Petrogenesis and magma evolution of Tertiary basalts from the Langjunshan area in the Jiurui mineralization district,Jiangxi Province:Evidence from pyroxene and feldspar

Tertiary volcanic rocks were found recently during deep drilling in Jiurui district. A detailed mineralogical study has been carried out in this study for these rocks. The results show that the volcanic rocks are all basalts,which can be divided into two eruption cycles. They are mainly composed of clinopyroxene and feldspar,and also contain a small amount of other minerals like apatite and irontitanium oxides. The mineralogical characteristics indicate that the nature of the magma in the early cycle( the lower layer) belongs to shoshonite series,while the nature of the magma in the late cycle( the upper layer) belongs to tholeiite series. Comparison of clinopyroxene composition in the two cycles of volcanic rocks indicates that the magma evolution in the early cycle is towards calcium rich,manganese rich and aluminium poor and alkali poor direction,while the magma evolution in the late cycle is towards titanium rich,manganese rich and aluminium poor,calcium poor and chromium poor direction. The volcanic rocks in the early cycle contain abundant plagioclase phenocrysts which show reverse and oscillatory zoning patterns,and abundant clinopyroxene phenocrysts which show normal,reverse and oscillatory zoning patterns. The plagioclase were commonly replaced by alkaline feldspar in feldspar matrix in the rocks. It can be infered that the magma in the early cycle has gone through multi-stage magmatic mixing process. The estimated temperature and pressure of clinopyroxene in the early cycle volcanic rocks are 1124 ~ 1157℃ and 1. 23 ~ 1. 42 GPa,at depth of 41 ~ 47 km. It can be inferred that these rocks were formed during the crystallization differentiation of relatively primitive magma at a relatively shallow level. According to the F1-F2 discrimination diagram and Ti O2-AlZdiagram of the clinopyroxene,the tectonic environment of the volcanic rocks shows an obvious association of arc affinity. It can be speculated that the volcanic rocks were likely generated in a back-arc extensional basin environment.