碧口群火山岩岩石成因研究

新元古代(846~776Ma)碧口群火山岩喷发于大陆板内裂谷环境。该火山岩系以基性火山岩为主,酸性火山岩次之,中性火山岩少见。根据岩石地球化学数据,碧口群裂谷基性熔岩总体上属于低Ti/Y(<500)岩浆类型。元素和同位素数据表明,碧口群基性熔岩的化学变化不是由一个共同的母岩浆的结晶分异作用所产生。它们极有可能是源于地幔柱源(εNd(t)≈+3,87Sr/86Sr(t)≈0.704,La/Nb≈0.7)。地壳混染作用对于碧口群裂谷基性熔岩的形成有重要贡献。我们的研究揭示,碧口群火山岩存在空间上的岩石地球化学变化。东部红岩沟和辛田坝—黑木林地区的碧口群基性熔岩以拉斑玄武岩为主,产生于幔源石榴子石稳定区的高度部分熔融。相反,西部白杨—碧口地区的碧口群基性熔岩的母岩浆则是形成于幔源的尖晶石-石榴子石过渡带:碱性熔岩是产生于部分熔融程度较低的条件下,拉斑玄武质熔岩则是产生于部分熔融条件较高的条件下。它们经受了浅层位辉长岩质(cpx+plag±ol)分离作用,化学变异较大。

Petrogenesis of the Bikou Group volcanic rocks

The Neoproterozoic (846-776 Ma) Bikou Group volcanic rocks erupted in an intracontinental rift setting in the Yangtze craton. The volcanic succession comprises a thick pile of mafic volcanic rocks, subordinate silicic and rare intermediate volcanic rocks. On the basis of geochemical data, the Bikou Group rift-related mafic lavas generally belong to a low Ti/Y (<500) magma type. Elemental and isotopic data suggest that the chemical variations of the Bikou Group mafic lavas cannot be explained by crystallization from a common parental magma. The Bikou Group mafic lavas most likely originated from a mantle plume source (εNd(t)≈+3, 87Sr/86Sr(t)≈0.704, La/Nb≈0.7). Crustal contamination has also contributed significantly to the formation of the Bikou Group rift-related basic lavas. Our data show that spatial geochemical variations exist in the Bikou Group rocks. The Bikou mafic lavas are dominantly tholeiitic in composition in the eastern Hongyangou and Xintianba-Heimulin areas, and were generated by high degrees of partial melting in the garnet stability field. In contrast, in the western Baiyang-Bikou area the parental magmas were generated by lower degrees of partial melting for the alkaline lavas and by higher degrees of partial melting for the tholeiitic lavas from a mantle source in the spinel-garnet transition zone. These magmas were then subjected to shallow level gabbroic (cpx+plag±ol) fractionation, which led to larger chemical variations.