阿尔泰造山带花冈岩时空演变、构造环境及地壳生长意义－以中国阿尔泰为例

The Altay orogen, which stretches across the boundary areas among China, Russia, Kazakhstan and Mongolia, is a major unit of the Central Asia Orogenic Belt. Numerous granitoid plutons occur in the orogen. This paper has summarized the spatial and temporal evolution of the plutons and discussed their tectonic setting and implications for crustal growth. According to zircon ages available, these plutons can be divided into three periods, namely early-middle Paleozoic, late Paleozoic and Mesozoic. The early-middle Paleozoic plutons can be subdivided into 470~440 Ma (middle-late Ordovician) and 425~360 Ma (late Ordovician-Late Devonian), the late Paleozoic int0 355~318 Ma (early Carboniferous) and 290~270 Ma (early Permian), and the Mesozoic mainly into 245~190 Ma (middle-late Triassic-early Jurassic). The early-middle Paleozoic (470~360 Ma) plutons that occur widely are mainly of calc-alkaline Ⅰ type subjected to various degrees of deformation, in which 470~440 Ma plutons occur as gneissic bodies. They were formed during syn-accretion orogenic processes, from the early active continental subduction (470~440 Ma) through continuous subduction/back-arc extension (420~380 Ma) to collision (380~360 Ma). The early Carboniferous plutons occur in the southern part of the orogen, and some of them are alkaline and show circular or irregular shape with insignificant deformation. They were emplaced in a late (or post) orogenic setting. The carly Permian plutons mainly occur in the southern part of the orogen, with a few seen in the central part. They are mostly of circular shape and show very weak or no deformation except for a few deformed ones in the Ertix slip zone. The rock types are characterized by Ⅰ and A types in association with mafic intrusions including dykes, displaying a post-orogenic extensional setting. Theearly Mesozoic plutons, showing circular or irregular shape without deformation, are characterized by high-differentiation Ⅰ-type and S-type, associated with rare metal deposits. They were emplaced in an intercontinental setting. Nd isotopic mapping of the plutons indicates that eNd (t) values are lower (-4 to + 2) in central Altay and higher (+1.4 to+6) in southern Altay; correspondingly, t(subscript DM) values decrease from 1.6~1.1 Ga in central Altay to 1.0~0.5 Ga in southern Altay. These results demonstrate that central Altay contains widespread old continental basement, whereas southern Altay comprises a significant proportion of juvenile crust, suggesting that the crust grew southward. The horizontal (lateral) and vertical crustal growth contributed ca. 18%~28% and 7%~8%, respectively, to the area of Chinese Altay. The authors' studies also suggest that the Altay orogen preserved original accretional crustal structures during Mesozoic, without large-scale vertical tectonic super-position. The Altay orogen experienced formation of an active continental margin, splitting of the margin to form a back-arc ocean and the final closing of the back-arc ocean. It is also held that the CAOB underwent not only accretional processes but also back-arc opening and closure of microcontinent margins, which might have been a common process in the central Asian accretionary orogen.