Ocean Island Basalts Research Articles

Scapolite metagabbros of the Xambica Suite: A Tonian OIB magmatism in the crustal evolution of the Araguaia Belt

The geological evolution of the Araguaia Belt (AB), an important Neoproterozoic orogen located in central-north Brazil, is marked by different stages in its tectonic evolution. These stages are characterized by various magmatic events represented by felsic alkaline plutons, ophiolitic suites, mafic-ultramafic intrusions, S-type granites, diabase dykes and tholeiitic basalts distributed along its 1200 km N–S long. One of these events was identified in the Xambioá region, in the NW of Tocantins/SE of Pará state, in which dozens of bodies of scapolite metagabbros and amphibolites were identified through mapping and fieldwork, which were gathered in the Xambica Gabbroic Suite (XGS). Field relations and structural observations reveal that they were intrusive bodies in the sedimentary successions of the Estrondo Group before metamorphism. We present complementary studies involving detailed petrographic, lithochemical and geochronological analyses to characterize these rocks and contextualize this magmatic event in AB evolution. The petrographic study allowed to classify the rocks of the XGS into metagabbros with plagioclase and clinopyroxenes relicts, scapolite-hornblende metagabbro, garnet-scapolite metagabbro, amphibolites, and rarely listwanite and steatite. Geochronological dating of zircon crystals from this suite of rocks was performed using two different techniques. Single zircon Pb-evaporation dating of a metagabbro (XL-02) yielded the age of 819 ± 5 Ma. U–Pb zircon dating by LA-MC-ICP-MS of a metagabbro (BV-01) and an amphibolite (XL-09) gave 822 ± 2 Ma and 826 ± 26 Ma, respectively. Two other metagabbro samples (XL-02 and XB-08) yielded concordant ages of 861 ± 21 Ma and 873 ± 15 Ma. The younger concordant ages are interpreted as the crystallization of the gabbroic protoliths in the Tonian, while the older concordant ages are related to the presence of inherited zircon. The geochemical signature of metagabbros and amphibolites is similar to that of ocean island basalts (OIB) related to an intraplate tectonic environment, characterizing an extensional tectonic phase in AB evolution, which differs from other mafic events already recognized in AB of a MORB or tholeiitic signatures. Similar Tonian age are reported in basic to intermediate volcanic secession in the Goiás Magmatic Arc representing a contemporary collisional tectonic event, which reveals a link in the evolution between these two geotectonic units.

Slab window–related magmatism as a probe for pyroxenite heterogeneities in the upper mantle

Abstract New high-precision trace-element analyses of magmatic olivines point to a pyroxenite-dominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (<~5%) melts of pyroxenite and peridotite.

RETRACTED-Late Permian to early Triassic gabbro in North Lhasa, Tibet: evidence for plume – subduction-zone interaction of the Palaeo-Tethys ocean

AbstractThe Palaeo-Mesozoic geodynamic evolution of the Tangjia–Sumdo accretionary complex belt, which separates the North and South Lhasa Terrane, remains controversial. Moreover, the lack of geological records restricts the understanding of the evolution of the Sumdo Palaeo-Tethys Ocean from the middle Permian until the middle Triassic. Here we present zircon U–Pb geochronology, whole-rock geochemistry and Sr–Nd–Hf isotopic compositions of the Yeqing gabbro. Zircon U–Pb geochronology yields ages from 254 ± 1 to 249 ± 1 Ma. In situ Hf isotopic analyses yield ϵHf(t) values of −0.2 to +6.3. These samples have high TiO2 (3.69 wt %) and P2O5 (0.78 wt %) contents, with typical patterns like ocean island basalt (OIB). Besides, they are classified as high-Nb basalts (HNBs) based on the high content of Nb (45.3–113.5 ppm). Whole-rock Sr–Nd isotopic compositions are similar to OIB, with initial 87Sr/86Sr of 0.7047–0.7054, 143Nd/144Nd of 0.512526–0.512647 and ϵNd(t) of 0.3–2.7. These signatures suggest that the Yeqing gabbro is mainly derived from low-degree melting of the garnet lherzolite mantle. Based on field observations of HNBs intruding into the continental margin and their geochemical characteristics, we infer that the Yeqing gabbro was generated in a subduction environment. Combined with the regional geology of the subduction environment and the evolution of oceanic islands in the Sumdo Palaeo-Tethys Ocean, we propose that the Yeqing gabbro may represent a product of the asthenosphere upwelling through a slab window produced by subduction of seismic ridge in the Sumdo Palaeo-Tethys Ocean, called plume – subduction-zone interaction, during the late Permian to early Triassic.

