Download A petrologic case for Eocene slab break

Petrologic case for Eocene slab breakoff during the
Indo-Asian collision
Matthew J. Kohn*

Christopher D. Parkinson 
Department of Geological Sciences, University of South Carolina, Columbia, South Carolina
29208, USA
ABSTRACT
The Greater Himalayan Sequence is the metamorphic core of
the Himalaya and has been a focus of considerable study, yet its
petrologic evolution remains controversial. Pre-Oligocene metamorphism was nearly obliterated by Miocene metamorphism and
melting, which many workers ascribe to shear heating, unusually
high concentrations of radioactive elements accompanying burial,
and/or several kilometers of Miocene exhumation due to extensional faulting. Sparse Oligocene to Eocene ages are often assigned
to a tectonically unspecified event. An alternative slab-breakoff
model (subduction of Greater Himalayan rocks to ;100 km depth
in the Eocene followed by buoyant extrusion due to decoupling of
the oceanic lithosphere) has also been proposed based on rare Eocene eclogites, but without explanation of Miocene melting or metamorphic petrogenesis. We argue that slab breakoff readily explains
the Eocene eclogites, Miocene partial melts, and late Eocene K-rich
magmas in southeastern Tibet, and that metamorphic and plutonic
ages help define the timing and rates of breakoff and extrusion.
This model implies that (1) much of the Greater Himalayan Sequence was subducted to depths greater than commonly considered, (2) fluid-absent, decompression melting at 30–35 km depth
was the consequence of as much as 100 km of extrusion, rather
than radioactive heating or a smaller, crustal level extensional or
erosional event, and (3) eruption of Eocene, K-rich Tibetan Plateau
lavas has no implication for topography of the Tibetan Plateau.
MODELS
The Greater Himalayan Sequence consists mainly of felsic migmatitic schist and gneiss, calc-silicate, and quartzite, and extremely rare
metamorphosed basaltic flows and sills. Protolith and inheritance ages
are Proterozoic to early Paleozoic (DeCelles et al., 1998). The most
obvious metamorphism occurred during partial melting ca. 20 Ma, but
rare relict metamorphic minerals, textures, and isotope ages as old as
35–55 Ma attest to earlier Himalayan metamorphism (e.g., see Hodges,
2000; de Sigoyer et al., 2000; Kaneko et al., 2001; Catlos et al., 2002).
The pressure-temperature (P-T) conditions of the early Cenozoic metamorphic stage are poorly known, although later Miocene reequilibration is well documented at 7–10 kbar and 600–700 8C (Guillot, 1999;
Fig. 1). Curiously, a suite of K-rich basaltic rocks also erupted in southeastern Tibet at 35–40 Ma (Chung et al., 1998; Fig. 1), i.e., coeval
with early metamorphism of the Greater Himalayan Sequence, but the
rocks are ordinarily only considered in tectonic models of Tibet.
Four main tectonic models for Greater Himalayan metamorphism
focus on different petrologic features and chronologies, particularly
with respect to heat sources and the partial melting event ca. 20 Ma.
Keywords: Greater Himalayan Sequence, Himalaya, Indo-Asian collision, slab breakoff.
INTRODUCTION
Collision between India and Asia commenced ca. 55 Ma (Klootwijk et al., 1992), ultimately creating the Himalaya and Tibet. The
most deeply buried rocks now exposed are the Greater Himalayan Sequence, a relatively thin (;7 km, on average), coherent slab of crystalline supracrustal rocks bounded above and below by the South Tibetan detachment system and the Main Central thrust, respectively
(Fig. 1). Many disparate models of Himalayan tectonics are based on
the petrology of Greater Himalayan rocks, especially focusing on either
extremely rare Eocene eclogites, or abundant early Miocene partial
melts. We argue that disparate petrologic elements that have been used
to support different models are explainable by continental subduction
followed by Eocene slab breakoff. That is, of existing tectonic models,
slab breakoff is most functional petrologically. In particular, (1) Miocene dehydration melting of the Greater Himalayan Sequence was likely forced by rebound of subducted continental crust out of the lithospheric mantle, (2) petrologic evolution of the Greater Himalayan
Sequence was not restricted to the upper 35 km as commonly assumed,
and (3) Eocene K-rich volcanic rocks in Tibet are genetically related
to Greater Himalayan Sequence evolution. The slab-breakoff hypothesis is easily tested, and has major implications for Indo-Asian
tectonics.
