HR: 16:20h
AN: V42G-02    [PDF]
TI: Deformation Structures and Geochronology of the Deformation-Metamorphism Event of the South Sulu High-Pressure Metamorphic belt
AU: * Xu, Z
EM: xuzhiqin@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, CAGS, Beijing 100037, China, Beijing, 100037 China
AU: McWilliams, M
EM: mac@pangea.stanford.edu
AF: Dep. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States, Standford, 94305 United States
AU: Qiu, H
EM: qhj@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, CAGS, Beijing 100037, China, Beijing, 100037 China
AU: Yang, J
EM: yangjsui@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, CAGS, Beijing 100037, China, Beijing, 100037 China
AU: Liou, J G
EM: liou@pangea.Stanford.EDU
AF: Dep. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, United States, Standford, 94305 United States
AB: The present structure of the Sulu HP-UHP belt in east China is a product of subduction and exhumation, resulting in a foliation parallel to the trend of the HP-UHP metamorphic belt, SE-ESE dips of 20-30­‘, with NW-SE stretching lineation. The Sulu HP-UHP belt has two parts: a southern stacked set of HP tectonic slices overlain by a northern stacked set of UHP slices. Previous studies indicate that the northern UHP slices is the product of Indosinian continental deep subduction and exhumation. SHRIMP ages indicate UHP metamorphism at 220-245 Ma and exhumation at 202-220 Ma. The HP belt is further subdivided into high-pressure (HP) and low-temperature (LT) slices in the southeast and the HP and medium-temperature (MT) slices in the northwest. The HP/LT slices have P-T estimates of 0.70-0.85 GPa and 300-360­‘, and contain characteristic minerals such as winchite, barroisite, phengite and aragonite. These rocks are strongly sheared and individual tectonic slabs are separated by mylonitize zones. During exhumation, HP-LT rocks underwent continuous retrograde metamorphism from blueschist through laucophane-greenschist to greenschist facies. Retrographic metamorphic minerals are developed on regional strong penetrative shearing foliations and have inferred P-T conditions at 350-410 ­‘ and 0.2-0.4 GPa. The HP-MT belt is composed of kyanite-topaz quartzite (500 to 600 ­‘) kyanite-bearing albite-quartz schist, marble, phosphatic marble and phosphorite. The HP-MT rocks experienced retrograde metamorphism from epidote-amphibolite to greenschist facies (350 to 410­‘ and 0.2 to 0.4 GPa). The LPO of quartz from mylonitized gneiss and schist constrains LPO models of the prism \{1010\} $<$a$>$ and base \{0001\}$<$a$>$, which reflects the MT (450-650 ­‘) and LT (< 400 ­‘)shear strain conditions. Shear strain and LPO of quartz indicate that shearing vector was from ESE to WNW. Three biotite samples yielded a weighted mean (WM) age of 232.5 $\pm$ 2.4 Ma and an isochron age of 232.9 $\pm$ 2.5 Ma, a plateau age of 253.9 $\pm$ 2.6 Ma and an isochron age of 254.0 $\pm$ 2.7 Ma and a plateau age of 243.9 $\pm$ 2.6 Ma and an isochron age 244.2$\pm$2.6 Ma. Two muscovite samples have a WM age of 214.2 $\pm$ 2.3 Ma and an isochron age of 213.8 $\pm$ 2.7 Ma and a WM age of 218.7 $\pm$ 2.3 Ma and an isochron age of 218.8 $\pm$ 2.4 Ma. Two K-feldspar samples yield WM ages of 173.1 $\pm$ 2.0 and 180.5 $\pm$ 2.0 Ma and an isochron age of 180.5 $\pm$ 2.2 Ma, and one hornblende has a WM age of 208.5 $\pm$ 2.2 Ma and an isochron age of 207.5 $\pm$ 2.6 Ma. All are the products of ductile shear and retrograde metamorphism. These ages represent deformation-retrograde events during exhumation. In contrast with the UHP ages of 242-220 Ma and the exhumation ages of 200-220 Ma, the HP metamorphic rocks apparently were exhumed since 250 Ma. Protolith for the HP rocks was Neoproterozoic metavolcanic basement of the Yangtze plate, whereas the UHP protolith was Meso- to Neoproterozoic paragneiss and granitic gneiss of the Yangtze plate. We infer that prior to 250 Ma, both continental basement slabs had been subducted northwards beneath the north China plate simultaneously. After reaching 20-30 km depth, HP metamorphic rocks were formed and soon after exhumed, while the incipient UHP rocks continued to be subducted to about 100 km depth and were later exhumed and appear at the surface with HP slabs.
DE: 8100 TECTONOPHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting