Garnet-spinel Transition Zone Research Articles

Petrogenesis and Tectonic Setting of the Baluogenguole Mafic Dykes, Zongwulong Belt: Implications for Evolution of the Northern East Paleo-Tethys Ocean

The late Paleozoic tectonic setting of the Zongwulong Belt (ZWLB), a significant unit located in the northern Qaidam margin, Qinghai province, remains uncertain. Diabase dykes in the western part of the Zongwulong Belt offer insights into this issue. Field investigations reveal that the dips of the dykes are almost vertical, and they have sharp boundaries with the host rocks. These dykes consist of plagioclase, clinopyroxene, and opaque minerals exhibiting a characteristic porphyritic texture and massive structure. Zircon U-Pb dating of the dykes yields a weighted 206Pb/238U age of 289 ± 1 Ma. The dykes exhibit relatively high concentrations of TFeO, K2O + Na2O, and TiO2, while the SiO2 and MgO concentrations are relatively low. They display relative light-over-heavy REE enrichment, and lack negative Nb-Ta and Eu anomalies. The dykes underwent negligible crustal contamination, and experienced extensive fractional crystallization of olivine, clinopyroxene, and Fe-Ti oxides. Originating from the spinel–garnet transition zone at depths of approximately 75 km, the dykes result from garnet facies low-degree melting (5%–10%) in a continental rift setting. Combining these findings with regional geological data, we propose that the ZWLB likely experienced a continental rift in the west and exhibited a narrow oceanic environment in the east in the late Paleozoic period, potentially representing the most distant north branch of the East Paleo-Tethys Ocean.

Geochemistry of Precambrian dyke swarms in the Singhbhum craton, India: Implications for recycled crustal components in the mantle source

The Singhbhum craton, eastern India records multiple stages of emplacement of Precambrian dyke swarms with contrasting petrogenetic models proposed for their formation. In this study, we document elemental and Sr-Nd isotopic data for three major dyke swarms in the southern part of the craton, including the ca. 2.7 Ga Ghatgaon dyke swarm, the Early Proterozoic Keonjhar dyke swarm and the ca. 1.76 Ga Pipilia dyke swarm. Dyke compositions are dominated by basalt and basaltic andesite with minor andesite, showing trace element signatures typical of continental crustal rocks. Age-corrected Nd isotopic data for Ghatgaon (εNdt = −4.8 to + 4.6), Keonjhar (εNdt = −11.9 to + 3.8), and Pipilia (a single sample with εNdt = −8.8) dyke swarms display substantial variations. The lack of magma compositions that could indicate the presence of elevated mantle potential temperature among the rocks suggests melting regime was likely similar to the ambient mantle. The Dy/Yb and Dy/Dy* systematics of the rocks indicates melting occurred between spinel-stable depths and the spinel-garnet transition zone. The dominantly mafic compositions of the rocks and ubiquitous continental crustal trace element signature are best explained by peridotite source with recycled crustal components, probably in the form of pyroxenites. Our new Nd isotopic data, which argue against any simple secular evolution trend invoked in previous studies, indicate that crustal recycling was likely an episodic phenomenon rather than a discrete, single-stage process since the Archean. Geochemical modelling indicates that a sublithospheric mantle source with (10% or less) recycled crustal components satisfactorily explains the trace element variations of the dyke swarms.

Petrogenesis and tectonic affinity of Early Cretaceous potassic diorites in the northern Taihang Mountain, Trans-North China Orogen

• Early Cretaceous potassic diorites are identified within the central Trans-North China Orogen. • Lithospheric delamination and thermo-mechanical erosion explain lithospheric structures during the Early Cretaceous in the North China Craton. • Early Cretaceous magmatism in the study area was linked to slab sinking and rollback. • The subducted Izanagi plate may be a candidate for carbonated peridotite at ∼140 Ma. Mesozoic mantle-derived potassic magmatic rocks in the Trans-North China Orogen (TNCO) are important for constraining tectonic evolution. However, potassic intermediate plutons in this orogen are rarely reported in literature, despite the significance of such a potassic magmatic event being well understood. Here, we describe the petrography of the Longmen potassic diorites and present zircon U–Pb ages and elemental and Sr–Nd–Mg isotopic compositions for this pluton. The Longmen potassic diorites were emplaced during the Early Cretaceous (ca. 140 Ma), showing shoshonite features with marked depletion of high-field strength elements relative to large ion lithophile elements with a slightly negative Eu anomaly. It possesses restricted ranges of initial 87 Sr/ 86 Sr ratios (0.70592 to 0.70667), ε Nd (t) values (−15.86 to −13.26), and low δ 26 Mg (−0.37 to −0.26‰). Low-degree partial melting of the ancient and enriched mantle, characterised by amphibole-bearing lherzolite in the spinel–garnet transition zone, was the likely mantle source of the evaluated Longmen potassic diorites, resulting from fluid-related metasomatism and upwelling mantle (carbonated) peridotite. Minor crustal materials were present in the lithospheric mantle because of Eastern Block and Western Block subduction/collision during the Paleoproterozoic. Based on our data and previous studies, slab sinking and rollback of the Izanagi plate (Paleo-Pacific plate) may have created an ancient, enriched mantle beneath the East Asian continent, inducing lithospheric thinning and heterogeneity of the lithospheric mantle in the North China Craton and lithospheric extension via thermo-mechanical erosion at 145–110 Ma in the TNCO.

