Nb-Ta Trough Research Articles

Mantle xenoliths from Komsomolskaya kimberlite pipe, Yakutia: Multistage metasomatism

Minerals from > 200 mantle xenoliths from Komsomolskaya kimberlite pipe were studied by electron microprobe and LA-ICP-MS. They are metasomatised garnet and spinel peridotites containing phlogopite, amphibole and ilmenite with garnets (up to 12.5 wt% Cr2O3) and clinopyroxenes (up to 5 wt% Na2O) or rarer Fe-pyroxenites and A, B, C eclogites.Thermobarometry indicates that the lithospheric mantle beneath the Komsomolskaya pipe is layered. Heated porphyroclastic, deformed peridotites at the lithosphere base (7–6 GPa) are enriched in Fe. The cold group at 6.0–5.5 GPa (34 mW/m2) are depleted peridotites with sub-Ca garnets. Cpx-fertilized varieties belong to the middle part of the mantle section. Amphiboles range from Cr-hornblendes to edenites (2–6 GPa), showing K-Ti enrichment. Picroilmenites yield two pressure intervals from 6.5 to 5.0 GPa and from 5.0 to 4.0 GPa, forming two differentiation branches. Eclogites mainly occur in the lower part of the section with a peak at pressures of 4–6 GPa.Trace elements of melts that formed harzburgitic garnets-pyroxenes refer to oceanic MORB like melt interaction with peridotites. The sub-calcic S-type garnets are similar to subduction-related melts (S-type REE) with troughs in HFSE. Adakite-like hybrid metasomatism formed Na, Al-rich pyroxenes with peaks in Sr and HFSE. K-bearing pyroxenes and amphiboles refer to shoshonitic metasomatism. Trace elements for Cpx of re-fertilized mantle peridotites reveal high LREE, Nb-Ta troughs and peaks in Zr, Th, Sr, U. They are reacted to carbonatite –alkaline melts. Protokimberlite (essentially carbonatitic) interaction produced HFSE-enrichment. Type B eclogites show more subduction-related features with HFSE troughs while type A eclogites are closer to hybrid and peridotitic signatures. We suggest six types of major metasomatic agents. The 40Ar/39Ar ages of phlogopites vary in the 440–690 Ma range, with some at 1.6 Ga, suggesting multistage metasomatism.

Genesis and tectonic setting of an Early Palaeozoic skarn in the East Kunlun Orogenic Belt, China: constraints from adakitic rocks and garnet megacrysts

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) in the northern Tibet Plateau has undergone a long multi-stage history of drifting, accretion, and collision of the Proto- and Palaeo-Tethyan oceans. It has been addressed a trench-arc-back-arc basin system of the Proto-Tethys in the E-KOB, and the tectonic history of this major ocean has been well constrained by previous studies. However, the evolution of the back-arc basin still remains unclear. In this study, the Yugusayi monzodiorite, quartz-bearing syenite, and gradually contacted skarn rocks characterized by garnet megacrysts, in the North Qimantagh Belt were investigated. To reveal the tectonic setting and skarnization mechanism, geochemical and geochronological studies were conducted on the intrusions and garnet skarn, and the elemental compositions of the garnet megacrysts were also analysed. These monzodioritic rocks have high MgO (6.87–8.31 wt.%), Sr and La contents, low Y and Yb contents, and relatively high La/Yb (31–55) and Sr/Y (41–51) ratios, resembling to high-Mg adakites. They display right-sloping chondrite-normalized REE patterns, enriched in LILEs, and depleted in Ti with significant Nb-Ta troughs, which are similar to those of continental arc rocks. Zircons from monzodiorite and skarn samples have contemporaneous U-Pb ages of 459 ± 3 Ma and 454 ± 4 Ma with εHf(t) values of − 8.53 to − 4.64 and − 7.7 to + 12.8, respectively. In addition, the chemical zoning garnet megacrysts are composed mainly of grossular and andraditic grossular, and are characterized by spoon-shaped REE patterns. Based on our new data and the regional geology, it can be concluded that the Yugusayi intrusive rocks formed in a subduction-related tectonic setting and resulted in the skarnization. These data not only provide key constraints on the initial subduction of the Qimantagh back-arc basin’s oceanic crust, but also offer a unique opportunity to explore the Caledonian skarnization in the Qimantagh Belt.

Compositional characteristics of mineralised and unmineralised gneisses and schist around the Abansuoso area, southwestern Ghana

ABSTRACT Gold-bearing granitoid deposits have recently been discovered in the Birimian of Ghana but their mode of formation and ore genesis remain enigmatic. This study presents petrographic, and geochemical characteristics of mineralised and unmineralised (gold grade >0.05 and <0.05 g/t respectively) granitoids (now gneisses) and schists (metasedimentary) to evaluate their petrogenesis/provenance, and relationship to gold mineralisation in the Abansuoso area. The unmineralised rocks comprise biotite- and hornblende-biotite gneisses, sericite-quartz, carbonate-sericite, and biotite-quartz schist. The mineralised varieties are biotite-, muscovite gneiss, iron-carbonate-sericite, carbonate-sericite-quartz, chlorite-carbonate, and biotite-carbonate schist. The mineralised and unmineralised gneisses are both metaluminous and peraluminous. Both mineralised and unmineraised gneiss and schist show Nb-Ta trough, depleted LILE and enriched HFSE although widespread overall trace element concentrations for the mineralised rocks on UCC-normalised multi-element diagram, suggestive of their formation in an arc setting. This suggests coeval granitic plutonism and sedimentation with subduction-accretion during the Eburnean orogeny, hence, mineralisation may be orogenic-type.

