Very high-pressure orogenic garnet peridotites
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Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved January 4, 2007 (received for review August 23, 2006)
Abstract
Mantle-derived garnet peridotites are a minor component in many very high-pressure metamorphic terranes that formed during continental subduction and collision. Some of these mantle rocks contain trace amounts of zircon and micrometer-sized inclusions. The constituent minerals exhibit pre- and postsubduction microstructures, including polymorphic transformation and mineral exsolution. Experimental, mineralogical, petrochemical, and geochronological characterizations using novel techniques with high spatial, temporal, and energy resolutions are resulting in unexpected discoveries of new phases, providing better constraints on deep mantle processes.
Data on the composition of the subcontinental lithospheric mantle are essential for erecting realistic large-scale models of the Earth's geochemical and tectonic evolution (1). Our knowledge of mantle composition and petrochemical processes has been derived mainly from studies of xenoliths and xenocrysts in kimberlites, mantle-derived volcanic rocks, and experimental very high-pressure (VHP) phase equilibria, and from the interpretation of seismic tomographic images. Recent studies of orogenic peridotites provide additional insights regarding upper mantle processes at convergent lithospheric plate boundaries. It was found that many orogenic peridotites were derived from a depleted, metasomatized mantle or crustal cumulate, and later were subjected to subduction-zone VHP metamorphism (e.g., refs. 2–6). Some peridotites preserve a record of ultradeep origin revealed by mineral exsolution and the persistence of VHP polymorphs (6–14), and several perido tites contain dense hydrous magnesian silicates (DHMS) that are stable only at mantle depths (15, 16). It was also found that some garnet peridotites, and their host continental crust, underwent coeval subduction-zone VHP metamorphism under pressure–temperature (P–T) conditions characterized by low thermal gradients (≤5°C/km), based on sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages of zircon separates from both rock types (e.g., refs. 17–20). Furthermore, VHP experiments have revealed that numerous hydrous phases and nominally anhydrous minerals containing substantial amounts of H2O are stable under such conditions. Therefore, cold subduction zones are the principal sites of H2O recycling back into the mantle (for reviews, see refs. 21 and 22). Such findings have advanced our knowledge of the thermal structure of subduction zones and of the recycling of volatiles into the mantle.
These petrochemical findings lead to new challenges posed by critical tectonic questions: How were deep-seated (>200 km) mantle rocks transported to shallow depths? How were such peridotites incorporated into subduction-zone orogens? How can we distinguish the petrochemical/geochronological processes taking place in a mantle wedge setting from those affecting deeply subducted ultramafic rocks of the continental lithosphere?
In the spirit of synergy of 21st century science and technology, this article presents an overview of VHP metamorphism of garnet peridotites and poses new challenges for petrochemical and experimental studies of mantle-derived orogenic peridotites. Specifically, we describe differences in petrochemical features for mantle-wedge and subduction-zone processes through examination of micrometer-sized minerals, exsolution textures, and polymorphic transformations. A recent study of garnet nodules in the Western Gneiss Region of the Norwegian Caledonides (6) indicates that the interpretation of continental subduction depths >200 km for some VHP terranes may be incorrect, inasmuch as the deep-mantle origin of the peridotites occurred before emplacement in the subduction zone. In the following discussion, except for a few specific examples, we focus mainly on our own published and unpublished research in the Dabie–Sulu terrane of east-central China.
VHP Metamorphism
Physical Conditions of Metamorphism.
Since the initial discoveries of coesite in supracrustal rocks (23, 24), VHP metamorphism has become synonymous with that portion of eclogite-facies conditions within the P–T stability field of coesite. Understanding VHP tectonics is viewed as a significant undertaking of considerable importance, as underscored by the abundance of recent task groups, workshops, conference sessions, and books devoted to the subject. VHP metamorphism refers to the transformation of crustal rocks to assemblages containing the index minerals coesite and/or diamond at a minimum P > 2.7 GPa at T > 600°C (Fig. 1); such metamorphism is now well recognized in the geologic community (25, 26). The discovery of tracts of upper continental crust metamorphosed under VHP conditions has revolutionized our understanding of collisional orogenic belts. The subduction of sialic materials to mantle depths plays a crucial role in crust–mantle interactions at convergent plate junctions. One of the most significant orogenic processes is the formation and subsequent exhumation of VHP rocks subducted to depths of 150 km or more. Several new VHP terranes (Fig. 2) have recently been identified on the basis of partially preserved trace index minerals (e.g., coesite with or without diamond) in strong containers such as zircon and/or garnet.
