The coincidence of late Paleogene to Neogene shortening and crustal thickening with vigorous volcanic activity in the central Andes has long invited speculation about a causal relationship between magmatism and deformation. In aid of understanding this and related issues, we present here a new compilation of radiometric ages, geographic location and dominant rock type for about 1450 Cenozoic volcanic and subvolcanic centers in the southcentral Andes (14–28° S). This paper describes variations in the timespace distribution of volcanism from 65 to 0 Ma, with emphasis on the post-30 Ma period where Andean-style shortening deformation and volcanism were most intense. The central Andes are unusual for the abundance of felsic ignimbrites and their distribution is shown separately from the intermediate to mafic volcanic centers which are here termed the "arc association". Overall, the time-space patterns of volcanic activity for the ignimbrite and the arc association are similar but ignimbrite distribution is more patchy and more closely associated spatially with the plateau region. The distribution of volcanic activity as a function of longitude and age, as well as cumulative frequency curves of volcanic centers as a function of age reveal major differences in arc productivity, i.e., number vs. age of volcanic centers, from north to south along the arc. Eocene and early-mid Oligocene activity was confined to a narrow belt in the Precordillera. Post-30 Ma activity was shifted to the east and spread over a much broader area than earlier arcs, probably due to a shallower subduction angle. This phase of volcanism began at about the same time from north to south (ca. 25 Ma) but the peak activity shifted progressively southward with time. Cumulative frequency curves demonstrate that 50% of volcanic output north of 20° S accumulated by 16 Ma, whereas this level was reached for 20–23°, 23–26° and 26–28° S segments at about 12, 10 and 8 Ma, respectively. Plate reconstructions place the subducted part of the Juan Fernández Ridge beneath the arc at these latitudes between about 25 and 5 Ma, but age-frequency diagrams of volcanism show no evidence that ridge subduction influenced arc productivity. The spatial distribution of volcanism shows some influence by crustal structures on a local scale (tens of km), most notably the preferential clustering of volcanic centers at intersections of the frontal arc with NW-SE-trending lineament zones. However, on a regional scale the time-space distribution of volcanic centers and the distribution of active shortening domains in the central Andes varied independently. The evidence does not support the concept that Andean crustal thickening and plateau formation were preconditioned by thermal weakening of the crust. A comparison of volcanic output vs. shortening rates for latitude 19–22° S confirms that the onset of intense deformation in the Oligocene preceded that of volcanism by about 10 Ma, and the increase in volcanic activity at about 20–16 Ma has no expression in shortening rates. After plateau formation, however, beginning at about 10 Ma, both shortening rate and volcanic output increased together and reached their highest levels. This period experienced extremely large-volume ignimbrite eruptions from the Altiplano-Puna volcanic complex. The ignimbrite magmas represent an episode of widespread crustal melting, and it is likely that the rise in shortening rates reflects meltenhanced weakening of the crust. Variations in the location and width of the CVZ arc respond to changes in slab dip, but the complex distribution of volcanic centers in time and space shown in this study belies a simple relationship. A condition that must be met for correlation between surface volcanism and slab dip is a near-vertical ascent of magmas, both in the mantle wedge and through the crust. We conclude that in the Neogene arc of the central Andes, vertical ascent of magmas is disturbed by effects of crustal heterogeneity, intense deformation, lithospheric thickening and partial delamination, and crustal melting.
Variations of mineral chemistry and whole-rock compositions were studied in detail, at millimetre to centimetre intervals, in two vertical drill core profiles through the platiniferous UG2 chromitite layer in the western and eastern limbs of the Bushveld Complex, South Africa. Analytical methods included electron microprobe and LA-ICP-MS analyses of the main rock-forming minerals, orthopyroxene, plagioclase and interstitial clinopyroxene. One profile was also studied by synchrotron-source XRF. Statistical analysis of crystal size distribution of chromite was also performed at different levels in the chromitite layer and in adjacent silicate rocks. The results provide new evidence for chemical and textural late magmatic re-equilibration in the UG2 layer and in the silicate rocks at the contact zones. The chromite crystal size distributions imply extensive coarsening of that mineral within the main chromitite seam, which has erased any textural evidence of primary deposition features such as recharge or mechanical sorting of crystals, if those features originally existed. The mineral compositions in chromitite differ from those in adjacent silicate rocks, in general agreement with predictions of chemical re-equilibration with evolved, residual melt (the trapped liquid shift effect). In detail, the geochemical data imply, however, that the conventional trapped liquid shift model has shortcomings, due to the effects of material transport driven by chemical gradients between modally contrasting layers of crystal mush undergoing re-equilibration reactions. In the presence of such gradients, selective open-system conditions may hold for alkalis and hydrogen because of their higher diffusion rates in silicate melts. Differential mobility of components in the interstitial melt can also sharpen the original modal layering by causing minerals to crystallise in one layer and dissolve in another. Detailed trace element profiles by synchrotron XRF reveal an uneven vertical distribution of incompatible elements which implies that the permeability of the chromitite layer may have been significant, even at the latest stages of interstitial crystallization.
