龟山杂岩的形成时代、构造属性及变质条件:对秦岭-桐柏造山带构造演化的指示

The formation age,tectonic attributes and metamorphic conditions of the Guishan complex:implications for the tectonic evolution of the Qinling-Tongbai orogenic belt

秦岭-桐柏-大别造山带是华北板块和扬子板块碰撞拼合的产物。桐柏造山带作为古生代秦岭增生造山带与中生代大别碰撞造山带之间的纽带,记录了两大板块之间从洋壳俯冲、地体增生到最终的碰撞造山过程。本文选取桐柏造山带龟山杂岩为研究对象,对其中变沉积岩、斜长角闪岩和花岗质脉体进行了岩相学、矿物学和锆石U-Pb年代学研究。结果指示变沉积岩碎屑锆石年龄峰值主要为~411 Ma,次要年龄峰值为~490 Ma,最年轻四颗碎屑锆石的加权平均年龄为352±11 Ma。斜长角闪岩原岩结晶时代为498±18 Ma,花岗质脉形成时代为412.8±4.4 Ma,两者中变质锆石加权平均年龄为320±10 Ma,指示它们共同经历了早石炭世变质作用。变质峰期温压条件为T=571~632℃,P=0.59~0.76 GPa,属于中压相系角闪岩相变质。区域年代对比分析显示,龟山杂岩变沉积岩物源来自于北侧的北秦岭地体,属于弧前沉积,沉积时代下限为早石炭世早期。结合前人研究,本文认为在古秦岭洋向北的俯冲过程中,上述沉积岩与斜长角闪岩因构造作用拼贴在一起,于~320 Ma发生中压角闪岩相变质作用。该研究结果也指示了古秦岭洋最终闭合时间可能晚于早石炭世早期。

The Qinling-Tongbai-Dabie orogenic belt was formed as a result of the collision between the North China and Yangtze plates.The Tongbai orogen is located in a key area linking the Paleozoic Qinling accretionary orogen and the Mesozoic Dabie collisional orogen.It is an ideal place to unravel the tectonic evolution from the initial oceanic subduction,terrane accretion to the final continental collision between the two plates.Petrological,mineralogical and geochronological studies of meta-sedimentary rocks,amphibolites and granitic vein from the Guishan complex in the Tongbai orogen were carried out in this study.Meta-sedimentary rocks of the Guishan complex have a detrital zircon age spectrum with a peak at 411 Ma and subordinate peak age of 490 Ma.The youngest weighted mean age of 352±11 Ma constrained the lower limit of deposition age of the Guishan complex.U-Pb zircon analyses reveal protolith ages of 498±18 Ma for amphibolites and 412.8±4.4 Ma for granitic vein.Metamorphic zircon grains from amphibolites and granitic vein yielded ages of 320±10 Ma,suggesting an Early Carboniferous metamorphic event.The peak metamorphic condition of the amphibolites is 571~632℃and 0.59~0.76 GPa,which belongs to amphibolite facies of medium pressure facies series.The age spectrum of detrital zircons revealed the provenance of the meta-sedimentary rocks of the Guishan complex were derived from the North Qinling terrane and it developed as forearc basin during the early Early Carboniferous.However,it was subsequently incorporated with amphibolites into tectonic mélange by the northward subduction of the Paleo-Qinling Ocean.It also suggests that Paleo-Qinling Ocean did not close until early Early Carboniferous.