Palaeoproterozoic arc related supracrustal units from the Tasiilaq Region, SE Greenland: Insights into the convergence of the Rae and North Atlantic Cratons

The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record the collision and suturing of the Rae Craton and North Atlantic Craton during the 1.9 Ga Nagssugtoqidian orogeny. The original mineralogy and textures of the supracrustal rocks have been largely overprinted by amphibolite facies metamorphism, obscuring the original protoliths and complicating the interpretation of the early Earth surface environment in which these rocks formed. Here, we present new major and trace element data for the Schweizerland, Kuummiut and Isertoq Terranes, with the former two representing the Rae craton, and the latter the northern margin of the North Atlantic Craton. Supracrustal lithologies include metapelites, marbles, calc-silicate rocks and mafic to felsic volcanic rocks and/or shallow intrusive rocks. Pelites normalised to Post Archaean Australian shales have (sub)horizontal rare earth element patterns, with La/SmCN: 1.61 – 23.91 and Eu/Eu* 0.34 – 1.83 and Th/Sc 1. The volcanic rocks are enriched relative to the primitive mantle (PM) with parallel to sub parallel PM-normalised heavy rare earth elements. They have a suprasubduction zone signature with distinct Nb-Ta troughs. The amphibolites can be further divided into those that interacted with the crust; back-arc basalts with subtle Nb-Ta troughs; and fore-arc basalts with distinct Nb-Ta and Zr-Hf troughs and negative Ti anomalies. The marbles consist of a Mg-rich dolomite group with δ13C 0.48 to 0.91 ‰ and δ18O −6.13 to −7.57 ‰; and Mg-poor calcitic carbonates with δ13C −0.74 to 0.17 ‰ and δ18O −10.81 to −14.87 ‰. This isotopic composition suggests formation at ∼ 2050 Ma during the low δ13C excursion following the Great Oxidation Event. We propose that the supracrustals of the Tasiilaq region were deposited at 2050 Ma in a series of interconnected basins during dual subduction convergence of the North Atlantic and Rae cratons, with development of an island arc with back arc basin in the north, and a continental arc in the south. These basins were inverted during continental collision, thrust, and folded into the Archaean Kuummiut and Isertoq TTG-dominated terrane basement, before being subjected to the region-wide amphibolite facies metamorphism at ∼ 1840 Ma during the Nagssugtoqidian orogeny.

Provenance of Late Ordovician sedimentary rocks in the SE Yangtze block: Implications for deposition in an active continental margin

The Jiangnan Orogen separates the Yangtze block from the Cathaysia block to the southeast. Several competing models have been proposed for the evolution of both blocks and the intervening orogen, which mainly focus on aspects of igneous and metamorphic rocks. Limited research has been conducted on the sedimentary rocks in this region. Provenance of the Late Ordovician sedimentary rocks in the northeastern Jiangnan Orogen has been constrained based on systemic analyses of petrology, heavy mineral assemblages, geochemistry, and detrital zircon U-Pb ages, which provide vital clues for reconstructing the evolution of this belt during the early Paleozoic. Late Ordovician deposits in the northeastern Jiangnan Orogen consist of conglomerates, sandstones, siltstones, shales, calcareous mudstones, and limestones. These strata were deposited in a shallow to deep sea environment, showing water depth deepening to the northwest. The sandstone contains lithic graywackes and feldspar-lithic graywackes, with low compositional and textural maturities. They consist, on average, of quartz clasts (27%), feldspar clasts (18%), and lithic fragments (55%). The lithic fragments are angular, including andesites, rhyolites, tuffs, granites, slates, phyllites, quartzites, siltstones, and chert, which also occur in conglomerate form. Minor pyroxene, chromite, magnetite, and garnet grains occur in the sandstone. Geochemical analyses show that the sandstone and siltstone have widely variable Fe2O3T + MgO (4.43–12.05%), Cs, V, Ni, and Ti contents. They also exhibit a distinct negative Nb-Ta trough compared to the average upper continental crust and have light rare earth element (LREE) enrichment patterns with obvious Eu anomalies, similar to subduction-related rocks in an arc setting. Detrital zircon U-Pb ages demonstrate a mixed source from ca. 784 Ma and ca. 455 Ma. Paleocurrent indicators point to a southeastward origin. These results demonstrate that Late Ordovician sedimentary deposition occurred in a northwestward-facing active continental margin along the southeastern Yangtze block.

Early Cretaceous granitoids in the Southern Pamir: Implications for the Meso-Tethys evolution of the Pamir Plateau

Although it is generally accepted that the Pamir was formed by long-term terrane drifting and accretion to the south margin of the Asia, the tectonic evolution during the Mesozoic remains unclear. The voluminous Cretaceous intrusions in the Southern Pamir-Karakorum provide key information for understanding the Mesozoic evolution in this region. In this contribution, we report detailed petrography, ages, mineral geochemistry, whole-rock elemental and SrNd geochemistry and in situ zircon LuHf isotopic compositions of the Early Cretaceous granitoids from the Chinese Wakhan Corridor, Southern Pamir. These granitoids are composed primarily of granodiorite and minor monzogranite and host dioritic enclaves. The granitoids were emplaced at 107–102 Ma, based on their zircon UPb ages. They have 63–73 wt% SiO2, Mg# between 39 and 53 and ACNK mostly lower than 1.1, and are I-type granites. The granitoids display arc-like geochemical signatures characterized by LREE-enriched patterns, variable negative Eu anomalies (LaN/YbN = 11.38–23.27; Eu/Eu* = 0.36–0.74), and a significant NbTa trough (Nb/La = 0.24–0.41). Isotopically, the granitoids have negative whole-rock εNd(t) (−8.73–7.35) and zircon εHf(t) (−17.7–5.3) values. The coeval dioritic enclaves (ca. 104 Ma) contain 57–63 wt% SiO2, and have high Mg# (46–57), low Cr (6.86–79.50 ppm), and Ni (2.88–10.00 ppm). These enclaves exhibit arc-like trace element patterns (Nb/La = 0.34–0.43) with variable negative Eu anomaly (Eu/Eu* = 0.52–0.87), uniform whole-rock εNd(t) (−4.74–4.73), and a large range of zircon εHf(t) values (−13.3–3.7). In line with their rock association and geochemistry, we suggest that the dioritic enclaves were derived from melting of a phlogopite-bearing mantle source modified by subduction-related process, followed by fractional crystallization and later assimilation by granitic hosts. These Early Cretaceous granitoids were most likely generated by partial melting of the Precambrian lower crust with variable involvement of mafic magma parental to the dioritic enclaves. Our new data, combined with the previous studies, reveal that the widespread Early Cretaceous magmatism in the Southern Pamir-Karakorum were most likely generated by flat-slab subduction of the Meso-Tethys Ocean during the Cretaceous.

