BEBEDOURITE & UGANDITE
Here are two scarce intrusive igneous rocks from
southeastern Brazil’s Alto Paranaíba Igneous Province.
The two-dozen or so igneous intrusions in this complex are located between
Brazil’s São Francisco Craton and the Paraná 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
variety of peridotite)
overwhelmingly dominated by diopside clinopyroxene (CaMgSi2O6),
plus a significant alkaline content. So, another good rock name would be alkaline
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 Araxá, western Minas Gerais State, southeastern
Brazil.
Bebedourite (alkaline 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
Indaiá II Intrusion. This rock is a finely-crystalline alkaline
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 Indaiá 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.