Negative Nb Ta Anomalies Research Articles

Neoarchean arc basaltic magmatism and associated sulfide mineralization in the North China Craton: Evidence from the Taoke mafic-ultramafic complex in Shandong Province

The Neoarchean Taoke mafic-ultramafic complex is located in the eastern block of the North China Craton. We report the first geochronological and geochemical data for the complex, and use these data to unravel the fundamental tectonic-magmatic processes involved in the formation of this complex. The Taoke complex comprises a large elongated gabbronorite intrusion, a small sulfide-mineralized olivine websterite body in the center, and many dolerite dikes. The U-Pb isotope ages of magmatic zircon from the mafic and ultramafic intrusions are 2520.3 ± 6.5 Ma and 2537 ± 12 Ma, respectively, which are contemporaneous with abundant granitoids and boninitic basalts but ~620 myr older than the collision-related, granulite-facies metamorphism in the region. Like the coeval boninitic basalts, the Taoke mafic-ultramafic rocks are characterized by light-REE enrichments and negative Nb-Ta anomalies, which are characteristic of modern arc basalts. The εNd(t) values of the Taoke mafic-ultramafic rocks are from 1.0 to 3.3, within the range of the coeval basalts in the region. The εHf(t) values of zircon from the Taoke mafic-ultramafic complex are from 0.2 to 2.3. The integrated Nd-Hf isotope data indicate that crustal contamination is absent to negligible. No correlation between Th/Nb ratios and εNd(t) values are present, implying that the negative Nb-Ta anomalies of these rocks are not the result of crustal contamination but are original features of the magma. Our new data support the interpretation that the complex is a Neoarchean arc-type mafic-ultramafic complex. Liquid inversion using the La/Yb and Gd/Yb ratios of the associated dolerite dikes and coeval boninitic basalts indicates a garnet-bearing lherzolite mantle source. The depth of partial melting is estimated to be >85 km, which is within the range of mafic magma generation in modern arc settings. The results from this study demonstrate that by Neoarchean the style of arc basaltic magmatism already closely resembled the modern counterpart. Based on the presence of sulfide mineralization in the Neoarchean arc-type mafic-ultramafic complex at Taoke and the association of major magmatic Ni-Cu sulfide ore deposits with post-Archean arc-type mafic-ultramafic complexes elsewhere in the world, we suggest that exploration for this type of deposit in the Neoarchean arc-type mafic-ultramafic complexes including the Taoke complex in western Shandong, should continue.

Petrography and Geochemical Characterization of Dolerites from Figuil (Northern Cameroon) and Léré (Southwestern Chad)

This work presents the petrographic and geochemical data of the dolerite dykes crosscutting the Pan-African basement of Figuil (North-Cameroon) and Lere (South-West Chad) in order to approach their petrogenesis and their emplacement context. Two groups of dolerites have been highlighted by petrographic and geochemical studies. These groups were discriminated by their TiO2, Fe2O3, REE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I; group II, on the other hand, has higher MgO, Mg#, Sc, Ni and Cr contents. The mineralogical assemblage of these dolerites is made up by plagioclases, pyroxenes, olivine, oxides, amphibole, biotite and sometimes pyrite, calcite, apatite, epidote and chlorite. The behaviour of the major and trace elements suggest that studied dolerites have an evolution dominated by fractional crystallization. Most dolerite samples show higher REE concentrations and (La/Yb)N > 8.7, (Tb/Yb)N > 1.9 and Dy/Yb > 2 ratios characterizing a garnet-bearing mantle. The difference in incompatible elements between the two groups is explained by the degree of partial melting of the same source which becomes more important over time. Low (Ce/Yb)N values (3.3 - 11.58) also suggest relatively low partial melting degree of the source. Fractional crystallization process was possibly combined with minor crustal contamination as shown by enrichment of Th/Yb from group II to Group I that might be due to turbulent magma emplacement. The chemical compositions of these dolerites are similar to that of continental tholeiites with slightly moderate negative Nb-Ta anomalies which are attributed to crustal contamination of magmas. As other dolerites of Cameroon, continental tholeiitic signature of the studied dolerites is evidenced in geotectonic discrimination diagrams with Group II dolerite compositions falling within the field of tholeiitic basalts and group I within the field of alkali basalts.

Subduction-related petrogenesis of Late Archean calc-alkaline lamprophyres in the Yilgarn Craton (Western Australia)

We present a comprehensive petrographic, mineralogical and geochemical study of calc-alkaline lamprophyres (CAL) from the Archean Yilgarn Craton, Western Australia. Previous studies have shown that the emplacement age of CAL from the Eastern Goldfields Superterrane of the Yilgarn Craton is ~2684 to ~2640 Ma. A new Rb/Sr mica age for a CAL sample in the Western Yilgarn is ~2070 Ma. Both Archean and Proterozoic CAL analysed in this study display porphyritic textures and contain phenocrysts of amphibole, minor clinopyroxene and biotite in a fine-grained groundmass dominated by feldspar. High MgO, Ni and Cr abundances (up to 11.9 wt%, 373 and 993 ppm. respectively) are consistent with derivation of primitive magmas from a mantle source. Enrichment in H2O, reflected in the abundance of magmatic amphibole and mica, combined with high whole-rock LILE, Th/Yb ratios and negative Nb-Ta anomalies in trace element patterns are consistent with a source that was metasomatised by hydrous fluids analogous to those generated by Phanerozoic subduction-related processes. Chondritic ɛNd and ɛHf signatures and Archean mantle-like Sr isotope signatures of the Late Archean CAL indicate that the fluid metasomatism required to explain their volatile and trace-element enriched composition shortly preceded partial melting (i.e. there was insufficient time to develop enriched radiogenic isotopic signatures). The concurrence of apparently juvenile radiogenic isotopes and fluid-related trace element compositions requires a geodynamic scenario whereby dehydration of a subducted slab triggered metasomatism of the overlying mantle wedge. Our findings therefore support a subduction setting at ~2.6–2.7 Ga along the eastern margin of the Yilgarn Craton. The CAL from the Western Yilgarn have similar compositions but enriched Sr-Nd-Hf isotopes compared to those in the Eastern Goldfields Superterrane. This signature is consistent with melting of lithospheric mantle domains previously enriched by subduction-related metasomatism. Hence, our study suggests the presence of a subduction setting in the Western Yilgarn during the Archean, which is consistent with previous geodynamic reconstructions. However, the geodynamic trigger for the early Proterozoic event that generated CAL magmatism in the Western Yilgarn is currently unclear.

Baddeleyite U-Pb geochronology and geochemistry of Late Paleoproterozoic mafic dykes from the Kongling complex of the northern Yangtze block, South China

Baddeleyite LA-ICP-MS U-Pb geochronology, major and trace element geochemistry and Nd-Pb isotope analysis of Late Paleoproterozoic mafic dykes from the northern Yangtze block, South China craton are used to investigate their petrogenesis and tectonic implications. The dykes mainly consist of fine-grained dolerite. Baddeleyite U-Pb dating indicates that the dolerites formed at 1866 ± 21 Ma. These rocks are alkaline in composition with high TiO2 contents (2.21–3.17 wt%) and display LREE enrichments ((La/Yb)N = 6.0–6.4) with slight positive Eu anomalies (Eu/Eu* = 1.06–1.14). On primitive mantle normalized multi-element diagrams they generally show convex upward curves with weak negative Nb-Ta anomalies. All of the samples have similar Nd-Pb isotopic compositions with negative εNd(t) values ranging from −4.3 to −3.2, (206Pb/204Pb)i from 14.91 to 15.31, (207Pb/204Pb)i from 14.99 to 15.08, and (208Pb/204Pb)i from 34.75 to 35.58. Geochemical features such as EM1-like isotopic compositions suggest the dolerites were derived from partial melting of metasomatised subcontinental lithospheric mantle. The mafic dykes are inferred to have formed in a post-orogenic extensional setting. We suggest that the northern Yangtze block experienced a tectonic switch from convergence to extension at ~1.87 Ga. A comparison of Paleoproterozoic magmatic, metamorphic, and sedimentary events in the Yangtze block with the southern Siberian craton and northern Laurentia supports a Paleoproterozoic position of the Yangtze block adjacent to southern Siberia and northern Laurentia within the Columbia supercontinent.

Tracking the transition from subduction‐related to post‐collisional magmatism in the north Arabian–Nubian Shield: A case study from the Homrit Waggat area of the Eastern Desert of Egypt

Field and geochemical observations of the granitoids of the Homrit Waggat area in the central Eastern Desert of Egypt reveal two magmatic phases. The early phase of weakly deformed subduction‐related calc‐alkaline rocks includes tonalite and granodiorite. We name the later phase the Homrit Waggat Pluton (HWP); it includes undeformed syenogranite, alkali feldspar granite, and minor albitized granite. The tonalite and granodiorite have distinct negative Nb–Ta anomalies and lower alkalis, REE, Nb, Zr, and Hf than the HWP. The early magmatic pulse is a subduction‐related suite, likely generated by underplating of mantle‐derived magmas that triggered partial melting of mafic lower crust; mixing of these melts led to intermediate magma that further fractionated to tonalite and granodiorite. The HWP granites of the late magmatic pulse are transitional from a subduction‐related to an anorogenic within‐plate environment, plausibly generated by post‐collisional lithosphere delamination. Although the parent magma of the HWP was I‐type, extensive fractional crystallization produced residual liquids with A2‐type character. Albitized granites are found only along the outer margin of the HWP, and contacts with the alkali feldspar granite are gradational, suggesting fluid interactions at a late stage of crystallization. The original textures of the albitized granites are preserved, but their bulk composition was modified by the production of Na‐rich minerals and the removal of K, REE, and some trace elements by fluids.

