Are Strongly Peraluminous Magmas Derived from Pelitic Sedimentary Sources?

Abstract

Igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous (Ps). Such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks. The term S-type has been applied to many igneous rocks based largely or even solely upon their Ps compositions, neglecting or placing less emphasis on other criteria for sedimentary parentage in Chappell and White's I- and S-type classification system. The S-type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature (strongly weathered-pelitic, shaly) source. Application of Chappell's and White's criteria, together with others based upon isotopic and phase equilibria considerations, leads to the following conclusions about Ps igneous rocks worldwide: (1) very few Ps rocks are derived entirely from pelitic sources; (2) a majority are derived largely from intermediate to felsic crustal sources, including both immature sedimentary rocks (e.g., metagraywackes) and metaigneous rocks; and (3) some are the products of partial melting of metaluminous mafic (crustal or subcrustal) sources-Ps compositions of such magmas may be a primary feature or a result of fractional crystallization.