Geochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores (original) (raw)
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Geochemistry of Volcanic Rocks from Faial Island (Azores)
2008
Whole rock geochemistry of nine rocks from Faial Island (Azores) was used to characterize Faial Island volcanism. Studied rocks are lavas and were classified based on chemical data as basalts, hawaiites and trachyte. These rocks represent five stratigraphic units of island, from the base to the top: Ribeirinha Volcanic Complex, Cedros Volcanic Complex, Almoxarife and Capelo formations. The rocks belong to the sodic alkaline series, with the exception of the trachytic rock, that exhibits potassic affinity. Faial Island volcanism is characterized by low SiO2 contents (45 to 49 %), high TiO 2 (2 to 3 %) and P 2 O 5 (0.29 to 0.74 %). MgO shows values varying from medium to high (2 to 15 %). The trachyte has 62 % of SiO 2 , low MgO (0.42 %), TiO 2 (0.53 %) and P 2 O 5 (0.13 %) contents. Al 2 O 3 varies from low to high (11 to 18 %) and reflects the plagioclase abundance variation. Na 2 O+K 2 O values increase with fractionation and K 2 O/Na 2 O ratio is less than 0.54, with the exception of the trachyte (0.73). Ni content decreases with magmatic evolution, from 356 to 5 ppm. The presence of the cumulate phases in basalts of Almoxarife and Capelo formations explains the MgO and Ni high values. Almost all the samples show Rb, Ba, Ta, Nb, Zr enrichment and Th, Sm impoverishment. The trachyte shows Ba enrichment and K depletion. Zr positive anomaly and U, Sr, P, Ti negative anomalies are observed in the trachyte. The REE patterns are typical of oceanic island lavas, showing LREE enrichment relative to MREE and HREE. The REE patterns suggest a continuous fractionation from a common melt generated by low partial melting rates. The rocks of Faial Island are the result of fractional crystallization and derived from a magma, which has mixed characteristics, mainly reflecting PREMA and EM II reservoirs, with minor contribution of HIMU.
Frontiers in Earth Science, 2018
The interaction between magmatic and tectonic processes in ocean intraplate volcanism yields insights into the ascent and transport of magmas. Many oceanic intraplate studies lack a temporal component and do not consider changes in tectonic regime and/or magmatic processes during the evolution of magmatic systems. The eastern part of the Azores archipelago formed under the influence of both an intraplate melting anomaly and a system of ultraslow spreading rift axes. The majority of recent submarine and subaerial eruptions in the Azores occur along volcanic rift zones and thus melt transport and volcanic processes are largely controlled by tectonic processes, and are therefore ideally suited to study volcano-tectonic interactions. Here, we investigate how variable the magmatic and tectonic processes are in space and time and how they interact. We present new bathymetric, geophysical, geochemical, and Sr-Nd-Pb isotope data from Faial Island and the surrounding seafloor, providing insights into the interaction between the asthenospheric melting anomaly and extensional lithospheric stresses. The bathymetry reveals large submarine volcanic rift zones on the western flank of Faial, including that of the 1957-1958 Capelinhos eruption. Based on absolute ages and seismic imagery, we develop a relative chronology of the magmatic evolution of the submarine rift zones. Their preferred WNW-ESE orientation implies that the stress field has not changed within the last ∼1 Ma. We can however show that melt productivity progressively decreased with time. Compositionally different magma batches fed distinct volcanic rift zones and edifices, suggesting that changes in the melting regime occur on a small spatial scale and that the distribution of compositionally similar lavas is tectonically controlled. As melt supply decreases, the tectonic influence on volcanism increases with a stronger localisation of melts along tectonically controlled lineaments. The youngest mafic and intermediate melts (<10 ka) on Faial are exclusively erupted along single rift zones and cover a smaller area, whereas the older volcanism was more widespread.
Geological Magazine, 2013
Magmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized b...
Journal of Petrology, 2013
Corvo island is one of two Azorean islands located to the west of the Mid-Atlantic Ridge. Its small size and the existence of a single main eruptive center make it an ideal island for an in-depth study of the magmatic processes controlling its evolution. This study presents detailed petrographic and geochemical data for representative samples of the complete volcanostratigraphic sequence (Pre-, Synand Post-caldera stages), including lava flows, dikes and cumulate gabbroic xenoliths hosted in a Pre-caldera lava flow. Some lava flows and dikes contain large crystals that are not in equilibrium with the host magma. These crystals are antecrysts crystallized from progenitor magmas that have been reincorporated into the host lava before eruption. We note for the first time that the accumulation of antecrysts in Azorean magmas strongly affects the bulk composition of the rocks. Antecryst contents as high as 50^60% can result in whole-rocks with MgO contents of 15 wt %. This accumulation process obscures the magmatic processes responsible for the differentiation of the volcanic products. Accordingly, only antecryst-free samples are considered to identify the magmatic processes involved in the generation of the island. The least evolved antecryst-free rock was used as the closest composition to the primary magma for modeling of major element (MELTS) and trace element variations; our results establish a petrogenetic relationship between Corvo antecryst-free lava flows, dikes and cumulate xenoliths, all of which are products of polybaric fractional crystallization. Pre-and Post-caldera products are related to an $15 km deep magma chamber, whereas the Syn-caldera evolved products are related to a shallower magma chamber. These results suggest that Corvo island developed from a complex magmatic plumbing system in which magmas and their crystal cargo, derived from variable depths, have interacted and been tapped throughout the evolution of the island.
