Crustal Contamination Processes Research Articles

Ancient high Pt/Os crustal contaminants can explain radiogenic 186Os in some intraplate magmas

The origin of variations in 186Os/188Os ratios amongst mantle-derived basaltic and komatiitic lavas remains controversial, with opposing models arguing for deep core-mantle versus shallow mantle sources. Crustal contamination has generally not been favored due to the low Os contents of such sources, meaning that variations in 186Os/188Os would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant 186Os/188Os variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic 186Os/188Os ratios are correlated with 187Os/188Os ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have δ186Os ([{(186Os/188Ossample[t]/186Os/188OsPM(t)) − 1} × 1000], where the modern primitive mantle [PM] 186Os/188Os is 0.1198388) values ranging between 0.04 to 0.15 for the ∼2.7 Ga Stillwater Igneous Complex, −0.05 to 0.17 for the ∼1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the ∼0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modeled through high sulfide-melt partitioning (typically >8000) and emphasize the role of S-saturation and HSE scavenging. Considering the high sulfide-melt partitioning and accounting for high silicate melt to sulfide melt ratios (R-factors), it is possible to explain the variations in 186Os–187Os in layered intrusions using calculated Os isotope crustal evolution growth models. These calculations indicate that <4% of ancient high Pt/Os crustal contributions can explain the composition of the chromitites and peridotites that were examined. Our observations are consistent with published models for chromitite genesis that invoke either crustal melt-primitive melt mixing, or cumulate assimilation. A crustal origin for radiogenic 186Os is a possible cause for 186Os/188Os ratio variations observed in some komatiites. It is more difficult to explain radiogenic 186Os/188Os measured in Hawaiian lavas by crustal contamination processes. Instead, ancient high Pt/Os oceanic crust, shallow mantle sources such as metasomatic sulfide, or metal-rich large low-shear wave velocity provinces at the core-mantle boundary, all remain valid explanations.

Petrogenesis of basaltic dikes from the Manjo area (Western Cameroon): insights into the Paleozoic magmatism at the northern margin of the Congo craton in Cameroon

A suite of basaltic dikes with a N60–80° E strike cross-cut the Paleo- to Neoproterozoic basement rocks in the Manjo area (southern continental part of the Cameroon Volcanic Line) along the northern border of the Congo craton in Cameroon. The dikes show a tholeiitic/transitional affinity as revealed by whole-rock and mineral chemistry data. Plagioclase and clinopyroxene are the dominant phases, with subordinate olivine and opaque oxides. The Manjo dikes have low MgO (5.2–7.4 wt%), Cr (119–274 ppm), and Ni (27–114 ppm) indicating that the samples are evolved. All basaltic dikes are mildly light lanthanide element enriched with Lan/Ybn values of 4.1–5.9. The Zr/Nb (4–10), Ba/Nb (6–27), and La/Nb (0.7–1.0) ratios are similar to those of basaltic rocks found in intraplate tectonic setting. Initial 87Sr/86Sr and 143Nd/144Nd isotope ratios range from 0.70472 to 0.70539 and from 0.512178 to 0.512377 (eNdi = + 1.0 to + 4.9), respectively. The Pb isotope compositions are in the ranges 206Pb/204Pb = 19.121–19.861 and 207Pb/204Pb = 15.624–15.703. The observed variations in major and trace elements cannot be fully attributed to plagioclase and clinopyroxene (± olivine) fractionation but indicate different parental magmas. The geochemical and isotopic data indicate no or minor crustal contamination processes in the petrogenesis of Manjo dikes. Trace element modeling suggests that the dikes were likely derived by partial melting of a spinel peridotite mantle source. The mildly enriched incompatible element contents of the Manjo dikes are similar to those of Paleozoic and Mesozoic mafic rocks recorded in Western Cameroon. The enrichment processes of the mantle sources of Manjo dikes may have occurred during Proterozoic subduction at the northern border of the Congo craton in Cameroon. The magmatism in the Manjo area resulted from the partial melting of this old, enriched continental lithospheric mantle induced by extensional processes during the Paleozoic.

Geochemical Features and Petrogenesis of Gökçeada Volcanism, Çanakkale, NW Turkey

Gökçeada Island, which is situated west of Biga Peninsula, has widespread magmatism with variable ages. Lower-Middle Eocene Dağiçitepe volcanics are the oldest volcanic unit in the island and consist of lavas, tuff-tuffites. They are influenced by alteration and almost all minerals, except quartz, are transformed into other minerals. Lower Oligocene Gökçeada andesitic lava/ domes exhibiting hypocrystalline porphyric texture, are the products of NE-SW trending domes/ cryptodomes. The phenocrysts assemblages consist of plagioclase, hornblende, clinopyroxene ± biotite and quartz. Middle Miocene Eşelek volcanics, which occur as lavas and pyroclastic rocks, exhibit hypocrystalline porphyric and intersertal textures. They are composed of plagioclase, hornblende and clinopyroxene crystals. Rhyolitic Dağiçitepe volcanics and andesitic Gökçeada lava/domes have calc-alkaline, andesitic Eşelek volcanics have tholeiitic character. They have geochemical features similar to subduction-related magmas. Lower-Middle Eocene Dağiçitepe volcanics are the products of syn-collisional magmas that have undergone processes of crustal contamination due to thickened crust. Whereas, Lower Oligocene Gökçeada andesitic lava/domes are the products of post-collisional magmas and were derived from metasomatized lithospheric mantle. Middle Miocene Eşelek volcanics were also derived from lithospheric mantle but, the mantle source generating Eşelek volcanics were relatively depleted over time. Geochemical data demonstrate the decreasing role of subduction signature and crustal contamination during the genesis and evolutionof Gökçeada volcanics from Lower-Middle Eocene to Middle Miocene.

Petrogenesis of the Solanas gabbro-granodiorite intrusion, Sàrrabus (southeastern Sardinia, Italy): implications for Late Variscan magmatism

The igneous complex of Solanas is a small, composite calc-alkaline intrusion emplaced at ~ 300 Ma in the Sarrabus region (southeastern Sardinia), and consists of olivine gabbronorites, amphibole gabbros, microgabbros, quartz diorites, tonalites, amphibole granodiorites, and biotite granodiorites. Thermobarometry calculations indicate that the Solanas rocks were emplaced at mid-to-upper crustal levels (0.6–4.0 kbar). The intermediate and silicic rocks are metaluminous to weakly peraluminous and are enriched in large ion lithophile elements. The range in the initial Sr and Nd isotopic compositions is small throughout the intrusion despite a large range in silica contents (46.3–73.6 wt% SiO2). The isotopic signatures, mineralogy, and geochemistry suggest that the quartz diorites, tonalites, and granodiorites derived from fractional crystallisation and crustal contamination processes starting from different mafic parental magmas. The origin of tonalites and granodiorites is compatible with removal of plagioclase, hornblende, biotite, apatite and zircon starting from a quartz dioritic magma. The mafic rocks range in composition from primitive to relatively evolved (Mg# 49–70). The olivine gabbronorites and amphibole gabbros have petrographic and geochemical features of arc cumulates derived from different basaltic magmas. The microgabbros have geochemical characteristics similar to high-alumina basalts with fractionated rare-earth element patterns (LaN/YbN = 4.3–6.0), enrichment in large ion lithophile elements (e.g., Rb, Ba, U, and K) and depletion in Nb and Ta compared with the primitive mantle. These characteristics are consistent with partial melting of a mantle source that was enriched by subduction-related fluids.

Approach to trace hidden paleo-weathering of basaltic crust through decoupled Mg[sbnd]Sr and Nd isotopes recorded in volcanic rocks

On Earth, chemical weathering of basaltic rocks not only modulates the climate change, but also plays a crucial role on the ocean oxygenation and ecosystem evolution. Records of paleo-weathered crusts are a challenge to identify because they are obscured by subsequent alteration processes and buried by the newly formed volcanic lavas. However, late formed volcanic rocks theoretically could record previously weathered crust via their assimilation and fractional crystallization (AFC) processes. Here, we present a systematic study on major, trace elements and Sr-Nd-Mg isotopes in a suite of Changbaishan early formed basalts and late erupted trachytes from Northeast China. Detailed evidence of field observation, geochemical signature and rhyolite-MELTS simulations suggest that trachytes mainly originated from deep fractional crystallization of basaltic melts. The low abundant trachytes from the stratigraphic layer above the basalts display decoupled MgSr and Nd isotopes. δ26Mg and 87Sr/86Sr in trachytes range from −0.27 up to +0.94‰ (the heaviest isotopic composition ever reported for Mg in the igneous rocks) and 0.70490 to 0.71065, respectively, which are significantly elevated relative to that of their mantle source. However, they preserved mantle source-like 143Nd/144Nd (0.512584 to 0.512656) ratios. These unusual characteristics cannot be explained by thermal diffusion, fractional crystallization or simple crustal contamination processes. Specifically, δ26Mg correlates positively with SiO2 and 87Sr/86Sr, and negatively with MgO, CaO, Al2O3, Sr and Nb/U indicators. The highly elevated Mg and Sr isotopes but limited Nd isotopic changes in trachytes require a component to be the relatively younger weathering residue of basaltic rocks. Taking this component as a wall rock endmember, the well-developed above correlations can be best modelled by AFC processes. This reflects that some trachytes in the aftermath of AFC processes almost fully lost their primary source characteristics, but inherited the intense chemical weathering signature of basalts. This event probably occurred in the middle Miocene as studies of paleosols showed humid and warm climate during that period after the initial eruption of Changbaishan basalts. Therefore, our study as a specific case for the first time provides an efficient way to explore paleo-weathering of basaltic crusts buried in the Earth's surface, through comprehensive investigating Mg, Sr and Nd isotopes of volcanic rocks developed in the late stage of a volcano cone.

