Abstract

Most anorthosites of the massif type crystallized in the episode 1.7-1.2 Gyr. The magmas that produced most anorthosites were dry, as shown by high-temperature mineralogy and anhydrous mineral assemblages in contact aureoles. The overall chemistry of anorthosites and residua is broadly tholeiitic and consistent with derivation from the mantle. It is proposed that hyperfeldspathic (plagioclase-supersaturated) liquids were generated by quasi-isothermal extraction of mafic minerals from tholeiitic magma enroute to and at the site of emplacement, and that such kinetic process was uniquely permitted in an environment of aborted continental rifting. Anorthositic rocks may have much to say about the episodic versus continuous geochemical evolution of the Earth's mantle. --Modified journal abstract.

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