P–T conditions of VHP mafic–ultramafic rocks. (Left) (A), P–T fields of VHP metamorphism, “forbidden-zone” (17), and stability of coesite and diamond; (B), P–T time paths for Dabie–Sulu eclogite and garnet peridotites. (Right) Zoned zircon domains with SHRIMP U–Pb ages for Sulu paragneiss.
Distribution and peak metamorphic ages of recognized VHP terranes worldwide (modified after ref. 27).
In Situ VHP Metamorphism.
The volumetrically predominant rocks of VHP terranes are felsic gneisses and schists, many of which lack obvious evidence of mantle-depth metamorphism. Recent observations (27) indicate that not all garnet peridotites and eclogites are fault-bounded, as was previously thought; some such VHP rocks preserve evidence that their contacts with gneissic rocks have retained structural coherence throughout subduction, metamorphism, and exhumation. Mineralogical indicators of VHP metamorphism have been found in a variety of wall rock lithologies, including gneisses, quartzites, and marbles (27–30). Detailed studies of mineral compositions in Dabie felsic gneisses and schists show that they were metamorphosed together with intercalated coesite-bearing eclogite and garnet peridotite bodies under similar P–T conditions.
Evidence of mantle-depth metamorphism is typically preserved as rare mineral inclusions and relict phase assemblages within host rocks that later reequilibrated under crustal conditions. Among the various types of evidence, zircons from VHP rocks provide the most useful information with regard to the P–T time path of a subduction complex, inasmuch as this mineral is extremely stable and resistant over a wide range of conditions. During growth stages, individual zircon zonal domains may include and preserve inclusions of minerals in equilibrium with the matrix phase assemblage. For instance, zircons that crystallized at mantle depths in equilibrium with garnet display characteristic heavy rare-earth element (REE) depletions and lack an Eu anomaly, whereas those grown at crustal depths in equilibrium with plagioclase have pronounced heavy REE enrichments and a marked negative Eu anomaly (31, 32). Consequently, identification of mineral inclusions and characterization of REE patterns of zoned zircons have been used in conjunction with ion microprobe U–Pb dating to elucidate the P–T time paths for some VHP terranes (e.g., refs. 33 and 34).
New isotopic ages support the hypothesis that Dabie–Sulu eclogites, garnet peridotites, and the surrounding wall rocks were subjected to coeval VHP metamorphism at 220–240 Ma. Metamorphic overgrowths on zircons from eclogites and country rock gneisses and schists yield virtually identical U–Pb Triassic ages (e.g., refs. 19, 35, and 36), demonstrating that all units were metamorphosed at the same time. Zircon separates from Dabie–Sulu VHP rocks retain low-P mineral-bearing inherited cores, VHP mineral-bearing (e.g., coesite) mantles, and rims that contain low-P minerals such as quartz and plagioclase (37, 38). Ion microprobe U–Pb analyses of these zoned zircons have identified three discrete age groups, shown schematically in Fig. 1: (i) the latest Proterozoic protolith ages (>680 Ma) in the inherited cores, (ii) a culminating VHP metamorphic event in the coesite-bearing mantles at 220–240 Ma, and (iii) a late amphibolite-facies retrogressive overprint in rims at 210 ± 10 Ma. The presence of anomalously low δ18O in VHP minerals, not only in coesite-bearing eclogites but also in the wall rocks, suggests that both Dabie–Sulu mafic–ultramafic and felsic rocks remained in mutual contact throughout subduction and that the entire terrane underwent Triassic VHP closed-system metamorphism (e.g., refs. 39 and 40).
Mantle-Derived and Crust-Hosted Garnet Peridotites.
