CHROMITITES ASSOCIATED WITH LAYERED COMPLEXES IN BRAZIL: TESTING "ONE-FIT-ALL" GENETIC MODELS
CESAR F. FERREIRA-FILHO
Instituto de Geociências, UnB, 70910-970 Brasilia, DF, Brazil.
Presented by ALCIDES N. SIAL
Chromitites represent a special case of cumulate rock where chromite is the only cumulus phase. Formation of chromitites thus requires that phase relations of the appropriate system are somehow changed to allow the system to fall into the chromite stability field. Several "one-fit-all" models were proposed to explain the origin of chromitite layers.
Mineral chemistry data were collected for three layered intrusions in Brazil; Bacuri Complex (Amapá), Ipueira-Medrado Sill (Bahia) and Niquelândia Complex (Goiás). They have distinct igneous stratigraphy, thus providing opportunity to look at chromitites formed in different environments.
At the Bacuri Complex, most of the chromite is concentrated in a single few meters-thick chromitite layer located at the base of the Ultramafic Zone (UZ) in direct contact with the underlying Lower Mafic Zone. Cryptic variation data is consistent with extensive fractionation within the UZ. The stratigraphic position of the main chromitite strongly supports a model for its origin associated with a major new influx of primitive magma, and mixing with more fractionated resident magma.
At the Niquelândia Complex, chromitites consist of several few centimeters-thick layers restricted to a 20 meters-thick horizon within an estimated 3 km-thick Ultramafic Zone. Detailed cryptic variation data indicate that the 20 meters-thick interval marks a slight reversal of the fractionation path. The data support a model for its origin associated with new influx of primitive magma, and mixing with slightly more fractionated resident magma.
At the Ipueira-Medrado Sill, a 5-8 meters-thick massive chromitite layer (MCL) is hosted by a 200-300 meters-thick layered intrusion. The MCL is located at the transition from dynamic open system to mainly closed system magma chamber. The most primitive compositions are observed at the MCL. Cryptic variations are the opposite to what is expected as the result of a new influx of primitive magma. Re-Os and Sm-Nd data indicate strong crustal contamination suggesting that chromite crystallization was triggered by changes of physical conditions associated to crustal contamination.
The data indicate that the chromitite layers are always associated with major changes in the magmatic chamber. However, a single genetic model does not apply to the three examples investigated. (May 24, 2002).
* E-mail: cesarf@unb.br
Publication Dates
-
Publication in this collection
09 Oct 2002 -
Date of issue
Sept 2002