Early Neoproterozoic (ca. 913–895 Ma) arc magmatism along the central–western Korean Peninsula: Implications for the amalgamation of Rodinia supercontinent

Neoproterozoic arc magmatism in or around the Korean Peninsula provides important insights into the history of the Rodinia supercontinent. Here we report the petrology, zircon U-Pb ages, geochemistry, and Hf isotope data from the Early Neoproterozoic (ca. 913–895 Ma) igneous complexes (gabbro, diorite and granitic dyke) in the northern part of the Gogunsan Islands along the southwestern margin of the Hongseong–Imjingang Belt in the central–western Korean Peninsula. The gabbro and diorite- yielded zircon U–Pb ages of ca. 913 Ma, and ca. 909–899 Ma, respectively. The granitic dykes within this complex show ages of ca. 903–895 Ma. The gabbro-diorite, and granite belong to the subalkaline series, and are characterized by large ion lithophile element (LILE) enrichments, Nb–Ta troughs, and depletion in P and Ti anomalies similar to magmatism in the continental arc setting. The initial 176Hf/177Hf ratios and εHf(t) values obtained of zircon grains are in the ranges of 0.282132–0.282268 and –2.7 to 2.3 for gabbro, 0.282010–0.282363 and –6.4 to 6.0 for diorite, and 0.282071–0.282285 and –5.0 to 2.6 for granitic dykes, suggesting juvenile crust production with mixing of older crust materials. In combination with previously published U-Pb ages and geochemical features of Late Paleoproterozoic, Mesoproterozoic, and Early Neoproterozoic rift-related rocks in and around the central–western Korean Peninsula, the Early Neoproterozoic arc magmatism reported in this study provides clear evidence for a tectonic transition from the disruption of the Columbia supercontinent to the amalgamation and disruption of Rodinia supercontinent.

Geochemistry of Dalma metavolcanic Suite from Proterozoic Singhbhum Mobile Belt, Eastern India: Implications for Petrogenesis and Tectonic Setting

ABSTRACT The Dalma metavolcanic belt is composed of ultramafics at the base, tuffs on top and basalts in-between. It extends for about 200kms east-west in the middle of the Proterozoic Singhbhum Mobile Belt. Ultramafics are chiefly composed of actinolite, hornblende, relic pyroxene and olivine. The main mineral constituents of basalts are actinolite, hornblende, chlorite, epidote, clinozoisite and plagioclase whereas; tuffs are composed of xenocrysts of biotite, quartz, k-feldspar, plagioclase and iron oxides. Ultramafic samples have high MgO (&amp;gt; 21 wt.%) and low SiO2 (&amp;lt; 42 wt.%), Al2O3 (&amp;lt; 10 wt.%) and Na2O+K2O (&amp;lt; 1 wt.%). CaO shows large variation in these samples from 3.91 to 7.77 wt. %. In studied basalt samples SiO2, TiO2, Al2O3, MgO and alkalis (Na2O+K2O) show variation like (45.7-48.3 wt.%), (0.64-0.97 wt.%), (9.3-13.9 wt.%), (8.5-13.6 wt.%) and (0.95-4.64 wt.%) respectively. In studied tuffs the SiO2, TiO2, Al2O3, MgO and alkalis show variation from (48.9-49.8 wt.%), (0.7-0.8 wt.%), (13.6-13.9 wt.%), (8.4-9.1 wt.%) and (2.39-2.85 wt.%) respectively. In general, studied samples show light rare earth element depleted patterns and flat patterns for heavy rare earth elements similar to MORBs on chondrite normalized REE diagrams. The overall enrichment of LILEs (e.g., Rb, K and Th) and depletion of HFSEs (e.g., Nb-Ta trough) on primitive mantle normalized multi-element diagrams are similar to subduction zone or islamd arc basalts. Ratios like Ti/Y, Ti/Zr, Zr/Nb and Ti/V are also similar to those found in arc related volcanic suites. Sm/Yb vs La/Sm diagram indicates generation of melts from the spinel lherzolite mantle source. On tectonic setting discrimination diagrams like Zr vs Zr/Y, Zr vs Ti and Ti vs V majority of studied samples clearly show island arc tectonic affinity.

