High Abundances Of Incompatible Elements Research Articles

A Mantle Plume Connection for Alkaline Lamprophyres (Sannaites) from the Permian Tarim Large Igneous Province: Petrological, Geochemical and Isotopic Constraints

AbstractThe origin of lamprophyres associated with large igneous provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed subcontinental lithosphere. Here, we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim LIP in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite, and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite, and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O + K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g. La and Ce) relative to P and high-field-strength elements (e.g. Ti). They show a relatively restricted range of δ66Zn values between 0.22‰ and 0.46‰ with an average of 0.37 ± 0.04‰ (2SE, n = 10), which is marginally heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from −0.97 to +5.62, and δ26Mg from −0.36‰ to −0.17‰ (n = 8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2–FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe–Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and rhyolite–MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle source. Metasomatism and related formation of amphibole-bearing metasomatized mantle may be linked to sublithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during a later stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains.

Origin and significance of Early Miocene high‑potassium I-type granite plutonism in the East Anatolian plateau (the Taşlıçay intrusion)

The Early Miocene high-K I-type plutonic rocks constitute the early products of the Neogene to Quaternary magmatism, and the youngest exposed intrusions in the East Anatolian plateau. Here we deal with the petrogenesis of the Early Miocene Taşlıçay intrusion covering an area of ∼62 km2. The intrusion comprises leucogranite, and minor gabbro-monzodiorite and rhyolite porphyry. U-Pb dating of zircons from the leucogranite, monzodiorite and rhyolite porphyry yielded identical igneous crystallization ages of ∼19 Ma (Early Miocene). According to the modified alkali-lime index, the leucogranite and the rhyolite porphyry are alkali-calcic, while the gabbro-monzodiorite is transitional calcic to calc-alkalic. On variation diagrams, the gabbro-monzodiorite and the leucogranite as well as rhyolite porphyry form distinct bimodal groupings, whereby the leucogranite display well-defined linear differentiation trends, in contrast to the gabbro-monzodiorite. The leucogranite has relatively fractionated rate earth element (REE) patterns with concave-upward shapes and significant negative Eu anomalies; middle REEs are hardly fractionated with respect to heavy REEs. The gabbro-monzodiorite is characterized by high abundances of incompatible elements, slightly fractionated chondrite-normalized REE patterns with feebly negative Eu anomaly. The rhyolite porphyry is compositionally similar to the leucogranite. The geochemical features imply a fractionating mineral assemblage of hornblende, plagioclase and biotite for the leucogranite, and hornblende and ± plagioclase for the gabbro-monzodiorite. All the rock types display a narrow Sr and Nd isotopic variation (initial 87Sr/86Sr = 0.7053 to 0.7065; initial εNd = −0.5 to −3.8). The leucogranite and rhyolite porphyry exhibit gradually slightly higher initial 87Sr/86Sr and lower initial 143Nd/ 144Nd ratios relative to the gabbro-monzodiorite. Similarly, δ18O and initial εHf values of zircons suggest slightly increasing amount of crustal component from the leucogranite to the rhyolite porphyry. The gabbro-monzodiorite is probably related to partial melts from the slightly enriched lithospheric mantle. The magmas of the leucogranite and the rhyolite porphyry, on the other hand, probably resulted from the remelting of a middle- to high-K basic to intermediate rocks, compositionally similar to the gabbro-monzodiorite, and assimilated gradually slightly increasing amount of old high-silica crustal material. Several lines of evidence such as (i) presence of the well-developed dike swarm of rhyolite porphyry at the north eastern margin of the intrusion, (ii) exhumation of the intrusion at the earth’s surface by Middle Miocene, (iii) widespread apatite fission tract ages between 22 and 16 Ma from literature, and (iv) the absence of the exposed intrusions younger than the Early Miocene suggest that the Early Miocene represents probably a time of continental extension and exhumation in Eastern Anatolia and NW Iran.

Petrogenesis of mafic and felsic rocks from the Comei large igneous province, South Tibet: Implications for the initial activity of the Kerguelen plume

The role of the Kerguelen plume in breaking up eastern Gondwana is so far unclear. Recently, the initial magmatic record of the plume has been identified in the Comei igneous province, Tethyan Himalaya. In this contribution, we investigated a group of basaltic intrusions (Juelongpu and Zhaxikang) and felsic lavas (Sangxiu) from the Comei igneous province. Laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb dating indicates that these mafic and felsic rocks were emplaced in the Early Cretaceous (ca. 140-137 Ma). Probably originating from the Kerguelen plume, the Juelongpu and Zhaxikang mafic rocks are characterized by high incompatible element abundances, high Nb/Y ratios, and oceanic-island basalt- like rare earth element and trace-element patterns. The Sangxiu felsic samples share A-type affinities and were derived by partial melting of crustal rocks. These mafic and felsic rocks represent a magmatic response to the initial activity of the Kerguelen plume. Their ages (ca. 140-137 Ma) are older than the timing of initial breakup of eastern Gondwana (ca. 137-136 Ma), indicating that the plume must have been incubating underneath the supercontinent before the breakup. Recent plate reconstructions coupled with our geochemical and geochronological constraints suggest that the Kerguelen plume head was resident under the Comei area of Greater India at ca. 140 Ma. Our results, in conjunction with previous studies, support the viewpoint that the Kerguelen plume played an active role in breaking up eastern Gondwana.

