桂西德峨地区基性岩的岩石成因及地质意义

Petrogenesis and geological significance of mafic rock in De′e area of the western Guangxi

桂西德峨地区广泛发育基性岩,为查明区内基性岩的岩石学、岩石地球化学特征及形成时代,探讨岩浆演化及其构造环境、岩浆活动与成矿关系。对德峨地区的基性岩进行了元素地球化学分析,得出其具有较低的SiO2(42.52%~48.09%)、 Mg#值(0.34~0.50)、 Cr和Ni含量(分别为21.77×10-6~202.2×10-6和47.87×10-6~122×10-6),较高的Fe2O3(11.89%~17.06%)和Al2O3(14.39%~17.04%)含量,暗示其为幔源岩浆演化的产物,并且在上升的过程中遭受少量的地壳混染。结合其不相容元素比值(如La/Sm和Sm/Yb),得出其岩浆起源于浅部地幔尖晶石橄榄岩相5%~10%部分熔融。LA-ICP-MS锆石U-Pb同位素测年结果显示,龙保山辉绿岩的谐和年龄为259.9±1.1 Ma,小帽上辉绿岩的谐和年龄为258.6±2.3 Ma,老山神玄武岩的谐和年龄为261.0±2.4 Ma。综合该地区的基性岩石的年代学、元素地球化学特征,峨眉山大火成岩省的产出范围可能延伸到本区,为重新界定峨眉山大火成岩省的分布范围提供新的直接资料。

The mafic rocks are extensively distributed in the De′e area of the western Guangxi. The geochemistry analyses and zircon U-Pb dating by LA-ICP-MS technology provide an effective approach to find out the relationship of the petrography, geochemistry and the age of these rocks, and further to understand the relation among the magmatic evolution, tectonic environment, magmatic activity and the metallogenic processes. The rocks contain low SiO2 contents(42.52%-48.09%), Mg# values(0.34~0.50), Cr and Ni contents(21.77×10-6 to 202.2×10-6 and 47.87×10-6 to 122×10-6)respectively), and high Fe2O3(11.89%-17.06%)and Al2O3(14.39%-17.04%)contents, implying the evolution and suffering crustal contamination. Combined with the ratios of incompatible elements(such as La/Sm and Sm/Yb), it is concluded that the magma originates from 5%-10% partial melting of the shallow spinel peridotite mantle. The zircon grains from Longbaoshan dolerites, Xiaomaoshang dolerites and Laoshanshen basalts yield U-Pb concordant age of 259.9±1.1 Ma, 258.6±2.3 Ma and 261.0±2.4 Ma respectively. Our researches suggest a larger distribution of ELIP than previous finding and provide a direct evidence for refining the extent of the ELIP.