西藏南部极低级变质岩及其地质与资源意义

doi:10.13745/j.esf.sf.2024.1.66

摘要/Abstract

摘要：

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。

关键词: 极低级变质作用, 极低级变质岩, 西藏南部, 伊利石结晶度(Ic), 地质与资源意义

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

中图分类号:

P588.34
P313

毕先梅, 莫宣学, 刘艳宾. 西藏南部极低级变质岩及其地质与资源意义[J]. 地学前缘, 2024, 31(1): 201-210.

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.

图/表 3

图1 青藏高原及邻区构造单元简图(a)与青藏南部地区地质简图(b)(b位置见图1a)(据文献[51⇓-53]修改)

Fig.1 Simplified tectonic map of the Tibetan Plateau and adjacent regions (a) and simplified geological map of the southern Tibetan Plateau (b, location see a). Modified after [51⇓-53].

表1 研究区黏土矿物 XRD测试结果

Table 1 XRD analytic results of clay minerals in the studied areas

图2 研究区伊利石Ic(Δ2θ)与时代协变图解

Fig.2 Bivariate diagram of illite Ic(Δ2θ) vs. age in the studied area

参考文献 56

[1]	沈其韩, 耿元生, 宋会侠. 近70年中国变质岩石学-变质地质学的研究进展[J]. 地球科学, 2018, 43(1): 1-23.
[2]	沈其韩, 耿元生, 宋会侠. 加强极低级变质作用研究[J]. 岩石矿物学杂志, 2018, 37(2): 342-348.
[3]	KÜBLER B. Les Argiles, indicateurs de métamorphisme[J]. Revue de I’Institut Francais du Pétrole, 1964, 19: 1093-1112.
[4]	ROBINSON D. Diagenesis and low-temperature metamorphism: introduction[J]. Mineralogical Magazine, 1985, 49(352): 301-303. DOI URL
[5]	FREY M. Very low-grade metamorphism of clastic sedimentary rocks[M]//FREY M. Low temperature metamorphism. New York: Chapman and Hall, 1987: 9-58.
[6]	BLENKINSOP T G. Definition of low-grade metamorphic zones using illite crystallinity[J]. Journal of Metamorphic Geology, 1988, 6(5): 623-636. DOI URL
[7]	DE CARITAT P, HUTCHEON I, WALSHE J L. Chlorite geothermometry: a review[J]. Clays and Clay Minerals, 1993, 41(2): 219-239. DOI URL
[8]	FREY M, ROBINSON D. Lowgrade metamorphism[M]. Oxford: 1999: 10-226.
[9]	MERRIMAN R J, FREY M. Patterns of very low-grade metamorphism in metapelitic rocks[M]//FREY M, MERRIMAN R J. Low-grade metamorphism. Oxford: Blackwell Science, 1999: 61-107.
[10]	赵宗溥. 沸石相质疑[J]. 岩石研究, 1983, 4: 123-133.
[11]	董申保. 中国变质作用及其与地壳演化的关系[M]. 北京: 地质出版社, 1986: 1-233.
[12]	董申保, 沈其韩, 孙大中, 等. 中国变质地质图说明书[M]. 北京: 地质出版社, 1986: 1-37.
[13]	任磊夫, 陈芸菁. 从黏土矿物的转变讨论沉积成岩到变质过程中的阶段划分[J]. 石油与天然气地质, 1984, 5(4): 325- 334.
[14]	张立飞. 陕北三叠系延长统浊沸石的成因及形成条件的理论计算[J]. 岩石学报, 1992, 8(2): 145-153.
[15]	赵孟为. 划分成岩作用与埋藏变质作用的指标及其界限[J]. 地质论评, 1995, 41(3): 238-244.
[16]	朱光, 徐嘉炜, FLETCHER C W N. 应用X射线衍射分析胶北蓬莱群板岩中变质作用[J]. 地质与勘探, 1994, 30(2): 42-49.
[17]	索书田, 游振东, 周汉文. 极低级变质作用和极低级变质带综述[J]. 地质科技情报, 1995, 14(1): 1-8.
[18]	索书田, 祁向雷, 毕先梅. 右江中生代极低级变质带的变质变形过程[J]. 地质科技情报, 1996, 15(4): 65-72.
[19]	索书田, 毕先梅, 赵文霞, 等. 右江盆地三叠纪岩层极低级变质作用及地球动力学意义[J]. 地质科学, 1998, 33(4): 395-405.