High crustal diversity preserved in the lunar meteorite Mount DeWitt 12007 (Victoria Land, Antarctica)

AbstractThe meteorite Mount DeWitt (DEW) 12007 is a polymict regolith breccia mainly consisting of glassy impact‐melt breccia particles, gabbroic clasts, feldspathic clasts, impact and volcanic glass beads, basaltic clasts, and mingled breccia clasts embedded in a matrix dominated by fine‐grained crystals; vesicular glassy veins and rare agglutinates are also present. Main minerals are plagioclase (typically An>85) and clinopyroxene (pigeonites and augites, sometimes interspersed). The presence of tranquillityite, coupled with the petrophysical data, the O‐isotope data (Δ17O = −0.075), and the FeOtot/MnO ratios in olivine (91), pyroxene (65), and bulk rock (77) indicate a lunar origin for DEW 12007. Impactites consist of Al‐rich impact‐melt splashes and plagioclase‐rich meta‐melt clasts. The volcanic products belong to the very low titanium (VLT) or low titanium (LT) suites; an unusual subophitic fragment could be cryptomare‐related. Gabbroic clasts could represent part of a shallow intrusion within a volcanic complex with prevailing VLT affinity. DEW 12007 has a mingled bulk composition with relatively high incompatible element abundances and shows a high crustal diversity comprising clasts from the Moon's major terranes and rare lithologies. First‐order petrographic and chemical features suggest that DEW 12007 could be launch‐paired with other meteorites including Y 793274/981031, QUE 94281, EET 87521/96008, and NWA 4884.

New Petrological and Geochemical Data of the Nephelinitic Lavas and Geodynamic Implications of Mount Etinde (Cameroon)

Mount Etinde is a Recent (<1 Ma) strombolian-type volcano located on the southern flank of Mount Cameroon. Mount Etinde lavas are distinguished on the basis of the mineralogical compositions of their phenocrysts: olivine-pyroxene, olivine-melilite and clinopyroxene-nepheline. Some magnetite and ilmenite occur as inclusions in these early phases. Mafic mineral composition indicates that fractionation involved only limited Fe-enrichment. Oscillatory, normal and sectorial zoning in clinopyroxene relates to the differentiation and fractional crystallization of the magma. Based on the primitive mantle-normalized trace and rare earth element patterns, all nephelinites have high abundances of incompatible elements (103 < La < 281; 131 < Ce < 503), with negative anomalies for high field strength element Ti and low Nb/Y (0.1 - 0.2) and Rb/Y (<0.03) ratios, suggesting derivation from a similar source. Textural characteristics and mineral chemical data, as well as whole-rock compositions, suggest that the nephelinitic lavas may have been derived from basaltic magma from a heterogeneous lithospheric mantle. Geochemical modeling of major and trace element variations indicates that the Etinde lavas could not have been produced by only fractional crystallization. Pneumatolytic reaction probably affected the pyromagmas (basaltic magma composition) and was responsible for the nephelinite rocks at lower oxygen fugacity (fO2).

MORB differentiation: In situ crystallization in replenished-tapped magma chambers

The differentiation of mid-ocean ridge basalt (MORB) is investigated with a focus on intermediate- to fast-spreading ridges and two recently proposed differentiation mechanisms: (i) differentiation in replenished-tapped-crystallizing (RTX) magma chambers, and (ii) chromatographic element separation during melt-rock reaction in the lower crust. There is compelling evidence in the petrology and geochemistry of MORB indicating that magma chambers at mid-ocean ridges behave as open systems, as required on thermal grounds in locations where a steady-state magma chamber exists. It has recently been suggested that the commonly observed over-enrichment of more-to-less incompatible elements during MORB differentiation can be explained by such an RTX model. However, the petrology of samples from the lower oceanic crust suggests an alternative mechanism could produce this over-enrichment. Clinopyroxene crystals in oceanic gabbros are commonly strongly zoned in incompatible elements with crystal rims apparently having grown from melts with very high incompatible element abundances. Elevated Zr/LREE in clinopyroxene rims, which has been interpreted as indicating growth from a melt in which these elements had been fractionated from one another by melt-rock reaction (chromatographic separation), is shown to be more simply explained by post-crystallization diffusive fractionation. However, the high incompatible element abundances in crystal rims demonstrates that the interstitial melt in crystal mush zones becomes highly differentiated. Disaggregation of such mush zones is indicated by the crystal cargo of MORB and must be accompanied by the return of interstitial melt to the eruptible reservoir – a form of in situ crystallization. Both a magma chamber undergoing closed system in situ crystallization, and a RTX magma chamber in which crystallization occurs in situ, are shown to be capable of reproducing the differentiation trends observed in MORB. Simple stochastic models of the latter process suggest that significant variations of incompatible element abundances and ratios, at a constant MgO content, are likely to be generated from a single parental melt compositions. Additionally, parental melt compositions will generally be substantially more depleted than would be suggested if only fractional crystallization is considered. This has important implications for understanding the composition of the upper mantle. For example, the Sm/Nd of MORB are likely to be significantly lower than that of the Moho-crossing melt complicating analysis of the Nd-isotopic evolution of the upper mantle.

