Enrichment In LILE Research Articles

Late Triassic magmatic rocks in the southern East Kunlun Orogenic Belt, northern Tibetan Plateau: Petrogenesis and tectonic implications

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) documents successive subduction and accretion processes of the Paleo-Tethyan Ocean. However, the tectonic regime of the E-KOB at the late orogenic stage remains ambiguous. This study presents new results of whole-rock geochemistry and zircon U-Pb-Hf isotopes of the Xidatan pluton in the southern E-KOB. Detailed investigation reveals that the pluton is composed of quartz monzodiorite, host monzogranite, and igneous microgranular enclaves (IMEs). Zircon U-Pb dating results suggest that they are coeval magmatic rocks emplaced at ca. 206–203 Ma. Geochemical studies reveal that the quartz monzodiorite and IMEs share a common magma source, evidenced by consistent compositions of major elements, congruently enriched LREEs and LILEs, and consistently moderately depleted HFSEs and HREEs. Enriched to slightly juvenile zircon Hf isotopes (εHf(t) = −4.24 to +0.49) indicate that the primitive magmas of the quartz monzodiorite and IMEs involve the addition of mantle-derived magmas. The quartz monzodiorite and IMEs have identical ages, and consistent isotope features with the host monzogranite, together with the observation of ingestion of xenocrysts from host felsic rocks into the IMEs indicating that they were produced through the mixing of the mantle-derived mafic magmas and host felsic magmas. In contrast, The host monzogranite has moderately high SiO2 and Na2O+K2O, but low Fe2O3 T and MgO contents. It shows crust-like trace element patterns such as strong enrichments in LREEs and LILEs, but remarkable depletions in HREEs. Along with enriched zircon Hf isotopes (εHf(t) = −3.51 to −0.25), the host monzogranite was most likely to have originated from partial melting of lower crustal mafic rocks induced by underplated mantle-derived magmas. Considering abundant Late Triassic igneous rocks across the E-KOB, these new data provide further evidence that the Late Triassic magmatism in the E-KOB may be related to the delamination of thickened orogenic root under an extensional setting.

Mesozoic magmatic evolution of the Laiyuan complex: Tracing the crust-mantle and lithosphere-asthenosphere interactions in the central North China Craton

The Laiyuan complex in the central North China Craton (NCC) incorporating different magmatic suites offers an excellent opportunity to investigate the lithospheric evolution and cratonic destruction. However, the petrogenesis and tectonic implications of this magmatic suite remain debated due to lack of integrated studies. Here we evaluate the magmatism and tectonic setting assembling data from multidisciplinary investigations of the Laiyuan complex. The complex is composed of volcanic suites, granitoids, ultramafic-mafic intrusions, and dykes showing common features of enrichments in LREEs and LILEs and depletions in HFSEs. Detailed petrogenetic considerations suggest that crust-mantle and lithosphere-asthenosphere interactions contributed to the formation of various magmatic suites. The involvement of thickened lower crust and enriched lithospheric mantle in the source, and diverse magmatic processes including partial melting, fractional crystallization, and magma mixing have played a significant role in the petrogenesis of the Laiyuan complex. Furthermore, the lithosphere-asthenosphere interaction induced by thinning lithosphere and upwelling asthenosphere controlled the source variations from dolerites to lamprophyres. The complex formed in an extensional tectonic setting triggered by the subduction of the Paleo-Pacific Plate. The subduction, rollback, and stagnation of the Paleo-Pacific slab contributed to the modification of the lithospheric architecture of the North China Craton. A slow and gradual thermal-mechanical erosion occurred at the central North China Craton whereas the rapid and intense lithospheric delamination occurred at the eastern North China Craton contributing to different lithospheric evolution. Both of the mechanisms combined with the subduction of Paleo-Pacific slab played a significant role in the destruction of the North China Craton and the formation of various magmatic suites. An integrated model is proposed to describe the magmatic evolution of the Laiyuan complex. During Jurassic, the subduction of the Paleo-Pacific Plate reached beneath the central North China Craton. At 145–140 Ma, the fast slab rollback occurred and lead to hot asthenosphere upwelling and extensional setting in the central North China Craton inducing the crust-mantle interaction accounting for the petrogenesis for the formation of granitoids with MMEs (137–126 Ma), volcanic rocks (131–127 Ma), and felsic dykes (131–127 Ma). Through time, the lithosphere became substantially thin with the asthenospheric input increasing to form dolerite dykes at 125–117 Ma and lamprophyre dykes at 115–111 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.

