Zircon U-Pb and Lu-Hf isotope constraints on Archean crustal evolution in Southeastern Guyana Shield

The southeastern Guyana Shield,northeast Amazonian Craton,in the north of Brazil,is part of a widespread orogenic belt developed during the Transamazonian orogenic cycle(2.26-1.95 Ga)that includes a large Archean continental landmass strongly reworked during the Transamazonian orogeny,named Amapa Block.It consists mainly of a high-grade metamorphic granulitic-migmatitic-gneiss complex,of Meso-to Neoarchean age and Rhyacian granitoids and supracrustal sequences.For the first time,coupled U-Pb and Lu-Hf isotope data were obtained on zircon by LA-ICP-MS from five tectono-stratigraphic units of the Archean basement and one Paleoproterozoic intrusive rock,in order to investigate the main episodes of crustal growth and reworking.Whole-rock Sm-Nd isotope data were compared to the zircon Lu-Hf data.Three main magmatic episodes were defined by U-Pb zircon dating,two in the Mesoarchean(~3.19 Ga and 2.85 Ga)and one in the Neoarchean(~2.69-2.65 Ga).SubchondriticεHf(t)values obtained for almost all investigated units indicate that crustal reworking processes were predominant during the formation of rocks that today make up the Amapa Block.Hf-TDMC model ages,ranging from2.99 Ga to 3.97 Ga,indicate that at least two important periods of mantle extraction and continental crust formation occurred during the Archean in southeastern Guyana Shield,an older one in the Eoarchean(~4.0 Ga)and a younger one in the Mesoarchean(~3.0-3.1 Ga).The latter is recognized as an important period of crustal accretion worldwide.The recognition of an Eoarchean episode to the southeastern most part of the Guyana Shield is unprecedented and was not recorded by whole-rock Sm-Nd data,which were restricted to the Meso-Paleoarchean(2.83 Ga to 3.51 Ga).This finding reveals t hat continental crust generation in the Amazonian Craton began at least 500 Ma earlier than previously suggested by the SmNd systematics.

Late Cryogenian-Ediacaran magmatism in southern Borborema Province,NE Brazil:Ages,sources,petrogenesis,and tectonic setting

The Sergipano Orogenic System(SOS)in southern Borborema Province(NE Brazil)hosts voluminous Neoproterozoic plutonism related to the Brasiliano/Pan-African Orogeny.This paper presents comprehensive whole-rock geochemical data,titanite U-Pb ages,and the first combined zircon U-Pb and Lu-Hf isotope results for plutonic rocks from the MacururéDomain to constrain their sources,petrogenesis,and tectonic setting.Three magmatic episodes are recognized and record the evolutionary stages of the orogen.(i)Early-collisional magmatism(643–628 Ma)comprises gabbros and diorites with minor tonalites characterized by well-developed tectonic foliation and evidence of solid-state deformation.These rocks are magnesian,high-K calc-alkaline,LILE-and LREE-enriched and provide subchondriticε_(Hf)(t)values(6.5 to4.7)and Orosirian Hf-T^(C)_(DM)model ages(1.83–1.94 Ga).Such features indicate derivation from a lithospheric mantle source metasomatized by incorporating crustal components through subduction processes prior to magma generation,possibly related to the Rhyacian Orogeny(2.20–1.96 Ga).Extensive mixing/mingling between basaltic and crust-derived magmas took place at lower crustal depths,producing coeval hybrid diorites and quartz-diorites.(ii)Syn-collisional magmatism(630–624 Ma)encompasses biotite-and muscovite-bearing granodiorites and monzogranites,preserving their structures parallel to the schistosity of the country rocks.These rocks are leucocratic,weakly metaluminous to peraluminous,and contain abundant surmicaceous enclaves.Petrographic features and geochemical composition suggest an origin related to the partial melting of graywacke protoliths with a subordinate igneous component.(iii)Late-collisional magmatism(625–600 Ma)includes undeformed and isotropic monzonites and granodiorites,which truncate the regional foliation.These rocks are consistently metaluminous and magnesian,showing affinities with the high-K calc-alkaline to shoshonite series.Trace element modeling with subchondriticε_(Hf)(t)values(8.3 to4.1)and Paleoproterozoic Hf-T^(C)_(DM)model ages(1.77–2.03 Ga)demonstrate that reworking of ancient lower mafic crust played an important role at this time.The integration of our data with previously published results leads us to conclude that the geodynamic evolution of the SOS along the western Gondwana margin is better explained by large-scale lithospheric extension followed by basin inversion and continental collision.