Norite and charnockites from the Venda Nova Pluton, SE Brazil: Intensive parameters and some petrogenetic constraints

Abstract

The Venda Nova Pluton (VNP) is a zoned ring structure emplaced in the southern portion of the Neoproterozoic Araçuai Belt, in Esp ırito Santo, Brazil. It is a slightly westward tilted cylinder-like intrusion, with an almost circular horizontal section. In the center of this structure, an off-centered gabbro-noritic core, surrounded by syeno-monzonitic rocks, intrudes an outer ring of charnockites and norite. These envelop the syeno-monzonitic and gabbro-noritic center, as a narrow discontinuous belt. While, in the core intrusion, mingling and mixing processes are widespread and well documented in the literature, in the outer ring, the norite and charnockite layers show predominantly homogeneous and isotropic internal structures. Nevertheless, smaller interaction zones between charnockites and norite denote a comparatively more restricted mingling process. The norite is a fine-grained rock with hypidiomorphic granular to intergranular texture. The charnockites are medium-grained and made up of: (a) orthopyroxene-tonalite, (b) orthopyroxene-quartz-diorite, and (c) orthopyroxene-granodiorite with hypidiomorphic granular to porphyritic textures. In all lithotypes both ortho-and clinopyroxene are replaced by hornblende and biotite. Two contrasting compositional sequences have been recognized, based on whole rock geochemistry: (1) a basic, with tholeiitic affinities (norite) and, (2) an intermediate, GEOSCIENCE FRONTIERSjournal homepage: www.elsevier.com/locate/gsf GEOSCIENCE FRONTIERS 3(6) (2012) 789e800 medium-K calc-alkaline, comprising the charnockites. Estimated crystallization temperatures, which have been calculated from micro-probe analysis of pyroxenes, range from 915 AE 25 C to 960 AE 50 C. Re-equilibration temperature (ilmenite-magnetite calibration) is around 600 AE 50 C. This indicates oxygen fugacities four order of magnitude below the FMQ-buffer and a reduced environment. Coeval pressure conditions estimated from the Al-content in hornblende range from 5.5 AE 0.6 kbar. Data obtained for the norite point toward an evolution from the partial melting of an anhydrous tholeiitic mantle magma. The charnockites may correspond to the differentiation of a calc-alkaline parental magma. The later could be the hybrid product from the contamination and mixing between the anhydrous tholeiitic magma with other deeper crustal sources. Further the magmatic system evolved through fractionation of orthopyroxene, clinopyroxene, plagioclase and ilmenite. Our results support the hypothesis for the evolution of the Venda Nova Pluton through a mantle-crust delamination process probably related to the collapse of the Araçuai orogeny. ª 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Diachronic collision, slab break-off and long-term high thermal flux in the Brasiliano–Pan-African orogeny: Implications for the geodynamic evolution of the Mantiqueira Province

“…However, in this time period (540-490 Ma), during the intrusion of the Magmatic Sequence III, the evidence for regional deformation related with convergence is insignificant (Pedrosa-Soares et al, 2008Valleriano et al, 2011;De Campos, 2014), but anorogenic igneous charnockites are widespread in the region (e.g. : Mendes et al, 2005;Mendes and De Campos, 2012;Gradim et al, 2014), further reinforcing the presented conclusions.…”

Charnockites and charnockites

“…Emslie, 1991) are interesting candidates as they often also preserve olivine together with the pyroxenes. Supported by petrographic and mineral chemical characteristics, Mendes and De Campos (2012) carries out a detailed characterization of intensive parameters, including re-equilibration temperatures, of Neoproterozoic charnockites associated with norites as part of the zoned Venda Nova pluton in SE Brazil.…”

Paleoproterozoic high-Mg low-Ti gabbro-granite series in eastern Sarmatia: Geochemistry and formation conditions

“…HighMg norites (often called boninite-norite complexes (Srivastava, 2008)) form in collision and postcollisional settings directly related to earlier subduction (Kemp, 2003;Mendes and De Campos, 2012) and in within-plate settings (Smithies, 2002;Srivastava, 2008;Srivastava and Gautam, 2009). The peak of their formation throughout the geologic history falls in the time span 2.5-1.8 Ga (Srivastava, 2008).…”

“…Sanukitoid series often form large massifs and are differentiated to rocks from dioritoids (diorites, quartz diorites, and monzodiorites) Russian Geology and Geophysics 57 (2016) to granodiorites and monzogranites , with subordinate tonalites. In contrast to them, intrusive boninite-like norite complexes are part of large layered plutons (Cadman et al, 1997;Hall and Hughes, 1993;Weaver and Tarney, 1981) and also occur as dike swarms (Srivastava, 2008;Srivastava and Gautam, 2009) or, seldom, as parts of small (3 × 10 km) occasional massifs (Mendes and De Campos, 2012;Smithies, 2002). The fact that boninite-like intrusive rocks can be similar in petrogenesis to sanukitoids was pointed out only by Smithies (2002).…”