Victor Fernandez Velázquez

Cretaceous Na‐Alkaline Magmatism from the Misiones Province (Paraguay): Its Relationships with the Paleocene Na‐Alkaline Analog from Asunción and Geodynamic Significance

Alkaline (sodic) volcanic rocks, i.e., ankaratrites‐melanephelinites, basanites‐tephrites, and phonolites, dated at 119 Ma and similar in composition to the 60‐Ma plugs of the Asunción Province, occur in Eastern Paraguay and belong to the Misiones Province. The age relationships confirm that the youngest volcanic events in Eastern Paraguay, at the central westernmost side of the Paraná basin, are represented by alkaline rock types of sodic affinity emplaced in late Early Cretaceous and Paleocene times. This sodic magmatism contrasts with the Early Cretaceous alkaline (potassic) magmatism of the region, and it is associated in space and time with the Paraná basin tholeiites. Geological and geophysical results for Eastern Paraguay indicate transtensional tectonics, with a NE‐SW regional extensional stress field. The geochemistry and Sr‐Nd‐Pb isotope systematics are consistent with a lithospheric mantle source(s) enriched in incompatible elements by metasomatic processes. Nd model ages suggest that these probably occurred during Meso‐ and/or Neoproterozoic times and may be regarded as precursors of both alkaline and tholeiitic magmas in Eastern Paraguay. Potential parents for the alkaline (sodic) liquids have been modeled in terms of small degrees of mantle partial melting. Multielemental plots of calculated mantle sources for these liquids from Asunción and Misiones contrast with the analog mantle sources for the Paraguayan alkaline (potassic) suite, confirming the view that popular geodynamic markers of this type remain implausible indicators of subduction. Our results support the view that the magma genesis and the emplacement of the alkaline magmatism in southeastern Paraguay, and even in northwestern Argentina and Bolivia, is related to and probably driven by reactivation of preexisting lithospheric discontinuities in the various South American blocks, which promoted local decompression melting of previously enriched mantle sources.

Relações tectônicas do magmatismo alcalino do Paraguai Oriental

The alkaline magmatism of Paraguay is distributed in six provinces (Alto Paraguay, Rio Apa, Amambay, Central, Misiones and Assuncao), with ages varying between Permian-Triassic and Eocene. These provinces, located in Eastem Paraguay, are frequently associated with major structural features, mainly faults, with magnetic and/or gravimetric expression. In this paper geologic and geophysical data are integrated in order to establish, as a first approximation, the tectonic controls for this alkaline magmatism.

A synthesis on the alkaline magmatism of Eastern Paraguay

Alkaline magmatism occurs in six distinct areas of Paraguay and forms bodies of variable size, shape, composition and age. The oldest rocks are found in the north and correspond to the Permo-Triassic Alto Paraguay Province (241 Ma). Four Early Cretaceous events can be distinguished in Eastern Paraguay: the Rio Apa and Amambay Provinces (139 Ma), both predating the tholeiites of the Serra Geral Formation, are located in the northern and northeastern regions, respectively; and the Central (126 Ma) and Misiones Provinces (118 Ma) in the central-eastern and southern regions, respectively. The youngest alkaline rocks are volcanic rocks in Asuncion of Tertiary age (59 Ma). Excluding the Alto Paraguay rocks influenced by the Amazonian craton, the emplacement of alkaline bodies is clearly controlled by a tectonic extensional regime that generated NW-SE-trending faults and grabens. Geochemically, Paraguayan alkaline rocks are predominantly miaskitic, with a potassic or sodic affinity, the former being characterized by strongly fractionated rare earth-elements, negative Ta-Nb-Ti anomalies, and high Sr and low Nd radiogenic isotopes. Sodic rocks have slightly positive Ta and Nb anomalies and are less enriched in Sr, approaching bulk earth values. Carbonatites behave similarly to the associated pre-tholeiites potassic rocks. The Sr-Nd-Pb isotope ratios suggest that two main mantle components were involved in the genesis of the Paraguayan rocks: an enriched mantle I component dominated the Early Cretaceous potassic magmatism, and an high 238U/204Pb ou high U/Pb component that was important for the late Early Cretaceous and Tertiary sodic magmatism. The close association of potassic and sodic suites, such as in the Asuncion-Sapucai-Villarrica graben, indicates that their parental magmas were derived from a heterogeneous subcontinental mantle, enriched with incompatible elements.

The Cretaceous Alkaline Dyke Swarm in the Central Segment of the Asunción Rift, Eastern Paraguay: Its Regional Distribution, Mechanism of Emplacement, and Tectonic Significance

A structural analysis of Cretaceous alkaline dykes swarm associated with the central segment of the Asuncion Rift is reported here. Dykes are generally single near-vertical tabular bodies, less than 5 m wide, although multiple and composite intrusions also occur. Many of these small bodies have been emplaced into Paleozoic sedimentary rocks and exhibit a regional NW-SE orientation pattern. Petrographical and geochemical data allow recognition of two different lineages of potassic dykes: a silica-undersaturated suite ranging from basanite to phonolite (B-P) and a silica-saturated suite ranging from alkali basalt to trachyte (AB-T). The morphological features, the regional en-echelon distribution, and the NW-SE orientation pattern suggest that the dykes were injected along fractures and faults, under a transtensional tectonic regime with σ1 NW/horizontal, σ2/vertical, and σ3 NE/horizontal. Detailed analysis, combining dyke petrography, orientation pattern, and relative chronology reveals a rotation from WNW toward NNW during dyke emplacement. In terms of the paleostress field orientation, the evidence indicates that the dykes were diachronically formed under a similar stress condition. Finally, the pattern of orientation documented for the Cretaceous alkaline dykes of the Asuncion Rift is consistent, temporally and spatially, with the phases of regional deformation that occurred during the process of the Atlantic Ocean opening.

The Colônia Impact Crater: Geological Heritage and Natural Patrimony in the Southern Metropolitan Region of São Paulo, Brazil

The Parelheiros district, located at the extreme southern end of the São Paulo metropolitan region, is historically famous for the occurrence of an unusual circular depression, denominated in the geological literature as Colônia Crater. This prominent geomorphological structure, with a 3.6-km diameter, formed by impact cratering, represents one of the few records of a violent and devastating geological process that shaped the Earth’s surface in the past. The crater lies within an environmental protection area and displays a remarkable landscape, as well as a rich and singular fauna and flora. With a wide geological and biological diversity, the Colônia impact crater is an extraordinary natural heritage and must be rigorously preserved to: (a) conserve its exceptional natural wealth in order to provide continuity to various scientific researches, (b) implement suitable land use and occupation programmes to maintain the crater as an environmental preservation area and (c) encourage the local community’s participation in the development of sustainable tourism in the region.