919 research outputs found
Plutonic xenoliths reveal the influence of cryptic melt-mush reaction processes in the plumbing system beneath St Vincent, Lesser Antilles Arc
No full textIn subduction zones, it is widely established that magmas are stored as crystal dominated mush within sub-volcanic plumbing systems. In these mush-dominated systems, it is likely that melt-mush reactions between migrating melts and the pre-existing mush influence the chemical evolution of magmas. However, melt-mush reactions and their effect on the chemical evolution of arc magmas can be elusive and cannot be constrained when studying erupted lavas in isolation. In this study, we focus on the island of St. Vincent in the Lesser Antilles volcanic arc because (1) the composition of erupted lavas has been interpreted to reflect simple fractional crystallisation, with minimal influence of other magmatic processes, and (2) an abundance of plutonic xenoliths (erupted fragments of crystal mush) can be found within the eruptive products. Thus, we are able to compare interpretations gleaned from the chemistry of erupted lavas with new observations of the crystal mush in the same magmatic system. To this end, textural analyses were undertaken on seventeen representative plutonic xenoliths from St Vincent, and four of these samples (two olivine gabbros, two hornblende-olivine gabbros) were studied in detail via element mapping, mineral trace element analyses and geochemical modelling. The chemical, textural and mineralogical characteristics of the olivine gabbros were best explained via fractional crystallisation in the mid-upper crust (~6–18 km depth). However, the hornblende-olivine gabbros (two of seventeen samples studied) contained clear textural evidence for melt-mush reaction in the mid-upper crust. The trace element compositions of minerals such as clinopyroxene in these two samples were best reproduced via assimilation-fractional crystallisation modelling, simulating melt-mush reactions, supporting the textural evidence. Our plutonic xenoliths reveal that in addition to fractional crystallisation, cryptic (i.e. not directly recorded in lavas) melt-mush reaction processes also contribute to magma chemical evolution, particularly influencing trace elements, within the sub-volcanic plumbing system. Textural evidence for melt-mush reaction is increasingly reported in plutonic xenoliths from other active arcs and exhumed arc crustal sections, suggesting that this process is ubiquitous in mush-dominated arc plumbing systems. Melt-mush reaction therefore represents an important process contributing to arc magma and arc crust trace element chemical diversity
Rb–Sr Geochronology (Igneous Rocks)
No full textThe preferential incorporation of Rb (following K) or Sr (following Ca) into different mineral structures, as well as their differing behaviors during closed system evolution of magmas by fractional crystallization, results in a wide range in Rb/Sr in mineral and whole rock compositions. The radioactive decay of87Rb to87Sr can then be exploited to generate an isochron and yield age information about a magmatic system, providing all the analyzed materials crystallized from a homogeneous parental melt, and the system has not been subsequently thermally or chemically disturbed. Some caution is required when applying the Rb–Sr chronometer to igneous systems; evaluation of the mobility of elements (especially Rb) and whether the strontium isotopic composition has been compromised by fluids (e.g., hydrothermal alteration and weathering) is required and the consequences of the relatively low closure temperature for Sr diffusion in many minerals, especially biotite, must be considered.</p
Geochronological constraints on granitic magmatism, deformation, cooling and uplift on Bornholm, Denmark
No full textCITATION: Waight, T. E., Frei, D. & Storey, M. 2012. Geochronological constraints on granitic magmatism, deformation, cooling and uplift on Bornholm, Denmark. Bulletin of the Geological Society of Denmark, 60:23-46, doi:10.37570/bgsd-2012-60-03.The original publication is available at https://2dgf.dk/publikationerU-Pb ages on zircon from 11 samples of granitoid and gneiss from the Danish island of Bornholm have been obtained using laser ablation – inductively coupled plasma mass spectrometry. These ages indicate that the felsic basement rocks were generated over a restricted period in the Mesoproterozoic at 1455 ± 10 Ma.
No evidence has been found for the presence of 1.8 Ga basement gneisses as observed to the north in southern Sweden and as inferred in previous studies. No distinction in age can be made between relatively undeformed granitic lithologies and gneissic lithologies within the errors of the technique. This indicates that granitic magmatism, deformation and metamorphism all occurred within a relatively restricted and contemporaneous period.
The granitic magmatism on Bornholm can thus be correlated to similar events at the same time in southern Sweden, Lithuania, and elsewhere in Baltica, and is therefore part of a larger magmatic event affecting the region. Argon and Rb-Sr ages on various minerals from a single sample of the Rønne Granite provide constraints on the cooling and uplift history of the basement in the region. Using recently published closure temperatures for each isotopic system a cooling curve is generated that illustrates a period of rapid cooling immediately after and/or during crystallisation.
This likely represents the period of emplacement, crystallisation, and deformation of the felsic basement. The modelled rate of post-emplacement cooling is highly dependent on the choice of closure temperature for Ar isotopes in biotite. Use of recently published values of around 450˚C defines a prolonged period of slower cooling (c. 4˚C per million years) over nearly 100 million years down to c. 300˚C and the closure temperature of Sr isotopes in biotite.
Use of older and lower closure temperatures defines curves that are more consistent with theoretical models. The low closure temperature of Sr isotopes in biotite explains much of the wide variation in previous age determinations using various techniques on Bornholm. There is no evidence in the geochronological data for disturbance during later tectonic events in the region.https://2dgf.dk/publikationer/bulletin/bulletin-vol-60-2012/Publisher's versio
The geology of Mount Turiwhate, Island Hill and Mount Tuhua, north Westland, New Zealand
No full textSeven petrologically distinctive granitoids are recognized on Mount Turiwhate, Island Hill and Mount Tuhua: the Turiwhate Granodiorite, the Summit Granodiorite and Paragneiss, the Fitzgerald Granodiorite, Island Hill Monzogranite, Kaniere Granodiorite, Milltown Granodiorite and Geologists Creek Monzogranite. The Summit Granodiorite and Paragneiss and Fitzgerald Granodiorite make up the ductilely deformed Wainihinihi Complex, which is likely to be an extension of the Fraser Complex.
Probable late Cretaceous basic dikes are present that are most likely part of the widespread lamprophyric dike swarm of this region, although none appear to be true lamprophyres. Hydrothermal alteration has affected some of the granitoids resulting in the presence of carbonate veins.
The Turiwhate Granodiorite shows many similarities to the I-type Separation Point suite, and the Summit Granodiorite may be part of the S-type Karamea suite of Tulloch (1983). The remainder of the granitoids show ambiguous I/S type characteristics and are considered to be part of the Rahu suite. No definitive ages have been determined for these granitoids
The geology of Mount Turiwhate, Island Hill and Mount Tuhua, north Westland, New Zealand
No full textSeven petrologically distinctive granitoids are recognized on Mount Turiwhate, Island Hill and Mount Tuhua: the Turiwhate Granodiorite, the Summit Granodiorite and Paragneiss, the Fitzgerald Granodiorite, Island Hill Monzogranite, Kaniere Granodiorite, Milltown Granodiorite and Geologists Creek Monzogranite. The Summit Granodiorite and Paragneiss and Fitzgerald Granodiorite make up the ductilely deformed Wainihinihi Complex, which is likely to be an extension of the Fraser Complex.
Probable late Cretaceous basic dikes are present that are most likely part of the widespread lamprophyric dike swarm of this region, although none appear to be true lamprophyres. Hydrothermal alteration has affected some of the granitoids resulting in the presence of carbonate veins.
The Turiwhate Granodiorite shows many similarities to the I-type Separation Point suite, and the Summit Granodiorite may be part of the S-type Karamea suite of Tulloch (1983). The remainder of the granitoids show ambiguous I/S type characteristics and are considered to be part of the Rahu suite. No definitive ages have been determined for these granitoids
The geology and geochemistry of the Hohonu Batholith and adjacent rocks, North Westland, New Zealand.
No full textThe Hohonu Batholith lies within the Buller terrane, immediately adjacent to the Alpine Fault and inland from Hokitika and Greymouth on the West Coast of the South Island of New Zealand. Detailed mapping has identified ten distinct granitoids intruded into Greenland Group metasediments. Four geochemical suites are recognized within the Hohonu Batholith.
Palaeozoic magmatism in the batholith is represented by the Summit Granite, which yields a Palaeozoic (381.2 Ma) age and displays affinites with granitoids of the Karamea Suite of Tulloch (1988a). The informal name Summit Granite suite is used to describe this pluton. The Summit Granite has acted as country rock and is intruded by two Cretaceous plutons. The poorly constrained Mount Graham Granite may also belong within the Summit Granite suite.
The Hohonu Batholith is dominated by the mid-Cretaceous (114-109 Ma) I-type Hohonu Super-suite, which is considered to encompass the previously defined Rahu Suite of Tulloch (1988a). The Hohonu Super-suite is characterized by relatively restricted radiogenic isotopic compositions with Sr(110) = 0.7062 to 0.7085 and εNd(110) = -4.4 to -6.1, and represents melting of a complex source combining depleted mantle-derived material, similar in composition to the source of the Early Cretaceous Separation Point Suite, and a complex, heterogeneous and largely unconstrained lower continental crustal component. A model is proposed whereby the Hohonu Super-suite was generated following the collapse and thinning of Western Province crust previously over thickened by the generation of the Median Tectonic Zone volcanic arc and its subsequent collision with the Western Province. Collapse of the over thickened crust is believed to be a consequence of the cessation of subduction along the Pacific Margin of the New Zealand portion of Gondwana and the subsequent removal of compressional forces maintaining crustal thickening. Rapid isothermal uplift of the thickened crustal root resulted in partial melting of the lower crust. Ambient temperatures in the lower crust were also raised by mafic underplating associated with isothermal uplift and adiabatic melting of the underlying mantle. Emplacement of the Hohonu Super-suite in an extensional environment is indicated by the intimate relationship between the Rahu Suite Buckland Granite and the Paparoa Metamorphic Core Complex, and the development of the extensional sedimentary basins of the Pororari Group. This extensional event is considered to predate and be unrelated to the separation of Australia and New Zealand and opening of the Tasman Sea.
Two suites are recognized within the Hohonu Super-suite in the Hohonu Batholith; the Te Kinga Suite and the Deutgam Suite. Geochemical contrasts between these two suites are attributed to melting at differing crustal depths, at varying water activities, and in equilibrium with different residual assemblages. The relatively mafic, meta1uminous, I-type compositions of the Deutgam Suite are ascribed to dehydration melting in equilibrium with an amphibolitic (plagioclase + amphibole) residue. Residual plagioclase retains Sr, Al2O3, Na2O and Eu and results in the low concentrations of these elements which characterize this suite. In contrast, the peraluminous high silica compositions of the Te Kinga Suite are attributed to water-saturated to under saturated melting in equilibrium with an eclogitic (garnet + amphibole residue) at greater depths in the crust. Residual garnet produces the HREE-depleted nature of the suite, and a lack of residual plagioclase contributes to the characteristically higher Sr, Al2O3, Na2O and Eu contents of the Te Kinga Suite.
Late Cretaceous magmatism in the Hohonu Batholith is represented by the French Creek Suite. This suite comprises the composite French Creek Granite, which displays geochemical and petrographic features typical of A-type granitoids, and associated hypabyssal rhyolite dikes. The alkaline magmatism of the French Creek Suite and the closely associated Hohonu Dike Swarm are intimately linked to extension during the opening of the Tasman Sea. The Hohonu Dike Swarm consists of predominately doleritic dikes, with subordinate camptonites and rare phonolites, concentrated on the Hohonu Ranges and Mount Te Kinga. Field evidence indicates that the Hohonu Dike Swarm and French Creek Granite are, at least partially, contemporaneous. The age of this activity is constrained by an 81.7 Ma SHRIMP age for French Creek Granite and is contemporaneous with the generation of the first oceanic crust in the Tasman Sea. A strong WNW-ESE trend within the Hohonu Dike Swarm parallels the line of Australia New Zealand break-up, and the alkaline compositions of both the dikes and the French Creek Granite are characteristic of emplacement into an anorogenic extensional environment. Consequently strong links are indicated between the opening of the Tasman Sea and genesis of the Hohonu Dike Swarm and French Creek Granite. Geochemical data are consistent with generation of French Creek Granite by prolonged fractionation of plagioclase and mafic phases from saturated and oversaturated members of the Hohonu Dike Swarm. Approximately 20% crustal contamination is also required to produce the isotopic compositions of French Creek Granite from the relatively depleted compositions of the Hohonu Dike Swarm.
Amphibolite-facies paragneisses, orthogneisses and metabasites of the Granite Hill Complex can be confidently correlated with similar rocks of the Fraser Complex. The dominance of metabasaltic rocks, distinct isotopic compositions and preliminary zircon inheritance studies indicate these gneisses are unlikely to represent metamorphic equivalents of the Greenland Group and intrusive granitoids as proposed for the Charleston Metamorphic Complex. Possible correlatives of the Fraser and Granite Hill Complexes may occur in Fiordland.
Poorly exposed Tertiary rocks along the north-west margin of the Hohonu Ranges are briefly described. These rocks are considered to represent material incorporated in a major fault zone along which the batholith has been uplifted and exposed during recent compression across the Alpine Fault
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