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Published online by Cambridge University Press: 17 February 2023
Where do all these huge volumes of noritic magma come from, representing the dolerites found globally associated with the Gondwana event. Although differing in chemical detail from one province to another, they all have a strong quartz–dolerite affinity and commonly show a large concentration of orthopyroxene primocrysts. Noritic magma can be produced in the lower crust but it is dissimilar to the rift-related dolerites. This is exemplified by the massive magmatic event at Sudbury, Ontario where an impacting bolide 1.85 Ga ago produced ~ 35,000 km3 of noritic magma in 3 minutes. Yet, a detailed chemical comparison of this magma with the Ferrar magmas shows a striking contrast on all counts. The Sudbury system shows no tight close variation in principal components as does the Ferrar. The Ferrar magmas, like other rift-related dolerites, are remarkable in their huge volumes, cohesive compositions, and relatively rapid generation and emplacement. The process giving rise to them cannot be tortuous and complex, but simple and straightforward, as in the production of the MORB of ocean ridges.
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