桐柏高压变质地体:对桐柏-大别-苏鲁高压/超高压变质带构造框架和俯冲/折返机制的制约

The Tongbai HP metamorphic terrane:Constraints on the architecture and subduction/exhumation of the Tongbai-Dabie-Sulu HP/UHP metamorphic belt

桐柏造山带位于秦岭造山带和大别-苏鲁造山带之间,是揭示秦岭-桐柏-大别-苏鲁巨型造山带中各地质体之间构造关系及地质演化差异的关键地区。桐柏高压变质地体主要由两个高压岩片(I和II)及其北侧的构造混杂岩带和南侧的蓝片岩-绿片岩带构成。高压岩片I以北、南两条榴辉岩带为代表,构成桐柏山背形构造的两翼,其峰期变质条件分别为530～610℃、1.7～2.0GPa和460～560℃、1.3～1.9GPa。高压岩片II以桐柏杂岩中的变质岩包体为代表,其峰期变质条件推测在<700℃、>1.2GPa的榴辉岩相范围内,而退变质条件为660～700℃、0.80～1.03GPa。U-Pb、Lu-Hf、Rb-Sr和Ar-Ar同位素年代学研究表明,高压岩片I的峰期变质时代为255Ma,冷却至白云母封闭温度的时代为238Ma;而高压岩片II的主期变质作用发生在232～220Ma,作为桐柏杂岩主体的片麻状花岗岩则侵位于140Ma。这说明,高压岩片I和II分属于两个时代不同的俯冲/折返岩片,当高压岩片II被俯冲到地壳深处并经受高压变质时,其上覆的高压岩片I已经折返到中上地壳的水平。这一结果验证了在西大别、东大别和苏鲁地区提出的高压/超高压岩石的穿时(或差异)俯冲/折返模型,同时说明华南大陆地壳最早的俯冲发生在晚二叠世,这也代表华北与华南陆块之间从洋壳俯冲转化为陆壳俯冲的时间。基于桐柏杂岩与北大别杂岩的可比性,认为桐柏高压变质地体相对低温低压的变质环境以及超高压岩石的缺乏缘于华南陆块的俯冲深度向西逐渐变浅,而早白垩世的构造挤出造成了桐柏-大别高压/超高压变质带东宽西窄的构造格局。

The Tongbai orogen is situated at the junction between the Qinling and Dabie-Sulu orogenes;hence it may hold a key to understanding the geological relationships and tectonic evolution of different terranes in the Qinling-Tongbai-Dabie-Sulu orogenic belt.The Tongbai high-pressure （HP） metamorphic terrane comprises two HP slices （I and II） and a tectonic mélange zone in the north and a blueschist-greenschist zone in the south.The HP slice I is represented by the northern and southern eclogite zones occurring on the two sides of the Tongbaishan antiform.The peak metamorphic conditions are estimated to be 530~610℃ and 1.7~2.0GPa for the northern zone and 460~560℃ and 1.3~1.9GPa for the southern zone.The HP slice II is represented by the metamorphic enclaves in gneissic granites in the Tongbai complex.Their peak metamorphic conditions are inferred to be within the eclogites facies field of 700℃ and 1.2GPa,and retrograde P-T conditions at 660~700℃ and 0.80~1.03GPa.U-Pb,Lu-Hf,Rb-Sr and Ar-Ar multichronometric data indicate the peak metamorphism of the HP slice I taking place at 255Ma,whereas cooling to the closure temperature of muscovite at 238Ma.By contrast,the metamorphic ages of the HP slice II are as young as 232~220Ma,whereas gneissic granites dominating the Tongbai complex were emplaced at 140Ma.This suggests that,when the HP slice II was subducted to deep levels and underwent HP metamorphism,the HP slice II overlain it might have been exhumed to mid-to upper-crustal levels.This testifies a diachronous （differential） subduction/exhumation model of different HP and ultrahigh-pressure （UHP） slices proposed for the western Dabie,eastern Dabie and Sulu regions.Furthermore,the above data demonstrate the commencement of subduction of the South China continental crust at the Late Permian,which also represents the transitional time of oceanic to continental subduction between the North and South China blocks.Based on the similarity between the Tongbai and North Dabie complexes,we consider that a lower P-T environment and the absence of UHP rocks in the Tongbai HP metamorphic terrane might be resulted from the shallower subduction of the South China block towards the west.The final west-narrowing architecture of the Tongbai-Dabie HP/UHP metamorphic belt is related to the Early Cretaceous extrusion towards the east.