HR: 13:40h
AN: V13C-01    [Abstracts]
TI: Chinese Continental Scientific Drilling (CCSD) Project: Progresses and Perspectives
AU: * Xu, Z
EM: xzq@ccsd.org.cn
AF: Key Laboratory for Continental Dynamics, MLR, Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Rd. #26, Beijing, 100037 China
AB: By employing modern deep drilling techniques, CCSD project is going to drill a 5000 m borehole at the Sulu ultrahigh pressure metamorphic belt. The scientific objectives are to reveal the 3-D deep materials constitutions of convergent plate margins, to improve our understanding the physical and chemical processes that are responsible for the formation and evolution of UHP metamorphic terrains worldwide and finally to establish a deep laboratory for long-term observation on present-day crustal activities. The main hole was drilled to a depth of 4400m and 3 satellite holes have been completed. Many rock types including the UHP eclogite, gneiss, schist, quartzite, mylonite and garnet-peridotite, and large numbers of petrological boundaries have been revealed. Various logging profiles (e.g. petrology and geochemistry)up to 4000 meters long have been established. Some important scientific progresses from CCSD cores and field investigations have been obtained: (1) coesite and other UHP mineral inclusions are found widely in the gneisses, schists, amphibolites, marble, quartzite, and eclogites from the CCSD borehole, satellite holes and surface, which indicate that a large amount of material has been subducted to a great depth (> 100km); (2) 5 tectonic units and major shear boundaries are well-defined in the first 2000m long, detailed and orientated tectonic profile. Geochemical data and the early structures suggest that a rifting setting at a passive continental margin for the formation of the protoliths that have experienced HP-UHP metamorphism; (3) garnet peridotites from PP1 have olivine slip system (100)[001], which is similar to that of the Alps Arami ultramafic body, Central Alps, suggesting a LT/UHP condition. Omphacite slip system (100)[010] from eclogites of CCSD borehole also implies a LT/UHP condition; (4) the elasticity wave and thermal conductivity profiles have provided an important petrophysical constraints on the seismic reflection and geothermal structure. The VSP seismic profile reveals the deep structure of the tectonic slices, and indicates a close relationships between the strong seismic reflection layers and large ductile shear zones; (5) UHP peak metamorphic age (240-220Ma) and amphibolite facies retrograde ages (220-200Ma) are determined by the SHRIMP U-Pb dating of zircons from coesite-bearing paragneiss and orthogneiss. Ar/Ar ages for the southern Sulu HP metamorphic rocks indicate that exhumation started at 250-240 Ma, earlier than that of the north Sulu UHP rocks (220-200Ma). We propose a new model for diachronous subduction and exhumation of the Sulu HP-UHP Metamorphic Belt; (6) the seismic reflective and the tomographic profiles across the Sulu UH-UHP metamorphic belt reveal that the Sulu HP-UHP metamorphic belt occurs as an arched and eroded slab, translucent reflection body of low velocity and low density reflectors, and the Tanglu fault and Xiangshui fault probably occur as mantle shear zones; (7) many CH4, CO2 and He anomalies are found in the borehole, and these anomalies are more abundant with depth; (8) bacteria and archaea were discovered in eclogite, amphibolite, and gneiss at depth of 529, 730, 1080, 1179, 1930, 2026 m, respectively. Microbes living in extreme conditions are present in the drilling cores identified by DNA analysis; and (9) heat producing element (HPE) analyses show that the first 2000 m borehole has a stepwise or sandwich-like HPE distribution resulted from the juxtaposition of low HPE mafic to ultramafic rocks with high HPE felsic rocks.
DE: 8102 Continental contractional orogenic belts
DE: 3660 Metamorphic petrology
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 0330 Geochemical cycles
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting