R. Y. Zhang

Global UHP Metamorphism and Continental Subduction/Collision: The Himalayan Model

Continental crust (density ~2.8 g·cm-3) resists subduction into the earths mantle (~3.3 g·cm-3) because of buoyancy. However, more than 20 recognized ultrahigh-pressure (UHP) terranes have been documented; these occurrences demonstrate that not only is continental crust subducted to depths as great as 150 km, but also that some supracrustal rocks were then exhumed to the earths surface. UHP terranes are composed of mainly supracrustal rocks that contain minor amounts of minerals such as coesite or diamond, indicative of P > 2.5 GPa. In general, quartzofeldspathic units are thoroughly back reacted, and only mafic eclogite lenses and boudins retain scattered UHP phases. These index minerals are restricted to micron-scale inclusions in chemically and mechanically resistant zircon, garnet, and a few other strong container minerals, and are difficult to identify by conventional petrologic studies. The continental rocks were subjected to UHP metamorphism at T ranging from ~700 to 950°C and P > 2.8 to 5.0 GPa, corresponding to depths of ~100 to 150 km. These UHP units were subsequently exhumed to crustal depths and subjected to intense hydration and amphibolite-facies overprint. Widespread Barrovian-type metamorphism in many collisional orogens may mask an earlier, higher-pressure metamorphic history. We suspect that coesite-bearing UHP rocks were once generated in the majority of exhumed collisional orogens. The recent finding of coesite inclusions in rare Himalayan eclogites and country rock gneisses is a typical example. We use the Himalayan model to illustrate UHP metamorphism and subduction of continental crustal rocks to mantle depths and later Barrovian-type overprint during exhumation. Himalayan UHP eclogites and adjacent gneisses were formed at mantle depths > 100 km at 46 to 52 Ma. These rocks were exhumed to crustal depths and subjected to Barrovian amphibolite- to granulite-facies metamorphism; associated magmatism occurred at 30 to 15 Ma. The Himalayan metamorphic belt was domally uplifted and the mountain-building process initiated since 11 Ma, when underthrusting of the Indian tectosphere beneath the Lesser Himalayas occurred.

Occurrences of intergranular coesite in ultrahigh-P rocks from the Sulu region, eastern China: Implications for lack of fluid during exhumation

Abstract Several coesite grains along garnet-omphacite grain boundaries were discovered in eclogites from Yangkou Beach of the Sulu region, eastern China. These grains exhibit variable extents of conversion of coesite to palisade- through mosaic- to granoblastic-quartz aggregates. The rare occurrence of intergranular and matrix coesite in ultrahigh-pressure (UHP) crustal rocks is related to the fast rate of exhumation and the low amount of fluid infiltration required during retrograde recrystallization. Sluggish reactions due to the lack of fluid in both UHP and retrograde metamorphism resulted in the occurrence of intergranular coesite in these eclogites, the preservation of gabbroic texture and relict igneous minerals, and the metastable persistence of low-P assemblages in the coesite-stability field.

Petrology, geochemistry and isotope data on a ultrahigh-pressure jadeite quartzite from Shuanghe, Dabie Mountains, East-central China