Middle Miocene forearc alkaline magmatism in Amami-Oshima Island, central Ryukyu Arc: implications for paleoreconstruction of Shikoku Basin

During the middle Miocene, forearc alkaline magmatism occurred in the Outer Zone of Southwest Japan and the northern Ryukyu Arc, resulting in the formation of forearc alkaline basaltic rocks with ocean island basalt (OIB)-like chemical signatures. In contrast, subduction-related magmatism was present in the central Ryukyu Arc. However, the southwestern margin of the forearc alkaline magmatism was poorly constrained in the Ryukyu Arc. We examined two basaltic dikes in the Chichibu accretionary complex of Amami-Oshima Island, the central Ryukyu Arc. The dikes cut massive basalt, reddish chert, varicolored shale, and grey chert. The chemical compositions of basaltic dikes are characterized by the enrichment of incompatible trace elements, possibly representing a low degree of partial melting from a deep mantle source. 40Ar/39Ar dating analyses indicate that the basaltic dikes yield ages of 16.37 ± 0.14 Ma and 16.51 ± 0.10 Ma. The paleomagnetic analyses on stepwise thermal demagnetizations allowed extracting the direction of stable magnetizations with unblocking temperatures of 450–575 °C for the two dikes [(Dec, Inc) = (138.1°, − 13.3°), (124.0°, − 24.9°)]. The magnetization could be primary, acquired either as part of a secular variation or a geomagnetic excursion during reversed polarity chron/subchron. The 40Ar/39Ar ages and paleomagnetic directions within the error range imply that they intruded almost simultaneously during C5Cn.2r (16.532–16.434 Ma). The age and trace element patterns of the basaltic dikes are comparable to those of Miocene alkaline basaltic rocks, which resulted from forearc alkaline magmatism during the initial subduction of the young and warm Shikoku Basin. The discovery of alkaline basaltic dikes on Amami-Oshima Island suggests that the distribution of middle Miocene forearc alkaline magmatism may extend to the central Ryukyu Arc. Hence, the northern end of the Kyushu-Palau Ridge (i.e., southern end of Shikoku Basin) could have been located south of Amami-Oshima Island around 16.5–16.4 Ma, then moved eastward to the current location.Graphical

Long-term core–mantle interaction explains W-He isotope heterogeneities

The isotopic characteristics of ocean island basalts have long been used to infer the nature of their source and the long-term evolution of the Earth's mantle. Anticorrelation between tungsten and helium isotopic signatures is a particularly puzzling feature in those basalts, which no single process appears to explain. Traditionally, the high 3He/4He signature has been attributed to an undegassed reservoir in the deep mantle. Additional processes needed to obtain low 182W/184W often entail unobserved ancillary geochemical effects. It has been suggested, however, that the core feeds the lower mantle with primordial helium, obviating the need for an undegassed mantle reservoir. Independently, the tungsten-rich core has been suggested to impart the plume source with anomalous tungsten isotope signatures. We advance the idea that isotopic diffusion may simultaneously transport both tungsten and helium across the core-mantle boundary, with the striking implication that diffusion can naturally account for the observed isotopic trend. By modeling the long-term isotopic evolution of mantle domains, we demonstrate that this mechanism can account for more than sufficient isotopic ratios in plume-source material, which, after dynamical transport to the Earth's surface, are consistent with the present-day mantle W-He isotopic heterogeneities. No undegassed mantle reservoir is required, bearing significance on early Earth conditions such as the extent of magma oceans.

Basaltoids of the ostrogorsk complex: geology, age, geochemistry, geodynamics (Western Sikhote-Alin)

This review-analytical article presents the most important geological, geochronological and geochemical characteristics of the basaltoids of the Ostrogorsk complex. Some Early Miocene datings of the age of Ostrogorsk basaltoids on the State Geological Map of the third generation (sheet M-53) were not considered due to methodological considerations and, for greater objectivity, they are given by the author in this article. The basaltoids of the Ostrogorsk complex, like the alkaline basalts of the Ayakit complex, belong to intraplate rift basalts. The spectra of rare earth and rare elements in both complexes are very similar. The composition of basaltoids of the Ostrogorsk and Ayakit complexes is close to the composition of oceanic island basalts and recycled components, which suggests that they have a single mantle-crustal source.