Geochemistry and Mineralogy of Basalts from the South Mid-Atlantic Ridge (18.0°–20.6°S): Evidence of a Heterogeneous Mantle Source

The South Mid-Atlantic Ridge is a typical slow-spreading ridge that represents a modern example to understand mantle composition and the evolution of mid-ocean ridge magmatism. In this paper, we investigate basalt samples dredged from four locations along the South Mid-Atlantic Ridge ranging from 18.0° to 20.6°S. The basalts belong to the tholeiitic series and exhibit normal mid-ocean ridge basalt (N-MORB) geochemical features with variable enrichments of Rb, Th, U, and Pb and depletions of Ba and Sr relative to primitive mantle. Some samples have negative Nb–Ta anomalies whereas others have positive Na–Ta anomalies to average N-MORBs. Plagioclase phenocrysts, microphenocrysts, and microlites occur in the in the matrix; phenocrysts and microphenocrysts are bytownite and labradorite in composition. Olivine phenocrysts are forsterite (Fo87 to Fo96). Chemical zoning in phenocrysts are interpreted to record crystal fractionation and magma mixing. Cores of plagioclase phenocrysts have higher anorthite values (An72–83) and estimated crystallization temperatures (~1180–1240 °C) that may suggest a xenocrystic origin. The lower anorthite proportions of rims of plagioclase phenocrysts (An65–71) and microphenocrysts (An54–72) yield lower estimated crystallization temperatures of ~1090–1120 °C and ~980–1060 °C, respectively. Rims of plagioclase phenocrysts and microphenocrysts may be generated in different environments such as magma chambers or magma channels, respectively. The basalt samples probably originated from partial melting of a depleted mantle spinel lherzolite source with a minor contribution of enriched materials possibly derived from the Saint Helena plume and subcontinental lithospheric mantle in the asthenosphere. Variable compositions of the basalt samples suggest heterogeneous mantle that includes depleted and enriched components at the South Mid-Atlantic Ridge between 18.0°–20.6°S.

280–310 Ma rift-related basaltic magmatism in northern Baoshan, SW China: Implications for Gondwana reconstruction and mineral exploration

The temporal and spatial variations of Late Paleozoic basaltic lavas in Baoshan, the northern part of a Gondwana-derived micro-continental block called Sibumasu, are important for Gondwana reconstruction. Magmatic zircon crystals from three selected dolerite dykes in the Baoshan region yield U-Pb ages from ~295 to ~310 Ma. These new ages, together with previous zircon U-Pb ages for this type of rock, define a protracted (~30 myr) episode of basaltic magmatism from ~310 to 280 Ma in a small area of this region, which is inconsistent with the typical temporal-spatial distribution of mantle plume magmatism. The trace element compositions of the Baoshan dolerite dykes and associated lavas are similar to arc basalts as well as continental flood basalts worldwide, showing light REE enrichments and negative Nb-Ta anomalies. Mixing calculations using the Sr-Nd-Hf isotope data of the Baoshan mafic rocks indicate that their intriguing trace element characteristics can be explained by contamination of mantle-derived magmas with crustal materials. Our new data, together with the lack of Late Paleozoic are-related calcalkaline rocks and granitoids in the Sibumasu block and other contemporaneous Gondwana-derived micro-continental blocks, strongly support the premise that the 310–280 Ma basalts and dolerites in the Baoshan region are the products of continental rift-related magmatism rather than arc magmatism. Based on the temporal correlation of the 310–280 Ma rift-related magmatism in several related Gondwana-derived micro-continents (Sibumasu, South Qiangtang, Lhasa and Himalaya), plus other independent constraints such as paleoclimate biotas and paleolatitudes from the literature, we provide an improved model for the configuration of the Gondwana supercontinent in the Early Permian. Based on the results from this study, we conclude that further investment in the exploration of magmatic Ni-Cu sulfide deposits associated with the 310–280 Ma mafic-ultramafic intrusions in northern Baoshan and the other related Gondwana-derived micro-continental blocks is warranted.

