Crustal Reworking Processes Research Articles

Generation of the Early Cretaceous granitoid in the Dazeshan region, Jiaodong Peninsula: Implications for the crustal reworking in the North China Craton

Lower crust-derived granitic rocks provide constraints on the crustal reworking process and consequently give hints on the destruction mechanism of the cratons. The North China Craton (NCC) underwent extensive crustal melting in the Mesozoic. This study investigated granitic intrusions in the Dazeshan region of the Jiaodong Peninsula. Whole-rock major and trace element analyses and zircon U-Pb ages coupled with Hf isotopes were used to reveal the crustal reworking processes. Zircons separated from the quartz porphyry, Linglong granite, rhyolite porphyry, and biotite granite showed weighted mean 206Pb-238U ages of 119.2 ± 1.0 Ma, 140.2 ± 1.0 Ma, 120.6 ± 0.5 Ma, and 119.9 ± 0.7 Ma, respectively. The quartz porphyry, rhyolite porphyry, and biotite granite had high silica contents (SiO2 = 74–77 wt.%) but low MgO, Co, and Ni concentrations. The calculated εHf(t) values for the rhyolite porphyry and the biotite granite ranged from −18.3 to −20.0 and −17.8 to −20.2, respectively. These geochemical features imply ancient crust sources. The quartz porphyry showed distinct primitive mantle-normalized rare earth element (REE) patterns and was characterized by lower ΣREE content and lack of pronounced negative Eu anomalies. Whole-rock and zircon Dy/Yb ratios showed no correlation with whole-rock Rb/Sr ratios and zircon Hf contents, reflecting limited effects of crystal fractionation. The Ba/La ratios were also high (>150), but the Sr/Y and La/Yb ratios were low (Sr/Y < 50; La/Yb < 15). These features likely indicate that the quartz porphyry was generated by water-fluxed melting without differentiation. The rhyolite porphyry and biotite granite shared many geochemical similarities, denoting a unified source. The high La/Yb (>30) but low Sr/Y (<20) ratios and apparent negative Eu anomalies indicated plagioclase fractionation. Decreased zircon Dy/Yb with increasing Hf concentrations reflected noticeable amphibole fractionation. These two suites had fairly low Ba/La ratios. These data together point toward an identical source: dehydration melting of a relatively thickened crust. These melts experienced crystal fractionation after extraction. We propose that the intrusions were generated by the underplating of water-rich mafic magma, which provided both fluid and heat and finally induced two kinds of melting.

Geochemistry and petrogenesis of the Paleoproterozoic ortho-gneisses and granitoids of the Banded Gneissic Complex, central Rajasthan, NW India: Implications for crustal reworking processes

Geochemistry and petrogenesis of the Paleoproterozoic ortho-gneisses and granitoids of the Banded Gneissic Complex, central Rajasthan, NW India: Implications for crustal reworking processes

Crustal maturation and cratonization in response to Neoarchean continental collision: The Suizhong granitic belt, North China Craton

The late Archean (3.0–2.5 Ga) is a pivotal period, when properties of the continental crust significantly changed and a fundamental shift of geodynamic processes took place. As the major felsic component in early Archean (> 3.0 Ga) crustal records, tonalite-trondhjemite-granodiorite (TTG) suites are compositionally different to the present-day felsic upper continental crust. But it remains unclear how and to what degree the felsic part of the continental crust evolved during the late Archean. The 250 km long Neoarchean Suizhong granitic belt in the North China Craton (NCC) is mainly composed of K-rich granitoids and compositionally distinct to the TTG-dominated early Archean upper continental crust. This belt is a natural laboratory to explore how the early Archean upper continental crust evolved in the late Archean. Combined with our new data (zircon U-Pb ages, bulk-rock geochemistry and zircon Hf isotopes), we provide an overview of the temporal and compositional features of the Neoarchean Suizhong granitic belt. Granitic magmatism in the Suizhong granitic belt mainly took place within 100 Myrs around the end of the Archean, and these rocks are primarily K-rich granitoids with minor TTGs and mafic magmatic enclaves (MMEs). The K-rich granitoids include sanukitoid-like granites and potassic granites; the former are hybrids between melts from Mesoarchean enriched mafic crust and enriched mantle, and the latter are melts from Hadean–Mesoarchean crustal lithologies. The volumetrically minor TTGs were sourced from Mesoarchean low-K mafic crust. MMEs are either cumulates or inclusions of captured enriched mantle melts. Overall, these Neoarchean granitoids are essentially intracrustal reworking products of ancient continental nuclei and are compositionally and temporally similar to syn- and post-collisional magmatism in Phanerozoic collisional orogenic belts. The Neoarchean crustal reworking process was most likely to be induced by amalgamation of micro-continents through collision. Through the collision-induced reworking, a variety of K-rich granitoids were introduced to the Neoarchean upper continental crust. As a result, layering of felsic upper and mafic lower crust was developed, and the maturity of Archean continental crust was significantly enhanced. Collisional orogenesis played a critical role in maturing the Archean continental crust and initial cratonization of the NCC during the late Archean.

