江南造山带乌石垄岩体的成因研究及成矿意义

Genesis and metallogenic significance of the Wushilong pluton in the Jiangnan Orogenic Belt

乌石垄岩体是江南造山带东北端一个具有代表性的成矿岩体。地质特征表明乌石垄岩体沿太平复式向斜轴部侵入,主要岩石类型为花岗闪长岩,次为二长花岗岩。锆石U⁃Pb定年显示,花岗闪长岩年龄在148.9±2.0 Ma~148.0±2.5 Ma之间,显示花岗闪长岩形成于燕山早期。地球化学特征显示,两类岩石均属高钾钙碱性和过铝质岩石。花岗闪长岩具有高SiO_(2)(>65%),高Sr/Y比值(样品平均值>21.7)和(La/Yb)_(N)(样品均值>15.33),高Al_(2)O_(3)(均值>15%),低MgO(<1.86%),低Yb(<2.50×10^(-6))和Y(<23×10^(-6)),低Sr(292.38×10^(-6)~424.60×10^(-6))含量,与典型的埃达克质岩相比存在一定差异;而二长花岗岩具有较低Al_(2)O_(3)(<13.5%),低Yb(1.29×10^(-6)~2.42×10^(-6)),低Sr(30.99×10^(-6)~44.10×10^(⁃6)),而高Y(10.42×10^(-6)~129.75×10^(-6)),低Sr/Y比值(0.34~2.97),低(La/Yb)_(N)(3.32~7.41)等特征,故其不是埃达克岩。花岗闪长岩的稀土元素配分图呈一致的右倾型,具微弱的Eu负异常,Ba总体弱亏损,P、Nb、Sr、Ti表现出较强亏损;二长花岗岩的稀土元素配分图呈较平坦的海鸥型,具有显著的负Eu异常,Ba、Nb、P、Sr、Ti表现出更显著的亏损。主体岩性为Ⅰ型花岗岩,次为A型花岗岩。花岗闪长岩岩浆源自壳幔混源,形成于陆内造山期后的挤压向拉张转换构造背景中;二长花岗岩岩浆主要来自壳源,形成于持续拉张构造背景中。地球化学特征显示该岩体具有较好的成矿潜力。

The Wushilong pluton is located at the northeastern end of Jiangnan Orogenic Belt,it is representative metallogenic pluton which is one of the largest plutons in Silurian in the district.Its geological characters show that Wushilong pluton intrudes along the axis of Taiping syncline.The main lithology is granodiorite,followed by adamellite.The LA⁃ICP⁃MS U⁃Pb dating results of the granodiorite intrusion is from148.9±2.0 Ma to 148.0±2.5 Ma.It shows that granodiorite was formed in the early Yanshan Period.The geochemistry of the granodiorite and adamellite show that they are potassium,high calc⁃alkaline and peraluminous rocks.The granodiorite has some similar features of adakites,but there is some differences:high SiO_(2)(>65%),High Sr/Y(sample mean>21.7)and(La/Yb)N(sample mean>15.33)ratios,high Al_(2)O_(3)(sample mean>15%),low MgO(<1.86%),low Yb(<2.50×10^(⁃6))and Y(<23×10^(⁃6))values,low Sr(292.38×10^(⁃6)to 424.60×10^(⁃6)),etc.However,the adamellite has no similar features of adakites:low Al_(2)O_(3)(<13.5%),low Yb(1.29×10^(⁃6)to 2.42×10^(⁃6)),Low Sr(30.99×10^(⁃6)to 44.10×10^(⁃6))and Y(10.42×10^(⁃6)to 129.75×10^(⁃6))values contents,low Sr/Y(0.34~2.97)and low(La/Yb)_(N)(3.32~7.41)ratios,etc.The REE partition map of the granodiorite is characterized by right⁃wing pattern,which is with P,Nb,Sr and Ti are strongly depleted.However,with weak negative Eu and Ba anomaly.The REE partition map of the adamellite is characterized by seagull type,which is with remarkable negative Eu anomaly,and more strong depletion in Ba,P,Nb,Sr and Ti.The granodiorite is I⁃type granite and the adamellite is A⁃type granite.These features show the granodiorite might originated from crust⁃mantle mixed source formed in the structural background of the transformation from extrusion to tension in the Early Yanshanian Period and the adamellite might mainly originated from crust source formed in the middlle Yanshanian extrusional tectonic setting,which may be closely related to the subduction of the paleo⁃Pacific Plate.The adamellite is richer in Mo than granodiorite.Geochemical characteristics show that the pluton has the metallogenic potential.