Subcontinental Lithospheric Mantle Source Research Articles

1.79–1.75 Ga mafic magmatism of the Siberian craton and late Paleoproterozoic paleogeography

The paper presents new geological, geochronological, geochemical, and Nd isotopic data on the late Paleoproterozoic dolerites of the Siberian craton. U-Pb baddeleyite ages of the Doros dolerites (Aldan shield, south-eastern Siberia) and East Anabar dolerites (Anabar shield, northern Siberia) are 1757 ± 4 and 1774 ± 6 Ma, respectively. This new geochronological data together with published 1.76–1.75 Ga ages for Timpton-Algamay dolerites of the Aldan shield and Chaya dolerites of the Baikal uplift (southern Siberia) revealed a 20 my difference between this magmatism in the north and south Siberia. The Doros dolerites in their chemical compositions correspond to sub-alkaline basaltic andesites, but the East Anabar dolerite is chemically close to basaltic trachyandesite. The Doros dolerites demonstrate negative and close to zero ɛNd(t) values (from −7.0 to +0.1), which correlate well with SiO2 and Mg#. All Doros dolerites have Nb–Ta and Ti anomalies in multielement spectra. The geochemical and Nd isotopic data suggest that the Doros dolerites have been formed by the mixing of mantle and crustal material. The East Anabar dolerites are characterized by a negative ɛNd(t) value of −3.7, negative Nb–Ta anomaly in multielement spectra, high concentrations of TiO2 and P2O5. The 1775 Ma East Anabar dolerites could be generated from a subcontinental lithospheric mantle source, possibly with some mantle plume interaction (possibly centred at the convergence with the 1.76–1.75 Ga fanning Aldan swarm). Geochemistry and Nd isotope systematics of all 1.78–1.75 Ga mafic dykes and intrusions of the Siberian craton indicate the subcontinental lithospheric mantle source or mantle source contaminated by crustal material. Geochronological data from the 1.79–1.75 Ga magmatic rocks of Siberia and other continents suggest continuous magmatism over this interval in some continents, but a series of short magmatic events/pulses (from one to four) separated by intervals of quiescence in other continents. We locate the analysed dykes and sills on new 1750 Ma and 1790 Ma global paleogeographic reconstructions. Analysis of 1.79–1.75 Ga geochemical data on mafic intrusions from Late Paleoproterozoic continents suggests the prevalence of subcontinental lithospheric mantle source for the mafic intrusions over the pure mantle plume source.

The enriched Variscan lithosphere of NE Iberia: data from postcollisional Permian calc-alkaline lamprophyre dykes of Les Guilleries

Post-collisional mafic dykes crosscut the Paleozoic metamorphic basement and late-Variscan plutons in Les Guilleries massif (Catalan Coastal Ranges, NE Iberia). The predominance of mafic phenocrysts, porphyritic texture, abundant amphibole, high MgO and volatile content, together with crustal-like trace-element patterns indicate that the dykes correspond to calc-alkaline lamprophyres, mainly spessartites. Their enrichment in LILE, HFSE and REE and initial Sr-Nd isotopic compositions (87Sr/86Sri between 0.70851 and 0.71127, epsilon Ndi between -5.23 and -4.63) are consistent with an enriched subcontinental lithospheric mantle source. U-Pb ages of matrix titanite crystals yield concordia ages of 262±7Ma, congruent with crosscutting relationships. Postmagmatic processes are evidenced by intense chloritization and albitization of the lamprophyres, together with systematic variations of Na2O vs SiO2, K2O, CaO, Ba, Rb, Cs, Pb, Sr, Tl, and Zn, and possibly the removal of F. The geochemical and geochronological data support an orogenic geochemical affinity, in accordance with the transitional tectonic regime between Variscan compression/transpression and post-collisional transtension/extension, related to the fragmentation of Pangea and thinning of the lithosphere. The lamprophyre dykes studied could represent the youngest pulse of Variscan orogenic magmatism and, therefore, mark its end in NE Iberia before the onset of the generalized Triassic extension.

New geochronological and geochemical constraints on petrogenesis and tectonic setting of the Loe-Shilman carbonatite complex, Northwest Pakistan

The Loe-Shilman carbonatite complex in northwest Pakistan comprises carbonatite and ultrapotassic silicate rocks. The age and environment of emplacement of these rocks have long been controversial, and therefore, impeded our understanding on the regional tectonic setting. New whole-rock geochemistry, Sr-Nd isotopic and zircon petrochronological data provide age and petrogenetic information on the carbonatite and ultrapotassic silicate rocks of the Loe-Shilman complex. Zircon U/Pb data yield 206Pb/238U weighted mean ages of 90.6 ± 1.0 Ma (n = 33) for the carbonatite and 89.3 ± 1.0 Ma (n = 30) for the ultrapotassic rocks, which indicates that the two lithologies were emplaced simultaneously during a single tectonic/thermal event. The Nd isotope data of the ultrapotassic rocks is homogenous (εNd(i) = −2.5 to −2.9) and overlap εNd(i) of the carbonatite (−2.6 to −2.8), indicating derivation from a common, enriched mantle source. Despite this temporal overlap, whole-rock geochemical data, however, indicates that generation of both rock types by fractional crystallization or liquid immiscibility is unlikely and that the carbonatite and ultrapotassic rocks are actually products of two independent melts generated by partial melting of an enriched sub-continental lithospheric mantle (SCLM) source at the same time. Trace element data further indicate that the ultrapotassic rocks are products of fractional crystallization of a melt produced by a low degree partial melting of an enriched mantle source, within the garnet stability field. Moreover, the ultrapotassic rocks and carbonatites show association with anorogenic and within-plate plume type tectonic settings, coeval with juxtaposition of the northwestern margin of the Indian plate over the Réunion hotspot mantle-plume during Late Cretaceous time.

Radiogenic (Sr-Nd-Hf-Os) isotope data for neogene volcanic rocks from the rif–betic orogen and lanzarote (Canary Islands) in the western Mediterranean

In this communication we present new radiogenic Sr-Nd-Hf-Os isotope data for Neogene volcanic rocks from the Rif-Betic Orogen and Lanzarote Island in the western Mediterranean along with some previously published data. Major and trace element data for the Rif samples are provided as a data supplement. Geological context and petrographic details for equivalent rocks have been published previously but the new data, especially the Hf and Os isotope data should be of use for future appraisals of the distribution of sub-crustal mantle domains in the region. Upon inspection, it would appear that alkali basalts, shoshonites and lamproites sample very different sub-continental lithospheric mantle sources in Spain and Morocco whereas there are close similarities between Lanzarote basalts and mantle xenoliths from the Atlas Mountains in Morocco.

Late Jurassic magmatism and associated Au–Cu polymetallic mineralization in the North China Craton: Constraints from U–Pb–Hf–Sr–Nd isotopes of the Xiayingfang intermediate-felsic igneous rocks

The Xiayingfang Au–Cu polymetallic deposit (22.5 Mt. @1.9 g/t Au, 0.54% Cu) in the northern margin of the North China Craton consists of the Xiajinbao and Dongliang ore blocks. It is characterized by multiple magmatic rocks, including rhyolitic tuff, granite porphyry, diorite porphyry, quartz porphyry, and monzonite porphyry. The Xiajinbao and Dongliang mineralization occurs mainly as auriferous quartz-sulfide veins in granite porphyry and diorite porphyry, respectively.Rhyolitic tuff, granite porphyry, diorite porphyry, quartz porphyry, and monzonite porphyry at Xiayingfang yielded zircon U–Pb ages of 162.3 ± 1.3 Ma (MSWD = 1.4), 161.8 ± 1.5 Ma (MSWD = 1.3), 162.7 ± 1.7 Ma (MSWD = 1.4), 162.9 ± 1.7 Ma (MSWD = 2.4), 162.7 ± 1.6 Ma (MSWD = 0.7), respectively, suggesting their Late Jurassic emplacement. The rhyolitic tuff, granite porphyry, quartz porphyry, and monzonite porphyry are characterized by high aluminum saturation index (ASI = 1.0 to 1.3), positive P2O5 vs. SiO2 correlation, negative Eu anomaly (δEu = 0.29 to 0.78), low average Nb/Th (3.05) and Nb/U (8.80) ratios, low Mg# values (14 to 42), negative zircon εHf(t) (–8.7 to –10.1) and high initial 87Sr/86Sr value (0.712 for granite porphyry), indicating a sedimentary origin. In contrast, the diorite porphyry is characterized by high Sr/Y (29.2 to 53.2), low Y (9.7 to 15.0 ppm) and Yb (1.4 to 1.8 ppm), indicating an adakite-like affinity. Moreover, its listric-shaped normalized REE patterns with no Eu anomalies, low ASI (0.8–0.9), zircon εHf(t) (–7.7 to –11.1), εNd(t) (–10 to –16) and initial 87Sr/86Sr (0.705 to 0.706), high Nb/Th (9.25), Nb/U (30.6) ratios, Mg# values (46 to 54), indicating a dominantly sub-continental lithospheric mantle (SCLM) source, which may have evolved to adakite-like signatures through amphibole fractionation. The mineral assemblages of paragenetic bornite, enargite and electrum at Xiayingfang reveals the highly oxidized condition of ore-related magma, as supported by the magnetic susceptibility (20 to 65 × 10–3SI unit) and average zircon Ce4+/Ce3+ ratio (352) of the diorite porphyry, and clearly different from those barren Xiayingfang porphyries (magnetic susceptibility below 1 × 10–3SI unit, and low average zircon Ce4+/Ce3+ ratio of 128). This reveals that the diorite porphyry may have been ore-causative at Xiayingfang, which is also supported by its clear ore-related alteration (illite, carbonate, and chlorite) and close spatial relations with the orebodies. Mineral Exploration programs in this area should therefore target the diorite porphyries instead.

