点苍山变质杂岩新生代变质-变形演化及其区域构造内涵

Cenozoic deformation-metamorphic evolution of the Diancang Shan metamorphic complex and regional tectonic implications

点苍山变质杂岩体是哀牢山-红河韧性剪切带四个变质杂岩体之一,遭受了多期多阶段变质-变形作用改造。本文重点针对点苍山杂岩的新生代变质-变形作用,尤其是以富铝质高级变质岩即夕线石榴黑云片麻岩和侵位于其中的糜棱岩化细晶花岗质岩石开展了深入研究。对夕线石榴黑云片麻岩的显微构造分析与矿物共生组合研究,确定了高角闪岩相和低角闪岩相变质矿物共生组合,分别为:石榴石(Grt)+夕线石(Sil)+钾长石(Kfs)+黑云母(Bi)+斜长石(Pl)±石英(Q)和夕线石(Sil)+白云母(Ms)+黑云母(Bi)+石英(Q)。对其中的变质锆石进行SHRIMP U-Pb测试,获得了新生代三个阶段的变质作用年龄,即54.2±1.7Ma、31.5±1.5Ma和27.5±1.2Ma.本文还深入研究了侵位于高级变质岩中的一个花岗岩质糜棱岩的宏观与显微构造特点,其LA-ICP-MS年龄为24.4±0.89Ma,代表着同剪切就位花岗质岩浆侵位和结晶年龄。夕线石榴黑云片麻岩中变质锆石从2150～27Ma多期多阶段表观年龄的发育,表明点苍山变质杂岩体具有复杂的构造演化史。点苍山杂岩的多阶段新生代构造-热演化归咎于印度-欧亚板块会聚与碰撞作用(约54Ma)、造山后伸展作用(大约40～30Ma)和沿着哀牢山-红河剪切带大规模左行走滑变形作用(约27～21Ma)。

The Diancang Shan complex is one of the four metamorphic complexes along the Ailao Shan-Red River （ASRR） ductile shear zone. The complex experienced a long-lasting history of multiple deformation and metamorphism during its tectonic evolution since the Proterozoic. With the Cenozoic deformation-metamorphic evolution as the major topic of study, this paper maiuly focuses on the high grade metamorphic aluminous rocks, i.e. the sillimanite-garnet-biotite gneiss, and a fine-grained granite intruding the high grade metamorphic rocks. Through microstructural analysis and paragenetic association study, an upper amphibolite facies assemblage of garnet ＋ sillimanite ＋ K-feldspar ＋ biotite ＋ plagioclase ＋ quartz and a lower pressure amphibolite facies assemblage of sillimanite ＋ muscovite ＋ biotite ＋ quartz were recognized in the high grade metamorphic aluminous rocks. SHRIMP U-Pb dating of metamorphic zircons from the high grade gneiss reveals Cenozoic growth of the zircons during three stages of metamorphism, i.e. at 54. 2 ＋ 1.7Ma, 31.5 -＋ 1.5Ma and 27. 5 ＋ 1.2Ma. At the same time, a mylonitized fine-grained granite hosted in the high grade metamorphic rocks was studied macroscopically and microscopically to constrain the timing of metamorphism and shear deformation of the high grade gneisses, and therefore of shearing along the ASRR shear zone. LA-ICP-MS U-Pb dating of magmatic zircons gives an age of 24.4 ＋ 0. 89Ma, being representative of magmatic crystallization of the syn-shearing granite. From the apparent age populations of metamorphic zircons from the high grade gneiss, it is shown that the metamorphic rocks from the Diancang Shan area has muhiple stages of Cenozoic thermal-tectonic evolution, which is attributed to the Indian-Eurasian plate convergence and plate collision （ca. 54Ma）, post-collisional extension （ca. 43Ma to 30Ma） and large scale left-lateral shearing along the Ailao Shan-Red River shear zone （ca. 27 -21Ma）.