Lu-Hf Data Research Articles

Evolution of the 3.65–2.58 Ga Mairi Gneiss Complex, Brazil: Implications for growth of the continental crust in the São Francisco Craton

The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex: Eoarchean (ca. 3.65–3.60 Ga), early Paleoarchean (ca. 3.55–3.52 Ga), middle-late Paleoarchean (ca. 3.49–3.33 Ga) and Neoarchean (ca. 2.74–2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.

Neoarchaean crustal evolution along the eastern flank of Nallamalai Shear Zone, southern India

ABSTRACT The Southern Granulite Terrane (SGT) in India is composed of Archaean to Proterozoic crustal blocks and intervening shear/suture zones. The Nallamalai shear zone lies along the eastern flank of the Shevaroy Block. This study is focused on the eastern part of this shear zone where the basement rocks are dominantly composed of charnockite, quartzo-feldspathic gneiss and metagabbro, all of which preserve original magmatic texture. Zircon U-Pb LA-ICPMS analysis on the charnockite samples yielded an age of 2680 to 2500 Ma for the protolith emplacement followed by metamorphism at 2520 to 2450 Ma. Magmatic zircon grains in the quartzo-feldspathic gneiss and metagabbro yielded weighted mean ages of ca. 2557 Ma and ca. 2583 Ma with metamorphism at ca. 2518 Ma. Zircon Lu-Hf data suggest that the protolith of metagabbro was sourced mainly from the recycled Mesoarchaean crustal component. The ƐHf (t) of charnockite (−0.4 to 4.1) and quartzo-feldspathic gneiss (0.5 to 6.0) represent a combined source of the mantle as well as recycling crust. All the rocks underwent amphibolite to granulite facies metamorphism and petrological studies and pseudosection modelling shows a similar P-T range of 7–8 kbar and 650–830°C. Results from the present study suggest that crustal evolution along the eastern flank of the Nallamalai Shear Zone involved melting of older crustal components (Mesoarchaean age) with significant juvenile input (ƐHf (t) = ~+6.0) within a continental arc setting.

Evidence for change in crust formation process during the Paleoarchean in the São Francisco Craton (Gavião Block): Coupled zircon Lu-Hf and U-Pb isotopic analyses and tectonic implications

The continental crust growth/evolution processes and the tectonic regime through Eo- and Paleoarchean times are enigmatic due to the scarcity of preserved crust. The Gavião Block, São Francisco Craton (Brazil), contains exposed and well-preserved Eo-Paleoarchean crust remnants, providing a rare opportunity to investigate these issues. Here, we describe new U-Pb ages and Hf isotope data of 3.51–3.4 Ga tonalites and diorites from the Gavião Block and compare these with previously published Lu-Hf data from Hadean/Eo-Paleoarquean zircons from the Gavião Block and other primitive cratons. The Eo- to Paleoarchean evolution of the Gavião Block is registered by ca. 360 Myr of continuous magmatic events from ca. 3.66 Ga to 3.30 Ga. From the available Hf data, we interpret that each of the events younger than 3.6 Ga registers a new juvenile addition that assimilated older crust, whereas the rocks older than 3.6 Ga are exclusively formed through the reworking of a Hadean, and to a less extent early Eoarchean crust. The shift in the crust generation process with the input of juvenile material into the Gavião Block has been documented within the ∼ 3.8–3.5 Ga time in other primitive cratonic complexes including the Wyoming, Pilbara, Kaapvaal, Slave, Singhbhum, and Yilgarn. As documented in these other cratons, our data suggest that a shift in the Hf isotope record to rocks younger than 3.6 Ga reflects a transition from stagnant-lid to mobile-lid tectonics in the crust formation process of the São Francisco Craton. This change in the geodynamic regime appears to have been global at ca. ∼ 3.8–3.5 Ga and facilitated the extraction of juvenile melts, crustal reworking, evolved magmatism, and the production of stabilizing melt-depleted lithospheric mantle.

Diamictitic iron formation (DIF) deposits of the Neoproterozoic Nova Aurora Iron District (Macaúbas Group, Southeast Brazil)