Petrology and geochemistry of the Tertiary alkaline intrusive rocks at Doupov, Doupovské hory Volcanic Complex (NW Bohemian Massif)

*Corresponding author Field observations, petrography, geochemical data and the results of K–Ar dating are presented for Oligocene (c. 30–29 Ma) alkaline intrusions at the former township of Doupov (Duppau) in the central part of the Doupovske hory Volcanic Complex (DHVC). The Doupov Intrusive Complex (DIC) is a very limited in area (< 2 km 2 ) but is petrographically highly variable. Several phaneritic rock types have been identified, including clinopyroxenite (both the clinopyroxenite-dominated breccia and xenoliths in younger intrusive phases), essexite, sodalite-bearing monzodiorite, sodalite monzosyenite to nepheline–sodalite syenite, and highly mafic to felsic nephelinolites (melteigite–ijolite–urtite). These rocks are accom panied by vent breccias, and a thermal and metasomatic aureole with zones of pervasive phlogopitization. Phaneritic intrusions and their host rocks are cut by thin aphanitic dykes ranging from mafic alkaline lamprophyres to felsic trachytes and phonolites. All the intrusions are characterized by undersaturated alkaline compositions with high abundances of incompatible elements. Geochemical variations and also subtle differences in the isotopic compositions of Sr and Nd suggest that the less undersaturated essexitic rocks and sodalite syenitoids do not form a common evolutionary line with highly undersaturated nephelinolites. Nephelinolites are isotopically close to the “European Asthenospheric Reservoir” or the “Common Mantle Reservoir” compositions and interaction of their parental magmas with the continental crust was negligible. Magmatic evolution of nephelinolites with “dry” mineral assemblages was dominated by fractionation of clinopyroxene accompanied at the early stage by olivine, and later by titanite, magnetite and apatite. Essexite is isotopically more different though still within the field of mafic lavas from DHVC and other volcanic complexes in Central and Western Europe. Either its parental magma originated in heterogeneous mantle with more distinct lithospheric signature or this magma reacted with the continental crust during its ascent and fractionation. Sodalite monzosyenite displays significantly higher 87 Sr/ 86 Sr ratios and its parental magma probably interacted even more intensively with the crustal rocks. The K–Ar ages of phaneritic intrusive rocks from Doupov are c. 30–29 Ma. Intrusions are older than the tephrite–basanite and foidite lavas forming the upper parts of the DHVC. The rock association resembles that accompanying many carbonatite intrusions around the world. However, brecciation and high-temperature metasomatic overprint including the extensive phlogopitization may be related to ijolitic magma. Nevertheless, the presence of carbonatite at some deeper level cannot be excluded.

Miocene basaltic magmatism in the Goldfield-Superstition volcanic province, central Arizona: Geochemistry, mineralogy, and petrology

