A Field Study of the Liquid Line of Descent of Hydrous Alkaline-rich Magmas at Elevated Pressures (0.5-1.0 GPa): The Dariv Alkaline Intrusive Complex

了解长英质岩浆高度多样化的性质的起源，无论是作为流纹岩喷发还是作为大型地壳上部岩石侵位，对于研究产生富含二氧化硅的大陆地壳的机制至关重要。普遍的共识是，强烈的所谓过铝花岗岩至少在一定程度上来自于变质沉积原岩的白云母和黑云母脱水熔融。然而，所谓的偏铝质花岗岩的起源并不那么确定。该项目旨在通过野外研究来研究碱性岩浆的含水液体下降线(LLD)以及含水分离结晶和部分熔融在偏铝碱性花岗岩形成中的相对作用。虽然碱性花岗岩在体积上没有钙质碱性花岗岩那么重要，但它们的成岩作用特别重要，因为它们往往与经济上重要的矿化有关。我们将研究在达里夫弧杂岩(蒙古西部阿拉泰地区)中新发现的一处曝露。该项目将研究层状层序，范围从以磷云母为主的超镁铁质/镁铁质到中等(石英)二长岩和(石英)正长岩，在空间上与大型碱长花岗岩有关。据我们所知，类似完整的碱性深成层序在以前的地质文献中从未被描述过。特别是，蒙古出露的超镁铁质岩石组合目前仅被描述为来自碱性玄武岩岩浆的包体。从超镁铁质岩石到长英质岩石的完全暴露为研究晶体分馏作为碱性长石花岗岩形成的可能机制提供了一个无与伦比的机会。

Understanding the origin of the highly diverse nature of felsic magmas, erupted as rhyolite or emplaced as large upper-crustal batholiths, is crucial to the investigation of the mechanisms that generate the silica-enriched continental crust. General consensus exists that strongly so-called peraluminous granites are derived, at least to some significant extent, from muscovite- and biotite-dehydration melting of metasedimentary protholiths. The origin of so-called meta-aluminous granites, however, is less certain. This project aims to investigate the hydrous liquid line of descent (LLD) of an alkaline magma and the relative roles of hydrous fractional crystallization versus partial melting in the formation of meta-aluminous alkaline granites through a field-based study. While alkaline granites are volumetrically less significant than calk-alkaline granites, their petrogenesis is of particular importance as they are often associated with economically significant mineralization. We will study a newly discovered exposures in the Dariv arc complex (Altai region of western Mongolia). The project will investigate a layered sequence ranging from phologopite-dominated ultramafic/mafic to intermediate (quartz-) monzonite and (quartz-) syenites rocks is spatially associated with large alkali-feldspar granite bodies. A similarly complete alkaline plutonic sequence has, to our knowledge, never been described in the geological literature before. In particular, the ultramafic rock assemblages exposed in Mongolia are currently only described as xenoliths from alkaline basaltic magmas. The complete exposures from ultramafic to felsic rocks provide an unrivaled opportunity to investigate crystal fractionation as a possible mechanism of alkali-feldspar granite formation.