Eclogite resembling metamorphic disequilibrium assemblage formed through fluid-induced metasomatic reactions

Equilibrium omphacite-garnet-bearing mafic rocks have been classified as eclogites, either pristine or retrogressed, that were formed at great depths in the lithosphere. Here we report a unique natural example of eclogite resembling assemblage in disequilibrium formed through fluid-induced metasomatic reactions under the amphibolite to granulite facies. Primarily, the amphibolized protolith experienced a garnet-amphibolite facies metamorphism at ~ 500–700 °C and ~ 0.8–1 GPa. Subsequently, CO2 fluid induced fracturing and dissolution-reprecipitation reactions occurred at peak metamorphic conditions of ~ 700 °C and ~ 1 GPa. Occasional omphacite-albite assemblage, which gradually replace diopside-oligoclase symplectite adjacent to albite veins along fractures, indicates fluid-induced coupled dissolution-reprecipitation disequilibrium reactions. Here the albite-omphacite assemblage is in local equilibrium at least on 1 mm length scale, during cooling, below ~ 600 ºC and ~ 1 GPa, within the amphibolite facies conditions. The results from this study clearly suggest that disequilibrium garnet-omphacite assemblage in mafic rocks could be formed by crustal reworking processes below granulite facies conditions, and their textural equilibrium is an important criterion while defining eclogite facies.

Crustal growth and reworking of Archean crust within the Rhyacian domains of the southeastern Guiana Shield, Brazil: Evidence from zircon U–Pb–Hf and whole-rock Sm–Nd geochronology

The southeastern Guiana Shield, northern Amazonian Craton, is part of a Paleoproterozoic orogenic belt that was built up during the Transamazonian orogenic cycle (2.26–1.95 Ga). This cycle includes large segments of Rhyacian juvenile crust and some reworked Archean terranes. The geology in this region consists mainly of Paleoproterozoic granulitic-migmatitic-gneissic complexes, deformed and metamorphosed metavolcanic and metasedimentary rocks, and granitoids (granitic and TTG magmatism). Three tectonic domains are distinguished in the Brazilian territory of the southeastern Guiana Shield. They are known as the Amapá Block, Lourenço Domain, and Carecuru Domain. The Amapá Block is a Meso–Neoarchean continental block that was intensely reworked during the Transamazonian orogeny. The other two domains represent Rhyacian landmasses, the evolution of which involved several stages of subduction of oceanic lithosphere in magmatic arc environments. There are also relics of reworked Archean continental crust, the formation of which was followed by a collisional stage of tectonic accretion of the magmatic arcs. Whole-rock Sm–Nd and U–Pb zircon geochronology have confirmed the juvenile character of much of this Transamazonian orogenic belt. However, for the Lourenço and Carecuru domains, Nd isotopic signatures indicate the participation of Meso–Neoarchean crustal material in the sources of the magmatic rocks. Combined zircon U–Pb and Lu–Hf isotopic analyses by LA-ICP-MS were performed on eleven Rhyacian granitoids and orthogneisses from the Lourenço and Carecuru domains. The aim was to verify the extension of the influence of the Archean continental crust in the adjacent Paleoproterozoic domains. The main magmatic episodes were identified in the Lourenço Domain (~2.17–2.18, 2.14 and 2.12–2.09 Ga) and Carecuru Domain (2.14 Ga) by U–Pb zircon geochronology. The Lu–Hf isotope data point to the predominance of crustal reworking processes (ƐHf(2.2–2.1 Ga) < 0; 67% of zircon crystals) during the formation of Lourenço and Carecuru domains. Hf model ages were found to be mostly Archean (98.4%), even for zircon grains that have positive ƐHf(2.2–2.1 Ga) values. For the terrane at the border of the Lourenço and Carecuru domains with the Amapá Block, assimilation of Archean crust of different ages and proportions in a magmatic arc environment accounts for the Hf–Nd isotopic signatures and Hf model ages of Rhyacian magmatism. In the northwestern part of the Lourenço Domain, more than 100 km north of the Amapá Block, the Hf–Nd isotopic signatures and Hf model ages indicate the participation of Archean crustal material, either as continental fragments and/or through incorporation of continental sediments in island arc environments, similar to what has been recorded for some Birimian terranes of the West African Craton in Ghana.

