The Mesozoic-early Cenozoic Tectono-magmatic Evolution Of The Nadanhada Terrane

This thesis conducts a systematic study on the petrology,zircon U-Pb geochronology,whole-rock geochemistry,whole-rock Sr-Nd-Hf isotopes and zircon Hf isotopes of the Mesozoic to the early Cenozoic igneous rocks in the Nadanhada Terrane of northeast?NE?China,and obtains the following achievements:1)testify the existence of the Mesozoic to the early Cenozoic magmatism,and further ascertain its stage,rock association and petrogenesis;2)limit the initial time of the Paleo-Pacific subduction in NE China,and further discuss the accretion-emplacement history of the Nadanhada Terrane;and 3)propose a preliminary temporal and spatial evolution model of the subduction of the Paleo-Pacific oceanic plate beneath the Eurasian Plate.The igneous rocks studied in this thesis formed in different tectonic settings,including two groups,i.e.,1)various stages of igneous rock association formed in an active continental margin setting and 2)OIB-type complex formed in an ocean intraplate environment.The former group consists of the Early Jurassic?187¹74 Ma?volcanic rocks,the Early Cretaceous?133¹06 Ma?igneous rocks,the Late Cretaceous?95⁶6 Ma?acid intrusive rocks,and the Eocene?⁵4 Ma?granodiorite.The newly discovered Early Jurassic volcanic rocks are mainly composed of calc-alkaline andesite-dacite-rhyolite-rhyolitic crystal tuff,Nb-enriched basaltic andesite-andesite,and high-Mg andesite.Among them,the calc-alkaline rocks are enriched in light rare earth elements?LREEs?,and depleted in heave rare earth elements?HREEs?and high field strength elements?HFSEs?,suggesting an origin via partial melting of crustal materials.Besides,the Nb-enriched basaltic andesite and andesite are enriched in Nb,Ti and HFSEs,and they were derived from partial melting of shallow?spinel phase?mantle peridotite that had been modified by slab-derived melts.In addition,the high-Mg andesite is enriched in LREEs,large ion lithophile elements?LILEs?,and HFSEs.They were derived from partial melting of garnet+spinel lherzolite that had been modified by fluids derived from ocean curst?+oceanic sediment?and terrigenous sediment.The Cretaceous igneous rocks include intermediate-acid volcanic rocks,acid intrusive rocks and mafic intrusive rocks.Among them,the intermediate-acid volcanic rocks are enriched in LREEs,and depleted in HFSEs.They were derived from partial melting of mafic materials within lower crust level.Additionally,the acid intrusive rocks are enriched in LREEs,and depleted in HREEs,classified as I-type and highly fractionated I-type granites,which were derived from partial melting of thickened lower crust and normal mafic lower crustal materials,respectively.Moreover,the mafic intrusive rocks are gabbroic rocks,which show weak LREEs and HREEs differentiation degree with an enrichment in HREEs and a depletion in HFSEs,indicating they were generated from partial melting of spinel lherzolite that had been modified by slab-derived fluids.The newly discovered Eocene granodiorites are enriched in LREEs and LILEs,and depleted in HREEs and HFSEs.They are I-type granitoids in nature and are derived from partial melting of thickened lower crust,which then underwent fractional crystallization during magma evolution.The OIB-type igneous rocks formed in the Middle Jurassic?169¹67 Ma?,consisting of pillow basalt,pyroxenite,gabbro,plagioclasite,and plagiogranite.The basalts are enriched in Ti O₂,light rare earth elements?LREEs??La/Sm_n=2.12?,and Nb(averages:Nb/Th_pm=1.46,Nb/La_pm=1.31).The rocks are depleted in heavy rare earth elements?HREEs??Gd/Yb_n=2.03?and exhibit high?_Nd?t??+5.6 to+5.8?and?_Hf?t??+9.0 to+9.1?values,indicating the basalts were derived by a low degree of partial melting?<5%?of peridotite in the garnet stability field.The intermediate-mafic intrusive rocks are geochemically similar to the basalts,most of them are enriched in LREEs and Nb,depleted in HREEs,and show low Zr/Nb ratios and high?_Nd?t??+7.2 to+8.2?and?_Hf?t??+8.0 to+10.3?values,indicating they were derived from a common source and are the products of fractional crystallization of the OIB-like basalts.The Early Jurassic to Eocene igneous rocks reported in this thesis are all closely related to the Paleo-Pacific subduction beneath the Eurasian continental plate.Based on the analysis of the rock association and petrogenesis of rocks with different ages,the important geodynamic implications behind it have been revealed.The first recognition of the Early Jurassic calc-alkaline volcanic rocks,Nb-enriched basaltic andesite-andesite and high-Mg andesite,limit the initial timing of the Paleo-Pacific subduction in NE China.The OIB-type igneous rocks were intruded by the Early Cretaceous granites with a continental arc affinity,suggesting the existence of mantle plume-related magmatism within the Paleo-Pacific Ocean.Subsequently,through a long period of the westward subduction Paleo-Pacific plate,the OIB-type igneous rocks along with those oceanic sedimentary strata accreted to the Nadanhada Terrane before the Early Cretaceous.Since then,multi-stages of magmatic events occurred in the Nadanhada Terrane,the corresponding rock products consist of 1)the Cretaceous intermediate-acid volcanic rocks,I-type and highly fractionated I-type granitoids,and mafic intrusive rocks;and 2)the Eocene I-type granodiorites.They were both formed in an active continental margin setting,suggesting that the Paleo-Pacific oceanic plate continues to subduct beneath the Eurasian continent during the Cretaceous to Eocene.