南秦岭造山带中生代构造演化的变质岩石学约束

Based on an integrated study of field investigation, isotope geochronology and metamorphic petrology on the Foping Barrovian metamorphic zones in the South Qinling orogen, the following questions will be resolved in the present project: (1) the distribution of the Foping Barrovian metamorphic zones and their relations to basements and cover sequences in the area, (2) the metamorphic conditions, metamorphic field gradients and P-T paths of different metamorphic zones, (3) the metamorphic ages of the Barrovian metamorphic zones and their relations to the emplacement of magmatic rocks, and (4) the Mesozoic tectonic evolution of the South Qinling orogen and its difference with the collisional mechanism of the Tongbai-Hong'an-Dabie-Sulu orogen. The data will be used to explore a tectonic model of 'hard-collision' in the east and 'soft-collision' in the west between the North and South China blocks along the Qinling-Tongbai-Hong'an-Dabie-Sulu orogenic belt. This research has potential significance for understanding the dynamic process of the huge Central China Composite Orogen between the North and South China continents.

选择南秦岭造山带中发育的佛坪递增变质带为研究对象，通过野外地质调查、高精度同位素定年和变质岩石学研究来查明：(1)递增变质带的分布及其与基底和盖层的关系；(2)不同变质带的变质条件、变质场梯度及P-T轨迹；(3)递增变质带的变质时代及其与岩浆侵位的关系；(4)南秦岭造山带中生代的构造演化及其与桐柏-红安-大别-苏鲁造山带碰撞体制的差异。进而探索华北-华南陆块沿秦岭-桐柏-红安-大别-苏鲁造山带东部“硬碰撞”、西部“软碰撞”的构造模式。这项工作对分割我国南北大陆的巨型中央造山带复合造山动力学的研究具有重要的科学意义。

本项目选择南秦岭造山带中的佛坪递增变质带为研究对象，通过详细的野外地质调查、高精度同位素定年和变质岩石学研究，取得如下主要成果：（1）确定南秦岭造山带主要由新元古代岩浆岩和沉积岩构成的前寒武纪基底和古生代盖层组成，古生代地层中缺少~970-950 Ma的碎屑锆石年龄峰值，推测其物源区来自于扬子陆块北缘；（2）佛坪递增变质带可划分出绢云母-绿泥石带、黑云母带、石榴石带、十字石±蓝晶石带和矽线石带，变质温度从石榴石带的~450 ℃递增至矽线石带的~800 ℃，地热梯度为~25-38 ºC/km；（3）佛坪递增变质带的主期变质时代为~215-210 Ma，晚期高温变质叠加的时代为~205-200 Ma，与两期变形花岗质岩体和岩脉的侵位时间近于一致，代表区域上由挤压向伸展构造体制的转变；（4）认为南秦岭中低温递增变质带主要形成于陆陆“软碰撞”阶段的地壳加厚时期，晚期地壳伸展引发岩浆底辟侵位，导致相对高温的矽线石带以及佛坪片麻岩穹隆的形成。本项研究对揭示碰撞造山带中递增变质带的形成及片麻岩穹隆的成因均具有重要的启示。