MELTING AND FLOW OF LOWERMOST CONTINENTAL CRUST, EAST LAKE ATHABASCA AREA, SASKATCHEWAN

The East Lake Athabasca region exposes a large area (>20,000 km2) of HP-granulite facies rocks that record more than 1 b.y. of deep crustal evolution. The overwhelming characteristic of these deep crustal rocks is their compositional and structural heterogeneity. Migmatitic rocks are common, and represent a protracted record of lower crustal melting, ductile flow, and dramatic strain partitioning. In situ, high-resolution geochronology, and direct linking of monazite and zircon to deformation fabrics and melting reactions, is critical for interpreting the significance of the migmatites and related lower crustal flow. One common migmatitic rock is a Grt-rich Sil + Ky + Qtz + Kfs ± Opx ribbon mylonite (“white gneiss”), interpreted to represent the migmatitic and restitic product of intense high-T partial melting. Grt-production occurred via a fluid-absent Bt-dehydration melting reaction (e.g. Bt + Pl + Qtz → Grt ± Opx + Kfs + Melt) involving a meta-igneous protolith. High-resolution EMP monazite geochronology reveals a population of low-Th, Eu-enriched, Ca-depleted Mnz included in high-Grs Grt rims, interpreted to record the breakdown of Pl and growth of Grt during high-P metamorphism at 2.58-2.5 Ga. Contours of Grs-in-Grt and Al-in-Opx for pseudosections modeled in the NaCKFMASH + melt system suggest that high-Grs annuli grew at pressures in excess of 1.4-1.5 GPa following UHT-melting at 1.0-1.1 GPa. Data are most consistent with lower crustal emplacement of an igneous protolith subjected to extensive partial-melting driven by emplacement of a mafic intra-plate during sub-horizontal fabric development (channel flow?). Elsewhere in the region, a large mafic dike swarm intruded tonalite gneiss and underwent partial melting at 1.9 Ga. Flow during the syn-kinematic melting event occurred along sub-vertical fabrics that facilitated export of partial melt and injection of new dikes. The 1.9 Ga event was recorded in the white gneiss as Mnz rims and incipient low-Grs Grt-growth with little to no melting. Deformation was dominated by folding and up-right cleavage development. For the lower crust, in general, dramatic flow events appear to be genetically linked to melting events. The location and character of flow, however, is related to the location and geometry of fertile rocks at the time of HT-metamorphism.