ALBERT

Description: Post-collisional magmatism in the southern Iberian and northwestern African continental margins contains important clues for the understanding of a possible causal connection between movements in the Earth's upper mantle, the uplift of continental lithosphere and the origin of circum-Mediterranean igneous activity. Systematic geochemical and geochronological studies (major and trace element, Sr–Nd–Pb-isotope analysis and laser 40Ar/39Ar-age dating) on igneous rocks provide constraints for understanding the post-collisional history of the southern Iberian and northwestern African continental margins. Two groups of magmatic rocks can be distinguished: (1) an Upper Miocene to Lower Pliocene (8·2–4·8 Ma), Si–K-rich group including high-K (calc-alkaline) and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene (6·3–0·65 Ma), Si-poor, Na-rich group including basanites and alkali basalts to hawaiites and tephrites. Mafic samples from the Si–K-rich group generally show geochemical affinities with volcanic rocks from active subduction zones (e.g. Izu–Bonin and Aeolian island arcs), whereas mafic samples from the Si-poor, Na-rich group are geochemically similar to lavas found in intraplate volcanic settings derived from sub-lithospheric mantle sources (e.g. Canary Islands). The transition from Si-rich (subduction-related) to Si-poor (intraplate-type) magmatism between 6·3 Ma (first alkali basalt) and 4·8 Ma (latest shoshonite) can be observed both on a regional scale and in individual volcanic systems. Si–K-rich and Si-poor igneous rocks from the continental margins of southern Iberia and northwestern Africa are, respectively, proposed to have been derived from metasomatized subcontinental lithosphere and sub-lithospheric mantle that was contaminated with plume material. A three-dimensional geodynamic model for the westernmost Mediterranean is presented in which subduction of oceanic lithosphere is inferred to have caused continental-edge delamination of subcontinental lithosphere associated with upwelling of plume-contaminated sub-lithospheric mantle and lithospheric uplift. This process may operate worldwide in areas where subduction-related and intraplate-type magmatism are spatially and temporally associated.

Description: Olivine major and trace element compositions from 12 basalts from the southern Payenia volcanic province in Argentina have been analyzed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The olivines have high Fe/Mn and low Ca/Fe and many fall at the end of the global olivine array, indicating that they were formed from a pyroxene-rich source distinct from typical mantle peridotite. The olivines with the highest Fe/Mn have higher Zn/Fe, Zn and Co and lower Co/Fe than the olivines with lower Fe/Mn, also suggesting contributions from a pyroxene-rich source. Together with whole-rock radiogenic isotopes and elemental concentrations, the samples indicate mixing between two mantle sources: (1) a pyroxene-rich source with EM-1 ocean island basalt type trace element and isotope characteristics; (2) a peridotitic source with more radiogenic Pb that was metasomatized by subduction-zone fluids and/or melts. The increasing contributions from the pyroxene-rich source in the southern Payenia basalts are correlated with an increasing Fe-enrichment, which caused the olivines to have lower forsterite contents at a given Ni content. Al-in-olivine crystallization temperatures measured on olivine–spinel pairs are between 1155 and 1243°C and indicate that the magmas formed at normal upper mantle (asthenospheric) temperatures of ∼1350°C. The pyroxene-rich material is interpreted to have been brought up from the deeper parts of the upper mantle by vigorous asthenospheric upwelling caused by break-off of the Nazca slab south of Payenia during the Pliocene and roll-back of the subducting slab beneath Payenia. The pyroxene-rich mantle mixed with peridotitic metasomatized South Atlantic mantle in the mantle wedge beneath Payenia.

Description: The igneous forearc basement along the Pacific coast of northern Central America (between southern Mexico and Costa Rica) comprises a highly tectonized accretionary assemblage of igneous and ultramafic rocks. Volcanic and gabbroic rocks with primitive arc geochemical signatures formed between ∼100 and ≥180 Ma and are interpreted to have originated by arc magmatism resulting from subduction of the Pacific–Farallon plate. Geochemically enriched ocean island basalt (OIB)-like units are interpreted as accreted seamounts and islands of a hotspot track, which was active between ≥220 and 100 Ma and originated from a hotspot located in the central Pacific. Based on their combined Pb, Nd and Hf isotopic compositions an affiliation of these rocks with the Caribbean Large Igneous Province or the present-day Galápagos hotspot appears unlikely. Rocks of similar age and geochemistry are exposed on the Santa Elena Peninsula in Costa Rica, suggesting that a similar forearc basement is accreted to the continental Chortis Block from southern Mexico to Costa Rica.

