中国阿尔泰造山带花岗岩Pb同位素组成特征:幔源成因佐证及陆壳生长意义

Pb Isotopic Composition of Granitoids from the Altay Orogen (China): Evidence for Mantle-derived Origin and Continental Growth

为进一步对阿尔泰造山带花岗岩进行物源示踪研究，本文选择几个较典型的同造山和后造山不同类型的花岗岩以及相伴生的基性岩进行长石Pb同位素的测定。结果显示花岗岩^204Pb／^204Pb范围为17．997～18．921，平均值为18．269；^207Pb／^204Pb范围为15．460～15．599，平均值为15．528；^208Pb／^204Pb范围为37．661～38．262，平均值为37．954；其弘值为9．19～9．71，集中于9．30～9．60，与典型的壳源花岗岩明显不同。在源岩判别图解上，主要落在洋岛玄武岩和岛弧玄武岩的范围内，所有点远离上地壳、下地壳和深海沉积物，其源区性质类似于洋岛玄武岩和岛弧玄武岩，与花岗岩同时代的伴生基性岩Pb同位素也具有相似的特征，说明两者可能具有相似的物源特征，即幔源组分。这与报道的Sr、Nd同位素的特征相一致，进一步证明阿尔泰花岗岩具有幔源组分。这种特点与其他造山带（如华南、喜马拉雅）明显不同，显示阿尔泰花岗岩的特殊性。该研究从另一个侧面证明中亚造山带存在一定规模的显生宙陆壳生长。

In the study of the sources of the Altay granitoids, Pb isotopic composition of feldspar from various typical syn-orogenic and post-orogenic granites and mafic intrusions from the Altay orogen are analyzed. The results show that the granites have ^206Pb/^204Pb values between 17. 997 and 18. 921 （averaging 18. 269）, ^207Pb/^204Pb values between 15. 460 and 15. 599 （ （averaging 37. 954） and μ values from these samples are mostly located in the averaging 15. 528）, ^208Pb/^204Pb values between 37. 661 and 38. 262 9.19 to 9.71 （mainly from 9.30 to 9.60）. In various lead diagrams, fields of oceanic island volcanic rocks and arc volcanic rocks, and all are far away from the upper crust, lower crust and pelagic sediments area. These suggest that the sources of the granitoids are similar to those of oceanic island volcanic rocks and arc volcanic rocks. In addition, the Pb isotopic composition of marie intrusions being coeval with the granitoids has similar features. This indicates that the granitoids and marie rocks have sources with the same or similar （mantle-derived） features. The result suggests that juvenile mantle-derived compositions have contributed to the formation of granites. This is consistent with previous studies related to Sr and Nd isotopes. Clearly, these granitoids differ from those from other orogens, such as the South China orogen and Himalayan orogen. This study provides new evidence for continental crustal growth in the central Asian orogenic belt during the Phanerozoic.