High-pressure metamorphic rocks in the Borborema Province, Northeast Brazil: Reworking of Archean oceanic crust during proterozoic orogenies

Arc accretion and crustal reworking from late Archean to Neoproterozoic in Northeast Brazil

“…Nevertheless, crustal reworking was intense in the eastern block area, practically obliterating the Archean protolith record. A second high-grade metamorphic event -the seventh recorded event -is indicated by 614-593 Ma old zircon grains and rims around the Archean zircon cores from the amphibolite samples 39 . Moreover, 604 Ma old K-feldspar-rich granitic intrusions and 566 Ma pegmatite veins suggest a more restricted Neoproterozoic partial melting when compared to the large volume of neosome generated during the Rhyacian.…”

“…This confirms that the 2.65 Ga tholeiitic intrusions were subsequently torn apart during 2.0 Ga and 600 Ma tectonothermal events and are now present as isolated boudins. The 2.9 Ga inherited zircon grains were captured or assimilated by 2.65 Ga tholeiitic magma during its ascent and emplacement in the 2.9 Ga host tonalite basement 39 . Furthermore, as there are no fine-grained felsic veins intruding the amphibolite, it is unlikely that the 2.65 Ga zircon grains obtained in the mafic lenses came from other sources 39 .…”

“…The ultramafic pyroxenite lenses contain 2.7-2.6 Ga Archean and 2.3 Ga Paleoproterozoic zircon crystals with distinct morphological features like oscillatory zoning ( Fig. 2C), typical of magmatic zircon 39 . Furthermore, pyroxenite samples present 2.0 Ga and ca.…”

“…These migmatites comprise Archean tonalitic gneisses that contain granitic Proterozoic leucosomes and alkali granite dikes. The mafic-ultramafic rocks comprise amphibolites and pyroxenites that are present as boudinaged bodies within the Archean migmatitic complex, which are further oriented parallel to the leucosomal layers of the host migmatites 39 . The overall outcrop pattern suggests that these mafic-ultramafic rocks were originally emplaced as dykes, intruding the host migmatitic gneisses.…”

“…The ultramafic pyroxenites show relict cumulate texture, and re-equilibration to cummingtonite-grunerite-rich rocks, with varying proportions of chlorite, serpentine and magnetite. Amphibolites comprise massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase + clinopyroxene in symplectitic texture, typical of retrograded high-pressure rocks 39 . The Itajá Block is composed of Paleoproterozoic K-feldspar-rich orthogneiss, and wehrlite intrusions that occur as elongated boudins (<100 m) in the host orthogneiss; minor amphibolite and supracrustal rocks also appear.…”