Neoarchean and Rhyacian TTG-Sanukitoid suites in the southern S?o Francisco Paleocontinent,Brazil:Evidence for diachronous change towards modern tectonics

The southern portion of the Sao Francisco Palaeocontinent in Brazil is denoted by Archean nuclei and Paleoproterozoic magmatic arcs that were amalgamated during Siderian to Orosirian orogenic processes(ca.2.4-2.1 Ga).New isotopic U-Pb in zircon and Sm-Nd whole rock combined with major and trace element composition analyses constrain the crystallization history of the Neoarchean Piedade block(at ca.2.6 Ga)and the Paleoproterozoic Mantiqueira Complex(ca.2.1-1.9 Ga).These therefore display quite different magmatic histories prior to their amalgamation at ca.2.05 Ga.Sm-Nd and Rb-Sr isotopes imply a mixed mantle-crustal origin for the samples in both units.A complete Palaeoproterozoic orogenic cycle,from subduction to collision and collapse,is recorded in the Piedade Block and the Mantiqueira Complex.Rhyacian to Orosirian subduction processes(ca.2.2-2.1 Ga)led to the generation of coeval(ca.2.16 Ga)TTG suites and sanukitoids,followed by late(2.10-2.02 Ga)high-K granitoids that mark the collisional stage.The collisional accretion of the Mantiqueira Complex against the Piedade Block at 2.08-2.04 Ga is also recorded by granulite facies metamorphism in the latter terrane,along the Ponte Nova suture zone.The collisional stage was closely followed by the emplacement of within-plate tholeiites at ca.2.04 Ga and by alkaline rocks(syenites and enriched basic rocks)at ca.1.98 Ga,marking the transition to an extensional tectonic regime.The discovery of two episodes of TTG and sanukitoid magmatism,one during the Neoarchean in the Piedade Complex and another during the Rhyacian in the Mantiqueira Complex,indicates that the onset of subduction-related melting of metasomatized mantle was not restricted to Neoarchean times,as generally believed,but persisted much later into the Paleoproterozoic.

A perspective on potassic and ultrapotassic rocks:Constraints on the Paleoproterozoic late to post-collisional event in the São Francisco paleocontinent

The late-to post-collisional stage in orogenic systems is characterized by the coeval existence of bimodal potassic to ultrapotassic magmatic activity related to partial melting of an enriched lithospheric mantle together with crustal derived melts.In this paper,we present new whole rock geochemical analyses combined with zircon and titanite U–Pb and zircon Hf isotopic data from potassic to ultrapotassic rocks from six plutons that occur within the Archean Itacambira-Monte Azul block(BIMA),to discuss their petrogenesis and the tectonic implications for the São Francisco paleocontinent.The new U–Pb ages range from ca.2.06 Ga to 1.98 Ga and reveal long-lasting potassic magmatism within the BIMA,which is within the late-to-post-collisional stage of the São Francisco paleocontinent evolution.The ultrapotassic rocks are compatible with a fluid-related metasomatized mantle source enriched by previous subduction events,whereas the potassic rocks are bimodal and have a transitional shoshonitic to A-type affinity.These rocks have a hybrid nature,possible related to the mixing between the mafic potassic/ultrapotassic rocks and high temperature crustal melts of the Archean continental crust.Our results also show an increase of within-plate signature towards the younger potassic magmas.The participation of an important Archean crustal component in the genesis of these rocks is highlighted by the common and occasionally abundant occurrence of Archean inherited zircons.The Hf isotopic record shows that most of the zircon inheritance has dominantly subchondriticεHf(t)values,which fits a crustal reworking derivation from a similar Eo-to Paleoarchean precursor crust.However,the presence of juvenile 2.36 Ga zircon inheritance in an ultrapotassic sample reveal the existence of a hidden reservoir that is somewhat similar to the described for the Mineiro Belt in southern São Francisco paleocontinent.

