Subcontinental Lithospheric Mantle Research Articles

Magma Chemistry and Tectonic Controls of Volcanic Activity in the Southern Ural Area during Early Carboniferous Time

Early Carboniferous (359.3–323.4 Ma) volcanic complexes are widespread in the Southern Ural tectonic province, a fragment of the western (in present-day coordinates) segment of the Central Asian Orogenic Belt. Here, the Lower Carboniferous sequences crop out within the following N–S-trending tectonic zones (from west to east): Magnitogorsk, Ui River, East Ural, Transural, and Valeryanovka. We describe and discuss the geology and geochemistry of the Early Carboniferous volcanic complexes on the basis of published and newly obtained data, with implications for paleo-tectonic models. The western zones are dominated by bimodal rhyolite-basalt series, with the basalts relatively enriched in Ti and Zr but depleted in Nb. The volcanics of the Valeryanovka zone belong to the typically evolved calc-alkaline series, with the derivatives depleted in Ti and Nb. Almost all of the selected groups of volcanics bear geochemical signatures transitional between those of subduction-related and intraplate igneous rocks. The relative enrichment of the volcanics of the East Ural and Transural Zones may be interpreted as a result of a contribution from asthenospheric mantle and/or from subcontinental lithospheric mantle. The volcanics of the Valeryanovka zone reveal features common to subduction-related series of the Andean type. The data obtained allow us to compare the Early Carboniferous geodynamic settings in the western zones with the modern setting of the Northeastern Pacific, whereas the geodynamic setting of the Valeryanovka zone resembles that at the western margin of South America.

Subcontinental lithospheric mantle discontinuities beneath the Eastern Himalayan Plate Boundary System, NE India

SUMMARY We use P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave group velocity dispersion data to model the shear wave velocity (Vs) structure of subcontinental lithospheric mantle (SCLM) discontinuities beneath northeast (NE) India. The most prominent SCLM discontinuity is the Hales Discontinuity (H-D) observed beneath the Eastern Himalayan Foreland Basin (Brahmaputra Valley) and Shillong Plateau. The P-to-SV converted phase from the H-D (Phs) is a positive amplitude arrival at ∼10–12 s and has positive moveout with increasing ray-parameter. From joint inversion, the H-D is modelled at a depth range of 90–106 km, with 11–12 per cent Vs increase beneath the Brahmaputra Valley. Beneath the Shillong Plateau the H-D is at a depth range of 86–99 km, with 6–10 per cent Vs increase. An intralithospheric discontinuity (ILD) has been identified in the Shillong Plateau station P-RFs, as a positive amplitude PILDs phase, arriving at 8–8.5 s. This is modelled at a depth range of 66–75 km with Vs increase of 2–9 per cent. We construct 2-D profiles of depth-migrated common-conversion-point stack of P-RFs to distinguish the SCLM discontinuity arrivals from crustal phases. 3-D spline-interpolated surface of the H-D has been constructed to visualize its lateral variations. We use xenolith data from the Dharwar Craton, which has similar geological age, petrology and seismic structure as the Shillong Plateau, to petrologically model the SCLM H-D and ILD Vs structure in NE-India. From the calculated Vs structure we conjecture that the H-D is a petrological boundary between mantle peridotite and kyanite-eclogite, with its origin as metamorphosed paleosubducted oceanic slab, similar to other global observations. We further speculate that the shallower ILD could be formed as a contact between frozen asthenosphere-derived metasomatic melts within the SCLM.

Time-space variations in the East African Rift magmatism: the role of different mantle domains

The East African Rift System (EARS) is the classic example of an active continental rift where extensional tectonics and lithospheric thinning have been closely associated to the generation of large volumes of magmas and represents the environment with the largest range of erupted magma types all over the world. The geochemical signature of erupted magmas testifies the involvement of different mantle domains and depths (i.e., subcontinental lithosphere, asthenosphere and deeper mantle sources). Our aim is to investigate the variable contribution of different mantle domains in the genesis of the EARS magmas through space and time, considering not only the geochemical signature of erupted magmas but also the geochemical message of mantle xenoliths. The main goal is to provide a large-scale view of the common process driving the origin of magmas in the EARS beyond the local peculiarities linked to specific settings. To this aim, we screened an exhaustive geochemical database of basalts and mantle xenoliths from the EARS, and we report original trace element and Sr-Nd isotope data of new samples collected from the Main Ethiopian Rift and Turkana depression. The data were subdivided according to spatial and temporal criteria. From a spatial point of view, the samples were ascribed to five groups, namely: Afar, Ethiopia, Turkana, Eastern Branch, and Western Branch; from a temporal point of view, the magmatic activity of the EARS was subdivided into three main temporal intervals: 45-25 Ma, 25-10 Ma and 10-0 Ma. The geochemical and radiogenic isotope (Sr, Nd, Pb) signature of the selected basalts denotes the variable contributions of a mantle plume, a more depleted asthenospheric mantle (DMM), and different SubContinental Lithospheric Mantle (SCLM) domains, depending on their temporal and spatial distribution. The geochemistry of the selected basalts shows a marked correspondence with the compositional heterogeneity of mantle xenoliths, whose isotopic systematics (Sm-Nd, Re-Os) indicates the formation of the local SCLM in the Archean and during the Pan-African orogeny. Both SCLM domains contributed significantly to magma genesis in the Western Branch (whose signature points towards a contribution of the Pan-African lithosphere) and Eastern Branch (which is also affected by Archean SCLM domains) magmas. The contribution of the SCLM generally increases with time, possibly related to an increase of the geothermal gradient in response to the arrival and flattening of the plume head at the base of the lithosphere and later extension, thinning and shallower melting. Our interpretation supports a pivotal role of the different SCLM domains in magma genesis that is able to fully explain the large compositional heterogeneity of the EARS basalts and represents a reasonable alternative to the putative presence of multiple mantle plumes or a heterogeneous mantle upwelling.

Discovery of Early Tonian Calc‐alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India: Implications of Proterozoic Sub‐continental Lithospheric Mantle

AbstractReports of shoshonitic rocks in Precambrian terrains are relatively rare. Pl‐Grt amphibolites and Hbl‐Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex (CGC) show calc‐alkaline and shoshonitic characteristics. Relict porphyritic, sub‐ophitic and poikilitic textures are noted in these rocks. Their parent magma was emplaced during the waning phase of the regional metamorphism. Geochemically, these metamafics are similar to the Group III potassic and ultrapotassic rocks of Foley et al. (1987). The magma was derived from the metasomatized subcontinental lithospheric mantle (SCLM). Subduction‐related sediment melts metasomatized the SCLM. Compositionally, the SCLM is a metasomatized phlogopite‐amphibole‐spinel‐bearing harzburgite. 1%–5% batch melting of the SCLM could produce the parental magma of the mafic granulites. Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling. Peak metamorphic assemblage (M1: Grt‐Cpx‐Pl‐Qz) in garnetiferous amphibolite equilibrated at 740°C and 8.7 kbar. The Cpx‐Pl corona appeared around the garnet during decompression (M2: 655°C, 6 kbar). The Hbl‐Pl symplectites around garnet formed during isobaric cooling (M3: 580°C and 5.9 kbar). The emplacement of shoshonitic magma and subsequent decompression happened at the slab break‐off stage of continental collision (∼990 Ma).

Tectono-magmatic evolution during the extensional phase of a Wilson Cycle: a review of the Alpine Tethys case and implications for Atlantic- type margins

Despite the recent progress in seismic imaging and dynamic modelling, the tectono-magmatic evolution during late orogenic extension and rifting is at present little understood. Answering questions such as when and where magma forms, how it percolates and how it is emplaced requires access to magmatic rocks from different crustal and mantle levels, which is impossible in modern rift systems. An alternative, presented in this study, is to use remnants of fossil extensional systems exposed in collisional orogens. Here we review the Permian to Early Cretaceous tectono-magmatic evolution recorded in the Alpine realm of Western Europe. While most of the previous studies reviewed the geochemical and petrological characteristics of the magmatic rocks, here we rely on a holistic approach to link the magmatic evolution to extensional processes on a large scale. We synthesize our observations on the timing and location of magma production in a Wheeler diagram. Our study shows that two major magmatic events can be defined: the first one took place during the Permian and was linked to post- orogenic extension of the Variscan belt. It affected a large portion of Western Europe, where it was responsible for significant sub-aerial volcanic activity, for the emplacement of massive magmatic additions at the base and within the crust, and for the depletion of the lower subcontinental lithospheric mantle. The second included several pulses of magmatic activity related to the multiphase rifting event that led to the successive opening of the Meliata-Vardar, Alpine Tethys and southern North Atlantic (proto-)oceanic domains. In the Alpine Tethys, the first rift-related magmatic activity was linked to melt infiltration within the subcontinental lithospheric mantle during the so-called necking stage. Basalts were emplaced only later, during and after mantle exhumation. Their chemical signature reflects the interaction between melts derived from Depleted Mid-oceanic ridge basalt Mantle (DMM) and the previously refertilized/infiltrated subcontinental mantle. Late alkaline magmatic activity post- dated breakup. A main result of this study is that the tectono-magmatic evolution of extensional systems cannot be explained by simple models using ß-factors or mantle temperatures alone but also needs to integrate the inheritance acquired during previous tectonic phases.

