Abstract Associated with subordinate potassic rocks, Mesozoic to Cenozoic sodic alkaline rocks are represented by four distinct provinces in Eastern Paraguay: Alto Paraguay, Central, Misiones, and Asunción; additional Rio Apa and Amambay provinces are exclusively of potassic affinity. Alkaline magmatism is clearly controlled by a tectonic extensional regime that generated NW-SE-trending faults and grabens. It varies in composition, with the intrusive sodic rocks mainly composed of foid syenites and quartz-bearing syenites and the potassic ones of syenites, syenogabbros, and gabbros, all of which are accompanied by fine-grained hypoabissal and volcanic counterparts. In the Central Province, potassic and sodic rocks occur together and are represented by two distinct magmatic series: basanites-phonolites and alkali basalts-trachytes/trachyphonolites. Ultramafic sodic rocks (ankaratrites and nephelinites) are uniquely found in the Misiones and Asunción provinces. In general, the sodic rocks exhibit LILE enrichment, high field strength elements depletion, and Nb-Ta positive anomalies in the mantle's normalized incompatible elements (IE) distribution. Chondrite-normalized rare earth element (REE) diagrams point to strong REE enrichment and high light/heavy rare earth element fractionation in different subparallel patterns. Conversely, the potassic rocks display negative anomalies of Nb-Ta and are fractionated in REE. In the initial Sri versus Ndi diagram, the sodic rocks approach to bulk earth, whereas the potassic ones plot into the Sr-enriched quadrant. Sr-Nd-Pb isotope data suggest that a high U/Pb mantle component played an important role in the genesis of the Late Early Cretaceous and Tertiary sodic magmas, while an enriched mantle component dominated the Early Cretaceous potassic magmas. The close association of potassic and sodic rocks implies that their parental magmas derived from a subcontinental mantle variable in composition enriched in IE. Paraguay affinity NWSEtrending NW SE trending grabens quartzbearing quartz bearing syenogabbros gabbros finegrained fine grained counterparts Province series basanitesphonolites basanites phonolites basaltstrachytes/trachyphonolites. basaltstrachytestrachyphonolites basalts trachytes/trachyphonolites. trachytes trachyphonolites basalts-trachytes/trachyphonolites ankaratrites nephelinites general depletion NbTa Nb Ta mantles s IE (IE distribution Chondritenormalized Chondrite (REE lightheavy light heavy patterns Conversely diagram Srenriched Sr quadrant SrNdPb Nd Pb UPb U basaltstrachytes basaltstrachytes/trachyphonolites trachytestrachyphonolites trachytes/trachyphonolites

ABSTRACT: This paper reviews general aspects of alkaline-carbonatitic rocks of Brazilian, Paraguayan and Bolivian terrains. Although 30 such occurrences are known in literature, only the major ones have been thoroughly investigated. The carbonatites are of Cretaceous age, with two well-defined Lower Cretaceous and Upper Cretaceous generation episodes. A clear tectonic control by ancient structural features such as archs, lineaments and faults characterizes most cases. The rocks exhibit a large compositional variation, in decreasing orders of abundance from calciocarbonatites to magnesiocarbonatites to ferrocarbonatites. In some complexes, they form multistage intrusions. C-O isotopes indicate that, in general, the carbonatites were affected by post-magmatic processes associated with the topographic level of emplacement and low-temperature H 2 O-CO 2 rich fluids responsible for the increased amount of heavy carbon and oxygen. Sr-Nd isotopic compositions similar to those of coeval alkaline silicate rocks, ranging from depleted to enriched mantle sources, have been influenced by two distinct metasomatic events in Proterozoic at 2,0-1.4 Ga and 1.0-0.5 Ga. Sr-Nd-Pb-Os data seem related to an isotopically enriched source, their chemical heterogeneities reflecting a depleted mantle that was metasomatized by small-volume melts and by fluids rich in incompatible elements. Fractional crystallization and liquid immiscibility are believed to be the most effective processes in the formation of the Cretaceous carbonatites, with minor contribution of crustal contamination. Pb isotopic ratios yield evidence that HIMU and EM I mantle components played an important role in the genesis of the carbonatitic magma.

Columnar joints, resembling those in igneous rocks, are present in quarz sandstones of various formations (Mesozoic to Cenozoic) in Eastern Paraguay. New data concerning the feld relationships and the geochemical characteristics constrain the processes responsible for the generation of columnar fracture patterns in sedimentary rocks. The columns are mostly normal to bedding and occur at several different stratigraphic levels. The host rocks are cut by volcanic necks or plugs, acting as heat sources and the development of columnar jointing may be controlled by the presence of magmatic rocks. Alternatively, thermal waters, circulating through fractures and favorable sedimentary layers, are responsible for the remplacement of the sandstones by opal. As a result of the strain originated by the contraction of opal, a system of joints was developed. It is also possible that the local tectonic stresses have been developed and contributed to the fragmentation processes.

La disyunción columnar, similar a esas formadas en rocas ígneas, son comunes en areniscas cuarzosas en formaciones Mesozoicas y Cenozoicas del oriente de Paraguay. Nuevos datos vinculantes a sus relaciones de campo y a sus características geoquímicas refuerzan los procesos responsables en la generación de las fracturas poligonales en estas rocas sedimentarias. Las columnas son principalmente normales a los estratos y ocurren en diferentes niveles estratigráficos. La roca de caja es cortadas por cuellos y conductos volcánicos, los que actúan como fuentes de transmisión de calor controlando así el desarrollo de la disyunción columnar en las rocas sedimentarias que se encuentran en sus cercanías. De manera complementaria, la percolación de aguas termales a través de las fracturas y contactos entre las capas sedimentarias, favorecen el reemplazamiento de las areniscas por ópalo. Como resultado del esfuerzo originado por la contracción del ópalo, se desarrolla un sistema de empalmes conjugados con diseño poligonal. Es posible también considerar que el desarrollo de centros de tensión localizados contribuyan al desarrollo del proceso de fragmentación.