The upper Oligocene San Rafael intrusive complex (Eastern Cordillera, southeast Peru), host of the largest-known high-grade tin deposit

The upper Oligocene San Rafael intrusive complex (SRIC), located at the southwest margin of the Eastern Cordillera of southeast Peru, is an exceptional case study to investigate the petrogenesis of a large composite granitic pluton associated with the largest known high-grade Sn deposit (>1 Mt. Sn). The granitic rocks are quasi-continuously exposed over more than 1.4 km vertically, from the cupola at surface to deeper levels in the underground mine workings, thus providing access to the upper part of a large pluton. We describe new field relationships and petrographic observations of the main granite varieties of the SRIC, and we report quantitative mineral analyses, whole rock geochemistry (major and trace elements, radiogenic Sr, Nd, and Pb isotopes), and in situ UPb dating and trace element analysis of zircons. The SRIC is dominantly composed of a biotite-cordierite -bearing K-feldspar megacrystic granite, which contains comagmatic enclaves and dismembered dikes of fine-grained and medium-grained granites, and subordinate lamprophyres. While the granitic rocks are widely affected by variable degrees of pervasive hydrothermal alteration, the least altered samples are characterized by moderately fractionated peraluminous S-type compositions, with similar enrichments in incompatible elements (Li, B, Ga, Rb, Sn, Cs, Th, and U), relatively high Zr/Hf and Nb/Ta ratios, and strongly crustal Sr, Nd, and Pb isotopic signatures. In situ UPb dating and trace element analysis of zircons indicate that the granitic magmas crystallized contemporaneously within age uncertainty at 24.02 ± 0.25 Ma (megacrystic granite), 24.17 ± 0.28 Ma (fine-grained granite), and 24.26 ± 0.26 Ma (medium-grained granite) at Ti-in-zircon temperatures from 638° to 840 °C (avg = 759 ± 49 °C). The lamprophyres have high contents of lithophile elements (Li, Rb, Cs, Sn, W) while showing more primitive SrNd isotopic compositions, indicating a metasomatized subcontinental lithospheric mantle source contaminated by crustal assimilation and/or mixing with granitic magmas. Tin contents of the main San Rafael granites are moderate (3–40 ppm; avg. = 13 ppm Sn) and fall in the same range of values as reported for “tin granites” from the Central Andean tin belt. Based on our petrological and geochemical data, we conclude that the SRIC results from multiple injection and mingling/mixing of several types of coexisting felsic and mafic magmas in a shallow-crustal reservoir. The granitic magmas were largely derived from partial melting of metasedimentary protoliths and hybridization with mantle-derived mafic melts, in a regional geodynamic setting dominated by anomalous asthenospheric heat flow beneath the southern Peruvian Altiplano during the Oligocene to Miocene. Based on the moderately fractionated peraluminous character of the SRIC, we conclude that the granitic pluton was a passive host for Sn mineralization and only provided the structural focusing for metal-rich magmatic fluids derived from a deeper part of the granitic magma reservoir. We propose a model in which the injection of high-temperature mafic melts into a more reduced, fractionated granitic magma reservoir triggered late-magmatic oxidation, crystal mush reactivation, and exsolution (second boiling) of metal-rich magmatic fluids. The latter ascended toward the upper part of the crystallized granitic pluton and were focused along subvertical dilational structures, where they cooled down and mixed with downwelling meteoric waters, thus generating the world-class San Rafael Sn (Cu) deposit.

Petrogenesis and tectonic settings of Proterozoic mafic magmatism from the northern Indian Shield and the Himalaya: possible role for interaction of mantle plume with the subcontinental lithospheric mantle

Abstract Northern Indian Shield and the western Himalaya have an impressive record of mafic magmatism. The Aravalli Craton preserved 2.3 Ga komatiitic (picritic) and 2.1–1.8 Ga tholeiities. Gwalior and Betul belts preserved 2.1 and 1.5–1.2 Ga tholeiites, respectively. Western Himalaya has preserved 2.1–1.8 Ga tholeiites in Garhwal and Himachal regions. Studied rocks depict enriched rare earth elements, large ion lithophile elements and depleted high field strength elements. Whereas komatiites/picrites represent higher degrees of partial melting ( c. 35–40%) at higher temperatures ( c. 1500°C), tholeiites represent lower degrees of partial melting ( c. 10%) at lower temperatures ( c. 1200°C). Our results indicate interaction of mantle plume with variably enriched subcontinental lithospheric mantle sources, causing generation of these varied magmatic suites of rocks. Whereas the higher temperature komatiitic/picritic melts from the Aravalli region appear to have been generated closer to the plume head, the lower temperature tholeiitic melts from the shield region and western Himalaya were generated towards the plume margins. Different terrains of the study have undergone plume tectonics causing the development of the rift valleys. The majority of these developed into aulacogens, except for the Aravalli basin, which developed into deeper marine facies.

Geodynamic Setting and Cu-Ni Potential of Late Permian Xiwanggou Mafic-Ultramafic Rocks, East Kunlun Orogenic Belt, NW China

Several magmatic Cu–Ni sulfide deposits have recently been explored along the deep Middle Kunlun fault related to the extension of the East Kunlun orogenic belt in Qinghai Province, NW China. The Xiwanggou mafic–ultramafic rocks associated with Cu–Ni sulfide mineralization are first to be dated as late Permian compared to most of the deposits formed during late Silurian–early Devonian in this region. The Xiwanggou complexes located in the junction area between the East Kunlun and West Qinling belts, are composed of gabbros, olivine-gabbros, pyroxenites, olivine-pyroxenites, and peridotites. The Cu–Ni mineralization are mainly hosted in the olivine-pyroxenites and pyroxenites, whereas the sulfide-poor mineralization distributed in gabbros and olivine-gabbros. Zircon LA-ICP-MS U–Pb dating of the gabbro and olivine-pyroxenite revealed their crystallized ages of 250.8 ± 0.8 Ma and 257.3 ± 0.7 Ma, respectively. The trace element characteristics of the Xiwanggou fertile mafic-ultramafic rocks shows the enrichments in Sr, Rb, Th, Ba and light rare earth elements, and depletions in Nb and Ta, which are associated with the slab derived fluid input and dehydration melting of amphiboles. Meanwhile, Sr–Nd and Hf isotopic compositions of the gabbro [εNd(t) = 0.66–1.18; εHf(t) = 5.2–12] and olivine-pyroxenite [εNd(t) = −1.09 – −0.43; εHf(t) = 5.4–17.7] show that the magma was mainly derived from the metasomatized portions of subcontinental lithospheric mantle (SCLM) source in the mantle wedge. The magma primarily experienced dehydration melting processes of amphiboles and subsequently underwent hydrated melting in the overlying mantle wedge and relatively reduced background. The cool subduction process of the Anemaqen oceanic lithosphere maybe trigger large melting in the mantle wedge resulting in a relative low-Ni content in the melt. The transpressional windows formed by the right-lateral strike-slip shearing action of the Wenquan and South Kunlun faults in the South Kunlun forearc belt created a significant conduit for the magma ascending. The thermometer of Fe and Ni exchange between coexisting olivine and sulfide melt indicates the magma were yielded in a temperature range of ca. 1200–1300°C and an oxygen fugacity range of ca. –10.57 to –8.98 (log unit), which suggested that the parental magma of the Xiwanggou complex derived from a relatively reduced source favoring Ni relative to Fe in the melt. The intermediate sulfide segregation from the melt resulted in a medium tenor potential for the Xiwanggou complex.

Petrology and zircon U-Pb geochronology of mafic - intermediate dykes in the West-Central Taurides: Implications for magma source during the Late Precambrian – Early Palaeozoic

ABSTRACT Cadomian magmatic activity is represented by a mafic – intermediate dyke suits in the Karacahisar dome (West – Central Taurides) formed by the southward subduction of the Proto-Tethyan ocean beneath North Gondwana. These dyke suites are classified as diabase, andesite and dacite. They display calc-alkali, high-K calc-alkali and shoshonitic character, and intruded into the siliciclastic rocks deposited in a back-arc rift setting. In this study, we present new zircon U-Pb ages, Sr-Nd isotopic and geochemical data to better understand the petrological evolution of the Cadomian magmatism. Zircon U-Pb concordia ages of 551.3 ± 3.5 Ma, 557.9 ± 3.7 Ma and 569 ± 7.2 Ma were obtained for these dykes. On the N-MORB normalized spider diagrams, mafic – intermediate dykes exhibit enrichment in large ion lithophile elements (LILE: Sr, K, Rb, Ba, Th), relatively less enrichment in light rare earth elements (LREE: La, Ce and Nd) and depletion in high field strength (HFS: Tb, Ti, Y and Yb) elements. Dyke samples have significant depletions of Ta and Nb relative to neighbouring LILE and LREE and display a significant subduction component. On the chondrite normalized REE diagrams, a weak Eu anomalies are observed, and this indicates some plagioclase accumulation. Dyke samples fall into the volcanic arc (island arc and/or continental arc), and back-arc fields on tectonic setting discrimination diagrams. The εNd(i) values of dykes range from −1.47 to 1.76, and Nd depleted-mantle model (TDM) ages vary between 1.32 and 1.81 Ga, and show enriched mantle or metasomatized subcontinental lithospheric mantle source signatures. The primary magmas of the dykes were derived from amphibole- and garnet-bearing mantle, and were affected by fractional crystallization, mixing and assimilation processes during ascent through the upper crust. All available data show that this magmatic activity developed in a back-arc rift setting subsequent to Cadomian arc magmatism in the West-Central Taurides.

Halogen heterogeneity in the subcontinental lithospheric mantle revealed by I/Br ratios in kimberlites and their mantle xenoliths from South Africa, Greenland, China, Siberia, Canada, and Brazil

Abstract To investigate halogen heterogeneity in the subcontinental lithospheric mantle (SCLM), we measured the concentrations of Cl, Br, and I in kimberlites and their mantle xenoliths from South Africa, Greenland, China, Siberia, Canada, and Brazil. The samples can be classified into two groups based on halogen ratios: a high-I/Br group (South Africa, Greenland, Brazil, and Canada) and a low-I/Br group (China and Siberia). The halogen compositions were examined with the indices of crustal contamination using Sr and Nd isotopes and incompatible trace elements. The results indicate that the difference between the two groups was not due to different degrees of crustal contamination but from the contributions of different mantle sources. The low-I/Br group has a similar halogen composition to seawater-influenced materials such as fluids in altered oceanic basalts and eclogites and fluids associated with halite precipitation from seawater. We conclude that the halogens of the high-I/Br group are most likely derived from a SCLM source metasomatized by a fluid derived from subducted serpentinite, whereas those of the low-I/Br group are derived from a SCLM source metasomatized by a fluid derived from seawater-altered oceanic crust. The SCLM beneath Siberia and China could be an important reservoir of subducted, seawater-derived halogens, while such role of SCLM beneath South Africa, Greenland, Canada, and Brazil seems limited.

