Back-arc Magmatism Research Articles

Back-arc magmatism of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt: evidence from the Ordovician Nanpingxueshan Nb-enriched gabbros

ABSTRACT The West Kunlun Orogenic Belt (WKOB) in the northwestern Tibetan Plateau records key information about the tectonic evolution of the Proto-Tethys Ocean. To further constrain the Early Palaeozoic tectonic evolution of the WKOB, we report petrography, geochronology, whole-rock geochemistry, and Sr-Nd-Hf-O isotope data of the newly identified Ordovician Nanpingxueshan gabbros in the southern WKOB. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-MC-ICP-MS) U-Pb zircon dating reveals that the gabbroic intrusion was emplaced during 476–471 Ma. Geochemically, the gabbroic rocks exhibit a tholeiitic signature and can be subdivided into two groups, i.e. normal arc-like gabbros (AGs) and Nb-enriched gabbros (NEGs). The AGs have TiO2 (1.20–1.64%), P2O5 (0.18–0.25%), and Nb (4.60–7.64 ppm) contents, fractionated REE patterns ((La/Yb)N = 2.64–4.11), and Nb-Ta depletion (Nb/La = 0.37–0.50). Compared with AGs, the NEGs have higher TiO2 (1.74–2.68%), P2O5 (0.34–0.55%), and Nb (10.79–14.08 ppm) contents and Nb/La ratios of 0.45–0.53, which are similar to typical Nb-enriched arc basalts. Isotopically, the AGs have depleted whole-rock εNd(t) values of 3.22 to 4.04 and negative to positive zircon εHf(t) values of −1.8 to 7.8, while the NEGs exhibit slightly lower whole-rock εNd(t) values of 0.69 to 3.18, zircon εHf(t) values of −7.8 to 4.6, and mostly δ18O values greater than those of normal mantle zircon. Integrated analyses reveal that the AGs and NEGs can be attributed to a similar mantle source metasomatized by different proportions of subducted sediment-derived melt during the subduction process. Our new data, combined with the results of previous studies, suggest that the southward subduction of the Proto-Tethys Ocean under the WKOB initiated at ca. 530 Ma. The long-term southward subduction led to the formation of the massive Early Palaeozoic accretionary complex in the South Kunlun Terrane, while trench retreat led to migration of the arc magmatism northward from the Mazar – Tianshuihai Terrane to the South Kunlun Terrane, as well as back-arc extension in the Mazar – Tianshuihai Terrane.

Late dry back-arc magmatism recorded as dolerite dike intrusion in the Mado Megamullion, Shikoku Basin

Late dry back-arc magmatism recorded as dolerite dike intrusion in the Mado Megamullion, Shikoku Basin

Devonian to Early Carboniferous Retreating—Advancing Subduction Switch in the Northwestern Patagonia Accretionary Orogen: U‐Pb and Lu‐Hf Isotopic Insights

AbstractIn this contribution, we present new early middle Devonian igneous and metaigneous units with a major juvenile magmatic source input in the North Patagonian Massif, which were discovered through U‐Pb and Lu‐Hf zircon analyses. Afterward, we assessed their tectonic implications for northwestern Patagonia and then for southern South America, combining our results with available database information consisting of igneous crystallization ages and isotopic data of the Devonian to early Carboniferous magmatic units, tectonic‐metamorphic analyses, and thermochronologic record. This study allows for distinguishing retreating and advancing subduction switching in northwestern Patagonia (38°30′ to 44°S) and a contrasting coetaneous evolution for basement outcrops exposed further north (27°30′ and 37°30′S). The early middle Devonian (400–380 Ma) northwestern Patagonian magmatism is characterized by widespread magmatism and positive εHf–εNd linked to forearc and backarc magmatism that evolved within a retreating subduction stage. A tectonic switching toward advancing orogeny stage began in the late Devonian, evidenced by a lull in magmatic activity with a negative εHf–εNd trend, possibly contemporaneous with the first tectonic‐metamorphic event in western Patagonia. An early Carboniferous magmatic gap, followed by the subsequent development of the main foliation in the basement during the Carboniferous‐Permian period, denotes the acme of this contractional stage. In contrast, the Devonian period in the northern segment is characterized by mostly negative εHf–εNd values, reverse shear zone activity in the foreland, and an inboard magmatism migration, evidencing a compressive tectonic setting that changed to an extensional configuration in the early Carboniferous with widespread arc magmatism development.

