Rock Sm-Nd Research Articles

Tracking subduction-related metasomatism of the subcontinental lithospheric mantle using Ca-, O-, and H-isotopes

Mantle xenoliths provide effective records of the metasomatic processes that affect continental lithosphere evolution, such as interaction with subducted components or modification via small-degree melts. Correlations between major/trace element geochemistry with stable and radiogenic isotope compositions can help constrain the source and timing of this metasomatism. We report new δ18O, δ44/40Ca, and δD values for twelve kimberlite-hosted mantle xenoliths from the Slave Craton (NWT, Canada), which show varying degrees of metasomatism. The δ18O values of olivine (δ18Ool = +5.33 ± 0.13‰; 1σ; n = 12) overlap average mantle values. Clinopyroxene and garnet δ18O values (δ18Ocpx = +5.31 ± 0.10‰; δ18Ogrt = +5.37 ± 0.23‰; 1σ) extend below those reported in most mantle peridotites and are strongly correlated with clinopyroxene δ44/40Ca (avg. = +1.00 ± 0.10‰; 1σ) and garnet δ44/40Ca (avg. = +1.18 ± 0.19‰; 1σ) respectively, extending from typical mantle values to low δ18O and high δ44/40Ca values. In general, Δ18Ocpx-ol and Δ18Ogrt-ol (ranging from −0.19‰ to +0.19‰ and from −0.56‰ to +0.35‰, respectively) are lower than expected equilibrium values at mantle temperatures. Strong negative correlations are found between δ18Ogrt and Δ18Ogrt-ol and garnet major and trace element composition (Na2O, H2O, La/YbN). Furthermore, phlogopite-bearing kelyphitic rims have δD values (avg. = −126 ± 13‰; 1σ) lower than typical mantle values. Whole rock Sm-Nd model ages and oxygen isotope diffusion modeling suggest that metasomatism occurred during the Mesozoic, shortly before kimberlite entrainment, consistent with indications from diamond-forming fluids from the Slave craton. The combined low δ18O, δD, and high δ44/40Ca signature of the mantle peridotite xenoliths, along with the age constraints, suggest the metasomatic fluid/melt is sourced from a recycled oceanic crust component related to Mesozoic subduction in western North America.

Magmatic evolution of the Schiel Alkaline Complex, Bushveld large igneous province, South Africa

The Paleoproterozoic Schiel Alkaline Complex (SAC) in the Limpopo Belt is a composite pluton that coevally formed with the layered intrusion of the Bushveld large igneous province. The SAC is mainly composed of alkaline felsic rocks, pyroxenite, gabbro, syenodiorite/−gabbro, and minor carbonatite and gabbroic dikes. Here we present a new geochemical, geochronological, and isotopic study of the felsic and (ultra)mafic rock suites of the western SAC. The whole rock SmNd isotope study of the felsic rocks revealed εNd2059 Ma values between −6.5 and − 3.8, while those of the syenodiorite/−gabbros and gabbros vary from −7.9 to −8.8, and − 4.3 to +1.1, respectively. The pyroxenites have εNd2059 Ma values of +7.4 to +2.7. The LuHf isotope study displays εHf2059 Ma values of −15.9 to −9.0 for the syenites, alkali granites and syenodiorite/−gabbros, and significantly higher εHf2059 Ma of −3.1 to +3.4 for the gabbros. LA-SF-ICPMS UPb zircon dating yields 2079 ± 10 Ma and 2088 ± 30 Ma for the syenodiorite/−gabbros, and 2086 ± 51 Ma and 2107 ± 30 Ma for the gabbros. In conjunction with published data from the eastern SAC, we propose a close petrogenetic link between the mafic and felsic rocks of the SAC. The positive εNd and εHf values of the pyroxenite and gabbro favor a depleted mantle source, while variable εNd and εHf values, and trace element ratios of the gabbros are ascribed to crustal contamination during magma ascent. The felsic rocks, however, mainly formed by fractional crystallization from the gabbros as revealed by major and trace element correlation trends, isotopic data and thermodynamic modelling.

The Khida terrane – Isotopic evidence for Paleoproterozoic to Neoarchean basement in the eastern Arabian Shield

Pre-Cryogenian crystalline rocks occur only in the eastern part of the Arabian Shield, around Jabal Muhayil in the Khida sub-terrane. These occurrences have been mapped and investigated using U-Pb zircon geochronology, bulk rock Sm-Nd and feldspar Pb isotopes. The previously unrecognized ca. 1.87 Ga Libab gneiss complex contains granitic rocks with inherited zircon up to ca. 2.5 Ga and has both Nd and Pb isotopic evidence for a Neoarchean precursor. The ca. 1.67 Su’ayra alkali feldspar granite similarly requires a Neoarchean precursor but lacks older zircon inheritance. The areally and topographically dominant Muhayil anorthosite cannot precisely be dated but has a minimum age of ca. 750 Ma based on a cross cutting gabbro and granodiorite, the latter also having ca. 1.7 Ga zircon cores. Highly retarded Pb isotopes in the anorthosite require long-term residence in a near-zero U/Pb reservoir, considered most consistent with a ca. 1.7 Ga emplacement age, though a previously suggested genetic link with the Su’ayra granite remains entirely speculative. The consistently ancient isotopic signature that occurs in the younger (850 – 750 Ma) studied samples suggest that these were influenced by Neoarchean continental crust during emplacement of the Siham magmatic arc, which developed along the southern margin of the Afif composite terrane prior to its amalgamation with the Asuir terrane to its west. Correlation with terranes in Yemen showing similar ages and isotopic character suggest a contiguous crustal block separate from the Azania microcontinent that was juxtaposed along the northward extension of the ophiolite-decorated Bayhan suture during Cryogenian closure of the Mozambique Ocean in the northern part of the East African Orogen. Whether the Neoarchean continental crust rifted off of Azania and then re-amalgamated or forms an entirely separate crustal domain remains to be tested.