The Variscan subduction inheritance in the Southern Alps Sub-Continental Lithospheric Mantle: Clues from the Middle Triassic shoshonitic magmatism of the Dolomites (NE Italy)

Although often speculated, the link between the Middle Triassic shoshonitic magmatism at the NE margin of the Adria plate and the subduction-related metasomatism of the Southern Alps Sub-Continental Lithospheric Mantle (SCLM) has never been constrained. In this paper, a detailed geochemical and petrological characterization of the lavas, dykes and ultramafic cumulates belonging to the shoshonitic magmatic event that shaped the Dolomites (Southern Alps) was used to model the composition and evolution of the underlying SCLM in the time comprised between the Variscan subduction and the opening of the Alpine Tethys. Geochemical models and numerical simulations enabled us to define that 5–7% partial melting of an amphibole + phlogopite-bearing spinel lherzolite, similar to the Finero phlogopite peridotite, can account for the composition of the primitive Mid-Triassic SiO2-saturated to -undersaturated melts with shoshonitic affinity (87Sr/86Sri = 0.7032–0.7058; 143Nd/144Ndi = 0.51219–0.51235; Mg # ~ 70; ~1.1 wt% H2O). By taking into account the H2O content documented in mineral phases from the Finero phlogopite peridotite, it is suggested that the Mid-Triassic SCLM source was able to preserve a significant enrichment and volatile content (600–800 ppm H2O) for more than 50 Ma, i.e. since the slab-related metasomatism connected to the Variscan subduction. The partial melting of a Finero-like SCLM represents the exhaustion of the subduction-related signature in the Southern Alps lithosphere that predated the Late Triassic-Early Jurassic asthenospheric upwelling related to the opening of the Alpine Tethys.

Crystallization and Segregation of Syenite in Shallow Mafic Sills: Insights from the San Rafael Subvolcanic Field, Utah

AbstractExposed plumbing systems provide important insight into crystallization and differentiation in shallow sills beneath volcanic fields. We use whole rock major element, trace element and radiogenic isotopic compositions, along with mineral geochemical data on 125 samples to examine the conditions of melt differentiation in shallow sills from the exposed 4-Ma-old San Rafael subvolcanic field (SRVF), Utah. The field consists of ∼2000 dikes, 12 sills and 63 well preserved volcanic conduits. Intrusive rocks consist of mainly fine-grained trachybasalts and coarse-grained syenites, which are alkaline, comagmatic and enriched in Ba, Sr and LREE. Within sills, syenite is found as veins, lenses, and sheets totally enveloped by the basalt. The SRVF intrusions have geochemical signatures of both enriched sub-continental lithospheric and asthenospheric mantle sources. We estimate partial melting occurred between 1·2 and 1·9 GPa (50–70 km), with mantle potential temperatures in the range 1260–1326 ± 25°C, consistent with those estimated for volcanic rocks erupted on the Colorado Plateau. Geobarometry results based on clinopyroxene chemistry indicate that (1) basalt crystallized during ascent from at least 40 km deep with limited lithospheric storage, and (2) syenites crystallized only in the sills, ∼1 km below the surface. San Rafael mafic magma was emplaced in sills and started to crystallize inward from the sill margins. Densities of basalt and syenite at solidus temperatures are 2·6 and 2·4 g/cc, respectively, with similar viscosities of ∼150 Pa s. Petrographic observations and physical properties suggest that syenite can be physically separated from basalt by crystal compaction and segregation of the tephrophonolitic residual liquid out of the basaltic crystal mush after reaching 30–45% of crystallization. Each individual sill is 10–50 m thick and would have solidified fairly rapidly (1–30 years), the same order of magnitude as the duration of common monogenetic eruptions. Our estimates imply that differentiation in individual shallow sills may occur during the course of an eruption whose style may vary from effusive to explosive by tapping different magma compositions. Our study shows that basaltic magmas have the potential to differentiate to volatile-rich magma in shallow intrusive systems, which may increase explosivity.

