Garnet texture and chemical zoning: a clue for the history of the Mayombe (Congo-Brazzaville)
Résumé
Structural and geochronological data in the Mayombe belt
show evidence of two orogenic events: Eburnean (c. 2000
Ma) and Pan-African (c. 600 Ma). However, the distribution,
amount and P-T conditions of Pan-African reworking of the
older Eburnean basement is a matter of debate. To solve this
question, a detailed petrological study of the metamorphic
history of the Mayombe belt has been undertaken.
Garnet is a key mineral for understanding the tectonometamorphic
history of the Mayombe since it preserves a
record of P-T conditions during its growth, expressed by
chemical zoning, and it may also display some inclusion
trails which can give kinematic indications. Field, textural
and chemical analyses of garnets from the metamorphic rocks
of the Mayombe belt reveal the existence of (i) magmatic and
metamorphic garnets, (ii) normal and reverse growth zoning,
(iii) narrow diffusion zoning along cracks and rims, and (iv)
multistage garnets recording metamorphic overgrowths
around magmatic cores.
Growth zoning of the metamorphic garnets shows evidence
for one major tectono-thermal cycle, whose age is under
investigation using isotopic techniques. Modification of
magmatic garnet by diffusion, and the development of
chlorite around garnet crystals, along cracks and in the
associated matrix may have been the main effect of the Pan-
African overprint on the Eburnean basement.