阿尔泰造山带花岗岩时空演变、构造环境及地壳生长意义——以中国阿尔泰为例

Spatial and temporal variations of granitoids in the Altay orogen and their implications for tectonic setting and crustal growth:perspectives from Chinese Altay

阿尔泰造山带横跨中、俄、哈、蒙四国边界,是中亚造山带主要组成部分,发育大量的花岗岩等侵入体。本文研究总结这些岩体的时空演变、成因类型和构造环境,并探讨其增生造山和地壳生长意义。依据锆石年龄,这些岩体可大致分为早中古生代的470～440Ma(中晚奥陶世)和425～360Ma(晚志留世—晚泥盆世)、晚古生代的355～318Ma(早石炭世)和290～270Ma(早二叠世)以及早中生代245～190Ma(中晚三叠世—早侏罗世)3个阶段5个期次,其中425～360Ma花岗岩可进一步细分为425～390Ma和380～360Ma两个峰期。早中古生代(470～360Ma)花岗岩体分布广泛,主要为钙碱性I型,多具不同程度变形,其中470～440Ma岩体变形极强(片麻岩体)。它们为同造山俯冲增生产物,形成于活动陆缘俯冲(470～440Ma)、继续俯冲弧后盆地伸展(420～390Ma)到聚合碰撞(380～360Ma)的过程中。早石炭世岩体发育于造山带南部,为不变形圆形状或不规则状,具典型碱性花岗岩特征,为晚(后)造山产物。早二叠世岩体主要发育于阿尔泰造山带南部,少量分布于造山带内部,多为圆形,不变形,少量变形岩体集中在额尔齐斯构造带内,成因类型以I、A型为特点,伴生有大量基性岩脉(体),显示为后造山底侵伸展环境。早中生代岩体为不变形圆形或不规则状,具有高分异I型和S型花岗岩特征,伴有稀有金属矿产,具有板内环境特点。花岗岩体同位素填图显示,阿尔泰中部块体岩体具有较低的εNd(t)值和老的Nd同位素模式年龄(1～1.3Ga),暗示存在古老地壳基底;由北向南εNd(t)值增高,模式年龄变年轻,显示陆壳向南生长,其中水平和垂向生长率分别为18%～28%和7%～8%。中生代时期阿尔泰造山带保留水平增生结构,没有发生大规模构造块体垂向叠覆。阿尔泰造山带经历了古陆缘构造演化,奥陶纪—志留纪陆缘俯冲,泥盆纪陆弧及陆缘边缘裂解、弧后盆地形成,晚泥盆世最终洋盆闭合及早石炭世各块体拼合的演化过程。该研究表明增生造山带中同样存在构造演化的阶段性;中亚增生造山作用不仅具有弧前增生,而且还存在陆缘裂解再拼合作用。

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 into 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 early 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.The early 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 εNd（t） values are lower （-4 to ＋2） in central Altay and higher （＋1.4 to ＋6） in southern Altay; correspondingly,tDM 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 superposition.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.