Geological and hypogene mineralization characteristics of the hematite‐rich Pelusa IOCG prospect, Antofagasta Region, Northern Chile

AbstractThe Japón, Tailandia and China Sur deposits in the Pelusa mineralized area correspond to a hematite‐dominant IOCG clan within the coastal Cordillera in the Antofagasta Region, Northern Chile, and their mineralization occurs in basaltic to andesitic volcanic rocks of the Jurassic La Negra Formation. In addition to geological and mineralogical descriptions, bulk chemistry of the host rocks, microthermometric and sulfur isotropic analyses were performed to elucidate ore‐forming characteristics of the three orebodies. Chemical data of the host rocks indicate a subalkaline calc‐alkaline affinity with enrichment of the incompatible LILEs, displaying a typical pattern of igneous rocks formed in subduction zones as a general characteristic of the Jurassic‐Lower Cretaceous magmatic arc. Chalcopyrite is the predominant hypogene copper mineral, that variously occurs as veinlets, disseminations and breccia matrix. Fluid inclusions data indicate formation temperatures of the hypogene mineralization ranging mostly from 186 to 377°C, with a high‐salinity range of 30.1 and 45.0 wt% NaCl. The sulfur isotopic ratios (δ34SCDT) of pyrite and chalcopyrite present a narrow range of values from −10.6 to −7.5‰. The negative values in the limited range imply that sulfide minerals of the Pelusa area were formed under a relatively oxidizing and/or neutral pH condition, compared with other Chilean IOCG deposits.

Gabbroic Rocks in Mawat Ophiolite Complex, NE Iraq: Geochemical Constraints on their Tectonogenesis

The Mawat Ophiolite Complex located in north-eastern Iraq represents part of the Iraqi Zagros Suture Zone, located at the border between the Arabian-Iranian plates. It consists of peridotite, gabbro, subvolcanic sheeted dykes and volcanic pillow lava. The geochemical signatures of the gabbro show a significant variation in major elements concentration, low concentrations of TiO2 (0.06-0.36 wt.%), Na2O (0.44-0.98 wt.%), K2O (0.024-0.48 wt.%), P2O5 (0.002-0.009 wt.%) and moderate variations in SiO2 (46.13-53.77 wt.%) with wide ranges and high concentration of Al2O3 (13.87-22.18 wt.%), Fe2O3 (0.04-2.95 wt.%), FeO (2.68-8.76 wt.%), MgO (8.09-14.28 wt.%) and CaO (11.08-17.04 wt.%). The flat REE patterns of the present rocks represent island arc tholeiite (IAT) tectonics and a subduction-related environment. The main features of the magma generated in the supra subduction zone are the enrichment of LILEs and depletion of the HFSEs and the strong negative Nb anomaly. Geochemical evidence indicates that the tholeiitic gabbros were generated in an arc-related tectonic setting. The tectonomagmatic diagrams for gabbros and the very low Ti content propose that the gabbroic rocks of Mawat Ophiolite have island arc tholeiitic (IAT) and possibly boninitic affinities which have a related to the supra-subduction zone (SSZ).

Petrogenesis and tectonic setting of TTG rocks from the Katoro area, Sukumaland Greenstone Belt, NW Tanzania: Evidence from zircon U-Pb geochronology, Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd isotopes

The northern part of the Tanzania Craton is mainly comprised of several low-grade metamorphic granite-greenstone belts that are exposed on the east and south shores of Lake Victoria. Due to their gold-endowment, these greenstone belts have been the main focus of research and were interpreted to form in various tectonic settings involving back-arc basin, island arc or continental arc, and oceanic plateau. However, the TTG gneiss, as the dominant component of Archean crust, has received far less attention from the scrutiny of modern analytical methods than its counterpart, the mafic volcanics of the greenstone belts. Here, we present a comprehensive geochemical dataset including zircon U-Pb age, zircon Lu-Hf isotope results, as well as bulk-rock geochemical compositions and Sm-Nd isotopic analyses for TTG suites from the Katoro area in northwestern part of Tanzania. Zircon U-Pb dating for three TTG samples yielded 207Pb/206Pb ages of 2688 ± 19 Ma, 2717 ± 19 Ma and 2695 ± 15 Ma, respectively, and the results fall within the peak crustal growth period in northern part of the Tanzania Craton. The Katoro TTGs have relatively high contents of SiO2 (70.77–74.69 %) and Na2O (4.74–5.16 %), high ratios of Sr/Y (59.49–167.25) and (La/Yb)N (17.27–50.72), but low MgO (0.24–0.70 %) and K2O (1.71–3.26 %) contents, low Nb/Ta ratios (8.50–16.60), and most of them belong to the trondhjemitic series rocks. They are characterized by strong enrichment of LILEs and LREEs with slightly positive Eu anomalies (Eu/Eu*=1.05–1.38), but depleted in Nb-Ta and Ti, as well as heavy REEs and Y. They have positive εHf(t) values (average εHf(t) = +1.90 ± 0.50 or + 2.07 ± 0.33) and εNd(t) values (+1.19 to + 2.40) but lower than the contemporaneous depleted mantle, which is inconsistent with a subducting slab signature, and it is proposed that the Katoro TTGs most likely originated from partial melting of rutile-free, garnet- and amphibole-bearing lower mafic crust of an oceanic plateau. The results presented here suggest that the tectonic evolution in this part of Tanzania Craton needs to invoke a scenario in which crustal growth caused by subduction-related modification of an oceanic plateau.

