Parana flood basalt volcanism primarily limited to ～ 1 Myr beginning at 135 Ma: New ~(40)Ar/~(39)Ar ages for rocks from Rio Grande do Sul, and critical evaluation of published radiometric data

摘要

Ar-40/Ar-39 step heating analyses were carried out on seven rocks (five basalts, an andesite and a rhyolite) from the southern Parana Province (similar to 28 degrees S-30 degrees S); they yield plateau/isochron ages of 135-134 Ma, in good agreement with published step heating data on rocks from the same area. Critical review of laser spot isochron ages for rocks from the Province, ranging from similar to 140 to 130 Ma, are shown to be unreliable estimates of crystallization ages, as the rocks were substantially altered; step heating results on three of these rocks thought to yield good plateau ages, are shown to be incorrect, as a result of a technicality in dating procedures followed. U-Pb ages on zircon and baddeleyite separated from a variety of rock types (similar to 30 degrees S-23 degrees S) fall in the range 135 to 134 Ma. All reliable Ar-40/Ar-39 and U-Pb ages indicate volcanism was sharply focused, initiated at 135 Ma, and similar to 1 Myr in duration; no variation of age with either latitude or longitude is noted, Scrutiny of published Ar-40/Ar-39 ages on the Florianopolis dykes shows they cannot be used as reliable crystallization ages. U-Pb work shows that this dyke swarm was formed coevally with the main part of the Parana province. Most of the published Ar-40/Ar-39 ages on the Ponta Grossa dyke swarm are unreliable; a few ages appear reliable and suggest the magmatic event in this area, may have postdated the main Parana pulse by 1-2 Myr. A single Ar-40/Ar-39 age from a high-Nb basalt in the southernmost part (similar to 34 degrees S) of the Parana at similar to 135 Ma, highlights the need for further radiometric work on other areas of this flood basalt province. The Parana Province postdates the time of the Jurassic Cretaceous bound-ary by similar to 10 Myr. (C) 2017 Elsevier B.V. All rights reserved.