Earth and Planetary Science Letters, 1990
We report the Sr, Pb, and Nd isotopic composition of thirty-six intercalated extracaldera silicic ignimbrites and basaltic lavas of the Miocene Hogarzales, Mogan, and Fataga Formations, Gran Canaria, Canary Islands. The aims are to constrain petrogenetic models for the silicic volcanics, and determine mantle source characteristics and temporal variations between 14.2 and -12.1 Ma. Feldspars from the extracaldera silicic ignimbritcs are identical in isotopic composition to coeval extracaldera basaltic lavas, supporting a fractional crystallization model for the evolved lavas from parental Hogarzales basalts. STSr/S6Sr ratios range from 0.70306 to 0.70341, 2°6pb/2°4pb from 19.32 to 19.90, Z°7pb/2°4pb from 15.56 to 15.65, and 2°spb/Z°4pb from 38.82 to 39.65. 143Nd/144Nd ratios are nearly constant at 0.512913 ± 15. The source of Gran Canaria magmas is heterogeneous on small scales of both time and distance. Isotope-isotope and isotope-incompatible element plots suggest mixing between well-mixed, slightly enriched mantle (similar to PREMA as defined by [1]) and the HIMU mantle component. The proportion of HIMU component (low STSr/S6Sr, high Z°6Pb/2°4pb) increases upsection. Stratigraphic patterns in major, trace element, and isotopic compositions may be explained by the influx of a geochemically distinct "Fataga" magma into the Tejeda magma chamber, which mixed with and/or finally completely displaced existing "Lower Mogan" magmas . Alternatively, mixing of these two end members could occur in the mantle, prior to injection into the chamber. There is no evidence of lithospheric/asthenospheric contamination in the late-stage shield magmas on Gran Canaria.
Contributions to Mineralogy and Petrology, 2016
Furnas is one of three active central volcanoes on São Miguel Island, Azores, and is considered to be one of the most hazardous in the archipelago. In this study, the pre-eruptive magma plumbing system of the 10 young (<5 ka), intra-caldera, sub-Plinian, trachytic eruptions of the Upper Furnas Group (UFG) is investigated via whole rock major and trace element geochemistry, mineral chemistry, thermobarometry, and petrogenetic modelling. The main aim of this work is to elucidate the petrogenesis of the Furnas trachytes, constrain the P-T-fO 2 conditions under which they evolve, and investigate the temporal evolution of the magma plumbing system. Results indicate that the trachytes are derived predominantly from extended fractional crystallisation of alkali basalt parental magmas, at depths between ~3 and 4 km. Communicated by Jochen Hoefs.
Journal of Volcanology and Geothermal Research, 2006
Analytical data (major, minor and trace elements) for samples from Pico Island (Azores archipelago) representative of the three volcanic complexes comprising the island demonstrate that the overwhelming majority of the rocks are basalts (∼78%), followed by subordinate hawaiites (∼20%). Only one lava flow, erupted in 1718 AD, has a more evolved character (mugearite and benmoreite). Samples define mainly a sodic alkaline series, but a few rocks show transitional and sub-alkaline characteristics. Fractional crystallization is the main process to produce the more evolved nature of the magmas, occurring within a complex plumbing system and involving liquids with different degrees of melting. Pico, like other islands of the Azores archipelago, was constructed in an intraplate tectonic setting. Compositionally, the mantle plume beneath the Azores is considered to be the result of mixing between HIMU and DM reservoirs, with a lowest contribution of an EM reservoir.
Contributions to Mineralogy and Petrology, 1995
Petrogenetic modeling of the Recent lava succession of Santa Barbara and Pico Alto volcanoes and associated basaltic lavas indicates that there are two discrete lava series present, one erupted from the axial rift linking the two central volcanoes and one associated with monogenetic cones scattered around the flanks of Santa Barbara. The felsic lavas of both volcanoes are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage including essential amphibole. Trace element abundances in the felsic lavas, particularly those of Sr and REE, cannot be reconciled with an origin through partial melting of basaltic material at the base of the volcanic pile. The difference between the comenditic and pantelleritic differentiation trends of Santa Barbara and Pico Alto is attributed primarily toJO 2 control of the crystallizing assemblage, probably related to thermal dissociation of magmatic water in the Santa Barbara magma chamber. This effect may be augmented by minor differences in parent basaltic compositions, the Pico Alto pantellerites being derived from the rift basalts whereas the Santa Barbara comendites are derived from the off-rift basalts. A compositional gap between 54 and 64% silica content in the lavas is not present if the suite is extended to include co-magmatic hypabyssal xenoliths, leading to the in%rence that the gap in this and other bimodal suites results solely from a relative inability of magma of intermediate composition to erupt.