Petrological and trace element geochemical features of the Okete Volcanics,western North Island,New Zealand

The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al₂O₃, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.     

Lithospheric control on late Cenozoic magmatism at the boundary of the Tuva-Mongolian Massif,Khubsugul area,northern Mongolia

Late Cenozoic lavas from the western wall of the Khubsugul rift trough were erupted within the Tuva-Mongolian Massif with a pre-Vendian basement, and the lavas in the eastern wall of the trough were erupted within Early Caledonian terranes. The composition of the lavas was determined to vary across the strike of the boundary of the Tuva-Mongolian Massif. The western wall of the trough is dominated by hawaiites and contains subordinate volumes of basanites and much lower amounts of olivine tholeiites and basaltic trachyandesites. The eastern wall contains, in addition to hawaiites, widespread olivine tholeiites and basaltic andesites with subordinate amounts of basaltic trachyandesites. The boundary zone contains practically all rock types (except basaltic andesites) in roughly equal proportions. The trace-element simulations of the partial melting processes demonstrates that the basaltic magmas were produced mainly by 0.5–5% partial melting of garnet lherzolite, with the probable mixing with partial melts derived from spinel lherzolite. The main factor controlling the compositional variations of the lavas was likely the variable depths of their derivation due to variations in the lithosphere thickness at the boundary of the Tuva-Mongolian Massif. Based on the assumption that the source of the magmas was relatively homogeneous and on the results of simulations with the use of experimental data on peridotite melting, we concluded that the asthenospheric sources of the basaltic magmas occurred at depths of 75 ± 10 km (24.6 ± 3.2 kbar) beneath the Tuva-Mongolian Massif and at 60 ± 12 km (20.1 ± 3.8 kbar) beneath the Early Caledonian terranes.     

吉林双辽七星山新生代玄武岩的特点及其成因探讨

本文通过岩石学、稀有元素及同位素地球化学等方面的研究,确认吉林双辽七星山火山是在国内含超镁铁岩包体的火山中唯一喷发于早第三纪的钠质系列碱性玄武岩火山群。该火山群中部三座山所产富橄碧玄岩系幔源原生岩浆直接喷发于地表的产物,并携有大量超镁铁岩包体;东部和西部五座山中的碱性橄榄玄武岩和粒玄岩同样来源于上地幔,但曾经历过一定程度的结晶分异作用。文中根据本区的特点和新的参数,重新计算了原生岩浆的几个判别标准。     

The relationship between the hawaiites and basalts of the itcha Volcanic Complex,central British Columbia

Volumetrically subordinate alkaline mafic lava flows form a late capping stage over the earlier felsic lavas that form the shield of the Itcha Volcanic Complex (IVC), of the Anahim Volcanic Belt (AVB) in central British Columbia (B.C.). The mafic capping stage of the IVC is dominated by hawaiites which are the earliest of the mafic lavas, and are succeeded by alkali olivine basalts (AOB) and then by basanites. The alkali olivine basalts can be subdivided into high-, intermediate- and low-MgO AOB groups, all of which share similar HFSE ratios (e.g. Nb/Zr) with the hawaiites. High Al contents and Sr/Zr ratios indicate that hawaiites and Fe-rich evolved AOB were derived from primitive AOB parental magmas by crystal fractionation of a wehrlitic assemblage at pressures on the order of 8 to 10 kbar. High Si and low Fe contents indicate that the majority of the evolved AOB lavas, however, do not represent an intermediate stage in the liquid line of descent to hawaiites, but were most likely produced by gabbroic fractionation from primitive AOB magmas at relatively low pressures. The parental magmas of the majority of these lavas were distinct from those of the observed high-MgO basalts, having higher HFSE contents and being more Si-under-saturated. The high Al, high Sr/Zr signature of high-pressure fractionation of a clinopyroxene-dominated assemblage in the IVC is shared by hawaiites of other alkaline volcanic suites of the Canadian Cordillera, such as the Edziza Volcanic Complex in northern B.C. and appears to be a feature of hawaiites in many localities, including Hawaii and Iceland. Viscosities calculated for both high- and low-pressure crystal fractionation models suggest that aphyric hawaiites are residual liquids escaped from a wehrlitic crystalline network, at elevated pressures, possibly at the base of the crust. Editorial responsibility: T.L. Grove     

