Primitive Mantle-normalized Trace Element Patterns Research Articles

Bimodal volcanism in the Hegau region (SW Germany): Differentiation of primitive melilititic to nephelinitic rocks produces evolved nosean phonolites

Peculiar bimodal volcanism in the Hegau region (SW Germany) comprises two contrasting SiO2-undersaturated rock suites.(I)Primitive olivine melilitites and melilite-bearing olivine nephelinites (12–9 Ma) are characterized by high MgO, CaO, Fe2O3, TiO2, Ni, V, F, moderate alkalis, Al2O3, P2O5, Ba, Nb, Zr, and low SiO2, Rb, Pb, and U concentrations. The rocks are composed of forsteritic olivine, diopsidic clinopyroxene, melilite, perovskite, Cr-bearing oxyspinel, F- and Ba-rich mica, and fluorapatite. In rare cases, they contain coeval coarse-grained ijolite patches generated by rapid in-situ fractionation in small melt pockets.(II)Evolved nosean phonolites (14–11 Ma) comprise high alkalis, Al2O3, SiO2, Rb, Nb, Zr, U, Pb, S, and low MgO, CaO, Fe2O3, TiO2, P2O5, Ba, Ni, and V concentrations, and contain abundant Ba-bearing alkali feldspar and nosean-haüyne-sodalitess macrocrysts, aegirine-augitic clinopyroxene, as well as accessory apatite, titanite, zircon, and pyrochlore.The melilititic–nephelinitic rocks formed by low degrees of partial melting of a carbonated amphibole ± phlogopite-bearing garnet wehrlite in the uppermost asthenosphere or in the thermal boundary layer and occur also in neighbouring regions. However, their coexistence with evolved nosean phonolites is a unique and so far, unexplained feature in the southern Central European Volcanic Province.Thermodynamic modelling implies that removal of 11–19% oxyspinel, 4–10% olivine, 42–57% clinopyroxene, <3% mica, <9% feldspathoids, <8% feldspar from melilititic–nephelinitic parental melts at upper crustal conditions (∼200 MPa) results in significant amounts of phonolitic residues that are compositionally similar to the exposed nosean phonolites. Strongly negative P and Ti anomalies and a trough of MREE in primitive mantle-normalized trace element patterns of phonolites indicate additional fractionation of titanite and apatite, consistent with the mineralogy of coarse-grained (nepheline) syenitic cumulates, which are present as enclaves in both rock suites. The modelling results suggest crystallization of the (nepheline) syenite cumulates between 1050 and 800 °C and ascent and eruption of the phonolite residues at no less than ∼900 °C with complete solidification of the observed assemblage at >750 °C. Significant crustal assimilation during fractionation appears unnecessary to explain the mineralogical, mineral chemical, and geochemical characteristics of the phonolites. Prolonged upper crustal differentiation of the magmas in one case (nosean phonolites) and fast ascent of primitive melts in the other (olivine melilitites and melilite-bearing olivine nephelinites) can be explained by stress field changes from an extensional to a more compressive regime, the magma ascent becoming thereby increasingly structurally controlled and supported by brittle deformation.

Petrogenesis and Geodynamic Implications of Cretaceous Nb-Enriched Mafic Dykes in the East Kunlun Orogen, Northern Tibet Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd Isotopes

There is a magmatic lull period in the East Kunlun orogen (EKO) during the Jurassic to the Cretaceous. However, due to the lack of records of magmatic activity restricts our understanding of the late Mesozoic magmatic-tectonic evolution of the EKO. Herein, an integrated study of geochronology, whole-rock geochemistry and Sr-Nd isotopes were conducted for the Cretaceous mafic dykes in the EKO, Northern Tibet Plateau, to reveal their petrogenesis and geodynamic implications. LA-ICP-MS Zircon U-Pb dating reveals that the studied mafic dykes comprising diabase and diabase porphyry emplaced at ca. 80.9 ± 0.8 Ma. The Cretaceous mafic dykes have low contents of SiO2 (46.36 wt.%~47.40 wt.%) but high contents of MgO (6.79 wt.%~7.38 wt.%), TiO2 (1.91 wt.%~2.13 wt.%), Nb (12.4~18.3 ppm) and Nb/U ratio (31~39), resembling Nb-enriched mafic dykes. They exhibit chondrite-normalized rare earth element (REE) and primitive mantle-normalized trace element patterns, remarkably similar but not identical to the oceanic island basalts (OIB). The moderate REE fractionation ((La/Yb)N = 3.55~5.37), weak negative Eu anomalies (δEu = 0.87~0.97) and relative enrichment of Rb, Ba, K, as well as high contents of Cr and Ni and slightly depleted Sr-Nd isotopes (εNd(t) = −0.18~1.33), suggest that the studied dykes originate from a partial melting of spinel lherzolite and a little of garnet which was previously modified by subducted sediments. Combined with other evidence, we propose that the studied Cretaceous Nb-enriched mafic dykes in the Northern Tibet Plateau were formed in the intraplate setting, which may be a partial melting of the enriched mantle in the lower lithosphere caused by the activity of the East Kunlun strike-slip fault.