Ultramafic–mafic and granitoids supra‐subduction magmatism in the southern Ashanti volcanic belt, Ghana: Evidence from geochemistry and Nd isotopes

Geochemical and Nd isotope data are presented for Palaeoproterozoic ultramafic to mafic rocks and granitoids which are associated with volcanic rocks in the southern part of the Ashanti greenstone belts of Ghana. The Prince's Town granitoids display subduction‐zone geochemical features, with some showing signatures similar to high‐SiO2 adakites (HSA). Their initial εNd (2.1 Ga) values range from −1.01 to +2.92, and they have TDM2 of 2.17–2.51 Ga. The gabbros show slightly LREE‐depleted and ‐enriched patterns, Th–U troughs, negative Nb, Zr–Hf and Ti anomalies, spikes in Sr and Pb, have εNd (2.1 Ga) values ranging from −1.23 to +5.23, and TDM2 values from 2.08 to 2.57 Ga. The Ahama ultramafic body is characterized by two groups of pyroxenites. Both groups show LREE enrichment patterns, display Th–U and Nb–Ta troughs, minor or pronounced negative Zr–Hf and Ti anomalies. However, the Group I pyroxenites exhibit minor Ce anomalies and lower total REE contents (28.1–32.8 ppm) whereas Group II pyroxenites show significant negative Ce anomalies and relatively higher total REE contents of 57.1–124 ppm. The pyroxenites show negative initial εNd (2.1 Ga) values (−0.58 to −5.68) and have TDM2 values of 2.52 to 2.75 Ga. The pyroxenitic and gabbroic rocks were most likely originated from a subduction‐related lithospheric mantle. The Aketakyi ultramafic–mafic complex is made up of mainly cumulate dunite, harzburgite, pyroxenites, and gabbro. The rocks display LREE‐depleted to flat patterns with low total REE contents (1.71 to 22.3 ppm) and demonstrate a supra‐subduction affinity. They show high positive initial εNd (2.1 Ga) values (+3.69 to +4.93), and TDM2 values of 1.99 to 2.04 Ga, which suggest that they were derived from depleted mantle magmas and were juvenile at their time of formation. The Nd isotopic data provides evidence for a possible contamination of the juvenile Birimian crust of the southern Ashanti belt by some amount of a pre‐Birimian (or Archaean?) crustal material. The trace element and isotopic signatures as well as the field relations of the studied granitoids and mafic–ultramafic rocks show they were derived through supra‐subduction‐related magmatism during the Palaeoproterozoic.

An Ediacaran back-arc basin preserved in the Transversal Zone of the Borborema Province: Evidence from geochemistry, geochronology and isotope systematics of the Ipueirinha Group, NE Brazil

The Ipueirinha Group is a ca. 75 km-long by 13 km-wide succession of metavolcano-sedimentary rocks situated in the westernmost Transversal Zone, Borborema Province, NE Brazil. It is mostly composed of marine terrigenous sedimentary rocks represented by immature metarhythmites and quartzites, local metaultramafites, rhyolitic metavolcaniclastic rocks and rare lenses of metamarl. The metavolcaniclastic rocks show highly fractionated chondrite-normalized REE patterns (LaN/YbN = 3.68–8.94) with slight to moderately negative Eu anomalies (Eu/Eu* = 0.72–0.94), incompatible trace element signatures similar to those of volcanic arc-related rocks, including a Nb-Ta trough, and negative ƐNd(t) ranging from −3.4 to −12.2. Metarhythmites show LREE enrichment (LaN/YbN = 4.83–15.32), moderate Eu anomalies (Eu/Eu* = 0.68–0.81), and incompatible element signatures consistent with provenance from a continental arc. Nd isotope data indicate important detrital contribution from juvenile sources for these metarhythmites, as reflected by the ƐNd(t) values between −3.6 and 0.3. Detrital zircon LA-ICP-MS U-Pb data reveal that the main sources of detritus for the metarhythmites are Neoproterozoic rocks (maximum age peak at ca. 845 Ma and younger zircons at ca. 636 Ma). Hf isotope data obtained from detrital zircons show evidence for an important period of juvenile magmatism in the Borborema Province from 895 to 834 Ma (ƐHf(t) = 2.0–5.7), and suggest that the Neoproterozoic crustal evolution of this province involved mixing between older sources with negative ƐHf(t) and younger juvenile sources. Metaultramafites associated with the Ipueirinha Group show ƐNd(t) ranging from −1.3 to −8.5 and TDM = 1.33–1.64 Ga. The whole unit was submitted to four deformation phases and to heterogeneous metamorphic conditions from lower to upper greenschist facies. Additional geochronological data obtained for a granitic sill (578 ± 13 Ma; zircon U-Pb) along with monazite U-Pb data from a chloritite sample (595 ± 29 Ma) indicate that strike-slip deformation, metamorphism and syn-orogenic melt generation coexisted in the Ipueirinha Group during the Late Ediacaran. We suggest that the Ipueirinha Group comprises a Cryogenian/Ediacaran back-arc basin to the Tamboril-Santa Quitéria magmatic arc, which is one of the few records of initial convergence tectonics in the West Gondwana margin of the Borborema Province.

