MORB-like Source Research Articles

The noble gas signature of the 2021 Tajogaite eruption (La Palma, Canary Islands)

Here, we characterize the temporal evolution of volatiles during the Tajogaite eruption by analyzing the elemental (He-Ar-CO2-N2) and isotopic (He-Ar-Ne) composition of fluid inclusions (FI) in phenocrysts (olivine+pyroxene) identified in erupted lavas. Our 2021 lava samples identify substantial temporal variations in volatile composition. We show that, during the 2021 Tajogaite eruption, the He-CO2-N2 concentrations in FI increased since October 15th; this increase was accompanied by increasing 40Ar/36Ar ratios (from ∼300 to >500), and paralleled a major shift in bulk lava chemistry, with increasing Mg contents (Mg#, from 47 to 52 to 55–59), CaO/Al2O3 (from 0.65 to 0.74 to 0.75–0.90), Ni and Cr, and decreasing TiO2, P2O5 and incompatible elements. The olivine core composition also became more forsteritic (from Mg# = 80–81 to Mg# = 84–86). Mineral thermobarometry and FI barometry results indicate that the eruption was sustained by magmas previously stored in at least two magma accumulation zones, at respectively ∼6–12 km and 15–30 km, corroborating previous seismic and FI evidence. We therefore propose that the compositional changes seen throughout the eruption can be explained by an increased contribution - since early/mid-October - of more primitive, less-degassed magma from the deeper (mantle) reservoir. Conversely, Rc/Ra values (3He/4He ratios corrected for atmospheric contamination) remained constant throughout the whole eruption at MORB-like values (7.38 ± 0.22 Ra, 1σ), suggesting an isotopically homogeneous magma feeding source. The Tajogaite He isotope signature is within the range of values observed for the 1677 San Antonio lavas (7.37 ± 0.17Ra, 1σ), but is more radiogenic than the 3He/4He values (>9 Rc/Ra) observed in the Caldera de Taburiente to the north. The 3He/4He ratios (6.75 ± 0.20 Ra, 1σ) measured in mantle xenoliths from the San Antonio volcano indicate a relatively radiogenic nature of the mantle beneath the Cumbre Vieja ridge. Based on these results and mixing modeling calculations, we propose that the homogeneous He isotopic signatures observed in volatiles from the Tajogaite/San Antonio lavas reflect three component mixing between a MORB-like source, a radiogenic component and small additions (6–15%) of a high 3He/4He reservoir-derived (>9Ra) fluid components. The simultaneous occurrence of high 3He/4He (>9Ra)- and MORB-like He signatures in northern and southern La Palma is interpreted to reflect small-scale heterogeneities in the local mantle, arising from spatially variable proportions of MORB, radiogenic, and high 3He/4He components.

Petrogenesis of the Girnar Complex in the Deccan Traps Province, India

AbstractThe c. 66 Ma Girnar Complex of the Deccan Traps consists of various mafic to silicic rocks and provides a unique setting for understanding the dynamics of magma chamber processes. The intrusive alkaline rocks in Girnar are diorites/monzodiorites and contain syenite/nepheline syenite veins. Several (alkaline) mafic dykes cut the alkaline rocks. These rocks are surrounded by intrusive tholeiitic gabbro and extrusive basaltic flows and a silicic (SiO2 > 65 wt%) ring dyke at the outer margin of the complex. The tholeiitic gabbro and basaltic lavas have relatively flat light rare earth element (REE) patterns (La/SmN ~ 0.8 to 1.8) with positive Eu anomalies of 1.1–1.3. The tholeiitic basalts formed by 3–5% partial melting of an incompatible element depleted mid-ocean ridge basalts (MORB)-like source. Accumulating c. 20–30% of (olivine, orthopyroxene, clinopyroxene, and plagioclase) crystals from the tholeiitic melt generated the tholeiitic gabbro. The alkaline rocks display enriched light REE patterns (La/SmN ~ 5.0 to 7.8) consistent with (~9%) partial melting of a spinel lherzolite source, different from the MORB-like source for tholeiitic rocks. The subsequent 58–72% fractional crystallization of olivine, clinopyroxene, plagioclase, orthoclase, spinel, and apatite mineral assemblage formed the alkaline rocks. Reverse and oscillatory zoning in plagioclase (An30 to An50) phenocrysts of the silicic rocks suggest that convective movement of crystals in a magma chamber led to initial crystallization of high-An plagioclase in the hotter zone of the magma chamber, followed by addition of low-An plagioclase in cooler regions of the magma chamber. Pressure estimates from shattered (xenocryst or antecryst) clinopyroxene crystals in silicic rocks give a minimum value of 450 MPa. Ti-in-quartz geothermometry gives a crystallization temperature between 690°C and 1080°C. The distinct isotopic composition of the silicic rocks (87Sr/86Sri = 0.7204–0.7275, εNd(i) = −6.8 to −7.3, 206Pb/204Pbi = 18.74–19.02, 207Pb/204Pbi = 15.76–15.79, 208Pb/204Pbi = 39.63–40.03, εHf(i) = −6.0 to −7.3) further indicates that the silicic rocks assimilated 7 to 9% granitic basement rocks. Overall, therefore, the Girnar Complex started to form with the emplacement of mafic rocks (both alkaline and tholeiitic) in the central part along a reactivated fault lineament and silicic rocks along the concentric marginal fault. Petrography, mineral chemistry, and whole-rock geochemical and Sr-Nd-Pb-Hf isotope ratios of the Girnar Complex rocks indicate that despite the close spatial association of tholeiitic gabbro and basalt, alkaline rocks, and silicic rocks, these rocks originated from multiple sources and evolved in a complex magma drainage and storage network in a continental setting of the Deccan Large Igneous Province (LIP).

Zircon U Pb geochronology and petrology of the tholeiitic gabbro from the Kovanlık (Giresun) area: Constraints for the Late Cretaceous bimodal arc magmatism in the Eastern Pontides Orogenic Belt, NE Turkey

Mafic intrusions of the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey are found rarely in varying ages, sizes and compositions. Of these, tholeiitic gabbro stocks, namely Kovanlık gabbro, consist of pyroxene gabbro and minor hornblende gabbro with a mainly granular texture. The pyroxene gabbro contains plagioclase, clino- and ortho-pyroxene, olivine, and Fe Ti oxides, whereas the hornblende gabbro includes mainly plagioclase, hornblende, rarely biotite and Fe Ti oxides. The clinopyroxene compositions of the pyroxene gabbro exhibit arc-related tholeiitic magmas. The Kovanlık gabbroic rocks have high U/Yb zircon ratios, implying continental arc-related tectonic settings. Zircon U Pb dating on the Kovanlık gabbro yielded a concordia age of ~92 Ma (Turonian). Petrochemically, the gabbroic rocks have a tholeiitic character and show arc- and MORB-like signatures. The gabbroic rocks show positive Rb, Ba, K, Pb, Sr, Ti and negative Nb, Ce, Nd, Hf, Zr anomalies in N-MORB normalized diagrams. The chondrite-normalized REE plots of the rocks show flat to feeble right-dipping REE patterns (La N /Lu N = 0.94–2.56) with positive Eu anomalies (Eu N /Eu* = 1.15–2.14; except for two pyroxene gabbro samples). The gabbroic rocks have 87 Sr/ 86 Sr (i) ratios (0.704533 to 0.706265), εNd (i) values (4.04–4.85), 206 Pb/ 204 Pb (18.56–18.68) and 207 Pb/ 204 Pb (15.61–15.64) ratios. Low Nb/La and Nb/Y ratios, as well as high Sr/Yb and Ba/Rb ratios, indicate that source regions were modified by slab-derived fluids and/or melts. The obtained zircon dating, isotopic and geochemical data, as well as regional geology, revealed that the tholeiitic mafic magma derived from a spinel-lherzolite mantle at a relatively shallow depth, and the Kovanlık gabbro in the EPOB represents one of the first mafic intrusions of Late Cretaceous bimodal arc magmatism related to the roll-back of the northwards subducting Tethys oceanic plate. • The Kovanlık tholeiitic gabbro formed in a continental arc with a MORB-like source. • The zircon U Pb dating of the gabbro gave a concordia age of ~92 Ma (Turonian). • The parent magma originated by partial melting of a spinel lherzolite mantle source. • The gabbro evolved by fractional crystallization and lesser crustal assimilation. • Cretaceous bimodal arc magmatism is related to the rollback of the subducting slab.

The early Eocene (48\xa0Ma) Qaladeza trondhjemite formed by wet partial remelting of mafic crust in the arc-related Bulfat Igneous Complex (Kurdistan, Iraq): constraints on the timing of Neotethys closure

Events in the tectono-magmatic framework of the Eocene Bulfat Igneous Complex in the Kurdistan region of northeastern Iraq are revealed through the integrated field, petrographic, geochemical and zircon petrography and U–Pb geochronology of the Qaladeza trondhjemite sheets that cut gabbros of the complex. The trondhjemite sheets are variably deformed, with the cataclasis of large plagioclase feldspars. The trondhjemites are albitic, and geochemical features such as strong enrichment of the light REE relative to the heavy REE and low Ti and REE content show that they did not form as late differentiates of a MORB-like source, but instead by hydrous partial melting of mafic rocks, possibly also involving some more evolved crustal source as well. Qaladeza trondhjemite oscillatory-zoned magmatic zircon has an age of 48.0 ± 0.6 Ma (95% confidence). These domains have been etched to form voids in the zircons, the walls of which have thin coatings of Th- and Ca-bearing species. Subsequent partial recrystallization of the zircons obliterated the voids and increased the Th/U of the recrystallized zircon, which mostly formed at 38.74 ± 0.74 Ma. The 48 Ma domains relate to recycling of mafic crust in a proto-arc tectonic setting and are congruent with Eocene proto-arc magmatic activity within the Neotethyan oceanic basin. The 39 Ma domains relate to later tectonic-induced recrystallisation events coeval with the closure of Neotethys resulting from the late Eocene collision between the Arabian plate and Eurasia in Kurdistan.

Cretaceous Kuching accretionary orogenesis in Malaysia Sarawak: Geochronological and geochemical constraints from mafic and sedimentary rocks

