冀北大囫囵古元古代花岗岩地球化学特征、岩石成因及其构造意义

Geochemistry,Petrogenesis and Tectonic Significance of the Paleoproterozoic Granites in Dahulun,North Hebei Province

冀北西部陆块东北缘局部地区发现古元古代花岗岩体,由于缺少系统研究,其地球化学特征及地质意义尚不清晰。本文对冀北大囫囵古元古代花岗岩的锆石U-Pb年代学、元素地球化学和Sr-Nd-Pb同位素进行了系统研究。结果显示:两个花岗岩样品的锆石U-Pb年龄为(1855±7)Ma和(1844±8)Ma。岩石高硅(w(SiO_(2))为71.58%~74.01%)、高钾(w(K_(2)O)为6.44%~7.07%,K_(2)O/Na_(2)O为3.68~4.20),高铝饱和指数(A/CNK为1.12~1.27),为钾质强过铝质岩石;花岗岩富集轻稀土和大离子亲石元素,亏损重稀土和高场强元素,结合高TFeO/(TFeO+MgO)值(0.74~0.80)和高10000 Ga/Al值(3.36~3.55)及较高的锆石饱和温度(840~873℃),确定其具有A 2型花岗岩特征,暗示其形成于碰撞造山后伸展构造体制。花岗岩样品具有较高的Sr初始值((^(87)Sr/^(86)Sr)i为0.7130~0.7162)、负的ε_(Nd)(t)值(-8.0~-7.6),Nd二阶段模式年龄T_(DM 2)为2.96~2.94 Ga,(^(206)Pb/^(204)Pb)t、(^(207)Pb/^(204)Pb)t、(^(208)Pb/^(204)Pb)t值分别为14.976~15.178、15.192~15.228和35.413~36.626。综合研究表明,大囫囵钾质强过铝质花岗岩岩浆源自约2.95 Ga的中太古代下地壳物质部分熔融,形成于华北克拉通东部陆块和西部陆块碰撞造山后的伸展构造环境。

Paleoproterozoic granites were discovered in localized areas of the northeastern margin of the landmass in the western part of north Hebei Province.However,systematically studies on their geochemical characteristics and geological significance are still unclear.Zircon U-Pb geochronology,geochemistry,and Sr-Nd-Pb isotopes analysis on the Paleoproterozoic granites in the Dahulun area,north Hebei Province,have been systematically studied.Two granite samples have been dated to(1855±7)Ma and(1844±8)Ma,and are rich in silicon(w(SiO_(2))=71.58%-74.01%)and potassium(w(K_(2)O)=6.44%-7.07%),with K_(2)O/Na_(2)O ratios from 3.68 to 4.20 and A/CNK values from 1.12 to 1.27,displaying characteristics of strongly peraluminous,relatively potassic granites.They are enriched in light rare earth elements and large ion lithophilic elements,depletion in heavy rare earth elements and high field strength elements.Additionally,they show high TFeO/(TFeO+MgO)(0.74-0.80),and 10000×Ga/Al ratios(3.36-3.55)and high zircon saturation temperature(840-873℃),which are typical characteristics of A 2-type granite in a post-collisional extensional setting.All sample have high initial^(87)Sr/^(86)Sr values of 0.7130-0.7162 and negativeε_(Nd)(t)values of-8.0 to-7.6,with two-stage model ages(T_(DM2))ranging from 2.96 to 2.94 Ga.The(^(206)Pb/^(204)Pb)t,(^(207)Pb/^(204)Pb)t and(^(208)Pb/^(204)Pb)t ratios were 14.976-15.178,15.192-15.228 and 35.413-36.626,respectively.The comprehensive study shows that the Dahulun granitic magma originated from the partial melting of the~2.95 Ga Mesoarchean lower crust and was produced in an extensional tectonic setting following the collision between the Eastern and Western blocks of the North China Craton.