Petrogenesis of Early Cenozoic Sarıcakaya–Nallıhan Volcanism in NW Turkey: Implications for the Geodynamic Setting and Source Characterization of the Balkanatolia Magmatic Realm

Sarıcakaya–Nallıhan Volcanism was generated within the Balkanatolia Magmatic Realm between 48 and 44 Ma (by 40Ar–39Ar age determination) and is represented by three different volcanic units all displaying subduction-related geochemical signatures, such as depletion in HFSE and enrichment in LREE and LILE. The first unit (V1) consists of nepheline-normative, olivine basalts with OIB-like affinity. The second (V2) and third (V3) units are represented by more evolved compositions such as basaltic-andesitic, andesitic, and dacitic-rhyolitic lavas. Even the most basic lavas have elevated Mg# values (62–69), and they are far from representing the true mantle melts. Source characterization of Sarıcakaya–Nallıhan Volcanism reveals that there might be two possible mantle sources for the primary melts of the lavas: (i) metasomatized peridotitic mantle fluxed by sedimentary melts, or (ii) accreted mélange. The direct melting of the mélange-like lithologies is a more favorable mechanism for the Middle Eocene (44–40 Ma) magmatism in Balkanatolia since the Hf–Nd trace element, Nd isotopic systematics and petrological modelling efforts supported the latter. Overall, Early Cenozoic magmatism within this realm was characterized, first (58–44 Ma) by contractional and later (44–40 Ma) by extensional tectonics and the late-stage magmatic phase in the area was possibly controlled by melting of accreted mélange-like lithologies. The presented data indicate that mélange melting might be much more common than envisaged for the magmatism in the Alpine–Himalayan orogenic belt.

Petrogenesis and Tectonic Implications of Late Carboniferous Intrusions in the Tuwu-Yandong Porphyry Cu Belt (NW China): Constraints from Geochronology, Geochemistry and Sr–Nd–Hf Isotopes

The Tuwu-Yandong porphyry Cu belt is located on the southern margin of the Dananhu island arc in eastern Tianshan, constituting the largest Cu metallogenic belt in Northwest China. Two episodes (~334 Ma and ~317 Ma) of porphyry Cu-Mo mineralization in the belt have been recognized, associated with Early and Late Carboniferous felsic intrusions, respectively. The Carboniferous intrusions, therefore, provide a unique opportunity to investigate tectono-magmatic-metallogenic evolution of the belt. New LA–ICP–MS zircon U–Pb dating indicates that the mineralization-related and post-mineralization intrusions (granodiorite porphyry, gabbro, and granite porphyry) were formed at 321.8 ± 3.1 Ma, 313.5 ± 1.2 Ma, and 309.8 ± 2.5 Ma, respectively. The zircon trace element shows that the granodiorite porphyry (Ce4+/Ce3+ ratios, avg. 129, median = 112, n = 15) was likely derived from a more oxidized (and hydrous) magma source than that of the gabbro (Ce4+/Ce3+ ratios, avg. 74, median = 40, n = 15) and granite porphyry (Ce4+/Ce3+ ratios, avg. 100, median = 91, n = 15), being favorable for porphyry copper mineralization. The granodiorite porphyry shows an adakitic affinity (e.g., high Sr/Y ratios and low Y contents) and has high εNd(t) (6.4–6.7), εHf(t) (11.4–14.3), and Mg# values (47.4–58.1) and low (87Sr/86Sr)i (0.703804–0.703953), suggesting that the melt was derived from partial melting of a subducted oceanic slab followed by mantle peridotite interaction. The gabbro exhibits higher Al2O3 (16.5–17.4 wt.%), Cr (107–172 ppm), and Ni (37–77 ppm) contents and εNd(t) (6.6–7.2), εHf(t) (11.6–15.9), and Mg # (53.3–59.9) values, while it has lower (87Sr/86Sr)i values (0.703681–0.703882) than the granodiorite porphyry, indicating a depleted mantle source. The granite porphyry exhibits an affinity with non-fractionated I-type granites and possesses higher SiO2 (71.1–72.0 wt.%) contents, lower but positive εNd(t) (4.8–5.2), εHf(t) (10.3–13.0), and Mg # (38.7–41.0) values, and higher (87Sr/86Sr)i (0.704544–0.704998) than the granodiorite porphyry and gabbro, together with young Nd and Hf model ages, suggesting that the parental magmas originated from the partial melting of a juvenile lower crust. The enrichment in LREEs and LILEs (e.g., Ba, U, K and Sr) and depletion in HFSEs (e.g., Nb, Ta, and Ti) indicate that these intrusive rocks formed in the subduction zone. With the integration of previous studies, it can be inferred that the northward flat subduction of the Kangguer ocean slab at ca. 335–315 Ma caused the formation of the adakites and associated porphyry Cu mineralization in the Tuwu-Yandong belt. After the prolonged flat subduction, slab rollback may have occurred at ca. 314–310 Ma, followed by a “quiet period” before the final closure of the ancient Tianshan Ocean along the Kangguer Fault in this belt.

