The geochemistry and tectonic significance of pre-metamorphic minor intrusions of the Central Metasedimentary Belt, Grenville Province, Canada

Abstract

More than 250 samples collected from dykes and sills intruded into the Grenville Supergroup have been analysed, by XRF techniques, for their major and trace element contents. The REE contents of 43 of the samples have been determined by INAA. Using these data, the rocks have been identified and placed in seven suites, each characterized by its own trace-element signature. There are four tholeiitic suites; the low zirconium suite, which comprises a bimodal series of basalts and minor dacites, the intermediate zirconium suite, which is made up of basalts, andesites, dacites, and rhyolites, the high zirconium suite consisting of high iron basalts, and rare highly fractionated tholeiites which comprise basalts characterized by high Mg number, high Ni and Cr contents, and with strongly fractionated trace-element patterns. In addition, a calc-alkaline suite made up of basalts and andesites, a transitional suite of andesites and trachyandesites, and an ultra-potassic suite of leucitites have been identified. On chondrite-normalized multi-element plots all of the suites show negative Nb and Ti anomalies. Similar ranges of rock types, with similar anomalies, occur within volcanic arcs and back-arc basins formed at collisional plate margins where the Nb and Ti anomalies are interpreted as subduction signatures, and also in some continental flood basalt sequences in which the anomalies are commonly suggested to result from crustal contamination. The geology of the Central Metasedimentary Belt leads us to favour an interpretation in which the minor intrusions were emplaced in supracrustal rocks deposited upon attenuated continental crust in a back-arc basin during a collisional orogeny.