Beyond Subduction and Plumes: A Unified Tectonic-Petrogenetic Model for the Mexican Volcanic Belt

The Mexican Volcanic Belt (MVB) is a major linear belt of Miocene to present-day volcanism in southern Mexico. Its origin has been controversial, although the majority opinion views it as a volcanic arc related to the subduction of the Cocos plate under the North American plate. Both calc-alkaline and alkaline volcanism characterize the belt; the latter has been previously cited as indicative of the role of a mantle plume. Here we present objections to these explanations, and conclude on the basis of geological, geochemical, and geophysical data that the MVB is unrelated to subduction or to a mantle plume, and is instead a rift-like structure experiencing active extension. Calc-alkaline or alkaline geochemistry of magmas is not useful for inferring tectonic setting, but reflects source parameters and petrogenetic processes. For the MVB, calc-alkaline geochemistry suggests crustal contamination, and the OIB-like geochemistry suggests an enriched mantle source. Our proposal of a heterogeneous mantle beneath the MVB comprising “normal” mantle and metasomatic, enriched veins, can explain the close association in space and time of calc-alkaline and alkaline volcanism throughout the belt.     

Geochemistry and Petrogenesis of Neoarchean Metamorphic Mafic Rocks in the Wutai Complex

Neoarchean metamorphic mafic rocks in the lower and the middle Wutai Complex mainly comprise metamorphic gabbros, amphibolites and chlorite schists. They can be subdivided into three groups according to chondrite normalized REE patterns. Rocks in Group #1 are characterized by nearly flat REE patterns （Lan/Ybn=0.86-1.3）, the lowest total REEs （29-52 ppm）, and weak negative to positive Eu anomalies （Eun/Eun=0.84-1.02）, nearly flat primitive mantle normalized patterns and strong negative Zr（Hf） anomalies. Their geochemical characteristics in REEs and trace elements are similar to those of ocean plateau tholeiite, which imply that this group of rocks can represent remnants of Archean oceanic crust derived from a mantle plume. Rocks in Group #2 are characterized by moderate total REEs （34-116 ppm）, LREE-enriched （Lan/Ybn=1.76-4.34） chondrite normalized REE patterns with weak Eu anomalies （Eun/Eun=0.76-1.16）, and negative Nb, Ta, Zr（Hf）, Ti anomalies in the primitive mantle normalized spider diagram. The REE and trace element characteristics indicate that they represent arc magmas originating from a sub-arc mantle wedge metasomatized by slab-derived fluids. Rocks in Group #3 are characterized by the highest total REEs （61-192 ppm）, the strongest LREEs enrichment （Lan/Ybn=7.12-16） with slightly negative Eu anomalies （Eun/Eun=0.81-0.95） in the chondrite normalized diagram. In the primitive mantle normalized diagram, these rocks are characterized by large negative anomalies in Nb, Ta, Ti, negative to no Zr anomalies. They represent arc magmas originating from a sub-arc mantle wedge enriched in slab-derived melts. The three groups of rocks imply that the formation of the Neoarchean Wutai Complex is related to mantle plumes and island-arc interaction.     

Contemporaneous eruption of calc-alkaline and alkaline lavas in a continental arc (Eastern Mexican Volcanic Belt): chemically heterogeneous but isotopically homogeneous source

Nearly contemporaneous eruption of alkaline and calc-alkaline lavas occurred about 900 years BP from El Volcancillo paired vent, located behind the volcanic front in the Mexican Volcanic Belt (MVB). Emission of hawaiite (Toxtlacuaya) was immediately followed by calc-alkaline basalt (Río Naolinco). Hawaiites contain olivine microphenocrysts (Fo_67–72), plagioclase (An_56–60) phenocrysts, have 4–5 wt% MgO and 49.6–50.9 wt% SiO₂. In contrast, calc-alkaline lavas contain plagioclase (An_64–72) and olivine phenocrysts (Fo_81–84) with spinel inclusions, and have 8–9 wt% MgO and 48.4–49.4 wt% SiO₂. The most primitive lavas in the region (Río Naolinco and Cerro Colorado) are not as primitive as parental melts in other arcs, and could represent either (a) variable degrees of melting of a subduction modified, garnet-bearing depleted mantle source, followed by AFC process, or (b) melting of two distinct mantle sources followed by AFC processes. These two hypotheses are evaluated using REE, HFSE, and Sr, Os and Pb isotopic data. The Toxtlacuaya flow and the Y & I lavas can be generated by combined fractional crystallization and assimilation of gabbroic granulite, starting with a parental liquid similar to the Cerro Colorado basalt. Although calc-alkaline and alkaline magmas commonly occur together in other areas of the MVB, evidence for subduction component in El Volcancillo magmas is minimal and limited to <1%, which is a unique feature in this region further from the trench. El Volcancillo lavas were produced from two different magma batches: we surmise that the injection of calc-alkaline magma into an alkaline magma chamber triggered the eruption of hawaiites. Our results suggest that the subalkaline and hawaiitic lavas were formed by different degrees of partial melting of a similar, largely depleted mantle source, followed by later AFC processes. This model is unusual for arcs, where such diversity is usually explained by melting of heterogeneous (enriched and depleted) and subduction-modified mantle.     

