Oceanic island Pb: Two-stage histories and mantle evolution

Leads in basaltic suites from seven oceanic islands form linear arrays on²⁰⁶Pb/²⁰⁴Pb versus²⁰⁷Pb/²⁰⁴Pb diagrams. These arrays are more reasonably interpreted as secondary isochrons than as mixing lines, because of their systematic relationship. Separate two-stage histories calculated for the leads from each island indicate that the source materials for the magmas were derived from a single primary reservoir with present²³⁸U/²⁰⁴Pb of 7.91 ± 0.04 by secondary enrichment in U/Pb at different times from 2.5 to 1 Ga ago. This is confirmed by a plot of isochron slope versus intercept, on which the points describing each island's Pb-Pb array all lie very near a single straight line. The isochrons for the Canary Islands and Hawaii, at least, are significantly different. The²⁰⁸Pb/²⁰⁴Pb versus²⁰⁶Pb/²⁰⁴Pb relationships are less coherent. The lead isotopic characteristics are consistent with a model in which lead in the oceanic island magmas is derived from ancient subducted oceanic crust. In particular, this explains the close relationship between lead in mid-ocean ridge and oceanic island basalts without invoking mixing.     

Westward younging disposition of Philippine ophiolites and its implication for arc evolution

Abstract   The different ophiolite complexes in the Philippine island arc system define a progressive younging direction westward. This resulted from the clockwise rotation of the Philippine island arc system during its north-westward translation in the Eocene resulting in its western boundary colliding with the Sundaland–Eurasian margin. As a consequence of this interaction, ophiolite complexes and mélanges accreted into the Philippine island arc system along its western side. A new ophiolite zonation with four belts is proposed that takes into consideration the observed spatial and temporal relationships of the exposed oceanic lithosphere slices. With progressive younging from east to west, Belt 1 corresponds to Late Cretaceous complete ophiolite complexes with associated metamorphic soles along the eastern Philippines, whereas Belt 2 includes Early to Late Cretaceous dismembered ultramafic-mafic complexes with mélanges exposed mainly west of eastern Philippines. Belt 3 is defined by Cretaceous through Eocene to Oligocene ophiolite complexes emplaced along the collision zone between the Philippine Mobile Belt and the Sundaland–Eurasian margin. Belt 4 corresponds to the ophiolite complexes emplaced along continental margins as exposed in the Palawan and Zamboanga–Sulu areas. This proposed zonation hints that the whole Philippine Mobile Belt, except for the strike-slip fault bounded Eocene Zambales ophiolite complex in Luzon, is underlain by Cretaceous proto-Philippine Sea Plate fragments. This is contrary to the previous models that consider only the eastern margin of the Philippines to contain proto-Philippine Sea Plate materials.     

Uranium as an indicator of the origin of the Tethyan ophiolites

The distribution of uranium in minerals and rocks of the ophiolitic sequences from the Northern Apennines (Italy) and the Hellenides (Greece) of the Tethyan mountain belt has been investigated by fission-track mapping. Whole-rock U abundances, ranging from 11 to 74 ppb in ultramafic rocks, from 13 to 54 ppb in gabbroic rocks, from 73 to 281 ppb in basalts and from 279 to 900 ppb in spilites, partially reflect the post-crystallization history of the rocks. Among the fresh relics of primary rock-forming minerals, clinopyroxene has the highest U content. The estimated original U concentration of ultramafic rocks is about 1 – 10 ppb. These rocks do not appear to represent the primary undepleted mantle material. U distribution in the analyzed suites of ophiolites is consistent with a model for alpine-type ophiolites, where harzburgites and lherzolites of the basal ultramafic zone represent the residuum after extraction of oceanic tholeiitic magma. The segregated basaltic liquid either extruded rapidly as basalts or underwent magmatic differentiation in a quiet environment producing mineral accumulates as troctolites and gabbros.     

