极低级变质作用(very low-grade metamorphism)是当代地球科学的前沿研究课题之一。极低级变质岩(very low-grade metamorphic rocks)既是地质过程的记录,又在资源环境方面有实用意义。西藏南部广泛分布着处于成岩-极低级变质-低级变质阶段的岩石。它们记录了从新特提斯到青藏高原形成发展的诸多信息,并为在西藏南部寻找油气资源提供了有用的线索。本文对采自西藏南部不同地点和构造单元的71件珍贵的富含黏土矿物的岩石样品进行了XRD分析,获得了71套以伊利石结晶度(Ic)为主的黏土矿物数据。数据表明,这些岩石的伊利石结晶度(Ic)在0.21°~1.61°(Δ2θ)范围内,分别落在低级成岩区、高级成岩区、极低级变质区、低级变质区。不同构造单元显示不同的变质特点。北喜马拉雅碳酸盐台地P₂,J-K,K₂均属极低级变质,N显示早期成岩阶段特征。拉轨岗日被动陆缘盆地发育P₁,T₁,T₂,T₃,J₁,J-K,K₁,K₂地层,绝大多数均遭受极低级变质,总的趋势是随着时代变新变质程度逐渐降低,具有埋藏变质的特点。也有部分样品的Ic落在成岩带内,提供了寻找油气的线索。从雅鲁藏布蛇绿(混杂)岩带中的沉积岩样品获得两组数据:一组相当于成岩阶段;另一组相当于极低级变质。初步解释,前者可能反映蛇绿岩形成时的古环境条件,后者可能代表蛇绿岩碰撞定位阶段的记录。日喀则弧前盆地中K₂(日喀则群)虽然有较强的褶皱变形,但变质程度不高,属高级成岩带,可能说明晚白垩世弧前盆地沉积并未卷入俯冲作用。冈底斯岩浆弧中侵入岩、火山岩、沉积岩均发育,沉积岩的伊利石结晶度(Ic),落入低级变质-极低级变质-成岩作用带的较宽范围,并显示埋藏变质与岩浆作用叠加的特点。西藏南部成岩阶段沉积岩-极低级变质岩中还蕴藏着有关石油天然气的信息。特别是,白垩纪中期发生的全球性的缺氧事件,也影响了西藏南部地区,已经在岗巴-定日盆地发现典型的黑色页岩,厚度巨大。前人研究还认为,定日-岗巴被动大陆边缘、羌塘周缘前陆盆地、冈底斯弧背盆地具有良好含油气前景。本文对西藏南部极低级变质作用和极低级变质岩做了初步研究,期望今后有更深入的研究。

Very low-grade metamorphism is one of the contemporary topics of frontier research since very low-grade metamorphic rocks contain important information on the geological processes related to oil and gas resources. Diagenetic sedimentary rocks, very low-grade metamorphic rocks and low-grade metamorphic rocks are widely distributed in southern Tibet, and these rocks preserve critical information on the evolution of the Neotethys and the Tibetan Plateau, as well useful clues for the exploration of oil and gas resources. This paper presents 71 data sets on illite crystallinity (Ic) and other parameters of clay mineral-bearing rock samples collected from different locations and tectonic units in southern Tibet. The illite crystallinity (Ic) of these rocks is in the range of 0.21°-1.61°(Δ2θ), and falls in the lower diagenetic, higher diagenetic, very low-grade metamorphic and low-grade metamorphic zones respectively. The different tectonic units display diverse metamorphic characteristics. Strata of P₂, J-K, K₂ in the North Himalayan carbonate platform all underwent very low-grade metamorphism, but N strata show the characteristics of early diagenetic stage. Strata of P₁, T₁, T₂, T₃, J₁, J-K, K₁ and K₂ are developed in the Laguigangri passive marginal basin, most of which experienced low-grade metamorphism, and show patterns of normal burial with increasing metamorphism from younger to older strata. The Ic of some samples also falls in the diagenetic zone, which provides clues for finding oil and gas. Two sets of data were obtained from sedimentary rock samples in the Yarlung Zangbo ophiolites. One is in the diagenetic stage, and another group corresponds to very low grade metamorphism. Preliminary interpretation suggests that the former may reflect the paleo-environmental conditions at the time of ophiolite formation, whereas the latter may represent the tectonic position of the ophiolite during collision. The strata of K₂ in Xigaze fore-arc basin are only in the high diagenetic grade, presumably indicating that Late Cretaceous strata in the fore-arc basin were not involved in subduction. Intrusive rocks, volcanic rocks and sedimentary rocks are all developed in the Gangdisê magmatic arc. The illite crystallinity (Ic) of the sedimentary rocks falls within a wide range of low-grade metamorphic-very low-grade metamorphic-diagenetic zone, and shows the characteristics of burial metamorphism superimposed by magmatism. Also, the very low-grade metamorphic rocks and diagenetic sedimentary rocks in southern Tibet contain information about oil and gas. In particular, the worldwide oceanic anoxic event that took place in the middle of the Cretaceous also affected southern Tibet, and very thick beds of typical black shale were found in the Gamba-Tingri basin. Previous studies also suggested that the Tingri-Gamba passive continental margin, Qiangtang peripheral foreland basin and Gangdese back-arc basin have good prospects for oil and gas exploration. The preliminary study on very low-grade metamorphism and very low-grade metamorphic rocks in southern Tibet in this paper opens scope for further detailed studies in future.