Evolution of arc crust and relations between contrasting sources: U-Pb (age), Nd and Sr isotope systematics of the ophiolitic terrain of SW Norway

The U/Pb dating of ophiolite and arc complexes in the Caledonides of SW Norway has demonstrated that these spatially associated rocks are also closely related in time. A sequence of tholeiitic island arc volcanics, and an unconformably overlying sequence of calc-alkaline volcanics have been dated as 494 ± 2 Ma (2σ) and 473 ± 2 Ma respectively. Ophiolitic crust formed both prior to, and during the first 10 Ma after the tholeiitic island arc volcanism. Boninitic and island arc tholeiitic dyke swarms intruded the ophiolites soon after they formed and represent a second phase of spreading-related magmatism in the ca 20 Ma period that separated the tholeiitic and the calc-alkaline island arc volcanism. The magmatism ended with the formation of alkaline, ocean island basalt (OIB)-like magmas. Quartz dioritic and S-type granitic plutons, dated to 479 ± 5 Ma and 474 +3/−2 Ma respectively, intruded into the base of the arc crust during and subsequent to the boninitic magmatism, and at the time when calc-alkaline volcanic centres developed. The quartz dioritic and the granitic rocks contain inherited zircons of Precambrian age which prove the involvement of a continental source. This together with the geology of the terrain and the geochemistry of these plutons suggests that the granitic magmas were partly derived from subducted clastic sediments. The Sr and Nd isotope systematics indicate that the same continental source was a component in the boninitic and the calc-alkaline magmas. While the calc-alkaline magmas may have gained this continental component at a crustal level by assimilation, both geology and isotope systematics suggest that the continental component in the boninitic rocks was introduced by source contamination – possibly by a direct interaction between the mantle source and the S-type granitic magmas. A modified mid ocean ridge basalt-like mantle source was the principal source during the earliest and the main crust forming stage. This source became replaced by an OIB-like source during the later stages in the evolution of this ancient arc. Received: 27 June 1994 / Accepted: 16 September 1996     

Constraints on the origin of alkaline and calc-alkaline magmas from the Tuxtla Volcanic Field,Veracruz, Mexico

 Lavas erupted in the Tuxtla Volcanic Field (TVF) over the last 7 Ma include primitive basanites and alkali basalts, mildly alkaline Hy-normative mugearites and benmoreites, and calc-alkaline basalts and basaltic andesites. The primitive lavas are silica-undersaturated, with high concentrations of both incompatible and compatible trace elements, variable La/Yb with constant Yb at 6 to 8 times chondritic, and low Sr and O and variable Pb and Nd isotopic ratios. The primitive magmas originated by increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar, in the garnet stability field. Another group of alkali basalts and hawaiites has lower Ni and Cr concentrations and higher Fe/Mg ratios, and was derived from the primitive group by crystal fractionation at pressures of several kbar. Incompatible trace elements in these silica undersaturated lavas show depletion in high field strength elements (HFSE) relative to large ion lithophile elements, similar to subduction-related basalts. Ba/Nb ratios are nearly constant and thus the HFSE depletion cannot be the result of a residual HFSE-bearing phase in the source, but could be the result of generation from a source contaminated by fluids or melts from the subducted lithosphere. The silica-saturated mugearites and benmoreites, and the calc-alkaline basalts and basaltic andesites, were erupted only between 3.3 and 1.0 Ma. These have incompatible element concentrations generally lower than in the silica-undersaturated lavas, and thus could not have been derived by crystal fractionation from the silica-undersaturated alkaline magmas. Magmas parental to the silica-saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas. Melting may have been promoted by an influx of fluid from the subducted lithosphere. Trace element and Sr, Nd, Pb and O isotopic data suggest that three components are involved in the generation of TVF magmas: the mantle, a fluid from the subducted lithosphere, and continental crust. TVF alkaline lavas are similar to those erupted in the back-arc region of the MVB and Japan, and show characteristics similar to alkaline magmas erupted in the southern Andean volcanic arc. These low degree melts reach the surface along with calc-alkaline lavas in the TVF due to an extensional stress field that allows their passage to the surface. Received: 15 September 1994/Accepted: 14 February 1995     

