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2021 ◽  
Author(s):  
◽  
Simon James Barker

<p>Recent work has shown that silicic volcanism can be abundant in intra-oceanic subduction settings, and is often associated with large explosive caldera-forming eruptions. Several major petrogenetic questions arise over the origin and eruption of large amounts of silicic magma at these relatively simple subduction settings. This study has investigated the geochemistry of pyroclasts collected from four volcanoes along the Kermadec arc, a young (<2 Myr) oceanic subduction zone in the southwest Pacific. Raoul, Macauley and a newly discovered volcano (here informally named 'New volcano') in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all erupted dacitic to rhyolitic pumice within the last 10 kyr. For Raoul, New volcano and Healy, whole rock major element compositions fall with a limited compositional range. In contrast, pumice dredged from around Macauley caldera covers a wide compositional range indicating that there have been multiple silicic eruptions, not just the Sandy Bay Tephra exposed on Macauley Island. Distinctive crystal populations in both pumice samples and plutonic xenoliths suggest that many of the crystals did not grow in the evolved magmas, but were mixed in from other sources including gabbros and tonalites. Such open system mixing is ubiquitous in magmas from the four Kermadec volcanoes studied here. Silicic magmas, co-eruptive mafic enclaves and previously erupted basalts show sub-parallel REE patterns, and crystal composition and zonation suggests that mafic and silicic magmas have a strong genetic affiliation. Examination of whole rock, glass and mineral chemistry reveals that evolved magmas can be generated at each volcano through 60-75% crystal fractionation of a basaltic parent. These findings are not consistent with silicic magma generation via crustal anatexis, as previously suggested for the Kermadec arc. Although crystallisation is the dominant process driving melt evolution in the Kermadec volcanoes, the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected by small scale chemical variations between eruption units. Larger scale chemical trends reflect the position of the volcanoes along the arc, which in turn may reflect structural changes in the subduction zone and variations in sediment influx.</p>


2021 ◽  
Author(s):  
◽  
Simon James Barker

<p>Recent work has shown that silicic volcanism can be abundant in intra-oceanic subduction settings, and is often associated with large explosive caldera-forming eruptions. Several major petrogenetic questions arise over the origin and eruption of large amounts of silicic magma at these relatively simple subduction settings. This study has investigated the geochemistry of pyroclasts collected from four volcanoes along the Kermadec arc, a young (<2 Myr) oceanic subduction zone in the southwest Pacific. Raoul, Macauley and a newly discovered volcano (here informally named 'New volcano') in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all erupted dacitic to rhyolitic pumice within the last 10 kyr. For Raoul, New volcano and Healy, whole rock major element compositions fall with a limited compositional range. In contrast, pumice dredged from around Macauley caldera covers a wide compositional range indicating that there have been multiple silicic eruptions, not just the Sandy Bay Tephra exposed on Macauley Island. Distinctive crystal populations in both pumice samples and plutonic xenoliths suggest that many of the crystals did not grow in the evolved magmas, but were mixed in from other sources including gabbros and tonalites. Such open system mixing is ubiquitous in magmas from the four Kermadec volcanoes studied here. Silicic magmas, co-eruptive mafic enclaves and previously erupted basalts show sub-parallel REE patterns, and crystal composition and zonation suggests that mafic and silicic magmas have a strong genetic affiliation. Examination of whole rock, glass and mineral chemistry reveals that evolved magmas can be generated at each volcano through 60-75% crystal fractionation of a basaltic parent. These findings are not consistent with silicic magma generation via crustal anatexis, as previously suggested for the Kermadec arc. Although crystallisation is the dominant process driving melt evolution in the Kermadec volcanoes, the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected by small scale chemical variations between eruption units. Larger scale chemical trends reflect the position of the volcanoes along the arc, which in turn may reflect structural changes in the subduction zone and variations in sediment influx.</p>


Lithos ◽  
2021 ◽  
Vol 394-395 ◽  
pp. 106170
Author(s):  
Ding-Jun Wen ◽  
Xiu-Mian Hu ◽  
Jian-Sheng Qiu ◽  
Jin-Hai Yu ◽  
Rui-Qiang Wang ◽  
...  