The best-known Neoproterozoic glaciation-related iron deposits are the Rapitan-type banded iron formation (BIF) formed dominantly by chemical-sedimentary processes. We present a detailed study on a distinct type of glaciation-related iron deposit: the iron-rich metadiamictites of the Nova Aurora Iron District, comprising over 20 billion tons of iron oxide-rich rocks: the diamictitic iron formations (DIF >15 wt% Fe) and ferruginous metadiamictites (FD: 5–15 wt% Fe). Drill core logging, field mapping and analytical data revealed the protoliths, sediment sources, depositional environment, and ore enrichment processes on the iron district, and were used to discuss differences between the studied DIF and classic Rapitan-type BIF. U–Pb age spectra and Lu–Hf data for detrital zircon grains revealed a wide range of sediment sources and a maximum depositional age around 879 Ma. These data support correlations of the Nova Aurora iron deposits with the Cryogenian Macaúbas rift, a basin filled by glaciomarine successions related to the Sturtian glaciation event. The matrices of prevailing hematite-rich metadiamictites comprise (in vol %): hematite (7–55), quartz (17–57), muscovite (2–40), carbonate (<1–30), biotite (<1–18), chlorite (<1–23), epidote (<1–7), magnetite (1–5), and traces of tourmaline (<3), apatite (<3) and zircon (<1), with hematite, mica and stretched quartz outlining the metamorphic foliation. Clasts are free of iron-rich rocks. Hematite reconcentration (>50 wt% Fe) occurs in local mylonitic shear zones and tightly crenulated bands after gangue minerals removal during deformation. Late hydrothermal alteration formed magnetite-rich metadiamictite in high-strain zones. From base to top, the stratigraphic type-section of the Nova Aurora Iron District shows barren metadiamictite with quartz-rich matrix and graded-bedded quartzite lenses, followed by hematite-rich-matrix metadiamictite, then passing to muscovite-rich-matrix metadiamictite gradually richer in magnetite and sulfide, enclosing metapelite lenses. This section presents a fining-up graded-bedded succession with clasts size decreasing upwards and recurrent load structures, typical of debris flows and turbiditic sedimentation. The hematite/mica, Fe/Al, Fe/Ti and Fe/REE ratios, as well as the biotite/muscovite and silicate/sulfide ratios, which accompany variations in the oxidizing conditions, decrease to the top. Accordingly, the early protoliths were quartz-rich diamictites that passed to iron-rich diamictites and then to clay-rich, and under more reducing conditions, sulfide-bearing diamictites. Sedimentological and geochemical evidences suggest ferrous-iron accumulation followed by iron oxide precipitation from seawater when the conditions changed from anoxic to oxidizing by the input of oxygen-rich waters from deglaciation, followed by increasing reducing conditions accompanying marine transgression. These processes took place in a relatively restricted sector of the glaciomarine basin, probably a graben of the Cryogenian Macaúbas rift. Differences in relation to the classic Rapitan-type iron formations are the coarse-grained (ruditic) texture and the massive structure, the high-energy sedimentary environment, and the absence of true BIF in the Nova Aurora DIF deposits.

Ocean Plate Stratigraphy of a long-lived Precambrian subduction-accretion system: The Wutai Complex, North China Craton

Ocean Plate Stratigraphy (OPS) is the travelogue of the oceanic lithosphere from its birth at the Mid Ocean Ridge to its demise at the subduction zone in convergent plate margins. Here we present an OPS succession preserved onland in the Wutai Complex of the North China Craton (NCC), representing a prolonged subduction-accretion-collision system and continental growth during the Neoarchean-Paleoproterozoic. The OPS succession in Wutai includes metabasalts and banded iron formation representing the pelagic sequences together with intercalated metacarbonates. Phyllite, greenschist, and mica schist represent the hemipelagic sequence. Quartzite and conglomerate correspond to the turbidite sequence, with a progressive southward accretion towards a magmatic arc in the north. We present results from petrological, geochemical, zircon U-Pb, REE, and Lu-Hf studies from representative rocks in the OPS sequence of the Wutai Complex. The trace elements including the rare-earth element (REE) data of the metabasalt show transitional features between normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB). The REY (lanthanide elements and Y) data of the banded iron formation (BIF), greenschists, and phyllite suggest the interaction of high-temperature hydrothermal fluids and seawater. Magmatic zircon grains from the quartzite show 206Pb/207Pb upper intercept age of 2443 ± 21 Ma, whereas zircon grains in the phyllite show upper intercept age of 2494 ± 32 Ma, with minor xenocrysts with older ages of 2726 ± 46 Ma and 2672 ± 48 Ma. Zircon grains in the metaconglomerate show upper intercept age of 2569 ± 32 Ma. Those from the siliceous and intercalated bands in the BIF yield upper intercept ages of 2583 ± 34 Ma and 2524 ± 49 Ma. The zircon Lu-Hf data on zircon grains from the BIF, phyllite, quartzite, and metaconglomerate all show positive εHf(t) values (+1.3 to +7.4) in the range of depleted mantle to new crust, suggesting Neoarchean intra oceanic arc subduction. Combined with the zircon U-Pb and Hf isotopic data previously reported from the Wutai Complex, we infer the following four tectonic evolution stages: ca. 2800 Ma subduction initiation, ca. 2750–2300 Ma arc-accretionary complex, ca. 2300–2100 Ma back-arc rifting, and ca.1900–1750 Ma arc-continent collision. The metavolcanics and part of the metasedimentary rock assemblages in the Wutai Complex represent a deformed and dismembered OPS sequence generated through northward subduction and closure of the Wutai Ocean.