Reworking of aged mafic crust in a Palaeoarchaean layered complex inferred from coupled Sm-Nd and Lu-Hf isotope systematics, Stolzburg Complex, Barberton Greenstone Belt, South Africa

The Stolzburg Complex is a prominent and well-preserved Palaeo- to Mesoarchaean layered ultramafic-mafic complex in the vicinity of the Barberton Greenstone Belt. Whole rock Sm-Nd, Lu-Hf and 142Nd isotopic data obtained from a variety of lithologies, and augmented by major and trace element geochemistry, are presented to examine petrogenesis, magma source composition, and mantle differentiation as well as the geodynamic setting of emplacement. Geochemically, all samples are characterized by unfractionated normalized trace element patterns (La/Luchond. = 0.7–2.3, La/Gdchond. = 0.6–1.7, Sm/Luchond. = 0.9–1.4) associated with weak HFSE anomalies (Nb/[0.5Th + 0.5La]PRIMA = 0.5–2.3). Trace element concentrations vary as a function of mineralogy, but differences in trace element compositions are unsystematic across lithologies. The coherence of trace element characteristics is consistent with a common mantle source that underwent moderately large degrees of melting at mantle pressures within the spinel stability field. Whole rock Sm-Nd and Lu-Hf data yield well-defined apparent isochrons corresponding to ages of 3367 ± 62 (n = 12, MSWD = 1.5) and 3396 ± 36 (n = 11, MSWD = 2.1), respectively. Initial Hf and Nd isotopic compositions vary from +3.3 to +5.7 and + 0.9 to +1.7, respectively, indicative of derivation from slightly to moderately depleted mantle source(s). Isochron ages and a regressed initial Hf isotopic composition of +4.0 ± 0.9 (at 3.4 Ga) contrast with an established U-Pb emplacement age (zircon, titanite) of 3.25 Ga and εHf values for zircon of −2.5 to +3.0. Hydrothermal alteration or weathering, incorporation of evolved pre-existing (continental) crust and fractionation by high-pressure phases prior to melt extraction can all be ruled out to have significantly affected isotope systematics. Protracted magma chamber activity and non-synchronous emplacement of unrelated gabbroic magmas also fail to fully account for the discrepancy between whole rock and mineral age and isotopic data. Instead, the coherence in apparent isochron ages and variability of initial Hf and Nd isotopic composition is adequately explained by reworking (partial or near-complete remelting) of aged mafic-ultramafic (likely oceanic) crust in staging chambers with compositions indistinguishable from newly formed magmas. Thus, apparent isochrons represent mixing lines and do not record timing of crystallization. A subset of samples was also analyzed for 142Nd isotopic compositions to trace preserved mantle heterogeneities caused by early Archaean crust-mantle differentiation. The results (μ142Nd = −2.3 ± 2.2 to +1.7 ± 2.1) are not resolvable from the modern mantle value, indicating that 142Nd-enriched or depleted mantle reservoirs had not remained isolated from convective homogenization in the asthenosphere, or the volume of mantle underlying the layered complex was too small to sample mantle heterogeneities. The Stolzburg Complex was possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous Ca-metasomatism as documented by exposed rodingites.

Constraints on Paleoproterozoic crustal growth from Birimian Supergroup lavas of the Bui belt (Ghana) in the West African Craton

The volcano-metasedimentary Bui belt in Ghana is one of the lesser known Birimian belts of the West African Craton that provide constraints on Paleoproterozoic crustal growth. This study presents the first major, trace element and Nd-Hf isotope data for the mafic suite of the Bui belt to constrain their petrogenesis and geodynamic setting. Although alteration is prevalent, the volcanic, volcaniclastic and plutonic rocks show tholeiitic signatures, with Mg# 35–65. Chondrite-normalised REE patterns are mainly flat, with slight LREE enrichment or depletion, and multi-element diagrams show Th and LREE depletion relative to other immobile incompatible trace elements. These features suggest derivation from a moderately depleted mantle source. Whole rock Sm-Nd isotope data yields an errorchron at ca. 2.2 Ga, with an initial εNd of +3.4 but the Lu-Hf isotopic system appears to be disturbed. The lack of arc related geochemical signatures (absence of negative Nb-Ta and Ti peaks, a nearly flat to depleted REE pattern) in the Bui mafic suite, together with the volcanic-sedimentary rock association, point towards magma generation in an oceanic plateau or divergent setting similar to a mid-ocean ridge, contrasting with the subduction setting that has been proposed for most other instances of Birimian magmatism in the West African Craton.

Osmium isotopes in peridotite xenoliths reveal major mid-Proterozoic lithosphere formation under the Transantarctic Mountains

