Very low-grade metamorphic rocks in southern Tibet and their significance on geological processes and resources

doi:10.13745/j.esf.sf.2024.1.66

Abstract

Abstract:

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.

Key words: very low-grade metamorphism, very low-grade metamorphic rocks, southern Tibet, illite crystallinity (Ic), geological and resource significance

CLC Number:

P588.34
P313

BI Xianmei, MO Xuanxue, LIU Yanbin. Very low-grade metamorphic rocks in southern Tibet and their significance on geological processes and resources[J]. Earth Science Frontiers, 2024, 31(1): 201-210.

Figures/Tables 3

References 56

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