塔里木大火成岩省瓦吉里塔格霞石岩稀有气体同位素特征及其地质意义

Noble gas isotopic characteristics and geological significance of Wajilitag nephelinites in Tarim large igneous province

稀有气体被广泛用作地球化学示踪剂,本文对塔里木大火成岩省西北缘瓦吉里塔格霞石岩中的橄榄石和辉石单矿物进行了稀有气体同位素测定。结果表明,瓦吉里塔格霞石岩中的橄榄石和辉石单矿物具有较低的~3He/~4He值(分别为2.0~2.4 Ra和0.65~0.85 Ra)和略高于大气值的^(40)Ar/^(36)Ar值(342.3~651.7),反映了由古板块俯冲导致的较低的He、Ar同位素比值特征。研究表明,早中古生代南天山洋向南俯冲到塔里木板块之下,将富U或富~4He以及含有大气组分的流体带入到深部地幔,在塔里木地幔柱的作用下地幔源区发生低程度部分熔融产生霞石岩岩浆。

Noble gases are widely used as geochemical tracers. In this study, noble gas isotopes of olivine and pyroxene in nephelinites from Wajilitag of Northwest Tarim large igneous province were analyzed. The results show low 3He/4He ratios （2.0~2.4 Ra and 0.65~0.85 Ra, respectively） and low 40Ar/36Ar ratios （342.3~651.7, slightly higher than atmospheric value）. This feature can be interpreted as being influenced by the paleo-subduction. It is inferred that the south Tianshan oceanic plate subducted southward beneath the Tarim plate in the early Paleozoic and the subducting plate brought the fluid enriched with U or 4He and atmospheric components into the deep mantle. Furthermore, the nephelinite magma was produced by low degree partial melting of the mantle source region with the involvement of the mantle plume.