西秦岭天水地区车河花岗岩研究

发布时间：2019-02-25 13:26

【摘要】：研究区位于北秦岭造山带的西端,是北秦岭和祁连造山带的重要结合部位。车河岩体是一套产出于天水地区早古生代草川铺岩体内的似斑状花岗岩体,岩体内发育少量暗色微粒包体,是壳幔混合的重要标志之一,但是前人对壳幔岩浆的混合机制及暗色微粒包体的成因问题缺乏深入研究。因此本文通过对车河岩体的野外地质勘察、岩相学、岩石学及室内岩石主量、微量元素地球化学分析,并结合锆石U-Pb测年技术,研究探讨了车河岩体的形成时代、成因、构造环境及其地质意义,主要研究成果如下:1、车河岩体寄主花岗岩SHRIMP U-Pb锆石年龄测试分析的结果显示,寄主岩石中岩浆锆石的加权平均年龄为(210.9±1.7)Ma(MSWD=1.74),因此,此套似斑状花岗岩的结晶时代为晚三叠世,是早中生代印支期花岗岩体,与草川铺岩体不是同时期形成的简单复式岩体,而是由多期花岗岩形成的复式岩体。2、车河花岗岩矿物组成中有角闪石和榍石,无石榴子石和白云母等富铝矿物,SiO2含量较高(72.57%~74.13%),K2O/Na2O=1.35~1.47,K2O+Na2O含量较高(8.02%-8.6%),Al2O3(13.57%~13.91%),A/CNK=0.99-1.07,为准铝-弱过铝质型花岗岩,10000Ga/Al介于2.05-3.97;在微量元素上,车河花岗岩具有贫Ba、Ti、P和Sr的A型花岗岩特点,但是Sr含量相对于典型A型花岗岩较高,且在其REE分布中负铕异常微弱,个别可至正异常,因此,车河花岗岩既有A型花岗岩特征,又显示部分I型花岗岩特征,但又与二者相区别。3、车河岩体内发育少量暗色微粒包体,包体形态各异,内部发育针状磷灰石和被暗色微粒矿物包裹的石英斑晶,微量元素Nb-Ta、Zr-Hf呈槽状分布,是岩浆混合作用的重要证据。4、对暗色微粒包体中发育的少量高Na单斜辉石包裹体研究显示,暗色微粒包体的形成可以简单概括为2个阶段。第一阶段为辉石等矿物的早期分异阶段:由于高温基性岩浆与较低温的酸性岩浆混合,致使混合后的高温基性岩浆快速结晶分异,以单斜辉石为代表的偏基性矿物组合在此阶段开始结晶;第二阶段为暗色微粒包体结晶阶段:基性岩浆在经历了岩浆混合结晶分异后,残余的偏基性与偏酸性岩浆一起上侵,开始结晶的单斜辉石随着偏基性岩浆一起上侵,此后偏基性岩浆在某一位置较快速冷凝结晶,从而导致单斜辉石被角闪石等矿物所包裹。5、对高Na单斜辉石的研究显示,壳幔岩浆混合的深度至少应在地表69km以下,这一厚度代表了晚三叠世时期研究区最低地壳厚度,由此表明,研究区在晚三叠世时已经历了碰撞造山活动,大陆地壳厚度相当于现在的喜马拉雅山地区地壳厚度。6、综合区域地质背景和构造环境图解分析得出,早中生代车河花岗岩应形成于后碰撞时期的拉张伸展环境。
[Abstract]:The study area is located at the western end of the North Qinling orogenic belt and is an important combination site between the North Qinling orogenic belt and the Qilian orogenic belt. The Chehe rock body is a set of porphyry-like granites occurring in the early Paleozoic Cao-Chuan-Pu rocks in Tianshui area. A small number of dark particulate inclusions are developed in the rocks, which is one of the important signs of crust-mantle mixing. However, the previous studies on the mixing mechanism of crust and mantle magma and the origin of dark particle inclusions are lacking. Based on the field geological investigation, lithofacies, petrology, laboratory petrology and geochemical analysis of trace elements of the Che rock mass, and combining with zircon U-Pb dating technique, the formation age and genesis of the Chehe rock mass are studied and discussed in this paper. The main research results of the tectonic environment and its geological significance are as follows: 1. The SHRIMP U-Pb zircon age test results of the host granite of the Chehe pluton show that, The weighted average age of magmatic zircons in host rocks is (210.9 卤1. 7) Ma (MSWD=1.74). Therefore, the crystalline age of the porphyry-like granite is late Triassic, and it is an early Mesozoic Indosinian granite. The Caochuanpu rock mass is not a simple complex rock mass formed at the same time, but a composite rock mass formed by multi-stage granite. 2. The Chehe granite minerals are composed of amphibole and atracite, without aluminite-rich minerals such as garnet and dolomite. The content of SiO2 is high (72.57%), the content of K _ 2O / Na _ 2O 1.35 ~ 1.47% K2O is higher (8.02% -8.6%), the content of Al2O3 (13.57%) is 13.91%, A / N _ K _ (0.99-1.07), quasi-aluminum-weakly peraluminous granites, the content of SiO2 is higher (74.13%), the content of K _ 2O is higher (8.02% -8.6%). 10000Ga/Al ranged from 2.05-3.97; In terms of trace elements, Chehe granite has the characteristics of A-type granite which is poor in Ba,Ti,P and Sr, but the content of Sr is higher than that of typical A-type granite, and the negative europium in its REE distribution is very weak, a few of them can be positive anomaly, so the content of Sr is higher than that of typical type A granite. The Chehe granite has A-type granite characteristics, but it also shows some I-type granite characteristics, but it is different from the two. 3, a small number of dark particulate inclusions have been developed in the Chehe granite body, and the inclusion forms are different. There are acicular apatite and quartz porphyry encased by dark particle minerals. The trace element Nb-Ta,Zr-Hf distributes as a trough, which is an important evidence of magmatic mixing. Studies on a small amount of high Na clinopyroxene inclusions developed in dark particle inclusions show that the formation of dark particle inclusions can be summarized into two stages. The first stage is the early differentiation stage of pyroxene and other minerals: the mixture of high temperature basic magma and lower temperature acidic magma results in rapid crystallization differentiation of mixed high temperature basic magma. The partial base mineral assemblage represented by clinopyroxene began to crystallize at this stage. The second stage is the dark particle inclusion crystallization stage: after the magmatic mixed crystallization differentiation, the residual metabase invaded with the acid magma, and the monoclinopyroxene, which began to crystallize, invaded along with the partial basic magma. Since then, the partial basic magma crystallized rapidly at some point, resulting in clinopyroxene encased by amphibole and other minerals. 5. The study of high Na monoclinopyroxene shows that the mixing depth of crust-mantle magma should be at least below the surface 69km. This thickness represents the lowest crustal thickness in the study area during the late Triassic period, indicating that the study area experienced collision orogeny during the late Triassic, and that the continental crust thickness is the same as the crust thickness of the Himalayas. 6. Based on the analysis of regional geological background and tectonic environment, it is concluded that the early Mesozoic Che granites were formed in the extensional and extensional environment during the post-collision period.
【学位授予单位】：长安大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P588.121

【相似文献】