Geomorphology and Geochemistry of Back Arc Basins in the Havre Trough, Southwest Pacific

Magmatic Activity ◽

Element Geochemistry ◽

Southwest Pacific ◽

Digital Terrain ◽

Show Evidence ◽

Back Arc ◽

Subduction System

The Havre Trough back arc system located behind the Kermadec Arc, in the southwest Pacific, is a classic example of an intra-oceanic back arc system. Subduction driven magmatism is focused at the arc front, and melting in the back arc is accompanied by back arc rifting. This study examines the deep back arc basins of the southern Havre Trough. Compared to the well-studied Kermadec Arc front volcanoes, the back arc basins remain poorly explored, yet are important features in understanding key structural and geochemical dynamics of the subduction system. The back arc is characterised by areas of deeper basins and constructional cross-arc volcanic edifices, which had previously been attributed to ‘rift regime’ and ‘arc regime’, respectively. In this study, recently acquired multibeam data was used to produce digital terrain maps that show individual basins within the Havre Trough that host a range of different morphological features, such as elongated ridges, nearly-flat basin floors, and small volcanic cones. Lavas dredged from the 10 basins were analysed, eight of which sample the rift regime and two sample the arc regime. The back arc basin lavas are basalts to basaltic-andesites and show fractionation of olivine + pyroxene ± plagioclase mineral assemblages. Olivine phenocrysts were tested for chemical equilibrium and predominantly show that crystallisation occurred in equilibrium with host melts. However, petrographic features such as dissolution and zoning within plagioclase show evidence of multistage magmatic evolution. Whole rock trace element geochemistry reveals trace element characteristics typical of volcanic arc lavas, such as enrichments in large ion lithophile elements (LILE) and Pb relative to high field strength elements (HFSE). From west to east, the back arc basin lavas show a decrease in NbN/YbN, consistent with trench perpendicular flow and progressive melt extraction towards the volcanic front. There is also a broad correlation between NbN/YbN and distance along the strike of the subduction zone. This may suggest a component of trench parallel flow of the mantle wedge, with increasing depletion northwards, although further evidence is needed to rule out pre-existing mantle heterogeneity. Ba/Th values, which trace the addition of slab-derived aqueous fluids, decrease with distance from the arc front. This indicates that the aqueous fluid component becomes less prominent with increasing distance from the arc front. Conversely, the basin lavas exhibit broadly increasing LaN/SmN values with distance from the arc front. As LaN/SmN can be used to trace the deep subduction component, i.e. sediment melt contribution, greater LaN/SmN suggests increasing contribution of a sediment signature away from the arc front. The parameters that measure recycled component flux are comparable between rift and arc regimes, so it is unlikely that increased volatile fluxing leads to the larger concentrations of magmatic activity displayed in arc regimes. Gill volcano (arc regime) has similar to higher NbN/YbN than lavas from adjacent basins, suggesting increased magmatic activity may in part relate to pockets of more fertile mantle. This study shows that back arcs and associated volcanism can be complicated, further research is integral in determining mechanisms for voluminous magmatic activity spread throughout the back arc.

Geochemistry, petrography, and zircon U–Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon–Tanana terrane

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2013-0004 ◽

2013 ◽

Vol 50 (8) ◽

pp. 826-846 ◽

Cited By ~ 10

Author(s):

Cynthia Dusel-Bacon ◽

Warren C. Day ◽

John N. Aleinikoff

Keyword(s):

Trace Element ◽

Ocean Basin ◽

Geochemical Characteristics ◽

Alkaline Basalt ◽

Metamorphic Complex ◽

Element Geochemistry ◽

East Central ◽

Metaigneous Rocks ◽

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

The Petrology, Geochemistry and Geochronology of Back-Arc Stratovolcanoes in the Southern Kermadec Arc-Havre Trough, SW Pacific

10.26686/wgtn.17060132 ◽

2021 ◽

Author(s):

◽

Alexander Zohrab

Keyword(s):