Petrology,volume and age relations of alkaline and saturated peralkaline volcanics from Terceira,Azores

Four volcanoes form Terceira, one of the islands of the Azores group; three contain both basaltic and peralkaline and one only peralkaline rocks. A recently active basaltic fissure zone trends NW-SE across the island.The rocks fall into the alkaline olivine basalt suite although some young basalts are of transitional affinity. The geochemistry shows two general basaltic series: 1) undersaturated, found in lavas of the oldest volcano and in some recent fissure zone basalts and hawaiites; 2) saturated, found in the younger basaltic lavas.Since the emergence of Terceira there has been a contemporaneity of basalt and salic peralkaline lavas. The younger rocks show a bimodal composition distribution, the most voluminous compositions being alkali olivine basalt and comendite with negligible volume in the benmoreite-trachyte range. Two processes appear viable for the derivation of voluminous oversaturated peralkaline rocks: 1) partial melting of upper mantle material giving small magma batches of contrasting composition or 2) fractionation from a transitional basaltic parental magma.Now at Department of Geology, Victoria University of Wellington, New Zealand.     

Tertiary basalts and peridotite xenoliths from the Hessian Depression (NW Germany), reflecting mantle compositions low in radiogenic Nd and Sr

A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. ¹⁴³Nd/¹⁴⁴Nd ratios and ⁸⁷Sr/⁸⁶Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.     

Alkaline hybrid mafic magmas of the Yampa area,NW Colorado,and their relationship to the Yellowstone mantle plume and lithospheric mantle domains

The Yampa volcanic field (late Miocene) consists of about 70 outcrops of monogenetic cinder cones, lavas, dykes, volcanic necks and hydrovolcanic pyroclastic deposits and is situated in the most northerly part of the Rio Grande rift. Contemporaneous extension in this part of the rift was small, but there is geological and geophysical evidence that, by the late Miocene, the area was underlain by hot asthenosphere convected by the Yellowstone mantle plume. The Yampa rocks are mafic and chemically diverse, including basanites, alkali basalts, potassic trachybasalts, hawaiites and shoshonites. About half the rocks bear the xenocryst suite feldspar, pyroxene, Fe–Ti oxide, amphibole, biotite. There is a tendency for xenocryst-free rocks to be the most mafic, interpreted to indicate that the xenocrysts are cognate, and represent cumulate material from fractional crystallization of the magmas in deep crustal magma chambers. The elemental and isotopic (Nd and Sr) variations can be modelled by mixing variable proportions of partial melts of local lithospheric mantle with an OIB end-member formed by partial melting of asthenosphere. The OIB end-member appears to have the elemental and isotopic composition of typical Northern Hemisphere OIB, in particular the plume-derived basanites of Loihi seamount, Hawaii. The OIB end-member at Yampa is interpreted to have been derived from mantle convected in the Yellowstone mantle plume.     

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     

Petrology and geochemistry of the Banks Peninsula volcanoes,South Island,New Zealand

Within the volcanic sequence of the twin volcanoes of Lyttelton and Akaroa, Banks Peninsula, New Zealand a number of different magma series have been distinguished.An early series of hawaiites (McQueens Valley Formation) was erupted about 32 m.y. ago and is of transitional or mildly tholeiitic chemistry. Stratigraphically above the McQueens Valley Formation, but unconformably overlain by the main volcanic dome sequence, is a unit of rhyolite (Gebbies Pass Rhyolites) which is not directly related to the earlier or later basaltic volcanism. The rhyolite was probably formed during intracrustal melting which was related to the rise of basaltic magma into the crust.Between 12 and 9.7 m.y. a large volcanic dome, composed mainly of hawaiite, was built at Lyttelton. Dykes, which intrude the Lyttelton volcanic sequence, range in composition from basalt to trachyte. Late, mildly alkalic, basaltic flank flows (7.5–5.8 m.y.) occur in several areas and they, and the differentiated rocks of the dyke swarm can be related by a crystal fractionation model which has been quantitatively tested.Following construction of the Lyttelton dome a second larger dome was built at Akaroa between 9 and 7.5 m.y. The rocks of the Akaroa Volcano are principally hawaiites but rocks ranging in composition through to trachyte also occur. The differentiated rocks of the Akaroa volcano have derived from the basaltic rocks by a crystal fractionation controlled process, operating during ascent through the crust.None of the Banks Peninsula basalts appear to have derived from primitive (pyrolitic) mantle material, but progressive changes in the chemistry of the basalts with time implies that the mantle source regions were evolving geochemically as partial melting proceeded. Later lavas tend to be more alkalic and to have lower MgO/FeO ratios than earlier lavas. The volcanic rocks of the Banks Peninsula volcanoes were derived by fractional removal of olivine, plagioclase, clinopyroxene, magnetite and apatite from ascending basaltic magma batches. Variations between the suites reflect differences between the parental magma batches.     

