Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

Geochemistry and origin of late archean volcanic rocks from the rhenosterhoek formation,dominion group,South Africa

The Rhenosterhoek Formation, composed chiefly of subaerial volcanic rocks, is part of the Dominion Group (∼ 2.8 Ga) which rests unconformably on older continental crust in South Africa. Chemical compositions of volcanic rocks from three drillcores of this formation do not show systematic differences either between or within cores. Major and immobile trace-element contents indicate that the volcanics are basaltic andesites and andesites with calc-alkaline affinities. Incompatible trace-element distributions are similar to those of modern andesites from evolved island arcs or continental-margin arcs.Geochemical modeling indicates that the basaltic andesites and andesites can be produced by open-system fractional crystallization with olivine, clinopyroxene and plagioclase as principal liquidus phases. Up to 10% contamination with continental crust is also allowed by the data. This succession does not appear to have formed in an oceanic or in a continental-margin arc, but may have formed adjacent to a continental-margin arc system in an incipient foreland basin on the Kaapvaal Craton.     

The Dir-Utror metavolcanic sequence,Kohistan arc terrane,northern Pakistan

The Dir-Utror volcanic series forms a NE–SW trending belt within the northwestern portion of the Kohistan island arc terrane in the western Himalayas of northern Pakistan. The Kohistan arc terrane comprises a diverse suite of volcanic, plutonic, and subordinate sedimentary rocks of late Mesozoic to Tertiary age, developed prior to and after suturing of the Indo-Pakistan and Asiatic continental blocks. The Dir-Utror volcanic series near Dir is dominated by basaltic-andesite and andesite, with subordinate basalt, high-MgO basalt, dacite, and rhyolite. Porphyritic textures are dominant, with less common aphyric and seriate textures. Plagioclase is the dominant phenocryst in mafic to intermediate rocks, K-feldspar and quartz phenocrysts predominate in the dacites and rhyolites. Chlorite, epidote, albite, and actinolite are the most common metamorphic phases; blue-green amphibole, andesine, muscovite, biotite, kaolinite, sericite, carbonate, and opaques are widespread but less abundant. Phase assemblages and chemistry suggest predominant greenschist facies metamorphism with epidote-amphibolite facies conditions attained locally.Whole rock major element compositions define a calc-alkaline trend: CaO, FeO, MgO, TiO₂, Al₂O₃, V, Cr, Ni, and Sc all decrease with increasing silica, whereas alkalis, Rb, Ba, and Y increase. MORB-normalized trace element concentrations show enrichment of the low-field strength incompatible elements (Ce, La, Ba, Rb, K) and deep negative Nb, P, and Ti anomalies—patterns typical of subduction related magmas. Mafic volcanic rocks plot in fields for calc-alkaline volcanics on trace element discrimination diagrams, showing that pre-existing oceanic crust is not preserved here. All rocks are LREE-enriched, with La=16–112×chondrite, La/Lu=2.6–9.8×chondrite, and Eu/Eu*=0.5–0.9. Dacites and rhyolites have the lowest La/Lu and Eu/Eu* ratios, reflecting the dominant role of plagioclase fractionation in their formation. Some andesites have La/Lu ratios which are too high to result from fractionation of the more mafic lavas; chondrite-normalized REE patterns for these andesites cross those of the basaltic andesites, indicating that these lavas cannot be related to a common parent.The high proportion of mafic lavas rules out older continental crust as the main source of the volcanic rocks. The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is mafic relative to normal continental margins. Andesites with high La, La/Lu, K₂O, and Rb may be crustal melts; we suggest that garnet-rich high-pressure granulites similar to those exposed in the Jijal complex may be restites formed during partial melting of the crust.     

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     

Variation in the geochemistry of mantle sources for tholeiitic and calc-alkaline mafic magmas,Western Sunda volcanic arc,Indonesia

