EVIDENCE FOR MAGMA DEFROSTING AND MELT EXTRACTION IN THE ASHLAND PLUTON, NORTHERN CALIFORNIA AND SOUTHERN OREGON

Magma defrosting, heating and partially melting a crystal-rich, weakly mobile or immobile magmatic mush, has gained wide support from volcanological studies because it provides a mechanism by which crystal-rich (monotonous) dacites may erupt or by which crystal-poor felsic magmas may be extracted from a mush. Studies of the latter type of defrosting call on a conjugate, cumulate plutonic part of the system from which the felsic melt was extracted. Petrographic and geochemical features of defrosting are well described for eruptive rocks, it is less clear which features of cumulate plutonic rocks provide evidence for defrosting and melt extraction. The Jurassic Ashland pluton is a tilted intrusive complex in which the quartz monzodioritic (QMD) central zone of the pluton displays crystal fragmentation, widespread mineral resorption, and local concentrations of refractory minerals, all of which are consistent with partial melting of a magmatic mush. In addition, hornblende and plagioclase in QMD samples are not in exchange equilibrium with a melt of bulk-rock composition, indicating that most QMD rocks are cumulates. The QMD unit is intruded and overlain by biotite granite. In the biotite granite unit, plagioclase core compositions are identical to plagioclase in the underlying QMD, some plagioclase cores contain inclusions of resorbed titanite, and alkali feldspar displays subhedral to anhedral core zones in poikilitic crystals. Hornblende and biotite in the biotite granite display enrichments in Ti and Nb relative to hornblende and biotite in QMD, indicating that the biotite granite did not evolve via fractional crystallization of the QMD magma. Instead, the high Ti and Nb contents reflect significant partial melting of titanite in the QMD mush, thereby releasing high field strength elements to the resultant (defrosted) biotite granite melt. The biotite granite magmas are thus interpreted as products of defrosting and melt extraction from the QMD; this melt extraction explains the cumulate compositions of most QMD samples. Widespread, intrusion-wide mingling and mixing of mainly intermediate magmas is thought to be the heat source for defrosting.