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    湘北华容地区桃花山花岗岩体年代学及地球化学特征

    Zircon SHRIMP dating and geochemical characteristics of Taohuashan granites in Huarong County, northern Hunan

    • 摘要: 提要:分布于湖南北部华容隆起中的桃花山岩体侵位于中元古代冷家溪群中,由早期细粒少斑状角闪石黑云母花岗闪长岩、细中粒斑状黑云母二长花岗岩、中细粒少斑状二云母二长花岗岩至末期细粒黑(二)云母二长花岗岩等四期侵入体组成。对该岩体中部的中粒斑状黑云母二长花岗岩进行锆石SHRIMP U-Pb测年,获得203Pb/238U加权平均年龄为(140.3±3.8)Ma(2σ)(n=7,MSWD=2.3),代表岩体侵位年龄,表明其成岩时代为晚侏罗世。岩石地球化学特征SiO2=71.10% ~73.00%,K2O/Na2O=1.09~1.66,属过铝质高钾钙碱性系列;岩石明显富集大离子亲石元素,亏损高场强元素,Rb/Sr=0.23~1.95;∑REE较高(175.1~259.9),Eu为弱负异常(δEu=0.55~0.82),(La/Yb)N=13.52~43.71;低εNd(-8.1~-10.1),高T2DM(1.59~1.75 Ga)。综合研究表明,桃花山花岗岩,属壳源含白云母过铝花岗岩类(MPG),为华南前寒武纪基底重熔而成;其形成的构造背景应为碰撞造山作用晚期或结束时期,陆内碰撞造山作用增温减压体制下的产物,同时也标志着湘北地区在140 Ma后全面转入陆内伸展阶段。

       

      Abstract: Abstract: The Taohuashan granitic pluton in northern Hunan which intruded into Middle Proterozoic Lengjiaxi Formation is composed of early fine-grained sparsely-porphyritic hornblende biotite granodiorite, fine- and medium-grained porphyritic biotite monzonitic granite, medium- and fine-grained sparsely-porphyritic two-mica monzonitic granite and late fine-grained biotite-two-mica monzonitic granite. Zircon SHRIMP dating of the granites yields an age of (140.3±3.8) Ma, belonging to Late Jurassic. The granites belong to the peraluminous High-K calc-alkaline series with SiO2 being 71.10%~73.00% and K2O/Na2O 1.09~1.66. The rocks are enriched in large-ion lithophile elements and depleted in high field-strength elements, with the data Rb/Sr=0.23~1.95, ∑REE=175.1~259.9, δEu=0.55~0.82, (La/Yb)N=13.52~43.71, εNd=-8.1~-10.1), T2DM=1.59~1.75 Ga. Studies show that the Taohuashan granites belong to muscovite-bearing peraluminous granitoids whose materials were derived from Precambrian basement. The rocks were formed with the weakening of the stress in late collision period, suggesting that northern Hunan has entered into an extensional period since 140Ma BP.

       

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