Petrological Characteristics And Tectonic Significance Of Late Yanshanian Period Granites In Qushan Island,Zhoushan,Zhejiang

The Qushan island,located in the northeast of Zhejiang province,is mainly affected by a series of NNE trending faults.The granite lithosome of Qushan island is mainly exposed in the northwest of Qushan island.The Qushan island granites are mainly composed of syenogranite,monzogranite,and granite porphyry,with minor adamellite.Field geological survey shows that the rock mass is zonal with medium-coarse syenite as the central facies and fine-grained monzonitic granite as the transition facies,and granite porphyry as the marginal facies.The rocks are characterized by high silicon,alkali-rich,and magnesium-calcium-poor.They are quasi-aluminum-weakly peraluminous granites and belong to the high potassium calc-alkaline series with A/CNK's ratio of molecules is between 0.96 and 1.17,which indicates that the magma source is related to the partial melting of the shell source material.The petrographic and geochemical characteristics indicate that most of the Qushan island granites are highly differentiated I-type granites,showing a right-leaning distribution pattern of the light rare earth enrichment(LREE)and the heavy rare earth(HREE)flat on the standardized distribution curve of chondrites of rare earth elements,and the negative anomaly of Eu(0.13~0.89)from weak to strong appears in the center phase,transition phase and edge phase.These granites are rich in a large amount of ionophile elements K,Rb,Th,U(LILEs)and relatively deficient with high field strength elements Nb,Ta,P,Ti(HFSEs)as well as Ba and Sr.Most of the granites are characterized by low Sr and high Yb,and belong to Nanling type granites.The above characteristics indicate that the Qushan island granites should be formed in normal or thin crustal thickness environment.LA-ICP-MS zircon U-Pb dating results show that the age of medium monzogranite,medium syenogranite,medium-coarse syenogranite and allgovite is respectively 100.5±1.7Ma,100.0±1.1Ma,98.0±1.2Ma and 100.2±1.6Ma,which belong to late Yanshanian.The petrogeochemical data and the tectonic environment discrimination maps show that the Qushan island granites were formed in the post-orogenic extensional environment and the extensional background should be related to the subduction of the ancient Pacific plate after subduction of the Eurasian continent.