TITLE:
Geochemistry and Petrography of Alkaline rocks from Monte Santo Alkaline Intrusive Suite, Western Araguaia Belt, Tocantins State, Brazil
AUTHORS:
Rúbia Ribeiro Viana, Gislaine Amorés Battilani
KEYWORDS:
Alkaline Rocks, Tocantins Structural Province, Brazil
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.2 No.3,
June
13,
2014
ABSTRACT: The Monte Santo
Alkaline Intrusive Suite (MSAIS) is an association syenite foid, nepheline
syenite and syenite. The MSAIS rocks are intruded in metapelites of the Rio do
Coco meta-volcanic-sedi- mentary Sequence and are abundant pegmathoid veins
cutting all of them. The mineral paragenesis is represented by aegirina,
arfvedsonite, albite and nepheline, crystallized during the initial phase of
crystallization. A late magmatic phase show nepheline, perthite, calcite and
biotite, and a hydrothermal phase allowed for the formation of cancrinite,
sodalite, analcime and natrolita associated with altered nepheline. The
geochemical analyses showed metaluminous and medium to high potassium characteristics,
being classified as miaskitic rocks, according to agpaicity and the Na + K >
1/6Si indexes. However, the mineralogical assemblage suggests a low to medium
agpaitic composition, which can be related to a transition from miaskitic to
agpaitic crystallization regime. The rare earth elements showed depletion in
heavy rare earth and a strong negative Eu anomaly and enrichments in the some
lithophile elements, suggesting a differentiated pattern later, which can be
associated to metasomatic alterations.