北大巴山超基性、基性岩墙和碱质火山杂岩形成时代的新证据及其地质意义  

New Evidence for the Ages of Ultramafic to Mafic Dikes and Alkaline Volcanic Complexes in the North Daba Mountains and Its Geological Implication

作　　者：向忠金[1,2] 闫全人[3] 宋博[4] 王宗起[5] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国科学院地质与地球物理研究所,北京100029 [3]中国科学院大学地球科学学院,北京100049 [4]中国地质调查局西安地调中心,西安710054 [5]中国地质科学院矿产资源研究所,北京100037

出　　处：《地质学报》2016年第5期896-916,共21页Acta Geologica Sinica

基　　金：国家自然科学基金青年基金项目(编号41102141);中国地质科学院地质所基本科研业务费项目(编号J1211);中国地质调查局项目(编号12120113067500)联合成果

摘　　要：北大巴山早古生代地层广泛出露一套超基性、基性岩墙和碱质火山杂岩(包括碱性玄武岩和粗面岩),为研究北大巴山早古生代构造演化提供了重要的载体。本文通过利用锆石U-Pb定年和金云母^(40)Ar/^(39)Ar同位素定年方法对它们的形成时代进行了系统研究,结合地球化学特征探讨它们的成因及构造意义。通过对岚皋县和镇坪县的两个辉绿岩墙开展锆石U-Pb定年,分别获得它们的年龄为399±1Ma和451±4 Ma,是研究区目前已报道的有关超基性、基性岩墙最年轻和最老的就位时代。地球化学分析结果表明,它们的主量元素与区内其他基性岩墙位于同一演化趋势线上,微量和稀土元素地球化学特征相似,类似于OIB的特征,表明它们来自相同的地幔源区。因此,研究区内基性岩墙侵入事件最早开始于晚奥陶世中期(约450Ma),结束于早泥盆世晚期(约400 Ma),期间经历了多次岩浆侵入活动。对与碱性玄武岩共生的火山碎屑岩中金云母晶屑进行^(40)Ar/^(39)Ar同位素定年,获得年龄446±3Ma,代表了火山喷发时间,表明碱性玄武质岩浆喷发活动与基性岩浆侵入事件大致同时发生。对粗面岩进行SHRIMP锆石U-Pb定年虽未获得准确的岩浆结晶年龄,但较年轻锆石年龄(165±3Ma和229±2Ma)的存在,暗示其形成时代可能属于中生代。系统的年代学研究表明,北大巴山地区早古生代地层中的超基性、基性岩墙和碱性玄武质火山杂岩为同期岩浆活动的产物,最早开始于450Ma,经历了多期火山喷发和岩浆侵入活动(450~400 Ma),结束于早泥盆世晚期(~400Ma)。而粗面岩与上述岩石不是同一期岩浆活动产物,研究区不存在双峰式火成岩组合,它们可能形成于中生代,属于南秦岭中生代岩浆活动的产物,为南秦岭、北大巴山中生代成矿作用提供物源。A large volume of ultramafic to mafic dikes and alkaline volcanic complexes,widely occurring in the Early Paleozoic strata in the North Daba Mountains,provides a good opportunity to explore the nature of mantle and tectonic evolution of the South Qinling and the North Daba Mountains.This study discussed the formation ages of these rock assemblages using zircon U-Pb dating and phlogopite ^（40）Ar/^（39）Ar isotopic dating and their genesis and implications for the tectonics.Zircon U-Pb dating yielded two ages of 399±1Ma and 451±4Ma for the Langao and Zhenping diabases,which are the youngest and oldest emplacement ages for the basic dikes.Geochemical data show that major elements of Langao and Zhenping diabases and other mafic dikes have been plotted on the same tendency line and their trace elements and rare earth elements features are also identical.These features are similar to OIB signature,indicating that they originated from a same mantle source.Therefore,the emplacement of ultramafic to mafic dikes started at Katian of Late Ordovician（~450 Ma）and ended at Emsian of Early Devonian（~400Ma）,during which several magmatic intrusions occurred.^（40）Ar/^（39）Ar isotopic dating yielded the age of 446±3 Ma for the phlogopites from the volcaniclastic rocks,which represents the age of volcanic activity.This indicates that the alkaline basaltic magma erupted at the same time as the mafic magma intrusive event occurred.Although the no precise ages of the trachytes have been obtained,the younger zircon ages（165±3Ma and 229±2 Ma）imply that they may be formed in Mesozoic.Systematic dating results show that the ultramafic to mafic dikes and alkaline basalts originated from the same magmatic source,with magmatic activity starting at 450 Ma and ending at ~400 Ma.The trachytes were not the products of the same magmatic event mentioned above because no bimodal volcanic rock association occurred in the study area,but the products of Mesozoic magmatism in the South Qinling orogenic belt.They provided ma

关 键 词：金云母 锆石U-Pb定年 碱性玄武岩 基性岩墙 北大巴山 

分 类 号：P597.3[天文地球—地球化学] P588.1[天文地球—地质学]