Magma Defrosting: Evidence from Plutonic Rocks

Abstract

Abstract The concept of defrosting, heating and partially melting a crystal-rich, weakly mobile or immobile magma (magmatic mush) has gained wide support from volcanological studies. This process has been called on as a means to promote eruption of crystal-rich (monotonous) dacites and also to permit extraction of felsic magmas, which may accumulate in the upper crust and erupt as crystal-poor rhyolite or trachyte. Most studies of the latter type of defrosting call on a conjugate plutonic part of the system from which the felsic melt was extracted. Although petrographic and geochemical features of defrosting are well described for eruptive rocks, few studies have investigated petrographic and geochemical features of plutons that indicate defrosting. The Jurassic Ashland pluton is a tilted intrusive complex exposed in southern Oregon and northern California, USA. The central part of the pluton, quartz monzodiorite (QMD), displays petrographic features (crystal fragmentation, widespread resorption, local concentrations of refractory minerals) characteristic of partial melting. This unit is intruded and overlain by biotite granite, which displays plagioclase cores identical in composition to plagioclase in the underlying QMD, resorbed titanite cores in plagioclase, and local subhedral to anhedral cores of alkali feldspar in poikilitic alkali feldspar crystals. Hornblende and biotite in the biotite granite display enrichments in Ti and Nb relative to these minerals in QMD, negating fractional crystallization as a petrogenetic process. Instead, these high Ti and Nb contents reflect significant partial melting of titanite in the QMD, thereby releasing high field strength elements to the defrosted biotite granite melt. The biotite granite magmas are thus interpreted as the melt-rich products of defrosting caused by intrusion of mafic–intermediate magmas into the QMD magma column.