Journal of Petrology | Volume 27 | Number 4 | Pages 915-944 | 1986
© Oxford University Press 1986
research-article |
Petrogenetic Aspects of Acid and Basaltic Lavas from the Paraná Plateau (Brazil): Geological, Mineralogical and Petrochemical Relationships
1Istituto di Mineralogia e Petrologia, Padova University Italy
2Istituto di Mineralogia e Petrografia, Trieste University, Piazzale Europa, I, Trieste Italy
3Instituto Astronômico e Geofisíco, S
o Paulo University Brazil
4Instituto de Geociências, Rio Grande do Sul Federal University Brazil
Received June 7, 1985; Revised typescript accepted December 19, 1985
ABSTRACT
The acid volcanics (Lower Cretaceous) of the Paraná basin cover an area of about 150000 km2 and are represented by dominant rhyodacites and subordinate rhyolites. They may be divided into two main types, characterized respectively by relatively low and relatively high contents of Ti, P, and other incompatible elements (La, Ce, Zr, etc.), i.e. the Palmas acid volcanics (PAY) and Chapecó acid volcanics (CAV), respectively. PAV are widespread in the southern Paraná basin and are closely associated with basaltic and andesitic rock-types similarly characterized by low Ti, P, and other incompatible elements. In contrast, CAV are dominant in the northern Paraná basin, where they are closely associated with basalts containing high Ti, P, and other incompatible elements.
The generation of the Palmas and Chapecó acid melts appears to be in part consistent with crystal fractionation processes, starting from the associated basic rocks and accompanied by crustal contamination. However the relative absence of intermediate rock-types (‘silica gap’: 54–56 to 63–65 wt. per cent), and the confinement of the acid volcanics towards the continental margin suggests that a model involving lower crustal basic material of significantly different composition in the northern and southern Paraná basin may be a more plausible alternative. In this preferred model the basic parent material may be represented by mafic granulites of different compositions, or by basalts trapped at the crust-mantle discontinuity and corresponding in composition to the contrasting low- and high-TiO2 basalts that flooded the Paraná basin in Lower Cretaceous times. The melting of these underplated materials may explain the close geochemical relationships between fissure acid volcanics and the closely associated basalt types (e.g., Ethiopia, Paraná). The beginning of the major rifting related to continental break-up should therefore correspond to the stage when the melting process affected the lower part of the continental crust.
*Reprint requests to E. M. Piccirillo
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