) at ~ 1.2 Ga, and later proposed that collision was followed by
strike–slip transport northward along the Laurentian margin (Tohver
et al., 2004). Based on similarities in Pb isotopes, Loewy et al. (2003)
interpreted 1.3–1.0 Ga basement in the present central and southern
Appalachians as an allochthonous block transferred to Laurentia from
Amazonia during collision at ~ 1.0 Ga. With the limited pieces of the
puzzle there is strong support for the interaction of Amazonia with
some portion of the eastern Laurentian margin during the assembly
of Rodinia. Some reconstructions also depict the Rio de la Plata craton
of South America (Fig. 11) near eastern Laurentia during the assembly
of Rodinia (e.g., Dalziel et al., 2000; Li et al., 2008; Weil et al., 1998).
Representing the oldest cratonic block in present South America, the
Rio de la Plata is largely covered by younger sediments and definition
of its boundaries is subject to discussion (Oyhantçabal et al., 2011;
Rapela et al., 2011).
The record of the breakup history of South American elements of
Rodinia is sparse in comparison to eastern Laurentia (Chew et al.,
2010). Neoproterozoic (774 ± 6 Ma) orthogneisses associated with
Mesoproterozoic basement in Argentina may represent early stages of
a north-migrating rift event between Amazonia and Laurentia (Baldo
et al., 2006). Mišković et al. (2009) likewise equated ~ 752–691 Ma
A-type granites in the Eastern Cordillera of Peru with Amazonia–
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
A)
Plume head impact
X
Y
X’
B)
Y’
X’
X
Lithospheric expansion (dome) above plume head
0
Depth (km)
333
200
Lithospheric
Plume Head
400
0
150
200
Ambient asthenosphere
o
Excess Temperature C
600 0
500
1000
1500
2000
Distance (km)
C)
Y’
Y
Depth (km)
0
200
400
0
150
Ambient
asthenosphere
200
o
Excess Temperature C
6000
500
1000
1500
2000
Distance (km)
Fig. 10. Schematic model of the possible events that triggered the breakup of Rodinia, modified from Sobolev et al. (2011). A plume head impacted Rodinia producing lithospheric
expansion above the plume head, followed by lithospheric spreading and erosion that evolved into rifting. Excess temperature represents temperature above ambient asthenosphere
provided by the mantle plume, which in modern conditions is about 200 °C, above ambient mantle.
Laurentia rifting. Orthogneisses from the Dom Feliciano belt that
borders the Rio de la Plata craton indicate a magmatic event between
850 and 750 Ma that was suggested to be related to rifting
(Oyhantçabal et al., 2009, and references therein), although recent
geochemical data indicate a continental arc setting for these rocks
(Lenz et al., 2013). Evidence for a Cryogenian intracontinental rift
volcano-sedimentary sequence in South America is even more equivocal. Despite the presence of epicontinental and passive margin-type
Neoproterozoic basins thought to have formed in response to extension,
datable volcanic rocks associated with these basins are rare to absent
(Teixeira et al., 2007). The Puncoviscana basin in northwest Argentina
comprises a N 1000 m thick sequence of turbiditic metasedimentary
rocks, containing minor trachytes and basalts (Ježek and Miller, 1987;
Keppie and Bahlburg, 1999; Omarini et al., 1999). Immature siliciclastics
and interlayered volcanic flows, intruded by mafic dikes, that occur near
the base of the sequence were suggested to represent rifting from the
Laurentian margin at 750 Ma (Omarini et al., 1999), although Franz
and Lucassen (2001) pointed out a lack of age control and limited geochemical data that would support this interpretation. Alternatively,
Keppie and Bahlburg (1999) interpreted the Puncoviscana Formation
as a foreland basin deposited during the formation of a magmatic arc
prior to the Early Cambrian Pampean orogeny.
5.4.2. Possible African elements
The Kalahari craton of present-day Africa (Fig. 11) has also been
connected with Laurentia during Rodinian assembly, with the ~ 1.2–
1.0 Ga Namaqua-Natal belt of the Kalahari commonly shown facing
the Grenville belt of southern Laurentia (present-day Texas) (Dalziel,
1991; Hoffman, 1991; Li et al., 2008). Paleomagnetic data are inconclusive, however, and allow for alternative models that call for either
wide latitudinal separation between Kalahari and southern Laurentia
(Dalziel et al., 2000; Hanson et al., 2004), or no interaction between
the two (Cordani et al., 2003; Evans, 2009; Pisarevsky et al., 2003). Li
et al. (2008) recognized the basis for the separation noted above at
~1110 Ma, but maintained that the APWPs allow for the continents to
have merged by ~1000 Ma. Although few reconstructions put Kalahari
adjacent to eastern Laurentia, the geological similarity between
Neoproterozoic successions on the Kalahari margin and the Mount
Rogers and Grandfather Mountain Formations is notable. The Port
Nolloth Group, located in the Gariep belt on the western margin of
the Kalahari craton (Alchin et al., 2005; Frimmel et al., 1996; Jacobs
et al., 2008; Macdonald et al., 2010a) rests on Mesoproterozoic and
Paleoproterozoic rocks of the Namaqua-Natal belt. Sedimentation
initiated in the Rosh Pinah graben with coarse siliciclastic and bimodal
volcanic rocks followed by laterally discontinuous diamictites of the
334
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
Baltica
in
North America
g
ar
n
ia
t
en
M
ur
La
West Africa
Tanzania
Sao Luis
Amazonia
Congo
Sao Francisco
Rio Apia
Kalahari
Rio de la Plata
Fig. 11. Present-day location of cratonic blocks (in gray) that may have interacted during assembly and breakup of Rodinia. See text for discussion.
