摘要:
江西省龙南地区离子吸附型稀土成矿花岗岩出露广泛,然而由于缺乏精确的同位素年代学依据,致使对各岩体的侵位时代、岩石成因等方面的认识存在分歧。本文对足洞、牛坑及半坑花岗岩的风化壳(或基岩)样品进行了LA-MC-ICPMS锆石U-Pb定年,获得206Pb/238U加权平均年龄分别为:(168.2±1.2) Ma、(168.3±1.7) Ma和(209.75±0.86) Ma,表明足洞和牛坑花岗岩体的侵位时代一致,均形成于燕山期,晚于寨背—关西岩体(~195 Ma),更晚于印支期侵位的半坑岩体。足洞—牛坑岩体的稀土配分类型为重稀土型,岩石学、矿物学方面具有相似性,可能为同源岩浆同期分离结晶的产物;寨背—关西岩体和半坑岩体的稀土配分类型均为轻稀土型,岩石学、矿物学方面具相似性,可能为同源岩浆不同期次形成的产物。而足洞—牛坑岩体与寨背—关西岩体具有不同的稀土矿物组合、稀土配分模式和微量元素特征(寨背—关西花岗岩风化壳的Zr/Hf比值(20~60)大于足洞—牛坑岩体(<20),且Zr/Hf比值与Nb/Ta比值正相关),可能来自不同的岩浆源区。
Abstract:
Granites related to ion adsorption type rare earth ore deposits are widely exposed in Longnan area, Jiangxi Province. However, there are different views concerning the emplacement age and petrogenesis of the granites due to the lack of accurate isotopic geochronologic data. In this paper, the authors conducted research on the zircon U-Pb chronology of the weathering crust or granite samples from Zudong, Niukeng and Bankeng granites by using Laser Ablation-Multicollector Inductively Coupled Plasma-Mass Spectrometry (La-MS-ICPMS). Three weighted average206Pb/238U ages obtained are (168.2±1.2) Ma, (168.3±1.7) Ma and (209.75±0.86) Ma respectively. These data show that the emplacement ages of the Zudong and Niukeng granites are consistent with each other in consideration of the errors, suggesting the Yanshanian period. The ages are younger than the age of the Zhaibei-Guanxi granite (~195 Ma), and are much younger than the age of Bankeng granite which was emplaced during the Indosinian period. The Zudong and Niukeng granites have similar petrologic and mineralogical characteristics as well as heavy rare earth distribution patterns, implying that they might be products of the same stage of the comagmatic evolution. The Zhaibei-Guanxi and Bankeng granites also have similar petrologic and mineralogical characteristics and light rare earth distribution patterns, suggesting that they might be products of different stages of the comagmatic evolution. A comparison between the Zudong-Niukeng granite and the Zhaibei-Guanxi granite shows that they have different rare earth minerals, rare earth distribution patterns and trace elements characteristics (the Zr/Hf ratios of the weathering crust samples from the Zhaibei-Guanxi granite (20~60) are higher than those of the Zudong-Niukeng granite (<20), and they are positively correlated with the Nb/Ta ratios). It is believed that these granites originated from different magmatic source areas.
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