Tonian granitic magmatism of the Borborema Province, NE Brazil: A review

Tonian granitoids, today augen-gneisses and migmatites, showing crystallization ages ranging from 870 Ma to 1000 Ma occur in the Borborema Province, NE Brazil. The majority of them have ages within the 925–970 Ma interval. Few intrusions with ages of ∼1.0 Ga and <900 Ma occur in the Transversal and South subprovinces. The Tonian granitoids constitute the most expressive magmatic rocks of the Cariris Velhos event. The studied granitoids (herein CVG -Cariris Velhos granitoids) intrude slightly older bimodal (but mostly felsic) volcanic successions and metasedimentary sequences in the Transversal and South subprovinces. Tonian granitoids are unknown in the North subprovince. The CVG comprise mainly coarse-grained augen-gneisses of granite to granodiorite composition. Fe-rich biotite (annite) is the main mafic mineral phase, constituting up to 15% of the modal composition. Garnet, muscovite and tourmaline occur as accessory phases in many plutons.The CVG augen–gneisses have high SiO2 (>71%) and alkali contents, they vary from slightly peraluminous to slightly metaluminous, and from slightly magnesian to typical ferrroan rocks. In the migmatized orthogneisses the SiO2 contents are usually <70%. Trace element variations in the CVG are extensive, reflecting the migmatization recorded in some plutons and/or distinct sources. They are Ca-, Sr- and Nb-poor, showing variable Ba (100–1260 ppm), Rb (164–400 ppm) and Zr (144–408 ppm) contents, and high abundances of Y (>40 ppm). The chondrite normalized REE patterns are characterized by strong to moderate negative Eu anomalies (Eu/Eu* = 0.23–0.70). In general, the spidergram patterns show deep troughs at Ti, P, Ba and Sr and less pronounced Nb–Ta troughs. These patterns are similar to those reported for anorogenic granites evolved from mixtures of magmas from both crustal and mantle sources. The CVG exhibit TDM model ages ranging from 1.9 to 1.1Ga, with slightly negative to slightly positive ƐNd(t) values, suggesting the involvement of distinct proportions of mantle and crustal components in the source of their protoliths.There is no consensus in the literature about the tectonic setting of the CVG ie they have been related to either continental margin magmatic arc, with possible back–arc association, or extention-related setting, with generation of A-type granites. However, all the available geochemical data suggest that the CVG represent extension related magmatism. The geochemical signature associated to bimodal volcanism, including pyroclastic rocks, with similar ages, and absence, up to now, of evidence for metamorphism of Tonian age, support the hypothesis of extension - related magmatism.

Geochemistry and tectonic setting of the Paleoproterozoic metavolcanic rocks from the Chirano Gold District, Sefwi belt, Ghana

Major and trace elements, including rare earth elements (REEs) data are presented for metavolcanic rocks of the Paleoproterozoic Birimian Sefwi belt to determine the geochemical characteristics as well as the possible tectonic setting of emplacement of these rocks. In order to accomplish the aim of the study, the petrographical characteristics of the rocks were examined coupled with analysis of the rocks for their whole-rock major and trace elements contents by ICP-AES and ICP-MS methods respectively. The rocks have been classified as basalt/basaltic andesites and dolerites based on their textural and mineralogical compositions. It is observed that the rocks have suffered various degrees of alteration evident in the overprinting of primary minerals such as pyroxenes and plagioclase by chlorite, epidote, sericite and others. Generally, the rocks are moderately deformed and may have experienced at least greenschist metamorphism. The basalt/basaltic andesites are derivative magmas [Mg# (20–51), Cr (10–220 ppm) and Ni (5–137 ppm)], and show flat REE to fractionated REE patterns with (La/Sm)N = 1.36–3.90, (La/Yb)N = 1.17–5.32 and strong negative to non-existent Eu anomalies (Eu/Eu* = 0.51–1.03). N-MORB-normalised multi-element diagrams show that the rocks have geochemical patterns characterised by enrichment in LILE relative to HFSE and in LREE relative to HREE. The basalt/basaltic andesites exhibit characteristics of subduction zone-related magmas, such as pronounced negative Nb–Ta anomalies, slightly negative Hf and variable negative Ti anomalies. The dolerites do not vary much from the basalts and basaltic andesites. The MgO and Fe2O3 values of the dolerite range from 2.97 to 6.93 and 5.98 to 14.35 wt.% respectively, corresponding to Mg#s of 38–62. LREEs enrichment over HREEs with (La/Sm)N ranging from 0.61 to 4.61 and (Gd/Yb)N ranging from 0.99 to 2.91 is also typical of these rocks. The dolerites also exhibit quite invariable Eu anomalies (Eu/Eu* = 0.81–1.00) and display a pronounced Nb–Ta trough and a minor negative Ti anomaly, suggesting arc characteristics. The metavolcanic rocks from the study area generally exhibit subduction-related setting characteristics with evidence of a sub-lithospheric contamination.

Two episodes of Paleoproterozoic metamorphosed mafic dykes in the Lvliang Complex: Implications for the evolution of the Trans-North China Orogen

Tectonic evolution of the Trans-North China Orogen (TNCO) at ∼2.2–1.8Ga is of great importance considering the prolonged controversial issue of when and how the Eastern and Western blocks were assembled to form the coherent basement of the North China Craton. We identified three types of mafic dykes in the Lvliang Complex, of which one was unmetamorphosed and two were subjected to amphibolite facies metamorphism. Zircon LA-ICP-MS U-Pb dating of six metamorphosed mafic dykes yields magmatic crystallization ages of 1919±18Ma, 1939.6±8.2Ma, 1949.9±9.6Ma, 1944±17Ma, 2116±15Ma and 2116±13Ma, indicating two major phases of mafic dykes emplacement at ∼2.11Ga and ∼1.94Ga, respectively. In situ zircon Hf isotope data of the ∼2.11Ga samples vary in large ranges (over fifteen epsilon units) with the highest ɛHf(t) value approaching the depleted mantle array and the lowest value plotting onto the evolution line of the ∼3.1Ga crust, indicating assimilation of depleted mantle-derived magma by old crustal material. For the 1.94Ga samples, almost all the ɛHf(t) values are positive with abundant data near or equal to the contemporary depleted mantle implying limited crustal assimilation. Geochemical data suggest that, most of the ∼2.11Ga dykes are alkaline whereas almost all the ∼1.94Ga dykes are subalkaline. The ∼2.11Ga dykes show variable LREE enrichment (La/Yb=1.9–10.3), prominent Nb-Ta troughs, high Zr contents and Zr/Y ratios while the ∼1.94Ga dykes display no pronounced fractionation between LREE and HREE, and also significant Nb-Ta-Ti negative anomalies, but low Zr contents and Zr/Y ratios. According to the Hf isotopic and the geochemical features, we suggest that the ∼2.11Ga dykes were probably derived from a sub-continental lithospheric mantle in a continental rift with some asthenospheric contribution, whereas, the ∼1.94Ga dykes were arc-related with little crustal contamination during forming and ascending process and the Nb-Ta troughs were probably attributed to subduction-related fluids and melts. Together with the previous researches of the TNCO and other parts of the North China Craton (NCC), we tend to think that there was a whole rift–subduction–collision cycle during the period of ∼2.2–1.85Ga and the subduction process initiated at ∼1.94Ga at least.