Geochronology, petrology and Sr-Nd-Hf-S isotope geochemistry of the newly-discovered Qixin magmatic Ni-Cu sulfide prospect, southern Central Asian Orogenic Belt, NW China

The Qixin mafic-ultramafic complex is present in the southern margin of the Central Asian Orogenic Belt in northern Xinjiang where many magmatic Ni-Cu sulfide deposits occur. To determine the temporal association of the newly-discovered Ni-Cu prospect associated with the Qixin complex with the known major magmatic Ni-Cu sulfide deposits in the region, as well as the origin of sulfide mineralization in the Qixin complex, we have carried out geochronological, petrological and geochemical studies of this complex. Gabbro, olivine gabbro, norite, olivine gabbronorite, troctolite, websterite and lherzolite are the major rock types of this complex. Two drill cores completed to date show two types of sulfide mineralization associated with the complex: (1) disseminated sulfides that occur near the base of bodies of lherzolite and troctolite; (2) blotchy-massive sulfides in the hornfels country rocks. The disseminated sulfides are composed of pyrrhotite, pentlandite and chalcopyrite, whereas the blotchy-massive sulfides in the country rocks are composed of pentlandite, violarite, pyrrhotite and chalcopyrite. Zircon crystals from the Qixin olivine gabbronorite yield a U-Pb age of 286.5 ± 2.5 Ma, which is among the oldest post-subduction magmatic Ni-Cu sulfide deposits in the region. The TiO2/Al2O3 ratios of Cr-spinel and clinopyroxene from the Qixin complex are similar to those from the older, subduction-related mafic-ultramafic intrusions in the region. This and other arc-like geochemical characteristics such as whole-rock negative Nb-Ta anomalies coupled with low initial 87Sr/86Sr (0.7036–0.7057), positive εNd(t) (+2.3 to +6.0) and high εHf(t) of zircon (+8.3 to +15), indicate that the parental magma of the Qixin complex was derived from a mantle source modified by pre-Permian metasomatic processes, similar to other contemporaneous mafic-ultramafic intrusions in the region. Mixing calculations using Sr-Nd-Hf isotopes indicate that the parental magma for the Qixin mafic-ultramafic rocks experienced no more than 15 wt% contamination with siliceous country rocks. The δ34S values of different sulfide assemblages of the Qixin Ni-Cu prospect are similar and vary from 1.1 to 1.7‰, which are within the range of typical mantle values as well as that of the country rocks, providing no direct evidence for addition of external sulfur. However, the role of external sulfur in sulfide mineralization at Qixin still cannot be ruled out, as forward modelling indicates that fractional crystallization plus crustal contamination involving siliceous country rocks free of S could not have induced sulfide saturation in the Qixin magma. This may also be true for the contemporaneous magmatic Ni-Cu sulfide deposits in the region. Based on the results from this study, we suggest that fractures in the solid country rocks nearby the Qixin complex and the ultramafic bodies of the complex are preferred targets if exploration continues.

Diabase Sills in the Outer Zone of the Emeishan Large Igneous Province, Southwest China: Petrogenesis and Tectonic Implications

Compositionally and texturally zoned diabase dykes and sills occur in the outer zone of the Emeishan large igneous province (ELIP) in the southern Guizhou Province, Southwest China. Based on the detailed petrology, whole rock geochemistry, zircon U-Pb geochronology and Hf isotopes and clinopyroxene mineral compositions studies, we investigate a representative diabase sill in the Luodian region with a view to understanding its petrogenesis and tectonic implications. Formed as composite zoned sub-volcanic intrusion, the diabase sill is characterized by gabbros and diabases in the inner zone and amygdaloidal diabases sporadically in the chilled zone within the upper sill margin. The diabasic and gabbroic rocks are composed of quartz-free and quartz-bearing groups. The quartz-free group rocks have low SiO2 (45.7 wt.%-49.5 wt.%), moderate MgO (5.66 wt.%-7.88 wt.%), high TiO2 (2.54 wt.%-3.65 wt.%), and Ti/Y values (536-747), corresponding to high-Ti type rocks. The quartz-bearing group rocks have higher SiO2 (49.8 wt.%-51.7 wt.%) and lower MgO (4.23 wt.%-4.74 wt.%), higher TiO2 (3.16 wt.%-3.63 wt.%), but lower Ti/Y values (399-419) than the quartz-free group ones, and thus belong to the low-Ti type. Both groups of rocks are enriched in LREE and LILE with negative Nb-Ta anomalies, and show broad tholeiitic affinity. The precursor magma of the high-Ti rocks might have originated from a source composed of mantle plume and subcontinental lithosphere mantle components, with minor crustal contamination during ascending. The magma of the low-Ti rocks was produced by mingling of the high-Ti diabasic rocks with minor injected intermediate-acidic magma plugs or blebs, suggesting magma mingling as one of the effective ways to change the high-Ti to low-Ti rocks of the ELIP. The diabasic sill underwent a rapid cooling event probably in response to a rapid tectonic uplift event, which probably occurred in the waning stage of ELIP during transition between the Middle and Late Permian after the domal uplift induced by the mantle-plume or in the Late Jurassic.