Alkaline Lavas in the Volcanic Front of the Western Mexican Volcanic Belt: Geology and Petrology of the Ayutla and Tapalpa Volcanic Fields

The Plio-Quaternary Ayutla and Tapalpa volcanic fields in thevolcanic front of the western Mexican Volcanic Belt (WMVB) containa wide variety of alkaline volcanic rocks, rather than onlycalc-alkaline rocks as found in many continental arcs. Thereare three principal rock series in this region: an intraplatealkaline series (alkali basalts and hawaiites), a potassic series(lamprophyres and trachylavas), and a calc-alkaline series.Phlogopite-clinopyroxenite and hornblende-gabbro cumulate xenolithsfrom an augite minette lava flow have orthocumulate textures.The phlogopite-clinopyroxenite xenoliths also contain apatiteand titanomagnetite and probably formed by accumulation of mineralsfractionated from an augite minette more primitive than thehost. The intraplate alkaline series is probably generated bydecompression melting of asthenospheric mantle as a result ofcorner flow in the mantle wedge beneath the arc. Alkaline magmasmay be common in the WMVB as a result of prior metasomatism(during Tertiary Sierra Madre Occidental magmatism) of the Mexicansub-arc mantle. Generation of the more evolved andesites anddacites of the calc-alkaline series is due to either combinedassimilation and fractional crystallization (AFC) or magma mixing.The preponderance of alkaline and hydrous lavas in this regiondemonstrates that these lava types are the norm, rather thanthe exception in western Mexico, and occur in regions that arenot necessarily associated with active rifting. KEY WORDS: arc basalt; subduction; alkali basalt; minette; hawaiite; metasomatism     

Geochemical variation within the northern Ryukyu Arc: magma source compositions and geodynamic implications

The major and trace element and Pb–Sr–Nd isotopic compositions of Quaternary mafic lavas from the northern Ryukyu arc provide insights into the nature of the mantle wedge and its tectonic evolution. Beneath the volcanic front in the northern part of the arc, the subducted slab of the Philippine Sea Plate bends sharply and steepens at a depth of ∼80 km. Lavas from the volcanic front have high abundances of large ion lithophile elements and light rare earth elements relative to the high field strength elements, consistent with the result of fluid enrichment processes related to dehydration of the subducting slab. New Pb isotopic data identify two distinct asthenospheric domains in the mantle wedge beneath the south Kyushu and northern Ryukyu arc, which, in a parallel with data from the Lau Basin, appear to reflect mantle with affinities to Indian and Pacific-type mid-ocean ridge basalt (MORB). Indian Ocean MORB-type mantle, contaminated with subducted Ryukyu sediments can account for the variation of lavas erupted on south Kyushu, and probably in the middle Okinawa Trough. In contrast, magmas of the northern Ryukyu volcanic front appear to be derived from sources of Pacific MORB-type mantle contaminated with a sedimentary component. Along-arc variation in the northern Ryukyus reflects increasing involvement of a sedimentary component to the south. Compositions of alkalic basalts from the south Kyushu back-arc resemble intraplate-type basalts erupted in NW Kyushu since ∼12 Ma. We propose that the bending of the subducted slab was either caused by or resulted in lateral migration of asthenospheric mantle, yielding Indian Ocean-type characteristics from a mantle upwelling zone beneath NW Kyushu and the East China Sea. This model also accounts for (1) extensional counter-clockwise crustal rotation (∼4–2 Ma), (2) voluminous andesite volcanism (∼2 Ma), and (3) the recent distinctive felsic magmatism in the south Kyushu region. Received: 30 November 1999 / Accepted: 20 July 2000     

Hf-Nd isotope evidence for contemporaneous subduction processes in the source of late Archean arc lavas from the Superior Province, Canada

