The role of the crust in the magmatic evolution of the island of Lipari (Aeolian Islands,Italy)

Volcanic rocks on the island of Lipari show the entire range of Sr, Nd, Pb isotopic compositions displayed by other islands in the Aeolian archipelago. The rapid isotopic evolution of subaerial volcanic rocks on Lipari towards crustal values together with the appropriate isotopic composition of the neighbouring Calabrian crust (Serre) indicate that many geochemical characteristics observed in the lavas can be attributed to contamination and mixing with crustal materials and melts. Interpretation of the data is complicated by the fact that underplating onto the crust-mantle boundary and the specific lithologies present in the crustal section differ underneath each individual sector of the island. In the central and northern parts of the island, metapelitic rocks were incorporated to provide the more radiogenic Sr isotopic compositions of some lavas. The products from M. Guardia in the southern part of Lipari, where activity is restricted to the last 30–40 ka, bear geochemical similarities to the island of Vulcano, where it is proposed that considerable remobilization of the crust took place in the presence of mafic mantle-derived melts. On Lipari the petrogenetic processes of magma mixing and assimilation dominate over fractional crystallization, and the observed increase of K₂O over Na₂O can be correlated with contributions from metapelitic crustal lithologies. It is suggested that the variability in isotopic composition and the budget of alkalis (Na₂O versus K₂O) in the lavas can be explained by invoking a heat source from an intruding asthenospheric MORB-type mantle into a cooler lithospheric crust/mantle during the opening of the Tyrrhenian basin.     

西天山托云盆地及周边中新生代岩浆活动的岩石学、地球化学与年代学研究

天山西南部白垩纪-老第三纪发育的托云盆地及其周边出露的岩浆岩是一套完整的碱性岩浆岩系列，包含了苦橄质玄武岩、玄武岩、碧玄岩、碱玄岩（橄榄玄武岩、黑云母辉长二长岩、辉长辉绿岩、辉石橄榄岩）和响岩等多种岩石类型。野外工作显示有火山喷出岩和侵入岩两种不同的产状。年代学结果指示岩浆岩形成于120-50Ma间，为晚白垩世-老第三纪盆地形成演化阶段岩浆活动的产物。分离结晶作用是岩浆演化和岩浆系列形成最主要的因素，托云岩浆岩大致经历了结晶分异过程的两个阶段：早期苦橄质岩浆中橄榄石、尖晶石的结晶分离，表现为MgO和微量元素Cr含量随SiO2含量增加大幅度的降低；晚期主要是单斜辉石、斜长石和钛铁矿等矿物的结晶分异，以CaO、FeO、TiO2等随SiO2含量增加大幅度的降低为特点。苦橄质岩石的出现指示了地幔较高温熔融事件的存在，进而为托云盆地地幔柱的存在提供了有力的证据。无论如何，碱性岩浆的活动表明托云盆地形成的大地构造背景是大陆主动裂谷环境，对应的深部背景为区域性的地幔柱构造。首次发现的响岩是结晶分异作用的最终产物。响岩较极端地指示了岩浆结晶分离过程对岩浆演化的巨大影响。托云岩浆岩的同位素特征指示其源区是一个接近于PREMA地幔，但微量元素特征显示其受地壳流体交代改造的特点。岩浆岩的Nd同位素TDM集中在250～600Ma之间，反映了一个古生代时期形成的新生岩石圈地幔，与新疆北部地区的晚古生代新生岩石圈地幔的事实相符。     

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province,southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant ¹⁴³Nd/¹⁴⁴Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO₂ at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.     