OIB-type mafic–ultramafic rocks in South Guangxi, South China: A record of interaction between Paleo-Tethys subduction and Emeishan large Igneous Province

Late Permian ultramafic–mafic igneous rocks in the Pingxiang area, Guangxi, South China include wehrlite, gabbro, diabase, and amygdaloidal basalt. Geochemical characteristics document interaction between the Emeishan Large Igneous Province (ELIP) and the mantle wedge related to subduction of the Paleo–Tethys. Gabbro and basalt show chondrite–normalized rare earth elements (REEs) and primitive mantle–normalized trace element patterns that parallel ocean island basalts (OIB) and mafic intrusions from the ELIP. Melts in equilibrium with clinopyroxenes, in both wehrlite and gabbro, have REEs and trace element compositions similar to OIB and high–Ti mafic rock of the ELIP. The clinopyroxenes also show a rift–related compositional trend and cover those from the ELIP rocks. These compositional features, along with their moderate initial 87Sr/86Sr and εNd(t) values (−0.8 to +3), are consistent with derivation from an enriched mantle plume source. This is the first time that ultramafic–mafic rocks are reported beyond the established extent of the ELIP and could represent an offshoot from the main mantle plume or indicate that the ELIP plume was considerably bigger than current proposals but has been significantly denuded by subsequent erosion. In addition, the Pingxiang rock units display slightly enriched εNd(t) values and slight negative Nb, Ta, and Zr relative to OIB rocks, indicating an enriched component in the mantle source. Formation of the ELIP spatially and temporally overlapped with subduction of the Paleo–Tethys ocean and interaction between the plume and mantle wedge accounts for the compositional diversity of the ELIP. Such interaction is probably common along the southwestern–southern margin of the Yangtze Block.

Cryptic crustal growth identified through Variscan post-collisional lamprophyre-granite composite dykes, French Massif Central

The processes that control crustal growth by addition of new mantle-derived material to the crust and the recycling of crustal components that were introduced into the mantle during subduction remain poorly understood in collisional orogens. This is largely due to the fact that the sub-orogenic mantle has, in many instances, been fertilized by the introduction of crustal components during previous subduction events, and lacks the typical depleted mantle signature. Here, we present a study of lamprophyric–granitic composite dykes from the northern margin of the post-collisional Variscan Aigoual granite, French Massif Central (FMC). At Aigoual, lamprophyres and granites have gradational contacts and similar geochemical characteristics which indicate that the two were co-magmatic. UPb dating of zircon yields crystallization ages between 311 ± 3 Ma and 313 ± 3 Ma for dykes, identical within uncertainty with the ages of 312 ± 3 Ma and 313 ± 3 Ma obtained for the emplacement of the Aigoual granite. The lamprophyres are metaluminous to slightly peraluminous and display enrichment in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K, LILE, LREE) and have crustal isotopic signatures in both radiogenic (Sr, Nd and Hf) and stable (O) isotope systems. This dual crust/mantle signature of the lamprophyres can be interpreted to reflect partial melting, within the spinel stability field, of an enriched mantle that was metasomatised by the addition of subducted sediments. The granites are peraluminous, high-K and have shoshonitic affinities. Their primitive mantle-normalized trace-element patterns are similar to the lamprophyres and the isotopic compositions of the two rock types overlap. The high Cr, Ni, Fe, and Mg contents of the granites are consistent with a mantle component in the magmas. Our results suggest that between 65 and 80 wt% of material in the lamprophyres and granites was derived from the depleted mantle, and the rest from recycled crustal material. Although blurred from an (conventional) isotopic point of view, this magmatism contributed to the crustal growth via the post-collisional magmatism of the FMC. Coupled with their high preservation potential, post-collisional sites might thus represent significant sites of crustal growth.

Trace Element Evidence of Subduction-Modified Mantle Material in South Mid-Atlantic Ridge 18–21°S Upper Mantle

Mid-ocean ridge basalts (MORBs), produced at mid-ocean ridge where the continents and subduction zones are distant, are the product of partial melting of the upper mantle and their chemical composition can provide information about the mantle itself. The geochemical characteristics of MORBs enable us to be more informed about the geological processes of the upper mantle below the mid-ocean ridge, and assist us in understanding mantle heterogeneity and geodynamic processes. In this paper, new data of major elements, trace elements, and Nd-Hf isotopes of south mid-Atlantic ridge (SMAR) 18–21°S MORBs are presented. TAS diagram shows that the samples belong to subalkaline basalt compositional field. Trace elements (e.g., (La/Sm)N = 0.49–0.79) show that the samples are N-MORBs. However, the primitive mantle-normalized trace element patterns showed that the studied samples were clearly enriched in Rb, U, Pb, and other fluid-mobile elements. Meanwhile, the trace element ratios, such as Nb/U and Ce/Pb, are also significantly different from the typical N-MORB. Combined with the Nd-Hf isotopic composition, we propose that these anomalies are not related to continental crust material, delaminated subcontinental lithospheric mantle (SCLM), recycled sediments, direct supply of mantle plume, nor are they the result of subduction directly affecting the mantle source, but are caused by the incorporation of mantle material modified by subduction.

