Enriched Mantle Research Articles

Age and Chemostratigraphy of the Finlayson Lake District, Yukon: Implications for Volcanogenic Massive Sulfide (VMS) Mineralization and Tectonics along the Western Laurentian Continental Margin

Abstract The Yukon-Tanana terrane in the Finlayson Lake district, Yukon, represents one of the first arc–back-arc systems that formed adjacent to the Laurentian continental margin in the mid-Paleozoic. Back-arc rocks contain many large and high-grade volcanogenic massive sulfide (VMS) deposits. This study integrates U-Pb zircon geochronology, lithogeochemistry, and Hf-Nd isotopes to establish precise controls on tectonomagmatic activity adjacent to the western Laurentian margin in the Late Devonian to Early Mississippian. High-precision chemical abrasion- (CA-) ID-TIMS U-Pb zircon geochronology defines coeval arc (ca. 363.1 to 348 Ma) and back-arc (ca. 363.3 to 355.0 Ma) magmatism in the Finlayson Lake district that intruded continental crust of Laurentian affinity (e.g., Snowcap assemblage). Mafic and felsic rocks display geochemical and isotopic characteristics that are consistent with being formed from mixtures of depleted asthenosphere and enriched lithospheric mantle sources. These melts variably entrained Laurentian continental crust via high-temperature crustal melting due to basaltic underplating. The high-temperature back-arc felsic magmatism occurs at specific time periods coinciding with VMS deposits and supports previous genetic models for VMS mineralization that suggest elevated heat flow and hydrothermal circulation were due to regional-scale rift-related magmatism rather than from local subvolcanic intrusions. The short timescales and transient nature of tectonomagmatic events in the Finlayson Lake district suggest that rapid and complex subduction initiation of oceanic and continental crust fragments facilitated coeval compression, extension, and magmatism in the arc and back-arc regions. We thus reevaluate the presently accepted tectonostratigraphic framework of the Finlayson Lake district and suggest revised interpretations that shed light on VMS depositional environments and a possible broader association with the ca. 358 Ma Antler Orogeny. Results of this study have implications for incipient tectonics, magmatism, and mineralization along the western Laurentian continental margin and other orogenic belts globally.

The dynamics of the Sorol Trough magmatic system: Insights from bulk‐rock chemistry and mineral geochemistry of basaltic rocks

The Yap trench‐island arc system, where three tectonic plates (i.e., the Philippine Sea Plate, Pacific Plate, and Caroline Plate) interact, has undergone a relatively complicated geological evolution. East of the system, the Caroline Plateau are being subducted, and the plateau has been rifted into east and west Caroline ridges by the development of the Sorol Trough. However, until now, knowledge of the nature of the basement of the Sorol Trough is lacking, which limits our understanding of the complete tectonic evolution of the Yap island arc‐trench system and the Caroline Plateau. In this study, we performed petrological and mineral geochemical (in situ major element compositions and trace element compositions) analysis of newly acquired subducting basalt samples from the Sorol Trough segment, which is being subducted along the Yap Trench, and K‐Ar age of the Sorol basalts is 23.8 ± 0.7 Ma. The results show that phenocryst minerals of basalt from Sorol Trough are mainly olivine and plagioclase, with minor clinopyroxene microlites. The forsterite (Fo) values of the olivine phenocrysts in these rocks vary from 84.97 to 86.85, and plagioclase (Pl) phenocrysts are predominantly labradorite, with a few andesine plagioclase microlites. Bulk‐rock chemical compositions confirm that these rocks are intraplate alkaline basalts and show ocean island basalt (OIB)‐like geochemical characteristics. The mantle source of these basement basalts could be peridotite, and these Sorol Trough basalts may have been formed by 5–10% partial melting of mantle spinel lherzolite. The potential temperature of the mantle beneath the Sorol Trough was inferred to be 1,331–1,393°C, shows that there is no thermal anomaly in the mantle source. In addition, the crystallization temperatures of the clinopyroxenes (1,068–1,263°C) and plagioclase (702–1,216°C) and their compositional characteristics indicate that the magmatic processes have a conspicuously fast rate of magma upwelling. In conclusion, the basalts in the Sorol Trough were the products of low‐degree partial melting of a relatively enriched mantle source that passively upwelled as the Sorol Trough separated the Caroline Ridge into its western and eastern parts.

Discovery of the Late Jurassic-Early Cretaceous Lamprophyres in Western Songliao Basin of Northeast China and Their Constraint on Regional Lithospheric Evolution

Contrary to the commonly accepted notion that the lithosphere in NE China thinned from the Late Jurassic through to the Early Cretaceous period, we report the discovery of a thickening episode in the backdrop of this long-term thinning. A series of lamprophyre dikes have been recently discovered in the Tuquan Basin of the western Songliao Basin that have been dated to 156.0 ± 2.3 Ma, 132.9 ± 1.2 Ma, and 126.2 ± 2.5 Ma by using the zircon U–Pb technique. These lamprophyres are subdivided into biotite orthoclase lamprophyre (BOL) from the Late Jurassic and quartz magnetite lamprophyre (QML) from the Early Cretaceous. The BOL and QMLs are shoshonite and calc-alkaline in series, are characterized by large amounts of FeOT, TiO2, MgO, and Mg#, and are rich in LREEs and LILEs but poor in HREEs and HFSEs. They have high ratios of (La/Yb)N, La/Ta, La/Nb, Th/Y, Ba/Nb, Ba/Ta, and Ba/Th, and low ratios of Zr/Ba, La/Sm, and Nb/Zr. These features collectively point to the derivation of dike magmas from the partial melting of the enriched lithospheric mantle that had been previously metasomatized by subduction-related fluids. The BOL has high ratios of Rb/Sr (0.42) and K/Yb*1000 (28.3), and low ratios of Ba/Rb (13.5) and Dy/Yb (2.35), suggesting the derivation of magma from a high degree of partial melting of the phlogopite-bearing lherzolite mantle in the spinel–garnet transition zone at a depth of about 60 km. The QMLs have low ratios of Rb/Sr (0.02–0.06) and K/Yb*1000 (8.13–19.73), and high ratios of Ba/Rb (17.6–42.6) and Dy/Yb (3.48–4.09), indicating that the magmas were derived from a low degree of partial melting of the lherzolite mantle in the garnet zone at a depth of ca. 85 km. The younger QML (126.2 ± 2.5 Ma) has a lower Dy/Yb ratio (3.48–3.92) than the older QML (132.9 ± 1.2 Ma), with a Dy/Yb ratio of 4.09 implying that the younger magma was formed at a shallower depth of the mantle (<85 km) than the older one. These observations indicate that in 156–132 Ma, the lithosphere thickened by approximately 25 km at a rate of approximately 1.0 km/Myr. This is used to propose a model of geodynamic evolution in three stages.

Noble gas isotope systematics in the Canary Islands and implications for refractory mantle components

Noble gas isotope systematics of ocean island basalts (OIB) provide evidence for relatively undegassed and primitive mantle sources. These OIB sources partly derive from the deep mantle by virtue of their distinctiveness from mid-ocean ridge basalts (MORB), which dominantly sample upper mantle. New helium, neon and argon isotope data are presented for Canary Islands lavas, carbonatites and cumulate and mantle xenoliths confirming 3He/4He ratios that are the same or lower than MORB, but that are heterogeneous (∼3–9.5RA) within and between islands in the archipelago. Neon and Ar isotope systematics for lavas are mostly within the range of air compositions. Harzburgite xenoliths from Lanzarote, which are interpreted to represent ancient refractory mantle residues, have distinct He-Ne-Ar isotope systematics from lavas or cumulate xenoliths. The harzburgites are characterized by forsteritic olivine (>Fo91) with uniformly low-3He/4He (6.6 ± 0.2RA), high 40Ar/36Ar (630–4900), and have Ne isotope compositions that range between air values or that are similar or more nucleogenic than depleted MORB mantle (DMM). Similar refractory mantle peridotites have been discovered as xenoliths at other OIB localities and are likely to be distinct from continental lithospheric mantle. Refractory mantle (RM), which by virtue of its high melting temperature is difficult to partially melt, may be a significant component in the convecting mantle and has potential to impart a cryptic noble gas signature to partial melts in intraplate, divergent and convergent margin settings. In this sense, RM may represent a ‘sixth mantle component’ after DMM, high-µ (high 238U/204Pb; HIMU), enriched mantle endmembers (EMI, EMII) and the ‘focus zone’ (FOZO). An RM-type component may be presented in the older eastern Canary Islands and possibly in recent rejuvenated volcanism from Teide (Tenerife). Canary Island intraplate volcanism samples multiple mantle components, confirming that the most gas-rich mantle sources involved in magmatism dominate OIB noble gas compositions.

