Petrology and Sr-Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province
Petrology and Sr-Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province
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DOI：
10.1093/petrology/egu067
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发表时间：
2014-12
影响因子：
3.9
通讯作者：
Xun Wei;Yigang Xu;Chuan-Lin Zhang;Jian-xin Zhao;Yue‐xing Feng
中科院分区：
文献类型：
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作者：
Xun Wei;Yigang Xu;Chuan-Lin Zhang;Jian-xin Zhao;Yue‐xing Feng
层状镁铁质侵入岩（LMI）零星分布在中国西北部早二叠世塔里木大火成岩省（LIP），并被众多同生岩脉横切。小海子韦尔长石岩体主要由橄榄石(Fo69-75)、单斜辉石(Mg#=75-84)、积间斜长石(An53-86)和Fe-Ti氧化物组成。岩相学和矿物成分都表明橄榄石和单斜辉石的结晶早于斜长石和铁钛氧化物。岩脉为碱性玄武岩至粗安岩，Mg# 较低（35-39）。相对于其他类似的不相容元素，污染最少的岩脉表现出强烈的稀土元素 (REE) 分馏、Nb 和 Ta 的富集以及 Pb 的贫化，与洋岛玄武岩 (OIB) 具有很强的相似性。这与它们的正 epsilon Nd-i 值 (4.3-4.8) 一起，与来自丰富的软流圈地幔源的推导一致。斜辉石中的单斜辉石显示出向上凸的球粒陨石标准化 REE 图案。与这些单斜辉石平衡的熔体具有与横切岩脉非常相似的微量元素组成，表明小海子韦尔岩侵入体和岩脉具有相似的地幔来源。小海子斜长石岩体的特点是单斜辉石与斜长石分离物之间的Sr-Nd同位素不平衡：单斜辉石的Sr-87/(86)Sri(0.7038-0.7041)和epsilon Nd-i(1.0-1.9)分别低于和高于间积斜长石的比例（Sr-87/Sr-86(i) = 0.7042-0.7043，ε Nd-i = 0.4-1.0）。单斜辉石分离的 Sr-87/(86)Sri 和 epsilon Nd-i 分别与 Zr/Nb 正相关和负相关，这意味着小海子韦尔岩侵入体形成过程中受到不同程度的地壳污染。随着斜长石 Ca 含量的增加，Sr-87/(86)Sri 增加，ε Nd-i 减少，表明较高 An 的斜长石受到更高程度的污染。这可以通过湍流岩浆上升（ATA）过程中大陆地壳的同化来解释。然而，该 ATA 模型未能解释单斜辉石分离物的 Mg# 和 epsilon Nd-i 之间的正相关性。小海子LMI的同位素不平衡更可能是在涉及太古代-新元古代基底和碳酸盐作为污染物的同化和分异结晶过程中产生的。
Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi wehrlite intrusion is composed mainly of olivine (Fo69-75), clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi wehrlite intrusion and dykes. The Xiaohaizi wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.