Paleo-and Mesoarchean TTG-sanukitoid to high-K granite cycles in the southern São Francisco craton,SE Brazil

The generation of the continental crust is widely accepted to have taken place predominantly in the Archean,when TTG magmatism associated with greenstone-belt supracrustal succession development was typically followed by emplacement of high-K granites before crustal stabilization.This study focuses on the Campos Gerais complex(CGC),which is an Archean granite-greenstone belt lithological association in a tectonic window located in the southwesternmost portion of the São Francisco craton(SFC).The CGC is an important segment of Paleo-to Mesoarchean continental crust to be integrated into paleogeographic reconstructions prior to the transition into the Paleoproterozoic.This investigation reports field relationships,28 major and trace element compositions,U–Pb(zircon)geochronological results,and Hf and Sm–Nd isotope data for orthogneiss and amphibolite samples.The results indicate that the CGC records a complex Archean crustal evolution,where voluminous 2.97 Ga TTG tonalites and trondhjemites(ε_(Nd)(t)=-4.7;T_(DM)=3.24 Ga)were followed by 2.89 Ga sanukitoid tonalite production(ε_(Nd)(t)=-1.9;T_(DM)=3.02 Ga),broadly coeval with the development of the Fortaleza de Minas and Pitangui greenstone-belts.These events are interpreted to represent the initial stage of an important subduction-accretion tectonic cycle,which ended with the emplacement of 2.82–2.81 Ga high-K leucogranites and migmatization of the TTG-sanukitoid crust,with hybrid and two-mica,peraluminous compositions(ε_(Nd)(t)=-8.0 to-8.6;T_(DM)=3.57–3.34 Ga).The presence of inherited zircons with207Pb/206Pb ages of 3.08 Ga,3.29 Ga,3.55 Ga and 3.62 Ga indicates that the Mesoarchean tectonic processes involved reworking of Meso-to Eo-archean crust.Renewed TTG magmatism took place at ca.2.77 Ga represented by juvenile tonalite stocks(ε_(Nd)(t)=+1.0 to-1.5;T_(DM)=2.80–2.88 Ga)which intrude the TTG-greenstone belt association.Crustal stabilization was attained by 2.67 Ga,allowing for the emplacement of within-plate tholeiitic amphibolites(ε_(Nd)(t)=-3.1;T_(DM)=2.87 Ga).The CGC shows important tectonic diachronism with respect to other Archean terrains in the southern São Francisco craton,including an independent Meso-to Neoarchean crustal evolution.

Rhyacian-Orosirian tectonic history of the Juiz de Fora Complex:Evidence for an Archean crustal reservoir within an island-arc system

The southern São Francisco Paleocontinent(SFP)comprises Archean nuclei and Paleoproterozoic complexes encompassing magmatic arcs juxtaposed during a Rhyacian to Orosirian orogenic event.The Juiz de Fora Complex(JFC)represents an imbricated thrust system that comprises orthogranulites with a wide compositional range formed in an intra-oceanic setting during the Siderian to the Orosirian and later accreted to the southeastern margin of the SFP.Here we report new petrological,geochemical,whole-rock Nd and Sr data,as well as zircon U–Pb ages from felsic and mafic orthogranulites from the JFC.The new data is combined with a regional compilation that enables an evaluation of the interaction between magmatism and orogenetic episodes in the context of the consolidation of São Francisco Paleocontinent during the Rhyacian–Orosirian.Pre collisional Island Arc tholeiites(IAT),Tonalites-Tron dhjemites-Granodiorites(TTGs)and sanukitoid magmatism occurred from 2200 Ma to 2085 Ma.This was followed by post-collisional magmatism,which is represented by hybrid granitoids coeval with the emplacement of E-MORB basic rocks.Crustal signatures for the Rhyacian to Orosirian evolution are highlighted by the dominance of negativeεNd(t)associated with Meso-to Neoarchean Nd TDMmodel ages as well as inherited zircon grains from the hybrid granitoids.The JFC is extensively highlighted in the literature as a primitive intra-oceanic arc,but here we propose the reworking or recycling of ancient crustal segments within the mature arc stage of the JFC,suggesting a Mesoarchean crustal source involved in the JFC evolution.