Basaltic lavas form part of the Miocene (∼20.5 to 18 Ma) Goldfield-Superstition silicic large igneous province in central Arizona. Most of the basalt erupted early in the development of this southern Basin and Range volcanic province, and only small amounts of basalt co-erupted with the silicic volcanism. We examined 50 samples of basalt from lower, middle, and upper stratigraphic positions of the province to establish basalt petrogeneses, the characteristics of basalt sources, relationships among compositionally different lavas, and the igneous processes that relate various basalt types. We base our study largely on major and trace elements, mineral compositions, and a small data set for Sr, Nd, and Hf isotopes. Lower-section basalts include the Weekes Wash basalts, which are transitional between alkalic and tholeiitic (SiO2 49–51 wt. %) and with MgO 8.1–10.2 wt. %. They are associated with lavas that are andesitic (SiO2 ∼59–62 wt. %; MgO 8–2.5 wt. %) or are seemingly andesitic due to alteration and felsic xenocrysts. Weekes Wash basalts have incompatible-element abundances that correlate positively with MgO. Their87Sr/86Sr ratios are ∼0.705. Olivines, rarely fresh, have Fo86–89 cores, and clinopyroxenes have Mg#s 86–89. Overlying the Weekes Wash are the Cottonwood Spring basalts, which are alkalic (SiO2 ∼45.5–47.5 wt. %), and can be categorized into subgroups defined by lower and higher incompatible-element abundances, or low Ti-P and high Ti-P (e.g., Ba ∼1100 versus 1800 ppm; La ∼75 versus 110 ppm). The Cottonwood Spring basalts are the closest to primary lavas we observed (MgO 10.1–11.8 wt. %), and their87Sr/86Sr ratios are ∼0.705–0.706. Their olivines are Fo86–89, and their clinopyroxenes have Mg#s 86–90. Overlying the Cottonwood Spring basalts is the Apache Gap Fe-Ti-enriched basalt (TiO2 ∼2.6 wt. %), which has the lowest MgO (∼5.6–7.5 wt. %) and incompatible-element abundances observed for any basalts in the province (e.g., Ba ∼400 ppm; La ∼25 ppm). All lower-section basalts have primitive-mantle normalizations showing Nb-Ta negative anomalies. Middle-section basalts erupted among silicic lava and pyroclastic flows from ∼19 to 18.5 Ma, and they compositionally resemble Weekes Wash basalts. Upper-section (post-18.5 Ma) lavas are the Willow Springs hawaiite (∼9.5 wt. % MgO;87Sr/86Sr ∼0.705) and Black Mesa basanite (SiO2 ∼44 wt. %; MgO ∼8 wt. %; CaO 14.7 wt. %;87Sr/86Sr ∼0.706). A Nb-Ta anomaly is clear in the hawaiite but weak in the basanite, as the basanite has the highest Nb and Ta observed (∼60 and ∼3 ppm; lowest Zr/Nb, ∼4 versus all others >7). Relevant interpretations are the following. Absence of ultramafic mantle xenoliths in Goldfield-Superstition basalts suggests that magmas occupied crustal reservoirs. The two subgroups of the Cottonwood Spring basalts attest to small-scale trace element and isotopic heterogeneities in lithospheric mantle sources that, based on Nb-Ta and Ce versus Ce/Yb modeling, had subduction zone characteristics and were garnet bearing. For Weekes Wash basalts, the decreasing incompatible-element abundances with decreasing MgO is consistent with a hybrid origin by the low-Ti-P subgroup of Cottonwood Spring basalts having assimilated lower crust. Trace-element modeling based on assimilation fractional crystallization (AFC) demonstrates that mixing Cottonwood Spring basalt with ∼50 percent (partial) melts of lower-crust pyroxenite in proportions from 80:20 to 50:50 yields Weekes Wash basalt compositions. Apache Gap lava represents AFC processes of high-Ti-P Cottonwood Spring basalts in a crustal reservoir, enabling Fe-Ti enrichment after ∼60 percent crystallization, as estimated by mass balancing, of mainly clinopyroxene and plagioclase. Upper-section hawaiite models as a differentiate after ∼50 percent crystallization of mainly clinopyroxene and plagioclase from a composition resembling low-Ti-P Cottonwood Spring basalt. Black Mesa basanite originated in a garnet-bearing lithospheric mantle source generally similar to those for Cottonwood Spring basalts, but additionally carbonitized (e.g., basanite has SiO2-undersaturation; high CaO) to yield magma relatively enriched in Nb and Ta. Basanite undersaturation is consistent with a smaller percent of source melting at the close of Goldfield-Superstition basaltic magmatism compared to initial source melting. This study of the Goldfield-Superstition volcanic province demonstrates that magmas produced from Miocene lithospheric mantle spanning ∼2.5 million years took various paths, from erupting as nearly primitive lavas from their lithospheric sources, to interacting with lower crust where they assimilated, differentiated, and provided heat to create a silicic large igneous province. Finally, the basalts reflect some changes in source and melting characteristics over the time of their emplacements, but they sustained lithospheric-source and ancient subduction characteristics throughout.

Caracterização de vulcânicas adakíticas e cálcio-alcalinas no greenstone belt do rio Itapicuru, Bahia: petrogênese e implicações geodinâmicas

