Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
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塔里木地幔柱的组成：来自中国西北部早二叠世小海子岩脉中单斜辉石前晶的约束
DOI：
10.1016/j.lithos.2015.05.010
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发表时间：
2015-08
期刊：
影响因子：
3.5
通讯作者：
Yue-Xing Feng
中科院分区：
文献类型：
--
作者：
Yi-Gang Xu;Zhen-Yu Luo;Jian-Xin Zhao;Yue-Xing Feng
在中国西北部塔里木火成岩省，大量碱性玄武岩岩脉在岩心和正长岩侵入体中横切早二叠世小海子韦尔岩。一个玄武岩岩脉含有丰富的单斜辉石粗晶，具有强吸收结构。这种结构不平衡与大晶 (Mg# = 80–89) 和主岩脉 (Mg# = 39，对应于与岩脉平衡的单斜辉石 Mg# = 73) 之间的化学对比一致，表明它们不是斑晶。单斜辉石巨晶的特点是 TiO2（0.26–1.09 wt.%）、Al2O3（1.15–3.10 wt.%）和 Na2O（0.16–0.37 wt.%）含量较低，与地幔橄榄岩中的不同，但与同一地区层状镁铁质侵入体中的相似。单斜辉石大晶体和小海子堆积体韦尔岩中的单斜辉石定义了一致的成分趋势，并具有相同的微量元素模式，表明这些晶体的彗差成因。因此，大晶体不可能是岩浆系统外来的异晶。相反，它们是从祖岩浆中结晶出来的前晶体，并在入侵之前重新融入到宿主岩脉中。高Mg#（80-89）单斜辉石粗晶的87Sr/86Sri（0.7035-0.7037）和εNdi（4.5-4.8）明显低于和高于斜辉石中单斜辉石各自的比例（Mg# = 75–84,87Sr/86Sri= 0.7038–0.7041，εNdi= 1.0–1.9）。这种同位素差异可以通过小海子岩体形成过程中进行的同化和分级结晶（AFC）过程来解释。从这个意义上说，单斜辉石宏观晶体记录了未受污染的塔里木羽流熔体的成分。
Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80–89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The clinopyroxene macrocrysts are characterized by low TiO2(0.26–1.09 wt.%), Al2O3(1.15–3.10 wt.%) and Na2O (0.16–0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The clinopyroxene macrocrysts and the clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenocrysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The87Sr/86Sri(0.7035–0.7037) and εNdi(4.5–4.8) of the clinopyroxene macrocrysts with high Mg# (80–89) are apparently lower and higher than their respective ratios of the clinopyroxenes in the wehrlites (Mg# = 75–84,87Sr/86Sri= 0.7038–0.7041, εNdi= 1.0–1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts.
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