Tectonomagmatic evolution of southwestern Laurentia: Insights from zircon U-Pb geochronology and hafnium isotopic composition of the Red Bluff Granite Suite, west Texas, USA

AbstractWe provide laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) and high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb ages and Hf isotopic compositions of zircons from the Red Bluff Granite Suite and mafic dikes in the Franklin Mountains of El Paso, Texas, USA. Granitoids exposed in the Franklin Mountains were previously divided into five magmatic stages based on cross-cutting relationships. Major and trace element compositions showed that these granitoids are ferroan, alkaline, and A2 type. Homogeneity in the whole-rock geochemistry suggests that the granite stages are genetically related and share similar petrogenetic histories. Weighted mean zircon 206Pb/238U dates from the older magmatic stage 1 alkali-feldspar quartz syenite and stage 2 alkali-feldspar granite are 1112.36 ± 0.35 and 1112.46 ± 0.37 Ma, respectively. The weighted mean εHf(t) values varying from +5.3 to +7.2 are similar to those of other regional ca. 1.1 Ga magmatic rocks throughout southwestern Laurentia. Geochemical characteristics, petrological modeling, and enriched Hf isotopic composition suggest fractional crystallization of a basaltic magma that was produced by melting of an enriched mantle reservoir. However, zircon inheritance ages of ca. 1.3 Ga and 1.26–1.15 Ga are consistent with a minor contribution from felsic crustal basement. Our data and regional geology are consistent with a post-collisional slab break-off that facilitated asthenospheric upwelling and partial melting of the enriched mantle, possibly subcontinental lithospheric mantle, extending from Llano Uplift, Texas, in the southeast to California to the northwest. Magma thus generated upon differentiation produced ferroan and A-type granitoids.

Assessing crustal contamination in the 28 to 18 Ma Dulce dike swarm with Nd-Sr isotopic data, southwestern Colorado

Alkaline to subalkaline mafic dikes in the 28 to 18 Ma Dulce swarm were emplaced in a zone of incipient extension from southern Colorado into northern New Mexico on the northeastern boundary of the San Juan Basin. The 87Sr/86Sr ratios for the dikes are 0.70503 to 0.70584, akin to most post-28 Ma mafic rocks across the northern San Juan Basin. These data are consistent with melting of metasomatized subcontinental lithospheric mantle with little to no crustal contribution as revealed by the geochemical and Sr-Nd isotopic signatures of most 28–0.6 Ma mafic rocks in the region. Time-corrected εNd(t) values of −4.1 to −7.4 for rocks in the Dulce swarm, however, indicate that magma production involved the crust. A previous hypothesis for Dulce magmas was contamination of lithospheric mantle melts with up to 45% mafic lower crust ± 0.5% upper crust. In this investigation, six new whole-rock Sr and Nd isotopic analyses were combined with published data to further investigate the contamination of lithospheric mantle melts with different crustal reservoirs. The Nd isotope signatures of the Dulce swarm offer evidence for the long-term involvement (∼10 Ma) of lower crust in the production of rift-related mantle magmas. Isotopic mixing curves support previous hypotheses for the contamination of lithospheric mantle melts with 10 to 40 percent lower mafic crust. This provides further insight into regional variations in mantle magmas produced after 28 Ma in the Four Corners region that likely triggered crustal melting related to caldera complexes in the western San Juan Mountains.

Paleoproterozoic Shoshonite Mafic Associations of the Irkut Block (Sharyzhalgai Uplift, Southwest Siberian Craton): U–Pb Age and Conditions of Zircon Crystallization

Abstract —The paper presents data on the composition and age of mafic rocks of the shoshonitic series in the Irkut block of the Sharyzhalgai uplift (southwest of the Siberian Сraton). According to the U–Pb dating of magmatic zircon, the formation of monzodiorites of the Poludennyi massif and gabbro-dolerites in the endo- and exocontact zones of the Toisuk pluton occurred at 1.87 and 1.86–1.85 Ga, respectively. The intrusion of mafic magmas and their underplating into the basement of the crust under postcollisional extension resulted in the near-coeval mafic and granitoid magmatism in the Irkut block between 1.87 and 1.84 Ga. The Paleoproterozoic mafic associations belong to the shoshonitic series and are characterized by enrichment in incompatible elements, including Zr, and low negative εNd(T) values. These geochemical and isotopic characteristics point to the derivation of magma from a long-lived enriched-mantle source, such as the subcontinental lithospheric mantle. The crystallization of zircon from the last portions of the evolved mafic melt is evidenced by low zirconium saturation temperatures (710–965 °C) and zircon enrichment in U and Th with increasing Th/U, reflecting the growth of concentrations of highly incompatible elements in the residual melt.