Petrogenesis and tectonic implications of the Late Triassic intrusions in the North China Craton: Case study on the Huata complex in the western Yanshan

This paper presents geochemical and age data for Late Triassic lamprophyre, syenite, and granite in the Huata area of the western Yanshan belt of the North China Craton, to provide constraints on the origin and tectonic settings of these Late Triassic igneous rocks. The zircons from the Huata lamprophyres and syenites yielded a mean U Pb age of 203 and 206 Ma, respectively. The lamprophyres are enriched in incompatible elements, with (La/Yb) N = 30.57–33.89, (Gd/Yb) N = 3.54–3.64, and Sr-Nd-Hf isotopic signatures ( 87 Sr/ 86 Sr (initial) = 0.7053–0.7061, εNd(t) = −11.11 to −10.50, εHf(t) = −9.33 to −7.70), consistent with them originating from the low-degree melting of previously metasomatized lithospheric mantle in the garnet-stable area. The syenites display similar incompatible elements to the lamprophyres, but have relatively low (La/Yb) N = 14.26–17.23 and (Gd/Yb) N = 1.81–2.15 ratios. Their Sr-Nd-Hf isotopic compositions ( 87 Sr/ 86 Sr (initial) = 0.7045–0.7050, εNd(t) = −12.10 to −11.35 and εHf(t) = −8.64 to −6.81) were also similar to the lamprophyres, and they are considered to have been generated from the low-degree melting of lithospheric mantle in the garnet-spinel transition zone. The granites have major and trace element patterns and Sr Nd isotopes quite different from the syenites, but are similar to the crust-derived felsic rocks, being enriched in large ion lithosphere elements and depleted in high field strength elements. They are enriched in Nd isotope (εNd(t) = −16.63 to −15.18), and are inferred to have been generated by lower crust anataxis. The presence of igneous rocks in the Huata district indicates that the northern margin of the NCC had already entered a non-orogenic regime by the late Late-Triassic. • Lamprophyres and syenites derive from a metasomatized mantle. • Granites originate from the modified lower continental crust. • The tectonic regime transformed from post-collisional to intraplate at 206 Ma.

Petrogenesis and geodynamic mechanisms of the Late Cretaceous magmatic ‘flare-up’ in the southern Lhasa Terrane, Tibet

Exploring magmatic ‘flare-up’ events can be used to unravel dynamic Earth processes including plate subduction and crustal growth. However, the cause and mechanism of magmatic ‘flare-up’ remain unresolved. The abundant magmatic records that are preserved in the Lhasa Terrane on the southern Tibetan Plateau make it an important region to study the episodic nature of the secular magmatism. This paper presents geochronological, mineralogical, and geochemical data that describes the Dazhuka diorites in the central-western and Sangri granites in the eastern parts of the southern Lhasa Terrane. Zircon LA–ICP–MS U Pb data indicate that these rocks were generated around ~95–90 Ma. Both are enriched in large ion lithophile elements and depleted in high field strength elements, indicating geochemical affinity with subduction-related magmatic rocks. The Dazhuka diorites have low SiO 2 contents (54.49–56.72 wt%), with moderate Mg# (41.7–42.7) and rare earth element fractionation ((La/Yb) N = 6.9–8.6), and homogeneous whole-rock 87 Sr/ 86 Sr i (0.704204–0.704480), ε Nd (t) (+3.5 − +4.1), and zircon ε Hf (t) (+10.2 − +13.3). These features suggest that the Dazhuka diorites originated in the spinel-garnet transition zone within the lithospheric mantle of a mantle wedge environment. The Sangri granites have high SiO 2 (64.36–69.54 wt%), Al 2 O 3 (14.27–15.61 wt%), and (La/Yb) N (4.6–49.6) and Sr/Y (34.7–119.3) ratios, indicating geochemical affinity with adakites. Low MgO (1.3–2.22 wt%) and depleted Sr-Nd-Hf isotopes (whole-rock 87 Sr/ 86 Sr i = 0.704099–0.705453, ε Nd (t) = +2.9 − +3.5, zircon ε Hf (t) = +7.1 − +9.3) suggest that the granites were derived from oceanic crust. The types of magmatic rocks that are distributed over the central-western and the eastern parts of the southern Lhasa Terrane differ, with higher magmatic temperatures in the east as compared to the central-west. These results indicate that different areas in the southern Lhasa Terrane may have experienced different geodynamic processes, with oceanic ridge subduction in the eastern part of the southern Lhasa Terrane and normal oceanic plate subduction in the central-western part. These different types of subduction processes are likely to have caused the magmatic ‘flare-up’ and variation in the magmatism of southern Tibet. • Both Dazhuka diorites and Sangri granites formed in early Late Cretaceous. • Dazhuka diorites were derived from the lithospheric mantle. • Sangri granites with adakitic affinity were sourced from the oceanic crust. • Multiply Neo-Tethys Ocean plate subduction processes result in magmatic ‘flare-up’.