Geochemical evidence of mixing between A‐type rhyolites and basalts from Southern Lebombo, South Africa: Implications for evolution of the Northern Karoo Igneous Province

The ~183 Ma Karoo Continental Flood Basalt (CFB), southern Africa, formed during a period of major crustal extension prior to Gondwana breakup. To explain the ‘lithospheric contamination’ observed in its whole‐rock chemical and isotopic data various models have been proposed, such as different subcontinental lithospheric mantle (SCLM) sources or interaction between a mantle plume and SCLM or subducted material. A re‐assessment of data in the literature from the Northern Karoo Province, combined with new petrographic, whole‐rock chemical and Sr‐Nd isotopic data of basaltic and dacite/trachydacite samples from the southern Lebombo suggest that chemical diffusion between low‐Ti basaltic/picritic magmas and high‐SiO2 rhyolite parent melt may have caused this ‘lithospheric contamination’. Such slow chemical diffusion, which is posited to have developed during formation of this CFB, can explain the bimodal Karoo basalt‐rhyolite association, the wide variation in SiO2 content (~62–78 wt%) in the dacite/rhyolites, and variable 87Sr/86Sr (183 Ma) and ɛNd (183 Ma) isotope ratios of the Karoo picrites and basalts (0.7037 to 0.7102; 3.58 to −33.52, respectively) and dacites/rhyolites (0.7032 to 0.7152; 0.44 to −17.19). The intermediate incompatible trace element and isotopic compositions of the Karoo high‐Ti basalt/picrite suggest that these rocks could have evolved by magma mixing through chemical diffusion. Our geochemical model for the northern Karoo CFB province involves existence of a voluminous hot (>1,000°C) low‐Ti basaltic magma chamber with remnant picritic melts at shallow crustal depth (~4 km) during major crustal extension. Magma storage was relatively protracted, resulting in significant volumes of partial melt of the surrounding granitoids of the Kaapvaal Craton under relatively anhydrous condition, leading to generation of A‐type high‐SiO2 rhyolite parent magma at pressure of ~1.5 kbar and temperature of ~930°C. This was followed by chemical diffusion of selected elements [LREE, HFSE, and some LILE (K, Rb, Ba, Pb)] and self‐diffusion of Sr and Nd isotopes in these two melts, which generated the observed litho‐type variations in Karoo CFB.

Post-collisional potassic rocks in Western Kunlun, NW Tibet Plateau: Insights into lateral variations in the crust-mantle structure beneath the India-Asia collision zone

The collision between India and Asia created the most significant orogenic system of the Cenozoic. Although numerous collision-related igneous rocks formed, many post-collisional rocks were emplaced but have received less attention. Recently, geophysical investigations and field observations have suggested significant lateral variations in crust-mantle structure beneath the collisional zone. However, there have been few attempts so far to reveal the possible relationship between the lateral variations in crust-mantle structure and the widely distributed post-collisional rocks. To reveal the possible linkage, we investigate the Pulu volcanic rocks in Western Kunlun, NW Tibet Plateau, through a combined analysis involving whole-rock geochemistry and Sr-Nd-Pb isotopes, and mineralogy. The similar profiles (e.g., Fo and An values) of olivine, pyroxene, and plagioclase phenocrysts, along with relatively consistent whole-rock isotopic compositions argue against mixing and significant crustal contamination. The sub-continental lithospheric mantle source (SCLM) was enriched by the input of sedimentary materials as indicated by Sr-Nd-Pb isotopic compositions (87Sr/86 Sr(t) < 0.708884, 143Nd/144Nd > 0.512344, and 204Pb/207Pb < 15.6831). The Pulu volcanic rocks in Western Kunlun display high Rb/Sr ratios (> 0.05) and low Ba/Rb ratios (〈23), indicating an amphibole-free phlogopite residual source at a depth of >70 km. In addition, high LREE/HREE (LaN/YbN > 49) and Nb/Ta ratios (> 20), and olivine geothermometer confirm a garnet-bearing mantle source at >970 °C under eclogite-facies conditions. In contrast to other post-collisional rocks formed under low pressure and widely variable temperatures associated with the delamination and thinning of SCLM in the eastern and southern Tibet Plateau, post-collisional rocks in Western Kunlun formed under the high pressure and a narrow range of temperature are attributed to lithospheric fractures at great depths before delamination. Such contrasting P-T conditions of the post-collisional rocks in Western Kunlun and eastern and southern Tibet Plateau record lateral variations in crust-mantle structures beneath the India-Asia collisional zone.

A telltale signature of Archean lithospheric mantle in the Paraná continental flood basalts genesis

The origin of the Paraná Continental Flood Basalts (PCFB), which constitute one of largest magmatic provinces in the world, remains a very controversial subject, particularly whether melts are sourced in the mesosphere (deep mantle plume), asthenosphere and/or lithosphere. In an effort to scrutinize those competing hypotheses, new measurements of Os, Nd, Sr and Pb isotopes, along with major and trace elements are presented for low-TiO2 tholeiites from the province. This suite occurs discreetly in the central-north of the province (termed Ribeira magma-type), whose lithospheric structure is characterized by higher P- and S- wave velocities, and high electrical resistivity. The initial 187Os/188Os isotopic compositions in Ribeira lavas, ranging from 0.10660 to 0.12575, are more unradiogenic than the other magma-types of the PCFB, as well as the estimates of the contemporary Depleted Mantle and are lower than any osmium isotopic ratio yet reported for continental flood basalts. These remarkably unradiogenic 187Os/188Os ratios preclude significant continental crust contamination and require the involvement of an ancient subcontinental lithospheric mantle source that evolved in a very low Re/Os environment. The fact that the Ribeira rocks occur on a peculiar lithospheric geophysical structure and have a unique unradiogenic osmium isotope signature reveals for the first time the existence of Archean lithosphere concealed by the Paraná Basin, inserted in the Paranapanema fragmented lithosphere. On the other hand, the other magma-types of the province, whose osmium isotopic signature is similar to the modern fertile mantle, occur close to more electrically conductive lithospheric structures. Thus, osmium isotopic information integrated with recent crustal and upper mantle geophysical soundings provides crucial information about the PCFB mantle sources. To account for the combined Os, Nd, Sr and Pb isotopic compositions of the Ribeira rocks, we propose that the primary melts formed from the heterogeneous lithospheric mantle (including Archean lithosphere fragments) that was variably hybridized by melts derived from recycling of eclogites related to multiple Neoproterozoic-Cambrian suture zones that surround the Paraná Basin and subduction processes.