Quaternary lavas of the normal island-arc basalt—andesite—dacite association in the islands of Java and Bali range from those belonging to tholeiitic series over Benioff-zone depths of ～ 150 km to high-K calc-alkaline series over Benioff-zone depths of 250 km. More abundant and diverse calc-alkaline lavas are found over intermediate Benioff-zone depths. On average, basaltic lavas become slightly more alkaline (largely due to increased K contents) with increasing depth to the Benioff zone. Levels of incompatible minor and trace elements (K, Rb, Cs, Ba, Nb, U, Th, light REE) show a corresponding increase of almost an order of magnitude.Low average Mg-numbers (～ 0.52) and Ni and Cr abundances (15–25 and 35–60 ppm, respectively) of basaltic lavas suggest that few lavas representing primary mantle-derived magma compositions are present. Calculated primary basaltic magma compositions for most tholeiitic and calc-alkaline volcanic centres are olivine tholeiites with 15–30% ol. The single high-K calc-alkaline centre considered yielded transitional alkali olivine basalt—basanite primary magma compositions. These calculated magma compositions suggest that the percentage of mantle melting decreases with increasing depth to the Benioff zone (from >25 to <10%), while the corresponding depth of magma separation increases from ～ 30 to 60 km.Calculation of REE patterns for basaltic magmas on the basis of peridotitic mantle sources with spinel lherzolite, amphibole lherzolite or garnet lherzolite mineralogy, and model REE levels of twice chondritic abundances, indicates that change in the conditions of magma genesis alone cannot explain the observed change in light-REE abundances of basaltic lavas with increasing depth to the Benioff zone. Complementary calculations of the REE levels of mantle sources required to yield the average tholeiitic, calc-alkaline and high-K calc-alkaline basaltic magma indicate that light-REE abundances must increase from 2–3 to 7–8 times chondrites with increasing depth to the Benioff zone. The percentages of mantle melting favoured on REE evidence are lower than those indicated by major-element considerations.The observed variation in incompatible element geochemistry of mantle magma sources is thought to be related directly or indirectly to dehydration and partial-melting processes affecting subducted oceanic crust. The possible nature of this relationship is discussed.     

On the origin of andesite in the northern Mariana Island Arc: Implications from Agrigan

Magmatic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt. Basaltic products predominate, but the ratio of andesites to basalts increases with time up to an event of voluminous andesitic pyroclastic ejection accompanied by caldera-collapse; post-collapse lavas are entirely basaltic. Moderate iron-enrichment is demonstrated for the volcanic suite, with indications of a progressive, pre-caldera decrease in iron-enrichment; post-caldera lavas display a return to moderate Fe-enrichment. Overall, the lavas are enriched in the LIL elements (K, Rb, Ba, Sr) and depleted in Ti, Mg, Cr, and Ni. From the oldest to the youngest pre-caldera volcanic sequence, the LIL elements increase 3-6X while Ca and Mg decrease by 50% or more. Approximately constant K/ Rb (430±60) and ⁸⁷Sr/⁸⁶Sr (0.7032–0.7034) indicate consanguinity of the basalts and the andesites. Cumulate plutonic xenoliths, common in the lavas, are composed of mineral phases also encountered as phenocrysts. The following order of crystallization is indicated: olivine; anorthite-bytownite; clinopyroxene; orthopyroxene and titanomagnetite. Co-existing xenolithic olivines (Fo_74–83) and plagioclase (An_88–96) are typical of calc-alkaline island-arc assemblages and contrast with assemblages in the tholeiites from the Mariana Trough to the west. The relatively fayalitic composition and low abundances of Ni in olivines and Cr in clinopyroxenes indicate equilibrium with an already-fractionated liquid. These data, along with structural evidence, high Ca in the olivines, and comparison of the observed assemblages with experimental studies, suggests that these xenoliths formed as crystal cumulates at the floor of a shallow ( 7 km) crustal magma chamber.Major element modeling studies using the separation of observed xenocrystic and phenocrystic phases from assumed parental liquids reproduce the observed temporal and geochemical variations in the lavas. Trace element modeling parallels this evolution with the exception of Cr and Ni in the andesites. An extensive (16.3 km³) gabbroic body is required by this modeling to be present beneath Agrigan to produce the inferred volumes of the various lithologies preserved in the volcano's evolution. The sum of stratigraphic, geochemical, and isotopic evidence on Agrigan supports the derivation of calc-alkaline andesite by the removal of about 75% solids from a high-alumina basalt accompanied by a process of K and Rb enrichment, such as volatile-transfer. Considerations of ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and ³He/⁴He isotopic data indicate that the source region of these parental liquids lies in the mantle, not subducted crust. In the northern Marianas, the model of a shallow-crustal origin for andesite is preferred over one requiring andesite generation in the deeper mantle and/or subducted slab.     

