Nomenclature and petrochemistry of the peralkaline oversaturated extrusive rocks

Four-hundred and twenty-one analyses of quartz-normative, peralkaline, extrusive rocks have been collected from the literature and from unpublished sources and are used to examine chemical variation in this group of rocks. Comparisons are particularly made between the full body of data and the variations recorded in the non-hydrated obsidians alone byMacdonald andBailey (1973). It is argued that the compositions of the magmas which formed these obsidians and those which subsequently crystallised were similar as regards the major oxides SiO₂, Al₂O₃, FeO + Fe₂O₃, Na₂O and K₂O. Marked variations in the abundances of the minor oxides CaO and TiO₂ are shown to be a result of geographical location. Small but significant differences in the distribution of Al and Fe as a function of normative quartz can be recognised between various pantelleritic suites. A new classificatory scheme is proposed, based on the iron (as FeO) and Al₂O₃ contents. This is simpler than previously employed normative classifications, is more applicable to crystalline rocks, and, happily, in 95 % of cases gives the same rock name as the normative system.     

Geochemistry of basaltic and gabbroic rocks from the West Mariana basin and the Mariana trench

This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).     

Radiometric ages for some rocks from Snares and Auckland Islands,Campbell Plateau

On the basis of K/Ar muscovite and biotite ages, and of Rb/Sr whole-rock, muscovite, and feldspar ages, the last cooling of granites from Snares and Auckland Islands on the Campbell Plateau took place in early Late Cretaceous times. The original emplacement is unlikely to have been older than Late Jurassic. These results from the basement rocks of the Campbell Plateau conform with data from Fiordland and southern Stewart Island in southern New Zealand and from Ford Range and other localities in West Antarctica. Two gabbros from Auckland Island yield Miocene K/Ar ages.     

Neogene calc-alkaline extrusive and intrusive rocks of the karalar-yesiller area

The Karalar-Ye?iller area lies on the estern flank of the Kazda \(\bar g\) i massif in northwestern Anatolia (Asiatic Turkey), and includes about 600 square kilometers, of which approximately 80% is covered by rhyodacite-quartz latite extrusive rocks, and a comagmatic granodiorite-quartz monzonite batholith, and stocks, all of middle Miocene age. Extrusive rocks consist chiefly of rhyodacite lava flows of the Hallaçlar Formation and quartz latite-rhyodacite ash-flow tuff, lava, and mudflow deposits of the overlying Dede Tepe Formation. These volcanic rocks lie on a basement composed, in asceding stratigraphic order, of: 1) pre-Permian Kalabak sequence, 2) Upper Triassic Halilar Formation, 3) post-Upper Triassic Ba \(\bar g\) burun Formation, and 4) allochthons of middle Permian: and Upper Jurassic limestone. Intrusion of a granodiorite-quartz monzonite batholith during the middle Miocene was accompanied or shortly followed by extensive alteration and base metal mineralization of the Hallaçlar and older rocks. Intrusion of six rhyodacitequartz latite stocks followed the main phase of hydrothermal alteration. At least one of these stocks may have contributed material to the Dede Tepe Formation of middle Miocene age. Field relations, petrologic, and geochemical data as well as radiometric age dates suggest that the intrusive and extrusive rocks are comagmatic. Parent magma is probably derived from partial melting of subducted oceanic crust and accompanying oceanic sediments. Geologic relations locally indicate assimilation of sialic crust by the grandiorite-quartz monzonite batholith.     

Remagnetization in tertiary rocks from the Faeroe Islands

Palaeomagnetic investigations of highly oxidized interbasaltic horizons and associated Lower Tertiary lavas from the Faeroe Islads suggest a long history of magnetization. The remanence directions are composed mostly of two deviating directions of opposite polarities aligned along an axis which palaeomagnetically appears to be younger than that of the original field. This, in addition to the observation that both the baking and baked rocks in most cases show almost identical directional trends upon demagnetization, questions the general validity of the baked contact test. It is suggested that to a major extent post-baking processes, such as low-temperature oxidation and precipitation of ferric oxides, have remagnetized the lavas as well as the tuff layers. Only two of the seven interbasaltic horizons investigated appear to have retained the ‘primary’ magnetization. It is concluded that besides a high bulk oxidation state (induced prior to, or as a result of baking) a “hornfelsic” nature of the baked sediments may be a necessary requirement for establishing primary palaeomagnetic directions in many volcanic regions.     

