Genesis Of The Precambrian Copper-rich Caraiba Hypersthenite-norite Complex, Brazil

Caraiba, the largest Brazilian copper deposit under exploitation, consists mostly of disseminated and remobilised bornite and chalcopyrite hosted in early Proterozoic norite and hypersthenite. The mafic igneous complex comprises multiple intrusions of dykes, veins and breccias of norites and hypersthenites, with minor proportions of amphibolised gabbronorite and peridotite xenoliths transported by the magma from deeper levels in the lithosphere. The country rocks are high-grade gneisses, granulites and metasediments. Compositions of plagioclase(An60-40) and orthopyroxene(En70-60) fall in a narrow range similar to the Koperberg Suite from the Okiep copper district, South Africa, and to that in many massif-type anorthosites. Whole-rock major and trace element geochemistry indicate a parental magma enriched in Fe, LREE, P, K, and Cu. Negative Nb anomalies on multi-element plots and fractionated REE patterns, along with sulphide sulphur isotopes in the range δ34S = -1.495 to + 0.643‰, suggest a primary mantle lithosphere source, although a lower crustal source for the gabbronorite and peridotite xenoliths cannot be excluded. Geochronological and field evidence indicate that both norite and hypersthenite are likely to have been emplaced during a major sinistral transcurrent (partly transpressional) shearing event associated with the waning stage of evolution of the early Proterozoic Salvador-Curaçá orogen. © 1995 Springer-Verlag.30535137

Significance of mineral composition variations in the Aegean Island Arc

Phenocryst mineral compositions in lavas from the different volcanics centres in the Aegean Island Arc are compared. The Aegean Arc is based on continental crust and is related to subduction at the Hellenic Trench, but is in a region of major lithosphere extension so that the crust is thinner along the central part of the arc than in the eastern sector, bordering Turkey, or in the western sector, by mainland Greece. There are differences in pyroxene, plagioclase and Fe-Ti oxide compositions between the centre and flanks of the arc. Minerals from the central volcano (Santorini) tend to be less magnesian and/or calcic, and the titanomagnetites more Ti-rich; however, hornblende is uncommon. These differences apply even in lavas of equivalent major-element composition. The mineral data indicate that magma temperatures were lower, oxygen fugacity lower and water contents lower in Santorini, yet surprisingly it was Santorini that suffered paroxysmal eruption. In combination with the differences in whole-rock geochemistry between the central and flanking sectors of the arc, it is suggested that whereas the magma source for much of the arc was rather refractory hornblende-bearing lithosphere, that for Santorini involved more fertile asthenosphere, uprising beneath the central part of the Aegean Arc as a consequence of the greater degree of lithosphere extension in the central part of the arc. © 1992

Petrogenesis Of The Late Proterozoic Curaçá Mafic Dyke Swarm, Brazil: Asthenospheric Magrnatisrn Associated With Continental Collision

The late Proterozoic Curaçâ mafic dyke swarm is one of the more alkaline swaams of the Brazilian Precambrian. Intea-dyke chemical variations are characterised by higher abundances of incompatible elements in dyke margins than in the dyke centres, a feature that may have resulted from flowage differentiation, from progressive evacuation of liquids from zoned magma chambers, or possibly through dynamic melting of an uprising diapir. Inter-dyke chemical differences are best interpreted by different extents of melting of a mantle source with a small modal proportion of garnet. This source was heterogeneous and had the trace element compositions of ocean island basalts with Dupal and non-Dupa' characteristics. The emplacement of the Curaçá dykes from 650-700 Ma may have been facilitated by the extensional tectonics perpendicular to the collision zone between the São Francisco craton and the Pernambuco-Alagoas massif during the evolution of the Sergipano orogen. The energy in the system was insufficient to promote much melting of, or interaction with, the continental lithosphere or overlying crust. © 1995 Springer-Verlag.531-3274

25. MINOR ELEMENT GEOCHEMISTRY OF SEDIMENTS AT SITE 328, FALKLAND OUTER BASIN AND SITE 329, FALKLAND PLATEAU, LEG 36, DEEP SEA DRILLING PROJECT

ABSTRACT Site 328, a deep-water site in the Malvinas Outer Basin, is composed of siliceous and zeolitic clays and claystones. Site 329, an equivalent shallow-water site on the nearby Falkland Plateau, is composed of siliceous and calcareous oozes and chalks deposited mostly above the CCD. A total of 72 samples from Site 328 and 74 from Site 329 has been analyzed for Ti, Fe, Mn, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pb, and Th in order to establish in detail the downhole geochemical variations at each site and to compare the geochemical behavior of trace elements in a deep water and a shallow water environment. There are significant short, medium-, and long-term geochemical variations at both sites, but particularly at Site 329, and wellmarked sympathetic and antipathetic relationships between many of the minor and trace elements. Many of the downhole variations correlate with lithology and stratigraphy. Geochemically the two sites are very different, not only in terms of mean element compositions, but also in the way the trace elements interrelate. These differences are examined in detail by use of correlation coefficient matrices. The results emphasize the fact that the sediment sources and depositional environments were fundamentally different at the two sites