Petrogenesis of the Yeonhwa ultrapotassic intrusions in the Yeongnam Massif—Evidence for enrichment of the Triassic continental lithospheric mantle beneath the Korean peninsula

The Triassic period is when abundant late-orogenic magmatism occurred in the Korean peninsula, but the characteristics of the lithospheric mantle at that time beneath the Yeongnam Massif have not been well-defined because of the scarcity of mafic intrusions. We report zircon U Pb and phlogopite 40 Ar/ 39 Ar ages, petrography, and geochemistry of the Triassic (227.6 Ma) ultrapotassic igneous rocks in the Yeonhwa I Mine (Yeonhwa Ultrapotassic Intrusions, YUI) emplaced into the Taebaeksan Basin in the Yeongnam Massif. The YUI occur as two stocks with different lithologies: phlogopite-rich and amphibole-rich, where the former underwent carbonate-melt interaction along the margin in contact with the carbonate formation. The two lithologies share similar compositional and isotopic characteristics; however, the phlogopite-rich YUI better retains its mantle-derived features, having idiomorphic high-Mg# (up to 91.9) phlogopite and diopside phenocrysts and bulk concentrations of high MgO (Mg# = molar 100 Mg/[Mg + Fe] > 70), Cr (208–1150 ppm), and Ni (48–219 ppm). The enrichment of the YUI in H 2 O (phlogopite-rich), silica contents (50.2–56.9 wt%), and large ion lithophile elements (LILE; K, Rb, Sr, Ba, Pb), together with depletion of high fields strength elements (HFSE; Ti, Zr, Nb, Hf, Ta), are typical of the ultrapotassic rocks in orogenic settings. The elemental and isotopic data imply that the YUI originated from partial melts of the phlogopite-bearing lithospheric mantle near the spinel-garnet transition zone. The extremely enriched isotopic ratios ( 87 Sr/ 86 Sr(t) = 0.70756 to 0.71227, ε Nd (t) = −8.82 to −11.0) suggest a highly enriched mantle source, possibly more enriched than the contemporary pelagic sediments. Considering that the Yeongnam Massif was in a subduction system during Permian-Triassic, not a continental collisional belt, the enrichment was inherited from a combination of both young, subduction-related and ancient, pre-existing metasomes in the continental lithospheric mantle. Our data confirm the presence of the enriched, volatile-rich lithospheric mantle beneath the Triassic Yeongnam Massif. This enriched origin contrasts with earlier ( c . 240–280 Ma) Yeongnam arc plutons that originated from the depleted asthenospheric mantle (ε Nd (t) ≥ +2). Since the enrichment is also characteristic of most of the c . 220–240 Ma igneous rocks around the Korean peninsula, such lithospheric mantle lithology was widespread during the Late Triassic. • The Yeonhwa Ultrapotassic Intrusions (YUI) intruded into the Yeongnam Massif at around 227.6 Ma. • Reaction with country rocks developed metasomatized margins of the intrusion. • The YUI were derived from the phlogopite-bearing sub-continental lithospheric mantle. • The Triassic Yeongnam Massif had the old lithospheric mantle further modified by the Permina-Triassic subduction zone.

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.

