羌塘中部晚三叠世岩浆活动的构造背景及成因机制——以红脊山地区香桃湖花岗岩为例  

Tectonic setting and petrogenesis mechanism of Late Triassic magmatism in Central Qiangtang,Tibetan Plateau: Take the Xiangtaohu pluton in the Hongjishan region as an example

作　　者：张修政[1] 董永胜[1] 李才[1] 邓明荣[1] 张乐[1] 许王 

机构地区：[1]吉林大学青藏高原地学研究中心,长春130061

出　　处：《岩石学报》2014年第2期547-564,共18页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41372066);中国地质调查局项目(1212011121243);吉林大学研究生创新基金资助项目(20121074)联合资助

摘　　要：羌塘中部晚三叠世岩浆活动,尤其是南羌塘北缘大面积花岗岩的形成背景及成因机制迄今尚未得到很好约束,这些问题对深入理解龙木错-双湖-澜沧江板块缝合带晚期构造演化具有重要意义。本文以羌塘西部香桃湖岩体为研究对象,进行了系统的岩石学、地球化学及同位素年代学研究。锆石的LA-ICP-Ms定年结果表明,香桃湖岩体的形成时代为213-211Ma,而非前人认为的晚侏罗世;地球化学特征显示,岩体以高SiO2（68.06%-74.05%）、较高K2O含量（3.06%-4.50%）和K2O/Na2O（1.05%-1.63%）以及低的P2O5含量（0.08%-0.14%）为特征,SiO2和P2O5具有明显负相关性,A/CNK=1.03-1.09;富集大离子亲石元素和轻稀土元素,强烈亏损Nb、Ti、Zr等高场强元素,同时具有P和Sr等元素的负异常;岩体的Th/U=5.19-9.36,Nb/Ta=8.13-10.06,具有高Sr（297×10^-6-465×10^-6,平均389×10^-6）低Yb（1.44×10^-6-3.02×10^-6,平均1.87×10^-6）的特征;岩体负铕异常不明显（Eu/Eu＊=0.61-0.87）,HREE分布平坦（Ho n≈Ybn）,且尾部具有微弱上翘趋势。综合以上特征,香桃湖岩体为一套准弱过铝质高钾钙碱性-钙碱性I型花岗岩,其岩浆源于下地壳角闪岩相-榴辉岩相变质基性火成岩的部分熔融。岩体中暗色淬冷包体的研究暗示了幔源基性岩浆的底侵作用,同时母岩浆亦经历了一定的岩浆混合作用;综合区域内时空演化格架以及大量相关地质事实,我们认为羌塘中部横跨龙木错-双湖-澜沧江板块缝合带分布的大规模晚三叠世岩浆活动均形成于后碰撞阶段伸展构造背景,俯冲板块的断离是这一期岩浆活动可能的成因机制。The tectonic setting and petrogenesis mechanism of Late Triassic granitoids,which widely exposed in the central Qiangtang Terrane,Tibet,remain unconstrained so far. A better understanding of such issues will help us to reveal tectonic evolution processes of the Longmu Co-Shuanghu-Lancang suture zone during the Late Triassic. This paper reports petrological characteristics, zircon U-Pb age,whole-rock geochemistry of analyzed samples from Xiangtaohu pluton,which is one of the representative Late Triassic pluton in the central Qiangtang Terrane. In situ zircon U-Pb LA-ICP-MS dating on the magmatic genetic zircons yielded an age of the 210. 9 ± 3. 9Ma and 212. 6 ± 1. 5Ma,representing the emplacement age of the pluton. Geochemically,the rocks are high-K calcalkaline to calc-alkaline series,and rich in alkali（ K 2 O ＋ Na 2 O = 5. 58%~7. 55%） and K（ K 2 O / Na 2 O = 1. 05%~1. 63%）,with the A / CNK values of 1. 03~1. 09,which fall into the range of slightly peralumnous rocks. The rocks are also characterized by enrichment of LREE and LILE（ Rb,Ba,Th,U,K and La） and depletion of Nb,Ti,Zr and P,which slightly negative Eu anomaly（ Eu/Eu＊= 0. 61~0. 87）. The elemental ratios of the Xiangtaohu pluton,such as Th / U（ 5. 19~9. 36）,Nb / Ta（ 8. 13~10. 06）, combined with the contents of high Sr（ average 389 × 10- 6）,low Yb（ average 1. 87 × 10- 6） and HREE-smooth pattern（ Ho N ≈ Yb N）,indicate the partial melting of magmatic rocks from lower crust,with amphibole eclogite as the restite. Macroscopic characteristics such as ubiquitous mafic microgranular enclaves（ MMEs）,long prisms of apatite and captured quarts in MMEs indicate that the origin of the pluton is closely related to magma mixing. Combined with the previous studies and correlative geological events,it is concluded that the Late Triassic magmatism in Central Qiangtang were formed in a post-collision extensional tectonics in response to slab break-off of the subducted Longmu Tso-Shuanghu Tethyan oceanic lithosphere beneath th

关 键 词：锆石U-PB定年 晚三叠世 后碰撞 板块断离 羌塘中部 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]