Volcanic suites from Wawa greenstone belts in the southern Superior Province comprise an association of typical late Archean arc volcanic rocks including adakites, magnesian andesites (MA), niobium-enriched basalts (NEB), and ‘normal’ tholeiitic to calc-alkaline basalts to rhyolites. The adakites represent melts from subducted oceanic crust and all other suites were derived from the mantle wedge above the subducting oceanic lithosphere. The magnesian andesites are interpreted to be the product of hybridization of adakite melts with arc mantle wedge peridotite. The initial ?_Hf values of the ∼2.7 Ga Wawa adakites (+3.5 to +5.2), magnesian andesites (+2.6 to +5.1), niobium-enriched basalts (+4.4 to +6.6), and ‘normal’ tholeiitic to calc-alkaline arc basalts (+5.3 to +6.4) are consistent with long-term depleted mantle sources. The niobium-enriched basalts and ‘normal’ arc basalts have more depleted ?_Hf values than the adakites and magnesian andesites. The initial ?_Nd values in the magnesian andesites (+0.4 to +2.0), niobium-enriched basalts (+1.4 to +2.4), and ‘normal’ arc tholeiitic to calc-alkaline basalts (+1.6 to +2.9) overlap with, but extend to lower values than, the slab-derived adakites (+2.3 to +2.8). The lower initial ?_Nd values in the mantle-wedge-derived suites, particularly in the magnesian andesites, are attributed to recycling of an Nd-enriched component with lower ?_Nd to the mantle wedge. As a group, the slab-derived adakites plot closest to the 2.7 Ga depleted mantle value in ?_Nd versus ?_Hf space, additionally suggesting that the Nd-enriched component in the mantle wedge did not originate from the 2.7 Ga slab-derived melts. Accordingly, we suggest that the enriched component had been added to the mantle wedge at variable proportions by recycling of older continental material. This recycling process may have occurred as early as 50-70 Ma before the initiation of the 2.7 Ga subduction zone. The selective enrichment of Nd in the sources of the Superior Province magmas can be explained by experimental studies and geochemical observations in modern subduction systems, indicating that light rare earth elements (e.g., La, Ce, Sm, Nd) are more soluble than high field strength elements (e.g., Zr, Hf, Nb, Ta) in aqueous fluids that are derived from subducted slabs. As a corollary, we suggest that the recycled Nd-enriched component was added to the mantle source of the Wawa arc magmas by dehydration of subducted sediments.     

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/La_pm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Yb_n=1–8), high Th/Nb_pm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO₂, Ta, P₂O₅, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Yb_n tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Yb_n of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs. Received: 2 June 1999 / Accepted: 29 December 1999     

Geochemistry, petrogenesis and tectonic setting of late Neoproterozoic Dokhan-type volcanic rocks in the Fatira area, eastern Egypt

 The Neoproterozoic Dokhan volcanics of the Fatira area in eastern Egypt comprise two main rock suites: (a) an intermediate volcanic suite, consisting of basaltic andesite, andesite, dacite, and their associated pyroclastic rocks; and (b) a felsic volcanic suite composed of rhyolite and rhyolitic tuffs. The two suites display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO₂ (54–76%), CaO (8.19–0.14%), MgO (6.96–0.04%), Sr (983–7 ppm), Zr (328–95 ppm), Cr (297–1 ppm), and Ni (72–1 ppm). They are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and show strong affinity to calc-alkaline subduction-related rocks. However, their undeformed character, their emplacement temporally and spatially with post-orogenic A-type granite, and their high Zr/Y values suggest that their emplacement follow the cessation of subduction in eastern Egypt in an extensional-related within-plate setting. Major and trace element variations in the intermediate volcanics are consistent with their formation via partial melting of an enriched subcontinental lithospheric mantle source followed by a limited low-pressure fractional crystallization of olivine and pyroxene before emplacement. The LILE enrichment relative to HFSE is attributed to the inheritance of a subduction component from mantle material which constituted the mantle wedge during previous subduction events in eastern Egypt. The evolution of the whole volcanic spectrum was governed mainly by crystal/melt fractionation of amphibole, plagioclase, titanomagnetite, and apatite in the intermediate varieties and plagioclase, amphibole, biotite, Fe–Ti oxides, apatite, and zircon in the felsic varieties. At each stage of evolution, crystal fractionation was accompanied by variable degrees of crustal contamination. Received: 28 June 1998 / Accepted: 25 August 1999     

