Petrological And Tectonic Studies On Metasandstones Of Chencai Group In Zhejiang Province

Chencai Group metamorphic rocks outcrop in the northeastern margin of the Cathaysia Block and might contain important information about both tectonic regime of the Cathaysia Block and the collision history between the Yangtze and Cathaysia blocks, which can provide key clues for tectonic evolution of South China. The forming age and tectonics of the Chencai Group metamorphic rocks have been still disputed as far.. In the past, Chencai group rocks were deemed as Mesoproterozoic metamorphic basements of the Cathaysia Block, and have completed studies on chronology and metamorphism. Previous studies focused mainly on gneiss and amphibolite, but almost no metasandstone. In recent years, based on geologic background studies on regional mineralization under the Mineral Resources Potential Evaluation in East China, Chencai Group metamorphic rocks were first classified into ascribed early Paleozoic subduction-accretionary complex including typical ocean stratigraphy system, and its metasandstones may belong to exotic blocks from old continental crust, or matrix of the subduction accretionary complex. Therefore, it will have important scientific significances to studies metasandstones for understanding both compositions and tectonic nature of Chencai Gruop and tectonic evolution of the Cathaysia Block.Metasandstone samples from Xiabuxi, Cuixi and Jianxi of the Chencai area are studied on their Petrology and geochemistry as well as chronology of their detrital zircons. The main new achievements are as follows:(1) the protolith of Chencai Group metasandstones is flysch, which consis of feldspar-quartz sandstone and quartz sandstone and were formed in in active continental margin and arc setting;(1) the detrital zircons have a wide age spectral range(1710Ma~3620Ma), indicating that they derived from Archean-Proterozoic provenance and deposited at the initial stage of Paleoproterozoic.On the other hand, to explore further tectonic setting of metasandstones, their assoicated amphibolites at Xiabuxi and Cuixi are also determined. According to their geochemical compositions, these amphibolites can be subdivided into three species:(1) inner-plate Nb-rich Ocean Island Basalts(OIB),(2) Nb-enriched Island Arc Basalts(IAB), and(3) Nb-depelted Island Arc Basalts, the two latters formed in subduction zone. Combined with occurrence of marble associated with amphibolites, it is inferred that Chencai Group contains oceanic island-seamount association which belong to typical ocean stratigraphy system. Metamorphic zircons from amphibolite are dated at 424.6±2Ma, but their rock-forming age can not be obtained.Chencai Group metasandstones deposited within active continental margin and island arc settings during Paleoproterozoic, and were metamorphised and melanged together with typical ocean formations system such as OIBs and IABs during early Paleozoic. It proves that Chencai Group rocks do not belong to regional metamorphic basements, and might be subduction- accretionary complex.