Crustal contamination in early Basin-and-Range hawaiites of the Los Encinos Volcanic Field,central México

The Los Encinos Volcanic Field (LEVF) consists of Miocene (10.6–13.6 Ma) hawaiitic volcanic necks and lava-capped mesas that crop out sparsely over an area of 11,500 km² at the eastern margin of the Mexican Basin and Range Province (BRP). The LEVF rocks are similar to other early extensional hawaiites from the southern BRP, and provide numerous contrasts with younger basanites and alkali basalts that erupted during the Quaternary at the Ventura and Santo Domingo Volcanic Fields about 100 km to the south. A suite of 18 LEVF hawaiites was studied in thin section, and analyzed for mineral compositions, whole-rock major and trace element compositions, and Sr, Nd, and Pb isotopic ratios. All samples contain the stable minerals plagioclase (An_53–64), olivine (Fo_61–88), clinopyroxene, titanomagnetite, and minor biotite. Most samples also contain a complex assemblage of resorbed and reacted xenocrysts and megacrysts. Some of these minerals appear to have crystallized slowly from related, but more differentiated magmas, but other xenocrysts were clearly derived from lower-crustal, high-grade orthogneisses and paragneisses that are found as large xenoliths in the nearby Quaternary volcanic fields. Quartz xenocrysts are especially common in many hawaiites (up to 3.9 vol.%) and show a wide range of reaction styles. One sample contains microxenoliths of sillimanite- and quartz-bearing paragneiss with fine-grained domains that are interpreted as coronal structures related to original garnet xenocrysts. The geochemical effects of crustal contamination in the LEVF hawaiites vary widely. Five samples appear to be essentially uncontaminated (type U). Aside from being somewhat differentiated from a more primitive parent, the type-U samples can be used to infer the geochemistry of the mantle that was melting during the early stages of basin-and-range rifting. Type-U samples range up to ε_Nd=+7.6 and down to ⁸⁷Sr/⁸⁶Sr = 0.70286 and ²⁰⁶Pb/²⁰⁴Pb = 18.74, compositions that are more extreme than any of the nearby Quaternary volcanic rocks. The other 13 samples are divided into two contamination types, A and B. Both types show trends toward higher ⁸⁷Sr/⁸⁶Sr (to 0.7040) and ²⁰⁶Pb/²⁰⁴Pb (to 18.98), lower ε_Nd (to +3.1), and elevated Yb, which appear to reflect bulk or AFC-style contamination by granulites, particularly garnet-bearing paragneisses. Type-A hawaiites also show selective enrichments in Cs, Rb, Th, Sb, U, Pb, K, and Si. These elements were probably transferred into the type-A hawaiitic magmas through mixing with low-degree partial melts from deep-crustal granulites. The enrichments of these elements in type-A hawaiites complement the depletions of many of these same elements in high-grade granulites worldwide and provide insight into the origin of those depletions. Mixing models between type-U hawaiites and paragneiss xenoliths indicate that up to 45% of the Pb found in type-A hawaiites is crustally derived. In comparison with more mafic Quaternary basanitic rocks from the volcanic fields to the south, which carried large peridotite and granulite xenoliths to the surface, the LEVF hawaiites are relatively differentiated and megacryst rich, but free of large xenoliths, and show a wide variety of petrographic and geochemical evidence for crustal contamination. These differences probably reflect the slow and interrupted ascent of the LEVF hawaiites during early stages of basin-and-range extension in the Miocene, when the crust had a somewhat lower density and the entire lithosphere was relatively thick and cool. We argue that Quaternary basanites were able to ascend significantly faster through the thinner, hotter, and more fractured and extended lithosphere, whose crust was made denser by mafic intrusions during the preceding magmatic episode. Consequently the Quaternary basanites rose without stagnating and interacting with crustal lithologies, and without losing their entrained peridotite xenoliths. Received: 29 September 1993 / Accepted: 16 May 1994     