Abstract In the Dabie ultrahigh-pressure terrane of east-central China, coesite-bearing jadeite quartzites occur locally as intercalated layers with marble and mafic eclogite. This rock assemblage is, in turn, enclosed within quartzofeldspathic gneisses. Metamorphic parageneses and kelyphitic textures reveal a multistage metamorphic evolution and complex exhumation history. The primary peak metamorphic assemblage consists of jadeite + garnet + coesite + rutile ± apatite. Minor coesite and coesite pseudomorphs occur as inclusions in jadeite and garnet. Three stages of retrograde assemblages are observed in the jadeite quartzites. Stage A is represented by the polymorphic transformation of coesite to quartz aggregates. Stage B is characterized by formation of coronas around jadeite porphyroblasts consisting of an inner layer of oligoclase + amphibole and an outer layer of albite ± aegirine—augite. The last stage (stage C) involved total replacement of jadeite and most garnets by taramitic amphibole + albite + aegirine-augite. Peak metamorphic P-T conditions were > 26 kbar at 660°C and are consistent with the estimates from the adjacent coesite-bearing eclogites. The jadeite quartzites display clockwise P-T path that matches those of the adjacent eclogites. Major and trace element data suggest that the protolith of the jadeite quartzite could have been an albitized siltstone enriched in Na and depleted in K and Ca. The highly negative present-day eNd value (−24.7) indicates a very old age for the protolith. Its late Archean model age (TDM) of 2.58 Ga is among the oldest so far identified for rocks from the Dabie UHPM terrane. Concordant field relations and petrogenetic considerations suggest that all mafic, politic, carbonate and gneissic rocks have experienced in-situ UHP metamorphism during Triassic continental collision between the Sino-Korean and Yangtze cratons.

Petrographic Characteristics and Metamorphic Evolution of Ultrahigh-Pressure Eclogites in Plate-Collision Belts

Ultrahigh-pressure eclogites are metabasic rocks that have equilibrated at pressures within the coesite P-T stability field. This paper reviews their occurrences and petrographic characteristics in exhumed ultrahigh-pressure metamorphic terrenes within certain major Phanerozoic continental plate—collision belts. Unequivocal identification of ultrahigh-pressure eclogites depends on the presence of relict coesite or of polycrystalline quartz pseudomorphs after earlier coesite. However, preservation of such distinctive petrographic evidence is at best sporadic, leading to serious problems over whether or not particular eclogite samples have experienced “peak” pressures consistent with coesite stability. Accordingly, we also review here other petrographic, mineral-assemblage, and mineral-chemistry features that may aid in the identification of ultrahigh-pressure eclogites. In addition, we discuss the quantitative evaluation of P-T conditions for formation based on various reaction equilibria, including the do...

Coesite inclusions in dolomite from eclogite in the southern Dabie Mountains, China: The significance of carbonate minerals in UHPM rocks

Abstract Coesite inclusions have been discovered in dolomite from carbonate-bearing eclogite in the Dabie UHP terrane. The eclogite boudins are enclosed in calc-silicate rocks that contain quartz aggregates after coesite in the matrix. The carbonate-bearing eclogite is composed of garnet, coesite or quartz, dolomite, magnesite, omphacite, rutile, ±minor phengite, apatite, and zircon, and was formed at ~760 °C, >28 kbar, and 0.01 < XCO₂ <0.1. In addition to coesite inclusions in dolomite, inclusions of coesite and quartz pseudo-morphs after coesite are abundant in garnet. One inclusion of quartz + calcite aggregates after coesite and aragonite (?) was identified in garnet. Radial fractures developed in garnet around the coesite inclusions but are absent in the dolomite. Dolomite contains about 14 mol% ankerite component, whereas magnesite contains about 2 mol% calcite and 23 mol% siderite. This is the first reported occurrence of coexisting magnesite + dolomite + possible aragonite + coesite in a single eclogitic sample and is consistent with experimentally determined stabilities of these phases at mantle conditions. Systematic mineral parageneses of eclogite, marble, and country-rock gneisses support the previous conclusion that supracrustal rocks were subducted to mantle depths of > 100 km.

Exsolution Lamellae in Minerals from Ultrahigh-Pressure Rocks

Exsolution microstructures have recently been identified in ultrahigh-pressure (UHP) ultramafic and eclogitic minerals from several orogenic belts, including the Western Gneiss Region, Norway; Alpe...

