Monte Santo suite, an example of Ediacaran-Cambrian deformed alkaline rocks in the Araguaia Belt, Central Brazil. Implications for Western Gondwana evolution

Abstract

Deformed alkaline rocks and carbonatites (DARCs) can be important tectonic indicators of regions that have experienced two separate events of rifting and collision, but can also represent alkaline magmas that were synkinematically emplaced along major structures and were therefore generated and deformed in the same geologic event. The Monte Santo Suite consists of two foliated and variably deformed massifs (Estrela and Monte Santo) that crop out in the Araguaia Belt and are characterized by igneous and subsolidus microstructures. In this study, we use the chemistry, mineralogy and age of the Monte Santo Suite to understand its genesis and the relations between its emplacement, the Brasiliano-Pan-African orogeny and the final evolution of Western Gondwana. The intrusions are predominantly nepheline syenite and alkali feldspar syenite. Pyroxene varies in composition from omphacite to aegirine-augite to aegirine and has mixed igneous and metamorphic characteristics. Biotite varies from phlogopite to siderophyllite to annite and presents primary, reequilibrated and neoformed trends. Amphibole varies from hastingsite to taramite. Nepheline occurs as both high-temperature and low-temperature varieties. The REE and trace element patterns display parallel trends with significant bulk content variations, positive anomalies for Nb, Ta, Zr and Hf, and negative anomalies for Ba, Sr, P and Ti in multielement diagrams, The Zr/Hf ratios for all the rocks of the massifs are similar and whole-rock εNd values range from −10.1 to −3.0. Trace element and isotopic data suggest that these magmas experienced extensive fractional crystallization, crustal contamination, and evolved from the same source, probably an enriched mantle. The Monte Santo Suite crystallization age is dated to 545 Ma, when these magmas were generated and deformed synkinematically with shearing along major regional faults related to the Transbrasiliano Lineament. Thus, this suite is an example of alkaline rocks generated and deformed in the same geological event throughout Western Gondwana.