Geochemical evolution of basaltic flows from Dongargarh Supergroup, Bastar Craton, Central India

Composition of basalts in magmatic arcs influenced by the subducting lithosphere, mantle wedge, dehydration of oceanic crust, and/or crustal assimilation beneath the arc. In this paper, we compiled earlier published geochemical data of Dongargarh basalts to decipher the genesis of volcanic rocks. SiO2 vs (FeO + MgO) plot of basalt suggests the volcanic rocks are tholeiitic in composition. Primitive mantle and REE normalized plots indicate either the source was enriched mantle or a possible interaction of depleted magmatic source with the Paleoarchean continental crust in the Bastar Craton. The primitive mantle normalized diagram shows a negative anomaly of Nb, Ti, and Ta indicates subduction-related magmatism. In addition to the basalt composition, variation diagrams for tectonic settings represent the continental arc-related magmatism. From the available geochemical data of basalts and earlier studies on Dongargarh volcanic, there was an oceanic ridge that was subducted beneath the continental plate. The source of Pitepani basalts was significantly enriched in HFSE and REE as compared to mid-oceanic basalts. Thus the study finds the volcanic rocks are part of enriched mantle source that formed in the subduction-related magmatism.

Evolution of the Sumdo Paleo-Tethyan Ocean: Constraints from Permian Luobadui Formation in Lhasa terrane, South Tibet

Reconstructing the evolution of the Sumdo Paleo-Tethyan Ocean (SPTO) is important for understanding the transition from the Paleo-Tethyan system to the Neo-Tethyan system. The existence of the SPTO is inferred from the presence of an (ultra-)high pressure metamorphic belt and dismembered ophiolite bodies that separate the southern Lhasa and northern Lhasa terrane. This paper presents new geochronological and geochemical data for the Luobadui Formation in the Lhasa terrane. The geochemical difference between the Lower Luobadui volcanics and the Upper Luobadui volcanics suggests a source transition from a depleted mantle source to an enriched mantle source, which indicates continental-arc subduction. The integrated geological evidence suggests that the SPTO initially opened during the late Carboniferous (ca. 304 Ma), as verified by the earliest record of oceanic crust. Our new data for the Luobadui volcanic rocks, together with the metamorphic and sedimentary records, support northward subduction of the SPTO lithosphere beneath North Lhasa, most likely starting during the Early Permian (ca. 275 Ma) and lasting until the Middle Permian (ca. 260 Ma). The main closure of the SPTO occurred during Middle–Late Triassic (ca. 240–220 Ma), as recorded by medium-pressure metamorphism and syn-collisional magmatism. The Wilson cycle of the SPTO is hereby reconstructed, providing significant insights into the amalgamation of the Tibetan Plateau.

Geochronological and Geochemical Constraints on the Petrogenesis of Lamprophyre from the Giant Weishan REE Deposit in China

The Weishan REE deposit is located in the southwest of the Luxi Terrane of the North China Craton (NCC), where a large number of lamprophyre dikes are spatially exposed with the deposit. Here, we report petrology, geochemistry and zircon U-Pb geochronology data for the lamprophyre of the Weishan REE deposit in order to develop constraints for the determination of the petrogenesis, magma source and evolution of the lamprophyre and the tectonic environment. LA-LCP-MS zircon U-Pb dating shows that the crystallization age of the lamprophyre is 125 ± 0.86 Ma. The geochemical data suggest that these lamprophyres have high levels of Al2O3, K2O, MgO and alkalis, moderate level of Na2O and low levels of SiO2, Fe2O3 and TiO2, and that they are enriched with LREEs (La, Ce) and LILEs (Rb, Ba) and depleted with regard to HREEs and HFSEs (Nb, Ta, Ti). They displayed negative εHf(t) values of −14.98 to −9.03, TDM1 ages of 1.1–1.4 Ga and TDM2 ages of 1.7–2.1 Ga, which suggest that the magma source originates from an enriched mantle. Low Rb/Sr and high Dy/Yb ratios suggest that the enriched mantle source was partially melted at the amphibole-bearing lherzolite garnet-facies. The high Ba/Th and Sr/Th ratios indicate that the enriched source was derived from subduction dehydration fluids of the oceanic crust. We propose that the mafic dike intrusions are consistent with an Early Cretaceous alkaline magma emplacement in an extensional setting, in which the magma was not contaminated by crustal material during its emplacement.