Geochronological and geochemical constraints for the metavolcanosedimentary succession of the Nyong Complex, northwestern margin of the Congo craton: Implications for depositional age and tectonic setting of associated banded iron formations

The Mewongo area of the Nyong Complex is located in the northwestern part of the Congo craton in southwestern Cameroon. The metavolcanosedimentary succession in this area comprises banded iron formations (BIFs), interlayered with amphibolites and gneisses and intruded by metamorphosed dolerite dykes. Within the framework of detailed field investigation, we report whole-rock geochemical data for the BIFs, metadolerites, and gneisses to constrain their provenance and tectonic setting, and present LA-ICP-MS U-Pb-Hf isotopic data for zircons from amphibolite and gneiss to assess the depositional age of BIFs from the Nyong Complex and understand the crustal evolution at the northwestern margin of the Congo craton. The geochemical composition of the Mewongo BIFs indicates a contribution of hydrothermal fluids, derived via submarine alteration of volcanics, and seawater. Geochemical and LA-ICP-MS U-Pb detrital zircon data for the gneisses reveal felsic protolith with ages between 2.97 and 2.65 Ga. Our zircon U-Pb-Hf isotope data suggest that a significant growth of juvenile crust occurred at ca. 2.7–2.2 Ga, followed by a reworking event at ca. 2.1 Ga with only minor juvenile crust addition. The metadolerites, cutting the metavolcanosedimentary succession, range in composition between tholeiitic and calc-alkaline basalts. They can be subdivided into two groups based on their trace element signature. The group I metadolerites have OIB-like geochemical composition (e.g., enriched LILE, HFSE, and LREE), while the group II metadolerites exhibit MORB-like geochemical signature (e.g., flat HFSE and HREE patterns). Metadolerites belong to two different so far undated magmatic events on the Congo craton that were previously recognized on the adjacent Ntem Complex.LA-ICP-MS U-Pb zircon dating of amphibolites, combined with chemical composition of metadolerites and gneisses, indicate deposition of the metavolcanosedimentary sequence at ca. 2.1 Ga in a pro-foreland basin sandwiched between the West and East Gabonian blocks, at the time when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny. The Nyong Complex BIFs were therefore deposited in a deep, open-marine setting during the ca. 2.1–2.0 Ga time interval.

Middle-late Miocene rift-related magmatism in Guaymas, Sonora, Mexico at the eastern margin of the Gulf of California: Petrogenetic implications associated to the Pacific-north American plate boundary

Miocene volcanism in the Guaymas region at the eastern margin of the Gulf of California, in coastal Sonora, NW Mexico, was dominated by arc magmatism. However, the presence of rift-related magmatism is also exposed. Rocks associated with subduction of the Farallon plate beneath North America plate are of Oligocene-Miocene ages, while the magmatic rocks with ages between ca. 12.5 to ca. 6 Ma occur in a post-subduction tectonic setting. This period is sometimes referred to as the proto-Gulf phase. The proto-Gulf magmatic rocks are exposed as part of a volcanic igneous plumbing system, herein named Guaymas Group. The Guaymas Group has an age range between ∼12 and 10 Ma and is defined by two magmatic events: an intermediate compositional pulse (Unit 1) and a felsic pulse (Unit 2).The lithological units from the Guaymas Group have similar geochemical features, exhibiting LILE and LREE enrichment and Nb, Ta and Ti depletion. These rocks show a transitional to tholeiitic composition affinity that differs from the arc magmatism from coastal Sonora. The geochemical data suggest that assimilation and fractional crystallization (AFC) processes played an important role in the geochemical variations observed in volcanic and subvolcanic rocks. The geochemical characteristics of the Guaymas Group rocks also could indicate an origin by decompression melting of a mantle source metasomatized by subduction processes.Finally, considering the geological, geochronological and geochemical study, we suggest that the emplacement of the Guaymas Group rocks is contemporaneous with the oblique rifting environment associated with an incipient Pacific-North American plate boundary.

Ca. 2.1 Ga low-δ18O gabbro-diorite association in southern North China Craton: Implications for an intraplate rifting

