豫南新县岩体地球化学、年代学和Hf同位素特征及地质意义

Petrogeochemistry,Geochronology and Hf Isotopes of the Monzogranite from Xinxian,Southern Region in Henan Province

新县岩体位于秦岭-大别造山带上,对其进行系统的岩石地球化学探讨有助于加深对秦岭-大别造山带中生代构造-岩浆演化及地球动力学背景的认识.岩石地球化学分析结果显示,SiO_2含量为74.94%~78.16%,K_2O含量为3.87%~5.86%,Na_2O含量为3.82%~4.24%,Al_2O_3含量为12.38%~13.57%,具有高钾钙碱性和过铝质(A/CNK=1.05~1.11)特征.岩石稀土总量较低(42.67×10^(-6)~110.45×10^(-6)),轻稀土元素相对富集,重稀土元素相对亏损,具有Eu的负异常,岩体富集Rb、U、Hf和Y,亏损Ba、Nb、Ta和Ti,显示出I型高分异花岗岩的特点.3个期次岩石的U-Pb年龄分别为153.4±1.1Ma、146.4±1.6Ma和131.6±1.8Ma.锆石Hf同位素分析结果表明,新县岩体的ε_(Hf)(t)值为-24.5^-20.3,位于地幔演化线之下,t_(DM2)值为2.68~2.93Ga,指示其源于扬子板块北缘的古老地壳,其组成类似于扬子板块北缘TTG型岩浆岩.结合区域地质背景,新县岩体形成于扬子陆块与华北陆块碰撞造山后的陆内伸展环境,对应的地球动力学背景为晚侏罗世-早白垩世地壳从挤压收缩向区域性伸展的构造体制大转换过程和大规模的地壳伸展和岩石圈减薄时期.

Xinxian monzogranites are located at the Qinling-Dabie orogenic belt,and a systematic geochemical study can facilitate further studies on the Mesozoic tectono-magmatic evolution and the geodynamics background of Qinling-Dabie orogenic belt.This paper presents geochemistry,zircon U-Pb geochronology,and Hf-isotopic data of the rocks,with the aim of constraining their petrogenesis,sources and tectonic implications.The geochemical analysis shows that Xinxian monzogranites are characterized by SiO_2=74.94%-78.16%,K_2O=3.87%-5.86%,Na_2O=3.82%-4.24%,Al_2O_3=12.38%-13.57%,A/CNK=1.05-1.11;the rocks are enriched in LREE,depleted HREE and negative Eu anomalies,enriched in Rb,U,Hf,Y and depleted Ba,Nb,Ta,Ti,which indicates that Xinxian monzogranites are peraluminous,high-K calc-alkaline Ⅰ-typegranite.The three stages yield LA-ICP-MS zircon U-Pb ages 153.4±1.1Ma,146.4±1.6Ma and 131.6±1.8Ma respectively.According to zircon Hf isotope analysis,theε_（Hf）（t）values of the granite range from-24.5to-20.3,which locates below the depleted mantle line.The t_（DM2） values range from 2.68 to 2.93 Ga,which indicates the monzogranites resulted from the lower crust of Yangtze Block.The component of the monzogranite is similar to that of Neoproterozoic TTG magmatic rocks.Xinxian monzogranites were emplaced after the major collisional period of the North China block and Yangtze block,in the postcollisional tectonic setting under the regional extension mechanism.The geodynamics setting is the transformation of tectonic regime of the Late Jurassic-Early Cretaceous and large-scale lithosphere delaminating thinning under post stretching mechanism.