METAMORPHISM OF MAFIC AND FELSIC IGNEOUS ROCKS WITH EMPHASIS ON PALEOZOIC METAMORPHISM IN SW NORTH CAROLINA: KEY TO DEFINING GRANULITE FACIES METAMORPHISM AND MINERAL-CHEMICAL CONSTRAINTS ON PEAK P-T CONDITIONS

The strict definition of the granulite facies (GF) by Eskola (1920, cited in and discussed by the SCMR of the IUGS: http://www.bgs.ac.uk/SCMR/docs/scmr_meta_r2.pdf), requires co-existing orthopyroxene (Opx), clinopyroxene (Cpx), and plagioclase. In general, rocks of basaltic composition are best suited to defining facies, based on both their widespread distribution and their tendency to develop distinct mineral assemblages that persist over broad pressure-temperature (P-T) regimes. Other compositions may develop this definitive GF assemblage, but metabasalts appear most sensitive to this reaction.

In the eastern Wayah Bald and western Franklin 7.5" quadrangles, SW NC, metabasaltic rocks interlayer with both metasedimentary and felsic metaigneous (Trimont Ridge Complex, TRC) rocks; only locally do they comprise distinct map-scale bodies. SHRIMP zircon dates in some proximal TRC felsic gneiss are ~1.1 Ga basement (Hatcher et al., 2004, GSA Mem. 197, p. 525); mafic intrusive ages are uncertain, and could be variable. Regardless, numerous dates have confirmed the peak GF metamorphism as Paleozoic (Ordovician; e.g. Moecher et al., 2004, J. Geol. p. 289). The progression from the amphibolite facies appears continuous (e.g., Eckert et al., GSA Bull. 101, p. 1434). TRC felsic gneiss, in many areas, is associated with metamafic rocks in various geometries, including intimately interlayered. This K-deficient, oligoclase-dominated leucotonalite likely formed from a noncontinental source (Eckert and Mohr, 1986, GSA abs. 18, p. 219). Some or all of these metamafic rocks may be age- and/or source-equivalent with the TRC basement, or potentially might have intruded later. Three metamafic rocks from which major-element bulk chemistry was determined (Eckert, 1984, MS, Univ. S. Carolina) all plot in oxide diagrams as tholeiitic basalt, proximal to mid-Atlantic-ridge basalt as likely mafic cumulates; one of these occurs within the TRC. All 3 metabasalts also fall in or near the ocean-floor basalt field in Pearce (1976, Jour. Pet. 17, p. 15) discriminant plots, again consistent with a noncontinental affinity. Most peak P-T estimates in the GF, by thermobarometry in both Grt-Opx/Cpx and metapelitic assemblages, are ~750-850°C, ~7-8.5 kbars. These noncontinental igneous rocks were entrained and buried deeply along the early convergent margin of Laurentia.