Ridge Subduction: Unraveling the Consequences Linked to a Slab Window Development Beneath South America at the Chile Triple Junction

AbstractThe subduction of an active spreading center generates a clear signature in the temporal evolution of subduction zones. It disrupts the typical arc‐type magmatism and intraplate seismicity, enhances the emplacement of backarc plateau lava and profoundly change the tectonics and topographic relief. These distinct observations are commonly linked to a slab window opening and mantle upwelling. The Chile Triple Junction provides the ideal setup to study the mid‐ocean ridge subduction process where both sides of the spreading center continue to subduct. Here, we use 2‐D numerical petrological‐thermomechanical modeling to focus on transient geodynamic processes caused by mid‐ocean ridge subduction. Model results show slab separation along the ridge axis with the opening of a slab window. During the opening, partial melts from the spreading center migrate toward the subcontinental mantle and high temperatures in the forearc are predicted. The temporal evolution of the modeled temperature is consistent with observed heat flow data, and with magmatism and high‐temperature metamorphism recorded in Chilean forearc rocks. Such migrated partial melts might explain the low viscosity inferred and low seismic velocity anomalies imaged in the slab window beneath South America, and the common geochemical signature of the Chile Ridge, the Taitao Ophiolite and the backarc magmatism. Following slab separation, our models suggest forearc uplift and changes in the stress regime, processes which are consistent with deformation records. Summarizing, our model of the geodynamic evolution of the Chile Ridge subduction provides a consistent framework that explains diverse records of magmatism, metamorphism, deformation and mantle physical properties.

Identification of the Late Devonian back-arc magmatism in the Chinese Eastern Tianshan

Identifcation of subduction to back-arc tectonic transitions is critical to the study of accretionary orogens. Some new geochronological and geochemical data for the Late Devonian (380–361 Ma) Kezier mafic–felsic complex and Tuwu granitic pluton in this study have well recorded such transition feature in the Chinese Eastern Tianshan orogen. The Kezier gabbro is classified as high-Al basalt given their high Al contents (A12O3 > 17 wt%). They were originated from a depleted mantle fluxed by fertile mantle components at shallow depths under hydrous condition. Both the Kezier and Tuwu granites show geochemical characteristics of high total alkali, Ga/Al, HFSE, low CaO, and strongly negative Ba, Sr, and Eu anomalies, demonstrating a close affinity with A-type granites. The granites were probably generated by high temperature melting (zircon saturation temperature up to 916 °C) of crustal rocks under a low-pressure (<0.8–1.0 GPa) condition. These rocks can help identify the Late Devonian back-arc extensional event in the Eastern Tianshan. This extensional event contemporaneous with the onset of the earliest arc magmatic records southward rejuvenating. The back-arc extensional with crustal high-temperature melting event can be reconciled with the Kangguer Ocean rollback, which forced the arc rifting occur on the Dananhu fore-arc side during the Late Devonian.

The Middle Jurassic Cheshmeh-Ghasaban High-Mg gabbro (Hamedan, NW Iran): A plume-influenced intracontinental rifting event?

The Jurassic mafic to felsic magmatism affecting the older Ediacaran-to-Cambrian basement of the Sanandaj-Sirjan Zone of Iran has been traditionally interpreted as the product of arc and/or back-arc magmatism related to the early stages of Neo-Tethys subduction beneath Iran in the early Jurassic. Recent works and new compositional and geochronological data have started challenging this commonly accepted model in favor of scenarios involving continental rifting, mantle plume activity, and/or passive margin formation. In the Hamedan area of the central sector of the Sanandaj-Sirjan Zone, the Jurassic Cheshmeh-Ghasaban gabbro (ca. 165 Ma) is a key formation to better understand the tectono-magmatic framework of the whole area. Our new data, combined with the existing literature, suggest a transitional to alkaline OIB-like compositional character for this gabbro similar to the nearby but slightly younger (ca. 145 Ma) Panjeh and Ghalaylan basaltic complexes (in the Songhor-Ghorveh area). When integrated with the existing geochemical data of Jurassic mafic rocks from the central Sanandaj-Sirjan Zone, our results point to a scenario of intracontinental rifting, possibly involving the upwelling old metasomatized (by Proto-Tethys subduction?) mantle or mantle-plume activity.

