Subduction zones: subduction zones

The circum-Mediterranean anorogenic Cenozoic igneous province

Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa

Petrogenesis and tectonic significance of the ~850 Ma Gangbian alkaline complex in South China: Evidence from in situ zircon U-Pb dating, Hf-O isotopes and whole-rock geochemistry

850-790 Ma bimodal volcanic and intrusive rocks in northern Zhejiang, South China: A major episode of continental rift magmatism during the breakup of Rodinia

Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkaline magmas and the significance of the Daly Gap occurring at local and regional scales. Incompatible element vs incompatible element diagrams display smooth positive trends; the isotope ratios of the silicic rocks (Sr/Sr  0 70406---0 70719; Nd/Nd  0 51274---0 51279) encompass those of the mafic rocks. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modelling of incompatible vs compatible elements excludes the possibility that peralkaline rhyolites are generated by melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks (AFC) starting from trachytes; the latter have exceedingly low contents of compatible elements, which precludes a derivation by basalt melting. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. This process generated a zoned magma chamber whose silicic upper part acted as a density filter for mafic magmas and was preferentially tapped; mafic magmas, ponding at the bottom, were erupted only during post-caldera stages, intensively mingled with silicic melts. The large number of caldera depressions found in the northern Ethiopian rift and their coincidence with zones of positive gravity anomalies suggest the occurrence of numerous magma chambers where evolutionary processes generated silicic peralkaline melts starting from mafic parental magmas. This suggests that the petrological and volcanological model proposed for Gedemsa may have regional significance, thus furnishing an explanation for the large-volume peralkaline ignimbrites in the Ethiopian rift.

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Relationships between mafic and peralkaline silicic magmatism in continental rift settings: A petrological, geochemical and isotopic study of the Gedemsa volcano, Central Ethiopian rift

New Sr–Nd–Pb isotopic ratios and trace element data for volcanic mafic rocks outcropping along a E–W transect in southern Italy, from Mt. Vulture to Neapolitan volcanoes, are reported. The variation of LILE/HFSE, HFSE/HFSE and radiogenic isotopes along this transect indicates that all of these volcanoes contain both intra-plate and subduction-related signatures, with the former decreasing from Mt. Vulture to Campanian volcanoes. New data are also reported for the Paleocene alkaline rocks from Pietre Nere (Apulia foreland), which show isotopic ratios mostly overlapping the values for Mediterranean intra-plate volcanoes as well as the Eocene–Oligocene alkaline mafic lavas from the northern Adria plate. Pietre Nere provides evidence for an OIB mantle composition of FOZO-type, free of subduction influences, that is present beneath the Adria plate (Africa) before its collision with Europe. After this collision, and formation of the southern Apennines, westward inflow of mantle from the Adria plate to the Campanian area occurred, as a consequence of slab break off. Interaction of subduction components with inflowing Adria mantle generated hybrid sources beneath the Vulture–Campania area, which can explain the compositional features of both Mt. Vulture and the Campanian mafic rocks. Therefore, mafic magmas from these volcanoes represent variable degrees of mixing between different mantle components.

Trace element and isotopic variations from Mt. Vulture to Campanian volcanoes: constraints for slab detachment and mantle inflow beneath southern Italy

