Magma Mixing And Petrogenesis Of Granite And Associated Mafic Microgranular Enclaves From Lingshan Pluton, Gan-Hang Belt

Many late Mesozoic volcanic-intrusive complexes bearing enclaves occur in the Gan-Hang magmatic-tectonic belt?GHTB?,SE China.Here we present the Lingshan granitic pluton and associated mafic microgranular enclaves?MMEs?as an example in the middle segment of the GHTB to investigate their petrogenesis and a possible origin model.Zircon LA-ICP-MS U-Pb ages indicate coeval formation for the MMEs and the host syenogranite in Early Cretaceous?¹33 Ma?.The syenogranite exhibit enrichment in Na₂O+K₂O,REE,HFSE and Ga,and high Ga/Al ratio and high contents of Zr+Nb+Ce+Y.the syenogranite show enrichment in LILEs,depletion in Sr,Ba,P,Eu and Ti,high zircon saturation temperature?797⁸37??and is belong to alkaline series.These geochemical features classify the host syenogranite as belonging to the A-type granitoids.The syenogranite shows a narrow range of?_Nd?t?values?-3.3^-2.51?,and their zircon grains are characterized by negative initial?_Hf?t?values,with most values concentrated in the range of-3.9 to-1.3,and Mesoproterozoic T_DM2 ages for both Nd and Hf isotopes.These features imply that the Lingshan syenogranite was probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks,without significant additions of mantle-derived magma.The MME in Lingshan granite has geochemical characteristics of mineral assemblage,trace elements and isotopic composition similar to that of host granite.The MMEs show spheroidal shapes,K-feldspar xenocrysts,acicular apatites,and fine grain structure,which suggest that the MMEs are magmatic origin,rapid crystallization and mingled with the host felsic magma.However,no matter what degree of mixing,MME and its host granites have very similar Sr-Nd-Hf isotopic characteristics,and the presence of Mg-rich amphibole nuclei in MME indicates that the magmatic mixing is not completely complete.It shows that the initial magma of MME has the same Sr-Nd-Hf isotopic composition as the host granite.In addition,the contents of Rb and Zr in MME is significantly higher than that in the host granite,indicating that the initial MME magma was not derived from the more basic depleted mantle or lithospheric enriched mantle.Therefore,we propose a"stacking and remixing"model to explain the formation of Lingshan MME.That is to say,Lingshan MME is formed by the process:the cumulates formed by the host granitic magma quenched at the walls of the magma conduits is brought into the granitic magma chamber,and then mixed with the granitic magma.The color rate of MME in Lingshan granite shows a series from deep to shallow.The elements content of MMEs show a liner variation trend and change in a large range comparing to their host granite,such as relatively lower SiO₂ contents?48.95⁶7.36wt.%?.These features provide evidences for the magma mixing processes between MME mafic magma and host felsic magma.Therefore,MME is subjected to different intensity of magma mixing.For Lingshan MME,the degree of magma mixing is not directly related to the volume,so the injection of MME magma is a multi-stage process.Lingshan A-type granites and other Late Mesozoic A-type granites in the GHTB constitute an A-type granite belt.In the Y+Nb vs.Rb tectonic discriminant diagram,these A-type granites fall within the scope of the intraplate granite,and,Lingshan granites fall within the scope of A2 type granites in Nb-Y-3Ga classification diagrams.Combined with previous studies,it is shown that the GHTB is in the extensional tectonic setting in the Late Mesozoic.This stretching environment was caused by the“roll back”of the Paleo-Pacific plate.