Rare earth contribution to the origin of Hawaiian lavas

Rare earth abundances were determined by neutron activation in twenty Hawaiian lavas and one diabase of known chemical and mineralogical compositions. These results demonstrate a systematic relationship between the absolute or relative rare earth abundances and the petrochemistry of these rocks. Three distinct lava groups are recognized. These correspond to: (1) tholeiites, (2) alkali series, (3) nepheline-melilite basalts.Based on rare earths: a) The hawaiites and mugearite of the alkali series represent residual melts derived from alkali olivine basalts, most likely by fractional crystallization; the trachyte, however, seems to have a more complicated history. b) Fractional crystallization models linking nephelinites or alkali olivine basalts to tholeiites are possible. However, production of these three lava groups, independently, by various degrees of partial melting of the mantle is equally likely and cannot be distinguished from these fractional crystallization models. c) Daly limestone syntexis hypothesis to produce the nephelinites is unlikely.     

Mantle metasomatism and magma formation in continental lithosphere: Data on xenoliths in alkali basalts from the Makhtesh Ramon,Negev desert,Israel

Mantle xenoliths (lherzolites, clinopyroxene dunites, wehrlites, and clinopyroxenites) in the Early Cretaceous volcanic rocks of Makhtesh Ramon (alkali olivine basalts, basanites, and nephelinites) represent metasomatized mantle, which served as a source of basaltic melts. The xenoliths bear signs of partial melting and previous metasomatic transformations. The latter include the replacement of orthopyroxene by clinopyroxene in the lherzolites and, respectively, the wide development of wehrlites and olivine clinopyoroxenites. Metasomatic alteration of the peridotites is accompanied by a sharp decrease in Mg, Cr, and Ni, and increase of Ti, Al, Ca contents and ³⁺Fe/²⁺Fe ratio, as well as the growth of trace V, Sc, Zr, Nb, and Y contents. The compositional features of the rocks such as the growth of ³⁺Fe/²⁺Fe and the wide development of Ti-magnetite in combination with the complete absence of sulfides indicate the high oxygen fugacity during metasomatism and the low sulfur concentration, which is a distinctive signature of fluid mode during formation of the Makhtesh Ramon alkali basaltic magma. Partial melting of peridotites and clinopyroxenites is accompanied by the formation of basanite or alkali basaltic melt. Clino- and orthopyroxenes are subjected to melting. The crystallization products of melt preserved in the mantle rock are localized in the interstices and consist mainly of fine-grained clinopyroxene, which together with Ti-magnetite, ilmenite, amphibole, rhenite, feldspar, and nepheline, is cemented by glass corresponding to quartz–orthopyroxene, olivine–orthopyroxene, quartz–feldspar, or nepheline–feldspar mixtures of the corresponding normative minerals. The mineral assemblages of xenoliths correspond to high temperatures. The high-Al and high-Ti clinopyroxene, calcium olivine, feldspar, and feldspathoids, amphibole, Ti-magnetite, and ilmenite are formed at 900–1000°. The study of melt and fluid inclusions in minerals from xenoliths indicate liquidus temperatures of 1200–1250°C, solidus temperatures of 1000–1100°C, and pressure of 5.9–9.5 kbar. Based on the amphibole–plagioclase barometer, amphibole and coexisting plagioclase were crystallized in clinopyroxenites at 6.5–7.0 kbar.     

