Crustal Contamination Processes Research Articles

Abanico East Formation: petrology and geochemistry of volcanic rocks behind the Cenozoic arc front in the Andean Cordillera, central Chile (33°50'S)

The stratigraphy, chemistry and age of rocks assigned to the eastern portion of the Abanico Formation exposed along the El Volcán river valley, Principal Cordillera east of Santiago (30º50'S/70º12'-70º5'W), are reported and discussed. This ca. 3,300 m thick succession is mainly composed of basalts, basaltic andesites and volcaniclastic rocks. 40Ar/39Ar radiometric dates on plagioclase from the lava flows yield Oligocene-lower Miocene ages with a maximum age of 34.3 ±0.4 Ma for the lower part and a plateau age of 21.4±1.0 Ma for the upper part of the succession. The lava flows show calc-alkaline affinities and have chemical characteristics that are typical of arc volcanic rocks erupted in an active continental margin. A temporal chemical evolution in the sequence is indicated by upward increases in concentrations of LILE and LREE elements and LaN/YbN ratios. This pattern can be attributed to increasing contributions of fluids derived from the subducted lithosphere with time. A chemical comparison of these rocks with Oligocene-lower Miocene volcanic rocks from the Cerro Abanico and Chacabuco areas on the western border of the Principal Cordillera, east of Santiago, and at the northern end of the Central Depression reveals west to east compositional variations. From west to east these variations include: (1) increasing LILE and LREE concentrations, LaN/YbN ratios and Sr and Nd initial isotopic ratios, and (2) decreasing LILE/HFSE and LREE/HFSE ratios. These pattern can be attributed to a west to east decrease in the contribution of slab derived fluids and increase in the influence of crustal contamination processes.

Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations

In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites, we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calc-alkaline, and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic plagiogranites; however, the compositions of the coexisting experimental minerals significantly deviate from those of the granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the sheeted dike complex, starting at temperatures exceeding 850°C. The newly formed melt can either crystallize to form oceanic plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated.

Hafnium isotope variations in Bure volcanic rocks from the northwestern Ethiopian volcanic province: a new insight for mantle source diversity

This paper presents new hafnium isotope data for the Bure volcanic rocks from the northwestern Ethiopian plateau to constrain the nature of mantle sources within the broad mantle upwelling. The 176 Hf/ 177 Hf values range from 0.282901 (eHf = 4.79) to 0.283206 (eHf = 15.71) for the Oligocene transitional tholeiite and 0.282915 (eHf = 5.45) to 0.283069 (eHf = 10.85) for the alkaline basalts; these ratios are distinct from MORB but typical to OIB sources. The eHf values correlate positively with eNd. The scattered variation between eHf and contamination indices (such as Nb/La and Ce/Pb) for the transitional tholeiites implies that most tholeiitic basalts are affected by crustal contamination processes. Components 1 (C1) and 2 (C2), which are proposed to be the endmember components for the Afar mantle plume, have a Hf isotopic signature of eHf = 15-17 and 5-9, respectively. The diversity of these compositions relies on the proportion of recycled materials in the deep source, its formation age, and proportion of sediments relative to oceanic crust in the recycled materials.

Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review

AbstractNeogene to Quaternary volcanic/magmatic activity in the Carpathian-Pannonian Region (CPR) occurred between 21 and 0.1 Ma with a distinct migration in time from west to east. It shows a diverse compositional variation in response to a complex interplay of subduction with rollback, back-arc extension, collision, slab break-off, delamination, strike-slip tectonics and microplate rotations, as well as in response to further evolution of magmas in the crustal environment by processes of differentiation, crustal contamination, anatexis and magma mixing. Since most of the primary volcanic forms have been affected by erosion, especially in areas of post-volcanic uplift, based on the level of erosion we distinguish: (1) areas eroded to the basement level, where paleovolcanic reconstruction is not possible; (2) deeply eroded volcanic forms with secondary morphology and possible paleovolcanic reconstruction; (3) eroded volcanic forms with remnants of original morphology preserved; and (4) the least eroded volcanic forms with original morphology quite well preserved. The large variety of volcanic forms present in the area can be grouped in a) monogenetic volcanoes and b) polygenetic volcanoes and their subsurface/intrusive counterparts that belong to various rock series found in the CPR such as calc-alkaline magmatic rock-types (felsic, intermediate and mafic varieties) and alkalic types including K-alkalic, shoshonitic, ultrapotassic and Na-alkalic. The following volcanic/subvolcanic forms have been identified: (i) domes, shield volcanoes, effusive cones, pyroclastic cones, stratovolcanoes and calderas with associated intrusive bodies for intermediate and basic calclkaline volcanism; (ii) domes, calderas and ignimbrite/ash-flow fields for felsic calc-alkaline volcanism and (iii) dome flows, shield volcanoes, maars, tuffcone/tuff-rings, scoria-cones with or without related lava flow/field and their erosional or subsurface forms (necks/ plugs, dykes, shallow intrusions, diatreme, lava lake) for various types of K- and Na-alkalic and ultra-potassic magmatism. Finally, we provide a summary of the eruptive history and distribution of volcanic forms in the CPR using several sub-region schemes.

Mesoproterozoic olivine gabbronorites of the Bashkirian anticlinorium, the South Urals: Parental melts and specifics of magma evolution

This paper is devoted to detailed study of picritic rocks (olivine melanogabbronorites) and comag� matic gabbrodolerites from sills and dikes in the central part of the Bashkirian meganticlinorium. These rocks are ascribed to the KamaBelsk magmatic province (K BP) that was formed in the eastern East European Platform (EEP) in the Mesoproterozoic time. The study of minerals (EMPA, SIMS), rocks, and their oxygen isotope compositions showed the contribution of crustal contamination, fractional crystallization and cumu� lus processes in their formation. The geochemical indicators of crustal contamination (Nb/Nb*, (Nb/La)n , δ 18 O, and others) show strong variations, which indicates uneven crustal contribution in the parental melts during rock formation (10-25%). The study of weakly contaminated (δ 18 O = 5.3‰) olivine melanogab� bronorites (MgO = 22.55 wt %) from the small Ishlya�1 subvolcanic body, which contain subordinate amount of cumulus (24%), highmagnesian olivine ( Fo91.3), and highCr spinel (cr# 0.67), as well as HREE depleted clinopyroxenes, allowed us to retrieve the composition of parental melt. The latter contained about 20 wt % MgO and was formed by 19-26% melting of mantle source (potential mantle temperature Tm of 1530-1545°C). Geochemical characteristics of KBP reflect the formation of primary melts by melting of mantle column at different depths, mixing of the melts, and significant contamination by crustal material. The dominant role in the formation of the rocks of the Ishlya area and Mashak Complex was played by derivatives of spinel peri� dotites, while the rocks of the Bakal-Satka area were derived from garnet peridotites. The melts of olivine melanogabbronorites crystallized within a pressure range from 7 to 2 kbar and tempera� ture from 1470°C to 1000-950°C at close to QFM. The comparison of model crystallization trend of parental melt with compositions of basic and ultrabasic rocks of the province showed that the observed com� positional diversity was provided by the accumulation of clinopyroxene-plagiolcase (dolerites and gabbrod� olerites) and olivive-plagioclase (picrites and picrobasalts) cumulates. Obtained estimates of conditions of magma generations with Tm significantly exceeding (at 150-165°C) that of asthenosphere indicate that the Mesoproterozoic magmatism in the eastern part of EEP was related to the ascent of deepseated plume. Pre� liminary results of comparative analysis of petrological-geochemcial characteristics of KBP and Mesoprot� erozoic basic rocks of Northern Greenland (Upton et al., 2005; Kalsbeek and Frei, 2006) revealed significant similarity of these withinplate manifestations, which in both cases bear signs of plume nature and distinct crustal contamination.

Sources of granitoids in the Tuva-Mongolian microcontinent and surrounding structures: Evidence from Pb, Nd, and O isotopic systematics

Newly obtained data on Pb isotopic ratios in feldspars from Early Proterozoic granitoids of the Sangilen block of the Tuva-Mongolian microcontinent and Caledonian structures surrounding it, considered together with earlier data on the O and Nd isotopic systems, indicate that ancient (approximately 2 Ga) crustal material mixed in the sources of granites of the Tuva-Mongolian microcontinent with younger juvenile material. Positive ɛNd values of granitoids from the Bashkimugor and Chgargalant massifs are accounted for by processes of crustal contamination during the interaction of the melt with crustal material. Similar Nd isotopic characteristics of granitoids in the Khoromnug pluton were caused by the melting of the Late Riphean crust. In granitoids of the massifs in the Kaakhem and Eastern Tannuola zones of the surrounding Caledonian structures, the involvement of juvenile material from oceanic crust increases away from the Sangilen block. Granites in the junction zone between the Tuva-Mongolian microcontinent and surrounding structures display evidence of the presence (up to 10–20%) of an ancient crustal component, and the melting history of granitoids in the Eastern Tannuola zone is dominated by an Early Paleozoic juvenile component in combination with material similar to the Vendian ophiolites of the Agardag-Erzin zone. An increase in the δ18O value, the 206Pb/204Pb ratio, and the TNd(DM) values within a single complex (from older to younger granitoid phases) is explained by the systematic involvement of crustal material in the melting processes.

The origin of geochemical trends and Eoarchean (ca. 3700 Ma) zircons in Mesoarchean (ca. 3075 Ma) ocelli-hosting pillow basalts, Ivisaartoq greenstone belt, SW Greenland: Evidence for crustal contamination versus crustal recycling

The Mesoarchean Ivisaartoq greenstone belt consists of tholeiitic pillow basalts, picrites, boninites, gabbroic to dioritic dykes and sills, actinolite schists and serpentinites. In addition, the belt includes volumetrically minor siliceous volcaniclastic sedimentary rocks, cherts, biotite schists, and quartzitic gneisses. Siliceous volcaniclastic rocks yield an average U–Pb zircon age of 3075 ± 15 Ma, and pillow basalts and gabbros yield a Sm–Nd errorchron age of 3080 ± 160 Ma, constraining the maximum age of the belt. The Ivisaartoq rocks underwent high-temperature seafloor hydrothermal alteration, resulting in extensive epidote-diopside calc-silicate metasomatism. Th–REE–Nb systematics of the least metasomatized volcanic rocks are consistent with a supra-subduction zone geodynamic setting, displaying a trend projecting from mid-ocean ridge basalt (MORB) field to island arc basalt (IAB) field. This trend is interpreted as reflecting a transition from the depleted upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration. Many pillow basalts in the Ivisaartoq greenstone belt contain numerous millimetre- to centimetre-long white polycrystalline ellipsoidal inclusions, called ‘ocelli’, set in a dark green mafic matrix (basaltic host). On the basis of new major and trace element, Nd isotope, and petrographic data, we re-interpret the origin of these inclusions and suggest that they were derived from the melting of hydrated mafic to ultramafic rocks in the lower oceanic crust during magma–crust interaction. Some pillow basalts contain rare small inherited magmatic zircons revealing Eoarchean (3706 ± 8 to 3717 ± 4 Ma) 207Pb 206Pb ages. In the least metasomatized pillow basalts, and gabbros and diorites there is a strong negative correlation between initial Nd isotopic composition ( ε Nd = + 0.76 to + 3.10) and depleted mantle model ages ( T DM = 3100–3800 Ma). Post-magmatic metasomatic alteration can be ruled out as the cause of this negative correlation, suggesting the contamination of the Ivisaartoq rocks, or their mantle source, by Eoarchean (~ 3700 Ma) continental crust or sediments derived from such crust. Four possible processes of crustal contamination might have taken place during or prior to the emplacement of the Ivisaartoq belt: (1) interaction with continental crust during magma ascent; (2) mixing between the depleted upper mantle and recycled sediments derived from Eoarchean continental crust; (3) mixing between the depleted upper mantle and delaminated lower continental crust; and (4) assimilation of continentally-derived sediments during eruption. It is unlikely that recycled Eoarchean zircons and their host rocks could have survived through multiple events of partial melting in the Mesoarchean upper mantle. A tectonic slice of metasedimentary rocks exposed between the Ivisaartoq belt and the Eoarchean (3600–3870 Ma) Isukasia terrane contains magmatic zircon grains up to 3800 Ma old. Accordingly, we argue that Eoarchean zircons are likely to have been derived from the protoliths of these metasedimentary rocks. The negative correlation between initial ε Nd and T DM in pillow basalts, gabbros and diorites, however, can be better explained by contamination of the mantle source through hydrous melts and/or fluids, derived from Mesoarchean subducted (recycled) sediments, than by contamination at the surface.

Remnants of a Mesozoic Rift in a Subducted Terrane of the Alpujarride Complex (Betic Cordilleras, Southern Spain)

Amphibolite bodies with eclogite relics are interlayered at different levels in the Ojen nappe (Alpujarride Complex) below the Ronda peridotites of the Betic Cordilleras in southern Spain. We present an integrated structural, petrological, and geochemical study of such amphibolites, from which it is possible to deduce a gabbroic origin for the amphibolite protoliths, and we propose that their intrusion into the continental crust took place in a rift setting of Jurassic to lower Cretaceous age. The geochemical data are consistent with basaltic magmas produced by partial melting of an enriched or transitional mantle source (E- or T-MORB [mid-ocean ridge basalt]) and reflect plagioclase accumulation and weak crustal contamination processes. The structures associated with the rifting stages are obliterated by deformation and metamorphism related to subduction at depths of 60 km and later exhumation of the amphibolite bodies during the complex convergence of the Iberian and African plates from upper Cretaceous onward. The recognition of this Mesozoic rift is of great interest in studies of the tectonic evolution of the Betic Cordilleras and places new constraints on paleogeographic reconstructions of the western Mediterranean. We suggest that such a Mesozoic rift would correspond to the transform seaway linking the Neo-Tethys realm with the Atlantic Ocean.

Geochemistry and geochronology of mafic rocks from Bamenda Mountains (Cameroon): Source composition and crustal contamination along the Cameroon Volcanic Line

Mafic rocks from the Bamenda volcanic province along the Cameroon Volcanic Line have been dated from 17 to 0 Ma. Associated with some trachytes and rhyolites, this volcanism covers a period of more than 25 Ma. The studied rocks are basalts to mugearites. Most of them have been contaminated by continental crust during their transit to the surface. The oldest rocks are the most contaminated. One group of samples shows high Eu, Sr and Ba contents. This characteristic is not due to crustal contamination process, but has a mantle source origin. We argue that these characteristics have been acquired by mixing of melts formed by partial melting of mantle pyroxenites with melts formed in mantle peridotites. Such pyroxenites have been observed as mantle xenoliths in the Adamaoua province, and their chemical and isotopic compositions are consistent with such a model.

Sr and Pb Isotope Micro-analysis of Plagioclase Crystals from Skye Lavas: an Insight into Open-system Processes in a Flood Basalt Province

Crystals in many magmatic rocks are heterogeneous in terms of their isotope composition. Detailed crystal-isotope stratigraphy (CIS) studies have shown that crystals act as reliable recorders of their magma source composition and of the pathways of magma interaction, and successfully identify the end-members involved in magmatic systems. In this paper we have analysed cores and rims and single plagioclase crystals from samples of the Skye Volcanic Centre, part of the British Palaeogene Igneous Province. The isotope analyses reveal that plagioclase crystals from different volcanic samples through thelava succession in Skye (Skye Main Lava Series, Preshal More Basalts and Big-Feldspar Lavas) have larger Sr and Pb isotopic ranges than the isotopic ranges found in hundreds of whole-rock analyses. The large Sr and Pb isotopic variation in plagioclase crystals in Skye is due to variable degrees of crustal contamination of the parental magmas during fractional crystallization and during the ascent of such magmas to the surface. The isotope variation of plagioclase crystals in the early lavas (Skye Main Lava Series) reflects assimilation-fractional crystallization processes dominated by the lower crust, whereas plagioclase crystals from later erupted magmas (Big-Feldspar Lavas and Preshal More basalts) show assimilation of upper crust superimposed on previous lower crustal contamination processes. The variability in Sr and Pb isotopes documented in different crystals within the same rocks shows that the crystals have been aggregated in magmas from different sites of storage and differentiation during the ascent of the magmas to the surface.

The ca. 350 Ma Beja Igneous Complex: A record of transcurrent slab break-off in the Southern Iberia Variscan Belt?

We report the results of an isotopic study of the large gabbro-dioritic Beja Igneous Complex (BIC) in the boundary between the highly contrasting Ossa–Morena (OM) and South Portuguese (SP) Zones of the Southern Iberian Variscan orogen. This boundary is interpreted as a major suture zone materialized by discontinuous, scattered strips of mafic/ultramafic rocks (the so-called Beja–Acebuches ophiolite complex, BAOC), and by mélange deposits of Middle to Late Devonian age in the Pulo do Lobo accretionary prism (PLAP). The Beja gabbro was interpreted either as part of the ophiolite-like units, or as a broadly arc-related massif reflecting the northward subduction of oceanic lithosphere. U–Pb zircon (ID-TIMS) dating of two diorites and a granodiorite points to igneous emplacement ages of 350 ± 2 Ma (Serpa), 352 ± 2 Ma (Torrão), and 353 ± 4 Ma (São Pedro), respectively, whereas a felsic dyke yields a slightly younger age of 345 ± 2 Ma. These results show that published Ar/Ar dates do not represent igneous crystallization ages, but merely reflect regional cooling below ca. 500 °C, at least 10 Ma after the major intrusive event, probably as a result of uplift of the OMZ side of the suture zone relative to the subsiding SPZ. 87Sr/ 86Sr 350 and εNd 350 display a large range of values (from 0.7041 to 0.7093 and from + 4.0 to − 6.1, respectively) which documents a rather complex petrogenetic history, with an important role played by crustal contamination processes. The more primitive Sr and Nd isotope signatures are measured in the mafic cumulates, while radiogenic Sr and unradiogenic Nd isotope compositions occur in the more evolved rock-types. The broad trend of decreasing εNd 350 with decreasing Sm/Nd and increasing SiO 2 concentration is reminiscent of crustal assimilation combined with fractional assimilation (AFC). εNd values of flasergabbros and associated cumulates ascribed to the ophiolite-like unit in the Guadiana valley are close to zero or even slightly negative, irrespective of the age chosen for the correction for in situ radioactive decay of 147Sm in the range 350–500 Ma, assumed to encompass the true geological age. This observation conflicts with a truly oceanic derivation of these rocks, which could rather represent early stage, deformed and metamorphosed equivalents of the Beja gabbro. Most metabasalts of the same area have positive, albeit variable εNd, consistent with mildly depleted mantle sources and/or significant crustal assimilation. Only a few samples reach highly positive values ( εNd 350–500 ca. + 8) similar to those typical for N-MORBs, as do the Acebuches amphibolites near Aracena ( εNd 350–500 ca. + 9). These data suggest that the BAOC is not only disrupted but also composite, since the gabbroic rocks are not cogenetic with the basalts. Based on the Late Devonian age of the collision between the OMZ and SPZ, the emplacement of the Beja gabbro cannot be related to an active subduction of oceanic lithosphere. Instead, the generation in the mantle, and emplacement in the overlying continental crust of copious volumes of mafic magmas with concomitant LP-HT metamorphism is tentatively ascribed to a process of transcurrent slab break-off that followed oblique subduction and collision.

AGE AND GEOCHEMISTRY OF MIDDLE TO LATE CARNIAN BASALTS FROM THE ALAKIRÇAY NAPPE (ANTALYA NAPPES, SW TURKEY): IMPLICATIONS FOR THE EVOLUTION OF THE SOUTHERN BRANCH OF NEOTETHYS

The geochemical features of spilitic basalts and the radiolarian fauna of associated pelagic sediments have been studied from two different sections of the Alakircay Nappe of the Antalya Nappes, SW Turkey. The first section in Cukurkoy is located in the eastern part of the Antalya Gulf and includes thick spilitic basalts, overlain by an alternation of mudstone, marl and pelagic cherty limestone. Radiolarian data from the cherty limestone in this unit reveals a middle Carnian age for this section. The second section in Yaylakuzdere is situated in the western part of the Antalya Gulf. In this section, thick pillow basalts are overlain by a succession of limestone/cherty limestone and shale beds. The age of the limestones interlayered with the pillow basalts was assigned to the late Carnian, whereas the basal part of the overlying cherty limestones was dated as latest Carnian/earliest Norian by conodont and radiolarian faunas. The pillow lavas in the Cukurkoy and Yaylakuzdere regions consist of within-plate type alkaline basalts and display multi-element pattern similar to typical oceanic island basalts (OIB). Low La/Yb, Zr/Y and La/Nb < 1 ratios are also indicative for an OIB-like deep mantle source. These alkaline basalts have not suffered interactions with a subducted slab and/or continental crust due to presence of high HFSE abundance, the lack of depletion in Nb and Ta that are characteristics of subduction and/or crustal contamination processes. Based on this data, it can be concluded that alkaline volcanism in these two regions of the Antalya Nappes have been probably generated by a small OIBtype mantle plume during the middle - late Carnian time interval, in the advanced stages of rifting of the Antalya Nappes successions in the southern branch of Neotethys. This data reveals the generation of a rift basin before middle Carnian for this ocean.

Collision‐Driven Slab Breakoff Magmatism in Northwestern Anatolia, Turkey

Postcollisional Eocene magmatism in northwestern Anatolia produced two E‐W‐trending linear belts of plutons along and north of the Izmir‐Ankara‐Erzincan suture zone (IAESZ), whose geochemical features and age relations support a slab breakoff model for their petrogenetic oinevolution. The suture zone granitoids (SZGs) in the southern belt have ages around 54–48 Ma, are intrusive into blueschist rocks of the IAESZ, and are composed of diorite, quartz diorite, granodiorite, and syenite. The Marmara granitoids (MGs) in the northern belt are slightly younger (48–35 Ma), intrusive into the Paleozoic‐Mesozoic crystalline basement rocks of the Sakarya continent, and composed of monzogranite, granite, and granodiorite. Both SZGs and MGs have moderately to highly evolved medium‐ to high‐K calc‐alkaline compositions and are predominantly metaluminous I‐type granitoids. Nd‐Sr isotope systematics ($$^{87}\mathrm{Sr}\,/ {}^{86}\mathrm{Sr}\,_{( \mathrm{t}\,) }=0.70624{\mbox{--}} 0.70704$$; $$^{143}\mathrm{Nd}\,/ {}^{144}\mathrm{Nd}\,_{( \mathrm{t}\,) }=0.512430{\mbox{--}} 0.512439$$) and trace element compositions of the SZGs and MGs suggest a metasomatized lithospheric mantle source modified by the Late Cretaceous subduction event for their parental melts. Partial melting of this mantle lithosphere was facilitated by asthenospheric upwelling and associated thermal perturbation in response to a slab breakoff experienced by the Tethyan oceanic lithosphere subducted beneath the Sakarya continent. Mantle‐derived melts were modified by crustal contamination, assimilation, and fractional crystallization processes as they migrated through the overlying crust. Stronger depletion of the MG rocks in Eu, Ba, Sr, and P and their higher contents of Pb, K, Ni, and SiO2 in comparison to the SZG rocks suggest greater amounts of crustal contamination during the evolution of their magmas rising through the Sakarya continental crust. The geochemistry, petrogenesis, and geochronology of the subparallel SZG and MG belts along and north of the IAESZ provide a case study of the evolution of slab breakoff magmatism in an Alpine‐style collision zone.

ヘリウム同位体比によって追跡されるマグマ分化中の地殻汚染プロセスついて：マリアナ島弧―海溝系における例

ヘリウム同位体比によって追跡されるマグマ分化中の地殻汚染プロセスついて：マリアナ島弧―海溝系における例

Interaction between basic and acid magmas during the latest stages of the post-collisional Variscan evolution: Clues from the gabbro–granite association of Ota (Corsica–Sardinia batholith)

The intrusive association of Ota (post-Variscan Corsica–Sardinia batholith) consists of mafic rocks and granites displaying mingling relations. The mafic rocks are olivine gabbros to quartz-diorites, and the granites are subsolvus and characterised by biotite as the only mafic phase. Minor amounts of amphibole-bearing granodiorites to granites develop along the contacts between mafic rocks and granites. The rather high initial ε Nd (+ 4.5) of the most primitive mafic sample indicates that parental basic melts had an extensive contribution from a depleted mantle source. In the mafic rocks, the decrease of initial ε Nd (to + 2.1) is associated with an increase of Th and U concentrations in clinopyroxene, thus indicating that the Ota basic melts underwent a process of crustal contamination. Associated biotite granites have low initial ε Nd (+ 0.4) and high Th and U concentrations. The chemical evolution of Ota basic melts most likely occurred through fractional crystallisation and concomitant contamination with the acid magma that gave rise to associated biotite granites. The incompatible element signature of late-stage anhedral amphibole from variably evolved mafic rocks is similar, thus suggesting that the whole Ota mafic sequence recorded the contamination process, possibly by percolation within the gabbroic crystal mush of a melt contaminated by the acid magma. Hybridisation between basic and acid magmas is also documented by contact amphibole-bearing granitoids. In particular, major and trace element zoning of amphibole from granodiorites reveals that such a hybridisation was associated with reactions between crystals from the basic magma (amphibole and clinopyroxene) and the acid melt. The occurrence of chemically homogeneous amphiboles in associated granites suggests that “contact” hybridisation comprised also a process of gradient-induced chemical diffusion.

Isotope geochemistry and FOZO mantle component of the alkaline–carbonatitic association of Fuerteventura, Canary Islands, Spain

Carbonatites are very rare in oceanic environments, where they have been reported only at the Canary and Cape Verde islands in the Atlantic Ocean. In the Canary archipelago, calciocarbonatites occur only on the island of Fuerteventura, in clear spatial and temporal association to alkaline silicate rocks, ranging from ultramafic to salic, and comprising alkaline clinopyroxenites, melteigites–ijolites–urtites, nepheline-bearing gabbros and nepheline–syenites, together with their volcanic equivalents. Selected isotopic (Sr, Nd and Pb) data from representative samples of this alkaline–carbonatitic association show a very restricted range of isotopic compositions, with those of the calciocarbonatites overlapping those of the alkaline silicate rocks. Despite the fact that all these rocks display cumulate textures and cover a wide range of SiO 2 and Sr, Nd, Pb contents, initial isotopic compositions are approximately constant and mantle-like, indicating that evolution of the alkaline–carbonatitic association of Fuerteventura must have taken place in the absence of crustal contamination processes. Regarding the mantle source for the alkaline–carbonatitic association, its depleted initial Sr ( 87Sr/ 86Sr = 0.7032–0.7036) and Nd ( 143Nd/ 144Nd = 0.5128–0.5129) compositions, fall on the same trend as the East African young (< 40 Ma) carbonatites. In 143Nd/ 144Nd and 87Sr/ 86Sr vs. 206Pb/ 204Pb diagrams, the alkaline–carbonatitic association of Fuerteventura shows FOZO compositions for Sr and Nd, and slightly higher (towards HIMU) Pb compositions. Furthermore, these data fit the projection of the mantle plane defined by Sr, Nd and Pb isotopic compositions of the DMM, HIMU and EMI components, this fit not being found in sources containing crustal components. In view of this, we propose that the alkaline–carbonatitic association of Fuerteventura was derived mainly from a heterogeneous mantle plume with a deep-seated (FOZO) signature, mixed with HIMU component.

Proterozoic mafic–ultramafic intrusions in the Arunta Region, central Australia: Part 1: Geological setting and mineral potential

Proterozoic mafic–ultramafic intrusions of the Arunta Region record a protracted period of magmatism during the evolution of this geologically complex and tectonically long-lived terrane in central Australia. New U–Pb zircon geochronology data highlight the episodic emplacement of the mafic–ultramafic systems. Five major events of dominantly tholeiitic mafic magmatism have been recognised at ∼1810 Ma, ∼1780 Ma, ∼1690 Ma, and ∼1635 Ma, and a much younger event of probably early Palaeozoic age. A sixth event at ∼1135 Ma has alkaline-ultramafic affinities. Chondrite- and mantle-normalised multi-element patterns of rocks with melt-like compositions have refined the correlations of the magmatic systems indicated by the geochronological framework. Continental margin environments (e.g. subduction or back-arc related) are generally indicated for the magmatic events at ∼1810 Ma, ∼1780 Ma, and ∼1635 Ma, whereas the mafic magmatism at ∼1690 Ma and in the early Palaeozoic is more characteristic of intracontinental settings (e.g. extensional rifts). The intrusions occur in proximity to major province-wide faults. Differential vertical movements along these faults provide the opportunity to examine geological processes at crustal depths ranging from ∼5 km to 25 km (2 kb to 8 kb). The intrusions form large homogeneous mafic granulite bodies, granulite bodies interfolded with felsic units, contaminated gabbroic sheets, stacked sequences of high-level doleritic sills, small pods and laterally extensive sheets of amphibolite, and rare plug-like ultramafic bodies. Metamorphic overprints range from granulite to sub-amphibolite facies, with a concentration of high-grade mafic granulite bodies in the central Arunta Region attributed to deeper levels of crust uplifted north of the Redbank Thrust. Mafic rocks in the intrusions of high-metamorphic grade are dominantly two-pyroxene mafic granulites with high clinopyroxene to orthopyroxene ratios and variable amounts of minor hornblende, biotite, quartz, and garnet. In contrast, gabbroic rocks in the lower-grade intrusions of the western Arunta Region contain more orthopyroxene, alkali feldspar, quartz, and Fe–Ti oxides. LREE-enrichment trends with decreasing 147Sm/ 144Nd ratios and initial ɛ Nd values (+1.5 to −4.7) indicate that felsic crustal contamination processes were particularly important during the evolution of the latter intrusions. Incompatible trace-element trends show that the Arunta intrusions fall into two major geochemical groups that for the first time highlight geographical differences in mineral prospectivity. 1. A S-rich group (∼1200 to 300 ppm S: Andrew Young Hills intrusion, Mount Hay Granulite, Mount Chapple Metamorphics) from the western and central Arunta regions that has potential for basal Ni–Cu–Co-sulphide associations. 2. A relatively S-poor (<300 ppm S), slightly more primitive group (Attutra Metagabbro, Mordor Complex) from the eastern Arunta Region that has greater potential for stratabound PGE-sulphide associations. Some potential also exists for structurally controlled hydrothermal deposits of Cu–Au ± PGEs ± Ag ± Pb spatially associated with mafic–ultramafic rocks (Riddock Amphibolite). The major challenges for finding massive Ni–Cu–Co sulphides in the lower parts of prospective S-saturated intrusions (e.g. Andrew Young Hills, Mount Hay Granulite) are to determine the pre-deformational geometries of the bodies and to locate favourable mineralised environments, such as embayments in basal contacts and feeder conduits, concealed by thin Cainozoic alluvial deposits.

A geochemical and Nd isotopic study of Barberton komatiites (South Africa): implication for the Archean mantle

The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al 2O 3/TiO 2 and CaO/Al 2O 3 ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb) N ratios. In contrast, the Al-depleted komatiites show significantly lower Al 2O 3/TiO 2 ratios of 8–12, highly variable CaO/Al 2O 3 (0.19–2.81) ratios combined with (Gd/Yb) N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. 147Sm/ 144Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite ε i,Nd (3.45) values cluster at +1.9±0.7. Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites. The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al 2O 3/TiO 2, (Gd/Yb) N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb) PMN, (Sm/Yb) PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites.

B/Nb and δ 11B systematics in the Phlegrean Volcanic District, Italy

Boron concentration and isotopic compositions were determined in representative products from the Phlegrean Volcanic District (PVD), which includes Campi Flegrei (CF), and the islands of Procida and Ischia. The most primitive products (from Procida and Ischia islands) are moderately enriched in B (4.6–12 ppm), whereas more evolved products of CF and Ischia Island are more enriched (21–118 ppm). The content of B is positively correlated with Nb, Zr, Th, La and, generally, with all incompatible elements. δ 11B values are generally lower and more uniform in samples from CF (−6.8 to − 10.6‰) compared with those from Procida (−3.6 to −8.5‰) and Ischia (−2.8 to −8.4‰) islands. Overall, B-enrichments relative to fluid-immobile elements of PVD are only slightly higher than those observed in mid-ocean ridge basalts and there is no significant correlation between δ 11B and B/immobile element ratios. For CF samples including the Campanian Ignimbrite (CI) and most post-CI products, δ 11B is negatively correlated with 87Sr/ 86Sr, with extreme compositions represented by evolved products of the CI (higher δ 11B, lower 87Sr/ 86Sr) and less evolved shoshonite (lower δ 11B, higher 87Sr/ 86Sr). In contrast, samples from Procida, Ischia, and pre-CI products and one young shoshonite from CF define a distinct correlation with primitive trachybasalt as the high δ 11B, low 87Sr/ 86Sr end-member, and young shoshonites as the low δ 11B, high 87Sr/ 86Sr end-member. The overall interpretation of the geochemical and isotopic data suggests three conclusions: (1) Geochemical and isotopic variations of post-CI products from CF can be explained by mixing/mingling between at least two distinct magmas, i.e. the CI and the young shoshonite (Minopoli 1). (2) The isotopic trend described by Procida trachybasalts, Ischia samples, pre-CI samples from CF and Pigna St. Nicola shoshonite is a mantle trend suggesting the involvement in their genesis of a distinct component depleted in B, with low δ 11B and high 87Sr/ 86Sr. A similar component has been invoked to explain the differences between Stromboli potassic lavas and more typical calc-alkaline rocks of the Aeolian Islands. This component is consistent with ‘sediments or melts thereof’ introduced into the mantle by subduction, but depleted in B due to metamorphic reactions accompanying subduction. (3) The displacement from the mantle trend of CI and Minopoli 1 shoshonite magmas is probably due to crustal contamination processes.

The tholeiitic dyke swarm of the Arraial do Cabo peninsula (SE Brazil): 39Ar/ 40Ar ages, petrogenesis, and regional significance

We present 40Ar/ 39Ar age determinations and chemical and Sr–Nd isotopic data for the tholeiitic dyke swarm cropping out at Arraial do Cabo peninsula at the eastern end of the Rio de Janeiro coastline. The age determinations indicate that the swarm emplaced approximately 55 Ma ago and thus is similar in age to the K-rich alkaline dykes and plugs also found in the peninsula. The dykes are basalts and basaltic andesites that belong to low Ti tholeiitic series. One dyke group is basaltic in composition, has relatively low Zr/Nb (8.9±1.8), roughly flat mantle-normalized incompatible element patterns, relatively low initial 87Sr/ 86Sr (0.70434–0.70426), and high εNd (55) (+2.3 to +3.8). These dykes chemically resemble enriched midocean ridge basalts (MORB). A second dyke group is formed by more evolved basaltic andesites with higher Zr/Nb ratios (12.7±2.1), negative Nb anomalies in the mantle-normalized patterns, higher initial 87Sr/ 86Sr (0.70548–0.70613), and lower εNd (55) (−1.8 to −2.2). Chemical and isotopic data exclude the possibility that the two dyke types are comagmatic or related to each other through crustal contamination processes. The genesis of the Arraial do Cabo tholeiites was likely caused by extension-related melting of a largely incompatible element-depleted mantle, with no trace of the enriched component in the roughly coeval, K-rich mafic alkaline magmas of the Serra do Mar province.