Geochemistry and zircon U–Pb ages of early Ordovician syenites from the Inexpressible Island, Antarctica and tectonic implications

The Ross Orogenic Belt is in the Antarctica Transantarctic Mountains. North Victoria Land Granite Harbour Intrusive complex (GHI) records the tectonic-magmatism evolution of Ross orogeny. Extensively developed post-collisional granites around this margin of early Paleozoic magmatism can provide insights into the growth of continental crust through accretionary orogenesis. We provide geochemical and geochronological data from syenites from Terra Nova Bay, north Victoria Land in order to constrain its tectonic evolution and setting. The syenite belongs to the potassium-alkaline, calc-alkaline series and is characterized by high concentrations of rare Earth elements and large ion lithophile elements (LILE), and low content in high field strength elements (Nb, Ta, P, Ti). The petrographic and geochemical signatures show a possible island-arc granite affinity. LA-ICP-MS zircon U-Pb dating results suggest that the Inexpressible Island syenite was emplaced at ca. 471.8 ± 1.8 Ma and 477.3 ± 1.7 Ma, respectively. Zircon εHf(t) values range from −7.4 to −9.1; average −8.2 and whole-rock εNd (t) values range from −8.5 to −10.3, indicating that formed by the partial melting of the lithospheric mantle enriched with subduction slab fluids and subcontinental lithosphere. Whereas, the syenite has a strong positive Eu anomaly and a positive Sr anomaly, suggesting that plagioclase cumulate crystallization occurred in the magma source area. Furthermore, through integration with previous studies, we suggest that syenite is a result of the melting zone of an older previously subduction enriched layer of the subcontinental lithospheric mantle (SCLM). To enable syenite emplacement we suggest a tectonic-magmatic model that invokes alternating phases of extension and contraction in the overriding plate. Finally, we report the youngest age of (post-orogenic) magmatism occurred during extension in the overriding plate ca. 478–471 Ma.

Petrogenesis and relationship with REE mineralization of the quartz syenite from Chishan and Longbaoshan alkaline complex, southeastern North China Craton: Insights from zircon U–Pb geochronology, element, and Sr–Nd–Pb–Hf isotope geochemistry

Mesozoic alkaline complexes associated with the rare earth element (REE) mineralization are developed in southeastern North China Craton (NCC), and they recorded some important information about the lithospheric thinning and destruction of the NCC. Zircon U–Pb dating results reveal that syenitic rocks from the Chishan and Longbaoshan alkaline complexes were emplaced at ca. 126.2–123.7 Ma. These syenitic rocks have an arc-like affinity with enriched LREEs and LILEs (e.g., Ba and Th) and depleted HFSE (e.g., Nb and Ta). However, they plot within the extensional alkaline–calc-alkaline field in the lg (CaO/(K2O + Na2O)) versus SiO2 discrimination diagram and plot within the fields of the divergent plate and within-plate settings in the NbN versus ThN discrimination diagram, implying an extensional tectonic setting. Thus, we suggest that the syenitic rocks should form in an extensional tectonic settings related to rollback and retreat of the subducting Paleo-Pacific plate. These syenitic rocks have similar Sr–Nd–Pb–Hf isotope compositions ((87Sr/86Sr)i = 0.7062 to 0.7101, εNd(t) = −8.2 to −15.0, εHf(t) = −13.5 to −9.6), which are the same as the enriched subcontinental lithospheric mantle (SCLM) of the NCC. Taking into account not only our new data but also previously published data regarding the evolution of NCC in the Mesozoic, we conclude that the syenitic rocks from the Chishan and Longbaoshan complexes are derived from partial melting of the lithospheric mantle triggered by the Paleo-Pacific plate subduction. Moreover, the geochemical and Sr–Nd–Pb isotopic similarity between the Chishan REE ores, Longbaoshan altered rocks, and the syenitic rocks implied that they are homologous products of Early Cretaceous and that the mineralization shows inheritance to the magmatic hydrothermal evolution.