Late Neoproterozoic mafic sills in the Suizhou area of the South Qinling block: constraints for the tectonic evolution of the northern margin of the Yangtze craton

ABSTRACT Late Neoproterozoic gabbroic-diabasic sills are widely distributed in the eastern part of the South Qinling block. Here, a comprehensive geochronological and geochemical study of these mafic sills has been conducted. LA-ICP-MS zircon U-Pb results indicate that the gabbroic-diabasic sills were formed at 632 ± 2 Ma. The sills consist mainly of tholeiitic basaltic rocks with relatively low TiO2 contents (0.81–1.07 wt.%) and high Fe2O3 T/MgO ratios. These rocks display relatively flat to moderate light rare earth elements (LREE) enrichment patterns and positive Eu anomalies (Eu/Eu* = 1.15–1.41). On the primitive mantle normalized spider diagram, the samples show E-MORB-like geochemical features. All of the studied samples display similar Sm-Nd isotopic compositions with negative εNd (t) values ranging from −6.2 to −5.5. Most clinopyroxenes from the mafic sills have relatively low Al/Ti ratios similar to those of typical rift-related cumulates. Therefore, Whole-rock and mineral geochemical features of the studied samples suggest that the mafic sills might have been derived from partial melting of a metasomatised subcontinental lithospheric mantle source in a continental rift setting. It can be advocated that the break-up of the Rodinia supercontinent in the northern part of the Yangtze craton might have been a long-lasting process. By comparison of the coherent Neoproterozoic magmatic events in the northern Yangtze craton and the Tarim block, it can be suggested that the Yangtze craton might have been adjacent to the Tarim block in the reconstruction model of the Rodinia supercontinent.

The middle Eocene high-K magmatism in Eastern Iran Magmatic Belt: constraints from U-Pb zircon geochronology and Sr-Nd isotopic ratios

ABSTRACT Intrusive rocks are well-exposed in the south Birjand around the Koudakan is herein compared to previously studied outcrops along the middle Eocene to late Oligocene Eastern Iran Magmatic Belt. This pluton is composed mainly of monzonite, quartz-monzonite, and granite with high-K calc-alkaline to shoshonitic affinities. The U-Pb zircon geochronology from monzonite and quartz-monzonite reveals the crystallization ages of 40.96 ± 0.48 to 38.78 ± 0.78 Ma (Bartonian). The monzonite, quartz-monzonite, and granite rocks show similar REEs and trace element patterns, as well as limited variations in εNd(i) and 87Sr/86Sr(i) ratio, suggesting that they are a comagmatic intrusive suite. The chondrite and primitive mantle normalized rare earth and trace element patterns show enrichment in the light rare earth elements, K, Rb, Cs, Pb, Th, and U and depletion in heavy rare earth elements, Nb, Zr, and Ti. The εNd(i) and 87Sr/86Sr(i) values range from +1.32 to +1.68 and 0.7044 to 0.7047, respectively, identical to island-arc basalt composition. The whole-rock Nd model age (TDM) for the intrusive rocks range between 0.69 and 0.73 Ga. These geochemical and isotopic signatures indicate a subduction-related sub-continental lithospheric mantle source for these rocks. Our new geochemical, isotopic, and geochronological studies integrated with previously published data indicate that the middle Eocene to late Oligocene magmatism in eastern Iran was formed in a post-collisional tectonic environment. We suggest the northeastward subduction of the Neo-Tethys ocean beneath the Lut block and the eastward subduction of the Sistan ocean beneath the Afghan block caused mantle wedge to be metasomatized by slab components. At a later stage, a collision between the Lut and Afghan blocks was accompanied by the lithospheric delamination, and the subsequent asthenospheric upwelling led to the melting of the metasomatized sub-continental lithospheric mantle and the generation of middle Eocene to late Oligocene magmatism in the Eastern Iran Magmatic Belt.

The age and geochemistry of the Bardkish syenite, northwest Iran: Syenite formation during Neo‐Tethyan subduction

AbstractSHRIMP U–Pb dating of zircon from two representative samples of the Bardkish syenitic pluton, Urumieh plutonic complex, northwestern Iran, yielded a Late Cretaceous weighted mean 206Pb/238U age of 91.1 ±0.8 Ma (Turonian). The pluton is characterized by high K2O content, Ti–Nb–Ta depletion, enrichment in light rare earth elements and large ion lithophile elements compared to high field strength element, Mg‐rich biotite and calcic amphibole, suggesting a shoshonitic affinity. Geochemical features of the Bardkish syenite are consistent with derivation by fractional crystallization of initial mafic parent magmas. It is most likely that primary magma of the syenite originated from a metasomatized subcontinental lithospheric mantle source with minor crustal component, and underwent a fractional crystallization process during its emplacement. The estimates of trapping pressures and temperatures suggest that crystallization began at the middle crustal level (less than 11 km) near 700–730 °C. The generation of the Bardkish syenite could be related to the subduction of Neo‐Tethyan oceanic crust beneath the Central Iranian microcontinent and it possibly marks a major magmatic episode prior to the final closure of the Neo‐Tethyan ocean in northwest Iran.

