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

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.