A Haplogranite Melt Component In Many Mafic Granite Magmas

We distinguish between silicate melts and magmas, where the latter may contain a solid crystalline component in a fraction that permits free movement of the bulk magma under geological conditions. This is the precise sense in which Scrope introduced the term magma into petrology in 1862. Many more mafic granite magmas were not complete melts and comprised a haplogranite melt containing crystals residual from partial melting. The low-temperature granites (GSA Spec. Pap. 389, pp. 1-10) form from these magmas.

Ion probe studies show that the zircon in many granites includes grains with an older core rimmed by zircon precipitated from the melt - both parts were magmatic! The restite model of White and Chappell proposed that many granites solidified from magmas that comprised varying proportions of a felsic partial melt and a solid restite component residual from the partial melting. The presence of older zircon in the low-temperature granites means that the bulk magma never existed at a temperature high enough to be completely molten. Inherited zircons are representative of other mineral components that were not precipitated from the melt, which may either be original mineral components of the magma or else formed by reaction between such minerals and the melt. This is illustrated by related volcanic rocks, which may be strikingly similar to the granites in bulk composition. These volcanic rocks comprise phenocrysts in a haplogranite groundmass that resulted from partial melting. In the equivalent granite the pyroxene phenocrysts of the volcanic rock are largely or completely modified to biotite ± hornblende, and the plagioclase phenocrysts are represented by cores of crystals with zoned more Na-rich rims. The quartz phenocrysts are part of more abundant quartz crystals in the granite and the Or component of the volcanic groundmass occurs as K-feldspar crystals and in biotite of the granite.