A FIELD AND GEOCHEMICAL STUDY OF THE HYDROUS FRACTIONATION OF AN ALKALINE PRIMITIVE ARC MELT: THE DARIV IGNEOUS COMPLEX, WESTERN MONGOLIA

The origin of felsic rocks constituting the upper continental crust is uncertain as both partial melting in the lower continental crust and hydrous fractionation of Si-poor cumulates can generate similar Si-enriched derivative melts. In order to understand the relative importance of these two fractionation mechanisms, it is essential to study field relations where the connection between cumulates/restites and derivative liquids can be made. Whereas field studies documenting the importance of hydrous fractionation for tholeiitic/calc-alkaline basalts in the formation of typical granodiorite, tonalite and granites exist, the origin of a more alkaline sequence dominated by monzonite and (alkali-) granites has up until now not been investigated in the field. Here, we present results from a combined field, petrological, & geochemical study of a paleo-arc alkaline fractionation sequence from the Dariv Range of the Mongolian Altaids documenting an alkaline rich liquid line of descent (LLD). In Dariv, a complete hydrous alkaline fractionation sequence ranging from phlogopite-bearing ultramafic & mafic cumulates through quartz-monzonites & alkali-feldspar granites is exposed, which so far has been documented only in fragmentary phlogopite-bearing ultramafic xenoliths. Several late stage fine grained mafic dykes representative of mantle-derived melt compositions parental to the cumulate and derivative rock sequence are characterized by high Na₂O & K₂O contents, enrichments in LILE & Pb, and depletions in Nb & Ta typical for arc magmas. We consider the rocks to represent an alkali-rich hydrous fractionation sequence defined by the appearance olivine + clinopyroxene ± Fe-Ti oxides --> phlogopite + apatite --> k-feldspar + plagioclase --> amphibole + quartz. To quantify the LLD of the crystallizing magma, we present a fractionation model, which incorporates a stepwise subtraction of cumulates of a fixed composition determined by cumulate analyses. The amounts of fractionated cumulates are constrained from geochemical fractionation indices such as Mg# (molar Mg/(Mg+Fe)) and verified with field observations. Our study demonstrates that the fractionation of phlogopite-rich cumulates from a primitive alkaline arc magma is capable of producing alkali-rich derivative granitoids.