Island Arc Andesites Research Articles

Uranium on Oceanic Side of Circum-Pacific Mobile Belt: ABSTRACT

Asymmetrical continental margin orogenic belts have superimposed mineralization belts also zoned asymmetrically toward low temperature in the foreland, suggesting orogenic redistribution of crustal metals. The quantity of uranium concentrated into deposits is maximum in a final forelandward zone. High-temperature Th-U concentrations are less prominent in rearward tectonic zones. Unfavorability of tectonic zones oceanward of the batholith zone, for low-temperature uranium, predicted in 1970, is reviewed as a test of the metallo-tectonic concept, after 12 years of intensive exploration during the price peak. Of 19 Pacific-margin countries containing island-arc, back-arc-flysch-basin, pluton-in-flysch, or batholith tectonic zones, three aggregate six uranium deposits (four of low temperature), nine prospects, and perhaps ten occurrences. Two more countries have one prospect each. One vein is in island-arc andesite. Two disseminations in basin flysch may represent submarine exhalations. Several high temperature veins are associated with alkaline plutons in flysch, and several high temperature replacement disseminations with porphyry copper/molybdenum deposits are known. Many uneconomic high temperature replacement disseminations in alkaline granite are known in the batholith zone. One economic dissemination is in a granite pluton contact zone and two supergene impregnations are in regolit over granite batholith. Several impregnations occur in basin sandstones derived from and overlying granite. Strongest uranium mineralization in oceanward tectonic zones was associated with cratonization of oceanic rocks. End_of_Article - Last_Page 968------------

Trace-element geochemistry of Archean volcanic rocks and crystal growth in southwestern Abitibi Belt, Canada

Trace-element geochemistry on 176 controlled samples in three adjoining Abitibi (∼2.7 b.y.) volcanic piles confirms the fundamental concept of Archean volcanic cycles composed of lower tholeiitic and upper calc-alkalic parts. The volcanic piles, each as much as 16 km thick, are either unicyclic or multicyclic. In the latter case, the cycles display secular geochemical trends. All the volcanic rocks studied are indicated to have come from mantle sources. A fundamental feature of Abitibi magma genesis is the simultaneous presence of both depleted and undepleted mantle sources. Abitibi tholeiitic basalts closely resemble modern mid-ocean ridge basalts. Abitibi calc-alkalic andesites and alkalic rocks are very similar to modern oceanic island-arc andesites and to some modern volcanic-arc high-K rocks, respectively. The principal constraints to geodynamic processes are interpreted in terms of pulsating migrating mantle diapirism—a type of “hot-spot tectonics” mechanism involving a layered mantle responsible for the early tholeiitic (depleted mantle source) and later calc-alkalic (undepleted mantle source) parts, respectively, of the volcanic cycles.

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO2 contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An95 to An60 in the basalts and An60 to An50 in the andesites, but, within each group, they are not correlated with SiO2 or Na2O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO2 variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.

Geochemical investigation of some volcanic rocks from the Philippines

Ten tholeiitic basalts and twelve low-Si andesites from the Philippines were analyzed for Si, Ti, Al, Fe, Mg, Ca, Na, K, Rb, Sr, Hf, Co, Sc and Cr, and seven REE (La, Ce, Sm, Eu, Tb, Yb and Lu). Philippine tholeiitic basalts are similar to most island-arc tholeiites and are relatively similar to continental tholeiites in terms of major-element contents and trends in K/Rb, Rb/Sr, K/Sr and Ca/Sr ratios. Compared to most submarine tholeiites, they are more strongly enriched in Rb relative to K and Sr, and enriched in Sr relative to K and Ca, and seem to show only slight effects of plagioclase fractionation at relatively low temperatures. Average REE distribution pattern of Philippine tholeiitic basalt differs slightly from Formosan tholeiite, but differs strikingly from Japanese and oceanic-ridge tholeiites. These contrasts suggest that Philippine tholeiitic basalts originated from an environment different from Japan and oceanic ridges, and probably at pressures nearly as great as those characteristic of Formosan tholeiites. Philippine low-Si andesites are enriched in light REE and have distribution patterns nearly parallel to Iwo-Jima (Japan) andesites and similar to Bougainville low-Si andesites, suggesting that Philippine low-Si andesites probably formed in a condition similar to most circum-Pacific island-arc andesites.

Genesis of lavas of the Taupo Volcanic Zone, North Island, New Zealand

The Taupo Volcanic Zone forms part of the Taupo-Hikurangi subduction system, and comprises five volcanic centres: Tongariro, Taupo, Maroa, Okataina and Rotorua. Tongariro Volcanic Centre is formed almost entirely of andesite while the other four centres contain predominantly rhyolitic volcanics and later fissure eruptions of high-Al basalt. Estimated total volume of each lava type are as follows: 2 km3 of high-Al basalt ( 10,000 km3 of rhyolite and ignimbrite (> 97.4%). The location of the andesites and vent alignments suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island-arc andesites such as those of Tonga; in particular by having higher contents of the alkali elements, light REE and Sr and Pb isotopes. This suggests some crustal contamination, and it is considered that this may occur beneath the wide accretionary prism of the subduction system. Amphibolite of the subduction zone will break down between 80 and 100 km and a partial melt will rise. A multi-stage process of magma genesis is then likely to occur. High-Al basalts are thought to be derived from partial melting of a garnet-free peridotite near the top of the mantle wedge overlying the subduction zone, locations of the vents controlled largely by faults within the crust. Rhyolites and ignimbrites were probably derived from partial melting of Mesozoic greywacke and argillite under the Taupo Volcanic Zone. Initial partial melting may have been due to hydration of the base of the crust; the “water” having come from dehydration of the downgoing slab. The partial melts would rise to form granodiorite plutons and final release of the magma to form rhyolites and ignimbrites was allowed because of extension within the Taupo graben. Dacites of the Bay of Plenty probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma, but those on the eastern side of the Rotorua-Taupo area were more likely formed by a higher degree of partial melting of the Mesozoic greywacke-argillite basement. This may be due to intrusion of andesite magma on this side of the Taupo volcanic zone.

Volatiles in submarine volcanic rocks from the spreading axis of the East Scotia Sea back-arc basin

The volatile content of glassy pillow rims from East Scotia Sea back-arc basin (BAB) lavas are unlike those of mid-ocean ridge (MOR) pillow-rim glasses, although non-volatile compositions of the two rock groups overlap. The East Scotia Sea samples have three to ten times greater water contents and nearly twice the average CO 2 and Cl contents of MOR samples; F contents are similar. S contents are only one-third those from MOR samples. H 2O and CO 2 contents of glassy pillow rims from Mariana island arc andesites are similar to those in the BAB lavas studied. Nevertheless, volatiles in the East Scotia Sea BAB magmas are probably not directly derived from the subducted slab, because there is no seismic evidence that the slab extends within 200 km of the spreading axis of the East Scotia Sea. Available data do not preclude the possibility that the magmas were contaminated by seawater prior to eruption or that the mantle under the East Scotia Sea spreading center is volatile-rich. The volatiles may have been added to the mantle during an earlier period of subduction, perhaps during the initial formation of the East Scotia Sea basin.

Trace Elements in Continental-Margin Magmatism: Part 1. Trace Elements in the Clarno Formation of Central Oregon and the Nature of the Continental Margin on Which Eruption Occurred

The Clarno Formation is an Eocene to lower Oligocene volcanic and volcaniclastic formation in central Oregon. It was apparently formed in response to subduction of Pacific Ocean crust under the western margin of North America. Clarno flow rocks, which constitute at least 50% of the formation, are compositionally intermediate between flow rocks typical of continental margins and those typical of island arcs. The dominant rock type is andesite, in a range from rhyolite to basalt, and the SiO 2 frequently distribution is unimodal about a mean of 60%. The principal phenocryst in the basalts and andesites is clinopyroxene. Abundances of lithophile elements (K, Rb, and Ba) in the basalts are very similar to average abundances in calc-alkalic basalts of island arcs. Lithophile abundances in Clarno andesites, however, are generally greater than those of island-arc andesites and less than those of andesites from continental margins. The Clarno Formation may be inferred to have formed on a crust intermediate between oceanic and continental; a thickness of 20 to 30 km can be estimated from K 2 O contents and also from Rb-Sr relationships.