*Corresponding author. E-mail: [email protected].
Figure 1. Pressure-temperature (P-T) diagram showing most important metamorphic reactions and P-T conditions of Greater Himalayan Sequence rocks (GHS; diagram after Spear et al., 1999; data
from O’Brien et al., 2001; Sachan et al., 2001; Lombardo et al., 1999;
Hodges, 2000). Final equilibration conditions for GHS vary along
strike and are clearly reset, but overall imply melting conditions of
at least 8–12 kbar and 750–800 8C. Inset shows locations of GHS,
eclogites, and K-rich lavas. PE—Pakistan eclogite; IE—Indian eclogite (likely window through Tethyan sediments into underlying
Greater Himalayan rocks); TE—Tibetan eclogite; NE—Nepal eclogite; MCT—Main Central thrust; STDS—South Tibetan detachment
system.
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; July 2002; v. 30; no. 7; p. 591–594; 2 figures.
591
We refer to these models as (1) wedge accumulation, (2) shear heating,
(3) decompression melting, and (4) slab breakoff. In wedge accumulation, Greater Himalayan rocks simultaneously accreted and eroded in
a wedge over the past ;50 m.y. (e.g., Huerta et al., 1998), and high
concentrations of heat-producing elements ultimately led to melting. In
the shear-heating and decompression-melting models, Greater Himalayan rocks had been previously thickened and were close to their
melting point. Melting was catalyzed ca. 20 Ma by initial Main Central
thrust movement in the shear-heating model (Harrison et al., 1998), but
by #20 km of extensional exhumation along the South Tibetan detachment system in the decompression-melting model (Harris and Massey, 1994). The slab-breakoff model is based principally on two Eocene
coesite localities and mechanical plausibility, not on subsequent Miocene petrogenesis (O’Brien et al., 2001; Chemenda et al., 2000). No
model considers the K-rich volcanic rocks in southern Tibet.
ECLOGITES
Eclogites are extremely rare in the Himalaya (Fig. 1). Coesite, a
dense polymorph of SiO2 stable only at P . 27 kbar (depths .;90
km), occurs in eclogites in Kaghan, Pakistan and Ladakh, India
(O’Brien et al., 2001; Sachan et al., 2001). Assignment of these eclogites to the Greater Himalayan Sequence is arguable, but lithologies
and ages are appropriate (Baldwin et al., 1998; O’Brien et al., 2001).
Retrograde eclogites that are indisputably Greater Himalayan occur in
the Arun valley in eastern Nepal, similar to retrograde eclogite documented along strike in the nearby Kharta region of Tibet (Lombardo
et al., 1999; Fig. 1). All eclogites are metamorphosed basaltic sills or
flows, and are stratigraphically contiguous with the surrounding felsic
rocks.
Ages for the coesite-bearing eclogites are 45–55 Ma (Tonarini et
al., 1993; de Sigoyer et al., 2000; Kaneko et al., 2001). The other
eclogites have not been dated directly, but Catlos et al. (2002) found
45 6 2 Ma monazites in a Greater Himalayan gneiss from the region.
Poor preservation of Eocene high-pressure or ultrahigh-pressure metamorphism could well reflect: (1) paucity of appropriate bulk rock compositions—such as metabasites—which may retain eclogitic or other
high-P and/or ultrahigh-P assemblages, and (2) obliteration of earlier
matrix minerals and textures during Miocene melting and thermal
reequilibration.
