The Early Cretaceous volcanic rocks of southern Uruguay comprise mafic and felsic volcanics. The position of these outcrops at the southern edge of the Paraná–Etendeka continental flood basalt province provides an opportunity to investigate possible lateral variations in both mafic and more evolved rock types towards the margins of such an area of plume-related magmatism. The mafic lavas are divided into two compositionally distinct magma types. The more voluminous Treinte Y Trés magma type is similar to the low-Ti basalts of the Paraná flood basalt province. The Santa Lucía magma type is a distinct and rare basalt type with ocean-island basalt type asthenospheric affinities (high Nb/La, low 87Sr/86Sri). The felsic volcanics are divided into two series, the Lavalleja Series and the Aigüa Series. The Lavalleja Series are chemically and isotopically similar to the Paraná–Etendeka low-Ti rhyolites, and are considered to be related to the Treinte Y Trés lavas by extensive fractionation and crustal assimilation. The Aigüa Series have low 143Nd/144Ndi and low 87Sr/86Sri and unlike the rhyolites of the Paraná, are interpreted as melts of pre-existing mafic lower crust that subsequently underwent extreme fractionation. The differences observed in the felsic suites may be linked to differences in the volumes of the associated basalts and the amounts of extension.
The ultrapotassic magmatism of southern Italy (the Roman province) is well known, and recently these highly unusual lavas have been explained in terms of subduction‐related processes. Less well studied are the coeval calc‐alkaline to potassic rocks of the nearby Aeolian Islands, which are situated above a Benioff zone and are therefore demonstrably related to recently active subduction. On a number of geochemical diagrams the Roman and Aeolian provinces define continuous trends, which may be accommodated in a single petrogenetic model involving mixing of three isotopically and elementally distinct components. Two of these are subduction‐related: first, a high Sr/Nd, high Th/Ta component derived largely from basaltic ocean crust and, second, a component with extremely high Th/Ta, but relatively low Sr/Nd derived largely from subducted sediments. These are mixed with mantle wedge material which, prior to subduction, was characterised by highly radiogenic Pb isotope ratios, and is therefore comparable to the mantle source of Mount Etna volcanism. Thus it would appear that midplate tholeiitic to Na‐alkalic magmatism and continental margin calc‐alkaline to ultrapotassic magmas were derived from mantle sources which, prior to subduction, had similar isotopic signatures. This observation has important implications for the potential involvement of trace element and isotope enriched (OIB‐like) mantle in the genesis of subduction‐related volcanism.
Wall rock assimilation and fractional crystallization (AFC) as Bowen has pointed out in 1928, should be considered together because the heat required for assimilation can be provided by the latent heat of crystallization of the magmas. This process (AFC) was discussed by several authors, who sought to reproduce observed trends on isotope-isotope and isotope-trace element diagrams. In general, these models were loosely constrained because of the pratical difficulties in handling large numbers of elements and in exploring the relationships between different variables for any data set. In this work, the approach has been to consider many trace elements and isotope ratios simultaneously, to calculate the best fit surface through the analytical data for suites of related samples and to evaluate the relationships between variables in the AFC model for the range of solutions which are consistent with the best-fit surface. This new analysis of the AFC model is illustrated with selected samples from a detailed section through basalts and rhyolites of the Serra Geral Formation in the Paraná Basin sequence. Results appear to be inconsistent with the bulk assimilation of average crustal compositions, but show excellent agreement with the trace element pattern of typical upper crustal melts.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)