UNRAVELLING THE CONNECTION BETWEEN METAMORPHOSED LIMESTONES, MARBLE BRECCIAS, ‘CARBONATITES', FENITES AND CIRCA 1040 MA SYENITE INTRUSIONS ALONG THE CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE OROGEN, NEAR LAKE CLEAR, ONTARIO

Ongoing 1:50 000 scale geological mapping in the Lake Clear area of Ontario has lead to improved understanding of long-standing controversial questions regarding the origin of alkalic syenite, fenite and ‘carbonatitc’ (SFC) rocks (ca 1040 Ma) within the hanging wall of the Central Metasedimentary Boundary Thrust Zone (CMBtz), as highlighted in this presentation. The broad surface distribution of SFC rocks in the Lake Clear area, compared to elsewhere along the CMBtz, is likely due to the shallow nature of the CMBtz there.

4 types of calcite-dominated rocks occur in the SFC package: A) calcite and minor dolomite marbles derived from chemical sediments (distal to CMBtz), B) white marble tectonic breccias derived from A (proximal to CMBtz), C) tectonic breccias (metasomatized) with pink calcite and abundant diopside, and D) buff or pink calcite veins and segregations with diopside-amphibole±apatite rims derived from either carbonatitic liquids or immiscible segregation from syenite melts. Types C and D occur with syenites and quartz syenites exhibiting a variety of textures, colours and grain sizes, as well as extensive fenitization and pyroxenite veining of host rocks. Together these rock types form the SFC package. Local differences in the syenites (km scale) may be related in part to host composition (e.g. corundum-bearing rocks occur near nepheline syenite host rocks). Late pegmatites along the CMBtz are typically uraniferous, but those in the SFC are enriched in Th and locally in P and/or Mo. If the source fluids were oxiding, then U may have partitioned from Th and moved to higher crustal levels.

The SFC package is attributed to an influx of fluids and heat between 1090 and 1035 Ma resulting from upwelling of asthenospheric mantle due to crustal delamination. This resulted in syenite production, with a) early syenites (1090-1070 Ma) being regionally distributed and forming discrete low U-Th, magnetic plutons; b) localization of subsequent foid-bearing syenites (ca 1050 Ma, low U-Th, magnetic) along the CMBtz; and c) emplacement of the fluid-rich SFC package (ca 1040 Ma, less magnetic, high Th) which interacted extensively with its 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.