扬子地台北缘白勉峡组和三湾组火山岩形成构造环境及岩石成因的地球化学约束

依据中基性火山岩主量和微量元素地球化学特征的差异,白勉峡组可分两部分,一部分火山岩TiO_2大于1%,变质程度较高,主要分布在下段;另一部分火山岩TiO_2小于1%,变质程度较浅,主要分布在上段.下段火山岩属拉斑玄武岩系列,上段主体属钙碱系列,稀土总量高(∑REE=83.4～180.8μg/g),轻重稀土分异较低(LREE/HREE=2.17～5.85),有弱的Eu负异常(δEu=0.79～1.01),微量元素原始地幔蛛网图显示有弱的Nb、Ta亏损,具有板内火山岩的地球化学特点,形成于板内裂谷环境.上段火山岩稀土总量低(∑REE=40.3～82.4μg/g),轻重稀土分异较高(LREE/HREE=2.3～7.6),无Eu负异常(δEu=0.90～1.11),微量元素原始地幔蛛网图发育明显的Nb-Ta槽和Zr-Hf槽,Ti、Sr发育较强的低谷,具有典型岛弧玄武岩的地球化学特点,形成于岛弧或大陆边缘弧环境.三湾组玄武岩和安山岩稀土元素分配型式呈LREE亏损的左倾型或呈近平坦型,类似于N-MORB,明显不同于白勉峡组,岩石组合和地球化学特点类似于弧后盆地火山岩.火山岩及相关侵入岩LA-ICPMS锆石U-Pb定年及元素及Sr-Nd同位素地球化学研究揭示,白勉峡组下段火山岩形成时代可能为1144Ma,其源区为与洋岛玄武岩类似的软流圈地幔源区,部分熔融发生在石榴子石二辉橄榄岩稳定区,岩浆在演化过程中经历了一定分离结晶作用(分离结晶矿物为斜长石+单斜辉石)和地壳混染作用.白勉峡组上段火山岩形成时代可能为437Ma,有可能跨到晚古生代,其源区为受俯冲作用改造的富集地幔区,部分熔融亦发生于石榴子石二辉橄榄岩稳定区.三湾组中基性火山岩源于N-MORB近似的亏损地幔源区.白勉峡组下段代表中元古代末板内拉张事件的地质记录,白勉峡组上段和三湾组目前的火山岩样品可能代表了古生代同一洋陆转化的地质记录.     

Generation of calc-alkaline andesite of the Tatun volcanic group (Taiwan) within an extensional environment by crystal fractionation

The Pliocene–Pleistocene northern Taiwan volcanic zone (NTVZ) is located within a trench-arc–back-arc basin and oblique arc–continent collision zone. Consequently the origin and tectonic setting of the andesitic rocks within the NTVZ and their relation to other circum-Pacific volcanic island-arc systems is uncertain. Rocks collected from the Tatun volcanic group (TTVG) include basaltic to andesitic rocks. The basalt is compositionally similar to within-plate continental tholeiites whereas the basaltic andesite and andesite are calc-alkaline; however, all rocks show a distinct depletion of Nb-Ta in their normalized incompatible element diagrams. The Sr-Nd isotope compositions of the TTVG rocks are very similar and have a relatively restricted range (i.e. I_Sr = 0.70417–0.70488; εNd_(T) = +2.2 to +3.1), suggesting that they are derived directly or indirectly from the same mantle source. The basalts are likely derived by mixing between melts from the asthenosphere and a subduction-modified subcontinental lithospheric mantle (SCLM) source, whereas the basaltic andesites may be derived by partial melting of pyroxenitic lenses within the SCLM and mixing with asthenospheric melts. MELTS modelling using a starting composition equal to the most primitive basaltic andesite, shallow-pressure (i.e. ≤1 kbar), oxidizing conditions (i.e. FMQ +1), and near water saturation will produce compositions similar to the andesites observed in this study. Petrological modelling and the Sr-Nd isotope results indicate that the volcanic rocks from TTVG, including the spatially and temporally associated Kuanyinshan volcanic rocks, are derived from the same mantle source and that the andesites are the product of fractional crystallization of a parental magma similar in composition to the basaltic andesites. Furthermore, our results indicate that, in some cases, calc-alkaline andesites may be generated by crystal fractionation of mafic magmas derived in an extensional back-arc setting rather than a subduction zone setting.     