The Kuching zone, the most important tectonic boundary in Borneo, separates the Sibu zone in Central Sarawak from the Schwaner Mountains granitoids in SW Borneo. It potentially provides a key window for probing the paleo-Pacific subduction process. This paper presents new geochronological, elemental and SrNd isotopic data for the Pakong-Serabang and Serian mafic rocks, along with detrital zircon UPb age-data for the associated greywacke in the Sarawak Kuching zone. These mafic rocks are dated at ~77–98 Ma and show distinctive geochemical signatures. The Pakong- Serabang mafic rocks are subalkaline basalt with SiO2 = 46.22–51.75 wt%, TiO2 = 0.49–2.42 wt% and MgO = 6.30–10.54 wt%. They exhibit MORB-like PM-normalized patterns with depletion in LILEs and HFSEs. Their 87Sr/86Sr(t) and εNd(t) range from 0.70285 to 0.70728 and + 7.4 to +9.4, respectively, originating from a slab-derived fluid-modified MORB-like source. The late Cretaceous Serian Volcanics are marked by SiO2 of 51.52–57.90 wt%, MgO of 3.50–5.06 wt% with mg-number = 36–46 and show arc-like geochemical signatures with (Nb/La)n = 0.37–0.47. They have 87Sr/86Sr(t) = 0.71188–0.71417 and εNd(t) = −11.1 ~ −7.3, and were derived from a mantle wedge with the input of recycled sedimentary components. The detrital zircon grains from the associated sandstones give similar age-spectra to those in East Peninsular Malaysia and SE Vietnam. Our data revealed a Cretaceous Kuching trench-arc system. In combination with other geological observations, it is concluded that NW Borneo to the southwest of the Sarawak Kuching zone was a part of the Indochina/East Peninsular Malaysia fragment in SE Asia prior to the Jurassic and re-activated as an active continental margin in the Cretaceous. Widespread accretionary orogenesis along the Kuching zone likely initiated at or before the early Cretaceous and ended no earlier than the latest Cretaceous in response to the westward subduction of the paleo-Pacific slab.

Geochemistry and U–Pb zircon ages of the metamafic-ultramafic rocks of the Riacho dos Machados metavolcanosedimentary sequence: Evidence of a late Rhyacian back-arc basin during the assembly of São Francisco-Congo paleocontinent

The São Francisco paleocontinent comprises Archean nuclei and Paleoproterozoic magmatic arcs which were amalgamated during Siderian to Orosirian thermal-tectonic events (ca. 2.4–2.0 Ga). Located in the Porteirinha domain, one of these Archean segments, the Riacho dos Machados metavolcanosedimentary sequence encompasses metamafites and metaultramafites intercalated with metasedimentary rocks. The metamafites clustered in the Type I category are tholeiitic, present MORB-like affinities and flat REE patterns with enrichment in La, Rb, and Cs. The metamafites from Type II are calc-alkaline, showing arc signatures with enrichment in LILE (Cs, Ba, U, Rb, K) and LREE, and depletion in the HFSE. The associated metaultramafites were classified as high-Mg ultramafic rocks similar to Barberton-Type komatiites, with enrichment in LILE and LREE, Al2O3/TiO2< 16 and Gd/Yb(N) > 1. U–Pb (LA–SF–ICP-MS) zircon analyses were carried out on the Type II hornblendite, unraveling a concordant crystallization age of 2071 ± 9 Ma. Inherited zircons yield a discordant age of 2922 ± 22 Ma and a lower intercept at 473 ± 48 Ma. The hybrid magmatism of the studied metamafites coupled with the inherited zircon grains, indicate that this metavolcanosedimentary sequence was developed in an intra-continental back-arc basin. The rocks from Type I have been originated from MORB-like source with a slightly input of subduction-derived fluids. In its turn, Type II samples represent melts more influenced by the fluids from the dehydrated slab. This assumption implies that the São Francisco paleocontinent, in the region of the Porteirinha block and surroundings, were under an accretionary stage at the late Rhyacian. During this orogenic process the metamorphic PT conditions reached lower-amphibolite facies. Moreover, this thermal-tectonic event in likely responsible for the auriferous fluid percolation of the Riacho dos Machados Gold Mine.

Petrology and geochemistry of the Lattan Mountain magmatic rocks in the Sanandaj–Sirjan Zone, west of Iran

Igneous rocks are distributed in the Lattan Mountain area, the center part of Sanandaj–Sirjan Zone, west Iran. Based on the mineralogy and geochemical composition, these are subvolcanic, volcanic, and plutonic rocks. It includes basalts, andesitic basalt, and andesite, with porphyritic to microlithic porphyry and vitrophyric textures as well as dolerite, diorite, gabbro, and microdiorite with a varied granular, microgranular, intersertal, and intergranular textures. The chemical compositions indicated to calc-alkaline to transitional nature, enrichment in LIL elements (Rb, Ba, Th, U, and Pb), and depletion in Ti and Zr, as it is evident in spider diagrams normalized to a primitive mantle. In addition, samples have enrichments of LREE relative to HREE. The Rb-Sr whole-rock isochron of these magmatic rocks shows an age of 152 ± 14 Ma (late Jurassic). The initial 87Sr/86Sr (0.7047 to 0.7051) and 143Nd/144Nd ratios range from (0.512534 to 0.512710) and eNdt = − 0.1 to + 2.2 that indicated to BSE composition. Our results suggest that the magmas for the magmatic rocks were derived from metasomatized enriched MORB-like sources. These rocks were formed in an island arc setting during subduction and closure of the Neotethys oceanic lithosphere beneath the Iran microplate about 152 Ma.

