U-Pb provenance fingerprints of metavolcanic-sedimentary successions of the Mineiro belt:Proxies for the continuity of plate tectonics through the Paleoproterozoic

We document new U-Pb detrital zircon LA-MC-ICP-MS data for seven metavolcanic-sedimentary successions and metasedimentary sequences and reassess additional dates of five siliciclastic samples toward their tectonic significance in the context of the Mineiro belt,Southern São Francisco Craton.This belt represents a crustal segment of the 2.47–2.00 Ga Minas Orogen,classically known by its Siderian and Rhyacian juvenile rocks with important implications in the Earth’s geodynamics.The new and compiled detrital provenance constraints unravel the long-lived magmatic and sedimentary history of the studied basins,lasting ca.230–220 Myr.The maximum depositional dates around 2.1 Ga reflect the renewed sediment budget with the subsequent metamorphic episode ca.2.0 Ga.Most of the unmixed relative probability diagrams are consistent with sourcing from the Siderian and Rhyacian arcs of the Mineiro belt,determining a detrital provenance change in time and space for the precursor basins.Alternative potential sources could be the youngest rocks of the Mantiqueira and Juiz de Fora terranes that constitute the other segments of the Minas Orogen,given the age match.The overall detrital fingerprints determine the study basins resumed mainly in Rhyacian fore-arc and/or back-arc settings,i.e.,akin to a subductionrelated system that evolved to a collisional(foreland)environment.Few samples show fingerprints of primary extensional settings,determined by major Archean detrital populations sourced from areas outside the Mineiro belt beside the Paleoproterozoic detritus.The working model considers the collage between the Mineiro belt and the ancient foreland around 2.10 Ga and eventual interaction with other crustal segments of the Minas Orogen,generating the ca.2.0 Ga metamorphism over the metasedimentary samples.The more complete isotopic repository in detrital and igneous zircon grains for the studied supracrustal successions and the associated rocks allows new insights into the Rhyacian–Orosirian dynamics of the Minas orogeny.In a broader perspective,the juvenile nature of the Mineiro belt reinforces the paradigm of uninterrupted continental growth during the Paleoproterozoic Earth.

Recycling process and proto-kimberlite melt metasomatism in the lithosphere-asthenosphere boundary beneath the Amazonian Craton recorded by garnet xenocrysts and mantle xenoliths from the Carolina kimberlite

Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite,Pimenta Bueno Kimberlitic Field,which is located on the southwestern border of the Amazonian Craton.We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite.Mantle xenoliths are mainly clinopyroxenites and garnetites.Some of the clinopyroxenites were classified as GPP–PP–PKP(garnet-phlogopite peridotite,phlogopite-peridotite,phlogopite-K-richterite peridotite)suites,and two clinopyroxenites(eclogites)and two garnetites are relicts of an ancient subducted slab.Temperature and pressure estimates yield 855–1102℃ and 3.6–7.0 GPa,respectively.Clinopyroxenes are enriched in light rare earth elements(LREE)(La_(N)/Yb_(N)=5–62;Ce_(N)/Sm_(N)=1–3;where N=primitive mantle normalized values),they have high Ca/Al ratios(10–410),low to medium Ti/Eu ratios(742–2840),and low Zr/Hf ratios(13–26),which suggest they were formed by metasomatic reactions with CO_(2)-rich silicate melts.Phlogopite with high TiO_(2)(>2.0 wt.%),Al_(2)O_(3)(>12.0 wt.%),and FeOt(5.0–13.0 wt.%)resemble those found in the groundmass of kimberlites,lamproites and lamprophyres.Conversely,phlogopite with low TiO_(2)(<1.0 wt.%)and lower Al_(2)O_(3)(<12.0 wt.%)are similar to those present in GPP-PP-PKP,and in MARID(mica-amphibole-rutile-ilmenite-diopside)and PIC(phlogopite-ilmenite-clinopyorxene)xenoliths.The GPP-PP-PKP suite of xenoliths,together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton.The Sr-Nd isotopic ratios of pyrope xenocrysts(G3,G9 and G11)from the Carolina kimberlite are characterized by high ^(143)Nd/^(144)Nd(0.51287–0.51371)and eNd(+4.55 to+20.85)accompanied with enriched ^(87)Sr/^(86)Sr(0.70405–0.71098).These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites.Based on Sr-Nd whole-rock compositions,the Carolina kimberlite has affinity with Group 1 kimberlites.The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9±5.4 Ma(2σ),which represents the cooling age after the proto-kimberlite melt metasomatism.Therefore,we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle(garnetites and eclogites);with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.

Experimental evidence of pressure effects on spinel dissolution and peridotite serpentinization kinetics under shallow hydrothermal conditions

Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained.Here we present two constant temperature(230℃)time-series of natural peridotite(77.5%olivine;13.7%enstatite;6.8%diopside;2%spinel)serpentinization experiments:at 13.4 MPa;and 20.7 MPa.Al-enriched lizardite was the main secondary mineral in all runs after olivine(olv)and orthopyroxene(opx)serpentinization(without any detectable brucite,talc or magnetite),while primary spinel and diopside partially dissolved during the experiments.Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite,as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine,while the opposite is true for Mg and Fe,with homogenization of serpentines compositions after 40 days.The Ni/Cr ratios of serpentines,however,remain diagnostic of the respective primary mineral.Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day^(-1)(0.26 mmol·day^(-1))and 0.26 wt.%·day^(-1)(0.13 mmol·day^(-1))at 13.4 and 20.7 MPa,respectively.Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al,which accelerates olivine serpentinization while delays orthopyroxene dissolution.Additionally,time-dependent increase in solid products masses suggests rock volume expands linearly 0.37%±0.01%per serpentine wt.%independently of pressure.Mass balance constrains suggests olv:opx react at~5:2 and~3:2 M ratios,resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series(13.4 MPa)and high-pressure series(20.7 MPa),respectively.Elevated starting peridotite olv:opx ratio(7.94:1)therefore indicates orthopyroxene serpentinization is~3.3x and~5.4x faster than olivine at 13.4 MPa and 20.7 MPa,respectively.This contradicts previous assumptions that olivine should dissolve faster than orthopyroxene at experimental conditions.Finally,serpentinization-derived fluids develop pH>10 and become enriched in H_(2),CH_(4),Ca^(2+)and Si within 6 weeks.Aqueous silica concentrations are highest after 5 days(265.75 and 194.79µmol/kg)and progressively decrease,reaching 13.84 and 91.54µmol/kg at 13.4 and 20.7 MPa after 40 days,respectively.These concentrations are very similar to the low-silica(M6)and high-silica(Beehive)endmembers of the Lost City Hydrothermal Field(LCHF).Beyond fluid characteristics,serpentinization products and conditions analogous to the LCHF suggest similar mechanisms between our experiments and natural processes.Our results demonstrate constant temperature serpentinization of a common protolith leads to distinct serpentine and fluid compositions at different pressures.Although additional data is necessary,recent studies and our experiments suggest peridotite serpentinization rates at 230℃rapidly decrease with increasing pressures at least up to 35 MPa.Whether pressure directly influences olivine and orthopyroxene serpentinization kinetics or indirectly controls reaction rates due to spinel dissolution under hydrothermal conditions deserves further investigation.

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.

Linking sedimentary provenance,climate and tectonics in the Neoproterozoic SeridóBelt,Borborema Province(NE Brazil)

The SeridóBelt,at the extreme northeast of Brazil,is one of the several Neoproterozoic mobile belts related to the Brasiliano/Pan-African Orogeny in Western Gondwana.The lithostratigraphy of the SeridóBelt comprises basal quartzites and metaconglomerates(Equador Formation)overlain by hornblende-bearing paragneisses,calc-silicate rocks and marbles(Jucurutu Formation)and biotiteschists(SeridóFormation).We investigated the sedimentary provenance of these rocks based on an integration of compiled whole-rock geochemistry,Sm-Nd isotopes,U-Pb detrital zircon ages and a new set of whole-rock trace elements analyses.Equador Formation quartzites have major and trace elements patterns that resemble cratonic sandstones,with a typical felsic provenance consistent with the isotopic composition of plutonic Archean to Paleoproterozoic basement rocks.Geochemical proxies suggest that these rocks were deposited in fresh waters under anoxic conditions and moderate to intense weathering conditions(CIA=58–79).A shift in provenance led to an increase in intermediate-mafic sources during the deposition of the Jucurutu and Seridóformations.Trace elements from rock types in these units are nearly identical,yet they differ on the contents of some high field-strength elements(e.g.,Sc,V and Ni),which are higher in the SeridóFormation than average pelites.Geochemical proxies show similar depositional environments,with brackish to marine conditions for the Jucurutu and Seridóformations under anoxic to euxinic conditions.Weathering conditions are consistently lower in the Jucurutu Formation(CIA=58–66)in comparison with the SeridóFormation(CIA=60–88).Compiled U-Pb data suggest maximum depositional ages of 633±11 Ma and 633±6 Ma for Jucurutu and Seridóformations,respectively,suggesting deposition following the ca.645–635 Ma Marinoan glaciation.We compared the geochemistry of the SeridóFormation schists with currently exposed basement rocks and showed a strong affinity with Neoproterozoic arc-related intermediate rocks,which is in agreement with their slightly radiogenic signature(averageε_(Nd)(630 Ma)=–1.68).When comparing with other pelitic sequences on the western Borborema Province,most of them fit the same sources and weathering conditions of the SeridóFormation schists and Jucurutu Formation paragneisses.We propose that the SeridóBelt represents an intracontinental rift basin(Equador Formation)that was overlain by fine-grained magmatic arc-related sediments(Seridóand Jucurutu formations)during the post-Marinoan deglaciation and sea level rise.Similar pelitic deposits in western Borborema Province suggest a large marine-influenced continental depositional system that was mostly sourced from magmatic arcs of the West Gondwana Orogen.

