An igneous clast from the Northwest Africa 773 (NWA 773) clan of lunar meteorites formed by silicic volcanism on the Moon. The clast was identified in Northwest Africa 2727 (NWA 2727), which is included in the NWA 773 clan. Over 80 mode% of the clast consists of silica + plagioclase + K-Ba-feldspar. The silica phases cristobalite, tridymite, and quartz are all present in the clast, indicating rapid cooling at low pressure in agreement with a volcanic setting. This clast is characterized as a dacite on the basis of mineral modes and whole-rock chemical composition. Olivine and pyroxene in the clast have high Fe/Mg ratios (olivine Fe# > 99, pyroxene Fe# > 99; with Fe# = molar Fe/(Fe + Mg) × 100), suggesting crystallization from a residual liquid after fractionation of more magnesian silicates. The clast is similar in some respects to ferroan gabbro alkaline-phase-ferroan (FG/ARFe) clasts that are inferred to be co-magmatic with olivine gabbro (OG) and other mafic lithologies of the NWA 773 clan. However, the high silica concentration and dominance of apatite as the main Ca-phosphate (no merrillite was identified) are distinct from the FG/ARFe clasts. Thus, the dacite clast probably crystallized in a magmatic setting that was independent of the OG and FG/ARFe lithologies. The mafic major element composition, young age, and high KREEP-content of the NWA 773 clan have been used previously to infer an origin from the Procellarum-KREEP terrane (PKT) on the nearside of the Moon. Several candidates for silicic volcanism/plutonism have been identified in the PKT (e.g., Gruithuisen Domes, Hansteen alpha, Lassell Massif). The presence of the dacite lithology provides additional support for an origin in or around PKT for meteorites of the NWA 773 clan, and for complex igneous activity in PKT region.

中文翻译：

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月球陨石中记录的月球硅质火山作用形成铁锰铁矿

来自西北非洲773（NWA 773）月球陨石的火成岩碎片，是由月球上的硅质火山作用形成的。在西北非洲2727（NWA 2727）中发现了这种爆炸物，这被包括在NWA 773家族中。至少80％的碎屑由二氧化硅+斜长石+钾长石组成。地壳中存在二氧化硅相方石英，鳞石英和石英相，表明在低压下迅速冷却与火山岩形成一致。根据矿物模式和全岩石化学成分，该碎屑被表征为泡翠。碎屑中的橄榄石和辉石具有高的Fe / Mg比率（橄榄石Fe＃> 99，辉石Fe＃> 99; Fe＃=摩尔Fe /（Fe + Mg）×100），表明在分馏后从残留液体中结晶更多镁质硅酸盐。该碎屑在某些方面与铁硼辉长岩碱性相铁（FG / ARFe）碎屑相似，后者推测与橄榄石辉长岩（OG）和NWA 773氏族的其他镁铁质岩性共岩浆。然而，高浓度的二氧化硅和磷灰石作为主要的Ca-磷酸盐（未鉴定到绢云母）占优势，与FG / ARFe碎屑不同。因此，the榴石碎屑可能在与OG和FG / ARFe岩性无关的岩浆环境中结晶。NWA 773氏族的镁铁质主要元素组成，年龄和较高的KREEP含量已被用来推断月球附近Procellarum-KREEP地层（PKT）的起源。在PKT中已经确定了几种硅质火山作用/岩性作用的候选物（例如Gruithuisen Domes，Hansteen alpha，Lassell Massif）。