Detrital zircon U-Pb and Hf isotope signature of Carboniferous and older strata of the Yukon-Tanana terrane in Yukon, Canadian Cordillera: Implications for terrane correlations and the onset of Late Devonian arc magmatism

Abstract The Yukon-Tanana terrane in Yukon, Canada, records Late Devonian (ca. 366–360 Ma) rifting and the onset of latest Devonian–Carboniferous arc and back-arc magmatism (ca. 360–325 Ma) in the Northern Cordillera. Detrital zircon U-Pb and Hf isotope analyses indicate that the metasedimentary basement of the Yukon-Tanana terrane was sourced in northwestern Laurentia. Sandstones in Late Devonian–Carboniferous successions generally have robust Late Devonian–Mississippian age peaks, and their Hf isotope signatures are characterized by strongly negative εHft values in Late Devonian zircons followed by progressively more juvenile εHft values in Carboniferous zircons. This Hf isotopic “pull-up” reflects the melting of Precambrian crust related to regional extension in the Late Devonian, followed by progressively more juvenile magmatism as the arc matured through the Carboniferous. Paleozoic rocks of the Tracy Arm terrane in southeastern Alaska, USA (formerly Yukon-Tanana south), have been compared with the Yukon-Tanana terrane in Yukon. Detrital zircons from the metasedimentary basement to the Tracy Arm terrane have distinct Precambrian populations that indicate sources along a different segment of the Laurentian margin compared to basement of the Yukon-Tanana terrane. Magmatism in the Tracy Arm terrane ranges from 440 Ma to 360 Ma and is characterized by an Hf isotopic “pull-down” in the Silurian to Early Devonian, followed by a “pull-up” in the Middle to Late Devonian and a second “pull-down” in the Late Devonian to early Mississippian. Thus, the Yukon-Tanana and Tracy Arm terranes record distinct pre-Carboniferous histories. Interactions between these two terranes are suggested by the influx of exotic early Mississippian clasts and detrital zircons on the Tracy Arm terrane that match sources in the Yukon-Tanana terrane.

Back-Arc Magmatism in the Cadomian Basin of NW Iran: Ortho-Amphibolites from the Alam Kandi Area

Back-Arc Magmatism in the Cadomian Basin of NW Iran: Ortho-Amphibolites from the Alam Kandi Area

Hot, Wide, Continental Back-arcs Explain Earth’s Enigmatic mid-Proterozoic Magmatic and Metamorphic Record

Higher than average thermobaric ratios (temperature/pressure) of metamorphic rocks and abundant ‘dry’ ferroan magmatism including massif anorthosite suites are two enigmatic features of the mid-Proterozoic (1.85–0.85 Ga) that have unclear origins. It has been proposed that elevated mantle temperatures due to insulation under the Columbia supercontinent, and/or to plate slowdown, combined with thin lithosphere, led to high continental geothermal gradients, high-temperature metamorphism, and an increase in dry, ferroan magmatism. Geodynamic modelling predicts that continental subduction zones at mid-Proterozoic mantle potential temperatures (80–150 °C hotter than at present) would exhibit key differences to the Phanerozoic, critically, extensive slab rollback combined with greater volumes of decompression melting of the asthenosphere would lead to wide regions of back-arc magmatism. We posit that these hot, wide continental back-arcs can effectively explain the abundance of ferroan magmatism, anorthosite suites, and high T/P metamorphism. Our model negates the need for extra mantle heating from supercontinental insulation or plate slowdown and shows that the tectonic regime of the mid-Proterozoic was a transitional phase between those of the Archean (likely comprising peel-back tectonics and episodic subduction) and the Phanerozoic (comprising deep continental subduction), and which could have resulted solely from secular cooling of the mantle.