The generation and evolution of the Archean continental crust:The granitoid story in southeastern Brazil

The Archean Eon was a time of geodynamic changes.Direct evidence of these transitions come from igneous/metaigneous rocks,which dominate cratonic segments worldwide.New data for granitoids from an Archean basement inlier related to the Southern S?o Francisco Craton(SSFC),are integrated with geochronological,isotopic and geochemical data on Archean granitoids from the SSFC.The rocks are divided into three main geochemical groups with different ages:(1)TTG(3.02–2.77 Ga);(2)mediumto high-K granitoids(2.85–2.72 Ga);and(3)A-type granites(2.7–2.6 Ga).The juvenile to chondritic(Hf-Nd isotopes)TTG were divided into two sub-groups,TTG 1(low-HREE)and 2(high-HREE),derived from partial melting of metamafic rocks similar to those from adjacent greenstone belts.The compositional diversity within the TTG is attributed to different pressures during partial melting,supported by a positive correlation of Dy/Yb and Sr/Zr,and batch melting calculations.The proposed TTG sources are geochemically similar to basaltic rocks from modern island-arcs,indicating the presence of subduction processes concomitant with TTG emplacement.From~2.85 Ga to 2.70 Ga,the dominant rocks were K-rich granitoids.These are modeled as crustal melts of TTG,during regional metamorphism indicative of crustal thickening.Their compositional diversity is linked to:(i)differences in source composition;(ii)distinct melt fractions during partial melting;and(iii)different residual mineralogies reflecting varying P–T conditions.Post-collisional(~2.7–2.6 Ga)A-type granites reflect rifting in that they were closely followed by extension-related dyke swarms,and they are interpreted as differentiation or partial melting products of magmas derived from subduction-modified mantle.The sequence of granitoid emplacement indicates subduction-related magmatism was followed by crustal thickening,regional metamorphism and crustal melting,and post-collisional extension,similar to that seen in younger Wilson Cycles.It is compelling evidence that plate tectonics was active in this segment of Brazil from~3 Ga.

Shoshonitic Magmatic Series and the High Ba-Sr Granitoids:A Review with Emphasis on Examples from the Neoproterozoic Dom Feliciano Belt of Southern Brazil and Uruguay

Fractional crystallization of parental magmas of shoshonitic or silica-saturated,ultrapotassic affinity,with variable amount of concurrent crustal assimilation,may result in granitic and syenitic rocks.Typical plutonic members of the shoshonitic series are monzonites and quartz monzonites,whilst syenites and quartz syenites are the dominant plutonic products of the ultrapotassic series.Lamprophyric magmas are commonly found in association with both series and are frequently part of coeval mingling/mixing systems.Ultrapotassic and shoshonitic primary magmas,including lamprophyric ones,are derived from amphibole-phlogopite-bearing mantle sources produced by previous,subduction-related metasomatism.Acidic and intermediate rocks can be derived from such parental magmas,generally through AFC processes.Shoshonitic-like granitoids,which have not clear relation with intermediate or basic shoshonitic rocks,or are produced dominantly by crustal melting,should be named high-Ba-Sr granitoids.This study focuses mainly on Neoproterozoic shoshonitic and silica-saturated ultrapotassic rock associations formed in post-collisional settings from southern Brazil and Uruguay.The source of magmas,their evolution,the role played by crustal contamination in modifying pristine geochemical signatures and their tectonic control are discussed based on elemental and Sr-Nd isotope geochemistry.The main features of plutonic rocks related to the shoshonitic series are their potassic,silica-saturated alkaline character,predominance of monzonitic to syenitic compositions,high Sr and Ba contents,monotonous,light REE-enriched patterns,and moderate HFSE contents.Syntectonic shoshonitic and high Ba-Sr granitoids within shear zones show lower alkali,LREE,HFSE,and Sr contents than those formed away from the main deformation sites.Plutonic rocks related to the extended silica-saturated ultrapotassic series are mostly syenites,alkalifeldspar granites and lamprophyres with K_(2)O/Na_(2)O ratios above 2.The typical values of^(87)Sr/^(86)Sri for shoshonitic plutonic rocks are 0.706–0.708,ranging from 0.704 to 0.710.Theε_(Nd)(t)values are negative and vary from 0 to-24.Crustal contribution tends to increase^(87)Sr/^(86)Sri and decreaseε_(Nd)(t)values,depending on protolith isotope signature,melting conditions and volume of assimilated material.Ultrapotassic rocks,on the other hand,show higher^(87)Sr/^(86)Sri ratios,from 0.709–0.711 up to 0.720.Geochemical evidence,including Sr-Nd isotope data,indicates that the shoshonitic and ultrapotassic rocks discussed in this study were formed from OIB-like sources with strong influence of previous subduction,probably a phlogopite,K-amphibole bearing veined mantle.Lithological variability in ultrapotassic-shoshonitic associations is interpreted to result from(i)variation of source composition,(ii)different melt fractions from similar sources,(iii)mixing-mingling,fractional crystallization,and assimilation processes.

Preface

Post-collisional settings have long been discussed, both in the aspect of nomenclature, tectonic significance, sources of magmatism, and their relations to pre-collisional and intraplate settings. To many authors, it should be synonymous with lateorogenic or late-collisional settings because the setting itself is defined as the period following the main impact of the colliding landmasses. On the other hand, terms like post-tectonic should be avoided when referring to post-collisional associations.