Mid-Neoproterozoic magmatism in the South Qilian Belt, NE Tibetan Plateau and its tectonic implications

AbstractWidely distributed Mid-Neoproterozoic mafic rocks of the Qilian – Qaidam – East Kunlun region record the tectonic evolution of the northeastern Tibetan Plateau. This study presents whole-rock geochemistry, zircon U–Pb geochronology and Hf isotopes for the Xialanuoer gabbros of the central South Qilian Belt (SQB). Zircon laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) U–Pb dating indicates that the gabbros were emplaced at ca. 738 Ma, indicating they are contemporaneous with mafic magmatism elsewhere in the northeastern Tibetan Plateau. The gabbros have low SiO2, Cr and Ni contents and Mg# values, are relatively enriched in light rare-earth elements (LREEs) and depleted in high-field-strength elements (HFSEs; e.g. Nb and Ta), have no positive Zr or Hf anomalies and have relatively high Nb/Ta but low Nb/La ratios. These data indicate that the Xialanuoer gabbros formed from calc-alkaline basaltic magmas that were originally generated by the partial melting of an enriched mantle of type-I (EMI-type) enriched region of the lithospheric mantle, underwent little to no crustal contamination prior to their emplacement, and have within-plate basalt geochemical affinities. Combining these data with the presence of widespread contemporaneous continental rift-related magmatism and sedimentation in the North Qilian, Central Qilian, South Qilian, Quanji, North Qaidam and East Kunlun regions suggests that the northeastern Tibetan Plateau underwent Mid-Neoproterozoic continental rifting, which also affected other Rodinian blocks (e.g. Tarim, South China, Australia, North America and Southern Africa).

Decoupling of Mg from Sr–Nd isotopic compositions in Variscan subduction-related plutonic rocks from the Bohemian Massif: implications for mantle enrichment processes and genesis of orogenic ultrapotassic magmatic rocks

Decoupling of Mg from Sr–Nd isotopic compositions in Variscan subduction-related plutonic rocks from the Bohemian Massif: implications for mantle enrichment processes and genesis of orogenic ultrapotassic magmatic rocks

Mingling with minimal mixing: Mafic-silicic magma interactions in the Oamikaub ring complex, Namibia

The ca. 560 Ma Oamikaub ring complex (ORC) is a member of the Goas Intrusive Suite in the Damara Belt of Namibia. The volumetrically dominant granitic rocks contain a variety of microgranular magmatic enclaves (MEs), commonly with lobate and cuspate borders against their enclosing host rocks. Some also have mafic selvages and evidence for internal melt segregation. These features indicate magma mingling and deformation during host-magma flow and emplacement, and a small degree of chemical mixing with the host magmas. The spatial diversity and mineralogical characteristics of the MEs show that mechanical hybridism occurred prior to mingling at emplacement level. Despite one group of MEs forming near-linear arrays in chemical plots, the mineralogical and isotopic data show that these do not form a mixing series. A second group represents samples of chilled mafic magmas that chemically resemble the mafic rocks that form the earliest-emplaced components of the ORC. However, their fine-grained textures and isotopic characteristics show that these were a separate, later pulse of mafic magmas. There were multiple episodes of mingling and the younger pulses of mafic ORC magmas were generally more hybridised with evolved crust. Magma mingling and limited marginal hybridisation of MEs produced impressive outcrop features. However, apart from initial, source-level hybrid formation, magma mixing had little or no role in producing compositional variations in either the MEs or their host rocks. Globally, such relationships are common in enclave suites from granitic rocks, and their study can provide insights into processes and materials in magma source regions. In the present case, enriched mantle melting, crustal melting (of mafic to silicic rocks) and fractionation of tholeiitic parental magmas may all have played their parts in forming the ME suite of the ORC.

Crustal maturation and cratonization in response to Neoarchean continental collision: The Suizhong granitic belt, North China Craton

The late Archean (3.0–2.5 Ga) is a pivotal period, when properties of the continental crust significantly changed and a fundamental shift of geodynamic processes took place. As the major felsic component in early Archean (> 3.0 Ga) crustal records, tonalite-trondhjemite-granodiorite (TTG) suites are compositionally different to the present-day felsic upper continental crust. But it remains unclear how and to what degree the felsic part of the continental crust evolved during the late Archean. The 250 km long Neoarchean Suizhong granitic belt in the North China Craton (NCC) is mainly composed of K-rich granitoids and compositionally distinct to the TTG-dominated early Archean upper continental crust. This belt is a natural laboratory to explore how the early Archean upper continental crust evolved in the late Archean. Combined with our new data (zircon U-Pb ages, bulk-rock geochemistry and zircon Hf isotopes), we provide an overview of the temporal and compositional features of the Neoarchean Suizhong granitic belt. Granitic magmatism in the Suizhong granitic belt mainly took place within 100 Myrs around the end of the Archean, and these rocks are primarily K-rich granitoids with minor TTGs and mafic magmatic enclaves (MMEs). The K-rich granitoids include sanukitoid-like granites and potassic granites; the former are hybrids between melts from Mesoarchean enriched mafic crust and enriched mantle, and the latter are melts from Hadean–Mesoarchean crustal lithologies. The volumetrically minor TTGs were sourced from Mesoarchean low-K mafic crust. MMEs are either cumulates or inclusions of captured enriched mantle melts. Overall, these Neoarchean granitoids are essentially intracrustal reworking products of ancient continental nuclei and are compositionally and temporally similar to syn- and post-collisional magmatism in Phanerozoic collisional orogenic belts. The Neoarchean crustal reworking process was most likely to be induced by amalgamation of micro-continents through collision. Through the collision-induced reworking, a variety of K-rich granitoids were introduced to the Neoarchean upper continental crust. As a result, layering of felsic upper and mafic lower crust was developed, and the maturity of Archean continental crust was significantly enhanced. Collisional orogenesis played a critical role in maturing the Archean continental crust and initial cratonization of the NCC during the late Archean.

