BEBEDOURITE & UGANDITE
Here are two scarce intrusive igneous rocks from
southeastern Brazil's Alto Parana’ba Igneous Province. The
two-dozen or so igneous intrusions in this complex are located between Brazil's
S‹o Francisco Craton and the Paran‡ Basin. These igneous bodies have
unusually alkaline-rich chemistries (resulting in the presence of rare rocks)
and date to between 80 and 90 million years (Late Cretaceous). They
appear to have been generated by hotspot (mantle plume) activity underneath the
South American continental lithosphere (see Gibson et al., 1995). The Trinidade-Martin
Vaz Hotspot has been identified as the culprit (now located in the western
parts of the South Atlantic Ocean).
First is bebedourite, a scarce variety of
ultramafic intrusive igneous rock. It's a pyroxenite (a >90% pyroxene)
overwhelmingly dominated by diopside clinopyroxene (CaMgSi2O6),
plus a significant alkali content. So, another good rock name would be alkali
clinopyroxenite. Other minerals reported from this bebedourite are
phlogopite mica, magnetite, perovskite, apatite, calcite, and ilmenite.
The sample comes from the type locality of bebedourite
- a ring of pyroxenites surrounding the carbonatite core of the Serra
Negra-Salitre Complex (a.k.a. Serra Negra-Salitre Carbonatite
Complex; Serra Negra-Salitre Alkaline Igneous Complex; Serra Negra-Salitre
Massif) of southeastern Brazil. Published isotopic dates show that the
Serra Negra-Salitre Complex is 83 to 84 million years old (mid-Late
Cretaceous).
Locality:
north-northeast of Arax‡, western Minas Gerais State, southeastern Brazil.
Bebedourite (alkali clinopyroxenite) (6.3 cm across) from the Late Cretaceous-aged
Serra Negra-Salitre Complex of southeastern Brazil.
The second rock is a ugandite from the Indai‡
II Intrusion. This rock is a finely-crystalline alkalic hypabyssal
intrusive having relatively small olivine phenocrysts set in a groundmass of
diopside clinopyroxene, phlogopite mica, spinel, perovskite, nepheline, and
glass.
Locality:
satellite intrusion immediately east-northeast of the Indai‡ I Intrusion, near
Rio Perdizos, 20 km north of Monte Carmelo, east of Sucuri, western Minas
Gerais, State, southeastern Brazil.
Ugandite
(field of view ~2.1 cm across)
Some info. provided by:
Tony Peterson (pers. comm.)
Gibson et al. (1995) - The Late Cretaceous impact of
the Trinidade mantle plume: evidence from large-volume, mafic, potassic
magmatism in SE Brazil. Journal of Petrology 36: 189-229.