Genetic pattern of the Albian volcanic rocks in the Ziruco area, Northern Tibet: Implications for A-type granites

Genetic pattern of a continuous volcanic series of andesites, dacites, and rhyolites from the Ziruco area, northern Tibet was debated in this paper to evaluate the later evolution of A-type granites. We present new zircon UPb ages, whole-rock geochemical, zircon Hf isotopic, and whole-rock SrNd isotopic data for these magmatic rocks. Our analysis shows that the zircon UPb ages of the andesites, dacites, and rhyolites are ca. 105.4 Ma, ca.105.3 Ma, and ca.106.8 Ma, respectively. The andesites have high Nb contents (16.3–24.9 ppm) and Nb/U ratios (22.7–32.4), and chondrite normalized REE and primitive-mantle normalized trace element patterns similar to Nb-rich basalts. They were most likely derived from the mantle that had been metasomatized by melts derived from subducted slab and sediment, and then underwent fractional crystallization. The dacites and rhyolites have high Ga/Al ratios (3.05–3.70 and 2.93–3.43, respectively), REE (226–256 and 195–285 ppm, respectively), Nb (30.0–34.4 and 20.3–24.4 ppm, respectively) and Zr contents (443–513 and 147–163 ppm, respectively), and zircon saturation temperatures (856–886 °C and 823–837 °C, respectively), which are similar to A-type granites. The dacites were most likely formed by partial melting of underplated basaltic lower crust with ilmenite and zircon, followed by slight fractional crystallization. The rhyolites were most likely formed by fractional crystallization of the dacite magmas. The minerals of fractional crystallization probably include feldspar, apatite, zircon, and titanite. This continuous volcanic series in Ziruco area may be related to break-off of the Meso-Tethyan slab in northern Tibet during the late Early Cretaceous. Our studies indicate that fractional crystallization is important later evolutionary process for A-type granites from the crustal source area, and it mainly controls the geochemical characteristics of significant negative Ba, Sr, P, Ti, and Eu anomalies of A-type granites from the crustal source area.

Olivines and Their Melt Inclusions in Potassic Volcanic Rocks Record Mantle Heterogeneity beneath the Southern Tibet

Abstract Constraints on mantle heterogeneity are important for understanding geochemical differentiation and recycling in Earth’s interior. Alkali basalts are of particular interest in this context due to the clear signatures of recycled components that they yield. Previous studies have focused more on oceanic basalts than on continental mafic rocks because the chemical and isotopic characteristics of the latter are often modified by crustal contamination; however, high-forsterite olivine crystals and olivine-hosted melt inclusions may record the compositions of primitive melts before extensive crustal contamination, providing valuable insights into mantle heterogeneity. Here we report the results of high-precision major-, minor-, and trace-element and Pb isotope analyses of olivine phenocrysts and melt inclusions from the Sailipu potassic alkali basalts from southern Tibet, which constrain the mantle source of continental potassic alkali basalts. The minor- and trace-element compositions of the olivine phenocrysts (e.g. Ni and Ca contents and 100 × Mn/Fe, 10 000 × Zn/Fe, and Ni/Co ratios) suggest that the primary magmas were derived from a phlogopite-bearing pyroxenite source in which peridotite residues were also present. The low- and high-Li contents of the olivine phenocrysts suggest the recycling of continental crustal material in their mantle source. Comparing the major-element compositions (e.g. SiO2, TiO2, Al2O3, MnO, CaO, and Na2O contents), FCMS and FCKANTMS parameters, and pseudo-ternary projections of the primary Sailipu magmas with experimental data suggests that the primary magmas were partial melts of olivine-poor or carbonated olivine-poor lithologies with a small amount of peridotite residue. The negative Nb and Ta and positive Pb anomalies seen in primitive mantle-normalized trace-element patterns and the extremely low Nb/U ratios of the melt inclusions suggest that recycling of continental crust was the main process involved in enriching the mantle source of the Sailipu magmas. The heterogeneous Pb isotopic compositions of the melt inclusions (208Pb/206Pb = 2.089 – 2.158; 207Pb/206Pb = 0.851 – 0.858) are consistent with the presence of peridotite residue in this source. The subduction of Indian continental crust introduced enriched material (including carbonatites) into the mantle.

Petrogenesis of the late Tonian arc-related Um Balad gabbro-diorite complex (Egypt) and insight into its spatially related orogenic gold mineralization

ABSTRACT The gabbro-diorite complex of the Um Balad prospect hosts lode gold mineralization. The complex is dated at 723 ± 4 Ma using the LA-ICP-MS U-Pb zircon method and is correlated with the late Tonian-early Cryogenian subduction-related magmatic stage during the evolution of the Arabian-Nubian Shield. The gabbro-diorite complex evolved through the crystallization of a calc-alkaline magma and the subduction signature of this magma is verified by primitive mantle-normalized trace element patterns that show enrichment in large ion lithophile elements, U, and Th relative to high field strength elements as well as negative Nb and Ti anomalies. The high La/Ybcn (3.1–9.4) and Ta/Yb (0.12–0.41) ratios are consistent with a continental arc rather than an oceanic arc system. Amphibole chemistry indicates that this complex might have crystallized under a moderately oxidizing condition from a hydrous magma (>6 wt% water content) at temperature and pressure estimates of about 800°C and 3 kbar, respectively. Gold mineralization in the Um Balad prospect is confined to structurally controlled massive quartz±carbonate veins and surrounding alteration halos. Alteration in the prospect is represented by localized sericitization and carbonation as well as pervasive chlorite-sericite alteration. The alteration halos are characterized by enrichment in K and Rb and depletion in Ca and Sr compared to their host rocks. The veins of the prospect are related to lower order extensional fractures associated with the regional first order transpressional Najd Fault System. The high Fe contents of the gabbro-diorite complex represent a suitable chemical trap for gold through sulfidation of the host rocks. Supergene alteration resulted in the formation of goethite in association with atacamite and chrysocolla. Free mill gold is associated with these supergene phases, which were deposited in near neutral to slightly alkaline conditions.