Geochemistry of mafic dykes in the Antarctic Peninsula continental-margin batholith: a record of arc evolution

Mafic dykes of the Antarctic Peninsula continental-margin arc are compositionally diverse, comprising calc-alkaline (dominant), shoshonite, tholeiite, and OIB-like varieties. Their compositions give information about different mafic magma sources tapped during arc evolution. The compositional groups represent partial melts of at least five distinct mantle sources: a low-ɛNd subduction-modified, garnet-bearing, lithospheric mantle (older calc-alkaline); a high-ɛNd subduction-modified, garnet-bearing, lithospheric mantle (shoshonites); a high-ɛNd subduction-modified, spinel-bearing, asthenospheric mantle (younger calc-alkaline); E-MORB-like spinel-bearing asthenosphere depleted by a previous melting event (tholeiites); and within-plate non-subduction modified, garnet- and spinel-bearing, asthenosphere (OIB-like). Slab-derived fluids, subducted sediment, and arc crust also contributed to the magmas. Consideration of previous work in the light of our new compositional and geochronological data enables presentation of a summary of arc evolution. For most of the Cretaceous and Tertiary, the tectonic regime of the Antarctic Peninsula arc was transtensional, and calc-alkaline magmas intruded. An oceanic spreading centre collided with the trench during the Late Cretaceous and induced tectonic changes which caused tapping of different magma sources. A pulse of shoshonitic, tholeiitic, and OIB-like mafic magmatism resulted. Three ridge-trench collisions are now recognized during the history of the arc, in Mid–Late Jurassic, Late Cretaceous, and Early–Mid Tertiary times. Received: 13 January 1997 / Accepted: 5 December 1997     

Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania

Miocene to Pleistocene calc-alkaline volcanism in the East Carpathianarc of Romania was related to the subduction of a small oceanbasin beneath the continental Tisza–Dacia microlate. Volcanicproducts are predominantly andesitic to dadtic in composition,with rare basalts and rhyodacites (51–l71% SiO₂; mg-number0.65–0.26) and have medium- to high-K calcalkaline andshoshonitic affinities. Mg, Cr and Ni are low in all rock-types,indicating the absence of primary erupted compositions. Detailedtrace element and Sr, Nd, Pb and 0 isotope data suggest thatmagmas were strongly crustally contaminated. Assimilation andfractional crystallization (AFC) calculations predict the consumptionof 5–35% local upper-crustal metasediments or sedimentsfrom the palaeo-accretionary wedge. Variations in the isotopiccomposition of the contaminants and parental magmas caused variationsin the mixing trajectories in different parts of the arc Themost primitive isotopic compositions are found in low-K dacitesof the northern Cdlimani volcanic centre and are interpretedas largely mantle derived. A second possible mantle reservoirof lower ¹⁴⁹ Nd/¹⁴⁴ Nd and lower ²⁰⁶ Pb/²⁰⁴ Pb is identifiedfrom back-arc basic calc-alkaline rocks in the south of thearc Both magmatic reservoirs have elevated isotopic characteristics,owing either to source bulk mixing (between depleted or enrichedasthenosphere and <1% average subducted local sediment) orlower-crustal contamination. KEY WORDS: Carpathians; assimilation; calc-alkaline; Sr-Nd-Pb-0 isotopes; laser flurination     

Young basalts of the central Washington Cascades, flux melting of the mantle, and trace element signatures of primary arc magmas