Rare metal enrichment of the Tianbao trachytic complex, North Daba Mountains (South Qinling): Insights from textures and geochemistry of trachytes and Nb-REE minerals

South Qinling hosts abundant alkaline igneous rocks characterized by rare metal mineralization, which, however, is not well understood, particularly for those extrusive alkaline magmatic systems. This study focuses on the Tianbao trachyte Nb(-REE) deposit located in the North Daba Mountains (South Qinling) and investigates textures and geochemistry of ore-bearing trachytes and Nb-REE minerals to reveal relative contributions of magmatic and hydrothermal processes to rare metal enrichment. Low K2O/Na2O ratios (0.26–1.64), depletions of Ba, Sr, and enrichments of Nb, Ta, Zr, Hf in the Tianbao trachytic complex resemble geochemical signatures of rifting-related alkaline igneous rocks, highlighting an intraplate rifting origin. Positive εNd(t) values (3.1 – 3.6) and constrained La/Yb (21.2 – 35.8) and Nb/Zr (3.7 – 4.8) ratios indicate that they might have originated from mixing of the asthenospheric mantle and metasomatized lithospheric mantle and formed due to protracted magmatic differentiation of coeval mafic magmas. Niobium and REEs in the Tianbao trachytic complex primarily originated from an enriched mantle source, and then upgraded to initial enrichment due to intense differentiation of the trachytic magmas. Niobium and REEs in the trachytic magmas were firstly incorporated into magmatic mineral phases (e.g., biotite, niobian titanite, niobian ilmenite and apatite) under relatively reduced (below the Ni-NiO oxygen fugacity buffer) conditions. With the differentiation of trachytic magmas, niobium, REEs and anion ligands (F-, CO32– and PO43-) got saturated in the late-stage alkaline melts and then entered the exsolved fluids. Ca-rich components should have been leached from the magmatic minerals during the hydrothermal stage and reacted with these ligands to develop Ca-rich hydrothermal mineral phases, including fluorite and calcite veins, allanite and apatite overgrowth under relatively oxidized (between the Ni-NiO and hematite-magnetite oxygen fugacity buffer) conditions. The remaining Nb and REEs finally precipitated as the economic minerals (e.g., allanite-(Ce), loparite-(Ce) and niobian Y-rich zircon) along fractures. Mass balance calculations indicate that these hydrothermal economic mineral phases account for the majority of Nb-REE reserve in the trachytic complex, and thus, the hydrothermal process might have played the key role in leading to Nb and REE concentrating to ore grade.

Evidence for a primitive deep mantle component in the source of Marquesas Islands Lavas from Os isotope and highly siderophile element abundance systematics