滇西墨江西部石炭、二叠纪火山岩

墨江西部出露有石炭纪和晚二叠世两套火山岩,它们呈平行的两条带分布。石炭纪火山岩以拉斑玄武岩为主,少量流纹岩,它们是由上地幔源区10%部分熔融形成的原始岩浆,经橄榄石、辉石和斜长石不同程度结晶分异作用形成。其喷发环境可能是一个发育在大陆边缘的张性海盆地。晚二叠世火山岩较为复杂,熔岩与火山碎屑岩交互发育,熔岩主要属于拉斑系列,还含有少量钙碱性系列岩石;形成于陆缘火山弧环境。     

滇东南建水地区高镁火山岩包体的成因和构造背景

在滇东南建水地区发现产于峨眉山玄武岩中的高镁火山岩包体,这对于地幔柱的形成演化具有重要研究意义.对这些包体进行了锆石U-Pb年代学、地球化学和岩矿分析.高镁火山岩包体具斑状结构,致密块状构造,斑晶主要为贵橄榄石和透辉石.13颗锆石U-Pb LA-ICP-MS加权平均年龄为259±2Ma(MSWD=1.9),显示与寄主岩石同期形成.包体岩石具有高镁(Mg~#=68～75)、低硅(SiO_2=45.11%～45.93%)特征,轻稀土元素(LREE)、高场强元素(HFSE)富集而重稀土元素(HREE)亏损,属于亚碱性、拉斑玄武岩系列,具有板内玄武岩(IPB)特征.火山岩包体的原始岩浆起源于石榴子石二辉橄榄岩低程度部分熔融的产物,岩浆演化过程中发生了橄榄石和单斜辉石的分离结晶作用,在侵位上升过程中未受明显的地壳混染作用.该高镁火山岩的存在,显示地幔柱除了垂直上升运动外,在地球深部不同的边界还有多次侧向扩展移动,表明滇东南晚二叠世存在峨眉山地幔柱的一个分支-地幔枝活动.     

Partial melting below Tubuai (Austral Islands, French Polynesia)

Process identification diagrams based on trace element data show that mafic lavas from Tubuai, including alkali basalts, basanites, analcitites and nephelinites, result from different degrees of partial melting of an isotopically homogeneous mantle source. Our fractionation-corrected data are consistent with a batch melting model or a dynamic melting model involving a threshold value for melt separation close to 1% and degrees of melting ranging from 5–8% (alkali basalts) to 1.5–3% (nephelinites). The relative source concentration pattern, calculated using an inverse numerical method, shows an enrichment in highly incompatible elements. We propose that the Tubuai lava suite was derived from a two-stage partial melting process. Melting first affected the plume material located within the transition zone between garnet and spinel domains, producing alkali basalts and basanites. Then, the melting zone migrated upwards to the base of the overlying spinel-bearing lithospheric mantle, producing highly silica-undersaturated lavas. The lower lithosphere had previously been enriched by intrusion of pyroxenite veins representing plume-derived melts which percolated away from the main magma conduits. Received: 11 June 1996 / Accepted: 8 January 1997     

Extreme source heterogeneity and complex contamination patterns along the Cameroon Volcanic Line: New geochemical data from the Bamoun plateau

We investigated mafic and felsic volcanic rocks from the Bamoun plateau, a magmatic province located north of Mount Cameroon, in the continental part of the Cameroon Volcanic Line (CVL). Basalts and dacites were probably emplaced more than 40 Ma ago, while basanites represent very young volcanic eruptions. Among the basalts, some of them have suffered crustal contamination during their uprise through the continental crust, and their primary trace element and isotopic compositions have been slightly modified. The formation of the dacites was also accompanied by some crustal contamination. Non-contaminated rocks show that the oldest magmas are transitional basalts formed by relatively high degrees of partial melting of a moderately enriched mantle source, probably containing pyroxenites. Recent basanites were produced by very low partial melting degrees of an enriched mantle source with HIMU composition, but different from the source of the nearby Mount Cameroon lavas. The mantle beneath the CVL is thus very heterogeneous, and the tendency towards more alkaline mafic-ultramafic compositions in the youngest volcanic manifestations along the CVL seems to be a general feature of all CVL.     

