Tectonic controls on magmatism associated with continental break-up: an example from the Parana-Etendeka Province

The high- and low-Ti basalts of the Parana-Etendeka province were primarily derived from old, trace element-enriched source regions in the lithospheri c mantle, and they are associated with dyke swarms of different orientation s. These swarms appear to reflect different amounts of extension, and it is inferred that the high- and low-Ti magma types were characterised by diffe rent melt generation rates of similar to 0.15 km(3) yr(-1) and similar to 0 .4 km3 yr(-1),respectively [Stewart et al. Earth Planet. Sci. Lett. 143 (19 96) 95-109]. There is probably a gap of similar to 2 Myr between the end of the main phase of CFB magmatism and the oldest rocks on the adjacent ocean floor. A simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension. Melting in the mantle is assumed to be co ntrolled by the dry peridotite solidus in the asthenosphere and the hydrous (0.2% H2O) peridotite solidus in the lithosphere. For a maximum beta of 4 and a duration of extension of 10 Myr, the derivation of melt from the asth enosphere by dry peridotite melting depends primarily on potential temperat ure (T-p) and is relatively insensitive to the thickness of the MEL, while the converse is the case for melt derived from the lithosphere by hydrous p eridotite melting. For a T-p of 1450 +/- 50 degrees C inferred from the cru stal thickness estimates along the Rio Grande Rise and Walvis Ridge, the mo del successfully generates 2-4 km of lithosphere-derived melt before produc ing significant volumes of asthenosphere-derived melt. It is concluded that increases of melt volume with time can be generated by decompression melti ng of the mantle lithosphere. Critically, in areas of significant melt gene ration within the mantle lithosphere during extension and break-up, there i s likely to be a gap in the volcanic record between the end of melt generat ion in the lithosphere and the onset of melting in the underlying asthenosp here. No such gap is present if all melts are generated within the mantle p lume, and thus these models may in principle be tested in the geologic reco rd. (C) 2000 Elsevier Science B.V. All rights reserved.