Geochemical implications for the alteration of the Dras volcanic rocks from the ophiolites of Indus Suture Zone,Kashmir Himalaya

The Dras volcanic rocks form a part of the ophiolite belt along the Indus Suture Zone in the Kashmir Himalaya. These volcanic rocks have suffered alteration as in any other ophiolite zone. Three types of alterations spilitisation, submarine weathering and ridongitisation were suggested. The spilitic mineralogy appears to be secondary and must have developed due to the reaction of these rocks with hot sea water. Depletion of MgO and CaO and enrichment of K₂O of these rocks relate to the submarine weathering at lower temperatures. Rodingitisation effect is reflected in the chemistry of some rocks with enriched CaO and depleted SiO₂. The trace elements — Co, Cr, Ni, Cu, Pb and Rb do not show any considerable changes during alteration.     

Geochemistry of upper cretaceous volcanic rocks from the pontic chain,northern turkey

Preliminary data on major elements, Cs, Ba, Rb, Pb, Sr, REE, Y, Th, U, Zr, Ht, Sn, Nb, W, Mo, Cr, V, Sc, Ni, Co and Cu contents for eight samples coming from the Upper Cretaceous volcanic belt of the Pontic Chain (Northern Turkey) are reported. SiO, versus K₂O relationship shows that the analyzed samples belong to the calc-alkaline and shoshonite series. The calc-alkaline rocks appear to represent two distinct magma types one close in composition to typical island are calc-alkaline magmas and one with high incompatible elements concentration and tractionated heavy REE patterns which suggest a genesis by partial melting at high pressure with a garnet bearing residue. Shoshonitic rocks show Na₂O/K₂O close to one, high incompatible elements concentration, and TiO₂%. Al₂O₃%, Ni and Co contents, Ni/Co and V/Ni ratios and REE patterns similar to typical island are andesites which suggest for these rocks similar genetical processes as the island are calc-alkaline magmas.     

Geochemistry of some volcanic rocks from south-eastern sicily: Rare earth and other trace element distribution

Some trace element data for volcanic rocks found at different levels, from Tertiary to Holocene, in south-eastern Sicily (Iblean Plateau and Mt. Etna) are presented and discussed in the present paper in order to better the information about the origin and relationships of the various rock types. Four groups of volcanic rocks have been recognized on the basis of their major element chemistry: 1) low-K tholeiites, 2) associated alkali basalts to nephelinites of the Iblean Plateau (Upper Pliocene to Lower Pleistocene), 3) the basal subalkaline lavas of Mt. Etna, and 4) the alkalic suite rocks that make up the bulk of the volcano. The distribution of Rb, Sr, Ni, Cr, Co, Cu, REE, Th and Sc suggests:

1. an origin of the Iblean magmas by a different degree of partial melting of a Rb-poor and possibly slightly hetereogeneous mantle;
2. quite distinct source compositions for the Etnean magmas, relative to those of the Iblean area, on the basis of their Rb and Sr contents;
3. an origin of the alkalic rocks of Mt. Etna from independently generated magma(s) rather than by crystal fractionation of the Etnean subalkaline magmas or of a magma having the geochemical features of the Iblean alkali basalts; evidence for this is given by the distribution features of the incompatible elements showing an origin for these rocks from compositionally different parent magmas and/or an evolution under widely variable environmental conditions;
4. the primary character for the chemical differences observed in some of the Etnean subalkaline rocks that can be accounted for by different physico-chemical conditions at their source rather than by crystal fractionation processes.

     

Rare earth element and other trace element distribution,and the origin of the iblean magmas

In the Iblean region, southeast Sicily, a sequence of subaqueous and subaerial volcanics is interlayered in sedimentary levels, Upper Miocene to Lower Pleistocene in age. These rocks range from low-K tholeiites to basanites.Rare earth elements (REE) have been determined by instrumental neutron activation analysis in five samples, and other trace elements (Li, Rb, Sr, Co, Cr, Cu, Ni) by atomic absorption spectrophotometry in thirteen samples, already analyzed also for major elements.The tholeiites differ systematically from rocks of the alkalic suite for elements like Li, Sr and light REE (Sr < 200 ppm, Ce ? 15 ppm in the former; Sr > 500 up to 2000 ppm, Ce > 150 ppm in the latter), while Ni, Co, Cr and heavy REE ranges overlap in the two rock suites.The results agree in indicating different degrees of partial melting, probably at different levels in a heterogeneous mantle, as responsible for the origin of most of the rocks found in the Iblean region: the tholeiites should have been formed at relatively shallow depth by fusion of large proportions of a depleted mantle, while increasingly undersaturated volcanics of the alkalic suite have been probably generated at greater depth by partial melting of decreasing amounts of mantle material.     