Petrogenesis and metallogenesis of the Qieganbulake carbonatite-related phosphate deposit associated with the mafic–ultramafic–carbonatite complex in the Kuluketage block, northeastern Tarim Craton

AbstractThe Qieganbulake deposit associated with a mafic–ultramafic–carbonatite complex in the Kuluketage block is not only the world’s second-largest vermiculite deposit, but also a medium-size carbonatite-related phosphate deposit. Field observations, radiometric dating results and Sr–Nd–Hf isotopes reveal that the parental magmas of the carbonatite and mafic–ultramafic rocks are cogenetic and formed synchronously at c. 810 Ma. Geochemical characteristics and Sr–Nd–Hf–S isotopes ((87Sr/86Sr)i = 0.70581–0.70710; ϵNd(t) = −0.20 to −11.80; ϵHf(t) = −7.5 to −10.3; δ34S = +0.7 ‰ to +3.0 ‰ (some sulfides with high δ34S values (+3.2 to +6.6) were formed by late hydrothermal sulfur)), in combination with mineral compositions and previous research, strongly indicate that the Qieganbulake mafic–ultramafic–carbonatite complex formed via extensive crystal fractionation/cumulation and liquid immiscibility of a carbonated tholeiitic magma, possibly derived from partial melting of an enriched subcontinental lithospheric mantle previously modified by slab-released fluids and sediment input in a continental rift setting. The coupled enriched Sr–Nd isotopic signatures, in combination with previous research, suggest that the enriched subcontinental lithospheric mantle could have been metasomatized by asthenospheric mantle melts to different degrees. The Qieganbulake carbonatite-related phosphate ores were the products of normal fractional crystallization/cumulation of P–Fe3+ complex enriched carbonatite magma in high oxygen fugacity conditions, which was generated by liquid immiscibility of CO2–Fe–Ti–P-rich residual magma undergoing high differentiation.

Large-scale replacement of ancient mantle lithosphere during supercontinent assembly: Evidence from the South China Craton

The modification and destruction of the sub-continental lithospheric mantle (SCLM) has been identified in numerous studies. However, what principally controls the susceptibility and extent of destruction of the SCLM and how this leads to the corresponding variation of the overlying crust remain long-standing issues. Here we provide an integrated study of petrology, mineralogy, and geochemistry, including highly siderophile element (HSE) abundances and whole-rock and/or mineral Os-Hf-Sr-Nd isotopic compositions of mantle peridotite xenoliths from Hainan Island, together with a compiled dataset from the eastern South China Craton (SCC) to investigate the mechanism and timing of replacement and destruction of the SCLM. The mantle xenoliths from the eastern SCC are overall more fertile than cratonic lithospheric mantle, and have generally experienced various extents of metasomatic enrichment, including carbonate and silicate melt metasomatism. In addition, the absence of Archean ages and compositional or temporal stratification of the SCLM beneath the eastern SCC indicates that the Archean lithospheric mantle may have been replaced/reworked and has essentially vanished, hence the present underlying lithospheric mantle is a relatively new and young lithospheric mantle. The formation time of the SCLM underneath Hainan Island constrained by both the ReOs and LuHf isotopic systems is late Mesoproterozoic (∼1.2 Ga), largely consistent with the Meso- to Neoproterozoic peak of whole-rock TRD ages (0.9–1.3 Ga) of the refractory mantle peridotites in the compiled dataset. This age distribution can be reconciled with the assembly time of the Rodinia supercontinent, indicating a close linkage between the replacement of large-scale lithospheric mantle via mantle convection erosion and regional continental amalgamation. Meanwhile, the relative location to the plate margins influences the preservation of the cratonic mantle. The SCLM located at the plate margin (e.g., eastern SCC compared to western SCC), where destructive processes frequently occur, is susceptible to destruction, whereas that within the plate interior is likely to remain relatively intact.

Geochronology and geochemistry of the Panjal Traps from the southern Pir Panjal Range, Kashmir, India

The eruption of the Panjal Traps is associated with rifting along the Tethyan margin of Gondwana. A CA-ID-TIMS zircon U-Pb age of basalt collected from the southern Pir Panjal Range (SPPR) yielded a weighted-mean 206Pb/238U age of 288.2 ± 0.3 Ma. The rocks from the SPPR can be divided into three compositionally distinct groups (basalt: SiO2 < 52 wt%; Mg# = 56–36, basaltic andesite: SiO2 = 52–60 wt%; Mg# = 32–48, and silicic: SiO2 > 64.0 wt%; Mg# = 27–23) and are chemically correlated with rocks from the eastern Kashmir Valley rather than the northern Pir Panjal Range (NPPR). The Sr-Nd isotopes (87Sr/86Sri = 0.70860–0.71104; εNd(t) = −6.1 to −4.4) of the mafic rocks are similar and indicate that their magmas were derived from a subduction modified subcontinental lithospheric mantle (SCLM) source. The silicic rocks were derived by partial melting of the continental crust (87Sr/86Sri = 0.71022–0.71093; εNd(t) = −8.7 to −7.2) and their discovery in the SPPR indicates that silicic volcanism was far more extensive than previously thought. The 187Os/188Osi isotopic ratios of basalts from different regions of the Panjal Traps range from 0.1230 to 0.2832. Two samples from the NPPR have ratios of 0.1230 and 0.1256 that are slightly higher than primitive upper mantle at the time of eruption whereas the remaining samples show enriched values (0.1343–0.2832). The new data suggest that it is likely that the mantle source of the Panjal Traps transitioned from a SCLM source to a sublithospheric source over time as seafloor spreading developed.

