PARTIAL MELTING OF APATITE-BEARING CHARNOCKITE, GRANULITE, AND DIORITE - MELT COMPOSITIONS, RESTITE MINERALOGY, AND PETROLOGIC IMPLICATIONS

Melting experiments (P = 6.9 kbar, T = 850-950 degrees C, NNO<fO(2)<HM ) were done on mafic to felsic charnockites, a dioritic gneiss, and a felsic garnet granulite, all common rock types in the Grenville baseme nt of eastern North America. A graphite-bearing granulite gneiss did n ot melt. Water (H2O+ = 0.60 to 2.0 wt %) is bound in low-grade, retrog rade metamorphic minerals and is consumed during the earliest stages o f melting. Most melts are water-undersaturated. Melt compositions rang e from metaluminous, silicic granodiorite (diorite starting compositio n) to peraluminous or weakly metaluminous granites (all others). In ge neral, liquids become more feldspathic, less silicic, and less peralum inous and are enriched in FeO, MgO, and TiO2 with increasing temperatu re. Residual feldspar mineralogy controls the CaO, K2O, and Na2O conte nts of the partial melts and the behavior of these elements can be use d, particularly if the degree of source melting can be ascertained, to infer some aspects of the feldspar mineralogy of the source. K-feldsp ar, a common restite phase in the charnockite and granulite (but not t he diorite) should control the behavior of Ba and, possibly, Eu in the se systems and yield signatures of these elements that can distinguish source regions and, in some cases, bulk versus melt assimilation. Apa tite, a common restite phase, is enriched in rare earth elements (REE) , especially middle REE. Retention of apatite in the restite will resu lt in steep, light REE-enriched patterns for melts derived from the di orite and charnockites.