内蒙古南炮台花岗斑岩成因与构造环境:锆石U-Pb年代学、Hf同位素和全岩元素组成制约

Petrogenesis and Tectonic Setting of the Nanpaotai Granite Porphyry in Inner Mongolia:Constraints from Zircon U-Pb Geochronology,Hf Isotopes,and Whole-rock Geochemistry

华北板块北缘中段多伦地区记录了强烈的早白垩世早期岩浆作用,构造环境的确立有助于解决其是受控于蒙古—鄂霍次克构造体系还是古太平洋板块俯冲上的争议。本文对多伦地区南炮台花岗斑岩进行了LA-ICP-MS锆石U-Pb年代学、Hf同位素和全岩元素组成研究。结果显示,花岗斑岩岩浆锆石^(206)Pb/^(238)U加权平均年龄为(136±1)Ma,代表南炮台花岗斑岩形成于早白垩世早期。锆石εHf(t)值变化于-8.1~+1.9之间,Hf同位素地壳模式年龄(TDMC)为1044~1680 Ma。南炮台花岗斑岩属弱过铝质钾玄岩系列,富SiO2(70.53%~72.72%)、富碱(K_(2)O+Na_(2)O=9.09%~9.48%)、贫MgO(0.19%~0.39%)、CaO(0.55%~0.67%)和P2O5(0.19%~0.39%),富集轻稀土元素和大离子亲石元素(K、Cs和Rb)、亏损高场强元素(Nb、Ta和Ti)及Sr、P元素,其(La/Yb)N介于22.10~57.67之间,Eu负异常(δEu=0.38~0.52)。综合地球化学特征,表明南炮台花岗斑岩为板内非造山伸展构造环境下的产物,属A型花岗岩,原始岩浆起源于古老下地壳物质的部分熔融,且存在幔源物质加入。结合区域构造演化,认为多伦地区早白垩世早期岩浆作用与蒙古—鄂霍次克洋闭合后的某个伸展环境有关,指示蒙古—鄂霍次克构造体系向南东的影响范围已波及至华北板块北缘中段内蒙古东南部地区。

Intensive magmatism in early Early Cretaceous is recorded in the Duolun area,which is located in the middle segment of the northern margin of North China Plate.Interpretation of the tectonic setting can aid to resolve the controversy whether the magmatism is controlled by the Mongol-Okhotsk tectonic activities or by the subduction of the Paleo-Pacific Plate.This paper presents the LA-ICP-MS zircon U-Pb geochronology,Hf isotopes,and whole-rock geochemistry of the Nanpaotai granite porphyry in Duolun.Zircons U-Pb dating on the granite porphyry shows a weighted mean age of 136±1 Ma,indicating that it was formed in early Early Cretaceous.The zirconεHf(t)values vary between-8.1 and+1.9,with Hf isotopes crustal model ages(TDMC)ranging from 1,044 to 1,680 Ma.The Nanpaotai granite porphyry belongs to slightly peraluminous shoshonite series,enriched in SiO2(70.53%-72.72%),total alkali(K_(2)O+Na_(2)O=9.09%-9.48%),but poor in MgO(0.19%-0.39%),CaO(0.55%-0.67%),and P2O5(0.19%-0.39%).It is also enriched in LREE and LILEs(e.g.,K,Cs and Rb),depleted in HFSEs(e.g.,Nb,Ta and Ti),Sr and P elements,with(La/Yb)N ratio varying between 22.10 and 57.67,and showing a negative Eu anomaly(δEu=0.38-0.52).Comprehensive geochemical evidence indicate that the Nanpaotai granite porphyry was formed in the intraplate anorogenic extensional tectonic environment and belongs to A-type granite.The primary magma mainly derived from the partial melting of the ancient lower crust,and there was mixing of mantle-derived components.Combined with the regional tectonic evolution,it is considered that the early Early Cretaceous magmatism in Duolun is related to an extensional tectonic environment after the closure of Mongol-Okhotsk Ocean.This suggests that the influence of the Mongol-Okhotsk tectonic system along the southeast direction had extended to Southeastern Inner Mongolia in the middle segment of the northern margin of North China Plate.