The Marquesas islands are an age-progressive volcanic chain in French Polynesia formed by partial melting above a mantle plume. Previous studies have demonstrated the presence of an enriched mantle (EM2) endmember mixed with a high-3He/4He (up to 14.4 RA) component in Marquesas post-shield lavas. In contrast, Marquesas shield lavas sample a dilute high-μ (HIMU) type component with lower 3He/4He (∼8 RA). We report new incompatible trace element and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances and osmium isotope data for bulk rocks from the islands of Fatu Hiva, Hiva Oa and Nuku Hiva, for which He, Sr, Nd and Pb isotope compositions have been reported previously (Castillo et al., 2007). As the HSE are chiefly hosted in sulfide phases in the mantle and all but Re are compatible during partial melting, they offer information on both sulfide-rich and melt-depleted mantle lithologies within the Marquesas plume. Primitive mantle-normalized HSE abundances show depletions in Ru and Pd, and fractionated (Ir/Os)N in fourteen of the seventeen analyzed samples. Ruthenium depletions and Ir-Os fractionations are likely caused by parental magma fractional crystallization. Palladium depletions are not easily explained by magmatic or secondary alteration processes and are instead likely to be a feature of the source mantle which was partially depleted in the more incompatible HSE by previous melting events within the Marquesas plume. Osmium isotopic systematics are mildly radiogenic relative to chondrites and are identical within uncertainties for shield-building Fatu Hiva (γOs = +2.9 ± 1.6; 2 SD; n = 7) and post-shield Hiva Oa (+2.7 ± 2.2; n = 9), showing no evidence for significant assimilation of radiogenic oceanic crust. The osmium budget of Marquesas lavas is controlled by a volumetrically dominant peridotite endmember that is incompatible element depleted while the Sr, Nd, and Pb budgets are strongly influenced by a volumetrically minor, incompatible element enriched EM2 component. The Nd-Sr-Os isotope systematics of the lavas can be explained by mixing of 0.3–0.7% of an EM2 type melt into the Marquesas source. Marquesas lavas offer further support that the high-3He/4He “focus zone” (FOZO) mantle has 187Os/188Os mildly more radiogenic than the depleted upper mantle, similar to estimates of the primitive mantle. Osmium isotope systematics offer a useful tool with which to characterize more depleted components within the ocean island basalt-source mantle that may be difficult to detect using lithophile isotope signatures.

Ediacaran magmatism and rifting along the northern margin of the Tarim craton: Implications for the late Neoproterozoic Rodinia configuration and breakup

Abstract The Tarim craton in modern Central Asia was an important component of the supercontinent Rodinia in the Neoproterozoic, although its paleogeography in Rodinia during that era is still controversial. Here, we present new stratigraphic, geochemical, and geochronological data from the Neoproterozoic sedimentary and volcanic rock successions along the northwestern margin of the Tarim craton and discuss the significance of these data and our interpretations for its tectonic evolution and paleogeographic position within Rodinia. The Lower Ediacaran sedimentary sequence (Sugetbrak Formation) in northwest Tarim includes terrestrial and shallow-marine clastic rocks intercalated with two discrete basaltic lava flows near the top. The Upper Ediacaran sedimentary sequence conformably overlying the volcanic and clastic rocks consists mainly of stromatolitic dolomite (Chigebrak Formation), representing a transgressive shallow-marine environment. Previous U-Pb zircon dating of the basaltic lava flows has constrained the timing of their eruption in the early Ediacaran (615 Ma). Detrital zircon U-Pb dating of a feldspar-quartz-sandstone unit situated between the two lava flows revealed an oldest age of 2517 ± 18 Ma and a youngest age of 612 ± 6 Ma, with a majority of zircon grains (n = 42) dated at 891–754 Ma (Tonian). A quartz-sandstone unit above the upper lava flow revealed an oldest age of 2724 ± 15 Ma and a youngest age of 607 ± 8 Ma, with a missing age group of 891–800 Ma. These data and observations indicate: (1) a major switch in the depositional setting from a terrestrial (synrifting) to shallow-marine environment following the eruption of the upper lava unit; and (2) an abrupt disappearance of the source rocks of the 891–800 Ma zircons and sediments from the provenance of the post-615 Ma (postrifting) sedimentary sequence. The basaltic rocks have low SiO2 and MgO but high total Fe2O3 and TiO2 contents (2.34–3.19 wt%), analogous to high-Ti basalts and continental flood basalts. Their Ti/V ratios (65–88), low Th/Nb ratios (~0.1), and high TiO2/Yb ratios (~1.1) are similar to those of ocean-island basalt (OIB). Combined with their Sm/Yb and La/Sm ratios and Sr-Nd-Pb-Hf isotope values, we infer that magmas of the Sugetbrak basalts were likely derived from partial melting of an enriched mantle source (EM I) in a transitional spinel-garnet lherzolite field. This petrogenetic evolution was a result of mantle plume–influenced rift magmatism during the Ediacaran breakup of Rodinia. The Central Tianshan terrane, which was attached to the northern Tarim craton until the Ediacaran Period, was the provenance of 891–800 Ma (Tonian) zircons in the synrift sedimentary succession. As the Central Tianshan terrane broke away from the Tarim craton after ca. 615 Ma, Tonianaged zircons were no longer available to the depocenter of the postrift sedimentary sequence. The transition from rifting to drifting between the Tarim craton and the Central Tianshan terrane marked the final breakup of Rodinia, a global event that was possibly driven by mantle plume activities coeval with the development of the Central Iapetus magmatic province in Laurentia during the Ediacaran.

