湘东北团山背金矿区辉绿岩脉年代学、地球化学及其地质意义

Geochronology and geochemistry of the Diabase Dikes in the Tuanshanbei gold deposit, northeastern Hunan, China and their geological implications

为确定湘东北团山背金矿区辉绿岩脉的形成时代、岩浆源区特征和形成时的大地构造背景,对采自该区的辉绿岩样品进行了LA-ICP-MS锆石U-Pb定年、岩石地球化学及Sr、Nd、Pb同位素分析。结果表明,辉绿岩形成于印支晚期(225 Ma),具有低SiO_(2)(44.56%~45.91%)、高MgO(9.49%~10.60%)及高Mg^(#)值(66.0~67.5)特征;富集K、Rb、Ba、Sr等大离子亲石元素,相对弱亏损Ta、Nb、P等高场强元素;轻、重稀土元素分馏较明显(La_(N)/Yb_(N)为6.83~8.73),稀土元素配分模式呈右倾的轻稀土元素富集型;微量和稀土元素分布特征与洋岛玄武岩(OIB)相似。辉绿岩具有高的(^(87)Sr/^(86)Sr)_(i)值(0.706 52~0.711 05)和低的ε_(Nd)(t)值(-1.64~-0.16);(^(208)Pb/^(204)Pb)t为38.4085~38.8981,(^(207)Pb/^(204)Pb)t为15.6439~15.6644,(^(206)Pb/^(204)Pb)t为18.0933~18.4207。岩石地球化学特征指示团山背辉绿岩岩浆形成于富集地幔(EM II)的部分熔融,源区为尖晶石-石榴子石二辉橄榄岩,部分熔融程度约为15%,与陆壳物质之间的同化混染作用较弱,同时岩浆演化过程中发生了弱的橄榄石、单斜辉石和磷灰石的分离结晶作用。综合前人研究成果认为,团山背辉绿岩形成于陆内碰撞造山后的伸展构造背景。基性岩脉与区内主要金矿床矿脉(体)的交切关系及近年来区内金矿床成矿年代学数据表明,浏阳-醴陵官庄地区北东向矿脉(体)类金矿床形成时代主要为加里东期,北西向矿脉(体)类金矿床主要形成于印支期。

In order to constrain the age, magma source and tectonic background for the formation of diabase dikes in the Tuanshanbei gold deposit in the northeastern Hunan, studies of the LA-ICP-MS zircon U-Pb dating, petrogeochemistry and Sr, Nd, Pb isotopes of samples from diabase dikes in the deposit have been carried out. The results indicate that the diabase was formed in the late Indosinian period with an age of 225 Ma. The diabase samples are characterized with low contents of SiO_(2)(44.56%~45.91%), high contents of MgO(9.49%~10.60%), and high Mg^(#)values(66.0~67.5), enriched in large ion lithophile elements(K, Rb, Ba, Sr), and relatively depleted in high field strength elements(Ta, Nb, and P), relatively obvious fractionation between LREEs and HREEs(La_(N)/Yb_(N)= 6.83~8.73), and LREE-enriched right declined REE distribution patterns. Their distribution characteristics of trace elements and rare earth elements are similar to those of the ocean island basalt(OIB). The diabase samples have high(^(87)Sr/^(86)Sr)_(i)values varying from 0.706 52 to 0.711 05, low ε_(Nd)(t) values varying from-1.64 to-0.16, and Pb isotopic compositions including(^(208)Pb/^(204)Pb)tof 38.4085~38.8981,(^(207)Pb/^(204)Pb)tof 15.6439~15.6644, and(^(206)Pb/^(204)Pb)tof 18.0933~18.4207. Petrogeochemical characteristics show that the magma of diabase dikes were derived from a small degree of partial melting of spinel-garnet lherzolite in the enriched mantle(EM II), with the partial melting degree of about 15%. Weak assimilation and contamination occurred between the original magma and continental crustal materials. In addition, weak fractional crystallization of olivine, clinopyroxene and apatite took place in the basaltic magma evolution process. Combining with previous research results, it is believed that these late Indosinian diabases in the Tuanshanbei deposit were formed in an extensional tectonic setting after the intra-continental collisional orogeny stage. The intersecting relationships between the basic dykes and the main orebodies of gold deposits and data of metallogenic chronology of gold deposits in recent years show that the NE-trending vein(body) type gold deposits in the Liuyang-Liling area were mainly formed in the Caledonian period, while the NW-trending vein(body) type gold deposits in the Liuyang-Liling area were mainly formed in the Indosinian period.