Chronology,Geochemistry And Tectonic Implications Of Neoproterozoic Granitoids In The Eastern Margin Of Songnen-Zhangguangcai Range Massif

This thesis mainly studies on the geochronology,whole-rock geochemistry and zircon Hf isotopic geochemistry of the Neoproterozoic granitoids within the eastern margin of the Songnen–Zhangguangcai Range Massif(SZRM)of NE China.Our aims are to(1)constrain the timing of the emplacement of these granitoids,(2)provide insights into their petrogenesis,and(3)yield information on the tectonic setting of the magmatisms.Taking into account previous paleomagnetic data,our dating results and comparisons with detrital zircon age populations in Precambrian rocks from the Tarim,North China and southern Siberia cratons as well as northeastern Gondwana,this thesis discusses the tectonic affinity between the SZRM and Tarim Craton,and the correlation between Neoproterozoic magmatisms within the SZRM and the evolution of the Rodinia supercontinent.The main achievements are as follows:1.Neoproterozoic magmatism within the SZRM can be subdivided into two stages,i.e.,917–911 and 850–841 Ma.The zircon U–Pb dating indicates that the Neoproterozoic magmatism within the SZRM can be subdivided into two stages: 917–911 and 850–841 Ma.The 917–911 Ma magmatism consists of a suite of gneissic syenogranites and monzogranites,which exposed limitedly in the Jinlin area of the eastern part of the Lesser Xin'an Range.The 850–841 Ma magmatism is composed of a suite of granodiorites,which exposed limitedly in the Hongqi area.The granodiorites exhibit a granoblastic texture,and a gneissic structure.2.Petrogenesis and tectonic setting of the Neoproterozoic granitoidsThe 917 Ma syenogranite and 911 Ma monzogranite samples were collected from the same pluton,and have similar geochemical characteristics.They contain high concentrations of Si O2,K2 O,and Al2O3,low concentrations of TFe2O3,Mg O,and Y(7.26–7.33 ppm),are enriched in the LREE and the LILE,are depleted in the HREE and the HFSE(e.g.,Nb,Ta,and Ti),and have weakly positive Eu anomalies,similar geochemically to high-K adakitic rocks.They have zircon ?Hf(t)values of –4.4 to +1.5 and TDM2 ages of 1915 Ma to 1592 Ma,respectively,suggesting that they were derived from a primary magma generated by partial melting of ancient thickened lower crustal material,and formed in an orogenic setting.In comparison,the 841 Ma granodiorites contain high concentrations of Al2O3 and K2 O,and relatively high concentrations of TFe2O3 and HREE,and have negative Eu anomalies and zircon ?Hf(t)values of –4.7 to +1.0 as well as TDM2 ages of 1875 to 1559 Ma.Taking into account the presence of inherited zircons within the granodiorites,we conclude that the primary magma for 841 Ma granodiorites could be generated by partial melting of ancient crustal material,and formed in an anorogenic setting.3.Tectonic attribution of the SZRM and its correlation with the Rodinia supercontinentThe two stages of Neoproterozoic granitoid magmatisms within the SZRM are corresponding to the assembly and break-up of the Rodinia supercontinent.Taking into account previous paleomagnetic data,our dating results,and comparisons with detrital zircon age populations in Precambrian rocks from the Tarim,North China and southern Siberia cratons as well as northeastern Gondwana,we conclude that the SZRM has an affinity to the Tarim Craton and that the SZRM,together with the Tarim Craton,was separated from the Rodinia Supercontinent during the late stage of of the Neoproterozoic.