Oxidized mantle sources of HIMU- and EM-type Ocean Island Basalts

Oxygen fugacity (fO2) is a fundamental variable in igneous petrology with utility as a potential tracer of recycled high Fe+3/ΣFe surficial materials in the sources of mantle-derived lavas. It has been postulated that ocean island basalts (OIB) have elevated fO2 relative to mid-ocean ridge basalts (MORB) owing to higher average Fe+3/ΣFe in their source regions. To examine this issue, trace-element systematics of olivine grains are reported from OIB lavas with HIMU (high-μ; Mangaia, Canary Islands), enriched mantle (EM; Samoa; São Miguel in the Azores Islands) and depleted MORB mantle (DMM; Pico in the Azores Islands) Sr-Nd-Pb-Os isotope signatures, in order to constrain the fO2 of each magmatic system. Despite sampling distinct mantle reservoirs based on radiogenic isotope systematics, these OIB suites show similar fO2, ranging from +0.8 to +2.6 ΔFMQ, with an average of +1.5 ± 0.8 ΔFMQ, largely higher than MORB using the same oxybarometry method at +0.6 ± 0.2 ΔFMQ. The studied OIB show no correlation between fO2 and bulk rock isotopic ratios or calculated parental magma compositions. The lack of correlations with isotopic signatures likely results from radiogenic isotope signatures being hosted in volumetrically minor trace element enriched mantle lithologies, while Fe+3/ΣFe ratios that act as the primary control on fO2 are controlled by the volumetrically dominant mantle component. Higher fO2 in many OIB relative to MORB implies that they sample a uniformly high Fe+3/ΣFe plume source mantle that may be the result of either a common oxidized oceanic crust-rich reservoir parental to many modern plume lavas, or preservation of un-degassed and oxidized mantle domains formed shortly after core formation.

Iron isotope compositions of subduction-derived rocks: Insights from eclogites and metasediments of the Münchberg Massif (Germany)

The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe3+/ΣFe = 0.06 to 0.30). Their δ56Fe values (+0.00 to +0.17‰; mean + 0.08 ± 0.01‰) mostly resemble those of MORB (+0.07 to +0.17‰). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier δ56Fe values (+0.09 to +0.22‰) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10‰) are typical for pelitic sediments.It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that δ56Fe values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Münchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic δ56Fe values throughout the subduction-exhumation cycle, whereas δ56Fe values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history.

Late Paleozoic Shoshonitic Magmatism in the Southwestern Middle Tianshan (Tajikistan) of the Southwestern Altaids: Implications for Slab Roll-Back With Extensional Arc-Related Basins After Flat Subduction

Paleozoic magmatic rocks are widespread in the western Middle Tianshan. Their petrogenesis can provide important insights into the geodynamic evolution of the southwestern Altaids. Here, we present an integrated study of U–Pb zircon geochronology and geochemical and Lu–Hf isotopic compositions for the Late Paleozoic shoshonitic Chorukhdairon pluton and genetically and spatially related quartz porphyry in the southern Chatkal–Kurama terrane, western Middle Tianshan. The Chorukhdairon pluton mainly comprises monzodiorite and quartz monzodiorite (first phase), quartz monzonite (second and main phase), monzogranite (third phase), and leucomonzogranite (fourth phase). LA–ICP–MS zircon dating yielded magma crystallization ages of 294–291 Ma and 286 Ma for the Chorukhdairon pluton and quartz porphyry, respectively. All the rocks possess high K2O content (3.29–5.90 wt.%) and show an affinity with shoshonite series rocks. They display similar trace element compositions characterized by the enrichment of large ion lithophile elements (e.g., Rb, Th, U, and K) and depletion of high-field strength elements (e.g., Nb, Ta, P, and Ti), compatible with typical arc magmatism. Combined with zircon Lu–Hf isotopic data, we suggest that the Chorukhdairon pluton was produced by partial melting of the enriched mantle, followed by fractional crystallization of pyroxenes, amphibole, plagioclase, biotite, and accessory Fe–Ti oxides, apatite, and zircon. The quartz porphyries are similar to highly fractionated I-type granitic rocks, and their parental magma could result from the mixing of different batches of mantle-derived magmas or magmas derived from the mantle and juvenile lower crust. Considering the continuousness of the Middle Carboniferous to Early Permian magmatism in the western Middle Tianshan and other regional geological data, we suggest that the Chorukhdairon pluton and related quartz porphyry probably formed in an oceanic subduction setting. Furthermore, the temporal and spatial evolution of the Paleozoic magmatism imply that the flat-slab subduction that was induced by the subduction/accretion of seamounts probably occurred beneath the Middle Tianshan during the Middle Devonian to Early Carboniferous, after which the southeastward slab roll-back occurred during the Middle Carboniferous to Early Permian. The late slab roll-back was responsible for the southeastward arc magmatism migration and magmatic flare-up in the Chatkal–Kurama terrane, western Tianshan, and led to the formation of arc-related extensional basins and significant crustal growth in the southwestern Altaids.

