LITHOGEOCHEMISTRY APPLY TO BASALTIC FLOWS SEPARATION FROM THE PARANA IGNEOUS PROVINCE IN THE STATE OF PARANA

Abstract

The Lower Cretaceous Paraná Magmatic Province is composed of basalt, basaltic andesite and felsic lavas grouped into high-Ti and low-Ti suites. Our study focuses on high-Ti basalts located nearby the Pitanga city in Paraná state. We correlate geochemical data obtained for samples from different basaltic flows which in turn were characterized by physical volcanological parameters. Three flows were recognized in the field work. The lithogeochemical data show a compositional range that cannot be associated with a single volcanic episode. In addition, the studied samples were correlated with the Paranepanema magma-type, as opposed to the Pitanga type as displayed on the more recent version of the geological map of the Paraná Basin. The results show how lithogeochemical data allow to refine the stratigraphy depicted by physical volcanology, updating the amount and the basalt series in the studied area.

Keywords: Volcanology; basaltic flows; lithogeochemistry.

Introduction

The Paraná Magmatic Province (PMP) represents the largest magmatic event of the Lower Cretaceous associated with the breakup of the Western Gondwana.This event is composed of basalts and basaltic andesites and, with minor felsic rocks (Bellieni et al., 1984; Peate, 1997). The basaltic lavas are grouped into two types: high TiO₂ (TiO₂ > 2 wt.%) and low TiO₂ (TiO₂ < 2 wt.%). The high-TiO₂ lavas, mainly located in the northern PMP, are divided in four magma types: Ribeira, Pitanga, Paranapanema, and Urubici. The latter is located in the south of the province (Peate et al., 1992; Peate, 1997). This study focuses in high-TiO₂ basalts in the northern PMP, located nearby the Pitanga city in the Paraná state. This study demonstrates that geochemical data can be used as a tool for a stratigraphic correlation of volcanic flows.

Methods

The materials of this study are volcanic rocks sampled nearby the Pitanga city. The methodology consisted of field work, macroscopic petrography and lithogeochemical analysis. The field work described lithofacies, delimited stratigraphic sections, and allowed sampling of exposed volcanic rocks of distinct flows. These flows are delimited in the field work by the combination of distinct lithofacies and lithofacies associations. The macroscopic petrographic analysis determined textures and structures belonging to the specific lithofacies (e.g., a aphyric and amygdaloidal basalt with spheric and millimeter-size amygdales). The fieldwork was carried out in the Pitanga and Barra Bonita regions in the state of Paraná, specifically at road cuts on the PR 239 highway and at the Dellai Mineração and Pedreira São Judas Tadeu quarries. The lithogeochemical data correspond to analyses of massive core of flows.  The major elements were obtained by inductively coupled plasma atomic emission spectrometry, while trace element concentrations, including rare earth elements, were obtained by inductively coupled plasma mass spectrometry.

Results

A total of 24 points and 64 samples of basalts and related rocks were collected in the study area. The stacking of the basalt flows reveals three flow sections separated by breccias within the whole area, hereby named F1, F2, and F3 from bottom to top of the stratigraphic profile (Figure 1).

The first flow (F1) consists of a layer of basalt (~12m), with ellipsoidal and flattened vesicles (5-8% vol) scattered at the base. The vesicles range from millimeters to centimeters in size and are totally or partially filled with quartz, zeolite, and/or calcite. The core of F1 consists of massive, homogeneous basalt with few or no vesicles. There are some areas of rounded, millimetric vesicles concentrated and limited to the core of the flow, rising to the top. The top of the F1 crust consists of vesicular basalt with flattened, ellipsoidal, centimetric, and scattered vesicles, which make up 10 to 12% of the volume. The second flow (F2) is separated from the first by a breccia layer composed of vesicular basalt clasts, clay minerals, secondary mineralization (like zeolite and chalcedony), and sediments. Due to the interaction structures between lava and sediments, this breccia layer corresponds to a peperite. The base and core of F2 are similar to F1, with the latter showing areas of vesicle concentration in a cylinder-shaped. The top is marked by rounded vesicles (8-10% vol), millimetric to centimetric, filled (total or partially) with celadonite and quartz. The third flow (F3) is also separated from the second by a layer of peperite with vesicular basalt clasts, clay minerals, and secondary mineralization, but these with angular to blocky clasts. The base of F3 is composed of basalt with centimeter-sized vesicles (10-15% vol), scattered, ellipsoidal, filled with quartz, zeolite, and/or calcite. The core is composed of homogeneous, fractured basalt with no vesicles. The top of the flow consists of centimeter-sized vesicles, scattered, flattened, ellipsoidal, and filled totally or partially with quartz, zeolite, and/or calcite.

Lithogeochemical data of 18 samples are plotted on the TAS diagram (Figure 2). These samples are classified as basalt and basaltic andesite. According to the bivariate diagrams (Figure 3), there is a negative correlation between MgO and CaO (10.78 to 7.71 wt.%) within each of the flows. The TiO₂ content remains roughly unchanged (1.98 to 2.39 wt.%) when compared to the MgO variation. These samples belong to the Paranapanema suite (Zr/Y< 7; Sr<400 ppm, Figure 3), despite being discriminated as Pitanga basalts in the more recent geological map of the Paraná Basin. In F1, the MgO concentration varies from 3.64 wt.% to 6.35 wt.%, representing a variation of 74%, in F2 from 4.11 wt% to 6.25 wt%, variantion of 52%, and in F3 from 4.54 wt% to 5.93 wt%, with 30% of variation. The concentrations of Fe₂O₃^T varying in F1 by 12%, in F2 by 19% and in F3 by 15%, while CaO varied by 33% in F1, 22% in F2 and 10% in F3, and Sr variation in F1 by 44%, F2 by 56% and F3 by 10%.These variations of each flow should not be expected if eruptions of each of them were related with the same volcanic pulse along the study area. These chemical differences suggest a more complex lava emplacement with the existence of successive flows within each of the three flow sections, meaning that different flow sections (i.e., F1, F2 and F3) may contain basalt flows erupted from the different magma pulse.

Conclusions

According to fieldwork data and physical volcanology, three flow sections separated by peperite layers were delimited and characterized in the study area. The samples collected from each section show significant variations in the chemical composition, such as MgO, Fe₂O₃^T, SiO₂, CaO, Na₂O+K₂O, Sr and Zr/Y. These variations are observed internally in each volcanic section, as well as between different sections. Such variations are difficult to explain assuming that each flow section derives from single eruptive events. Therefore, these three flow sections must represent a more complex emplacement scenario, suggesting successive eruptive episodes from different pulse along a certain time interval separated by peperite layers. In other words, each flow section delimited by physical volcanology does not represent a single volcanic flow but may contain different flows that amalgamated in different parts of the study area due to paleotopographic controls. Also, the high-Ti basalts in the study area are related with the Paranapanema magma type as opposed to the Pitanga one firstly expected to occur nearby the homonomous city. These results contribute to the understanding of the extension and emplacement conditions of the high-Ti basalts within northern PMP.