Petrogenesis of Neoarchean (2.80–2.75 Ga) Jagannathpur volcanics and the Ghatgaon and Keshargaria dyke swarms, Singhbhum craton, eastern India: Geochemical, Sr[sbnd]Nd isotopic and Sm[sbnd]Nd geochronologic constraints for interaction of enriched-DMM derived magma with metasomatised subcontinental lithospheric mantle

Petrogenesis of Neoarchean dyke swarms and dispersed volcanic successions in the Singhbhum craton can potentially constrain mantle heterogeneity. Their arc-like trace element contents, that have a mismatch with their intracratonic extensional setting, were interpreted to propose widely varied genesis from either an enriched mantle (EM I) or wholly from a metasomatised subcontinental lithospheric mantle (SCLM). In order to test the type of enriched source(s) of magma to the dykes swarms and volcanics, geochemical modeling, Sr and Nd isotopic analyses and geochronology were conducted. The Jagannathpur amygdular volcanics, one such unmetamorphosed volcanic succession, yielded an SmNd isochron age of 2799 ± 67 Ma (2σ) with Ndi = 0.50895 ± 0.00006, MSWD = 0.36 (n = 16) and εNd(T) = −1.12 ± 0.50. As the eruptive age is indistinguishable within error to the Ghatgaon (2762.4 ± 2.0 Ma, 2752.0 ± 0.9 Ma) and the Keshargaria (2800.2 ± 0.7 Ma) dyke swarms, the dykes are interpreted as coeval feeder conduits to the volcanics. Enriched LILE and LREE, Nb/Th and Zr/Th ratios coupled with highly radiogenic present-day Sr isotopic ratios which have mantle-like 87Sr/86Sri ratios (0.7031–0.7055) and negative εNd(T) values (between −1.8 and − 5.6 for the dykes and − 1.3 and − 1.4 for the Jagannathpur volcanics) indicate that the source mantle components underwent enrichment processes likely by pre-2800 Ma subduction. The primitive mantle-normalized trace element patterns for these dykes and volcanics have pronounced negative Nb-Ta-Ti anomalies and their arc-like geochemistry suggest interaction with metasomatised SCLM. Generation of primitive magmas has been modeled by two end member components involving 18% partial melt from enriched-DMM (depleted MORB mantle) [100–70%] and interaction with low degree (5%) partial melt of metasomatised SCLM [0–30%]. These magmas ponded at Moho depths and underwent assimilation-fractional crystallization (AFC) forming the diverse dykes and basaltic andesitic-andesitic volcanics. We link the geophysically imaged Mid Lithospheric Discontinuity to depth of metasomatised SCLM beneath the Singhbhum craton that formed ≥2800 Ma ago.

Melting of metasomatically enriched lithospheric mantle – Constraints from Pan-African monzonites (Damara Orogen, Namibia)

The 560.8 ± 2.4 Ma old Okamutambo Pluton is the easternmost intrusion of the syn-collisional Otjimbingwe Alkaline Complex (OAC; Damara Belt, Namibia) and consists of mafic to intermediate alkaline rocks that belong to the monzodiorite-syenite series. The studied samples are potassic (K2O/Na2O > 1) and have moderately high magnesium (MgO: 5.7–2.6 wt%), nickel (Ni: 66–26 ppm), and chromium (Cr: 223–62 ppm) concentrations. LILE (Ba: 2425–1243 ppm; Sr: 1359–941 ppm) and HFSE (Zr: 447–202 ppm, Nb: 32.8–16.4 ppm, Hf: 4.7–9.4 ppm, Ta: 1.2–2.0 ppm) contents are also high. Strontium and Nd isotope data reveal the existence of two magmatic suites indicating a multi-source origin. Group 1 monzonites-quartz monzonites have moderately evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7066 to 0.7073; initial εNd: −3.5 to −5.0) and radiogenic Pb isotope ratios (206Pb/204Pb: 17.65–18.02; 207Pb/204Pb: 15.62–15.67; 208Pb/204Pb: 38.19–38.32). In contrast, group 2 monzodiorites-syenites display more evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7088 to 0.7090; initial εNd: −6.7 to −7.1) but similar Pb isotope ratios (206Pb/204Pb: 17.63–17.82; 207Pb/204Pb: 15.64–15.66; 208Pb/204Pb: 38.21–38.37). Differentiation involved AFC processes in group 1 monzonites-quartz monzonites whereas group 2 monzodiorites-syenites were modified by fractional crystallization. Although second-order processes were operative, high total alkali contents, incompatible trace element concentrations in excess of bulk crustal values, and evolved isotopic compositions of the most primitive samples are source-controlled and provide insight into the origin of the parental magmas. With reference to experimental data from the literature, it is inferred that the Okamutambo alkaline rocks represent evolved melts that were generated through melting of enriched lithospheric mantle (phlogopite-lherzolite). The observed negative Nb-Ta and Ti anomalies and positive Pb anomalies in primitive mantle-normalized trace element patterns are in line with a mantle source that contains a recycled crustal component. Isotopic compositions indicate that mantle enrichment is an ancient feature that might be linked to Proterozoic subduction. At 561 Ma, the geodynamic regime in the Damara Orogen was mainly characterized by compression during convergence and continental collision between the Congo and Kalahari cratons which is difficult to reconcile with the generation of the OAC as alkaline magmatism is commonly associated with extensional tectonic regimes. The OAC is, however, associated with a major suture zone that may have been involved in localized transtensional tectonics during oblique flat subduction and thus enabled the generation and ascent of mantle-derived alkaline melts.