Trace Element ◽

Fractional Crystallisation ◽

Element Geochemistry ◽

Rock Composition ◽

Volcanic Arc ◽

Phenocryst Assemblage ◽

Origin And Evolution ◽

Depleted Mantle ◽

The Kermadec Arc-Havre Trough (KAHT) is widely regarded as a classical example of an intra-oceanic arc-back-arc system, where subduction-driven arc magmatism is focused at the Kermadec volcanic arc-front, and magmatism within the Havre Trough back-arc system results from decompression-related melting. In detail, however, the Havre Trough has not been well-studied, and data for very few lavas have been reported. Recent mapping undertaken in the southern Havre Trough has resulted in the discovery of several prominent submarine stratovolcanoes, Gill Seamount, Rapuhia Seamount and the related Rapuhia Ridge, Yokosuka Seamount, and Giljanes Seamount, situated in the middle of deep rifts and on elevated crustal plateaux. The origin and evolution of these stratovolcanoes is unknown. The first detailed dataset of whole rock major and trace element geochemistry, mineral chemistry, and ⁴⁰Ar/³⁹Ar isotope data, for lavas erupted from these volcanoes is presented here, and used to investigate the processes that drive volcanism in the Havre Trough back-arc. ⁴⁰Ar/³⁹Ar ages obtained from back-arc stratovolcanoes range from ca. 1167 - 953 ka for Gill Seamount, and ca. 107 - 50 ka for Rapuhia Ridge. These ages overlap with known ages for arc-front lavas, indicating that both back-arc and arc-front volcanism are coeval. These ages are all significantly younger than the inferred initation of Havre Trough rifting ca. 5 - 6 Ma. Lavas analysed from Gill Seamount and Rapuhia Ridge are basaltic to basaltic-andesitic in whole rock composition and contain a phenocryst assemblage of olivine ± orthopyroxene + clinopyroxene ± plagioclase. Lavas from Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount range from andesitic to dacitic in composition, and have a phenocryst assemblage consisting primarily of plagioclase ± clinopyroxene ± amphibole ± Fe-Ti oxides ± apatite. Variations in mineral assemblages and whole rock compositions of the lavas are consistent with crystal fractionation of their respective phenocryst phases. The more evolved compositions of Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount, all sited on an elevated crustal plateau, are inferred to result from prolonged assimilation + fractional crystallisation (AFC) in the mid- to upper- crust. Mineral compositions provide additional evidence for fractional crystallisation, and most crystals are inferred to have crystallised in equilibrium with their host melt. However, compositions of some olivine phenocrysts in Gill Seamount and Rapuhia Ridge indicate multiple populations of olivine, suggesting their magmatic systems were open to contributions from secondary processes. Variations in Or content in plagioclase crystals for a given lava suite suggests the sample suites crystallised from melts with different starting K₂O compositions. Elevated ratios of Nb/Yb in the mafic Gill Seamount and Rapuhia Ridge lavas indicate the back-arc volcanoes and ridges originated from a less depleted mantle than that present underneath the Kermadec volcanic arc-front, likely a consequence of trenchward advection of mantle within a suprasubduction wedge and/or partial melting of a fusible enriched mantle component. All whole rock samples from these back-arc volcanoes have trace element characteristics that resemble those of typical volcanic arc magmas, indicating that they are variably modified by subducting plate-derived components despite their rear-arc setting. However, the extent of fluid enrichment is less than that at the Kermadec volcanic arc-front. Elevated REE patterns and (La/Sm)N ratios suggest the subduction-component modifying back-arc volcano magmas is dominated by subducting sediment. This sediment component is not consistent with aqueous fluid transfer or bulk mixing, but by the addition of a sediment-derived partial melt with residual accessory phases monazite + zircon + rutile. HFSE/REE fractionated trace element patterns overlap for unmodified basalts from Gill Seamount and Rapuhia Ridge, and Rumble V Ridge back-arc constructional volcanism to the south. This suggests that a similar mechanism triggers constructional back-arc volcanism at both locations in the southern Havre Trough, likely a consequence of thermal anomalies inferred to be present in the mantle wedge (Todd et al. (2011)).

