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Reflection attributes of paragneiss in the upper crust

Science China Earth Sciences 2008年第1期51卷 1-10页

作者： YANG WenCai XU ZhiQin YU ChangQing Institute of Geology Chinese Academy of Geological SciencesBeijing 100037China

The integrated study of the geological and seismic reflection data from the drilling area of CCSD has discovered that the density and the P-wave velocity of orthogneiss are almost the same as that of the paragneiss in the area; but the orthogneiss and the paragneiss hold different reflection attributes. The strong seismic reflector packes coinciding spatially with the paragneiss suites have implied that the paragneiss buried in the metamorphic crust itself can cause bone-like seismic reflector sets. The P-wave velocity of paragneiss shows little apparent difference with that of the orthogneiss; but its transverse wave velocity is lower, with the Vp/Vs ratios being high. The paragneiss has partially inherited the layering structures and textures of the protolithe of sedimentary rocks, hence shows strong heterogeneity and anisotropy, that is why the paragneiss are able to produce the bone-like reflectors in the upper crust. The low transverse wave velocity of paragneiss often means weak shear resistance, which will further cause cracks or fractures in the rock, consequentially increase its porosity and permeability during tectonic movements, and form the paragneiss reservoirs of low-permeability zones for gases uplifted from the deeper crust. Because the paragneiss in the crustal metamorphic basement can cause the seismic reflectors, seismic reflection sections are able to provide information about the paragneiss under certain prerequisites.

关键词： seismic reflection method metamorphic basement paragneiss crust reflectors

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Ultrahigh-pressure and Retrograde Metamorphic Ages for Paleozoic Protolith of paragneiss in the Main Drill Hole of the Chinese Continental Scientific Drilling Project(CCSD-MH),SW Sulu UHP Terrane

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Acta Geologica Sinica(English Edition) 2006年第3期80卷 336-348页

作者： LIU Fulai XU Zhiqin XUE Huaimin ZHOU Kaifu Institute of Geology Chinese Academy of Geological Sciences Beijing 100037

Laser Raman spectroscopy and cathodoluminescence （CL） images show that most zircon crystals separated from paragneiss in the main drill hole of the Chinese Continental Scientific Drilling Project （CCSD-MH） at Maobei, southwestern Sulu terrane, contain low-pressure mineral-bearing detrital cores, coesite-bearing mantles and quartz-bearing or mineral inclusion-free rims. SHRIMP U-Pb dating on these zoned zircons yield three discrete and meaningful age groups. The detrital cores yield a large age span from 659 to 313 Ma, indicating the protolith age for the analyzed paragnelss is Paleozoic rather than Proterozoic. The coesite-bearing mantles yield a weighted mean age of 228 ± 5 Ma for the UHP event. The quartz-bearing outmost rims yield a weighted mean age of 213 ± 6 Ma for the retrogressive event related to the regional amphibolite facies metamorphism in the Sulu UHP terrane. Combined with previous SHRIMP U-Pb dating results from orthogneiss in CCSD-MH, it is suggested that both Neoproterozoic granitic protolith and Paleozoic sedimentary rocks were subducted to mantle depths in the Late Triassic. About 15 million years later, the Sulu UHP metamorphic rocks were exhumed to mid-crustal levels and overprinted by an amphibolite-facies retrogressive metamorphism. The exhumation rate deduced from the SHRIMP data and metamorphic P-T conditions is about 6.7 km/Ma. Such a fast exhumation suggests that the Sulu UHP paragnelss and orthogneiss returned towards the surface as a dominant part of a buoyant sliver, caused as a consequence of slab breakoff.

关键词： SHRIMP U-Pb dating zircon mineral inclusions Paleozoic protolith paragneiss Chinese Continental Scientific Drilling Project

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Mineral inclusions of zircon and UHP metamorphic evidence from paragneiss and orthogneiss of pre-pilot drill-hole CCSD-PP2 in north Jiangsu Province,China

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Chinese Science Bulletin 2001年第12期46卷 1037-1042页

作者： LIU Fulai XU Zhiqin YANG Jingsui S Maruyama J.G.Liou I.Katayama H.Masago Chinese Academy of Geological Sciences Beijing 100037China Department of Earth&Planetary Science Tokyo Institute of TechnologyTokyo 152-8551Japan Stanford University Stanford CA 94305USA

Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectros-copy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and or-thogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.

关键词： coesite zircon laser Raman cathodoluminescence orthogneiss and paragneiss pre-pilot drillhole CCSD-PP2.

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