Lithology of EM1 Reservoir Revealed by Fe Isotopes of Continental Potassic Basalts

AbstractThe origin of EM1 (Enriched Mantle 1) reservoir, initially defined by the ocean island basalts (OIBs) with extremely low 143Nd/144Nd and 206Pb/204Pb, has been long debated, because melting of the ambient refractory peridotite along with the EM1 component will dilute the “EM1 fingerprints” recorded in these rocks. Comparing to the OIBs, Cenozoic potassic basalts from northeast China, the typical EM1‐type basalts in continental region, are formed at a lower‐degree melting, and therefore have the chance to preserve more information of the EM1 component. Here high‐precision Fe isotopes of these potassic basalts are reported to constrain the source lithology. Their δ57Fe (0.15–0.28‰) are positively correlated with the K2O, SiO2, K/U, and Rb/Y, and negatively correlated with the εNd and δ26Mg, forming binary mixing arrays. One endmember is the inferred EM1 reservoir, whereas the other is the local lithospheric mantle. Major elemental compositions of the melts released from the EM1 component resemble those sediment‐derived experimental melts. Combining with their heavier Fe isotopes and higher Zn/Fe ratios relative to those mid‐ocean ridge basalts (MORBs), an eclogitic source of these potassic basalts is therefore proposed to account for these features. Differing from the most conventional thinking of the metasomatized, phlogopite‐bearing lithospheric mantle, we argue that the EM1 component in the source of continental potassic basalts are composed of ancient subducted crustal materials (i.e., recycled sediment ± oceanic crust). This deep EM1 component will transform into eclogite and release high‐SiO2 potassic melts when ascending to the shallow asthenosphere.

Petrogenesis of mafic rocks from northwest Iran (Piranshahr) and comparison with northeast Iraq ophiolites: Implications for slab window magmatism in an evolving Neotethys arc

AbstractThe Piranshahr ophiolite is located in northwest Iran. The Mawat ophiolite in northeast Iraq is the western continuation of the Piranshahr ophiolite. The whole rock geochemistry of mafic rocks from the Piranshahr ophiolite (in the Gerdikavalan regions) is studied and compared with mafic rocks from the Mawat ophiolite in northeast Iraq (in the Hasanbag, Walash and Naopurdan regions). Two groups can be identified among the ophiolitic basalts in northwest Iran and northeast Iraq. The first one is represented by Late Cretaceous calc‐alkaline basalts, displaying typical active continental margin signatures and thought to have been derived from the melting of enriched mantle sources modified by continental crust and subduction fluids. The second group is characterized by Eocene–Oligocene tholeiitic basalts, displaying signatures intermediate between mid‐ocean ridge basalt (MORB) and oceanic island basalt (OIB) and back‐arc basin (BAB) like magmas and suggested to have been derived from the melting of mixed asthenospheric‐lithospheric mantle sources in a slab window. We propose that the subduction signature is acquired by interaction of the mantle advected through the slab window in the upper part of the subducting plate.

Protolith origin of eclogites from the North Qaidam UHP metamorphic Belt, NW China: Implications for the breakup of the Rodinia supercontinent

The North Qaidam ultrahigh-pressure metamorphic belt (NQUB) is distinguished by the widespread distribution of eclogites. However, the protolith nature of these eclogites remains controversial. This study reports new zircon LA-ICP-MS U–Pb ages and whole-rock geochemistry as well as Sr–Nd isotopic data on the Yuka and Dulan eclogites in the NQUB. Zircon U–Pb dating results of these eclogites indicate that the protolith and metamorphic ages are 831–809 Ma and 439–433 Ma. Based on distinct geochemical features, the samples can be classified into high-Ti/Y type (Group 1) and low-Ti/Y type (Group 2 and Group 3). The Group 1 eclogite protoliths are geochemically similar to oceanic island basalts (OIB) and likely originated from garnet-spinel peridotite mantle sources. The protoliths of Group 2 eclogites have a similar mantle source to those of the Group 1 protoliths, but experienced a higher degree of fractional crystallization and crustal contamination. The Group 3 eclogites have N-MORB-like affinity, and may be derived from shallower mantle sources in the spinel stability field. The differences in mantle sources of enriched (Group 1 and Group 2) and depleted (Group 3) eclogite protoliths can be ascribed to varying extents of mixing between enriched plume and depleted MORB-type mantle components at various melting depths. Our results suggest that the Neoproterozoic protoliths of eclogites in the NQUB belonged to a part of widespread Neoproterozoic rift-related mafic magmatism in the Altun-Qilian-Kunlun Orogenic Belt. These rocks could be the products of mantle plume activities that caused the breakup of Rodinia. Most Neoproterozoic mafic rocks in the NQUB experienced early Paleozoic HP/UHP metamorphism.

