Navegando por Autor "Stevenson, Ross"
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Item Evolution of Siderian juvenile crust to Rhyacian high Ba-Sr magmatism in the Mineiro Belt, southern São Francisco Craton.(2018) Moreira, Hugo Souza; Seixas, Luís Antônio Rosa; Storey, Craig Darryl; Fowler, Mike; Lasalle, Stephanie; Stevenson, Ross; Lana, Cristiano de CarvalhoPlutonic rocks from the Mineiro Belt, Brazil record a delayed onset of the transition from TTG to sanukitoid-type magmatism (high Ba-Sr), starting during the Siderian magmatic lull when little juvenile magma was added to the continental crust. Rocks mostly belong to the calc-alkaline series, meta- to peraluminous and originally “I-type”, meaning that oxidized magmas were formed by partial melting of subducted material. The temporal distribution and apparent secular changes of the magmas are consistent with the onset of subduction-driven plate tectonics due to an increase of the subduction angle and opening of the mantle wedge. New isotopic analyses (Sm-Nd whole rock and Lu-Hf in zircon) corroborate the restricted juvenile nature of the Mineiro Belt and confirm the genetic link between the Lagoa Dourada Suite, a rare ca. 2350 Ma high-Al tonalite-trondhjemite magmatic event, and the sanukitoid-type ca. 2130 Ma Alto Maranhão Suite. U-Pb dating of zircon and titanite constrain the crystallisation history of plutonic bodies; coupled with major and trace element analyses of the host rocks, they distinguish evolutionary trends in the Mineiro Belt. Several plutons in the region have ages close to 2130 Ma but are distinguished by the lower concentration of compatible elements in the juvenile high Ba-Sr suite.Item Geochemistry, Nd isotopes and U–Pb geochronology of a 2350 Ma TTG suite, Minas Gerais, Brazil : implications for the crustal evolution of the southern São Francisco craton.(2012) Seixas, Luís Antônio Rosa; David, Jean; Stevenson, RossThe Paleoproterozoic is considered the main period of crustal growth in the South American continent, with voluminous production of granitoid suites emplaced during the onset of the ca. 2.2–2.1 Ga Trans- Amazonian orogeny. This work documents and interprets mineral chemistry, whole rock geochemistry, Nd isotopes and U–Pb geochronology data for the high-aluminium Lagoa Dourada TTG suite. This suite constitutes a hitherto unknown episode of Paleoproterozoic (ca. 2350 Ma) juvenile felsic plutonism in the southern São Francisco craton, Minas Gerais, Brazil. It is also a rare example of juvenile continental crust production in the global crustal age-gap between 2.45 and 2.2 Ga. The suite is composed of metaluminous to slightly peraluminous, low-Mg#, low-K and high-Ca, biotite-hornblende to hornblende-biotite (± garnet-magnetite) tonalites and biotite trondhjemites, varying from ∼62 to 73 wt.% SiO2. The trace element geochemistry of the suite is characterized by mildly depleted Nd isotope compositions [εNd(t) = +1.0 to +2.1], low large-ion-lithophile element (LILE, i.e., Rb, Ba, and including the highly incompatible Th) and heavy Rare Earth element (REE, Yb < 1.00 ppm) contents. The suite is also characterized by high Sr/Y ratios (≥41 up to 81), high (La/Yb)N ratios (≥12 up to 46), and positive Eu/Eu* anomalies. These data are consistent with the derivation of the suite from a tholeiitic metabasaltic source, which had a short crustal residence time prior to melting. The chemical diversity of the suite is attributed mainly to the fractional crystallization of hornblende and accessory minerals, coupled with plagioclase accumulation in the tonalitic parental melt. Emplacement of new batches of magma could explain samples which diverge from the proposed liquid line of descent of the suite. The composition and age of the Lagoa Dourada suite suggest a link with the accretion of greenstone belt packages in an intra-oceanic tectonic setting.Item Petrology of the high-Mg tonalites and dioritic enclaves of the ca. 2130Ma Alto Maranhão suite: evidence for a major juvenile crustal addition event during the Rhyacian orogenesis, Mineiro Belt, southeast Brazil.(2013) Seixas, Luís Antônio Rosa; Bardintzeff, Jacques-Marie; Stevenson, Ross; Bonin, BernardCombined field observations, petrography, mineral chemistry, geochemistry, Nd isotopes and U–Pb zir-con data are presented for the ca. 2130 Ma Alto Maranhão suite. The suite, composed by allanite-bearingbiotite hornblende tonalites and commingled dioritic mafic magmatic enclaves (MME), occupies an esti-mated area of >300 km2in the southern Quadrilátero Ferrífero region, Minas Gerais, Brazil. Tonalitesand commingled MME are medium-K rocks, with high-Mg (Mg-number ≥ 0.46) and high-Cr (≥55 ppm)contents. They are also enriched in large-ion lithophile elements (LILE, Sr, Ba), some high-field-strengthelements (HFSE, Th, U), and light rare earth elements (REE), but depleted in Nb, Ta, and heavy REE. TDMmodel ages for tonalites and MME are similar and range from 2.3 to 2.4 Ga. The εNd(t) values are groupedaround chondritic values (=0 ± 1.0). Field relations and geochemical data indicate that tonalites and com-mingled dioritic MME crystallized as synchronous independent magma pulses, with limited mixing. Bothmagmas are compositionally equivalent to high-Mg andesites in modern subduction zones, implying thatthe suite was issued from the partial melting of a mantle wedge metasomatised to different degrees byslab-derived melts. Geobarometry (aluminium-in-hornblende) indicates crystallization at mid-crustaldepths (i.e. 0.59–0.42 GPa). The igneous suite evolved by fractional crystallization of hornblende, biotiteand accessory minerals (zircon, apatite, allanite and Fe–Ti oxide). The ubiquitous occurrence of horn-blende and allanite in tonalites and dioritic MME suggests that this mineral pair was largely responsiblefor the observed changes of light REE/heavy REE and Eu/Eu* ratios within the members of the suite. Thegeographical extent (>1000 km2) and Paleoproterozoic ages of the Alto Maranhão suite (ca. 2130 Ma) andassociated granitoids of the Mineiro Belt (2.1–2.2 Ga) constitute a major event of juvenile addition to thecontinental crust in the context of the southern São Francisco craton, and the ca. 2.2–2.1 Ga Rhyacianorogenesis of the South American platform. The age and general geochemical characteristics of the AltoMaranhão suite and its emplacement after a ca. 2.35 Ga juvenile, tholeiitic-source derived high-Al TTGsuite, indicate a tectonic setting akin to that of Late-Archaean high-Mg granitoids.