Geology and geochemical constraints on the origin of the Mesoarchean granitoids from Carajás province, Amazonian craton

The Carajás province (CP) is the largest preserved Archean core of the Amazonian craton. In this paper we present a detailed work about the Tucumã area located in the northwestern portion of the Rio Maria domain of the CP, where more detailed studies had not yet been carried out. The new geological and geochemical data obtained allowed the identification of Mesoarchean granitoids of different natures that resemble typical late-Archean granitoids (TTG, leucogranites, sanukitoids and hybrid granites) identified in several cratons around the world. The most extensive unit in the Tucumã area is the Leucomonzogranitic batholith which is intrusive in a tonalite crust of TTG affinity, that exhibits an older N-S structural pattern commonly found in oldest rocks (greenstone belt sequence). The batholith is coeval with small granitoid intrusions that occur as lenses controlled by NE-SW and E-W direction anastomosed shear zones. Apart from the tonalite, these granitoids show affinity with the calc-alkaline series and are individualized in four units. The porphyritic granodiorite is a magnesium-rich unit that has moderate K2O/Na2O ratio and higher levels of Ba, K and Th, indicating similarities with the Transitional TTG suites (Yilgarn craton). Equigranular granodiorites are equivalent of sanukitoid suites; they show enrichment in Sr and compatible elements (Mg, Cr, and Ni) and depletion in HREE. Hybrid granitoids were identified as High-HFSE granites (medium- and high-Ba) that share some geochemical affinities with Closepet granite of the Dharwar craton. The Leucomonzogranite is enriched in LILE and exhibit negative Eu anomalies, which indicate crustal reworking processes. This granitoids are syntectonic bodies that underwent a progressive deformation processes that started before the magma crystallization was complete, under high temperatures (>500 °C) conditions compatible with those of the amphibolite facies. Two phases of magmatism are recorded in the RMD, which would be responsible for the formation of these granitoids. The first phase (2.98-2.92 Ga) is related to formation of a thick TTG crust from parcial melting of an oceanic plateau or a thickened mafic crust. The second magmatism phase (~2.87 Ga) initiated due to thermal events in a collisional setting, where both crustal rework processes and generation of juvenile magmatism occurred at the same time, resulting in the formation of heterogeneous plutons, at different crustal levels that were emplaced by mechanisms analogous to diapirs.

Crust-mantle mixing and crustal reworking of southern Tibet during Indian continental subduction: Evidence from Miocene high-silica potassic rocks in Central Lhasa block

Collisional zones are commonly considered as important regions for crustal reworking, but the reworking mechanism remains debated. The well-known Himalayan-southern Tibetan orogen, built by India-Asia collision and convergence, has the thickest continental crust on Earth and is therefore an ideal region for studying crustal reworking during collisional orogenesis. Here we revisit the Miocene high-silica potassic rocks (trachytes) in the Konglong area of the central Lhasa block, southern Tibet. Integrated studies of geochronology, mineral compositions, bulk-rock major- and trace-element geochemistry, and Sr-Nd-Pb-Hf-O isotopes unequivocally indicate that the Konglong trachytes formed by mixing between enriched mantle-derived ultrapotassic and thickened ancient crust-derived magmas. Combined with post-collisional magma mixing recently identified in the southern Lhasa block, we suggest that magma underplating and subsequent mantle-crust interaction (i.e., the matter and energy transfer from the mantle to the crust) has been a common and important crustal reworking process in southern Tibet during Indian continental subduction. This process may be related to Indian plate flat subduction and subsequent foundering during the post-collisional stage. In combination with the nature of Cenozoic magmatism in the Himalaya block, we suggest that in addition to partial melting of the subducted continental crust, magma underplating and subsequent crust-mantle mixing beneath the obducted continent has also played an important role in crustal reworking of the collisional zone.

