HR: 14:36h
AN: V13C-05    [Abstracts]
TI: The metamorphic evolution of the eclogitic rocks from the 100-2000m core of Chinese Continental Scientific Drilling, China
AU: * You, Z
EM: zdyou@263.net
AF: Faulty of Earth Sciences,China University of Geosciences, Lumo Rd. #388, Wuhan, 430074 China
AU: Su, S
EM: susg@cugb.edu.cn
AF: School of the Earth Sciences and Resources,China University of Geosciences,Beijing, Xueyuan Rd. #29, Beijing, 100083 China
AU: Liang, F
EM: lfh@ccsd.org.cn
AF: Key Laboratory for Continental Dynamics of MLR,Institute of Geology,CAGS, Baiwanzhuang Rd. #26, Beijing, 100037 China
AU: Zhang, Z
EM: zzm@ccsd.org.cn
AF: Key Laboratory for Continental Dynamics of MLR,Institute of Geology,CAGS, Baiwanzhuang Rd. #26, Beijing, 100037 China
AB: On the basis of petrographic study of the metamorphic rocks from 100-2000m in the main hole of the Chinese Continental Scientific Drilling (CCSD) it is revealed that the major lithological types are: (1)Eclogites and garnet clinopyroxenites; (2)Eclogitic gneisses; (3)Garnet peridotites; (4)Biotite (hornblende) two-feldspar gneiss (granitic gneiss) and (5) Fault breccia and mylonites. Most of the eclogites and eclogitic gneisses had a long residence history in the crust before the deep subduction of the continental slab. However the garnet clinopyroxenites are closely related with garnet peridotites which occur within 607-783m of the drill hole. Based on their mineralogical and petrochemical characteristics they were ultramafic-mafic complexes incorporated into the subducted slab from the overlying mantle wedge during the subduction of the continental slab and had undergone ultrahigh"Cpressure metamorphism (UHPM). The eclogitic gneisses, equivalent partly to paragneiss as called by some geologists, are UHPM rocks of intermediate acidic protolith associated with crustal eclogites. The granitic gneisses are of diverse origin, most of them are products of decompressive melting of the retrogressive amphibolite and biotite gneiss derived from eclogitic rocks. The metamorphic evolution of the eclogitic rocks can be divided into 3 major stages. The first (M1) is the ultrahigh pressure metamorphism (UHPM) as evidenced by coesite inclusions within garnet and omphacite grains in eclogites. The second (M2) is a retrogressive stage sequence during which most of the UHPM rocks were turned to rocks of high-pressure eclogite facies, amphibolite facies and then epidote amphibolite facies. At this stage the eclogites are characterized by the growth of symplectites and coronas. The eclogitic gneiss is retrograded to be biotite (hornblende) plagioclase gneiss and epidote biotite (amphibole) gneiss. The granitic rocks are turned to biotite hornblende two feldspar gneiss (so-call orthogneiss) due to partial melting or K-metasomatism of the HP supercritical fluid. Some of them are allanite (Ce)"Cbearing, the La/Ce=0.42­¦0.72. The allanite are zonal and are rimmed by epidote, representing the later overprint of epidote amphibolite facies retrometamorphism. The third stage (M3) is characterized by the formation of tectonic breccia and mylonites in response to the brittle and ductile-brittle deformation related with uplift. The matrix of these tectonites containing chlorite, actinolite and calcite indicates the greenschist facies of metamorphism (M3). All of the above-mentioned 3 stages of metamorphism are records of the tectonic processes surpassed by these eclogitic rocks. They delineate a clock-wise metamorphic PTt D path of the Donghai UHP terrane which is comparable in pattern with those of Dabieshan. The metamorphic evolution and the successive deformational events found in the 100­¦2000m core from the CCSD main hole confirms that during the collision of the two (Yangtze and North China) plates voluminous crustal materials including granitic rocks can deeply be subducted to mantle depth and then rapidly returned back to the earth surface. Acknowledgement: This work is sponsored by the Major State Key Project to CCSD: 2003CB716501 and major project from NSFC: 40399142.
DE: 8100 TECTONOPHYSICS
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