New geochemical data for andesites and dacites of the Maria Preta Unit, Rio Itapicuru greenstone belt, revealed two distinct geochemical groups: one of adakitic affinity and another calc-alkaline. The adakitic suite, localized to the west in the unit, is characterized by high values of Sr/Y, Ti/Y N >;1; it shows enrichment in LFSE (Low Field Strength Elements) relative to HFSE (High Field Strength Elements) and moderate rare earth elements fractionation. The calc-alkaline suite shows low Sr/Y ratios, lower rare earth elements fractionation and negative Eu anomalies. It also shows high abundances of incompatible elements (Y, Zr, Th) and high Cr and Ni concentrations. The most likely mechanism of formation for the calc-alkaline rocks is partial melting of mantle peridotite hydrated by fluids released from the subducting slab, followed by plagioclase fractionation. The two suites cannot be linked to each other by fractional crystallization and have probably derived from distinct magmas (different initial Nd ratios). The adakite geochemical features require partial melt of hydrous metabasalts at depths high enough to stabilize garnet+hornblende as the residual phase. This suite does not show any significant interaction with the mantle wedge as inferred from its low Cr and Ni abundances when compared to Cenozoic adakites. Sm-Nd isotope data indicated high positive εNd(T) for the adakites (+4.8 to +2.5) and slightly lower values for the calc-alkaline suite (εNd(T) +1.9 to +1.4). These data indicate a juvenile magma input to form the UMP volcanics. If the two volcanic suites are of the same age, then their geographical position suggests subduction of the oceanic slab to the west.

Mechanisms for incompatible-element enrichment on the Moon deduced from the lunar basaltic meteorite Northwest Africa 032

The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb–Sr and Sm–Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb–Sr and Sm–Nd ages. Rubidium–Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial 87Sr/ 86Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an 87Rb/ 86Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium–neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial ε Nd of +9.71 ± 0.74 corresponding to a source region with 147Sm/ 144Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts. Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.

Syn-extensional granitoids in the Menderes core complex and the late Cenozoic extensional tectonics of the Aegean province

AbstractThe Miocene granitoid plutons exposed in the footwalls of major detachment faults in the Menderes core complex in western Anatolia represent syn-extensional intrusions, providing important geochronological and geochemical constraints on the nature of the late Cenozoic magmatism associated with crustal extension in the Aegean province. Ranging in composition from granite, granodiorite to monzonite, these plutons crosscut the extensional deformation fabrics in their metamorphic host rocks but are foliated, mylonitized and cataclastically deformed in shear zones along the detachment faults structurally upward near the surface. Crystallization and cooling ages of the granitoid rocks are nearly coeval with the documented ages of metamorphism and deformation dating back to the latest Oligocene–early Miocene that record tectonic extension and exhumation in the Menderes massif. The Menderes granitoids (MEG) are represented by mainly metaluminous-slightly peraluminous, high-K calc-alkaline and partly shoshonitic rocks with their silica contents ranging from 62.5 to 78.2 wt%. They display similar major and trace element characteristics and overlapping inter-element ratios (Zr/Nb, La/Nb, Rb/Nb, Ce/Y) suggesting common melt sources. Their enrichment in LILE, strong negative anomalies in Ba, Ta, Nb, Sr and Ti and high incompatible element abundances are consistent with derivation of their magmas from a subduction-metasomatized, heterogeneous sub-continental lithospheric mantle source. Fractional crystalization processes and lower to middle crustal contamination also affected the evolution of the MEG magmas. These geochemical characteristics of the MEG are similar to those of the granitoids in the Cyclades to the west and the Rhodope massif to the north. Partial melting of the subduction-metasomatized lithospheric mantle and the overlying lower-middle crust produced the MEG magmas starting in the late Oligocene–early Miocene. The heat and the basaltic material to induce this partial melting were provided by asthenospheric upwelling caused by lithospheric delamination. Rapid slab rollback of the post-Eocene Hellenic subduction zone may have peeled off the base of the subcontinental lithosphere, triggering the inferred lithospheric delamination. Both slab retreat-generated upper plate deformation and magmatically induced crustal weakening led to the onset of the Aegean extension, which has migrated southward through time.

The Somuncura Large Igneous Province in Patagonia: Interaction of a Transient Mantle Thermal Anomaly with a Subducting Slab

The Oligo-Miocene Somuncura province is the largest (∼55 000 km2) back-arc mafic volcanic field in Patagonia, and one of Earth's largest with no clear link to a hotspot or major extension. Major and trace element and Sr–Nd–Pb isotopic data suggest involvement of a plume-like component in the mantle magma source mixed with hydrous, but not high field strength element (HFSE)-depleted components, from a disintegrating subducting plate. Magmatism is attributed to mantle upwelling related to disturbances during plate reorganization, possibly at a time when the South America plate was nearly stationary over the underlying mantle. Melting was enhanced by hydration of the mantle during Paleogene subduction. Crustal contamination was minimal in a refractory crust that had been extensively melted in the Jurassic. Eruption began with low-volume intraplate alkaline mafic flows with depleted Nd–Sr isotopic signatures. These were followed by voluminous ∼29–25 Ma tholeiitic mafic flows with flat light and steep heavy rare earth element (REE) patterns, intraplate-like La/Ta ratios, arc-like Ba/La ratios and enriched Sr–Nd isotopic signatures. Their source can be explained by mixing EM1–Tristan da Cunha-like and depleted mantle components with subduction-related components. Post-plateau ∼24–17 Ma alkaline flows with steep REE patterns, high incompatible element abundances, and depleted Sr–Nd isotopic signatures mark the ebbing of the mantle upwelling.

