【作者】 查显锋；

【导师】 董云鹏；

【作者基本信息】 西北大学 ， 构造地质学， 2010， 硕士

【摘要】 佛坪隆起位于东、西秦岭交接的最狭窄部位,北邻商丹缝合带,南接勉略缝合带,属南秦岭构造带(秦岭微板块),也是华北板块和扬子板块挤压作用最为强烈的地区之一。该区以前寒武纪变质基底的穹状出露和印支期花岗岩的广泛侵入为主要地质特征,区内岩石组合可划分为结晶基底、过渡基底、外围盖层岩系及中酸性侵入岩体,古老的基底主要出露于佛坪县城及其北侧的龙草坪一带。佛坪隆起遭受了多期次、多层次的构造变形作用的改造,主要表现为早期以基底为主的塑性变形及晚期的脆-韧性变形。该区现今的主体构造主要表现为一系列东西向展布的褶皱构造,反映了强烈的南北向挤压作用。县城隆起和龙草坪隆起位于两个背斜的核部。佛坪地区的变质特征显示,以佛坪隆起核部为核心,麻粒岩相-高角闪岩相-低角闪岩相-高绿片岩相-低绿片岩相变质带呈同心环状向外展布的空间分布规律。其变质条带的这种同心环状展布,也揭示了其“穹窿”构造特征。其中麻粒岩主要出露于佛坪县城南约500m河东岸,呈条带状和透镜状产出,其矿物组成以透辉石、斜长石、角闪石、黑云母及石榴子石为主。该区混合岩化作用发育,根据其产出特征和构造形态认为其成因系深部岩浆作用所致。本文在对佛坪隆起基底及其周围地区构造面理和线理的系统测量和统计的基础上,通过古应力场恢复,进而揭示区域构造应力场分布规律,探讨佛坪隆起的形成机制。研究结果表明,佛坪隆起的主体及其北部龙草坪隆起区外围地层中的构造片理呈外倾的封闭环状展布,在平面上呈东-西长、南-北短的椭圆形,揭示了县城隆起和龙草坪隆起总体具有“改造穹窿”的构造特征,而这种改造来自于印支期的近南-北向挤压构造体制。根据该区构造形迹特征,对其产状进行测量和统计,并判断其运动学指向,从而恢复隆起区古构造应力场。古应力场分析表明,县城隆起及龙草坪隆起区的两期构造具有类似的分布规律和特点：第一期构造最大主应力(σ1)呈以隆起核部为核心的高角度外倾,最小主应力(σ3)则表现为近于水平放射状展布；第二期构造最大主应力(σ1)总体呈现近水平的南-北或北东-南西向展布,最小主应力(σ3)多为东-向或北西-南东向伸展。揭示了县城隆起与龙草坪隆起具有相同或类似的成因机制和构造演化过程。隆起区主要遭受两期构造变形的叠加改造,即早期垂向隆升和后期南-北挤压作用。第一期构造是由于扬子板块向北俯冲,导致岛弧区在主体挤压作用下叠加深部岩浆上涌,使得岛弧基底发生穹状隆升,佛坪基底抬升主要生成于这种穹状隆起,它与该区岩浆侵入活动密切相关；第二期构造是由于扬子板块和秦岭微板块沿勉略带的碰撞造山作用,使得原岛弧区遭受近南北向挤压,形成近东西向的构造形迹,并叠加改造了先期穹隆构造的位态。佛坪穹隆是在早期深部岩浆作用下发生近垂直的穹状隆升,并叠加了后期南北向挤压构造才奠定了其基本构造面貌。该区岩浆岩中锆石U-Pb定年结果为212±1.5Ma,代表了岩浆岩主体结晶时代,稍晚于秦岭造山带主碰撞期,其成因可能为秦岭造山期后的下地壳拆沉作用。测得混合岩中变质锆石的U-Pb年龄为182.3±1.1Ma,代表了一期由部分熔融作用导致的混合岩化作用,表明秦岭造山带已经发生构造体制转换,进入伸展构造阶段。由于燕山期秦岭造山带受到巴山弧形逆冲推覆构造及汉南地块向北东向挤入,处于秦岭造山最狭窄地带的佛坪地区遭受最强烈的挤压作用,并持续隆升,并进一步剥露地表。更多还原

