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Magma Transfer Zone, Pedreiras Silveira and Pelotense, Rio Grande do Sul, Brazil

 

dyke

Roberto Weinberg, Fatima Bitencourt, Edinei Koester and students


Monash University and Universidade Federal do Rio Grande do Sul

 

 

 

This page describes two quarries close to each other, close to the town of Pelotas, and part of the Neoproterozoic Pelotas Batholith, Dom Feliciano Belt in Rio Grande do Sul. They are comprised of a heterogeneous, banded and layered granite, formed by multi-pulse granitic magmas and with numerous blocks of earlier magmatic pulses and cut by later pegmatitic dykes. Earlier pulses, preserved as blocks inside later magmas, are: a) dioritic migmatites with peritectic Hbl, b) banded granitoids with a weak gneissosity and c) a coarse porphyritic granite. These blocks are enclosed in medium- and fine-grained, layered grey granite with a number of curious spotted textures. Spots are white and surround grains of magnetite in some localities or titanite in other localities. This complex sequence suggests that these quarries marked the passageways of anatectic melts formed by the melting of pre-existing granitoids/diorites at depth. This quarry also has a magnetite granite which has complex relations with pegmatites suggesting contemporaneity between the two.

 

 

Magnetite granite with complex relations with pegmatites
magnetite granite mingling
Pegmatite with individual grains interlocking with mafic magnetite granite. Pegmatite is connected directly and grades into dykelets of felsic granite in the magnetite granite with irregular curved and diffused margins. 		magnetite granite mingling
Pegmatite is connected directly and grades into dykelets of felsic granite in the magnetite granite with irregular curved and diffused margins.
magnetite granite mingling
Pegmatite is connected directly and grades into dykelets of felsic granite in the magnetite granite with irregular curved and diffused margins. 		magnetite granite mingling
Pegmatite with individual grains interlocking with mafic magnetite granite. Pegmatite is connected directly and grades into dykelets of felsic granite in the magnetite granite with irregular curved and diffused margins.
magnetite granite mingling
Coarse granite irregular margins at the grain scale, grains interlocking with mafic magnetite granite. Two bands of pegmatite are separated by a mingled layer of magntite mafic granite and leucogranite. 		magnetite granite mingling
Pegmatite dyke with irregular curved margins contrasting with a felsic granite dyke with straight angular intrusive relationships with same mafic granitoid.

 

banded granite
Banded granite (black arrows) intruded by late pegmatitic granite. 		banded granite
Detail.
banded granite
Detail of magmatic banding. Felsic magma with rounded, melanocratic margins. 		banded granite
Pegmatite dyke in a shear zone dragging banded granite.
magmatic thrust plane
Magmatic thrust plane developed above a mafic enclave. Schlieren above the thrust have asymmetries indicative of top-to-left. On the lower left (under the hammer), magmatic banding is conformable with the margin of the enclave and truncated by the shear plane. Syn-magmatic flow and thrusting.
porphyritic granite xenolith
Angular blocks of porphyritic granite inside grey granite. 		Igneous xenoliths
Small igneous xenoliths inside grey granite.
Igneous xenoliths
Eroded blocks of igneous xenoliths inside grey granite (black arrows, two of the arrows mark the margins of a large more mafic xenolith). These xenoliths are interpreted to represent the recycling of older granitoids, possibly remelting of an igneous source. 		gneissic xenolith
Xenoliths of banded igneous rocks with gneissosity (above lens cap).
Migmatites Pelotense
Migmatites after an intermediate protolith with peritectic Hbl. 		Migmatites Pelotense
Same.
Migmatites Pelotense
Detail showing peritectic Hbl. The leucosome veinlet that runs below the coin is rooted on a migmatite patch rich in peritectic Hbl. Interpretation: melt extraction.