Lower Oligocene Calc-Alkaline Spessartitic Lamprophyres from Central Iran (East of Anarak Area); an Evidence from the Eastern Branch of Neotethys Subduction-Related Mantle Enrichment

The Lower Oligocene Kal-e-kafi (East of Anarak, Central Iran) lamprophyres occur as stocks and dikes, which cross-cut the Eocene volcanic and Cretaceous sedimentary rocks. The predominant minerals of these lamprophyres are hornblende (magnesiohastingsite) and clinopyroxene (diopside) phenocrysts set in a fine- to medium-grained matrix of the same minerals plus plagioclase (labradorite to bytownite), sanidine, apatite, and magnetite. Secondary minerals are chlorite, magnetite, calcite, and epidote. Petrography, mineral chemistry, and whole rock compositions classify these rocks as calc-alkaline lamprophyre, in general, and spessartite in particular. These samples have intermediate compositions (SiO2 ~ 58 wt %). The chondrite-normalized REE patterns and primitive mantle-normalized multi-element spider diagram of Kal-e-kafi lamprophyres are remarkably parallel and suggest that these dikes and stocks were derived from the same parental magma and underwent similar melt extraction. These rocks are enriched in alkalies, large-ion lithophile elements (e.g., Rb, Ba, K), and light rare-earth elements (e.g., La, Ce), and exhibit moderate to high fractionation in LREE patterns, with an average La/Lu ratio of 112. The large amount of hydrous fluids coming from the subducted slab rather than sediments caused to the enrichment and metasomatism of subcontinental lithospheric mantle source. Crustal contamination and assimilation of host rocks also played role in the genesis of these lamprophyres. Geochemical characteristics of the studied rocks suggest that parental magma have been derived from partial melting of a metasomatized amphibole-bearing spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism to explain enrichment in volatiles of the mantle, and the lamprophyric magmatism of the Kal-e-kafi area in Lower Oligocene times. Several tectonomagmatic discrimination diagrams indicate that the Kal-e-kafi lamprophyres occurred during postcollisional period of lithospheric extension.

Cambrian mafic and granitic intrusions in the Mazar-Tianshuihai terrane, West Kunlun Orogenic Belt: Constraints on the subduction orientation of the Proto-Tethys Ocean

The West Kunlun Orogenic Belt (WKOB) carries key information about the evolution of the Proto-Tethys Ocean. In order to have a better understanding of the early Paleozoic tectonic evolution of the WKOB and to unravel the process of the Tarim assemblage to Gondwana, we report field observations, petrography, ages, chemical and Sr-Nd-Hf isotopic compositions of the newly identified Cambrian mafic and granitic intrusions in the Mazar Terrane-Tianshuihai terrane (MZT-TSHT) at the interior of the WKOB. Our zircon U–Pb dating reveals that the mafic and granitic rocks in Tianshuihai Terrane (TSHT) emplaced at ca. 530 Ma, and that the mafic rocks in the Mazar Terrane (MZT) emplaced at ca. 500–490 Ma. Whole-rock major and trace element geochemistry defines a tholeiitic signature for these mafic rocks. Isotopically, the TSHT mafic rocks have depleted Nd isotope compositions with initial εNd(t) isotopes ranging from 2.74 to 3.60, whereas the MZT mafic rocks exhibit enriched whole-rock Nd isotope compositions with εNd(t) values ranging from −12.90 to −9.71. The distinct chemical and isotopic signatures argue that the Cambrian mafic rocks in the TSHT were derived from a depleted mantle source in an initial subduction setting, whereas the mafic rocks from MZT were derived from an enriched sub-continental lithospheric mantle source in an arc setting. The Cambrian granites in the TSHT show typical fractionated I-type granite elemental signatures, in line with their Nd-Sr-Hf isotope compositions. These Cambrian granites were most likely generated by partial melting of Mesoproterozoic (0.9–1.3 Ga) crustal sources due to underplating of the coeval mafic magma. Our results in combination with previous studies suggest that the Cambrian mafic and granitic rocks in the MZT-TSHT were generated by the southward subduction of the Proto-Tethys Ocean initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive early Paleozoic accretionary wedge in WKOB. The Proto-Tethys Ocean finally closed at ca. 440 Ma as demonstrated by the amphibolite facies metamorphism of the accretionary complex and the middle to late Devonian molasses unconformably overlying the accretionary complex. This process caused the blocks in East Asia, including the Tarim, Qaidam and North Qilian, Yangtze and Indochina, to dock at the northern fringe of the Eastern Gondwana.