桂西南早中生代流纹岩年代学、地球化学特征及地质意义

印支期桂西南地区处于多板块构造交汇地带,其岩浆构造演化存在很大的争议.对桂西南十万大山盆地两侧酸性火山岩进行了系统的锆石年代学、全岩地球化学及Sr-Nd同位素地球化学研究.结果表明,十万大山盆地两侧酸性火山岩LA-ICP-MS锆石U-Pb年龄为240.5～248.4 Ma,形成于早三叠世.岩性主要为流纹岩,具有高SiO...

Geochronology and Geochemistry of Early Mesozoic Rhyolite in Southwestern Guangxi and Its Geological Significance

The Indosinian southwestern Guangxi was located in the intersection of multiple tectonic plates, with it’s structure and evolution still being controversial. In this paper, systematic studies were carried out on zircon geochronology, whole-rock major and trace elemental, and Sr-Nd isotopic data of the felsic volcanic rocks on both sides of the Shiwandashan basin in southwestern Guangxi. LA-ICP-MS zircon U-Pb dating of the felsic volcanic rocks yielded 240.5-248.4 Ma, suggesting that the rocks were formed in the Early Triassic. The felsic volcanic rocks are mainly rhyolitic, characterized by high SiO₂, K₂O and Al₂O₃, and low CaO and MgO, belonging to high-K calc-alkaline and strongly peraluminous series with A/CNK=1.17-1.25. The rhyolite is enriched in Rb, Th, U, Zr and Hf, but depleted in Sr, Nb, Ti and P in the primitive mantle-normalized spider diagram. The chondrite-normalized REE patterns show slightly right inclining, LREE/HREE enrichment patterns, and moderate negative Eu anomalies (δEu=0.44-0.73). The rocks have relatively high (⁸⁷Sr/⁸⁶Sr)_i(0.706,21-0.719,51) and low ε_Nd(t) (-10.67 to -9.72), with model age of 1.85-1.96 Ga, suggesting a Paleoproterozoic crustal-derived magma source. Based on the previous studies, the rhyolite was emplaced at late/post-collisional setting after the main Indosinian collision between the South China and Indochina blocks.