Eocene magmatism in the Himalaya: Response to lithospheric flexure during early Indian collision?

Abstract Eocene mafic magmatism in the Himalaya provides a crucial window for probing the evolution of crustal anatexis processes within the lower plate in a collisional orogen. We report geochemical data from the earliest postcollision ocean-island basalt–like mafic dikes intruding the Tethyan Himalaya near the northern edge of the colliding Indian plate. These dikes occurred coeval, and spatially overlap, with Eocene granitoids in the cores of gneiss domes and were likely derived from interaction between melts from the lithosphere-asthenosphere boundary and the Indian continental lithosphere. We propose that these mafic magmas were emplaced along lithospheric fractures in response to lithospheric flexure during initial subduction of the Indian continent and that the underplating of such mafic magmas resulted in orogen-parallel crustal anatexis within the Indian continent. This mechanism can explain the formation of coeval magmatism and the geologic evolution of a collisional orogen on both sides of the suture zone.

Silica-rich melts originating from melt-hydrated peridotite reactions

Interactions between melts and hydrated peridotite may control production of silica-rich melts, fluids and magmas in the upper mantle and may be predominant mechanism for producing felsic crust in shallow ultramafic settings of rocky planets. This research investigated composition of olivine-hosted mineral, melt and multi-phase inclusions in dunite xenoliths from the 1800–1801 Kaupulehu flow of Hualalai Volcano. The host olivine (forsterite Fo 82–89 , mol. %) appears with mineral inclusions of crystalline chromite having compositions similar to those found in alkaline series rocks. Abundant silica-rich glasses (SiO 2 = 63 ± 2.9 wt%, Al 2 O 3 = 21 ± 1.2 wt%; K 2 O = 4.3 ± 0.5 wt%, Na 2 O = 3.6 ± 0.7 wt%, 38 analyses) found in the olivine-hosted melt and multiphase inclusions are interpreted as quenched silicate liquids entrapped due to nephelinite melt percolation in hydrated harzburgite in the presence of free aqueous fluid at pressures of 0.7 to 0.1 GPa and a melt-to-peridotite mass ratio of 0.2 to 0.3. This work also re-interprets the metasomatic origin of global examples of silica-rich glass inclusions in peridotite xenolith minerals according to results from recent experiments on melt-hydrated peridotite interaction processes. As a planetary process, production of silica-rich melts and fluids within series of associated magmas and their crystallized differentiates can occur due to reaction of primitive mafic (e.g., ocean island basalt, OIB) melts with hydrated peridotite (or serpentinite) in the presence of aqueous fluids at pressures below 1 GPa. • Si-rich glasses are produced due to reaction of mafic melts with hydrated peridotite.

Discovery of Ultra‐Depleted Melt Inclusion in Late Cretaceous Intracontinental Basaltic Andesites in South China: Implications for Recycling of Lower Oceanic Crust

AbstractIncompatible element ultra‐depleted melt (UDM) has occasionally been found as olivine‐hosted inclusion in plume‐related oceanic island basalts and mid‐oceanic ridge basalts and is genetically linked with the involvement of lower oceanic gabbroic crust. However, it has rarely been reported in intracontinental magmatism, and the contribution of such a recycled component to the origin of intracontinental magmas remains unclear. Here, we first report UDM inclusions hosted in olivine from Late Cretaceous intracontinental basaltic andesites in South China. These UDM inclusions are characterized by low SiO2 (46.4–47.9 wt%) and extremely high Al2O3 (18.47–21.79 wt%) contents, positive Sr‐Eu anomalies, and low 207Pb/206Pb and 208Pb/206Pb ratios. Such geochemical features resemble those of high‐pressure melts derived from young lower oceanic gabbro. These results, in combination with geochemical modeling, plate reconstruction, and seismic topography, suggest that tearing or fragmentation of the subducted paleo‐Pacific slab during Late Cretaceous may trigger melting of the refractory lower oceanic cumulates. Our results demonstrate that lower oceanic gabbroic crust could also play an important role in the generation of intracontinental magmatism, which may previously have been underestimated.

