Geochemical and isotopic evidence for the petrogenesis and emplacement tectonics of the Serra dos Orgaos batholith in the Ribeira belt, Rio de Janeiro, Brazil
Résumé
The Serra dos Orgaos batholith in the State of Rio de Janeiro (Brazil)
is a NE-SW-trending elongated body that occupies ca. 5000 km(2) in plan
view. It is a foliated intrusion, especially at its borders and is
crosscut by syn-magmatic shear zones, with foliations that are
moderately-to steeply-dipping to the northwest and moderately-to
shallow-dipping in the center and to the southeast, in a configuration
of a large laccolith. It was emplaced between 560 and 570 Ma, during an
extensional episode that was part of a series of events that comprise
the Brasiliano Orogeny in SE Brazil, and which includes deformation,
metamorphism and granite intrusion during the interval between 630 and
480 Ma. The two main rock types in the batholith are biotite-hornblende
monzogranite, and biotite leucogranite, with subordinate tonalite,
granodiorite, diorite, quartz diorite (enclaves), aplite and pegmatite.
Harker-type diagrams help show two rock groups with similar trends of
evolution: a dioritic and a granitic. The first one is tholeiitic,
whereas the second is calc-alkaline, with medium-to high-K calc-alkaline
affinity and metaluminous to slightly peraluminous character. In both
groups strong decrease in Al2O3, MgO, FeOT and CaO relative to silica
contents are observed, which is compatible with trends of fractional
crystallization involving clinopyroxene and/or hornblende, plagioclase,
opaque minerals, apatite, microcline and biotite. The Sr and Nd isotopic
data suggest recycling of a Paleoproterozoic crust as an important
petrological process to generate the batholith rocks. Geothermometry
(amphibole composition) and geobarometry (saturation in zircon and
apatite) indicate that most of the batholith solidified at mid to lower
crustal levels at about 750 degrees C and between 5 and 5.5 kbar. We
consider that Serra dos Orgaos crustal protoliths underwent melting
caused by the interaction with hotter mafic magma at the base of the
crust. These two magmas, with distinct initial compositions and
rheology, probably underwent mixing and mingling. This process continued
during the rise of the magma through the crust, which was accompanied by
magmatic differentiation. The main feature that characterizes the
post-collisional Serra dos Orgaos granite magmatism is the connection
with high angle ductile shear zones of continental scale and presence to
a greater or lesser extent of mafic magmas.
Domaines
Géochimie
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