Insights from the Alabama Hills into Mesozoic Magmatism and Tectonics in Eastern California

Abstract New zircon U-Pb ages for the Alabama Hills Granite in Owens Valley, eastern California, range from 103 to 102 Ma, nearly 20 Ma older than previously published zircon ages. The data preclude previously implied links between the pluton and the adjacent Late Cretaceous Mount Whitney Intrusive Suite. Geochronologic and isotopic data indicate a connection between the Alabama Hills Granite and leucogranites to the northwest on the Sierra Nevada crest, as well as a pluton to the southeast in the Coso Range. We refer to these units as the Kearsarge plutons. The suite was intruded from 103 to 100.5 Ma with S87r/S86ri=0.7045 to 0.7060 and εNdi=−2.4 to -4.5 and has distinctive enrichments in high field strength, middle, and heavy rare earth elements, as well as negative Eu anomalies and high Y/Sr. We suggest the Alabama Hills block could not have moved more than 10 km dextrally relative to the Sierra Nevada batholith since the Middle Jurassic and is thus a suitable piercing point for offsets across Owens Valley. The Kearsarge plutons, like other markers on either side of Owens Valley, support approximately 65–75 km of dextral offset across the valley. The suite’s location east of other middle Cretaceous suites, coupled with its mantle-like isotope geochemistry, suggests it could represent backarc magmatism, perhaps controlled by preexisting shear zones or fractures.

Golpayegan Metamorphic Complex (Sanandaj–Sirjan Zone, Iran) as Evidence for Cadomian Back-Arc Magmatism: Structure, Geochemistry and Isotopic Data

Golpayegan Metamorphic Complex (Sanandaj–Sirjan Zone, Iran) as Evidence for Cadomian Back-Arc Magmatism: Structure, Geochemistry and Isotopic Data

Evolution of arc magmatic cycles from the Carboniferous to the Early Cretaceous in the western paleomargin of Gondwana, north of the Andes

This work presents a considerable volume of new and compiled data indicating that arc magmatism in the western paleomargin of Gondwana began in the Carboniferous and continued during the Permian and Early Triassic. Subsequently, the magmatism reactivated during the Early and Middle Jurassic due to the subduction of the Farallon Plate under the continental paleomargin. The arc pluton belts are distributed from the edge of the paleomargin toward the interior of the continent in the same orientation as the slab (west–east direction). During the Carboniferous, between ca. 333 Ma and ca. 300 Ma, magmatism formed small calcic metaluminous gabbro and leucotonalite plutons of tholeiitic to calc-alkaline affinity on the western margin of Gondwana. Later, the second belt of arc plutons formed during the Permian/Triassic (between ca. 300 Ma and ca. 234 Ma) and are represented by metaluminous to peraluminous calc-alkaline to high-K calc-alkaline batholiths and stocks of heterogeneous composition (granites, granodiorites, diorites, quartz monzonites, and monzonites), which were intruded by dikes and minor granite bodies during the Middle Triassic. Between ca. 214 Ma and ca. 186 Ma, peraluminous plutons of batholithic dimensions of monzogranitic to syenogranitic composition developed in the back-arc. Between ca. 197 Ma and ca. 186 Ma, back-arc magmatism occurred, while a new magmatic cycle began along the arc axis. At the end of the Jurassic, the magmatic arc cycle ended in the northwestern paleomargin of Gondwana (ca. 164 Ma). The intrusion ca. 159 Ma of porphyritic bodies of alkaline andesitic basalts toward the edge of the continental margin suggests the strangulation and collapse of the subduction zone in the mantle. To the west, off the continental margin, a new arc magmatic cycle began over a different continental terrane ca. 171 Ma and extended to ca. 138 Ma, giving rise to a belt of calcic to calcic-alkaline plutons emplaced in the Ordovician metamorphic (Anacona Terrane), Triassic (Tahamí Terrane), and Upper Jurassic (Tierradentro Orogen) rocks. The assemblage amalgamated to the western margin of Gondwana in this period.