Geochronology and geochemistry of the Manxin ophiolitic mélange in the Changning‐Menglian Suture Zone, southwest China: Implications for the tectonic evolution of the Proto‐Tethys Ocean

To date, the evolutionary history of the Proto‐Tethys Ocean that separated the Gondwana and Laurasia continents remains ambiguous. The Changning‐Menglian Suture Zone assumingly represents the Proto‐ and Palaeo‐Tethys suture in the Sanjiang area (the Yunnan Province, southwestern China). The Palaeozoic Proto‐Tethys Ocean in the Manxin area in the southmost Changning‐Menglian Suture Zone is widely recognized, but the timing and formation mechanism of the Manxin ophiolitic mélange are poorly understood. In this study, we present petrographical, geochronological and geochemical data of the Manxin ophiolitic mélange situated in the Changning‐Menglian Suture Zone. We found that this ophiolitic mélange is composed of pillow basalt, metamorphic gabbro, diabase dike, and altered peridotite. The mafic rocks could be divided into two groups based on geochemical characteristics: (1) Mid‐ocean ridge basalt (MORB) [formed at 446.0 ± 3.2 Ma and exhibiting depleted light rare earth element (LREE) patterns and high ƐNd(t) (+5.7 to +8.4)] Trace element and isotope analyses indicated that the MORB was derived from a spinel‐facies depleted mantle. (2) Ocean island basalt (OIB) [crystallized at 460.2 ± 6.0 Ma]. ƐNd(t) (+5.7) and significant enrichments in large‐ion lithophile element (LILE) and light rare earth element (LREE) were identified, while the trace element and isotope systematics suggested that the OIB rocks originated from partial melting of the garnet‐spinel lherzolite mantle. Overall, we clearly constrained the timing and nature of the Manxin ophiolitic mélange using the proposed data, thus further supporting the notion that the Manxin ophiolites represent remnants of the Proto‐Tethys Ocean.

Coexistence of Carboniferous oceanic island basalts with Permian supra‐subduction zone ophiolites in the Changning–Menglian accretionary wedge: Implication for tectonic reconstruction

This paper reports two types of basalt that have different ages (Carboniferous and Permian) but occur next to each other in the northern part of the Changning–Menglian accretionary wedge, southwestern China. We use the geochronology and geochemical data to evaluate the tectonic evolution of the Palaeo‐Tethys during this period. Zircon grains from the mafic‐ultramafic rocks associated with the Permian basalts in the Xiaomengtai area yield a U–Pb age of 281 Ma and εHf(t) values from +9.2 to +12.8. The Permian mafic‐ultramafic rocks and the associated basalts are all characterized by normal mid‐ocean ridge basalt (N‐MORB)‐like chondrite‐normalized rare earth elements patterns, moderately negative Nb‐Ta anomalies in the mantle‐normalized immobile incompatible trace element patterns, and positive εNd(t) values from +4.2 to +6.5, which are consistent with the geochemical characteristics of mafic‐ultramafic rocks in supra‐subduction zone (SSZ)‐type ophiolite. On the contrary, the Carboniferous basalts, which are associated with marine carbonates, are characterized by light REE enrichments, slightly positive Nb‐Ta anomalies, and εNd(t) values from +2.8 to +4.0. These features are similar to those of typical oceanic island basalts (OIBs) worldwide. Modelling results using REEs show that the parental magmas for the Carboniferous OIBs and the Permian basalts were likely derived from mantle peridotites at the depths of garnet and spinel stability, respectively, consistent with the formation depth of these two different types of basalt globally. The occurrence of these two different types of mafic‐ultramafic rocks with significantly different ages in the same area supports the view that they are the remnants of the accreted Palaeo‐Tethys oceanic crust. The Carboniferous OIBs are considered to be parts of an OIB‐carbonate seamount chain that originally formed in the southern part of the Palaeo‐Tethys. The Permian mafic‐ultramafic rocks are regarded as fragments of SSZ‐type ophiolites that were present in the northern part of the Palaeo‐Tethys. These different pieces of oceanic crust were accreted to the Simao–Indochina continental Block by subduction between the Late Permian and the Triassic.

