华北克拉通北缘少郎河地区约2.5 Ga的二长花岗岩及地质意义

本文首次在华北克拉通北缘少郎河地区发现新太古代末期(约2.5 Ga)二长花岗岩，并对其进行了锆石U Pb定年、主微量和锆石Hf同位素原位微区分析。研究表明，这套二长花岗岩形成年龄为(2 527±25) Ma，属于新太古代末期。岩石整体为块状构造，局部呈似片麻状构造，中粗粒结构，并可见暗色包体。主量元素显示其富Na和K，低P，A/CNK=0.80～0.96，属于高钾钙碱性系列。微量元素方面，富集轻稀土元素并基本无Eu异常，富集Rb、Sr、Ba等大离子亲石元素，亏损Nb、Ti、Ta、P等高场强元素。锆石Hf同位素分析结果显示二长花岗岩εHf(t)值在-4.0～4.8，均值为-0.6，对应的两阶段模式年龄(TDM2)在2 712～3 221 Ma，暗示二长花岗岩可能为中新太古代新生下地壳熔融后结晶分异的产物。结合近年来少郎河地区新发现太古宙绿岩带和TTG岩石，提出在该地区存在前寒武基底，存在形式可能为一古老的微陆块。二长花岗岩的形成是新太古代末期(约2.5 Ga)华北陆块克拉通化的重要标志。虽然二长花岗岩地化特征具有俯冲背景的特点，我们依然认为在新太古末期华北微陆块之间仅有有限的和小规模的弧陆或陆陆的俯冲和碰撞，与板块构造的体制有较大的差别。

The ~2.5 Ga monzogranite in the Shaolanghe area of the northern margin of the North China Craton and its geological significance.

In this study, the ~2.5 Ga late Neoarchaean monzogranites have been identified for the first time in the Shaolanghe area of the northern margin of the North China Craton (NCC). Analyses of in situ zircon U Pb ages, Hf isotopic data and whole rock major and trace element contents have been performed on the monzogranites. U Pb dating of magmatic zircons indicates that the monzogranites formed in 2527±25 Ma, in the late Neoarchaean. Their massive structures contain partly gneissic like and medium coarse grain structures with sporadic dark inclusions. The rocks have high Na and K, low P, a A/CNK ratio of 0.800.96, and belong to the high K calc alkaline series. The monzogranites exhibit enriched light rare earth elements (LREEs) with no δEu anomalies, and are enriched in large ion lithophile elements (LILEs) (e.g., Rb, Sr and Ba), and depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, P and Ti). The zircon εHf(t) value and two stage Hf depleted mantle model age (TDM2) range from 4.0 to 4.8 and 2712 to 3221 Ma, respectively, suggesting that the monzogranites were derived from fractional crystallization after melting of the juvenile Mesoarchaean and Neoarchaean lower crust. Relating to the newly discovered greenstone belt and TTG igneous rocks of the Archean in the Shaolanghe area in recent years, this paper proposes that there exists a Precambrian basement in the Shaolanghe area, possibility in the form of an ancient microblock. The formation of the monzogranites is an important symbol of cratonization at the end of the Neoarchaean in the NCC. Although the geochemical characteristics of the monzogranites indicate their probable association with subduction, we consider such subduction or collision between arc and continent or two continents limited and at a small scale in the late Neoarchaean, quite different from the tectonic plate system.