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.