Island arc volcanism in the Greater Antilles persisted for >70 m.y. Y/Yb is>1, indicating that garnet and amphibole were insignificant as residual phases and that melting occurred predominantly within from Middle Cretaceous to Late Eocene time. During the initial 50 m.y., lavas in central Puerto Rico shifted from predominantly relatively dry spinel lherzolite. Yb concentrations, which provide constraints on degree of melting, are consistent with a narrow range island arc tholeiites (volcanic phase I, Aptian to Early Albian, 120–105 Ma), to calc-alkaline basalts (phase II, Late Albian, from 30 to 35% melting in volcanic phase I, but with a much broader range from 25 to 40% melting during phase III. It seems 105–97 Ma), and finally to high-K, incompatible-element-enriched basalts (phases III and IV, Cenomanian–Maastrichtian, likely that such high degrees of melting were attained through a combination of flux-related melting and buoyancy-driven pressure97–70 Ma). Following an island-wide eruptive hiatus, geochemical trends were reversed in the Eocene with renewed eruption of calcrelease fusion. Nb abundances, which reflect degree of incompatible element enrichment compared with fertile MORB mantle (FMM), alkaline basalts (phase V, 60–45 Ma). Progressive increases in large-ion lithophile elements (LILE)/light rare earth elements are low in volcanic phase I, consistent with >2% low-degree pressure release melting of source material in the back-arc region (LREE), LILE/high field strength elements (HFSE), LREE/ HFSE, and HFSE/heavy rare earth elements (HREE) characterize before entry into the arc melting zone. Subsequent lavas from phases II and III have N-MORB-like or higher Nb abundances, indicating the compositional evolution of the first four volcanic phases. The shift in trace element compositions is mirrored by increasing radiogenic that (1) back-arc processes peaked in intensity during the first 10−20 m.y. and later declined in significance, and/or (2) the content of the lavas. Pb /4 values, representing deviations of Pb/Pb from the Northern Hemisphere Reference Line (NHRL), degree of incompatible element enrichment gradually increased as a result of subduction of a thickening accumulation of pelagic sediment. range from −20 to almost +2·0 in phases I and II, and up to +25 in phase III. Similarly, Nd values decrease slightly from Isotope mixing models indicate that the proportion of authigenic pelagic sediment incorporated into Puerto Rican basalts increased +8 to almost +6 between volcanic phases I and III. Finally, initial (i) Sr/Sr values in phase I basalts have a narrow range from negligible levels in phase I to as high as 2% in phases III and IV. Although the absolute magnitude of the sediment component from 0·7033 to 0·7040, near the upper limit of altered mid-ocean ridge basalt (MORB), whereas values from phases III and IV increased progressively, a narrow range of Th/La in mafic endmembers indicates that the terrigenous contribution remained uniform basalts have a broader range from 0·7034 to 0·7044. N-MORBnormalized incompatible element distribution patterns of Puerto throughout volcanism, consistent with the insular setting of the eastern Greater Antilles Arc. Rican volcanic rocks have uniformly flat HREE segments and

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Secular Geochemistry of Central Puerto Rican Island Arc Lavas: Constraints on Mesozoic Tectonism in the Eastern Greater Antilles

Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ultramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO2, alkalies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is highly magnesian (up to Fo%=96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indicate a genesis of ultramafic ejecta by thermal metamorphism of a siliceous dolomitic limestone, probably with input of chemical components from potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive equivalents of lavas or cumulates crystallized from variably evolved ultrapotassic magmas. One sample of the former group has major element composition resembling ultrapotassic rocks with kamafugitic affinity. Some cumulitic rocks have exceedingly high abundances of Th (81–84 ppm) and light rare-earth elements (LREE) (La+Ce=421–498 ppm) and extreme REE fractionation (La/Yb=288–1393), not justified by their modal mineralogy which is dominated by sanidine, leucite and nepheline. Finegrained phases are dispersed through the fractures and within the interstices of the main minerals. Semiquantitative EDS analyses show that Th and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the high concentration of incompatible trace elements in the whole rocks. The interstitial position of these phases and their association with fluorite support a secondary origin by deposition from fluorine-rich fluids separated from a highly evolved potassic liquid. The Nd isotopic ratios of the cjecta range from 0.51182 to 0.51217. 87Sr/86Sr ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster around 0.7105±3. This indicates either the occurrence of several isotopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires selective assimilation of host rocks in order to explain isotopic and geochemical characteristics of lavas and xenoliths. The new data indicate that the evolutionary processes in the potassic magmas of the Alban Hills were much more complex than envisaged by previous studies. Interaction of magmas with wall rocks may be an important process during magmatic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during the latest stages of magmatic evolution. Such a process was able to produce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurrence of xenoliths with kamafugitic composition points to the existence of this type of ultrapotassic magma at the Alban Hills.

Mineralogical and geochemical study of granular xenoliths from the Alban Hills volcano, Central Italy: bearing on evolutionary processes in potassic magma chambers

Tsai-Way Wu

Papers

Relationships between mafic and peralkaline silicic magmatism in continental rift settings: A petrological, geochemical and isotopic study of the Gedemsa volcano, Central Ethiopian rift

Trace element and isotopic variations from Mt. Vulture to Campanian volcanoes: constraints for slab detachment and mantle inflow beneath southern Italy

Secular Geochemistry of Central Puerto Rican Island Arc Lavas: Constraints on Mesozoic Tectonism in the Eastern Greater Antilles

Mineralogical and geochemical study of granular xenoliths from the Alban Hills volcano, Central Italy: bearing on evolutionary processes in potassic magma chambers

Evolution of Calc-alkaline Magmas in Continental Arc Volcanoes: Evidence from Alicudi, Aeolian Arc (Southern Tyrrhenian Sea, Italy)