Although widespread ca. 2.2–2.1 Ga igneous activities were identified through the North China Craton (NCC), their petrogenesis and implications for the Paleoproterozoic tectonic regime of the NCC are still under hot debate. Here we present the 2.18–2.14 Ga gabbro-diorite plutons in the Xiong'ershan Taihua Complex, southern North China Craton. They are dominantly gabbros with minor as gabbroic-dioritic rocks. The samples exhibit transitional geochemical characteristics from tholeiitic to calc-alkali series with enrichment in LREEs (e.g., La/Yb N = 1.90–13.9) and LILEs (e.g., Rb, Ba and Sr) but depletion in HFSE (e.g., Nb, Ta and Ti), corresponding to affinity of arc-related magmatism. The high Fe 2 O 3 (8.90–14.91 wt%), CaO (6.78–9.81 wt%) contents, and Ti/Y ratios (179–635), low Th concentrations (0.11–4.08 ppm), and consistent La/Nb and Th/Nb ratios, preclude significant crustal contamination during magma emplacement. The samples show negative zircon ε Hf (t) (−7.3–−1.6) and whole-rock ε Nd (t) (−4.0–+0.9) values with much older model ages ( T DM Hf = 2811–2606 Ma and T DM Nd = 3069–2711 Ma) than their formation ages, along with high La/Nb (1.69–6.21) and low La/Ba (0.01–0.05) ratios, indicating a refertilized Archean lithospheric mantle source. Considering that zircons of 2.18–2.14 Ga gabbro-diorite in the Xiong'ershan area lack metamict features and exhibit originally low but homogeneous δ 18 O values in the mantle source (3.04‰ to 5.07‰, 4.21‰ on average), we prefer they derived from preexisting 18 O-depleted mantle source, which might be a highly heterogeneous metasomatized SCLM by subducted-fluid during the Archean cratonization. The consistent Sm/Yb and La/Sm ratios indicate ∼5–30% partial melting of the garnet-spinel dominant source region. Combing the low δ 18 O signature and within-plate (high Zr, Zr/Y, Th/Hf and Ta/Hf) geochemical features, we suggest a thermal anomaly linked to a rift setting could account for the mantle melting. The intraplate ca. 2.1 Ga low-δ 18 O magmatism possibly resembles those during the Neoproterozoic glaciations associated with major dispersal periods of Rodinia. To better understand the Paleoproterozoic tectonic evolution of the NCC, we have also comprehensively compiled all the published ages and geochemical data of the 2.2–2.1 Ga mafic-felsic igneous rocks. The mafic suites can be divided into two groups based on ages and geochemical affinities. Mafic rocks of Group 1 (2.19–2.12 Ga) belong to rift-related basalts, together with spatially associated 2.19–2.13 Ga A-type granitic rocks and volcano-sedimentary associations, likely indicating an intraplate rift setting. Whereas mafic rocks of Group 2 (2.15–2.06 Ga) show geochemical affinity of island arc calc-alkaline basalts, together with coeval 2.18–2.05 Ga I-S type granitic associations, implying an arc-related setting and recording a possible tectonic transition from extension to subduction-collision. The rifting events in the NCC during 2.2–2.1 Ga may coincide with the global lithospheric break-up of the supercontinent, and the subsequent tectonic transition could correspond to development of the globally orogenesis at 2.1–1.8 Ga. • The Paleoproterozoic low-δ 18 O gabbro-diorite plutons were formed at 2.18–2.14 Ga. • The mafic association was derived from an ancient metasomatized SCLM source. • They were formed in an intraplate rifting system. • The extensive 2.2–2.1 Ga magmatism in the NCC corresponds to global breakup event.

Relics of 2.6–2.5 Ga oceanic crust from the ultramafic-mafic complex of Goa, western India: Magmatic response to a progressive subduction system

Subduction initiation in the Early Earth and the onset of plate tectonics are topics of wide interest in understanding the formation and evolution of continents and supercontinents on our planet. Here we report results from integrated petrological, geochemical and geochronological (Zircon U Pb ages) studies on an ultramafic-mafic complex from the Bondla-Usgao-Durigini (BUD) sectors of Goa to address its petro-tectonic evolution and correlation with Neoarchean crustal growth processes and cratonization events. The ultramafic-mafic cumulate sequence from BUD sectors of Goa comprising dunite, peridotite, troctolite, olivine gabbro and gabbro represents a dismembered crustal section of oceanic lithosphere formed either in a MOR or SSZ fore-arc extensional regime. The geochemical attributes marked by boninitic-IAT trend, depleted to enriched mantle compositions, selective enrichment in fluid-mobile LILE and LREE over HFSE enunciate a multistage petrogenetic evolution associated with a SSZ fore-arc tectonic setting. The ocean-continent transition featured by the geochemical signatures can be translated in terms of oceanic crust formation in a juvenile extensional fore-arc in a SSZ setting, its maturation and accretion at active continental margin during Neoarchean ocean basic closure and arc-continent collision. The studied ultramafic-mafic suite fingerprints the transition of depleted mantle wedge to a LILE-LREE enriched metasomatized one through influx of subduction components and hydrous arc magmatism typical of SSZ oceanic lithosphere formed above a nascent subduction zone. The magmatic precursors of the studied lithologies were derived from a transitional depleted to enriched mantle regime marked by hydration and metasomatism of a depleted MOR-type mantle by the influx of subduction-derived fluids. Zircon grains in the peridotite sample yield positive εHf(t) of +3.2 to +7.6, together with T DM C of 2610–2877 Ma suggesting depleted mantle source. The gabbro sample shows positive εHf(t) values of +3.7 to +6.3 and T DM C age of 2731–2868 Ma also attesting to depleted mantle source. Zircon U Pb ages of 2602.42 ± 0.82 Ma for peridotite and 2479–2684 Ma for gabbro represent the magma emplacement time and Neoarchean-early Paleoproterozoic melt activity. The highly variable ages from zircon grains in the studied samples might suggest multiple stages of metasomatism and melt-rock interaction in a long-lived sub-continental lithospheric mantle. The tectonic evolution can be equated with Neoarchean global subduction-accretion-collision and arc-continent amalgamation events. This study deciphers a 2.6–2.5 Ga Neoarchean oceanic crust formation and magmatic response to subduction initiation, arc maturation and accretion processes correlatable with global crustal growth processes and amalgamation of western Dharwar Craton (WDC) and Antongil-Masora blocks into the Greater Dharwar Craton (GDC). • Ultramafic-mafic cumulate sequence from Goa represents dismembered SSZ oceanic crust. • Depleted to enriched melt composition reflects mantle heterogeneity. • Paleo-ocean closure and arc accretion culminated to GDC formation. • 2.6–2.5 Ga age coincides with Neoarchean global subduction-accretion event.