Geochemical Constrains on Petrogenesis and Tectonic Setting of Volcanic Rocks from the Baimianxia and Sanwan Formations in the Northern Margin of the Yangtze Plate

On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units: high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which occurs in the lower part with high-grade metamorphism, but the latter is less than 1% and crops out in the upper part with low-grade metamorphism. The high-TiO2 unit is dominated by tholeiitic lavas showing high rare earth element (REE) contents (ΣREE?=?83.4–180.8?μg/g), high light/heavy REE (LREE/HREE) ratios (LREE/HREE=2.17–5.85) and weak negative Eu anomaly (Eu=0.79–1.01). Its trace element patterns display weak Nb-Ta anomalies with respect to Th, K, La, Ce, showing within-plate basalt affinities. In contrast, the low-TiO2 unit is characterized by low REE contents, low LREE/HREE ratios, and pronounced Nb-Ta anomalies, indicating typical arc or continental arc signature. Chondrite-normalized REE patterns of basalts and andesites from the Sanwan Formation are flat or LREE depletion, which is very similar to normal mid-oceanic basalt. Therefore, we suggest that these lavas should be formed in a back-arc basin setting. Sr-Nd isotopic data of the basalt in the lower part suggest that the rocks would have been formed in ~1144?Ma. Based on the geochemical and isotopic features of the basalts, we suggest that these rocks in the low part of the Baimianxia Formation should originate from an asthenospheric oceanic-island basalt-like mantle source, which may be produced by partial melting of garnet lherzolite, and significantly underwent fractional crystallization and crustal or lithospheric mantle contamination en route to the surface. However, laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating of the basalt sample from the upper part of the Baimianxia Formation gives a 437 Ma, indicating a Early Paleozoic age. The geochemical analysis in this paper suggests that they may originate from an arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in the garnet stability field. The samples of basalts and andesites in the Sanwan Formation show they are derived from depleted mantle source similar to normal mid-oceanic basalt. Finally, we can conclude that the lavas in the lower part of the Baimianxia Formation represent the geological records of rift-related volcanism in the middle Proterozoic, which is commonly considered to be the precursor of continental breakup and followed by oceanic basin forming from Neoproterozoic to early Paleozoic. Whereas, the lavas in upper part of the Baimianxia Formation and Sanwan Formations may have been generated by the oceanic and continental conversion that occurred in the early Paleozoic.     

大陆的起源

太阳系固体星球都有类似的核-幔-壳结构,但唯独人类居住的地球具有长英质组成的大陆壳.太古宙大陆克拉通主要由英云闪长岩(Tonalite)-奥长花岗岩(Trondhjemite)-花岗闪长岩(Granodiorite)为主的TTG深成侵入体变质而成的正片麻岩和由基性-超基性酸性火山岩及少量沉积岩变质的表壳岩(绿岩)组成....     

Diverse mantle and crustal components in lavas of the NW Cerros del Rio volcanic field,Rio Grande Rift,New Mexico

Products of Pliocene (2–4 Ma) mafic to intermediate volcanism in the northwestern Cerros del Rio, a dominantly mafic volcanic field in the Española Basin of the Rio Grande Rift (RGR), range from 49% to 63% SiO₂ and exhibit diversity in silica saturation, trace-element patterns, and isotopic compositions. Tholeiites, which are largely confined to west of the Rio Grande, have trace-element abundances that resemble those of oceanic basalts, but with mild depletions in Nb and Ta, and high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd, and high δ¹⁸O compared to typical OIB. They are regarded as asthenospherically-derived magmas contaminated with continental crust. Alkali basalts and hawaiites erupted from vents east of the Rio Grande are geochemically distinct, having generally higher overall incompatible-element abundances, but with pronounced depletions in K, Rb, Nb and Ta with respect to Th and LREE. Spatially-associated benmoreites, mugearites and latites (collectively termed “evolved” lavas) have similar trace-element characteristics to the mafic mildly-alkaline compositions, but are typically not as depleted in K. Hawaiites and evolved lavas exhibit a good negative correlation of ¹⁴³Nd/¹⁴⁴Nd with SiO₂, due to interaction with lower continental crust. The most silicic “evolved” lavas carry the highest proportions of crustal material, and consequently have higher K/Th than the related hawaiites. Several (mostly mafic) lavas contain abundant crustally-derived resorbed quartz xenocrysts in O-isotope disequilibrium with the host magma. The δ¹⁸O values of xenocrystic quartz range over 4‰, indicating a variety of quartz-bearing crustal contaminants beneath the Española Basin. The hawaiites, with their unusual combination of trace-element enrichments and depletions, cannot be generated by any process of fractionation or crustal contamination superposed on a common mantle source type (oceanic or arc-source). It is a regional mantle source type, inasmuch as it was also present beneath NW Colorado during the mid-late Cenozoic. We argue that the hawaiite source must have originally existed as arc-source mantle enriched in LILE, generated during Mesozoic to early Cenozoic subduction at the western margin of North America. This arc-source mantle lost K, Rb and Ba, but not Th or LREE, prior to magmagenesis. Selective element loss may have occurred during lithospheric thinning and uprise of hydrated phlogopitebearing peridotite-possibly as a thermal boundary layer between lithosphere and asthenosphere — to shallow mantle depths, with consequent conversion of phlogopite to amphibole (an inferior host for K, Rb and Ba). We suggest that this occurred during the early extensional phase of the northern RGR. Further extension was accompanied by partial melting and release of magma from this source and the underlying asthenosphere, which by the Pliocene was of oceanic type. The hawaiite source mantle is the product of a long history of subduction succeeded by lithospheric extension of the formerly overriding plate. Similar chemical signatures may have developed in the mantle beneath other regions with comparable histories.     

