广东三水盆地火山岩: 地球化学特征及成因--兼论火山岩性质的时空演化和南海形成的深部过程

位于中国东南部的三水盆地、珠江口盆地、雷琼半岛和北部湾地区广泛分布新生代火山岩。火山岩的形成时间具有从内陆向沿海变新的特点,早第三纪三水和珠江口盆地火山岩具有由玄武岩与粗面岩-流纹岩构成的双峰式特点。其中玄武岩和粗面岩的微量元素和稀土元素的配分形式相似,富集大离子亲石元素并且有相似的εNd(T)同位素组成(2.34～6.4),说明它们来自相同的地幔源区,为同源岩浆演化的产物。玄武岩和粗面岩经历了不同的结晶分异过程,其中玄武岩在较深部岩浆房中经历橄榄石和单斜辉石为主的分离结晶作用,而粗面岩则是在浅部岩浆房中由玄武岩浆分异形成的过渡性岩浆再经过强烈的钾长石和斜长石、以及磷灰石的结晶分异形成的。晚第三纪珠江口盆地和北部湾火山岩、雷琼半岛第四纪火山岩则由碱性和拉斑玄武岩构成。这些火山岩的形成时间和地球化学和同位素特征表明它们经历了连续的软流圈地幔上涌和部分熔融过程,受控于自晚中生代以来的地幔柱构造。南海的形成是地幔柱活动引起的地幔上涌和大陆裂解作用的结果。

GEOCHEMISTRY AND PETROGENESIS OF CENOZOIC VOLCANIC ROCKS FROM SANSHUI BASIN: IMPLICATIONS FOR SPATIAL AND TEMPORAL VARIATION OF ROCK TYPES AND CONSTRAINTS ON THE FORMATION OF SOUTH CHINA SEA

Cenozoic volcanic rocks are widely distributed in southeastern China, such as in Sanshui basin, Zhujiangkou basin, Leiqiong Peninsula and Beibuwan Gulf. Volcanic rocks within the Sanshui basin display bimodal characteristics and are composed of basalts and rhyolite/trachyte. Basalts and trachytes have similar distributive patterns of rare earth elements and trace elements, and Nd isotopes (ε Nd(T)=2.34~6.4). These suggest that they were generated from enriched mantle sources. Basalts were storied in deep magma chamber and experienced crystallization of olivine and clinopyroxene, while the felsic rhyolite and trachyte were derived from basalts that were input in shallow magma chambers, then by extensive fractionation of feldspar and apatite from transitional intermediate andesitic magma. Spatially, volcanism are younger and rock types are getting simpler from southeastern toward southern South China Sea, and are dominated by tholeiite and alkaline basalts. Their geochemical and isotopic features and variations in both space and time suggest that they were generated from continuing melting of upwelling asthenosphere mantle, similar to the case with mantle plume. The formation of the South China Sea might be associated with plume activity and resulted from continental rifting process.