Mo isotopes archive oceanic sediments in post-orogenic lithospheric mantle

The super-chondritic Mo isotopic composition of the continental crust has been attributed to contributions from subducted oceanic slabs and/or sediments, which introduce isotopically light Mo to the mantle. It has been hypothesized that dehydrated oceanic slabs subduct into the deep mantle and contribute to the genesis of ocean island basalts (OIBs). However, OIBs have similar average δ98/95Mo values (−0.22‰± 0.04‰; 2SE; n = 87) to the depleted mantle. Subducted oceanic sediments may create a light-δ98/95Mo reservoir in the subcontinental lithospheric mantle, but this has not been investigated in detail. Understanding the Mo isotopic systematics of the continental crust and mantle requires an understanding of the Mo isotopic composition of lithospheric mantle-derived volcanic rocks. The lithosphere in the Tibetan region has experienced multiple stages of oceanic subduction and is thus ideal for studying magmas derived from sediment-modified lithospheric mantle. Mafic ultrapotassic volcanic rocks (UPVRs) from the Sailipu area on the southern Tibetan Plateau have high Mg# values (67.1–72.5) and enriched Sr–Nd isotopic compositions and are primitive magmas derived from the lithospheric mantle. These rocks have higher Ce/Mo ratios (225–422) and lighter δ98/95Mo values (−0.45‰ to −0.13‰) than the depleted mantle, mid-ocean ridge basalts (MORBs), and OIBs, as well as high εHf(t) values at given εNd(t) values compared with the terrestrial array (ΔεHf = 4.59–5.73). The involvement of Ca-poor pelagic sediments rather than the lower continental crust and hydrated oceanic slab best accounts for the high Ce/Mo ratios and light δ98/95Mo values of the studied rocks. The Sailipu UPVRs also have higher Nb/Ta (18.9–20.8) and Th/La (2.6–3.4) ratios than arc-related igneous rocks and MORBs, suggesting that their source was phlogopite-bearing lithospheric mantle with a metasomatic vein component (rutile + monazite ± lawsonite). We propose a two-stage geodynamic process to explain the formation of the Sailipu UPVRs: (1) a metasomatic vein component with high Th/La and Ce/Mo ratios formed in the lithospheric mantle during multiple stages of oceanic subduction; (2) high mantle temperatures related to convective thinning of the lithosphere resulted in melting of the vein component and the ambient mantle, which produced hybrid melts of various mantle domains during the post-orogenic stage. The Sailipu UPVRs have enriched Sr–Nd isotopic compositions and high Th/La and Ce/Mo ratios, similar to lamproites that are widespread in the Tethyan orogenic realm from the Mediterranean to the southern Tibetan Plateau. Given that UPVRs with light δ98/95Mo values have also been identified in the Western Alps, we propose that the isotopically light Mo component is widespread in Tethyan lithospheric mantle. This isotopically light Mo in the lithospheric mantle may be a complementary end-member to the isotopically heavier continental crust. The recycling of subducted oceanic sediments has a key role in the deep recycling of Mo.

Origin of the Songpan-Garze Terrane on the northeastern Tibetan Plateau, eastern segment of the Tethyan tectonic domain: A Middle Permian−Early Triassic intracontinental rifting system

The initiation of a continental rift or the birth of an oceanic basin, and their long-term evolution, are challenges in the understanding of the evolution of regional and global tectonics. Rifts develop within the interior or margins of plates, essentially as extensional structures, and they have been studied widely since the discovery of plate tectonics. The eastern segment of the Paleo-Tethys traversed central Asia, its structural features, timing, geological setting, and geodynamic mechanisms are poorly understood. The Songpan-Garze Terrane (SGT) on the northeastern Tibetan Plateau has been regarded as either a remnant of the Paleo-Tethys Ocean or a continental rift basin. Different interpretations of the SGT lead to various tectonic reconstructions between Gondwana and Laurasia. Here, sedimentology, geochemistry, and geochronology have been applied to the southern and northern parts of the SGT to reconstruct the initiation of a Permian rift system. In this area, for the first time, field investigation reveals that the SGT is characterized by a continuous succession of Permian−Lower Triassic terrestrial-neritic deposits and ocean island basalt (OIB)-type basalts, and that it developed as an intracontinental rift basin in the Yangtze Block. Zircons from gabbros with OIB-type geochemical signatures intruded in the southern SGT yield U-Pb ages of 263−254 Ma, constraining the timing of the initial rifting. Proterozoic felsic rocks collected from the southern SGT yield U-Pb ages of 761−865 Ma, and they are petrologically and geochemically comparable to Neoproterozoic basement rocks along the western margin of the Yangtze Block. The Middle−Late Permian volcano-sedimentary sequences of the SGT and their geochemistry and geochronology are comparable to rift sequences in the Panxi region of the western Yangtze Block. Thus, it has been proposed that the SGT originated as an intracontinental rift within the Yangtze Block during the Middle Permian−Early Triassic, and that it continued to deepen as a rift basin through the Middle−Late Triassic.

