PYROCLASTIC BRECCIAS AND RELATED DEPOSITS OF THE POÇOS DE CALDAS ALKALINE COMPLEX, MG/SP, SE-BRAZIL
ARTUR D. ALVES AND HANS D. SCHORSCHER
Instituto de Geociências, USP, São Paulo, SP.
Presented by ANTONIO C. ROCHA-CAMPOS
Different pyroclastic breccias and related deposits represent important petrogenetic indicators of the magmatic-stratigraphic and tectonic evolution of the Poços de Caldas Alkaline Complex (PAC), the largest in South America and one of the 10 largest such complexes worldwide. PAC is a Cretaceous subcircular volcanic-subvolcanic caldera within Precambrian basement rocks at the eastern border of the Paraná Basin exhumed by differential erosion. It comprises ultrabasic through intermediate alkaline igneous, pyroclastic, volcaniclastic and minor sedimentary and basement rocks; PAC-magmatism lasted from ³ 92 to ~ 76 Ma b.p.
The oldest breccias in the PAC W-NW sector are of extrusive ankaratrites. Abundant CO2-rich fluid discharge caused prominent vesiculation, hydraulic fracturing, brecciation and carbonatization of these lava flows. Different pyroclastic, mixed and reworked breccias formed successively during construction of the main regional volcano of miaskitic phonolites and nepheline syenites (with AI. of 0.85-1.15, and giannettite as the most common rare-metal silicate), phonolitic tuffs, mud and debris-flows, until caldera-collapse and emplacement of discontinuous phonolite ring-dikes.
Post-caldera-collapse breccias formed through more local magmatic-tectonic, volcaniclastic and hydrothermal events, with or without temporal and/or spacial superposition. Fragments of typical aphanitic, microporphyritic phonolites, which cut the former nepheline syenites as shallow-subvolcanic dikes, occur associated with carbonate rock, ankaratrite, phonolite, pyroxenite, peridotite, granitic basement and Paraná Basin sedimentary rock fragments in intrusive to extrusive breccias bearing phlogopite-booklets in the phonolitic, locally carbonatized or pyritized matrix; lateral and vertical granulometric grading and interlayering with phonolitic surges and tuffisites and carbonatite flows also occur. These breccias were intruded by phonolite dikes; possibly coeval agpaitic eudialite nepheline syenites (Pedra Balão and Morro do Serrote type) occur as regionally separated minor intrusions.
At the end of regional phonolite and nepheline syenite magmatism breccia pipes formed at foci of magmatic-phreatic explosions and suffered low-grade U-Th-REE-Zr-F-Mo mineralization through strong potassic and pyritic hydrothermalism. Local carbonatization of these breccias suggests nearby carbonatite intrusions (Morro do Ferro type). Finally, at about 76 Ma, dikes of ultramafic ultrapotassic biotite lamprophyres with abundant upper mantle xenoliths intruded the breccia pipes. (December 14, 2001).
Publication Dates
-
Publication in this collection
09 Oct 2002 -
Date of issue
Sept 2002
