Abstract

A vast region of upper Paleozoic to Middle Jurassic (300-150 Ma) silicic magmatic rocks that erupted inboard of the Gondwana margin is a possible Phanerozoic analogue to the extensive Middle Proterozoic (1500-1350 Ma) silicic magmatic province that underlies much of the southern mid-continent of North America. Like the North American rocks, the Gondwana silicic magmas appear to be melts of crust that formed about 200-300 m.y. earlier. In the North American case, this older crust formed and was accreted to the continent during a major period of crustal formation (1700-1900 Ma), whereas in the Gondwana case, the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the Paleozoic. In both cases, basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older (and previously melted) crust. The similarities between the North American and Gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas, ponded at the base of the preheated crust, caused extensive crustal melting that inhibited upward passage of the basalts. In both cases, silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes, and before breakup of the supercontinent. By analogy with the polar wander curves for Gondwana, the granite-rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles.

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