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首页> 外文期刊>Contributions to Mineralogy and Petrology >Phlogopite- and clinopyroxene-dominated fractional crystallization of an alkaline primitive melt: petrology and mineral chemistry of the Dariv Igneous Complex, Western Mongolia
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Phlogopite- and clinopyroxene-dominated fractional crystallization of an alkaline primitive melt: petrology and mineral chemistry of the Dariv Igneous Complex, Western Mongolia

机译：碱性原始熔体的金云母和斜辉石为主的分步结晶：蒙古西部达里夫火成岩复合体的岩石学和矿物化学

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摘要

We present field relationships, petrography, and mineral major and trace element data for the Neo-proterozoic Dariv Igneous Complex of the Altaids of Western Mongolia. This unique complex of high-K plutonic rocks is composed of well-exposed, km-scale igneous intrusions of wehrlites, phlogopite wehrlites, apatite-bearing phlogopite clinopyroxenites, monzo-gabbros, monzodiorites, and clinopyroxene-bearing monzonites, all of which are intruded by late stage lamprophyric and aplitic dikes. The biotite-dominated igneous complex intrudes depleted harzburgitic ser-pentinite. The observed lithological variability and pet-rographic observations suggest that the plutonic rocks can be ascribed to a fractionation sequence defined by olivine + clinopyroxene ± Fe-Ti oxides -> phlogopite + apatite -> K-feldspar + plagioclase -> amphibole + quartz. Notably, phlogopite is the dominant hydrous mafic mineral. Petrogenesis of the observed lithologies through a common fractionation sequence is supported by a gradual decrease in the Mg# [molar Mg/(Fe_(total) + Mg) x 100] of mafic minerals. Crystallization conditions are derived from experimental phase petrology and mineral chemistry. The most primitive ultramafic cumulates crystallized at <0.5 GPa and 1,210-1,100 °C and oxygen fugacity (fO_2) of +2-3 AFMQ (log units above the fayalite-quartz-magnetite buffer). Trace element modeling using clinopyroxene and apatite rare earth element compositions indicates that the dominant mechanism of differentiation was fractional crystallization. The trace element composition of a parental melt was calculated from primitive clinopyroxene compositions and compares favorably with the compositions of syn-magmatic lamprophyres that crosscut the fractionation sequence. The parental melt composition is highly enriched in Th, U, large ion lithophile elements, and light rare earth elements and has a pronounced negative Nb-Ta depletion, suggestive of an alkaline primitive melt originating from a subduction-imprinted mantle. Comparison with a global compilation of primitive arc melts demonstrates that Dariv primitive melts are similar in composition to high-K primitive melts found in some continental arcs. Thus, the high-K fractionation sequence exposed in the Dariv Igneous Complex may be a previously unrecognized important fractionation sequence resulting in alkali-rich upper crustal granitoids in continental arc settings.
机译：我们介绍了蒙古西部阿尔泰山脉的新元古代达里夫火成岩复合体的田间关系，岩相学，矿物主要和微量元素数据。这种高K型深成岩的独特复合体由暴露良好的千米级火成岩，白云母，金云母，白云母，磷灰石，金云母，辉长岩，辉石，辉长岩，辉闪石和斜辉石的辉石组成，所有这些都是侵入岩通过后期的墨生性和顶生堤防。黑云母为主的火成岩侵入了贫化的哈兹伯格型蛇纹石。观察到的岩性变化和岩相学观察表明，该古生岩可归因于橄榄石+斜辉石±铁钛氧化物->金云母+磷灰石-> K长石+斜长石->角闪石+石英所定义的分馏序列。值得注意的是，金云母是主要的含水镁铁质矿物。镁铁质矿物的Mg＃[摩尔Mg /（Fe_（总计）+ Mg）x 100]逐渐降低，可以通过共同的分馏序列实现岩性的成岩作用。结晶条件来自实验相岩石学和矿物化学。最原始的超镁铁质晶体在<0.5 GPa和1,210-1,100°C时结晶，氧逸度（fO_2）为+ 2-3 AFMQ（铁橄榄石-石英-磁铁矿缓冲液上方的对数单位）。微量元素建模，使用次生辉石和磷灰石稀土元素组成表明，主要的分化机理是分步结晶。母体熔体的痕量元素组成是由原始的次氯环己烯组成计算得出的，可与横切分馏序列的同岩浆岩萤组成进行比较。母体熔体成分高度富含Th，U，大型离子亲石元素和轻稀土元素，并且Nb-Ta明显为负，表明碱性原始熔体源自俯冲烙印地幔。与原始弧熔体的整体汇编的比较表明，Dariv原始熔体的成分与在某些大陆弧中发现的高K原始熔体相似。因此，在达里夫火成岩中暴露的高K分馏序列可能是以前无法识别的重要分馏序列，导致大陆弧环境中富含碱的上地壳花岗岩。

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