Timescales and rates of intrusive and metamorphic processes determined from zircon and garnet in migmatitic granulite, Fiordland, New Zealand

Abstract Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland, New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30 ± 0.13 to 115.70 ± 0.18 Ma (CA-ID-TIMS) and 121.4 ± 2.0 to 109.8 ± 1.8 Ma (SHRIMP-RG). Zircon dates in host and garnet reaction zone do not define distinct populations. In addition, the dates cannot be readily grouped based on external morphology or internal CL zoning. Zircon trace-element concentrations indicate two distinct crystallization trends, clearly seen in Th and U. Garnet occurs in selvages to the leucosome veins and in the adjacent garnet reaction zones. In selvages and host orthogneiss, garnet is generally 0.5 to 1 cm diameter and euhedral and is 0.1 to 0.5 cm diameter and subhedral in garnet reaction zones. Garnet Sm-Nd and Lu-Hf dates range from ca. 115 to 101 Ma (including uncertainties) and correlate with grain size. We interpret the CA-ID-TIMS zircon dates to record the age of magma emplacement and the SHRIMP-RG dates to record a range from igneous crystallization to metamorphic dissolution and reprecipitation and/or local Pb loss. Zircon compositional trends within the garnet reaction zone and host are compatible with locally isolated melt and/or separate intrusive magma batches for the two samples described here. Dates for the largest, ~1 cm, garnet of ~113 Ma record growth during metamorphism, while the smaller grains with younger dates reflect high-temperature intracrystalline diffusion and isotopic closure during cooling. The comprehensive geochronological data set for a single location in the Malaspina Pluton illustrates a complex and protracted geologic history common in granulite facies rocks, estimates lower crustal cooling rates of ~20 °C/m.y., and underlines the importance of multiple chronometers and careful textural characterization for assigning meaningful ages to lower-crustal rocks. Numerous data sets from single locations, like the one described here, are needed to evaluate the spatial extent and variation of cooling rates for Fiordland and other lower crustal exposures.

Insights into sedimentary provenance and the evolution of the Potiguar Basin, NE Brazil, using U–Pb ages and Lu–Hf isotopes in detrital zircons

U–Pb and Lu–Hf data obtained from detrital zircon of 21 samples from four wells were used to determine the source terrains of the Cretaceous sedimentary rocks of the Pendência, Pescada and Alagamar Formations (rift and post-rift sections) of Potiguar Basin, northeast Brazil. More than 1500 U–Pb concordant ages allowed the determination of the main age peaks in Paleoproterozoic (ca. 2.2–2.0 Ga and 1.9–1.7 Ga) and Neoproterozoic (ca. 0.62–0.55 Ga), matching the ages of rocks from the basement adjacent to the basin (northern portion of Borborema Province). εHf values obtained from 61 zircon grains range from −0.53 to −8.86 for Paleoproterozoic and from 7.67 to −25.43 for Neoproterozoic zircons, consistent with published data from rocks of the adjacent basement. Neoproterozoic zircons shows εHf values more negative from bottom to top, and presents a trend in U–Pb age vs εHf where the older grains show juvenile signature and the younger, continental crustal signature. This trend mimics the evolution of granitoid generation in the basement adjacent to the Potiguar Basin during the Neoproterozoic orogeny. The age pattern varies vertically in most wells, reflecting the contribution of distinct source areas following the rift evolution. Furthermore, the age pattern in the wells located at southeast of the grabens and horsts system of the onshore Potiguar Basin show Paleoproterozoic zircons concentrated in a single peak age, few or no zircon grains of 1.8–1.6 Ga and lesser amount of 0.6–0.7 Ga zircons, in contrast with the wells located at northwest, with Paleoproterozoic ages distributed in more than one peak, significant presence of 1.8–1.6 Ga zircons and proportionally more 0.6–0.7 Ga zircon grains. It suggests that this system acted as a barrier to sediment transport, delimiting the basin in two sectors fed by two different sources, one being the northeast portion of Borborema Province and the other, the northwest portion, both in the rift and post-rift sections. Such differences can result in distinct quality for the potential reservoirs in these sectors and would be an important guide for hydrocarbon exploration in Potiguar basin.

The effects of Antarctic alteration and sample heterogeneity on Sm-Nd and Lu-Hf systematics in H chondrites

Long-lived radioactive isotope systematics, such as Sm-Nd and Lu-Hf, are useful tools as important chronometers and tracers for chemical differentiation processes. Even though Antarctic meteorites include rare meteorites such as ungrouped meteorites, the effects of Antarctic alteration on the Sm-Nd and Lu-Hf systems in chondrites have not yet been evaluated in detail. Moreover, the heterogeneity of Sm-Nd and Lu-Hf data in bulk chondrites prevents the determination of precise average Sm-Nd and Lu-Hf values (e.g., for individual chondrite groups). To examine the effects of Antarctic alteration and sample heterogeneity on the Sm-Nd and Lu-Hf isotope systematics, ten Antarctic H chondrites (HCs) and three HCs from hot deserts were characterized for their modal abundances, elemental abundances, and Sm-Nd and Lu-Hf isotopic compositions. Regardless of the classical weathering index for Antarctic meteorites and the normalized Rb abundance used as a chemical alteration indicator in this study, the modal and elemental abundances in Antarctic HCs appear to be in good agreement with those in non-Antarctic HCs. The Sm-Nd and Lu-Hf isotopic compositions of the characterized H chondrites fall within the range measured for both HC falls and for falls of other chondrite groups, except in the case of the most heavily altered sample. Consequently, the effects of Antarctic alteration processes on the Sm-Nd and Lu-Hf systematics in HCs appear to be limited, except in the case of Asuka 09516. The latter meteorite exhibits severe mineralogical and chemical alteration, with considerable losses of even the rare earth elements (REEs), which are considered relatively immobile. The 147Sm/144Nd, 143Nd/144Nd, 176Lu/177Hf, and 176Hf/177Hf of bulk HCs correlate with their P/Mg and Y/Mg. Furthermore, the Lu-Hf ratios correlate strongly with their P/Ca and Y/Ca as well as their P/Mg and Y/Mg. Thus, the distribution of the elements between constituent minerals in ordinary chondrites (OCs) may control the heterogeneity observed for the bulk Sm-Nd and Lu-Hf data. In this context, the weight ratio of Ca-phosphates to Ca-pyroxene, or at least that of Ca-phosphates to silicates, may be a key factor leading to the observed elemental and isotopic variations. This observation indicates that the nugget effect of Ca-phosphates in OCs as the result of insufficient homogenization or terrestrial alteration leads to the heterogeneities displayed by the Sm-Nd and Lu-Hf data. Moreover, it also indicates that the use of equilibrated OCs for the determination of Sm-Nd and Lu-Hf data is affected more by sample heterogeneity, especially with respect to Ca-phosphates, than is the case for unequilibrated OCs, based on the re-distribution of REEs during thermal metamorphism on their parent bodies. This study demonstrates that Antarctic meteorites commonly preserve their original Sm-Nd and Lu-Hf isotopic compositions as much as chondrite falls, although exceptions are possible in the case of severe alteration. Similar to previous studies, we recommend the use of unequilibrated chondrites, for which the re-distribution of REEs is less extensive, for the determination of well-constrained average Sm-Nd and Lu-Hf isotopic compositions for individual chondrite groups as well as their robust Chondritic Uniform Reservoir values.

