Poikilitic texture of plagioclase in the Quxu batholith of the Gangdese belt, southern Tibet: Implications for magma recharge process

The Gangdese belt in southern Tibet is an excellent location for studying the growth and evolution of continental crust as it has undergone the Mesozoic subduction-accretionary orogeny and the Cenozoic collisional orogeny. The Early Eocene Quxu batholith, located at central part of the Gangdese belt, consists of granite, granodiorite, diorite, and gabbro, etc. Lots of chronologic and geochemical studies have been done for the Quxu batholith. However, there are still debates about whether the formation of the Quxu batholith is controlled by magma recharge. Besides, the detailed processes of magma recharge are unclear. The wide distribution of magmatic microgranular enclaves in the granodiorite such as enclave dikes and dispersed enclaves, indicates the important role of magma recharge during the formation of the Quxu batholith. Poikilitic texture, with amphibole embedded in plagioclase, is developed in enclaves. Here, we will focus on the poikilitic texture of plagioclase and the chadacrysts ( e. g., amphibole) and attempt to track the processes of magma recharge through backscattered-electron imaging, mineral energy spectrum scanning, and mineral electron probe analysis. Analytical results reveal that the crystallization temperature and pressure of the amphibole chadacrysts are the highest (783 similar to 853 degrees C, 0.23 similar to 0. 45GPa), followed by the amphiboles in the groundmass of enclaves (781 similar to 808 degrees C, 0.21 0. 31GPa), and then the amphiboles in the granodioritic host rocks (769 similar to 802 degrees C, 0. 18 similar to 0. 26GPa). In addition, plagioclase with poikilitic texture develops obvious discontinuous zoning, and the sodic core indicates the existence of crystal mush. Based on these observations, we establish a tentative model to explain the process of magma recharge and the formation of poikilitic texture. Mafic magma rises along the fractures in the lower part of felsic crystal mush and quenches, leading to the formation of numerous small grains of amphibole. Subsequently, as the mafic magma enters the upper part of crystal mush, it breaks up into small droplets and captures plagioclase crystals from the felsic crystal mush. The plagioclase xenocrysts enclose amphiboles crystallized at deep level and produces the poikilitic texture.