A Jurassic-Cretaceous island arc in the Ladakh-Himalayas

The Ladakh Mesozoic ophiolite belt (western Himalaya) contains a pile of volcanic thrust sheets (Dras unit) which differ significantly in structure and composition from the ophiolitic mélange zones. The Dras unit is composed of pillow lavas, doleritic sills, very irregular basaltic (?basaltic andesites) and dacitic flows intercalated with pyroclastics, volcanoclastic sediments and radiolarian cherts. According to fossil evidence, this volcanism must have been active between Upper Jurassic and Upper Cretaceous.The presence of relict primary minerals, such as magnesiochromite, clinopyroxene, hastingsitic hornblende and Ti-magnetite as well as distinctive bulk chemistries, suggests that the volcanics belong to island arc tholeiite and to calc-alkaline rock series, typical of present island arcs in the Caribbean and Pacific.Model calculations incorporating probed phenocryst phases indicate that in addition to olivine, clinopyroxene and plagioclase, amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt. The latter process must have taken place at about 1000°C and at moderate depth of 5–15 km within or underneath the island arc. Today, hornblende-bearing mafic cumulates appear in the vicinity of Kargil within and close to the Dras volcanics.In a Sr-evolution diagram, the Dras volcanics have yielded a “pseudo-isochron” with a low initial ratio of 0.7035 ± 0.0003, which is in the same range as the mean of modern island arc volcanics. However, a geologically unrealistic age of 263 m.y., is obtained from the slope of this isochron.The upper mantle is regarded as the source material for the island arc tholeiitic magmas. Enrichment in K, Ba, Sr and LREE supports the involvement of components derived from dehydration or incipient melting of subducted Tethyan oceanic crust in the mantle.     

Magma genesis in the lesser Antilles island arc

¹⁴³Nd/¹⁴⁴Nd,⁸⁷Sr/⁸⁶Sr and REE results are reported on volcanic rocks from the islands of Dominica and St. Kitts in the Lesser Antilles. Particular attention is given to the lavas and xenoliths of the Foundland (basalt-andesite) and the Plat Pays (andesite-dacite) volcanic centres on Dominica. Combined major and trace element [2] and isotope results suggest that the bulk of the andesites and dacites on Dominica, and by analogy in the rest of the arc, are produced by fractional crystallisation of basaltic magma. The differences in the erupted products of the two volcanoes do not appear to be related to any significant differences in the source rocks of the magmas.Along the arc⁸⁷Sr/⁸⁶Sr ratios range from 0.7037 on St. Kitts, to 0.7041–0.7047 on Dominica, and 0.7039–0.7058 on Grenada [5], and these are accompanied by a parallel increase in K, Sr, Ba and the light REE's. Moreover, compared with LIL-element-enriched and -depleted rocks from MOR and intraplate environments, the basic rocks from the Lesser Antilles are preferentially enriched in alkaline elements (K, Ba, Rb, Sr) relative to less mobile elements such as the rare earths.¹⁴³Nd/¹⁴⁴Nd varies from 0.51308 on St. Kitts, to 0.51286 on Dominica, and 0.51264–0.51308 on Grenada [5], and all these samples have relatively high⁸⁷Sr/⁸⁶Sr ratios compared with the main trend of Nd and Sr isotopes for most mantle-derived volcanic rocks. Alkaline elements and⁸⁷Sr appear to have been introduced from the subducted ocean crust, but the results on other, less mobile elements are more ambiguous — island arc tholeiites (as on St. Kitts) do not appear to contain significant amounts of REE's, Zr, Y, etc., from the subducted oceanic crust, but such a contribution may be present in more LIL-element-enriched calc-alkaline rock types.     

Effective elastic thickness of island arc lithosphere under Japan

Abstract Using topography and observed gravity anomalies, we have estimated the effective elastic thickness as a measure of strength of Japanese island arc lithosphere. The thickness is found to range from about 3 km to >20 km. The thickness seems to be controlled primarily by the thermal state of the lithosphere. The higher the heat flow, the thinner is the elastic plate. However, several areas show significant deviations. The smaller effective elastic thickness in the northern Ryukyu arc than that inferred from heat flow may be attributed to the stress regime. In Japan, extensional tectonics are going on only in the Ryukyu arc region. Shallow subducting slab under the south-western Japan frontal arc probably increases the effective thickness by several kilometers. The determined effective elastic thickness suggests that when we consider vertical movements in the volcanic arc, we should take account of topographic and subsurface loading over a few hundred kilometers. However, if the dip of the slab is shallow, the flexural responses of the underlying slab, not only that of the island arc lithosphere, should be taken into account for the compensation, as is the case of the south-western Japan frontal arc.     

