Very high-pressure orogenic garnet peridotites J. G. Liou*, R. Y. Zhang, and W. G. Ernst Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved January 4, 2007 (received for review August 23, 2006) Mantle-derived garnet peridotites are a minor component in many very high-pressure metamorphic terranes that formed during con- tinental 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. ata on the composition of the These petrochemical findings lead to the index minerals coesite and/or dia- subcontinental lithospheric new challenges posed by critical tectonic mond at a minimum P Ͼ 2.7 GPa at mantle are essential for erect- questions: How were deep-seated (Ͼ200 T Ͼ 600°C (Fig. 1); such metamorphism ing realistic large-scale models km) mantle rocks transported to shallow is now well recognized in the geologic D depths? How were such peridotites in- of the Earth’s geochemical and tectonic community (25, 26). The discovery of evolution (1). Our knowledge of mantle corporated into subduction-zone oro- tracts of upper continental crust meta- composition and petrochemical pro- gens? How can we distinguish the morphosed under VHP conditions has cesses has been derived mainly from petrochemical/geochronological pro- revolutionized our understanding of col- studies of xenoliths and xenocrysts in cesses taking place in a mantle wedge lisional orogenic belts. The subduction kimberlites, mantle-derived volcanic setting from those affecting deeply sub- of sialic materials to mantle depths plays rocks, and experimental very high- ducted ultramafic rocks of the continen- a crucial role in crust–mantle interac- pressure (VHP) phase equilibria, and tal lithosphere? tions at convergent plate junctions. One from the interpretation of seismic tomo- In the spirit of synergy of 21st century of the most significant orogenic pro- graphic images. Recent studies of oro- science and technology, this article pre- cesses is the formation and subsequent genic peridotites provide additional sents an overview of VHP metamorphism exhumation of VHP rocks subducted to insights regarding upper mantle pro- of garnet peridotites and poses new chal- depths of 150 km or more. Several new cesses at convergent lithospheric plate lenges for petrochemical and experimental VHP terranes (Fig. 2) have recently boundaries. It was found that many oro- studies of mantle-derived orogenic perido- been identified on the basis of partially genic peridotites were derived from a tites. Specifically, we describe differences preserved trace index minerals (e.g., depleted, metasomatized mantle or in petrochemical features for mantle- coesite with or without diamond) in crustal cumulate, and later were wedge and subduction-zone processes strong containers such as zircon and/or subjected to subduction-zone VHP through examination of micrometer-sized garnet. metamorphism (e.g., refs. 2–6). Some minerals, exsolution textures, and poly- morphic transformations. A recent study peridotites preserve a record of ultra- In Situ VHP Metamorphism. The volumetri- of garnet nodules in the Western Gneiss deep origin revealed by mineral ex- cally predominant rocks of VHP ter- Region of the Norwegian Caledonides (6) solution and the persistence of VHP ranes are felsic gneisses and schists, indicates that the interpretation of conti- many of which lack obvious evidence polymorphs (6–14), and several perido- nental subduction depths Ͼ200 km for of mantle-depth metamorphism. Recent tites contain dense hydrous magnesian some VHP terranes may be incorrect, in- observations (27) indicate that not all silicates (DHMS) that are stable only at asmuch as the deep-mantle origin of the garnet peridotites and eclogites are mantle depths (15, 16). It was also peridotites occurred before emplacement fault-bounded, as was previously found that some garnet peridotites, and in the subduction zone. In the following thought; some such VHP rocks preserve their host continental crust, underwent discussion, except for a few specific exam- evidence that their contacts with coeval subduction-zone VHP metamor- ples, we focus mainly on our own pub- gneissic rocks have retained structural phism under pressure–temperature lished and unpublished research in the coherence throughout subduction, meta- (P–T) conditions characterized by low Dabie–Sulu terrane of east-central China. Յ morphism, and exhumation. Mineralogi- thermal gradients ( 5°C/km), based on cal indicators of VHP metamorphism sensitive high-resolution ion microprobe VHP Metamorphism have been found in a variety of wall (SHRIMP) U–Pb ages of zircon sepa- Physical Conditions of Metamorphism. Since rock lithologies, including gneisses, rates from both rock types (e.g., refs. the initial discoveries of coesite in su- 17–20). Furthermore, VHP experiments pracrustal rocks (23, 24), VHP meta- have revealed that numerous hydrous morphism has become synonymous with Author contributions: J.G.L. and R.Y.Z. designed research; phases and nominally anhydrous miner- that portion of eclogite-facies conditions 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. als containing substantial amounts of within the P–T stability field of coesite. The authors declare no conflict of interest. H2O are stable under such conditions. Understanding VHP tectonics is viewed Therefore, cold subduction zones are as a significant undertaking of consider- This article is a PNAS Direct Submission. Abbreviations: DHMS, dense hydrous magnesian silicates; the principal sites of H2O recycling back able importance, as underscored by the P–T, pressure–temperature; REE, rare earth element; into the mantle (for reviews, see refs. 21 abundance of recent task groups, work- SHRIMP, sensitive high-resolution ion microprobe; TEM, and 22). Such findings have advanced shops, conference sessions, and books transmission electron microscopy; VHP, very high pressure. our knowledge of the thermal structure devoted to the subject. VHP metamor- *To whom correspondence should be addressed. E-mail: of subduction zones and of the recycling phism refers to the transformation of [email protected]. of volatiles into the mantle. crustal rocks to assemblages containing © 2007 by The National Academy of Sciences of the USA 9116–9121 ͉ PNAS ͉ May 29, 2007 ͉ vol. 104 ͉ no. 22 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0607300104 Downloaded by guest on September 29, 2021 SPECIAL FEATURE: PERSPECTIVE 6.0 G 211± 4 Ma enrichments and a marked negative Eu Peridotite G anomaly (31, 32). Consequently, identifi- Eclogite Sulu (A) cation of mineral inclusions and charac- 5.0 zone 160 terization of REE patterns of zoned Dabie- zircons have been used in conjunction 4.0 Forbidden Diamond with ion microprobe U–Pb dating to 231± 4 Ma Graphite o 120 elucidate the P–T time paths for some 5 C/km G Dry EC e > 680 Ma VHP terranes (e.g., refs. 33 and 34). 3.0 Lw-EC G Coesit rtz (A) Qua 231±4 Ma New isotopic ages support the hypoth- Depth km 80 esis that Dabie–Sulu eclogites, garnet Pressure [GPa] Amp-EC Protolith age: > 680 Ma 2.0 Ep-EC S peridotites, and the surrounding wall Qtz-bearing inherited core rocks were subjected to coeval VHP BS S UHP metamorphic age: 231±4 Ma HGR metamorphism at 220–240 Ma. Meta- (A) 40 1.0 > 680 Ma Coe-bearing mantle Retrograde metamorphism: 211±4 Ma morphic overgrowths on zircons from EA AM GR GS Qtz-bearing rim eclogites and country rock gneisses and 210 ± 4 Ma (B) 0 Exhumation rate: > 5 km/Ma schists yield virtually identical U–Pb 200 400 600 800 1000 Triassic ages (e.g., refs. 19, 35, and 36), Temperature [ oC] demonstrating that all units were meta- morphosed at the same time. Zircon Fig. 1. 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 separates from Dabie–Sulu VHP rocks and garnet peridotites. (Right) Zoned zircon domains with SHRIMP U–Pb ages for Sulu paragneiss. retain low-P mineral-bearing inherited cores, VHP mineral-bearing (e.g., coes- ite) mantles, and rims that contain quartzites, and marbles (27–30). De- to the P–T time path of a subduction low-P minerals such as quartz and pla- tailed studies of mineral compositions complex, inasmuch as this mineral is gioclase (37, 38). Ion microprobe U–Pb in Dabie felsic gneisses and schists show extremely stable and resistant over analyses of these zoned zircons have that they were metamorphosed together a wide range of conditions. During identified three discrete age groups, with intercalated coesite-bearing eclogite growth stages, individual zircon zonal shown schematically in Fig. 1: (i) the and garnet peridotite bodies under simi- domains may include and preserve in- latest Proterozoic protolith ages (Ͼ680 lar P–T conditions. clusions of minerals in equilibrium with Ma) in the inherited cores, (ii) a culmi- Evidence of mantle-depth meta- the matrix phase assemblage. For in- nating