Basaltic lavas from the Three Sisters and Dalles Lakes were erupted from two isolated vents in the central Washington Cascades at 370–400 ka and 2.2 Ma, respectively, and have distinct trace element compositions that exemplify an important and poorly understood feature of arc basalts. The Three Sisters lavas are calc-alkaline basalts (CAB) with trace element compositions typical of most arc magmas: high ratios of large-ion-lithophile to high-field-strength elements (LILE/HFSE), and strong negative Nb and Ta anomalies. In contrast, the Dalles Lakes lavas have relatively low LILE/HFSE and no Nb or Ta anomalies, similar to ocean-island basalts (OIB). Nearly all Washington Cascade basalts with high to moderate incompatible element concentrations show this CAB or OIB-like compositional distinction, and there is pronounced divergence between the two magma types with a large compositional gap between them. We show that this trace element distinction can be easily explained by a simple model of flux-melting of the mantle wedge by a fluid-rich subduction component (SC), in which the degree of melting (F) of the peridotite source is correlated with the amount of SC added to it. Distinctive CAB and OIB-like trace element compositions are best explained by a flux-melting model in which dF/dSC decreases with increasing F, consistent with isenthalpic (heat-balanced) melting. In the context of this model, CAB trace element signatures simply reflect large degrees of melting of strongly SC-fluxed peridotite along relatively low dF/dSC melting trends, consistent with derivation from relatively cold mantle. Under other conditions (i.e., small degrees of melting or large degrees of melting of weakly SC-fluxed peridotite [high dF/dSC]), either OIB- or MORB (mid-ocean ridge basalt)-like compositions are produced. Trace element and isotopic compositions of Washington Cascade basalts are easily modeled by a correlation between SC and F across a range of mantle temperatures. This implies that the dominant cause of arc magmatism in this region is flux melting of the mantle wedge. Received: 2 March 1999 / Accepted: 18 August 1999     

Alkaline and calc-alkaline lavas near Los Volcanes,Jalisco, Mexico: geochemical diversity and its significance in volcanic arcs

A remarkably diverse suite of lavas erupted during the late-Pliocene at the volcanic front of the western Mexican Volcanic Belt near the town of Los Volcanes, Jalisco. This region is much closer to the Middle America Trench than the main axis of Quaternary andesite-dacite stratovolcanoes, and volcanism occurred in a complex tectonic regime involving both subduction of the young Rivera Plate and transverse crustal extension of the Jalisco structural block. The variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc-alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the Mexican Volcanic Belt. Other alkaline varieties intermediate between these extremes include absarokites, trachybasalts and trachyandesites. Phlogopite, amphibole and apatite are common phenocryst phases; whole-rock compositions show a wide range of alkali contents (e.g. K₂O of 1.0–8.6 wt.%), and typically contain >5 wt.% MgO. MgO, Ni, V and Cr show little systematic decrease with increasing SiO₂, suggesting that these lavas have evolved from primitive, mantle derived magmas with a wide range of SiO₂ contents. Strong enrichments in incompatible trace elements are observed in all of the lavas (Sr 700–5100 ppm, Ba 470–4800, Ce 22–325 ppm, Zr 90–700 ppm), as is the relative enrichment of large ion lithophile (Ba, Sr, Rb) and light rare-earth elements (La, Ce) over the high field strength elements (Ti, Zr) which is typical of magmas in volcanic arcs. This enrichment pattern suggests that these magmas come from source regions which contain incompatible element-rich phases such as phlogopite, amphibole and apatite. The petrological and geochemical features of the lavas which occur in the Los Volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins. The complex tectonic regime in western Mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub-arc mantle.     

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension     

Experimental constraints on the origins of primitive potassic lavas from the Trans-Mexican Volcanic Belt

Primitive magmas in the Trans-Mexican Volcanic Belt (TMVB) span a wide geochemical range that includes calc-alkaline basalt and basaltic andesite, potassic shoshonites, and intraplate alkaline basalts, indicating that the subarc mantle wedge is chemically heterogeneous. The aim of this study is to experimentally constrain the origins of potassic lavas that have erupted along the volcanic front in the TMVB. We used a piston-cylinder apparatus to determine the P–T–H₂O near-liquidus phase relations for two primitive potassic lavas: a hornblende trachybasalt (shoshonite) from Cerro La Pilita in the central TMVB and a high-K calc-alkaline basalt from Ayutla in the western TMVB. Experiments were conducted at mantle pressures (0.8–2.5 GPa) and temperatures (1,100–1,400 °C) with 1.5–6 wt% H₂O. Results show that both samples were last equilibrated with an olivine + clinopyroxene assemblage at upper mantle pressures. Integrating our results with trace element characteristics, we conclude that the potassic magmas formed by a complex, multistage process in which melts from the hottest part of the mantle wedge either reequilibrated with clinopyroxene-rich veins in the shallow upper mantle or caused melting of such veins by advective heating. We combine our results with previous experiments on TMVB lavas to provide an along-arc perspective of melt equilibration depths in the mantle wedge. The results suggest that although melts may initially form deep in the wedge, they commonly reequilibrate with heterogeneous mantle at shallower depths. Primitive, medium-K basaltic andesites in the TMVB form by reequilibration with harzburgite, which we infer to be a common lithology in the upper mantle, whereas some potassic magmas like the ones studied here form through reequilibration with or melting of veins of olivine + clinopyroxene ± phlogopite. Though somewhat rare at the volcanic front relative to the more abundant medium-K volcanic rocks, the potassic magmas are an important lava type for revealing mantle chemical heterogeneities.     

