【作者】 闫臻；

【导师】 李继亮； 王宗起；

【作者基本信息】 中国科学院研究生院（地质与地球物理研究所） ， 构造地质学， 2002， 博士

【摘要】 陕西太白—凤县和甘肃西和—礼县一带的西秦岭造山带可分为三个构造单元:北侧的岛弧带、南侧的增生楔和夹于其间的弧前盆地。后者是西秦岭的主体,控制了晚古生代地层的发育和多个大型铅锌矿、金矿床的生成。 岛弧带由李子园群(包括丹凤群、罗汉寺群)的变质火山系组成。火山岩系属于基性→中性→酸性岩石演化序列。岩石地球化学进一步分析研究表明,火山岩系形成于岛弧为主的大地构环境。玄武岩为低钾—中钾玄武岩,以亚碱系列为主,轻稀土富集,具有Eu负异常,Nd明显正异常,Cr元素含量高出标准岛弧拉斑玄武岩的2～3倍。增生楔由分布于徽县、成县、留坝、石泉一带的原划志留系浅变质陆源碎屑岩系夹碳酸盐岩、硅质岩及基性岩、火山岩块构成,发育时限为晚古生代—三叠纪。弧前盆地主要由分布于漳县、成县、礼县、凤县、太白一带的泥盆系、石炭系和二叠系组成。 西秦岭的晚古生界分布于上述岛弧带和增生楔之间,属于弧前盆地内的充填物,主要为泥盆系的大草滩群、舒家坝群、西汉水群和铁山群,沿着李子园群南缘呈带状展布。大草滩群分布于盆地北侧,是一套晚泥盆世巨厚的红色、灰绿色陆源粗碎屑岩;舒家坝群是一套中—晚泥盆世的浅变质碎屑岩及少量碳酸盐岩,位于盆地中部;西汉水群出露于盆地的南部,以板岩、千枚岩及砂岩为主,夹灰岩块和生物礁;铁山群为晚泥盆世浅海相碳酸盐岩及碎屑岩组合,主要见于盆地西南缘。 盆地内的断裂构造主要为北西向断裂。断裂两侧地层的岩石组合及其沉积相变、砾岩组成和特征、滑塌褶曲、生物组合及断裂带内地层的综合分析显示,这些断裂为一系列多期活动的同沉积断裂系。它们将盆地分割成多个呈叠瓦状排列的次级盆地,具有背驮型增生特征。深部热液沿着盆地基底同生断裂发生广泛喷流作用,为生物礁的形成提供了物质和能量,从而在其周围形成大量热水喷流沉积岩、生物礁及铅锌矿床。 古水流方向分析显示出西秦岭晚古生代弧前盆地是由多个逆冲断隆和深水盆地组成的。盆地内的沉积物主要来自北侧岛弧带,岩石碎屑组成、地球化学特征共同表明,盆地内充填物源区的大地构造环境为岛弧。 盆地内沉积相变规律明显。平面上,从北向南呈现出由浅水到深水的变化,即陆缘斜坡冲积扇→浅海相、生物礁→半深海、深海相。这一现象揭示泥盆纪时西秦岭具有北高南低的大陆斜坡古地理格局。由南向北及山西向东均表现为超覆,且由北向南表现为同期异相。盆地内碎屑组成具有向北变粗的特征,这与盆地内充填物垂向变化相一致,表现为一进积型盆地充填序列,显示了多级水下冲积扇互相叠加特征。 弧前盆地内的铅锌矿床与西汉水群同时期形成,严格受同沉积断裂构造控更多还原

【Abstract】 The Western Qinling Orogenic belt in the Taibai-Fengxian and Xihe-Lixian areas can be subdivided into three units structurally from north to south, which are the island-arc, forearc basin and accretionary wedge, respectively. The forearc basin developed in the Late Paleozoic mainly controls sedimentation and some larger lead-zinc and gold deposits in the western Qinling.Stratigraphically, the island arc is dissected into the Liziyuan Group, the Danfeng Group and the Luohansi Group. The metavolcanic rocks include basic, intermediate and acidic rocks, and their geochemistry demonstrates that these igneous rocks generated in an island arc. Where, the basalts are subalkaline series charactered by low-medium potassium, with enriched LREE, negative Eu anomaly, and positive Nd anomaly. Cr-content of volcanic rocks is 2-3 times higher than that of island arc tholeiite all over the world. In addition, the lightly metamorphosed accretionary wedge in the areas of Huixian, Chengxian, Liuba and Shiqun is dominated by terrigenous sediments with carbonatite, chert, mafic and volcanic rocks. The age of the wedge is the Late Palaeozoic to the Trassic, while previous work suggested that it is the Silurian.The Upper Paleozoic between the island arc belt and accretionary wedge are mainly the sediments filled in the fore arc basin. The fillings in the forearc basin were subdivided into the Dacaiotan Group, the Tieshan Group, the Shujiaba Group and the Xihanshui Group, previously. They outcropped along the southern margins of the Liziyuan Group. The Dacaotan Group, the Upper Devonian, is close to the island arc complex, and composed of a suite of red and gray-green thick and coarse terrestrial clastics. The Shujiaba Group, the Mid-Upper Devonian, is located in the middle of the basin, is mainly fine-grained clastics with a few intercalations of limestone. The Xihanshui Group, which distributes in the southern of the basin, is mainly slates, phyllites and sandstones with carbonatite and reef blocks. The Tieshan Group, the Upper Devonian, just outcrops in the southwest of the basin, is carbonatite and clastic rocks, and deposited in the shallow -sea environment.The faults in the basin are mainly NW trend. The sedimentary characteristics, slump folds, biological assemblages in both sides of and within those faults demonstrate that they were syn-sedimentary faults with multi-period activities. They separated the forearc basin into several sub-basins, which imbricate in the background of a forearc basin with sedimentary characteristics of the piggyback basin. The deep hydrothermal fluid erupted along the syn-sedimentary faults, supported nutrition andenergy for the reef, and resulted in hydro thermal-sedimentary rocks, reef and lead-zinc deposits along these faults.The sedimentary facies in the basin varies from the continental slope alluvial fan, to shallow-sea reef facies, and then to deep-water from north to south, which implies that there was a continental slope in the Devonian in the west Qinling. The strata overlap to north and to east respectively. Additionally, the coeval sedimentary facies in north and south are significantly different. The elastics become more and more coarser to north in the basin as well as upward coarsing. These features indicate prograding fillings followed by overlaps of the different fans underwater. The paleocurrent analyses show that the forearc basin is composed of thrust-ramp-basins and deep-water basins. The provenance of the fillings in the basin is the island arc in the north.The lead-zinc deposits were synchronous with the Xihanshui Group in the early stage of development of the forearc basin. They were strongly constrained by syn-sedimentary faults and then modified by the hydrothermal fluids. The gold deposits distributed in the north of the basin resulted from the tectonic activities and magmatism in the later stage of the basin evolution, and occurred at the top of the lead-zinc deposits spatially. The scales of lead-zinc deposits in the south of the basin are larger than that of the gold-deposits. The Pb-Zn deposits in the west of the basin are larger than those in the east, while the Gold deposits in the west of the basin are smaller than those in the east. Mineralizing ages of these deposits become younger and younger to west.更多还原