A comparative study of the petrochemistry of the tholeiitic rocks of India

A comparative study of the petro-chemistry of the tholeiitic rocks (Cuddapah, Gwalior, Rajmahal and the Deccan traps) is made employing various crystallization parameters such as the differentiation index (D. I.) and the solidification index (S. I.). The D. I. values of the upper Deccan traps are much higher than those of the lower traps, rocks of intermediate composition being rare. The S. I. values of the Deccan traps allow them to be classified into three categories i.e., (1) the lower trap with higher S. I. values, (2) the differentiated upper division traps, (3) the unusual rock types of Western India. Murata’s alkali-silica diagram, based on all the available analyses of the traps (Pre-Cambrian to Eocene), indicates that, following the chemical definition, it is not possible to group the entire upper Deccan trap flows under the tholeiites. All the Precambrian traps are not tholeiitic either. There is a clear relationship between optical and petro-chemical characters. Attention is drawn to the difficulties in the application of Barth’s «f-norm » to trace the cooling history of the rocks crystallizing at a later stage.     

Ar and Sr isotopes of mantle-derived rocks from the Japanese Islands

⁴⁰Ar-³⁰Ar and Sr isotope analyses have been made for ultramatic nodules in basaltic rocks from Itinome-gata and the Oki-Dōgo Island, northeast and southwest of Japan respectively, in order to examine the state of the upper mantle under the Japanese Islands. ⁴⁰Ar-³⁰Ar analyses have revealed that ultramafic nodules from Oki-Dōgo Island were involved in events which caused Ar redistribution in these rocks and probably were related to their formation not older than several tens of millions of years ago. These nodules are relatively depleted in both Rb and Sr contents together with some other incompatible elements compared to similar nodules from the other regions in the world. This may be typical for ultramafic nodules from the Japanese Islands. Ultramafic nodules show mostly a little higher⁸⁷Sr/⁸⁶Sr ratios (0.704–0.706) than those of host basaltic rocks (0.703–0.705) indicating the accidental origin for these nodules. Present results also suggest both horizontal and vertical inhomogeneities of the uppermost part of the mantle with respect to Sr isotopes on a regional scale. Hence, the ultramafic nodules in volcanic rocks may not always directly represent the upper mantle materials from which the magma is produced at present. Such ultramafic nodules might be relatively old material from the uppermost part of the mantle under the Japanese Islands.     

O, S, Sr, and Pb isotope variations in volcanic rocks from the Northern Mariana Islands: implications for crustal recycling in intra-oceanic arcs

Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges ofδ¹⁸O = +5.7 to +6.6 (‰ SMOW) andδ³⁴S = +2.0to+20.7 (‰ CDT). Chemically evolved andesites and dacites with meanδ¹⁸O = +6.3 ± 0.2 are slightly enriched in¹⁸O with respect to unfractionated basalts of<53%SiO₂ with meanδ¹⁸O = +6.0 ± 0.1. This¹⁸O enrichment can be explained in terms of differentiation of parental mafic magmas havingδ¹⁸O values between +5.7 to +6.2‰ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherentδ¹⁸OSiO₂ trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO₂; therefore, the meanδ³⁴S value of ca. +11‰ for the Mariana lavas is considered to be representative of their source region.The enrichment of¹⁸O and³⁴S in Mariana Arc parental magmas relative to ocean floor basalts withδ¹⁸O ca. + 5.7‰ andδ³⁴S = ca.0.3‰ is attributed to the recycling of¹⁸O- and³⁴S-rich crustal components (sediment withδ¹⁸O = ca. + 25‰ and seawater sulfate withδ³⁴S = ca. +20‰ into the upper mantle source region for these arc magmas. This interpretation is consistent with enrichments of radiogenic Sr and Pb in the same lavas relative to ocean-floor basalts erupted either side of the arc, which are presumed to share a common upper mantle source. This enrichment is considered to reflect the mixing of two components, one having a typical upper mantle composition and the other having a more radiogenic character similar to that of western Pacific pelagic sediments.     

