柴北缘果可山地区三叠纪侵入岩锆石年代学、成因及其形成环境探讨

发布时间：2018-05-01 09:40

本文选题：柴北缘 + 石英闪长寄主岩　；参考：《合肥工业大学》2017年硕士论文

【摘要】：柴达木盆地北缘果可山地区石英闪长岩中发育大量的暗色微粒包体,石英闪长寄主岩及其暗色微粒包体锆石U-Pb定年结果分别为247 Ma和250 Ma,在误差范围内二者形成时代一致,均为柴北缘三叠纪早期(早、中三叠世)岩浆活动产物。石英闪长寄主岩属准铝质-弱过铝质系列(A/CNK=0.95~1.02),具有I型花岗岩特征。寄主岩富集轻稀土元素和大离子亲石元素,相对亏损重稀土元素和高场强元素,显示出弧岩浆岩特征。寄主岩具有平坦的重稀土元素配分曲线,暗示角闪岩可能为主要的源区残留相矿物。岩石学实验表明其可能由低钾的玄武质岩石(角闪岩)部分熔融形成,而不是中-高钾玄武岩、英云闪长岩、变硬砂岩部分熔融形成。寄主岩MgO含量(2.04%~2.44%)和Mg#值(48.9~51.9)均高于变玄武岩/变泥岩在1~4 GPa压力下部分熔融形成的熔体,暗示其形成过程中有幔源物质的加入。寄主岩锆石低的Ti温度和高的Ce4+/Ce3+比值,反映其结晶自低温、高氧逸度岩浆,暗示岩浆形成过程中可能有富水熔体/流体的加入;另外,寄主岩La/Nb比值(2.27~2.64)与活动大陆边缘区高La/Nb比值(2.0)特征相吻合,并在花岗岩构造环境判别图解上均落在火山弧环境区域,表明寄主岩可能是活动大陆边缘环境下,由下地壳角闪岩部分熔融产生的岩浆和幔源岩浆混合而成。暗色微粒包体岩性为闪长质岩石;呈椭圆状、透镜状和不规则状产出,与寄主岩石界限截然,部分暗色微粒包体具有弱的拉长现象或发育反向脉。暗色微粒包体具有岩浆结构,矿物颗粒明显小于寄主岩,发育矿物不平衡结构(斜长石包含有早期结晶形成的角闪石微晶)和短柱状-针状磷灰石。另外,暗色微粒包体其Nb/Ta比值(9.50~10.33)和Zr/Hf比值(27.65~36.92)接近于大陆地壳(Nb/Ta=11,Zr/Hf=33)。综合以上表明,闪长质暗色微粒包体为基性岩浆注入酸性岩浆与之发生不彻底混合的产物。结合区域地质背景分析,认为柴北缘果可山地区石英闪长寄主岩及其暗色微粒包体形成于古特提斯洋向北俯冲的活动大陆边缘弧环境。
[Abstract]:A large number of dark particle inclusions have been developed in quartz diorites in the northern margin of Qaidam Basin. The U-Pb dating results of quartz diorite and its dark grain-inclusion zircon are 247Ma and 250Ma, respectively. The age of their formation is the same within the error range. They are the products of early Triassic (early and Middle Triassic) magmatic activity in the northern margin of Qaihai. Quartz diorite is a kind of quasialuminum-weak peraluminous series (A / CNK _ (0.95N) 1.02), which is characterized by I-type granite. The host rocks are enriched in light rare earth elements and large ion lithophile elements, and are relatively depleted of heavy rare earth elements and high field strength elements, showing the characteristics of arc magmatic rocks. The host rock has a flat distribution curve of heavy rare earth elements, suggesting that amphibolite may be a major source of residual facies minerals. The petrological experiments indicate that it may be formed by partial melting of low potassium Black Tortoise rocks (amphibolite), rather than medium-high potassium basalt, dolomite diorite, and hardened sandstone. The MgO content and Mg# value of host rock are higher than that of melts formed by partial melting of metamorphic rocks / metamorphic rocks under 1 GPa pressure, indicating that there are mantle source materials in the formation process of the host rocks. The low Ti temperature and high Ce4 / ce _ 3 ratio of zircon from host rock indicate that the crystallization of the host rock is from low temperature and high oxygen fugacity magma, suggesting that there may be water-rich melt / fluid in magma formation. The La/Nb ratio of host rock (2.27 / 2.64) coincides with that of high La/Nb ratio (2.0) in active continental margin, and falls in volcanic arc environment on the distinguishing diagram of granite tectonic environment, indicating that the host rock may be active continental margin environment. Magma derived from partial melting of amphibolite in the lower crust is mixed with mantle magma. The diorite rocks are ellipsoidal, lenticular and irregular, and have a distinct boundary with host rocks. Some of the inclusions have weak elongations or develop reverse veins. The dark particle inclusions have magmatic structure, and the mineral particles are obviously smaller than the host rocks. The mineral disequilibrium structure (plagioclase contains hornblende microcrystals formed in early crystallization) and short columnar acicular apatite. In addition, the Nb/Ta ratio of dark particle inclusions is 9.50 ~ 10.33) and the Zr/Hf ratio is 27.65 ~ 36.92), which is close to that of NB / Ta _ (11) Zr / HF ~ (33) in continental crust. The above results indicate that the diorite dark color inclusions are the product of incomplete mixing of basic magma injected with acid magma. Based on the analysis of the regional geological background, it is considered that the quartz diorite host rock and its dark particle inclusions were formed in the active continental margin arc environment of the PaleoTethys ocean subducting northward in the Guokeshan area of the northern margin of Qaidam.
【学位授予单位】：合肥工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P597.3;P588.12

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