Garnet-bearing ultramafic rocks are widespread as a minor but significant component of the Dabie–Sulu VHP terrane (e.g., ref. 41). Although most surface exposures are heavily serpentinized, fresh samples from quarries and drill-hole cores of the Chinese Continental Scientific Drilling Project consist of garnet lherzolite, harz bur gite with or without minor wehrlite, and dunite. Dabie–Sulu garnet perido tites are classified as mantle-derived (type A) or crust-hosted (type B) on the basis of structural, geochemical, and isotopic characteristics (Table 1 and Fig. 3). Type B igneous intrusions occur as minor ultramafic cumulates associated with dominant metagabbroic layers of various compositions, whereas type A peridotites represent depleted, metasomatized mantle fragments, some of which contain minor eclogite and garnet clinopyroxenite pods. Members of both types were subjected to Triassic subduction-zone VHP metamorphism; some were metamorphosed at mantle depths under P–T conditions (<5°C/km) involving pressures up to ≈6.7 GPa and T < 950°C (15, 41–43) (Fig. 1).
Dual origin of Dabie–Sulu garnet peridotites in Triassic VHP terrane
A schematic model for Triassic subduction of the Yangtze beneath the Sino-Korean cratons, showing the tectonic setting for mantle-derived (type A) and crustal-hosted (type B) garnet peridotites (for details, see ref. 2).
Morphologies of zoned zircon grains, and SHRIMP U–Pb isotopic analyses obtained from their cores and rims, provide important constraints regarding whether or not the mantle-derived garnet peridotites experienced subduction-zone metamorphism. Most zircons from Chinese garnet peridotites are of the rounded isometric form without inherited cores, implying a metamorphic origin. SHRIMP U–Pb dating of zircons from both the peridotites and the enclosed eclogite lenses in the Sulu region has yielded VHP metamorphic ages of 220–240 Ma (18–20, 44); the data are consistent with VHP ages of 230 ± 10 Ma for the country rocks. However, reconnaissance study of Hf isotopic and U–Pb upper-intercept ages of zircons yield Early Proterozoic, even Archean, model ages for certain other Sulu garnet peridotites (44), suggesting that some bodies had long resident times in the mantle wedge before involvement in the Triassic subduction.
Microstructures of VHP Minerals
Exsolution of Mineral Lamellae.
Studies of microminerals and exsolution structures have revealed numerous preserved, deep-seated features formed at much higher P than values estimated using conventional (e.g., Grt–Opx) geobarometers (27, 45). The best example may be the electrifying report of micrometer-sized FeTiO3 rods and plates of chromite in olivine from the Alpe Arami garnet lherzolite in the Central Alps (7). Dobrzhinetskaya et al. (7) hypothesized that the exsolved FeTiO3 lamellae were originally a high-P perovskite polymorph of ilmenite and that the precursor phase formed at 10–15 GPa (300–450 km). From the abundance, morphology, crystallography, and topotaxy of these oxides, they argued that the inferred very high solubility of highly charged cations (Ti and Cr) reflects previously unrecognized mantle conditions in recovered rock samples. Despite considerable controversy, subsequent experiments under these conditions have confirmed that >1 vol % of TiO2 can be accommodated in olivine (46, 47). The additional observation of exsolved Ca-poor pyroxene displaying antiphase domains in diopside (8, 48, 49) also supports the idea of an extremely deep origin (>300 km) for the Alpe Arami garnet peridotite.
Precursor majoritic garnet was postulated based on the identification of pyroxene lamellae exsolved from garnet in a Norwegian orogenic garnet peridotite (9, 10). Multistage processes for formation of the majoritic garnet nodules subsequently were described for the hypothesized ≈180 km depth of origin of the Otroy peridotite (10). Experimental simulation of the exhumation path of mantle material shows that high-T (1,400°C) decompression of lherzolite from 14 to 12 GPa results in exsolution of interstitial blebs of diopside and Mg2SiO4 (wadsleyite) lamellae from a parental majoritic garnet (47, 50). Later, research involving critical analyses of REE concentrations in minerals and other geochemical characteristics, as well as recalculation of the volume of exsolved pyroxene inclusions in garnet, led researchers to reinterpret the origin of Otroy garnet peridotite (6). Evidently, Archean (≈3.5 Ga) deep-seated mantle peridotites containing majoritic garnets underwent exsolution during upwelling from a depth of 350 km or more and were subjected to extensive partial melting; the residue formed a garnet-bearing cratonic root. These lithospheric mantle fragments of Proterozoic age (≈1.8–1.4 Ga) were then incorporated into subducting sialic crust during the Caledonian continental collision at ≈400 Ma. The conclusion of presubduction exsolution was based on detailed Re-Os and Nd-Sm age data and on REE concentrations and partitioning between exsolved pyroxenes and the host garnet.