Mantle source and magma oxygen fugacity of Permian Ni--Cu sulfide-bearing mafic-ultramafic intrusions in the Inner Mongolia, North China: Controls on the sulfide saturation of magmas in the Andean-style continental convergent margins

Permian Erbutu and Kebu Ni--Cu sulfide-bearing mafic-ultramafic intrusions in the central Asian orogenic belt (CAOB) formed in the Andean-style continental convergent margin adjacent to the northern margin of the North China Craton (NCC). The rocks of the two intrusions show arc-like affinity on the primitive mantle-normalized trace element patterns and have εNd(t) ranging from −8.7 to −4.2, which are interpreted to be related to derivation from the metasomatized, enriched, subcontinental lithospheric mantle (SCLM) beneath the NCC. Using the olivine-orthopyroxene-spinel oxygen barometer, the magma fO2 of the Erbutu and Kebu intrusions was calculated to be from FMQ-0.8 to FMQ + 0.2, and from FMQ + 1.3 to FMQ + 2.0, respectively. The sulfides in the rocks have δ34SCDT ranging from +5.3 to +7.5‰ for the Erbutu intrusion and from +2.0 to +4.5‰ for the Kebu intrusion, much higher than that of the MORB mantle (δ34SCDT = −1.5 to 0.6‰). The rocks of the Erbutu and Kebu intrusions have δ13C ranging from −26.1 to −20.4‰ and − 26.4 to −20.9‰, respectively, similar to that of organic carbon in the crust (δ13C = −28 to −20‰), but much lower than that of the MORB mantle (δ13C = −7 to −5‰). These may indicate the addition of externally derived sulfur (S) and organic carbon into the magmas, which would increase S contents and decrease S solubility of the magmas, respectively, triggering sulfide saturation of the magmas. It is noted that the sulfide saturation of the Erbutu intrusion was mainly related to the addition of externally derived sulfur in the magmas as sulfur is generally present as S2− species in the low-fO2 magmas. Both intrusions have trace element patterns and εNd(t) values nearly identical to those of the Permian Elitu volcanic rocks in the same region, indicating that they were likely genetically linked. The Elitu volcanic rocks show negative correlation of (Gd/Yb)N with >3.0 wt% MgO, which is interpreted as the primary magmas of the rocks may have experienced early garnet fractionation at high pressure (e.g., 0.8 to 2.0 GPa). The calculated thickness of the crust in this region is ~56 km, equivalent to ~1.7 GPa in pressure. The primary magmas of the Erbutu and Kebu intrusions were produced by partial melting of the enriched SCLM beneath the thickened crust and are likely Ni-enriched, which is also crucial to the economic Ni--Cu sulfide mineralization in the Andean-style continental convergent margin in the CAOB.

Geochronology and Geochemistry of the 890 Ma I-Type Granites in the Southwestern Yangtze Block: Petrogenesis and Crustal Evolution

The tectonic evolution of the southwestern Yangtze Block during the Early Neoproterozoic period is still controversial because of the limited quantities of 1 000-860 Ma magmatic rocks. In this study, our new LA-ICP-MS zircon U-Pb dating results demonstrate that the Yanbian granodiorites in the southwest Yangtze Block were emplaced at 894.6±7.4 Ma, representing the product of an 894 Ma magmatism. The Yanbian granodiorites are metaluminous to weak peraluminous with A/CNK values of 0.8-1.1, resembling I-type granitoids. They are characterized by right-inclined REE patterns with moderate to insignificant negative Eu anomalies (δEu=0.6-0.9). Their primitive mantle-normalized trace element patterns are characterized by depletion of Nb, Ta and Ti and weakly enrichment of Th. Considering the positive whole-rock eNd(t) (+5.8 to +6.8), we propose that these granodiorites originated from the partial melting of juvenile mafic lower crust. The Yanbian I-type granitoids have low Y and Nb contents similar to volcanic arc igneous rocks in the Y-Nb plot for tectonic discrimination. In conclusion, Early Neoproterozoic Yanbian granodiorites have generated in a compression setting in an active continental margin. Together with previous studies from the southwestern Yangtze Block, we suggest that the 894 Ma subduction-related Yanbian granodiorites represent the early stage of subduction at the southwestern margin of the Yangtze Block.

Platinum-group element geochemistry of the volcanic rocks associated with the Jaguar and Bentley Cu–Zn volcanogenic massive sulfide (VMS) deposits, Western Australia: implications for the role of chalcophile element fertility on VMS mineralization