Research Article| September 01, 2008 Tourmaline B-isotopes fingerprint marine evaporites as the source of high-salinity ore fluids in iron oxide copper-gold deposits, Carajás Mineral Province (Brazil) Roberto Perez Xavier; Roberto Perez Xavier * 11Departamento de Geologia e Recursos Naturais, Instituto de Geociências, Universidade Estadual de Campinas, 13083-970 Campinas (SP), Brazil *E-mail: xavier@ige.unicamp.br. Search for other works by this author on: GSW Google Scholar Michael Wiedenbeck; Michael Wiedenbeck 22GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Robert B. Trumbull; Robert B. Trumbull 22GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Ana M. Dreher; Ana M. Dreher 33Companhia de Pesquisa de Recursos Minerais/Serviço Geológico do Brasil, Av. Pasteur 404, 22290-240 Rio de Janeiro (RJ), Brazil Search for other works by this author on: GSW Google Scholar Lena V.S. Monteiro; Lena V.S. Monteiro 11Departamento de Geologia e Recursos Naturais, Instituto de Geociências, Universidade Estadual de Campinas, 13083-970 Campinas (SP), Brazil Search for other works by this author on: GSW Google Scholar Dieter Rhede; Dieter Rhede 22GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Carlos E.G. de Araújo; Carlos E.G. de Araújo 11Departamento de Geologia e Recursos Naturais, Instituto de Geociências, Universidade Estadual de Campinas, 13083-970 Campinas (SP), Brazil Search for other works by this author on: GSW Google Scholar Ignacio Torresi Ignacio Torresi 11Departamento de Geologia e Recursos Naturais, Instituto de Geociências, Universidade Estadual de Campinas, 13083-970 Campinas (SP), Brazil Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (9): 743–746. https://doi.org/10.1130/G24841A.1 Article history received: 02 Mar 2008 rev-recd: 16 May 2008 accepted: 10 Jun 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Roberto Perez Xavier, Michael Wiedenbeck, Robert B. Trumbull, Ana M. Dreher, Lena V.S. Monteiro, Dieter Rhede, Carlos E.G. de Araújo, Ignacio Torresi; Tourmaline B-isotopes fingerprint marine evaporites as the source of high-salinity ore fluids in iron oxide copper-gold deposits, Carajás Mineral Province (Brazil). Geology 2008;; 36 (9): 743–746. doi: https://doi.org/10.1130/G24841A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Carajás Mineral Province in northern Brazil contains a variety of world-class (>100 Mt ore) iron oxide copper-gold (IOCG) deposits, including the only Archean examples of this deposit class (e.g., the Igarapé Bahia/Alemão and Salobo deposits). Tourmaline of schorl-dravite composition, a common gangue mineral in these deposits, precipitated shortly prior to and after the ore assemblage. A boron isotope study of texturally different tourmaline from three IOCG deposits (Igarapé Bahia, Salobo, and Sossego) using secondary ion mass spectrometry (SIMS) provides new evidence in the long-standing debate of magmatic versus non-magmatic sources for the high salinity (up to 50 wt% NaCl equiv.) of ore fluids in these deposits. Values of δ11B from 14‰ to 26.5‰ for the Igarapé Bahia and Salobo deposits confirm marine evaporite–derived brines in the ore fluids, whereas lower δ11B values for the Igarapé Bahia deposit (5.8‰ to 8.8‰) suggest that these fluids may have mixed with an isotopically different hydrothermal fluid, or one that had a mixed boron source. More variable and isotopically lighter boron in tourmaline from the Sossego deposit (−8‰ to 11‰) is attributed to mixed sources, including light boron leached from felsic intrusive and volcanic host rocks, and heavy boron derived from marine evaporites. The boron isotope data indicate that the characteristic high salinity of the ore fluids in the Carajás Mineral Province was acquired by the interaction of hydrothermal fluids with marine evaporites. For IOCG deposits that contain tourmaline as a common gangue mineral, boron isotopes offer a valuable tool to constrain the high-salinity source problem, which is a critical issue in metallogenesis of IOCG deposits worldwide. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Basanite intrusions from the Early Cretaceous Erongo complex, Namibia, have compositions consistent with near-primary mantle melts derived from a depth of at least 100 km. These rocks provide a key reference for the mantle component(s) involved in breakup-related magmatism in this region. Initial Sr–Nd–Pb isotope ratios of the Erongo basanites and associated tephrites and phonotephrites (87Sr/86Sr = 0·70425–0·70465; εNd = +1·8 to +2·7; 206Pb/204Pb = 18·63–18·91) are independent of the degree of differentiation and correspond closely to an estimated range for the Tristan plume at 130 Ma. Incompatible trace element ratios also overlap with ratios of ocean island basalt (OIB) from the South Atlantic islands of Tristan da Cunha, Gough and Inaccessible associated with the modern Tristan hotspot. The Tristan plume signature of Erongo basanite–tephrite intrusions is shared by at least six other Early Cretaceous mafic alkaline complexes in Namibia, whereas the associated flood basalts in general lack a plume signature. We attribute the contrast in mantle sources for the flood basalts and alkaline complexes to their relative timing with respect to lithospheric thinning. Thick lithosphere during the main flood basalt event prevented direct melting of the Tristan plume and magmas were generated mostly from the lithosphere. The alkaline complexes intruded later, when the lithosphere was sufficiently thinned to allow decompression melting of the underlying plume mantle.
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