宁芜盆地花岗岩类的锆石U-Pb年龄、同位素特征及其意义

Zircon U-Pb Ages and Isotopic Characteristics of the Granitoids in the Ningwu Basin, China, and their Significance

宁芜盆地中花岗岩类侵入体出露零星,以石英二长岩和花岗岩为主,研究程度薄弱。本文对盆地内主要的5个花岗岩类侵入体牛迹山岩体、霍里岩体、姑山岩体(姑山铁矿床钻孔中)、小石山岩体和娘娘山岩体进行了年代学、锆石Lu-Hf同位素、Nd-Sr同位素分析研究。这5个花岗岩类侵入体的LA-ICPMS锆石U-Pb年龄依次为128.0±1.7Ma、126.4±1.3Ma、129.8±1.6Ma、130.1±1.5Ma、128.3±1.8Ma,形成于126～130Ma之间,均为早白垩世岩浆活动的产物,与姑山、娘娘山旋回火山岩活动时限相当,略晚于玢岩型铁矿的成矿时代。Nd-Sr同位素指示,宁芜盆地花岗岩类源于交代地幔源区,其性质接近于EMI型富集地幔,其形成与古板块的俯冲交代作用有关;Lu-Hf同位素指示,宁芜盆地花岗岩类与成矿的闪长(玢)岩在岩浆起源和演化上存在联系。盆地中的花岗岩类形成于中国东部岩石圈减薄高峰期的地球动力学背景,为典型张性构造应力体制的产物。

Granitic intrusions in the Ningwu basin,outcropping sporadically,consist mainly of quartz monzonite and granite.Few works have been done on these granitic intrusions.This paper presents a detail study of some granitic intrusions by means of LA-ICPMS zircon dating,Lu-Hf zircon isotope,and Sr-Nd isotope analyses.The U-Pb zircon ages for five granitic intrutions are 128.0±1.7 Ma for Niujishan pluton,126.4±1.3 Ma for Huoli pluton,129.8±1.6 Ma for Gushan pluton （sampling from drill core in the Gushan iron deposit,）,130.1±1.5 Ma for Xiaoshishan pluton,and 128.3±1.8Ma for Niangniangshan pluton.These date indicate that the granitic intrusions emplaced in 126~130Ma,suggesting that they were the products of magmatic activity in early Cretaceous.The ages are similar to those of the Gushan and Niangniangshan Formations volcanic rocks and slightly younger than the mineralization ages of the porphyrite iron deposits in the Ningwu basin.The Nd-Sr isotope results suggest that the granitic intrusions derived from the metasomatic mantle,compositionally closing to the EMI,which has relationship with the subduction of the Pacific Plate.The Lu-Hf isotope results imply that the granitic plutons have genetic and evolutional relationship with the mineralized diorites or porphyry diorites.The granitic plutons in the Ningwu basin formed in an extensional geodynamic setting,and were products of the lithospheric thinning in eastern China.