In-situ Lu[sbnd]Hf geochronology of garnet, apatite and xenotime by LA ICP MS/MS

LuHf geochronology is a powerful method to constrain the temporal evolution of geological systems. Traditional application of this dating method requires time-consuming chemical separation of the parent (176Lu) and daughter (176Hf) isotopes that is commonly accompanied by loss of textural context of the analysed minerals. In contrast, In-situ (laser-ablation based) LuHf geochronology offers a number of advantages including rapid analysis with high spatial resolution, as well as control on textural relationships of the analysed mineral. However, laser-ablation based LuHf geochronology has been hindered by isobaric interferences of 176Yb and 176Lu on 176Hf that have effectively masked reliable determination of 176Lu and 176Hf. We present a methodology that resolves these interferences using LA-ICP-MS/MS (laser ablation tandem inductively coupled mass spectrometry) and NH3 gas to separate Hf from Lu. Both Lu, Yb, and Hf react with NH3 to form a variety of product ions. By measuring high order reaction products (e.g. Hf(NH)(NH2)(NH3)3+), we demonstrate that 176Hf can be measured interference-free from 176Lu and 176Yb with sufficient sensitivity to yield useful geochronological age data.The novel in-situ LuHf technique has been successfully applied to a variety of Palaeozoic and Precambrian-aged garnet, apatite and xenotime samples, including published reference materials. The resulting age uncertainties are as low as ~0.5% (95% conf. interval). The technique has the potential to obtain spatially-resolved LuHf ages in garnet-bearing samples that would be difficult to obtain by conventional techniques. The method also offers the opportunity for rapid “campaign style” geochronology in complex terrains that record poly-metamorphic histories. In apatite, the expected higher closure temperature of the LuHf system compared to the commonly used UPb system allows high-temperature thermal history reconstructions. In addition, LuHf dating of apatite allows dating of samples with low U and high common Pb (e.g. mafic and low-grade metamorphic rocks and ore deposits). Furthermore, apatite tends to incorporate little to no common Hf, allowing single grain ages to be calculated, which opens new doors for detrital provenance studies. In situ LuHf dating of xenotime offers an additional avenue to UPb dating, and may be particularly beneficial to dating of rare earth element ore deposits that often have complex temporal records of development.

Mantle-like Hf[sbnd]Nd isotope signatures in ~3.5 Ga greenstones: No evidence for Hadean crust beneath the East Pilbara Craton

The geological setting of extensive ~3.5 Ga greenstone sequences in the Pilbara Craton (Australia) has been controversial for many years. Models range from a purely oceanic origin to emplacement over a pre-existing, possibly Hadean, sialic basement. Here we present new isotopic (NdHf) and trace element data for a geographically extensive suite of komatiitic-tholeiitic metavolcanic rocks which comprise much of the Pilbara Supergroup. Most of the samples have flat to mildly enriched normalised trace element patterns, often associated with a slight NbTa depletion. A small subset of samples, represented throughout the stratigraphy, show strong LREE enrichment and pronounced HFSE anomalies (low Nb/Th, high La/Nb and Zr/Y) either requiring a different magma source or the addition of an enriched component (possibly pre-existing continental crust). Despite this compositional diversity, the SmNd and LuHf data for all samples define ~3.5 Ga isochrons and yield homogeneous initial ratios (εNd = 1.15 ± 0.19; εHf = 2.29 ± 0.65) suggesting a magma source — or sources — slightly more depleted than the putative primitive (chondritic) mantle but consistent with recent observations of other primordial, deep mantle reservoirs. Published SmNd data for the Dresser Formation obtained during the Pilbara Drilling Project appear to be affected by alteration processes and are considered unreliable for the estimation of an accurate initial isotopic composition.The data for the enriched samples are inconsistent with assimilation of known examples of locally exposed continental crust as a mechanism responsible for their trace element characteristics. If an alternate crustal contaminant is invoked it cannot be much older than the basaltic magmatism itself. As a consequence, we see no evidence for an extensive Hadean (>4.0 Ga) sialic basement in the region.

