AFC Modelling Research Articles

Famatinian magmatism in the SW Gondwana margin: A review focused on the Sierras Pampeanas (Argentina)

The Famatinian orogeny along the proto-Pacific margin of Gondwana, from Venezuela to northeastern Patagonia, was of the accretional type during the Paleozoic (Ordovician to Silurian). Information gathered from the Sierras Pampeanas (Argentina) reveals magmatism (plutonism and volcanism) between 490 and 465 Ma, with a peak at 473–467 Ma, similar to other regions along the margin. Metamorphic events in the middle crust occurred between 484 and 465 Ma, and at ca. 440 Ma. Recent reviews propose a lithospheric extension phase (>480 Ma) before contraction episodes (ca. 470 and 440 Ma). Magmatism transitioned from peraluminous to metaluminous intrusions, with prolonged mafic magmatism throughout. Mafic intrusions show a broad spectrum of isotope compositions (87Sr/86Srt = 0.705 to 0.711 and ƐNdt = +5 to −6) but with general trends toward isotopically enriched magmas. The main arc consists of tonalite-granodiorite magmas (ca. 480–470 Ma) with <25% crustal contamination but crustal-like isotopic signatures (87Sr/86Srt = 0.707 to 0.711 and ƐNdt = −4 to −6). The wide isotopic variations in granodiorite-monzogranite (and volcanic equivalents) (87Sr/86Srt = 0.705 to 0.711 and ƐNdt = +3 to −6) suggest two origins: differentiation and contamination of tonalite-granodiorite magmas (AFC model) for the most evolved, and differentiation from juvenile mafic additions for the isotopically juvenile ones. Regardless of depth (∼1–6 kbar) and position within the magmatic arc, analyzed areas display north-south striking and steep magmatic fabrics. These fabrics may have rotated by no more than 20–28° from its original position. The petrogenetic model involves isotopically diverse sub-arc mantle sources. Famatinian magmatism ceased abruptly around 465 Ma (magmatic lull), correlating with arcs along the SW Gondwana margin, indicating significant geodynamic changes in the subduction regime.

Metasomatized mantle under the Eastern Mexican Alkaline Province: evidence from the Oligocene Rincón Murillo Gabbro, Sierra de San Carlos-Cruillas, NE México

The Cenozoic Sierra de San Carlos-Cruillas (SSCC) magmatic complex belongs to the Eastern Mexican Alkaline Province (EMAP). The SSCC mainly consists of plutonic and subvolcanic bodies displaying diverse lithological compositions. These rocks exhibit both OIB- and arc-like geochemical characteristics. In this contribution, we present new petrographic, geochemical, isotopic, and geochronological data of the Rincón Murillo Gabbro (RMG), situated in the southern part of the SSCC. The RMG is nepheline normative and displays an OIB-like intraplate character. Laser ablation ICP-MS U-Pb analyses on titanites yielded Oligocene (Rupelian) crystallization ages of 31.8 ± 1.8 and 31.0 ± 1.7 Ma. The 87Sr/86Sr(i) (0.703400–0.704159) and 143Nd/144Nd(i) (0.512732–0.512815) ratios indicate a mantle origin. Trace element data indicate the participation of different enriched mantle components in the source. One component is related to previous subduction processes, generating the HFSE-depleted arc-like magmas of San José Gabbro in the northern part of SSCC. The second component is related to deep mantle melts with a slightly carbonatitic signal and HFSE-enriched fluids derived from the subducted Hess conjugate plateau generating different enriched sources in the upper mantle under the SSCC. The passage, eclogitization, detachment and foundering of the Hess plateau gave rise to an asthenospheric upwelling that triggered the low to moderate partial melting degrees (3–15%) of an amphibole-bearing garnet peridotite, generating the OIB-like parental magmas of RMG, located in the southern part of SSCC. Two-component mixing, and AFC models indicate that variable degrees of crystal fractionation and minor crustal assimilation are involved in the evolution of Rincón Murillo Gabbro.

Origin and Petrogenesis of Magmatism in Collision-Related Environments: Evidence from the Melikler Volcanics on the Kars Plateau-Turkey in the Turkish-Iranian High Plateau

Abstract The temporal distribution of enriched source components and magmatism in continental collision zones provides critical information about mantle dynamic processes in collision-related environments. This paper presents petrology, mineralogy, K-Ar ages and whole-rock major and trace elements, as well as Sr-Nd-Pb-Hf isotopic compositions of Melikler volcanism in Kars Plateau (KP) in the East Anatolia Collision Zone, NE Turkey, with the aim to understand the role of the subducting slab, the origin of magmatism and the geodynamic evolution in the collision-related environments. Our K-Ar dating results show the Melikler volcanism erupted between 5.29 and 1.7 Ma and allows us to divide it into an early (5.29–2.53 Ma) and a late (2.24–1.7 Ma) stage. Major-trace element abundances, isotopic compositions, EC(R) AFC (energy-constrained recharge, assimilation, and fractional crystallisation) and MELTS model calculations of both stages indicate that the least evolved samples were not affected by significant crustal contamination and fractional crystallisation. More evolved samples of the late stage underwent AFC processes with up to 6–9% crustal assimilation; however, those of the early stage were differentiated from a parental magma composition via AFC (up to 2–7.5% crustal assimilation) and experienced magma replenishment at pressure of 0.5 kbar; thus, both early and late stages have experienced open system conditions. The least evolved samples of both stages across the KP have arc-enriched geochemical and isotopic signatures, characterised by prevalent negative Nb–Ta anomalies and moderately radiogenic Sr, unradiogenic Nd-Hf and highly radiogenic Pb isotopic compositions. These primary melts could be derived from a depleted MORB mantle source metasomatised by sediment melt from the subducting Neotethys oceanic slab. Combined trace elemental and isotopic modelling results suggest that the least evolved samples of the early stage were formed by 2–4% melting of an amphibole-bearing garnet lherzolitic mantle source, which was metasomatised by 0.3–0.5% contribution of subducted slab component with a ratio of sediment melt/AOC (altered oceanic crust) melt about 90:10. A depleted lherzolitic mantle source containing apatite and garnet through inputs of 0.6–0.8% melts derived from the subducted oceanic slab, with 5–10% partial melting degree, could produce the least evolved samples of the late stage. Thermobarometric calculations reveal that the least evolved samples of the late stage are derived from the lithosphere-asthenosphere boundary at a depth of 77–82 km; in contrast, those of the early stage are produced from the lithosphere at a depth of 66–69 km. Literature data and the findings obtained from this study indicate that the onset of the Arabian-Eurasian collision may have occurred in the Oligocene and lithospheric dripping caused by the hard collision that occurred around the Late Miocene-Early Pliocene may produce the Melikler volcanic rocks.

Geochemical Characterization of Intraplate Magmatism from Quaternary Alkaline Volcanic Rocks on Jeju Island, South Korea

The major and trace elements of Quaternary alkaline volcanic rocks on Jeju Island were analyzed to determine their origin and formation mechanism. The samples included tephrite, trachybasalts, basaltic trachyandesites, tephriphonolites, trachytes, and mantle xenoliths in the host basalt. Although the samples exhibited diversity in SiO2 contents, the relations of Zr vs. Nb and La vs. Nb indicated that the rocks were formed from the fractional crystallization of a single parent magma with slight continental crustal contamination (r: 0–0.3 by AFC modeling), rather than by the mixing of different magma sources. The volcanic rocks had an enriched-mantle-2-like ocean island basalt signature and the basalt was formed by partial melting of the upper mantle, represented by the xenolith samples of our study. The upper mantle of Jeju was affected by arc magmatism, associated with the subduction of the Pacific Plate beneath the Eurasian Plate. Therefore, we inferred that two separate magmatic events occurred on Jeju Island: one associated with the subduction of the Pacific Plate beneath the Eurasian Plate (represented by xenoliths), and another associated with a divergent setting when intraplate magmatism occurred (represented by the host rocks). With AFC modeling, it can be proposed that the Jeju volcanic rocks were formed by the fractional crystallization of the upper mantle combined with assimilation of the continental crust. The xenoliths in this study had different geochemical patterns from previously reported xenoliths, warranting further investigations.

Petrological and Geochemical Studies on the Si-Undersaturated Rocks of the Mount Cameroon: Genesis of the Camptonite and Nephelinite at the Cameroon Hot Line

The Cameroon hot line is dominated by magmatic rocks. The variations of magma and chemistry are generally due to the difference of physical conditions and chemistry in the magma source region during the ascent of magma. The Mt Etinde and the Mt Cameroon, both edifices belong to the Cameroon Hot line, have a particularity some rare rocks such as camptonite and nephelinite. The relationship between the silica undersaturated rocks in the both edifices is characterized by the lateral variation appear through the petrography of the different rocks. The concerned geochemical data allow to compare the Mount Etinde nephelinite and Mount Cameroon camptonite where the differentiation process reflects geochemical affinities from a basaltic magma source on the Cameroon hot line. The compatible elements between the Mount Etinde nephelinite and the Mount Cameroon camptonite and basalt correlate with the difference in modal compositions of mineral phases. The lateral variation of major and trace element contents in the Mount Cameroon camptonite and Mount Etinde nephelinite seem to be related to the difference in the fractional crystallization processes of mineral phases, the difference in the partial melting processes and the metasomatism source rich in volatile. The silica-undersaturated character of the camptonite and nephelinite could be attributed to assimilation of carbonate rocks within depth-level magma chambers. Trace element AFC modelling revealed that the parental magmas of both edifice volcanic rocks were mostly affected by fractional crystallisation coupled with metasomatism process in Ca rich source.

Generation of the Mt Kinabalu Granite by Crustal Contamination of Intraplate Magma Modelled by Equilibrated Major Element Assimilation with Fractional Crystallization (EME-AFC)

AbstractNew geochemical data are presented for the composite units of the Mount Kinabalu granitoid intrusion of Borneo and utilised to explore the discrimination between crustal- and mantle-derived granitic magmas. The geochemical data demonstrate that the units making up this composite intrusion became more potassic through time. This was accompanied by an evolution of isotope ratios from a continental-affinity towards a slightly more mantle-affinity (87Sr/86Sri ∼0·7078; 143Nd/144Ndi ∼0·51245; 206Pb/204Pbi ∼18·756 for the oldest unit compared to 87Sr/86Sri ∼0·7065, 143Nd/144Ndi ∼0·51250 and 206Pb/204Pbi ∼18·721 for the younger units). Oxygen isotope ratios (calculated whole-rock δ18O of +6·5–9·3‰) do not show a clear trend with time. The isotopic data indicate that the magma cannot result only from fractional crystallization of a mantle-derived magma. Alkali metal compositions show that crustal anatexis is also an unsuitable process for genesis of the intrusion. The data indicate that the high-K units were generated by fractional crystallization of a primary, mafic magma, followed by assimilation of the partially melted sedimentary overburden. We present a new, Equilibrated Major Element -Assimilation with Fractional Crystallization (EME-AFC) approach for simultaneously modelling the major element, trace element, and radiogenic and oxygen isotope compositions during such magmatic differentiation; addressing the lack of current AFC modelling approaches for felsic, amphibole- or biotite-bearing systems. We propose that Mt Kinabalu was generated through low degree melting of upwelling fertile metasomatized mantle driven by regional crustal extension in the Late Miocene.

Mantle driven cretaceous flare-ups in Cordilleran arcs

Continental arcs display episodic magmatism characterized by flare-ups and lulls. Models published to explain these patterns invoke (1) upper plate crustal processes driven by internal feedback; (2) episodic mantle melting processes, or (3) external lower plate tectonic events.This study addresses the role of mantle magmas during flare-ups in Mesozoic Cretaceous continental arcs using geochronological and geochemical data for three Cretaceous arc segments: the western Peninsular Ranges Batholith (wPRB), the Peruvian Coastal Batholith (PCB), and the Chilean Coastal Batholith (CCB).In all three arc segments, bedrock zircon age patterns defining a flare-up from ~125 to 90 Ma characterized by gabbro to granite units with Sri < 0.705, ƐNd from 0 to +7.5, 208Pb/204Pb from 38.2 to 38.7, and 206Pb/204Pb from 18.3 to 18.7. These values project well towards a depleted mantle source. Areal measurements show that gabbro forms ~18% (wPRB), ~24% (PCB), and ~10% (CCB) of exposed plutonic material. AFC modeling indicates that these magmas have experienced fractional crystallization with only minor crustal assimilation (<20–30%), implying that the great majority of these magmas are mantle-derived. Thus the cause of these flare-ups must be episodic mantle processes: crustal melting was not required for triggering the flare-up, and only played a secondary role in modifying melt compositions. It remains unclear if the episodic mantle processes reflect internal feedback(s) or external tectonically driven processes.

Intrusion of shoshonitic magmas at shallow crustal depth: T–P path, H2O estimates, and AFC modeling of the Middle Triassic Predazzo Intrusive Complex (Southern Alps, Italy)

The multi-pulse shoshonitic Predazzo intrusive complex represents an ideal igneous laboratory for investigating the chemical and physical conditions of magma emplacement in a crustal context, since numerical models can be constrained by field evidence. It constitutes the most intriguing remnant of the Middle Triassic magmatic systems of the Dolomitic Area (Southern Alps), preserved by the Alpine tectonics. Predazzo Intrusive Complex comprises silica saturated (pyroxenites/gabbros to syenites), silica undersaturated (gabbros to syenites), and silica oversaturated (granites and syenogranites) rock suites. In this paper, we modeled its emplacement and evolution with a multiple thermo-/oxy-barometric, hygrometric, and EC-AFC approach. At odds with what proposed in literature but according to the field evidence, the emplacement of the Predazzo Intrusive Complex occurred at shallow depth (< 6 km). In this context, the different pulses differed slightly in bulk water content, but shared a common thermal regime, with temperatures between 1000 and 1100 °C and ~ 600 °C at low-to-moderate oxidizing conditions (− 0.1 to + 0.7 ΔFMQ). The interaction between the intrusion and the shallow crustal rocks was minimal, with Sr and Nd isotopic compositions indicating an average of 5–6% assimilation of crust. A thermo- and oxy-barometric comparison with the nearby Mt. Monzoni also enabled to speculate about the solidification time of the intrusion, which we infer took place over about 700 ka.

Melt/rock reaction at oceanic peridotite/gabbro transition as revealed by trace element chemistry of olivine

Several recent studies have documented that reactions between melt and crystal mush in primitive gabbroic rocks (via reactive porous flow) have an important control in the formation of the lower oceanic crust and the evolution of MORBs. In this context, olivine-rich rocks can form either by fractional crystallization of primitive melts or by open system reactive percolation of pre-existing (possibly mantle-derived) olivine matrix. To address this question, we performed in-situ trace element analyses (by LA-ICP-MS) of olivine from the Erro-Tobbio ophiolite Unit (Ligurian Alps), where mantle peridotites show gradational contacts with an hectometer-scale body of troctolites and plagioclase wehrlites, and both are cut by later decameter-wide lenses and dykes of olivine gabbros. Previous studies inferred that troctolites and olivine gabbros represent variably differentiated crystallization products from primitive MORB-type melts. Olivines in the three rock types (mantle peridotites, troctolites, olivine gabbros) exhibit distinct geochemical signature and well-defined elemental correlations. As expected, compatible elements (e.g. Ni) show the highest concentrations in peridotites (2580–2730ppm), intermediate in troctolites (2050–2230ppm) and lowest in gabbros (1355–1420ppm), whereas moderate incompatible elements (e.g. Mn, Zn) show the opposite behaviour. By contrast, highly incompatible elements like Zr, Hf, Ti, HREE are variably enriched in olivines of troctolites, and the enrichment in absolute concentrations is coupled to development of significant HFSE/REE fractionation (ZrN/NdN up to 80). AFC modelling shows that such large ZrN/NdN ratios in olivines are consistent with a process of olivine assimilation and plagioclase crystallization at decreasing melt mass, in agreement with textural observations. In-situ trace element geochemistry of olivine, combined with microstructural investigations, thus appears a powerful tool to investigate reactive percolation and the origin of olivine-rich rocks in the lower oceanic crust.

Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

40Ar–39Ar age, whole-rock chemical, and Sr–Nd isotope data are presented for the post-collisional, Eocene (51.3–44.1Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal, and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded, and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K, and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr, and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042–0.7051), high positive εNd(t) values (+0.20 to +3.32), and depleted mantle Nd model ages (TDM1=0.63–0.93Ga, TDM2=0.58–0.84Ga). The bulk-rock chemical and Sr–Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element modelling suggests that the parental magma(s) of the volcanic rocks represent mixtures of melts derived by low-degree (~5–10%) partial melting of spinel–lherzolite (40–85%) and garnet–lherzolite (15–60%) mantle sources. Sr–Nd isotopic modelling also suggests that a 25–35% lower crustal component was added in the parental magmas; AFC modelling additionally indicates minor upper crustal contamination during the evolution of the volcanic rocks. In conclusion, integration of the geochemical, petrologic, and isotopic data with regional geology suggests that the analcime-bearing and -free volcanic rocks evolved from parental magma(s) derived from melts of a subcontinental lithospheric mantle and lower crustal sources.

DIFFERENTIATION PROCESSES in LATE CRETACEOUS ULTRAPOTASSIC VOLCANICS AROUND AMASYA

Late Cretaceous lithologies around Amasya region are represented by Pontide fore-arc basin units which corresponds a volcanoclastic sequence. This sequence has the products of alkaline ultrapotassic magmatism accompanying calcalkaline lavas which are abundant along Pontide arc. The ultrapotassic rocks which are classified as leucitite, minette and trachyte based on their mineralogical composition, occur as dikes, stocks and rarely lava flows as to be comprised by the Late Cretaceous Volcanoclastic Succession (LCVS). Fractional crytallization accompanied by assimilation (AFC) is a low pressure processes able to differentiate ultrapotassic parental melts to various compositions in a continental margin tectonic setting.The trachytes are the youngest and the most evolved members of LCVS. Therefore we performed AFC modelling using the most primitive minette sample as starting composition and calculated the fractionation trends based on the theoretical mineralo- gical compositions. We also used the Triassic metapelitic basement rocks of Central Pontides as assimilant. The AFC modelling results imply that it is possible to produce trachytes by adding Central Pontide basement rocks up to 5 %, begining from the most primiti- ve phonolitic sample of Amasya. However the differentiation of leucitite and minette is able to be explained by neither fractional nor assimilation processes.

Unravelling the complex interaction between mantle and crustal magmas encoded in the lavas of San Vincenzo (Tuscany, Italy). Part II: Geochemical overview and modelling

This work reports a geochemical overview and modelling of the lavas erupted ~4.4Ma ago at San Vincenzo (Tuscan Magmatic Province, TMP). Although these lavas cover a relatively small area (~10km2), they show very large geochemical variations caused by the interaction of mantle-derived and crustal-anatectic magmas. The lavas consist of peraluminous rhyolites (87Sr/86Sr(i) up to 0.726) hosting primarily variably sized magmatic enclaves with shoshonite/latite compositions (87Sr/86Sr(i) down to 0.708). New whole-rock data for a large shoshonite enclave show high concentrations of LREE, LILE, and tetravalent HFSE, coupled with pentavalent HFSE depletions and enrichments in compatible elements such as Cr and Co. The chondrite-normalised REE pattern is strongly fractionated and characterised by a negative Eu anomaly (Eu/Eu*=0.79). Hybridisation and AFC models suggest that the shoshonite enclave is the result of 12% rhyolite contamination of a mantle-derived magma akin to the potassic trachybasalt/shoshonite lavas of Capraia Island (~4.6Ma; TMP), following an 18.5% assimilation of Late Triassic metasediments (13% evaporite and 5.5% carbonate) and 56% fractionation of clinopyroxene (39%), plagioclase (10%), and biotite (7%).Each rhyolite sample is characterised by mineral-scale isotopic disequilibria (e.g., 87Sr/86Sr(i)=0.711–0.726), glass inclusions with large K2O/Na2O variations (1.1–3.4) and a poli-thermobarometric history of crustal melt production at eutectic conditions. A multi-parametric approach accounting for K2O/Na2O (1.3–2.2), 87Sr/86Sr(i) (0.713–0.725), Sr (104–311ppm) and Rb (294–403ppm) whole-rock variations, allowed us to divide the anatectic (A) rhyolites into five groups (A1, A2.1, A2.2, A2.3, A3). Group A1 shows the highest 87Sr/86Sr(i) ratios and the lowest values of Sr, K2O/Na2O and Rb. It is related to A2.1 and A3 rhyolites by positive K2O/Na2O–Rb and K2O/Na2O–FeO correlations. These three rhyolite groups crop out in the south of San Vincenzo and are interpreted to derive from mixing and ascent of peraluminous magma batches locally extracted at progressively higher temperatures from the heterogeneous Paleozoic metamorphic basement. The A2.3 rhyolites show the highest Rb concentrations (up to 403ppm) and inverse Sr–Rb and K2O–FeO correlations, suggesting reactions involving the incongruent melting of a biotite-bearing crustal source, which was previously depleted in low-temperature melting components such as muscovite and albite. Most of the A2.3 rhyolites crop out in the northern SVVC area and appear spatially and geochemically related to the A2.2 rhyolites by similarly low 87Sr/86Sr(i) ratios. However, A2.2 rhyolites show lower Rb contents (296–316ppm) and geochemical correlations (e.g., 87Sr/86Sr(i)–Rb, Sr–Rb, MgO–FeO) clearly indicating hybridisation (mixing) with the shoshonite mantle-derived magma (Rb=109–122ppm).Finally, we propose a four-stage model for the magma plumbing system of San Vincenzo, produced by the uprising of a mantle-derived magma in a normally faulted (and doubled) crustal basement characterised by two sections of Paleozoic siliciclastic rocks separated by Late Triassic evaporite-carbonate sequences.

Genesis of mugearites and benmoreites of Nemrut Volcano, eastern Turkey: Magma mixing and fractional crystallization of trachybasaltic melt

Presents detailed mineralogical and phase characteristics of trachybasalt, mugearite, trachydacite, and comendite samples from Nemrut Volcano in eastern Turkey, estimates of mineral formation conditions, and analyses of glasses from melt inclusions in olivine and rock matrix. Based on the analysis of the mineralogy and geochemistry of the rocks and mass balance calculations, the most feasible models of mugearitic and benmoreitic magma formation were proposed. The crystallization conditions of olivine, feldspar, and Fe-Ti oxides (titanomagnetite and ilmenite) were determined. Titanomagnetite and ilmenite were formed under the following conditions: 960-922°C, \(\Delta \log _{f_{O_2 } } \) NNO from −1.54 to −0.73 in the mugearite, 940-890°C, \(\Delta \log _{f_{O_2 } } \) NNO from −1.46 to −0.79 in the benmoreite, 870-845°C and \(\Delta \log _{f_{O_2 } } \) NNO from −2.11 to −1.82 in the trachydacite, and 705-667°C, \(\Delta \log _{f_{O_2 } } \) NNO from −2.48 to −2.18 in the comendite. Feldspars crystallized at 1150-950°C in the trachybasalt, 920-800°C in the benmoreite, and 760-720°C in the comendite. The temperature of melt inclusion entrapment in olivine (Fo75-40) from the trachybasalt, mugearite, benmoreite, and trachydacite was estimated as 1270-860°C. Except for the trachybasalt, partly resorbed phenocrysts and/or xenocrysts were observed in all the samples, which indicates their formation under nonequilibrium conditions. Mass balance calculations for rock compositions (FC, AFC, and FCA models) and mineralogical observations suggest that the magmas or melts of mugearitic and benmoreitic compositions could be produced by the fractional crystallization of trachybasaltic melt (mass fraction of melt F = 0.63–0.79), which assimilated a small amount of crustal material, as well as by the mixing of trachybasaltic (F = 0.16–0.45) and trachydacitic (F = 0.45–0.58) magmas in the presence of excess olivine, plagioclase, magnetite, and apatite (totaling 10–24 wt %). Pre-caldera comendites are enriched in Fe (4–5 wt % FeOtot) and trace elements compared with post-caldera comendites (2–3 wt % FeOtot). The analysis of the geochemical data and mass balance calculations indicated that the post-caldera benmoreitic magma could not be produced by the fractionation of trachybasaltic melt. This magma and corresponding post-caldera benmoreites have anomalously low Ba (46–54 ppm) and Sr (203–269 ppm) contents, which could not be obtained in the models of fractional crystallization of trachybasaltic melt accompanied by crustal assimilation. The compositions of post-caldera benmoreites and hybrid rocks of trachydacitic composition showing evidence for magma mixing (presence of xenocrysts from benmoreitic and comenditic magmas and compositionally variable glasses) were best reproduced by mixing trachybasaltic (F = 0.7-0.5) and low-Fe comenditic (F = 0.3–0.5) melts. Magma chambers with low-Fe comenditic melts appeared during the post-caldera stage owing to the fractional crystallization of pre-caldera trachytic and trachyte-comenditic magmas. Perhaps, the repeated eruptions of low-Fe comendites in the caldera and “rift” zone of Nemrut Volcano were related to the injection of benmoreitic magma into these chambers.

El volcanismo jurásico superior de la Formación Río Damas-Tordillo (33°-35,5°S): antecedentes su sobre petrogénesis, cronología, proveniencia e implicancias tectónicas.

The uppermost Jurassic continental and volcanic deposits of the R&iacute;o Damas-Tordillo Formation represent an interval of intense continental deposition within the Jurassic to Early Cretaceous dominantly marine environment of the Mendoza-Neuqu&eacute;n back-arc basin. Stratigraphic and geochronological data indicate that progressive emersion of the arc and forearc domain, disconnecting the back-arc region from the Pacific Ocean, occurred during occurred during the Late Jurassic and probably the Early Cretaceous (~160-140 Ma). This change in the margin configuration induced a marine regression and the subsequent deposition of continental material in the back-arc basin. The most likely source of the sediments would have been the Jurassic arc, located west of the back-arc basin. The maximum depositional age of 146.4&plusmn;4.4 Ma obtained from a red sandstone immediately below volcanic rocks confirms recent Tithonian maximum depositional ages assigned to the R&iacute;o Damas-Tordillo Formation, and suggests that the volcanic rocks, overlain by marine fossiliferous Tithoninan-Hauterivian sequences, should have erupted within a short time span during the Late Jurassic. Volcanism was probably facilitated by the presence of extensional structures related to the formation of the back-arc basin. Elemental and isotopic data, along with forward AFC models, suggest a depleted sub-arc asthenospheric mantle source for the volcanic rocks and the fractionation of olivine and plagioclase, along with small volumes of lower crust assimilation, as the main processes involved in the magmatic evolution. It is not possible to establish a different source and petrogenetic conditions for the R&iacute;o Damas-Tordillo Formation and the magmatism in the arc domain located further west, at the present-day Coastal Cordillera.

Age and composition of Cu–Au related rocks from the lower Yangtze River belt: Constraints on paleo-Pacific slab roll-back beneath eastern China

Abstract Cu–Au mineralization in the Yangtze River belt is closely related to the formation of adakitic rocks. The origin of these rocks is still controversially discussed. In this study, we report zircon U–Pb ages, major-trace element and Pb–Sr–Nd isotopes of Cu–Au related adakitic rocks from the Anqing and Tongling areas of the lower Yangtze River belt (LYRB). Geochemically, the rocks show similarities to adakites derived from slab melting. Statistical evaluation of existing zircon U–Pb data from the LYRB reveals a protracted magmatic activity from 148 Ma to 106 Ma with peak activity at ~ 139 Ma. Cu–Au related magmatism tends to decrease in age from the southwest (Gan-Hang belt) to the northeast (LYRB). This trend was probably imposed by a roll-back process of the paleo-Pacific plate. The Cu–Au related adakitic rocks in the Gan-Hang belt and the middle and lower Yangtze River belt show systematic variation in geochemical and Pb–Sr–Nd isotopic compositions, pointing to a change in magma composition with time. The rocks have features in common with Cenozoic slab-derived adakites, but have experienced different extents of melt peridotite interaction and complex crustal assimilation processes. AFC modeling shows that input of crustal material increased with time. Combined with the decrease of estimated copper reserves, it is concluded that this records a diminishing influence of slab-derived components during the course of the paleo-Pacific subduction process.

Late Triassic tholeiitic magmatism in Western Sicily: A possible extension of the Central Atlantic Magmatic Province (CAMP) in the Central Mediterranean area?

Late Triassic basaltic rocks crop out in the Lercara area in Western Sicily. Major and trace element composition, as well as Sr–Nd isotopic ratios (87Sr/86Sri=0.7074−0.7076; εNdi=from −0.69 to −1.09) of the Lercara rocks shows many similarities with Large Ion Lithophile Elements (LILE)- and Light Rare Earth Elements (LREE)-rich tholeiitic basalts of the Central Atlantic Magmatic Province (CAMP), that erupted during the Mesozoic fragmentation of the Pangea supercontinent and subsequent opening of the Central Atlantic Ocean. The geochemical features of the Lercara igneous rocks, together with the spatial distribution of the ~200Ma old CAMP rocks are unlikely to be associated with the arrival of a thermal anomaly in the form of a mantle plume and are more compatible with adiabatic melting of passively upwelling sub-lithospheric mantle. The original melts variably interacted with lower crustal rocks before reaching the surface. AFC modeling suggests two distinct differentiation paths including either simple mixing or assimilation-fractional crystallization processes involving lower crustal rocks. These interactions with continental crust indicate that an ocean basement most probably had not yet formed.

Mineralogy, geochemistry and petrogenesis of the Khopoli mafic intrusion, Deccan Traps, India

The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (eNdt = −9.0 to −12.7; 87Sr/86Sr = 0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm3, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.

Devonian volcanism in the Minusa basin in the Altai–Sayan area: geological, geochemical, and Sr–Nd isotopic characteristics of rocks

Based on geological data and the geochemical and isotopic (Sr, Nd) parameters of the Devonian volcanic associations of the Minusa basin, the main regularities of volcanism development are considered, the composition of magmatic sources is studied, and the geodynamic mechanisms of their involvement in rifting are reconstructed. The early stage of formation of the Minusa basin was characterized by intense volcanism, which resulted in differentiated and, more seldom, bimodal volcanic complexes composed of pyroclastic rocks and dolerite sills. At the late stage, only terrigenous deposits accumulated in the basin. It has been established that the basites are similar in composition and are intermediate in geochemical characteristics between intraplate rocks (OIB) and continent-marginal ones (IAB). The basites, like OIB, have high contents of all lithophile elements, which is typical of enriched mantle sources, and, like IAB, show negative anomalies of Nb, Ta, Ti, and, to a smaller extent, Rb, Th, Zr, and Hf, selective enrichment in Pb and Ba (and, sometimes, Sr), and a weak REE differentiation (7 < (La/Yb)N < 17). In contrast to the basins in other segments of the Devonian Altai–Sayan rift area, the igneous associations in the Minusa basin are characterized by a worse expressed geochemical inhomogeneity of rocks and lack of high-Ti (>2 wt.% TiO2) basites. The Sr and Nd isotope compositions of the Minusa basites deviate from the mantle rock series toward the compositions with high radiogenic-strontium and low REE contents.This points to the melting of a mantle substratum (PREMA-type) and carbonate-rich sedimentary rocks, which were probably assimilated by basaltic magma. The correlations between the contents of trace incompatible elements in rocks with SiO2 = 53–77 wt.% testify to the assimilation of crustal substrata by parental basaltic melts and the subsequent differentiation of contaminated magmas (AFC model). We propose a model for the formation of primary melts with the simultaneous participation of magmatic sources of two types: plume and fluid-saturated suprasubductional, localized beneath the active continental margin.

Heterogeneous Zircon Cargo in Voluminous Late Paleozoic Rhyolites: Hf, O Isotope and Zr/Hf Records of Plutonic to Volcanic Magma Evolution

Voluminous rhyolitic lavas and ignimbrites (c. 34 000 km3) were formed in the NE German Basin in a post-collisional setting at c. 295 Ma. Trace elements, eHf and δ18O have been measured in dated magmatic and inherited rhyolitic zircons from three drill cores across the basin. Magmatic zircons crystallized in two stages: the first stage in less differentiated and isotopically heterogeneous magmas and the second stage in more differentiated and isotopically homogeneous magmas. The first and second stages can be related to the crystallization of zircons within a heterogeneous pluton and in an evolved, silicic magma that was later erupted. Some zircons crystallized entirely during the evolved magma stage and provide good resolution for identifying and characterizing processes happening shortly before eruption. The isotopic compositions of the magmatic zircons constrain the proportions of juvenile and reworked materials involved in the formation of voluminous silicic magmas in a post-collisional tectonic setting. The advantage of studying the volcanic rocks from the NE German Basin is that their petrogenesis involved a relatively simple, two-source interaction during magma production, which permits quantitative estimation of the amount of each source component. AFC modelling shows that the first stage zircons crystallized from magmas containing 5–80% of the juvenile component, whereas the final rhyolitic magmas contained 30–40% of this material. The inherited zircons have Hf model ages of 1·9–2·2 Ga, suggesting that much of the local basement was initially derived from the mantle at that time and that it was subsequently reworked at around 1·5 Ga. Similar model ages are a feature of Baltica-derived sediments and the implication is that such sediments underlie large areas of the NE German Basin. The lack of any record of Avalonian basement in the NE German Basin may indicate that both the sedimentary cover and the underlying basement are part of Baltica.

Geochemical, isotopic and single crystal 40Ar/39Ar age constraints on the evolution of the Cerro Galán ignimbrites

The giant ignimbrites that erupted from the Cerro Galan caldera complex in the southern Puna of the high Andean plateau are considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone. Major and trace element analyses of Cerro Galan ignimbrites (68–71% SiO2) that include 75 new analyses can be interpreted as reflecting evolution at three crustal levels. AFC modeling and new fractionation corrected δ18O values from quartz (+7.63–8.85‰) are consistent with the ignimbrite magmas being near 50:50 mixtures of enriched mantle (87Sr/86Sr ~ 0.7055) and crustal melts (87Sr/86Sr near 0.715–0.735). Processes at lower crustal levels are predicated on steep heavy REE patterns (Sm/Yb = 4–7), high Sr contents (>250 ppm) and very low Nb/Ta (9-5) ratios, which are attributed to amphibolite partial melts mixing with fractionating mantle basalts to produce hybrid melts that rise leaving a gravitationally unstable garnet-bearing residue. Processes at mid crustal levels create large negative Eu anomalies (Eu/Eu* = 0.45–0.70) and variable trace element enrichment in a crystallizing mush zone with a temperature near 800–850°C. The mush zone is repeatedly recharged from depth and partially evacuated into upper crustal magma chambers at times of regional contraction. Crystallinity differences in the ignimbrites are attributed to biotite, zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near-eutectic crystallization occurs at temperatures as low as 680°C just preceding eruption. 40Ar/39Ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the Galan ignimbrite erupted in more than one batch including a ~ 2.13 Ma intracaldera flow and outflows to the west and north at near 2.09 and 2.06 Ma. Episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic magmas, whose changing chemistry reflects their evolution over a steepening subduction zone, could trigger the eruptions of the Cerro Galan ignimbrites.