EVALUATION OF SOIL GEOCHEMISTRY DATA OF CANCA AREA (GÜMÜŞHANE, TURKEY) BY MEANS OF INVERSE DISTANCE WEIGHTING (IDW) AND KRIGING METHODS- PRELIMINARY FINDINGS

In Northwest Anatolia, widespread magmatism developed due to collision between Anatolide-Tauride platform and Sakarya continent during Late Cretaceous-Early Tertiary period. The granitoids in Biga Peninsula are products of post-collisional magmatism following the convergence of the northern branch of Neotethyan Ocean and developed in two different stages as Eocene and Oligo-Miocene. Eocene Karabiga, Gureci, Kuscayir and Dikmen granitoids are granite and diorite-granodiorite; Oligo-Miocene Sarioluk, Yenice, Kestanbol, Eybek, Evciler, Camyayla and Alankoy granitoids are diorite, granodiorite, monzonite and Q-monzonite in composition. Metaluminous and peraluminous granitoids have similar geochemical variations and exhibit post-collisional geochemical signatures. Trace element patterns are almost similar to those observed in upper crust and GLOSS (Global Subducting Sediment) patterns with depletion in high field strength (HFS) elements (Nb, Ta, Ti, Zr, Hf). But, Oligo-Miocene Sarioluk, Yenice-Cakiroba, Kestanbol, Evciler, Camyayla, Alankoy and Eocene Karabiga, Gureci and Kuscayir granitoids have higher Th and U contents relative to upper crust and GLOSS. Dikmen, Yenice-Hamdibey, Yenice-Eskiyayla and Eybek granitoids have lower Th content. Geochemical variations indicate that partial melting and fractional crystallisation- crustal contamination processes are effective in their genesis and evolution. Trace element ratios also indicate subduction signatures in their genesis and Rb/Ba, Rb/Sr ratios suggest mantle melting rather than crustal melting. Accordingly, post-collisional Biga Peninsula granitoids were derived from a previously metasomatised lithospheric mantle source, which was enriched during northward subduction and closure of the northern branch of Neo-Tethys Ocean beneath the Sakarya continent, since variations in Rb, Cs, Th, La and Sm reveal that lithospheric mantle was mesomatised by both aqueous fluids and sediment melts.

Geochemistry and geochronology of the mafic dikes in the Taipusi area, northern margin of North China Craton: Implications for Silurian tectonic evolution of the Central Asian Orogen

The Taipusi area in the Bainaimiao Arc Belt is located in the northern margin of the North China Craton, at the southern margin of the middle Central Asian Orogenic Belt. It is characterized by large exposures of mafic dikes. In this contribution, we present first-hand whole-rock major and trace elements, zircon U–Pb geochronology and in situ trace element geochemistry data for these mafic rocks, which reveal their petrogenesis and tectonic evolution. These mafic dikes display varied compositions of \(\hbox {SiO}_{2}\) (49.42–54.29%), \(\hbox {TiO}_{2}\) (0.63–1.08%), \(\hbox {Al}_{2}\hbox {O}_{3}\) (13.94–17.60%), MgO (4.66–10.51%), \(\hbox {Fe}_{2}\hbox {O}_{3}\) (1.59–3.07%), FeO (4.60–6.90%), CaO (4.57–8.91%), \(\hbox {Na}_{2}\hbox {O}\) (1.61–4.26%), \(\hbox {K}_{2}\hbox {O}\) (0.92–2.54%) and \(\hbox {P}_{2}\hbox {O}_{5}\) (0.11–0.29%). They are mainly of high-K calc-alkaline series with indistinct Eu anomalies, enriched in large ion lithophile elements (e.g., Rb, Ba, K and Sr) but depleted in high field strength elements (e.g., Nb, P and Ti). These suggest that the crystallizing magma was derived from enriched mantle altered by metasomatic fluids in a subduction setting with imprints of active continental margin features. The high concentrations of Hf, U, Th, Pb and Y, pronounced positive Ce but slightly negative Eu anomalies in zircons indicating that the magma underwent a fractional crystallization and crustal contamination process, with medium to high \(f\hbox {O}_{2}\). Zircon LA–ICP–MS U–Pb dating yielded concordant ages of 437–442 Ma for these mafic dikes, which is consistent with the early Paleozoic volcanic arc magmatic activity in the Bainaimiao area. Hence, we conclude that the Bainaimiao Arc Belt is a continental arc formed by the southward subduction of the Paleo-Asian ocean during early Paleozoic.

Pyroxene standards for SIMS oxygen isotope analysis and their application to Merapi volcano, Sunda arc, Indonesia

Measurement of oxygen isotope ratios in common silicate minerals such as olivine, pyroxene, feldspar, garnet, and quartz is increasingly performed by Secondary Ion Mass Spectrometry (SIMS). However, certain mineral groups exhibit solid solution series, and the large compositional spectrum of these mineral phases will result in matrix effects during SIMS analysis. These matrix effects must be corrected through repeated analysis of compositionally similar standards to ensure accurate results. In order to widen the current applicability of SIMS to solid solution mineral groups in common igneous rocks, we performed SIMS homogeneity tests on new augite (NRM-AG-1) and enstatite (NRM-EN-2) reference materials sourced from Stromboli, Italy and Webster, North Carolina, respectively. Aliquots of the standard minerals were analysed by laser fluorination (LF) to establish their δ18O values. Repeated SIMS measurements were then performed on randomly oriented fragments of the same pyroxene crystals, which yielded a range in δ18O less than ±0.42 and ±0.58‰ (2σ) for NRM-AG-1 and NRM-EN-2, respectively. Homogeneity tests verified that NRM-AG-1 and NRM-EN-2 do not show any crystallographic orientation bias and that they are sufficiently homogeneous on the 20μm scale to be used as routine mineral standards for SIMS δ18O analysis. We subsequently tested our new standard materials on recently erupted pyroxene crystals from Merapi volcano, Indonesia. The δ18O values for Merapi pyroxene obtained by SIMS (n=204) agree within error with the LF-derived δ18O values for Merapi pyroxene but differ from bulk mineral and whole-rock data obtained by conventional fluorination. The bulk samples are offset to higher δ18O values as a result of incorporation of mineral and glass inclusions that in part reflects crustal contamination processes. The Merapi pyroxene SIMS data, in turn, display a frequency peak at 5.8‰, which allows us to estimate the δ18O value of the primary mafic magma at Merapi to ~6.1‰ when assuming closed system differentiation.