Eruptive history and petrology of Mount Drum volcano,Wrangell Mountains,Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km³ of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO₂, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in ⁸⁷Sr/⁸⁶Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks. Received March 13, 1993/Accepted September 10, 1993     

87Rb-87Sr studies of waters in a geothermal area: The Cantal,France

We have determined K, Rb and Sr concentrations and⁸⁷Sr/⁸⁶Sr ratios in fresh surface waters, a rain water sample and five geothermal waters from the Cantal volcanic area in the Massif Central, France. A comparison with appropriate rock types of the region showed no apparent chemical and isotopic fractionation occurring in the fresh water-surface rock system. The thermo-mineral water results suggest that all springs discharge dissolved Sr from the following contributors: Hercynian granito-metamorphic basement, lacustrian sediments underlying the volcano, Miocene-Pliocene volcanic rocks of basaltic to rhyolitic composition.     

87Rb-87Sr constraints on the genesis and evolution of the Cantal continental volcanic system (France)

We have investigated 24 whole rocks and mineral separates of five different rock types from the Cantal shield volcano in France, applying high-precision Rb-Sr techniques. The chemical and isotopic systematics suggest the distinction of two series throughout the different rock classes, one practically uncontaminated, the other seriously influenced by wall rock assimilation. The first group comprises basalts and intermediate rocks with⁸⁷Sr/⁸⁶Sr= 0.70340–0.70382. The second group in addition includes rhyolites and the corresponding⁸⁷Sr/⁸⁶Sr ratios vary between 0.70421 and 0.71270. The data of mineral separates support the hybridization hypothesis and possibly suggest an original⁸⁷Sr/⁸⁶Sr ratio of 0.7028 for the magma source region. Moreover they provide internal isochron ages which place a period of extensive volcanic activity at 8.1–8.8 m.y. ago in accord with K-Ar ages of volcanic rocks from the center of the Cantal volcano.     

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO₂ shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial ¹⁴³Nd/¹⁴⁴Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNd_t (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.     

Electric conductivity of young basaltic rocks of Central and South-East Slovakia

Summary The electric conductivity of basaltic rocks of the final volcanic phase of the Alpine-Carpathian orogenesis was studied in the temperature interval of 200–1000°C. The results obtained are compared with the chemical and modal composition of the rocks and with the content of trace elements (Cr, Co, Ni, V). The statistical treatment of a set of 11 rocks types indicated that the electric conductivity is mostly affected by the modal composition of the rock in the temperature interval of 200–600°C, whereas the effect of trace elements can be seen distinctly in the interval of 600–1000°C.     

Origin of volcanic rocks from Stromboli (Italy)

Quaternary volcanic rocks of Stromboli (Italy) can be divided into older calc-alkaline and younger shoshonitic series. The SiO₂ contents of the rocks range from 50% to 61% but the majority of them are basalts. The rocks show systematic variations in chemical composition which correlate with the volcanic stratigraphy, such that, at a given SiO₂ content, K and other incompatible elements such as REE increase with decreasing age. In addition, the La/Yb ratio increases while the K/Rb, K/Ba, Zr/Ce and Zr/Nb ratios decrease towards the top of the volcanic pile. On the other hand, the abundances of transition elements, V, Co, Sc and Zn, like most major elements are broadly similar in comparable rocks of different ages. It is suggested that the parent magmas were derived by partial melting from upper mantle peridotite enriched in incompatible elements by fluids released from the descending oceanic lithosphere. The temporal chemical variations may probably be related to the lengths of time during which fluids were in contact with the upper mantle source.     