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

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

Early cretaceous tectonic setting of eastern Australia: Evidence from the subduction-related Morton Igneous Association of Southeast Queensland

Early Cretaceous magmatic rocks of the central east coast of Australia are recognized as a lithologically, geochemically, and geochronologically distinctive rock package herein named the Morton Igneous Association. The association embraces small plutons and hypabyssal intrusions within the Jurassic and Lower Cretaceous Nambour and Maryborough basins. It also includes volcanics of the Grahams Creek Formation, basal infill of the Maryborough Basin with an indicated volume exceeding 0.25 M km3, representing a much more substantial igneous assemblage. Plutonic rocks vary from hornblende-rich gabbro to granodiorite, and syenite. The Grahams Creek Formation consists of volcanic to volcaniclastic rocks of basaltic to dacitic composition. The association is UPb zircon dated at 145–140 Ma, and is characterized by distinctive subduction-related trace element compositions typical of magmatic rocks from continental arc settings. Oxygen isotope and εHf compositions obtained from zircon are consistent with a largely mantle source for the host magmas. Trace element data for the 130–95 Ma Whitsunday Igneous Province, of more northerly location on the east Australian coast, are consistent with a continental back-arc association. The MIA and Whitsunday Igneous Province represent small parts of an extensive Cretaceous active margin system developed for east Gondwana, mainly located on continental crust comprising northern Zealandia which is now separated from east Australia, largely submerged and little documented. East Australia records both older (145–140 Ma) continental arc magmatism and younger (132–95 Ma) continental back-arc magmatism attributed to progressive Cretaceous roll-back of a consuming plate boundary to east Gondwana.

Carbon flux and alkaline volcanism: Evidence from carbonatite-like carbonate minerals in trachytes, Ulleung Island, South Korea

Abstract Carbon flux metasomatism in the subduction environment is an important process, but it remains poorly understood. The paucity of exposed lower crust and upper mantle rocks in continental arcs renders xenoliths a major target for studying the slab-derived carbon cycle. This study of the carbonate phases in volcanic rocks from three drill cores in Ulleung Island, South Korea, sheds light on the interaction of carbon flux in the upper mantle and lower crust in a back-arc setting. The volcanic rocks from Ulleung Island range in composition from trachybasalt to trachyte and contain abundant euhedral pseudomorphic carbonate grains, ulvöspinel-hosted and biotite-hosted carbonate-silicate melt inclusions, and irregular carbonate globules. Integrated petrographic and geochemical studies of a variety of phenocrysts, carbonate phases, and carbonate-silicate inclusions in biotite and ulvöspinel indicate that recharging of carbon flux affected magma evolution. Carbon and oxygen isotopes of the pseudomorphic carbonate grains overlap with mantle values, indicating a carbonatite-like origin of the carbonate phases. The (MgO, FeO, CaO)-rich silicates in ulvöspinel-hosted silicate inclusions and pseudomorphic carbonate grains likely represent a primary melt, which formed from the partial melting of carbonated eclogite of the subducted slab within the mantle wedge beneath Ulleung Island. A petrogenetic model is proposed to illustrate that the crystal mush in the magma chamber was intruded by carbonate-rich liquids and caused alteration of cumulate crystals to generate the euhedral pseudomorphic carbonate grains. The extrusive magma captured those pseudomorphic grains and erupted to form the trachybasalt-trachyte units. The observed carbonate phases and their geochemical characteristics indicate that carbon flux metasomatism played a fundamental role in this back-arc magmatism.

