Morfologia e assinatura geoquímica de zircão da suíte sanukitoide Rio Maria, Província Carajás: implicações petrológicas

Abstract

This research involved the morphological and compositional study of zircons from granodioritic rocks of Sanukitoide Rio Maria suite, Carajás Province. For this study were chosen five regions inserted in this Province, namely: Rio Maria and Bannach, type areas of the Rio Maria Sanukitoide rocks; Ourilândia do Norte, containing correlated granodioritic rocks to sanukitoids Rio Maria; São Felix do Xingu, where there are still poorly studied granodiorite rocks similar to sanukitoides, besides Trairão Granodiorite, situated in the Pau D'Arco region, belonging to the Guarantã suite and geochemically distinct from sanukitoide rocks. Zircon crystals were studied with the aid of secondary electron images (ES), cathodoluminescence (CL), semiquantitative analysis by spectroscopic energy dispersive (EDS) using a scanning electron microscope (SEM), aiming to define morphological features and characteristics geochemical signatures for zircons from each group of rock, compare the typological aspects between them and reaffirm the importance of zircon in petrological studies and SEM-CL-EDS methodology as a support tool for these purposes. The morphological study was conducted in zircons from two groups of rock. In the first one, made up of zircons from Rio Maria Sanukitoide Suite, were selected one hundred ten zircons and in the second one, represented by zircons of Trairão Granodiorite, twenty nine crystals. Zircons from the first group have euhedral shapes, subordinately subhedral, well defined standard zoning, well-developed and preserved cores, thin luminescent edges, suggesting compositional change at the end of crystallization, and rare to moderate presence of F-apatite inclusions. The zircons of Trairão Granodiorite showed euhedral the subhedral forms and well defined oscillatory zoning. The presence of F-apatite inclusions is recurring in this sample, both at the cores or at the edges of the crystals, truncating or taking place in parallel with the growth zones, suggesting simultaneous crystallization of both minerals. For the typological study were selected two groups of zircons. The first, made up of zircons of the Rio Maria sanukitoides, falls mostly within S18 type, with rare occurrences in P4 type. The second, represented by zircons of the Guarantã Granodiorite, correlates with S3 and S8 types. Semiquantitative ESD analysis performed in zircons from the five studied regions were compared and interpreted in specific geochemical diagrams. Zircons of Rio Maria and Ourilândia do Norte sanukitoides showed the lowest Nb contents (1.0-1.8%), followed by the zircons of Bannach and Xingu regions (1.8-2.5%) and Trairão Granodiorite (dominantly between 2.2 to 3.3%). Zircons from Rio Maria and Ourilândia do Norte sanukitoides showed higher Zr/Nb ratios, between 30 and 50, and zircons from Trairão Granodiorite the lower ratios, predominantly between 17 and 23. Zircons from Bannach and Xingu sanukitoides presented intermediate Zr/Nb ratios, between 23 and 32. The Sr versus Zr/Nb diagram shows a well defined negative trend, with zircons of Trairão Granodiorite more enriched in Sr (1.5 to 2.4%) and zircons from the Rio Maria and Ourilândia of Norte sanukitoides most depleted (0.6 to 1.6%), with partial overlap between them. Zircons from Bannach and Xingu sanukitoides showed intermediate Sr content (1.2 to 1.7%) and Zr/Nb (23 and 32) and plotted between the previous in the diagram. The study showed morphological, typological and compositional differences between the zircons of sanukitoides rocks of southeastern Para, and between these and the zircons of the Trairão Granodiorite. According to Pupin (1980) scheme, these differences allowed to frame the zircons of the sanukitoid rocks and the Trairão Granodiorite into two distinct groups, suggesting its crystallization occurred in different environments and temperatures.