Lead isotope constraints on the mantle sources involved in the genesis of Mesozoic high-Ti tholeiite dykes (Urubici type) from the São Francisco Craton (Southern Espinhaço, Brazil)

ABSTRACT: The first results of Pb isotope compositions of the high-Ti Mesozoic dykes of the Southern Espinhaço are presented. The results do not show large variations and are significantly more radiogenic than the Pb isotope compositions of the high-Ti tholeiites from the Paraná Continental Flood Basalts. The data combined with published geochemical and Sr-Nd isotope results rule out crustal contamination processes in the genesis of the dykes, requiring magma generation in metasomatized subcontinental lithospheric mantle with the involvement of HIMU-type and carbonatite components. The magmas may have been also derived from a mantle source containing ~4 - 5% of pyroxenite and ~1% of carbonatite melts, agreeing with published Os isotope compositions of high-Ti rocks from the Paraná Continental Flood Basalts. These metasomatizing agents could be responsible for mantle source refertilization, as was also proposed in the literature to explain the characteristics of xenoliths of the Goiás Alkaline Province, which also occurs in the border of the São Francisco Craton. Additionally, to evaluate the risks of Pb contamination during sample preparation for analysis, several experimental tests were accomplished, which indicate the need of sawed surface removal and a careful washing of small-sized rock fragments before powdering, especially for rocks with [Pb] &lt; 7 µg/g.

Age, geochemical characteristics and petrogenesis of Cenozoic intraplate alkaline volcanic rocks in the Bafang region, West Cameroon

The origin of the volcanism in the Cameroon Volcanic Line and the nature of its mantle sources are still highly controversial. We present major and trace element compositions as well as Sr–Nd–Pb–Hf isotopic results on mafic and intermediate lavas from the Bafang area in the central part of the Cameroon Volcanic Line. The lavas range from basanites and basalts to hawaiites and mugearites with an alkaline affinity and were emplaced between 10 and 6Ma ago. The evolution from basalts and basanites to more differentiated rocks involved fractionation of olivine, clinopyroxene and Fe–Ti oxides, but the isotopic compositions show that crustal contamination processes affected some magmas during their ascent in the crust. Basalts and basanites originated from a garnet-bearing mantle source and their differences are mostly due to variable degrees of partial melting. The isotopic composition of the uncontaminated samples imply the participation of three distinct mantle components, the depleted MORB mantle (not dominant), an enriched component and a Pb radiogenic component similar to the source of the Mount Cameroon. Combined with previously published isotopic data from the Cameroon Volcanic Line, our new results indicate that the source of the volcanism mostly reside in the lithospheric mantle and is different from what can be expected from the melting of a mantle accreted from or modified during the emplacement of the St. Helena mantle plume.

The 2.31 Ga mafic dykes in the Karelian Craton, eastern Fennoscandian shield: U–Pb age, source characteristics and implications for continental break-up processes

Major- and trace-element geochemistry, Sm–Nd isotopic and U–Pb geochronological (ID TIMS, baddeleyite) data are presented for Paleoproterozoic mafic dykes in the Karelian Craton, eastern Fennoscandian shield. Mafic dyke swarms recognized in the Lake Upper Kuito area in the Western Karelian terrane of the Karelian Craton include (i) NE-trending ca. 2450Ma gabbronorite dykes, (ii) NW-trending ca. 2310Ma dolerite dykes, and (iii) NNW-trending ca. 2130Ma continental MORB-type tholeiitic dykes. Each swarm belongs to a distinct intraplate igneous event and indicates specific mantle melting and extensional processes.The ca. 2.31Ga doleritic dykes have high-Fe, high-Ti tholeiitic series affinity and characterized by high concentrations of lithophile trace elements. The enrichment of LREE and unfractionated HREE patterns together with depletion of Nb on multi-element diagrams and initial ɛNd values of +0.5 to +0.8 are characteristic for ca. 2.31Ga dolerites. The geochemical modelling results suggest that these features could be originated via combination of fractional crystallization and crustal contamination processes, and do not indicative of a mantle source enrichment. It also indicates long-term storage of melts in an intracrustal magma chamber and a relatively slow ascent to the surface. The primary melts of ca. 2.31Ga dolerites possibly originated by partial melting of a DM-type mantle source in the spinel peridotite stability field.In contrast to previous ca. 2.45Ga event studied ca. 2.31Ga dolerites do not indicate high-temperature mantle-plume induced mantle melting. On the other hand, the ca. 2.31Ga dykes have some similarities with syn-breakup ca. 2.13Ga continental MORB-type tholeiites, although there is no evidence for the break-up of the continental lithosphere of the Karelian Craton at ca. 2.31Ga. Thus ca. 2.31Ga dykes could indicate the existence of relatively stable extensional tectonic environment and an unsuccessful attempt of break-up of the continent.The ca. 2.31Ga dykes together with previously studied Runkaus basalts and Taivalkoski dykes could be considered as a component of distinct relatively wide-spread Kuito-Taivalkoski igneous event within the ca. 2.2–2.4Ga “crustal age gap” period in the eastern Fennoscandian shield. This probably indicate that the number and intensity of endogenous events of age 2.4–2.2Ga are greatly undervalued. This also argues against a decrease of endogenic activity and magmatic “slowdown” during ca. 2.4–2.2Ga period.

Geochemistry and petrogenesis of lava flows around Linga, Chhindwara area in the Eastern Deccan Volcanic Province (EDVP), India

Based on systematic three-tier arrangement of vesicles, entablature and columnar joints, three distinct quartz normative tholeiitic lava flows (I, II and III) were recognized in the area around Linga, in the Eastern Deccan Volcanic Province (EDVP). Each of the flows exhibits intraflow chemical variations marked by high Mg#-low Ti, and low Mg#-high Ti contents. The MgO (4.27–7.74wt.%), Mg# (23.45–41.89) and Zr (161.5–246.3ppm) of Linga flows suggest an evolved chemistry marked by fractional crystallization and crustal contamination processes. Positive Rb and Th anomalies, negative Nb anomalies, relative enrichment of LILE–LREE with respect to Nb, Nb/Th:3.71–6.77 indicate crustal contamination of magma by continental materials through magma–crust interaction during melt migration and contributions from sub-continental lithospheric mantle (SCLM). Negative K, Sr and Ti anomalies corroborate an intracontinental, rift-controlled tectonic setting for the genesis and evolution of Linga basalts. Chondrite-normalized REE patterns reflect low HREE abundances and prominent LREE/HREE, MREE/HREE fractionation thereby pointing towards partial melting of garnet peridotite mantle source. Nb, Zr, Y variations suggest 10–15% partial melting of mantle source for the derivation of parent tholeiitic melt that suffered crystal fractionation of phenocrystal phases and subsequent liquid immiscibility. Critical evaluation of Srinitial and Ndinitial (65Ma) isotopic compositions (87Sr/86Srinitial between 0.705656 and 0.706980 and 143Nd/144Ndinitial between 0.512523 and 0.512598) suggests that these basalts were derived from an enriched mantle (∼EM I–EM II) source. The εSr (21.84–41.27) and εNd (−0.28 to 1.10) isotopic signatures defined by higher εSr and lower εNd fingerprint a plume-related source. Positive and negative values of εNd indicate an isotopically heterogeneous mantle source marked by mixing of depleted (DM) and enriched mantle (EM I–EM II) components at the source region and together with 87Sr/86Srinitial ranging from 0.705656 to 0.706980 suggest two stage contamination of parent magma which is much similar to that of Poladpur, Toranmal, Mhow, Chikaldara flows. Ba/Y versus 87Sr/86Sr and Nb/Y versus Rb/Y variations show an Ambenali–Poladpur contamination trend for the Linga basalts thereby suggesting the role of upper continental granitic crust as the contaminant of these flows through magma–crust interaction during melt migration. The lava flows of Linga are geochemically correlatable with the Poladpur flows of southwestern and Toranmal flows of northern Deccan and show genetic coherence with the basalts of Jabalpur, Seoni, Chakhla–Delakhari of eastern Deccan.

Petrology, mineral chemistry and Sr–Nd–Pb isotopic compositions of granitoids in the central Menderes metamorphic core complex: Constraints on the evolution of Aegean lithosphere slab

Plutonic associations in the central Menderes metamorphic core complex are suitable rocks in order to understand the magma forming processes in extended terrains. Syn-extensional Salihli and Turgutlu granitoids have granodioritic composition and contain monzonitic and monzodioritic microgranular enclaves. They are transitional metaluminous/peraluminous and high-K calc-alkaline in character and are located on I- and S-type transition. Salihli and Turgutlu granodiorites are geochemically similar to each other while their microgranular enclave chemistry is in contrast with low SiO2 and high Mg#values. Mineral chemistry data from granodiorites and mafic microgranular enclaves confirm their shallow emplacement at about 7km. Geochemical modelling suggests that syn-extensional granitoids were derived from the mixing of mantle and lower crustal components, which were finally modified by a significant amount of upper crustal contamination and fractional crystallization processes at shallow crustal levels. Early-Middle Miocene syn-extensional granitoids across the Aegean region form a magmatic belt associated with roll-back of the Aegean lithosphere slab. Roll-back induced magmatism together with ductile deformation in western Turkey ceased after cooling of the Salihli granodiorites at 12.2Ma. But core-complex related magmatism was continuous in the Cycladic metamorphic core complex during Late Miocene and was followed by an active arc volcanism in the southern Aegean. Such abrupt change from ductile to brittle mode of extension in western Turkey can be explained by opening of a slab window on the Aegean lithosphere slab, which would lead to upwelling of fertile subslab asthenospheric mantle, forming transitional and finally OIB-type basalts.

The Aguilar pluton (23°12′ S–65°40′ W; NW Argentina): Petrological implications on the origin of the Late Jurassic intraplate magmatism in the Central Andes

The Late Jurassic Aguilar pluton is located in NW Argentina, about 300–400 Km east of the Tarapacá basin, representing the backarc basin linked to the Jurassic volcanic arc. This small-size and compositionally heterogeneous pluton intruded the metasedimentary rocks of the Ordovician Santa Victoria Group, along the Cobres-Salinas Grandes lineament. A revision of published geochemical data in the light of new field and petrological results, allows us to propose a model concerning the petrogenesis and emplacement mechanisms of Aguilar pluton and to discuss its geodynamic setting. The pluton is mainly composed of metaluminous and nearly peraluminous granitoids, showing the geochemical characteristics of ferroan granites. The volumetrically subordinate mafic rocks are both ne- and hy-normative, and their primary magmas were generated by partial melting of a pristine Proterozoic mantle. Aguilar rocks display a rather limited range in (87Sr/86Sr)i, compared to the entire rift-related plutonic suite, i.e., 0.703198–0.704601, and εNdt from −1.06 to 3.82, calculated at 149 Ma. Fractional crystallization of mantle-derived magmas and crustal contamination processes explain the evolution to produce strongly silica-oversaturated magmas, which emplaced in the continental crust. The petrological data indicate that magma emplacement and cooling occurred at rather shallow depth. Multiple injections of magma batches into the magma chamber caused mingling and mixing processes early in the crystallization history. The Aguilar pluton is one of the several igneous complexes whose formation was associated with the extensional tectonics active during Late Jurassic-Early Cretaceous in NW Argentina. Based on the geological position and the igneous rocks affinity, we exclude that the Late Jurassic magmatism was generated in an orogenic setting and envisage that it was linked to the early extensional phase that preceded the Cretaceous continental rifting, related to the break–up of the South America-Africa continents.

Preservation of magmatic signals in metavolcanics from Wedel Jarlsberg Land, SW Svalbard

AbstractThe purpose of this study is to determine the role of metamorphism and thereby identify the preserved magmatic signature in metavolcanics from Wedel Jarlsberg Land in southwestern Svalbard. Samples have been collected from late Precambrian metavolcanics occurring within metasedimentary rocks of the Sofiebogen Group, as well as dikes cutting older metasedimentary rocks of the Deilegga Group. The volcanic rocks were metamorphosed under greenschist facies conditions during the Caledonian Orogeny. To investigate the role of metamorphism, we present petrography, major and trace element geochemistry, and use factor analysis as a tool to identify correlations that correspond to primary magmatic signals.The metavolcanics are classified as subalkaline basalt to basaltic andesite and they contain relicts of primary clinopyroxene and plagioclase. The metamorphic minerals are actinolite, secondary plagioclase, chlorite and minerals belonging to the epidote group. Major element variations are highly scattered with no obvious trends observed. The HFSE and REE show strong trends attributed to fractional crystallization. The LILE, Th and La show elevated contents in some samples.Factor analysis shows that the HFSE and REE are well correlated. The LILE form a separate well correlated group, while the major elements are not correlated, except for Na2O, Fe2O3 and CaO. The lack of correlation for major elements, as well as the lack of observed fractional crystallization trends between these elements suggests that they were modified by metamorphism. The strong correlation of HFSE and REE reflects the original geochemical signal generated by magmatic processes. The correlation of the LILE is consistent with their elevated composition implying the influence of crustal contamination processes, and though some variability is likely superimposed due to metamorphism, the primary magmatic record is not completely destroyed. We conclude that the HFSE and REE are not influenced by metamorphic processes and therefore provide robust records of magmatic processes.

Late Neoproterozoic volcanics and associated granitoids at Wadi Ranga, south Eastern Desert, Egypt: A transition from subduction related to intra-arc magmatism

The late Neoproterozoic of Wadi Ranga area, south Eastern Desert was characterized by extensive magmatism consisting predominantly of calc-alkaline older granitoids, Ranga volcanics and granite porphyry intrusions forming a belt of volcanic-intrusive complexes. Geological studies of the Ranga volcanics revealed two sequences of volcanic suites: the older mafic sequence composed essentially of basalt/basaltic andesite lava flows and the associated pyroclastics, while the younger felsic one represented by pitchstone porphyry. Geochemically, the older granitoids are exhibit low K-calc-alkaline characters and most likely formed in a convergent margin (arc) tectonic environment. Basalt/basaltic andesites are compositionally uniform with tholeiitic affinity. They have trace element characteristics of arc related volcanics with low Nb (0.2–0.9ppm), Zr/Nb ratios of 40–81 with an average 54 compared to 32 in average N-MORB, signify a convergent margin setting with a depleted mantle wedge. Low Mg# (45 to 60 with an average 51), Co and Ni contents may be indicative of a non-plume-influenced mantle wedge. Compositionally, these mafic volcanics are commensurate with modern intra-arc basalts. REE modeling indicates that the magma from which these mafic volcanics were formed was generated by about 17–30% partial melting of a depleted mantle that composed mainly of spinel lherzolite in an extensional marginal basin. Geochemical characters of major, trace and REE patterns of the granite and pitchstone porphyries proved their formation from the same magma source which is originally different from the magma producing the mafic volcanics. The relative enrichment in K2O and LREE of the pitchstone porphyry along with the depletion in CaO and MgO relative to granite porphyry probably resulted from crustal contamination processes.

Heterogeneous mantle sources feeding the volcanic activity of Mt. Karacadağ (SE Turkey)

The volcanic activity of Mt. Karacadağ (SE Anatolia) is divided into three major stages: Siverek Stage (∼11–2.7Ma), Karacadağ Stage (∼1.9–1.0Ma) and Ovabağ Stage (0.4–0.01Ma). The magmas are mildly alkaline mafic rocks, mostly basanites, hawaiites and alkali basalts. Detailed geochemical investigation indicates a continuous variation of composition with time, with the oldest products (Siverek Stage) being characterized by average lowest HFSE (Ti, Hf, Zr, Nb, Ta), 143Nd/144Nd, Nb/U, Ta/Yb, Nb/Nb* and the highest ΔQ, La/Nb, Ti/Nb, Zr/Nb, Ba/Nb, Th/Ta, K/La, and the youngest products (Ovabağ Stage) at the opposite end of the trend. The overall incompatible element content of the Karacadağ volcanic rocks resembles closely average HIMU-OIB compositions, with the oldest samples deviating more strongly from typical compositions of “anorogenic” magmas.We interpret these geochemical variations with a process of partial melting of a chemically and mineralogically heterogeneous mantle source rather than with process of variable crustal contamination at shallow depths. During the first stages of mantle melting the volumes with lowest solidus temperature (e.g., the amphibole and phlogopite-rich metasomes, particularly abundant in the Arabia mantle xenolith suite) contributed significantly to the partial melts. “Average” peridotitic matrix is involved in partial melting processes only when these metasomatic volumes start to be exhausted, producing mantle melts with geochemical composition resembling average “anorogenic” mildly alkaline sodic rocks, common in the circum-Mediterranean area.

The magmatic feeding system of the Campi Flegrei caldera: Architecture and temporal evolution

To develop a model of both the structure and evolution of the Campi Flegrei caldera (CFc) magmatic feeding system, geochronological, geochemical and Sr, Nd, Pb and B isotopic data of representative volcanic products of the past 15 ka have been combined with geophysical and melt inclusion literature data, structural setting and dynamics of the resurgent caldera. According to previous petrological data, the CFc magmatic feeding system consists of a deep reservoir, in which mantle-derived K-basaltic parental magmas differentiate to shoshonite, latite and trachyte, through combined crustal contamination and fractional crystallization processes, and shallow reservoirs where the evolved magmas further differentiate and mingle/mix before eruptions. The Sr, Nd, Pb, and B isotope data allow recognition of three distinct magmatic components. One component is believed to be residual magma from the Neapolitan Yellow Tuff (NYT) caldera forming eruption. The NYT component ( 87Sr/ 86Sr of 0.70750–53, 143Nd/ 144Nd ratio of ca. 0.51246, 206Pb/ 204Pb of ca. 19.04 and δ 11B of ca. –7.9‰), has been the most prevalent component over the past 15 ka being mixed, in most cases, with the other two components. One of these other components is best recognized in the Minopoli 2 magma, first erupted 10 ka ago. Minopoli 2 magma is shoshonitic in composition and is the most enriched in radiogenic Sr ( 87Sr/ 86Sr of ca. 0.70860) and unradiogenic Nd and Pb ( 143Nd/ 144Nd ratio of ca. 0.51236, 206Pb/ 204Pb of ca. 18.90), and is characterised by δ 11B value of ca. –7.32‰. The third component is trachytic in composition and has higher 206Pb/ 204Pb (ca. 19.08), lower 87Sr/ 86Sr (ca. 0.70726) and δ 11B (−9.8‰) and higher 143Nd/ 144Nd (ca. 0.51250), with respect to the NYT component. This third component is best recognized in the Astroni 6 magma and did not appear until ca. 4 ka. The identified isotopically distinct magmatic components were erupted in different sectors of the CFc. During both I (< 14.9–9.5 ka) and II (8.6–8.2 ka) epochs of volcanic activity, magmas similar to the NYT component, and those resulting from mixing between Minopoli 2 and NYT components were erupted from vents located mostly on the marginal faults of the NYT caldera. During the III epoch (4.8–3.8 ka) magmas either similar to NYT, or resulting from mixing between Astroni 6 and NYT components were erupted from vents located along faults bordering the La Starza resurgent block and, subordinately, the NYT caldera. Moreover, magmas resulting from mixing between Minopoli 2 and NYT components were erupted from vents located along NE–SW regional faults activated during caldera resurgence. The inferred present structure of the feeding system is characterised by a deep reservoir, whose top is at about 8 km depth, that hosts shoshonitic–trachytic magmas. Remnants of the NYT magma reside at shallower depth in different sectors of the crust underlying CFc, and were sometimes intercepted by volatile-rich magmas of deep provenance during the three epochs of CFc volcanic activity.

Relationship between monogenetic magmatism and stratovolcanoes in western Mexico: The role of low-pressure magmatic processes

A large Quaternary monogenetic volcanic field is present in the western part of the Trans-Mexican Volcanic Belt. It is composed by mafic-intermediate scoria cones and silicic domes that are arranged in two NNW–SSE alignments. These mark the north and south borders (Northern Volcanic Chain and Southern Volcanic Chain, SVC) of the San Pedro–Ceboruco graben. The products of this monogenetic volcanic field span a large range of compositions (from basalt to rhyolite) and magma affinities (from sub-alkaline to Na-alkaline), defining different magmatic groups. Mafic and silicic monogenetic centres from the north alignment also coexist with two stratovolcanoes (Ceboruco and Tepetiltic) and sometimes punctuate their flanks. Whole-rock analyses indicate the existence of 4 different types of primitive magmas (Na-alkaline, High-Ti, Low-Ti/SVC and sub-alkaline) which have evolved independently by low-P magmatic processes. Despite the relatively small size and simplicity of the monogenetic magmatism, open-system processes have modified the geochemical and isotope composition of erupted products. The negative correlation between Sr isotope ratios and MgO contents observed for Southern Volcanic Chain and High-Ti groups points to crustal interaction via AFC processes, involving upper granitic crust and mafic lower crust respectively. In contrast, the large variability in Nd-isotopic ratios, combined with low and less variable 87Sr/ 86Sr, shown by the most mafic compositions of the High-Ti group is mostly due to mantle source heterogeneities. Low-Ti and Na-alkaline compositions are only slightly modified by crustal contamination processes and their whole-rock geochemistry reflects the complex nature of the western Mexico sub-arc mantle. It is therefore apparent that a combination of mantle source processes plus crustal assimilation has generated complex geochemical and isotopic characteristics in the western part of the Trans-Mexican Volcanic belt. Despite the presence of monogenetic cones on the flanks of stratovolcanoes, limited magma interaction between monogenetic and polygenetic magmatism has been recognised only at Ceboruco, possibly producing the chemical variability of post-caldera lavas. Indeed, mafic magma feeding High-Ti monogenetic systems might represent the possible mafic end-member which triggered the Ceboruco caldera-forming event. This may have important implications for other explosive systems in which monogenetic magmatism is associated with stratovolcanoes. A geographic/tectonic control is also suggested by the geochemical data. Na-alkaline compositions are only found in the northern part of the Northern Volcanic Chain. Parental magmas of both the High-Ti and Low-Ti monogenetic series, erupted between the Ceboruco and Tepetiltic stratovolcanoes, were modified by lower crust AFC processes possibly favoured by the stress regime. Indeed, the presence of a local left-hand step over along the northern main fault systems between the two stratovolcanoes might inhibit free uprising of monogenetic mafic magmas. The preferential alignment of stratovolcanoes and monogenetic volcanic vents parallel to the northern main fault systems and the possible mixing between High-Ti mafic monogenetic magmas and more evolved Ceboruco magmas suggests that, under the predominance of regional stress, the influence of central volcanic vents on monogenetic magmatism might be more complex than simple control of vent directions and might favours magma mixing processes.

Fluid Inputs to Magma Sources of St. Vincent and Grenada (Lesser Antilles): New Insights from Trace Elements in Olivine-hosted Melt Inclusions

St.Vincent and Grenada, known for the presence of high Mg-basalts, are two of the southernmost islands of the Lesser Antilles arc, located at the boundary between the Atlantic and Caribbean Plates. We report here secondary ionization mass spectrometry measurements of trace elements (V, Sr, Y, Zr, Nb, Ba, La to Eu) in basaltic melt inclusions trapped in olivine (Fo90^86) within high-MgO basalts. Combined with previous data on light elements and stable isotopes obtained on the same sample set, these results allow us to decipher the nature of the slab-derived fluids that modified the mantle wedge beneath these two islands. The melt inclusions exhibit trace element patterns that encompass those of the whole-rocks, but with a larger range of variability at Grenada. On the whole, such patterns are trace element enriched but not very fractionated compared with mid-ocean ridge basalt and display typical Ba and Sr enrichment. The variations of mobile/immobile element ratios (e.g. K/La, Sr/ Y) are poorly reconciled with crustal contamination processes but rather track the involvement of early aqueous fluids, which are also responsible for significant increases in Cl/F, B and Cl recorded by the melt inclusions. The variations of immobile element ratios such as La/Sm and Nb/Y cannot be accounted for by variable extents of melting alone, but also imply the influence of a hydrous sediment-derived melt for St. Vincent, whereas the by far larger enrichments of La and Nb in the Grenada samples require contributions of solute-rich fluids extracted at high temperature and pressure from both subducted oceanic crust and sediments. Moreover, a few Grenada melt inclusions display unusual Zr enrichment (up to 1274 ppm), a rare feature in basaltic melts that possibly involves the localized dissolution of zircon. The comparison of trace element distribution patterns in the melt inclusions (MI) and whole-rocks indicates the primary composition of the MI and the interaction of the melt with mantle paragenesis after their entrapment in olivine.