位于三江南段双江勐库地区的临沧花岗岩,主体岩性为黑云母二长花岗岩,其次为黑云母花岗闪长岩和碱长花岗岩.通过野外地质调查,在勐库热水塘附近黑云二长花岗岩中发现了与其紧密共生的暗色镁铁质微粒包体(MME)—闪长岩.本文对暗色闪长岩包体进行了岩相学观察、LA-ICP-MS锆石U-Pb定年和全岩主量元素分析.暗色闪长岩包体LA-ICP-MS锆石U-Pb年龄为230.9±1.2Ma,与中细粒黑云二长花岗岩LA-ICP-MS锆石U-Pb年龄(229.2±0.8Ma)基本一致.岩石地球化学特征表明,黑云二长花岗岩富K 2 O和Na 2 O,Na 2 O>K2 O,富Al 2 O3,铝饱和指数A/CNK平均为1.17;闪长岩K 2 O和Na 2 O含量中等,Na 2 O>K2 O,富Al 2 O3和MgO,铝饱和指数A/CNK平均为0.77.二者稀土配分曲线为右倾的轻稀土富集型,二者均富集大离子亲石元素而亏损高场强元素Nb、Ta等,均具有相对较高的Mg#(黑云二长花岗岩30.50～61.41;闪长岩58.58～67.34).中细粒黑云二长花岗岩Cr、Ni含量(平均值分别为46.96×10-6和11.21×10-6)小于闪长岩Cr、Ni含量(平均值分别为197.62×10-6和75.68×10-6).综合研究表明,该地区花岗质岩浆的形成很可能与地幔流体作用引发的地壳部分熔融与壳幔岩浆混合作用密切相关,形成于陆陆碰撞-后碰撞的构造背景,暗示保山地块与思茅地块在230Ma已经进入了陆陆碰撞-后碰撞的地质时期.%The Lincang granites in Mengku,Shuangjiang,western Yunnan consist,lithologically,of biotite monzogranite,biotite granodiorite and alkali feldspar granite. The dark mafic microgranular diorite enclaves (MME)were discovered in biotite monzogranite near Reshuitang,Mengku during our field geological survey. The present paper focuses on petrographic observation,LA-ICP-MS zircon U-Pb dating and whole-rock major elemental analysis of the dark diorite enclaves. The LA-ICP-MS zircon U-Pb age of 230. 9 ± 1. 2 Ma for the dark diorite enclaves is roughly consistent with that of 229. 2 ± 0. 8 Ma for the medium- to fine-grained biotite monzogranite. Geochemically,the biotite monzogranite is characterized by the enrichment of K 2 O and Na2 O contents (Na 2 O >K2 O)and Al 2 O3 ,and an average A/ CNK ratio of 1. 17,while the dark diorite has moderate contents of K 2 O and Na2 O (Na 2 O > K2 O),enrichment of Al 2 O3 and MgO,and an average A/ CNK ratio of 0. 77. Both the biotite monzogranite and dark diorite display the right-leaning LREE distribution patterns. They are all enriched in large ion lithophile elements (LILE)and depleted in the high field strength elements (HFSE)Na and Ta,and show relatively higher Mg# values (30. 50 to 61. 41 for biotite monzogranite and 58. 58 to 67. 34 for dark diorite, respectively). The Cr and Ni contents in biotite monzogranite range in 46. 96 × 10-6 and 11. 21 × 10-6 ,respectively, which are lower than those in dark diorite (an average ranging in 197. 62 × 10-6 and 75. 68 × 10-6 ,respectively). The formation of the granitic magmas in the study area may well be closely related with the partial crustal melting caused by mantle fluid action and crust-mantle magmatic mixing in the continental-continental collision to post-collisional tectonic settings,implying that the Baoshan and Simao blocks were in the continental-continental collision to post-collisional periods at the age of 230 Ma.
展开▼