Siderian to Rhyacian evolution of the Juiz de Fora Complex: Arc fingerprints and correlations within the Minas-Bahia Orogen and the Western Central Africa Belt

The Juiz de Fora Complex (JFC) is one of the basement units of the Occidental Terrane, Central Ribeira belt, in the Neoproterozoic framework of Western Gondwana, regarded as part of the reworked border of the São Francisco Paleocontinent. The JFC comprises Paleoproterozoic granitoid rocks with TTG, calc-alkaline, sanukitoid, and tholeiite-type geochemical compositions. They exhibit complex deformational fabrics associated with two episodes of granulite-facies metamorphism, occuring in the early Orosirian and the Ediacaran. The new U-Pb (SHRIMP) isotopic ages and Lu-Hf (LA-MC-ICPMS) data combined with available lithogeochemical, whole-rock Sm-Nd and Rb-Sr isotopic data constrain the tectonic evolution of the JFC to a Siderian-Rhyacian evolved arc system. Five samples were selected for U-Pb geochronology and Lu-Hf analysis on zircon grains: a low-K tholeiite, one TTG, two sanukitoids, and a mafic tholeiite. Combined whole-rock Sm-Nd, Rb-Sr isotopes, and zircon Lu-Hf isotopic data suggest the following evolution: an initial moderately juvenile (2445 Ma) low-K tholeiitic magmatism (εNd(t) of − 0.1; εHf(t) from − 8 to + 3), followed by a moderately juvenile to evolved (2200–2182 Ma) coeval TTG-sanukitoid pulse (εNd(t) from − 0.6 to − 4.8; εHf(t) from − 11.7 to + 2.5), and eventually by a moderately juvenile (2130 Ma) tholeiitic magmatism (εNd(t) = − 0.05). The integrated information provides preliminary petrogenetic insights for these magmatic episodes, such as interaction between mantle sources and supracrustal subducted material, melting of the oceanic slab, and crustal assimilation. A ca. 2035 Ma collisional granulite event recorded in metamorphic rims and neoformed zircon grains in leucosomes, possibly marks the amalgamation of the JFC onto the Mantiqueira Complex. Geochronological and isotopic correlations with coeval terranes of the Minas-Bahia Orogen in the Brazilian and African counterparts are addressed for tectonic implications at a broader scale.

A multi-proxy study of the Cerro Piche Graben - A Lower Jurassic basin in the central North Patagonian Massif, Argentina

The volcano-sedimentary succession of the Cerro Piche Graben, Río Negro province, Patagonia (Argentina), was studied U–Pb and Lu–Hf in zircon. As well, a stratigraphic, sedimentological, palynology and a review of the paleoflora content was made. The combined stratigraphic and geochronological study on the volcano-sedimentary succession indicates that the Cerro Piche Graben was developed during the Early Jurassic and allow us to propose the Cerro Piche Formation. This formation is composed of a lower volcanic succession (named the Loma Blanca Member) that is restricted to the tectonic depression and that was probably emplaced during the Sinemurian (ca. 191 Ma), and an upper sedimentary succession (named the El Tono Member) that was unconformably deposited over the lower sequence during the Pliensbachian (ca. 183 Ma). The sedimentological study of the upper sequence allows to recognition of nine sedimentary cycles related to the evolution of an alluvial fan. The palynofacies interpretations reflect intermittent exposure to weathering and/or intense reworking of organic components as a result of a dynamic sedimentary system with a high terrestrial input, sediment reworking, and/or intermittent subaerial exposure. The paleoflora preserved in this sedimentary succession is composed exclusively of conifers and assigned to Cupessinoxylon sp., and probably corresponds to an arboreal paleocommunity. The U–Pb data on Early Jurassic detrital zircon grains from the El Tono Member indicate a maximum depositional age (MDA) of 185 Ma for this sequence, and the Lu–Hf data support a reworked continental crust for the zircon source. . Additionally, the U–Pb and Lu–Hf data obtained during this work together with data from the North Patagonian Massif supports the recognition of four magmatic cycles, called C0, C1 (including C1i source), C2 and C3. The C1 (172–180 Ma) and the C3 (155–162 Ma) cycles indicate mixing of juvenile mantle-derived and reworked crustal components in the parental magma of zircons and the C0 (182–192 Ma) cycle, which includes the distal volcanism identified in the Cerro Piche Graben, the C1i (172–180 Ma) and C2 (166–169 Ma) cycles seemingly represent three episodes of crustal reworking.

