Slow cooling in the metamorphic cores of Grenvillian large metamorphic core complexes and the thermal signature of the Ottawan orogenic lid

Abstract

ABSTRACT Prolonged slow cooling (average 1–3 °C/m.y.) of Ottawan phase granulite-facies gneisses (peak temperature ≥850 °C ca. 1090–1080 Ma) through the argon closure temperatures (TC) of hornblende ca. 980–920 Ma and biotite ca. 890–820 Ma in the western Grenville Province and in an inlier in the central Appalachians is well established, but its tectonic setting has not been systematically investigated. Here, the case is made that this slow cooling occurred in the suprasolidus cores of large metamorphic core complexes that were exhumed during mid-Ottawan (ca. 1050 Ma) extensional orogenic collapse. The ductile midcrustal metamorphic cores of the large metamorphic core complexes are overlain across gently dipping extensional detachments by a brittle-ductile cover composed of upper orogenic crust, parts of which preserve evidence of relict pre-Ottawan fabrics and peak prograde Ottawan temperatures of <500 °C (TC of Ar in hornblende), collectively implying thermal, structural, and rheological decoupling across the detachments. Slow average rates of cooling of the orogenic midcrust for >150 m.y. imply an anomalously hot upper mantle and mask short periods of more rapid cooling indicated by analyses of retrograde diffusional mineral zoning patterns. It is suggested that these slow average rates of cooling, coupled with slow average rates of exhumation of ≤0.1 km/m.y. modeled for one data set, were a result of decompression melting of rising asthenosphere and emplacement of voluminous mafic intrusions within or at the base of the crust, which reduced the buoyancy of the residual thinned lithosphere. This process is compatible with either delamination of subcontinental lithospheric mantle or slab rollback. The high-strain extensional detachments of the large metamorphic core complexes are sites of amphibolite-facies retrogression, suggesting a feedback between ingress of hydrous fluid, which was likely derived from beneath the detachment during crystallization of migmatite, and strain. Extensional juxtaposition of the hot midcrust (T >850 °C) and cooler cover (T <500 °C) across the detachments led to conductive heating of the base of the cover, locally raising its temperature above 500 °C, as recorded by amphibolite-facies metamorphism and young cooling ages. The slow cooling and exhumation of Grenvillian large metamorphic core complexes contrast with much faster rates in smaller metamorphic core complexes in other settings (e.g., North American Cordillera). The slow rates of these processes in large metamorphic core complexes are attributed to the prolonged high temperature and low viscosity of their metamorphic cores due to proximity of the asthenosphere, and to the intrusion of voluminous asthenospheric mafic magmas that both advected heat and reduced lithospheric buoyancy.