宝鸡—天水地区北秦岭早古生代变质作用与岩浆活动

发布时间：2018-03-14 08:40

本文选题：北秦岭造山带　切入点：榴闪岩　出处：《西北大学》2017年硕士论文　论文类型：学位论文

【摘要】：秦岭造山带是由多期俯冲-碰撞造山和随后多阶段隆升成山形成的复合型造山带。很多岩石地球化学以及同位素地球化学数据表明北秦岭是独立于华北板块在洋岛的基础上形成的微陆块。近年来一个突破性进展是在北秦岭东部即陕西-河南交界地区以及桐柏地区的秦岭群高级变质岩中发现存在大量退变榴辉岩和高压麻粒岩,其中部分退变榴辉岩锆石包裹体中出现柯石英。这些发现证明北秦岭东部和桐柏地区的确在500-480 Ma发生过深俯冲作用,随后俯冲板块抬升形成退变榴辉岩。这些HP-UHP岩体在官坡、松树沟、富水、寨根等地均呈面状出露,且具有大致相同的变质年龄,部分学者提出北秦岭作为一个整体都经历了这次俯冲事件。可惜的是,这些岩体都只在北秦岭东部地区被发现,纵然北秦岭西部地区已陆续报道发现蛇绿岩证明了洋壳的存在,但一直未曾见有HP-UHP岩体在北秦岭西部地区存在。本论文首次在北秦岭西部宝鸡眉县地区发现了榴闪岩体,该榴闪岩主要由石榴石和角闪石组成,含少量石英、透辉石和绿帘石。石榴石变斑晶都具有明显由退变质扩散作用形成的环带结构,绿帘石变斑晶具有与榴辉岩中绿帘石大致相同的XFe, CaO和AL2o3含量。同时,石榴石变斑晶核部还含有Amp+Cpx+Qtz包裹体,该包裹体矿物组合暗示其退变前应该是榴辉岩,而榴闪岩是在流体活动过程中退变质作用形成的。榴闪岩中锆石的离子探针SIMS定年获得很好的U-Pb谐和年龄(443.9±1.1 Ma)和206Pb/238U的加权平均年龄(443.2±2.5 Ma)。这些锆石具有亏损的Hf同位素和类似地幔的氧同位素组成。因遭受退变质作用的强烈影响,其εHf(t)值和δ18O值有较宽的分布范围(εHf(t)=2.4-14.7;δ180=4.25-6.39%。),其亏损地幔模式年龄为0.48-0.95 Ga。所以,锆石的Hf-O同位素特征指示榴闪岩的原岩主要来自新元古代时期的亏损地幔玄武质岩石。结合北秦岭东部地区榴辉岩发生退变质作用时代间隔可以大致判断北秦岭西部的榴闪岩的峰期变质时代可能为470-480 Ma。山此发现,北秦岭东西部俯冲碰撞事件时限上存在差异,北秦岭俯冲碰撞作用可能是沿商南-宝鸡一线自东向西发生的。结合北秦岭地区变质岩变质作用过程的研究,对北秦岭西部的岩浆岩成因进行了讨论。在宝鸡及周边地区采集六块闪长质以及花岗质样品,其CL图像具有明显的震荡环带,具有岩浆锆石的特征,未发现明显的核边结构。通过对锆石进行U-Pb年龄测试,发现研究区内的岩浆岩主体分为两个时期:～450 Ma和～430 Ma。两期岩浆岩中锆石的Hf同位素特征以及其中一个样品的锆石O同位素特征说明岩浆形成过程中并没有非常古老的地壳物质参与,而是相对年轻的具有亏损地幔特征的中新元古代洋壳物质熔融的结果。结合北秦岭眉县地区早古生代时期的变质岩的研究,认为这两期岩浆岩可能是俯冲板块在470-480 Ma经历超高压后发生折返,在折返过程中俯冲板块减压发生部分熔融形成的。
[Abstract]:Qinling Mountains orogenic belt is composed of multi period subduction and subsequent multi stage compound orogenic belt formed by uplift Chengshan. Many geochemical and isotopic geochemical data show that North Qinling Mountains is independent of micro continental blocks in North China plate based on the formation of oceanic island. In recent years a breakthrough in North Qinling Mountains in eastern Qinling Mountains group Shaanxi - Henan border area of high-grade metamorphic rocks in Tongbai area and found the existence of a large number of retrograded eclogites and granulites, which part of coesite eclogite zircon inclusions. These findings show that the north east of Qinling Mountains and Tongbai area is in the 500-480 Ma occurred in deep subduction and subsequent subduction uplift the formation of eclogite. These HP-UHP rock in guanpo, Pine Valley, rich water, etc. were Walled root surface exposed, and has a metamorphic age of roughly the same, some scholars put forward North Qinling Mountains as a whole have experienced the subduction event. Unfortunately, these rocks are only in the eastern region of North Qinling Mountains was found, even though the north western region of Qinling Mountains has been reported that the ophiolite proved that oceanic crust exists, but has not seen there in the north western region of Qinling Mountains. This is the first time to rock HP-UHP in Baoji Meixian area of western North Qinling Mountains found eclogite diorite, the garnet amphibolite is mainly composed of garnet and amphibole composition, containing a small amount of quartz, diopside and epidote. Garnet crystals have obvious change spot zoning structure by retrograde diffusion effect, epidote porphyroblasts with approximately the same with eclogite the green shade stone XFe, CaO and AL2o3 content. At the same time, garnet porphyroblast core containing Amp+Cpx+Qtz inclusions, the inclusions of minerals that the degeneration of the former should be eclogite and garnet amphibolite in fluid activity The formation of Cheng Zhongtui metamorphism. SIMS ion microprobe zircon garnet amphibolite dating U-Pb concordant age good (443.9 + 1.1 Ma) and 206Pb/238U (mean age 443.2 + 2.5 Ma). The loss of oxygen isotope has zircon Hf isotope and similar mantle composition. Due to strong impact metamorphism the role of the epsilon Hf (T) values and 18O values have a wide distribution range (Hf (T) =2.4-14.7; Delta 180=4.25-6.39%.), the depleted mantle model age is 0.48-0.95 Ga. so, Hf-O isotopic characteristics of zircon indicate garnet amphibolite protolith mainly from Neoproterozoic basaltic depleted mantle rock. North eastern region of Qinling Mountains eclogite metamorphism with the time interval can largely determine the north western Qinling Mountains garnet amphibolite peak metamorphic age of 470-480 Ma. may find this mountain, North Eastern Qinling Mountains subduction collision event time deposit The difference in the North Qinling Mountains collision may be along the Shangnan Baoji line from east to west. Combined with the study of metamorphic rocks occur metamorphism in the north of Qinling Mountains, North Western Qinling Mountains magmatite in the collection are discussed. Six pieces of dioritic and granitic samples in Baoji and the surrounding areas, the CL image has oscillatory zoning obvious, with characteristics of magmatic zircon, obvious nuclear edge structure not found. Through the test of zircon U-Pb age, found the main magmatite in the study area is divided into two periods: ~ 450 Ma and ~ 430 Ma. zircon Hf isotopic characteristics of two magmatite and the zircon O isotopic characteristics of a samples of magma formation process is not very old in the crust, but relatively young has depleted mantle characteristics of the Neoproterozoic oceanic crust melting results. Combined with the North Qinling Mountains During the period of the early Paleozoic Meixian area of metamorphic rocks, that these two magmatite may be subduction exhumation of the UHP happened at 470-480 Ma, during exhumation of subducted plate decompression partial melting to form.

【学位授予单位】：西北大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P534.4

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