Enriched mantle one (EMI) type carbonatitic volcanism in Namibia: Evidence for a concentrically-zoned Etendeka plume head

The Etendeka large igneous province in central Namibia is believed to be caused by widespread melting of the Tristan/Gough mantle plume head between ∼137 and 123 Ma ago. To explain the observed compositional variations of the Etendeka flood basalts, a laterally-zoned plume head has been proposed. Here we present new (major and trace element and Sr-Nd-Pb-O-C isotope) geochemical data from the Okorusu and Messum carbonatitic and silica-undersaturated rocks (both 127Ma). Okorusu carbonatites, located at the far eastern end of the Etendeka province, have a Gough-type enriched mantle one (EM1) composition, consistent with derivation from a common source with the northern Etendeka flood basalts, Walvis Ridge and Gough (southern) hotspot subtrack of the southern Atlantic Guyot Province including Gough Island. The Messum basanite, erupted directly after the Etendeka event near the central coast of western Namibia, has a different EM1 type flavor (with more radiogenic Nd, less radiogenic Sr and thorogenic Pb isotopes), similar to the Doros, Tafelkop and Horingbaai formations of the Etendeka flood basalts. Combining our new findings with published data from flood basalts, carbonatites and silica-undersaturated rocks from the region, we propose a concentric zonation model for the postulated plume head with the isotopically Gough-type EM1 plume mantle enclosing a blob of Doros-type EM1 plume mantle.

Oceanic Zircon Records Extreme Fractional Crystallization of MORB to Rhyolite on the Alarcon Rise Mid-Ocean Ridge

Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.

Nature of the Late Cretaceous mantle source beneath the western Lhasa terrane, southern Tibet: Insights from the newly discovered mafic intrusion

Recent studies indicate that the mantle source of eastern Lhasa terrane was still depleted during the late stage of oceanic subduction (100–50 Ma). However, the mantle source beneath the western Lhasa terrane during this period was poorly constrained mainly due to the rare exposure of mafic rocks in this region. Here, we report the detailed petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb isotopic data for recently identified gabbroic rocks in the Taruocuo area, western Lhasa terrane, southern Tibet. The zircon U-Pb dating results indicate that the Taruocuo gabbroic rocks were emplaced during the Late Cretaceous (~90 Ma). These gabbroic rocks have low SiO2 (50.9–55.0 wt%), and moderately high MgO (4.10–5.60 wt%) contents with arc-like trace-element patterns, such as enrichment in large ion lithophile elements, light rare earth elements, and depletion in high field strength elements. Meanwhile, they have high Th/La, low Ba/La values, and exhibit relatively enriched and uniform Sr–Nd–Hf–Pb isotopic compositions (87Sr/86Sr(i) = 0.7048–0.7058, εNd(t) = −1.8 to −1.2, εHf(t) = +3.1 to +3.8, and 206Pb/204Pb = 18.60 to 18.66). Such geochemical features of the Taruocuo gabbroic rocks suggest that they would originate from an enriched and fertile mantle source that was generated by the metasomatic reaction between the mantle wedge peridotite and the subducted Neo-Tethyan oceanic sediment-derived melts. Taking the regional tectonic evolution into consideration, we favor the model of Neo-Tethyan oceanic-ridge subduction having taken place to account for the Late Cretaceous (~90 Ma) magmatic event in the Lhasa terrane. Under such geological background, partial melting of this enriched mantle would have produced the Taruocuo gabbroic rocks in the western Lhasa terrane. Given that the Late Cretaceous Taruocuo gabbroic rocks have the relatively enriched Sr–Nd–Hf isotopic compositions, distinctive from the contemporaneous mafic rocks in the eastern Lhasa terrane that have the depleted Sr–Nd–Hf isotopic compositions, we propose that the mantle source beneath the Lhasa terrane was probably isotopically segmented during the Late Cretaceous (~90 Ma).