滇西腾冲新生代花岗岩:成因类型与构造意义

Cenozoic granitoid in Tengchong,western Yunnan:Genesis type and implication for tectonics.

滇西腾冲地区位于喜马拉雅造山带东构造结的东南弧形构造带内,发育的新生代花岗岩,记录了大量印度-亚洲大陆碰撞的时间信息和东部碰撞带区域构造演化及大陆动力学信息。本区新生代花岗岩可划分出二长花岗岩、正长花岗岩、白云母花岗岩和白云母钠长花岗岩等四种主要岩相类型。最早期侵位的二长花岗岩和白云母花岗岩,^(40)Ar/^(39)Ar年龄为66～58Ma,大规模侵位的正长花岗岩和二长花岗岩,同位素年龄集中在两个时段,即54～52Ma和43～41Ma。二长花岗岩和正长花岗岩ASI(铝饱和指数)接近于1,属偏铝到过铝之间的高钾钙碱性花岗岩,白云母(钠长)花岗岩ASI变化于1．02～2．63,属过铝到强过铝花岗岩;这些花岗岩的K_2O/Na_2O＞1,且K_2O/Na_2O比值和SiO_2含量依次增加;微量元素含量前者具相对高Sr、Ba而低Rb,后者具明显的高Rb和异常的低Sr、Ba,其中,白云母花岗岩以异常高Y为特征,白云母钠长花岗岩以异常高Rb为标志;稀土元素前者REE配分型式具有右倾的LREE富集型,负Eu异常明显,后者具有“燕式”REE配分型式,暗示不同类型之间有着不尽相同的岩浆源岩或熔融机制。这些新生代花岗岩的岩浆序列和岩石组合揭示,青藏高原东部地区碰撞造山经历了复杂的演化历程:(1)印度大陆与亚洲大陆于66～59Ma发生对接并强烈碰撞,导致地壳大幅度加厚和地壳深融;(2)经过大约5Ma的应力调整,于54～52Ma发生碰撞高峰期后的短暂张弛和正长花岗岩浅成侵位;(3)于43～41Ma,张弛加剧,地壳伸展,伴随着二长花岗岩和正长花岗岩的形成发育。

Cenozoic granitoid developed in Tengchong, an area located in the SE arc structural belt of east structural knot of Himalayan orogen, recorded lots of information about the time of India-Asia continental collision, structural evolution and continent dynamics in the area. In term of the lithofacies and intrusion age, the granitoid can be subdivided into monzonite granite, syenogranite, muscovite granite and muscovite-albite granite. The ^40Ar/^39 Ar data indicate that ages of the firstly intrusived monzonite granite and muscovite granite range from 66Ma to 58Ma, and ages of the syenogranite and monzonitic granite, which intrusived with a large scale, range between 54 -52Ma and 43 -41Ma. According to Aluminium Saturation Index （ASI） of the granite, we can conclude that monzonitic granite and syenogranite, which ASI value approaches to 1, belong to metaluminous-peraluminous high K calc-alkaline series, and muscovite（-albite） granite（ ASI： 1.02 - 2.63） are peraluminous-strongly peraluminous granite. K2O/ Na2O value of all the granitoid exceed 1, but from syenogranite, monzonitic granite to muscovite （-albite） granite, K2O/Na2O value and SiO2 content increase gradually. The syenogranite and monzonitic granite are rich in Sr, Ba and poor in Rb, with light-REE enriched and strongly negative Eu anomalies. In constrast, muscovite（-albite） granite are characterized by obviously higher B.b, unusually lower Sr and Ba, and swallow-shaped REE pattern, suggesting that they either originated from different magma reservoir or experienced different melt process. According to magma sequence and rock assemblage, we can conclude ： （ 1 ） during 66 - 59Ma, contact and strong collision between India and Asia continent happened, resulting in crust thickening greatly and melting in deep part; （2） after 5Ma stress adjusting, short-lived relaxation and hypergene emplacement of syenogranite occurred between 54Ma and 52Ma; （3） during 43 - 41Ma, crust extension appeared, accompanying with development of monzonite granite and syenogranite.