A petrogenetic model for the igneous complex in the Spanish Peaks region,Colorado

Twenty representative rocks ranging from lamprophyric to granitic composition, from the Spanish Peaks igneous Complex, south-central Colorado, were analyzed for Sr isotopic compositions and their concentrations of K, Rb, Sr and Ba. The various igneous rocks from this Cenozoic complex do not have a comagmatic relationship from the evidence of their Sr isotopic compositions. Due to the generally low Sr⁸⁷/Sr⁸⁶ isotopic ratios, the possibility of the highly radiogenic underlying Precambrian basement as the source of magma generation can be ruled out. The sources for the magmas of this igneous complex must be in the upper mantle or the lower crust. Model calculations using elemental distribution coefficients and assumed mantle materials suggest that the abundant lamprophyric magmas in this region could be derived from a phlogopite-bearing hornblende peridotite by a small degree of partial melting (<5%) at lower pressure environment (<50 km). Other possibilities for lamprophyric magma generation were also examined. The slightly higher Sr⁸⁷/Sr⁸⁶ ratios observed in the granitic rocks are interpreted as reflecting the nature of this source-the lower crust. Alternatively, they may suggest a limited contamination of the original liquid by upper crustal material. For the entire igneous complex, mixing of two independent magmas, lymprophyric and granitic, is suggested to be the mechanism responsible for the complicated and diverse chemical characteristics.     

Effects of basement composition and age on silicic magmas across an accreted terrane-Precambrian crust boundary,Sierra Madre Occidental,Mexico

Trace element and isotopic compositions of mid-Tertiary siliceous magma sequences from two localities of the Sierra Madre Occidental, northern Mexico, display differences that reflect the composition and age of the basement through which they erupted. The crust beneath the section at San Buenaventura is thicker and more evolved and forms part of the North American basement, while that under El Divisadero consists of allochthonous terranes of island arc/oceanic? crust accreted during the Mesozoic.The volcanics are highly differentiated and range in composition from basalt to rhyolite (SiO₂=50–76%). Those erupted through the accreted terranes display a small range of isotope ratios and have lowest initial (age-corrected) Sr isotope ratios (>0.7044) and the highest Nd (<0.5126) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb ∼18.9). Isotope ratios of the continental suite are more variable and form an array which trends away from that of the accreted terrane suite toward compositions more typical of old crust (to ⁸⁷Sr/⁸⁶Sr ∼0.710 and ¹⁴³Nd/¹⁴⁴Nd ∼0.5123). The volcanics in the continental zone are relatively more enriched in moderately incompatible elements compared with those within the accreted terranes (Ce/Yb=25–45 vs. 13–33, respectively), but are depleted in some highly incompatible elements such as U and Rb (e.g., Th/U=3.8–7.5 vs. 2.5–4.0, respectively). Those higher in the stratigraphic sections have higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, and Th/U ratios, and lower ¹⁴³Nd/¹⁴⁴Nd ratios than those lower in the sections.The data have implications for the nature of the sources and the petrogenesis of these volcanics. The isotope ratios of both suites fall between those of mafic magma compositions from the Sierra Madre Occidental, and intermediate and felsic lower crustal xenoliths in northern Mexico and the southwestern USA. The relationship between the isotope ratios of the sequences and the age of the basement, combined with the fact that the overall data set forms well-defined isotopic arrays, demonstrates the strong effects of the crust on the chemistry of the silicic magmas. In the continental suite, isotope ratios covary with Th/Pb and U/Pb ratios, approaching the compositions found in the intermediate and felsic granulite facies xenoliths, strongly indicating that they are not anatectic melts of the lower crust but rather reflect interaction between mantle-derived basaltic parental magmas and the crust. Crustal contributions appear to be large, on the order of 20–70%. The small range of isotope ratios in the accreted terrane suite appears to reflect interaction of the basaltic parent with relatively juvenile crust whose isotopic composition is similar to the mantle-derived magmas. High Th/U and Th/Rb ratios indicate that the crustal contamination occurs in the lower crust. Moreover, the less radiogenic ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios in the continental suite indicate that the depletion in highly incompatible elements in the continental lower crust is an old feature. The secular changes in the isotope ratios within the stratigraphic sections indicate increasingly shallow crustal contributions with time, initially by predominantly mafic deep lower crust and later by more felsic middle crust. Using lavas from outside of the two heavily sampled stratigraphic sections, the differences in the isotopic compositions between volcanics erupted through the accreted terranes and the continental basement help to delineate the location of the boundary.     

Magma sources for Mesozoic anorogenic granites of the White Mountain magma series,New England,USA

The magma sources for granitic intrusions related to the Mesozoic White Mountain magma series in northern New England, USA, are addressed relying principally upon Nd isotopes. Many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts. Sm–Nd and Rb–Sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 Ma. The possibility of crustal sources is further examined with analyses of representative older crust including Paleozoic granitoids which serve as probes of the lower crust in the region. Multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations (<1 in _Nd). Overall, Mesozoic granite plutons range in _Nd [T] from +4.2 to -2.3, with most +2 to 0, and in initial ⁸⁷Sr/⁸⁶Sr from 0.7031 to 0.709. The isotopic variations are roughly inversely correlated but are not obviously related to geologic, geographic, or age differences. Older igneous and metamorphic crust of the region has much lower Nd isotope ratios with the most radiogenic Paleozoic granitoid at _Nd [180 Ma] of -2.8. These data suggest mid-Proterozoic separation of the crust in central northern New England. Moreover, the bulk of the Mesozoic granites cannot be explained as crustal melts but must have large mantle components. The ranges of Nd and Sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources. Crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation. The results indicate production of anorogenic granites from mantle-derived mafic magmas.     

Elemental fingerprints of isotopic contamination of hebridean Palaeocene mantle-derived magmas by archaean sial

One of the major puzzles presented by the geochemistry of the Palaeocene plateau lavas of Skye and Mull (N.W. Scotland) is that, although a very strong case can be made that the magmas are variably isotopically contaminated by Archaean Lewisian continental crust, little evidence has been gleaned to date from their major- and trace-element compositions to illuminate this hypothetical process. The combined results of published Sr-, Nd- and Pb-isotope studies of these lavas allow the basalts and hawaiites to be divided into three broad groups: essentially uncontaminated; contaminated with granulite-facies Archaean crust; contaminated with amphibolite-facies Archaean crust. Members of each group show distinctive chondrite-normalised incompatible-element patterns. The processes which gave rise to isotopic contamination of these lavas also affected the abundances and ratios of Ba, Rb, Th, K, Sr and light REE in the magmas, whilst having negligible effects on their abundances and ratios of Nb, Ta, P, Zr, Hf, Ti, Y and middle-heavy REE. Because such a wide range of elements were affected by the contamination process, it is postulated that the contaminant was a silicate melt of one or more distinctive crustal rock types, rather than an aqueous or similar fluid causing selective elemental movements from wall rocks into the magmas. As previous experimental and isotopic studies have shown that the Skye and Mull basic magmas were not constrained by cotectic equilibria at the time when they interacted with sial, the compositions of the contaminated lavas have been modelled in terms of simple magma-crust mixtures. Very close approximations to both the abundances and ratios of incompatible elements in the two groups of contaminated basalts may be obtained by adding 15% to 20% of Lewisian leucogneisses to uncontaminated Palaeocene basalt. Nevertheless, major-element constraints suggest that the maximum amount of granitic contaminant which has been added to these magmas lies between 5% and 10%. These estimates may be reconciled by postulating that the contaminants were large-fraction cotectic partial melts of Lewisian leucogneisses, leaving plagioclase residua. A corollary of this hypothesis is that it is necessary to postulate that the magma chambers where the sialic contamination occurred were, in fact, dykes or (more probably) sills. The very large surface-to-volume ratios of such magmas bodies would permit the systematic stripping, by partial melting, of the most-easily-fusible leucogneisses and pegmatites from the Lewisian crust, whilst failing to melt its major rock types. A present-day analogue to this situation may be the extensive sill-like magma bodies detected by geophysical methods within the continental crust beneath the Rio Grande Rift, southwestern U.S.A.     

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

The Rhön area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.     

Mixed Caledonian appinite magmas: implications for lamprophyre fractionation and high Ba-Sr granite genesis

Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic ⁸⁷Sr/⁸⁶Sr and nonradiogenic ¹⁴³Nd/¹⁴⁴Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant juvenile additions to the crust. Received: 26 September 1995 / Accepted: 5 June 1996     

Arc dacite genesis pathways: Evidence from mafic enclaves and their hosts in Aegean lavas

Mafic enclaves are commonly found in intermediate arc magmas, and their occurrence has been linked to eruption triggering by pre-eruptive magma mixing processes. New major, trace, Sr–Nd and U–Th isotope data of rocks from Nisyros in the Aegean volcanic arc are presented here. Pre-caldera samples display major and trace element trends that are consistent with fractionation of magnetite and apatite within intermediate compositions, and zircon within felsic compositions, and preclude extensive hybridization between mafic and felsic magmas. In contrast, post-caldera dacites form a mixing trend towards their mafic enclaves. In terms of U-series isotopes, most samples show small ²³⁸U excesses of up to  10%. Mafic enclaves have significantly higher U/Th ratios than their dacitic host lavas, precluding simple models that relate the mafic and felsic magmas by fractionation or aging alone. A more complicated petrogenetic scenario is required. The post-caldera dacites are interpreted to represent material remobilized from a young igneous protolith following influx of fresh mafic magma, consistent with the U–Th data and with Sr–Nd isotope constraints that point to very limited (< 10%) assimilation of old crust at Nisyros. When these results are compared to data from Santorini in the same arc, there are many geochemical similarities between the two volcanic centers during the petrogenesis of the pre-caldera samples. However, striking differences are apparent for the post-caldera lavas: in Nisyros, dacites show geochemical and textural evidence for magma mixing and remobilization by influx of mafic melts, and they erupt as viscous lava domes; in Santorini, evidence for geochemical hybridization of dacites and mafic enclaves is weak, dacite petrogenesis does not involve protolith remobilization, and lavas erupt as less viscous flows. Despite these differences, it appears that mafic enclaves in intermediate Aegean arc magmas consistently yield timescales of at least 100 kyrs between U enrichment of the mantle wedge and eruption, on the upper end of those estimated for the eruptive products of mafic arc volcanoes. Finally, the data presented here provide constraints on the rates of differentiation from primitive arc basalts to dacites (less than  140 kyrs), and on the crustal residence time of evolved igneous protoliths prior to their remobilization by mafic arc magmas (greater than  350 kyrs).     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

Geochemical and Sm–Nd isotopic study of titanite from granitoid rocks of the eastern Dharwar craton,southern India

Titanite occurs as an accessory phase in a variety of igneous rocks, and is known to concentrate geologically important elements such as U, Th, rare earth element (REE), Y and Nb. The differences in the abundances of the REEs contained in titanite from granitoid rocks could reflect its response to changes in petrogenetic variables such as temperature of crystallization, pressure, composition, etc. Widespread migmatization in the granodiorite gneisses occurring to the east of Kolar and Ramagiri schist belts of the eastern Dharwar craton resulted in the enrichment of the REEs in titanite relative to their respective host rocks. A compositional influence on the partitioning of REEs between titanite and the host rock/magma is also noticed. The relative enrichment of REEs in titanite from quartz monzodiorite is lower than that found in the granodioritic gneiss. Depletion of REE and HFSE (high field-strength elements) abundances in granitic magmas that have equilibrated with titanite during fractional crystallization or partial melting has been modelled. As little as 1% of titanite present in residual phases during partial melting or in residual melts during fractional crystallization can significantly lower the abundances of trace elements such as Nb, Y, Zr and REE which implies the significance of this accessory mineral as a controlling factor in trace element distribution in granitoid rocks. Sm–Nd isotope studies on titanite, hornblende and whole rock yield isochron ages comparable to the precise U–Pb titanite ages, invoking the usefulness of Sm–Nd isochron ages involving minerals like titanite.     

