河南省崤山东部老里湾岩体年代学、地球化学及岩石成因

摘要

The Laoliwan deposit in Luoning County of Henan Province is a newly discovered large-sized Ag-Pb-Zn deposit in a shallow covered area of eastern Xiaoshan.The mineralization of the deposit is closely related to the Laoliwanintrusion.In order to find out diagenetic age,source component,and petrogenesis of the Laoliwan intrusion,the authors carried out LA-ICP-MS zircon U-Pb dating and analysis of geochemistry and Lu-Hf isotopes.Four granite porphyry samples from the Laoliwan intrusion yielded weighted mean 206 Pb/238U ages of (133 ± 1) Ma (MSWD =0.58),(133 ± 1) Ma(MSWD =1.20),(134 ± 1) Ma(MSWD =1.16),and (137 ± 1) Ma(MSWD =0.45),resPectively,indicating that the Laoliwan intrusion was formed during Early Cretaceous.The Laoliwan intrusion is mainly composed of granite porphyry,and the rocks have high content of SiO2(70.48％ ～ 72.75％) and K2O (5.90％ ～7.01％),very high K2O/Na2O ratios (7.68 ～ 12.89),and relatively high A/CNK ratios (0.97 ～ 1.25,1.11 on average),thus belonging to a high-K calk-alkaline series.The total rare earth elements (REE) values of the Laoliwan intrusion vary from 231 × 10-6 to 278 × 10-6,and its REE distribution patterns are characterized by intermediate-weak negative Eu anomalies (Eu/Eu * values ranging from 0.59 to 0.78,0.68 on average) and strong fractionation between light REEs and heavy REEs with (La/Yb) N values varying from 26.7 to 36.5.The Laoliwan intrusion is enriched in large ion lithophile elements such as Rb,Ba,Th,U and K and LREEs but strongly depleted in high field strength elements such as Nb,Ta,Zr,Hf,P and Ti.The granite porphyry has varying zircon εHf(t) values (-43.0 ～-5.1,-20.0 on average) and Hf two-stage model ages (1.52 ～ 3.85 Ga,2.45 Ga on average).The geochemical characteristics and zircon Hf isotopic composition suggest that magmatic source of the Laoliwan intrusion was dominated by the Meso-Neoarchean and Paleoproterozoic crustal materials with a small amount of Paleoarchean and Eoarchean crustal materials,and that an intense crust-mantle interaction occurred during the magmatism.The magma mixing between basaltic magma generated by underplating and felsic magma formed by partial melting of the ancient crustal materials was the primary mechanism responsible for the Early Cretaceous Laoliwan intrusion.%近年在河南省洛宁县的崤山东部浅覆盖区发现了老里湾大型银铅锌矿床,该矿床与矿区内的老里湾岩体有密切的成因联系.为了查明老里湾岩体的形成时代、源区组成及岩石成因,对其开展了LA-ICP-MS锆石U-Pb定年、岩石地球化学和锆石Lu-Hf同位素分析.老里湾岩体4件花岗斑岩样品的206 Pb/238U年龄加权平均值分别为(133±1)Ma(MSWD =0.58)、(133±1)Ma(MSWD=1.20)、(134±1)Ma(MSWD=1.16)和(137±1)Ma(MSWD=0.45),表明老里湾岩体形成于早白垩世.岩体的主要岩石类型为花岗斑岩,岩石具有高的w(SiO2)(70.48％ ～72.75％)和w(K2O)(5.90％ ～7.01％),非常高的K2O/Na2O比值(7.68 ～ 12.89)和较高的A/CNK值(0.97 ～ 1.25,平均1.11),属于高钾钙碱性系列.老里湾岩体的稀土元素总量介于231×10-6 ～278×10-6,轻、重稀土元素分馏强烈,其(La/Yb)、值介于26.7 ～36.5之间,岩石具有中等到弱的负铕异常(Eu/Eu*值介于0.59 ～0.78,平均0.68).老里湾岩体富集Rb、Ba、Th、U、K等大离子亲石元素和轻稀土元素,强烈亏损Nb、Ta、Zr、Hf、P、Ti等高场强元素.老里湾岩体锆石的εHf(t)介于-43.0～-5.1(平均-20.0),Hf同位素两阶段模式年龄为1.52～3.85 Ga(平均2.45Ga).老里湾岩体的源区主要为中-新太古代和古元古代的地壳物质,并有少量的古太古代和始太古代地壳物质,岩浆形成过程中发生了较为强烈的壳幔相互作用,古老地壳物质部分熔融形成的岩浆与底侵的玄武质岩浆混合是老里湾早白垩世花岗斑岩形成的主要机制.