Heavy halogen compositions of lamprophyres derived from metasomatized lithospheric mantle beneath eastern North China Craton

Abstract Halogens and other volatiles are widely recycled into the deep mantle by subduction and are key components to metasomatize the sub-continental lithospheric mantle (SCLM). Lamprophyres are well known to be rich in volatiles and are important for understanding the halogen characteristics of the meta-somatized SCLM and/or the mobilization of halogens during the ascent of such volatile-rich, low-degree partial melts. The North China Craton (NCC) hosts lamprophyre dikes coeval with extensive thinning of the eastern NCC in the Mesozoic and generated from lithosphere metasomatized by multiple-stage subduction components. Here we report bulk-rock heavy halogens (Cl, Br, and I) of 16 lamprophyres from the eastern NCC. The bulk-rock halogen concentrations are overall very low (Cl = 58–170 μg/g, Br = 285–559 ng/g, and I &amp;lt;5 ng/g), comparable with depleted Mid-Ocean ridge basalts (N-MORBs). Volatile-rich minerals (e.g., amphibole and biotite) are abundant (20–30 vol%) in these lamprophyres, however, electron probe microanalyses (EPMA) data indicate that amphiboles are mainly rich in OH and F but display very low Cl concentrations (0.01–0.04 wt%). The bulk rock and amphibole data consistently indicate low abundances of heavy halogens in the lamprophyres, which is difficult to reconcile with the remarkable enrichment of fluid-mobile large ion lithophile elements such as Ba, Rb, and K. Based on low Cl/Nb and Br/Nb but high Ba/Nb and K/Nb ratios, the low halogen concentrations likely resulted from extensive volatile loss (&amp;gt;90%) during melt ascent. The low Cl concentrations in early-stage amphiboles (Mg# 60–64) further indicate that such loss would have occurred before amphibole crystallization at a depth of ~15 km. We thus propose that crystallization of early olivines and pyroxenes and reaction with surrounding mantle rocks likely induced volatile saturation and exsolution, leading to strong partitioning of the halogens into the exsolved aqueous volatile phases and thus the extensive loss of halogens from the rising melt. These results reveal that significant volatile loss of halogens not only occurs during surficial low-pressure eruption but also at much deeper levels in the crust, as also identified for some kimberlites. Consequently, it would be difficult to constrain the primitive halogen components of the mantle sources via lamprophyres or similar magmas.

Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge

The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low 206Pb/204Pb isotope signature combined with high 207Pb/204Pb and 208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled 143Nd/144Nd and 176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.

Neo-Tethyan subduction triggered Eocene–Oligocene magmatism in eastern Iran

AbstractEocene–Oligocene magmatic rocks are well exposed in the region south of Birjand, eastern Iran. The ages, geochemistry and petrogenesis of these rocks are important to understand eastern Iran’s magmatic and geodynamic history during the Cenozoic. Detailed field investigations show that numerous intrusive, intermediate to felsic units are intruded into a thick sequence of Eocene–Oligocene lava flows and their accompanying pyroclastic rocks. The volcanic rocks are mainly basaltic andesitic to rhyolitic, whereas intrusive rocks are characterized by dioritic to granitic composition. Previously compiled U–Pb geochronological data indicate that Eocene–Oligocene magmatism in eastern Iran formed continuously from ∼46 Ma to ∼25 Ma. Our new zircon U–Pb data reveal crystallization ages of 43.6 ± 0.4 Ma to 39.5 ± 0.6 Ma, consistent with the upper end of this age range. Geochemically, the igneous rocks have high-K calc-alkaline to shoshonitic signatures. Rare-earth and trace element patterns show enrichment in LREEs, K, Rb, Cs, Pb, Th and U and depletion in HFSEs such as Nb, Zr and Ti, typical of a subduction-related environment.87Sr/86Sr(i)and ϵNd(i)values range from 0.7051 to 0.7064 and −0.1 to +0.2, respectively. We postulate that the Cretaceous northeastward subduction of the Neo-Tethyan oceanic lithosphere underneath the Iranian Plateau caused sub-continental lithospheric mantle (SCLM) metasomatism by slab-derived fluid components. Subsequently, slab roll-back of the Neo-Tethyan oceanic lithosphere associated with asthenospheric upwelling led to lithospheric thinning and melting of the metasomatized SCLM. The resulting parental magmas probably interacted with upper continental crust during magma ascent to form Eocene–Oligocene magmatism in eastern Iran.