Enriched mantle source for the Central Atlantic magmatic province: New supporting evidence from southwestern Europe

Remnants of the Central Atlantic magmatic province (CAMP), emplaced ca. 201Ma during the rifting phases leading to Pangaea breakup, are still preserved in southwestern Europe (SWE) in the form of sills, dykes and lava flows. Low–Ti (TiO2 0.48–1.46wt.%) tholeiitic basalts and basaltic andesites crop out as sills only in the Pyrenean area, as dykes (especially the Messejana–Plasencia dyke) from central Spain to the Atlantic coast, and as lava flows within sedimentary basins in Southern Portugal. Here we present new geochemical data (major and trace elements, mineral chemistry and combined Sr–Nd–Pb–Os analyses) on 132 samples, aiming to investigate the mantle source of these rocks and correlate them with magmatism from other areas of the CAMP. Crustal-like signatures in incompatible element patterns (Nb–Ta troughs, Pb peaks, generally shared by most CAMP rocks) and the enriched Sr–Nd–Pb isotopic characters (87Sr/86Sr200Ma 0.70529–0.70657; 143Nd/144Nd200Ma 0.51238–0.51225; 206Pb/204Pb200Ma 18.15–18.48; 207Pb/204Pb200Ma 15.57–15.68; 208Pb/204Pb200Ma 37.99–38.52) apparently argue in favor of crustal assimilation playing an important role in the evolution of these magmas. However, the low initial 187Os/188Os values (0.1298±0.0056) as well as the restricted geochemical variations shown by SWE-CAMP rocks over such a large area limit the crustal assimilation of various Iberian lithologies to small amounts. We thus locate this enrichment in the mantle source, in the form of upper and lower crustal material recycled during earlier subduction-related events. This process, while imparting crustal signatures to incompatible elements and Sr–Nd–Pb isotopes, would not alter the Os isotopic signature, dominated by the peridotite. The mixed contribution of 3–7% of local upper (pelitic) and lower (felsic granulitic) crust is sufficient to enrich a depleted mantle source, which can be either the sub-SWE lithosphere or the upper depleted asthenosphere. Similar processes of crustal recycling within the upper mantle have been recognized to be responsible for the mantle source enrichment in other areas of the CAMP (Eastern North America). Geochemical correlations of the here studied tholeiites with CAMP rocks from other areas inscribe European basalts within the main pulse of CAMP magmatism.A subset of samples from Southern Portugal (here defined high-Sr dykes) shows different major and trace element geochemistry (e.g. Sr and CaO enrichment, SiO2 depletion) as well as more radiogenic 87Sr/86Sr200Ma (0.70669–0.70749) and Pb isotopic ratios (e.g., 206Pb/204Pb200Ma 18.55) at similar 143Nd/144Nd200Ma (0.51232–0.51224). This reflects a different magmatic evolution for these rocks, dominated by the late-stage assimilation of 10–20% local carbonates.

Petrology and geochemistry of the ultramafic-mafic Mawpyut complex, Meghalaya: a Sylhet trap differentiation centre in northeastern India

The present article describes, for the first time, petrological and geochemical details of the Mawpyut differentiated complex which is related to the Sylhet trap located at Jaintia Hills district, Meghalaya, northeastern India. The Mawpyut complex occurs as an arcuate body that intrudes into the surrounding Shillong Group rocks. The complex in general contains ‘ultramafic’ and ‘mafic’ rocks, as well as minor syenitic veins that postdate the main units. The lithotypes correspond to cumulate and noncumulate units. The cumulate unit is represented by olivine clinopyroxenite, clinopyroxenite, plagioclase-bearing ultramafic, olivine gabbronorite, mela-gabbronorite, melagabbro, orthopyroxene gabbro, and gabbro, all with a pronounced cumulus texture. The noncumulate unit is marked by gabbro, monzonite, monzodiorite, and quartzsyenite. The use of several major and trace element variation diagrams suggests that magmatic differentiation led to the formation of cumulate and noncumulate units. In chondrite-normalized REE diagrams the cumulate rocks show flat LREE and MREE patterns and a moderate positive Eu anomaly (in plagioclase-bearing ultramafics) due to plagioclase cumulation. The rocks of the noncumulate unit show a strongly fractionated REE pattern and no Eu anomaly. The noncumulate mafic rocks are geochemically comparable to high-phosphorous/high-titanium basalts (HPT) indicative of low pressure fractional crystallization. In a primitive mantle-normalized multielement diagram some of the cumulate rocks show pronounced negative anomalies for K and P, indicating anorogenic mafic magmatism in a within-plate setting. The rocks of the noncumulate unit show a slight negative anomaly for Yb and a Nb–Ta trough, indicating a subduction-related signature that perhaps is inherited from subducted sedimentary rocks incorporated during crustal contamination of the derived magma (left after crystal cumulation) with country rocks. Various trace element ratios for the cumulate mafic rocks indicate parent EMI/EMII/HIMU sources with a very limited crustal signature. The noncumulate mafic rocks (corresponding to the derived evolved magma) indicate EMI/EMII/HIMU sources with a pronounced crustal contamination. The Sr–Nd isotopic compositions of the Mawpyut samples typically plot in the continental flood basalt field, with an affinity to the EMII source. The isotopic compositions of the noncumulate rocks also clearly indicate crustal contamination. We suggest that partial melting (involving garnet in the residue) of the enriched mantle source EMI/EMII/HIMU could have derived the parental melt; this melt, in turn, underwent assimilation and fractional crystallization to produce the variety of cumulate-noncumulate lithologies of the Mawpyut complex. Copyright © 2013 John Wiley & Sons, Ltd.

