南东帕米尔热斯卡木地区中新世高锶低钇花岗岩:对新生代构造演化的制约

The Miocene adakitic rocks in the Reskam area,SE Pamir:Implications for the Cenozoic tectonic evolution

碰撞后岩浆作用是探索岩石圈物质组成、反演深部地球动力学过程的重要对象。近来,笔者等所在课题组在南东帕米尔热斯卡木地区新识别出一套新生代高锶低钇花岗岩,并分析了该花岗岩的锆石U-Pb年龄、全岩主微量元素和Sr—Nd同位素及锆石Lu—Hf同位素组成。锆石LA-MC-ICPMS U-Pb定年显示,这些岩浆岩为中新世岩浆活动的产物(12.0±0.3 Ma)。元素地球化学显示,样品具有高SiO_(2)(72.14%~74.35%)和K_(2)O(3.78%~5.25%)含量,低MgO(0.13%~0.50%)和Mg#(18~35),高Sr(363×10^(-6)~754×10^(-6)),低Y(3.41×10^(-6)~16.4×10^(-6))和Yb(0.327×10^(-6)~0.903×10^(-6)),从而高Sr/Y(27.1~188)和(La/Yb)N值(18.9~210),与典型Adakite地球化学特征一致。同位素方面,样品具有显著富集的锆石ε_(Hf)(t)(-10.1~-5.4)和全岩ε_(Nd)(t)(-8.33~-6.39)值。综合笔者等及前人研究成果,热斯卡木地区中新世高锶低钇花岗岩是加厚下地壳部分熔融的结果。欧亚大陆碰撞以来,区内地壳显著增厚、高原快速隆升。~12 Ma,由于增厚地壳局部岩石圈重力不稳发生垮塌,软流圈上涌使加厚古老下地壳发生部分熔融,形成该时期的高锶低钇花岗岩岩浆。

Objective:In order to provide constraints for the Miocene tectonic evolution and deep dynamic process in the SE Pamir area,the age of the intrusive rocks in the Reskam area is redetermined and the petrogenesis and magmatic processes are discussed.Methods:Combined with field work and microscope observation,analysis of the LA-MC-ICP-MS zircon U-Pb isotopic dating,XRF major elements,ICP-MS trace elements,MC-ICP-MS Sr-Nd isotopes,LA-MC-ICPMS zircon Lu-Hf isotopes are carried out.Results:Zircon LA-MC-ICP-MS U-Pb dating results show that these magmatic rocks were emplaced at Miocene(12.0±0.3 Ma).Geochemically,they have high SiO_(2)(72.14%~74.35%)and high K_(2)O(3.78%~5.25%)contents,with low MgO(0.13%~0.50%)and Mg#value(18~35).They exhibit high Sr(363×10^(-6)~754×10^(-6)),low Y(3.41×10^(-6)~16.4×10^(-6))and Yb(0.327×10^(-6)~0.903×10^(-6)),thus high Sr/Y(27~188)and(La/Yb)N(18.95~210)ratios,in keeping with the typical adakites.In terms of isotopes,they have significantly enriched in situ zirconε_(Hf)(t)(-10.1~-5.4)and whole-rockε_(Nd)(t)(-8.33~-6.39)values.Conclusions:The Reskam monzogranite was formed in the Miocene(~12 Ma)and has the characteristics of Adakite.The continuous subduction of the Indian plate significantly thickened the crust in the Pamir area.At around 12 Ma,due to the tectonic transformation of the Karakoram fault,the local detachment of the thickened lithospheric mantle in the Pamir area resulted in tectonic extension,triggering the upwelling of the asthenosphere and partial melting of the thickened ancient lower crust.