北山独龙包地区晚石炭世陆缘弧岩浆作用——花岗闪长岩锆石U-Pb年龄及地球化学证据

The Continental Arcs Magmatic at the Dulongbao Area in Beishan Orogenic Belt in Late Carboniferous:Evidences from Zircon U-Pb Dating and Geochemistry of the Granodiorite

本文报道了北山地区独龙包南花岗闪长岩的LA-ICP-MS锆石U-Pb年龄与全岩地球化学,以确定其形成时代、岩石成因及构造环境。花岗闪长岩中的锆石呈短柱状或近等轴粒状,具典型的振荡环带,Th/U值介于0.42~0.75之间,指示岩浆成因。对岩浆锆石的定年结果显示,独龙包南花岗闪长岩形成于303.4±2.1 Ma。该期花岗闪长岩的Si O2含量为63.73%~69.12%,Na_2O=3.69%~4.03%,K_2O=2.23%~2.73%,全碱(Na_2O+K_2O)为6.08%~6.44%,Al2O3含量较高(14.83%~15.89%),K2O/Na2O为0.58~0.74,A/NCK=0.91~0.99;该区花岗闪长岩富集轻稀土(LREEs)和大离子亲石元素(LILE)Rb、Th、U、Pb,亏损重稀土元素(HREEs)和高场强元素(HFSE)Nb、Ta、P、Ti,且轻重稀土元素分馏较强,(La/Yb)N变化于4.77~6.35,具有弱的负铕异常,δEu为0.79~0.94。上述特征表明,该期花岗闪长岩属于准铝质钙碱性I型花岗岩,岩浆源区为下地壳,受到地幔物质的混染,形成构造背景为陆缘弧。结合邻区特征,得出红石山—百合山—蓬勃山蛇绿岩带代表的洋盆于晚石炭世可能发生了南北双向俯冲。

Objectives： The Late Carboniferous Dulongbao granodiorite pluton located in the north of Hongshishan—Baiheshan—Pengboshan ophiolite mlangen belt have important geological significance for constraining the tectonic evolution of the Beishan Orogenic Belt,but no report on isotope and tectonic setting about granites.Methods： Based on the field work,through the microscope observation,the whole-rock geochemical analysis and the LA-ICP-MS zircon U-Pb dating of the granodiorite.Results： Zircon grains from these granodiorite are euhedral subhedral in shape,display fine-scale oscillatory growth zonong with high Th/U ratios of 0.42～ 0.75,indicating a magmata origin. U-Pb dating results show that the pluton was emplaced with the age of 303.4±2.1 Ma. The granodiorite have high SiO2contents of 63.73% ～ 69.12%and Al2O3 contents of 14. 83% ～ 15. 89%,Na2O concentrations of 3. 69% ～ 4. 03%,and K2O concentrations of 2.23% ～ 2.73%,with（ Na2O＋K2O） concentrations of 6.08% ～ 6.44% and K2O/Na2O ratios of 0.58 ～ 0.74,A/NCK values of 0.91 ～ 0. 99,Mg#values of 48. 27 ～ 50. 93. They are enriched in light rare earth elements（ LREEs） and large ion lithophile elements（ LILE）,such as Rb,Th,U,Pb,and depleted in heavy rare earth elements（ HREEs）and high field strength elements（ HFSE）,such as Nb,Ta,P,Ti,with（ La/Yb）Nratios of 4.77 ～ 6.35 and weak negative Eu anomalies（ δEu = 0.79 ～ 0.94）.Conclusions： These granodiorites chemically belong to metaluminous and cacl-alkaline I-type ones,and theirprimary magma could be mixed by mantle and crust,formed under the tectonic setting of continental arc in late Carboniferous. Combining neighborhood characteristics, We propose that the ocean basin represented by Hongshishan—Baiheshan—Pengboshan ophiolite mlangen belt may have subducted to north and south in late Carboniferous.