Minimal Crustal Contamination Research Articles

Slab break-off of the Kalamaili oceanic slab revealed by the latest Carboniferous mafic–ultramafic rocks in eastern North Tianshan (NW China)

Identifying the tectonic transition from oceanic subduction to collision is crucial for tracking the final stage evolution of ancient orogenic belts. In this study, we present new geochronological and geochemical data for the Mozbaysay mafic–ultramafic complex in the Balikun area, eastern North Tianshan of the southern Central Asian Orogenic Belt. This complex had intruded into the late Carboniferous volcano-sedimentary rocks and is comprised mainly of hornblende gabbro and lherzolite. Zircon U-Pb ages of the hornblende-gabbros reveal that this complex was emplaced at ca. 305 Ma. Geochemical analyses suggest these mafic–ultramafic rocks are characterized by slight enrichment of light rare earth elements (LREEs) and relatively depleted heavy rare earth elements (HREEs), resembling enriched mid-ocean ridge basalt (E-MORB). They also exhibit restricted (87Sr/86Sr)i ratios (0.702396–0.704295) and εNd(t) values (+7.0 to +9.1), indicative of a depleted mantle source with minimal crustal contamination. Incompatible element ratios (i.e., Nb/Ta, Zr/Hf, Rb/Nb, and Ba/Nb) suggest the involvement of subducted slab-derived aqueous fluids in their mantle source. These collectively indicate that the parental magmas of the Mozbaysay mafic–ultramafic rocks may have been generated by a mixed mantle source consisting of the E-MORB-like asthenospheric mantle, subcontinental lithospheric mantle (SCLM), and hydrous fluids from subducted slab. Furthermore, a slab break-off model is proposed to explain the generation of these latest Carboniferous mafic–ultramafic rocks. Integrating these findings with regional geological data, we propose that the tectonic transition from subduction (slab roll-back) to collision (slab break-off) along the Kalamaili suture zone occurred at ca. 305–300 Ma.

Geochemical characteristics of Neoproterozoic metavolcanic rocks of Ariab Auriferous Volcanogenic Massive Sulfide deposit, Red Sea hills, North-East Sudan

The Ariab volcanogenic massive sulfide (VMS, ∼118.3 Mt with >2 Moz Au recovered) district is part of the Ariab-Arbaat belt located in the Haya terrane, Red Sea Hills, Northeast Sudan. The gold-bearing gossan and VMS deposits are mainly hosted by Neoproterozoic bimodal volcanic succession. The petrogenesis, age dating, metallogenic evolution, and tectonic environment as well as the relationship between the metals occurrences and hosting materials remain poorly constrained and controversial. The study aims to investigate the geochemical characteristics of metavolcanic rocks of Ariab Auriferous Volcanogenic Massive Sulfide. XRF and ICP-MS techniques were employed to analyze major, trace, and rare earth elements content. The metavolcanics composition of Ariab VMS hosting rocks ranging from basalt to rhyolitic lava with variable pyroclastics. The chemical affinity varies from sub-alkaline tholeiitic to calc-alkaline nature, raising an argument against the possibility of fractional crystallization. The Ariab VMS metavolcanics were likely formed in the intra-oceanic arc tectonic environment. Ariab mafic lava is characterized notably by high Cr and Ni concentrations. The negative and positive anomalies in trace element profiles are typically ocean island basalt patterns, implying minimal crustal contamination. The geochemical results of dacitic and rhyolitic rocks reveal an assimilation-fractional-crystallization process, including the possibility that plagioclase-rich restite was formed in the parent magma source with a significant contribution from the old crust. The mobility of elements such as Zn, Cu, Pb, Ag, Au, As, and Co complicates the determination of their primary abundances. The partitioning behavior of REEs, the geochemical signatures of chalcophile elements, and their associations with VMS deposits, underscores the complex interaction between igneous processes and subsequent hydrothermal alteration.

Magmatic evolution during proto-oceanic rifting at Alu, Dalafilla and Borale Volcanoes (Afar) determined by trace element and Sr-Nd-Pb isotope geochemistry

Continental rifting and associated magmatism can eventually result in the formation of new ocean basins. However, the characteristics of magmatism in the latest stages of rifting are poorly understood. The Erta-Ale volcanic segment (EAVS) in the Danakil Depression of Afar, Ethiopia, provides a unique natural laboratory in which to investigate how magma generation evolves during the shift from continental rifting to oceanic spreading. Here we present new trace element data combined with Sr-Nd-Pb isotope ratios for three volcanoes, Alu, Dalafilla and Borale, in the north of the EAVS. These data shed light on the changes in melt production and storage that occur at this late stage in the rifting cycle. Elevated Ce/Pb and ΔNb (33–48, 0.25–0.47 respectively) of the basalts, alongside Sr-Nd-Pb isotope geochemistry indicate the presence of a HIMU component, supplied by the Afar plume, together with contamination by the crust. Melting conditions, estimated using the trace element ratios, Smn/Ybn, Dyn/Ybn and Cen/Smn, indicate that magmas were primarily derived from spinel lherzolite (85–90%) with minor garnet lherzolite (10–15%) with a melt fraction of ~4%. Melt-mantle equilibrium depths are estimated to be on the order of 64 to 83 km, shallower than that previously inferred within Afar. We suggest that this is likely a result of the increased plate thinning beneath the EAVS compared to other parts of Afar. Basaltic volcanics are found to exhibit heterogeneous Sr-Nd-Pb isotope compositions whilst more evolved rocks (i.e., SiO2 ≥52 wt%) exhibit consistent radiogenic compositions. This indicates that homogenisation of all melt compositions occurs prior to or during melt differentiation, with the latter process occurring rapidly in the upper crust with minimal crustal contamination. Overall whilst the Afar plume appears to be the dominant mantle component in the volcanic rocks, the melt characteristics and magmatic storage conditions beneath the EAVS shows variability that is likely controlled by a dynamic interplay between rifting and mantle processes.

Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu ± Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry

Abstract Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2·74 Ga, ~2·70 Ga, and ~ 2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4·5 ± 0·3, 4·2 ± 0·6, and 4·3 ± 0·4, and δ18O ± SD values of 5·40 ± 0·11 ‰, 3·91 ± 0·13 ‰, and 4·83 ± 0·12 ‰, respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as ΔFMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ –0·3 ± 0·6 to ΔFMQ +0·8 ± 0·4 and to ΔFMQ +1·2 ± 0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1·6 ± 0·3 and ΔFMQ +2·6 ± 0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ΣS) in primary apatite yielded ≥ΔFMQ −0·3 and ΔFMQ +1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3·3 ± 1·3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0·8 ± 0·4) to moderately (~ΔFMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.

Geochemical evidence for a Ganderian arc/back-arc remnant in the Nashoba Terrane, SE New England, USA

New geochemical data including Sm/Nd isotopic data show evidence for an early Paleozoic arc/back-arc complex in the Nashoba terrane of southeastern New England. The Nashoba terrane lies between rocks of Ganderian affinity to the northwest and Avalonian affinity to the southeast. It consists of early Paleozoic mafic to felsic metavolcanic and metasedimentary rocks that were intruded by intermediate to felsic plutons and metamorphosed to upper amphibolite facies conditions in the mid-Paleozoic. Major and trace element geochemical data indicate that the early Paleozoic igneous rocks contain a mix of arc, MORB, and alkaline signatures, and that the terrane formed as a primitive volcanic arc/back-arc complex built on thinned continental crust. Amphibolites have +4 to +7.5eNd(500) values consistent with formation in a primitive volcanic arc with minimal crustal contamination. Intermediate and felsic gneisses have eNd(500) values between +1.2 and –0.75 indicating a mixture of juvenile arc magmas and an evolved (likely basement) source. Depleted mantle model ages of 1.2 to 1.6 Ga point to a Mesoproterozoic or older age for this source. Metasedimentary rocks yielded –6 to –8.3 eNd(500) values and 1.6 to 1.8 Ga model ages, indicating an isotopically evolved source (or sources) that included Paleoproterozoic or older material. The eNd(500) values and model ages of the intermediate and felsic and metasedimentary rocks indicate that the basement to the Nashoba terrane is Ganderian rather than Avalonian. The Nashoba terrane therefore represents a Ganderian arc/back-arc complex similar to the Cambrian Penobscot arc/back-arc seen in Maritime Canada and Newfoundland, and particularly in the Annidale and New River terranes of southern New Brunswick. This correlation has not previously been recognized in southeastern New England. The Ganderian affinity of the Nashoba terrane also extends Ganderia farther SE in New England than previously established and indicates that the Nashoba terrane did not originate as a separate oceanic arc/back-arc complex or microcontinent.

Two suites of gabbros in the Buem Structural Unit, of the Pan-African Dahomeyide orogen, southeastern Ghana: Constraints from new field and geochemical data

The Buem Structural Unit (BSU) of the Pan-African Dahomeyide orogenic belt, southeastern Ghana, is characterized by the occurrence of clastic and chemical sedimentary rocks, volcanic rocks and mafic-ultramafic rocks. The mafic rocks, comprising mainly gabbros occur in smaller proportion within the BSU. Representative samples of the gabbros have been analyzed petrographically and geochemically to infer their mode of emplacement, petrogenesis and tectonic setting. Two distinct gabbro types have been identified from petrography and whole-rock major and trace elements geochemistry, namely the B1 and B2 gabbros. The gabbros are generally phaneritic, holocrystalline and composed of plagioclase and clinopyronexe which have been replaced either completely or partially by sericite, chlorite and epidote. On the whole, B1 gabbros are deformed, altered, alkaline in nature, show no significant effect of crustal contamination and has affinity to OIB/E-MORB. The B2 gabbros, are relatively less deformed, subalkaline, akin to N-MORB and show arc signatures with minimal crustal contamination. The geochemical characteristics of the B1 gabbros are similar to the Buem volcanic rocks. The similarity of the B1 gabbros to the volcanic rocks may suggest a similar source and tectonic setting. Thus, the B1 gabbros may be related to rifting and emplacement at the eastern margin of the West African Craton (WAC). The effect of minimal crustal contamination and the arc nature of the B2 gabbros may suggest subduction related magmatism. Taken together, the occurrence of the gabbros may be related to rifting and subduction-collision at the eastern margin of the WAC.

Source and evolution of the alkaline Pilanesberg Complex, South Africa

The Pilanesberg Complex (South Africa) is one of the world's largest but least studied alkaline complexes. It consists of trachytes, phonolites, syenites and nepheline syenites (foyaites) and the preservation of the volcanic carapace makes it unique among the larger alkaline complexes. The intrusive history of the Pilanesberg Complex shows similarities to the Greenland Kangerlussuaq Intrusion, and our new whole-rock major and trace element analyses, combined with existing data, show that the complex belongs to the Sr-rich type of evolved alkaline rocks, more similar to the complexes of the Kola Peninsula than Ilímaussaq. Despite the absence of mafic lithologies, comparison with experimental studies shows that the parental magma was most likely an alkali basalt. Significant iron enrichment is caused by an early stage of fractionation involving clinopyroxene and amphibole rather than olivine and plagioclase, reflecting water-rich compositions and intermediate levels of oxygen fugacity. This fractionation trend has led to strong enrichment in Sr and Ba, but only moderate levels of Y and middle to heavy rare earth elements, and minimal Eu-anomaly. Late-stage water-rich fluids caused significant autometasomatism in most units. New U-Pb dating of titanite constrains the age of the Pilanesberg Complex as 1395 +10/−11Ma. Initial 87Sr/86Sr isotope ratios around 0.7028 (εSr1395=−1) are typical for a moderately depleted mantle source, unlike the local lithospheric mantle. The combination of an enriched trace-element signature and depleted isotopic characteristics is evidence for small degrees of partial melting. Epsilon Sr values are similar to those reported for other alkaline complexes worldwide, emplaced in crust with contrasting geological histories; this likely reflects lithospheric metasomatism shortly before magmatism and minimal crustal contamination.

Origin of Permian extremely high Ti/Y mafic lavas and dykes from Western Guangxi, SW China: Implications for the Emeishan mantle plume magmatism

Late Permian mafic flows and dikes are prominent features in and around the Western Guangxi region in southern China. Based on petrographic, geochemical and Sr-Nd isotopic data, the western Guangxi mafic rocks are geochemically akin to the Emeishan large igneous province (ELIP) high-Ti basalts, except that they possess extremely elevated Ti/Y ratios (750–2000). The Dy/Yb and Ti/Y vs. Dy/Dy∗ covariations of the mafic rocks indicate a garnet-controlled magmatic differentiation of a mafic melt at relatively great depth. The limited εNd(t) range from +0.41 to +1.81 also suggests minimal crustal contamination of such a melt. Geochemical modeling using TiO2/Yb vs. Nb/Yb and Zr/Y vs. Nb/Y projections indicate that the parental melts of the western Guangxi mafic rocks formed at a low degree (<5%) of partial melting at or over 3.5GPa, consistent with a deep mantle plume source under a thick continental lithosphere. Thus, the Guangxi extremely high Ti/Y mafic rocks most likely represent a part of outer zone of the ELIP plume magmatism. Results of this study reinforce the previously proposed temporal and spatial distribution of the ELIP.

Petrogenesis of the A-type, Mesoproterozoic Intra-caldera Rheomorphic Kathleen Ignimbrite and Comagmatic Rowland Suite Intrusions, West Musgrave Province, Central Australia: Products of Extreme Fractional Crystallization in a Failed Rift Setting

The Pussy Cat Group rhyolites of the Mesoproterozoic west Musgrave Province of central Australia, a constituent part of the Bentley Supergroup, were deposited during the c. 1085–1040 Ma Ngaanyatjarra Rift and Giles events, and are related to the Warakurna Large Igneous Province. This study focuses on the two silicic components of the Pussy Cat Group, the Kathleen Ignimbrite and the Rowland Suite. These silicic rocks are A-type, metaluminous (to slightly peraluminous) rhyolites and are enriched in the rare earth elements (REE) relative to average crustal abundances. The rhyolitic Kathleen Ignimbrite records an explosive caldera fill-sequence and contains, amongst others, a thick (≤500 m), initially subaqueously emplaced, rheomorphic, intra-caldera ignimbrite unit, whereas the Rowland Suite consists of a number of mineralogically and geochemically related porphyritic rhyolites that intrude throughout the Pussy Cat Group. Whole-rock geochemistry, Rb–Sr, Sm–Nd and in situ zircon Lu–Hf isotope data are indicative of a dominantly mantle-derived source for the magmas that formed the Pussy Cat Group rhyolites. Secondary ion mass spectrometry U–Pb dating of these units yields ages of 1062 ± 8, 1071 ± 5, 1076 ± 5, and 1078 ± 5 Ma. The magmas that formed these units were formed by extreme fractional crystallization of a mantle-derived basaltic magma, with minimal crustal contamination, during a failed intra-plate extensional rifting event. This involved three main stages of fractional crystallization: early fractionation of plagioclase, olivine, clinopyroxene and magnetite from a basaltic magma to reach an intermediate composition, subsequent fractionation of plagioclase, K-feldspar and quartz to form a proto-Rowland Suite-type magma at mid- to upper-crustal levels that migrated into the shallow upper crust and formed a magma chamber, and final fractionation of quartz, K-feldspar, plagioclase, magnetite and biotite ± minor REE-enriched accessory phases from the Rowland Suite magma resulting in the evolved Kathleen Ignimbrite magmas. This final phase of fractionation generated the most evolved silicic rock suite identified to date within the entire west Musgrave Province. The new petrographic, geochronological, geochemical, and isotopic data presented within this study indicate that these two units are coeval and comagmatic, suggesting a common source for the Kathleen Ignimbrite and the entire Rowland Suite. In addition, these data suggest that the crystal-rich, porphyritic rhyolite intrusions of the Rowland Suite represent a primitive cumulate end-member of the magmatic system, whereas the varying crystal-poor to crystal-rich Kathleen Ignimbrite eruption sequence represents the evolved and highly fractionated end-member of the system that formed thorough the evacuation of a shared or at least partly linked, compositionally zoned and differentiated source magma chamber or chambers.

