The St. Anthony complex, northwestern Newfoundland : a petrological study of the relationship between a peridotite sheet and its dynamothermal aureole

摘要

The St. Anthony Complex consists of a suite of alkaline basaltic rocks (the Ireland Point Volcanics), greenschist and epidote amphibolite facies metasedimentary and metabasic rocks (the Goose Cove Schist), amphibolite to transitional granulite facies metabasites (the Green Ridge Amphibolite), an assemblage of dunite, troctolite, gabbro, and anorthosite (the Long Ridge Metagabbro), and a lherzolite-harzburgite massif (the White Hills Peridotite). These rocks are interpreted as a partial ophiolite and an underlying dynamothermal aureole formed during its tectonic emplacement. -- The overall metamorphic grade increases from the base of the St. Anthony Complex to the peridotite contact. Lithological and structural discontinuities occur within the aureole, with the epidote amphibolite to amphibolite facies transition marked by a metasomatic mylonite zone. Temperatures of 850ﾟC to 950ﾟC have been calculated for the amphibolites and peridotites in the contact zone, while the transition from greenschists to undeformed volcanics corresponds to temperatures of 300ﾟC to 350ﾟC. This indicates an overall thermal gradient of 750ﾟ to 800ﾟC/km. Pressure estimates range from 7 to 10 kb near the peridotite contact to 2 kb or less at the greenschist-volcanic transition. -- The lithological evidence and the pressure and temperature estimates suggest that the aureole is composite, formed by the juxtaposition of progressively shallower units during the dynamothermal metamorphism which accompanied the tectonic emplacement of the peridotite. Thermal models indicate that heating by conduction and friction contributed to the high contact temperatures. The evidence is consistent with a model involving westward transportation of oceanic lithosphere over tholeiitic gabbros, alkaline basalts, and turbidites to form the St. Anthony Complex, before final emplacement along late thrust faults as part of the Hare Bay Allochthon. -- Comparison of the St. Anthony Complex with other peridotite-aureole complexes shows general similarities in geological setting, lithology, and metamorphic grade. It is concluded that the formation of dynamothermal aureoles beneath transported peridotite is a normal process accompanying the early stages of tectonic peridotite emplacement.