A new ca. 1.73 Ga mafic magmatic event in the Indian Shield: Evidence from an in-situ SIMS U-Pb baddeleyite date and geochemistry of the mafic intrusions within the Gwalior basin, Bundelkhand craton

A suite of mafic sill intrusions is present in the Gwalior sedimentary basin of the Indian Shield, and is herein dated. An obtained in-situ SIMS U-Pb baddeleyite date of 1733 ± 23 Ma, represents a new mafic magmatic event in the Bundelkhand craton, in particular, and in the Indian Shield, in general. Gwalior mafic sill samples are typically doleritic in nature and show ophitic or sub–ophitic textures. Their geochemical characteristics classify them as sub-alkaline high-iron tholeiitic basalts. Geochemistry suggests emplacement of these mafic sills in a within-plate setting and derived from a melt originated from a subduction-zone-modified lithospheric mantle (SZLM) source in spinel-garnet transition zone. Mafic magmatic/LIP events of the same age (i.e., ca. 1.73 Ga) are known only from the North China, North Australia and Siberian cratons. However, the date uncertainties would allow matches over the range of 1756–1710 Ma, and additional blocks that could be relevant include Laurentia, West Africa, East Antarctica, and Sao Francisco cratons. Based on the new U-Pb Gwalior sill age and other available ages on the basement rocks and sedimentary units, it is suggested that the Gwalior sediments were deposited between 1980 and 1710 Ma.

Quaternary Collision-Zone Magmatism of the Greater Caucasus

Abstract The Greater Caucasus mountains (Cavcasioni) mark the northern margin of the Arabia–Eurasia collision zone. Magmatism in the central part of the Greater Caucasus began in the Pleistocene, up to ~25 Myr after initial collision. This paper presents bulk-rock and Sr–Nd–Pb isotope geochemistry from 39 Quaternary volcanic rock samples (<450 Ka) recovered from the Mt. Kazbek (Kasbegui) region of the Greater Caucasus, Georgia, to assess the sources and magmatic evolution of these lavas and the possible triggers for melting in the context of their regional tectonics. Compositions are dominantly calc-alkaline basaltic andesite to dacite (57–67 wt % SiO2). Although the lavas were erupted through thick continental crust, there is little evidence for extensive modification by crustal contamination. Trace element and isotopic systematics indicate that the lavas have supra-subduction zone signatures, most likely reflecting derivation from a lithospheric source that had been modified by melts and/or fluids from material subducted before and during the collisional event. Mass-balance modelling of the Sr–Nd isotope data indicates that the lavas require significant input from a subducted slab, with deep-sourced fluids fluxing the slab into the source region. In contrast with published data from Lesser Caucasus magmatism, data from the Mt. Kazbek region suggest that a compositionally distinct sediment source resides beneath the Greater Caucasus, producing characteristic trace element and Pb isotopic signatures. Two distinct compositional groups and therefore primary liquids can be discerned from the various volcanic centres, both derived from light rare-earth element enriched sources, but with distinct differences in Th/Yb and Dy/Yb ratios and Pb isotopes. Rare-earth element modelling of the lava sources is consistent with 3–4% melting starting in the garnet peridotite and continuing into the spinel facies or, potentially, sited in the garnet-spinel transition zone. Small-scale convection related to mantle upwelling provides a plausible mechanism for Greater Caucasus magmatism and explains the random aspect to the distribution of magmatism across the Arabia–Eurasia collision zone.

Geochemical composition of dykes along the Cameroon Line (CL): Petrogenesis and similarities with the Central Atlantic Magmatic Province

We report whole-rock geochemistry and Sr–Nd–Pb isotopic compositions of mafic dykes intruded in the Precambrian granito-gneissic basement complex, exposed at Nyos, Batibo, Dschang and Foumban on the Cameroon Line. The dykes are alkaline (Batibo), transitional (Foumban), and subalkaline (Nyos, Batibo and Dschang) with SiO2 of 45–54 wt% and MgO of 2–9 wt%, similar to dykes reported in other areas of the Cameroon Line (CL) and the Central Atlantic Magmatic Province (CAMP). The abundances of rare earth elements (REE) and the Primitive Mantle normalised patterns for the Nyos, Batibo and Dschang dykes are similar to those of MORB, indicating that the dykes formed at shallower depths by a higher degree of partial melting relative to the Foumban dykes and the alkaline lavas of the CL. The transitional basaltic dykes with steeper REE patterns have their sources at deeper levels in the lithospheric mantle, possibly the garnet-spinel transition zone and were generated by a lower degree partial melting of the lithospheric and plume components. The Nyos and Batibo subalkaline dykes show similar isotopic compositions with a spectrum extending from depleted (DMM-like) to enriched (EM1-like) mantle, indicating the similarity in their source components. The Dschang dykes show distinct isotopic characteristics with relatively unradiogenic Nd-Pb isotope compositions compared to the Batibo and Nyos dykes. The Foumban transitional dykes with characteristic wide ranges in Sr-Nd-Pb isotopic compositions reveal varying contributions from enriched mantle components (EM1 and EM2) in addition to its plume signature similar to those of CL lavas. The Nyos and Batibo dykes alongside other dykes on the CL have low TiO2 abundances (<2 wt%), negative PM-normalised Nb-anomalies, and moderately to strongly enriched REE patterns, and isotopic composition that overlaps with those of CAMP, suggesting a similar lithospheric origin.

