【作者】 王涛；

【导师】 王宗起；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2008， 博士

【摘要】 位于南秦岭陕西—甘肃交界地区的康县、徽县、略阳和留坝一带的“白水江群”(包括大河店组、舟曲组、迭部组和和大堡组)长期被认为属于扬子板块北缘的被动大陆边缘。通过野外地质调查和室内综合分析,结果表明大河店组、舟曲组、迭部组和大堡组及其南部的三河口群共同组成了南秦岭增生杂岩带。南秦岭增生杂岩带主要由基质和岩块组成,根据基质和岩块的特征划分为三个亚带。北亚带指白水江群大河店组由深海盆地相的泥岩和炭硅质岩,以及碳酸盐岩块体、硅质岩块体和超基性岩块体组成;中亚带指白水江群舟曲组和迭部组,主要由巨厚的浊积岩组成,形成于海底扇和深海盆地环境,由细砂岩、粉砂岩、泥质粉砂岩和泥岩及其夹含薄层状灰岩,以及碳酸盐岩块体和碎屑岩块体组成;南亚带指白水江群大堡组和三河口群,由深海盆地相的泥岩、炭硅质岩和夹碳酸盐岩块体、硅质岩块体、基性和超基性火山岩块体组成。在南秦岭增生杂岩带中,通过对基质在碎屑组分、重矿物组合、地球化学特征和碎屑锆石的年代学研究,表明其大地构造背景并非前人所认为的属于扬子板块被动大陆边缘,而是属于活动大陆边缘环境。在南秦岭增生杂岩带北亚带和南亚带中,分布有基性和超基性火山岩块体。地球化学分析表明这些火山岩具有蛇绿岩、洋岛火山岩、岛弧火山岩的特征,形成于大洋板内和岛弧环境。利用SHRIMPⅡU—Pb锆石定年获得楼房沟铁镁质岩的形成年龄为210Ma;云台、蚂蝗峡铁镁质岩的形成年龄为667Ma、771Ma和778Ma;云台和佛田坝获得辉长—辉绿岩块体的形成时代为445Ma和450Ma。结合前人在勉略带中测得铁镁质岩的形成年龄为新元古代,确定南秦岭洋盆为原始古大洋,而非晚古生代形成的再生有限洋盆。在南秦岭增生杂岩带的形成时代研究中,前人在北亚带的大河店—虞关等地灰岩和硅质岩块体中发现寒武纪的小壳化石和中泥盆世的珊瑚化石,而课题组在泥岩和炭硅质板岩的基质中发现了二叠纪的放射虫;在中亚带的同一灰岩块体中发现志留纪到中、晚泥盆纪的珊瑚、牙形刺化石,在碎屑岩中获得志留纪几丁虫、虫颚和孢子等化石,以及对基质进行碎屑锆石年代学研究中获得大量晚古生代早期的锆石年龄数据;在南亚带炭硅质板岩块体中发现了奥陶纪笔石,并在三河口群和西乡群硅质岩中发现石炭纪放射虫。本文在灰岩块体中发现志留纪的珊瑚和海百合茎等化石,在基质中获得石炭纪—二叠纪的放射虫化石,以及楼房沟超基性岩块时代为晚三叠纪。这些古生物和年代学证据表明,南秦岭增生杂岩带形成于晚古生代—三叠纪。基于本文研究成果和综合区域资料分析,南秦岭包括岛弧带、弧前盆地和增生杂岩带;根据岛弧火山岩、岩浆岩的形成时代,以及增生杂岩带中基质和岩块的形成时代,表明南秦岭为原始古大洋,在泥盆纪发生南北双向俯冲增生,直到三叠纪末整个洋盆才全部闭合。更多还原

【Abstract】 The ’Baishuijiang Group’ in the western part of the southern Qinling(Kangxian, Huixian counties of Gansu province,and Lueyang,Mianxian and Liuba counties of Shannxi province) consists mainly of fine-grained clastic rocks,carbonate blocks and volcanic rock blocks with metamorphism at different degrees,and has been traditionally inferred as a stratum of the passive margin of northern Yangtze plate. Based on the primary field investigation and mapping,and integrative analysis of sedimentology,paleontology,structure geology and geochemistry,the ’Baishuijiang Group’ and the Mian-Lue belt together composed an accretionary complex of the sourthern Qiling.The accretionary complex belt of southern Qinling is divided into three subbelt according to the characteristic of the matrix and blocks.The northern subbelt(the Dahedian formation) is composed of abyssal mudstone and carbonaceous siliceous slate, and carbonate,chert and volcanic blocks;the central subbelt(The Zhouqu formation and Diehu formation)contains huge thick turbidit which formed the trench-depositional environment and a few limestone blocks;the southern subbelt (The Dabao formation and Sanhekou group)comprised abyssal mudstone and carbonaceous and siliceous slate,and limestone,chert and volcanic blocks.The analyse of petrology,heavy mineral assemblage,geochemistry and clastic zircon dating of the matrix shows that they originated from active continental margin,rather than from a passive continental margin.In southern subbelt and northern subbelt,the volcanic rock blocks include Ultrabasic,basic,and intermediate volcanic rocks,and have undergone different degree of metamorphism in different structural position.Geochemical characteristics indicate that the igneous rocks formed within-plate setting and from the mantle,which is similar to oceanic island arc and seamount.SHRIMP U-Pb analyses for zircon grains showed that the mafic rock blocks in the Loufanggou area of the northern subbelt were formed in the Mesozoic(210±5Ma).SHRIMP dating results yielded ages of 667～778 Ma to the mafic blocks and gabbro and diabase blocks was dated at 445Ma and 450Ma in Yuntai and Fotianban in southern subbelt.The ages of mafic and basic rock blocks and the data of ophiolitic and mafic blocks of Mian-Lue subbelt in the early papers,indicate that Qinling ocean was a proterozoic ocean basin,rather than the so-called Devonian regenerated ocean basin. In accretionary complex of the southern Qiling,many fossils were found in matrix and rock blocks,the fossils contain the Cambrian small shell fossils and middle Devonian Coral fossils in limestone and chert blocks and Permian radiolarian in mudstone and argillite slate of Matrix in northern subbelt,and Silurian to middle Devonian Coral and conodonts in limestone blocks and Silurian chitinozoas, scolecodonts and spores in clastic rocks in central subbelt,and Devonian Coral fossils in limestone blocks and graptolites in carbonaceous and siliceous slate in southern subbelt.La-ICP-MS clastic zircon analyses in matrix obtain the age of early Devonian.In the region,late Paleozoic radiolarian were found in San hekou group and Xixiang group in cherts.Therefore,according to ages of the fossils found in matrix and blocks,the formation age of accretionary complex of southern Qinling is the Permian-Triassic,rather than the the Silurian as previously inferred.Based on this research results and comprehensive analysis of regional data,the Qinling orogenic belt includes the island-arc belt of northern Qinling,the forearc basin of middle Qinling and an accretionary complex of sourthern Qinling from Shangdan fault to Kangxian-Mianlue fault.The ages of volcanic rocks,matrix and limestone blocks in accretionary complex indicate the Qinling ocean existed from proterozoic to trassic,and the subduction occurred to north in Ordovician and south in Devonian.The closure of the whole Qinlng ocean basin was late Trassic.更多还原