Abstract
The Steel Mountain complex in western Newfoundland is one of several possible peri-Gondwanan basement slivers that have been emplaced along the northwestern margin of the Appalachian Orogen. The complex includes feldspathic anorthosite, gabbronorite, and magnetite-ilmenite ore. Megacrysts of plagioclase (andesine), orthopyroxene (hypersthene) and amphibole (hornblende) were analyzed by electron microprobe and they are of similar composition to the groundmass minerals. The ferromagnesian megacrysts have exsolution lamellae of Fe-Ti oxides, magnesio–hornblende, and clinopyroxene. The hornblende megacrysts and the lack of plagioclase in the exsolution lamellae are unusual for Proterozoic anorthosites and result from unusual abundance of water in the parental magma. Whole-rock Nd-Sm isotope determinations give an isochron age of ~1.2 Ga. Ar-Ar dating of biotite rims on hornblende shows an early Ordovician plateau age and the final heating step suggests a ~0.8 Ga age for the amphibole itself. Taken together with published zircon geochronology, these data suggest a provenance in the southern peri-Gondwanan Appalachians.
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