Geochemistry of Archaean spinifex-textured peridotites and magnesian and low-magnesian tholeiites

Major and trace element (Rb, Sr, Ba, Zr, Y, Nb, Ni, Co, V, Cr) data are presented for 11 spinifex-textured peridotites (STP) and a number of high-magnesian and low-magnesian tholeiitic basalts. The STP, representing high-magnesian liquids, come from the Yilgarn Block of Western Australia, Munro Township in the Abitibi Belt of Canada and one sample from the Barberton area of South Africa. All of the basaltic samples come from the Yilgarn Block.The STP and high-magnesian rocks are considered to belong to the komatiite suite (1, 2) despite their low CaO/Al₂O₃ ratios. It is argued that the high values (about 1.5) reported for this ratio from the Barberton area can be explained by a combination of factors, viz. garnet separation, Al loss or Ca addition during metamorphism. The processes can be evaluated using CaO/TiO₂, Al₂O₃/TiO₂ ratios, the REE group and trace elements (e.g. Y, Sc). It would appear that most STP from other Archaean belts do not have abnormal CaO/Al₂O₃ ratios.The STP display close to chondritic ratios for Ti/Zr, Zr/Nb, Zr/Y, and TiO₂/Al₂O₃ and are considered to represent liquids produced by large amounts of partial melting of the Archaean mantle. The data suggest that virtually all phases other than olivine were removed by melting during the production of STP liquids. In the STP, Ti/V, Ti/P ratios are non-chondritic, suggesting original depletion and/or incorporation into the core.For lower levels of partial melting, including mid-ocean ridge basalts (MORB) non-chondritic ratios are exhibited by Zr/Y, TiO₂/Al₂O₃, TiO₂/CaO, suggesting controlling phases in the residue for Y, Ca, Al. It is apparent that for STP, Cr is not being controlled, indicating the absence of chromite in the residual. However, at about 15% MgO the data suggest that chromite becomes a residual phase.The transition metals, with the exception of Mn, have higher abundances in Archaean basaltic rocks than in MORB. This is interpreted as being mainly due to more extensive partial melting of the mantle in the Archaean, as a result of higher temperatures.It is suggested that the generation of STP liquids with about 32% MgO is due to upwelling mantle diapirs which probably originated at depths greater than 400 km and at temperatures in excess of 1900°C.Modern equivalents to Archaean greenstone sequences are lacking. The closest tectonic analogue would be the development of oceanic crust within a rifted continental block.     

Petrology and geochemistry of potassic rocks in the Gölcük area (Isparta, SW Turkey): genesis of enriched alkaline magmas

The Lower Pliocene volcanic rocks occurring in the Gölcük area of SW Turkey exhibit alkaline major element trends with a general potassic character. The development of volcanism can be divided into 2 major stages such as trachytic ancient lavas/domes and tephriphonolitic, trachyandesitic to trachytic Gölcük eruptions (ignimbrites, lava/dome extrusions, phreatomagmatic deposits, and finally, young domes). Volcanic rocks consist primarily of plagioclase, clinopyroxene (which ranges in composition from diopside to augite and are commonly zoned), biotite, and phlogopite. Amphibole phenocrysts are restricted to the pyroclastic deposits. Pseudoleucites are also seen only in the lava/dome extrusions. Oxides and apatites are common accessory phenocryst phases. As would be expected from their potassic–alkaline nature, the volcanic rocks of the Gölcük area contain high amounts of LILE (Ba, Sr, Rb and K), LREE, and Zr. Concentrations of compatible elements such as Cr, Ni and V are very low, possibly indicating fractionation of olivine and clinopyroxene. Correlation of SiO₂, Rb/Sr and MgO with ⁸⁷Sr/⁸⁶Sr (0.703506–0.704142) exhibit an increasing trend in the direction of crustal contamination. However, the isotopic compositions of Sr are not as high to indicate a high level of crustal contamination. Geochemical data are consistent with the derivation of Gölcük volcanic rocks from a metasomatized and/or enriched lithospheric mantle source during crustal extension in the area. This metasomatism was probably occurred by fluids released from the northward subduction between African and Eurasian plates during Tertiary, as the Gölcük volcanic rocks display features of island-arc magmas with having high Ba/Nb (>28) ratios, and Nb and Ti depletions. Lower Pliocene volcanism in the Gölcük was response to extensional tectonics.     

Major and trace element provenance signatures in stream sediments from the Kando River, San'in district, southwest Japan

Abstract Basement rocks in the catchment of the Kando River in southwest Japan can be divided into two main groups. Paleogene to Cretaceous felsic granitoids and volcanic rocks dominate in the upstream section, and more mafic, mostly Miocene volcanic and volcaniclastic rocks occur in the downstream reaches. Geochemically distinctive Mount Sambe adakitic volcanic products also crop out in the west. X‐ray fluorescence analyses of major elements and 14 trace elements were made of two size fractions (<180 and 180–2000 µm) from 86 stream sediments collected within the catchment, to examine contrasts in composition between the fractions as a result of sorting and varying source lithotype. The <180 µm fractions are depleted in SiO₂ and enriched in most other major and trace elements relative to the 180–2000 µm fractions. Na₂O, K₂O, Ba, Rb and Sr are either depleted relative to the 180–2000 µm fractions, or show little contrast in abundance. Sediments from granitoid‐dominated catchments are distinguished by greater K₂O, Th, Rb, Ba and Nb than those derived from the Miocene volcanic rocks. Granitoid‐derived <180 µm fractions are also enriched in Zr, Ce and Y. Sediments derived from the Miocene volcanic rocks generally contain greater TiO₂, Fe₂O₃*, Sc, V, MgO and P₂O₅, reflecting their more mafic source. Sediments containing Sambe volcanic rocks in their source are marked by higher Sr, CaO, Na₂O and lower Y, reflecting an adakitic signature that persists into the lower main channel, where compositions become less variable as the bedload is homogenized. Normalization against source averages shows that compositions of the 180–2000 µm fractions are less fractionated from their parents than are the <180 µm fractions, which are enriched for some elements. Contrast between the size fractions is greatest for the granitoid‐derived sediments. Weathering indices of the sediments are relatively low, indicating source weathering is moderate, and typical of temperate climates. Some zircon concentration has occurred in granitoid‐derived <180 µm fractions relative to 180–2000 µm counterparts, but Th/Sc and Zr/Sc ratios overall closely reflect both provenance and homogenization in the lower reaches.     

The chemistry and origins of Proterozoic low-potash,high-iron,charnockitic gneisses from Tromøy,south Norway

New major and trace element data for 79 acid-intermediate charnockitic gneisses (the Tromøy gneisses) and 16 associated metabasites from the island of Tromøy show that this part of the 1200–900-m.y. Sveconorwegian zone is occupied by rocks of unusual composition. Overall values for K and Rb are the lowest yet reported for any granulites, and K/Rb ratios are very high. Cs and Th are also low and, abnormally for granulites, so are Ba, Sr and Zr. Ba/Sr ratios are similar to those in other suites, but K/Ba and K/Sr are higher. These features may partially be reflecting unusual pre-metamorphic lithologies, but it is considered more likely that they are largely the product of metamorphically induced depletion processes involving metasomatism. There is some indication that the Na₂O/CaO and normative Ab/An ratios may also have been modified during metamorphism.Data for the presumed relatively immobile elements Cr, Co, Ni and V support an igneous origin for the Tromøy gneisses, but the presence of a paragneiss component cannot be ruled out. A characteristic of the gneisses is their high iron content, and spatial and temporal considerations point towards a genetic link with the iron-rich, intrusive rapakivi suites of Finland, Sweden and south Greenland. If the Tromøy gneisses do represent material of this type, it would seem to follow that potash fractionation has been extreme.     

Geochemistry of Cenezoic volcanic rocks, Baja California, Mexico: Implications for the petrogenesis of post-subduction magmas