We report whole-rock major, trace, and platinum-group element (PGE) geochemistry of volcanic rocks from the Teutonic Bore complex that hosts the Jaguar and Bentley Cu–Zn volcanogenic massive sulfide (VMS) deposits. This study aims to understand their sulfide saturation history and chalcophile element evolution during differentiation of the Jaguar and Bentley magmas, and investigate the role of chalcophile element fertility on the formation of VMS deposits. The fractionated primitive mantle–normalized trace element patterns, with negative Nb and Ti anomalies of basalts, andesites, dacites, and rhyolites from Jaguar and Bentley, are similar to each other. The trace elements and PGE show continuous variations when plotted against fractionation indices such as Yb, which can be explained by a two-stage fractional crystallization model: stage 1 Rayleigh fractionation of plagioclase + clinopyroxene + Cr-spinel, and stage 2 the fractional of plagioclase + clinopyroxene + magnetite + 0.1 wt% sulfide liquid. Dolerites, which postdate the mineralization, differ from the other rock types and require a different magma source. Andesite and basalt are the most PGE-enriched lithologies in Jaguar and Bentley. The PGE behave incompatibly in the early stage of magma differentiation at < 4 ppm Yb, whereas they abruptly decrease at > 4 ppm Yb, indicating sulfide saturation at this point. When Pd/MgO and Pd/Pt are used as chalcophile element fertility indicators, the andesite before sulfide saturation (< 4 ppm Yb) is as fertile as the magmas associated with porphyry Cu-only deposits. In contrast, the andesite after sulfide saturation and other lithologies are characterized by markedly depleted fertility similar to those of barren suites. This suggests that sulfide-undersaturated andesite, and probably basalt, may have been a significant source for Cu in the Jaguar and Bentley Cu–Zn VMS deposits. However, the Au fertility of the Jaguar and Bentley andesite must have been low and not enough to form Au-rich VMS deposits because their Pd/MgO and Pd/Pt values are 5–10 times lower than those of andesite and dacite from the modern Au-rich seafloor massive sulfide deposits. This can be explained if ore formation occurred shortly after sulfide saturation. If the amount of sulfide melt to precipitate was small, Au, with its high partition coefficient into immiscible sulfide melts, would have been largely stripped from the silicate melt, whereas Cu, with its lower partition coefficient, would be little affected. Our study shows that chalcophile element fertility may play an important role in the formation of VMS deposits, especially in controlling the Au contents of the ore, if the magmatic-hydrothermal component is the dominant source for metals in VMS systems.

Formation and stabilization of the Dunhuang Block, NW China: Constraints from the Late Paleoproterozoic A-type granites of the Dunhuang Complex

The geochemical composition and temporal evolution of A-type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust. Moreover, these aspects may provide an indication of when orogenic processes ended and craton formation/stabilization began in Early Precambrian blocks. In this study, we report the results of zircon U–Pb ages, Hf isotope data and whole-rock geochemistry for the Proterozoic Huoyanshan potassic granites in the Dunhuang Block, northwest China. Zircon U–Pb dating reveals that the Huoyanshan granites were emplaced at ca. 1791–1786 Ma, representing a Late Paleoproterozoic magmatism event. The granites have high SiO2 (70.71–72.20 wt%), total alkali contents (K2O + Na2O = 8.81–8.96 wt%; K2O/Na2O = 1.57–1.66), TiO2/MgO ratios (2.19–2.57), low CaO (0.77–1.27 wt%), MgO contents (0.12–0.16 wt%), and Mg# values (7.72–8.67), with A/CNK ratios of 0.96–1.04, pointing to alkaline and metaluminous to weakly peraluminous affinities. These granites are characterized by high total rare-earth element concentrations (∑REE = 419–755 ppm) and variable (La/Yb)N values (8–24) with significantly negative Eu anomalies (Eu/Eu* = 0.31–0.42). The primitive mantle-normalized trace element patterns suggest that the Huoyanshan plutons are depleted in Ba, Nb, Ta, Sr, P, and Ti, but enriched in Rb, Th, and Pb. Additionally, the granites have a high zirconium saturation temperature of 855–897 °C, with an average of 878 °C. These geochemical characteristics indicate that Huoyanshan potassic granites are an aluminous A-type granite, and further classified as A2-type. Together with the zircon Hf isotope data (εHf(t) = −11.21 to −2.76), indicates that these granites were generated from a crustal source. The heterogeneous Hf isotope compositions in zircons are principally caused by the disequilibrium melting of Zr-bearing sources. In addition, two-stage Hf model ages of zircons from the granites are ca. 2.6–3.1 Ga and plot on the average crustal evolution trend of Archean tonalite–trondhjemite–granodiorite (TTG) gneiss (~2.5 to 3.0 Ga) in the Dunhuang Block. Thus, we propose that A-type granites in the study area were generated from the disequilibrium partial melting of Archean tonalitic to granodioritic rocks under high-temperature and low-pressure conditions according to their low Al2O3 + CaO + Na2O + K2O (molar) values and high 10000 × Ga/Al values (>2.9). Subsequently, the magma experienced prominent fractional crystallization of K-feldspar, plagioclase, orthopyroxene, and/or magnetite. Our results, withavailabledata, suggest that the Huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting, and serve as a precursor for the formation and stabilization of the Dunhuang Block during the Late Paleoproterozoic. Furthermore, it is probable that the post-collisional magmatism event at ca. 1.79 Ga in the Dunhuang Block was closely related to the assembly of the Columbia supercontinent.

