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High-Temperature/Low-Pressure Metamorphism in a Continental Rift in the Northern Margin of the South China Block

He Qiang;Zheng Yongfei;CAS Key Laboratory of Crust-Mantle Materials and Environments,School of Earth and Space Sciences,University of Science and Technology of China;  
The formation of high-temperature(HT)/low-pressure(LP) metamorphic rocks requires high thermal gradients of 30 ℃/km. It is intriguing which tectonic setting is responsible for such geological processes. This paper presents a summary of our petrological and geochemical studies on metagranite and metabasalt from the northern margin of the South China block,which were formed during breakup of Rodinia supercontinent in the middle Neoproterozoic. The results demonstrate that continental rifts are the most plausible setting for the production of HT/LP metamorphic rocks. The HT/LP metamorphism is mainly recorded in alumino silicates-bearing metagranites,in which metamorphic andalusite and sillimanite were produced by muscovite dehydration reaction. Metamorphic P-T conditions of 1.0-3.5 kbar and 560-660 ℃ were obtained from the petrology of aluminosilicates-bearing peak mineral assemblages in combination with pseudosection calculations. The metamorphic andalusite shows very negative δ18 O values in O isotope disequilibrium with magmatic zircon,further demonstrating that it is the metamorphic product after magma crystallization. The U-Pb dating of metamorphic titanite yields concordant ages of 751±11 Ma for the HT/LP metamorphism,consistent with the peak age of the Rodinia breakup. The metabasalt shows island arc basalts-like trace element distribution patterns,indicating that its source was generated by metasomatic reaction of the mantle wedge peridotite with fluids derived from the subducting oceanic crust. Therefore,the mantle source was formed during the Grenvillian assembly of Rodinia supercontinent. In this regard,the continental rifting that resulted in the supercontinental breakup was developed in the former subduction zone. By comparing heat flow required to form the metamorphic peak mineral assemblages with that provided by heat producing elements in the metagranites,it appears that anomalously high heat flow was indeed delivered from the asthenospheric mantle to the continental rift,leading to the HT/LP metamorphism during the Rodinia breakup.
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