Petrology and crustal evolution of the Tartarugal Grande Granulitic Complex - Northeastern Amazonian Craton

Abstract

The Tartarugal Grande Complex (TGC) is an association of high-grade metamorphic rocks with Archean and Paleoproterozoic protoliths, located at the border of the Archean Amapá and Rhyacian Lourenço domains, in southeastern Guiana Shield, northeast of the Amazonian Craton. It consists of felsic granulites, and aluminous leucogneisses and rare mafic granulites. Both granulites and gneisses are affected by thrust and transcurrent shear zones along NW-SE trend. Migmatization with quartz-feldspathic neosomes, with or without orthopyroxene, and garnet-rich neosomes are present in felsic granulites and leucogneisses, respectively. The gneisses have high silica contents and are peraluminous due to the presence of biotite, garnet, and cordierite, indicating the derivation from metasedimentary sequences. The felsic granulites have medium to high silica content and geochemical signature of calc-alkaline magmatic arc series. The mafic granulites are small bodies embedded in the felsic granulites and leucogneisses. They display geochemical affinities with tholeiitic series and represent ancient diabase dykes. The metamorphic paragenesis in the granulite facies is represented by Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx + Cpx + Bt ± Hbl (enderbitic granulite), Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx ± Bt (charnoenderbitic granulite); Mc-mesoperthite + Qtz + Pl + Opx + Bt (charnockitic granulite), Pl (An60)+Opx + Cpx ± Hbl ± Grt (mafic granulite), and Qtz + Mc + Pl ± Bt ± Grt ± Crd (leucogneiss). The presence of orthopyroxene, clinopyroxene and hornblende in equilibrium in the granulites allows estimating temperatures at 800° ± 20 °C. In addition, non-extensive anatexis under anhydrous conditions produced charnockitic neosomes in the granulites and garnet-bearing leucogranite neossomes in the paragneisses. The presence of cordierite in leucogneisses indicates low-medium pressure conditions around 6–7 kbar. The partial or total replacement of pyroxenes by hornblende and/or biotite, garnet by biotite, clinopyroxene by titanite, and myrmekite intergrowths, indicate retrometamorphism and cooling during exhumation to higher crustal levels. The U–Pb LA-ICP-MS dating on zircon from a charnoenderbitic granulite yielded an age of 2082 ± 5 Ma for the crystallization of the igneous protolith of the granulites. Metamorphic zircons from an enderbitic granulite yielded a U–Pb LA-ICP-MS age of 2045 ± 14 Ma, which corresponds to the high-grade metamorphic episode that has affected the region at the end of the Rhyacian. The age of the protoliths is not constrained but 207Pb/206Pb dates at 2.57–2.70 Ga indicated inherited crystals and/or xenocrystals from Archean source. In agreement with previous works, we conclude that the TGC consists of calcium-alkaline magmatic rocks and pelitic sedimentary sequences formed during the development of a Rhyacian magmatic arc at the edge of the Archean Amapá Block. This was followed by a thermo-tectonic event at the end of Rhyacian, around 2.05 Ga, under high-grade conditions reaching the granulite facies, with restricted anatexis. Records of the tectonic exhumation of these terranes were defined by partial disequilibrium of some mineral phases in amphibolite facies. This Late-Rhyacian thermo-tectonic event is contemporaneous to the ultra-high temperature metamorphism (2.09–2.03 Ga) of the Bakhuis Granulite Belt in Suriname. The thermal differences between these two high-grade metamorphic terrenes may be related to the different crustal levels where they settled. Late-Rhyacian asthenospheric upwelling by crustal stretching during oblique continental collision seems to be the more plausible scenario as previously proposed, even if a slab break-off scenario cannot be ruled out.