Melt Migration and Interaction in a Dunite Channel System within Oceanic Forearc Mantle: the Yushigou Harzburgite–Dunite Associations, North Qilian Ophiolite (NW China)
Melt Migration and Interaction in a Dunite Channel System within Oceanic Forearc Mantle: the Yushigou Harzburgite–Dunite Associations, North Qilian Ophiolite (NW China)
复制标题
DOI：
10.1093/petrology/egaa115
复制
发表时间：
2021-01
影响因子：
3.9
通讯作者：
Xiang Zhou;Jianping Zheng;Yibing Li;Hui Zhu;W. Griffin;S. O’Reilly
中科院分区：
文献类型：
--
作者：
Xiang Zhou;Jianping Zheng;Yibing Li;Hui Zhu;W. Griffin;S. O’Reilly
鱼石沟蛇绿杂岩(北祁连缝合带，中国西北部)中的方辉橄榄岩-纯橄榄岩组合为俯冲带上方的熔体迁移和熔岩相互作用提供了新的线索。在详细的岩相学和现场分析的基础上，提出了描述大洋弧地幔熔体通道形成历史的三阶段模型。在第一阶段，高度熔融提取之后是小体积熔体的广泛渗透，形成了构成玉石沟杂岩大部分的方辉橄榄岩。因此，玉石沟方辉橄榄岩具有高耐火主量元素组成(橄榄石Fo=91·1~93·2，尖晶石Cr#[原子100×Cr/(Cr+Al)]=55·2~62·9，全岩Mg#=90·5~92·0，Al_2O_3=0.29~0.74 wt%，CaO=0.35~0.54 wt%，Na2O=0.02~0.05wt%)，轻稀土元素(LREE)略有富集。第二阶段，大量集中熔体渗入地幔楔体，形成两种类型的纯橄榄石，高铬尖晶石(主要为63·1~73·6)和低铬尖晶石(23·6~33·7)。低铬纯橄榄岩中尖晶石包裹体明显较少，橄榄石的Fo值(88·7~90·1)明显低于高铬纯橄榄岩(Fo=·9~90·5)，而间隙硫化物的形态丰度较高。两组纯橄榄岩表现出相似的极低的二氧化钛(高铬纯橄榄岩为10)，这表明两组纯橄榄岩的渗透熔体存在明显的差异。低铬纯橄榄岩是富铝、富S的无水玄武岩熔体的堆积体，高铬纯橄榄岩则是方辉橄榄岩与含碳碱性水合硅酸盐岩浆反应形成的。这些熔体的汇流最终形成了沟道化的纯橄榄岩网络，后来的岩浆活动受限于这些通道。铬铁矿结晶后的脉动熔体供给在两种纯橄榄岩中的尖晶石周围形成了弥漫的筛状织构边缘，但方辉橄榄岩中这种边缘很少见。第三阶段以毫米级脉状导管记录的流体控制交代作用为标志，两种纯橄榄石中均含有碳酸盐(白云石和菱镁矿)、角闪石、金云母和复合晶体组合。CH4-N_2-石墨为主的流体包裹体广泛存在于所有岩性中，记录了最后一次交代事件中的挥发分。因此，玉石沟杂岩提供了多批次熔体或流体与岩石圈地幔楔形二粒岩通道系统之间相互作用的详细例子。这项研究的结果进一步表明，豆荚状铬铁矿的形成需要熔体的参与，熔体的性质和来源可能是不同的。
Harzburgite–dunite associations in the Yushigou ophiolitic complex (North Qilian suture, NW China) offer insights into melt migration and melt–rock interaction above a subduction zone. Based on detailed petrographic and in situ analyses, we propose a three-stage model to describe the history of melt channel formation in oceanic arc mantle. In the first stage, high-degree melt extraction was followed by widespread infiltration of small-volume melts, which formed the harzburgite that makes up most of the Yushigou complex. The Yushigou harzburgites thus have highly refractory major-element compositions (olivine Fo = 91·1–93·2, spinel Cr# [atomic 100 × Cr/(Cr + Al)] = 55·2–62·9, whole-rock Mg# = 90·5–92·0, Al2O3 = 0·29–0·74 wt%, CaO = 0·35–0·54 wt%, and low Na2O = 0·02–0·05 wt%) but slight enrichment in the light rare earth elements (LREE). In the second stage, high-volume focused melts infiltrated into the mantle wedge and produced two types of dunite, with high-Cr# (mainly 63·1–73·6) versus low-Cr# (23·6–33·7) spinel. In the low-Cr# dunites, spinels have significantly fewer inclusions and lower Fo values (88·7–90·1) in olivine than in the high-Cr# dunite (Fo = 89·9–90·5), together with higher modal abundances of interstitial sulfides. The two groups of dunite show similar extremely low TiO2 (10 in high-Cr# dunite), indicating distinct differences in the infiltrating melts. The low-Cr# dunite is a cumulate from an anhydrous Al- and S-enriched basaltic melt, whereas the high-Cr# dunite was produced by reaction of harzburgite with a carbon-bearing, alkaline hydrous silicate magma. The confluence of these melt migrations finally formed a channelized dunite network, and later magmatic activity was restricted to these channels. Pulsated melt supply after chromite crystallization formed pervasive sieve-textured rims around spinel in both kinds of dunite, but these are rare in the harzburgite. The third stage was marked by fluid-dominated metasomatism recorded by millimeter-scale veined conduits, which contain carbonate (dolomite and magnesite), amphibole, phlogopite and compound crystal assemblages in both kinds of dunite. CH4–N2–graphite-dominated fluid inclusions are widespread in all lithologies, recording the volatiles transported during the last metasomatic event. The Yushigou complex thus provides a detailed example of interaction between multiple batches of melt or fluid and a lithospheric mantle wedge dunitic channel system. The results of this study further suggest that the formation of podiform chromite requires melt participation, and the nature and origins of the melts can be diverse.