Description: Intraplate volcanism was widespread and occurred continuously throughout the Cenozoic on the New Zealand micro-continent, Zealandia, forming two volcanic endmembers: (1) monogenetic volcanic fields; (2) composite shield volcanoes. The most prominent volcanic landforms on the South Island of New Zealand are the two composite shield volcanoes (Lyttelton and Akaroa) forming the Banks Peninsula. We present new Ar-40/Ar-39 age and geochemical (major and trace element and Sr-Nd-Pb-Hf-O isotope) data for these Miocene endmembers of intraplate volcanism. Although volcanism persisted for similar to 7 Myr on Banks Peninsula, both shield volcanoes primarily formed over an similar to 1 Myr interval with small volumes of late-stage volcanism continuing for similar to 1 center dot 5 Myr after formation of the shields. Compared with normal Pacific mid-ocean ridge basalts (P-MORB), the low-silica (picritic to basanitic to alkali basaltic) Akaroa mafic volcanic rocks (9 center dot 4-6 center dot 8 Ma) have higher incompatible trace element concentrations and Sr and Pb isotope ratios but lower delta O-18 (4 center dot 6-4 center dot 9) and Nd and Hf isotope ratios than ocean island basalts (OIB) or high time-integrated U/Pb HIMU-type signatures, consistent with the presence of a hydrothermally altered recycled oceanic crustal component in their source. Elevated CaO, MnO and Cr contents in the HIMU-type low-silica lavas, however, point to a peridotitic rather than a pyroxenitic or eclogitic source. To explain the decoupling between major elements on the one hand and incompatible elements and isotopic compositions on the other, we propose that the upwelling asthenospheric source consists of carbonated eclogite in a peridotite matrix. Melts from carbonated eclogite generated at the base of the melt column metasomatized the surrounding peridotite before it crossed its solidus. Higher in the melt column the metasomatized peridotite melted to form the Akaroa low-silica melts. The older (12 center dot 3-10 center dot 4 Ma), high-silica (tholeiitic to alkali basaltic) Lyttelton mafic volcanic rocks have low CaO, MnO and Cr abundances suggesting that they were at least partially derived from a source with residual pyroxenite. They also have lower incompatible element abundances, higher fluid-mobile to fluid-immobile trace element ratios, higher delta O-18, and more radiogenic Sr but less radiogenic Pb-Nd-Hf isotopic compositions than the Akaroa volcanic rocks and display enriched (EMII-type) trace element and isotopic compositions. Mixing of asthenospheric (Akaroa-type) melts with lithospheric melts from pyroxenite formed during Mesozoic subduction along the Gondwana margin and crustal melts can explain the composition of the Lyttelton volcano basalts. Two successive lithospheric detachment/delamination events in the form of Rayleigh-Taylor instabilities could have triggered the upwelling and related decompression melting leading to the formation of the Lyttelton (first, smaller detachment event) and Akaroa (second, more extensive detachment event) volcanoes.

Description: Little is known about the effects that subducting an oceanic large igneous province (LIP) has on the petrogenesis of submarine arc volcanoes and their geochemical composition. The southern Kermadec arc represents a rare example where an LIP—the Hikurangi Plateau—is currently subducting and where its effect on mantle composition, element recycling and arc volcanism can be studied. We present mineral chemistry and whole-rock major and trace element, and Sr–Nd–Pb isotope data from samples recovered from the southern Kermadec arc volcanoes Rumble II East and Rumble II West, together with shipboard gravity and magnetic measurements. The Rumble II volcanoes (including a volcanic cone ∼10 km further west) form an ∼23 km long arc–backarc transect located ∼250 km north of New Zealand above the subducting Hikurangi Plateau. Although only a short distance apart, rocks from the two volcanoes have different mineral and whole-rock geochemical compositions. Lavas from Rumble II East are predominantly basaltic and contain primitive olivine phenocrysts (≤Fo91), high-Mg# clinopyroxene (≤96) and anorthitic plagioclase (≤An97). Geochemically these lavas are very diverse and cover a spectrum from low Th/Yb (〈0·15) at high Ba/Th (〉1014) to higher Th/Yb (〉0·15) at lower Ba/Th (〈844). This spectrum, together with 206Pb/204Pb and 143Nd/144Nd in the range of 18·74–18·83 and 0·51309–0·51298 respectively (at similar to slightly elevated 87Sr/86Sr), suggests a mantle wedge that has undergone previous melt extraction and significant fluid addition from the subducting Pacific Plate and that contains sediment and HIMU-type Hikurangi Plateau components. The geochemistry of the sediment–HIMU-type components is exemplified in an olivine pyroxenite (e.g. 206Pb/204Pb = 20·02; 87Sr/86Sr = 0·70516; 143Nd/144Nd = 0·5126). We propose that the olivine pyroxenite formed through melt or fluid–rock metasomatism and represents the first direct evidence of a near Moho arc mantle rock that shows the imprint from a subducting HIMU-type (Hikurangi) seamount. Conversely, lavas from Rumble II West and the cone ∼10 km to the west are generally more silica rich than Rumble II East lavas and mainly contain plagioclase with less ortho- and clinopyroxene + olivine phenocrysts. The low Ba/Th (〈470) and 206Pb/204Pb (〈18·74), a range of 143Nd/144Nd (0·51297–0·51307) and elevated Th/Yb (0·13–0·39) in these lavas can best be explained by minor sediment input into a less depleted mantle wedge. In addition, the geochemical composition of the Rumble II West lavas does not require involvement of a Hikurangi component, placing a spatial limit on Hikurangi material influencing regional melt generation beneath the backarc. Supported by a gravity model requiring two distinct magma chambers, the different geochemical compositions of Rumble II East and West lavas are inconsistent with a shared magma plumbing system. The different geochemical compositions of lavas from the two Rumble II volcanoes furthermore demonstrate that across-arc geochemical heterogeneities can occur within a few kilometres and may originate from both a geochemically heterogeneous mantle wedge and Moho transition layer, recording inherited geochemical heterogeneities beneath the volcanoes.

