Early evidence of pyrometamorphism and a late hydrothermal alteration related to Albian volcanism in the Pernambuco sedimentary basin, northeast Brazil

Abstract

Pyrometamorphic aureoles and late hydrothermal alterations linked to Albian alkaline volcanism of the Ipojuca Magmatic Suite are described for the first time in the Pernambuco Basin, northeast Brazil. Field features, petrography, scanning electron microscopy (SEM-WDS), reflectance spectroscopy and X-ray diffraction data were used to characterize the mineral assemblage related to these two geological features. The pyrometamorphic aureoles occur when trachybasalts and arcosian sandstones (upper Cabo Formation) are in contact and were classified as buchites, due to the mineral assemblage composed of mullite, spinel (hercynite), tridymite, pyrrhotite and montmorillonite. The sanidinite facies (high temperature and lower pressure) is attributed to the mineral assemblage described above. Regarding the late hydrothermal alteration, two main events occurred in shallow conditions (up to 1 km). The first event is marked by an argillic alteration along NE-SW and NW-SE faults and fractures. The mineral assemblage is composed of kaolinite, quartz and minor magnetite. This alteration is also characterized by a reflectance spectral signature demonstrated by the high crystallinity of kaolinite that could also indicate its hypogene origin, in temperatures ranging from 100 to 250 °C. The second event is characterized by amygdales and veinlets in basalts lava flows that are filled with smectite (e.g montmorillonite), the formation of which is attributed to alkaline hydrothermal fluids with temperatures between 75 and 150 °C. Both events have been associated with the same fluid source. However, the first event occurred at higher temperatures and with acidic fluids, while the second presented lower temperatures and alkaline fluids.