Osmium isotopes, whole rock and mineral geochemical data from peridotite xenoliths from two Miocene McMurdo Volcanic Group cinder cones in the Transantarctic Mountains (TAM) in Southern Victoria Land, Antarctica, reveal that the underlying mantle preserves evidence for major mid-Proterozoic lithosphere formation despite the crust being dominated by late Neoproterozoic-Ordovician (~0.65–0.47 Ga) rocks. The Hooper Crags xenolith suite is dominated by harzburgites with highly refractory olivine Mg# (up to 92.3) and depleted bulk rock major and platinum group element + Re abundances, with 187Os/188Os ratios indicating depletion in the mid-Proterozoic. Pipecleaner Glacier xenoliths, 18 km distant, are lherzolites with olivine Mg# (<91) and fertile major and platinum group element abundances, with Os isotope abundances defining an aluminochron that also indicates mid-Proterozoic depletion. Although exposed crust along this portion of Antarctica reveals only minor evidence for Proterozoic magmatism, the major episode of lithosphere formation indicated by the Os isotope data is supported by published bulk rock Sm-Nd isotope and zircon εHf mantle model ages of Neoproterozoic to Ordovician plutonic rocks. The heterogeneous circum-cratonic mid-Proterozoic mantle under Southern Victoria Land has therefore persisted on a Ga timescale, including through the formation and destruction of Rodinia and Gondwana supercontinents as well as extensive crustal melting and emplacement of the Ferrar large igneous province. This longevity may be due to the thick (>250 km) East Antarctic Craton lithosphere shielding the immediately adjacent circum-cratonic mantle from being affected by convective asthenosphere-driven erosion. This contrasts with mantle lithosphere accreted distally to the East Antarctic Craton (represented by the now-detached Zealandia continent), which did not attain extreme thickness and has therefore been more susceptible to tectonic reworking and lateral translation.

Palaeoproterozoic reworking of early Archaean lithospheric blocks: Rocks and zircon records from charnockitoids in Volgo-Uralia

The Volgo-Uralia segment, which constitutes one fourth of the East European Craton, is covered by sedimentary deposits. From geophysical studies and examination of thousands of drillcores, Volgo-Uralia has been recognised as a vast high-grade terrain with a complex crustal history extending from the Palaeoarchaean to the Palaeoproterozoic. Our recent studies are focused on the search for the oldest crust formation event by extracting whole rock Sm-Nd and zircon U-Th-Pb and Lu-Hf isotope information from samples recovered by drilling in southern Volgo-Uralia. Particular attention is devoted to the Kolyvan charnockitoid rock suite, which makes up several large areas of gneisses and granitoids of enderbite, charnockite and tonalite composition.The zircon from the granitoids show complex internal structures and consists of large magmatic cores with oscillatory zoning, surrounded by CL black-and-bright bands of metamorphic rims. The crystallisation age of the cores is defined as 3140 ± 7 Ma (SHRIMP) and 3127 ± 46 Ma (LA-ICPMS), while the CL-bright rims are dated at 1950 ± 25 Ma (LA-ICPMS). The ingressive recrystallisation of primary magmatic zircon correlates with depletion in REE, which is observed in each studied core-rim pair. No differences in O-isotopic compositions have been detected between the cores and the rims. δO18 values with an average of 5.8 ± 0.3‰ (1SD) implying that no supracrustal rocks were involved in the source of the Kolyvan melts. The Hf-isotope compositions of magmatic cores (−3 to −9 εHfT) and metamorphic rims (−14 to −28 εHfT), and their similar crustal model ages from 3.42 to 3.86 Ga indicate Eo- to Palaeoarchaean crustal sources for the charnockitic magmas. Sm-Nd model ages of ca 3.46 Ga for the Kolyvan rocks are consistent with the zircon Hf-isotope data and indicate a long crustal prehistory of a source of the Mesoarchaean magmas.We conclude that the Mesoarchaean Kolyvan suite rocks was formed by reworking of Eo- to Palaeoarchaean lithosphere, which probably had been widespread throughout Volgo-Uralia. The obtained geochemical and isotope data can be reconciled in a model of deep mantle-plume activity at 3.1 Ga causing mantle underplating, extension of the Palaeoarchaean crust and high-T magmatism.

The formation and evolution of the Moon’s crust inferred from the Sm-Nd isotopic systematics of highlands rocks

Ages determined for magnesian and ferroan anorthosite crustal rock suites overlap, suggesting they formed contemporaneously about 4.3–4.5 Ga. A notable exception is the Sm-Nd age previously determined on Mg-suite gabbronorite 67667 which is at least 100 Ma younger than the youngest ferroan anorthosite. New chronologic measurements of 67667 presented here yield concordant Sm-Nd and Rb-Sr mineral isochron ages of 4349 ± 31 Ma and 4368 ± 67 Ma, suggesting the sample is older than previous estimates. Furthermore, a whole rock Sm-Nd isochron of Mg-suite rocks from the Apollo 14, 15, 16, and 17 landing sites yields an age of 4348 ± 25 Ma, indicating that Mg-suite magmatism was widespread and roughly contemporaneous on the lunar nearside. Analysis of Sm-Nd internal isochron ages confirms that Mg-suite magmatism was restricted to a period between about 4.33 and 4.35 Ga at the Apollo 14, 15, 16, and 17 landing sites and was synchronous with magmatism at the Apollo 16 site associated with the ferroan anorthosite suite between 4.35 and 4.37 Ga. Magnesian- and ferroan anorthosite suite rocks with ages younger than ∼4.33 Ga appear to have experienced slow cooling in the deep lunar interior, so that the ages record when the samples cooled below the closure temperature of the Sm-Nd isotopic system and not the time they crystallized.The ages determined for Mg-suite and ferroan anorthosite suite rocks are concordant with the age determined for the formation of urKREEP of 4350 ± 34 Ma using the Sm-Nd isotopic systematics of 67667 and measurements completed on norite 78238, troctolite 76535, KREEP basalt 15386, and gabbronorite NWA 773. Crystallization ages of Mg-suite and FAS are also concordant with the average of 146Sm-142Nd ages previously determined for the formation of the mare basalt source region of 4333 ± 30 Ma. The similarity of ages for Mg-suite magmatism, ferroan anorthosite suite magmatism, urKREEP formation, and formation of the mare basalt source regions implies the processes that produced these rocks were petrogenetically linked. It also implies that both early-stage and late-stage lunar magma ocean cumulates formed over a relatively short duration of <40 Ma. Late and somewhat rapid solidification of a lunar magma ocean can account for the concordance of ferroan anorthosite suite rocks, urKREEP, and the mare basalt source regions. However, the major and trace element compositions of Mg-suite magmas preclude them from being a primary differentiation product of the lunar magma ocean. Instead, the Mg-suite could be produced as a result of mixing of magma ocean solidification products during density driven overturn occurring immediately after, or perhaps during, solidification of the lunar magma ocean. This scenario not only accounts for the chronology of the various rock suites, but is consistent with the petrogenesis of the Mg-suite that involves the interaction between pre-existing Mg-rich, plagioclase-rich, and urKREEP-rich cumulates of the magma ocean.