Petrogenesis and tectonic significance of the Baoxing granitic and mafic intrusions, southwestern China: Evidence from zircon U–Pb dating and Lu–Hf isotopes, and whole-rock geochemistry

Voluminous Neoproterozoic gabbros, gabbro-diorites, and granites are exposed in the Baoxing area along the western margin of the Yangtze Block. We present here zircon U–Pb and Lu–Hf isotope data, as well as whole-rock geochemical data for these rocks, which help constrain the tectonic characteristics and evolution of the western margin of the Yangtze Block. Zircons from these rocks have high Th and U contents and Th/U ratios (>0.2), and show striped absorption patterns, growth striations, or zoning, indicating a typical magmatic origin. Zircon U–Pb dating and Lu–Hf isotope results show that the gabbros formed at ca. 850Ma with εHf=+5.44 to +11.94, and a single-stage model age (THf) of 1.04Ga, suggesting a depleted-mantle-derived source. The gabbro-diorites formed at ca. 800Ma with major εHf=+3.99 to +14.62 and THf=0.97Ga, and minor εHf=+0.68 to +5.66 and THf=1.27Ga, again suggesting a depleted-mantle-derived source. The granites were emplaced at ca. 770Ma with εHf=+2.53 to +10.58 and THf=1.07Ga, indicating that they were derived from the partial melting of juvenile crust. The geochemical data show that the gabbros and gabbro-diorites are characterized by low SiO2 contents, high Mg# values and Cr, Co, and Ni contents, significant enrichment in light rare-earth elements and large-ion lithophile elements, depletion in high field strength elements (e.g., Nb, Ta, Zr, and Hf), and relatively high Nb/U, Ta/U, and Ce/Pb ratios. These data suggest that the rocks were derived from the partial melting of depleted lithospheric mantle that had been metasomatized by subduction-derived fluids or melts, and which underwent very minimal crustal contamination during magma evolution and ascent. The granites have high SiO2 and K2O, and low MgO contents, exhibit enrichment in Rb, Ba, Th, and Pb and depletion in Nb, Ta, and Ti, and have low Sr/Y and (La/Yb)N ratios and high Y and Yb contents, which together with their Hf isotopic compositions imply a juvenile crust source. Taking into account regional pre-750Ma arc-related magmatism and the subsequent extensional setting, it is proposed that a long-lived, active continental-margin setting may have evolved along the western margin of the Yangtze Block during the Neoproterozoic, and that the coeval granitic and mafic rocks in the Baoxing area were formed by the subduction of the western Yangtze Block.

Fluid origin, gas fluxes and plumbing system in the sediment-hosted Salton Sea Geothermal System (California, USA)

The Salton Sea Geothermal System (California) is an easily accessible setting for investigating the interactions of biotic and abiogenic geochemical processes in sediment-hosted hydrothermal systems. We present new temperature data and the molecular and isotopic composition of fluids seeping at the Davis-Schrimpf seep field during 2003–2008. Additionally, we show the first flux data for CO 2 and CH 4 released throughout the field from focused vents and diffuse soil degassing. The emitted gases are dominated by CO 2 (~ 98%) and CH 4 (~ 1.5%). By combining δ 13C CO2 (as low as − 5.4‰) and δ 13C CH4 (− 32‰ to − 17.6‰) with 3He/ 4He (R/Ra > 6) and δD CH4 values (− 216‰ to − 150‰), we suggest, in contrast to previous studies, that CO 2 may have a significant Sub-Continental Mantle source, with minimal crustal contamination, and CH 4 seems to be a mixture of high temperature pyrolitic (thermogenic) and abiogenic gas. Water seeps show that δD and δ 18O increase proportionally with salinity (Total Dissolved Solids in g/L) ranging from 1–3 g/L (gryphons) to 145 g/L (hypersaline pools). In agreement with elemental analyses, the isotopic composition of the waters indicate a meteoric origin, modified by surface evaporation, with little or no evidence of deep fossil or magmatic components. Very high Cl/Br (> 3,000) measured at many seeping waters suggests that increased salinities result from dissolution of halite crusts near the seep sites. Gas flux measurements from 91 vents (pools and gryphons) give a conservative estimate of ~ 2,100 kg of CO 2 and 11.5 kg of CH 4 emitted per day. In addition soil degassing measured at 81 stations (20x20 m grid over 51,000 m 2) revealed that 7,310 kg/d CO 2 and 33 kg/d CH 4 are pervasively released to the atmosphere. These results emphasise that diffuse gas emission from soil can be dominant (~ 75%) even in hydrothermal systems with large and vigorous gas venting. Sediment-hosted hydrothermal systems may represent an intermediate class of geologic methane sources for the atmosphere, with emission factors lower than those of sedimentary seepage in petroleum basins but higher than those of traditional geothermal-volcanic systems; on a global scale they may significantly contribute to the atmospheric methane budget.

