安徽狮子山矿田南洪冲岩体形成时代及成因机制研究

Geochronology and origin of Nanhongchong pluton in Shizishan ore-field, Anhui province

南洪冲岩体是狮子山矿田内一个隐伏岩体,通过 LA-ICPMS 锆石 U-Pb 定年,获得^(206)Pb/^(238)U 年龄为141.2±1.6Ma,与矿田内其它岩体形成时代较为接近,说明南洪冲岩体是晚侏罗—早白垩世岩浆活动的产物。岩体的岩性为花岗闪长岩,主量元素以富碱(Na_2O>K_2O)、准铝质和高钾钙碱性为特征;微量元素表现为 Rb、Ba、Sr 等大离子亲石元素(HLE)和 Th、U富集,Nb、Ta、Ti 等高场强元素(HFSE)亏损;LREE 与 HREE 分馏显著((La/Yb)_N=21.07～24.28),无明显的 Eu 异常(δEu=0.92～1.03)。岩体的(^(87)Sr/^(86)Sr)_i 值偏高(0.70792～0.70793),ε_(Nd)(t)值偏低(-9.5～-9.1),Nd 模式年龄为中元古代(1.67～1.71Ga),与铜陵地区,特别是狮子山矿田内岩浆岩的 Sr-Nd 同位素初始组成和 Nd 模式年龄相仿。上述研究表明,南洪冲花岗闪长岩与矿田内乃至铜陵地区的燕山期岩浆岩具有相近的地球化学特征,说明成岩物质起源于富集型的岩石圈地幔,在中国东部地壳由挤压向伸展-减薄过渡的构造转换期,岩浆上升侵位,以同化混染与分离结晶作用方式产生了大量岩体。但南洪冲花岗闪长岩的形成主要与分离结晶作用有关,反映了岩浆作用过程中存在一定差异。

Nanhongchong pluton in Shizishan ore-field, Anhui province, is spatially close to those Yanshanian plutons that have metallogenic relations with important copper-iron-gold-sulfur deposits. Zircon U-Pb LA-ICPMS dating yields an age of 141.2 ± 1.6Ma, which meets the ages of other plutons in the ore-field and suggests the magmatism event occurred between late Jurassic and early cretaceous. Nanhongchong pluton was metaluminous calc-alkaline granodiorite characterized by high alkali （ Na20 〉 K20）. Rb, Ba, Sr, Th and U were enriched, while Nb, Ta and Ti were depleted. REE showed strong LREE enrichment with （La/Yb）N ratios of 21.07 -24.28 and the absence of Eu anomaly with tSEu values of 0.92 - 1.03. Isotope geochemical study revealed initial STSr/S6Sr ratios were 0.70792 - 0. 70793 while 6Na （t） values were - 9.5 - - 9. 1, and Nd model ages （ 1.67 - 1.71Ga） were Mesoproterozoic. The characteristics above in accord with former relevant data indicate that the mixed magma derived from enriched mantle, intruded and fractionated in a crustal extension-thinning tectonic environment while assimilated a proportion of the crust. Particularly, the magma in Shizishan ore-field was rather evolved when Nanhongchong pluton formed and fractional crystallization was the dominant process then, which suggested moderately diverse magmatism.