北山柳园地区双峰山早泥盆世A型花岗岩的确定及其构造演化意义

北山柳园地区分布有大量的早中古生代花岗岩类岩石.柳园双峰山岩体具有高硅、高碱(AR=3.99～5.05,NK/A>0.85)、高FeOT/MgO比值和10 000×Ga/Al值、低Al2O3、贫CaO和MgO的特征,显示出准铝质、碱质花岗岩的特点;∑REE较高,LREE略富集,轻重稀土元素分馏不十分明显,Eu负异常明显;相对富集Rb、K、Pb等大离子亲石元素(LILE),强烈亏损Ba、Sr、P、Eu、Ti,弱亏损Ta、Nb等元素;同时具有较高的Rb/Nb和Y/Nb比值,显示了A2型铝质花岗岩的特征.采用LA-ICP-MS锆石U-Pb定年方法,获得双峰山岩体的206Pb/238U年龄为415±3 Ma(MSWD=1.5),代表该岩体的形成年龄,即双峰山岩体形成于早泥盆世.地球化学及Nd同位素特征综合分析显示,该岩体可能由幔源岩浆底侵导致上覆地壳物质(可能由洋壳和岛弧建造组成)部分熔融形成的花岗闪长质岩浆经进一步结晶分异作用形成,为该区较早的钙碱性花岗岩演化到后期的产物.岩体特征、年代学、地球化学和地质背景综合分析结果表明,该岩体形成于后造山或造山作用演化晚期阶段.双峰山早泥盆世A型花岗岩为目前北山地区发现的最老的A型花岗岩,这对探讨古生代花岗岩成因类型及岩浆演化具有重要的意义.

Identification of the Early Devonian Shuangfengshan A-type granites in Liuyuan area of Beishan and its implications to tectonic evolution

Lots of early-middle Paleozoic granitoids occur in Liuyuan area of Beishan. The Shuangfengshan granitic pluton in Liuyuan is characterized by high Si, alkali (AR = 3.99～.05, NK/A>0.85), FeO~T/MgO and 10 000× Ga/Al values, and low Al_2O_3, CaO and MgO content, showing metaluminous and alkali features. The granites show high REE, slight richness of LREE, a weak fractionation between LREE and HREE, and an evidently negative Eu anomaly. They are enriched in Rb, Th, Zr and Hf, strongly depleted in Ba, Sr, P, Eu and Ti, and slightly depleted in Ta and Nb. These rocks also have high Rb/Nb and Y/Nb ratios, implying the aluminous A_2-type granite. The LA-1CP-MS zircon U-Pb dating yielded a ~(206)Pb/~(238)U age of 415 ± 3 Ma(MSWD = 1.5), considered to be the emplacement age of the Shuangfengshan granitic pluton. All these geochemical features indicate that these granites probably resulted from the earlier calc-alkaline granitic magmas which might crystallized from differentiation of granodioritic magmas formed by partial melting of continental crust (oceanic crust and island arc) due to the underplating of the mantle-derived magma. Based on characteristics of outcrops, chronology, geochemistry and geological setting, the authors hold that the Shuangfengshan granitic pluton might be a post-orogenic or late stage orogenic pluton. The Early Devonian Shuangfengshan A-type granite is the oldest A-type granitic pluton ever found in Beishan, which provides important information for the researches on magmatic and tectonic evolution.