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 Amapá 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). Subchondritic ɛHf(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 Amapá Block. Hf-TDMC model ages, ranging from 2.99 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 that continental crust generation in the Amazonian Craton began at least 500 Ma earlier than previously suggested by the Sm-Nd systematics.

Late Carboniferous to early Permian partial melting of the metasedimentary rocks and crustal reworking in the Central Asian Orogenic Belt: Evidence from garnet-bearing rhyolites in the Chinese South Tianshan

Peraluminous granitic rocks provide important insights on crustal reworking processes. Here we study the garnet-bearing rhyolites of the Early Permian Xiaotikanlike Formation exposed in the Heiyingshan region located in the South Tianshan region in Xinjiang, NW China. Two layers of garnet-bearing rhyolites were recognized, which discordantly overlie the strongly deformed Late Carboniferous strata. The rocks generally display porphyritic texture with alkali-feldspar, quartz, plagioclase, biotite and garnet as the major phenocryst phases. Zircon LA–ICP-MS U–Pb analyses yield ages of 281.5±0.7Ma for the upper layer of rhyolite (ULR) and 299.9±1.9Ma for the lower layer (LLR). The ULR shows SiO2 in narrow range of 71.08 to 72.39wt.%, and Fe2O3 from 0.99 to 1.69wt.% with Fe2O3/FeO ranging from 1.68 to 2.68. The normalized trace element patterns are characterized by prominent troughs for Nb, Ta, Sr, P and Ti and relatively sloping rare earth element patterns. They have radioactive Sr–Nd isotopic compositions [(87Sr/86Sr)i=0.7128 to 0.7179 and εNd=−13.09 to −12.44] and negative εHf(t) values of −9.97 to −1.27. The LLR exhibits similar trace and rare earth element patterns, εNd (−12.68 to −11.70) and εHf(t) values (−7.56 to −2.6) to the ULR, but have higher (87Sr/86Sr)i=0.7166–0.7269 and lower Fe2O3/FeO ratios (0.17–1.73). Geochemical and isotopic characteristics indicate that these rhyolites are dominantly alkali-calcic and strongly peraluminous, which were most likely derived from muscovite dehydration melting of metagreywacke. We correlate this with the tectonic processes following the collision between the Central Tianshan–Yili Block and the Tarim Craton. Both the Zr saturation temperatures and Al2O3/TiO2 ratios suggest an increase in temperature of the source during 300Ma to 280Ma, which may be attributed to heat input from the underplating of mantle-derived basaltic magmas. The field relation between the rhyolite and the Late Carboniferous strata favors the model that the final collision culminated during Late Carboniferous before 300Ma. In combination with the occurrence of felsic lavas in other regions such as the Xiaotikanlike Formation from Wensu, Laohutai and Boziguo'er region, we define a >200km peraluminous magma belt in the central part of Chinese South Tianshan, which provides new insights into the crustal reworking process in the Central Asian Orogenic Belt.

Paleozoic intrusive rocks from the Dunhuang tectonic belt, NW China: Constraints on the tectonic evolution of the southernmost Central Asian Orogenic Belt