The Petrology, Geochemistry and Geochronology of Back-Arc Stratovolcanoes in the Southern Kermadec Arc-Havre Trough, SW Pacific

10.26686/wgtn.17060132.v1 ◽

2021 ◽

Author(s):

◽

Alexander Zohrab

Keyword(s):

Trace Element ◽

Fractional Crystallisation ◽

Element Geochemistry ◽

Rock Composition ◽

Volcanic Arc ◽

Phenocryst Assemblage ◽

Origin And Evolution ◽

Depleted Mantle ◽

The Kermadec Arc-Havre Trough (KAHT) is widely regarded as a classical example of an intra-oceanic arc-back-arc system, where subduction-driven arc magmatism is focused at the Kermadec volcanic arc-front, and magmatism within the Havre Trough back-arc system results from decompression-related melting. In detail, however, the Havre Trough has not been well-studied, and data for very few lavas have been reported. Recent mapping undertaken in the southern Havre Trough has resulted in the discovery of several prominent submarine stratovolcanoes, Gill Seamount, Rapuhia Seamount and the related Rapuhia Ridge, Yokosuka Seamount, and Giljanes Seamount, situated in the middle of deep rifts and on elevated crustal plateaux. The origin and evolution of these stratovolcanoes is unknown. The first detailed dataset of whole rock major and trace element geochemistry, mineral chemistry, and ⁴⁰Ar/³⁹Ar isotope data, for lavas erupted from these volcanoes is presented here, and used to investigate the processes that drive volcanism in the Havre Trough back-arc. ⁴⁰Ar/³⁹Ar ages obtained from back-arc stratovolcanoes range from ca. 1167 - 953 ka for Gill Seamount, and ca. 107 - 50 ka for Rapuhia Ridge. These ages overlap with known ages for arc-front lavas, indicating that both back-arc and arc-front volcanism are coeval. These ages are all significantly younger than the inferred initation of Havre Trough rifting ca. 5 - 6 Ma. Lavas analysed from Gill Seamount and Rapuhia Ridge are basaltic to basaltic-andesitic in whole rock composition and contain a phenocryst assemblage of olivine ± orthopyroxene + clinopyroxene ± plagioclase. Lavas from Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount range from andesitic to dacitic in composition, and have a phenocryst assemblage consisting primarily of plagioclase ± clinopyroxene ± amphibole ± Fe-Ti oxides ± apatite. Variations in mineral assemblages and whole rock compositions of the lavas are consistent with crystal fractionation of their respective phenocryst phases. The more evolved compositions of Rapuhia Seamount, Yokosuka Seamount and Giljanes Seamount, all sited on an elevated crustal plateau, are inferred to result from prolonged assimilation + fractional crystallisation (AFC) in the mid- to upper- crust. Mineral compositions provide additional evidence for fractional crystallisation, and most crystals are inferred to have crystallised in equilibrium with their host melt. However, compositions of some olivine phenocrysts in Gill Seamount and Rapuhia Ridge indicate multiple populations of olivine, suggesting their magmatic systems were open to contributions from secondary processes. Variations in Or content in plagioclase crystals for a given lava suite suggests the sample suites crystallised from melts with different starting K₂O compositions. Elevated ratios of Nb/Yb in the mafic Gill Seamount and Rapuhia Ridge lavas indicate the back-arc volcanoes and ridges originated from a less depleted mantle than that present underneath the Kermadec volcanic arc-front, likely a consequence of trenchward advection of mantle within a suprasubduction wedge and/or partial melting of a fusible enriched mantle component. All whole rock samples from these back-arc volcanoes have trace element characteristics that resemble those of typical volcanic arc magmas, indicating that they are variably modified by subducting plate-derived components despite their rear-arc setting. However, the extent of fluid enrichment is less than that at the Kermadec volcanic arc-front. Elevated REE patterns and (La/Sm)N ratios suggest the subduction-component modifying back-arc volcano magmas is dominated by subducting sediment. This sediment component is not consistent with aqueous fluid transfer or bulk mixing, but by the addition of a sediment-derived partial melt with residual accessory phases monazite + zircon + rutile. HFSE/REE fractionated trace element patterns overlap for unmodified basalts from Gill Seamount and Rapuhia Ridge, and Rumble V Ridge back-arc constructional volcanism to the south. This suggests that a similar mechanism triggers constructional back-arc volcanism at both locations in the southern Havre Trough, likely a consequence of thermal anomalies inferred to be present in the mantle wedge (Todd et al. (2011)).

Trace element geochemistry of the Meen-Dempster greenstone belt, Uchi subprovince, Superior Province, Canada: back-arc development on the margins of an Archean protocontinent

Canadian Journal of Earth Sciences ◽

10.1139/e00-007 ◽

2000 ◽

Vol 37 (7) ◽

pp. 1021-1038 ◽

Cited By ~ 17

Author(s):

Pete Hollings ◽

Greg Stott ◽

Derek Wyman

Keyword(s):

Trace Element ◽

Greenstone Belt ◽

Volcanic Rocks ◽

Tholeiitic Basalt ◽

Pyroclastic Flows ◽

Element Geochemistry ◽

Lree Enrichment ◽

Compositional Diversity ◽

Comprehensive trace element analyses of mafic and felsic volcanic rocks from the 2.85-2.74 Ga Meen-Dempster greenstone belt reveal a wide compositional diversity. The ~2.85 Ga Kaminiskag assemblage is dominated by mafic tholeiite characterized by predominantly unfractionated REE (La/Smn = 0.8-1.1). Rare intermediate and felsic pyroclastic flows (SiO2 = 62-74) with moderate to pronounced LREE enrichment are intercalated with the tholeiite. The Kaminiskag assemblage is overlain by the ~2825 Ma Meen assemblage, comprising dominantly dacitic tuffs and pyroclastic breccia, displaying enriched LREE (La/Smn = 3.7-7.2) and moderately fractionated HREE, in conjunction with pronounced negative Nb anomalies. Five distinct suites have been recognized in the ~2740 Ma Confederation assemblage: (1) tholeiitic basalt with flat to smoothly depleted REE, (2) tholeiite with flat to weakly depleted LREE in conjunction with pronounced negative Nb anomalies, (3) Fe-rich basalt with elevated Ti and P contents, LREE enrichment, and fractionated HREE, (4) LREE enriched basalt and andesite with negative Nb anomalies, and (5) dacite and rhyolite with enriched LREE, moderately fractionated HREE, and variable high field strength element anomalies. The geochemistry and geochronology of the Kaminiskag and Meen assemblages are consistent with the formation of an oceanic back arc (Kaminiskag assemblage), which formed the basement for a subduction-related arc complex (Meen assemblage) after a 15 Ma hiatus. The Confederation assemblage is interpreted to represent an Archean back arc, where the complex interplay of mantle sources allows for the eruption of tholeiite, subduction-modified tholeiite, ocean island basalt-like basalt, and subduction-related arc-type volcanic rocks. The recognition of back-arc basins within the Meen-Dempster greenstone belt emphasizes a continuity of crustal growth processes from the Archean to the present day.

Trace element geochemistry of the Sokoman Iron Formation

Canadian Journal of Earth Sciences ◽

10.1139/e77-136 ◽

1977 ◽

Vol 14 (7) ◽

pp. 1598-1610 ◽

Cited By ~ 42

Author(s):

B. J. Fryer

Keyword(s):

Trace Element ◽

Sea Water ◽

Iron Formation ◽

Element Geochemistry ◽

Element Abundances ◽

Rock Types ◽

Show Evidence ◽

Trace Element Contents ◽

Element Contents

Rare earth and other trace element data are presented for samples of the Sokoman Iron Formation, Labrador, and its associated sediments. The results show that the slates associated with the iron formation are typical in trace element contents compared to other argillaceous sediments except for the large Eu depletion characteristic of slates of their age. The iron formation, however, is fundamentally different in its trace element concentrations and patterns from those of the associated rocks. It is relatively enriched in the heavy REE and Eu and both the REE and Co, Cr, Sc, and Th concentrations bear no relationship to those of the slates and the dolomite.Trace element analyses of the various textural and mineralogic rock types in all cases substantiate the genetic conclusions of earlier workers based on field and petrographic observations. Silicate–carbonate facies samples show constant REE, Co, Sc, and Th distributions which are compatible with an origin as crystalline precipitates in equilibrium with sea water. Riebeckite-bearing iron formation is distinctive in that it reflects contamination by ordinary clastic material and (or) metamorphic solutions. Oxide facies rocks exhibit widely variable trace element abundances as is to be expected for rocks whose original trace element contents were controlled by adsorption processes. A group of iron-enriched oxide facies rocks show evidence of important heavy REE complexing associated with the migration of iron during diagenesis. Minor Ce anomalies in all facies of Sokoman Iron Formation indicate that oxidation of Ce to the +4 state was taking place at the time of iron deposition but probably not in close proximity to it.