川西老君沟和孟通沟花岗岩的地球化学特征、成因机制及对松潘-甘孜地体基底性质的制约  

Geochemistry and petrogenesis of Laojungou and Mengtonggou granites in western Sichuan,China:constraints on the nature of Songpan-Ganzi basement.

作　　者：赵永久[1] 袁超[1] 周美夫[2] 颜丹平[3] 龙晓平 李继亮[4] 

机构地区：[1]中国科学院广州地球化学研究所 [2]香港大学地球科学系 [3]中国地质大学地球科学与资源学院,北京100083 [4]中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2007年第5期995-1006,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金(项目编号40572043;40421303;40334044);中国科学院领域前沿项目(编号GIGCX-03-01)共同资助

摘　　要：老君沟岩体和孟通沟岩体是位于松潘-甘孜地体东部的中生代花岗岩,为碰撞过程中岩浆作用的产物,包含有地壳基底性质的重要信息。两岩体具有中等至较高的SiO_2含量(63.9～75.0wt%),具有准铝质和轻微过铝质(ACNK=0.86～1. 08)特征,属于中钾到高钾钙碱性系列。其中,老君沟岩体富Al_2O_3(14.2～16.0wt%)、K_2O(2.88～4.36wt%),贫MgO(0.38～1.98wt%),并具有高Sr(307～697ppm)低Y(12.2～21.0ppm)特征以及中等至较高的稀土分馏((La/Yb)_N>10),这些特征非常类似于下地壳熔融形成的埃达克岩。孟通沟岩体的主微量元素特征类似于老君沟岩体,但具有相对低的Sr(110～397 ppm)、稍高的Y(12.2～21.0ppm)和明显较低的Sr/Y比值(5～32)。两岩体均具有地壳特征的Nd-Sr同位素组成(ε_(Nd)(T)= -6.02～-3.11;I_(Sr)=0.7053～0.7075),排除了直接来自地幔岩浆的可能性,表明其可能来自于石榴石稳定区地壳物质的部分熔融。两岩体富含Rb(106～202ppm)、Cs(3.79～25.6ppm)而K_2O/Na_2O(0.56～1.12)比值较高,显示其可能来自黑云母的脱水熔融。由于黑云母的脱水熔融温度较高而剪切生热所能达致的温度较低,同造山的滑脱构造难以导致上述两个岩体的形成,地幔岩浆的底侵作用可能是导致下地壳物质部分熔融的主要原因。两岩体的源区成分和Nd模式年龄(T_(DM(Ⅱ))=1.23～1.44 Ga)均反映松潘-甘孜地体具有类似于扬子板块的中元古陆壳基底。The Mesozoic Laojungou and Mengtonggou granitic plutons in the eastern part of the Songpan-Ganzi terrane represent products of a major orogenic granitic magmatism and contain important information about the nature of the basement. These granites are intermediate- to high-K calc-alkaline, with intermediate to high SiO2 （ 63.9 - 75.0 wt% ） and metaluminous to mildly peraluminous characteristics （ ACNK = 0.86-1.08 ）. Rocks of the Laojungou pluton contain high A12 03 （ 14.2 - 16, 0 wt% ） and K2O （ 2.88- 4.36 wt% ）but low MgO （0.38 - 1,98 wt% ）. The Laojungou pluton has relatively high Sr （307 -697 ppm） and low Y （12.2 -21.0 ppm） with fractionated REE （ （La/Yb）N 〉 10） and relatively high Sr/Y ratios （22-55）, and resembles adakitic crustal melts. The Mengtonggou pluton is quite similar to the Laojungou pluton in most major and trace element characteristics, except for its relatively low Sr （110 - 397 ppm）, high Y （12.2 -21.0 ppm） and strikingly low Sr/Y ratios （5 -32）. The two plutons all exhibit crustal Nd-Sr isotope compositions （ SNd （T） = - 6.02 - - 3. 11 ; ISr = 0. 7053 - 0. 7075 ）, indicating that these granites were not mantle-derived, but most likely resulted from partial melting of lower crustal materials in the garnet stability field, The high Rb （ 106 - 202 ppm）, Cs （3.79 -25.6 ppm） contents and intermediate to high K2O/Na2O ratios （0, 56 - 1.12） of both plutons are consistent with their generation from dehydration of biotite. The relatively high temperature for biotite dehydration precludes shear-heating as the cause for the genesis of these plutons, and requires an underplating or intraplating scheme of mantle-derived magma to fuse the crustal materials. The two plutons possess similar, Mid-proterozoic TDM ages （ TDM（ I1 ） = 1. 23 - I. 44 Ga）, which together with their geochemical compositions, indicate that the Songpan-Ganzi terrane is underlain with the Yangzi-type basement.

关 键 词：松潘-甘孜地体 花岗岩 源区成分 ND模式年龄 下地壳部分熔融 底侵作用 基底性质 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]