Geochemistry of Late Permian basalts from boreholes in the Sichuan Basin, SW China: Implications for an extension of the Emeishan large igneous province

The emplacement of the Late Permian Emeishan large igneous province (ELIP) in the Upper Yangtze craton, SW China, is considered to be a crucial factor that triggered the end-Guadalupian biodiversity crisis. However, the contribution of ELIP remains debated because of its relatively modest known size. The restricted assessment of its areal extent is, in part, due to the lack of study of the potential volcanic rocks concealed beneath younger strata. Here we present the first geochemical and isotopic variations and detailed spatial distribution of Late Permian basaltic rocks penetrated by deep exploratory boreholes in the Sichuan Basin. The Late Permian basaltic rocks are mainly distributed in the western and northeastern parts of the Sichuan Basin and are positioned at the same stratigraphic level as the Emeishan flood basalts. Petrography, whole-rock trace elements and Sr-Nd-Pb isotopic compositions demonstrate that these rocks are alkaline basalts and have oceanic island basalt - like characteristics, indicating a similar origin as the Emeishan flood basalts. The trace element and isotope ratios suggest that the basaltic magmas in the western Sichuan Basin were derived by low degrees of partial melting of a garnet-bearing source. In contrast, the basalts in the northeastern part of the basin were generated by higher degrees of melting of a shallower mantle source in the spinel-garnet transition zone. Varying amounts of contamination by lower continental crustal melts are suggested by rough correlations between isotope and immobile trace element ratios. The spatial changes in composition and origin of the basaltic rocks in the Sichuan Basin can be interpreted as the result of varying extents of Emeishan mantle plume-lithosphere interaction under different lithospheric stress backgrounds, such that the reactivation of pre-existing basement faults controlled the emplacement of basaltic magma in the periphery of the ELIP. Our work reveals a substantially broader extent of the ELIP in buried sedimentary strata, which might have contributed to the abrupt end-Guadalupian climatic change.

Kersantites and associated intrusives from the type locality (Kersanton), Variscan Belt of Western Armorica (France)

Kersantite is the international designation for hypabyssal lamprophyres essentially made up of dark micas and plagioclase. Kersantites from the type locality (Kersanton hamlet) and the W-Armorican surrounding area are Serpukhovian/Bashkirian in age. They are systematically associated in the field with hypabyssal microgranodiorites, both types being intruded synchronously as syn-compressional magmatic bodies that postdated pre-tectonic Tournaisian/Visean dolerites. The kersantitic amphibole compositions can be used to constrain the crystallization pressures and oxygen fugacities (redox conditions much more oxidizing than the Ni-NiO buffer). The oscillatory zoning observed in clinopyroxene phenocysts is consistent with a magma chamber repeatedly refilled by mafic liquid. The melting source of the W-Armorican kersantites corresponded to a lithospheric mantle located in the spinel-garnet transition zone (75–85 km depth), highly metasomatized by fluids probably released by a subducted sediment-bearing slab. Microgranodiorites could result from intimate mixing between mantle- and crust-derived magmas, with cordierite as possibly melting phase. The emplacement of dolerites with anorogenic geochemical signature prior to kersantites/microgranodiorites implies upwelling of the asthenospheric mantle up to a relatively high level, probably as the result of an extension induced by either roll back of the subducted lithospheric slab or delamination of the overriding plate. The heat input associated to the long-lasted asthenosphere rise could have favored (1) the formation of early dolerites between 347 and 330 Ma; and then, (2) the genesis of syn-compressional kersantites and microgranodiorites between 330 and 310 Ma, once the overlying lithospheric mantle was metasomatized by crust-derived fluids. This magmatic sequence marked the beginning of the Avalonia-Armorica collision, just prior to the main Variscan compressional event in this zone at c. 315 Ma.

Geochemistry of 2.21 Ga giant radiating dyke swarm from the Western Dharwar Craton, India: Implications for petrogenesis and tectonic evolution