Geochronology and geochemistry of Late Carboniferous dykes in the Aqishan–Yamansu belt, eastern Tianshan: Evidence for a post-collisional slab breakoff

With aim of providing constraints on the Late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt (CAOB), an integrated study was conducted on the geochronological and geochemical data for dioritic, granitic and diabase dykes from the Aqishan–Yamansu belt in the eastern Tianshan, NW China. Zircon U-Pb dating indicates that the dioritic and granitic dykes were both emplaced in the Late Carboniferous (∼311 Ma and ∼315 Ma). The dioritic dykes show adakitic characteristics and have high Na2O and positive εHf(t) values (+12 to +17), which suggest an origin from partial melts of a subducted oceanic slab. The granitic dykes have high SiO2 and K2O contents and are characterized by enriched light rare earth elements (LREE) and slightly flat heavy rare earth elements (HREE), with negative Eu and Nb–Ta–Ti anomalies. These dykes are alkali-calcic and show geochemical features of highly fractionated I-type granites. Their positive εHf(t) values (+16 to +17) suggest that they were derived from a juvenile accreted oceanic crustal source. The coeval diabase dykes have low SiO2 and K2O contents but high TiO2, MgO and Mg# (54–59). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Ba/Th, slightly low Th/Ta ratios, and negative Nb-Ta anomalies imply a mantle source metasomatised by slab-derived fluids. Thus, these basic dykes were generated likely by partial melting of the upwelling asthenosphere mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous dykes were triggered by a post-collisional slab breakoff and the Aqishan–Yamansu belt was a continental arc formed by south-dipping subduction of the Kangguer oceanic plate.

From passive margin to continental collision: Geochemical and isotopic constraints for E-MORB and OIB-like magmatism during the neoproterozoic evolution of the southeast Brasília Belt

Metamorphic rocks derived from mafic rocks can be found in metamorphic belts, and their geochemical and isotopic characteristics may be valuable sources of information for interpretation of ancient geologic magmatic scenarios. The southern Brasília Belt, between the Amazonian and São Francisco cratons in Central Brazil exhibits different amphibolite rocks that were emplaced into Veríssimo Sequence and Araxá Group metasedimentary sequences, during the Neoproterozoic. The obtained results indicate that the studied amphibolite rocks from Pires do Rio-Catalão area represent three magmatic events associated with three magmatic environments: 1) a passive margin development at 979 Ma, a forearc/back-arc basin at ∼819 Ma, and a continental collision at ∼651 Ma.The first magmatic event, related to continental extension and passive margin development is represented by the emplacement at 979 ± 17 Ma of E-MORB low-K tholeiitic basalts with positive εNd(t) values between +4.4 and −2 and TDM between 1.26 and 1.82 Ga, related with the Veríssimo metavolcanosedimentary Sequence. The zircon populations include a high amount of metamorphic, inherited grains.The second magmatic event, related with an forearc/back-arc extension is represented by E-MORB basalts with negative Nb-Ta anomalies and slightly positive εNd(t) values of +0.78 to +2.71, cogenetic with the Araxá Group. An OIB-like basalt sample is also related to this stage and has a U-Pb age of 819.7 ± 6.3 Ma. εNd(t) values of +1.82 to +2.57, εHf(t) in zircon around-zero and TDM ages from 1.03 to 1.18 Ga.The third event is represented by syn- to late-tectonic OIB-like basaltic/gabbroic magmatism at 651 ± 6 Ma, with positive εNd(t) value of +2.57 and younger TDM age at 1.18 Ga, that can be associated to regional continental collision and closure of the Neoproterozoic ocean.Also, provenance studies into metasedimentary rocks support that passive and active margins could be successively superposed during the main thrusting tectonic event in the Brasiliano orogeny, but slices of different stratigraphic units preserve their depositional isotopic signature.These findings suggest that the occurrence of main mafic magmatic events over a timespan as long as the entire evolution of the Brasília Belt (1.0–0.6 Ma) might be a key to individualize superposed tectonic basins in Central Brazil.

Geochemistry and zircon U-Pb geochronology of Miocene plutons in the Urumieh-Dokhtar magmatic arc, east Tafresh, Central Iran