【Abstract】 Foping dome, which located in the junction of Eastern and Western Qinling, is belongs to South Qinling tectonic belts (Qinling micro-plate). This dome sites between Shangxian-Danfeng suture zone and Mianxian-Lueyang suture zone, and it is also one of the strongest compression areas by North China Block and Yangtze Block. Foping area is characterized by outcropping of Precambrian metamorphic basement and Indosinian granite. The rock assemblage in this area can be classified into crystalline basement, translation basement, caprock rocks, and intermediate-acid intrusive rocks. The crystalline basement mainly outcrops at Foping town and Longcaoping. It suffered multi-period and multilevel deformation in Foping area, and the structural characteristics here show a series of fold structure, which spreads nearly east-west and reflects a strong north-south compression. The Foping town dome and Longcaoping dome locate at the nuclear of two anticlines.The metamorphism in Foping area shows:the granulite facie-High-amphibolite facies-Low-amphibolite facie-High-greenschist facie-Low-greenschist facie spread in the shape of concentric rings and to the core of Foping dome in the plane, which can also proves the "Dome" structure features. The granulite facie rock shows lenticulars, mainly outcrops at about 500 meters to the town, east of river. The major minerals of these rocks are diopside, plagioclase, hornblende, biotite and garnet. The migmatization developed well in this region, which maybe caused by deep magmatism.The study of paleostress fields by systematic measurement and statistics on the tectonic foliation and lineation in the Foping dome and its adjacent areas. This can reveal the distribution of regional paleostress fields, by which we can discuss the genetic mechanism of the Foping dome. The results show that the foliation in Foping and Longcaoping uplifting and their adjacent stratum proves the circle features with outward trend, and longer in E-W, shorter in S-N in the plane. It proves that the view Foping uplifting is a thermal dome, which is transformed by latter tectonic, that is the nearly N-S compression tectonic in Indosian.The recovery of the paleostress field is based on systematic measurement of the occurrences and the judge of the kinematic meaning in the field. The characteristics of the paleostress fields in Foping County and it northern Longcaoping revealed that the similar paleostress fields. The first phase structure shows:the compression stress (al) generally proves outward trend with high angles, with the core of the core of the domes, and the tensile stress (σ3) generally proves nearly horizontal radial spreading. The second phase structure shows:the compression stress (σ1) generally spread nearly horizontal, N-S or NE-SW, and the tensile stress (σ3) generally mainly proves extension with E-W or NW-NE. It reveals that the town uplifting and Longcaoping uplifting shared the same of similar genetic mechanism and tectonic evolution. The uplifting area suffers two stages’deformations, which are vertical uplifting in the early stage and N-S compression in the latter stage.The first stage structure was caused by the northward subduction of Yangtze Plate. The uplifting of the basement of Foping dome was closely related with the deep magmatism and granite intrusion, which was generated by the subduction. The second stage structure was caused by the collision orogeny between Yangtze Block and Qinling Micro-Plate, which leaded the N-S compression in the island-arc zone, and it superposited on the early structures and formed a series E-W structures. The combination of regional tectonic interpretation and the analysis of principal stress field demonstrate that Foping dome underwent a sub-vertical uplifting period, which was superimposed by the latter tectonic of the N-S compressive.The result of U-Pb zircon dating shows that the main magmatic crystallization is 212±1.5Ma, which is later on the time of main orogenic period. The magma intrusion may be a result of delamination. We also get the dating 182.3±1.1Ma from the migmatites in this region, it reveals a migmatization in this period. The result proves that the structural transformation had occurred in Qinling belt and had been in the extensional tectonic stage. In Yanshania, because of the NE direction compression of Bashan arc thrust structure, Foping area, which sited the most narrow zone of Qinling orogeny, suffered the most intense compression. The Foping dome continued uplifting and was further exhumated to the surface.更多还原