Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio

Abstract Modal classifications of common volcanic rocks are expensive, difficult, or impossible to attain. As a consequence, these rocks are classified using the chemical composition. However, existing classificatory diagrams are unable to identify all 16 families of common volcanic rocks; the most...

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Main Authors: JOÃO O.S. SANTOS, LÉO A. HARTMANN
Format: Article
Language:English
Published: Academia Brasileira de Ciências 2021-07-01
Series:Anais da Academia Brasileira de Ciências
Subjects:
Online Access:http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0001-37652021000501103&tlng=en
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author JOÃO O.S. SANTOS
LÉO A. HARTMANN
author_facet JOÃO O.S. SANTOS
LÉO A. HARTMANN
author_sort JOÃO O.S. SANTOS
collection DOAJ
description Abstract Modal classifications of common volcanic rocks are expensive, difficult, or impossible to attain. As a consequence, these rocks are classified using the chemical composition. However, existing classificatory diagrams are unable to identify all 16 families of common volcanic rocks; the most used is the total alkali-silica (TAS) diagram that identifies six families. Rocks not in the TAS diagram are misclassified with other rock names; their names are evolving to extinction, e.g. latite and rhyodacite. Some diagrams use Na2O, which is a complicating element rather than discriminant. Na2O is present both in alkali feldspar and plagioclase making difficult the separation of the amount associated to either feldspar. Silicon, potassium, and calcium are the three major elements with highest variations among volcanic rocks. They are selected for use in two new diagrams confronting CaO/K2O ratio with K2O content (KCK diagrams). One diagram is designed for saturated (intermediate) rocks (quartz <5 vol.%) and the other for oversaturated (acid) volcanic rocks (quartz >5%). These diagrams are tested using compositions of volcanic rocks from Uatumã magmatism in the Amazon Craton and also by plotting compositions of rocks from type-localities and the world averages. The proposed limits between rock families agree with plotted rock compositions. The KCK diagrams are an alternative to existing diagrams to classify volcanic rocks.
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spelling doaj.art-fb39d84719784053aaab9c9574c15f5f2022-12-21T17:21:50ZengAcademia Brasileira de CiênciasAnais da Academia Brasileira de Ciências1678-26902021-07-0193310.1590/0001-3765202120201202Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratioJOÃO O.S. SANTOShttps://orcid.org/0000-0001-5524-7949LÉO A. HARTMANNhttps://orcid.org/0000-0001-7863-5071Abstract Modal classifications of common volcanic rocks are expensive, difficult, or impossible to attain. As a consequence, these rocks are classified using the chemical composition. However, existing classificatory diagrams are unable to identify all 16 families of common volcanic rocks; the most used is the total alkali-silica (TAS) diagram that identifies six families. Rocks not in the TAS diagram are misclassified with other rock names; their names are evolving to extinction, e.g. latite and rhyodacite. Some diagrams use Na2O, which is a complicating element rather than discriminant. Na2O is present both in alkali feldspar and plagioclase making difficult the separation of the amount associated to either feldspar. Silicon, potassium, and calcium are the three major elements with highest variations among volcanic rocks. They are selected for use in two new diagrams confronting CaO/K2O ratio with K2O content (KCK diagrams). One diagram is designed for saturated (intermediate) rocks (quartz <5 vol.%) and the other for oversaturated (acid) volcanic rocks (quartz >5%). These diagrams are tested using compositions of volcanic rocks from Uatumã magmatism in the Amazon Craton and also by plotting compositions of rocks from type-localities and the world averages. The proposed limits between rock families agree with plotted rock compositions. The KCK diagrams are an alternative to existing diagrams to classify volcanic rocks.http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0001-37652021000501103&tlng=enChemical classificationvolcanic rockssilica saturationUatumã volcanism
spellingShingle JOÃO O.S. SANTOS
LÉO A. HARTMANN
Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio
Anais da Academia Brasileira de Ciências
Chemical classification
volcanic rocks
silica saturation
Uatumã volcanism
title Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio
title_full Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio
title_fullStr Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio
title_full_unstemmed Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio
title_short Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio
title_sort chemical classification of common volcanic rocks based on degree of silica saturation and cao k2o ratio
topic Chemical classification
volcanic rocks
silica saturation
Uatumã volcanism
url http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0001-37652021000501103&tlng=en
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