西大别山腹地晚中生代安山岩U-Pb年代学和地球化学特征

U-Pb geochronology and geochemistry of Late Mesozoic andesite in the hinterland of the western Dabie Orogen,China

文章对西大别山腹地的仙居顶安山岩开展了激光剥蚀-电感耦合等离子体质谱法(laser ablation-inductively coupled plasma-mass spectrometry,LA-ICP-MS)锆石原位U-Pb同位素测年和岩石地球化学研究。该安山岩的w(SiO_(2))为59.98%~61.40%,w(K 2O)为5.03%~7.40%,轻重稀土分异明显((La/Yb)N为15.16~18.62),Eu异常不明显;岩石属于钾玄岩系列。通过锆石U-Pb同位素分析,获得26个测点表面年龄在2727~123 Ma之间,具有5组谐和的锆石,其206 Pb/238 U年龄加权平均值分别为:第1组,(2416±120)Ma(n=6);第2组,(730±13)Ma(n=5);第3组,(430±9)Ma(n=2);第4组,(224±15)Ma(n=3);第5组,(128±2)Ma(n=9)。通过锆石成因和区域地质分析认为:第1组年龄反映了大别地区新太古代物质基底;第2组年龄记录了扬子北缘新元古代强烈岩浆事件;第3组年龄暗示了古生代岛弧岩浆作用;第4组年龄记录了早中生代扬子与华北陆块碰撞拼合-变质作用;第5组最年轻的岩浆锆石年龄代表了安山岩形成年龄。仙居顶安山岩可能起源于西大别地区白垩纪下地壳拆沉后、受到流体/沉积物熔体改造的富集岩石圈地幔,形成于约128 Ma的陆壳伸展环境;其中俘获锆石记录了大量太古代—中生代U-Pb年龄信息,暗示了元古代—晚中生代多期构造-热事件。

Whole-rock major-trace elemental,whole-rock Sr-Nd and laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)zircon U-Pb isotopic analyses were carried out on the Xianjuding andesites within the hinterland of the western Dabie Orogen,in order to better understand their petrogenesis and geodynamic significance.The andesites are characterized by SiO_(2) contents of 59.98%to 61.40%,high K 2O contents of 5.03%to 7.40%,enriched in LREE with(La/Yb)N=15.1618.62 and unapparent Eu anomaly.They belong to shoshonitic series.In situ LA-ICP-MS U-Pb dating yielded 206 Pb/238 U ages ranging from 2727 Ma to 123 Ma for the zircons from the andesites.Twenty-six analyses were grouped into five clusters with weighted mean 206 Pb/238 U ages at(2416±120)Ma(n=6),(730±13)Ma(n=5),(430±9)Ma(n=2),(224±15)Ma(n=3),and(128±2)Ma(n=9).Combining regional geology,it is proposed that the oldest 206 Pb/238 U age cluster of(2416±120)Ma reflects the ancient material basement of the Dabie region,the age cluster of(730±13)Ma records the Neoproterozoic magmatic event in an extensional setting,the age cluster of(430±9)Ma reflects the Early Paleozoic magmatism,the age cluster of(224±15)Ma reflects the continental collision between Yangtze and North China blocks,and the youngest age cluster of(128±2)Ma represents the Early Cretaceous crystallization age for the Xianjuding andesites.The Cretaceous volcanic activity occurred not only in Beihuaiyang area but also in the hinterland of the Dabie Orogen.The Xianjuding andesites are likely to be derived by partial melting of metasomatic enriched lithospheric mantle,and be erupted in an extensional setting during ca.128 Ma.Its inherited zircons suggest multistage thermo-tectonic evolution of the western Dabie crust during Archaeozoic to Mesozoic.