Petrology, geochemistry and Sm-Nd systematics of the Paleoproterozoic Itaguara retroeclogite from São Francisco/Congo Craton: One of the oldest records of the modern-style plate tectonics

Paleoproterozoic retrogressed eclogite (retroeclogite) occurs in the Itaguara Sequence included in the suture zone formed by collision between the Archean Divinópolis and Campo Belo/Bonfim Complexes in the southern São Francisco Craton, which represents the South American counterpart of the African Congo Craton. The Itaguara retroeclogite contains scarce omphacite and phengite but abundant garnet porphyroblasts embedded in a fine-grained, amphibole, biotite and quartz-bearing matrix. The 2.20 ± 0.05 Ga eclogitization event (garnet and whole rock Sm-Nd isochronic age) of the E-MORB protolith (TDM ~ 2.47 Ga) is recorded by omphacite formation during high-pressure prograde stage in amphibole eclogite facies due to ~70 km depth subduction process. Amphibole eclogite facies metamorphic peak stage of 17–20 kbar and 600–700 °C occurred during ~2.1 Ga continental collision. Tectonic exhumation-related decompression during collision probably triggered partial melting of the eclogitic rock. Finally, decompression late stage estimated between 5 and 8 kbar and 550–650 °C under amphibolite facies overprint during orogenic collapse was responsible for appearance of kelyphitic reaction rims (symplectite) around garnet crystals. As its Paleoproterozoic contemporary analogues from Congo Craton, the Itaguara retroeclogite is one of the oldest records of the modern-style plate tectonics.

LA–ICP–MS zircon U[sbnd]Pb dating, Lu[sbnd]Hf, Sm[sbnd]Nd geochronology and tectonic setting of the Mesoarchean mafic and felsic magmatic rocks in the Sangmelima granite-greenstone terrane, Ntem Complex (South Cameroon)

The Ntem Complex represents the northwestern margin of Congo Craton in southern Cameroon. It consists mainly of charnockites, TTG suites, potassic grantitoids, minor supracrustal and mafic rocks. The latter include gabbronorite enclave or sill, gabbro and dolerite dykes. The metagabbro are basaltic to andesitic in composition and show geochemical features of low-La Archean tholeiitic basalts, and LREE-depleted (LaN/SmN < 1) akin to modern N-MORB. The doleritic dykes show high TiO2/P2O5 (7.50–11.47) and (La/Yb)N ratios, and low NbTa values similar to modern intraplate continental basalts and intermediate-La Archean tholeiitic basalt, least affected by contamination from granitic continental crust. The partial melting of low-to high-K mafic rocks of oceanic and continental arc affinity generated the charnockites and low−/medium-pressure TTGs. The potassic granites were formed by partial melting of charnockite or TTG, and inherited many geochemical features of their sources. New LA–ICP–MS UPb zircon dates suggest that the charnockites and TTG suites formed at ~3155–2850 Ma. The gabbro intrusion was dated at 2866 ± 6 Ma. Migmatisation of TTG-gneiss was coeval whit the regional high–grade metamorphism event at 2843 ± 7 Ma. The potassic granites were probably crystallized at ~2758 Ma. The garnet-whole rock SmNd age of 2744 ± 31 Ma may represent the age of the second anatexis event coeval with D2 deformation. Literature data indicate the disruption or open isotopic systems in the Ntem Complex around 2500–2600 Ma.

Chafalote Metamorphic Suite (Uruguay): Reflections on the evolution of the Punta del Este Terrane

The Chafalote Metamorphic Suite, located in the Uruguayan sector of the Punta del Este Terrane, southeasternmost Dom Feliciano Belt, comprises semipelitic, migmatitic metapelites, mafic granulites, and calc-silicate/amphibolitic gneisses. These supracrustal rocks occur as roof pendants in a granodiorite belonging to the Cerro Olivo Complex. The main structure of the Chafalote Metamorphic Suite is a composite S0/S1 banding, related to the main deformation event (D1) developed under upper-amphibolite/granulite facies conditions. This work presents an integrated study on the latter rocks, involving different geochronological methods (monazite U-Pb LA-ICP-MS and zircon U-Pb SHRIMP) and whole rock Sm-Nd isotope data in order to better understand the metamorphic evolution of the Chafalote Metamorphic Suite during the amalgamation of the SW Gondwana paleocontinent in the Neoproterozoic. The studied rocks record a peak metamorphic event (M2) occurred at around 660 Ma (monazite U-Pb ages), which was followed by decompression and partial melting (M3 event) at around 640 Ma (zircon U-Pb ages), probably during the late stages of the D1 event. The retrograde methamorphic event (M4) is related to the transition between the D1 and the development of NE-SW striking, low-grade sinistral shear zones (D2 event) and intrusion of syn-to post-orogenic Brasiliano granitoids of the Aiguá Batholith at around 600 Ma. The metamorphic conditions, together with geochronological and isotopic similarities, allow the correlation between the studied high-grade metasedimentary rocks and those from the Paso del Dragón Complex in Uruguay, and from the Várzea do Capivarita, Arroio Grande, and Telho Complex in Brazil. The latter rocks possibly share similar source areas and represent different portions of the Neoproterozoic fore-arc and/or back-arc basins developed in the Dom Feliciano Belt during the Brasiliano/Pan-African orogenic cycle.