Author(s):  
Rui Wang ◽  
Roberto F. Weinberg ◽  
Di-Cheng Zhu ◽  
Zeng-Qian Hou ◽  
Zhi-Ming Yang

The Yadong-Gulu Rift, cutting across the Gangdese belt and Himalayan terranes, is currently associated with a thermal anomaly in the mantle and crustal melting at 15−20 km depth. The rift follows the trace of a tear in the underthrusted Indian continental lithospheric slab recognized by high resolution geophysical methods. The Miocene evolution of a 400-km-wide band following the trace of the tear and the rift, records differences interpreted as indicative of a higher heat flow than its surroundings. In the Gangdese belt, this band is characterized by high-Sr/Y granitic magmatism that lasted 5 m.y. longer than elsewhere and by the highest values of εHf(i) and association with the largest porphyry Cu-Mo deposits in the Gangdese belt. Anomalously young magmatic rocks continue south along the rift in the Tethyan and Higher Himalayas. Here, a 300-km-wide belt includes some of the youngest Miocene Himalayan leucogranites; the only occurrence of mantle-derived mafic enclaves in a leucogranite; young mantle-derived lamprophyre dikes; and the youngest and hottest migmatites in the Higher Himalayas. These migmatites record a history of rapid exhumation contemporaneous with the exhumation of Miocene mafic eclogite blocks, which are unique to this region and which were both heated to &gt;800 °C at ca. 15−13 Ma, followed by isothermal decompression. We suggest that the prominent tear in the Indian lithosphere, sub-parallel to the rift, is the most likely source for these tectono-thermal anomalies since the Miocene.


2021 ◽  
Vol 48 (3) ◽  
Author(s):  
Alvaro Rodrigo Iriarte ◽  
Umberto Giuseppe Cordani ◽  
Kei Sato

The Real Cordillera granitoids are a suite of Triassic and Oligocene plutons located at the core of the Eastern Cordillera of the Central Andes of Bolivia. Its geotectonical setting, chemical and ore composition make them part of the so called “Inner Magmatic Arc” which differs from the actual “Magmatic Arc” located immediately to the west. U-Pb SHRIMP ages were obtained in order to constrain their crystallization ages. The Triassic group yielded the following results: 240 ± 2 Ma for the Huato granite, 230.7 ± 1.3 Ma for the Illampu granodiorite, 222.2 ± 2.4 Ma for the Huayna Potosí granite and 221.9 ± 1.5 Ma for the Taquesi granodiorite. For the Oligocene group we obtained two ages of 26.87 ± 0.26 and 26.88 ± 0.21 Ma both for the Quimsa Cruz granite. Mafic enclaves from the Illampu and Taquesi granodiorites report ages that were older than their respective granitoid hosts, yielding 234.1 ± 1.3 Ma and 227 ± 1.3 Ma, respectively. Secondary processes related to regional thermal anomalies and magmatic melt-enrichment, reset the K/Ar and U/Pb isotopic systems, producing: a) younger ages by Ar loss and b) older ages by U/Pb isotopic ratios reorganization. As noted in previous studies, the Zongo/Kuticucho Triassic granite yielded extremely high U enrichment in most zircon analysed, producing reset of U/Pb ratios, wide span in age ranges and reverse discordia curves that obscure its actual crystallization age. Relatively abundant zircon inheritance was found in these “cold” and inheritance-rich granitoids, with ages suggesting provenance from early Paleozoic metapelites that also recycled older sources. This relatively abundant xenocrystic inheritance records the influence of the Gondwanide orogeny (336-205 Ma) as an overall subduction arc environment, punctuated at its final stage with the imprint of a continental rifting (245-220 Ma).


2021 ◽  
Author(s):  
Filippo Mastroianni ◽  
Iacopo Fantozzi ◽  
Chiara Maria Petrone ◽  
Georgios E. Vougioukalakis ◽  
Eleonora Braschi ◽  
...  