Petrogenesis of Early Cretaceous alkaline basalts in the West Qinling: Constraints from olivine chemistry

Olivine crystals in alkaline basalts preserve important information about the Oxygen fugacity and thermal condition of the mantle‐derived magma. This paper presents new olivine mineral chemistry and whole‐rock geochemistry for Early‐Cretaceous alkali basalts in the Ganjia area, northeastern margin of the Tibetan Plateau. The olivine phenocrysts of Ganjia basalts have low Ni (74–2261 ppm) and high Ca contents (1217–4443 ppm), similar to the olivines in MORB, suggesting that they were crystallized from peridotite‐derived basaltic magma. Al‐in‐olivine/spinel thermometer results indicate crystallization temperatures of olivine around 1202 ± 46°C, corresponding to a mantle potential temperature of 1279 ± 47°C, with low oxygen fugacity of FMQ −1 to FMQ 0, suggesting a reduced mantle source. Whole‐rock geochemical compositions of the Ganjia basalts were characterized by low SiO2 contents (45.39%–48.70 wt%), high alkali contents (3.88–5.11 wt%), positive Nb and Ta anomalies, and enrichment in LREEs, HFSEs and LILEs, similar to those of OIB rocks, with high ratios of (Dy/Yb)N (1.38–1.54) and (La/Yb)N (5.37–7.42) ratios, indicating garnet residue. The above results suggest that the Early Cretaceous intraplate alkaline basalts in West Qinling were formed by a low degree of partial melting of peridotite and that no high‐temperature mantle plume existed. The Qinling Orogenic Belt may have been affected by the subduction of the Palaeo‐Pacific. We propose that the Ganjia basalts originated from the interaction between the upwelling asthenosphere and a lithospheric source.

Magmatic records of Gondwana assembly and break-up in the eastern Himalayan syntaxis, northeast India

The Indian sub-continent was an integral part of the Gondwana supercontinent with multiple magmatic episodes during the Gondwana assembly and break-up events. However, most of these vital records to understand the past magmatism were obliterated during the Himalayan orogeny due to the subduction of the Indian plate. In this contribution, we attempt to tackle this issue by investigating the Abor magmatic rocks from the eastern Himalayan syntaxis, which are likely to represent the leftover fragments of the eastern Gondwana continental margin. This study uses zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopic ratios data of the mafic intrusive and felsic volcanic rocks of the Abor magmatism. The mafic intrusive rocks have zircon ages of 500–473 Ma, while the felsic rocks yield ages of 145–132 Ma, indicating two temporally separated episodes of magmatism. The mafic intrusives are sub-alkaline/tholeiitic (Nb/Y < 0.65), with high TiO2 (1.63–3.42 wt%) and ocean island basalt to enriched-mid oceanic ridge basalt affinities. A relatively narrow range of initial 87Sr/86Sr (0.703887–0.705513), 143Nd/144Nd (0.511978–0.512118), and εNd(t) (-0.323–+2.43) of the mafic intrusives suggest fractional crystallization with negligible crustal contamination, generated by low degree (∼3–13 %) partial melting of a primitive mantle (garnet and spinel lherzolite). The felsic rocks display low MgO (0.38–1.17 wt%), CaO (1.06–5.31 wt%), LREE and LILE (Rb, K, Pb) enrichment, depletion in HREE, Sr, Nb, Ti, with strong negative Eu-anomaly (0.48–0.73), high initial 87Sr/86Sr (0.707878–0.717650), and negative εNd(t) (-14.35 to −9.21), suggesting A-type felsic magmatism. The older mafic intrusions were thus attributed to the events of the Gondwana assembly and were inferred to form in an extensional passive margin during the early Paleozoic. However, the younger felsic rocks were likely to have been generated by the interaction of the upwelling Kerguelen mantle plume and the pre-existing crust during the initiation of the eastern Gondwana break-up during the early Cretaceous. Our new findings reveal that two episodic magmatic events related to the eastern Gondwana assembly and the subsequent Gondwana break-up are responsible for the magmatism in the Siang window of eastern Himalayan syntaxis, northeast India.