Description: We report major and trace element X-ray fluorescence (XRF) data for mafic volcanics covering the 15-Ma evolution of Gran Canaria, Canary Islands. The Miocene (12-lS^Ma) and Pliocene-Quaternary (0-6 Ma) mafic volcanics on Gran Canaria include picrites, tholeiites, alkali basalts, basanites, nephelinites, and melilite nephelinites. Olivine±clinopyroxene are the major fractionating or accumulating phases in the basalts. Plagioclase, Fe-Ti oxide, and apatite fractionation or accumulation may play a minor role in the derivation of the most evolved mafic volcanics. The crystallization of clinopyroxene after olivine and the absence of phenocrystic plagioclase in the Miocene tholeiites and in the Pliocene and Quaternary alkali basalts and basanites with MgO〉6 suggests that fractionation occurred at moderate pressure, probably within the upper mantle. The presence of plagioclase phenocrysts and chemical evidence for plagioclase fractionation in the Miocene basalts with MgO〈6 and in the Pliocene tholeiites is consistent with cooling and fractionation at shallow depth, probably during storage in lower-crustal reservoirs. Magma generation at pressures in excess of 3-0-3-5 GPa is suggested by (a) the inferred presence of residual garnet and phlogopite and (b) comparison of FeO1 cation mole percentages and the CIPW normative compositions of the mafic volcanics with results from high-pressure melting experiments. The Gran Canaria mafic magmas were probably formed by decompression melting in an upwelling column of asthenospheric material, which encountered a mechanical boundary layer at ~ 100-km depth.

Description: Trace elements and Sr–Nd–Pb isotopes have been analyzed on sedimentary and igneous (metabasalt, metadiorite and metagabbro) samples from the Jurassic oceanic crust beneath Gran Canaria (Canary Islands). The igneous crust exhibits extreme heterogeneity in 87Sr/86Sr (0.7029–0.7052), 206Pb/204Pb (18.2–20.8) and 208Pb/204Pb (38.1–41.3). Leaching experiments indicate that seawater alteration has elevated the 87Sr/86Sr ratio but has not appreciably affected 143Nd/144Nd (0.51295–0.51306). An Sm–Nd isochron gives an age of 178 ± 17 Ma, which agrees with the age predicted from paleomagnetic data. Hydrothermal alteration near the ridge axis has increased 207Pb/204Pb (15.59−15.73), 208Pb/204Pb (as well as Δ7/4Pb and Δ8/4Pb), 238U/204Pb (μ) and Ce/Pb but has not appreciably changed 206Pb/204Pb. The large range in 206Pb/204Pb and 208Pb/204Pb reflects radiogenic ingrowth with μ being as high as 107. Portions of the Jurassic ocean crust have trace element and isotope characteristics within the range found at St Helena, the Atlantic type locality for the HIMU (high μ) mantle end-member. Evaluation of the published isotopic data for Gran Canaria volcanic rocks indicates that the isotopic composition of these melts primarily represents the composition of their mantle sources rather than crustal assimilation.

Description: Osmium concentrations and isotopic signatures were measured in 28 primarily Holocene basalts (22 of which have been analyzed for Sr–Nd–Pb isotope composition), two carbonatites and two mantle xenoliths from the Canary Islands, Selvagen Grande and Madeira in the eastern North Atlantic. 187Os/188Os ratios in the basalts range from 0.129 to 0.183. The Os isotope systematics indicate that the basalts fall into three petrogenetic groups: (1) a ‘radiogenic’ group with high 187Os/188Os from 0.152 to 0.183; (2) an ‘unradiogenic’ group with low 187Os/188Os from 0.129 to 0.138; (3) an ‘intermediate’ group with 187Os/188Os between 0.139 and 0.151. The Os isotope systematics of the radiogenic group samples are consistent with minor contamination of the basalts by marine sediment. All samples in the unradiogenic group contain mantle xenoliths, and the unradiogenic Os can be explained by bulk assimilation of ≤ 5% mantle peridotite in the form of disaggregated xenoliths. The radiogenic and unradiogenic groups are also characterized by higher 87Sr/86Sr and 208Pb/204Pb but lower 143Nd/144Nd than samples with similar 206Pb/204Pb from the intermediate group, which is interpreted to reflect interaction of plume magmas with the lithospheric mantle. The intermediate group samples are believed to represent the isotopic signature of the mantle plume. The Os isotopic composition of the Canary plume is among the most radiogenic found in ocean island basalts, comparable with the endmember HIMU islands Mangaia and Tubuaii, but at significantly lower 206Pb/204Pb. The radiogenic Os and moderate 206Pb/204Pb signature of the Canary plume is consistent with a plume which contains 25–35% of relatively young (∼1.2 Ga) recycled oceanic crust. Variable degree of mixing of the Canary Island plume source with shallow depleted asthenosphere containing a component of Paleozoic oceanic crust produces the limited range in Os isotopic signatures observed in the Madeira and Canary Island basalts despite a large range in 206Pb/204Pb isotopic composition.

Description: The subaerial portion of Gran Canada, Canary Islands, was built by three cycles of volcanism: a Miocene Cycle (8-5—15 Ma), a Pliocene Cycle (1-8-60 Ma), and a Quaternary Cycle (1-8-0 Ma). Only the Pliocene Cycle is completely exposed on Gran Canaria; the early stages of the Miocene Cycle are submarine and the Quaternary Cycle is still in its initial stages. During the Miocene, SiO2 saturation of the mafic volcanics decreased systematically from tholeiite to nephelinite. For the Pliocene Cycle, SiO2 saturation increased and then decreased with decreasing age from nephelinite to tholeiite to nephelinite. SiO2 saturation increased from nephelinite to basanite and alkali basalt during the Quaternary. In each of these cycles, increasing melt production rates, SiO2 saturation, and concentrations of compatible elements, and decreasing concentrations of some incompatible elements are consistent with increasing degrees of partial melting in the sequence melilite nephelinite to tholeiite. The mafic volcanics from all three cycles were derived from CO2-rich garnet lherzolite sources. Phlogopite, ilmenite, sulfide, and a phase with high partition coefficients for the light rare earth elements (LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were residual during melting to form the Miocene nephelinites through tholeiites; phlogopite, ilmenite, and sulfide were residual in the source of the Pliocene-Quaternary nephelinites through alkali basalts. Highly incompatible element ratios (e.g., Nb/U, Pb/Ce, K/U, Nb/Pb, Ba/Rb, Zr/Hf, La/Nb, Ba/Th, Rb/Nb, K/Nb, Zr/Nb, Th/Nb, Th/La, and Ba/La) exhibit extreme variations (in many cases larger than those reported for all other ocean island basalts), but these ratios correlate well with degree of melting. Survival of residual phases at higher degrees of melting during the Miocene Cycle and differences between major and trace element concentrations and melt production rates between the Miocene and Pliocene tholeiites suggest that the Miocene source was more fertile than the Pliocene-Quaternary source(s). We propose a blob model to explain the multi-cycle evolution of Canary volcanoes and the temporal variations in chemistry and melt production within cycles. Each cycle of volcanism represents decompression melting of a discrete blob of plume material. Small-degree nephelinitic and basanitic melts are derived from the cooler margins of the blobs, whereas the larger-degree tholeiitic and alkali basaltic melts are derived from the hotter centers of the blobs. The symmetrical sequence of mafic volcanism for a cycle, from highly undersaturated to saturated to highly undersaturated compositions, reflects melting of the blob during its ascent beneath an island in the sequence upper margin-corelower margin. Volcanic hiatuses between cycles and within cycles represent periods when residual blob or cooler entrained shallow mantle material fill the melting zone beneath an island.