Late Cenozoic volcanism in Baja California records the effects of cessation of subduction at a previously convergent, plate margin. Prior to 12.5 m.y., when subduction along the margin of Baja ceased, the predominant volcanic activity had a calc-alkaline signature, ranging in composition from basalt to rhyolite. Acidic pyroclastic activity was common, and possibly represented the westermost, distal edge of the Sierra Madre Occidental province. After 12.5 m.y., however, the style and composition of the magmatic products changed dramatically. The dominant rock type within the Jaraguay and San Borja volcanic fields is a magnesian andesite, with up to 8% MgO at 57% SiO₂, low Fe/Mg ratios, and high Na/K ratios. These rocks have unusual trace-element characteristics, with high abundances of Sr (up to 3000 ppm), low contents of Rb; K/Rb ratios are very high (usually over 1000, and up to 2500), and Rb/Sr ratios are low (less than 0.01). Furthermore, La_n/Yb_n ratios are high, consistent with derivation from a mantle source with fractionated REE patterns. ⁸⁷Sr/⁸⁶Sr ratios are less than 0.7048, and usually less than 0.7040, whereas the pre-12.5 m.y. lavas have ⁸⁷Sr/⁸⁶Sr ratios between 0.7038 and 0.7063. We have previously termed these rocks bajaites, in order to distinguish them from other magnesian andesites. Bajaites also occur in southernmost Chile and the Aleutian Islands, areas which also have histories of attempted or successful ridge subduction.It is proposed that the bajaite series is produced during the unusual physico-chemical conditions operating during the subduction of young oceanic lithosphere, or subduction of a spreading centre. During normal subduction, the oceanic crust dehydrates, releasing volatiles (water, Rb and other large-ion lithophile elements) into the overlying wedge. Subduction of younger crust will result in a progressive decrease, and eventual cessation of the transfer of volatiles when subduction stops. Thermal rebound of the mantle may cause the slab to melt, perhaps under eclogitestable conditions. The resulting melt will be heavy-REE-depleted, perhaps dacitic, but will otherwise inherit MORB-like Rb/Sr and K/Rb ratios. The ascending melt will react with the mantle to form the source of the bajaitic rocks. Furthermore, any amphibole in the mantle, stabilised during the higher PH₂O conditions of earlier subduction, will break down and contribute a high-K/Rb ratio component.The implications of this study are that firstly, the subducted slab does not contribute a highly fractionated REE component in most modern arcs (i.e. the slab does not melt); secondly, Rb has a very short residence time in the mantle, and its abundance in arc rocks is a direct reflection of the input from the dehydrating slab; and thirdly, bajaitelike rocks may provide recognition of attempted or successful ridge subduction in the geologic past.     

Fine chronology of volcanic processes using 238U-230Th systematics

The²³⁰Th-²³⁸U radioactive disequilibrium method was applied to the study of recent volcanic rocks from Costa Rica. Most samples are from the Irazu volcano. Some samples were dated by internal isochrons using the (²³⁰Th/²³²Th)-(²³⁸U/²³²Th) diagram, others were studied only by whole rock analyses. The evolution of the parent magma may be followed by the initial (²³⁰Th/²³²Th)₀ ratios of the rocks. A model involving a differentiating magma chamber that existed for 140,000 years under the Irazu volcano correlates well with the observations. Other volcanoes seem to be in earlier stages of their evolution. Continuing study may help to solve the tholeiitic to andesitic volcanism relationship.     

Isotope geochemistry and origin of calc-alkaline lavas from a caledonian continental margin volcanic arc

Analyses for major and trace elements, including REE, and Sr, Nd and Pb isotopes are reported from a suite of Siluro-Devonian lavas from Fife, Scotland. The rocks form part of a major calc-alkaline igneous province developed on the Scottish continental margin above a WNW-dipping subduction zone. Within the small area (ca. 15 km²) considered, rock types range from primitive basalts and andesites (high Mg, Ni and Cr) to lavas more typical of modern calc-alkaline suites with less than 30 ppm Ni and Cr. There is a marked silica gap between these rocks (< 62%) and the rare rhyolites (> 74%), yet the latter can be generated by fractional crystallization from the more mafic lavas. In contrast, variation in incompatible element concentrations and ratios in the mafic lavas can not be generated by fractional crystallization processes. Increasing SiO₂ is accompanied by increasing Rb, K, Pb, U and Ba relative to Sr and high field strength elements, increasing LREE enrichment and increasing _Sr calculated at 410 Ma, and by decreasing HREE, Eu/Eu*, Sm/Nd and _Nd (410). _Nd and _Sr are roughly anticorrelated and have more radiogenic compositions than the mantle array, in common with data reported elsewhere from this part of the arc. The correlation extrapolates up to cross the mantle array within the composition field of the contemporary MORB source, and extrapolates down towards the probable compositional range of Lower Palaeozoic greywackes, which may form the uppermost 8 km of the crust, or may be supplied to the source by subduction. One sample, however, lies within the mantle array, and closely resembles lavas from northwestern parts of the arc, where a mantle source with mild time-integrated Rb/Sr and LREE enrichment has been inferred. The lavas have relatively high initial ²⁰⁷Pb/²⁰⁴Pb for their ²⁰⁶Pb/²⁰⁴Pb, a feature which has been interpreted elsewhere as the result of incorporation of a sediment component into arc magmas. The systematic changes with increasing SiO₂ in isotopic and chemical parameters can be explained by mixing of a greywacke-derived component with depleted mantle. The various possible mixing mechanisms are discussed, and it is considered most likely that mixing occurred in the mantle source through greywacke subduction. The bulk of the Rb, K, Ba and Pb in the lavas is probably recycled from the crust, whereas less than some 40% of the Sr and Nd is recycled. The calc-alkaline chemical trends are solely a function of mixing with the sediment component.     

Explosive silicic volcanism in Iceland and the Jan Mayen area during the last 6 Ma: sources and timing of major eruptions

Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ¹⁸O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K₂O/SiO₂>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island.     

Hydrothermal alteration and tectonic evolution of an intermediate- to fast-spreading back-arc oceanic crust: Late Ordovician Solund-Stavfjord ophiolite, western Norway

Abstract The Solund‐Stavfjord ophiolite complex (SSOC) in western Norway represents a remnant of the Late Ordovician oceanic lithosphere, which developed in an intermediate‐ to fast‐spreading Caledonian back‐arc basin. The internal architecture and magmatic features of its crustal component suggest that the SSOC has a complex, multistage sea floor spreading history in a supra‐subduction zone environment. The youngest crustal section associated with the propagating rift tectonics consists of a relatively complete ophiolite pseudostratigraphy, including basaltic volcanic rocks, a transition zone between the sheeted dyke complex and the extrusive sequence, sheeted dykes, and high‐level isotropic gabbros. Large‐scale variations in major and trace element distributions indicate significant remobilization far beyond that which would result from magmatic processes, as a result of the hydrothermal alteration of crustal rocks. Whereas K₂O is strongly enriched in volcanic rocks of the extrusive sequence, Cu and Zn show the largest enrichment in the dyke complex near the dyke–volcanic transition zone or within this transition zone. The δ¹⁸O values of the whole‐rock samples show a general depletion structurally downwards in the ophiolite, with the largest and smallest variations observed in volcanic rocks and the transition zone, respectively. δ¹⁸O values of epidote–quartz mineral pairs indicate 260–290°C for volcanic rocks, 420°C for the transition zone, 280–345°C for the sheeted dyke complex and 290–475°C for the gabbros. The ⁸⁷Sr/⁸⁶Sr isotope ratios show the widest range and highest values in the extrusive rocks (0.70316–0.70495), and generally the lowest values and the narrowest range in the sheeted dyke complex (0.70338–0.70377). The minimum water/rock ratios calculated show the largest variations in volcanic rocks and gabbros (approximately 0–14), and generally the lowest values and range in the sheeted dyke complex (approximately 1–3). The δD values of epidote (?1 to ?12‰), together with the δ¹⁸O calculated for Ordovician seawater, are similar to those of present‐day seawater. Volcanic rocks experienced both cold and warm water circulation, resulting in the observed K₂O‐enrichment and the largest scatter in the δ¹⁸O values. As a result of metal leaching in the hot reaction zone above a magma chamber, Zn is strongly depleted in the gabbros but enriched in the sheeted dyke complex because of precipitation from upwelling of discharged hydrothermal fluids. The present study demonstrates that the near intact effect of ocean floor hydrothermal activity is preserved in the upper part of the SSOC crust, despite the influence of regional lower greenschist facies metamorphism.     

Occurrence of oceanic plagiogranites in the older tectonic zone,Southwest Japan

K, Rb and Sr concentrations and Sr isotopic compositions were determined for the Dai granitic rocks of trondhjemitic composition occurring in a serpentinite mass in the Nagato tectonic zone formed in the Late Paleozoic era, and for the granitic rocks of quartz dioritic composition recently dredged from the seamount of the Kyushu-Palao Ridge. Both granitic rocks are characterized by low abundances of K and Rb, low K₂O/Na₂O ratios, high K/Rb ratios, low Rb/Sr ratios and low initial⁸⁷Sr/⁸⁶Sr ratios. These characteristics suggest that strong similarities may exist between the Dai granitic rocks and the dredged granitic rocks, and that the Dai granitic rocks may be classified as oceanic plagiogranite. These oceanic plagiogranites may plausibly represent single-stage mantle-derived granites, possibly from the suboceanic mantle.