Strontium isotope ratios in volcanic rocks from the South Sandwich Islands

⁸⁷Sr/⁸⁶Sr ratios in the island-arc tholeiite series from the South Sandwich Islands cluster about a value of 0.7040. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The uniformity of the⁸⁷Sr/⁸⁶Sr values is consistent with the fractional crystallization relationship previously proposed for this suite. Though higher than values reported for ocean-floor tholeiites they are not significantly different from ratios reported for calc-alkaline island-arcs.     

Electrical conductivity of some rocks from the Indian subcontinent

Summary Results on electrical conductivity, chemical analysis and age of the rock samples from the Indian Subcontinent are presented. The old Precambrian Indian rocks tend to be less conductive than characteristic rocks of crustal and upper mantle structures.

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nuam mam uu mnm¶rt;mu, uu aaua u n¶rt;u u uu n¶rt; u ¶rt;uu. ma nauu n¶rt; u ¶rt;uu naam mn¶rt;m aamumuu n¶rt; u amuu.

     

Helium isotope geochemistry of some volcanic rocks from Saint Helena

³He/⁴He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, includingin vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited³He/⁴He ratio at St. Helena is 4.3–5.9 R_A. Helium isotope disequilibrium is present within the phenocrysts, with lower³He/⁴He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to -particle implantation from the surrounding(U + Th)-rich lavas.

A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave³He/⁴He= 7.1 R_A.³He/⁴He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to³He/⁴He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low²⁰⁶Pb/²⁰⁴Pb). Overall, the HeSrPb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere.³He/⁴He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 R_A). This suggests that the recycled materials were injected into the mantle within the last 10⁹ yrs.     

Geochronology and petrochemistry of Easter and Sala y Gomez Islands: implications for the origin of the Sala y Gomez Ridge

Easter and Sala y Gomez Islands lie along the Sala y Gomez Ridge, a broad zone of high topography and scattered seamounts extending east-southeast from the East Pacific Rise. K-Ar ages and major element abundances of volcanic rocks from these islands are used to test the fixed melting spot hypothesis for the origin of this feature. Poike volcano, the oldest center on Easter Island, was constructed in two episodes, occurring at 2.5 and 0.9 m.y. ago. Eruptive activity on Sala y Gomez was nearly contemporaneous with the earliest volcanism on Easter Island. No migration of volcanism with time is apparent along the Sala y Gomez Ridge. Basaltic rocks from Easter Island have tholeiitic affinities, while those dredged from the base of Sala y Gomez belong to an alkali olivine basalt series. Differences in basalt chemistry suggest that the volcanic rocks from the two islands formed from magmas that equilibrated at different pressures, and a model is presented which relates the character of the erupted liquids to magma segregation at the base of a progressively thickening lithosphere. The islands and seamounts comprising the Sala y Gomez Ridge do not fall on a small circle about the Nazca-mantle pole of absolute motion. Available evidence, therefore, does not support a fixed melting spot origin for Easter Island, Sala y Gomez, and the Sala y Gomez Ridge. Rather, it appears that the Sala y Gomez Ridge formed along a leaky fracture zone, and in response to a major re-orientation of spreading centers in this area.     

Preliminary paleomagnetic results on some Triassic rocks from East Greenland

Paleomagnetic measurements have been carried out on six samples of Early Triassic age and five samples of Middle Triassic age from East Greenland. The mean stable remanent magnetization directions obtained after alternating-field demagnetization tests give the virtual geomagnetic pole positions as: (1) 49°N, 158°E for the Middle Triassic and (2) 34° 30′N, 176°W for the Early Triassic. The Greenland Triassic paleomagnetic results have been compared with those for Europe and North America. It is inferred from this comparison that these preliminary results for Greenland do not conform with the requirements of a reconstruction based on a geometrical fit of the three landmasses.     

Origin of some calc-alkalic andesites in the Japanese Islands

Origin of calc-alkalic andesite in the Japanese Islands is reviewed on the basis of the recent trace element data and new experimental results. It is suggested that calc-alkalic andesites in the Japanese Islands have at least four different origins; (1) fractional crystallization with separation of magnetite of high-alumina basalt magma, (2) partial melting of hydrous upper mantle peridotite (for magnesian andesite), (3) fractional crystallization with separation of olivine and/or orthopyroxene of magnesian andesite magma and (4) mixing of dacitic and basaltic magmas. Emphasis is placed on the possible generation of primary magnesian calc-alkalic andesite magmas by direct partial melting of the upper mantle peridotite under hydrous conditions at depths between 40 and 60 km.     