Petrologic evolution of Palau,a nascent island arc

Palau Islands, 7°30′N, are the only emergent feature on the more than 2500‐km‐long Kyushu–Palau Ridge. Small islands are mainly uplifted reef carbonate. Larger islands are volcanic with basalt to dacite and rare boninite. Polymict breccia is abundant: sills, flows, and dykes are common but pillows are rare. Palau Trench samples include all types found on the islands as well as high‐Mg basalt. Volcanism began in the late Eocene and ended by early Miocene. All igneous rocks comprise a low‐K primitive island arc‐tholeiite series. None are mid‐ocean ridge basalts. Rare earth elements and high field‐strength elements indicate a depleted mantle source. Elevated large ion lithophile elements and light rare earth elements indicate influx of ‘dehydration fluid’. Ce/Ce* and Eu/Eu* ratios show no evidence for recycling of arc‐derived clastics. Plate reconstructions and paleomagnetic data suggest that the arc probably formed on the trace of a transform fault that migrated northward and rotated clockwise up to 90°. Episodes of transtension caused upwelling of hot mantle into depleted mantle and sheared altered rocks of the transform. Episodes of transpression may have initiated subduction of old seafloor with a thin cover of pelagic sediments deposited far from terrigenous sediment sources.     

An island arc origin for the Canyon Mountain ophiolite complex,eastern Oregon,U.S.A.

The Canyon Mountain ophiolite, Oregon, is exceptional in lacking sheeted dikes, basaltic pillow lavas, and sediments that are characteristic of many other ophiolites. Instead, the uppermost portion of the complex consists of a significant volume of plagiogranites, which, in addition to minor basalts, intrude a large section of keratophyres believed to be of volcanic origin. The trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite. It is suggested that the plagiogranites, basalts, and keratophyres comprise a sill complex. Both the plagiogranites and the keratophyres are similar, respectively, to low-K₂O plutonic and extrusive rocks of island arcs. The mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation, related to magmatism and diapirism above a Benioff zone. Radiometric age dates of plagiogranites confine the minimum age of the complex to the Early Permian. The Canyon Mountain ophiolite may thus be correlative with other fragments of a Lower Permian arc terrane throughout northeastern Oregon which were chaotically mixed during renewed subduction in middle to late Triassic time.     

Is the Vourinos Complex an island arc ophiolite?

A geochemical study has been undertaken on the Vourinos ophiolites, northern Greece, a complex long known for its unusual characteristics such as an environment of acidic rocks and a calc-alkaline chemical affinity. The Nd-Sr isotopic ratios and the Hf/Th and Ta/Th ratios are indicative of an island arc origin for Vourinos as opposed to the mid-oceanic ridge origin inferred for other ophiolites such as Inzecca, Corsica. Other data on trace elements confirm that the cumulative suite and the lavas originated from the same magma through a simple fractional crystallization process and show that this magma would have formed through partial melting of an already highly-depleted material. It is thus possible to distinguish ophiolites with MORB characteristics from island arc ophiolites such as the Vourinos Complex, the existence of the latter type imposing new constraints on the possible tectonic processes for emplacement.     