The occurrence of coesite-grade (ultrahigh-P) eclogites suggests
that the Himalaya are comparable to other collisional orogens (e.g.,
Kokchetav, Kazakhstan, and Dabie-Sulu, China) that include relicts of
.100 km continental subduction. In these terranes, ultrahigh-P mineralogies are only readily identifiable in uncommon rocks, such as eclogite, garnet peridotite, and whiteschist (Maruyama and Parkinson,
2000). In the predominant felsic gneisses, preservation of ultrahigh-P
minerals (e.g., coesite, diamond, jadeite) is restricted to micron-scale
inclusions in chemically and mechanically resistant zircons (e.g., Katayama et al., 2000). Clearly, coherent slabs, hundreds of kilometers
long and several kilometers thick, can undergo ultradeep burial (.100
km) and subsequent exhumation in subduction-collision zones (e.g.,
Maruyama and Parkinson, 2000) qualitatively similar to the Indo-Asian
collision. However, evidence for ultrahigh-P metamorphism may be
overlooked in felsic rocks due to melting and reequilibration during
exhumation.
MELTS
Partial melting textures are common in Greater Himalayan rocks
from Bhutan to Pakistan (e.g., see Guillot, 1999), and in central Nepal
melt textures occur throughout the sequence. Apparently wherever rock
compositions were appropriate, the Greater Himalayan Sequence partially melted. Geochronology on the resulting leucogranites indicates
that melting was almost simultaneous at 18–22 Ma over the length of
the Himalaya (Harrison et al., 1998), a critical observation that must
592
be explained in any tectonic model. Rock and mineral textures, trace
elements, and experiments definitively identify muscovite dehydration
melting as a primary cause at P-T conditions of at least 8–12 kbar and
750–800 8C (Inger and Harris, 1992; Harris et al., 1993; Patiño-Douce
and Harris, 1998; Fig. 1). Biotite dehydration-melting at higher temperatures is also possible (e.g., Ganguly et al., 2000). However, workers
dispute how the dehydration-melting reactions were crossed and how
chronologic simultaneity was attained, which gives rise to the three
different melting models (decompression melting, shear heating, and
wedge accumulation). Melting eradicated most earlier textures and
mineral assemblages, complicating evaluation of models and inferences
of Himalayan tectonics prior to ca. 20 Ma.
Eocene (35–40 Ma) K-rich lavas occur in southeastern Tibet
;300 km north of the Greater Himalayan Sequence, and were likely
sourced by enriched subcontinental lithosphere that was exposed to
asthenospheric upwelling (Chung et al., 1998). These lavas are broadly
coeval with early evolution of the Greater Himalayan Sequence. Other
K-rich volcanic rocks occur on the plateau (e.g., Miller et al., 1999),
but have ages #25 Ma. That is, no K-rich volcanic rocks older than
ca. 40 Ma have been found associated with the Indo-Asian collision.
INTERPRETATIONS
Understanding the Himalayan orogen requires assessing the fate
of the oceanic lithosphere that was formerly attached to Indian continental lithosphere. A Cretaceous to Eocene arc on the southern margin
of Asia assures northward subduction of oceanic lithosphere (Dewey
and Bird, 1970). However, gravity and teleseismic data suggest that
the oceanic slab is not attached to Indian lithosphere today (Jin et al.,
1996). The oceanic slab likely broke off and foundered some time in
the past while India maintained its northward course into Asia. Mechanical models of continental subduction (Davies and von Blanckenburg, 1995; Chemenda et al., 2000) predict that lithospheric failure
will occur at a depth of ;100–200 km. Lithospheric mantle may subduct, whereas buoyancy drives detached crustal slices upward, ultimately exposing eclogites (Maruyama et al., 1996). However, in the
Himalaya there is no consensus for either the relationship of buoyancydriven ascent to Greater Himalayan metamorphism and melting, or the
timing of slab breakoff (if it occurred). We believe that Eocene slab
breakoff best explains the petrologic evidence (Fig. 2).
In the slab-breakoff model, mechanical coupling between downgoing oceanic and Indian continental lithospheres between 55 and 45
Ma subducted the northern margin of India beneath Asia, producing
widespread Greater Himalayan eclogites. At a convergence rate of ;5
cm/yr, and a subduction angle of 308, Indian continental crust could
have been subducted to 250 km depth in 10 m.y., although petrologic
data require only ;100 km. Decoupling of continental and oceanic
lithospheres at 40–45 Ma caused: (1) rapid removal of oceanic lithosphere, initiating asthenospheric upwelling, (2) initial heating of subcontinental lithosphere to produce Tibetan K-rich lavas by 40 Ma, and
(3) buoyant rebound of the subducted Greater Himalayan rocks out of
the mantle lithosphere. By analogy, slab breakoff is now occurring in
the Banda arc of eastern Indonesia—subducted oceanic lithosphere has
separated from the Australian lithosphere at depths of 100–300 km,
causing rapid extrusion of formerly subducted sediments (Maruyama
et al., 1996). Breakoff in the Himalaya occurred no earlier than 45–55
Ma (the oldest Cenozoic ages on metamorphic minerals), and no later
than ca. 22 Ma (the oldest age of leucogranites). Possibly a younger
breakoff age could be accommodated by placing greater emphasis on
younger (ca. 25 Ma) K-rich lavas. Along-strike diachroneity is expected, although perhaps by as little as 5 m.y. (Davies and von Blanckenburg, 1995).
We propose that 50–100 km of exhumation of Greater Himalayan
rocks between 45 and 25 Ma was accommodated by thrust-sense and
GEOLOGY, July 2002
Figure 2. Model showing evolution of Greater Himalayan rocks during Indo-Asian collision. A: Between 55 and 45 Ma, Greater Himalayan Sequence was subducted beneath Asia. B: Slab breakoff of
oceanic portion of Indian plate at ca. 45 Ma forced mantle convection
above downgoing broken edge, exposing Greater Himalayan rocks
to increased mantle heat flow and strong buoyancy forces, and initiating melting of enriched continental lithosphere either within broken edge(s) of Indian plate, or within newly exposed subcrustal lithosphere of Asia. C: After loss of negatively buoyant oceanic
lithosphere, Greater Himalayan rocks were gravitationally extruded
from mantle, and K-rich lavas erupted in southern Tibet. D: By ca.
20 Ma, exhumation of Greater Himalayan rocks decreased pressure
to point of muscovite (6biotite) dehydration melting, leading to extensive melting of Greater Himalayan rocks. GHS—Greater Himalayan Sequence; STDS—South Tibetan detachment system; MCT—
Main Central thrust.
normal-sense slip on earlier fault systems now occupied by the Main
Central thrust and South Tibetan detachment fault systems. Increased
basal heat flow due to mantle upwelling may have maintained relatively
high temperatures during this rapid exhumation. Most important,
Greater Himalayan rocks crossed the muscovite dehydration-melting
reaction at 18–22 Ma at P $ 8–12 kbar. They were already hot because
GEOLOGY, July 2002
they had been partially subducted into the mantle and then exposed to
enhanced mantle heat flow after breakoff. This interpretation of the
cause of heating and melting departs critically from previous models,
which have either never tried to explain it (model 4), or have instead
interpreted melting to result from less radical, shallower level tectonic
processes coupled with local heat sources (models 1–3). Extensive
melts eradicated most prior Greater Himalayan high- and/or ultrahighP history, except in rare mafic eclogites. Apparently the entire Greater
Himalayan slab crossed the melting reaction(s) nearly synchronously,
which implies that slab breakoff occurred over a period of only a few
million years. Greater exhumation and higher temperatures in some
areas (e.g., eastern Nepal and Sikkim) caused some rocks to cross other
reactions to produce different key minerals (Fig. 1). Continued movement on the Main Central thrust and Southern Tibetan detachment system in the Miocene further exhumed the rocks.
Although we believe that our interpretation best explains petrologic observations, it raises serious stratigraphic, structural, and igneous issues. For example, there are no Eocene or Oligocene sediments
in the foreland that directly record the exhumation we propose. However, breakoff may yield erosion rather than deposition as slab pull is
eliminated (Davies and von Blanckenburg, 1995), and foreland sediments show a profound Oligocene unconformity (DeCelles et al.,
1998). Or, sedimentation might shift elsewhere, as possibly indicated
by Eocene through Oligocene clastic sediments in Pakistan and the
Indus Fan (Qayyum et al., 1997), lateral to the modern foreland. We
also recognize that there are no documented Greater Himalayan plutons
or South Tibetan detachment system and Main Central thrust structures
older than early Miocene. However, we question whether any plutons
would have been present in the Oligocene to record the early deformation, and whether structures would be preserved through widespread
melting. Major melting does not occur in the crust until dehydrationmelting reactions are crossed. Until Greater Himalayan rocks had exhumed to ;35 km depth ca. 20 Ma, there is no reason why melts
should have been present. The extensive melting that then occurred
would likely have obliterated any preexisting structures.
Harrison et al. (1999) showed that both shear heating and a pressure decrease are capable of explaining Greater Himalayan melting.
Our model includes both very large pressure decreases and thrust-sense
shearing, and so is consistent with their calculations of melt production.
We predict that regional Eocene eclogite facies metamorphism in
Greater Himalayan rocks is preserved as inclusion microassemblages
in zircon and possibly garnet in felsic rocks, and as rare eclogites in
the cores of mafic boudins and layers. The presence of these inclusions
and relicts and the metamorphic ages of host phases provide a test of
our model.
Our Himalayan tectonic model has major implications for Tibetan
Plateau topography and tectonism. Genesis of K-rich lavas has been
ascribed to lithospheric delamination (England, 1993), and Chung et
al. (1998) argued that the lavas in southeastern Tibet indicate Eurasian
plate delamination and rapid Eocene uplift. In contrast, slab breakoff
has no direct implications for Tibetan topography because enriched
subcontinental mantle is exposed via Indian plate dynamics. Thus we
ascribe no topographic significance to K-rich lavas in Tibet until, at
earliest, 25 Ma. Similarly, Nelson et al. (1996) described Greater Himalayan rocks as the extruded equivalent of modern partial melts in
southern Tibet. However, melts in southern Tibet, if they occur, may
result from high heat flow due to either lithospheric erosion or delamination, rather than rebound following slab breakoff. If extrusion drove
Greater Himalayan Sequence melting, then it may not have a Tibetan
equivalent.
CONCLUSIONS
Although many petrologic and tectonic models have been proposed for the Greater Himalayan Sequence, we believe that slab break593
off explains the greater body of petrologic observations. Petrologic
justification for slab breakoff has previously focused on scattered eclogites, the paucity of which throughout the Greater Himalayan Sequence
understandably engenders skepticism. Instead, other models have focused on widespread partial melting in support of different tectonic
models. As we have shown, partial melting is at least equally well
explained by extrusion of Greater Himalayan rocks from great depths
(to 100 km) following separation of the oceanic slab during the Eocene,
a mechanism that also explains Eocene K-rich magmas in Tibet. Thus,
slab breakoff is the most petrologically functional model to date, and
should be tested via examination of the mineralogy of microinclusions
in garnet and zircon.
ACKNOWLEDGMENTS
This work was funded by National Science Foundation grants to Kohn
(EAR-0073803) and Parkinson (EAR-0125867). We thank M. Brown and P.
DeCelles for incisive reviews, although they do not necessarily agree with our
model. Special thanks to Shige Maruyama for his valuable insights and advice
regarding ultrahigh-pressure metamorphism and tectonics.
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Manuscript received December 7, 2001
Revised manuscript received March 5, 2002
Manuscript accepted March 19, 2002
Printed in USA
GEOLOGY, July 2002