Anatetic Events And Their Tectonic Significance Of Precambrian Metamorphic Basement In Yunkai Area, Guangdong And Guangxi Provinces

The study area of Yunkai block is located in the southeast of Guangxi province and the west of Guangdong province,is the southwestern part of Wuyi-Yunkai Caledonian fold belt(Guangxi orogenic belt) in Cathaysia Block. Precambrian metamorphic basement widely outcropped which is composed mainly of Tiantangshan Group complex and Yunkai Group complex in Yunkai area,and Tiantangshan Group metamorphic basement has experienced intensive anatexis and migmatization. About the formation time of Yunkai metamorphic basement and the time of occurence of anatexis have not reached an agreement.Based on the field geologic observation?systematic sampling?petrographic investigation,a combined detailed study of petrology,rock geochemistry and ziron U-Pb geochronology for the samples from the Tiantangshan Group complex in Yunkai massif,aiming to elucidate the original and formation time of the metamorphic basement, the age of anatexis and its tectonic significance.(1) According to the field geologic observation in Yunkai area,we find Tiantangshan metamorphic intrusive rocks occurred anatexis besides Tiantangshan Group.(2) Most of the previous studies show that the Tiantangshan intrusive rocks formed in the Early Paleozoic, but we find part of Tiantangshan intrusive rocks formed in the Mesoproterozoic and the structural setting of volcanic arc through our research.(3) The original rock of Tiantangshan Group metamorphic basement are mainly sedimentary rocks,and include igneous rocks through protolith reconstruction research,part of them formed in the Mesoproterozoic and the structural setting of active continental margin and island arc. And we also find the Archean remnant crustal information in the Yunkai Precambrian metamorphic basement,the oldest age can up to 3.2 Ga.(4) We find Tiantangshan Group metamorphic basement and metamorphic intrusive rocks occurred anatexis twice by the field geologic observation and microscopic studies,the anatexis occurred in the Neoproterozoic(850±Ma) and early Paleozoic(448±Ma).The reason of the first phase anatexis is due to increasing temperature of crustal rocks which lead to partial melting of the basement rocks while the second phase anatexis is due to the penetration of foreign magma along the gneissic schistosity which caused the metasomatism melting with the basement rocks.(5) The bending and retreat of the oceanic crust which had subducted beneath the Cathaysia Plate leaded to upwelling of magma in the upper mantle which caused the increasing temperature of crustal rocks in the Neoproterozoic, and then the Tiantangshan Group metamorphic basement and metamorphic intrusive rocks occured the first phaseanatexis with partial melting.The split time of Yangtze Plate and Cathaysia Plate was later than 850 Ma. The penetration of foreign granitic magma caused by the that the switch of tectonic regime from arc-continental collision orogenesis along the gneissic schistosity caused the second phase anatexis in the early Paleozoic which means that the switch of tectonic regime occured in 448±Ma,the metamorphic event occurred in 240±Ma is a response to the Indosinian collision orogenesis in the South China plate.