Partial melting of apatite‐bearing charnockite, granulite, and diorite: Melt compositions, restite mineralogy, and petrologic implications

Authors:
  • JBJames S. Beard
    James S. Beard
  • GLGary E. Lofgren
    Gary E. Lofgren
  • ASA. Krishna Sinha
    A. Krishna Sinha
  • RTRichard P. Tollo
    Richard P. Tollo
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Abstract
Melting experiments (P = 6.9 kbar, T = 850–950°C, NNO < ƒ02 < HM) were done on mafic to felsic charnockites, a dioritic gneiss, and a felsic garnet granulite, all common rock types in the Grenville basement of eastern North America. A graphite‐bearing granulite gneiss did not melt. Water (H2O+ = 0.60 to 2.0 wt %) is bound in low‐grade, retrograde metamorphic minerals and is consumed during the earliest stages of melting. Most melts are water‐undersaturated. Melt compositions range from metaluminous, silicic granodiorite (diorite starting composition) to peraluminous or weakly metaluminous granites (all others). In general, liquids become more feldspathic, less silicic, and less peraluminous and are enriched in FeO, MgO, and TiO2 with increasing temperature. Residual feldspar mineralogy controls the CaO, K2O, and Na2O contents of the partial melts and the behavior of these elements can be used, particularly if the degree of source melting can be ascertained, to infer some aspects of the feldspar mineralogy of the source. K‐feldspar, a common restite phase in the charnockite and granulite (but not the diorite) should control the behavior of Ba and, possibly, Eu in these systems and yield signatures of these elements that can distinguish source regions and, in some cases, bulk versus melt assimilation. Apatite, a common restite phase, is enriched in rare earth elements (REE), especially middle REE. Retention of apatite in the restite will result in steep, light REE‐enriched patterns for melts derived from the diorite and charnockites.

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