The Study Of The Multi-tectonic Thermal Events In The Region Of Northern Sulu Orogen Belt By The Zircon Chronology

Sulu orogen belt was formed by the Triassic subduction and collision between the South China Block and the North China Block, is a natural laboratory to study the geodynamics of continental collision, although it is still on the controversy concerning the protolith’s composition, the formation era, the tectonic setting and the tectonic affinity in the core of Sulu orogen belt. This paper presents an integrated study of field geological, petrography, zircon chronology in the region of northern Sulu orogen belt. The results show that the northern Sulu orogen is mainly composed of predominant Noeproterozoic granitic gneisses with associated amphibolite, non-metamorphic Mesozoic granitoids intrusive rocks and small amounts of Precambrian metamorphic rock series. The Precambrian metamorphic rock series include of gneiss, amphibolite, shist, quartzite and pyroxenite with lenticular eclogite or garnet amphibolite which formed in retrograde stage. Mineral assemblages and substitute relations show that both the Neoproterozoic granitic gneisses and Precambrian metamorphic rock series suffered early eclogite-facies metamorphism and late amphibolite-facies retrograde metamorphism, and then suffered the post-collision partial melting and migmatization. The study of zircon U-Pb chronology proves that these grantic gneisses, whose protolith formed in Neoproterozoic (～753Ma), suffered UHP (ultrahigh-pressure) metamorphism at～230Ma and retrograde metamorphism at221-208Ma, and later anatexis at～176Ma. The zircons from gneiss which belongs to the Precambrian metamorphic rock series record early metamorphic ages of～2500Ma and late amphibolite-facies metamorphic ages of Y208Ma. As to the other rocks from the Precambrian metamorphic rock series, the zircons from amphibolites record metamorphic ages f1896-1837Ma and the pyroxenite’s zircons yield magmatic crystallization ages of～1895Ma and metamorphic ages of～1870Ma. The zircons from quartzite give inherited ages of2.8to2.0Ga with the peaks at2.80Ga,2.50Ga,2.30Ga and2.15Ga, and also give metamorphic ages of～1.84Ga. These ages show that the northern Sulu orogen belt generally record three magmatic thermal events (～1900Ma,～740Ma,～176Ma) and four tectonic thermal events (～2500Ma,1896-1837Ma,～230Ma,221-208Ma). Abased on the result above, we got the following conclusion:(1) The protolith of these granitic gneisses formed in the Neoproterozoic attributed to the rift magmatism during the breakup of the supercontinent Rodinia.(2) The Precambrian metamorphic rock series, in the core of Sulu orogen belt, was mainly composed of Archean gneiss, Paleoproterozoic pyroxenite and sedimentary rocks. And all of them suffered metamorphism in the Paleoproterozoic.(3) Both Precambrian metamorphic rock series and Neoproterozoic magmatic rocks suffered UHP meramorphism and late amphibolite-facies metamorphism in the Triassic period, and then experienced magmatism in the middle Jurassic.(4) The core of Sulu orogen belt was possibility as the result of the Triassic UHP metamorphism attributed to the subduction of the Yangtze block beneath the North China block because of a similar tectonic thermal history recording to Yangtze plate.