The Aurora volcanic field,California-Nevada: oxygen fugacity constraints on the development of andesitic magma

 The Aurora volcanic field, located along the northeastern margin of Mono Lake in the Western Great Basin, has erupted a diverse suite of high-K and shoshonitic lava types, with 48 to 76 wt% SiO₂, over the last 3.6 million years. There is no correlation between the age and composition of the lavas. Three-quarters of the volcanic field consists of evolved (<4 wt% MgO) basaltic andesite and andesite lava cones and flows, the majority of which contain sparse, euhedral phenocrysts that are normally zoned; there is no evidence of mixed, hybrid magmas. The average eruption rate over this time period was ∼200 m³/km²/year, which is typical of continental arcs and an order of magnitude lower than that for the slow-spreading mid-Atlantic ridge. All of the Aurora lavas display a trace-element signature common to subduction-related magmas, as exemplified by Ba/Nb ratios between 52 and 151. Pre-eruptive water contents ranged from 1.5 wt% in plagioclase-rich two-pyroxene andesites to ∼6 wt% in a single hornblende lamprophyre and several biotite-hornblende andesites. Calculated oxygen fugacities fall within –0.4 and +2.4 log units of the Ni-NiO buffer. The Aurora potassic suite follows a classic, calc-alkaline trend in a plot of FeO^T/MgO vs SiO₂ and displays linear decreasing trends in FeO^T and TiO₂ with SiO₂ content, suggesting a prominent role for Fe-Ti oxides during differentiation. However, development of the calc-alkaline trend through fractional crystallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the Ni-NiO buffer. Nor can fractionation of hornblende be invoked, since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron, together with a thermal stability limit that is lower than the eruption temperatures of several andesites (∼1040–1080°C; derived from two-pyroxene thermometry). Unless some progressive oxidation process occurs, fractionation of titanomagnetite or hornblende cannot explain a calc-alkaline trend in which all erupted lavas have oxygen fugacites ≥ the Ni-NiO buffer. In contrast to fractional crystallization, closed-system equilibrium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt. However, the eruption of nearly aphryic lavas argues against tapping from a magma chamber during equilibrium crystallization, a process that requires crystals to remain in contact with the liquid. A preferred model involves the accumulation of basaltic magmas at the mantle-crust interface, which solidify and are later remelted during repeated intrusion of basalt. As an end-member case, closed-system equilibrium crystallization of a basalt, followed by equilibrium partial melting of the gabbro will produce a calc-alkaline evolved liquid (namely, high SiO₂ and low FeO^T/MgO) with a relative f _{O 2} (corrected for the effect of changing temperature) that is similar to that of the initial basalt. Differentiation of the Aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large, stable magma chambers is consistent with the low eruption rate at the Aurora volcanic field. Received: 7 July 1995 / Accepted: 19 April 1996     

The Xiong'er volcanic belt at the southern margin of the North China Craton: Petrographic and geochemical evidence for its outboard position in the Paleo-Mesoproterozoic Columbia Supercontinent

