科研之友
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摘要
Basin formation from 1800 to 1640 Ma in northern Australia took place in a backarc extensional upper plate setting accompanied by lithospheric thinning and voluminous but episodic basaltic magmatism. West-dipping subduction combined with oceanward retreat of an associated magmatic arc drove these processes and commenced after the separation of Laurentia, precluding any connection with eastern Australia from 1800 Ma until basin closure and re-accretion of the arc ca. 1650-1640 Ma. Early rift-related basaltic magmatism generated km-thick piles of interstratified 1785-1775 Ma shallow-water sediment and tholeiitic lava whose low- and high-Ti compositions bear some resemblance to basalts erupted during the earlier stages of Gondwana breakup. Later basalts evolved in a western Pacific-style marginal marine basin and exhibit compositions consistent with melting of a subcontinental mantle source that had either been metasomatically altered during flat-slab subduction or contaminated with crustal material during an earlier subduction-related event. From 1780 to 1670 Ma, the crust and underlying lithospheric mantle rapidly thinned, and basaltic magmatism increasingly occurred in a turbidite-dominated deep marine environment with concomitant changes towards more subdued subduction-related signatures in the east and less enrichment in incompatible elements. Coincidently, granites emplaced from 1780 to 1670 Ma at deeper crustal levels were extensionally unroofed along with their country rocks. Backarc extension concluded no later than 1655 Ma by which time MORB-like magmas were being sourced directly from the asthenosphere and northern Australia had transitioned from an active to passive continental margin lying along the inboard side of the marginal sea. With ongoing subduction and consumption of oceanic crust to the east, Laurentia began to encroach upon both the arc and Australian margin, resulting initially in collapse of the backarc basin and then continent-continent collision by ca. 1620 Ma. These events represent an important milestone in assembly of the Nuna supercontinent and mark the end of one complete Wilson cycle from the time that the two continents first separated until their re-amalgamation 150 Myr later. Timeframes for the opening and closure of ocean basins in southern Laurentia are similar, raising the possibility that the same convergent margin and backarc extensional processes may once have extended along strike into ancestral North America.
- 出版日期2018-8