Insights from olivine chemistry into crustal magmatic processes and the mantle source lithology of basalts from Hainan Island, China

Olivine in ocean island basalts can provide unique insights into lithological heterogeneity (i.e., peridotite versus pyroxenite) in the mantle. Here we present high-precision major and trace element data for olivine crystals in basalts from Hainan Island, China, and use these data to investigate their mantle source. Many of the olivines are in disequilibrium with their whole-rock compositions (i.e., Mg# values), and the basalts contain both normally and reversely zoned olivine crystals, which are indicative of magma mixing. The decoupled Fo–NiO profiles and concave-down trends defined by data in a Fo–NiO diagram suggest that diffusive re-equilibration controlled the olivine chemistry. The olivine compositions were significantly affected by crustal magmatic processes, such as fractional crystallization, magma mixing, and diffusive re-equilibration. However, the lack of clear correlations between Mn/Fe, Mn/Zn, Zn/Fe, and Ga/Sc ratios and olivine Fo contents suggests these trace element ratios were not significantly affected by crustal processes, and thus preserve information about the mantle source. The Mn/Zn, 100 × Mn/Fe and 10,000 × Zn/Fe ratios of the examined olivines indicated that the Hainan basalts had a mixed pyroxenite-peridotite mantle source. This is consistent with the whole-rock geochemical data and previously reported isotope geochemistry data for the Hainan Island basalts. However, crustal magmatic processes also affected the compositions of the primitive olivines that crystallized in the mantle-derived magma. Therefore, careful assessment of post-melting processes using olivine textures and minor element chemistry needs to be undertaken before discriminating between pyroxenitic and peridotitic source components based on olivine chemistry.

Mesozoic tectonic transition from the Tethyan to Paleo-Pacific domains in South China: New insights from Mesozoic igneous rocks in the western Nanling Range

• The studied Mesozoic igneous in the Nanling Rang include post-orogenic ultramafic–mafic rock, subduction-related mafic rocks and other calc-alkaline intermediate-felsic rocks. • Those igneous rock assemblages in the Nanling Rang reflect the tectonic transition of the SCB. • The Nanling Rang was a Tethyan domain before ca. 170 Ma but a Paleo-Pacific domain after that. • The Mesozoic evolution of the SCB involved four stages: post-orogenic extension, slab foundering, and tectonic transition from subduction to rollback, followed by slab rollback. Mesozoic tectono-magmatic evolution of the Nanling Range in the South China Block (SCB) remains controversial, especially about the role of the Tethyan and Paleo-Pacific dynamics in the generation of Mesozoic igneous rocks. The Mesozoic magmatic events have produced a diverse variety of rocks, dominantly granites and spatially associated mafic–ultramafic rocks, lamprophyres and diorites. We present new zircon U-Pb ages and in-situ Hf isotope data, along with whole-rock elemental and Sr-Nd isotopic data for the Mesozoic igneous rocks from the western Nanling Range. The two ca. 200 Ma ultramafic–mafic rocks have low SiO 2 contents (40.00–44.94 wt%) and typical ocean island basalt (OIB)-like trace elemental affinities with positive Nb and Ta anomalies, positive ε Nd (t) (4.47 to 4.88) and variable ε Hf (t) values (–3.65 to 11.91), suggesting that they were generated by partial melting of an asthenospheric mantle source. The ca. 165 Ma Jiangyong lamprophyre dyke has relatively high SiO 2 content (51.70 wt%) and arc-like trace elemental affinities with significantly negative Nb and Ta anomalies and variable ε Hf (t) values (–9.10 to 2.65). They were derived from a lithospheric mantle source previously modified by subducted component, along with a contribution from the asthenospheric mantle. Granites from the ca. 146–137 Ma Taibao granitic pluton have high SiO 2 contents (63.41–66.66 wt%), variable (La/Yb) N and Sr/Y values, and negative to positive ε Hf (t) values (−4.16 to 0.86). These granites have compositions transitional from adakite to arc-type rocks and were likely generated by partial melting of Mesoproterozoic meta -igneous rocks, with an addition of a mantle-derived component. Compared with those granites, diorites of the ca. 99 Ma Eryue pluton exhibit moderate SiO 2 contents (59.90–60.30 wt%), a wide range of zircon ε Hf (t) values (−12.04 to 0.50) and negative ε Nd (t) values (–8.61 to –8.57), and they were probably formed by magma mixing of crust-mantle materials associated with back-arc extension. Our new data, together with other geological evidence, provide further insights for the evolution of the Tethyan and Paleo-Pacific tectonic domains during the Late Triassic to Late Cretaceous in the SCB. In the early stage, the Tethyan tectonic domain, ultramafic–mafic rocks from the Ludong and Fuchuan plutons in the Nanling Range was related to the post-orogenic extension. This domain was gradually changed to the Paleo-Pacific domain during the period from Late Triassic to Early Jurassic and the tectonic transition was accomplished at ∼ 170 ± 5 Ma. In the later stage, the Paleo-Pacific tectonic domain, the Jiangyong lamprophyre, Taibao granites and Eryue diorites were produced via the crustal thickening, localized extension, and large-scale lithospheric extension due to the advancing, slab-foundering, and rollback of the oceanic slab during the period from Middle Jurassic to Late Cretaceous.

