桐柏-大别造山带高压变质单元岩石Pb同位素组成

铅同位素组成对于研究构造分区与演化、块体相互作用以及识别地壳中不同块体的上、下层次关系等具有重要意义.桐柏-大别造山带高压变质单元岩石的全岩Pb同位素组成研究表明, 在该造山带不同区段, 高压变质岩系二云钠长片麻岩与榴辉岩具有相似的Pb同位素组成, 表现为上部地壳高放射成因的Pb同位素组成特征, 其中Pb同位素组成为: 206Pb/204Pb=17.599~18.310, 207Pb/204Pb=15.318~15.615, 208Pb/204Pb=37.968~39.143.大别和桐柏地区高压变质岩系Pb同位素组成的一致性进一步证明了大别地区与桐柏地区的高压变质岩系是可以相连的, 它们应属于同一构造单元.高压变质岩系Pb同位素比值总体高于超高压变质岩系, 验证了桐柏-大别造山带扬子俯冲陆壳从下部岩系到上部岩系Pb同位素比值呈规律增长这一Pb同位素化学特征.侵入于高压变质岩系中的面理化(含榴)花岗岩, 其Pb同位素组成与高压变质岩系相比相对较低, 而与超高压变质岩系及其中的面理化(含榴)花岗岩相似, 为: 206Pb/204Pb=17.128~17.434, 207Pb/204Pb=15.313~15.422, 208Pb/204Pb=37.631~38.122.这表明高压变质岩系和超高压变质岩系中的面理化(含榴)花岗岩具有相同的岩浆来源.结合面理化(含榴)花岗岩具有A型花岗岩的地球化学特征分析, 它们的岩浆物质可能来自超高压变质岩折返至中下地壳的减压退变和部分熔融. 

Pb Isotope Compositions of Metamorphic Rocks from High-Pressure Unit in Tongbai-Dabie Orogenic Belt

Pb isotope compositions play an important role in tectonic division and evolution, block interaction and identifying different block relations in the crust. Our study shows that Pb isotope compositions of the high-pressure (HP) metamorphic rocks in different parts of Tongbai-Dabie orogenic belt, consisting of two-mica albite gneisses and eclogites, are similar and characterized by upper crustal Pb isotope compositions. They have 206pb/204Pb= 17. 599 - 18. 310, 207pb/204Pb= 15. 318 - 15. 615, 208pb/204Pb=37. 968 - 39. 143. The Pb isotope compositions of the HP metamorphic rocks both in Tongbai and Dahie areas are consistent. It is suggested that the HP metamorphic rocks both in Tongbai and Dabie areas belong to the same lithotectonic unit. In general, the Pb isotope compositions of the HP metamorphic rocks are higher than those of the ultrahigh pressure (UHP) metamorphic rocks. It is evident that the Pb isotope compositions of the rocks from the subducted Yangtze continental crust in Tongbai-Dabie orogenic belt show a regular variation from lower crust to upper crust, from lower radiogenic abundances in the lower crust to higher abundances in the upper crust. The foliated garnet-bearing granites, in-truded into the HP metamorphic rocks in different parts of the orogenic belt have similar Pb isotope compositions. They have 206pb/204Pb= 17. 128 - 17. 434, 207pb/204Pb= 15. 313 - 15. 422 and 208pb/204Pb= 37. 631 - 38. 122, which are obviously different from the compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP metamorphic rocks. This shows that the granites both in the HP and UHP metamorphic rocks have a common magma source. The foliated garnet-bearing granites have the geochemical characteristics of A-type granites, suggesting that the magma of these granites in the UHP and HP metamorphic rocks were derived from the partial melting of the retro-metamorphosed UHP metamorphic rocks exhumed into the middle to lower crust.