青海锡铁山矿区滩间山群a-2岩段与d-2岩段变质岩原岩恢复及对比

Restoration and comparison of metamorphic protoliths in the a-2 and d-2 rock members of the Tanjianshan Group in the Xitieshan mining area,Qinghai Province

锡铁山铅锌矿床位于柴达木盆地北缘造山带中段,矿体主要赋存于滩间山群a-2岩段和b岩组绿片岩中,而在与a-2岩段岩性极其相似的d-2岩段中还未发现矿体。本文通过岩石学和元素地球化学方法对a-2、d-2岩段变质岩进行了原岩恢复及对比,得出以下结论:1)a-2岩段绿片岩较d-2岩段绿片岩,碳质成分含量较多,且碳酸盐化程度较高;2)a-2岩段大理岩成分较纯,方解石含量大于95%,而d-2岩段大理岩中多含石英、长石和绢云母矿物;3)2个岩段SiO_(2)含量的显著变化显示其源区多来源的特点,w(Al_(2)O_(3))-w(TiO_(2))二元组分关系特征反映2个岩段源区至少由2个端元组成,1个端元相对低Ti O_(2)和Al_(2)O_(3),另1个端元相对高Ti O_(2)和Al_(2)O_(3),且a-2岩段中TiO_(2)和Al_(2)O_(3)组分相对d-2岩段更加分散;4)2岩段变质岩的微量元素N-MORB标准化分布形式大致相同,均呈现大离子亲石元素K、Rb、Ba相对高场强元素Ta、Nb、P、Ti较强富集;5)两岩段变质岩的稀土元素球粒陨石标准化配分模式相近,均为右倾配分模式,轻稀土较重稀土富集,且轻稀土分异程度明显高于重稀土,Eu呈现明显负异常,Ce异常不明显,a-2岩段较d-2岩段∑REE、LREE、HREE含量较低,Eu负异常更加明显;6)a-2、d-2岩段变质岩原岩主要为砂质沉积岩、泥质沉积岩、粘土岩以及中-酸性凝灰岩,含少量火山岩。相比较而言,a-2岩段含砂质成分较多,d-2岩段则含少量基性成分,推测是由于物源区不同及沉积碎屑物运移距离远近所致,与d-2岩段沉积碎屑物相比,a-2岩段沉积碎屑物可能经历了较长距离的运移。

The Xitieshan lead-zinc deposit is located in the middle section of the northern orogenic belt of the Qaidam Basin,and the ore-bodies mainly occur in the greenschist of the a-2 rock member and b rock formation of the Tanjianshan Group.However,no ore-body has been found in the d-2 rock member,which is very similar in lithology to the a-2 rock member.In this paper,the restoration and comparison of metamorphic protoliths in the a-2 and d-2 members are carried out by petrology and element geochemistry methods,and draw the following conclusions:(1)Compared with the greenschist in the d-2 rock member,the greenschist in the a-2 rock member contains more carbonaceous components,and the degree of carbonation is higher;(2)The marble of the a-2 rock member is relatively pure,with a calcite content greater than 95%,while the d-2 rock member marble contains more quartz,feldspar and sericite minerals than a-2 rock member;(3)Significant changes in the SiO_(2) content of the two rock members show the multi-source characteristics of their sources.The characteristics of the w(Al_(2)O_(3))-w(TiO_(2))binary composition relationship reflect that the sources of the two rock members have at least two ends,one is relatively low in TiO_(2) and Al_(2)O_(3),the other is higher in TiO_(2) and Al_(2)O_(3),and the TiO_(2) and Al_(2)O_(3) components in the a-2 rock member are relatively more dispersed;(4)The N-MORB standardized trace element distribution of the metamorphic rocks in the two rock members is roughly the same,and both show that large ion lithophile element(K,Rb,Ba)are strongly enriched than high field strength element(Ta,Nb,P,and Ti);(5)The chondrite standardized distribution patterns of rare earth element in the metamorphic rocks of the two rock members are similar,and they are all right-leaning distribution patterns.The content and the degree of differentiation of LREE is significantly higher than HREE.Eu presents obvious negative anomalies and Ce anomalies not obvious,the content ofΣREE,LREE,and HREE in the a-2 rock member are lower than that in the d-2 rock member,and the negative Eu anomaly in the a-2 rock member is more obvious;(6)The metamorphic protoliths of the a-2 and d-2 members are mainly sandy sedimentary rocks,argillaceous sedimentary rocks,claystones and intermediate-acid tuffs,and contain a small part of volcanic rocks.In comparison,the a-2 member has more sandy components,while the d-2 rock member contains a small amount of basic components,which is probably due to the different sources and the migration distance of the sedimentary clastics.Compared with the d-2 rock member,it is possible that the sedimentary clastics of the a-2 rock member experienced a longer distance migration.