Early Svecofennian rift-related magmatism: Geochemistry, U-Pb-Hf zircon isotope data and tectonic setting of the Au-hosting Uunimäki gabbro, SW Finland

We characterise the geochemistry, zircon Lu-Hf composition, age and the structure of the Uunimäki gabbro (UGB) in south-western Finland to improve the understanding of i) the early Svecofennian (1.92–1.89 Ga) crustal evolution of the central Fennoscandian Shield, ii) the potential role of rift-related magmatism for the build-up of the Paleoproterozoic accretionary orogens and iii) evaluate, which geological features provide the primary control over the localization of an orogenic gold mineralisation. The zircon U-Pb geochronology defines an age of 1891 ± 5 Ma for the UGB, which is slightly older than most mafic intrusions in south-western Finland. The obtained chondritic initial zircon ɛHf values with E-MORB type geochemical affinity suggest a sub continental lithospheric mantle source for the UGB. The overall geochemistry indicates that the UGB magma as well as other E-MORB type rocks in the Pirkanmaa and Häme belts were formed in a rift-related environment in a fore-arc region at 1.89 Ga, predated by arc-type magmatism at ~1.90 Ga and back-arc magmatism at ~1.92 Ga in the Tampere belt. Slab retreat due to roll-back is suggested to cause the extension and related magmatism in the fore-arc region. Moreover, the timing and compositional and isotopic changes of early-orogenic magmatism are broadly compatible with intervals of extension and contraction, i.e., a tectonic switching model, and may provide a perspective to rapid build-up of Paleoproterozoic crust. Structural characterisation provides a framework where gold mineralisations are preferentially located within the high-strain north-eastern domain of the UGB, within fracture networks adjoining the high-strain zones. Our results indicate that neither the geochemical composition nor age of the intermediate-mafic intrusive host rocks play a major role in controlling the formation of gold mineralisation. By contrast, the localization of orogenic gold is controlled by localised structures (shear zones, fractures), and the variation in lithological composition of the intrusive host may contribute to the style of the mineralisation.

Coupling Between Lithosphere Removal and Mantle Flow in the Central Andes

AbstractThe central Andes is part of a Cordilleran orogen formed through continent‐ocean convergence. In contrast to the thickened crust, the mantle lithosphere below much of the orogen is anomalously thin. Additionally, the surface is characterized by widespread backarc magmatism and transient ∼100‐km‐wide basins that developed the over last 30 million years, with basins located systematically seaward of major backarc ignimbrite centers and basin formation predating the late Miocene magma/ignimbrite flare‐up. Using numerical models, we propose a novel mechanism whereby lithosphere removal is coupled with mantle flow. First, a small area of high‐density eclogitized lower crust initiates a gravitational instability, causing a localized basin at the surface that subsides and then uplifts. Foundering crust and adjacent lithosphere are entrained by subduction‐induced mantle flow, driving regional lithosphere removal and magmatism. The models demonstrate that mantle flow can amplify a local lithosphere instability to orogen‐wide lithosphere removal, rapidly eliminating accumulated mass in the orogen.

Geological, mineralogical and geochemical characteristics of Mississippian K-bentonites from southern Turkey: A correlation with coeval tephras from Gondwana-derived terranes

Five thin (5–15 cm) and yellowish green tephra (K-bentonite) layers are exposed along two sections in the south of Akşehir (Konya, southern Turkey) within the Early Carboniferous (Mississippian) red dolomite-dolomitic limestone succession of the Sultandağ Unit in the central Tauride Belt. K-bentonites exhibit typical porphyritic texture with euhedral plagioclase phenocrysts replaced by fine-grained white K-micas and sparry dolomite crystals. The intensively sericitized volcanic matrix has vitrophyric texture and contains hydroxidized euhedral pyrite crystals. Illite is the only clay mineral in all K-bentonite samples. Non-clay minerals are dolomite, calcite, quartz, pyrite, rutile and zircon. Illites have sheet-like flakes (1–4 μm sizes) showing well-developed orientations and partly slaty cleavage. Illite Kübler index values (KI, Δ°2θ) of K-bentonites reflect high-grade diagenetic to anchizonal conditions indicating temperatures that reached up to 200 °C. Illite d060 values (1.4990–1.5007 Å) corresponding to dioctahedral composition display differences in coexisting slates and shales of the same age. Illite polytypes are characterized by the association of 2M1, 1M and 1Md. The presence of 1M illites is indicative of a volcanic (tephra) origin. Immobile element compositions suggest that tephras have been derived from trachyandesitic volcanism. K-bentonites exhibit some differences from the coeval pelitic occurrences in the Hadim-Bozkır areas, S Turkey, and the Late Devonian-Early Carboniferous K-bentonites in Bartın and Zonguldak areas of the Istanbul-Zonguldak Terrane, NW Turkey. A geochemical evolution of the studied K-bentonites suggests that they were possibly originated from the Early Carboniferous (Mississippian) intra-continental back-arc magmatism at the northern margin of Gondwana-derived Tauride-Anatolide microplate.

