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

초록

Granitic crystal mush consists of felsic cumulate and interstitial melt. Thus, the whole-rockcompositions of fully crystallized granite depend on the proportions of cumulus felsic minerals and interstitialmelt. Here, we (1) report the petrographic and whole-rock major and trace element elements characteristics of theMoa, Bomun and Dongcheon Granite occurring at the eastern contacts of the Middle Yangsan Fault, South Koreaand (2) provide a case that the geochemical proxies used in tectonic discrimination may reflect felsic cumulate-meltseparation rather than tectonic setting where the granites formed. Granitic rocks in the study area are subdividedinto biotite granite (Moa and Bomun Granite) and alkali-feldspar granite (Dongcheon Granite). They are leucocraticmonzogranite-syenogranite-alkali-feldspar granite on the basis of modal compositions. Feldspars and biotite inthe alkali-feldspar granite have more evolved composition than those in the biotite granite. These leucocraticgranites have extremely high silica contents (74-78 wt.%), showing a continuous calc-alkalic trend evolving fromthe 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-arccrust. Our fractional crystallization models indicate that the biotite granite is felsic cumulate-dominant part whereasthe alkali-feldspar granite is frozen melt extracted from the crystallizing mush.

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