Trace elements in continental-margin magmatism: Part I. Trace elements in the Clarno Formation of central Oregon and the nature of the continental margin on which eruption occurred: Summary

The Clarno Formation is an Eocene to lower Oligocene volcanic and volcaniclastic formation in central Oregon. It was apparently formed in response to subduction of Pacific Ocean crust under the western margin of North America. Clarno flow rocks, which constitute at least 50% of the formation, are compositionally intermediate between flow rocks typical of continental margins and those typical of island arcs. The dominant rock type is andesite, in a range from rhyolite to basalt, and the SiO 2 frequently distribution is unimodal about a mean of 60%. The principal phenocryst in the basalts and andesites is clinopyroxene. Abundances of lithophile elements (K, Rb, and Ba) in the basalts are very similar to average abundances in calc-alkalic basalts of island arcs. Lithophile abundances in Clarno andesites, however, are generally greater than those of island-arc andesites and less than those of andesites from continental margins. The Clarno Formation may be inferred to have formed on a crust intermediate between oceanic and continental; a thickness of 20 to 30 km can be estimated from K 2 O contents and also from Rb-Sr relationships.

Nickel partitioning between olivine and silicate melt

Partitioning of Ni between olivine and silicate melt has been determined for compositions in the system Fo-Ab-An (1 atm) for temperatures ranging from 1250°C to 1450°C. Nickel concentrations were determined by electron microprobe; concentration levels in the liquids ranged from 0.1% to 0.5%. Platinum capsules or Pt wire loops were used as containers. Equilibrium was evaluated from kinetic considerations and by variation of run parameters; it was documented in one case by a bracketed reversal. No evidence was found for a dependence of the partition coefficient D (Ni in olivine/Ni in liquid) on Ni concentration. D is strongly dependent on melt composition, varying linearly with (1/MgO) at constant temperature. The intrinsic temperature dependence of D is small; the apparent temperature dependence reported in previous studies is largely related to the variation of melt composition with temperature. Our D values determined in the simple system Fo-An-Ab agree well with those reported by Leeman for natural (Fe-bearing) basalt systems. Overall variation of D in our system (and in natural basalts) can be expressed by the regression: D = (124/MgO) − 0.9 Our data are used to evaluate published Ni-MgO relationships in natural basalt series from Kilauea, Crozet, Cape Verde and Baffin Bay. A combination of olivine accumulation and fractional crystallization processes are sufficient to model these series. Using our data, unique “parental” liquids can be specified for each of these series; the MgO content of these liquids varies from 6% to 13%. Basalts with MgO contents greater than these “parental” liquids must be accumulative. The linear Ni-MgO trends, high absolute Ni concentrations, and large spread of Ni contents for the high-MgO basalts argue convincingly against their being “primary” liquids. Models such as those of O'Hara [6,13] and Clarke [24], based on the assertion of primary high-MgO liquids, must therefore be re-evaluated. Because of the high Si/O ratio and low MgO content of island arc andesites, the Ni partition coefficient D may be quite high. Therefore, the relatively low Ni content of such andesites may not be an argument against their derivation as direct partial melts of the mantle.

Andesites of the Tongariro volcanic centre, North Island, New Zealand

The Tongariro volcanic centre comprises four major andesite massifs, Kakaramea, Pihanga, Tongariro and Ruapehu, and four smaller cones and flows, Maungakatote, Pukeonake, Hauhungatahi and Ohakune. Older lavas from Kakaramea, Pihanga and Tongariro were erupted from a series of vents aligned NW-SE and more recent lavas from vents aligned NNE-SSW. The most voluminous lava types in the centre are labradorite and labradorite-pyroxene andesite containing phenocrysts of plagioclase (An 68—An 48), orthopyroxene (mainly bronzite) and clinopyroxene, in a fine to medium-grained groundmass. Smaller amounts of pyroxene andesite (from Pihanga), olivine andesite (from NNE-trending vents), and hornblende andesite also occur. Pigeonite surrounds orthopyroxene in some of the pyroxene andesites. Average major and trace element compositions of each petrographic type indicate that all lavas are “normal” calc-alkaline andesites and low-Si andesites. Low-Si andesites, represented mainly by pyroxene and olivine andesites, are typified by higher FeO, MnO, MgO, Sr, Cu, Ni, V, Sc and Cr and lower Al 2O 3, Na 2O, K 2O, Rb, Ba, Zr and Pb than “normal” andesites. Some low-Si andesites are regarded as phenocryst-rich cumulates. The location of the Tongariro andesites and alignments of eruptive vents suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island arc andesites such as those of Tonga by having higher Na 2O, K 2O, Rb, Ba and Zr. This suggests some crustal contamination, but the overall chemistry of the andesites is inconsistent with an origin by mixing, en route to the surface, of high-alumina basalt with either Mesozoic greywacke-argillite or with a partial melt of the greywacke-argillite equivalent in composition to Taupo rhyolite. A model is presented in which oceanic crust assimilates Mesozoic and Cenozoic sediments at the eastern side of the North Island, and is subducted to produce amphibolite. This amphibolite would subsequently break down, hydrating the overlying upper mantle and lower crust, and, in steeply dipping subduction zones such as that under Tongariro, would produce phlogopite eclogite below 90 km. The latter will partially melt at 150–200 km and the resultant magma will fractionate in the upper mantle or lower crust to produce andesite.

Andean Cenozoic volcanism: Magma genesis in the light of strontium isotopic composition and trace-element geochemistry

Subduction of the oceanic Nazca plate beneath the western edge of the South American plate is thought by most proponents of plate theory to have generated the tectonic forces and magmatic activity that have structured the Andean orogen. By this view, the Cenozoic andesitic magmas of the Andean stratovolcanoes originated within the oceanic crust of the descending plate or within the mantle adjacent to that plate. Accordingly, these Andean volcanic rocks should exhibit chemical and isotopic compositions similar to those of island-arc andesite of like origin. However, while major- and minor-element analyses reveal that the late Cenozoic volcanic rocks are chemically akin to those of the calc-alkalic suite of circum-Pacific island arcs, they exhibit significantly higher Sr/sup 87//Sr/sup 86/ ratios. The purpose of this paper is to examine these anomalously high isotopic ratios in the light of trace-element geochemistry and to develop from this a model for the genesis of the Cenozoic magma.

The oceanic affinities of some alpine mafic rocks based on their Ti-Zr-Y contents

Ophiolite-Chert complexes containing ultramafics, mafic dykes and pillow lavas, overlain by radiolarian cherts and other pelagic sediments are significant features of mountain belts, especially in the Alpine-Himalayan chain (Vine & Hess 1970), and in the Caledonides ( Bird et al. 1971 ). The sedimentology and fauna of the cherts and the similarity of the igneous succession with inferred oceanic igneous structure are consistent with the hypothesis that these complexes are tectonically emplaced fragments of oceanic crust and upper mantle (Vine & Hess 1970). Elsewhere in orogenic belts, mafic rocks of oceanic aspect are frequently found without the full ophiolite succession being present. Although these are broadly similar to oceanic mafic lavas it has not proved possible to establish the genetic affinities of such rocks by their major element chemistry ( Vallance 1969 ). It has recently been proposed that the genetic affinities of basaltic rocks may be distinguished by the ratios of the three elements, Ti, Zr and Y which have been shown to be insensitive to alteration affecting major elements during metamorphism up to greenschist facies ( Cann 1971 ; Pearce & Cann 1971 ). Five magma types (number of analyses given in parentheses) have been considered by Pearce & Cann; island arc andesites (12), island arc tholeiites (14), ocean island alkali basalts (8), ocean island tholeiite basalts (13) and mid-ocean ridge basalts (39). Using discriminant analysis they constructed functions in Ti, Zr and Y which distinguished these lava types. Analyses were by X-ray fluorescence on pressed powder pellets. On this basis, samples of supposed oceanic rocks

Ophiolite origin investigated by discriminant analysis using Ti, Zr and Y

Samples of rocks from four known or suspected ophiolite complexes were compared with five groups of Cenozoic volcanic rocks using their contents of Ti, Zr and Y. Triangular plots of Ti-Zr-Y and plots of Ti-Zr were constructed, and the problem was also approached using discriminant analysis. The elements Ti, Zr and Y were chosen because of their apparent invariance with respect to such secondary processes as weathering and metamorphism which have often affected ophiolitic rocks. The five groups of volcanic rocks nominated as standards were a group of ocean-floor basalts, one of Hawaiian tholeiites, one of alkali olivine basalts from Flores, Azores, one of island-arc andesites, and one of island-arc tholeiites from Japan. These could all be well discriminated using the three elements chosen. The four ophiolites selected included two, Troodos and Oman, which have been postulated on structural grounds to be fragments of ocean crust, and two others, from the Grossglockner region, Austria, and from the Othrys region, Greece, which were less well-defined. Most of the samples from all four ophiolites showed the greatest affinity with the ocean-floor basalt group, except for seven andesites from Greece. It is thus very probable that most of the samples measured were generated as ocean crust at spreading plate boundaries, and have been thrust into their present positions during continental collision.