Petrogenesis of Cenozoic mafic–ultramafic alkaline lavas from the Tigris volcanic field, NE Syria

AbstractMafic–ultramafic Quaternary lava flows form the Tigris volcanic field (covering 1750 km2) at the northeastern tip of Syria and extend into Turkey. This volcanic field occurs between the Euphrates graben and the Bitlis–Zagros collision suture that forms the boundary between the Arabian and Eurasian plates. The rocks are made up of labradorite, clinopyroxene, olivine and opaque phases. The Tigris lavas are compositionally restricted to basanites and alkali basalts, having a narrow range of major element compositions (SiO2, 42.2–48.2 wt%; MgO, 5.7–9.0 wt%, with Mg numbers ranging from 0.51 to 0.62; TiO2, 1.7–3.2 wt%), and are alkaline in nature. The rocks are enriched in HFS elements such as Zr (119–231 ppm), Nb (14–43 ppm) and Y (17–22 ppm). The REE patterns are strongly fractionated ((La/Yb)N= 10.6), indicative of a garnet-bearing source. The143Nd/144Nd isotopic compositions range from 0.512803 to 0.512908, and87Sr/86Sr from 0.70327 to 0.70403 (εNd= 3.2–5.3) suggesting strong affinities to ocean island basalts. Modelling using a variety of mantle source materials and different degrees of partial melting indicates that the magma was produced by a small degree of batch partial melting (F = 1.5%) of a primitive, garnet-lherzolite fertile mantle source. The overall petrological/chemical nature supports this interpretation. Shear heating at the base of the lithospheric mantle of the northern boundary of the Arabian plate, caused by a change in plate motion as the Arabian plate moved in a more easterly direction during the Plio-Quaternary, could represent a possible source of the heat necessary for partial fusion and magma generation. Adiabatic decompression and melting represents a more likely process for the generation of the Tigris magma. Elemental ratios such as K/P (4.6), La/Ta (12), La/Nb (0.90), Nb/Y (1.22) and Th/Nb (0.09) indicate that the magma was subjected to minimal crustal contamination.

Dynamic Magma Systems, Crustal Recycling, and Alteration in the Central Sierra Nevada Batholith: the Oxygen Isotope Record

Values of δ18O of zircon from the central Sierra Nevada batholith (SNB), California, yield fresh insight into the magmatic evolution and alteration history of this classic convergent margin batholith. Direct comparison of whole-rock and zircon (Zrc) δ18O provides evidence for modest (0.5‰), but widespread, alteration, which has complicated interpretation in previous whole-rock δ18O studies. Four discrete belts of δ18O values are recognized in the central Sierra. A small belt of plutons with relatively low δ18O(Zrc) values (5·2–6·0‰) intrudes the foothills, with a sharp increase of δ18O revealing the concealed Foothills Suture; high δ18O(Zrc) values (7·0–8·5‰) dominate the rest of the western SNB. East of the axis of the Sierra, δ18O is distinctly lower (6·75–5·75‰), and decreases monotonically to the Sierra Crest. A sharp 1‰ increase of δ18O in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-δ18O North American crust in the White Mountains and Owens and Long Valleys. Correlated O, Sr, and Pb isotope ratios reveal differences in magma generation between the western and eastern Sierra. The western Sierra experienced massive crustal recycling, with substantial melting and mobilization of accreted oceanic and volcanic arc rocks; crustal contamination affects many western SNB plutons. In contrast, the eastern Sierra was dominated by voluminous recycling of the lithospheric mantle and lower crust, with minimal crustal contamination. Batholith-wide shifts in δ18O occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, δ18O may be unchanged (Tuolumne); increase (Sonora and Whitney); or decrease (Sequoia and John Muir) with time. These trends show stable long-lived sources, or those where recycling and contamination may increase or decrease with time. Overall, δ18O reveals diverse magma system behavior at a range of scales in the Sierran arc.

The Euphrates volcanic field, northeastern Syria: petrogenesis of Cenozoic basanites and alkali basalts

AbstractThe Plio-Quaternary Euphrates volcanic field of NE Syria includes large discontinuous exposures of basanitic and basaltic lava flows (1200 km2in area). It represents the northern segment of the Cenozoic volcanic province of the Middle East and is located near the Bitlis collision suture. The rocks consist of olivine (15–20%), clinopyroxene (30–35%), plagioclase (45–55%) and opaque phases. Chemically, the rocks are largely ultrabasic (SiO238.2–45.5 wt%, MgO 8.7–13.0 wt% and average Mg number of 0.65). They are enriched in incompatible trace elements such as Zr (133–276 ppm), Nb (25–71 ppm) and Y (17–28 ppm). The REE patterns are strongly fractionated ((La/Yb)N= 19.6), indicative of a garnet-bearing source. The143Nd/144Nd isotopic compositions range from 0.512868 to 0.512940 (εNd= 4.5 to 5.9), and87Sr/86Sr from 0.70309 to 0.70352. These chemical and isotopic compositions reflect strong affinities to OIB. Elemental ratios such as K/P (3.4), La/Ta (13) and La/Nb (0.77), and the low SiO2values, suggest that the Euphrates magma was subjected to minimal crustal contamination. Petrogenetic modelling has been carried out using a variety of mantle source materials, different degrees of partial melting (0.1 to 10%), and a number of scenarios including metasomatized sources. Modelling suggests that the magma could have been produced as a result of a small degree of partial melting of either (1) a garnet-bearing depleted source enriched with a small addition of metasomatizing fluids, or (2) a garnet-bearing fertile source. The overall chemical and petrological characteristics are more consistent with the generation of the Euphrates magma by a small degree of partial melting (F = 1%) of a primitive, garnet-lherzolite mantle source, possibly containing a minor spinel component. The Neogene collision of the Arabian plate with Eurasia along the Bitlis suture resulted in reactivation (beneath the Euphrates basin) of deep-seated fractures, along which lavas may have penetrated the crust.

High Fe–Ti mafic magmatism and tectonic setting of the Paleoproterozoic Broken Hill Block, NSW, Australia

We present petrographic, geochemical (major, traces and rare earth elements (REE)) and isotopic (Sm–Nd and Rb–Sr) data from ca. 1685 Ma mafic rocks of the Willyama Supergroup in the Broken Hill Block of western NSW, Australia. The mafic rocks occur throughout the lower Willyama Supergroup stratigraphy and are interpreted here as shallowly emplaced sills that were metamorphosed to upper amphibolite and granulite facies during the Olarian Orogeny (ca. 1600–1580 Ma). Our data indicate that the metabasites originated as a result of variable degrees of partial melting of a depleted mantle source, but slightly enriched with incompatible elements compared to present day N-MORB. This was followed by simple crystal fractionation or by an assimilation-fractional crystallisation (AFC) process involving only small degrees of crustal assimilation (rate of assimilation to rate of crystal fractionation, r = 0.05–0.2). Crystal fractionation proceeded along a tholeiitic trend of extreme primary iron and titanium enrichment, leading to melts with up to 25 wt% of total iron as Fe 2O 3 and 4.2 wt% of TiO 2. Volumetrically minor intermediate rocks evolved from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Rasp Ridge and Hores/Potosi Gneisses) are not linked by fractional crystallisation to the mafic melt that produced the meta-igneous amphibolites and are products of the anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bimodal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Block (ca. 1685 Ma) was the extensional axis and depositional centre of an advanced stage intra-cratonic rift with relatively thin crust and lithosphere. Data from the neighbouring Olary Domain, in contrast, imply smaller degrees of partial melting, higher degrees of crustal contamination and the presence of a subcontinental lithospheric mantle source, which suggests relatively thicker lithosphere and places the Olary Domain on the rift margin. Active faulting during the rift stage, coupled with submarine sedimentation and an anomalous geothermal gradient driven by lithospheric thinning, provide an ideal theoretical environment for the formation of the Broken Hill Pb–Zn–Ag orebody.

Noble gases in corundum megacrysts from the basalts in Changle, Shandong Province, eastern China

This study presents noble gaseous data of the corundum megacrysts from the Cenozoic basalts in Changle, Shandong Province, eastern China. It is known that no noble gaseous data of corundum megacryst have been documented before. The 3He/4He ratios (1.13–7.37 Ra) of the corundums from Changle vary from atmosphere to MORB values; the 20Ne/22Ne (9.67–10.75) and 21Ne/22Ne (0.0280–0.0372) data define two linear trends on Ne three-isotope diagram, respectively, along the MFL and the correlation line between atmosphere and MORB; the 38Ar/36Ar (0.177–0.194) ratios, the 40Ar/36Ar (280.9–404.2) ratios and the 128–136Xe/132Xe ration with obvious 129Xe excess are generally higher than atmospheric component, but the 40Ar/36Ar ratios are much closer to atomospheric ratio. The isotopic compositions of noble gases (particularly for He and Ar) of the corundums are similar to those of pyroxene, anorthoclase megacrysts, and mantle-derived xenoliths from this area, and those of mantle-derived xenoliths from several areas in eastem China. Therefore, the noble gases trapped in the corundums probably are from mantle source, representing a ‘mixed fluid’ produced by the interaction between the lithospheric mantle and fluids releasing from the convective plate. Both the noble gas isotopic compositions and the oxygen isotopic compositions of the solid corundums are not the characteristics of crustal source. These suggest that the corundums crystallized from mantle-derived magmas with minimal crustal contamination.

High Fe-Ti mafic magmatism and tectonic setting of the Broken Hill Inlier, N.S.W. Australia

We present petrographic, geochemical (major, traces and REE) and isotopic (Sm-Nd and Rb-Sr) data from ca. 1685 Ma mafic rocks of the Willyama Supergroup in the Broken Hill Inlier of western NSW, Australia. The mafic rocks occur throughout the lower Willyama Supergroup stratigraphy and are interpreted here as shallowly emplaced sills that were metamorphosed to upper amphibolite and granulite facies during the Olarian Orogeny (ca. 1600-1580 Ma). Our data indicate that the metabasites originated by variable degrees of partial melting of a depleted mantle source, only weakly more enriched in incompatible elements compared to present day N-type MORB. This was followed by simple crystal fractionation or by an AFC process involving only small degrees of crustal assimilation (r = 0.05-0.2). Crystal fractionation proceeded along a tholeiitic trend of extreme primary iron and titanium enrichment, leading to melts with up to 25 wt % of total iron as Fe2O3 and 4.2 wt % of TiO2. Some intermediate rocks were derived from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Alma, Rasp Ridge and Potosi Gneisses) are not linked by fractional crystallization to the mafic melt that produced the meta-igneous amphibolites, and are products of anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bi-modal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Inlier (ca. 1685 Ma) was the extensional axis and depositional centre of an advanced stage intra-cratonic rift with relatively thin crust and lithosphere. Data from the neighbouring Olary Inlier, in contrast, that imply smaller degrees of partial melting, relatively thicker lithosphere and more crustal contamination, are consistent with placing this Inlier on a rift margin. Active faulting during the rift stage coupled with submarine sedimentation and an anomalous geothermal gradient driven by lithospheric thinning, provide an ideal theoretical environment for the formation of the Broken Hill Pb-Zn-Ag orebody.

Cenozoic volcanism in the Middle East: petrogenesis of alkali basalts from northern Lebanon

The Cenozoic volcanic field of the Akkar region in northern Lebanon consists of a thick succession (200 m) of basaltic lava flows, erupted at the junction between a restraining bend (the Yammouneh transform fault) and its northern extension (the Ghab transform) in Syria. Both faults are part of the Dead Sea transform fault system, which represents the boundary between the Arabian and African plates and the Levantine subplate. The lavas are made up of about 15–25 vol. % olivine (Fo79–84), 30–40 % clinopyroxene (salite), 40–50 % plagioclase (An58–67), and opaque Fe–Ti oxides (∼ 5 %). Geochemically, they exhibit a narrow range of SiO2 (44.6 to 47.0 wt %), and MgO (2.9 to 7.5 wt %), are relatively enriched in TiO2 (2.0 to 2.9 wt %), and are classified as alkali basalts. Mg-numbers range from 0.32 to 0.59, with an average of 0.47. The rocks are enriched in incompatible trace elements such as Zr (98–184 ppm), Nb (16–39 ppm) and Y (25–34 ppm). The REE patterns are fractionated ((La/Yb)N=8.2), and are generally parallel to subparallel. Such compositions are typical of those of HIMU-OIB and plume-related magmas. Elemental ratios such as K/P (2.9), La/Ta (21.8), La/Nb (0.80), Nb/Y (0.92) and Th/Nb (0.35), and the low average SiO2 content (46.1 wt %) suggest that the magma was subjected to minimal crustal contamination. This may be related to a rapid ascent of the parental magma, in agreement with the nature (mafic, oceanic crust-like) and the thickness (only about 12 km) of the crust of the Eastern Mediterranean region. Cenozoic volcanism in this region is interpreted to have occurred in association with an episode of localized extension, particularly at the junction between the Yammouneh restraining bend and the Dead Sea–Ghab Transform (that is, in a transtensional tectonic regime). The 143Nd/144Nd isotopic composition of the basaltic rocks of northern Lebanon ranges from 0.512842 to 0.512934 (εNd=4.0 to 5.8), and 87Sr/86Sr from 0.703317 to 0.703579, suggesting a HIMU-like mantle source. Modelling indicates that the magma was produced by a small degree of partial melting (F=2 %) of a primitive, garnet lherzolitic mantle source, possibly containing a minor spinel component.

Polybaric Evolution of Phonolite, Trachyte, and Rhyolite Volcanoes in Eastern Marie Byrd Land, Antarctica: Controls on Peralkalinity and Silica Saturation

In the Marie Byrd Land volcanic province, peralkaline and metaluminous trachytes, phonolites, and rhyolites occur in 18 large shield volcanoes that are closely associated in time and space. They are arrayed radially across an 800 km wide structural dome, with the oldest at the crest and the youngest around the flanks. Several lines of evidence suggest that these rocks evolved via opensystem, polybaric fractionation. We have used mass balance modeling of major elements together with trace-element data and mineral chemistry to help explain the evolution of this diverse suite of felsic rocks, which appear to have been generated coevally in isolated magma chambers, and erupted close to each other in patterns related to tectonic uplift and extension within the West Antarctic rift system. Isotopic and trace-element data indicate that this occurred with only minimal crustal contamination. We focus on volcanoes of the Executive Committee Range and Mount Murphy, where we find good representation of basalts and felsic rocks within a small area. Our results suggest that the felsic rocks were derived from basaltic magmas that differentiated at multiple levels during their passage to the surface: first to ferrogabbroic compositions near the base of the lithosphere, then to intermediate compositions near the base of the crust, and finally to felsic compositions in mid- to upper crustal reservoirs. The high-pressure history has been largely masked by low-pressure processes. The best indications of a high-pressure history are the mineral phases in cumulate nodules and their correlation with modeling results, with REE anomalies, and with the composition of an unusual gabbroic intrusion. Silica saturation characteristics are believed to have originated in magma chambers near the base of the crust, via fractionation of variable proportions of kaersutite and plagioclase. Development of peralkalinity in felsic rocks took place in upper crustal reservoirs by fractionating a high ratio of plagioclase to clinopyroxene under conditions of low pH2O. With increasing pH2O, the ratio plagioclase/clinopyroxene in the fractionated assemblage decreases and metaluminous liquids resulted. Crustal contamination seems to have had a role in suppressing peralkalinity, and was probably a factor in the origin of high-silica metaluminous rhyolite, but metaluminous rocks are uncommon. The volume and diversity of felsic rocks were probably enhanced by the structure of the lithosphere, the persistence of plume activity, and the immobility of the Antarctic plate. Mechanical boundaries at the base of the lithosphere and crust, and within the crust, appear to have acted as filters, trapping magmas at multiple levels, and prolonging the fractionation process. Final volumes would have been further enhanced by repeated refluxing of the same magma chambers, controlled by plume activity and plate immobility.

Helium isotope ratios in mafic phenocrysts and geothermal fluids from La Palma, the Canary Islands (Spain): implications for HIMU mantle sources

We report a comprehensive study of He isotope ( 3He/ 4He) variations on well-characterized lavas from La Palma, the current locus of activity of the Canary hotspot. Cogenetic olivine (OL) and clinopyroxene (CPX) phenocrysts from 11 basaltic lava flows representing all stages of the island’s evolution were analyzed by crushing in vacuo. Additionally, the sample set is supplemented by a CO 2-rich bubbling cold spring in Taburiente caldera. Phenocryst 3He/ 4He ratios vary between 6.3 to 8.9R A; both extremes occur in the 2 to 0.6 Ma shield-building Taburiente lavas. Historic flows vary between 7.0 to 7.8R A, whereas phenocrysts from a single submarine basement sill (4.0–2.9 Ma) have 3He/ 4He ratios of 8.3R A (OL) and 8.4R A (CPX). He isotopic equilibrium characterizes all the phenocryst pairs except one Taburiente lava. The absence of a correlation between 3He/ 4He and [He] contrasts with other ocean island basalts, e.g., Heard Island (Hilton et al., 1995) and, together with equilibrium in phenocrystic 3He/ 4He, suggests that the La Palma sample suite has 3He/ 4He ratios that have suffered minimal crustal contamination. The cold spring (geothermal) sample gives a 3He/ 4He ratio of 9.5R A, which is higher than any of the phenocrysts and in excellent agreement with a previously reported value for Taburiente caldera (Perez et al., 1996). The geothermal 3He/ 4He values indicate that La Palma is the first HIMU-like ocean island (where HIMU implies a distinctive mantle source with high time-integrated 238U/ 204Pb (μ) ratio) with reported 3He/ 4He ratios higher than the canonical value (8 ± 1R A) characteristic of depleted mid-ocean ridge basalt (MORB) mantle. We present two simple mixing scenarios that are compatible with the He–Pb isotope systematics of La Palma: a) a high- 3He mantle plume, extensively but variably depleted in its original He content, mixes with a HIMU source characterized by both radiogenic He and lead isotope compositions; and b) mixing between a composite plume, with high 3He/ 4He and radiogenic lead, and depleted mid-ocean ridge basalt mantle. The latter model is favored as it appears to account more readily for the mid-ocean ridge MORB-like He isotope characteristics of the lavas. Addition of radiogenic He—most likely produced in mantle melts frozen into the sub-Canarian mantle lithosphere—to resultant mixtures may subsequently occur in either model. Such a process can explain the absence of correlations between He and Pb isotope ratios in the lavas, and the lower 3He/ 4He values of the phenocrysts with respect to the geothermal fluids. We find no need to invoke a radiogenic growth/diffusion model (Hanyu and Kaneoka, 1998) to explain the He–Pb isotope relationships for La Palma. These mixing models may also be applicable to other HIMU islands: however, any high- 3He plume signature of St. Helena and Mangaia may be obscured due to the high proportion of the HIMU component at these localities and/or by addition of radiogenic He.