陕西洋县城山辉长岩体地球化学及锆石U-Pb年代学研究

发布时间：2018-09-05 06:03

【摘要】：新元古代是扬子板块岩浆活动极为发育的时期之一,其北缘汉南隆起区出露大面积的760-830Ma的各类火成岩,这一时期扬子板块北缘的成岩构造环境存在地幔柱源岩浆作用和岛弧岩浆作用两种不同的认识,差异主要由基性杂岩体的地球化学数据解释和认识不同而造成。汉南基性杂岩区西北部的毕机沟-城山岩带,东部毕机沟一带,辉长岩体上部层位发现有大型含钒钛磁铁矿床,研究程度较高,西部城山辉长岩体研究程度低,本文通过对城山辉长岩体的野外地质、元素地球化学特征、岩石类项和矿物组合特征、锆石U-Pb同位素年龄、锆石氧同位素组成研究,深入讨论该辉长岩体成因类型及汉南基性杂岩动力学机制。取得的主要研究工作进展包括:(1)城山岩体由紫苏辉长岩、辉长岩、含云母辉长岩组成,见少量含橄榄石辉长岩。与毕机沟岩区相比,岩石类型相对稳定,成分韵律层构造不发育,紫苏辉石、黑云母等暗色矿物呈现出弱定向排列,可见有由矿物粒度变化显示的韵律层构造,岩石学研究指示斜长石早于辉石结晶,指示其成岩岩浆为干熔浆。岩体周边发育有与其密切伴生的花岗闪长岩,浅、暗色矿物成略定向排列,呈条带状构造,主要有斜长石、单斜辉石和角闪石组成。(2)矿物学研究揭示,城山辉长岩体中斜长石的Ca含量变化较大,An:38-70,中长石为主,其次为钠长石和倍长石;单斜辉石主要为普通辉石;斜方辉石多为紫苏辉石。与含钒钛磁铁矿毕机沟岩体矿物成分的对比研究表明,造岩矿物类型一致,成分特征反映城山辉长岩体相当于结晶分异完善的毕机沟辉长岩体所揭示的堆晶层位的橄榄辉长岩带,或反映,城山辉长岩区距岩浆补给区远,靠近岩浆房边缘,结晶分异不甚发育。(3)岩石地球化学研究揭示,微量元素蛛网图中显示城山岩体富集Ba、K、Sr等大离子亲石元素(LILE),部分样品呈现不同程度亏损Nb、Ta、Zr、Hf和Th等高场强元素(HFSE),辉长岩中显示出弱的Ti负异常;城山辉长岩体中各类岩石均显示具LREE富集的右倾配分模式,部分岩石呈现出Eu正异常,显示成岩过程中伴有分离结晶和斜长石的堆晶,且Eu的异常与Zr、Hf的异常呈现正相关,即Eu越富集,Zr、Hf的亏损程度就越弱;结合常量元素成分主体沿拉斑玄武岩趋势线演化的特征,显现其成岩母岩浆为碱性玄武质熔浆。(4)城山辉长岩体三件样品进行LA-ICP-MS锆石U-Pb测年分析,结果显示城山辉长岩体辉长岩(15BJ-119)和紫苏辉长岩(15BJ-125)的成岩年龄分别为为781.5±4.0 Ma和783.1±3.7 Ma,揭示城山辉长岩体的成岩年龄约为782 Ma。与城山辉长岩体密切伴生的花岗闪长岩(15BJ-120)中成岩年龄780.4±6.0 Ma,并捕获有820.2±3.8 Ma的捕获锆石,指示其可能是研究区~820 Ma基性玄武质岩浆活动侵入结晶后,随后期构造运动发生部分熔融的产物。(5)对紫苏辉长岩(15BJ-125)中锆石氧同位素组成进行了初步研究,表明,城山辉长岩体矿物的氧同位素值δ18O(‰)介于4.52-7.14之间平均值δ18O(‰)为5.53,指示了成岩母岩浆没有受到地壳物质混染,由幔源玄武质溶浆上升侵位至下地壳后,在相对稳定和封闭环境下结晶成岩。(6)综合研究揭示城山辉长岩体是扬子北缘~820Ma玄武质岩浆大规模侵入之后又一次幔源岩浆活动产物,侵位年龄~782 Ma,该期岩浆成分具有钙碱性岩特征,含有相对高的K2O+Na2O,和LREE及大离子亲石元素,反映城山辉长岩是伴随Rodinia超大陆裂解,在扬子北缘发生进一步裂解,形成有~782Ma的大陆玄武岩浆侵入和早期结晶岩石部分熔融产生的中酸性岩浆活动。
[Abstract]:The Neoproterozoic is one of the most developed periods of the Yangtze plate magmatism. The Hannan uplift in the northern margin of the Yangtze plate exposed a large area of 760-830 Ma igneous rocks. During this period, there were two different understandings of mantle plume-derived magmatism and island-arc magmatism in the diagenetic tectonic environment of the northern margin of the Yangtze plate. The difference was mainly due to the geology of the basic complex. In the Bijigou-Chengshan rock belt in the northwest of Hannan basic complex area and in the Bijigou area in the east, a large-scale vanadium-bearing titanomagnetite deposit has been found in the upper stratum of gabbro-intrusive body. The study of Chengshan gabbro-intrusive body in the west is relatively low. In this paper, the field geology and elements of Chengshan gabbro-intrusive body have been studied. Geochemical characteristics, rock types and mineral assemblages, zircon U-Pb isotope age, zircon oxygen isotope composition, and in-depth discussion of the genetic types of the gabbro and the dynamics of the basic complex in Hannan have been carried out. The main research progresses achieved include: (1) Chengshan pluton is composed of purple peridotite gabbro, gabbro and mica-bearing gabbro. A small amount of olivine-bearing gabbro is found. Compared with Bijigou area, the rock type is relatively stable, the composition rhythm layer structure is not developed, dark minerals such as perilla pyroxene and biotite show a weak directional arrangement. It can be seen that there are rhythm