桐柏-大别山区高压变质相的构造配置

作为华北和扬子陆块间的碰撞造山带桐柏大别山区以发育高压、超高压变质带为特征，从南到北变质相从低级到高级，代表俯冲带深度不同的变质产物，整体形成高压变质相系列。不过现今各变质相岩石的分布极受后期地壳规模的伸展构造控制，大别杂岩的穹隆作用更使高压变质相带的空间分布复杂化。超高压变质岩今日多呈大小不等的块体嵌布于相对低压的大别杂岩之内，造山带根部物质的热软化，使许多深层地幔物质得以像挤牙膏一样挤出于大别杂岩内。它们之中广泛发育着减压退变质的显微结构，与大别杂岩内一些麻粒岩相表壳岩所保存的减压退变质证迹一样，同是挤出作用和碰撞后隆升的构造证迹。高压相系的发育使南桐柏山和大别山迥然不同于桐商（ 商丹） 断裂以北的北秦岭北淮阳变质带。新近发表的同位素年代学（４０Ａｒ ３９ Ａｒ） 资料：３１６ ～４３４ Ｍａ ，已证明北秦岭是古生代变质带，它与桐柏－ 大别印支期碰撞造山带差异甚大。这两个变质地温梯度差异甚大的变质地体的拼合，说明华北和扬子陆块碰撞的主缝合带是商丹－ 桐商断裂带

The tectonic distribution of high-pressure metamorphic facies in Tongbai - Dabieshan, central China

As a collisional orogenic belt between the North China and Yangtze continental blocks, its most important tectonic feature is the development of high pressure(HP) and ultrahigh pressure (UHP) metamorphic belts.In Dabieshan from south northwards there are epidote blueschist facies, greenschist facies,albite epidote amphibolite facies,amphibolite facies,granulite facies and at last HP and UHP eclogite facies rocks constituting the entire high pressure metamorphic facies series.These metamorphic facies with an increasing grade represent the different degree of metamorphism as a consequence of different depth of subduction or the depth of the continental crust concerned.However,the present spatial distribution of the metamorphic rocks of different baric types is highly controlled by the crustal extension process subsequent to the collision,moreover,the doming of the Dabie gneiss complex had resulted in the complexity in tectonic distribution of the metamorphic rocks.At present,the UHP metamorphic rocks often occur as lenses or disrupted layers with different size emplaced within the Dabie gneiss complex of relatively lower pressure.During the collision processes the thermal softening of the root zone of the orogenic belt had resulted in the extrusion of the rock materials from the lithospheric mantle and the lower crust like a tooth paste into the Dabie gneiss complex.The prominent nearly isothermal decompressive microstructures in the Dabieshan UHP metamorphic and its related HP rocks are visual evidences of the extrusion elevation which is responsible for the uplift and exhumation of the UHP metamorphic rocks posterior to the collision event.The ubiquitous migmatization provides good evidence for partial melting which had enhanced the rheology of the rocks in the lower crust and the extrusion elevation of the deep seated rock materials.The development of HP and UHP metamorphic facies rocks suggests a great difference between the south Tongbai-Dabieshan tectonic belt and the north Qinling metamorphic belt located to the north of the Tongbai-Shangcheng Fault(equivalent to the Shangnan-Danfeng Fault in the Shaanxi province).The available 40 Ar 39 Ar dating of the major metamorphic rocks in north Qinling and north Huaiyang has proved a Paleozoic metamorphism of 316～434 Ma ,which is greatly different from the mid Triassic HP and UHP metamorphism of 230～210 Ma,dated by previous researchers working in Dabieshan.Therefore,the amalgamation of these two metamorphic terranes with different ages and different baric types of metamorphism suggests that the major tectonic suture between the North China and the Yangtze continental blocks should be located along the Shangdan-Tongshang Fault.