Recognition of Late Cretaceous Hasanbag ophiolite-arc rocks in the Kurdistan Region of the Iraqi Zagros suture zone: A missing link in the paleogeography of the closing Neotethys Ocean

The Hasanbag igneous complex is situated near Sidekan, 100 km northeast of Erbil city, Kurdistan Region, within the Iraqi Zagros suture zone. It forms part of an ophiolite-bearing terrane referred to as the Upper Allochthon or Gemo-Qandil Group. The Hasanbag igneous complex consists predominantly of calc-alkaline basaltic andesites to andesites cut by microgabbro and diorite dikes, which previously were interpreted as a part of the Eocene Walash volcanic group. However, 40 Ar- 39 Ar dates on Hasanbag igneous complex kaersutite (magmatic hornblende) indicate an Albian–Cenomanian age (106–92 Ma). This reveals a previously unrecognized portion of the Late Cretaceous, Neotethyan ophiolite-arc complexes in the Iraqi Zagros suture zone that may well represent lateral equivalents of the ophiolite-arc sequences in Oman-Neyriz and Cyprus. Plagioclase in all rock types has been strongly albitized, such that vestiges of more calcic igneous plagioclase are rare. Other relict igneous silicates are kaersutite, augite (Wo 44.46 -En 46.43 -Fs 9.1 ), and rarely anorthoclase. The whole-rock geochemical variation can be explained by fractionation of plagioclase, clinopyroxene, hornblende, magnetite, and apatite but with superimposed alteration. Nb/Yb-Th/Yb relationships show that all the samples fall within the compositional field of arc-related rocks and above the field of the mid-ocean-ridge basalt (MORB)–ocean island basalt (OIB) mantle array. Hasanbag igneous complex samples show enrichment relative to normal (N)-MORB in the large ion lithophile elements and depletion in the high field strength elements, with characteristic Nb-Ta troughs. Hence, they were generated from the mantle wedge within a suprasubduction-zone setting influenced by volatile components from the downgoing slab. The Hasanbag igneous complex shows similar geochemical trends to arc rocks from Neyriz in southwestern Iran. We interpret the Hasanbag igneous complex as a remnant of the Late Cretaceous ophiolite-arc system (named here the Hasanbag arc complex) that developed within the Neotethys Ocean and was subsequently accreted to the Arabian plate during the Late Cretaceous to Paleocene. This represents the first time in Iraq that Late Cretaceous ophiolite-arc complexes have been identified in the Iraqi Zagros collision zone. The Hasanbag arc complex is unequivocally separated from the lithologically similar but younger Eocene–Oligocene Walash-Naopurdan arc-backarc complex, in the same general area but in a structurally lower thrust slice.

Mafic rocks from Erinpura gneiss terrane in the Sirohi region: Possible ocean-floor remnants in the foreland of the Delhi Fold Belt, NW India

A small isolated mafic body occurs to the south of Sirohi near village Daba within the Neoproterozoic Erinpura Granite in the southern sector of the Proterozoic Delhi Fold Belt in NW India. This mafic body occurs close to a 100 m wide NE–SW trending shear zone (Daba Shear Zone) which overprints the felsic rock fabrics. Further south, a small mafic body near village Kui was also sampled which forms the southern limit of the Phulad Ophiolite Suite which is a 300 km long major NE–SW trending lineament, described as Western Margin Fault. Some of the lithological components of the Daba mafic body show locally preserved magmatic fabric but completely transformed mineralogies under lower amphibolites facies metamorphic conditions where two-stage deformation has been inferred. Magnetic fabric analysis underlines a general correspondence of structural elements in both felsic and mafic lithologies. Binary correlations of Zr with other high field strength elements underline fractionation as the main process in the evolution of Daba and Kui rocks. Geochemical characteristics indicate subalkaline tholeiitic basalt affinity for these mafic rocks. The trace element characteristics, such as enriched LIL elements, high Th, absence of negative Nb anomalies and depletion in compatible elements in Daba samples suggest an enriched mantle source and lower degree of melting. The trace and rare earth element characteristics for Kui (Th anomaly, Nb–Ta trough and less spiked patterns, flat REE trends) indicate derivation from a refractory mantle source affected by fluids derived from subduction. Distinct differences in trace and REE characteristics between Daba and Kui can be interpreted in terms of different stages of ophiolite development.

Early Mesozoic Plutonism of the Cordillera de la Costa (34°–37°S), Chile: Constraints on the Onset of the Andean Orogeny

New isotopic ages constrain the onset of the Andean Orogeny in central Chile (34°–37°S). They define three pulses of Early Mesozoic magmatism in the Cordillera de la Costa: granitic magmatism at 225–220 Ma, bimodal magmatism at 210–197 Ma, and mafic magmatism at 170–150 Ma. Each pulse has distinct field, petrographic, geochemical, and Sr-Nd-Pb isotopic characteristics and crystallization conditions. Major magmatic sources are the regional Mesozoic subarc-subcontinental mantle and the Paleozoic Chilean continental crust, with the importance of juvenile contributions increasing with time. All three Early Mesozoic magmatic pulses display geochemical signatures that are characteristic of subduction-related magmas of magmatic arcs, such as a Nb-Ta trough and a positive Pb anomaly. The 210–197-Ma magmatism is peculiar for its bimodal nature, the occurrence of transitional A-type granitoids, the presence of fayalite granites, and its association with an episode of crustal extension. A contemporaneous accretionary wedge nevertheless indicates ongoing subduction at that time. The 210–197-Ma magmatic pulse thus was formed in an extensional suprasubduction zone environment. It marks the transition from the Gondwana Orogeny to Andean-style subduction magmatism. We argue that the onset of the Andean Orogeny was related to a reconfiguration of the tectonic conditions of plate convergence during overall near-continuous subduction and arc magmatism since the Paleozoic.

