Generation, Diversification, and Emplacement of the Rockford Granite, Alabama Appalachians: Mineralogic, Petrologic, Isotopic (C & O), and P--T Constraints

Abstract

The Rockford Granite is a plutonic suite of synmetamorphic, two-mica granitoids that intrude graphite-bearing, amphibolite-facies metasediments in the eastern Blue Ridge of the Alabama Appalachians. Geochemical classification indicates peraluminous granite and trondhjemite with subordinate granodiorite. The trondhjemites are products of pervasive, infiltrative Na-metasomatism; whereas the granites and granodiorites sufficiently retain primary traits for petrogenetic modelling. A significant proportion of the Rockford Granite magmatic phases may represent refractory restite, expressed as centimeter-size biotite clots, normal oscillatory zoned plagioclase with a subidiomorphic zone-boundary core, and rounded, zoned xenocrystic zircons. Major and trace element modelling indicate that the Rockford Granite could be a product of 40% equilibrium partial melting of a metagraywacke source producing felsic, granitic melt plus a restite assemblage of plagioclase49-quartz31-biotite14-apatite5 5-zircon0 5. The limited geochemical heterogeneity of the Rockford Granite is attributed to progressive restite-melt separation, reflected by systematic collinearity of calculated source and restite compositions in bivariate diagrams. Whole rock oxygen and reduced carbon isotope analyses support the metagraywacke source model for the Rockford Granite and also record regional isotopic exchange with metamorphic fluids from the volumetrically-dominant metapelites. Combined whole-rock oxygen and carbon isotope determinations both constrain granitoid source rock characteristics and measure the extent of C-O-H metamorphic fluid-rock interaction. Quantitative geothermobarometric estimates for the Rockford Granite and surrounding metasediments indicate kyanite-sillimanite facies series regional metamorphism, with a Barrovian geothermal gradient of 30°C/km. The prograde P-T path is abruptly terminated by nearly isothermal decompression, interpreted as due to diapiric intrusion and associated thermal doming. Rockford Granite and migmatite samples fall in the P-T regime where the maximum overlap between muscovite dehydration and minimum melt conditions exist suggesting that the former controlled anatexis. The tectonic setting and depositional environment of the host metasedimentary protoliths are interpreted as associated with opening of the lapetus Ocean. Structurally lowermost are interlayered amphibolites and quartzites, representing synrift volcanism and sedimentation, overlain by thick passive margin pelitic and graywacke deposits. The Rockford Granite contains a Grenville-age (l·0–1·2 Ga) inherited, xenocrystic zircon component interpreted to record crustal recycling via weathering of a Grenville gneiss source to produce metagraywacke protoliths, followed by regional metamorphism and anatexis to yield the granitic magmas. Barrovian regional metamorphism, associated crustal shortening, and anatexis in an Acadian continental-oceanic subduction regime best represents the tectonomagmatic setting of Rockford Granite genesis.