豫西五丈山花岗岩锆石U-Pb年龄、地球化学特征及地质意义

Zircon U-Pb ages and geochemical features of Wuzhangshan granite in Xiong’er Mountain, western Henan Province, and its geologic implications

五丈山花岗岩是熊耳山地区出露面积居第二位的酸性侵入岩,它的岩石成因及深部动力学过程研究不仅可为熊耳山区域构造演化提供新的证据,而且能为区域内生金属成矿动力学研究提供新的参考,还可为熊耳山地区地质找矿提供新的思路。对五丈山斑状黑云母二长花岗岩样品WZS01进行了LA-ICP-MS锆石U-Pb定年,27个测点中的18个测点的同位素比值位于n(207Pb)/n(235U)—n(206Pb)/n(238U)一致线上,它们的加权平均年龄为159.6±1.6 Ma。结合五丈山花岗岩的SHRIMP和LA-ICP-MS锆石U-Pb定年结果,认为它是晚侏罗世约160 Ma岩浆活动的产物。五丈山花岗岩160颗锆石的U-Pb年龄形成了一个从新太古代至晚侏罗世的单颗粒锆石年龄谱,8个峰值年龄依次为~2.8 Ga、~2.3 Ga、~1.9 Ga、~1.8 Ga、~1.7 Ga、~1.3 Ga、~178 Ma和~160 Ma,区域上均存在与之对应的岩浆热事件。五丈山花岗岩具有高硅、富碱高钾、贫镁低钙的特征,属于钾玄岩系列和高钾钙碱性系列。它富集大离子亲石元素,亏损高场强元素Nb、Ta,在微量元素蛛网图上显示明显的Nb、Ta、P及Ti负异常,稀土配分模式具有右倾平滑、轻稀土富集及重稀土亏损及无明显负Eu异常等特征。五丈山花岗岩具有高Sr、低Y的特征,属于高锶低钇中酸性岩(Adakite,有人音译为埃达克岩),它是加厚大陆下地壳的部分熔融形成的,部分熔融源区的残余相矿物包括石榴子石、金红石和角闪石,且无(富钙)斜长石残留。熊耳山地区在~160 Ma经历的岩石圈拆沉作用致使不同深度范围、不同成分的原岩近同时发生部分熔融,多种成因的锆石加入到深部流体内并与之快速上升,途中注入到未完全固结岩浆/流体库,在经历岩浆—流体活化后上侵至地壳浅部后形成了具有复杂单颗粒锆石年龄谱的五丈山花岗岩。另外一方面,深部流体也因熊耳山地区~160 Ma岩石圈拆沉而获得快速释放,其周缘~160 Ma的Au、Mo成矿作用得益于深部含矿流体的有效封闭,五丈山花岗岩周缘及深部仍具有巨大的内生金属成矿潜力。

Objectives:Wuzhangshan granite is the second area acid intrusion in Xiong’er Mountain,western Henan Province.Petrogenesis and deep process of Wuzhangsha granite are important to recognize tectonic evolution in Xiong’er Mountain area.It is significant not only to regional endogenous metllogenic dynamics,but also to geological exploration in Xiong’er Mountain.Methods:Based on detailed field obversations of Wuzhangshan granite in Xiong’er Mountain,western Henan Province,we have studied its features of the petrology,geochemistry and chronolgy.The formation age,partial melting source features,deep tectonic process,zircon age spectrum and regional mineralization potential of Wuzhangshan granite were discussed.Results:The zircon U-Pb dating method of LA-ICP-MS was used to confirm the age of porphyritic biotite monzonitic granite sample named WZS01 from Wuzhangshan granite.The results of 18 zircon spots in total 27 spots are concordia with n(207 Pb)/n(235 U)—n(206 Pb)/n(238 U)trendline,and their n(206 Pb)/n(238 U)weighted average age is 159.6±1.6 Ma.In combination with dating results of SHRIMP and LA-ICP-MS zircon spots from Wuzhangshan granite,it was formed^160 Ma in late Jurassic.The ages of total 160 U-Pb zircon spots of the Wuzhangshan granite formed a zircon age spectrum from late Archean to late Jurassic,and 8 age peaks are displayed,i.e.~2.8 Ga,~2.3 Ga,~1.9 Ga,~1.8 Ga,~1.7 Ga,~1.3 Ga,~178 Ma and^160 Ma,which recorded the corresponding regional magmatic thermal events.Samples of Wuzhangshan granite are characterized by higher Si O2 and alkali,enrichment of K2 O,and lower Mg O and Ca O.They fall into shoshonite series and high-K calc-alkaline series in the Si O2—K2 O diagram.These samples are enriched in large ion lithophile elements and depleted in high field-strength elements,which has formed troughs of Nb,Ta,P and Ti in spider diagram.The characteristics of LREE-enrichment and HREE depletion without obvious Eu negtive anomaly are shown in chondrite-normalized REE patterns.Samples of Wuzhangshan granite have higher Sr and lower Y,which has suggested that it is adakite.Conclusions:Wuzhangshan granite originated from partial melting of the thickened lower crust,and its residual phases of partial melting source included garnet,rutile and amphibole,and no plagioclase.Lithosphere delamination of Xiong’er Mountain area occurred in^160 Ma,which conduced different compositional source rocks at different depth range begin to melt partially.The polygenetic zircons were added into deep fluid,and ascended rapidly together.The activation of incomplete consolidation magma—fluid reservoir occurred after rejection of deep fluid,and the mixed deep fluid continued ascending to form Wuzhangshan granite with zircon age spectrum.On the other hand,huge deep fluid was drove and discharged rapidly during^160 Ma lithosphere delamination in Xiong’er Mountain area.The deep ore-bearing fluid was enclosed effectively in Xiong’er Mountain area because of^160 Ma mineralization of Au and Mo around Wuzhangshan granite.It has suggested that the area around Wuzhangshan granite and its deep part has large mineralization potential.