Contemporaneously emplaced submarine volcaniclastic deposits and pillow lavas from multiple sources in the island arc Brook Street Terrane, Southland, New Zealand

ABSTRACT Volcaniclastic and coherent volcanic rocks of the Permian island arc Brook Street Terrane crop out along the Southland coast, New Zealand, near Riverton. Extensive along-strike exposures reveal lateral variations in these submarine deposits and provide a rare opportunity to examine deposits of an arc environment within a thin slice of time. We describe pillow lavas, dikes, hydroclastic breccias, tuffs, lapilli tuffs and argillites, all hydrothermally altered and metamorphosed to prehnite-pumpellyite facies. Trace element geochemical data indicate multiple primitive island arc to MORB-like magma sources. Two separate volcanic centres apparently tapped different mantle domains in a source region transitional between the arc and back-arc. The centres’ proximity and inferred small sizes suggest they were satellite vents of a larger arc volcano. Tuff and lapilli tuff were formed as hyaloclastite during eruption of pillow lava, and redeposited by subaqueous mass flows. Peperitic contacts indicate that the lavas intruded unconsolidated volcaniclastic deposits. We infer that hydroclastic fragmentation associated with pillow emplacement produced debris that was then intruded by later pillows, with local peperite formation, as eruption persisted. The hydroclastic debris from these satellite vents was resedimented in turbidite fans with channels, and periodic turbidity currents delivered fine-grained material from the volcano’s main vent.

Pyroxene-bearing low- and high-HREE TTGs from the northeastern margin of the Kaapvaal Craton, southern Africa: Implications for Archean geodynamics

Archean tonalite-trondhjemite-granodiorite (TTG) rocks are generally amphibole- and/or biotite-bearing. Pyroxene-bearing granitoids of TTG affinity are less commonly reported. This study evaluates the range in composition and petrogenesis of pyroxene-bearing TTGs from a high-grade terrane along Archean cratonic margin. The Southern Marginal Zone high-grade terrane along the northeastern margin of the Kaapvaal Craton exposes prominent Mesoarchean pyroxene-bearing granitoids. The present study describes two groups of the granitoids in the Commissiedraai-Hugomond areas. The tonalite-trondhjemites with Fe-rich orthopyroxene (XMg = 0.46–0.49) dominate and occur as massive homogenous and banded varieties, the latter likely due to anatectic overprint. Enclaves of metabasites occur in both varieties. The darker layers of the banded variety are identical in mineralogy, texture and chemistry to the massive dark greenish-grey rocks. A less dominant group of clinopyroxene-rich massive tonalites occur in the Hugomond area. Clinopyroxene exhibit prominent orthopyroxene exsolution, with a pre-exsolution composition of XMg = 0.61–0.63. The spatially associated orthopyroxene-bearing and clinopyroxene-rich rocks are compositionally connected by a two-pyroxene-bearing variety with Mg-rich orthopyroxene (XMg = 0.56–0.59). No textures of pyroxene formation after earlier FeMg minerals occur in the studied rocks, whereas textures of pyroxene destabilization are indicative of late magmatic interaction with a residual melt. Pyroxenes in all rocks are primary magmatic. Biotite-quartz intergrowths and amphibole after pyroxenes are late magmatic in origin. Small garnet crystals occur locally in the orthopyroxene-bearing tonalite-trondhjemites and are related to metamorphic/anatectic overprint. The rocks are affected by late alkali metasomatism close to shear zones. Geochemical characteristics of the orthopyroxene-bearing tonalite-trondhjemites are comparable to low-HREE TTGs, while that of the clinopyroxene-rich tonalites are similar to high-HREE TTGs. The proposed petrogenetic model considers the formation of both groups of pyroxene-bearing granitoids via remelting of mafic sources in a thickened plume-related oceanic plateau. The high temperatures of melting, low water activity and oxidized conditions likely resulted in the formation of charnockitic assemblages, and is supported by phase equilibria modelling. Shallow level melting of a MORB-like source produced clinopyroxene-rich high-HREE TTG. Subsequent melting of a slightly enriched-MORB-like source occurred at relatively greater depths forming orthopyroxene-bearing TTG with low-HREE characteristics. The two-pyroxene-bearing rocks likely resulted from the mingling-mixing of the high- and low-HREE TTG end members. Implications of the results on Archean geodynamics are discussed together with a comparative exercise with similar rocks from other Archean terranes.

Early Neoproterozoic continental arc system at the central Jiangnan Orogen, South China: Geochronological and geochemical constraints on the key igneous rock-association

Abstract To better reveal the tectonic process of the Jiangnan Orogen in creating the united South China Block (SCB), a set of new geochronological, geochemical, and Sr-Nd-Hf-O isotopic data is herein presented for the coeval early Neoproterozoic mafic to acid rocks in the Lengjiaxi Group and its equivalents. Our data show that these rocks belong to subalkaline basalt, basaltic andesite, andesite, and dacite in composition, and can be geochemically classified into N-MORB- and arc-like mafic (Group 1 and Group 2, respectively), along with high-Mg andesitic-dacitic (Group 3) rocks. They yield zircon U-Pb ages of 837 ± 4 Ma, 832 ± 11 Ma, and 835 ± 7 Ma, respectively, suggesting their early Neoproterozoic (ca. 835 Ma) origin. Group 1 has SiO2 = 47.21–52.45 wt%, MgO = 7.36–10.07 wt% with Mg-number of 56–68, 87Sr/86Sr(t) = 0.7062–0.7136 and εNd(t) = +1.3–+9.4, along with N-MORB-like REE- (rare earth element) and PM (primitive mantle)-normalized patterns, suggesting its derivation from a refractory MORB-like source newly introduced by slab-derived fluid. The Group 2 samples are classified as tholeiitic basalt and basaltic andesite with SiO2 = 48.78–55.75 wt%, MgO = 7.70–12.74 wt%, Mg-number = 63–77, 87Sr/86Sr(t) = 0.7046–0.7098 and εNd(t) = –1.6 to –0.4. They exhibit similar “spiky” REE- and PM-normalized patterns to typical arc basalt, with pronounced enrichment in (large ion lithophile elements) and sharp depletion in (high field strength elements), probably inheriting from a sub-arc source modified by fluids and/or melts released from recycled sedimentary component. The Group 3 rocks show SiO2 = 57.74–66.30 wt%, FeOt = 6.22–8.59 wt%, Mg-number = 43–66, K2O+Na2O = 3.18–6.17 wt% with K2O/Na2O of 0.59–3.28, classified as high-Mg andesites and dacites. Diagnostic elemental and isotopic ratios of Group 3 include (La/Yb)cn = 7.7–11.4, (Gd/Yb)cn = 1.44–1.87, Eu/Eu* = 0.49–0.80, Nb/La = 0.27–0.42 and εNd(t) = –7.3 to –5.4, resembling to those of other early Neoproterozoic high-Mg intermediate rocks in the SCB, originating from a mantle wedge source with proportional addition of the recycled SCB sediment-derived components. In combination with the available data and geological observations, it is concluded that the N-MORB- and arc-like mafic, along with high-Mg andesitic rocks are coeval in the Lengjiaxi Group and its equivalents along the central Jiangnan Orogen and formed at ca. 847–832 Ma of the early Neoproterozoic period. The synthesis of these observations suggests the development of an early Neoproterozoic continental arc-basin setting. The arc-basin system westerly subducted and finally closed at ca. 830 Ma and the assembly of the Yangtze and Cathaysia blocks terminated at ca. 810 Ma along the central Jiangnan Orogen.

Early Neoproterozoic (∼840 Ma) slab window in South China: Key magmatic records in the Chencai Complex

It has been generally accepted that the Neoproterozoic amalgamation of the Yangtze and Cathaysia Blocks along the Jiangnan orogenic zone created the united South China Block (SCB). However, its final oceanic consumption time and subsequent assemblage process remain controversial. This paper presents a set of new geochronological, geochemical and Sr-Nd-Hf-O isotopic data for the early Neoproterozoic mafic to felsic igneous rocks in the previously-mapped Chencai Group and the associated equivalents. Our data show that the compositions of these rocks plot into the fields of the subalkaline and alkaline series, and they can be geochemically subdivided into five groups. Group 1 has SiO2 = 56.68–63.97 wt.% and MgO = 2.31–8.90 wt.% with Mg-number of 42–69, resembling the high-Mg intermediate rocks. Their 87Sr/86Sr(t) ratios range from 0.7022 to 0.7043, εNd(t) from +3.0 to +5.5, εHf(t) from +4.9 to +7.7 and δ18O from 4.78‰ to 5.57‰, most likely showing their petrogenetic relevance to the interaction of young slab-derived fluids with the overlying refractory mantle. Group 2 with SiO2 of 63.68–74.89 wt.% shows high Sr, Ba, Sr/Y and (La/Yb)cn, but low Y and Yb, resembling high-silica adakites. Its arc-like elemental signatures and highly positive εNd(t) -εHf(t) values, along with δ18O vaules of 4.95–5.39‰, suggest its derivation from the recently-ponded mafic lower crust with a mantle wedge source similar to Group 1. Group 3 is characterized by the arc-like basaltic and andesitic rocks with SiO2 = 45.06–63.53 wt.%, MgO = 3.38–12.99 wt.%, 87Sr/86Sr(t) = 0.7032–0.7047, εNd(t) = +2.0 ∼ + 5.9, εHf(t) = +3.2∼+7.8, originating from a MORB-like source newly modified by slab-derived fluids. Group 4 has SiO2 = 44.55–52.46 wt.%, MgO = 3.34–8.07 wt.%, Nb = 18.2–55.0 ppm and Nb/La = 1.01–1.76. These rocks can be classified as the high-Nb OIB-like alkaline mafic rocks, and interpreted as the product of the enriched component-modified asthenosphere. Group 5 has similar elemental signatures to the Nb-enriched basalt, with SiO2 = 44.52–48.80 wt.%, Nb/La = 0.38–0.81 and Nb = 8.2–17.4 ppm, and 87Sr/86Sr(t) = 0.7032–0.7044 and εNd(t) = +2.7∼+5.5, suggestive of an origin from a mantle wedge source recently modified by both slab-derived and OIB-like components. Our zircon U-Pb geochronological results for the representative samples of five groups indicate that all the five groups formed at 830–859 Ma with a cluster age of ∼830–845 Ma in the early Neoproterozoic, identical to the deposited time of the Lengjiaxi Group and its equivalents. The coexistence of the 830–859 Ma high-Mg intermediate (Group 1) and high-Si adakitic (Group 2) rocks, arc-like basaltic and andesitic (Group 3) rocks, and high-Nb (Group 4) and Nb-enriched (Group 5) mafic rocks in the Chencai Complex suggests an early Neoproterozoic arc-trench setting and the ongoing of subduction until ∼830 Ma. The initial assemblage of the Yangtze with Cathaysia Blocks along the whole Jiangnan Orogen might be roughly synchronous (∼820–830 Ma). An early Neoproterozoic slab window model at subduction termination (∼830 Ma) is proposed, representing an improved understanding of the petrogenesis of the early Neoproterozoic Chencai igneous rocks.

Fingerprints of the Paleotethyan back-arc basin in Central Hainan, South China: geochronological and geochemical constraints on the Carboniferous metabasites

Hainan of Southeast Asia has been regarded as a key area for understanding the Late Paleozoic tectonic regime and amalgamation process of the Indochina with South China Blocks that are not well constrained. This paper presents a set of new geochronological, elemental, and Sr–Nd isotopic data for the Paleozoic Bangxi and Chenxing metabasites in Central Hainan. The geochronological data show that the representative samples yield the 40Ar/39Ar plateau age of 328.1 ± 2.6 Ma and zircon U–Pb age of 330.7 ± 4.4 Ma, respectively. They are SiO2- and TiO2-poor, Al2O3-rich mafic rocks. The Chenxing samples are characterized by left-sloping chondrite-normalized REE and N-MORB-like multi-elemental patterns. The Bangxi samples have the E-MORB-like geochemical affinity. All samples show high e Nd(t) values ranging from +5.61 to +9.85. Such signatures suggest their origination of a MORB-like source with the input of subduction-derived components. Our investigation has verified the presence of the Carboniferous metabasites with both MORB- and arc- like geochemical affinities at the Bangxi-Chenxing area in Central Hainan. In combination with the available data from the Jinshajiang, Ailaoshan, and Song Ma suture zones, it is proposed for the development of a Carboniferous back-arc basin along the Ailaoshan–Song Ma and Central Hainan suture zones in response to the subduction of the Paleotethyan main Ocean.

A melt-focusing zone in the lithospheric mantle preserved in the Santa Elena Ophiolite, Costa Rica

The Santa Elena Ophiolite in Costa Rica is composed of a well-preserved fragment of the lithospheric mantle that formed along a paleo-spreading center. Within its exposed architecture, this ophiolite records a deep section of the melt transport system of a slow/ultra-slow spreading environment, featuring a well-developed melt-focusing system of coalescent diabase dikes that intrude the peridotite in a sub-vertical and sub-parallel arrangement. Here we present an integrated analysis of new structural data, 40Ar/39Ar geochronology, major and trace element geochemistry and radiogenic isotope data from the diabase dikes in order to elucidate the tectonic setting of the Santa Elena Ophiolite. The dikes are basaltic and tholeiitic in composition. Petrological models of fractional crystallization suggest deep pressures of crystallization of >0.4GPa for most of the samples, which is in good agreement with similar calculations from slow/ultra-slow spreading ridges and require a relatively hydrated (~0.5wt.% H2O) MORB-like source composition. The diabase dikes share geochemical and isotope signatures with both slow/ultra-slow spreading ridges and back-arc basins and indicate mixing of a DMM source and an enriched mantle end-member like EMII. The 40Ar/39Ar geochronology yielded an age of ~131Ma for a previous pegmatitic gabbroic magmatic event that intruded the peridotite when it was hot and plastic and an age of ~121Ma for the diabase intrusions, constraining the cooling from near asthenospheric conditions to lithospheric mantle conditions to ~10Ma. Our findings suggest a complex interplay between oceanic basin and back-arc extension environments during the Santa Elena Ophiolite formation. We propose an alternative hypothesis for the origin of Santa Elena as an obducted fragment of an oceanic core complex (OCC).

Are continental “adakites” derived from thickened or foundered lower crust?

The geochemical signatures of “adakites” are usually attributed to high-pressure (≥50 km) partial melting of mafic rocks, and accordingly the occurrence of adakitic magmas in continental settings is frequently used as an indicator of a thickened or foundered lower crust at the time of magma emplacement. These premises are built on experiments and modeling using an MORB-like source, but the probable source of continental “adakites” (i.e., continental lower crust) is compositionally different from MORB. To elucidate the effect of source inheritance and pressure on resultant melts, geochemical analyses and trace-element modeling have been carried out on Jurassic adakitic rocks from the northern part of the North China Craton. The results show that these continental adakitic melts can be generated at depths less than 40 km, and their “adakitic” signature is most likely inherited from their source rocks. Such conclusions can be applied to the Mesozoic adakitic magmas from the interior of the North China Craton. Only the “adakites” from collisional orogens (i.e., Tibet, Dabie UHP belt) require crustal melting at depths greater than 50 km, consistent with collision-induced crustal thickening in these areas. This study therefore highlights the importance of source composition when defining the formation conditions of magmatic rocks in general, and in particular questions the common use of “adakites” as an indicator of specific geodynamic situations.

Eocene supra-subduction zone mafic magmatism in the Sibumasu Block of SW Yunnan: Implications for Neotethyan subduction and India–Asia collision

Abstract Geochemical and isotopic data for metabasic rocks in NW Yunnan constrain the nature of the mantle source beneath the East Himalayan Syntaxis during the Neotethyan subduction and the timing of initial collision between India and Asia. Our results indicate that these mafic rocks crystallized at 50–55 Ma and were metamorphosed at ~ 39 Ma, contemporaneous with magmatic flare-ups and high-grade metamorphism in South Tibet. The mafic rocks in NW Yunnan are divisible into three groups. Group 1, from Nabang, shows geochemical affinity to a MORB-like source modified by the recycled component with e Nd (t) of + 5.1 to + 9.5 and e Hf (t) of + 5.1 to + 11.6, and resembles back-arc basin basalt. Group 2 is mainly from Jinzhouzhai and Tongbiguan to the east of Group 1, and is marked by high Al 2 O 3 , enrichment in LILEs and depletion in HFSEs, along with negative e Nd (t) (− 1.1 to − 5.6) and e Hf (t) (− 4.6 to − 0.9) values. Group 2 is interpreted to have originated from the lithospheric mantle modified by input from a slab-derived component in a continental arc setting. Group 3, from Nanjingli, is located a considerable distance east from Group 1, and exhibits negative e Nd (t) (− 6.7 to − 7.6) and e Hf (t) (− 7.1 to − 2.2), and is inferred to be derived from the Tengchong lithospheric mantle but with limited involvement of a slab-derived component. Overall Nd–Hf isotopic compositions show a sharp decrease from Group 1 to Group 2 and then a more subdued decrease to Group 3, with the three groups corresponding to an overall spatial distribution from west to east. Our results suggest that early Eocene magmatism in NW Yunnan represents the eastward continuation of the Gangdese magmatic belt and that the Neotethyan subduction continued until ~ 50 Ma followed by the India–Asia collision. It is proposed that at least two eastward-dipping subduction zones are involved in the Neotethyan suprasubduction system prior to 55 Ma. The sudden decrease in convergence rate in the early Eocene (55–50 Ma), together with the inferred positive buoyancy of the Tengchong lithosphere, stimulated rollback of down-going slab and induced melting of heterogeneous mantle sources to result in a flare-up in magmatic activity.

