Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen, southeastern Brazil : evidence from the polymetamorphic Carlos Chagas batholith.
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2017
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Abstract
From the earliest (ca. 630 Ma) pre-collisional plutons to the latest (ca. 480 Ma) post-collisional intrusions, the
Araçuaí orogen (SE Brazil) records an outstanding succession of granite production events in space and time.
The Carlos Chagas batholith (CCB) is the largest (~14,000 km2) granitic body ascribed to the collisional plutonism
(G2 supersuite) in the back-arc region of the Araçuaí orogen, to the east of the Rio Doce magmatic arc. A wide
range of monazite and zircon ages (N725Ma to ca. 490Ma) have been found in CCB granites, recording a rich history
of crustal recycling and inheritance, magmatic crystallization and anatexis. The CCB includes a dominant
granite richer in garnet than in biotite, in which three mineral assemblages can be identified: 1) Qz + Pl +
Kfs + Bt + Grt + Ilm ± Rt; 2) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil; and 3) Qz + Pl + Kfs + Bt + Grt +
Ilm+Sil+Spl. Rocks which contain mineral assemblage 2 and 3 all contain two generations of garnet. Textural
evidence for the presence of former melt, recognized in all studied CCB samples, includes: silicate melt inclusions
in poikiloblastic garnet, pseudomorphed thin films of melt surrounding both generations of garnet,
pseudomorphedmelt pools adjacent to garnet and biotite, and plagioclase and quartzwith cuspate-lobate shapes
occurring among matrix grains. Both generations of garnet crystals (Grt1 and Grt2) are unzoned in terms of major
element concentration, contain small rounded inclusions of Ti-rich biotite and, in addition, the Grt2 crystals also
contain inclusions of remnant sillimanite needles. Microstructural evidence, in combination with mineral chemistry,
indicates that the garnet crystals grew during two distinct metamorphic-anatectic events, as the peritectic
products of fluid-absent melting reactions which consumed biotite, quartz and plagioclase, in the case of Grt1,
and which consumed biotite, quartz, plagioclase and sillimanite in the case of Grt2. P–T pseudosections calculated
via Theriak-Domino, in combination with in situ U–Pb monazite and zircon dating, provide new constraints on
the thermal evolution of the back-arc region of the Araçuaí orogen. Data from assemblage 1 suggests P–T conditions
for the first granulite-facies metamorphic event (M1) at 790–820 °C and 9.5–10.5 kbar, while the assemblage
2 records P–T conditions for a second granulite-facies metamorphism (M2) of around 770 °C and
6.6 kbar. Monazite and zircon within garnets from the different assemblages give age peaks at 570–550 Ma
(M1) and 535–515 Ma (M2), recording two anatectic events in the CCB during a single orogenic cycle. The PT
conditions for these metamorphic events can be related to: i) M1, striking crustal thickening, probably involving
thrusting of the magmatic arc onto the back-arc region; and ii) M2, decompression related to the gravitational
collapse of the Araçuaí orogen.
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Keywords
Multiple anatectic event, Garnet, Granulite facies metamorphism
Citation
MELO, M. G. de et al. Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen, southeastern Brazil: evidence from the polymetamorphic Carlos Chagas batholith. LITHOS, v. 277, p. 51-71, 2017. Disponível em: <http://www.sciencedirect.com/science/article/pii/S0024493716303413>. Acesso em: 25 ago. 2017.