南秦岭黄羊山贫矿碱性岩体地球化学特征和成因及其与成矿岩体的对比研究:对区域稀土成矿的指示意义

Geochemical Characteristics and Petrogenesis of the REE-barren Huangyangshan Alkaline Intrusion in the South Qinling,Central China and the Comparative Study to the REE Mineralized Intrusions:Implication for the Regional REE Mineralization

南秦岭分布有一系列志留纪碱性岩体,其稀土矿化程度有明显差异,原因不明。本文聚焦研究较为薄弱的贫矿岩体(黄羊山岩体),开展了岩石学、元素地球化学和同位素年代学分析测试,以期通过与成矿岩体(庙垭+杀熊洞)对比揭示稀土成矿差异的原因。结果表明,黄羊山岩体(正长岩和碱性花岗岩)形成于(438.8±3.7)Ma,岩石具过碱质、高钾钙碱性-钾玄岩特征;具有右倾的球粒陨石标准化稀土配分曲线和Eu负异常;微量元素蛛网图显示Eu、Ba和Sr负异常但无高场强元素异常;具高的Zr和Nb含量、Ga/Al比值和锆石饱和温度(~830℃),与典型A型花岗岩类相似;ε_(Nd)(t)值为2.31~3.43,与区域上同期幔源基性岩类似。结合相关地球化学特征,约束其来源于新生的基性地壳,不同于来源富集地幔的同期成矿正长岩体。对比研究显示,贫矿岩体与成矿岩体的岩浆源区不同,这可能是导致南秦岭不同碱性岩体在地球化学特征及稀土含矿性上存在差异的关键因素。

The key factors controlling obviously different degrees of REE mineralization(fertile and barren)among a series of Silurian alkaline intrusions distributed in the South Qinling orogenic belt have long been poorly understood.In this paper,we have conducted a combined petrological,geochemical,and isotopic investigation of the REE-barren Huangyangshan alkaline pluton,in order to make a comprehensive comparison with REE-mineralized intrusions(the Miaoya and Shaxiongdong)and then to explore the reason why there is obvious difference among those intrusions in terms of the REE mineralization.Our results show that rocks(syenite and alkaline granite)of the Huangyangshan intrusion,which was formed in 438.8±3.7 Ma,have peralkaline,shoshonitic and high-K calc-alkaline characteristics.They have right-declined(LREE-rich)chondrite-normalized REE distribution patterns with negative Eu anomalies,obvious depletion of Eu,Ba,and Sr but weak HSFEs anomalies in the primitive mantle-normalized trace-elemental spider diagrams,and have high Zr and Nb contents,high Ga/Al ratios,and high zircon saturation temperatures(~830℃).They are similar to the typical A-type granite.The Huangyangshan alkaline rocks haveε_(Nd)(t)values of 2.31~3.43 which are similar to those of the contemporaneous mantle-derived mafic rocks in the region.In combination with related geochemical features of these rocks,it is constrained that their parental magmas could be derived from partial melting of juvenile crustal mafic rocks.They are different from the contemporaneous REE-mineralized syenite which was derived from enrich mantle(EM I).The comparison of barren and mineralized intrusions show that they were derived from different magma sources.Thus,it is believed that the different magma sources of different alkaline intrusions in the South Qinling orogenic belt could be key factors resulting in their different geochemical characteristics and different properties of the REE mineralization.