南秦岭武当群火山-沉积岩地球化学及地质意义

There have long been debates about when and by which processes the continental crust was created. Zircon U-Pb ages and Hf isotopes revealed that crustal evolution is different among continental blocks. South China was considered as part of supercontinent Rodinia. Neoproterozoic is an important period of crustal growth and reworking in the Yangtze Block. Neoproterozoic volcanic-sedimentary strata must have recorded the tectonic setting and early crustal evolution of the Yangtze Block. The Wudang Group, located at the northern margin of the Yangtze Block, consists of sedimentary and volcanic rocks. The former is composed of sandstone and siltstone. The volcanic rocks mainly comprise dacite, rhyolite and tuffes with minor basaltic and andesitic rocks.In this project, we will analyze the sedimentary phases, and use whole-rock chemical compositions of the volcanic and sedimentary rocks, as well as zircon U-Pb age-Hf-O isotopes to study the following questions: (1) ages and nature of the Wudang Group and their implications for the tectonic setting relative to the other Neoproterozoic sedimentary basins around the Yangtze Block; (2) crustal growth and evolution of the Yangtze Block in the Precambrian time, and similarities and differences with the other continents in the world; (3) spatial distribution and genesis of the Neoproterozoic low-δ18O magmas at the northern margin of the Yangtze Block.

大陆地壳的形成演化是地学研究的热点问题，研究表明全球各陆块地壳生长和演化并不完全一致。华南是Rodinia超大陆的重要组成部分，新元古代是扬子板块地壳增生和活化的重要时期，因此新元古代地层记录了扬子板块构造背景和早期陆壳形成演化。扬子北缘新元古代武当群是一套变火山-沉积岩组合，沉积岩以长石石英砂岩为主、夹粉砂质泥岩和泥质粉砂岩, 火山岩以中酸性英安-流纹岩和凝灰岩为主,夹少量玄武-安山质熔岩和火山碎屑岩。本项目拟在沉积岩岩相分析基础上，结合火山岩和沉积岩全岩地球化学，锆石年龄-Hf-O同位素，研究以下地质问题：(1)与扬子周缘其它新元古代火山-沉积地层比较，武当群的形成时代、属性和大地构造背景；(2) 扬子板块前寒武纪地壳增生和活化历史；(3) 扬子北缘新元古代低δ18O岩浆作用的分布和成因。上述研究将为扬子板块前寒武纪陆壳演化和新元古代构造属性提供新的认识。

(1) 扬子板块新元古代构造演化及其在Rodinia超大陆中的位置一直以来存在较大争议。扬子北缘南秦岭地区发育大规模火山-沉积岩组合，其沉积时限、沉积环境和物源研究，能够为上述科学问题提供约束。随县群沉积岩以长石石英砂岩为主，夹粉砂质泥岩和泥质粉砂岩。碎屑锆石U-Pb年龄峰值为700-850 Ma, 1800-1950 Ma，2400-2600 Ma，最小年龄为~700 Ma；εHf值变化范围较大(-22.5 ~ +11.1)。碎屑锆石年龄图谱和Hf同位素组成与南秦岭前寒武岩浆岩一致，反映近缘沉积特点。碎屑岩全岩稀土元素配分模式表现为轻稀土富集，并有Eu负异常；微量元素表现为高La/Sc (0.91-7.28)，Th/Sc (0.56-2.16) 和低 Cr/Th (0.94-5.28)。上述结果表明，沉积岩源区主要为岛弧型中酸性岩浆岩。(2) 扬子西缘和东南缘的同时期沉积岩，显示相似的碎屑锆石年龄和εHf分布特点，其峰值为~720-840 Ma，1800-2000 Ma和2400-2600 Ma，εHf值与相应地区岩浆岩εHf值一致，表明沉积岩源区为邻区岛弧岩浆岩。(3) 扬子板块和印度北缘具有相似的新元古代岩浆-沉积序列和碎屑锆石年龄分布图，显示二者具很强的亲缘性，推测均位于Rodinia超大陆边缘。