The Western Dharwar Craton (WDC) exhibits numerous exposures of NNW–SSE to NW–SE trending mafic dykes. Geochemistry of four 2.21 Ga NNW–NW trending dykes and 21 other similar trending dykes provides new insights into the geochemical behaviour, mantle source characteristics, and tectonic history of mafic magmatic event, which have also affected the Eastern Dharwar Craton (EDC). Based on modal mineralogy and texture, these dykes can be classified into dolerite, olivine dolerite, gabbro, and olivine gabbro. They show basaltic to basaltic‐andesitic composition and have sub‐alkaline tholeiitic nature. LaN/LuN ratio classifies the dykes into two groups: Group I, with values close to 2, shows relatively flat REE patterns while, Group II, with values 4–6, has inclined REE patterns. In general, both groups show LREE enrichment and most of the samples exhibit negative Nb–Ta and Ti anomalies, indicating crustal contamination. However, negative Zr–Hf anomaly and low Th/Nb values (0.14–0.73) noticed within the samples preclude significant crustal inputs. Petrogenetic modelling using batch melting equation suggests that the dykes fractionated from two distinct mantle melts and had slightly different sources, both within the spinel–garnet transition zone. The overall composition of these 2.21 Ga dykes is consistent with a mantle plume‐induced heterogeneous source that was previously modified by some ancient subduction event. Coeval dykes from EDC exhibit similar geochemical behaviour, mantle source characteristics, and tectonic history. These and other synchronous dykes from distant cratons, viz. Superior, Slave, Greenland, represent a 2.21 Ga large igneous province event, which is linked with the Sclavia/Superia supercraton.

Lithospheric modification at the onset of the destruction of the North China Craton: Evidence from Late Triassic mafic dykes

Mantle-derived magmatism provides important insights for understanding the mechanism of lithospheric thinning. Here we report the results of an integrated geochronological and geochemical study of Late Triassic mafic dykes in Eastern Hebei, northern North China Craton. In situ zircon U-Pb dating shows that the dykes were emplaced between 238 and 223 Ma; the coeval Gaojiadian and Mataizi dykes intruded Precambrian basement at 238–234 Ma and the Saheqiao dyke was emplaced into Neoarchean supracrustal rocks later at 223 ± 4 Ma (2s). Bulk-rock geochemistry indicates that the Late Triassic dykes in Eastern Hebei were produced by melting of ancient lithospheric mantle within the garnet-spinel transition zone (~70–80 km), heated by upwelling asthenosphere. This ancient lithospheric mantle had been metasomatized during previous subduction events. The Gaojiadian and Mataizi dykes resulted from higher degrees of partial melting at slightly lower pressures than the Saheqiao dyke. The melting depth of Late Triassic dykes in Eastern Hebei indicates that the intact ancient lithospheric mantle had been at least locally modified/thinned to ~70–80 km by the Late Triassic. The intrusion of these Late Triassic dykes took place at the onset of the lithospheric thinning of the North China Craton, caused by post-collisional extension after subduction and collision of neighboring blocks with the North China Craton.

Geochemistry of NW-SE trending Palaeoproterozoic mafic dyke intrusions in the Bundelkhand Craton, India and subcontinental lithospheric mantle processes

NW-SE trending mafic dykes in the Bundelkhand Craton of the Indian shield manifest Palaeoproterozoic igneous activity. These dykes are Fe-rich tholeiitic basalts with compositions varying from near primary melts to more evolved magmas (Mg#: 0.64–0.35) and show enrichment of large-ion lithophile and light rare earth elements relative to primordial mantle values. Despite very subtle variations of elemental abundances and similar structural trend, at least two groups are identified, mainly based on incompatible element ratios and rare earth element patterns. Group 1 samples are characterized by relatively low Ti/Y (av. 222 ± 22) and Ti/Nb (av. 609 ± 108), high Zr/TiO2 (av. 117 ± 15), more light to heavy rare earth element fractionation (La/Yb)CN: 2.7–5.1) and mild negative Eu anomaly (Eu* = 0.77 ± 0.10). Group 2 samples possess high Ti/Y (av. 334 ± 46) and Ti/Nb (av. 1349 ± 198), low Zr/TiO2 (av. 70 ± 7) and minor light to heavy rare earth element fractionation ((La/Yb)CN: 1.5–2.7) without any Eu anomaly (Eu* = 0.97 ± 0.04). Both groups show relative depletions in Nb, Sr and P while the Group 1 also shows Ti depletion. A third group is apparent but is less certain. Group 2 dykes constitute the 1.98 Ga Jhansi swarm, whilst the Group 1 and unclassified samples are likely to be of older age (~2.18–2.20 Ga and ~2.37 Ga) based on available U-Pb ages and palaeomagnetic considerations. Despite the compositions indicate different batches of magma, the dykes have near similar petrogenetic evolutionary pattern as if the entire population constitutes a single clan. Samples of both groups (a) do not indicate significant crustal contamination (b) derived from two different batches of magmas formed by ~10–12% fractional melting of mantle near the spinel-garnet transition zone and (c) evolved through initial fractionation of olivine and thereafter clinopyroxene and plagioclase became important fractionation phases. The more primitive compositions of both groups indicate mantle potential temperatures of ~1550–1600 °C suggesting thermal anomaly. The dyke magmas inherited variably enriched compositions as a result of interaction of upwelling mantle melts with metasomatised subcontinental lithospheric mantle. Low H2O-CO2-rich silicate melts/fluids are likely the dominant metasomatic agents to develop the Indian subcontinental lithosphere in the Archaean (c. 3.0 Ga).