Origin of the Late Cretaceous syenite from Yandangshan, SE China, constrained by zircon U–Pb and Hf isotopes and geochemical data

The Yandangshan syenite is a representative Late Cretaceous igneous pluton cropping out in SE China. U–Pb zircon dating using LA‐ICP‐MS yielded a crystallization age of 98±1 Ma for the syenite. Petrographically and geochemically of shoshonitic affinity, it is enriched in LREE and LILE, and has a pronounced Nb–Ta trough in the primitive mantle‐normalized trace element spider diagram. Zircon εHf(t) values vary from −3.04 to −7.71, displaying a unimodal distribution. The syenite also has high Sr [(87Sr/86Sr) i = 0.7086–0.7089], low Nd [εNd(t) = −6.57 to −7.64] isotopic ratios, plotting in the enriched mantle field on an εNd(t) versus (87Sr/86Sr) i diagram. We propose that the Yandangshan syenite was generated by pyroxene‐dominated high‐pressure fractional crystallization from basaltic magma that was derived from an enriched mantle source. Although coexisting Yandangshan rhyolites have Sr–Nd isotopic compositions similar to the Yandangshan syenite, they were not derived from the same source. Instead, the rhyolitic magma was produced by partial melting of crustal materials as a result of the underplating of basaltic magma. The crust‐like Sr–Nd–Hf isotopic signature of the Yandangshan syenite is ascribed to mantle sources that were enriched by subducted sediments. Formation of Yandangshan syenite may represent roll‐back of the subducting palaeo‐Pacific plate and migration of the arc front to the Yandangshan area at ∼98 Ma.

A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle

There is no consensus on the processes responsible for near-coeval formation of Archaean continental crust (dominantly tonalite–trondhjemite–granodiorite: TTG), greenstone belts dominated by komatiitic to tholeiitic lavas (KT), and sub-continental lithospheric mantle (SCLM). The Douglas Harbour domain (2.7–2.9 Ga) of the Minto Block, northeastern Superior Province, has two TTG suites, the western and eastern Faribault–Thury (WFT and EFT), with embedded KT greenstones. Tonalites of both suites have high light/heavy rare-earth element ratios (L/HREE), high large ion lithophile element (U–Th–Rb–Cs–La: LILE) contents, positive Sr–Pb anomalies, and negative Nb–Ta–Ti anomalies. Such typical Archaean TTG signatures are commonly explained by melting of subducted oceanic crust, but could also originate by melting the base of thick basaltic plateaux formed above mantle upwellings (plumes), leaving behind restites containing pyroxene, garnet, and rutile. Field relationships (in situ segregation veins), phase equilibria (hornblende stabilized at lower crustal pressure), petrography (corroded epidote and muscovite phenocrysts, rare plagioclase phenocrysts), and trace element models, all imply that FT tonalite to trondhjemite evolution reflects hornblende-dominated fractional crystallization, not partial melting of subducted crust. The geochemistry of parental FT tonalites can be modeled by 15–30% melting of FT tholeiitic metabasalts, with residues of eclogite, garnet-websterite, or hornblende-garnet websterite. A minor residual Ti-phase such as rutile is also needed to generate negative Ti–Nb–Ta troughs in the TTGs. However, large volumes of eclogitic restites complementary to TTG are not observed either at the base of Archaean crustal sections, or in the SCLM. Additional problems with slab-melting models include: (a) the rarity of lithologies and associations characteristic of active margins (ophiolites, andesites, blueschists, accretionary mélanges, molasse, flysch, high-pressure belts, and thrust-and-fold belts); (b) the need to deliver plume-derived KT melt through the slab; and (c) extracting enough TTG melt from a subducting slab in the time available (200–300 my). In the plateau-melting model, heat for crustal anatexis is supplied by ongoing KT magma derived from mantle upwellings. However, SCLM rocks differ from predicted 1-stage mantle melting residua; and the voluminous residual eclogites complementary to TTG generation somehow need to be removed. These two problems might solve one another if the dense crustal restites disaggregated and mixed into the underlying depleted mantle. Mantle melting slows upon exhaustion of Ca–Al-rich phases, with large temperature increases needed to extract more melt from harzburgite residua. Physical addition of delaminated crustal restites would refertilize the refractory mantle, allowing extraction of additional melt increments, and might explain the ultra-depleted and orthopyroxene-rich nature of the SCLM. A hybrid source composed of 10% eclogitic restite of EFT tonalite generation, mixed with harzburgitic residues from 25% melting of primitive mantle, yields model melts with trace element signatures resembling typical Munro komatiites. Variations in the mineralogy and geochemistry of the delaminated component might account for the diversity of komatiite types. Degassing of hornblende-rich delaminated restites would transfer LILE to surrounding depleted mantle and could generate boninites. Fusion of undepleted metabasalt sandwiched among denser restites could generate sanukitoids. Mantle melt pulses generated by catastrophic delamination events would underplate nascent TTG crust and trigger renewed crustal melting, followed by delamination of newly formed eclogitic restites, triggering additional mantle melting, and so on. I posit that delamination of crustal restites catalyzed multi-stage melting of the SCLM and maturation of the Archaean continental crust. Thus, Archaean crust and SCLM are genetically inter-linked, and both form above major mantle upwellings.