The Xiong'er volcanic belt, covering an area of more than 60,000 km² along the southern margin of the North China Craton, has long been considered an intra-continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the Paleo-Mesoproterozoic supercontinent Columbia. However, such interpretations cannot be accommodated by lithology, mineralogy, geochemistry and geochronology of the volcanic rocks in the belt. Lithologically, the Xiong'er volcanic belt is dominated by basaltic andesite and andesite, with minor dacite and rhyolite, different from rock associations related to continental rifts or mantle plumes, which are generally bimodal and dominated by mafic components. However, they are remarkably similar to those rock associations in modern continental margin arcs. In some of the basaltic andesites and andesites, amphibole is a common phenocryst phase, suggesting the involvement of H₂O-rich fluids in the petrogenesis of the Xiong'er volcanic rocks. Geochemically, the Xiong'er volcanic rocks fall in the calc-alkaline series, and in most tectono-magmatic discrimination diagrams, the majority of the Xiong'er volcanic rocks show affinities to magmatic arcs. In the primitive mantle normalized trace-element diagrams, the Xiong'er volcanic rocks show enrichments in the LILE and LREE, and negative Nb–Ta–Ti anomalies, similar to arc-related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge. Nd-isotope compositions of the Xiong'er volcanic rocks suggest that 5–15% older crust has been transferred into the upper lithospheric mantle by subduction-related recycling during Archean to Paleoproterozoic time. Available SHRIMP and LA-ICP-MS U–Pb zircon age data indicate that the Xiong'er volcanic rocks erupted intermittently over a protracted interval from 1.78 Ga, through 1.76–1.75 Ga and 1.65 Ga, to 1.45 Ga, though the major phase of the volcanism occurred at 1.78–1.75 Ga. Such multiple and intermittent volcanism is inconsistent with a mantle plume-driven rifting event, but is not uncommon in ancient and existing continental margin arcs. Taken together, the Xiong'er volcanic belt was most likely a Paleo-Mesoproterozoic continental magmatic arc that formed at the southern margin of the North China Craton. Similar Paleo-Mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of Laurentia, the southern margin of Baltica, the northwestern margin of Amonzonia, and the southern and eastern margins of the North Australia Craton, which are considered to represent subduction-related episodic outbuilding on the continental margins of the Paleo-Mesoproterozoic supercontinent Columbia. Therefore, in any configuration of the supercontinent Columbia, the southern margin of the North China Craton could not have been connected to any other continental block as proposed in a recent configuration, but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the North China Craton.     

The Harrat Al-Birk basalts in southwest Saudi Arabia: characteristic alkali mafic magmatism related to Red Sea rifting

Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~5 Ma. This volcanic field is almost monotonous and is dominated by basalts that include mafic–ultramafic mantle xenoliths (gabbro, websterite, and garnet-clinopyroxenite). The present work presents the first detailed petrographic and geochemical notes about the basalts. They comprise vesicular basalt, porphyritic basalt, and flow-textured basalt, in addition to red and black scoria. Geochemically, the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti, sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment. There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe–Ti oxides. The latter have abundant titanomagnetite and lesser ilmenite. There is a remarkable enrichment of light rare earth elements and depletion in Ba, Th and K, Ta, and Ti. The geochemical data in this work suggest Harrat Al-Birk basalts represent products of water-saturated melt that was silica undersaturated. This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts. Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites. The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe–Ti oxides, particularly ilmenite. Accordingly, the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths (e.g. Nemeth et al. in The Pleistocene Jabal Akwa Al Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region, SW Saudi Arabia 2014).     

An Ordovician intra-oceanic subduction system influenced by ridge subduction in the West Junggar,Northwest China

