Petrology And Geochemistry Of Early Paleozoic Igneous Rocks In The Lesser Xing'an-Zhangguangcai Ranges:Constrains On The Amalgamation History And Crustal Nature Of The Massifs

This thesis mainly studies on the geochronology and geochemistry of the early Paleozoic igneous rocks from the Lesser Xing'an-Zhangguangcai Ranges within the eastern Songnen-Zhangguangcai Range Massif,NE China,with the aim of revealing the early Paleozoic tectonic evolutionary history between the SongnenZhangguangcai Range Massif(SZM)and Jiamusi Massif(JM),the crustal nature,and crustal accretion and reworking processes in the SZM of the eastern Central Asian Orogenic Belt(CAOB).The main achievements are as follows:1.The early Paleozoic magmatisms within the eastern SZM can be subdivided into five stages at 516 Ma,~505–490 Ma,482–461 Ma,~460–450 Ma,and 435–420 MaZircon U-Pb dating results indicate that the early Paleozoic magmatisms within the eastern SZM can be subdivided into at least five stages at 516 Ma,~505–490 Ma,482–461 Ma,~460–450 Ma,and 435–420 Ma.Early Cambrian(516 Ma)intrusive rocks are limitedly exposed in the Tadong area of the southeastern margin of the SZM,and comprise a suite of tonalites.Middle-Late Cambrian(~505–490 Ma)igneous rocks widely occur in the Lesser Xing'an Range of the northern SZM,and include505–496 Ma monzogranites,and ~490 Ma monzonites,quartz monzonites and monzogranites.Only minor Middle-Late Cambrian(502–496 Ma)basaltic andesites and monzogranites occur within the Tadong area of the southeastern margin of the SZM.Ordovician(~482–450 Ma)magmatic events have been widely identified throughout the eastern SZM.Early-Middle Ordovician igneous rocks within theeastern SZM consist of a series of quartz monzodiorites,granodiorites,monzogranites,syenogranites,and rhyolites.Late Ordovician igneous rocks within the southeastern margin of the SZM are dominated by diabases,diorites,tonalites,monzogranites,dacites and rhyolites,whereas the coeval igneous rocks in the Xiaojingou area and Lesser Xing'an Range,relatively far from the suture belt,consist of minor amounts of interlayered andesites in the shallow-marine facies massive carbonate and siltstone units,and a great amount of rhyolites and alkali-feldspar granites.Silurian(435–420Ma)igneous rocks within the southeastern margin of the SZM are composed of minor basalts,basaltic andesites,andesites,and tonalites,the coeval igneous rocks in the northern SZM include minor granodiorites,monzogranites and dacites,whereas igneous rocks within the central eastern SZM(Shuguang area)are dominated by monzogranites.2.Reconstruction of the early Paleozoic tectonic evolution between the SZM and the JMEarly Cambrian magmatism within the southeastern margin of the SZM consists of Na-rich tonalite(516 Ma),its primary magma was generated by partial melting of subducted plagioclase-rich sediments with addition of slab-derived fluids.Middle-Late Cambrian basaltic andesites(502 Ma)and monzogranites(496 Ma)in this area,belong chemically to low-K tholeiitic and high-K calc-alkaline series,respectively,they are enriched in light rare earth elements(LREEs)and large ion lithophile elements(LILEs),and depleted in heavy rare earth elements(HREEs)and high field strength elements(HFSEs).All the above Cambrian igneous rocks within the southeastern margin of the SZM formed in an active continental margin setting associated with northwestward subduction of an oceanic plate between the southern SZM and the JM.In contrast,505–496 Ma K-rich monzogranites in the northern SZM have adakitic affinities,their magmas were generated by partial melting of thickened Paleoproterozoic and Mesoproterozoic lower crustal material,indicating that a compressional tectonic setting occurred in the northern SZM.However,the ~490 Ma monzonites,quartz monzonites,and monzogranites are geochemically similar to A-type granitoids,suggesting an extensional environment.Together with the existence of coeval granitoids with similar geochemical features in the northern JM,we propose that the formation of the Middle-Late Cambrian rocks was related to the collision between the northern SZM and the northern JM during the Cambrian,and the subsequent post-collisional