The petrogenesis of massif anorthosites: a Nd and Sr isotopic investigation of the Proterozoic of Rogaland/Vest-Agder,SW Norway

Sm-Nd and Rb-Sr isotopic analyses of charnockitic migmatite, augen gneiss, anorthosite-leuconorite and two acid plutons from the Rogaland and Vest-Agder districts of southwest Norway constrain their crustal residence ages, origin and evolution. The charnockitic migmatites, which are a major component of the metamorphic basement complex, represent the oldest and largest episode of accretion, in which new crust was derived 1.5–1.9 Ga ago from a mantle source of depleted Nd isotopic composition. The basement complex was intruded by a number of large anorthositic to granitic plutons during and after the Sveconorwegian orogenic period. Samples from the ca. 1050 Ma old, synorogenic Håland anorthosite-leuconorite massif exhibit substantial variation of initial _Nd of +2.1 to +4.4 at an anorthosite locality and –0.5 to +2.3 at a leuconorite locality, but display significant variation of initial ⁸⁷Sr/⁸⁶Sr ratio only between the localities (anorthosite mean=0.70369, leuconorite mean=0.70560). A model is proposed whereby the anorthosite and leuconorite were derived by major crustal contamination of, and fractional crystallization from, a picritic magma derived from isotopically-depleted mantle. Two younger acid intrusions, the 950 Ma old Lyngdal granodiorite and the 930 Ma old Farsund charnockite, both have initial Sr and Nd isotope ratios consistent with massive contamination of depleted-mantle-derived magma by old continental crustal material.     

Coupled Ce-Nd isotope systematics and rare earth elements differentiation of the moon

¹³⁸Ce/¹⁴²Ce and ¹⁴³Nd/¹⁴⁴Nd isotope ratios of lunar samples are determined to constrain the petrogenetic differentiation and evolution of the moon. High-precision Ce-Nd isotope data, well-defined Rb-Sr isochrons, and rare earth elements (REE) abundances of lunar samples show that unexpectedly low La/Ce ratios of evolved lunar highland samples are preserved from at least 3.9 Ga. Precise analysis of REE abundances indicates that the low La/Ce ratio results from a depletion of La relative to other REE. This depletion can be seen in pristine KREEP basalts and Mg-suite rocks from 3.85 to 4.46 Ga. As REE abundances of all these samples are controlled by the presence of a KREEP component, the depletion was probably inherited from a late crystallization sequence of the lunar magma ocean related to the production of the original KREEP component.     

Origin of Low-K Intermediate Lavas at Nekoma Volcano, NE Honshu Arc, Japan: Geochemical Constraints for Lower-Crustal Melts

Extremely low-K basaltic andesite to andesite lavas at Nekomavolcano, situated in the frontal volcanic zone of the NE Honshuarc, were produced from melts that originated in the lower crust.Multiple incompatible trace element model calculations suggestthat extremely low-K basalt found in the same arc is a naturalanalog for the source composition. However, fractional crystallization,magma mixing, and crustal contamination models of primary low-Kbasalt cannot reproduce the Nekoma chemical composition. Derivationof melts from an extremely low-K amphibolitic lower-crustalrock with the residual mineral phases hornblende, olivine, pyroxenes,plagioclase, and magnetite is plausible. Major element compositionsof Nekoma lavas are very similar to those of experimental meltsof amphibolite dehydration melting, which further support theproposal. Light rare earth elements are slightly enriched, buttotal rare earth element abundances are relatively low, suggestinga high degree of partial melting of the source. Ba/Th ratiosare low for frontal arc lavas, reflecting modification of theratio during partial melting. Zr/Hf and Nb/Ta ratios are significantlygreater than is usual for arc lavas, suggesting an anomaloussource composition. Markedly low K, Rb, Cs contents in the extremelylow-K lavas are attributed to an extremely low-K source. Underplatingof an extremely low-K basalt originating from a hydrous depletedmantle wedge could form such an amphibolite. In contrast, Ndand Sr isotope ratios fall close to Bulk Earth values, indicatingan isotopically enriched source. Hornblende-bearing rocks maypredominate in the lower crust of the NE Honshu arc, based onthe observation of crustal xenoliths. The presence of largelow-V_p regions at lower-crustal depths beneath the frontal arcis suggested by geophysical observations. These observationsfurther support lower-crustal melting beneath Nekoma as theorigin of the intermediate low-K lavas. KEY WORDS: amphibolite source; crustal melting; low-K andesite; Sr–Nd isotopes; trace element     

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.     

