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Master's Dissertation
DOI
https://doi.org/10.11606/D.44.2020.tde-19012021-150008
Document
Author
Full name
Viviana Marcela Monsalve Hernández
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2020
Supervisor
Committee
Janasi, Valdecir de Assis (President)
Florisbal, Luana Moreira
Oliveira, Davis Carvalho de
Title in English
The meaning of oxidized A-type post-orogenic granitic magmatism: Petrogenesis of the Itupeva Pluton, Itu Batholith, SE Brazil
Keywords in English
Antecrysts
Cogenetic
Crystal-rich magma reservoirs
MASH
Petrogenesis
Transcrustal magmatic system
Abstract in English
In this work, an integrated study of the petrography, mineral composition, whole-rock geochemistry and Sr-Nd isotopes was carried out for the Itupeva pluton. This pluton belongs to the Itu Batholith, which is the most expressive occurrence of granitic rocks with A-type affinity in the post-orogenic Itu Granite Province, São Paulo, Brazil, and it is mainly composed by granites with subordinate quartz monzodioritic rocks occurring as syn-plutonic dykes and microgranular enclaves. The granites are magnesian, high-K calc-alkaline and metaluminous to slightly peraluminous and classify as oxidized A-type granites. Important geochemical features are an enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), with negative Eu anomalies (Eu/Eu* = 0.47- 0.58), and a relative depletion in high field strength elements, reflected in negative Nb and Ti anomalies in spidergrams. The associated quartz monzodiorites are metaluminous and have REE-rich patterns that are subparallel to those of the granites, sharing with them some other key geochemical features such as enrichments in Rb, Ba, K and LREE, and depletions in Nb and Ti. Together with similar Sr- Nd isotope signatures, this is suggestive of a comagmatic character for granites and quarz monzodiorites. Macro and microstructural features such as the occurrence as synplutonic dikes and microgranular enclaves with resorbed and mantled quartz and K-feldspar, locally back-veined by granitic material indicate that the quartz monzodiorites crystallized from magma that was injected into and mingled with the granitic mushes at the emplacement level, locally inducing some remelting. The integrated petrology and elemental and isotopic geochemistry suggests that the granites and quartz monzodiorites were generated by the interaction of mantle-derived magmas and crustal melts in the MASH or "hot" zone at deep crustal levels. During ascension to shallower crustal levels these magmas entrained antecrysts (e.g., high and low-An plagioclase in quartz monzodiorites, high- Ba biotite in granites) derived from crystal-rich magma reservoirs. In this scenario, we suggest a magma evolution in interconnected crystal-rich magma reservoirs localized in different crustal levels. The compositional and mineralogical variation featured in the Itupeva pluton records multiple processes of magma evolution, including crystallization and accumulation, recharge and remobilization of crystal mushes, magma mixing and mingling, assimilation, and entrainment of antecrysts, which operated along a transcrustal magmatic system.
Title in Portuguese
The meaning of oxidized A-type post-orogenic granitic magmatism: Petrogenesis of the Itupeva Pluton, Itu Batholith, SE Brazil
Keywords in Portuguese
Antecristais
Cogeneticos
MASH
Petrogenesis
Reservatórios de magma
Sistemas magmáticos transcrustais
Abstract in Portuguese
Neste trabalho, um estudo integrado de petrografia, composição mineral, geoquímica de rocha total e isótopos Sr e Nd foi realizado para o Pluton Itupeva. Este plúton pertence ao Batólito Itu, que é a ocorrência mais expressiva de rochas graníticas com afinidade tipo A na Província Granítica Itu pós-orogênica, São Paulo, Brasil, e é composto principalmente por granitos, com rochas quartzo monzodioríticas subordinadas ocorrendo como diques sinplutônicos e enclaves microgranulares. Os granitos são magnesianos, cálcio-alcalinos de alto K, e metaluminosos a levemente peraluminosos e são classificados como granitos tipo A oxidados. As características geoquímicas importantes são um enriquecimento em elementos litófilos (LILEs) e elementos de terras raras leves (LREEs), com anomalias de Eu negativas (Eu / Eu * = 0,47-0,58) e um empobrecimento relativo em elementos de alta intensidade de campo (HFSEs), refletida em anomalias negativas de Nb e Ti. Os quartzo monzodioritos associados são metaluminosos e têm padrões ricos em REE que são subparalelos aos dos granitos, compartilhando com eles algumas outras características geoquímicas importantes, como enriquecimento em Rb, Ba, K e LREE, e empobrecimento em Nb e Ti. Junto com assinaturas isotópicas de Sr-Nd semelhantes, as observações são sugestivas do caráter comagmático para granitos e quartzo monzodioritos. Características macro e microestruturais, como a ocorrência de diques sin-plutônicos e enclaves microgranulares com quartzo e feldspato alcalino reabsorvidos e manteados, e a presença local de estruturas de back-veining, indicam que os quartzo monzodioritos cristalizaram de um magma que foi injetado e misturado com os mushes graníticos no nível de colocação, induzindo localmente alguma refusão. A petrologia integrada e a geoquímica elementar e isotópica sugerem que os granitos e quartzo monzodioritos foram gerados pela interação de magmas derivados do manto e fundidos crustais no MASH (melting, assimilation, storage, homogenization) ou "hot zone" em níveis crustais profundos. Durante a ascensão para níveis crustais mais rasos, esses magmas arrastaram antecristas (por exemplo, plagioclásio com alto e baixo teor de anortita nos quartzo monzodioritos, biotita de alto bário em granitos) derivados de reservatórios de magma ricos em cristais. Nesse cenário, sugerimos uma evolução do magma em reservátorios de magma interconectados ricos em cristais e localizados em diferentes níveis crustais. A variação composicional e mineralógica apresentada no plúton Itupeva registra múltiplos processos de evolução do magma, incluindo cristalização e acumulação, recarga e remobilização de mushes, mixing e mingling, assimilação e carreamento de antecristas, que operaram ao longo de um sistema magmático transcrustal.
 
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Publishing Date
2021-01-19
 
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