A mantle- and a lower crust-derived bimodal suite in the Yusufeli (Artvin) area, NE Turkey: trace element and REE evidence for subduction-related rift origin of Early Jurassic Demirkent intrusive complex

The Yusufeli area, in the Eastern Black Sea Region of Turkey, contains a crystalline complex that intruded into the Carboniferous metamorphic basement and is composed of two intrusive bodies: a gabbro-diorite and a tonalite-trondhjemite. The mafic body (45–57 wt% SiO₂) displays a broad lithological spectrum ranging from plagioclase-cumulate to quartz diorite. Primitive varieties of the body have Mg-number, MgO and Cr contents that are close to those expected for partial melts from mantle peridotite. Data are consistent with the magma generation in an underlying mantle wedge that was depleted in Zr, Nb and Ti, and enriched in large ion lithophile elements (K, Rb, Ba, Th). However, high Al₂O₃, CaO and generally low Ni (<65 ppm) contents are not in agreement with the unfractionated mantle-derived primitive magmas and require some Al₂O₃- and CaO-poor mafic phases, in particular, olivine and orthopyroxene. Absence of orthopyroxene in crystallization sequence, uralitization, and a common appearance of clinopyroxene surrounded by hornblende imply an anhydrous phase fractionated from highly hydrous (5–6%) parent. Geochemical modelling suggests derivation by 15–20% melting of a depleted-lherzolitic mantle. The tonalite-trondhjemite body (58–76 wt% SiO₂) ranges in composition from quartz diorite to granodiorite with a low-K calc-alkaline trend. Although LILE- and LREE- enriched characteristics of the primitive samples imply a metasomatic sub-arc mantle for their source region, low MgO, Ni and Cr concentrations rule out direct derivation from the mantle wedge. Also, lack of negative Eu anomalies suggests an unfractionated magma and precludes a differentiation from the diorites of mafic body, which show negative Eu anomalies. Their Na enrichments relative to Ca and K are similar to those of Archean tonalites, trondhjemites and granodiorites and Cenozoic adakites. However, they exhibit important geochemical differences from them, including low-Al (<15 wt%) contents, unfractionated HREE patterns and evolution towards the higher Y concentrations and lower Sr/Y ratios within the body. All these features are obtained in experimentally produced melts from mafic rocks at low pressures (≤5 kbar) and also widespread in the rocks of arc where old (Upper Cretaceous or older) oceanic crust is being subducted. Major and REE modelling supports formation of the quartz dioritic parent to the felsic intrusive rocks by 70% partial melting of a primitive gabbroic sample (G694). Therefore, once taking into account the extensional conditions prevailing in the Pontian arc crust in Early Jurassic time, former basic products (gabbros) seem to be the most appropriate source for the tonalite-trondhjemite body. Magmatic emplacement of stratigraphically similar lithologies in the Pulur Massif, just southwest of the Yusufeli, was dated to be 184 Ma by the ⁴⁰Ar/³⁹Ar method on amphibole, and is compatible with the initiation of Early Jurassic rifting in the region.     

Geochemistry and isotope systematics of calc-alkaline volcanic rocks from the Saar-Nahe basin (SW Germany) – implications for Late-Variscan orogenic development

