Petrogenesis And Geological Implications Of Miarolitic Alkali Feldspar Granites In East Ujimqin Banner, Inner Mongolia Of NE China

Northeastern (NE) China is the easternmost part of the Central Asian Orogenic Belt(CAOB), and is characterized by immense volumes of Phanerozoic granitic rocks.Traditionally, the granitoids are regarded as of late Paleozoic age, mainly distributed in theGreat Xing’an Range area, forming a NE-trending late Paleozoic magmatic belt along theNenjiang-Hegenshan fault. However, only limited precise age data for the granitic rocks inthis belt were previously available, which significantly hampers understanding of thepetrogenesis and geodynamic setting of these rocks. This paper presents geochemical andisotopic results of five miarolitic alkali feldspar granite plutons from the East UjimqinBanner in the southern Great Xing’an Range (GXR), aiming to determine theiremplacement ages and petrogenesis and further constraint the late Paleozoic tectonicevolution of the GXR. Petrographically, these alkali feldspar granites are composedprincipally of quartz and alkali feldspar, accompanied by minor amounts of plagioclase,biotite, hornblende and other accessory minerals. In addition, many samples containabundant miarolitic cavities, suggesting that they are emplaced at shallow levels withextensive fractional crystallization. New LA-ICP-MS zircon U-Pb age data demonstratethat these granites were emplaced during the period of315-300Ma. Geochemically, thesegranites have high SiO2(73.22%-77.36%) and K2O (3.89%-4.86%), low MgO(0.07%-0.49%), MnO (0.015%-0.09%), CaO (0.14%-0.60%) and P2O5(0.01%-0.08%),with A/CNK and δEu values of0.98~1.14and0.03~0.60, respectively. They invariably show enrichment in light rare earth elements (LREE) and significant negative Eu anomalies.All the granitic rocks have the characteristics of depletion in Ba, Nb, Ta, Sr and Eu, andenrichment in Zr, Hf, Rb, Th and U in the spidergram. All these features indicate that theyare peraluminous and highly fractionated calc-alkaline I-type granitic rocks. In addition, theεHf(t) values of zircons in the granites range from+7.6to+15.1, with two-stage Hf modelages (TDM2) varying from364Ma to835Ma, indicating that the primary magma wasderived from partial melting of newly accreted crust. According to the geochemical dataand previous geological investigations, we conclude that the miarolitic alkali feldspargranites were emplaced in an active continental margin setting, which was probably relatedto the northwestward subduction of the Paleo-Asian oceanic plate between Xing’an andSongnen terranes.