| Under the Mongol-Okhotsk Ocean and Paleo-Pacific Ocean tectonic domains,extensive Late Mesozoic volcanic rocks occurred in the Great Xing’an Range.Lots of researches have been carried out on these volcanic rocks,and proposed several models to explain their petrogenesis,such as mantle plume,post-collision extension,back-arc extension,ocean range subduction and flat subduction.This thesis mainly studies Late Mesozoic volcanic rocks in the Zhalantun-Zhalaiteqi,central segment of the Great Xing’an Range,and systematically analyze their petrological,geochemical,geochronological and zircon Hf isotopic characteristics.Combined with previous studies,this thesis tries to discuss temporal and spatial variation characteristics,petrogenesises and geodynamic setting of Late Mesozoic volcanic rocks in the central segment of the Great Xing’an Range,and better constrains the spatial and temporal range of Mongol-Okhotsk Ocean and Paleo-Pacific Ocean tectonic domains.According to zircon U-Pb dating results and previous geochronological studies,the Late Mesozoic magmatic events in the central Great Xing’an Range can chiefly be divided into four episodes:（1）early Late Jurassic（～158 Ma）;（2）late Late Jurassic-early Early Cretaceous（age peaks at～150 Ma and～138 Ma）;（3）middle-late Early Cretaceous（age peaks at～130 Ma and～126 Ma）,and（4）late Early Cretaceous（age peaks at～119 Ma and～112 Ma）.The early Late Jurassic dacites（～158 Ma）are calc-alkaline I-type rocks and yield zirconεHf（t）values of+1.96～+10.61,indicative of partial melts of juvenile crust.Contemporaneous A-type granites,alkaline granites,rift-related mafic volcanics and fault basin imply an extensional tectonic setting.The eastern segment of Mongol-Okhotsk Ocean has not been closed during the early Late Jurassic.Therefore,such episode of magmatic activities formed under a southward subduction related back-arc extensional tectonic setting.The late Late Jurassic-early Early Cretaceous volcanics（age peaks of～150 Ma and～138 Ma）comprise andesites,dacites and rhyolites.～150 Ma dacites are high K calc-alkaline I-type rocks and yield zirconεHf（t）values of+8.57～+12.38,indicative of partial melts of juvenile lower crust.～138 Ma andesite-dacites are high K calc-alkaline rocks and give zirconεHf（t）values of+2.42～+10.53,formed by partial melting of lithosphere mantle previously metasomatized by subduction-related fluids.Rhyolites are A-type rocks and exhibit zirconεHf（t）values of-6.70～+7.17,originated from partial melting of ancient crust materials with contaminations by mantle magmas.The late Late Jurassic-early Early Cretaceous molasse-like formation,compressional deformations and adakitic rocks suggest a compressional tectonic domain in the Great Xing’an Range.A flat subduction of the Mongol-Okhotsk Ocean was recorded during the late Late Jurassic-early Early Cretaceous and caused contemporaneous magmatisms under a compressional tectonic setting.The middle-late Early Cretaceous volcanics formed at two episodes of～130 Ma and～126 Ma.～130 Ma rhyolites are I-type rhyolites and give zirconεHf（t）values of4.29～10.24,indicative of partial melts of juvenile lower crust.Rhyolites and contemporaneous mafic volcanic rocks constitute bimodal volcanic lithologies,indicative of an extensional tectonic setting.～126 Ma volcanic rocks is composed of basaltic andesites,andesites,dacites and rhyolites,among which intermediate rocks are high K calc-alkaline rocks and yield zirconεHf（t）values of+3.84～+9.29 and positive Eu anomalies,which imply that these intermediate magmas were derived from depleted lithosphere mantle,and then assimilated plagioclase-rich accumulated plutons in the lower crust by melting,assimilation,storage and homogenization process（MASH）.The felsic rocks are I-type rhyolites and give zirconεHf（t）values of+4.84～+9.19,which were derived from partial melting of juvenile lower crust and then underwent fractional crystallizations of plagioclase and apatite.～126 Ma volcanic rocks in the Great Xing’an Range mainly outcrop in the east of the North-South Lineament,and involve MASH process between mantle magmas and plagioclase-rich lower crust,which is highly different from～130 Ma crust-mantle interactions.Combined with contemporary A-type rhyolites and metamorphic core complex,we propose that～126 Ma magmatisms were highly likely to be formed under a back-arc extensional tectonic setting caused by a westward subduction of Paleo-Pacific Ocean.The late Early Cretaceous volcanic rocks（age peaks of～119 Ma and～112 Ma）comprise andesites and rhyolites.Rhyolites exhibit shoshonitic rock series and zirconεHf（t）values of+5.47～+7.38,which were derived from partial melting of juvenile lower crust.Contemporary intrusive rocks comprise quartz diorite,syenogranite and granite porphyry,among which quartz diorite exhibits high K calk-alkaline rock series and zirconεHf（t）values of+5.58～+9.02,and was derived from partial melting of lithosphere mantle metasomatized by subduction-related fluids.Syenogranite is I-type granite and yield zirconεHf（t）values of-16.5～-19.9 and TDM2values of 1949 Ma～2132 Ma,which were originated from partial melting of ancient crust.Granite porphyry belongs to I-type rock series and give zirconεHf（t）values of+4.76～+7.36,indicative of partial melts of juvenile lower crust.The Mongol-Okhotsk Ocean has closed during the late Early Cretaceous.Therefore,the late Early Cretaceous magmatisms formed under a back-arc extensional tectonic regime caused by rollback of subducting Paleo-Pacific plate.Taken together,this thesis proposes that the North-South Lineament of Great Xing’an Range separates regions influenced by the Mongol-Okhotsk Ocean tectonic domain and the Paleo-Pacific Ocean tectonic domain.The middle-late Early Cretaceous（～130 Ma）northeast-trending bimodal magmatisms in the central Great Xing’an Range limit the spatial scope of influence by a southward subduction of the Mongol-Okhotsk Ocean tectonic regime.Collectively,the western areas of the North-South Lineament are dominated by Mongol-Okhotsk Ocean tectonic domain while the eastern areas of the North-South Lineament by the Paleo-Pacific Ocean tectonic domain. |