ABSTRACT The Tafresh plutons that include Ahmadabab diorite, Vasfonjerd monzonite, Mehrezamin diorite and Chahak diorite, located to the east of Tafresh city, north-central Iran, are part of Urumieh-Dokhtar magmatic arc. U-Pb dating of zircon grains provides emplacement ages of 22.3 ± 1 Ma for the Ahmadabad diorite, and tightly clustered ages of 22.2 ± 0.2 Ma, 21.3 ± 0.2 Ma, and 21.7 ± 0.4 Ma for Vasfonjerd monzodiorite, Mehrezamin diorite-monzonite, and Chahak diorite-monzonite plutons, respectively. These rocks are metaluminous to weakly peraluminous, calc-alkaline, and characterized by enrichment in light rare earth elements, Nb-Ta negative anomalies, and high LILE/HFSE ratios. Tafresh plutonic rocks originated from a parental magma source and experienced different degrees of partial melting. Geochemical signatures of Tafresh plutonic rocks, such as a wide range of Y/Nb (2.7–8.4) and low Zr/Nb (19.5–35.) ratios, Nb/Ta (11.46–18.15), argue for mantle–crust interaction during generation of Tafresh magmas. Relatively low Nb/La ratios further indicate that the lithospheric mantle played a significant role in melt generation. HREE signatures (i.e. decrease Dy/Yb with increasing SiO2) preclude substantial involvement of garnet either in the residue, both during partial melting and fractionation of the magma. The plutons are a product of final stages of subduction-related magmatism prior to the collision between the Arabian and Eurasian tectonic plates.

Contrasting whole-rock and mineral compositions of ore-bearing (Tongchang) and ore-barren (Shilicun) granitic plutons in SW China: Implications for petrogenesis and ore genesis

A common perception is that oxidized magma is critical for the formation of a porphyry Cu ± Mo ± Au ore deposit. We have used an ore-bearing pluton (Tongchang) and an ore-barren pluton (Shilicun) in the western rim of the South China block to test this idea and to determine other important controls on ore genesis. Zircon UPb ages indicate that the Tongchang and Shilicun plutons were emplaced at 36.3 ± 0.2 Ma and 35.2 ± 0.4 Ma, respectively, broadly coinciding with strike-slip faulting in the region and continental collision that occurred ~300 km to the west. These two plutons are all characterized by significant light REE enrichments and pronounced negative NbTa anomalies, similar to the bulk crust. Apatite separates from these two plutons all have elevated initial 87Sr/86Sr (Tongchang, 0.70690 to 0.70796; Shilicun, 0.70703 to 0.70726) and negative εNd(t) (Tongchang, −6.2 to −7.3; Shilicun, −4.5 to −5.8). The mean εHf(t) and δ18O of zircon from these plutons are −1.4 and 6.8‰ for Tongchang, and −0.3 and 6.5‰ for Shilicun. The Sr-Nd-Hf isotope compositions are all within the ranges of the Neoproterozoic mafic arc lower crust in the region. Whole-rock U-Nb-Ta systematics indicate that the Tongchang and Shilicun plutons originated from an amphibole-bearing source and a garnet-bearing source, respectively, implying a shallower and more H2O-rich mafic source for the former than the latter. Quartz trace element data indicate that the Tongchang pluton was emplaced at a shallower depth than the Shilicun pluton. Zircon Ce4+/Ce3+ and whole-rock V/Sc ratios, and apatite SO3 and MnO contents indicate that the parental magmas of both plutons are as oxidized as those of some porphyry Cu deposits in northern Chile, but the parental magma of the Tongchang pluton is less oxidized than that of the Shilicun pluton. Apatite Cl-F-OH systematics and higher abundant hydrous silicate minerals such as amphibole and biotite in the Tongchang pluton than the Shilicun pluton indicate that the parental magma of the Tongchang pluton has higher Cl and H2O content than that of the Shilicun pluton. Our results show that oxidized magma is important but this alone cannot make a porphyry Cu ± Mo ± Au ore deposit. High contents of H2O-Cl in magma and shallow depth of emplacement are also important.

Geochemistry and geochronology of orthogneisses across a major transcurrent shear zone (East Pernambuco shear zone, Borborema Province, Northeast Brazil): Tectonic implications

The East Pernambuco shear zone (EPSZ) is a dextral shear zone bounding the Rio Capibaribe and Pernambuco-Alagoas domains of the Neoproterozoic Borborema Province (NE Brazil). This study reports new geological data from orthogneisses across the central segment of the EPSZ. Two samples from the Bezerros orthogneiss to the north of the EPSZ yielded 207Pb/206Pb weighted mean ages of 2111 ± 7 Ma and 2104 ± 8 Ma. The c. 2.1 Ga age is taken as the age of crystallization of the protolith. It is undistinguishable from the age of the 2.13–2.09 Ga-old Vertentes Complex, which is the main Paleoproterozoic unit of the Rio Capibaribe Domain, and of orthogneisses that occur immediately south of the EPSZ. Like the Vertentes Complex, the Bezerros orthogneiss has a subduction-zone geochemical signature (e.g., pronounced negative Nb-Ta anomalies), suggesting intrusion in a magmatic arc setting. South of the EPSZ, a migmatized granitic orthogneiss and the Encruzilhada de São João orthogneiss yielded 206Pb/238U weighted mean ages of 645 ± 3 Ma and 639 ± 5 Ma, respectively. Ages of 615 ± 17 Ma and 618 ± 12 Ma obtained at the rims of zircon grains from Encruzilhada de São João and migmatized granitic orthogneisses, respectively, are interpreted as dating the acquisition of the low-angle gneissic fabric. The age of the EPSZ is bracketed by the 591 ± 4 Ma age of a monazite grain from the Bezerros orthogneiss and by the of 575 ± 17 Ma age of a young zircon grain in the Encruzilhada de São João orthogneiss. Rather than being a terrane boundary, the EPSZ more likely represents an intracontinental shear zone that nucleated at preexisting crustal heterogeneities.