Oxygen-isotope evidence for recycled crust in the sources of mid-ocean-ridge basalts

Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas and are believed to form by partial melting of a globally extensive reservoir of ultramafic rocks in the upper mantle. MORBs vary in their abundances of incompatible elements (that is, those that partition into silicate liquids during partial melting) and in the isotopic ratios of several radiogenic isotope systems. These variations define a spectrum between 'depleted' and 'enriched' compositions, characterized by respectively low and high abundances of incompatible elements. Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir, or any of a number of processes of intramantle differentiations. Variations in (18)O/(16)O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that (18)O/(16)O ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are consistent with control of the oxygen-isotope and incompatible-element geochemistry of MORBs by a component of recycled crust that is variably distributed throughout their upper mantle sources.

High-potassium, calc-alkaline I-type plutonism in the European Variscides: northern Vosges (France) and northern Schwarzwald (Germany)

Early Carboniferous high-K, calc-alkaline I-type plutonic rocks from the northern Vosges and Schwarzwald were studied for their chemical and Sr–Nd isotopic compositions. Intrusion relationships and mineralogical and chemical characteristics allow to distinguish four suites. The oldest intrusions are diorites (1), followed by a granodioritic (2) and a granitic (3) suite. These older granitoids (OG) and their contact metamorphic country rocks are cut by younger high-K to shoshonitic granitic plutons (YG) (4). Still later, peraluminous S-type granitic magmas intruded (not included in this study). Diorites (1) have SiO 2 between 46 and 61 wt.% and are characterized by relatively high Mg# of 62–38, low contents of Na 2O (2.3–4.0 wt.%), high abundances of incompatible elements (LILE, Nb, and P) and enriched Nd–Sr initial isotopic signatures [ ε Nd(I)=−1.7 to −2.8; 87 Sr / 86 Sr (I)=0.7046–0.7061]. Chondrite-normalized (cn) REE patterns are relatively flat [(La/Yb) cn=5.1–7.8; (Tb/Yb) cn=1.2–1.8] and show small negative Eu anomalies (Eu/Eu*=0.73–0.90). All these characteristics suggest an origin of the diorites from enriched lithospheric mantle sources. Compared with the diorites, the granodiorites (2) show higher ε Nd(I) (+0.5 to −0.4) but similar 87 Sr / 86 Sr (I) (0.7051–0.7053). High values of molar CaO/(MgO+FeO tot) combined with low Al 2O 3/(MgO+FeO tot) and K 2O/Na 2O ratios suggest an origin by dehydration melting from a metabasaltic to metatonalitic source. Radiogenic isotopic signatures of the older granites (3) are similar to those of the diorites [ ε Nd(I)=−1.8 to −2.5; 87 Sr / 86 Sr (I)=0.7048–0.7058]. Compared with the granodiorites the older granites show similar values of CaO/(MgO+FeO tot), but significantly higher ratios of Al 2O 3/(MgO+FeO tot) and K 2O/Na 2O pointing to a metagreywacke source. REE patterns of both the granodiorites and the granites are characterized by relatively low (Tb/Yb) cn ratios (1.2–1.7) excluding substantial amounts of garnet as a fractionating phase. Instead, the residues were probably dominated by amphibole and plagioclase, and possibly also pyroxene. The YG (4) have elevated abundances of large ion lithophile elements (K, Rb, Th, U, Ba, and Sr) and of some high field strength elements (Nb and P). Their isotopic signatures [ ε Nd(I)=−1.5 to −3.4; 87 Sr / 86 Sr (I)=0.7046–0.7060] are similar to those of the older granites. Relative to all OG, their REE patterns are characterized by higher ratios of (La/Yb) cn (11.8–38.9) and (Tb/Yb) cn (1.3–2.6) but lower values of Eu/Eu*. Combined with higher Mg# and lower abundances of Y, these characteristics point to an increasing role of garnet in the residues of the partial melts. Relatively low values of molar Al 2O 3/(MgO+FeO tot) and K 2O/Na 2O in combination with variable molar CaO/(MgO+FeO tot) ratios suggest that these magmas were derived from heterogeneous metasedimentary sources.