Numerous examples of garnet and clinopyroxene exsolution from Dabie–Sulu peridotites and eclogites have been hypothesized to have been derived from a majoritic garnet precursor formed at considerable mantle depths. Coarse-grained clinopyroxenes from Rizhao garnet clinopyroxenites contain up to 25 vol % exsolved garnet and 4 vol % ilmenite (Fig. 4 A). Petrologic and experimental studies suggest that the precursor of such intergrowths was majoritic garnet in which Ca2+Ti4+ → 2Al3+, Mg2+Si4+ → 2Al3+, and Na+Ti4 → Ca2+Al3+ in octahedral sites (14, 42, 43). Exsolved needles of pyroxene, rutile, and apatite along garnet (111) planes from the Yangkou eclogite layer within peridotite (11, 45, 51) are based on optical and SEM observations.
Similar exsolution lamellae, suggesting that a majoritic garnet precursor formed at depths >200 km, have also been reported in the Erzgebirge massif of Germany, another VHP terrane (13). Despite numerous reports of majoritic garnets, however, formation of VHP solid-solution phases (e.g., majorite) may have taken place either in the deep upper mantle, and then later been sequestered in the mantle wedge such as in Norway, or in a subduction zone such as in the Kokchetav of northern Kazakstan (see below). This problem remains to be satisfactorily investigated.
Supersilicic titanite was suggested as a precursor phase of coesite lamellae in titanite from a Kokchetav impure marble that formed at P > 6 GPa (52). Other exsolution lamellae of quartz or K-feldspar with or without phengite in diopside from diamond-bearing marble and gneiss, and quartz exsolution in eclogitic omphacite (53), together with biologic C-isotope signatures for microdiamonds (54), allow the conclusion that subducted crustal felsic and carbonate protoliths reached mantle depths >180 km. Transmission electron microscopy (TEM) identification of nanometric inclusions of aragonite (CaCO3) and magnesite (MgCO3) in microdiamonds, together with the experimental stability of these carbonates, further suggests that the diamond-bearing rocks of the Kokchetav massif were subducted to a depth of ≈190–280 km (55). Apparently, exsolution occurred during decompression/exhumation of VHP rocks. In addition, nanometer-thick (<2 nm) lamellae of α-PbO2-type TiO2 occur between multiple twinned rutile crystals in both diamond-bearing felsic rocks from the Erzgebirge (56) and coesite-bearing Dabie eclogites (57). The occurrence of these lamellae implies subduction of continental materials to a depth >200 km. These observations, together with inferred supersilicic titanite in Kokchetav marble, allow the conclusion that some continental supercrustal rocks have been subducted to depths of at least 300 km before being returned to the surface. These depths of metamorphism demonstrate that country rocks, although perhaps not as deeply buried as some garnet perido tites, have ascended from astonishing subduction depths.
Exsolution of Hydrous Phases.
K-bearing pargasite [KCa2(Mg,Fe)4AlSi6Al2O22(OH,F)2] lamellae in clinopyroxene inclusions within garnet megacrysts, and phlogopite lamellae in lherzolitic diopside from Sulu, show topotactic intergrowths and are confined to the cores of the host clinopyroxene (14, 58). These K- and OH-bearing exsolved phases suggest that the primary clinopyroxene may have incorporated a considerable amount of K2O and H2O under VHP conditions, as documented in clinopyroxene inclusions in Kokchetav zircons (59). This conclusion is consistent with VHP experiments demonstrating the solubility of K in clinopyroxene (60, 61).
