SLOW COOLING OF HOT ROCKS IN LARGE GRENVILLIAN METAMORPHIC CORE COMPLEXES, AND THE THERMAL SIGNATURE OF THE OTTAWAN OROGENIC LID

Slow cooling (≤3°C/M.y.) of Ottawan granulite-facies gneisses (peak T ≥750 °C at ~1090-1080 Ma) through the argon closure temperatures of hornblende (Hbl) at ~980-920 Ma and biotite at ~890-820 Ma in the western Grenville Province in Ontario, Quebec, and New York, and a Grenvillian inlier in New Jersey is well established. It occurs in partially retrogressed gneiss complexes with sub-horizontal fabrics that compose the cores of large metamorphic core complexes (LMCCs) formed during extensional orogenic collapse. The metamorphic cores are surrounded by remnants of the non-gneissic cover of the LMCCs, the Ottawan Orogenic Lid (OOL), composed of orogenic upper crust with steep pre-Ottawan fabrics and a variable Ottawan metamorphic overprint. Specifically, parts of the OOL exhibit pre-Grenvillian Hbl-Ar ages >1090 Ma, implying Ottawan metamorphic temperatures <500 °C and structural and thermal decoupling of the mid and upper orogenic crust across the extensional detachment of the LMCC; whereas elsewhere, Hbl-Ar ages of ~1050-1020 Ma indicate that cooling of the OOL through 500 °C occurred as the mid crust was undergoing retrogression and exhumation.

Slow cooling of the mid crust over 100 M.y. is indicative of anomalously hot lower crust or upper mantle, suggesting emplacement of a crustal underplate and/or rise of asthenosphere to the base of the crust. Both are compatible with delamination of sub-continental lithospheric mantle and its replacement by asthenosphere, or by mafic magmas derived from it. Amphibolite-facies retrogression in the cores of the LMCCs suggests heat flow was mediated by both conduction from below and downward advection of hydrous fluid along normal faults, their interaction resulting in local thermal coupling between the core and proximal cover, and providing a plausible explanation for the T estimates in the OOL >500 °C, and the Hbl-Ar ages of ~1050-1020 Ma, noted above, that at first sight appear incompatible with the cover.