SiO2-saturated potassic alkaline magmatism in the central Amazonian Craton, southernmost Uatumã-Anauá Domain, NE Amazonas, Brazil

Abstract

This paper approaches the record of SiO 2 -saturated potassic alkaline magmatism of Castanhal Quartz Monzonite, Mapuera Suite, and Ladeira da Vovó Quartz Syenite. These samples are located near the Northern border of the Amazon Basin. Such rocks show K 2 O + 2 > Na 2 O and K 2 O/Na 2 O < 2 values that confirm the potassic or shoshonitic character of these rocks. The Castanhal Quartz Monzonite contains less than 20% volume of quartz, which is also a characteristic of the shoshonitic or SiO 2 -satured potassic alkaline A-type magma signature observed on geochemical plots. Listric faults, representing the rifting phase of Amazon Basin formation, emplaced and reworked Ladeira da Vovó Quartz Syenite, which caused its granophyric texture, probably during the Tonian period. A group of 21 zircon crystals was extracted from a hornblende quartz monzonite and yields an average age of 1872 ± 6 Ma (MSWD = 2.4). However, an additional zircon crystal yielded a Trans-Amazonian age of 2062 ± 17 Ma. These potassic alkaline rocks of Orosirian (1872 Ma) age may correspond to a post-collisional setting. Dominantly negative εHf t values and Hf T DM ages reveal a large contribution of a mafic crustal component from Mesoarchean to Neoarchean age (2.95 -2.66 Ga), and a felsic crustal component from Neoarchean to later Siderian ages (2.51 -2.34 Ga). KEYWORDS:

Shoshonitic Magmatic Series and the High Ba-Sr Granitoids: A Review with Emphasis on Examples from the Neoproterozoic Dom Feliciano Belt of Southern Brazil and Uruguay

“…With ages between 1 875 and 1 770 Ma, these rock associations are post-collisional relative to the Svecofennian Orogeny and were formed by partial melting of subduction-modified mantle sources with negligible crustal contribution and subsequent differentiation dominated by fractional crystallization. Near the northern border of the Amazon Basin, Valério et al (2017) described shoshonitic affinity monzonitic and syenitic rocks of ca. 1 872 Ma associated to post-collisional settings and produced, according to those authors based mainly on Lu-Hf isotope data, by partial melting of the crust.…”