The Geochemistry of Lavas from the Gomores Archipelago, Western Indian Ocean: Petrogenesis and Mantle Source Region Characteristics

New mineral and bulk-rock analyses, as well as Nd, Sr and Pb isotope compositions are presented for lavas from Grande Comore, Moheli and Mayotte, thru of the four main islands of the Comores Archipelago in the western Indian Ocean, and these data an used to evaluate the petrogenesis, evolution and mantle source region characteristics of Comorean lavas. The typically silica-undersaturated, alkaline lavas from all three islands can be grouped into two distinct types: La Grille-type (LGT) lavas, which display strong relative depletions in K, and Karthala-type (KT) lavas, which do not. With the exception of the lavas erupted by La Grille volcano on Grande Comore, which exhibit the petrographic and geochemical characteristics expected of primary mantle-derived magmas, all Comorean lavas analysed have experienced compositional modifications after they segregated from their source regions. Much of this variation can be explained quantitatively by fractional crystallization processes dominated by the fractionation ofolivine and clinopyroxene. Semi-quantitative modelling shows that the consistent and fundamental difference in composition between K-depleted LGT lavas and normal KT lavas can be attributed to partial melting processes, provided amphibole is a residual mantle phase after extraction of LGT magmas at low degrees of melting. Low absolute abundances of the heavy rare earth elements in LGT magmas are interpreted to reflect partial melting within the garnet stability field In contrast, KT magmas, which do not show relative K depletions, are considered to be the products of somewhat larger degrees of partial melting of an amphibolefree source at comparatively shallower depths. Whereas the Nd and Sr isotopic compositions of Comorean lavas (which show a significant range: 87Sr/86Sr = 0.70319–0.70393; 143Nd/Nd = 0.51263–0.51288) bear evidence for a time-averaged depletion in incompatible elements, the high incompatible element abundances of the lavas are interpreted to reflect the effects of a recent mantle enrichment event. At depths well within the garnet stability field this mantle enrichment is interpreted to have taken the form of modal metasomatism with the introduction of amphibole (giving rise to the source of LGT magmas), whereas cryptic metasomatism took place at shallower levels (giving rise to the source of KT lavas). The Nd, Sr and Pb isotope signature of the majority of Comorean lavas (both LGT and KT) is proposed to be the result of predominant4 contributions from a somewhat heterogeneous source4 4 4 presentative of the ambient sub-Comorean mantle, comprising a mixture between a HIMU component and a component on the depleted portion of the mantle array (possibly the source of Indian Ocean MORB), with only limited contributions from an EM I plume component. The lavas erupted by Karthala volcano (the youngest Comorean lavas), however, have significantly different isotopic compositions from all other Comorean lavas (lower 143Nd/144Nd and higher 87Sr/86Sr), suggesting increased contributions from the EM I component.

Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania

This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle.

Compositional diversity in late Cenozoic mafic lavas in the Rio Grande rift and Basin and Range province, southern New Mexico

This study reports major- and trace-element compositions of late Cenozoic (5 Ma and younger) mafic volcanic rocks from southern New Mexico. Stratigraphic sampling is reported for two of the volcanic fields, the Potrillo and the Jornado del Muerto volcanic fields; reconnaissance sampling, for an additional seven fields. These are among the first chemical analyses for some of these volcanic fields, and they provide insight into the dynamics of melt generation in this continental rift zone. The lavas fall into two groups: (1) an alkaline suite of basanite, alkali basalt, and trachybasalt with low SiO2 and high abundances of incompatible elements (TiO2, Nb, Rb, and Sr); (2) a subalkaline suite of subalkali basalts with high SiO2 and low abundances of the incompatible elements. Compositions of volcanic rocks from the Geronimo volcanic field, the Taos Plateau volcanic field, and the transition zone in central New Mexico are similar to those reported in this study, confirming that these are general characteristics of lavas throughout the region. For many of the large fields, the volcanic rocks belong exclusively to one compositional group or the other. Furthermore, there appear to be no patterns to the distribution of these compositional groups; for example, the Potrillo volcanic field, which is composed exclusively of alkaline lavas, and the Jornado del Muerto volcanic field, which is dominantly subalkaline, both occur in the axis of the Rio Grande rift. We see no evidence for evolution in melt chemistry during the past 5 m.y. In the Potrillo volcanic field, for which we have the best documentation of stratigraphic changes in composition, incompatible elements increase upsection and are correlated with decreasing Mg number, suggesting that magmas underwent differentiation before their final ascent. Phenocrystic olivine and plagioclase, which are liquidus phases at low pressure, imply a shallow depth for this crystallization event. Processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution. Correlation between incompatible-element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types.

Petrological significance of high-pressure ultramafic xenoliths from ultrapotassic rocks of Central Italy

Two suites of ultramafic xenoliths have been found in ultrapotassic lavas from the 0.9 Ma old Torre Alfina volcano sited at the northern border of the Vulsinian district (Central Italy). One group of Xenoliths consists of spinel-bearing lherzolites, harzburgites, minor wherlites and dunites with a maximum size of 3–4 cm. Some samples contain discrete laths of phlogopite. A second class consists of phlogopite-rich, glass-bearing peridotites. The first suite displays textural characteristics such as triple points, deformed olivine with well developed kink banding and porphyroclastic textures indicating equilibration at high pressure. Pressure estimates give values in the range 1.3–2.5 GPa, corresponding to mantle depths in the area, where the present-day Moho is about 25 km deep. Equilibration temperatures have been estimated in the range between 950–1000°C. The chemical composition of some phases, such as the very high Fo contents of olivines (up to Fo94 in harzburgites), Mg content of orthopyroxenes and CrCr+Al ratios of clinopyroxenes and spinels, suggest that these xenoliths represent peridotites which suffered different degrees of partial melting before being incorporated into the Torre Alfina magma. On the other hand, the occurrence of phlogopite speaks for metasomatic events. The phlogopite-rich, glass-bearing xenoliths consist of phlogopite, olivine, clinopyroxene, rare orthopyroxene and glass. Apatite is the most common accessory. Olivine is present in both euhedral and strained crystals. A few relics of protogranular textures are also observed. Textural and chemical evidence suggests that these xenoliths represent mica-rich peridotites which have undergone phlogopite breakdown during rapid rise to the surface with the development of a K-rich liquid which reacted with mafic phases producing a rapid growth of olivine and, to a lower extent, pyroxene. Originally, these xenoliths may have represented intensively metasomatized upper mantle. However, a cumulitic origin from previous potassic magmatic events cannot be excluded. The host lavas have compositions intermediate between high-silica lamproite and Roman-type ultrapotassic rock. They have high abundances of incompatible elements and radiogenic Sr, coupled with high Mg content, MgOCaO, Ni and Cr. These features support a genesis in a residual upper mantle which has suffered partial melting with the extraction of basaltic liquids, followed by metasomatic events which caused an enrichment in incompatible elements and radiogenic Sr. The presence of mantle-derived ultramafic xenoliths in the torre Alfina lavas testifies for a rapid uprise of the magma which reached the surface without suffering fractional crystallization and significant interaction with the upper crust. Accordingly, the Torre Alfina lavas represent an unique example of primitive potassic liquid in Central Italy.

Geochemical variations in Andean basaltic and silicic lavas from the Villarrica-Lanin volcanic chain (39.5� S): an evaluation of source heterogeneity, fractional crystallization and crustal assimilation

At 39.5° S in the southern volcanic zone of the Andes three Pleistocene-recent stratovolcanoes, Villarrica, Quetrupillan and Lanin, form a trend perpendicular to the strike of the Andes, 275 to 325 km from the Peru-Chile trench. Basalts from Villarrica and Lanin are geochemically distinct; the latter have higher incompatible element abundances and La/Sm but lower Ba/La and alkali metal/La ratios. These differences are consistent with our previously proposed models involving: a) a west to east decrease in an alkali metal-rich, high Ba/La slab-derived component which causes an across strike decrease in degree of melting; or b) a west to east increase in the contamination of subduction-related magma by enriched subcontinental lithospheric mantle. Silicic and mafic lavas from the stratovolcanoes have overlapping Sr, Nd and O isotopic ratios. Silicic lavas also have geochemical differences that parallel those of their associated basalts, e.g., rhyolite from Villarrica has lower La/Sm and incompatible element contents than high-SiO2 andesite from Lanin. At each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing. Apparently crustal contamination was not an important process in deriving the evolved lavas. Basaltic flows from small scoria cones, 20–35 km from Villarrica volcano have high incompatible element contents and low Ba/La, like Lanin basalts, but trend to higher K/Rb (356–855) and lower 87Sr/ 86Sr (0.70361–0.70400) than basalts from either stratovolcano. However all basalts have similar Nd, Pb and O isotope ratios. The best explanation for the unique features of the cones is that the sources of SVZ magmas, e.g., slab-derived fluids or melts of the subcontinental lithospheric mantle, have varying alkali metal and radiogenic Sr contents. These heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing. This model is preferable to one involving crustal contamination because it can account for variable Sr isotope ratios and uniform Nd and Pb isotope ratios among the basalts, and the divergence of the cones from across-strike geochemical trends defined by the stratovolcanoes.

Incompatible-element enrichment in Archean basalts: A consequence of contamination by older sialic crust rather than mantle heterogeneity

High abundances of incompatible elements in MgO-rich Archean basalts from Western Australia, which were thought to have been derived from an enriched mantle source, are probably the result of contamination of primitive magmas by small amounts of older sialic crust. Because these lavas represent some of the more extreme cases of enrichment in incompatible elements observed in Archean basalts, it may be possible to explain much of the trace-element variation previously ascribed to a heterogenous Archean mantle by variable crustal contamination of mantle-derived magmas. Recent calculations of thermal erosion rates predict that high-temperature high-MgO magmas, abundant in the Archean, are most susceptible to contamination; therefore, a cautious reappraisal of isotopic and trace-element data from Archean greenstone belts is required before the real extent of Archean mantle heterogeneity can be established.