滇西点苍山杂岩中混合岩的地质年代学分析及其区域构造内涵

Geochronology and tectonic implication of migmatites from Diancangshan,Western Yunnan,China.

位于红河-哀牢山断裂带北延的点苍山杂岩由中部深变质杂岩、西侧中生界浅变质-未变质的火山-沉积岩系和东部叠加退变质岩带组成。中部深变质杂岩包括类片麻岩-长英质岩类、云母片岩、大理岩类和角闪质岩石。岩石普遍遭受角闪岩相变质作用和混合岩化作用的改造,形成条带状、条纹状、条痕状和角砾状等具有不同构造特点的混合岩。混合岩中有两种不同的脉体物质(花岗质岩脉),它们含有具不同特点的锆石颗粒,即由混合岩化作用改造形成的低 Th/U 比值锆石颗粒和由岩浆结晶作用形成的高 Th/U 比值锆石颗粒。对于锆石颗粒采用 SHRIMP U-Pb 测年,揭示出二者具有相近的年龄值,分别为842.5±9.9Ma与833±9Ma。它们的存在揭示出从混合岩化作用向岩浆作用的转变。混合岩的变形作用、变质作用特点分析,尤其是对于花岗质脉体的锆石年代学研究揭示出,点苍山深变质杂岩与扬子地块基底具有显著的相似性。结合扬子地块新元古代整体构造.岩浆活动性规律,揭示出由板块俯冲与消减引起,早于新元古代 Rodinia 裂解时期地幔柱上涌事件。

The Diancangshan complex, located at the northwestern extension along the Red River - Ailaoshan fault zone is constituted by three units with contrasting characteristics, a high grade metamorphic complex in the middle, Mesozoic low grade metamorphic volcanic-sedimentary sequence to the west, and a superimposed retrograde metamorphic belt to the east. The high grade metamorphic complex contains various gneisses, quartzo-feldspathic rocks, mica schists, marbles and hornblenditic rocks. Their primary rocks were transformed by amphibolite facies metamorphism and migmatization into migmatites with different characteristics. The rocks are dominated with banded, laminated, striaeted and brecciated structures. There are two types of migmatite veins with zircon grains of different characteristics. Zircon grains formed by migmatization have lower Th/U ratios and those formed by magmatic recrystallization have higher Th/U ratios. SHRIMP U-Pb dating of zircon grains from granitic veins in the migmatites gives similar ages, 842. 5± 9. 9Ma and 833 ± 9Ma respectively. The existence of the two types of migmatites reveals a transition from migmatization to magmatism, Deformation, metamorphism and geochronological evidences suggest rocks in the Diancangshan high grade metamorphic complex are similar to the basement rocks in the Yangtze Block. From the regional variation of Neoproterozoic tectono-magmatic activities in the Yangtze Block, it is suggested that the Neoproterozoic tectono-magmatic activities at Diancangshan and Yangtze plate are attributed to plate subduction, earlier than the distributed tectono-magmatic activities from plate boundaries to plate interior due to mantle plumb activities.