Tectonic switch of the north Yangtze Craton at ca. 2.0 Ga: Implications for its position in Columbia supercontinent

Two episodes of magmatism with zircon U-Pb ages of 2.46 Ga and 2.0 Ga were studied in the Jinpan Complex in the north Yangtze Craton of South China. The ca. 2.46 Ga gneissic granites with various chemical compositions were formed by remelting of different crustal materials in a collisional event. The occurrence of both negative and positive whole-rock εNd(t) and εHf(t) values in these rocks indicates the growth of juvenile crust and reworking of Archean crust during early Paleoproterozoic. The ca. 2.0 Ga trondhjemites have high (La/Yb)N and Sr/Y ratios as well as negative whole-rock εNd(t), εHf(t) and zircon εHf(t) values, suggesting that they were formed by remelting of thickened lower crust in a collisional orogen. The ca. 2.0 Ga A-type granites have high Zr, Nb, Y contents and Ga/Al, Y/Nb ratios, belonging to A2 type granite. Their negative whole-rock εNd(t), εHf(t) and zircon εHf(t) values are similar to those of the ca. 2.0 Ga trondhjemites. These evidences suggest that they were produced by remelting of the residue after extraction of earlier granitic magma in an extensional tectonic setting. The 2.0 Ga mafic dykes show significant enrichment in LILEs but weak negative Nb, Ta anomalies and various Nb/U ratios. This is consistent with a parent magma similar to continental flood basalts and experienced different degrees of crust contamination. The co-occurrence of 2.0 Ga trondhjemites, A-type granites and mafic dykes implies a tectonic switch from convergence to extension at ca. 2.0 Ga in the north Yangtze Craton. Based on the similarities of the ca. 2.0 Ga magmatic rock association and metamorphic records in the north Yangtze Craton and south Siberia, the position of the Yangtze Craton in Columbia supercontinent is proposed to be close to south Siberia.

Formation of Late Paleoproterozoic Gaositai Hornblendite in Northern North China Craton: Evidence from Zircon U-Pb Isotopes and Amphibole Trace Elements

Paleoproterozoic tectonic evolution of the northern North China Craton has been a hot research topic. We firstly identified a 1.85 Ga hornblendite from the Gaositai mafic–ultramafic complex, in northern Hebei. Systematic studies of petrology, zircon U-Pb geochronology, and in situ mineral major and trace elements of hornblendite are the key to revealing the petrogenesis of the Paleoproterozoic ultramafic rock and the tectonic evolution of northern North China Craton. LA-ICP-MS zircon U-Pb dating suggests the Gaositai hornblendite formed at 1851 ± 44 Ma. The late Paleoproterozoic ultramafic rocks, together with coeval post-collisional granites, formed a bimodal igneous assemblage. Both hornblende and its equilibrium melt compositions show strongly fractionated HREE patterns, relative enrichments in LREEs and LILEs, and depletions in HREEs and HFSEs. The phlogopite-bearing hornblendite magma could have originated from a hydrous garnet-facies mantle source metasomatized by slab-derived silicate melt. Furthermore, the variations of major and trace elements in hornblende from core to rim also reveal the mineral fractional crystallization and magma recharge. Zircon trace elements, melt composition equilibrium with hornblendes, and the bimodal igneous assemblage suggest that the generation of the Gaositai Paleoproterozoic hornblendite was likely the product of post-collisional extension related to the collision between eastern and western North China blocks.

Petro-geochemical features of the Bajocian island-arc volcanism in the Lesser Caucasus (Azerbaijan)

&#x0D; &#x0D; &#x0D; This article discusses the petro-geochemical features of the Bajocian volcanism in the Azerbaijan Lesser Caucasus. Jurassic volcanism, manifested with varying intensity in the Lok-Karabakh zone, took place in various geodynamic settings, depending on the alternation of extension and compression processes in the island arc. Under these conditions, in the Lesser Caucasus during Middle Jurassic Epoch, two volcanic associations were formed: the Bajocian basalt-rhyolite and the basalt-andesite-dacite-rhyolite of the Bathonian age. It was found that the volcanic rocks of the Lower Bajocian complex belong to the tholeiitic series, and the Upper Bajocian rocks belong to the calc-alkaline series. In the rocks of the association, light REE slightly prevail over heavy ones and form almost flat spectra, the normalized plots are characterized by the chondritic nature of the distribution of rare earth elements, and the lines are parallel to the spectrum of the distribution of rare earth elements in MORB. In such rocks particularly the europium ratio (Eu / Eu * = 0.81–1.21) approaches 1 and low La / Yb ratios are observed. In some samples of more basic rocks, the content of heavy REE increases. Such a distribution of REE in the studied volcanic rocks is common for basic rocks of the tholeiitic series in typical island arcs. In the analyzed single rhyolite sample, a negative Eu anomaly is observed (Eu / Eu * = 0.56). The volcanic rocks on the primitive mantle normalized multi-element plots are characterized by depletion in Ta, Nb and enrichment in LILE (Rb, Ba, Pb, U, Th), which is characteristic of island arc-related volcanic rocks of supra-subduction zones (SSZ). The rocks are also depleted in titanium, potassium, and phosphorus. It was concluded that, in terms of geochemical features, the Middle Jurassic igneous rocks were formed at the ensimatic island arc, which was the initial stage of the development of the island arc tectonic setting, replaced in the Upper Jurassic by ensialic subduction.&#x0D; &#x0D; &#x0D;

Geochemistry of arc alkaline magmatism of Java Island, Sunda Arc: a statistical review

