松潘甘孜造山带早侏罗世的后造山伸展:来自川西牛心沟和四姑娘山岩体的地球化学制约

Post-orogenic extension of Songpan-Garzê orogen in Early Jurassic:Constraints from Niuxingou monzodiorite and Siguniangshan A-type granite of western Sichuan,China

牛心沟岩体和四姑娘山岩体位于松潘甘孜造山带东部,分别为中生代中性和酸性侵入岩。牛心沟岩体具有中等的SiO2含量(53.7%～63.6%),较高的K2O(3.44%～5.06%)和全碱含量(K2O+Na2O=6.55%～8.80%),显示橄榄粗玄岩特征。牛心沟岩体的微量元素Ba(1280～2010μg/g)、Sr(1010～1660μg/g)含量很高,稀土元素分馏明显((La/Yb)N=37.4～67.3),Nb、Ta和Ti明显负异常,并缺乏明显的Eu和Sr负异常,因此属于高Ba-Sr系列范围。牛心沟岩体的Cr(37.6～193μg/g)、Ni(9.13～76.8μg/g)和V(57.1～126μg/g)等过渡元素含量变化较大,并与SiO2含量之间存在负相关关系,且岩体的几个不相容元素对的比值变化较大(Zr/Sm=2.54～29.7、Nb/La=0.13～0.38、Th/Nb=0.53～0.94),表明该岩体可能由幔源基性岩浆与壳源酸性岩浆混合形成。四姑娘山岩体的SiO2含量介于68.9%～73.6%之间,具有较高的全碱含量(K2O+Na2O=7.21%～9.38%)、高场强元素含量和较高的10000×Ga/Al比值(2.34～3.03),同时具有平坦的重稀土分布模式((Gd/Yb)N=0.79～1.43),明显的Eu负异常以及较高的Rb、Th和U含量,该岩体相对低的Ba、Sr、P和Ti含量表明它经历了相当程度的分离结晶作用,反映出A型花岗岩的特征。地球化学特征显示两岩体的形成均有幔源物质或新生地壳物质的参与,其中幔源岩浆的加入为松潘甘孜造山带大量中生代花岗岩的形成提供了热源和部分物源。地幔岩浆的上侵和四姑娘山A型花岗岩体的形成标志着松潘甘孜造山带至少在早侏罗世以前就已处于伸展构造体制。

The Mesozoic Niuxingou pluton （NXG） and Siguniangshan pluton （SGN） are located in the eastern part of Songpan-Garze^ orogen, China. The NXG is shoshonite, with intermediate SiO2 content （53.7% - 63.6% ） and metaluminous characteristics （A/CNK=0.66-1.00）. It contains very high Sr （1010-1660 μg/g） and Ba （1280 - 2010 μg/g） contents, with fractionated REE （（La/Yb） N = 37.4 - 67.3）, and relatively depleted Nb, Ta and Ti contents, and does not show significant negative anomalies of Eu and Sr, exhibiting a strong affinity to high Ba-Sr series. The contents of transition metals Cr （37. 6 - 193μg/g）, Ni （9. 13 - 76.8 μg/g） and V （57. 1 -126 μg/g ） varies greatly and negatively correlates to silicon content. Its highly variable Zr/Sm （2.54- 29.7）, Nb/La （0. 13 -0.38） and Th/Nb （0. 53 -0.94） ratios imply that it was probably produced by mixing of mantlederived mafic magma and crustal melt. The SGN has relatively high SiO2 （68.9% - 73.6% ）, intermediate Al203 （12. 5% - 15.7% ） and low MgO （0. 21% - 1.06% ） contents. It has high total alkali （K2O ＋Na2O =7.21% - 9.38% ）, HFSE and Rb, Th, U contents and possesses high 10000 × Ga/Al ratios （2.34 - 3.03）, with flat HREE pattern （（Gd/Yb） N =0.79 - 1.43） and distinct negative Eu anomaly （δEu =0.23 -0. 69）, showing characteristics of A-type granite. Geochemical characteristics indicate that both NXG and SGN contain mantle components, which provide heat and part of source material for the Mesozoic granites in Songpan-Garze^ orogen. The involvement of mantle-derived magma and occurrence of the SGN A-type granite （ 184. 8 ＋ 3.3 Ma） indicate that the Songpan-Garze^ orogen was under an extensional scheme at least before the Early Jurassic.