Twenty million years of post-orogenic fluid production and hydrothermal mineralization across the external Araçuaí orogen and adjacent São Francisco craton, SE Brazil

The Araçuaí orogen (AO) is a hot orogenic system that is known for its widespread and long-lived magmatism (630–480 Ma). Within the last c. 20 million years of its evolution, the AO saw the widespread development of hydrothermal systems in its lower-grade external fold-and-thrust belt and its Foreland Domain (eastern margin of the adjoining São Francisco craton). UPb crystallization ages for hydrothermal monazite, rutile, and xenotime indicate that fluid circulation along the entire AO occurred between 515 to ca. 495 Ma, largely overlapping with UPb ages of hydrothermal systems and mineralized zones in the adjacent São Francisco craton. Titanium-in-quartz thermometry suggests that most hydrothermal systems closed at temperatures of ca. 360 °C. The SmNd isotope composition of the vein monazite samples ranges from εNd500Ma −16.8 to −17.8, and one less-evolved sample with εNd500Ma of −5.9. The SmNd isotope compositions overlap with the bulk rock SmNd data for the Espinhaço Supergroup, suggesting that the hydrothermal veins in this study are mainly related to metamorphic fluids derived from the prograde dehydration of low-grade metasedimentary rocks. The UPb ages, SmNd isotope composition, and vein mineralogy are all consistent with a 20 My period of channelized fluid flow, characterized by an oxidizing, low pH, REE-, P-, and Ti-bearing aqueous metamorphic fluid. The expression of supercritical fluid flow in the core of the AO is given by the emplacement of large volumes of bare/mineralized pegmatite bodies and regional resetting of monazite ages in host rocks. On a regional context, this widespread fluid flow is associated with a high-T, low-P metamorphism that marks the beginning of the post-collisional decompression of the orogeny, which is associated with lithospheric mantle delamination. The reorganization of mass and heat, due to extensional collapse, resulted in multi-sourced fluid production and mineralization from deep to shallow crustal levels that affected an area of over 400.000 km2 along the eastern border of the São Francisco Craton.

Multi-proxy isotopic tracing of magmatic sources and crustal recycling in the Palaeozoic to Early Jurassic active margin of North-Western Gondwana

We trace source variations of active margin granitoids which crystallised intermittently over ~300 Ma in varying kinematic regimes, by combining zircon Lu-Hf isotopic data from Early Palaeozoic to Early Jurassic igneous and metaigneous rocks in the Mérida Andes, Venezuela and the Santander Massif, Colombia, with new whole rock Rb/Sr and Sm-Nd isotopic data, and quartz O isotopic data. These new data are unique in South America because they were obtained from discrete magmatic and metamorphic zircon populations, providing a high temporal resolution dataset, and compare several isotopic systems on the same samples. Collectively, these data provide valuable insight into the evolution of the isotopic structure of the continental crust in long-lived active margins.Phanerozoic active margin-related granitoids in the Mérida Andes and the Santander Massif yield zircon Lu-Hf model ages ranging between 0.77 Ga and 1.57 Ga which clearly define temporal trends that can be correlated with changes in tectonic regimes. The oldest Lu-Hf model ages of >1.3 Ga are restricted to granitoids which formed during Barrovian metamorphism and crustal thickening between ~499 Ma and ~473 Ma. These granitoids yield high initial 87Sr/86Sr ratios, suggesting that evolved, Rb-rich middle to upper crust was the major source of melt. Granitoids and rhyolites which crystallised during subsequent extension between ~472 Ma and ~452 Ma yield younger Lu-Hf model ages of 0.80 Ga–1.3 Ga and low initial 87Sr/86Sr ratios, suggesting that they were derived from much more juvenile, Rb-poor sources such as mafic lower crust and mantle-derived melts. The rapid change in magmatic sources at ~472 Ma can be attributed either to reduced crustal assimilation during extension, or a short pulse of crustal growth by addition of juvenile material to the continental crust. Between ~472 Ma and ~196 Ma Lu-Hf model ages remain mostly constant between ~1.0 and ~1.2 Ga. The large scatter and the absence of definite trends in initial 87Sr/86Sr ratios suggest that both mafic, Rb-poor, and evolved Rb-rich sources were important precursors of active margin magmas in Colombia and Venezuela throughout the Palaeozoic to the Early Jurassic.Previous studies have shown that the genesis of arc magmas may be stimulated by heat advection to the crust during the underplating of mantle derived melt, but the absence of permanent younging trends in Lu-Hf model ages from ~472 Ma to ~196 Ma suggests that very little new crust was generated during this period in the studied region. An overwhelming majority of the analysed igneous rocks yield zircon Lu-Hf model ages of >1 Ga which may be accounted for by documented local crustal end members of 1 Ga–1.6 Ga, and do not require contributions from the depleted mantle. Therefore, recycling of ~1 Ga and older crust was a dominant process in the north-western corner of Gondwana between ~472 Ma and ~196 Ma.This study shows that whole rock Sm-Nd and zircon Lu-Hf data can be interpreted similarly regarding the age of the source regions, whereas Rb-Sr and O isotope data from the same rocks yield valuable information regarding the geochemical nature of the source.

