Subalkaline low-Ti Mesozoic dykes from Região dos Lagos (RJ): geochemistry and 40Ar/39Ar geochronology

Author(s)

Karine Zuccolan Carvas

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto Astronômico e Geofísico (IAG/SBD)
• Defense date: 2016-12-14

Examining board members

Leila Soares Marques; Isabela de Oliveira Carmo; Sergio de Castro Valente

• Advisor: Leila Soares Marques

Abstract

The Região dos Lagos tholeiitic dykes, located in Rio de Janeiro State, encompass the Arraial do Cabo, Cabo Frio and Armação de Búzios towns and integrate the Serra do Mar Dyke Swarm. Their role in the South Atlantic opening processes is still poorly defined because of their peculiar geochemical and geochronological features. In this work, the petrogenesis and the geochronology of these intrusions were investigated in detail based on new data of elemental (major, minor and trace elements) and isotope (Sr, Nd and Pb) geochemistry along with a careful 40Ar/39Ar dating. The dykes are chemically represented by tholeiitic basalts, andesi tholeiitic basalts and transitional basalts, usually presenting TiO2 contents lower than 2% (LTi). The elemental geochemistry allows the recognition of two magmatic groups; one of them is more primitive and depleted in incompatible elements (group A), whereas the other one is more evolved and enriched in these elements (group B). The data also indicate that crustal assimilation, if existed, had a small role in the magmatic evolution of both groups, and point out similarities between group A and the LTi flows (Esmeralda) from the Paraná Magmatic Province (PMP). Group B, in turn, displays geochemical features very different from those of the LTi basalts from PMP. The Sr isotope compositions of group A are less radiogenic than those of the B one, but the opposite occurs with Pb, which makes the first group similar to the Horingbaai dykes from Namibia. The Pb isotope ratios indicate that A and B were originated from distinct mantle sources, which also differ from those related to PMP LTi flows. The isotope data are not compatible with assimilation-fractional crystallization processes or simple mixtures involving the classic mantle reservoirs. This behavior suggests origin in heterogeneous lithospheric subcontinental mantle, probably affected by long subduction periods during Western Gondwana amalgamation. The 40Ar/39Ar geochronological data point out that the presence of plagioclase sericitization and albitization in their rims may imprint the age of these events in the age spectra. Sericitized plagioclase grains of group A displayed ages of 106-108 Ma, while the albitized fractions provided ages of 96 Ma. Such results disagree with both the fresh plagioclase grains (maximum ages of 125-140 Ma), very affected by excess Ar due to pyroxene and apatite inclusions, and the 132 Ma age of the amphibole-biotite aliquots. This last result very probably corresponds to the dyke emplacement event, confirming the genetic relationship between group A and the PMP. Whole rock dating in a sericite-free group B dyke provided maximum age of 108 Ma, which would imply that A and B groups are not coeval. The similarity between the last age and the uplifting event of Southwestern Brazilian coast suggests that the dykes of group B could have been generated during such process, which also altered (sericitized) the plagioclases of the group A dykes. (AU)

• FAPESP's process: 14/22948-9 - Geochemical, isotopic and geochronological characterization of tholeiitic dykes of the Southeastern Coast of Brazil: geodynamical implications
• Grantee: Karine Zuccolan Carvas
• Support Opportunities: Scholarships in Brazil - Master