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浙江安吉坞山关杂岩体岩相学和岩石地球化学特征及岩石成因
摘要
位于钦—杭成矿带东段北缘的浙江安吉多金属矿区发育侵位序列复杂的坞山关杂岩体及与其有关的蚀变和矿化。为了探讨区内岩浆岩与成矿的关系,在野外调查的基础上,文章对该杂岩体的岩相学、岩石地球化学开展了研究,并籍此对其成因类型、源区特征进行了探讨。坞山关杂岩体由黑云母二长花岗岩、花岗闪长岩和正长花岗岩组成,前者具富碱、高钾特征,属准铝质-钾玄岩系列,花岗闪长岩具高钾特征,属准铝质-高钾钙碱性系列,正长花岗岩具高硅、低钙、贫镁、高FeO*/MgO值、富碱特征,属准铝-弱过铝质钾玄岩系列。该杂岩体的∑REE为(123.5~216.47)×10-6,稀土元素配分型式呈明显的右倾轻稀土元素富集型,Eu呈不同程度的负异常。微量元素蛛网图中,各岩石单元大离子亲石元素(尤其是Rb、Th、K)明显富集,Ba、Sr负异常明显,相对亏损高场强元素Nb、Ti、P。该杂岩体成矿元素w(Mo)、w(W)明显高于克拉克值,暗示其具有提供成矿物质的潜力。黑云母二长花岗岩为分异I型花岗岩,花岗闪长岩属I型花岗岩,正长花岗岩具有A型花岗岩的多数特征,但从杂岩体各岩石单元的成岩年龄相差不大、具地球化学亲缘性以及A型花岗岩不具高分异特征来考虑,将其归为高分异I型花岗岩更合理。La/Sm-La图解显示出,该杂岩体为平衡部分熔融作用所形成,其轻稀土元素富集、Ta-Nb、Ti、Eu、Sr、Ba负异常可能是源区为含角闪石族、金红石、斜长石作为难熔相遗留在残留物中所致。
In the Anji ore district located in northern Qinzhou-Hangzhou metallogenic belt,there exists theWushanguan complex bodyin whichthe rock units have complicatedintrusionsequences and are relatedto alteration and mineralization.In order to study the relationship between magmatic activity and mineralization,theauthors analyzedthe major andtrace elements concentrationandthe characteristics of petrogeochemistry and petrogenesis of Wushanguan complex body based on the results of field work and petrographic studies.The majorand trace elements concentrations were tested by XRFand ICP-MS.Wushanguan complex bodyis mainly com posed of biotite-monzonitic granite,granodiorite and syenogranite.The biotite-monzonitic granite is characterized by alkali-rich and potassium-high nature,and belongs to quasi-aluminous rock and shoshonite series.Thegranodiorite is characterized by high potassiumand belongs to quasi-aluminous rock and high-Kcalc-alkaline series.Syenogranite is si milar to A-type granite characterized by high Si O2,low CaO,low MgO,high FeO*/MgOratio,relatively rich alkali and high potassium,and belongs to metaluminous-weakly peraluminous rockand shoshonite series.The total concentrations of the rare earth elements of the complex range from123.5×10-6to 216.47×10-6.The rare earth elements distribution patterns of the complex are all of LREE-enrichedtype〔(La/Yb)N=7.47 ~23.93〕with different negative Eu anomalies(Eu/Eu*=0.38 ~0.86).Inthe pri mitive mantle normalized multi-element variation diagram,large-ionlithophile elements,such as Rb,Th and K,are abundant with obvious negative anomalies of Ba and Sr,andthe rocks are all relatively depletedinsome highfield strength elements such as Nb,Ti and P.Among the ore-forming elements,the element values of Mo andWare higher thantheir Clarke values obviously,that istosay,the complex hasthe potential of becoming a mineralization source.Accordingtothe characteristics of major elements andtrace elements,especiallythe discri mination diagrams,biotite-monzonitic graniteis separatelyfractionated I-type granites andthe granodiorite belongsto I-type granite.Syenogranite has some characteristics of highlyfractionated A-type granite,but there are several reasons suggestingthat it does not belongto A-type granite:firstly,A-type granites generally do not exhibitevidence of being strongly differentiated;secondly,on the whole,all the complex members have a si milar agewithin the experi mental error as well as have an affinityto each other in geochemistry,sothat all the rock unitsshould be formedin the same tectonic setting.Considering that biotite-monzonitic granite and granodiorite belong to I-type granite,it is more reasonable to assign moyite to highly fractionated I-type granite.The La/SmLa diagramshows that the complex body was formed by equilibriumfusion.Light rare earth elements enrichment and Ta-Nb-Ti,Eu,Sr and Ba negative anomalies probably resulted from hornblende group,when rutileand plagioclase stayedin residue in source region.
引文
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