Insight into Archean crustal growth and mantle evolution from multi-isotope U-Pb and Lu-Hf analysis of detrital zircon grains from the Abitibi and Pontiac subprovinces, Canada

Lu-Hf laser ablation – multi-collector – inductively coupled plasma – mass spectrometry (LA-MC-ICP-MS) analysis was conducted on ~1800 detrital zircon grains from successor basins of the Archean Abitibi and Pontiac subprovinces of Ontario and Quebec, Canada, and paired with previous U-Pb LA-MC-ICP-MS analyses of the same grains. Results are used to constrain the isotopic character of magmatic source domains of the zircon grains to establish the sedimentary provenance of the ~2690–2670 Ma successor basins, to provide constraints on terrane configurations and amalgamations at the time of basin formation, and to assess their significance for the record of crust-mantle growth in the region. The majority of results (95%) yield ɛHf values of + 1 to + 10 for ~2850–2675 Ma zircon, and clusters along compositions of the Archean depleted mantle (DM), which is based on projections of modern MORB compositions. Subordinate results, comprising ~2% of the data set, yielded values (ɛHf > +10) corresponding to extremely depleted mantle compositions, reflecting anomalously depleted sources in the ~2950–2670 Ma age range. The remaining 3% correspond to chondritic uniform reservoir (CHUR)-like to negative ɛHf values that reflect primitive sources and/or evolved magmas in zircon that crystallized in the ~3250–3050 Ma and ~2950–2670 Ma age ranges. While Neoarchean grains dominate the data set (~88%), approximately 12% are Mesoarchean. The Lu-Hf data collected on these zircon grains, when compared with published isotopic results, preserve signatures indicative of derivation from exotic crustal domains juxtaposed during ~2690–2670 Ma amalgamation of the southern Superior Province. Since depleted compositions are characteristic of Neoarchean and Mesoarchean zircon groups in the southern Superior Province, and sources include local and distal domains that were likely separated by many 100s of kilometers prior to amalgamation, it is inferred that a depleted upper mantle reservoir was not only well-established, but prevalent in the mantle below each of these areas during their construction. Based on the predominant Hf isotope signatures in the detrital zircon results and predicted isotopic trends produced by probable geodynamic mechanisms, crustal growth by direct differentiation from a depleted mantle reservoir is likely to have been moderated by subduction-accretion processes.

Middle Triassic volcanic rocks from the Gangdese belt, southern Tibet: petrogenesis and implications for Tethys tectonic evolution

ABSTRACT The voluminous Middle Triassic to Early Jurassic magmatic rocks in the southern Gangdese belt of Tibet have received wide attention in tracing the subduction history of the Yarlung-Zangbo Neo-Tethys Ocean (YZNTO) and Bangong-Nujiang Tethys Ocean (BNTO). Here we report a suite of volcanic rocks from the Gangdese arc belt and present results from petrological, geochemical and zircon U-Pb-Lu-Hf isotopic studies with a view to constrain their timing and magma source in relation to the Tethys tectonic evolution. Zircon U-Pb data show ages in the range of 237.2–235.0 Ma for the andesitic rocks, representing their eruption ages during the Middle Triassic. Zircon Lu-Hf data display markedly positive εHf(t) values of 11.44–15.99 with the two-stage model ages (TDM C) of 535–241 Ma, suggesting that the magma was derived from depleted mantle or juvenile crust components. Geochemically, the andesitic rocks show low Sr/Y (20.26–43.74) and La/Yb ratios (8.52–14.21) in the absence of Eu anomalies, enrichment in LILEs (K, U, Rb, Ba) and LREEs, and depletion of HFSEs (Nb, Ta, Ti, Zr) and HREEs, suggesting normal arc andesite with emplacement in a continental island arc setting. Accordingly, we infer that these rocks were formed by partial melting of a young oceanic crust and sediments in the transitional zone between plagioclase and garnet stability within subduction-related setting, followed by fractional crystallization of amphibole and apatite. In conjunction with previous studies on the Tethys tectonic evolution, we argue that the southern margin of the Gangdese belt during the Middle Triassic might be unrelated with the northward subduction of the YZNTO and southward subduction of the BNTO. Our results suggest that the Middle Triassic magmatic suites in the southern margin of the south Gangdese belt were generated through the southward subduction of the Zhikong-Sumdo Tethys Ocean.

Petrological and Lu–Hf age constraints for eclogitic rocks from the Pam Peninsula, New Caledonia

We report new petrological and geochronological data for eclogitic rocks of the Pouébo Eclogite Mélange (PEM) in the New Caledonian high-pressure/low-temperature (HP/LT) belt. Pseudosections were calculated to link newly determined Lu–Hf dates with P-T conditions corresponding to garnet growth, which started on the prograde path and continued into the eclogite facies to approximately 560 °C and 2.05 GPa, coinciding with omphacite formation. The Lu–Hf garnet dates of 38.27 ± 0.14, 36.5 ± 0.7, and 38.42 ± 0.15 Ma are weighted towards the early part of the garnet growth interval, and thus represent maximum ages of eclogite-facies conditions. These dates are indistinguishable from those reported earlier from Ar–Ar in white mica, which had been interpreted as cooling ages, and they are about 6 Myr younger than those from U–Pb in metamorphic zircon, which had been previously considered to date peak eclogite-facies metamorphism at 44 Ma. The new Lu–Hf dates overlap with Rb–Sr white mica ages of nearby metasomatic veins related to the blueschist-to-eclogite transition. The Lu–Hf and Rb–Sr data imply that eclogite-facies metamorphism in parts of the PEM lasted at least until 38 Ma and is thus younger than previously suggested. This prograde, near-peak event cannot be reconciled with concurrent (41–36 Ma) cooling previously inferred from the Ar–Ar dates. We conclude that both the 38.4–36.5 Ma Lu–Hf and Rb–Sr dates as well as similar Ar–Ar results record near-peak conditions, while the 44 Ma U–Pb zircon rims probably are related to prograde mineral reactions. The HP stage is followed by rapid exhumation and cooling at minimum rates of ~0.7 cm/yr and ~50 °C/Myr.