Petrogenesis of Cenezoic volcanic rocks from the Aegean island arc

The Aegean volcanic arc formed in response to northeasterly subduction of the Mediterranean sea floor beneath the Aegean Sea. The active arc lies over 250 km from the Hellenic Trench in a region which has suffered considerable extension and subsidence since the mid-Tertiary. Suites of samples from the different volcanic centres making up the arc have been studied geochemically in order to assess lateral variations and to constrain the contribution of crustal contamination and sediment subduction in their petrogenesis.Lavas from all the major volcanic centres exhibit typical calc-alkaline major-element characteristics, and show enrichment in light REE and LIL elements but low contents of HFS elements. The enrichment in light REE is greater in the eastern (Nisyros, Kos) and western (Milos, Poros, Methana, Aegina) sectors of the arc (Ce_n/Yb_n=4) than in the central Santorini sector (Ce_n/Yb_n=2). All lavas have significant negative Eu anomalies and many have slight negative Ce anomalies. Less coherence is observed in the abundances and ratios of the other LIL elements, compared with the REE, along the island chain.Whereas the effects of crystal fractionation are evident in the trace-element patterns of lavas from individual islands, and are particularly well marked for Santorini, it is clear that there are consistent differences in trace-element abundances and ratios in the lavas of the various islands which reflect compositional differences in the mantle source and/or in melting conditions. Lavas from the eastern and western sectors have much higher levels of Ba and Sr but relatively lower Th, K and Rb than those from Santorini. Although some geochemical features could be explained through involvement of a component of subducted sediment in the source regions of the volcanoes, other element abundances and ratios indicate that this component must be very small. Detailed consideration of the inter-island geochemical variations suggests a complex make-up of the underlying lithosphere, resulting from a long history of subduction. In the region of Santorini, where crustal stretching is greatest, the underlying asthenosphere may be involved in magma production.     

The geological diversity of island arc volcanism: Northeast Russia

Paleovolcanological and paleotectonic reconstructions developed for the continent-ocean transition zone in Northeast Asia demonstrate a high diversity of island arc volcanic settings. There are two main types of island arc volcanism recognized so far, (i) volcanic arcs of euliminary systems (VAES) and (ii) intrageosynclinal volcanic arcs, including areas of insular volcanism (IIV). The volcanic arcs of euliminary systems include the present-day Kuril-Kamchatka, Aleutian, and the Paleozoic- Early Cretaceous Taigonos volcanic arcs. The latter is considered to be a part of the Talovka-Taigonos euliminary system (TTES), an old double island arc system analogous to present-day systems, the Kuril-Kamchatka and Aleutian ones. Both the TTES and similar present-day euliminary systems are structural complexes that confine concentrically-zoned geosyncline areas on the side of the Pacific. The characteristic features of the VAES include a long history of evolution, stable (calc-alkaline) basalt-andesite composition of volcanic products, and transverse geochemical zonation. Geophysical evidence reveals the complicated processes of endogenous crustal accretion and destruction of continental crust within the VAES zones. The IIV follow the structural pattern of the corresponding geosynclinal system. Their evolution is relatively short, while the spatial position and the composition of their magmatic bodies may considerably vary at different stages of evolution of the geosynclinal systems. Most island arc zones are characterized by calc-alkaline volcanism, but potassium alkaline, alkali-ultrabasic, and ultrabasic rocks also occur in some structures. The settings of intrageosynclinal insular volcanism are diverse and include (a) volcanic overcompensation, (b) geoanticlinal uplift, and (c) volcanotectonic downwarping during the orogenic stage of geosynclines. The calc-alkaline volcanism of island environments in geoanticline zones is likely related to the endogenous accretion of continental crust within a geosyncline system. Intrageosynclinal island-arc volcanism is still very poorly understood. Investigation of this phenomenon is one of the urgent tasks of paleovolcanology.     

Implications of island arc rotations to the studies of marginal terranes

Paleomagnetic studies of rocks from the Bonin and Mariana Island arcs indicate that these island arcs have undergone substantial rotation and northward translation since their formation 40 to 45 Ma. These rotations are inconsistent with existing models of marginal basin and island arc formation. The data presently available suggest to us that the Mariana and Bonin island arcs rotated as one tectonic element at the margin of the Philippine Sea and Pacific plate. These observations demonstrate that large vertical axis rotations are present in the island arc environment, prior to any interaction with a continental landmass. Since many circum-Pacific marginal terranes have been assigned island arc origins, these pre-accretion rotations should be considered when interpreting paleomagnetic results for possible microplate reconstructions. Knowledge of the structural and rotational styles of oceanic pre-accretionary deformations may prove useful for separating these pre-accretion effects from those imposed by the accretionary process in future studies aimed at deciphering the geologic history of island arc marginal terranes.     

Mediterranean island arcs and origin of high potash volcanoes

Active volcanoes of the Mediterranean Sea are distributed along two arc structures: the Hellenic arc in the Aegean Sea and the Calabrian arc in the Tyrrhenian Sea. The active volcanoes in both arcs lie above earthquakes with focal depth greater than 100 km. The depth of these earthquakes increases generally northward reaching a maximum depth of about 200 km in the Aegean Sea and more than 300 km in the Tyrrhenian Sea.     

Genesis of the extremely low-K tonalites from the island arc volcanism

The Adachi-Medeshima pumice deposit of the Late Pleistocene age, found in Northeast Japan, contains extremely low-K tonalite fragments, having a wide variation in SiO₂ content but fairly uniform K₂O content (<0.1%). The tonalites coexist with essential dacite fragments. Mineralogical properties of the tonalites are the same as those of the pumice and dacite fragments. Sr isotope ratios of the tonalite, dacite fragments and pumice also fall within a narrow range. By assuming a Rayleigh fractionation model, the concentration of incompatible trace elements in cumulus phases was calculated, and the amounts of major elements were estimated from the fractionation ratios of each mineral and the volume ratio of crystal-liquid. The estimated element abundances of the cumulus phase show a MORB normalized pattern similar to that of the natural tonalite having cumulate textures. Thus, the tonalite represents the cumulus phase precipitated from dacite magma. Equilibrium temperatures and fO₂ of the tonalites, dacite fragments and pumice were estimated from coexisting magnetite-ilmenite pairs at 876°–796° C and –logfO₂ = 9.8–11.9, respectively. The fO₂ is among highest group yet reported. This is a case of extremely low-K tonalite which formed in the island arc system different from the oceanic environments.     

Crustal structure and origin of the northeast Japan arc

Abstract Northeast Japan is a typical island arc region and its topographic arrangement reflects the geophysical characteristics of the island arc system. However, the structural style of the arc is very complicated and varied due to the repeated superposing of faults and folds on to earlier structures.
Geotectonic events that involved creation of the fundamental framework of the island arc crust occurred in east Asia in the Late Jurassic to Early Cretaceous and were probably induced by accretion and collision tectonics. The fragmentation and subsequent displacement of the crust took place during the Early Neogene in response to the terrane collision and the change in oceanic plate motion, leading to the opening of the Japan Sea. Huge amounts of volcano-sedimentary rocks buried the tilted fault blocks of pre-Tertiary basement with the development of the island arc.     

The Troodos ophiolitic complex was probably formed in an island arc

The Troodos ophiolitic complex in Cyprus has been widely regarded as a fragment of oceanic crust that was created in a mid-oceanic ridge. However, about one-third of the analyzed rocks of the lower pillow lavas and sheeted complex in it follows a calc-alkalic trend. This strongly suggests that the massif was created as a basaltic volcano in an island arc with a relatively thin oceanic-type crust rather than in a mid-oceanic ridge. Other chemical features and the structure of the massif are also consistent with an island arc origin. Some other ophiolitic complexes also appear to have been formed in island arcs.     

Mechanism of magma ascent and deep feeding channels of island arc volcanoes

The paper discusses the mechanism of deep magma activity beneath island are volcanoes and similar structures on the basis of data from geophysical investigations in Kamchatka; the analyses of forces that cause the ascent of magma; and related phenomena that are due to hydrostatic forces. It is shown that the ascent of magma through the astnenosphere occurs most likely in magma columns with a diameter of approximately 700–2,000 m and with a velocity of about 0.8–3 m/year. A regular line of such columns spaced in Kamchatka at a distance of about 30 km gives rise to a chain of separate Etrge volcanoes or volcanic centers. Ultrabasic magmas are most likely accumulated near the M discontinuity, whereas the apparent place of andesitic magma accumulation is in the earth’s crust near the boundary between the basement and sediments.