Caracterização petrológica e condicionamento estrutural de alojamento do enxame de diques de lamprófiro de Resende (Cretáceo Superior), estados do RJ e MG

Abstract

The occurrence of lamprophyre dikes in the western portion of Resende (RJ) has been the subject of studies on tectono-magmatic evolution of the region during Meso-Cenozoic. These alkaline and ultrabasic shallow magmatic expressions comprise narrow dark dikes with exotic mineralogy, which integrate a regional Alkaline Magmatism and are inserted in the Poços de Caldas-Cabo Frio Alignment. Other magmatic and sedimentary entities also occur in the region, namely Massif Alcalino de Itatiaia (MAI) and Resende Basin, respectively. However, the evolution of these dikes is still not fully understood, and its real dimension and structural control active during its emplacement were not widely invested either. Thus, in this study new occurrences of lamprophyre dikes, petrography data, lithogeochemistry and, mainly, structural analyzes are presented. More than 30 new dikes have been found, all with physical characteristics corresponding to lamprophyres, configuring a dike swarm due to the wide spread and large number of bodies of the same type. Resende lamprophyres are thin dikes and sills that can reach kilometers in length. The rock is dark and formed by phenocrystals of anhydrous and hydrated ferromagnesian minerals with euhedral shape immersed in a fine groundmass with the same phases plus glass and minor amount of opaques and carbonate. TAS and alkalinity diagrams confirm the alkaline and silica-unsaturated character of these rocks. The high rates and wide variation of compatible elements (Ni, Cr, Sc, Sr) point to the presence of primary magmatic material and the occurrence of extensive fractional crystallization. Mantle and chondrite- normalized diagrams point to an enriched mantle as lamprophyre magmas source. Th/Yb and Ta/Yb ratios confirm a within-plate enriched mantle source with residual carbonate and phlogopite/amphibole as suggested by LILE enrichment and the presence of carbonate and phlogopite/amphibole as primary phase. The dikes are arranged in a main ENE-trending set, containing more than half of the dikes, and other secondary sets, with NW-SE and WNW-ESE directions. All dikes are sub-vertical, except for a single sill intrusive in local foliation plane. The dikes of the main set are straight and do not follow regional foliation, which has the same strike (ENE-WSW) but an average dip angle of 60°. Secondary sets, on the other hand, intrude previous fractures and present displacements following the same pattern and even dikes in zigzag that result in numerous dilations in the NNW-SSE orientation. An emplacement model was created for this dike swarm considering the dikes classified as good paleoindicators. Such model points to an extensional regime, with the a sub-horizontal least compressive stress axis in the NNW-SSE direction and the main compressive axis sub-vertical. These analyzes also fit into tectono- sedimentary models related to the Resende Basin, which would have been formed in multi-stages, with the first event in the Paleogene, an NNW-SSE extension that caused the edge faults and main lineaments. Besides that, main regional NE- to ENE-trending lineament ages at Rio de Janeiro range from 95 to 65 Ma. Thus, with the ages already published for some of the lamprophyre dikes in the region (between 84.0 and 72.0 Ma) and hierarchical configuration observed in the field, it is possible that this extensional regime was also active during this dike swarm emplacement, considered the first activity alkaline magmatism in the region