layer structures indicated by mineral grain size changes. Petrological studies indicate that plagioclase crystallizes earlier than pyroxene, indicating that the magma is magmatic. Dry magma. There are granodiorites closely associated with the granodiorite, light and dark minerals aligned slightly in a directional direction, with strip-like structure, mainly plagioclase, monoclinopyroxene and amphibole. (2) Mineralogical study reveals that the Ca content of plagioclase in the Chengshan gabbro varies greatly, with A:38-70 being the dominant medium feldspar, followed by albite and amphibole. The comparative study of mineralogical compositions between the Chengshan gabbro and the Bijigou pluton of vanadium-bearing titanomagnetite shows that the type of rock-forming minerals is the same, and the composition characteristics reflect that the Chengshan gabbro is equivalent to the olivine gabbro of the accumulative horizon revealed by the well-differentiated Bijigou gabbro. (3) Petrogeochemical study reveals that the Chengshan gabbro is rich in Ba, K, Sr and other heavy ion lithophilic elements (LILE), and some samples show different depletion of Nb, Ta, Zr, Hf and Th high field strength elements (HFSE). All kinds of rocks in Chengshan gabbro body show LREE enrichment right-dip distribution pattern, and some rocks show positive Eu anomaly, indicating separation crystallization and plagioclase accumulation during diagenesis, and Eu anomaly is positively correlated with Zr and Hf anomalies, that is, the more Eu enrichment, the more Zr and Hf are depleted. (4) LA-ICP-MS zircon U-Pb dating of three samples of Chengshan gabbro shows that the diagenetic ages of Chengshan gabbro gabbro (15BJ-119) and purple peridotite gabbro (15BJ-125) are respectively. The diagenetic age of the Chengshan gabbro is about 782 Ma. The diagenetic age of the granodiorite (15BJ-120) closely associated with the Chengshan gabbro is 780.4+6.0 Ma, and the trapped zircon with 820.2+3.8 Ma is captured, indicating that it may be the result of intrusion and crystallization of ~820 Ma basic basaltic magma. The oxygen isotope composition of zircon in the Zisu gabbro (15BJ-125) has been preliminarily studied. The results show that the oxygen isotope value of the Chengshan gabbro is between 4.52-7.14 and the average value of 8240 (6) The Chengshan gabbro is the product of mantle-derived magma activity after the large-scale intrusion of basaltic magma from the northern margin of Yangtze to 820 Ma. The emplacement age is ~782 Ma. The magma composition of this period is characterized by calc-alkaline rocks with relatively high K2 content. O+Na2O, LREE and large-ion lithophilic elements indicate that the Chengshan gabbro was accompanied by the Rodinia supercontinent cracking and further cracking occurred in the northern margin of Yangtze, resulting in the formation of intermediate-acid magmatism resulting from the continental basalt magma intrusion of ~782Ma and the partial melting of early crystalline rocks.
【学位授予单位】：成都理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P59;P588.124

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