Geology and petrology of the potassic and ultrapotassic rocks from the northern part of Senirkent (Isparta-SW Turkey): evidence of magma–carbonate wall-rock interactions

Potassic, ultrapotassic lava domes, flows, and volcano-sedimentary deposits including leucite-bearing and leucite-free grains crop out to the north of Senirkent (Isparta), at the apex of the Isparta Angle (southwestern Turkey). Ultrapotassic rocks are nepheline-foidolite, italite, leucitite, melilite leucitite, melilite-gehlenite leucitite, diopside leucitite, and leucitophyre with lowest SiO2 content (38.71–44.72 wt%, but the leucitophyre has 51.4 wt%), higher K2O (4.36–10.19 wt%), MgO (3.25–6.98 wt%), and K2O/Na2O ratio (2.65–32.8). Potassic rocks are phonolitic leucitites, sanidine-phyric phonolite, and trachytes with SiO2 (49.35–64.46 wt%), K2O (8.28–11.38 wt%), MgO (0.17–1.39 wt%), and K2O/Na2O ratio (2.25–11.8). Some leucitites, diopside leucitites, and phonolitic leucitites enclose partly carbonatite occurences or associated with carbonate-rich part including carbonatite globules, phlogopite, clinopyroxene, and/or leucite, melanite crystals embedded in glassy volcanic matrix and/or carbonate matrix. Primitive mantle-normalized incompatible element patterns display strong enrichments in LILEs (Cs, Rb, Ba, U, Th, Sr) relative to HFSEs (Nb, Ti) with distinct negative Nb–Ti anomalies, suggesting subduction-related lavas. Chondrite-normalized REE patterns are characterized by light REE enrichment (251–493 times chondrite for La) and flat heavy REE patterns (10–27 times chondrite for Yb). All the leucite-bearing and leucite-free rocks display similar, subparallel chondrite-normalized and primitive mantle-normalized trace element patterns, suggesting cogenetic origin. Mineralogical, petrological, and geochemical data suggest magma–carbonate wall-rock interaction and tapping of different parts of the melts within the magma chamber along the fault lines formed during neotectonic period and extension-related block faulting.

An early Devonian intra-plate bimodal volcanic suite in the Kyrgyz North Tianshan belt, the central Asian orogenic belt

A suite of bimodal volcanic rocks in the Kyrgyz North Tianshan belt is composed of totally ∼100-m-thick basalt and basaltic andesite layers of the Karakastek Formation and totally ∼700-m-thick rhyolite layers of the Kastek Formation, with minor amounts of interlayered felsic to intermediate tuff/ignimbrite. Zircon grains from a rhyolite sample have a weighted mean 206Pb/238U age of 416 ± 8 Ma, which is considered to be the crystallization age of the rhyolite. The zircon grains have δ18O ranging from +6.5 to +7.3‰. The calculated zircon saturation temperatures (TZr) range from 850 to 900 °C. Rhyolites have high total alkaline, Zr, Y and Nb contents and high Ga/Al similar to A1-type granitoids formed in an intra-plate setting. Mafic volcanic rocks have SiO2 ranging from 49.4 to 55.3 wt% and FeOt/MgO from 1.4 to 2.4, showing a tholeiitic trend. They have negative Nb and Ta anomalies on the primitive mantle-normalized trace element patterns and have εNd(t) of −1.5 to 0. There is a positive correlation between εNd(t) and ΔεHf(t) of the mafic volcanic rocks, indicative of crustal contamination. However, one least evolved basalt sample has Mg# of 57 with low Zr and Hf concentrations, indicating insignificant crustal contamination. Modeling results indicate that the mafic volcanic rocks may have been derived from a dry, garnet-bearing lherzolite mantle source. They have εHf(t) and εNd(t) varying along the “Island arc array”, which is interpreted as the mantle source was metasomatized by slab-derived melts rather than fluids. In addition, the rhyolites have εNd(t) of −2.4 to −2.9, much lower than the mafic volcanic rocks, and were thus unlikely produced by the fractionation of basaltic magmas. The rhyolites show Nd-Hf isotope decoupling, which is interpreted as a result of the anataxis of Precambrian lower crust associated with the breakdown of garnet. Therefore, the early Devonian bimodal volcanic suite in the Kyrgyz North Tianshan belt was considered to have formed in an intra-plate setting.

U-Pb geochronology, bulk-rock geochemistry and petrology of Late Cretaceous syenitic plutons in the Gölköy (Ordu) area (NE Turkey): Implications for magma generation in a continental arc extension triggered by slab roll-back

Late Cretaceous felsic plutons in the Eastern Pontides orogenic belt of Turkey are found in varying ages, sizes and compositions. The Late Cretaceous syenitic bodies, namely Direkli and Yeniköy Tepe plutons in the Ordu area, intruded into the Late Cretaceous volcanics and volcanoclastics, and compositionally consist of syenite and minor quartz-syenite with mainly granular and rarely porphyric, poikilitic and perthitic textures. The studied plutons contain K-feldspar, plagioclase, quartz, clinopyroxene, biotite and minor hornblende, and Fe-Ti oxide, apatite, and zircon. Mineral thermobarometer estimations imply that the syenitic plutons were crystallized in P-T conditions representing mid to shallow crustal levels. Petrochemically, these syenitic plutons show I-type, metaluminous-peraluminous transitional (A/CNK = 0.85–1.04) and shoshonitic features. LA-ICP-MS U-Pb zircon dating on the syenitic plutons yielded weighted mean ages of 78.5–72 Ma. Primitive mantle-normalized trace element patterns of the studied plutons are characterized by negative anomalies of Ba, Sr, Nb, Ta and Ti, and enrichment in Rb, Th, K, Ce and La. The chondrite-normalized rare earth element (REE) plots of the studied plutons show moderately enriched concave-shaped patterns (LaN/LuN = 10.3–10.7 for the Direkli pluton; LaN/LuN = 6.8–11.7 for the Yeniköy Tepe pluton) with negative Eu anomalies. The plutons have 87Sr/86Sr ratios (0.705368–0.706954), 143Nd/144Nd ratios (0.512600–0.512672), 206Pb/204Pb ratios (19.29–19.52) and 207Pb/204Pb ratios (15.65–15.67). The geochemical data, as well as regional geology, suggest that the Late Cretaceous syenitic plutons have formed in an extensional continental arc setting triggered by slab roll-back. The parental magma of syenitic plutons was derived from melts of lower crustal materials (metabasalt/metaandesite) and lithospheric mantle component.