A refractory mantle protolith in younger continental crust, east-central China: Age and composition of zircon in the Sulu ultrahigh-pressure peridotite

Zircons have been extracted from garnet peridotite and its wall rock (gneiss), which was intersected by the pre–pilot hole of the Chinese Continental Scientific Drilling project (CCSD-PP1) in the Sulu ultrahigh-pressure (UHP) metamorphic belt. The peridotitic zircons record early Mesozoic UHP metamorphism ( 206 Pb/ 238 U age of 223.5 ± 7.5 Ma), but their Hf isotope compositions indicate that the protolith of the peridotite is at least Mesoproterozoic in age (minimum depleted-mantle ages [ T DM ] of 1.4 Ga). Zircons from the gneiss also reflect the Mesozoic metamorphism, with a cluster of 206 Pb/ 238 U ages at 224.5 ± 11.5 Ma; a trail of discordant grains indicates a protolith age older than 800 Ma, consistent with T DM model ages of younger than 1.2 Ga. The peridotitic zircons have trace-element patterns similar to kimberlitic and/or carbonatitic zircons, while those from the gneiss have affinities with zircons from syenites/monzonites. The differences suggest that the metasomatic agents that affected the peridotitic zircons were derived from the asthenospheric mantle rather than from subducted continental crust. The strong depletion of the CCSD-PP1 peridotite in basaltic components, and the relatively unradiogenic Hf isotopic compositions (e.g., −16.3 to ∼−13.8 e Hf ) of the peridotitic zircons, indicate that the peridotitic body is a fragment of refractory Archean mantle that experienced Mesoproterozoic metasomatism and represents a tectonic intrusion into younger crust.

Characteristics and Tectonic Significance of the Late Proterozoic Aksu Blueschists and Diabasic Dikes, Northwest Xinjiang, China

A stratigraphically coherent blueschist terrane near Aksu in northwestern China is unconformably overlain by unmetamorphosed sedimentary rocks of Sinian age (∼600 to 800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, were derived from shales, sandstones, basaltic volcanic rocks, and minor cherty sediments. They have undergone multi-stage deformation and transitional blueschist/greenschist-facies metamorphism, and consist of strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, blueschist, and minor quartzite, metachert, and meta-ironstone. Metamorphic minerals of basaltic blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and compositions of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and 300 to 400° C. Many thin diabasic dikes cut the Aksu Group; they are characterized by high alkali, TiO2, and P2O5 contents and possess geochemical char...

Very high-pressure orogenic garnet peridotites

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.

Low δ18O, ultrahigh-P garnet-bearing mafic and ultramafic rocks from Dabie Shan, China

Abstract The Maowu mafic–ultramafic complex from the Dabie ultrahigh-pressure metamorphic belt of China is enclosed in coesite-bearing gneiss. The complex mainly consists of layered garnet ortho- and clinopyroxenites in addition to minor harzburgite, eclogite and rare omphacitite. The rocks contain pyrope-rich garnet, enstatite, diopside and forsterite and minor clinohumite, magnesite, chromite, rutile, monazite and apatite with rods of monazite. Thermobarometry and coesite inclusions in omphacite constrain peak metamorphism at about 700±50°C and 40–60 kbar. The δ 18 O (SMOW) values of garnet, olivine, orthopyroxene, clinopyroxene and omphacite from seven samples were determined by in situ analysis with an Ultraviolet Laser Microprobe. Except for omphacite, the δ 18 O data define a narrow range between +2.9 and +4.4 per mil; omphacite has the lowest δ 18 O values (+1.5±0.3). Furthermore, fractionation of 18 O and 16 O between coexisting mineral pairs (e.g., ΔGrt-Cpx, ΔOl-Cpx, and ΔGrt-Opx) is less than ±0.5 indicating high-temperature O-isotope equilibrium. High REE contents and extreme LREE-enrichment [(La/Yb)N up to 17 400] for whole rock, as well as low δ 18 O data of minerals suggest the ultramafic–mafic rocks were metasomatized by a LREE-enriched fluid and reacted with meteoric water at a shallow crustal level prior to ultrahigh-P metamorphism at upper mantle depths.