Early Cretaceous subduction-modified lithosphere beneath the eastern Qinling Orogen revealed from the Daying volcanic sequence in central China

New geochronological, geochemical and Sr-Nd-Hf isotopic data for the early Cretaceous Daying volcanic sequence, eastern Qinling Orogen, provides a probe into the nature of, and tectonic process operating in, the underlying Mesozoic mantle. The volcanic rocks in the sequence are composed of basaltic trachyandesite, pyroxene-andesite, trachyandesite and trachyte. They yield zircon U-Pb and whole-rock 40Ar/39Ar plateau ages of 134–124 Ma, roughly synchronous with the Cretaceous igneous rocks throughout the eastern Qinling and Dabie orogens. Clinopyroxene and plagioclase phenocrysts of the volcanic rocks exhibit simple normal zonation with higher mg-number and An content in the core relative to the rim. The amphibole is characterized by tschermakite and tschermakitic hornblende with mg-number of 50–67. The Daying volcanic rocks have variable SiO2 (54.27–63.44 wt%), Al2O3 (15.39–20.34 wt%), FeOt (5.23–9.94 wt%), and MgO (1.05–4.03 wt%) contents with low mg-number (26–49). They exhibit arc-like REE- and PM-normalized patterns with significant negative Nb-Ta anomalies, and highly- enriched Sr-Nd isotopic composition with (87Sr/86Sr)i ratios of 0.7066–0.7078 and εNd(t) values of −15.5 ∼ −11.0. Their zircons exhibit strongly negative εHf(t) values ranging from −27.3 to −17.2 and two-staged model ages from 2.90 Ga to 2.28 Ga. Such geochemical signatures suggest that the Daying volcanic rocks were derived from a hybridized source of the North China Craton enriched mantle with the input of some Yangtze crustal materials created by Triassic deep-subduction digestion. They also share similar eruption time, as well as elemental and Sr-Nd-Hf isotopic compositions, with those in the southern margin of the North China Craton, suggesting a northward subduction of the Yangtze block. It is proposed that the volcanic rocks are petrogenetically related to an early Cretaceous extensional event along the Qinling- Dabie Orogenic belt in response to unrooting of the thickened lithosphere.

Petrogenesis of plagiogranite and associated diorites and mafic rocks in the Habana–Matanzas ophiolites, northwestern half of central Cuba

Structural field relationships, petrography and geochemical features of plagiogranites and associated intermediate (diorite) to mafic rocks (basalt and gabbro) of the Habana–Matanzas ophiolites in northern Cuba suggest that these felsic and mafic rock units represent a boundary case between forearc and volcanic arc ophiolite types. Subduction-related basalts can be subdivided into back-arc, back-arc to forearc, forearc and volcanic arc types. The majority of the basalts plot in the oceanic arc (as well as joint oceanic arc–continental arc) field of the Th/Yb–Nb/Yb diagram. The samples that classify as volcanic arc basalts plot in the continental arc field of the Th/Yb–Nb/Yb diagram and within the island arc tholeiite and boninite fields in the V–Ti diagram. The Habana–Matanzas plagiogranites and associated basalts, diabases and gabbros display significantly negative Ta–Nb anomalies and are depleted in heavy rare earth elements. 40 Ar/ 39 Ar whole-rock plateau ages of these plagiogranites reveal minimum crystallization ages of 98.11 ± 0.50 Ma, providing the first important time constraint for suprasubduction zone oceanic crust formation on the island of Cuba. Based on our petrogenetic modelling and geochemical characterization, we conclude that magmas of the plagiogranites and associated intermediate rocks of the Habana–Matanzas ophiolites were derived from partial melting of highly depleted, pre-existing crustal rocks (basalt and/or gabbro). Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Protoliths and tectonic implications of the newly discovered Triassic Baqing eclogites, central Tibet: Evidence from geochemistry, Sr[sbnd]Nd isotopes and geochronology