Petrogenetic significance of rare-earth element behavior in the basement rocks of southern Obudu Plateau, Bamenda Massif, southeastern Nigeria

Rock samples representing various igneous and metamorphic rocks of southern Obudu Plateau were analyzed for rare-earth element ( REE ) behavior by ICP-MS. Results of the analyses indicate a range of REE abundances and distinctive patterns from highly fraetionated patterns with negative Eu anomalies in granitic rocks to relatively low abundances and less REE fractionated flat patterns with little Eu anomaly in some paragneisses, schists, enderbites and dolerites to unfractionated patterns with positive Eu anomalies in some paragneisses and charnockites. Over all, there are low to high ∑ REE contents with negative to positive Eu anomalies. The ratios of different parameters, especially La/Yb and Ce/Yb, show behaviors consistent with crustal to mantle derivation. The heterogeneity of REE abundances and REE patterns reflects mantle to crustal petrogenetic variations of different rock suites on the Plateau. The LREE content is higher than the HREE content in the highly differentiated rocks, as evidenced by their La/Yb,Ce/Yb and La/Sm ratios, which are normally higher in residual products than in primary melts. The dominantly intermediate nature of the source rock of the orthogneisses is suggested by the generally low ∑ REE. The granites enriched in LREE and depleted in HREE and some of the charnockites with negative Eu anomalies were probably formed by partial melting and crystallization.     

Source contributions to Devonian granite magmatism near the Laurentian border, New Hampshire and Western Maine, USA

Radiogenic isotope data (initial Nd, Pb) and elemental concentrations for the Mooselookmeguntic igneous complex, a suite of mainly granitic intrusions in New Hampshire and western Maine, are used to evaluate petrogenesis and crustal variations across a mid-Paleozoic suture zone. The complex comprises an areally subordinate monzodiorite suite [377±2 Ma; ε_Nd (at 370 Ma)=−2.7 to −0.7; initial ²⁰⁷Pb/²⁰⁴Pb=15.56–15.58] and an areally dominant granite [370±2 Ma; ε_Nd (at 370 Ma)=−7.0 to −0.6; initial ²⁰⁷Pb/²⁰⁴Pb=15.55–15.63]. The granite contains meter-scale enclaves of monzodiorite, petrographically similar to but older than that of the rest of the complex [389±2 Ma; ε_Nd (at 370 Ma)=−2.6 to +0.3; initial ²⁰⁷Pb/²⁰⁴Pb 15.58, with one exception]. Other granite complexes in western Maine and New Hampshire are 30 Ma older than the Mooselookmeguntic igneous complex granite, but possess similar isotopic signatures.

Derivation of the monzodioritic rocks of the Mooselookmeguntic igneous complex most likely occurred by melting of Bronson Hill belt crust of mafic to intermediate composition. The Mooselookmeguntic igneous complex granites show limited correlation of isotopic variations with elemental concentrations, precluding any significant presence of mafic source components. Given overlap of initial Nd and Pb isotopic compositions with data for Central Maine belt metasedimentary rocks, the isotopic heterogeneity of the granites may have been produced by melting of rocks in this crustal package or through a mixture of metasedimentary rocks with magmas derived from Bronson Hill belt crust.