The Dunhuang tectonic belt (DTB) is of great importance for understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt (CAOB). In this study, the temporal-spatial distribution, petrogenesis and tectonic setting of the Paleozoic representative intrusive rocks from the DTB were systematically investigated to discuss crustal evolution history and tectonic regime of the DTB during Paleozoic.Our results reveal that the Paleozoic magmatism within the DTB can be broadly divided into two distinct episodes of early Paleozoic and late Paleozoic. The early Paleozoic intrusive rocks, represented by a suite metaluminous-slight peraluminous and medium- to high-K calc-alkaline I-type granitoids crystallized at Silurian (ca. 430–410Ma), are predominantly distributed along the northern part of the DTB. They were probably produced with mineral assemblage of eclogite or garnet+amphibole+rutile in the residue, and were derived from magma mixing source of depleted mantle materials with various proportions of Archean-Mesoproterozoic continental crust. The late Paleozoic intrusive rocks can be further subdivided into two stages of late Devonian stage (ca. 370–360Ma) and middle Carboniferous stage (ca. 335–315Ma). The former stage is predominated by metaluminous to slight peraluminous and low-K tholeiite to high-K calc-alkaline I-type granitic rocks distributed in the central part of the DTB. They were also generated with mineral assemblage of amphibolite- to eclogite-facies in the residue, and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust. The later stage is represented by adakite and alkali-rich granite exposed in the southern part of the DTB. The alkali-rich granites studied in this paper were possibly produced with mineral assemblage of granulite-facies in the residue and were generated by partial melting of thickened lower continental crust. Zircon Hf isotopes and field distribution of those Paleozoic intrusive rocks reveal that both the Silurian and the late Devonian magmatic activities predominantly represent crustal growth processes in the DTB, accompanied by different degrees of reworking of pre-existing continental crust. However, the middle Carboniferous (ca. 335–315Ma) magmatic activity reflects a crustal reworking process. The Silurian and late Devonian intrusive rocks were most likely formed in the arc-related subduction zones, whereas, the middle Carboniferous intrusive rocks were possibly formed in a transitional tectonic setting from compression to extension, representing the final stage of Paleozoic orogeny in the DTB.These Paleozoic magmatic rocks further suggest that the DTB has reactivated from a stable block to an orogen and undergone two episodes (the early Paleozoic and the late Paleozoic) of orogeny during Paleozoic. It represents a Paleozoic accretionary orogen of the southernmost margin of the CAOB between the Tarim Craton and North China Craton, and tectonically extends northward to the Beishan orogen and westward to the eastern South Tianshan Belt.

Neoarchean arc magmatism and crustal growth in the north-eastern North China Craton: Evidence from granitoid gneisses in the Southern Jilin Province

Late Neoarchean granitoid gneisses are widespread in the Jiapigou–Jingyu area of the Southern Jilin Province (SJP) of the North China Craton (NCC). These rocks are composed of tonalitic–trondhjemitic–granodioritic–dioritic (TTGD) gneisses and preserve important information on the geodynamic evolution and Archean crustal growth along the northeastern margin of the NCC. Here we present LA–ICP–MS zircon U–Pb isotopic ages which reveal that the magmatic precursors of the tonalitic–trondhjemitic gneisses were emplaced at ∼2578–2574Ma, and the granodioritic–quartz dioritic magmas crystallized at ∼2546–2524Ma. The rocks were subjected to regional amphibolite facies metamorphism at ∼2510–2490Ma.Geochemically, the tonalitic–trondhjemitic gneisses can be subdivided into two subgroups. The magmas of high-Yb subgroup were derived by partial melting of dominantly ancient basaltic materials at the crustal level in the stable field of amphibole, clinopyroxene and significant amounts of garnet, and subsequently subjected to varying degrees of fractionation. Magmas of the low-Yb subgroup were generated by partial melting of dominantly juvenile basaltic materials with minor greywackes at the lower crustal level in the stability field of garnet and rutile, with a possible eclogitic residue. The magmatic precursors of granodioritic gneisses were produced by partial melting of subducted oceanic slabs and minor sediments, with the melts contaminated by the mantle wedge materials during their ascent. The parental magmas for the quartz dioritic gneisses were possibly generated by partial melting of a depleted mantle source that was significantly metasomatized by slab-derived fluids or melts.Combined with their petrogenesis, ∼2578–2574Ma tonalitic–trondhjemitic gneisses display positive zircon εHf(t2) values (+0.4 to +6.2) with TDM(Hf) ages of 2856–2643Ma, suggesting a crustal reworking process. On the other hand, ∼2546–2524Ma granodioritic to quartz dioritic gneisses have highly depleted zircon εHf(t2) values (+4.5 to +7.7) and TDM(Hf) ages of 2682–2550Ma that are close to their magmatic crystallization ages, indicating a significant crustal growth. Integrated with our recent studies on ∼2683–2654Ma metavolcanic rocks in Helong granite-greenstone belt and voluminous ∼2588–2536Ma metavolcanics in Jiapigou granite-greenstone belt, we infer that the Southern Jilin Province experienced Neoarchean subduction-related evolution history, with an Andean-type active continental margin along the northern margin of the NCC, and witnessed two major episodes of subduction-related crustal growth at ∼2.7Ga and ∼2.6–2.5Ga, with the latter period coupled with reworking of old crustal materials.