We report elemental and Nd–Sr isotopic data for three types of Ordovician volcanic and gabbroic rocks from the Sharburti Mountains in the West Junggar (Xinjiang), Northwest China. Gabbros and Type I lavas occur in the Early Ordovician Hongguleleng ophiolite whereas Type II and III lavas are parts of the Middle Ordovician Bulukeqi Group. Gabbros and Type I lavas are tholeiites with a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a crystallization sequence of plagioclase–clinopyroxene, suggesting formation at a mid-oceanic ridge. Type II lavas are Nb-enriched basalts (NEBs, Nb = 14–15 ppm), which have E-MORB-like REE patterns and Nb/Yb and Th/Yb ratios. They come from mantle metasomatized by slab melts. Type III lavas are further divided into two sub-types: (1) Type IIIa is tholeiitic to calc-alkaline basalts and andesites, with REE patterns that are flat or slightly LREE enriched, and with a negative Nb anomaly and Th/Yb enrichment, indicating that they were generated above a subduction zone; (2) Type IIIb is calc-alkaline basalts and andesites, which are strongly enriched in LREE with a marked negative Nb anomaly and Th/Yb enrichment, suggesting generation in a normal island-arc setting. The initial ⁸⁷Sr/⁸⁶Sr ratios of Type III lavas range from 0.70443 to 0.70532 and ?Ndt ranges from +1.5 to +4.5, suggesting that these melts were derived from mantle wedge significantly modified by subducted material (enriched mantle I (EMI)) above a subduction zone. Contemporary tholeiitic to calc-alkaline basalt–andesite and NEB association suggest that the NEBs erupted during development of the tholeiitic to calc-alkaline arc. We propose a model of intra-oceanic subduction influenced by ridge subduction for the Ordovician tectono-magmatic evolution of the northern West Junggar.     

Geochemical constraints on the origin of the Hegenshan Ophiolite,Inner Mongolia,China

The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44–54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72–76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc–marginal basin system that was later accreted to the southern margin of the Siberian Altaids.     

Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)

Geochemical studies of long-lived volcanic complexes are crucial for the understanding of the nature and composition of the subduction component of arc magmatism. The Pichincha Volcanic Complex (Northern Andean Volcanic Zone) consists of: (1) an old, highly eroded edifice, the Rucu Pichincha, whose lavas are mostly andesites, erupted from 1,100 to 150 ka; and (2) a younger, essentially dacitic, Guagua Pichincha composite edifice, with three main construction phases (Basal Guagua Pichincha, Toaza, and Cristal) which developed over the last 60 ka. This structural evolution was accompanied by a progressive increase of most incompatible trace element abundances and ratios, as well as by a sharp decrease of fluid-mobile to fluid-immobile element ratios. Geochemical data indicate that fractional crystallization of an amphibole-rich cumulate may account for the evolution from the Guagua Pichincha andesites to dacites. However, in order to explain the transition between the Rucu Pichincha andesites and Guagua Pichincha dacites, the mineralogical and geochemical data indicate the predominance of magma mixing processes between a mafic, trace-element depleted, mantle-derived end-member, and a siliceous, trace-element enriched, adakitic end-member. The systematic variation of trace element abundances and ratios in primitive samples leads us to propose that the Rucu Pichincha magmas came from a hydrous-fluid metasomatized mantle wedge, whereas Guagua Pichincha magmas are related to partial melting of a siliceous-melt metasomatized mantle. This temporal evolution implies a change from dehydration to partial melting of the slab, which may be associated with an increase in the geothermal gradient along the slab due to the presence of the subducted Carnegie Ridge at the subduction system. This work emphasizes the importance of studying arc-magma systems over long periods of time (of at least 1 million of years), in order to evaluate the potential variations of the slab contribution into the mantle source of the arc magmatism.     

Geochemical constraints on the genesis of the volcanic rocks in the southeast of Isfahan area, Iran

Late Miocene–Pliocene to Quaternary calc-alkaline lava flows and domes are exposed in southeast of Isfahan in the Urumieh Dokhtar magmatic belt in the Central Iran structural zone. These volcanic rocks have compositions ranging from basaltic andesites, andesites to dacites. Geochemical studies show these rocks are a medium to high K calc-alkaline suite and meta-aluminous. Major element variations are typical for calc-alkaline rocks. The volcanic rocks have SiO₂ contents ranging between 53.8% and 65.3%. Harker diagrams clearly show that the dacitic rocks did not form from the basaltic andesites by normal differentiation processes. They show large ion lithophile elements- and light rare earth elements (LREE)-enriched normalized multielement patterns and negative Nb, Ti, Ta, and P. Condrite-normalized REE patterns display a steep decrease from LREE to light rare earth elements without any Eu anomaly. These characteristics are consistent with ratios obtained from subduction-related volcanic rocks and in collision setting. The melting of a heterogeneous source is possible mechanism for their magma genesis, which was enriched in incompatible elements situated at the upper continental lithospheric mantle or lower crust. The geochemical characteristics of these volcanic rocks suggested that these volcanic rocks evolved by contamination of a parental magma derived from metasomatized upper lithospheric mantle and crustal melts.     

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     

Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle.