青藏高原东南缘金顶铅锌矿集区中新世沉积特征:大陆斜向碰撞带周缘前陆盆地沉积

Sedimentary features of the Miocene strata of the Jinding Zn-Pb ore concentration area,the SE Tibetan Plateau:Fillings of a peripheral foreland basin in the oblique collision belt

兰坪中-新生代盆地位于青藏高原东南缘的三江造山带中段,受印度-欧亚大陆斜向碰撞相关构造的强烈影响,导致地层序列及时代归属难辨,区域地质与成矿作用认识存在诸多分歧。先期完成的区域地质调查发现,在金顶铅锌矿集区内常见的含石膏岩地层并非前人认为的古新统云龙组的一部分,而是角度不整合于前古新统之上的一套新地层,暂定名为中新统金顶砂泥岩。系统的剖面测量和详细的沉积学研究表明,金顶砂泥岩从下到上由洪积相砾岩、辫状河三角洲相粉-细砂岩夹泥岩和砾岩以及湖泊相粉砂岩夹泥岩组成,石膏层位于砾岩之上。辫状河三角洲相中频繁出现灰绿色、灰黑色粉砂质泥岩及黄铁矿结核层,可能是频繁的区域性挤压作用造成的逆冲-走滑断层使上三叠统基底岩系中古油气藏破裂、泄露所致;该套地层内广泛发育的同沉积逆断层(视正断层)、滑脱褶皱和液化砂脉等软沉积物变形构造,指示沉积盆地发育期间地壳发生了NEE-SWW向缩短。金顶砂泥岩是印度-欧亚大陆碰撞背景下近东西向挤压构造诱发并控制的周缘前陆盆地沉积,时代为中新世。这一认识为深入理解大陆斜向碰撞带构造演化及大规模金属成矿作用提供了新的资料。

The Mes-Cenozoic Lanping Basin is located in the middle segment of the Sanjiang Orogenic Belt,the southeastern Tibetan Plateau.Owing to oblique collision between India and Eurasia,the strata of this basin were largely modified,which led to numerous controversies in its stratigraphic division and regional tectonics,as well as in the understanding on the related ore metallogeny in this area.Recently,completed geological mapping revealed that the gypsum-bearing sedimentary unit in the Jinding Zn-Pb ore concentration area does not belong to the Paleocene Yunlong Formation,but unconformably overlies the pre-Paleocene strata in this basin.Here,we term it the Jinding sandstone-and-mudstone complex.Detail section measurements and sedimentary study demonstrate that this complex is composed of a set of conglomerates,sandstones and mudstones.The ascending stratigraphic sequence of the complex consists of a set of pluvial facies conglomerates,braided river delta facies sandstones intercalated with mudstones and minor conglomerates,and lacustrine facies intercalations of sandstones and mudstones,among which the gypsum-bearing horizon is upon the pluvial facies conglomerates.Greenish and dark gray siltstones or mudstones and pyrite nodules are common in the braided river delta facies rocks,most likely indicating the intrusion of reduced-gas derived from paleo-oil-and gas-reservoirs of the Upper Triassic sedimentary rocks.The leakage of the reduced gas probably was caused by episodic faulting during the syn-sedimentary tectonism.Widespread syn-sedimentary soft deformation structures,such as reverse faults(pseudo-normal faults),slump folds,and liquefied sandy-dykes are common in the strata,indicating successive NEE-SWW direction crustal shortening during the sedimentation.The newly identified Jinding sandstone-and-mudstone complex most likely represents the fillings of a peripheral foreland basin in respond to the NEE-SWW compression due to the India-Eurasia oblique collision during the Miocene period.These new findings provide new evidence for further understanding on the tectonic evolution and related large-scale mineralization of the oblique collision belt.