New geochemical, U–Pb SIMS geochronology and Lu–Hf isotopic data in zircon from Tandilia basement rocks, Río de la Plata craton, Argentina: Evidence of a sanukitoid precursor for some Paleoproterozoic granitoids

In this paper new geochemical, geochronological and isotopic Lu–Hf data from the Siempre Amigos Hills (SA), western Tandilia, Argentina, is presented. Outcrops in this small area include representative lithologies of the basement Río de la Plata craton (RPC) known as Tandilia Terrane. The new geochemical data characterize a heterogeneous Paleoproterozoic crust with a similar U–Pb SIMS magmatic age (~2.1 Ga) represented by a suite of mafic (amphibolites, charnockitic granulites), intermediate and felsic compositions (diorites, granodiorites and granitic gneisses), as well as intrusive granitic rocks (with Neoproterozoic age of ~652 Ma in zircon). Geochemical features suggest processes of magmatic differentiation from a mafic igneous protolith associated with the ‘Camboriu orogeny’ of the Trans-Amazonian–Eburnean cycle. Some amphibolites may represent sanukitoid magmas similar to those formed at the Archean-Proterozoic transition. This paper examines the hypothesis of a sanukitoid precursor for RPC Rhyacian magmatism. The potassic and calc-alkaline character of magmatic series, together with negative Nb–Ta–Ti anomalies and REE patterns, are compatible with classical modern subduction-related magmas. Precise Lu–Hf analyses of zircons collected from a granodiorite and a gneiss indicate a predominantly juvenile source (+εHf), with minor crustal recycling (-εHf). An inheritance age is recorded in a single Neoarchaean zircon core from a granodiorite (2771 ± 15 Ma), similar to calculated maximum Hf TDM (2.8 Ga). Considering the new available data from this region of the RPC, a sustainable petrogenetic model is proposed to explain the presence of geochemical and isotopic evidence of sanukitoid magmatism which may have occurred after 2.7 Ga, which seems to constitute a magmatic series which is a precursor to the development of the Rhyacian magmatic arc in the Tandilia Terrane. Medium-to high-grade metamorphic conditions probably resulted from Paleoproterozoic subduction followed by the intrusion of huge volumes of Rhyacian calc-alkaline magmatism. This model is compatible with RPC configuration as a collage of terranes, and also consistent with Trans-Amazonian–Eburnean amalgamation of various microcontinents in the formation of Atlantica.

Age and deposition of the Fort Crittenden Formation: A window into Late Cretaceous Laramide and Cenozoic tectonics in southeastern Arizona

AbstractThe Upper Cretaceous Fort Crittenden Formation exposed in the Santa Rita and Huachuca Mountains of southeastern Arizona is a syntectonic deposit that has been associated with Laramide tectonic activity. However, the spatio-temporal relationships among Cretaceous sedimentation, magmatism, basement exhumation, and possible flat slab-related processes in the southern Laramide region remain poorly understood. Age controls for uplift and erosion of local topography and syntectonic deposition in response to deformation remain particularly poor. The Fort Crittenden Formation comprises 800–2500 m of locally derived fluvial to alluvial fan sedimentary rocks and records paleodrainage reorganization in response to active tectonics. Changes in sedimentary facies, provenance, and paleoflow suggest deposition in a tectonically partitioned intraforeland basin. New detrital zircon data constrain the timing of deposition of the Fort Crittenden Formation between ca. 86 Ma and ca. 76 Ma. The lack of depositional age zircons throughout the majority of the Fort Crittenden Formation is consistent with a magmatic lull in the Cordilleran arc between ca. 90 Ma and ca. 76 Ma. The overlying Salero Formation and Late Cretaceous intrusions are expressions of renewed magmatism in southeastern Arizona at ca. 75 Ma. New Lu-Hf data indicate that magmas evolved from contamination of old juvenile crust. When interpreted in conjunction with other regional data sets, our study indicates that the Laramide deformation front migrated eastward into southwestern New Mexico by 75 Ma. Thermal modeling of apatite fission track and (U-Th)/He data from granitic clasts are consistent with Late Cretaceous–Paleocene (ca. 76–55 Ma) heating related to magmatism and cooling and exhumation during the Eocene and Oligocene.