Petrology and geochemistry of the Yoro-Yangben Pan-African granitoid intrusion in the archaean Adamawa-Yade crust (Sw-Bafia, Cameroon)

Abstract: South of Bafia (Cameroon), the Pan-African Yoro-Yangben massif is emplaced in the Adamawa-Yade block, an archaean-paleoprotrozoic micro-continent that detached from the Congo craton during the early Neoproterozoic. This emplacement within a micro-continent raises the problematic of its (i) origin either by the melting of a subducted slab and the mantle wedge, or by the melting of the base of a thick crust or both, and (ii) bearing on the evolution of the Pan-African orogeny in Cameroon. Granodiorites, quartz monzonites, quartz diorites and adamellites occur as massive bodies crosscut by dykes of gabbros and granites. The texture varies progressively from granular bands at the core of the massif to foliated texture towards the border. Mineralogical composition is constant in all the rock types (Am+Bt+Pl+Kfs+Qtz+Op+Sph+Zrn+Ap), except the adamellites which are amphibole-free. Chemically, these potassic rocks are either magnesian and metaluminous, or ferroan and peraluminous. Quartz diorites, granodiorites and granites are High-K calc-alkaline whereas quartz monzonites, adamellites and gabbros are shoshonitic. Most incompatible elements (e.g. Nb, Zr) show an erratic variation with increasing silica contents. Granites rare earth elements (REE) patterns are highly fractionated with a positive Eu anomaly (Eu/Eu* = 2.50–2.04) compared to less fractionated REE patterns of adamellites, quartz monzonites, and granodiorites. The negative anomalies in U, Nb, Ta and Ti shown by gabbros, quartz diorites, quartz monzonites and granodiorites spider diagrams as Th, K, Pb, Zr and Hf positive anomalies for the adamellites and granites patterns are consistent with magmas derived from a subduction setting. The low 87Sr/86Sr(600) initial ratio (0.697–0.706) and eNdi values (−0.82706- 1.03707) of the rocks plot in the mantle array. However, a wide variation of the 87Sr/86Sr ratio values with constant eNdi suggests an enrichment in radiogenic 87Sr likely from an old crust displaying high Rb/Sr ratio. The variations of the La/Yb, Y/Nb and Zr/Nb ratios preclude the fractional crystallization process. Moreover, these characteristics, combined with the erratic (Zr, Nb) suggest particular magmatic sources and complex fusion processes to the Yoro-Yangben heterogeneous granitoids. The lower range of Ba/Nb values (130–135) and the occurrence of shoshonitic magmas likely point to the melting of an oceanic slab; while the higher range (225–230) indicates a blend of melts from oceanic slab and the associated sediments. Granite compositions are similar to melting liquids of metagreywackes. La/Sm and Sm/Yb ratios in gabbros, quartz monzonites and quartz diorite indicate an origin by a batch melting of phlogopite-amphibole-garnet lherzolite, whereas granodiorites and adamellites parent magmas may have originated from a batch melting of a metasomatized spinel lherzolite. Whole rock Sm-Nd and Rb-Sr results yield a unique age of 600 Ma, assigned to the collision time between the West-African and the Congo cratons. Thus, the Yoro-Yangben intrusion is part of the numerous Pan-African granitoids in Cameroon, South of Chad and East of Nigeria.

The late-Paleoproterozoic I- and A-type granites in Lüliang Complex, North China Craton: New evidence on post-collisional extension of Trans-North China Orogen

The late-Paleoproterozoic granitoids from Lüliang Complex can provide pivotal constraints on the amalgamation process between Eastern and Western blocks of North China Craton along the Trans-North China Orogen. LA-ICP-MS zircon dating gives emplacement ages of 1854 ± 20 Ma for the Huijiazhuang granite, 1830 ± 21 Ma for the Xiyupi granite vein and 1760 ± 20 Ma for the Dacaoping porphyritic granite, respectively. The Huijiazhuang granite and Xiyupi granite dyke have variable SiO2 (66.71–74.31 wt%), high K2O (5.09–6.35 wt%), low P2O5 (0.02–0.16 wt%), Al2O3 (13.92–15.31 wt%), right inclined REE patterns with medium negative Eu anomalies, enrichment in LILE, depletion in HFSE, especially Nb, Ta, consisting to high-K I-type granite in a post-collisional setting. The Sr/Y (7.36–59.95), εNd(t) (−5.7 to −4.1) with TDM (2381 Ma to 2570 Ma) from whole rock Sm-Nd isotope and εHf(t) (−9.6 to 2.3) with TCDM (2360 Ma to 3070 Ma) from zircon Lu-Hf isotope suggest that they are produced by partial melting of slightly thickened Neoarchean-Paleoproterozoic basement materials (including both meta-sedimentary and meta-igneous rocks). The Dacaoping porphyritic granites are characterized by high SiO2 (70.83–74.30 wt%), K2O (4.84–5.60 wt%), FeOT/(FeOT + MgO) (0.86–0.92), “seagull-type” REE pattern with strong negative Eu anomaly (δEu=0.16–0.35) and higher 10000∗Ga/Al (2.99–3.36), HFSE (Zr + Nb + Ce + Y = 378–583 ppm), showing an affinity of A2-type granite. They have low Sr/Y (1.17–8.62), εNd(t) (-6.1 to -6.4) with TDM (2690 Ma to 2776 Ma) from whole rock Sm-Nd isotope and εHf(t) (-7.9 to -5.2) with TCDM (2775–2938 Ma) from zircon Lu-Hf isotope, indicating a result from the melting of thinned Neoarchean calc-alkaline intermediate basement. Taking into account the temporal-spatial distributions of late-Paleoproterozoic rocks in the Trans-North China Orogen, it suggests a post-collisional extension occurred during 1.89–1.76 Ga and the crustis thinnedvisibly since 1.82 Ga.