Kaigas Formation, which contain gravel- to boulder-sized clasts of the
pre-Gariep basement (Alchin et al., 2005; Macdonald et al., 2010a).
The Kaigas has been interpreted as glaciogenic (e.g. Frimmel et al.,
1996), although this interpretation is questioned (Macdonald et al.,
2010a, 2010b). Overlying the Kaigas is the Rosh Pinah Formation,
comprising rhyolitic flows and volcaniclastic rocks interlayered with
siliciclastic and subordinate calcareous rocks. Rhyolite flows yielded
ages of 752 ± 6 Ma (Borg et al., 2003) and 741 ± 6 Ma (Frimmel
et al., 1996). Mafic rocks have also been identified from drill cores, and
geochemistry of the felsic and mafic rocks points to a continental rift
environment (Frimmel et al., 1996). The overlying Numees Formation
contains massive and stratified diamictites with dropstones, interpreted
as deposited during the ~716.5 Ma Sturtian glacial episode (Macdonald
et al., 2010a, 2010b).
A potential connection between the Kalahari margin and eastern
Laurentia is difficult to reconcile with the timing of incorporation of
Kalahari into Gondwana, as noted by Jacobs et al. (2008), and similarity
to the MRF may simply reflect rifting that was widespread across
Rodinia at ~ 750 Ma, along with the global effects of the subsequent
Sturtian glaciation. However, the stratigraphic resemblance to the
~750 Ma bimodal, rhyolite-dominated, volcanic rocks and interlayered
coarse clastic rift-basin sediments of the MRF, which rest unconformably
upon ~ 1.3–1.0 Ga plutonic basement and are overlain by glaciogenic
deposits of the Konnarock Formation, is striking.
in activity until extrusion of basalt and minor rhyolite at ~570–555 Ma,
during relatively rapid extension that led to continental breakup and
opening of the Iapetus Ocean.
New and existing geochemical data from rhyolites and basalts in the
Mount Rogers Formation suggest that a mantle plume triggered the
breakup of Rodinia, as they share geochemical signatures with recent
volcanic rocks from the East African Rift and Yellowstone–Snake River
Plain. The basaltic samples were produced by melting mantle sources
with deep depleted and recycled components common in mantle
plume samples (positive ΔNb) and the rhyolites appear to be melts
that resulted from the interaction of mantle-derived melts with the
lower crust.
The Cryogenian volcanic and plutonic rocks show a general pattern
of younging from southwest to northeast, and a regional unconformity
between Mesoproterozoic Grenville basement and the ~ 750–760 Ma
Mount Rogers and related rocks could signal pre-rift doming and uplift.
Together, the field and geochemical data are best explained by a mantle
plume that impacted Rodinia at ~765–700 Ma, triggering the regional
extension and intra-continental rifting of that preceded breakup of the
Laurentian margin.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.lithos.2014.08.006.
6. Conclusions
Field work and sample collection was supported by a Radford
University Faculty Seed Grant and the U.S. Geological Survey, National
Cooperative Geologic Mapping Program, under assistance Award Nos.
G11AC20134 and G13AC00108 to McClellan. Analytical work was
made possible through an RU College of Science and Technology Faculty
Research Grant to McClellan. The project was also supported by
GeoPrisms-NSF award EAR-1249412 to Gazel. Many thanks go to
Radford University students J. All, R. Balderas, M. Brett, M. Jessee, D.
During Neoproterozoic breakup of Rodinia, the eastern North
American (Laurentian) margin experienced an ~ 200 m.y. history of
rifting that occurred in two major pulses. The first episode, over the
period of ~765–680 Ma, involved intracontinental rifting characterized
by bimodal volcanism and intrusion of mafic dikes and A-type granitoid
plutons. In the region of this study, there appears to have been a hiatus
Acknowledgments
E. McClellan, E. Gazel / Lithos 206–207 (2014) 321–337
Sublett, and J. Yonts for assistance with field mapping, sample collection, and preparation. The original manuscript was much improved by
revisions and comments by S. Whattam and R. Ernst and the editorial
handling of A. Kerr. This manuscript is submitted for publication with
the understanding that the United States Government is authorized to
reproduce and distribute reprints for governmental use. The views
and conclusions contained in this document are those of the authors
and should not be interpreted as necessarily representing the official
policies, either expressed or implied, of the U.S. Government.
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