[20]	毕先梅, 索书田, 莫宣学, 等. 极低级变质作用的研究现状[J]. 地学前缘, 1998, 5(4): 302-306.
[21]	张继军, 毕先梅. 成岩-极低级变质作用及油气勘探意义[J]. 地学前缘, 1999, 6(2): 251-258.
[22]	燕守勋, 田庆久, 吴昀昭. 极低级变质作用及其研究方法[J]. 现代地质, 2002, 16(1): 37-44.
[23]	王河锦. 关于伊利石结晶度Kübler指数的误差计算[J]. 地质论评, 1998, 44(3): 328-335.
[24]	SHAN Y H, YANG H Y. Very low grade metamorphism of ultramafic fragments in basaltic tuffs from Hsuehshan Range, northern Taiwan[C]// Proceedings IGCP Project 294 international symposium, Xi’an. 1994: 76-91.
[25]	YANG H Y, LO Y M, HUANG T M. The pyrophyllite isograds in the metamorphic terrane of Taiwan[C]// Proceedings of IGCP Project 294 international symposium, Xi’an. 1994: 168-175.
[26]	CHEN C H. Application of K-mica crystallinity to the study of very low grade metamorphism in the central range of Taiwan[C]// Proceedings of IGCP Project 294 international symposium, Xi’an. 1994: 176-183.
[27]	DRISTAS J A, MASQUENLIN H, WEMMER K, et al. New insights into the Piedra de Afilar Formation, Canelones department of Uruguay: very-low-grade metamorphism, pressure solution, and age[J]. Journal of South American Earth Sciences, 2023, 123: 104237. DOI URL
[28]	ELLERO A, FRASSI C, GÖNCÜOĞLU M C, et al. Geological, structural and mineralogical approach to investigate the evolution of low- and very low-grade metamorphic units from the Intra-Pontide Suture Zone, central Pontides, Turkey[J]. Journal of Earth Science, 2021, 32: 1512-1527.
[29]	VERDECCHIA S O, COLLO G, ZANDOMENI P S, et al. Crystallochemical indexes and geothermobarometric calculations as a multiproxy approach to p-T condition of the low-grade metamorphism: the case of the San Luis Formation, eastern Sierras Pampeanas of Argentina[J]. Lithos, 2019, 324/325: 385-401. DOI URL
[30]	毕先梅, 莫宣学. 成岩-极低级变质-低级变质作用及有关矿产[J]. 地学前缘, 2004, 11(1): 287-294.
[31]	王河锦. 关于甚低级变质作用与近变质带研究中的几个问题[J]. 地质学报, 2022, 96(5): 1711-1723.
[32]	WILLNER A P, MASSONNE H J, BARR S M, et al. Very low- to low-grade metamorphic processes related to the collisional assembly ofavalonia in SE Cape Breton Island (Nova Scotia, Canada)[J]. Journal of Petrology, 2013, 54(9): 1849-1874. DOI URL
[33]	WANG H J, RAHN M, ZHOU J. Tectonothermal evolution of the Triassic flysch in the Bayan Har Orogen, Tibetan Plateau[J]. Tectonophysics, 2018, 723: 277-287. DOI URL
[34]	LOWEY G. Very low-grade metamorphism of the Dezadeash Formation (Jura-Cretaceous): constraints on the tectonometamorphic history of the Dezadeash flysch basin and implications regarding the tectonic evolution of the Northern Cordillera of Alaska and Yukon[J]. AIMS Geosciences, 2021, 7(3): 355-389. DOI URL
[35]	CORNO A, GROPPO C, BORGHI A, et al. To be or not to be Alpine: new petrological constraints on the metamorphism of the Chenaillet Ophiolite (Western Alps)[J]. Journal of Metamorphic Geology, 2023, 41: 745-765. DOI URL
[36]	VARGA A, PÁL-MOLNÁR E, RAUCSIK B. Revealing the mineralogical and petrographic signs of fluid-related processes in the Kelebia basement area (Szeged Basin, S Hungary): a case study of alpine prograde metamorphism in a permo-triassic succession[J]. Geofluids, 2023, 2023: 1-18.
[37]	莫宣学, 董方浏, 王勇, 等. 云南巍山-永平矿化集中区铜金多金属矿远景评价报告[R]. 北京: 全国地质资料馆, 2003. DOI: 10.35080/n01.c.120241.
[38]	段政, 邢光福, 廖圣兵, 等. 江南造山带东段九岭新元古代复式花岗岩源区性质的差异: 来自地球化学及锆石Hf同位素的制约[J]. 岩石学报, 2017, 33(11): 3610-3634.
[39]	CUNNINGHAM J K, GÓMEZ-FERNÁNDEZ F, GONZÁLEZ-MENÉNDEZ L, et al. Black shales and mesozonal quartz vein-hosted Au: the Truchas syncline, Spain and the Harlech dome, Wales, a comparative study[J]. Geological Journal, 2023, 58(1): 85-107. DOI URL
[40]	杨春, 戴金星, 陈汉林, 等. 浅变质岩热模拟实验及天然气成藏潜力[J]. 石油勘探与开发, 2009, 36(2): 200-207.
[41]	余川, 汪生秀, 汪威, 等. 大巴山北缘鲁家坪组变质作用及其对页岩气储层的影响[J]. 地质学报, 2020, 94(11): 3461-3470.
[42]	DECELLES P G, CARRAPA B. Differences between the central Andean and Himalayan orogenic wedges: a matter of climate[J]. Earth and Planetary Science Letters, 2023, 616: 118216. DOI URL
[43]	LEÃO M F, BARROSO E V, POLIVANOV H, et al. Weathering of metapelites from the Quadrilatero Ferrifero mineral province, southeastern Brazil[J]. Bulletin of Engineering Geology and the Environment, 2019, 78: 19-33. DOI
[44]	胡大千, 韩春元, 马瑞, 等. 内蒙古锡林郭勒地区上古生界极低级变质作用: 伊利石和镜质体反射率的证据[J]. 岩石学报, 2012, 28(9): 3042-3050.
[45]	袁晏明, 桑隆康, 李德威, 等. 青藏高原可可西里地区三叠系巴彦喀拉山群低级-极低级变质作用[J]. 地质科技情报, 2008, 27(3): 14-20.
[46]	袁晏明. 不冻泉地区三叠系极低级变质岩石学研究[D]. 武汉: 中国地质大学(武汉), 2008.
[47]	孙占营, 王瑜, 敬乐. 青海省达卡地区三叠系巴颜喀拉山群极低级变质作用[J]. 价值工程, 2020, 39(12): 206-207.
[48]	李宁, 陈永东, 魏龙. 青海牙扎康赛地区巴颜喀拉山群极低级变质作用及构造意义[J]. 西部资源, 2020(4): 52-53, 57.
[49]	BI X M, ZHANG J J, MO J. The 31st International Geological Congress Abstracts Volume[C]. 2000, Session 10-1: 1-23.
[50]	莫宣学, 潘桂棠. 从特提斯到青藏高原形成: 构造-岩浆事件的约束[J]. 地学前缘, 2006, 13(6): 43-51.
[51]	莫宣学. 青藏高原新生代碰撞-后碰撞火成岩[M]. 北京: 地质出版社, 2009.
[52]	潘桂棠, 丁俊, 王立全, 等. 青藏高原及邻区地质图(附说明书)[M]. 成都: 成都地图出版社, 2004.
[53]	潘桂棠, 李兴振, 王立全, 等. 青藏高原新生代构造演化[J]. 地质通报, 2002, 21(11): 701-707.
[54]	万晓樵, 刘文灿, 李国彪, 等. 白垩纪黑色页岩与海水含氧量变化: 以西藏南部为例[J]. 地学前缘, 2003, 30(1): 36-47.
[55]	李祥辉, 王成善, 伊海生, 等. 西藏中白垩世和始新世岩相古地理[J]. 中国区域地质, 2001, 20(1): 82-89.
[56]	王成善, 郑和荣, 冉波, 等. 活动古地理重建的实践与思考: 以青藏特提斯为例[J]. 沉积学报, 2010, 28(5): 849-860.