Multiple mantle melting events for two overlapping ca. 2.21-2.18 Ga mafic dyke swarms in the Dharwar craton, India

ABSTRACT Two distinct NW- to WNW-trending mafic dike swarms of the previously dated ca. 2.21 Ga Anantapur-Kunigal and ca. 2.18 Ga Mahbubnagar-Dandeli swarms, emplaced in the Dharwar craton, are studied in greater petrological detail. Petrography classifies these as dolerite, olivine-dolerite, and meta-dolerite. The bulk rock geochemistry of 39 new and 11 published samples of both dike swarms are compared to those of three ca. 2.21 Ga and two ca. 2.18 Ga samples previously dated through baddeleyite U-Pb geochronology. All 50 samples are sub-alkaline tholeiite basalt to basaltic andesite in compositions and both swarms have similar differentiation/crystallization trends. Nonetheless, based on the slightly enriched REE patterns of ca. 2.21 Ga samples, as opposed to practically flat ca. 2.18 Ga REE patterns; we propose that 41 samples constitute the ca. 2.21 Ga Anantapur-Kunigal dike swarm and 9 samples belong to the ca. 2.18 Ga Mahbubnagar-Dandeli swarm. Petrogenetic modelling suggests that each swarm was derived through different degrees and proportions of partial melting of both an ambient asthenospheric and metasomatized sub-continental lithospheric mantle (SCLM) and further differentiated through similar fractional crystallization processes. Both mantle melts segregated within the garnet to garnet-spinel transition zone. Geodynamic conditions suggest that dikes of both swarms were emplaced in a similar tectonic system within a single LIP, which was likely triggered through the heating by the same (pulsating?) mantle plume. Such an indirect influence from a mantle plume, not recorded in geochemical signatures, is supported by ~1576°C potential mantle temperature (Tp) estimates for more magnesian parents within both swarms. Available geochronological data on mafic dikes and Palaeopositions of the Dharwar and North American cratons during ca. 2.21–2.18 Ga are well supportive of their link to the Superia supercraton reconstruction.

Controls of mantle source and condition of melt extraction on generation of the picritic lavas from the Emeishan large igneous province, SW China

Like many continental flood basalt (CFB) provinces in the world, the source mantle compositions and melting conditions for generation of the Emeishan CFB are highly debated. We have carried out an integrated Sr-Nd-O-Mg isotopic study of the Emeishan picritic lavas to evaluate possible effects of mantle heterogeneity on the magma composition. Moreover, we have used PRIMELT3 (Herzberg and Asimow, 2015) to calculate the primary magma compositions and melting conditions for these picritic lavas. Picrites from Maoniuping, Tanglanghe and Wuguijing exhibit depleted Sr-Nd isotopic compositions (87Sr/86Sri = 0.7038–0.7050; εNd = 0.27–3.89), normal mantle-like δ26Mg (−0.27 to −0.2‰) and δ18O (5.0–5.4‰), which can be explained by partial melting of plume materials with negligible contribution of sedimentary carbonates. The Wulongba picrites are characterized by depleted Sr-Nd isotopic compositions (87Sr/86Sri = 0.7041–0.7045; εNd = 2.57–2.85), slightly lower δ26Mg (−0.36 to −0.23‰) and higher δ18O (5.6–5.7‰), which were attributed to involvement of minor sedimentary carbonates (magnesite) in their mantle source. The calculated mantle potential temperatures for the Emeishan picrites range from 1516 to 1596 °C. The initial melting pressures range from 4 to 5 GPa. The final melting pressures for the Wulongba picrites (2–3 GPa) are lower than those for the Tanglanghe, Maoniuping and Wuguijing picrites (>3 GPa). This suggests that the Wulongba picrites were generated in shallower garnet-spinel transition zone of the upper mantle. Picrites from other three localities are entirely generated in garnet stability field of the upper mantle.

