A preliminary study on the genesis of high-silica granite by incomplete separation of felsic cumulate-melt: leucocratic granites along the eastern contacts of the Middle Yangsan Fault, South Korea

Granitic crystal mush consists of felsic cumulate and interstitial melt. Thus, the whole-rock compositions of fully crystallized granite depend on the proportions of cumulus felsic minerals and interstitial melt. Here, we (1) report the petrographic and whole-rock major and trace element elements characteristics of the Moa, Bomun and Dongcheon Granite occurring at the eastern contacts of the Middle Yangsan Fault, South Korea and (2) provide a case that the geochemical proxies used in tectonic discrimination may reflect felsic cumulate-melt separation rather than tectonic setting where the granites formed. Granitic rocks in the study area are subdivided into biotite granite (Moa and Bomun Granite) and alkali-feldspar granite (Dongcheon Granite). They are leucocratic monzogranite-syenogranite-alkali-feldspar granite on the basis of modal compositions. Feldspars and biotite in the alkali-feldspar granite have more evolved composition than those in the biotite granite. These leucocratic granites have extremely high silica contents (74-78 wt.%), showing a continuous calc-alkalic trend evolving from the magnesian metaluminous to ferroan peraluminous fields. The major and trace element elements characteristics suggest that the leucocratic high-silica granites are a product of granitic crystal mush in the shallow volcanic-arc crust. Our fractional crystallization models indicate that the biotite granite is felsic cumulate-dominant part whereas the alkali-feldspar granite is frozen melt extracted from the crystallizing mush.