Trends and rhythms in carbonatites and kimberlites reflect thermo-tectonic evolution of Earth

Abstract Earth's thermo-tectonic evolution determines the way the planet's interior and surface interact and shows temporal changes in both trends and periodic rhythms. By sampling the subcontinental lithospheric mantle that represents the interface between the convecting mantle and the crust, carbonatite and kimberlite should be ideal rock types for documenting this evolution. The first-order secular rise of kimberlites over time has been noted by researchers, but there is much debate over how to interpret this trend, and their second-order variability has received less attention. We compiled a comprehensive global carbonatite database and compared it with an existing kimberlite one. We find that the numbers of carbonatites and kimberlites have similar increasing secular trends, with accelerated growth after ca. 1 Ga, and show the same periodic rhythms that have been synchronized to the supercontinent cycle since ca. 2.1 Ga. We link these trends and rhythms to the long-term change of Earth and the supercontinent cycle, both of which have altered the temperature of, and the subduction-recycled volatile flux into, the subcontinental lithosphere. Such consistent records in carbonatite and kimberlite behavior provide critical evidence for the synchronous thermo-tectonic evolution of the entire subcontinental lithosphere.

The Andaluca plutonic unit, Vinquis Intrusive Complex, Argentina: An assessment of mantle role in the genesis of Early Carboniferous weakly peraluminous A-type granites in the pre-Andean SW Gondwana margin

During the Early Carboniferous, significant intracontinental magmatism was developed in the retro-arc region of the SW margin of Gondwana between ca. 27° and 31°S. This magmatism consisted of metaluminous to weakly peraluminous A-type granites, strongly peraluminous A-type granites (A- to S- hybrid granitoids) and, in a lesser extent, (sub-)volcanic felsic (rhyolites and trachytes) and mafic (alkaline basalts and lamprophyres) rocks. The Vinquis Intrusive Complex (VIC) registers these two compositional varieties of A-type granites: the Vinquis Unit (VU) represents the strongly peraluminous granites and the Andaluca Unit (AU) the weakly peraluminous granites. The adjacent Zapata Intrusive Complex (ZIC) also contains metaluminous to weakly peraluminous A-type granites. The AU is a semi-elliptical plutonic body of approximately 60 km 2 located in the southwestern part of the VIC, intruding the Vinquis Unit. The AU comprises three dominant rock types: i) quartz monzonite, ii) syenogranite and iii) alkali feldspar granite that can be classified as F-rich (calculated F content: 1950–5700 ppm), ferroan (FeO t /(FeO t + MgO) = 0.86–0.95) and weakly peraluminous (ASI = 1.03–1.10) A-type (Zr + Nb + Ce + Y = 364–570 ppm; Ga = 20–23 ppm) granitoids (SiO 2 wt% = 67.8–75.4% wt%). This unit was emplaced in early Carboniferous time (U Pb zircon crystallization ages of 346 ± 3 Ma and 342 ± 3 Ma). The AU has 87 Sr/ 86 Sr t , εNd t and εHf t values ranging from 0.7092 to 0.7140, −2.6 to −3.6 and − 16.5 to −3.3, respectively. These isotopic suggest that the parental magma of AU dominantly involved variable mixtures of mantle-derived components (magmas/fluids) and peraluminous continental crust. The extremely felsic granites of the AU (SiO 2 > ~72%) with a strongly fractionated composition (Rb > 336 and Sr < 70 ppm and Eu/Eu* < 0.22) supports extensive fractional crystallization. The high contents of fluorine and HFSE in AU could be explained as derived from an F- HFSE- and alkali-rich parental mafic magma plus contributions from F-bearing minerals assimilated from the country rock. Likewise, the magmatic differentiation processes from these reduced mafic magmas could explain the ferroan character of the AU granitoids. We envision three stages in the origin of these rocks: (1) fluids and magmas from the asthenospheric mantle metasomatize and melt the subcontinental lithospheric mantle (SCLM), the alkali-rich mafic melts produced are emplaced at the base of the crust, transfer heat to the crust and melt it, generating strongly peraluminous A-type magmas; (2) continued introduction of alkali-rich mafic magmas into the deep crust produce extensive assimilation of the preheated crust in equilibrium with mafic cumulates (generated by reaction and/or fractionation); (3) the hot magmas so produced migrate into the middle-upper crust to produce ferroan weakly peraluminous A-type granitoids. • The Andaluca plutonic unit of the Vinquis Intrusive Complex crystallized between 346 ± 3 Ma and 342 ± 3 Ma. • Andaluca Unit shows a F-rich weakly peraluminous ferroan A 2 -type affinity. • The high contents of fluorine and HFSE in Andaluca Unit can be explained from an F-rich parental alkaline mafic magma. • The Andaluca Unit granitoids can be generated through combined crustal assimilation and fractional crystallization of magmas derived from the metasomatized subcontinental lithospheric mantle.

