PENECONTEMPORANEOUS SYENITIC-PHONOLITIC AND BASIC-ULTRABASIC-CARBONATITIC ROCKS AT THE POÇOS DE CALDAS ALKALINE MASSIF, SE BRAZIL: GEOLOGIC AND GEOCHRONOLOGIC EVIDENCE

Abstract

The large Pocos de Caldas alkaline massif, southeastern Brazil, is constituted mainly by felsic rocks (nepheline syenites, phonolites and subvolcanic tinguaites), associated with volcaniclastic basic-ultrabasic deposits. Critical appraisal of earlier K/Ar, Rb/Sr and microchemical ages (microprobe Th-U-total Pb determinations), combined with geological and paleomagnetic informations, can limit more adequately the age interval for the massif. A critical appraisal of the existing K/Ar data limit the age interval for the felsic rocks between 64 and 83 Ma, with a median value of 77 Ma. Earlier Rb/Sr data for various nepheline syenites result in isochron ages between 89 ± 8 and 83 ± 21 Ma (whole rock), while more recent determinations show 79 ± 7 Ma (internal isochron), with initial ratio of 0.70511 ± 0.00001. Nepheline syenites strongly affected by hydrothermal alteration were dated at 76 ± 2 Ma (Rb/Sr isochron), i.r. 0.7053 ± 0.0002. A phlogopite lamprophyre in the uranium open pit Osamu Utsumi mine yielded phlogopite Ar-Ar ages of 76 ± 1-2 Ma, close to a microprobe Th-U-total Pb age of thorite, 79 ± 3 Ma, found in carbonatite veins associated with lamprophyric-pyroxenitic dikes emplaced within nearby basement gneisses. These geochronologic data, together with geological-structural information and published magnetization directions, indicate that the felsic rocks were emplaced during a short time interval of perhaps 1-2 Ma, during the reverse 33r Campanian magnetization event (dated between 83 and 79.1 Ma). The vocaniclastic basic-ultrabasic deposits were emplaced, at least in part, during the following 33n magnetization event. All basic-ultrabasic occurrences in the district (the Vale do Quartel rocks, phlogopite lamprophyre dike in the open pit, pyroxenitic-carbonatitic dikes) seem to be related, and are in part somewhat younger than the felsic rocks or penecontemporaneous to them, partly accompanying the hydrothermal and mineralization event in the massif.