Penecontemporaneous syenitic-phonolitic and basic-ultrabasic-carbonatitic rocks at the Poços de Caldas alkaline massif, SE Brazil: geologic and geochronologic evidence

Abstract

The large Poços 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, atleast 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.