Tipologia de zircão: análise do método aplicado a rochas vulcânicas silícicas do Cráton Amazonas

Abstract

Zircon (ZrSiO4) is a very common accessory mineral in igneous rocks, with a high crystallization temperature (~910 °C), which makes it thermally resistant. Furthermore, it often presents growth zones and a composition rich in trace elements, such as U, Pb and Ti, key characteristics for petrology and geochronology research. Thus, the present work sought to characterize samples of silicic volcanic rocks, with known ages, from different parts of the Amazonas Craton, using petrography and zircon typology — a technique that chemically, thermally and petrogeneticallyclassifies a rock based on the morphology of its zircons), whole-rock geochemistry and mineral chemistry data, which were used to measure the quality of the paleotemperature results obtained by typology. Samples were analyzed from the Vila Riozinho Formation (2,0 - 1,97 Ga), the Sumuru Group (1,99 Ga), the Cachoeira da Ilha (1,96 a 1,98 Ga) and the Morais Almeida (~1,88 Ga) formation, and from the Colíder Group (1,82 -1,77 Ga). The samples from the Vila Riozinho, Cachoeira da Ilha and one from the Morais Almeida formation contained a Calcialkaline chemical layer with hybrid origin, part from the mantle and part crustal; while the samples from Sumuru and Colíder groups, as well as one sample from the Morais Almeida formation contained an alkaline layer, with its magma classified as hot, dry and of predominantly mantle origin. The average crystallization temperatures obtained by the typology for the various samples vary between 727,27 °C and 805,95 °C, while the temperatures calculated by total rock geochemistry, with models M and G, vary, respectively, from 526,09 °C e 1220,47 °C and from 642,45 °C to 848,45 °C. Although not precise, the typology temperature values showed good convergence with those of other chemical models, especially the G model. Finally, the results of the chemical and mineralogical composition of the samples showed excellent correlation with data from petrography and literature. The zircon typology proved to be an effective, cheap and easy to apply method.