The capturing and decompression of melt inclusions in olivine and ilmenite megacrysts from the Monastery kimberlite, South Africa

Ilmenite and olivine megacrysts from the 89 Ma Monastery kimberlite (Kaapvaal Craton, South Africa) captured abundant and large melt inclusions containing quenched Si-Mg-rich melt, calcite, spinel, perovskite, phlogopite, and serpentine. Textural observations and 3D X-ray tomography of ilmenite and olivine megacrysts show melt inclusion shapes, sizes and distribution patterns indicative of melt capture during primary crystal growth near the base of the subcontinental lithospheric mantle (SCLM). Patterns supporting secondary melt injection along fractures or veins, such as planar arrays of melt inclusions, are absent. Melt inclusions in olivine, in some examples reaching the dimension of centimetres, likely were captured in skeletal voids forming in fast growing, up to decimetre-sized olivine megacrysts. These large melt inclusions commonly decrepitated, forming apophyses, radial fractures, and veins, along which residual volatile- and Si-Mg-rich melt was extracted. We attribute the decrepitation of melt inclusions in olivine to the rapidly increasing difference between the melt pressure in the inclusions, captured at mantle depth, and the decreasing confining stress to which the host olivines were exposed during magma ascent and after emplacement. In ilmenite, melt inclusions up to ∼ 6 mm in diameter remained commonly intact during the kimberlite ascent from its mantle source to the shallow crust. The quenched silicate melt in olivine- and ilmenite-hosted melt inclusions, in some places preserved as unaltered hydrous and CO3-bearing glass, shows systematic major element compositional variations that suggest that this melt formed by similar fractionation and depletion processes, irrespective of the hosting megacryst phase. Apparent modal variations in quenched silicate melt, calcite, and oxide contents suggest that the melt batches captured as inclusions in ilmenite and olivine either record different evolution stages in the megacryst magma, or document compositional heterogeneities in this magma at the time of megacryst growth.

Triassic volcanism on the North margin of the North China Craton: Insights for lithospheric modification during closure of Paleo-Asian Ocean

As a key tectonic region linking the northern margin of the North China craton (NCC) and eastern Central Asian Orogenic Belt (CAOB), western Liaoning, with extensively distributed magmatic activity, provides natural samples to investigate the evolutionary history of the Paleo-Asian Ocean plate subduction to closure. The Middle Triassic (ca. 241 Ma) trachyandesites and Late Triassic (ca. 223 Ma) basaltic andesites were first identified from the Tiaojishan Formation, western Liaoning. They have variable SiO 2 contents (52.48–60.31 wt%) and alkaline or high-K calc-alkaline to shoshonite compositions, and belong to the metaluminous series. Middle Triassic trachyandesites are enriched in large ion lithophile elements (LILEs), high field strength elements (HFSEs), and light rare earth elements (LREEs) with high Nb/La ratios (0.63–0.76). They have homogeneous whole-rock Sr Nd ( 87 Sr/ 86 Sr i = 0.704684–0.704808, ε Nd (t) = −4.6 to −4.0) and variable zircon Hf isotopic signatures (ε Hf (t) = −10.3 to 8.2), indicating that they probably originated from the partial melting of the subcontinental lithospheric mantle with the addition of asthenosphere materials. In contrast, the Late Triassic basaltic andesites show arc-related geochemical affinities, such as LILEs and LREEs enrichment and HFSEs depletion. They have more enriched whole-rock 87 Sr/ 86 Sr i (0.704895–0.705394), ε Nd (t) (−16.1 to −14.0), and zircon ε Hf (t) (−13.1 to −8.8) than the Middle Triassic trachyandesites, suggesting that they probably represent partial melts of the subduction-related fluids-modified lithospheric mantle. The modification of the northern margin of the NCC lithospheric mantle is closely related to the final subduction of the Palaeo-Asian oceanic plate and the subsequent post-orogenic extension. • Triassic volcanic rocks are founded in Tiaojishan Formation, Western Liaoning. • Study area was in a post-orogenic extensional setting in the Middle-Late Triassic. • Ocean Subduction modified the North China Craton lithosphere in the Triassic.