Zircon geochronology and Sm–Nd isotopic study: Further constraints for the Archean and Paleoproterozoic geodynamical evolution of the southeastern Guiana Shield, north of Amazonian Craton, Brazil

The eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil. Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the ε Nd values and T DM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components. The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, T DM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and ε Nd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga. The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block.

Integrated Nd isotopic and U–Pb detrital zircon systematics of clastic sedimentary rocks from the Slave Province, Canada: evidence for extensive crustal reworking in the early- to mid-Archean

A combined U–Pb geochronology and Nd isotopic analysis of clastic metasedimentary rocks from the Archean Slave Province, Northwest Territories, Canada is presented. A series of clastic sedimentary rocks with deposition age of ∼3.13 Ga to ∼2.58 Ga was collected from the west-central Slave Province. These samples include conglomerates as well as finer sedimentary rocks such as greywacke, arkose and quartzite. Although it is generally agreed that the Nd model age ( T DM) of clastic sedimentary rocks represents the average sedimentary provenance age, T DM of samples studied here is generally older than the average U–Pb age of detrital zircons extracted from the same sample. Rather, there seems to be a better ∼1:1 relationship between the T DM and U–Pb age of the oldest zircon from each sample. A simple model relationship between U–Pb geochronology and T DM suggests that such correlation is a result of Archean crustal evolution of the Slave Province being dominated by crustal reworking (i.e. intracrustal processes) rather than addition of juvenile materials from the mantle. One exception is the period between ∼3.1 and ∼3.2 Ga, during which period a significant portion of detrital zircons have U–Pb ages exceeding the T DM, indicating a large input of juvenile material and/or decreasing level of crustal reworking. An alternative explanation for the U–Pb vs. T DM trend may be a mixing of older mafic juvenile crust (containing no zircon) and younger felsic juvenile crust. However, the general lack of extensive pre-2.8 Ga mafic crust in the Slave Province makes this scenario unlikely. The results of this study cannot constrain the change in the volume of continental crust at any given period of time, since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle. However, a dominance of crustal reworking during the early- to mid-Archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events. Since the majority of zircon may be lost during crustal reworking processes (unless they are preserved as an inherited core), absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early Earth.

A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie-Norseman area, Western Australia

Accurate dating of granites and greenstones from the Kalgoorlie-Norseman area shows that there is considerable overlap in time between the formation of granites and greenstones. However, within any given area, the oldest granites are always younger than the oldest greenstones. It is suggested that the long-lived (100 Ma) thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting, leading to the production of granites. The time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust. Anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma. Where this happens the basaltic magma ponds at the base of the crust, supplying additional heat and accelerating the melting process. Eventually the granitic melt layer becomes unstable and rises within the crust. Areas intruded by granite become topographic highs, with topographic lows forming above areas where light melt has been withdrawn from the lower crust. When the topographic highs protrude above sea-level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones. Thus the change from a basalt-dominated lower greenstone succession to a sediment and felsic volcanic-dominated upper whitestone sequence, seen in many granite-greenstone terrains, results from the intrusion of granitic magma into the upper crust. The crust of the Kalgoorlie-Norseman area formed in two distinct stages, separated by a time break of up to 700 Ma. The first stage involved the formation of primitive crust, probably above subduction zones. This primitive crust was characterized by low Rb/Sr and high Sm/Nd ratios. The second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents. This led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas. Melting reset the Rb/Sr, Sm/Nd and U/Pb systems, giving the impression that the second event, which occurred at ∼ 2.7 Ga, was a major crust-forming event. It was, in fact, a crustal reworking process although some basalt was added to the crust at this time. Although the two-stage history of the Kalgoorlie-Norseman area is clearly seen in the lead isotopes, and in zircons, it is not readily discernible from the Rb Sr and Sm Nd systematics, due to mantle-like ratios of those elements in the primitive crust.