125 YEARS OF ATTEMPTING TO UNDERSTAND THE ORIGIN AND TECTONIC SIGNIFICANCE OF LATE ALKALIC SUITE INTRUSIONS PRESENT ALONG THE LAURENTIAN MARGIN-COMPOSITE ARC BELT BOUNDARY ZONE IN THE GRENVILLE OROGEN, ONTARIO: A STATUS REPORT

For over a century, debate has raged over the age, significance and tectonic setting of syenitic and associated ‘carbonatitic' rocks present along the Laurentian Margin-Composite Arc Belt boundary (CMBtz) in the Grenville Orogen in Ontario. Key questions include their age (from 1290 to 1000 Ma), origin (magmatic, evaporitic, or metasomatic), and tectonic setting (continental rifting or collisional orogeny). Recent 1:50 000 scale geological mapping in the Lake Clear area of Ontario is leading to an improved understanding of these complicated rocks.

New observations suggest 3 pulses of syenite emplacement. 1) Older, igneous-textured ovoid biotite syenite (Burns Lake, Mt. St. Patrick) and alkalic gabbro (Woermke, Highland) plutons and mafic dikes of the Kensington-Skootamatta Suite emplaced between 1088 and 1073 Ma. 2) Medial linear heterogeneous bodies of monzodiorite, syenite and nepheline-syenite (Wolfe, Eleanor plutons), but which yield U-Pb zircon ages much younger than the ~1290 Ma previously suggested (1054.5±1.5 Ma, Tory Hill; 1042±3 Ma, Wolfe). 3) A late pulse at ~1033±1 Ma (Lorwell, Lake Clear) which produced texturally and compositionally diverse pyroxene syenites and associated pyroxene- and/or calcite-bearing metasomatic rocks variously referred to as skarns, fenites and ‘carbonatites’. The late syenites only occur proximal to the CMBtz. Confirmation of this timing is found near Cobden, where late syenites intrude and metasomatize straight gneisses of the CMBtz that formed between 1080-1060 Ma. Metasomatic fluids associated with the late syenites also resulted in the precipitation of REE and graphite along grain boundaries in older carbonate rocks.

These observations preclude an early rifting scenario, but are consistent with an influx of fluids and heat between 1090 to 1035 Ma due to upwelling of asthenospheric mantle from crustal delamination. This produced a) regionally distributed early syenites (1090-1070 Ma) that formed discrete plutons; b) subsequent foid-bearing syenites (ca 1050 Ma) localized along the CMBtz; and c) late syenites (ca 1035 Ma) which interacted extensively with their host rocks. This progression in syenite type may reflect decreasing depth to the mantle source and increasing interaction with the lower crust during the Ottawan stage of the Grenville orogeny.