Geochemical, SrNd isotopic and geochronological methods were used to reveal the protoliths and geodynamic implications of the newly discovered Triassic Baqing eclogites, eastern Qiangtang terrane, central Tibet. We assessed the mobility of trace elements, and it turned out that high field strength elements (HFSEs) and rare earth elements (REEs) were immobile in the high-pressure (HP) metamorphic process and reliably employed to discriminate the protoliths of the Baqing eclogites. Whole-rock geochemical data (especially immobile elements) suggested both arc-related (negative NbTa anomalies, high light (L) REEs/heavy (H) REEs ratios) and mid-oceanic ridge basalt (MORB)-related (without negative NbTa anomalies, relatively low REEs contents) characteristics, indicating the contribution of different proportions of subduction material with a backarc basin origin. According to the SrNd isotopic ratios (initial 87Sr/86Sr ratios: 0.7078–0.7086; εNd(t) values: −0.58 to +1.96), the protoliths of the Baqing eclogites originated from mantle which experienced continental crustal contamination in the subduction zone. Compared with nearby subduction-related magmatic rocks from eastern Qiangtang terrane, backarc basin was the most likely tectonic environment for the Baqing eclogite protoliths. The Jinsha Paleo-Tethyan Ocean southward subduction underneath East Qiangtang block (EQB) and rollback led to the formation of this backarc basin between the Late Permian and Early Triassic, and subsequent northward subduction of the backarc basin formed the Baqing eclogites in early Late Triassic (227–221 Ma).

Coupled Assimilation and Fractional Crystallization (AFC) and Mantle Plume Source(s) Contribution in the Generation of Paleoproterozoic Mafic Dykes of the Eastern Dharwar Craton, Southern India

Abstract The eastern Dharwar craton (EDC) of the southern Indian Shield hosts five geochronologically distinct Paleoproterozoic mafic dyke swarms emplaced at 2.37, 2.21, 2.18, 2.08 and 1.89 Ga. Trace element geochemical data available for these dykes display the ‘arc signals’ viz., negative Nb-Ta anomalies and elevated Zr/Nb, Th/Yb and Th/Ta values, which are conventionally interpreted to represent involvement of subduction in their genesis. It is shown that these ‘arc signals’ resulted from coupled assimilation and fractional crystallization (AFC) processes that modified these mantle-derived melts. Since, mafic dykes under study are highly evolved, an attempt has been made to estimate (using PRIMELTS2.xls software) the composition of the primary magma from the most primitive sample available from the 2.21 and 2.37 Ga swarms. The mantle potential temperature derived from the estimated primary magma compositions revealed anomalously hot mantle source regions compared to the known ambient upper mantle temperatures during Paleoproterozoic, thus implying the possible involvement of thermal plumes in their genesis.

Crustal melting above a mantle plume: Insights from the Permian Tarim Large Igneous Province, NW China

Low NbTa rhyolites from the Permian Tarim Large Igneous Province (LIP) represent the melting products of continental crust above a mantle plume. Zircon UPb dating reveals that the Kuchehe and Dawanqi rhyolites were erupted at 292.6 ± 0.5 Ma and 274 ± 2 Ma respectively. Both rhyolite groups are characterized by pronounced negative NbTa anomalies, crust-like trace element ratios and negative εNd-Hf(t) values, consistent with a crustal origin. Newly acquired data, combined with available data in the literature, clearly show that there is an increasing trend of zircon saturation temperature (TZr) from 794 to 827 °C to 886–936 °C for the early to middle stage (293–287 Ma), followed by a decreasing TZr of 803–908 °C for the late stage (287–274 Ma) of the Tarim magmatism. The temporal changes in TZr for the Tarim rhyolites are interpreted to reflect the crustal melting behavior above a mantle plume, such that fusible materials in the crust melt at an earlier stage and refractory materials melt at a later stage. The gradual decrease in TZr, towards the final stage of the Tarim rhyolitic magmatism, reflects the gradual ‘decay’ of thermal input from mantle plume to the lithosphere in the region. This thermal model is also applicable to describe the crustal melting behavior in the Chon Aike silicic LIP of Patagonia and the Snake River Plateau-Yellowstone Plateau volcanic provinces.

The Thickness of the Mantle Lithosphere and Collision-Related Volcanism in the Lesser Caucasus

The Lesser Caucasus mountains sit on a transition within the Arabia–Eurasia collision zone between very thin lithosphere (<100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from NW to SE in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb–Ta anomalies. Magmatic rocks from the SE, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic ⁸⁷Sr/⁸⁶Sr, and less radiogenic ¹⁴³Nd/¹⁴⁴Nd. Across the region, there is no correlation between SiO₂ content and Sr–Nd isotope ratios, revealing a lack of crustal contamination. Instead, ‘spiky’ mid-ocean ridge basalt normalized trace element patterns are the result of derivation from a subduction-modified mantle source, which probably inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent–continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the SE. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the SE must occur at greater depths in the mantle. Temperatures of melting below 1200°C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in NW Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran. The mid-lithosphere magma source has a distinct composition compared with the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation.