Low-pressure Fractional Crystallization Research Articles

Untangling differentiation in arc lavas: Constraints from unusual minor and trace element variations at Salak Volcano, Indonesia

Volcanic rocks from Salak Volcano in West Java display intriguing minor and trace element geochemical variations with silica. TiO 2 and P 2O 5 contents, Y, HFSE and REE concentrations are abnormally rich in the Central Vent Group (CVG) lavas (e.g. Y = 32–69 ppm; Yb, 3–6.5 ppm) and display striking positive correlations with SiO 2. This contrasts with rocks erupted at side vents (SVG) on the eastern and western flanks of Salak and with rock suites of most other Javan volcanoes where these elements remain relatively constant with increasing SiO 2. Modelling of major and trace element data indicate that low pressure fractional crystallisation exerts strong control on the composition of CVG lavas. HFSE and HREE data are inconsistent with magma mixing, and can be explained by incompatible behaviour during fractionation of plagioclase, clinopyroxene and Fe-Ti oxide ± orthopyroxene and olivine. The observed variations in K/Rb and Ba/Th ratios and correlation of 87Sr/ 86Sr with indices of differentiation necessitate assimilation of a low K/Rb, low Ba/Th, Sr-rich contaminant with 87Sr/ 86Sr of ~ 0.7048 during fractional crystallisation. For the eastern flank SVG, deep fractionation of a phase in which HFSE and HREE are compatible (e.g. amphibole) is implicated. By extension, this is also suggested to occur beneath the majority of Javan volcanoes. Radiogenic isotope ratios (Sr-Hf-Nd) of Salak lavas are similar to other Javan lavas. SVG rocks erupted from the eastern flank vent have significantly more primitive Hf-Nd isotope ratios than other Salak volcanic rocks and may represent the least contaminated (by arc crust or subduction input) Salak lavas.

Petrogenesis and tectonic significance of Jurassic IAT magma types in the Hellenide ophiolites as deduced from the Rhodiani ophiolites (Pelagonian zone, Greece)

The Rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units: the “lower” Ultramafic unit and the “upper” Volcanic unit, both bearing calcareous sedimentary covers. The Ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores, and represents the typical mantle section of supra-subduction zone (SSZ) settings. The Volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence, both including basalts, basaltic andesites, andesites, and dacites. Chemically, the Volcanic unit displays low-Ti affinity typical of island arc tholeiite (IAT) ophiolitic series from SSZ settings, having, as most distinctive chemical features, low Ti/V ratios (< 20) and depletion in high field strength elements and light rare earth elements. The rare earth element and incompatible element composition of the more primitive basaltic andesites from the Rhodiani ophiolites can be successfully reproduced with about 15% non-modal fractional melting of depleted lherzolites, which are very common in the Hellenide ophiolites. The calculated residua correspond to the depleted harzburgites found in the Rhodiani and Othrys ophiolites. Both field and chemical evidence suggest that the whole sequence of the Rhodiani Volcanic unit (from basalt to dacite) originated by low-pressure fractional crystallization under partially open-system conditions. The modelling of mantle source, melt generation, and mantle residua carried out in this paper provides new constraints for the tectono-magmatic evolution of the Mirdita–Pindos oceanic basin.

Physical volcanology and geochemistry of a Late Archaean volcanic arc: Kurnalpi and Gindalbie Terranes, Eastern Goldfields Superterrane, Western Australia

Late Archaean (2715–2704 Ma) calc-alkaline volcanic complexes in the Kurnalpi Terrane are identical in facies and geochemistry to modern intra-arc volcanic complexes. Preserved volcanic rocks and associated feldspathic sedimentary rocks represent proximal to medial submarine facies of submarine to locally emergent intra-oceanic arc-related volcanoes. The least-altered volcanic rocks are dominantly andesite but range from basalt and basaltic andesite to rare rhyolite, and are the product of magmas similar in composition to those of modern arcs. Intermediate magmas were most likely derived from large ion lithophile element (LILE) enriched mafic magmas by fractional crystallisation (with or without assimilation) processes. The Kurnalpi Terrane thus provides an example of a Late Archaean volcanic arc in which hydrated mantle wedge melting was the main mechanism for magma genesis. TTD-suite volcanism derived from slab melting was either absent or played a subordinate role. The adjacent Gindalbie Terrane contains younger (∼2692–2680 Ma) bimodal (basalt–rhyolite) volcanic complexes, with associated quartz-rich sedimentary rocks, and calc-alkaline intermediate-silicic volcanic complexes. Volcanosedimentary basement to the Gindalbie volcanic sequences is deeply eroded equivalents of the calc-alkaline volcanic complexes, their associated sedimentary rocks and younger basaltic sequences found in the Kurnalpi Terrane. Gindalbie basalt–rhyolite successions locally contain volcanic-hosted massive Cu–Zn mineral deposits. Rhyolites derived by low-pressure fractional crystallisation, or melting, of andesites are more abundant than in the Kurnalpi volcanic complexes. Facies associations and the local occurrence of distinctive incompatible element-enriched, mildly peralkaline (A-type) rhyolites are consistent with rifting of a mature arc system. The two terranes most likely represent volcanic evolution of the same Late Archaean volcanic arc but are different tectonic fragments that were welded together by ∼2660 Ma.

Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S)

Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47°S–71°30′W), and occur together with scarce trachytic lava flows. Whole-rock K–Ar ages between 3.98 and 3.08 Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic–felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A 1-type granitoid affinities and are characterized by high silica and alkali contents (60–68 wt.% SiO 2; 8.7–10.8 wt.% Na 2O + K 2O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts (( 87Sr/ 86Sr) o = 0.70488–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512645). Nevertheless, some of them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province (( 87Sr/ 86Sr) o = 0.70556–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A 1-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-arc tectonic settings. In Central Patagonia, these ∼ 3–4 Ma old alkaline intrusions occur aligned along a ∼ N160–170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area, related to major regional tectonics occurring in possible relation with the collision of one segment of the Chile Spreading Ridge with the trench.

Genesis of post-hotspot, A-type rhyolite of the Eastern Snake River Plain volcanic field by extreme fractional crystallization of olivine tholeiite

Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene rhyolites of the Yellowstone–Snake River Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km3 of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical signatures including high whole-rock FeOtot/(FeOtot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine ± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛNd = −4 to −6; 87Sr/86Sri = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the rhyolite lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO2) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s of km3 each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves. Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and rhyolite parent magmas. Interestingly, models of rhyolite genesis that involve large degrees of melting of Archean crust or previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO2), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the rhyolites and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing and fractional melting occur to limited degrees, but are not essential parts of the rhyolite formation process.

Magmatic evolution of the Puyehue–Cordón Caulle Volcanic Complex (40° S), Southern Andean Volcanic Zone: From shield to unusual rhyolitic fissure volcanism

Magmas erupted from Quaternary volcanoes of Southern Andes between 37° and 46° S latitude are mainly basaltic to andesitic. However, PCCVC (40° S) shows a singular magmatic evolution due to the abnormal evacuation of rhyolites, especially in the last 100 ka. In addition, PCCVC is the result of juxtaposing products from the NW-trending alignment of Cordillera Nevada caldera, Cordón Caulle fissure volcano and the Puyehue stratocone. Using 40Ar/ 39Ar and 14C geochronology it can be established that they evolved since ca. 500 ka as coeval but separated vents with a first stage of shield volcanism, followed by repeated collapses that formed an internal NW-elongated graben. From ca. 100 ka, volcanic activity occurred in both a fissure system (Cordón Caulle) and a central volcano (Puyehue). Holocene explosive eruptions, mainly in the Puyehue crater, accompanied the dome growing along a NW-trending fissure system. Last historical eruptions were in 1921 and 1960 when NW fissures of Cordón Caulle fed rhyodacitic lava flows. In 1960, the fissure eruption was triggered by a remote Mw: 9.5 thrust earthquake. Cordillera Nevada caldera presents a reduced compositional range (52–63% SiO 2) and geochemical features of low-pressure magma mixing and assimilation. Instead, Cordón Caulle and Puyehue volcanoes have a wide silica range (48–71% SiO 2) and an outstanding affinity, which can be modelled with initial high-pressure fractional crystallization, moderate magma mixing and subsequent low-pressure fractional crystallization from a common parental source. The exceptional magmatic evolution and eruptive style of PCCVC in Southern Andes could be related with the physics of the plumbing system, which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime.

Petrogenesis of Cretaceous adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province (eastern China): Implications for geodynamics and Cu–Au mineralization

Both adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province, eastern China are associated with Cretaceous Cu–Au mineralization. The Shaxi quartz diorite porphyrites exhibit adakite-like geochemical features, such as light rare earth element (LREE) enrichment, heavy REE (HREE) depletion, high Al 2O 3, MgO, Sr, Sr / Y and La / Yb values, and low Y and Yb contents. They have low ε Nd( t) values (− 3.46 to − 6.28) and high ( 87Sr / 86Sr) i ratios (0.7051–0.7057). Sensitive High-Resolution Ion Microprobe (SHRIMP) zircon analyses indicate a crystallization age of 136 ± 3 Ma for the adakitic rocks. Most volcanic rocks and the majority of monzonites and syenites in the Luzong area are K-rich (or shoshonitic) and were also produced during the Cretaceous (140–125 Ma). They are enriched in LREE and large-ion lithophile elements, and depleted in Ti, and Nb and Ba and exhibit relatively lower ε Nd( t) values ranging from − 4.65 to − 7.03 and relatively higher ( 87Sr / 86Sr) i ratios varying between 0.7057 and 0.7062. The shoshonitic and adakitic rocks in the Luzong area have similar Pb isotopic compositions ( 206Pb / 204Pb = 17.90–18.83, 207Pb / 204Pb = 15.45–15.62 and 208Pb / 204Pb = 38.07–38.80). Geological data from the Luzong area suggest that the Cretaceous igneous rocks are distributed along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China and were likely formed in an extensional setting within the Yangtze Block. The Shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of > 50 km (i.e., ∼1.5 GPa), possibly leaving rutile-bearing eclogitic residue. The shoshonitic magmas, in contrast, originated mainly from an enriched mantle metasomatized by subducted oceanic sediments. They underwent early high-pressure (> 1.5 GPa) fractional crystallization at the boundary between thickened (> 50 km) lower crust and lithospheric mantle and late low-pressure (< 1.5 GPa) fractional crystallization in the shallow (< 50 km) crust. The adakitic and shoshonitic rocks appear to be linked to an intra-continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China. Both the shoshonitic and adakitic magmas were fertile with respect to Cu–Au mineralization.

Origin and evolution of magmas on the Ontong Java Plateau

Abstract The Early Cretaceous Ontong Java Plateau (OJP) represents by far the largest igneous event on Earth in the last 200 Ma and yet, despite its size, the OJP’s basaltic crust appears to be remarkably homogeneous in composition. The most abundant rock type is a uniform low-K tholeiite, represented by the Kwaimbaita Formation on Malaita and found at all but one of the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill sites on the plateau and in the adjacent basins. This is capped by a thin and geographically restricted veneer of a slightly more incompatible-element-rich tholeiite (the Singgalo Formation on Malaita and the upper flow unit at ODP Site 807), distinguished from Kwaimbaita-type basalt by small but significant differences in Sr-, Nd- and Pb-isotope ratios. A third magma type is represented by high-Mg (Kroenke-type) basalt found in thick (&gt; 100 m) successions of lava flows at two drill sites (ODP Sites 1185 and 1187) 146 km apart on the eastern flank of the plateau. The high-Mg basalt is isotopically indistinguishable from Kwaimbaita-type basalt and may therefore represent the parental magma for the bulk of the OJP. Low-pressure fractional crystallization of olivine followed by olivine+augite+plagioclase can explain the compositional range from high-Mg Kroenke-type to Kwaimbaita-type basalt. The Singgalo-type basalt probably represents slightly smaller-degree, late-stage melting of an isotopically distinct component in the mantle source. Primary magma compositions, calculated by incremental addition of equilibrium olivine to aphyric Kroenke-type basalt glass, contain between 15.6% (in equilibrium with Fo 90 ) and 20.4% (Fo 92 ) MgO. Incompatible-element abundances in the primary OJP magma can be modelled by around 30% melting of a peridotitic primitive-mantle source from which about 1% by mass of average continental crust had previously been extracted. This large degree of melting implies decompression of very hot (potential temperature &gt;1500°C) mantle beneath very thin lithosphere. The initiation of an exceptionally large and hot plume head close to a mid-ocean ridge provides the best explanation for the size, homogeneity and composition of the OJP, but is difficult to reconcile with the submarine eruption of virtually all of the basalt so far sampled.

Crustal contribution to the genesis of Ethiopian plateau rhyolitic ignimbrites: basalt and rhyolite geochemical provinciality

The Ethiopian continental flood basalt province contains large volumes of felsic eruptive rocks (&gt;6 × 10 4 km 3 ) overlying, and sometimes interlayered with the upper parts of the flood basalt sequence. Rb–Sr isochron ages (30.60 ± 0.79 Ma for Lima Limo and 30.17 ± 0.54 Ma for Wegel Tena rhyolite sequences) coincide with the short, well-defined continental flood basalt pulse, indicating rapid emplacement of the rhyolites. Lima Limo rhyolites are characterized by relatively low TiO 2 (0.3–0.5%), Th (5–7 ppm) and Nb (48–68 ppm) concentrations, high Rb/Nb (1.7–3.0) and La/Nb (1.4–2.9) values, non-radiogenic initial 87 Sr/ 86 Sr ratios (0.70357–0.70464) and generally negative initial ε Nd values (+0.3 to −1.4). In contrast, Wegel Tena rhyolites are characterized by relatively high TiO 2 (0.5–1.0%), Th (10–17 ppm) and Nb (73–115 ppm) abundances, low Rb/Nb (0.8–1.3) and La/Nb (0.8–1.2) values, low initial 87 Sr/ 86 Sr ratios (0.70412–0.70469) and positive initial ε Nd values (+4.0 to +5.7). The origin of the rhyolite suites is attributed to low-pressure fractional crystallization of basaltic magmas, similar in composition to the exposed flood basalts, combined with assimilation of mafic crustal rocks. Assimilation–fractional crystallization modelling suggests that the genesis of Lima Limo rhyolites involved higher amounts of contamination by crustal material (&lt;14%) relative to those of Wegel Tena (&lt;6%).

The transition from alkaline to tholeiitic magmas: a case study from the Orosei-Dorgali Pliocene volcanic district (NE Sardinia, Italy)

During the Pliocene, simultaneously with the opening of the Tyrrhenian Sea, mafic magmas were erupted in NE Sardinia (Orosei-Dorgali area). These range from mildly alkaline with sodic affinity (about 80% of exposure) to tholeiitic (about 20%). The tholeiitic rocks (basaltic andesite) are slightly more evolved than the alkaline ones and show geochemical features (e.g., Mg#<63; Ni<150 ppm and Cr<270 ppm) different from typical primitive mantle liquids, suggesting low pressure fractional crystallization processes. Alkaline lavas (mainly hawaiite plus rare alkali basalt and mugearite) are commonly characterized by the presence of mantle xenoliths and have higher Mg# (up to 71), Ni (up to 340 ppm) and Cr (up to 420 ppm) than the tholeiitic rocks. Both alkaline and tholeiitic lavas show sub-parallel patterns in primitive mantle-normalized diagrams, with peaks at Ba, Pb and Sr and relatively low abundances of Nb and Ta, resulting in high Ba/Nb ratios (generally >20). Similar incompatible element ratios for both alkaline and tholeiitic rocks suggest different degrees of melting of a single mantle source. Mathematical modeling indicates ∼4–6% and ∼10–15% partial melting for alkaline and tholeiitic lavas, respectively. Trace element abundances of the Orosei-Dorgali volcanic rocks are typical of Plio-Pleistocene volcanic rocks of Sardinia but differ strongly from other Cenozoic anorogenic volcanic rocks of Europe. Similarly, Sr ( 87Sr/ 86Sr=0.70442–0.70455), Nd ( 143Nd/ 144Nd=0.512465–0.512558) and Pb ( 206Pb/ 204Pb=17.74–17.86; 207Pb/ 204Pb=15.53–15.60; 208Pb/ 204Pb=37.89–38.02) isotopic ratios are very unusual when compared with other Cenozoic European volcanic rocks. Trace element abundances and isotopic composition of the Orosei-Dorgali volcanic rocks suggest a lithospheric mantle origin.

Late collision rhyolitic volcanism in the north-eastern part of the Armenian Highland

Plio-Pleistocene acidic volcanism of the northeastern part of the Armenian highland is related to a continental collision zone as a result of convergence between Eurasia and Arabia lithosphere plates. The development of volcanism is divided into three stages of 10–17, 4.5–7.5 and 0.1–2.8Ma. A crustal origin model is proposed to explain the geochemical and petrogenetic aspects of the acidic volcanism formation. Spatially separated Ba-rich and Rb-rich geochemical types of rhyolites were revealed, indicating more evolved and primitive rhyolitic magma types, respectively. The variation interval of the primary 87Sr/86Sr ratio was found to be from 0.70438 to 0.70636. Age and lateral variation of geochemical and isotopic parameters were determined by differences of initial substrates and degrees of their partial melting. The mechanisms of rhyolitic magma evolution were the remelting of heterogeneous sialitic materials and low-pressure fractional crystallization in isolated magmatic chambers. These facts confirm the model of crustal anatexis origin of the eutectic melts. The eutectic rhyolitic magma was generated by an open-system melting in the presence of deep-horizon mobile K–Rb fluids, initiated by mantle resources.

Pétrologie de l'intrusion alcaline mésozoïque de la région d'Anemzi, Haut Atlas Central, Maroc

The Mesozoic Anemzi intrusion belongs to the peri-Atlantic Mesozoic alkali magmatism. It is composed of gabbros (with or without olivine), monzodiorites and quartz-bearing syenites. Dolerite dykes of alcaline affinity occur nearby. The alkaline rocks are intrusive into mid-Jurassic limestones. Liquidus phases crystallised following the characteristic order of alkali magmas, i.e. olivine + spinel, plagioclase, clinopyroxene and amphibole. Clinopyroxene and amphibole compositions are governed by Ti-tschermakite substitutions. Clinopyroxene and amphibole in syenites are enriched in aegirine and richterite components, respectively. The mineralogical and chemical evolution in the Anemzi intrusion is consistent with low-pressure fractional crystallisation from an alkali lava under low to moderate fO 2 ( MW + [1 − 2 log units]). However, the parental liquid shows some geochemical characteristics of transitional magma, such as a high SiO 2 content (48 wt%) and low La/Yb N ratio (5). Silica activity decreased from the parental magma (0.56) to 0.08 after fractionation of the gabbros and then increased to 1.0 at the end of the magmatic differentiation. Compared with rocks of similar ages, belonging to the peri-Atlantic Cretaceous alkaline magmatism, the Anemzi intrusion is distinguished by low REE contents ([ La/ Yb] N = ∼ 6) and transitional basalt(s) as parental magma. This magma was emplaced through reactivation of Palæozoic fault systems.

Origin of high-silica liquids at Stromboli volcano (Aeolian Islands, Italy) inferred from crustal xenoliths

High-silica igneous xenoliths (granophyre and obsidian fragments with SiO2 ca. 75 wt.%) and high-silica glass (SiO2 between ca. 64 and 75 wt.%) within glass-bearing tonalitic and dioritic xenoliths have been discovered at Stromboli. They are well beyond the silica range of the volcanic rocks erupted during the subaerial activity of the volcano. The granophyre and the obsidian fragments occur within the Petrazza pyroclastics (high-K andesite pumiceous scoriae) of the Paleostromboli I period ( 61 ka), whereas the glass-bearing tonalites and diorites are hosted in the Omo basaltic-andesite lava flows of the Paleostromboli II period (64–55 ka). The obsidian represents an extremely evolved liquid derived from low-pressure fractional crystallization of high-K calc-alkaline magmas of Stromboli, coupled with minor assimilation of upper crust or terrigenous sediments. The other studied high-silica products have an anatectic origin. The granophyre composition is compatible with a genesis by low-degree melting of leucotonalites similar to some Calabrian Arc lithotypes. Partial melting of crustal rocks from the Stromboli basement is also demonstrated by interstitial glass in the tonalite and diorite xenoliths. Textural evidence and the presence of variable glass compositions suggest that in these plutonic xenoliths the melt chemistry was controlled by dehydration melting of biotite and non-modal melting of local mineralogy. High-degree undercooling of granophyre melt and of the partially melted xenoliths is consistent with rapid rise of the host magmas leading to eruption. Whether partial melting was initiated during stoping and transport of xenoliths within the rising magma or in situ during magma storage within the crust has not been unequivocally established.

From basalt to dacite: origin and evolution of the calc-alkaline series of Salina, Aeolian Arc, Italy

The island of Salina comprises one of the most distinct calc-alkaline series of the Aeolian arc (Italy), in which calc-alkaline, high-K calc-alkaline, shoshonitic and leucite-shoshonitic magma series are developed. Detailed petrological, geochemical and isotopic (Sr, Nd, Pb, O) data are reported for a stratigraphically well-established sequence of lavas and pyroclastic rocks from the Middle Pleistocene volcanic cycle (430–127 ka) of Salina, which is characterized by an early period of basaltic volcanism (Corvo; Capo; Rivi; Fossa delle Felci, group 1) and a sequence of basaltic andesites, and andesites and dacites in the final stages of activity (Fossa delle Felci, groups 2–8). Major and trace element compositional trends, rare earth element (REE) abundances and mineralogy reveal the importance of crystal fractionation of plagioclase + clinopyroxene + olivine/ orthopyroxene ± titanomagnetite ± amphibole ± apatite in generating the more evolved magma types from parental basaltic magmas, and plagioclase accumulation in producing the high Al2O3 contents of some of the more evolved basalts. Sr isotope ratios range from 0.70410 to 0.70463 throughout the suite and show a well-defined negative correlation with 143Nd/144Nd (0.51275–0.51279). Pb isotope compositions are distinctly radiogenic with relatively large variations in 206Pb/204Pb (19.30–19.66), fairly constant 207Pb/204Pb (15.68–15.76) and minor variations in 208Pb/204Pb ratios (39.15–39.51). Whole-rock δ18O values range from +6.4 to +8.5‰ and correlate positively with Sr isotope ratios. Overall, the isotopic variations are correlated with the degree of differentiation of the rocks, indicating that only small degrees of crustal assimilation are overprinting the dominant evolution by crystal–liquid fractionation (AFC-type processes). The radiogenic and oxygen isotope composition of the Salina basalts suggests derivation from primary magmas from a depleted mantle source contaminated by slab-derived fluids and subducted sediments with an isotopic signature of typical upper continental crust. These magmas then evolved further to andesitic and dacitic compositions through the prevailing process of low-pressure fractional crystallization in a shallow magma reservoir, accompanied by minor assimilation of crustal lithologies similar to those of the Calabrian lower crust.

Geochemistry, petrogenesis and tectonic setting of late Neoproterozoic Dokhan-type volcanic rocks in the Fatira area, eastern Egypt

The Neoproterozoic Dokhan volcanics of the Fatira area in eastern Egypt comprise two main rock suites: (a) an intermediate volcanic suite, consisting of basaltic andesite, andesite, dacite, and their associated pyroclastic rocks; and (b) a felsic volcanic suite composed of rhyolite and rhyolitic tuffs. The two suites display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO2 (54–76%), CaO (8.19–0.14%), MgO (6.96–0.04%), Sr (983–7 ppm), Zr (328–95 ppm), Cr (297–1 ppm), and Ni (72–1 ppm). They are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and show strong affinity to calc-alkaline subduction-related rocks. However, their undeformed character, their emplacement temporally and spatially with post-orogenic A-type granite, and their high Zr/Y values suggest that their emplacement follow the cessation of subduction in eastern Egypt in an extensional-related within-plate setting. Major and trace element variations in the intermediate volcanics are consistent with their formation via partial melting of an enriched subcontinental lithospheric mantle source followed by a limited low-pressure fractional crystallization of olivine and pyroxene before emplacement. The LILE enrichment relative to HFSE is attributed to the inheritance of a subduction component from mantle material which constituted the mantle wedge during previous subduction events in eastern Egypt. The evolution of the whole volcanic spectrum was governed mainly by crystal/melt fractionation of amphibole, plagioclase, titanomagnetite, and apatite in the intermediate varieties and plagioclase, amphibole, biotite, Fe–Ti oxides, apatite, and zircon in the felsic varieties. At each stage of evolution, crystal fractionation was accompanied by variable degrees of crustal contamination.

Two-stage fractionation history of the alkali basalt-trachyte series of Sete Cidades volcano (São Miguel Island, Azores).

A comprehensive data set of mineral and whole-rock major element compositions of the Na-alkaline lavas and pumices erupted by the Sete Cidades volcano (Sao Miguel Island, Azores) is here presented. Mineralogy and geochemistry of the Sete Cidades extrusives indicate a low-pressure fractional crystallization, at relatively high fO2, as the main magmatic process generating the volcanic suite, which is mostly made up of basaltic-hawaiitic and trachytic rocks; very few mugearites are known on Sete Cidades emphasizing the possibility that residual liquids were able to leave the hawaiitic crystal mush only when the density had been lowered by compositional change or volatile exsolution ( i.e. at the trachytic stage). We mainly show how compositional variations within each group of silicates (and non-silicate) and the major-trace element covariances in the mineral structures through the alkali basalt-trachyte series are highly dependent on the sequence of crystallization and the correspondent magmatic environment (temperature, pressure, fO 2 ). Relatively primitive, slightly ne - normative basalts with MgO up to 14 % were tapped through flank eruptions indicating they were not held at any crustal levels, with rapid ascent through the plumbing system. Some of these basalts were held close to the Moho where they evolved by wehrlite fractionation to produce slightly hy - normative hawaiites with less than 7 % MgO. Slightly evolved basalts or hawaiitic magmas leaving the deep crustal reservoir were either erupted or were trapped in high level magma chambers to fractionate through towards metaluminous, q - + an - normative two-feldspar trachytes and thence to slightly peralkaline, one-feldspar, q - + ac - normative trachytes. Evidence of this liquid line of descent is also constrained by the hornblendite-kaersutite gabbro-diorite shallow plutonic roots of the volcano. Geothermobarometric and fO 2 calculations, performed on equilibrated pair of minerals (or mineral/liquid) generally agree with the proposed two-stage fractionation history of the Sete Cidades Na-alkaline magmas occurred within the relatively thick Azorean oceanic crust.

Exceptional REE-enrichment in apatite during the low pressure fractional crystallisation of alkali olivine basalt; an example from the British Tertiary Igneous Province

AbstractThe Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body of alkali-olivine basalt magma which differentiated at low pressure and in situ, from dolerite through teschenite to minor nepheline-syenite. The syenites occur as isolated pods and pegmatitic schlieren within the leucodolerite, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca-catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an accessory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typically low (σREE = 0·57–3·21 wt.% oxide), the highest REE contents occurring in irregular, deuterically altered rims and internal patches. The REE-enriched rims also have slightly elevated SiO2 contents at 0·81–0·95 wt.%, suggesting that the substitution scheme Ca2++P5+ ⇔ REE3++Si4+ was operating. These apatites have up to 0·08 wt.% Cl and 3·7 wt.% F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (σREE = 0·27–0·45 wt.%), coupled with low Na (&lt;0·12 wt.%) and low SiO2 (&lt;0·39 wt.%) contents. However, within the syenites two distinct populations of apatite exist. The first, most common, variety consists of unzoned, low-REE apatites (max. 3·1 wt.% σREE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum σREE content of 42 wt.%; this is by far the most REE-enriched natural fluor-apatite so far reported from the British Isles. The REE-enriched zones are often less than 3 μm wide, and have Na content up to 5·4 wt.% Na2O, implying that the substitution scheme Na+ + REE3+⇔2Ca2+ dominated over the more typical scheme involving Si4+ which operated in the dolerites and teschenite. Other zones are either variably enriched in Y (up 2·1 wt.% Y2O3) or Th (up to 0·85 wt.% ThO2). However, there is no correlation between Y and REE contents, suggesting that crystallographic factors were involved in apatite Y and REE partitioning. The REE-rich apatites have very low Cl content (&lt;0·04 wt.%), but high F concentrations (&lt;2·8 wt.%). It is believed that these strongly enriched apatites crystallised under disequilibrium conditions from isolated, variably REE-enriched domains, within the fluid-rich residual syenitic magma. These domains may have been generated by the prior crystallisation of monazite, Ca-catapleiite or zirconolite, which can be found as small inclusions within albite and interstitial analcime. The dynamic process of slumping of the denser teschenite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration of residual magmas.

Ti-rich Amphibole of Igneous Origin in MOR-Type Gabbroic Rocks (Northern Apennine Ophiolites, Italy): An Ion Microprobe Study

Two major hypotheses have been formulated for the origin of Ti-rich amphibole in MOR-type gabbroic rocks (e.g. Otten, 1984). In the first, amphibole derives from an igneous trapped liquid. This hypothesis implies that the parental magma had a relatively high volatile content, thus contrasting with the nearly anhydrous nature of MORB. The second hypothesis assumes a metamorphic origin, i.e. amphibole was originated by solid state reactions of igneous minerals with volatile-rich seawater-derived fluids. In the gabbroic rocks from Northern Apennine ophiolites, Ti-rich amphibole occurs both with igneous and metamorphic texture. The gabbroic rocks were formed by low-pressure fractional crystallization of N-MORB type liquids in a slowspreading ridge system (Tiepolo et al., 1997). Accessory Ti-rich amphibole with igneous texture is found as the last crystallizing mineral, in interstices between plagioclase and clinopyroxene and as rim around Fe-Ti-oxides. Morever, Ti-rich amphibole occurs as a major mineral in the most evolved, volumetrically subordinate, intrusive lithologies (albitites). These rocks consist of albitic plagioclase and Ti-rich amphibole (up to 30 % by volume), both showing euhedral to subhedral habitus, plus accessory Fe-Ti-oxides, apatite and zircon. This work aims to unravel the origin of Ti-rich amphibole with igneous texture. Amphibole from Mgand Fe-rich gabbroic rocks, as well as from albitite, has been analysed for major elements by electron microprobe, and for halogen (F, CI and H) and trace elements (REE, Y, Sr, Zr, Cr, V, Sc, Ti, Ba, K, Nb) by ion microprobe.

Late precambrian volcanism at Wadi Allaqi, south Eastern Desert, Egypt: evidence for transitional continental arc/margin environment

The Dokhan volcanics at Wadi Allaqi, situated in the south Eastern Desert of Egypt, range in composition from basaltic-andesite to dacite. Geochemically, they have a transitional character from low K-tholeiite to calc-alkaline with a relatively high Zr/Y ratio that characterises a continental arc/margin setting. The most basic sample has extremely low Mg# (40) and Ni (55 ppm) values, indicating significant fractionation of olivine and clinopyroxene and that it is not a direct partial melt of the mantle-peridotite. The examined andesites and dacites might have been formed, respectively, via low-pressure fractional crystallisation of basaltic-andesite and andesite melts with about 53% and 35% removal of mostly hornblende and plagioclase with little magnetite and biotite. The enrichment of LILE (supposed to be derived from the subducted slab) and the relative depletion of HFSE (Ta, P and Ti) seem to be inherited from the mantle source. Hence, the examined volcanics are affected by both slab-derived partial melts and aqueous fluids in their generation.

Geochemical characteristics of the Abor volcanic rocks, NE Himalaya, India: nature and early Eocene magmatism

The Abor volcanic rocks. exposed in the Siang window, NE Himalaya. occur as dismembered thrust-hound packages interbanded with fossiliferous shelf sediments of Upper Palaeocenc to Middle Eocene age. Their age is broadly contemporaneous with the age of collision between the Indian and Tibetan continents. They were. therefore, erupted in a convergent setting through channel ways in the leading edge of the Indian continent. The volcanic rocks occur beneath the Lesser Himalayan thrust sheets and were transported with them during their southward propagation. The Abor volcanic rocks were erupted in an early Palaeogene shallow basin located within marginal parts of the Himalayan fold-thrust belt south of the collision zone. They may thus represent foreland basin magmatism. The abundances of some major and large-ion-lithophile elements have been affected by alteration of the samples. Ti and some other trace elements remain unaffected however, and their abundances indicate that the volcanic rocks form a chemically coherent group of tholciites and alkalinc basalts. The tholeiitic and alkaline basalts reflect different degrees of melting, and low-pressure fractional crystallization involving olivine and plagioclase has played a significant role in the evolution of the basalts. On the basis of ratios of strongly incompatible elements, the two basalt types appear to have been generated from sources having similar character. This source is inferred to he enriched sub-continental mantle.