Rapakivi granites within Phanerozoic collisional orogens as a possible consequence of continental subduction and following exhumation of the Precambrian crust: Evidences from the Permian Jangart rapakivi in South Tien Shan collisional belt, Eastern Kyrgyzstan

摘要

All worldwide known rare occurrences of Phanerozoic A-type granites with typical rapakivi texture do not usually have any significant volume of contemporaneous basic rocks in magmatic association and occur only in those collisional orogens, which border ancient cratons and contain HP-UHP metamorphic terranes, suggesting connection of their petrogenesis with a deep continental subduction and subsequent exhumation of the ancient crust. Early Permian Jangart rapakivis and genetically related leucogranites and Li-mica granites widely spread within the Saryjaz syntaxis, which is distinguished by the shortest distance between the Tarim craton and Caledonian Kazakh continent, collided in the Late Carboniferous with the possibly deepest continental subduction of the Tarim under the South Tien Shan collisional belt. Rapakivi plutons occur within the remarkable huge (20 × 120 km) mega-melange (mega-breccia) terrane of diapiric (not tectonic) origin, which was identified by the latest geological data and deciphering of satellite imagery. According to geophysical data the most of subducted Tarim continental crust was trapped within lithospheric mantle and never exhumed. But in the Saryjaz syntaxis, where the collisional suture has been subjected to flecsure-like oroclinal bending due to sinistral strike-slip tectonics during oblique collision between the Tarim and Kazakh plates, the subduction channel was weakened and exhumation occurred. The suggested P-T-t path of exhumed slices of the subducted Tarim continental crust presumes a relatively long (around 20 Ma) residence time on a mantle depth as well as additional internal radiogenic heating and rapid (around 2 Ma) isothermal exhumation, which were favorable for extensive melting and subsequent rapakivi formation. Phanerozoic rapakivis originated from the same ancient lower crust granulite source rocks as their abundant Proterozoic counterparts but mechanism of heating and melting of those source rocks was radically different.