Phanerozoic Metamorphism Of Precambrian Metamorphic Basement In Southwestern Zhejiang

The South China Block was the largest part and played a key role during the amalgamation and evolution of continental Southeast Asia. The Cathaysia Block in the southeastern part of South China contains extensive Precambrian metamorphic basement terrains that were more or less reworked during the assembly and breakup of the Neoproterozoic Supercontinent, a Caledonian event during the Ordovician and Silurian, and the Indosinian orogenic episode from Late Permian to middle Tertiary. One of the main exposed areas for the Precambrian metamorphic basement of the Cathaysia Block, is in southwest Zhejiang Province. A combined study using petrographic investigation, electron-microprobe analysis (EMPA), geothermobarometer, LA-ICP-MS zircon U-Pb dating and trace elements, zircon Hf isotopes was carried out on the metamorphosed rocks in the metamorphic basement of the Cathaysia Block, southwestern Zhejiang Province. The formation and metamorphic ages of the rocks from the metamorphic basement of the Cathaysia Block were determined based on zircon U-Pb geochronology, and the metamorphic P-T-t parth were difined. Results from this study provided an important restriction for the tectonic evolution in South China and the Southeast Asia in the Phanerozoic.A combined study using LA-ICP-MS and SHRIMP U-Pb dating and trace elements was carried out on zircon from the metamorphosed rocks in the metamorphic basement. The result shows that the formation age of the Biotite amphibolite and Plagioclase amphibolite from Danzhu is 1850±9.3Ma. Theε_Hf(t) values of the older zircons were from -7 to -3, with two-stage model Hf ages (T_DM2^LC) of about 2.9 to 3.4 Ga, indicating that the source material was derived from anatexis and recycling of the Archean crust. Then, the Badu group was formed in Paleoproterozoic, and it is the oldest rock as far as our knowledge goes. The protolith age of the Garnet-bearing gneiss from Chencai Group is 870±36Ma, it indicate the upper limit of the formation time of Chencai Group.The Precambrian basement rocks in the Southwestern area of Zhejiang Province have been reworked by two periods of metamorphic events in the Phanerozoic. The strongest one with most incidences occurred during the Caledonian period, and the one with relatively lower metamorphic degree took place during the Indosinian period.As completely recorded in the Caledonian metamorphism, the progressive metamorphic phase (Ml) was dated at ca. 445 Ma, with metamorphic temperature of about 680℃and pressure of about 0.54GPa, the peak metamorphism (M2) was dated at ca. 430 Ma, with metamorphic temperature of about 780℃and pressure of about 0.8GPa, and the retrograde metamorphic phase occurred in ca. 420Ma, under the condition of 580～650℃and 0.4GPa. The ⁴⁰Ar/³⁹Ar age of Hornblende indicates that the rocks cooled down to～500℃, which is believed as the closure temperature of hornblende, in 415 Ma during the retrograde metamorphism. The P-T-t path established according to the alteration of the P-T conditions along with the metamorphic ages is in the form of clockwise, which suggests that the Caledonian metamorphism in Southwestern Zhejiang Province was developed in the tectonic environment of collision orogenesis.The newly formed metamorphic zircons from these metamorphosed basic rocks in Badu Group yielded U-Pb ages of 260—230Ma. The metamorphic temperature calculated using the Ti-in-zircon geothermometer ranged from 610 to 710℃, Amphibole-plagioclase geothermometer gived the similar result which ranged from 600℃to 690℃, indicating that the Cathaysia Block experienced an amphibolite facies metamorphism during the Indosinian.Results from this study provided an important restriction for tectonic in Cathaysia Block in the Phanerozoic. During Neoproterozoic to early Paleozoic, Cathaysia Block most likely is a part of passive continental margin in North Gondwana. And in the terminal stage of amalgamation of Gondwana continental, the extrusion of foreign landmass result in the Cathaysia Block deformation and metamorphism. Since late Paleozoic, these Blocks depart from Gondwana and moved northward, and amalgamated to southeastern Asian continent at different time. And cause the metamorphism in the cathaysia block during the Late Permian and Early Triassic times. It is indicated that 260—230 Ma was an period of active tectonic evolution in this area, possibly related to the oblique subduction of the proto-Pacific plate underneath South China. The subduction of the proto-Pacific plate underneath South China possibly resulted in crustal thickening of southeastern South China, and high amphibolite facies metamorphim, and triggered the extensive magmatic and tectonic activities in South China.