New data from other granites in western Maine include Pb isotope data for the Phillips pluton, which permit a previous interpretation that leucogranites were derived from melting heterogeneous metasedimentary rocks of the Central Maine belt, but suggest that granodiorites were extracted from sources more similar to Bronson Hill belt crust. Data for the Redington pluton are best satisfied by generation from sources in either the Bronson Hill belt or Laurentian basement. Based on these data, we infer that Bronson Hill belt crust was more extensive beneath the Central Maine belt than previously recognized and that mafic melts from the mantle were not important to genesis of Devonian granite magma.     

Crustal assimilation during turbulent magma ascent (ATA); new isotopic evidence from the Mull Tertiary lava succession,N. W. Scotland

Assimilation of crustal rocks with concomitant fractional crystallisation (AFC) is a well documented phenomenon in many igneous suites, but geochemical evidence from the Tertiary Mull lava succession suggests that in these magmas crustal contamination occurred by a distinctly different mechanism. Lavas from the lower half of the Mull Plateau group (MPG) can be divided into two broad sub-types; high (>8%) MgO basalts with elevated Ba and K; and lower MgO (<8%) basaltic-hawaiites with lower Ba and K. The lower crust and most of the upper crust beneath Mull is probably of Lewisian age. The Sr-, Nd-and Pb-isotope compositions of local Lewisian crustal samples yield the following ranges: ⁸⁷Sr/⁸⁶Sr=0.71002–0.72348, ¹⁴³Nd/¹⁴⁴Nd=0.51045–0.51058 and ²⁰⁶Pb/²⁰⁴Pb=14.0–14.6. Ten lavas have also been analysed and yield the following ranges: ⁸⁷Sr/⁸⁶Sr=0.7028–0.7042, ¹⁴³Nd/¹⁴⁴Nd=0.51214–0.51230 and ²⁰⁶Pb/²⁰⁴Pb=15.1–17.9. However, within this range, it is predominantly the more primitive mafic compositions, with elevated Mg, Ba and K, that show the lowest Nd- and Pb-, and the highest Sr-isotope values. Modelling of these isotopic results, in conjunction with major and trace element data, show that: (1) contamination by Lewisian lower crustal material does occur; (2) that the process involved was not one of assimilation with concomitant fractional crystallisation (AFC). The proposed contamination process is one whereby the hottest (most MgO rich) magmas have assimilated acidic partial melts of Lewisian lower crust during turbulent ascent (ATA) through thin, poorly connected dyke- and sill-like magma chambers. The chemical composition of the contaminated lavas can be modelled successfully through addition of 5% acidic Lewisian crust to an uncontaminated lava. In contrast, the more evolved magmas — which probably fractionated at sub-crustal levels — were either not hot enough to molt significant amounts of crust, or did not ascend turbulently because of their higher viscosity, and so are less contaminated with crust.     

浙江新昌复合式火成杂岩的岩石学、地球化学及其地质意义

浙江新昌复合式火成杂岩包括复合火山岩流和复合侵入杂岩。复合火山岩流由玄武岩和流纹岩/流纹质熔结凝灰岩组成,有的含少量安山质-英安质岩流和岩石包体;复合侵入杂岩由辉绿岩和花岗岩复合而成,含闪长质-石英闪长质岩石包体。它们在空间上紧密伴生,同位素年龄为96~113 Ma;地球化学上,它们统属钙碱性岩系,具有轻稀土元素和大离子亲石元素富集、高场强元素亏损等特点,稀土元素、微量元素配分型式基本相同,Sr=0.7069~0.7079,εNd (t ) = -2.3~-5.3,指示它们的微量元素和同位素发生过充分的交换。这些特点与浙闽沿海其他复合岩流和复合侵入杂岩的特点基本一致。它们形成在伸展构造背景,由起源于受消减作用影响的岩石圈地幔部分熔融而产生的玄武岩岩浆底侵,并与深熔的壳源花岗岩浆发生不同程度的岩浆混合而形成。新昌复合式火成杂岩的研究,为更深入研究中国东南部沿海地区晚中生代地球动力学环境变化和构造-岩浆作用提供了一个典型实例。