Alkaline magmatism develops in the rear arc area of Java, Sunda Arc, at different range of slab depth; ~270-580 km in central section and ~150 km in the east. We collate published geochemical data of volcanic rocks from four alkaline volcanoes (Muria, Lasem, Bawean, and Ringgit-Beser) and perform statistical analysis to evaluate geochemical characteristics of each suite. A set of major and trace elements is scaled and transformed using principal component analysis (PCA) and then followed by implementation of k-means algorithm to cluster the data points based on Euclidian distances. K-means clustering of the dataset suggests that Central Java alkalines are most elevated in K2O and total alkali. The algorithm further suggests that Muria samples can be clustered into two, owing to these components. These two clusters, however, are not well reflected on trace element-based clustering. Lasem volcanics show distinct cluster high in Na2O/K2O and SiO2, while Bawean samples are mixed into both Muria clusters. Ringgit-Beser alkalines show two distinct clusters tied to MgO and enrichment in Ba, Rb, and Sr. Our findings suggest that the potassium and LILE enrichment in these alkaline rocks is independent of slab depth and is most likely regulated by tectonic-related arc segmentation in Java subduction zone.

The Bazman and Taftan volcanoes of southern Iran: Implications for along-arc geochemical variation and magma storage conditions above the Makran low-angle subduction zone

Bazman and Taftan are two Late Miocene-Quaternary volcanoes of the Makran continental arc formed by the subduction of Arabian oceanic lithosphere beneath southern Eurasia. This study compares their bulk rock compositions as well as major and trace element contents of major mineral phases. Bazman is composed of a geochemically heterogeneous suite of low to medium-K basic to acidic compositions, while Taftan exposes more homogenous mainly andesitic rocks of medium to high-K affinity. Both volcanoes are characterized by the enrichment of LILE compared to HFSE and depletion of Nb, Ta, and Ti, reminiscent of subduction zone magmatism. At a given SiO2 content, the Taftan volcanics have higher concentrations of incompatible elements and, La/Yb, Th/Yb, and Sr/Y ratios (adakitic signature), here interpreted as being related to the more enriched mantle source, higher contribution of subducted sediments in magma genesis and thicker continental crust beneath Taftan. Thermobarometric calculations based on mineral chemistry (amphibole, orthopyroxene, clinopyroxene, and plagioclase) indicate pre-eruptive T = ∼1050–800 °C, P = 250–100 MPa and H2Omelt > 3.5 wt%. Bazman lavas record slightly lower P and T, and higher H2Omelt, suggesting a shallower magma reservoir and thus a more developed magma plumbing system. Trace element abundances in minerals show consistent distinctions between the two volcanoes, e.g., amphibole is REE-enriched in Taftan over Bazman samples. The features of trace element compositions and zoning patterns of minerals (amphibole, plagioclase and, pyroxene) along with available major element data are suggestive of both recharged magmatic systems and convective self-mixing magma chambers.

Jurassic subduction of the Paleo-Pacific plate in Southeast Asia: New insights from the igneous and sedimentary rocks in West Borneo

The tectonic affinity of West Borneo and its relationship with the East Asia continental margin are key issues for unraveling SE Asia tectonics. New zircon U-Pb and Hf-O isotopes and whole-rock elemental and Sr-Nd-Pb isotopic data of the igneous and associated sedimentary rocks in West Borneo document the development of Early (∼200–185 Ma) and Late (∼160–145 Ma) Jurassic arc-like igneous rocks. In the Kuching Zone and South Schwaner Mountains, the Jurassic mafic-intermediate igneous rocks are characterized by enrichment in LILEs, depletion in HFSEs, εHf(t) = +0.6 – +11.6, δ18O = 4.97–5.61‰, εNd(t) = −2.5∼+2.4, (206Pb/204Pb)i = 18.71–19.37 and (208Pb/204Pb)i = 38.75–39.58. They originated from a sediment-modified mantle wedge source in a supra-subduction zone setting. The Jurassic granitoids shared consistent elemental and Sr-Nd-Pb-Hf-O isotopic signatures with εHf(t) = +1.7∼ +15.6, δ18O = 4.97–6.01‰, εNd(t) = −1.1 ∼ +4.2, with Δ8/4 = 22.5–51.9 and Δ7/4 = 5.8–14.3, arguing for a juvenile crust-dominated origin. Detritus from the associated sedimentary rocks in the Kuching Zone and South Schwaner Mountains was largely derived from the nearby Jurassic magmatic arc. Their detrital zircon age patterns are comparable to those from the East Malaysia-Indochina and Sibu zone, but distinctive from those in SE Borneo. Integrating all data with those from regional surveys, the igneous and sedimentary rocks in the South Schwaner Mountains are suggested to commence in the earliest Jurassic or even Triassic. West Borneo might be the eastward extension of the Triassic Sundaland. The western Kuching Zone and western South Schwaner Mountains represent the southmost part of the active Jurassic Andean-type East Asian continental margin. The westward subduction of the paleo-Pacific plate was initiated around the earliest Jurassic at ∼200 Ma or earlier.