The root to the Galápagos mantle plume on the core-mantle boundary


 Ultra-low velocity zones (ULVZs) are thin anomalous patches on the boundary between the Earth's core and mantle, revealed by their effects on the seismic waves that propagate through them. Here we map a broad ULVZ near the Galápagos hotspot using shear-diffracted waves. Forward modelling assuming a cylindrical shape shows the patch is ~600 km wide, ~20 km high, and its shear velocities are ~25% reduced. The ULVZ is comparable to other broad ULVZs mapped on the core-mantle boundary near Hawaii, Iceland, and Samoa. Strikingly, all four hotspots where the mantle plume appears rooted by these ‘mega-ULVZs’, show similar anomalous isotopic signatures in He, Ne, and W in their ocean island basalts. This correlation suggests mega-ULVZs might be primordial or caused by interaction with the core, and some material from ULVZs is entrained within the plume. For the Galápagos, the connection implies the plume is offset to the west towards the base of the mantle. 

Molybdenum isotopes in mafic igneous rocks record slabmantle interactions from subarc to postarc depths

Abstract Arc volcanic rocks erupted above oceanic subduction zones generally have heavy molybdenum (Mo) isotopic compositions, which are attributed to contributions from oceanic slab–derived fluids with isotopically heavy Mo at subarc depths. Given that mafic igneous rocks with light Mo isotopic compositions have been rarely identified, it is unclear where light Mo isotope reservoirs reside, as these must exist due to oceanic subduction through geologic time. We present Mo isotope data from Mesozoic–Cenozoic mafic igneous rocks from the eastern North China craton, which were affected by continuous subduction of the oceanic slab. The Mesozoic mafic igneous rocks have island-arc basalt (IAB)–like features and high δ98Mo values of –0.15‰ to +0.09‰. In contrast, the Cenozoic mafic igneous rocks have ocean-island basalt (OIB)–like features and low δ98Mo values of –0.53‰ to –0.19‰. In addition, these two suites of rocks exhibit systematic differences in Sr-Nd isotopic compositions and fluid- and melt-mobile element contents, indicating that their mantle sources contained crustal components derived from different depths. The heavy Mo and weakly enriched Sr-Nd isotopic compositions and IAB-like trace-element signatures were inherited from slab fluids (including oceanic crust–derived aqueous solutions and seafloor sediment–derived hydrous melts) liberated at subarc depths. The light Mo and depleted Sr-Nd isotopic compositions and OIB-like trace-element signatures were inherited from dehydrated slab–derived melts (with rutile breakdown) at postarc depths. Therefore, Mo isotope systematics of mafic igneous rocks are a powerful means with which to trace the geochemical fluxes from subducting oceanic slabs to the mantle wedge at different depths.