&lt;p&gt;Kolumbo is the largest of twenty submarine volcanic cones, tectonically aligned in the transtentional Anydros basin, one of the most seismically active zones in the South Aegean Volcanic Arc, whose magmatism is related to the subduction of the African Plate beneath the Aegean microplate. Kolumbo explosively erupted in 1650 CE, causing the death of 70 people on Santorini, which is only 7 km SW of Kolumbo. Explorative cruises employing ROVs discovered a high temperature (220&amp;#176;C) hydrothermal field with CO2-rich discharges and accumulation of acidic water at the bottom of the crater (505 m b.s.l.), increasing the related hazard. A possible magma chamber was recognized below the crater at depth 9-6 km by seismic data [Dimitriadis et al. 2009]. Geochemical data [Klaver et al. 2016] suggest that Kolumbo have a different mantle source and storage system from Santorini. It is fundamental to understand the behaviour of this volcano, and how its storage and plumbing system works, to correctly assess risk for nearby islands.&lt;/p&gt;&lt;p&gt;We present petrographic, geochemical and isotopic data of samples collected during the cruises and by divers. Most samples represent the juvenile products of the 1650 CE activity, characterizing different magmas interacting before the eruption. They consist of white rhyolitic pumices with grey and black bands, also including basaltic-andesitic enclaves. Plagioclase, biotite, pyroxenes are the main mineral phases; olivine is found in the mafic enclaves. Minerals show quite complex zoning and a large compositional variability. Fresh lithic lavas were sampled; they also have amphibole and can be subdivided in three groups with distinctive petrographic textures that are well reflected in their different chemical compositions. They give information on the early history of the volcano and on how the rhyolitic magma could have been generated.&lt;/p&gt;&lt;p&gt;Our data suggest the presence of a complex storage system where the most evolved magma differentiated by assimilation and fractional crystallization, undergoing several inputs of mafic magmas. Early batches of new melts initially mixed with the resident ones, whereas later arrivals only mingled with the rhyolitic magma, thus possibly representing the final trigger of the eruption.&lt;/p&gt;


Author(s):  
Donnelly B. Archibald ◽  
Lauren M.G. Macquarrie ◽  
J. Brendan Murphy ◽  
Robin A. Strachan ◽  
Chris R.M. McFarlane ◽  
...  

Magmatic and tectonic processes can transport large volumes of magma generated in the deep crust as discrete pulses to shallower crustal depths, resulting in the incremental construction of large, composite batholiths over thousands to tens of millions of years. The Silurian to Early Devonian Donegal composite batholith in Ireland is a classic example of which regional geological syntheses and lithogeochemical data show that emplacement was syn- and post-kinematic with respect to the terminal phases (ca. 437−415 Ma) of the Caledonian orogeny. We used U-Pb dating of zircon and titanite to investigate the construction of the batholith over time. Imaging of these minerals reveals complex, zoned grains with distinct autocrystic (growth during pluton emplacement) and antecrystic (growth during lower crustal incubation) domains as well as xenocrysts (incorporated from wall rocks). To determine the ages of emplacement and of inherited domains, discrete growth zones were targeted for dating using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). Taken together, the zircon and titanite U-Pb isotopic data indicate that magmatism occurred over at least 30 m.y., between ca. 430 Ma and 400 Ma. Batholith emplacement is bracketed by the ca. 427−423 Ma Ardara pluton and the latest phases in the Main Donegal and Trawenagh Bay plutons (ca. 400 Ma). Although apparently volumetrically minor, U-Pb data from spatially associated mafic rocks (appinite suite, lamprophyre dikes, and mafic enclaves in granitoid plutons) yield ages ranging from ca. 431−416 Ma, which indicates ongoing mafic magmatism during emplacement of much of the Donegal composite batholith.


Geosciences ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 104 ◽  
Author(s):  
Alba Patrizia Santo

The Tuscany Magmatic Province consists of a Miocene to Pleistocene association of a wide variety of rock types, including peraluminous crustal anatectic granites and rhyolites, calcalkaline and shoshonitic suites and ultrapotassic lamproites. In addition to the magma types already recognised, the occurrence of a new, distinct magma type at Capraia and Elba islands and in mafic enclaves in the San Vincenzo rhyolites has been suggested by recent studies. This particular type of magma, represented by intermediate to acidic calcalkaline rocks showing high Sr, Ba, and LREE, is restricted to the northwestern sector of the province and to a time interval of about 8 to 4.5 Ma. New data obtained on rocks from Capraia Island have allowed for the verification of the occurrence of this new magma type, the exploration of its origin and a discussion of its possible geodynamic significance. The high-Sr-Ba andesite-dacite rocks occurring in the Laghetto area at Capraia display a composition that is intermediate between adakitic and calcalkaline rocks. It is suggested that they represent a distinct type of magma that originated at mantle pressure by melting of the lower continental crust, followed by mixing with other Capraia magmas. The geodynamic model that best explains the composition of the studied rocks is the thickening of the continental crust during continental collision, followed by extension that favoured melting of the lower crust.


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