extension during the Late Cambrian.Ordovician(~482–450 Ma)intermediate-basic to felsic igneous rocks in the eastern SZM belong chemically to medium-to high-K calc-alkaline series,and are characterized by enrichments in LREEs and LILEs,and depletions in HREEs and HFSEs,consistent with the features of igneous rocks generated in the active continental margin settings.These lines of evidence,together with the presence of residual Ordovician oceanic crust between the southern SZM and the JM,and the absence of ~480–420 Ma magmatic events in the JM,suggest that the Ordovician calc-alkaline igneous rocks within the eastern SZM formed under an active continental margin setting associated with northwestward subduction of an oceanic plate between the two massifs.Furthermore,the occurrence of 475–461 Ma K-rich monzogranites in the southeastern SZM which were derived from magmas generated by partial melting of thickened Mesoproterozoic lower crust,suggests that the southeastern SZM was in a compressional active continental margin setting during the Early-Middle Ordovician,whereas the Late Ordovician(460–450 Ma)rhyolites and alkali-feldspar granites with affinity to A-type granites in the Lesser Xing'an Range,far away from the subduction zone,reveal that they formed in a back-arc extensional setting.The Silurian Na-rich tonalites(426 Ma),calc-alkaline basalts,basaltic andesites,and andesites(420 Ma)within the southeastern margin of the SZM,together with the calc-alkaline granodiorites,monzogranites and dacites(~435 Ma)within the northern SZM,exhibit arc-like geochemical signitures(enrichments in LREEs and LILEs,and depletions in HREEs and HFSEs),suggesting that the northwestward continuous subduction beneath the southeastern SZM took place until the late Silurian.This suggestion,combined with the presence of ~425 Ma collision-related peraluminous monzogranites in the central eastern SZM and the previous researches,indicates that the collision between the southern SZM and the southern JM progressed from north(Shuguang area)to south(Tadong area)before the final collision occurred at the end of the early Paleozoic.3.The nature,and accretion and reworking processes of deep crust beneath the SZMThe early Paleozoic granitoids and felsic volcanic rocks in the eastern SZM have ancient zircon Hf two-stage model ages(TDM2 age peaks at ~1.8 Ga and 1.5–1.2 Ga),suggesting that deep crust beneath the eastern SZM could contain ancient Precambrian basement material.The spatial-temporal variations of zircon Hf isotopiccompositions for the early Paleozoic granitoids and felsic volcanic rocks reveal the lateral and vertical heterogeneity of ancient lower crust beneath the eastern SZM.Additionally,our zircon Hf isotopic compositions for these early Paleozoic igneous rocks also indicate that crustal growth within the eastern SZM mainly happened during the Paleoproterozoic and Mesoproterozoic,that the intense Mesoproterozoic crustal accretion had modified the Paleoproterozoic crust,and that the Paleoproterozoic crustal material occurred as residues beneath the eastern SZM.During the early Paleozoic,the reworking of the ancient lower crust beneath the eastern SZM produced the Early Cambrian to middle Silurian granitoids and felsic volcanic rocks in the study area.The Early-Middle Cambrian granitoids were generated by reworking of ~1.8 Ga crustal material,whereas the formation of Late Cambrian(496 Ma and ~490 Ma)granitoids in the study area resulted from the reworking of Mesoproterozoic(~1.5–1.2 Ga)lower crustal material.The Ordovician(~482–450 Ma)granitoids and rhyolites within the eastern SZM were generated by the reworking of dominant ~1.5–1.2 Ga and minor ~1.8 Ga lower crustal material.In contrast,the reworking of ~1.8 Ga crustal material resulted in the formation of435–426 Ma magmatisms.Furthermore,the early Paleozoic subduction of the Paleo-Asian Ocean plate beneath the eastern SZM and collision between the SZM and JM caused significant reworking of the ancient crust beneath the SZM,and further generated the widespread early Paleozoic magmatisms.The reworking of ancient crust played an important role in the early Paleozoic crustal evolution of the eastern CAOB.