Abstract

The Mont Albert mafic–ultramafic complex of central Gaspé, Quebec, is generally regarded as a partial ophiolite with an underlying metamorphic sole. The metamorphic rocks include a number of mineralogical and textural varieties of amphibolite, including some migmatite, as well as minor metasedimentary and quartzo-feldspathic gneisses. The most intriguing rocks in the complex are mafic to ultramafic garnet- and clinopyroxene-bearing amphibolites that are restricted to the vicinity of the peridotite contact. These rocks have unusual Fe-rich, Si-poor bulk compositions and may represent tholeiites chemically modified by interaction with fluid or melt before or during metamorphism. These amphibolites are not retrograde eclogites, since andesine was present throughout the metamorphic history, and the clinopyroxene is not omphacitic. Coexisting mineral compositions and temperature estimates overlap for core, rim, and matrix grains of all the major phases in the garnet–clinopyroxene amphibolites, implying equilibration in the range 750–800 °C at 8–9 kbar (1 kbar = 100 MPa). Garnet amphibolites lacking clinopyroxene yielded somewhat lower PT estimates of 600–700 °C and 6–7 kbar. Owing to complex field relationships, it is not clear whether or not these PT conditions resulted from evolution along a single PTt path.

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