Silicic lavas with no basal breccia: origin of the thinly jointed basal facies of low-Ti dacites in the Paraná-Etendeka Large Igneous Province

Large-volume silicic eruptions in large igneous provinces are unique in the geological record because there are no observed analogues. Their facies architecture do not strictly follow the diagnostic features proposed in the literature for rheomorphic ignimbrites and lava flows. The models proposed for their emplacement depend on conditions such as high temperature (> 950 °C) and low viscosity (< 10 6 Pa s) coupled with unusually high effusion rates. Low-Ti dacitic lavas from the Paraná-Etendeka Large Igneous Province (LIP) have a wide variety of morphologies and lithofacies on both sides of the Atlantic Ocean. A common facies association framework for the Caxias do Sul/Grootberg dacites (outcropping in Namibia and southern Brazil) include the presence of platy and thinly jointed facies with a columnar jointed massive core and an amygdaloidal upper facies, with an absence of basal breccia. In this work, we describe and interpret features observed in the basal portion of these silicic units, focusing on the thinly jointed facies. This basal facies includes what has been termed as “zebra-like banding.” Zebra-like banding is an apparent structure that originates from a fracture network which involves horizontal and long first-order fractures, oblique and short second-order fractures, oblique short to long third-order fractures, and intersection zones of fourth-order structures. First-order fractures are related to pure shear, the second- and third-order fractures are Riedel shears, and the fourth-order structures represent angular contacts between different sets containing first-, second-, and third-order fractures. The combination of first-, second-, and third-order fractures evolves to shear lenses. The zebra-banding” is caused by oxidation/reduction halos developing in the host rock during post-emplacement fluid circulation along the fractures, culminating in precipitation of silica polymorphs and zeolite. The fracture system, including all orders of fracture, formed during late to post emplacement stages, and accommodated pure and simple shear in a ductile–brittle basal zone during oscillations in effusion rate. Fracturing of the basal zone, rather than the formation of basal breccia, demonstrates stress accumulation in this part of the flow and helps to explain why the presence of basal breccia is not a diagnostic feature in distinguishing large-volume silicic lava flows from rheomorphic ignimbrites.