Field occurrences and Nd isotopic characteristics of the meta-mafic-ultramafic rocks from the Caozhuang Complex, eastern Hebei: Implications for early Archean crustal evolution of the North China Craton

Crust-mantle differentiation and growth of early crust are critical issues in the field of solid earth science and the proper understanding of how Earth evolved during Eoarchean-Paleoarchean greatly relies on these issues. The Caozhuang Complex, one of the oldest rock units in the North China Craton (NCC), is located in eastern Hebei and composed of the Caozhuang supracrustal rocks and the Huangbaiyu orthogneisses. The Caozhuang supracrustal rocks are considered to be the oldest sedimentary sequences in the NCC, into which the Huangbaiyu grey gneisses with ∼3.3 Ga zircon ages intruded. In previous studies, the Caozhuang supracrustal rocks are represented by fuchsite-bearing quartzites with ∼3.8 Ga detrital zircons and metamorphosed mafic rocks (amphibolites) with ∼3.5 Ga single-stage Nd model age relative to depleted-mantle (TDM(Nd)).Here we present our new study on meta-mafic–ultramafic rocks of the Caozhuang supracrustal sequence in accordance with detailed large-scale field mapping, petrology, elemental and Sm-Nd isotopic analyses, and then discuss the possible crust-mantle differentiation events that could take place during Eoarchean and Paleoarchean in the Caozhuang area.The meta-mafic-ultramafic rocks can be subdivided into two groups based on field occurrence. Group One rocks are amphibolites and hornblendites that occur as layers or slabs only in sedimentary sequences, which are typically accompanied by banded iron formations (BIFs). By contrast, Group Two never coexists with BIFs, and they mainly occur as amphibolites and hornblendites lenses or deformed dykes and sills in supracrustal sequences and orthogneisses, with blastophitic textures preserved. Sm-Nd isotopic analyses show that rocks from Group One have a Sm-Nd isochron age of 3580 ± 98 Ma and εNd(t) of +0.5 to +1.3, corresponding to TDM(Nd) of ∼3.7–3.5 Ga; while Group Two rocks yield a Sm-Nd isochron age of 3179 ± 90 Ma and εNd(t) between +4.6 and +6.6, corresponding to TDM(Nd) of ∼3.3–3.1 Ga. Geochemical characteristics of the two groups, such as moderate Mg#, negative correlation between Fe2O3T and MgO, and positive correlation between Ni and MgO, suggest they both experienced fractional crystallization of ferromagnesian minerals (dominated by olivine and clinopyroxene) during emplacement of magma(s). And combined with the flat HREEs patterns, and N-MORB-like trace element spider diagrams and positive εNd(t) values, it could be indicated that they could be partial melting products of ancient depleted mantle in the spinel stability fields. Based on the consistent whole rock Sm-Nd isochron ages with TDM(Nd), we suggest that two episodes of crustal growth occurred during ∼3.7–3.5 Ga in late Eoarchean and ∼3.3–3.1 Ga in late Paleoarchean in the Caozhuang area. The depleted characteristics of Group One indicate lithospheric mantle in the Caozhuang area might have been depleted before ∼3.7–3.5 Ga. And the Sm-Nd isochron ages of Group One basic volcanic rocks constrained that Caozhuang supracrustal sequence may have deposited during ∼ 3.7–3.5 Ga. Basic dykes or sills with TDM(Nd) of ∼3.3–3.1 Ga and time-equivalent TTG gneisses represent a significant thermal event in late Paleoarchean. The meta-mafic-ultramafic rocks in the Caozhuang Complex are an important window to reveal and understand the early crustal evolution of the NCC, which probably has global significance.

A sedimentary overlap assemblage links Australia to northwestern Laurentia at 1.6 Ga

The Columbia (Nuna) supercontinent existed from approximately 1.9 Ga to 1.3 Ga. Laurentia was part of Columbia, and the western edge of Laurentia (current coordinates) was likely proximal to a large landmass during parts of this interval. Here, we present detrital zircon ages of a Paleoproterozoic sedimentary succession in northern Yukon, Canada, that bear on the evolution of Columbia. The sedimentary succession is preserved as clasts within 1.60 Ga hydrothermal megabreccias. Analyses of detrital zircon reveal abundant 1.78–1.68 Ga zircon with evolved Hf isotope values (−16.1 < εHf(t) < +1.4). Sm-Nd isotope analysis on clasts yields εNdi from −5.3 to −5.5 and model ages from 2.4 to 2.2 Ga. The detrital zircon age distribution is strikingly similar to those from sedimentary megaclasts in the ca. 1.59 Ga Olympic Dam Breccia Complex on the Gawler Craton of Australia. The whole rock Sm-Nd ratios are consistent with derivation from the Gawler Craton. We propose that the sedimentary material contained in both breccia complexes was derived from an overlap assemblage deposited on Australia and Laurentia at ca. 1.6 Ga. This model supports a previous hypothesis that the Gawler Craton was connected to northwestern Laurentia at ca. 1.6 Ga, and that these regions shared a single hydrothermal province that is recognized in northwestern Laurentia as the Wernecke Breccia and in the Gawler Craton as the Olympic Dam Breccia Complex and associated IOCG deposits. The sedimentary overlap succession was deposited after collision between Australia and Laurentia. Australia was subsequently translated southward along the Laurentian margin, placing the Gawler Craton next to southwestern Laurentia and the Mt. Isa Inlier adjacent to northwestern Laurentia by 1.5 Ga.

Assessing the shock state of the lunar highlands: Implications for the petrogenesis and chronology of crustal anorthosites

Our understanding of the formation and evolution of the primary lunar crust is based on geochemical systematics from the lunar ferroan anorthosite (FAN) suite. Recently, much effort has been made to understand this suite’s petrologic history to constrain the timing of crystallisation and to interpret FAN chemical diversity. We investigate the shock histories of lunar anorthosites by combining Optical Microscope (OM) ‘cold’ cathodoluminescence (CL)-imaging and Fourier Transform Infrared (FTIR) spectroscopy analyses. In the first combined study of its kind, this study demonstrates that over ~4.5 Ga of impact processing, plagioclase is on average weakly shocked (<15 GPa) and examples of high shock states (>30 GPa; maskelynite) are uncommon. To investigate how plagioclase trace-element systematics are affected by moderate to weak shock (~5 to 30 GPa) we couple REE+Y abundances with FTIR analyses for FAN clasts from lunar meteorite Northwest Africa (NWA) 2995. We observe weak correlations between plagioclase shock state and some REE+Y systematics (e.g., La/Y and Sm/Nd ratios). This observation could prove significant to our understanding of how crystallisation ages are evaluated (e.g., plagioclase-whole rock Sm-Nd isochrons) and for what trace-elements can be used to differentiate between lunar lithologies and assess magma source compositional differences.

Tracing the fluid evolution of the Kiruna iron oxide apatite deposits using zircon, monazite, and whole rock trace elements and isotopic studies

The ore genesis of the Paleoproterozoic iron oxide apatite deposits in the vicinity of Kiruna in northern Sweden is poorly understood, despite a century-long mining history and 2500Mt of iron ore with grades of 30 to 70wt% Fe produced in the region to date. Zircon grains from the ore, recently dated at ca. 1874Ma, show very different appearances compared to zircon from surrounding host rocks (ca. 1880Ma) and related intrusions (ca. 1880 and ca. 1874Ma), particularly an inclusion-rich rim. In contrast, zircon from the host rocks, and a proximal granite intrusion, exhibit typical igneous growth zoning. Electron microprobe results show near stoichiometric composition for Zr, Si, and Hf in the host rock zircon grains. The ore zircon crystals have low analytical totals with significant concentrations of Ca, Fe, Y, and P and infrared spectroscopy showed several weight percent of water. These ore zircon grains further show Fe-rich inclusions, zones and/or veins in elemental X-ray maps, and light rare earth elements (LREE) enrichment. Transmission electron microscopy (TEM) shows that the LREE are not due to micro- or nano-inclusions in the zircon, but are likely hosted as LREE oxides in amorphous regions of the grains. Based on these characteristics, the rims on ore zircon grains are interpreted to be of hydrothermal origin. Uranium-Pb in monazite from the ore, measured by SIMS, suggests a secondary event influencing the area at ca. 1624Ma, a period of known geologic activity in Fennoscandia. Electron microprobe X-ray mapping of these monazite grains shows no zoning and relatively low U and Th concentrations.Stark contrasts are visible between the ore (depleted mantle influence) and host rocks (crustal influence) in the whole rock Lu-Hf and Sm-Nd data. The depleted mantle signature of the ore could be related to the Kiruna greenstone group as a potential source region for the iron. The Sm-Nd isotopic composition of monazite from the ore shows a crustal influence, and indicates that the younger event has not disturbed the whole rock Sm-Nd signature of the ore. The hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation, with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source.

Th-U-Pb zircon geochronology of the Palaeoproterozoic Hartley Formation porphyry by six methods, with age uncertainty approaching 1 Ma.

The Palaeoproterozoic Hartley Formation in the Olifantshoek Group was deposited in one of the rift-related Waterberg (sensu lato) red bed basins which formed on the Kaapvaal Craton after the 2.05 Ga Bushveld intrusions and coeval thermal event. The age of these basins is not well constrained due to the shortage of directly dateable rock types. The Hartley Formation contains rare quartz-porphyry lavas interbedded with the dominant basalts and these provide the means to date the formation by analyses of zircon. In this work zircon from one sample has been dated by six Th-U-Pb methods, namely Laser Ablation ICP Quadrupole Mass Spectrometry, Laser Ablation ICP High-resolution Mass Spectrometry, Laser Ablation ICP Multicollector Mass Spectrometry U-Pb (also Lu-Hf), Nordsim Ion probe U-Pb and Th-Pb; and Krogh method ID-TIMS. Our precise ages give a combined age of 1915.2 ± 1.1 Ma. Including one published ion probe date from the only other known occurrence of quartz porphyry, the results only agree if the quoted analytical errors are increased by 20%, which gives a combined result of 1915.6 ± 1.4 Ma. This is considered a reliable, precise and accurate age for the Hartley Formation and supersedes the published Kober method 207Pb/206Pb age of 1928 ± 4 Ma. The new Lu-Hf zircon data, supported by published whole rock Sm-Nd and Rb-Sr data, suggests that both the dominant basalts and the rare quartz porphyries of the Hartley Formation were derived from mafic source rocks which had been in the crustal domain from Archaean times. By contrast with the intracratonic rifts of the other Waterberg Basins, the Olifantshoek Supergroup reflects the development of a western passive margin as the Archaean Kaapvaal Craton rifted and drifted. This was followed by accretion of the Rehoboth Province along the Kalahari Line, accompanied by the development of the east-vergent Kheis Province thrust complex. This created a larger cratonic block against which the 1.2 Ga collisions of Namaqua-Natal terranes impacted. The Kheis Province now yields ~1.17 Ma cooling ages, reflecting the Namaqua collisions, but the true age of the Kheis event is still enigmatic.