Back-arc tectonic tempos: Records from Jurassic–Cretaceous basins in the eastern North China Craton

Magmatic and structural tempos have been recognized in continental magmatic arcs, such as Cordilleran and Andean arcs. However, it remains unclear whether similar tempos or episodicity occur in back-arc settings, knowledge of which would be crucial for understanding back-arc geodynamics. The western Liaoning (WL) region in eastern China contains well-preserved basins of Jurassic–Cretaceous age and provides an opportunity to investigate back-arc tectonic processes. We present stratigraphic, structural, and geochronological data from the WL region to constrain tectonic evolution during the Jurassic–Cretaceous. The results show four stages of rifting or extension during 190–168, 167–137, 136–97, and 96–70 Ma, indicating a duration of 20–40 Myr for each extensional stage. Aside from a magmatic lull at 96–70 Ma, volcanism was associated with each of the three earlier extensional stages. Each main period of volcanism had a duration of 10–20 Myr, with the main eruption events occurring on an average cycle of ~25 Myr. The Jurassic–Cretaceous extension and rifting were interrupted by three compressional events at 168–167, 137–136, and 97–96 Ma, as evidenced by angular unconformities, basin inversion, and structural shortening. Each episode of compression lasted for ~1 Myr, on a 30–40 Myr cycle. The results indicate tempos in both back-arc stress state and magmatism. The back-arc tectonic evolution was characterized by frequent alternation of long-lived extensional and short-lived compressional events, and the back-arc magmatism in all cases occurred in an extensional setting. The back-arc tectonic tempos can be explained by a slab-driven model involving control by alternating phases of slab rollback and retreat on back-arc stress states and magmatism.

Ca. 2.1 Ga Mahakoshal Supracrustal Belt: An allochthonous terrain in Central India Tectonic Zone

Back-arc magmatism is perhaps extremely rare in a Paleoproterozoic mobile tectonic regime. The present study provides a detailed account on the petrogenesis of basalts that are interleaved with marine sedimentary rocks in the Agori Formation, which overlies the Chitrangi Formation in the ~2.1 Ga Mahakoshal supracrustal belt within the Central India Tectonic Zone. The basalts are sub alkaline, tholeiitic in nature. They have moderate concentrations of TiO2 (~0.9 wt%), Nb (2.6–5.2 ppm), and Zr (59–105 ppm), Zr/Nb (18–24) and Zr/Y (3.1–3.6) ratios, mildly enriched chondrite normalized rare earth element patterns (LaN/YbN = 1.6–2.8), and negative Nb and Ti anomalies on a primitive mantle normalized trace element variation diagram. Post-magmatic low grade metamorphism, alteration and trace element mobility, and/or contamination by Archean continental crust, if any, cannot relate to these patterns. The positive initial εHf (2.1 Ga) + 2.24 to +5.15 and εNd(2.1 Ga) + 1.02 to +1.54 isotopic compositions in these basalts are variable, yet consistent with a juvenile mantle source. We ascribe the isotopic variability in the Agori basalts to reflect the general isotopic variability in the depleted mantle at 2.1 Ga. Consequently, the basalts were produced by decompression partial melting in the shallow upper mantle in a back-arc setting. Subduction zone magmatic processes produced the arc volcanics in the upper stratigraphic unit (Agori), whereas, a plume source generated the ferropicrites in the underlying Chitrangi Formation. The evolution of the Mahakoshal supracrustal belt predates the collision and interlocking of North and South India cratonic blocks, which resulted in the formation of Central India Tectonic Zone. Hence, it represents an allochthonous plume-arc accreted terrain in the CITZ.