北太行紫荆关基性岩体的成因：岩石学和地球化学证据  

Petrogenesis of the Zijingguan Mafic Pluton,Northern Taihang Orogen:Constraints from Petrology and Geochemistry

作　　者：刘玲[1] 陈斌[1] 刘安坤[1] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室地球与空间科学学院,北京100871

出　　处：《地球科学（中国地质大学学报）》2009年第1期165-178,共14页Earth Science-Journal of China University of Geosciences

基　　金：国家自然科学基金项目(No.40625005)

摘　　要：通过对北太行紫荆关晚中生代基性岩体的岩石学和地球化学特征研究,讨论其岩浆源区特征及成因.结果表明,紫荆关岩体主要由橄榄二辉石岩、角闪辉长岩和闪长岩等岩石组成.橄榄二辉石岩具有明显的堆晶结构,可能是堆晶成因,而角闪辉长岩和闪长岩可能是堆晶形成之后的残余岩浆的结晶.闪长岩中的斜长石显示复杂的结构和成分环带,相对富钠的核部见富钙斜长石叠加,后者又向富钠斜长石演化.闪长岩中的单斜辉石也显示复杂的成分环带,主要是最边部的MgO含量比紧邻的内侧辉石更高.这表明在闪长岩形成过程中有新的幔源高温富镁的岩浆注入.以上各岩石类型具有类似的地球化学性质,包括较强的稀土分异((La/Yb)N比值为5.76～23.13),高Sr/Y比值(50.95～146.13),富集K、Sr、Ba等大离子亲石元素和LREE,亏损Nb、Zr、Ti等高场强元素和HREE,Eu异常不明显.都显示富集的Sr-Nd同位素特征,ISr=0.7058～0.7066,εNd(t)=-13.8～-18.2.这些特征表明紫荆关基性岩体的母岩浆来源于EM1型富集地幔的部分熔融.橄榄辉石岩和角闪辉长岩具有类似的Nd同位素成分(主要为εNd(t)=-15.0～-16.6),但形成较晚的闪长岩的εNd(t)值却比较高(-13.8～-14.6),也支持新的幔源岩浆在晚期的注入.Through the petrological and geochemical study carried out for the Zijingguan mafic pluton （North Taihang）, this paper is to understand their magma sources and petrogenesis. The Zijingguan mafic rocks are mainly made up of olivine-bearing pyroxenites, hornblende gabbros and diorites. Cumulate textures can be seen in the olivine-bearing pyroxenites, suggesting a cumulus origin of them. Hornblende gabbros and diorites are probably crystallized products from the residual melts after the accumulation process. Plagioclases from diorites show complicated textures and compositional zoning, with relatively sodium-rich core mantled by calcium-rich plagioclase, the latter evolved to sodium-rich plagioclase towards the rim with descent of magma temperature. Similarly, clinopyroxenes from diorite show complicated compositional zoning, with the MgO abundances in the rim being slightly higher than that of immediate inner part of pyroxenes. These textures suggest that the magma chamber had experienced a process of magma recharge from the mantle source. The different rock types of the Zijingguan mafic pluton share many common geochemical features like highly fractionated REE patterns （（La/Yb）N = 5.76-23.13） and high Sr/Y ratios （50.95-146.13）, highly enriched LILE （K, Sr, Ba） and LREE, and depleted HFSE （Nb, Zr, Ti） and HREE, with minor Eu anomalies in the REE patterns. They show enriched Sr-Nd isotopic compositions, with Isr=0.7058-0.7066, εNd （t）= -13.8 to -18.2. These geochemical features suggest that the mafic pluton originated from partial melting of an EM1 type mantle source. Olivine-bearing pyroxenites have isotopic composition identical to that of hornblende gabbros （mostly εNd （t）=-15 to -16.6）, while the later-crystallizing diorites have a slightly higher εNd（t）=-13.8 to -14.6, which is in agreement with the model of magma recharge during magma evolution.

关 键 词：基性岩体 斜长石环带 SR-ND同位素 岩浆补充 太行山 

分 类 号：P581[天文地球—岩石学]