Early Neoproterozoic (∼850 Ma) back-arc basin in the Central Jiangnan Orogen (Eastern South China): Geochronological and petrogenetic constraints from meta-basalts

The Neoproterozoic mafic rocks of the Jiangnan Orogen in South China provide a critical geological record for unraveling regional tectonic history and testing different tectonic models. However, the geochronology, geochemistry and petrogenesis of >835Ma mafic igneous rocks in the Central and Southwestern Jiangnan Orogen have not been well reported so far, in contrast to the extensive study of such rocks in the Eastern Jiangnan Orogen. An integrated study of geochronology and geochemistry has been carried out for the basaltic rocks from NE Hunan and NW Jiangxi Provinces of the Central Jiangnan Orogen. SHRIMP, SIMS and LA-ICP-MS zircon U–Pb dating of three mafic samples yielded zircon U–Pb ages of 860±20Ma, 838±12Ma and 847±18Ma, respectively. This provides evidence for the development of early Neoproterozoic (∼850Ma) mafic lava along the Central Jiangnan Orogen. SiO2 ranges from 49.43wt% to 52.07wt% and MgO from 7.36wt% to 9.04wt% with mg-number of 68–57. Their chondrite-normalized REE patterns exhibit a left-sloping pattern with more depleted LREEs relative to HREEs. On the N-MORB-normalized multi-element patterns, they are characterized by enriched highly incompatible elements, depleted moderately incompatible elements relative to those in N-MORB, insignificant Nb–Ta depletion and remarkably positive Sr and weakly negative Ti anomalies, resembling those of modern Eastern Lau back-arc basin basalts. In addition, these rocks have positive ɛNd(t) values of 1.30–9.36 with initial 87Sr/86Sr ratios in the range of 0.70796–0.71126. The geochemical characteristics above indicate an origination of the MORB-like source modified by the flux of the slab-derived fluid and input of the recycled sediment-derived melt. This finding, together with other geological observations, suggests that the mafic rocks in the Central Jiangnan Orogen might be formed in a back-arc basin in the eastern SCB. In conjunction with other data, it is inferred that the closure of the back-arc basin along the Central Jiangnan Orogen is contemporaneous with that in the Fuchuan area.

Earliest Neoproterozoic (ca. 1.0 Ga) arc–back-arc basin nature along the northern Yunkai Domain of the Cathaysia Block: Geochronological and geochemical evidence from the metabasite

The tectonic nature of the Cathaysia Block during earliest Neoproterozoic period has been long disputed and attracted considerable attentions. One reason for this problem is rare exposure and few data for the igneous rocks of Grenvillian age in this Block. In this paper, a set of new U–Pb zircon geochronological, elemental and Nd isotopic data are presented for the metabasites in the Yunkai Group which shows WNW-trending extension with a length of more than 100km along the northern Yunkai Domain of the Cathaysia Block. The plagioclase amphibolite and metabasite from Shiwo (Beiliu) and Tantu (Xinyi) from the northern Yunkai Domain, representative of Groups 1 and 2, yielded the 206Pb/238U weighted mean ages of 997±21Ma and 978±19Ma, respectively, evidencing the presence of the Grenvillian mafic rocks within the Cathaysia Block. Based on the Nb, Nb/La and Nb/U, these samples can be geochemically subdivided into two groups (Groups 1 and 2). Group 1 has Nb/La=0.33–0.54 and Nb=1.71–2.85ppm, and exhibits similar geochemical signatures to those of Saunders island-arc. Group 2 gives Nb/La of 0.57–0.86 and Nb of 7.58–12.70ppm, resembling to those of Phanerozoic Nb-enriched basalts, and shows the geochemical affinity to the Okinawa back-arc basin basalt. Both groups have positive ɛNd(t) values, ranging from +3.84 to +5.36 and +3.93 to +6.55, respectively. These geochemical and isotopic characteristics suggest that Group 1 might have been originated from a MORB-like source modified by subduction-derived fluids. The source of Group 2 Nb-enriched basalt might be metasomatized by melt released from a newly subducted slab. In conjunction with other geological observations, it is identified for the Grenville-aged (ca. 1.0Ga) arc and back-arc basin along the northern Yunkai Domain of the Cathaysia Block, analog to the modern West Pacific. The South China Block probably located at the margin of Rodinia supercontinent between west Australia and east India.

Geochemistry and petrogenesis of mafic sills in the 1.1 Ga Umkondo large igneous province, southern Africa

The detailed petrogenesis of mafic sills occurring throughout southern Africa provides strong support for the development of an Umkondo large igneous province on the eastern margin of the Kalahari craton at 1.1Ga. The sills are most extensively developed in the Waterberg and Middelburg basins in northern South Africa and south-eastern Botswana. They are typical fractionated continental tholeiites with subophitic to ophitic dolerites, gabbros and gabbro-norites, and largely basaltic andesite in composition. The vast majority of the sample set defines one major geochemical subgroup, here referred to as the Mesoproterozoic Post-Waterberg sills A (MPWA sills), which is characteristically LREE enriched with relatively unfractionated HREEs, and with normalised incompatible element profiles similar to modern island arc andesites. A small number from the sample set define a minor subgroup (MPWB sills), which has so far only been recognised in the Middelburg basin, South Africa and which is characterized by fractionated HREEs. Both the major and trace element geochemical signatures of the MPWA sills are indistinguishable from the type Umkondo sills and less common lavas documented from Eastern Zimbabwe and mafic sills on the Grunehogna craton in present day Eastern Antarctica. This provides strong supporting evidence for an Umkondo large igneous province developed on the Kalahari craton at 1.1Ga. Despite crustal-type Sr–Nd isotopic signatures in the MPWA sills, bulk contamination by the continental crust is ruled out in favour of derivation from a primitive mantle-like asthenospheric source with a contribution from the subcontinental lithospheric mantle modified by a previous subduction event. The smaller MPWB magma type could represent a smaller degree melt at greater depth from a modified MORB-like source, although the relationship between the two subgroups remains unclear.

Geochemistry of picrites and associated lavas of a Devonian island arc in the northern Junggar terrane, Xinjiang (NW China): Implications for petrogenesis, arc mantle sources and tectonic setting

The Junggar terrane in China's Xinjiang Province sits adjacent to the Siberian craton and Kazakhstan block. A nearly 100-m thick series of high-Mg lava flows is found in a Devonian arc in the Junggar terrane. The highly porphyritic high-Mg lavas are stratigraphically in the lower part of the Middle Devonian Beitashan Formation, and are overlain by basalts and andesites. Based on the chemistry and phenocryst contents, the high-Mg lavas are classified into picrites and ankaramites. The former are characterized by a large amount of olivine phenocrysts (Fo 79–82) with minor clinopyroxene (En 44–50Fs 1–10Wo 44–48) and rare chrome spinel with Cr# (Cr/Cr + Al) values between 0.63 and 0.86, whereas clinopyroxene phenocrysts are dominant in ankaramite. The groundmass in the picrites and ankaramites consists of plagioclase, clinopyroxene, and Ti-magnetite. Glass is not preserved, but minor metal sulfides are present. Based on olivine-melt equilibrium, the primitive magmas had approximately 9 wt.% MgO, and the high-Mg lavas contain accumulated olivine (picrites) and clinopyroxene + olivine (ankaramites). All lavas have similar primitive mantle normalized trace element patterns, characterized by negative Nb, Ta and Ti anomalies, as typical for island arc volcanic rocks. These relationships suggest that the rock suites are co-magmatic. The Zr/Nb ratios (23–66) of the picrites and basalts resemble MORBs (10–66), suggesting MORB-like sources for the picrites. The picritic rocks have slightly LREE-enriched patterns with (La/Yb) n ranging from 1.6 to 3.5 and (La/Sm) n from 1.2 to 2.1. In contrast, basaltic rocks have flat to slightly LREE-enriched patterns with (La/Yb) n ranging from 1.2 to 5.7 and (La/Sm) n from 0.8 to 2.5, whereas andesitic rocks have modestly LREE-enriched patterns with (La/Yb) n ranging from 2.8 to 9.1 and (La/Sm) n from 1.7 to 2.9. The REE patterns for all rocks do not exhibit Eu anomalies. The rocks have similar ( 87Sr/ 86Sr) t (0.7033–0.7043, t = 385 Ma) and ε Nd( t) values (6.4–7.3), overlapping with the present day island arc field. Consequently, primary magmas were most likely generated in N-MORB-type mantle, which was modified by the addition of a fluid component derived from altered subducted oceanic crust as indicated by elevated Sr/Nd and low Th/Yb ratios. REE modelling suggests that the primary magmas were derived from garnet–spinel transition zone mantle.

Geochemistry of post-collisional mafic lavas from the North Anatolian Fault zone, Northwestern Turkey

Extensive magmatic activity developed at the northwestern part of the Anatolian block and produced basaltic lavas that are situated along and between the two segments of the North Anatolian Fault zone. This region is a composite tectonic unit formed by collision of continental fragments after consumption of Neotethyan ocean floor during the late Cretaceous. Northwestern Anatolian basalts and evolved lavas exhibit both tholeiitic and calc-alkaline characteristics. Mafic lavas are moderately enriched in LILE (except depleted part of Yuvacık and İznik samples) and depleted in HFSE (but not Zr, Hf) relative to primitive mantle values, suggesting derivation from a MORB-like mantle source that is unexpected in this subduction environment. Sr and Nd isotopes are close to the mantle array and vary beyond analytical error ( 87Sr/ 86Sr 0.70404–0.70546, 143Nd/ 144Nd 0.51270–0.51289). These geochemical features may result from two possible processes: (1) melting of a MORB-like mantle source that was modified by subduction-released fluids and melts or (2) modification of mafic liquids derived from a dominantly MORB-like source by crustal or lithospheric mantle material. Geochemical characteristics of the lavas (e.g., Ba/Rb, Rb/Sr, Ba/Zr, 87Sr/ 86Sr, Sr/P) vary systematically along the fault zone from east to west, consistent with a decrease in the degree of melting from east to west or a change in the nature of the source composition itself. Thus, the difference in incompatible elements and Sr–Nd isotopic ratios seems to result from small-scale mantle heterogeneity in a post-collisional tectonic environment.