Geochemical characterization and geodynamic implication of N-trending mafic dyke swarm from the western Dharwar craton and their possible link to the ca. 2.22 Ga large igneous province

Several N-trending mafic dykes are exposed in the western Dharwar craton (WDC) and they are thought to be coeval with the ca. 2.22 Ga N- to NNW-trending Kandlamadugu dyke swarm of the eastern Dharwar craton (EDC). Geo- chemical characterization of these dykes is presented here to understand their genetic aspects and likely correlation with their counterpart in the EDC. Petrographic examinations suggest mineralogical and textural variations from dolerite to metadolerite types. Geochemically they are classified either as sub-alkaline tholeiitic basalt or basaltic andesite. Geochemical variations suggest evolution of mantle melt and demonstrate prominent clinopyroxene fractionation, however, minor role of olivine, orthopyroxene and plagioclase fractionation cannot be discarded at initial stages of crystallization. Fractionation trends of trace elements suggest crystallization of accessory phases like ilmenite, apatite and zircon, at later stages. Although observed geochemical nature suggests a little effect of involvement of crust, however, its role in the genesis of the studied mafic dykes cannot be ignored. Conversely, it is suggested that they are derived from a melt generated in a sub-continental lithospheric mantle (SCLM), which was metasomatized during an ancient subduction event before its cratonization. Based on the petrogenetic models of incompatible trace elements, it is inferred that they were likely to be derived from a melt generated by a lower percentage of melting within the garnet or garnet-spinel transition zone. Their connection to the ca. 2.22 Ga large igneous province (LIP) indicates as an integral part of the ca. 2.22 Ga N- to NNW-trending Kandlamadugu dyke swarm of the EDC. The existence of a mantle plume, substantiated by mantle potential temperature (Tp) estimate, is well-supported by higher thermal regime in the upper mantle. Although there is no direct age data available for the studied mafic dykes, however, their geochemical similarities with the ca. 2.22 Ga Kandlamadugu swarm suggest that they are co-genetic and could be linked to the same event. The likely age correlation of the ca. 2.22 Ga Kandlamadugu swarm with mafic dykes of North Atlantic and Superior cratons, support their link with the Superia supercraton.

Continental extension of northern Gondwana margin in the Eastern Himalaya: Constraints from geochemistry and U–Pb zircon ages of mafic intrusives in the Siang window, Arunachal Himalaya, India

We report new U–Pb zircon age and whole-rock geochemical data from the Pangin mafic intrusive rocks of the Siang window, eastern Himalayas. These mafic rocks are medium to coarse-grained gabbros, consisting mainly of plagioclase and clinopyroxene with accessory phases (hornblende + Fe–Ti oxides) that retain granular and interlocking texture. Geochemically, they display enriched-mid oceanic ridge basalt (E-MORB) affinity characterized by moderate to slightly fractionated REE patterns marked by (La/Yb) N =2.65-3.99. Their geochemical characteristics suggest that the parental magmas of these rocks were formed by medium to higher degrees (∼12–28%) of partial melting similar to that of the asthenospheric mantle in the garnet-spinel transition zone. Magmatic zircons from two gabbros yield U–Pb ages of 521.50 ± 2.53 Ma and 568 ± 2 Ma. This new age reveals two pulses of Late Neoproterozoic and Early Cambrian mafic magmatism that are inconsistent with the temporal distribution of Paleozoic magmatism in the Siang window of the Eastern Himalayas. However, based on the results of this study and the correlation of continental extensional mafic magmatism in the Northwest Himalaya, we suggest that investigated mafic intrusive rocks might have been generated in an extensional tectonic environment during the long-lasting Pan-African orogenic cycle of the late Neoproterozoic to early Cambrian which ended with the formation of the Gondwana supercontinent.

Early Jurassic Mafic Intrusions in the Southern Youjiang Basin, SW China: Petrogenesis, Tectonic and Metallogenic Implications

A suite of mafic intrusions, composed of diabase and micro-gabbro outcrops in the Jingxi area of southern Youjiang Basin, SW China. This study conducts geochronological, geochemical, and Sr–Nd isotopic analyses on the mafic intrusions in Jingxi with the aim of determining their petrogenesis, tectonic setting, and metallogenic implications. Zircon U–Pb dating for the mafic intrusions yielded an age of 183 ± 3 Ma (MSWD = 2.3), which is coeval with the Carlin-like gold mineralization in the Youjiang Basin. The mafic intrusions are alkaline in composition and characterized by low TiO2 (1.25–1.87 wt %) contents and low Ti/Y ratios (410–550). They exhibit OIB-like patterns of trace element distribution and they have low (87Sr/86Sr)i ratios of 0.704341 to 0.705677, slightly negative εNd(t) values of −0.30 to −2.16, low La/Ta (11.57–15.66) and La/Nb (0.77–1.06) ratios, with [La/Yb]N = 6.52–10.63. The geochemical characteristics, combined with regional considerations, suggest that the mafic intrusions originated from partial melting of upwelling asthenosphere within the garnet-spinel transition zone, as a result of intracontinental back-arc extension triggered by the steep subduction of the Paleo-Pacific plate beneath the South China Block. Moreover, the new data not only suggest Early Jurassic magma was a possible heat source, but also support a magmatism-related model for the Carlin-like gold mineralization in the Youjiang Basin.