Late Carboniferous (300–290 Ma) calc-alkaline basalts, andesites, and rhyolites typical of volcanic arc settings occur in the intermontane Saar-Nahe basin (SW Germany) within the Variscan orogenic belt. The volcanic rock suite was emplaced under a regime of tensional tectonics during orogenic collapse and its origin has been explained by melting of mantle and crust in the course of limited lithospheric rifting. We report major, trace and rare-earth-element data (REE), and Nd-Pb-Sr-O isotope ratios for a representative sample suite, which are fully consistent with an origin closely related to plate subduction. Major and trace element data define continuous melt differentiation trends from a precursor basaltic magma involving fractional crystallization of olivine, pyroxene, plagioclase, and magnetite typical of magma evolution in a volcanic arc. This finding precludes an origin of the andesitic compositions by mixing of mafic and felsic melts as can be expected in anorogenic settings. The mafic samples have high Mg numbers (Mg# = 65–73), and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating derivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical characteristics of the near-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment in the large ion lithophile elements (LILE), negative Nb and Ti, and positive Pb anomalies relative to the neighboring REE, suggesting melting of a subduction-modified mantle. Initial _Nd values of −0.7 to −4.6, Pb, and ⁸⁷Sr/⁸⁶Sr_(t) isotope ratios for mafic and felsic volcanics are similar and indicate partial melting of an isotopically heterogeneous and enriched mantle reservoir. The enrichment in incompatible trace elements and radiogenic isotopes of a precursor depleted mantle may be attributed to addition of an old sedimentary component. The geochemical characteristics of the Saar-Nahe volcanic rocks are distinct from typical post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate subduction at the margin of the Variscan orogenic belt not obvious from the regional geologic context. Received: 3 August 1998 / Accepted: 2 January 1999     

Magmatic diversity of western Mexico as a function of metamorphic transformations in the subducted oceanic plate

This geochemical study of the Mexican subduction zone elucidates how metamorphic and dehydration reactions affecting the subducted oceanic plate at different depths can influence magmatic diversity. In the western Trans-Mexican Volcanic Belt, there is a narrow potassic volcanic front running in parallel to the Middle American Trench that becomes replaced by intraplate-like high-Nb rocks to the north, and by more typical calc-alkaline products to the southeast. Potassic rocks have high MgO and are enriched in incompatible trace elements, but have lower heavy rare earth element contents than more evolved calc-alkaline and high-Nb magmas, and slightly more enriched Sr, Nd and Pb isotopes. Potassic magmas also have higher Rb/Cs and Ba/Cs ratios than the calc-alkaline and high-Nb suites, and extend to unusually high Nb/Ta ratios that correlate positively with Rb/Ta, Zr/Ta, La/Ta and Gd/Yb. These chemical variations are inconsistent with different extents of melting of a peridotitic source, but are also incompatible with melting of a phlogopite-rich mantle (vein-plus-wall-rock relationship), unless mica is totally consumed during melting, and a titaniferous phase such as rutile remains in the residue together with garnet. This assemblage is unlikely in the source region of primitive hydrous magmas, but it is what would be expected during dissolution of phengite and monazite/allanite in the subducted slab, with the concurrent formation of an anhydrous rutile-bearing eclogite. The magmatic diversity of western Mexico can thus be explained by invoking contributions of chemically different subduction agents as a function of slab depth and residual mineralogy: a low-pressure/temperature aqueous fluid would induce melting of the peridotitic mantle wedge and form typical calc-alkaline volcanoes, whereas a deeper and hotter slab-derived melt (or supercritical liquid) would contribute to the formation of potassic magmas due to phengite/monazite/allanite disintegration. In this context, intraplate-like magmas derive from decompression melting of the upper mantle as a natural consequence of subduction geodynamics.     

准噶尔盆地西北缘中拐凸起石炭纪安山岩年代学、地球化学特征及其构造意义

为加深对中亚造山带西段石炭纪构造背景的认识,对准噶尔盆地西北缘中拐凸起金龙10井区石炭纪安山岩岩心样品开展岩相学、年代学和地球化学研究。安山岩的LA-ICP-MS锆石U-Pb定年结果显示,其结晶年龄为(322.4±1.1) Ma,属中石炭世。地球化学分析结果表明,研究区安山岩属于低钾-中钾钙碱性岩石系列,其稀土元素配分曲线均略向右倾,具微弱的Eu负异常;微量元素原始地幔标准化蛛网图显示整体相对富集Pb和大离子亲石元素Rb、Ba等,亏损高场强元素Nb、Ta、Zr等,具有消减带弧火山岩的特征。安山岩的Th/La、Th/Ce和Lu/Yb值均低于大陆地壳的平均值,而接近幔源岩浆的比值。研究区安山岩属于正常的岛弧火山岩,源于富水地幔楔,是由俯冲板片脱水产生的流体交代地幔楔发生部分熔融所形成的。结合区域地质背景并对比相近时期岩浆岩,综合认为研究区安山岩形成于大陆弧环境,为西准噶尔残余洋盆俯冲消减作用的产物,且残余洋盆在(322.4±1.1) Ma时还未完全关闭。