Mesozoic crustal melting and metamorphism in the U.S. Cordilleran hinterland: Insights from the Sawtooth metamorphic complex, central Idaho

In this study, we present whole-rock geochemistry and Sm-Nd data; zircon trace element, U-Pb, and Lu-Hf data; titanite U-Pb dating; and structural analysis of igneous and metasedimentary rocks of the Sawtooth metamorphic complex that provide insight into regional metamorphism, partial melting, and crustal thickening in the Idaho batholith segment of the Cordilleran orogen. Four magmatic events are revealed: (1) pre-tectonic felsic magmatism at ca. 156 Ma, (2) syn-tectonic mafic and felsic magmatism between ca. 100 Ma and ca. 92 Ma, (3) felsic magmatism concurrent with late-stage deformation at ca. 89−84 Ma, and (4) post-tectonic felsic magmatism at ca. 77 Ma. The multiple generations of felsic magmatism include a variety of sedimentary- and igneous-derived granitoids distinguished by zircon trace element compositions (e.g., U/Ce versus Th and Ce/Sm versus Yb/Gd) and were sourced from progressively more evolved crustal components as shown by Lu-Hf and Sm-Nd isotopic data. U-Pb data of metamorphic zircons and titanites from high-grade metasedimentary rocks suggest that regional metamorphism occurred from ca. 100−93 Ma, which was characterized by granulite-facies partial melting and concurrent growth of metamorphic zircons and garnets. The episodic magmatism in the Sawtooth metamorphic complex records pervasive melt migration in a hot, mid-crustal setting at ca. 100‒92 Ma and additional magma ascent in a cool, upper-crustal setting at ca. 77 Ma. The uplift of the Sawtooth metamorphic complex from mid- to upper-crust was likely caused by underthrusting at lower crustal levels coupled with erosion and thinning of the upper crust. This work suggests that the crust of the Cordilleran hinterland in the Idaho batholith region underwent significant thickening from ca. 100‒84 Ma, and a crust of Andean-like thickness was probably achieved by ca. 84 Ma. By ca. 77 Ma, the central Idaho crust started to thin likely due to mid-crustal flow and surface erosion. The new data from the Sawtooth metamorphic complex are consistent with the two major magmatic flare-ups in the Late Jurassic and Late Cretaceous in the U.S. Cordilleran orogen.

Late Paleoproterozoic post-collisional bimodal magmatism in the North China Craton: Insights from the Miyun gabbro-granite suite

Following the final cratonization of the North China Craton (NCC) through the assembly of major crustal blocks, the craton witnessed post-collisional magmatism during late Paleoproterozoic with the formation of diverse types of magmatic suites in the western and eastern margins of the Trans-North China Orogen (TNCO). Here, we investigate the anorogenic granitoid and associated gabbro from the Miyun complex located at the eastern margin of the TNCO. We present petrology, geochemistry, and zircon U-Pb, and Lu-Hf data on these rocks. The granite pluton is composed of a rapakivi phase with almost totally digested mafic magmatic enclaves, whereas the granite phase hosts gabbroic enclaves. The granitoids show high SiO2 content (71.28 to 78.83 wt%), with peraluminous A2 type affinity and within plate (rift-related) features. The magmatic zircon grains from the granite yielded 207Pb/206Pb weighted mean age of 1696 Ma. A similar crystallization age of 1683 Ma for the gabbro indicates coeval bimodal magmatism. Both rocks carry zircon xenocrysts with older ages that are consistent with the magmatic and metamorphic events recorded from the basement rocks. The εHf(t) values of the zircon grains from the granitoid are markedly negative (−7.9 to −5.0) with two-stage Hf crustal model ages (TC DM) ranging from 2688 Ma to 2855 Ma, indicating the involvement of Mesoarchean to Neoarchean crustal components in the magma source. In contrast, the zircon grains from gabbro show both positive and negative εHf(t) values, suggesting magma derivation from depleted mantle sources with minor juvenile crustal component. We envisage that lower crustal melting induced by mantle-derived mafic magmas during post-collisional rifting generated felsic magmas, into which the mafic magma was injected, resulting in magma mixing and mingling. In conjunction with the information from post-collisional magmatic suites in the adjacent locations including gabbro-anorthosite complex, and similar aged magmatic complexes within the TNCO, we propose a major post-collisional magmatic event in the NCC which involved extensive mantle upwelling and lower crustal melting.

Lu-Hf Isotopic Data of the Mb&amp;#233;-Sassa-Mbersi Tonalite (Central Cameroon Domain): Indicator of ca. 1.0 Ga Juvenile Tonian Magmatism in the Region

The Pan-African/Brasiliano orogenic belts are a part of numerous Neoproterozoic orogenic belts that belong to the long-lived orgenic cycle that distancing phase started at the Tonian around 1.0 Ga. The Tonian magmatism fairly documented in the Neoproterozoic belts of Borborema Province (NE-Brazil), seemed so far inexistent in the Central African Orogenic Belt (CAOB) although these two belts were geologically correlated. Through the Lu-Hf geochronological analysis on zircon of tonalite, the present work, coupled with the previous data, suggests the existence of a Tonian age magmatism in the Central Cameroon Domain of the CAOB although the latter is much reworked. The Nguesseck tonalite outcrops in the northern part of the Mbe-Sassa-Mbersi region, in the northern edge of Central Cameroon Domain of the CAOB and in the Tchollire-Banyo shear zone (TBSZ). The Lu-Hf data obtain on the zircon grains of this tonalite reveal juvenile HfTDM age of ca. 1.0 Ga. This age, combined to the previous geochemical data suggests that the protholites of this tonalite would have extracted from the source during the distancing phase (rifting and oceanization) of the Pan-african/Brasiliano orogeny at the early Neoproterozoic.