Early Paleoproterozoic (ca. 2.36 Ga) post-collisional granitoids in Yunnan, SW China: Implications for linkage between Yangtze and Laurentia in the Columbia supercontinent

The early Paleoproterozoic is an essential period for understanding the early tectonic evolution of the Yangtze Block and its reconstruction in the Columbia supercontinent, whereas these issues are largely unknown due to the scarce preservation of early Paleoproterozoic geological records. This study presents rarely reported early Paleoproterozoic granitoids from the southwestern Yangtze Block, with integrated studies of zircon U-Pb ages, Lu-Hf isotopic compositions and whole-rock geochemistry. Magmatic zircon U-Pb dating for two monzogranite samples yields weighted mean 207Pb/206Pb ages of 2359 ± 16 Ma and 2363 ± 16 Ma, providing unambiguous evidence for the presence of early Paleoproterozoic magmatism in the Yangtze Block. All the samples have high A/CNK values of 1.30–1.61, typical of S-type granite. These rocks have low total REE contents of 72.5–139.0 ppm with marked enrichment in LREEs ((La/Yb)N = 10.73–23.83) and conspicuously negative Eu anomalies (Eu/Eu* = 0.46–0.55). The rocks exhibit uniform primitive mantle-normalized trace element patterns with enrichment in Rb, Th, U and Pb and depletion in Sr, Nb, Hf and Ti. These dated zircon grains have consistent negative εHf(t) values of −6.2 to −2.8 and TDM2 ages of ca. 2.97–3.16 Ga, indicative of Mesoarchean crustal materials as their magma source. Considering the existence of the ca. 2.36 Ga regional high-grade tectono-metamorphic event, we propose that the ca. 2.36 Ga monzogranites were most likely generated in the early stage of post-collisional extension. Therefore, the southwestern Yangtze Block likely experienced a collision event during ca. 2.40–2.36 Ga, indicative of the involvement of the Yangtze Block in the ca. 2.5–2.3 Ga Arrowsmith orogeny of Laurentia. In addition, a series of accretionary events might also have occurred in the western Yangtze Block before the final assembly of the Columbia supercontinent, implying a tight spatial connection between the Yangtze Block and northwestern Laurentia within this global-scale supercontinent.

Petrogenesis of the two phases of intrusive rocks at Chodarchay, NW Iran: using trace and rare earth elements

The Chodarchay area is located 50 km east of the city of Zanjan, within the Tarom-Hashtjin subzone, in the Western Alborz structural zone of NW Iran. Igneous rocks in the area include intrusive granitic (phases I and II), and volcanic rocks. Geochemical characteristics of the granitic rocks were determined using their trace and rare earth element (REE) compositions. The phase I intrusive rocks at Chodarchay include quartz monzonite and quartz syenite, while the phase II mainly consists of granite and alkali granite. These intrusive rocks have SiO2 contents ranging between 60.11 and 69.14 wt%, high K2O + Na2O (8.20 to 10.21 wt%) and K2O/Na2O ratios of 1.24 to 2.56. Petrological and geochemical studies indicate that the granitoids have high-K to shoshonitic affinity, they are metaluminous to mildly peraluminous and characterize as I-type granitoids. The investigated granitoids (phases I and II) have fractionated REE patterns characterized by enrichment in light REEs and varying degrees of depletion in heavy REEs. Large-ion lithophile element (LILE) enrichment, high-field strength element (HFSE) depletion, and distinct positive Pb anomaly in the primitive mantle-normalized trace element patterns of the rocks imply subduction-related arc magmatic signatures. It is suggested that the granites formed in a volcanic arc to post-collision setting from a garnet spinel lherzolite source.

Ca. 2.0 Ga mafic dikes in the Kongling Complex, South China: Implications for the reconstruction of Columbia

Paleoproterozoic mafic dikes from the Kongling Complex in South China provide an ideal opportunity to examine mantle properties and tectonic evolution of the Yangtze Block and thus to understand the paleo-position of South China in the Columbia supercontinent. The 2.0 Ga dikes are the oldest mafic intrusions identified in the Yangtze Block by far. Samples from the dikes consist of fine to medium-grained dolerite and have variable SiO2 (43.48–52.61 wt%) and MgO contents (3.17–7.35 wt%). They are enriched in LREE and show slightly negative to positive Eu anomalies (Eu/Eu∗ = 0.85–1.06). Their primitive mantle-normalized trace element patterns are characterized by enrichment of large-ion lithophile elements (Rb, Ba, Pb) and depletion of high-field-strength elements (U, Nb, Ta, Ti). Rocks from the dikes have variable initial 87Sr/86Sr ratios (0.698874–0.710753), εNd values (−3.0 to +4.1) and zircon εHf values (−2.7 to +3.4). These geochemical features indicate that their parental magmas were formed by <10% partial melting of a spinel-bearing lherzolite mantle in a back-arc basin setting. Paleoproterozoic mafic rocks from South China, Northwest Laurentia, Northwest Australia and Siberia had different trace element ratios and crystallization ages, their mantles had different thermal states. Thus, we argue against the previous model that the four continents were assembled together in the Columbia supercontinent.