湘西南苗儿山岩体北段印支期花岗岩成因:来自地球化学、U-Pb年代学证据

Genesis of Indonisian granite in the northern section of Miaoershan pluton in Southwestern Hunan Province:evidence from geochemistry and U-Pb geochronology

苗儿山岩体北段处于湖南境内,南段主要位于广西境内,岩体总出露面积约1600 km2。该岩体按大地构造位置处于扬子地块东南缘江南造山带与华夏地块的结合部位,为晋宁期、加里东期、印支期和燕山期花岗岩组成的复式岩体,其中印支期花岗岩的成岩年龄为210.6±1.6 Ma。印支期花岗岩Si O2含量(74.72%)较高,铁镁质成分TFe O和Mg O含量(1.16%和0.24%)低,K/Na比值(1.41)大于1,铝饱和度(A/CNK)大于1,富集Rb、K、Th、U和稀土元素,强烈亏损Ba、Nb、Sr、P、Ti元素,Nb/Ta比值(3.98)、Zr/Hf比值(29.83)及δEu值(0.15)较小。分异指数(92.74)大,固结指数(2.47)。(87Sr/86Sr)i为0.725717,大于大陆地壳(87Sr/86Sr)i平均值0.719,εNd(t)(-10.8)较小。t2DM(两阶段Nd模式年龄)为1.87 Ga。印支期花岗岩的主微量元素特征、全岩Sr-Nd同位素特征共同显示其为过铝质壳源S型花岗岩,与典型的华南壳源花岗岩的特征相符。印支期花岗岩形成于碰撞挤压峰期之后挤压减弱、应力松弛的后碰撞构造环境。

Miaoershan pluton, of which the northern part is located in Hunan Province, while its southern part is mainly located in Guangxi province, exposed totally about 1600 km2. The pluton is a complex pluton composed of Jinning period, Caledonian, Indosinian and Yanshan period granites, and is located at the binding site of Jiangnan orogenic belt of southeastern margin of the Yangtze block with Cathaysia Block. The forming age of Indosinian granite is 210.6 ＋ 1.6 Ma. The Si O2 content（74.72%） of Indosinian granite is relatively high, while its femic components content（TFe O and Mg O（1.16% and 0.24%）） are low,and its K/Na ratio（1.41） is larger than 1, and its aluminum saturation（A/CNK） is larger than 1. The Indosinian granite is enriched in Rb, K, Th, U and rare earth elements, and strongly depleted in Ba, Nb, Sr, P, Ti, and its Nb/Ta ratio（3.98）, Zr/Hf ratio（29.83）, and δEu ratio（0.15） are small. The differentiation index（93.74） of Indosinian granite is large, while the consolidation index（2.47） is small. The（^87 Sr/^86 Sr）iof Indosinian granite is 0.725717, greater than the average（^87 Sr/^86 Sr）iof the continental crust（0.719）, and the εNd（t）is small（-10.8）. T2 DM（two stage Nd model age） is 1.87 Ga. The characteristics of major and trace elements, and the Sr-Nd isotopic characteristics of the Indosinian granite show that it is peraluminous crust source S type granite, consistent with typical characteristics of crust derived granite in South China. Indosinian granite is formed in the post-collision environment, of which the compression is weakening and the stress is relaxing, after the collisional compressional peak.