J. G. Liou

Coesite-bearing eclogite from the Dabie Mountains in central China

Coesite and Coesite pseudomorphs are recognized in eclogite from the Dabie Mountains, a collision zone between the Sino-Korean and Yangtze cratons in central China. This third known occurrence of Coesite in deep crustal rocks is within an Archean gneiss terrane and is the first locality where Coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location. Coesite-bearing eclogite is estimated to form at pressures of >28 kbar and temperatures from 600 to 710 °C. The Dabie Mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation. The preservation of Coesite in eclogite is found to be strongly controlled by the pressure-temperature-time path that coesite- bearing eclogite followed during its uplift and retrograde metamorphism.

An outline of the plate tectonics of China

China is principally a part of the Eurasian plate, but the margins of the Indian and Philippine Sea plates are involved in the Himalayas and in the Coastal Range of Taiwan, respectively. Within the Eurasian plate, the Cathaysian paleoplate is separated from the Angaraian paleoplate by the Junggar-Hegen suture, which contains Paleozoic ophiolites and rare blueschists. The three microplates of the Cathaysian paleoplate consist of Precambrian cratons and/or Phanerozoic accretionary fold belts. These coalesced Precambrian cratons record at least six stages of intense orogeny before cratonization. The Paleozoic to Cenozoic accretionary fold belts of China can be correlated with similar events now found in west Pacific-, Andean-, and Atlantic-type active continental margins. Ophiolites occupying many of these tectonic zones provide evidence for the age and igneous history of oceanic crust formed during the Paleozoic to Cenozoic. The presence of blueschist in some of these Chinese sutures reveals evidence of large-scale subduction and tectonic exhumation during consolidation of the Eurasian plate. Cenozoic collision of the Eurasian and Indian plates produced deformation and uplift of the Himalayas, strongly influencing the tectonics of western China. In contrast, Mesozoic-Tertiary evolution of eastern China is typical basin-range geology, similar to that of the western United States, which included development of deep sedimentary basins along with calc-alkaline plutonic and volcanic activity associated with crustal thinning and high heat flow. The complicated tectonic evolution of China is greatly illuminated by the presence of ophiolites and blueschists in Proterozoic to Tertiary convergent boundaries. These petrotectonic assemblages provide evidence of an extremely mobile history of plate movement in China.

Blueschists and eclogites of the world and their exhumation

High-P/T metamorphic belts were classified into types A and B according to their protoliths. The A-type (collision-type) blueschists possess passive-margin protoliths characterized by platform-type carbonates, bimodal volcanics, and peraluminous sediments. B-type (Cordilleran-type) blueschists consist of active continental-margin protoliths in an accretionary complex characterized by bedded chert, MORB and ocean-island basalts, reef limestones, and graywackes. The spatiotemporal distribution of blueschists and eclogites of the world was compiled; among 250 recognized high-P/T belts, about 20% belong to the A type and the rest to the B type. Most A-type zones lie in Europe and the Tethyan domain, include ultrahigh-pressure metamorphic terranes, and have metamorphic pressure up to 45 kbar. B-type zones occur mainly in the circum-Pacific orogenic belts and intracontinental orogens in Asia, and were recrystallized at P <12 kbar. Associated peridotites include garnet peridotite in the A type and strongly serpe...

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.

When continents collide : geodynamics and geochemistry of ultrahigh-pressure rocks

Preface. 1. Active Crustal Subduction and Exhumation in Taiwan C.H. Lin, S.W. Roecker. 2. Melting of Crustal Rocks During Continental Collision and Subduction A.E.P. Douce, T.C. McCarthy. 3. Rheology of crustal Rocks at Ultrahigh Pressure B. Stockhert, J. Renner. 4. Thermal Controls on Slab Breakoff and the Rise of High-Pressure Rocks During Continental Collisions J.H. Davies, F. von Blanckenburg. 5. Exhumation of Ultrahigh-Pressure Rocks: Thermal Boundary Conditions and Cooling History B. Grasemann, et al. 6. Active Tectonics and Ultrahigh-Pressure Rocks A.E. Blythe. 7. K-Ar (40Ar/39Ar) Geochronology of Ultrahigh Pressure Rocks S. Scaillet. 8. Geochemical and Isotopic Characteristics of UHP Eclogites and Ultramafic Rocks of the Dabie Orogen: Implications for Continental Subduction and Collisional Tectonics B.M. Jahn. 9. Stable Isotope Geochemistry of Ultrahigh-Pressure Rocks D. Rumble. 10. Tracing the Extent of a UHP Metamorphic Terrane: Mineral-Inclusion Study of Zircons in Gneisses from the Dabie Shan H. Tabata, et al. 11. H2O Recycling During Continental Collision: Phase-Equilibrium and Kinetic Considerations W.G. Ernst, et al. 12. Influence of Fluid and Deformation on Metamorphism of the Deep Crust and Consequences for the Geodynamics of Collision Zones H. Austrheim.

Contrasting plate-tectonic styles of the Qinling-Dabie-Sulu and Franciscan metamorphic belts

The Dabie Mountains are part of the >2000-km-long Qinling-Dabie-Sulu suture zone juxtaposing the Sino-Korean and Yangtze cratons. An eastern extension apparently crosses Korea and lies along the Japan Sea side of Honshu as the Imjingang and Sangun terranes, respectively; a northeastern segment may be present in Sikhote-Alin, Russian Far East. This orogenic belt records late Paleozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs in east-central China. Coesite and microdiamond inclusions in strong, refractory minerals of eclogite facies ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu area attest to profound subduction of a leading salient of the old, cold Yangtze craton, now recovered through tectonic exhumation and erosion. Northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high-pressure (HP) and UHP metamorphism documented in the Western Alps. In both regions, subduction of narrow prongs of continental material, UHP metamorphism, and return toward midcrustal levels of relatively lower density, buoyant microcontinental blocks resulted from delamination of these rocks from the descending, higher density, oceanic-crust-capped lithospheric plate. Such salients of continental crust represent an integral structural part of the downgoing slab, remain intact, and may be dragged to great depths before disengaging and rising differentially as coherent blocks. UHP parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700–900 ° C and 28–35 kbar or more. In contrast, Pacific-type HP metamorphic belts, as represented by the Franciscan Complex of western California, recrystallized under physical conditions up to 200–500 ° C, 10 ± 3 kbar. In this setting, voluminous quartzo-feldspathic and graywacke debris was carried downward on oceanic-crust-capped lithosphere, choking the subduction zone with incompetent material. Sited between both plates, and strongly adhering to neither, this buoyant, largely sedimentary complex decoupled at 25–30 km depth, and ascended toward the surface. In both Alpine-type intracontinental collision and Pacific-type underflow, light sialic material displaced dense mantle; thus, the return to midcrustal levels was propelled dominantly by body forces.

Regional ultrahigh-pressure coesite-bearing eclogitic terrane in central China: Evidence from country rocks, gneiss, marble, and metapelite

The Dabie Mountains represent a fault-bounded, coesite-bearing eclogite gneissic terrane within the collisional belt between the Sino-Korean and Yangtze cratons. The following geologic evidence indicates that this terrane has been subjected to regional ultrahigh-pressure metamorphism. (1) The country rocks have a metamorphic pressure-temperature evolution compatible with that of the eclogites, on the basis of the occurrence of quartz pseudomorphs after coesite, calcite pseudomorphs after aragonite, relict high-pressure assemblages in different rock types of the country rocks, and consistent, comparable compositional zonations of garnets in both country rocks and eclogites. (2) Peak metamorphic pressure-temperature conditions consistently decrease both from east to west in the collisional belt and from north to south in the Dabie Mountains, and clockwise pressure-temperature paths are common for eclogites over the region. (3) Layered eclogites interbedded with gneisses and amphibolites contain omphacite, kyanite, and garnet, indicating a minimum pressure of 20 kbar at 600 °C for formation of these rocks.

40Ar/39Ar geochronology and exhumation of high-pressure to ultrahigh-pressure metamorphic rocks in east-central China

New 40 Ar/ 39 Ar ages from rocks in the high-pressure and ultra-high-pressure (HP-UHP) metamorphic complex in the Hong9an and Dabie Mountains areas, east- Central China, document a two-phase cooling and exhumation history following Triassic continental collision and metamorphism. Phengite ages from blueschist through kyanite-bearing eclogite facies rocks in Hong9an record initial exhumation from the collision zone between 230 and 195 Ma. Biotite and hornblende ages from migmatites and eclogite-bearing gneisses from the Dabie Mountains record a cooling event between 128 and 117 Ma, corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons. The Triassic through Early Jurassic 40 Ar/ 39 Ar cooling ages corroborate U-Pb and Sm-Nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid. Emplacement of granitic melts within the HP-UHP sequences ∼80 m.y. after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial, rapid exhumation episode.

Coesite-Bearing Eclogites from the Dabie Mountains, Central China: Petrogenesis, P-T Paths, and Implications for Regional Tectonics

Coesite- and kyanite-bearing eclogites are abundant in the southern part of the Dabie Mountains (southern Dabie terrane-SDT). Two types of eclogites from the SDT (Types III and IV) were selected for detailed paragenetic study. Type III eclogites, most abundant in the northern part of the SDT, occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite + garnet + phengite + epidote + coesite + kyanite + carbonate + rutile + ilmenite. These minerals exhibit weak compositional zoning and contain few mineral inclusions. Type IV eclogites, mostly in the southern part of the SDT, occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite + garnet + glaucophane + kyanite + epidote + phengite + quartz + rutile + ilmenite. Garnets of Type IV eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite, phengite, epidote, quartz, and rutile in the core and omphacite, barroisite, and Mg-katophorite in the rim. Prograde blueschist facies (~400°C) assemblages were partially preserved in Type IV eclogites. The eclogitic assemblages of both types of eclogite have been partially or completely retrograded to amphibolite and greenschist facies assemblages. Parageneses and compositions of minerals from eclogites indicate that these rocks have undergone a clockwise P-T evolution path. Within the SDT, the temperatures, estimated according to