The petrogenesis of metamorphosed carbonatites in the Grenville Province, Ontario

Abstract

Veins and dikes of calcite-rich rocks within the Central Metasedimentary Belt boundary zone (CMBbz) in the Grenville Province of Ontario have been interpreted to be true carbonatites or to be pseudocarbonatites derived from interaction of pegmatite melts and regional Grenville marble. The putative carbonatites have been metamorphosed and consist mainly of calcite, biotite, and apatite with lesser amounts of clinopyroxene, magnetite, allanite, zircon, titanite, cerite, celestite, and barite. The rocks have high P and rare earth element (REE) contents, and calcite in carbonatite has elevated Sr, Fe, and Mn contents relative to Grenville Supergroup marble and marble mélange. Values of δ18OSMOW (9.9–13.3‰) and δ13CPDB (−4.8 to −1.9‰) for calcite are also distinct from those for marble and most marble mélange. Titanites extracted from clinopyroxene–calcite–scapolite skarns formed by metasomatic interaction of carbonatites and silicate lithologies yield U–Pb ages of 1085 to 1035 Ma. Zircon from one carbonatite body yields a U–Pb age of 1089 ± 5 Ma; zircon ages from two other bodies are 1170 ± 3 and 1143 ± 8 Ma, suggesting several carbonatite formation events or remobilization of carbonatite during deformation and metamorphism around 1080 Ma. Values of εNd(T) are 1.7–3.2 for carbonatites, −1.5–1.0 for REE-rich granite dikes intruding the CMBbz, and 1.6–1.7 for marble. The mineralogy and geochemical data are consistent with derivation of the carbonatites from a depleted mantle source. Mixing calculations indicate that interaction of REE-rich pegmatites with regional marbles cannot reproduce selected major and minor element abundances, REE contents, and O and Nd isotope compositions of the carbonatites.

Role of vein-phases in nanoscale sequestration of U, Nb, Ti, and Pb during the alteration of pyrochlore

“…The U-Pb dating of uraninite from the Faraday Mine in the Bancroft indicated ages of 1059-1020 Ma (Little, 1969); similar values were obtained for U-bearing titanite and zircon (1089-1035 Ma; Moecher et al, 1997), as well as sphene, monazite, and rutile (1074-1030 Ma; Mezger et al, 1993) in this area. Consequently, the samples in the present study are estimated to be of similar age of ~1 billion years.…”

Enigmatic association of the carbonatite and alkali-pyroxenite along the Northern Shear Zone, Purulia, West Bengal: A saga of primary magmatic carbonatite

“…Such mineralogical assemblage even under highest grade of metamorphism does not greatly change the mineralogical assemblages of carbonatite as exemplified by the carbonatites from East India, Bull's Run carbonatite in Natal, some Ontario examples and those of the Canadian Cordillera gneissic rocks (Natarajan et al, 1994;Viladkar and Subramaian, 1995;Scogings and Forster, 1989;Moecher et al 1997;Pell and Höy, 1989). So, the presence of triple junction can be attributed to the rheological changes/readjustment particularly in response to post crystallization deformational activities.…”

Age and isotopic geochemical characteristics of Archean carbonatites and alkaline rocks of the Baltic shield

“…An important indicator of the geodynamic settings related to the final stages of the GrenvilleSveconorwegian Orogeny is the association of carbonatite dikes and veins formed 1.17-1.09 Ga ago, which was discovered and studied within the CMB [Moecher et al, 1997].…”