辽宁鞍山弓长岭地区古太古代花岗岩研究进展——来自野外地质和锆石U-Th-Pb-Hf同位素的证据

The research progress of the Paleoarchean granite in the Gongchangling area: Evidence from geology and zircon U-Th-Pb-Hf isotopic composition

在早期地球(>2.6 Ga)的大陆地壳中,富钾花岗岩系所占的比例十分有限,对古老花岗岩的研究可为揭示大陆地壳演化过程提供重要制约。本文对出露于鞍山弓长岭地区的古太古代富钾花岗岩进行了详细的野外地质和锆石U-Th-Pb-Hf同位素研究,它们的主要矿物组合为钾长石(40%~60%)、斜长石(10%~30%)、石英(约30%)和少量黑云母及白云母,以包体形式与大规模中太古代(2.9 Ga)花岗岩共生,具有与后者相同的构造和变形特征。古太古代花岗岩中绝大多数岩浆锆石的Th、U含量分别为100×10^(-6)~600×10^(-6)、400×10^(-6)~1 500×10^(-6),且Th/U值较高(0.10~0.93),13个谐和数据点加权平均年龄为3 337±24 Ma(MSWD=1.5),代表其侵位时代。另有3.76~3.60 Ga的继承或残余锆石存在。锆石初始^(176)Hf/^(177)Hf_(i)值为0.280 391~0.280 857,一阶段亏损地幔模式年龄(t_(DM1))为3 856~3 327 Ma。Hf同位素组成的变化表明:(1)鞍本地区在古太古代晚期之前已经形成了一个具有多期岩浆作用记录的古老陆核;(2)太古宙大陆地壳的稳定化过程是不同步的,华北克拉通在2.5 Ga形成统一的大陆地壳之前,研究区很可能在古-中太古代就已经完成了区域陆壳的成分分异,并形成了稳定的陆壳结构。

The K-rich granite took limited volumes of the continental crust in the early earth(>2.6 Ga). Studies on ancient granite will provide great evidence for revealing the geological history of the crust. In this paper, we carry detailed studies on the geology and zircon U-Th-Pb-Hf isotopic composition of the Paleoarchean granite, which is exposed in the Anshan Gongchangling area of the North China Craton(NCC). The studied Paleoarchean granite, which is occurring as an enclave in the regional large-scaled of Mesoarchean(2.9 Ga) granite with similar tectonic feature to the latter, is composed of K-feldspar(40%~60%), plagioclase(10%~30%), quartz(30%) and minor mica. Most magmatic zircons from the Paleoarchean granite have varying Th and U contents of 100×10^(-6)~600×10^(-6)and 400×10^(-6)~1 500×10^(-6), respectively, with relatively high Th/U ratios(0.10~0.93). 13 concordant analysis yielded meaning weighted average age of 3 337±24 Ma(MSWD=1.5), interpreted as the emplacement time of the granite. And the inherited zircons formed at 3.76~3.60 Ga. The analyzed zircon grains yielded initial ^(176)Hf/^(177)Hf_(i) ratios of 0.280 391~0.280 857 with Hf depleted mantle modal ages(t_(DM1)) of 3 856~3 327 Ma. The differences of Hf isotopic compositions suggested that(1) an ancient continent nucleus with multiple magmatic records had been formed before the end of the Paleoarchean in the Anshan area;(2) the stabilization of the early crust should not be synchronous during the geological history. Crustal stabilization and differentiation had been carried out locally,early before the amalgamation of the continental basement of the NCC at ~ 2.5 Ga.