Radioactivity and geochemistry of granulites and some related rocks from the saxonian granulite complex

Summary The distribution of radioactive(Th, U, K), major and selected trace(Rb, Sr, Ba, Y, Zr, V, Cr, Ni) elements of granulites from the Saxonian Granulite Complex was studied. Similarly to the South Bohemian granulites, the Saxonian granulites can be divided according to the contents of their major and trace elements into two main groups, groupA containing mostly acid and subacid granulites (K ₂ O>2.5%, SiO ₂ >68%), and groupB containing mostly intermediate and basic granulites (K ₂ O<2.5%, SiO ₂ <68%). Statistically significant differences between groupsA andB were found for all major oxides and several trace elements(Rb, V, Cr, Ni). The Saxonian granulites follow the same calc-alkaline trend as the South Bohemian, granulitesA being placed mostly in the rhyolite field and granulitesB mostly in the dacite, andesite and basalt fields of this trend. The investigated granulites are characterized by a considerable scatter ofTh andU contents accompanied by very variableTh/U ratios; theTh andU concentrations of granulitesA are substantially lower than is usual for rocks of corresponding acidity.

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¶rt;a an¶rt;u a¶rt;uamu(Th, U, K) u ua ¶rt;u(Rb, Sr, Ba, Y, Zr, V, Cr, Ni) m aum n¶rt;a aaum na. naa, m u¶rt;aum n uu aam n aaum u ¶rt;u am aua, u u uu. aum n u uu ma a¶rt;um ¶rt; ¶rt;nn; nnA nua¶rt;ama a au¶rt; u au¶rt;aum (K ₂ O>2,5%, Si O ₂ >68%), nnB ¶rt;u u aum (K ₂ O<2,5%, SiO ₂ <68%). ¶rt; muunnau mm mamumuu m au ¶rt; a u u ¶rt; m ¶rt;u m(Rb, V, Cr, Ni). auaum n¶rt;¶rt;m um- m¶rt; a u -uaum;aumA a¶rt;ma a uum n, uaumB a a ¶rt;aum, a¶rt;um u aam n m m¶rt;a. ¶rt;aum — u unnA — aamum uu ¶rt;au da¶rt;uamu mTh uU.

     

Agrigan: An introduction to the geology of an active volcano in the Northern Mariana Island arc

Agrigan is the tallest (965 m a.s.l.) and largest (44 km²) of the volcanoes of the northern Mariana Islands. Its slopes are asymmetric to the east; a small caldera (4 km²) dominates the interior. The volcanic edifice has been disrupted along three sets of faults: 1) exterior slump faults, 2) radial faults, and 3) interior faults related to caldera-collapse. The rocks of the volcano are characterized by porphyritic clinopyroxene-olivine-plagioclase basalts and subordinate andesites. Cumulate xenoliths composed of Fo₈₁, An₉₅ and diopside are common in the basalts. Development of the volcano began with 3–4 km of submarine growth. The earliest recognizable flows are the result of fissural Hawaiian- and Strombolian-type eruptions. These were followed by the eruption of more viscous lavas from above the present summit. Flank eruptions of basalt and andesite preceded voluminous outpourings of andesitic pyroclastics contemporaneous with caldera-collapse. Subsequent magmatic resurgence is localized along a N10E rift zone. Violent ejection of lapilli and ash occurred in 1917.     

Volcanoes of the southwestern extension of the active Mariana island arc: New swath-mapping and geochemical studies

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back-arc basin intersects the line of arc volcanoes. They develop well-defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back-arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault-controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features.     

Temporal evolution of the Mariana arc during rifting of the Mariana Trough traced through the volcaniclastic record

The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled by Deep Sea Drilling Project (DSDP) site 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards from the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of arc tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected by fractional crystallization. This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3–4 Ma, ∼ 3–4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosphere. Parallel trends are not seen in the enrichment of incompatible high field strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of arc volcanism on the trench side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly due to melt extraction from the mantle source as it passed under the developing back-arc spreading axis, prior to melting under the volcanic front.