Similar exsolution lamellae of rutile plus sodic amphibole plus apatite in garnet from the Qaidam garnet peridotite of western China (Fig. 4 B and C) have been suggested (62) to represent decompression products from depths >200 km of supersilicic majorite crystals typified by high concentrations of Na2O (0.3 wt %) and hydroxyl (up to 1,000 ppm). Thus, in addition to DHMS and nominally anhydrous silicates, majoritic garnet and supersilicic clinopyroxene could be important reservoirs of H2O at mantle depths.
Polymorphic Transformations.
Intergrowths of ortho- and clinoenstatite lamellae are common within Chinese orogenic garnet peridotites. Clinoenstatite lamellae in orthoenstatite may have formed either by inversion from orthoenstatite or by a displacive transformation from VHP clinoenstatite during decompression (8, 12). Experiments indicate that orthoenstatite transforms to VHP clinoenstatite at P > 8 GPa, 900°, corresponding to a mantle depth of ≈300 km (e.g., refs. 63 and 64) (Fig. 5). The growth of high-P clinoenstatite in mantle-derived peridotite, and the inferred majoritic garnet precursor, may have formed at great depth in the mantle wedge long before insertion into the downgoing continental lithospheric plate, then recrystallized during subduction-zone metamorphism.
Phase transformations in enstatite. (Upper) TEM image of enstatite with clinoenstatite lamellae from Sulu garnet peridotite. (Lower) P–T path for such transformation. (For details, see ref. 12.)
Synergy of VHP Metamorphic Studies with Mineral Physics
The above review concludes that some sections of continental crust have reached subduction depths approaching or exceeding 200 km, involving passive-margin lithologies, including carbonate, felsic, pelitic, and minor mafic–ultramafic protoliths. Some peridotites now hosted in continental orogens may have been formed even deeper. However, the recognition of mineral exsolution in mantle-derived garnet peridotites resulting from decompression in a mantle-wedge setting, or due to exhumation in a subduction zone, remains to be determined, except in cases for which age constraints, such as those in the Western Gneiss Region, are conclusive. With the recent breakthroughs in VHP technology and new-generation synchrotron, neutron, and laser facilities for characterization of nano-sized materials, new in-depth research on VHP minerals and rocks is now within reach; such studies are just beginning (see refs. 65, 66, and 83). Three general fields of mineral physics research are highlighted below to illustrate some of the opportunities. These fields all have relevance to a fuller understanding of crust–mantle tectonics.
Identification of Nano-Sized Minerals.
A host of peculiar minerals and mineral compositions have been described in the Western Gneiss Region (38 minerals in all) (67) and in the Dora Maira massif (>7 minerals) (68) VHP rocks by conventional methods. Novel techniques developed for experimental studies with high spatial, temporal, and energy resolution should be applied for microanalysis of solid and fluid inclusions in naturally occurring VHP minerals, including unexplored trace but widespread opaque phases (e.g., Fe-Ni sulfides) in garnet peridotites and corundum-bearing garnetite (69). Ubiquitous micrometer-sized mineral and fluid inclusions in tough, rigid VHP mineral hosts, including zircon, diamond, garnet, and pyroxenes (e.g., refs. 65 and 70–72), require analytical electron microscopy to characterize their structures and compositions. For example, kokchetavite, a new hexagonal polymorph of K-feldspar, was discovered as a metastable phase together with α-cristobalite plus phengite plus siliceous glass with or without phlogopite/titanite/calcite/zircon as multiphase cloudy inclusions in clinopyroxene and garnet from a diamond-grade Kokchetav garnet-pyroxene rock (73). This problematic phase (2- to 7-μm-sized plates) can be misidentified as K-feldspar by conventional techniques and misinterpreted as an exsolved phase.
Focused ion beam techniques and TEM studies of microdiamonds from the Erzgebirge (74) have revealed numerous nanometric crystalline inclusions, including phases of known stoichiometries such as SiO2 and Al2SiO5 and minerals with different combinations of Si, K, P, Ti, Fe, and O2. These phases need to be investigated by employing synchrotron radiation. Such approaches are only now beginning to be applied
to VHP rocks. For exam-ple, metamorphic diamonds from the Erzgebirge have been examined using synchrotron infrared absorption,
Raman scattering, and fluorescence spectroscopy. The characteristic features of C
C and C
H bonds, molecular H2O, OH− and CO3
2− radicals, and N impurities all support the concept of diamond crystallization from a COH-rich supercritical fluid (66, 75).
Characterization of Mineral Exsolution and Phase Transformations.
Exsolution intergrowths are common in minerals of decompressed VHP rocks; however, the exsolution mechanisms are poorly understood. Each lamellae-bearing host mineral preserves information concerning the composition and physical conditions of formation of the homogeneous precursor phase, as well as a portion of the inferred P–T path during decompression. Compositional and structural characterization of lamellae–host mineral pairs will provide important new constraints on the physical conditions of crystallization/recrystallization. Such studies should guide subsequent experimentation to delineate the P–T conditions and mechanisms for the formation of the primary VHP minerals.
Experimental Phase Relations and Compositional Variations.
Experimental investigations of the KMASH, CMASH, and KNCMASH systems have revealed the possibility of occurrences of hydrous phases in VHP pelitic and peridotitic rocks (76–78). Experimental investigation of hydroxyl solubility in clinopyroxene, orthopyroxene, and olivine (79, 80) indicates that the OH content of these minerals is highly dependent on pressure and temperature. Many hydroxyl-bearing phases, such as OH-topaz and phase A, are only stable under mantle P–T conditions involving geotherms considerably less than 5°C/km. Because such conditions are essentially transient and inevitably are followed by a period of thermal relaxation, these phases have little chance of surviving T increase and P decrease on return to the surface through erosional and tectonic processes. However, one possibility for the preservation of these phases is as minute inclusions in high mechanical-strength, impervious containers like garnet, zircon, or diamond (69).
Interpretation of the occurrence of majoritic garnets in orogenic peridotites requires better experimental data, and some syntheses using natural peridotite samples have been accomplished (47, 51). For example, as shown in Fig. 6, the depths for the formation of majoritic garnet are based on reconnaissance investigation of the pseudobinary system MgSiO3–Mg3Al2Si3O12 (81). Although experiments involved testing the effects of Fe and Ca, the effect of Ti has not been explored. Solubilities of Ti, K, OH, and other trace elements in olivine, garnet, and pyroxene in mafic–ultramafic systems need to be examined, inasmuch as natural analogs exhibit numerous microstructures. These phases also contain minute inclusions as yet unidentified that may turn out to be VHP phases or DHMS previously synthesized only in diamond-cell or multianvil experiments (e.g., see ref. 82). Discovery of natural representatives of these synthetic phases would be a major step forward in understanding mantle processes, including the role of hydrous phases as storage sites for H2O.
Summary Statement
The search for mantle minerals in orogenic VHP garnet peridotites should be conducted with the same sophisticated techniques (microRaman spectroscopy, synchrotron x-ray diffraction, and high-resolution TEM) used by experimentalists to identify synthetic analogs. Such studies are needed to bridge the gap between mantle petrology and mineral physics. A review of recent progress involving such an experimental approach is detailed by Dobrzhinetskaya (83).
Acknowledgments
We thank Dave Mao, Larissa Dobrzhinetskaya, Harry Green, and Nick Sobolev for support and reviews, including review of a draft version of this manuscript. This work was supported by Stanford University and by National Science Foundation Continental Dynamic Program Grants EAR 00-03355 and 05-06901 (to J.G.L.).
Footnotes
- *To whom correspondence should be addressed. E-mail: liou{at}pangea.stanford.edu
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Author contributions: J.G.L. and R.Y.Z. designed research; J.G.L. and R.Y.Z. performed research; W.G.E. analyzed data; and J.G.L., R.Y.Z., and W.G.E. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
- Abbreviations:
- DHMS,
- dense hydrous magnesian silicates;
- P–T,
- pressure–temperature;
- REE,
- rare earth element;
- SHRIMP,
- sensitive high-resolution ion microprobe;
- TEM,
- transmission electron microscopy;
- VHP,
- very high pressure.
- © 2007 by The National Academy of Sciences of the USA