Evidence for fractionation of Quaternary basalts on St. Paul Island, Alaska, with implications for the development of shallow magma chambers beneath Bering Sea volcanoes

St. Paul Island is the youngest volcanic center in the Bearing Sea basalt province. We have undertaken a field, petrographic, and geochemical study of select St. Paul volcanic rocks in order to better understand their differentiation; specifically, to test the hypothesis that magmas erupted from individual Bering Sea basaltic volcanoes are not related by shallow-level processes such as crystal fractionation. Petrographically, all of the St. Paul volcanic rocks are olivine-, plagioclase-, and clinopyroxene-phyric. Textural features and modal contents of olivine phenocrysts, however, vary widely throughout the spectrum of basalt compositions. Although differing in size and abundance, olivine phenocrysts in all rock compositions are euhedral and commonly skeletal, suggesting rapid growth during ascent or eruptive quenching. None, however, display reaction textures with surrounding groundmass liquid. Compositionally, the St. Paul volcanic rocks are basalts and tephritic basalts and all have high contents of normative nepheline (8% to 16%). Concentrations of many major and incompatible trace elements display no clear correlations with bulk-rock SiO₂ and MgO contents or modal abundances of phenocrysts, suggesting that much of the compositional diversity of these magmas reflects variable mantle sources and degrees of partial melting. Similarly, chondrite-normalized REE patterns show variable degrees of light REE enrichment (La_n=70–90) that do not correlate with bulk-rock mg-numbers. In contrast, concentrations of compatible trace elements (Ni, Cr, and Co) are positively correlated with MgO contents and modal percentages of olivine phenocrysts. Maximum forsterite contents of olivine phenocryst cores in most St. Paul rocks decrease with decreasing bulk-rock mg-number and are similar to the calculated equilibrium range. This is evidence that the high mg-numbers are magmatic and do not result from olivine accumulation. Instead, major and compatible trace element mass balance calculations support derivation of the low mg-number lavas from the high mg-number lavas mainly by olivine fractionation, which, in turn, implies that St. Paul magmas may have temporarily resided in crustal magma chambers prior to eruption.     

Origin of basaltic magmas in the Mojave Desert area,California

The basaltic lavas erupted throughout the Mojave Desert are basanites (SiO₂<46%, normative nepheline>5%, and K₂O>1.5%), alkali-olivine basalts (SiO₂=46–48%; ne=0–5%; and K₂O=1.0–1.5%), and low-alumina, sub-alkaline basalts (SiO₂=48–51%; ne=0; K₂O<1.0%). One volcano, Pisgah Crater, erupted five times, with lava from each successive phase containing more silica and less potash than the one proceeding it. This compositional trend is the reverse of that expected from differentiation of a single alkalic magma, and therefore, may represent a succession of magmas tapped from a zone of continuing partial melting in the mantle.These lava compositions suggest that first melting was under high water pressure and was followed by relatively dry partial melting of gamet-orthopyroxene-clinopyroxene-olivine assemblages. The successive increase in silica and alkali decrease also requires that the partial melting zone move to shallower levels.All lavas sampled in the Mojave Desert area have compositions that can best be explained by the extraction of magma from such a rising melting zone, analogous to the mantle diapirs suggested by Green and Ringwood.     

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     

Early Cretaceous magmatism of Mount Hermon, Northern Israel

 Early Cretaceous (146–115 Ma) magmatism in the region of Mt. Hermon, Northern Israel, is part of an extensive Mesozoic igneous province within the Levant associated with the evolution of the Neotethyan passive margin of Gondwana. The initial stages of activity were characterised by the emplacement of tholeiitic dykes (146–140 Ma) which were uplifted and eroded prior to the eruption of a sequence of alkali basalts, basanites and more differentiated alkaline lavas and pyroclastics from 127 to 120 Ma. The latest stages of activity (120–115 Ma) were highly explosive, resulting in the emplacement of diatreme breccias. Trace element and Sr-Nd-Pb isotope data for the most primitive Early Cretaceous mafic igneous rocks sampled suggest that they were derived by mixing of melts derived by variable degrees of partial melting of both garnet- and spinel-peridotite-facies mantle sources. Though isotopically heterogeneous, the source of the magmas has many similarities to that of HIMU oceanic island basalts. Earlier Liassic (200 Ma) transitional basalts and Neogene–Quaternary (15–0 Ma) alkali basalts erupted within northern Israel also have HIMU affinities. The petrogenesis of the Early Cretaceous and Cenozoic basalts is explained by partial melting of a lithospheric mantle protolith metasomatically enriched during the Liassic volcanic phase, which may be plume-related. Received: 23 July 1998 / Accepted: 6 December 1999