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Search Results: 1 - 10 of 2790 matches for " Back-arc volcanism "
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The Cinotepeque Range of central El Salvador: Geology, magma origin, and volcanism
Rapprich V,Hradecky P
Bulletin of Geosciences , 2005, DOI: 10.3140/bull.geosci.2005.04.277
Abstract: The Cinotepeque Range is a geological block in NW El Salvador with a complicated volcanic history. Due to the absence of data concerning the geological basement, it remains unclear when volcanic activity started in this zone. The oldest rocks found in the Cinotepeque Range are rhyolitic basal ignimbrites produced from unknown sources. Volcanic activity then proceeded with the silicic pyroclastic products of calderas, the activity of which can be traced up to the Holocene. It is difficult to identify the exact sources of individual pumiceous deposits. Their potential candidates are the three calderas Ilopango Antiguo, Old Coatepeque, and Chilamatal. Later, extrusions of lava sheets of 'inferior' and 'superior' andesites, interrupted by the deposition of agglomeratic pyroclastic flows, called 'Rana', covered the majority of the landscape. The Rana pyroclastic flows were most probably produced from Texistepeque Caldera located between the towns of Santa Ana and Metapán. The youngest volcanism is represented in this area by monogenic volcanic cones. Source vents of these youngest volcanic products are situated mostly on faults that cut and displace all older volcanic rocks. Two different processes of magma origin occurred during the volcanic history of this part of El Salvador: a) during the first stage magma originated by flux melting at a subduction zone; b) during the next stage the decompressional melting in a back-arc environment occurred.
Ages and geochemistry of Mesozoic-Eocene back-arc volcanic rocks in the Aysén region of the Patagonian Andes, Chile
Parada,Miguel A.; Lahsen,Alfredo; Palacios,Carlos;
Revista geológica de Chile , 2001, DOI: 10.4067/S0716-02082001000100002
Abstract: eighteen new radiometric ages (fourteen 40ar-39ar, four k-ar), combined with previously published ages, confirm the existence of three main extensional back-arc volcanic events, previously defined by stratigraphic relationships, in chilean patagonia (aysén region). these three events developed during the middle jurassic -early cretaceous (160-130 ma). cretaceous (114-75 ma), and eocene (55-46 ma). based on distinct geochemical data and sr-nd isotopic characteristics of the back-arc volcanic rocks collected north and south of 46°30's, two mesozoic-eocene magmatic domains are recognized: northern magmatic domain (nmd) and southern magmatic domain (smd). most analyzed basalts and intermediate volcanic rocks of the nmd have alkaline affinities and depleted to slightly depleted sr-nd isotopic values similar to those derived from an asthenosphere-dominated source. the smd mafic volcanic rocks have a subalkaline character and more enriched sr-nd isotopic signatures, comparable to those derived from a lithospheric source. the felsic volcanic rocks of the smd have lower end values and slightly higher initial 87sr/86sr ratios than the nmd felsic rocks, suggesting a larger crustal contribution in the magma sources. the geochemical and isotopic distinction between nmd and smd felsic rocks could be influenced by the presence of paleozoic metamorphic rocks as basement of the volcanic rocks of the smd. moreover, the compositional distinction between basalts of both domains may correspond to differences in magnitude of extension, the nmd being the one where the extension would have been greater and, consequently, the lithosphere thinner
Generation of Back-Arc Basins as Side Effect of Shortening Processes: Examples from the Central Mediterranean  [PDF]
Enzo Mantovani, Marcello Viti, Daniele Babbucci, Caterina Tamburelli, Nicola Cenni, Massimo Baglione, Vittorio D’Intinosante
International Journal of Geosciences (IJG) , 2014, DOI: 10.4236/ijg.2014.510091
Abstract: The evolution of the Mediterranean area since the Oligocene-Lower Miocene has been driven by the convergence of the surrounding plates. This implies that the observed deformation pattern in that region must be the most convenient shortening pattern, i.e. the one controlled by the minimum action principle. To understand why the fulfilment of such condition has required a complex spatio-temporal distribution of major tectonic events, such as uplift, lateral displacement and bending of orogenic belts, consumption of large lithospheric domains and formation of back arc basins, it may be very useful to take into account a basic tectonic concept, which helps to identify the process that can minimize the resistance of tectonic forces. Such concept starts from the fact that the most convenient consumption process is the one that involves low buoyancy oceanic lithosphere (Tethyan domains). However, such process is highly favoured where the oceanic lithosphere is stressed by vertical forces, a situation that develops when orogenic wedges are forced to over thrust and load the oceanic domain to be consumed. This interpretation can provide plausible and coherent explanations for the complex pattern of the observed deformations. In this view, the generation of back arc basins is taken as a side effect of an extrusion process, as suggested by numerical and mechanical experiments.
Ages and geochemistry of Mesozoic-Eocene back-arc volcanic rocks in the Aysén region of the Patagonian Andes, Chile
Miguel A. Parada,Alfredo Lahsen,Carlos Palacios
Revista geológica de Chile , 2001,
Abstract: Eighteen new radiometric ages (fourteen 40Ar-39Ar, four K-Ar), combined with previously published ages, confirm the existence of three main extensional back-arc volcanic events, previously defined by stratigraphic relationships, in Chilean Patagonia (Aysén region). These three events developed during the Middle Jurassic -Early Cretaceous (160-130 Ma). Cretaceous (114-75 Ma), and Eocene (55-46 Ma). Based on distinct geochemical data and Sr-Nd isotopic characteristics of the back-arc volcanic rocks collected north and south of 46°30'S, two Mesozoic-Eocene magmatic domains are recognized: Northern Magmatic Domain (NMD) and Southern Magmatic Domain (SMD). Most analyzed basalts and intermediate volcanic rocks of the NMD have alkaline affinities and depleted to slightly depleted Sr-Nd isotopic values similar to those derived from an asthenosphere-dominated source. The SMD mafic volcanic rocks have a subalkaline character and more enriched Sr-Nd isotopic signatures, comparable to those derived from a lithospheric source. The felsic volcanic rocks of the SMD have lower eNd values and slightly higher initial 87Sr/86Sr ratios than the NMD felsic rocks, suggesting a larger crustal contribution in the magma sources. The geochemical and isotopic distinction between NMD and SMD felsic rocks could be influenced by the presence of Paleozoic metamorphic rocks as basement of the volcanic rocks of the SMD. Moreover, the compositional distinction between basalts of both domains may correspond to differences in magnitude of extension, the NMD being the one where the extension would have been greater and, consequently, the lithosphere thinner Edades y geoquímica de las rocas volcánicas del trasarco del Mesozoico-Eoceno en la región de Aysén de los Andes patagónicos, Chile. Diez y ocho nuevas edades radiométricas (catorce 40Ar-39Ar, cuatro K-Ar) junto con las ya publicadas confirman la existencia de tres eventos volcánicos (previamente definidos por relaciones estratigráficas) en la Patagonia chilena (Región de Aysén) durante el intervalo Mesozoico-Eoceno: Jurásico Medio-Cretácico temprano (160-130 Ma), Cretácico (114-75 Ma) y Eoceno (55-46 Ma). Sobre la base de las características geoquímicas e isotópicas de Sr y Nd de las rocas volcánicas estudiadas, se pueden reconocer dos dominios magmáticos Mesozoico-Eoceno: Dominio Magmático Norte (DMN) y Dominio Magmático Sur (DMS). Los basaltos y rocas intermedias del DMN tienen afinidades alcalinas y valores isotópicos de Sr-Nd deprimidos a moderadamente deprimidos similares a aquellos derivados de una fuente dominada por material as
Middle Miocene calc-alkaline volcanism in Central Patagonia (47°S): petrogenesis and implications for slab dynamics
Espinoza,Felipe; Morata,Diego; Polvé,Mireille; Lagabrielle,Yves; Maury,René C; de la Rupelle,Aude; Guivel,Christèle; Cotten,Joseph; Bellon,Hervé; Suárez,Manuel;
Andean geology , 2010,
Abstract: we present a chronological (k-ar), petrologic and geochemical study (major and trace elements, sr-nd isotopes) of middle miocene (ca. 16-14 ma) calc-alkaline rocks (basalts to andesites) extruded in the present-day back-arc region of central patagonia (zeballos volcanic sequence (zvs), 47°s). this magmatism started shortly after mafic plutonism ceased in the are region (ca. 16 ma, 200 km west), and ended ca. 2 my before the onset of volumi-nous slab tear-related back-arc alkaline basaltic magmatism (ca. 12 to pliocene). the studied calc-alkaline rocks have a typical subduction-related signature (high lile/hfse ratios, depletion in nb, ta and ti; ba/la >20; ta/hf <1.5; (87sr/86sr)o=0.70366-0.70402, εnd=+0,1±3,8). major and trace elements contents are consistent with their evolution by closed system fractional crystallization of a presumed parental liquid similar in composition to the most basic rock of the suite. moreover, a strong subducted sediment imprint is recognized (increasing th/hfse and decreasing ce/pb during differentiation). however, these rocks show striking similarities with volcanic complexes emplaced above areas where a gently dipping slab oceurs (high k contents; similar lree/hree, nb/zr, ba/nb; th/hf; th/ta, ta/hf<0.3), particularly the present-day andean flat-slab region and the neuquén basin during the late miocene. a comprehensive tectono-magmatic model is here presented to explain the generation and extrusion of these calc-alkaline magmas during the middle miocene. the development of a transient low-angle subduction and the resulting eastward migration of the volcanic front are then proposed. mixing between stored remnants of calc-alkaline magmatism and later primitive alkaline melts is envisioned as the most likely process accounting for the transitional signature (le., intermediate between calc-alkaline and alkaline, la/nb >1; tio2 <2 wt%) of some basalts extruded synchronously with genuine alkaline lavas in the neogene patagonian plateau lava
Volcanic stratigraphy and evidence of magma mixing in the Quaternary Payún Matrú volcano, andean backarc in western Argentina
Hernando,Irene R; Llambías,Eduardo J; González,Pablo D; Sato,Kei;
Andean geology , 2012,
Abstract: the payún matrú volcanic field is located in the payenia basaltic province of the recent back-arc of western argentina (35°s-38°s). this province is younger than 5 ma, and most of its volcanic activity took place since 2 ma. the payún matrú volcanic field contains two composite volcanoes, payún matrú and payún liso, and two basaltic fields in an e-w oriented zone, located east and west of the payún matrú volcano. payún matrú is the largest volcano of this volcanic field, and consists of a shield-shaped edifice with a circular summit caldera of 8 km in diameter. the composition of both composite volcanoes is alkaline and predominantly trachytic, having also minor intermediate lavas. the basaltic fields consist of basalts and trachybasalts, with clinopyroxene and abundant olivine as phenocrysts and also in the groundmass. textures indicating mixing and mingling processes, such as dusty plagioclases along with clear ones, biotite replaced by anhydrous minerals and two groundmasses with a fluid-fluid relationship, are common in the early pre-caldera stage of payún matrú and some post-caldera lavas. the latest post-caldera lavas are trachytic, with clean sanidine phenocrysts without disequilibrium textures. a remarkable characteristic of the payún matrú volcanic field is the fact that the payún matrú caldera is surrounded by basaltic fields at its base, while no basalts were erupted in the caldera region. we propose that the absence of basaltic lavas in the payún matrú volcano is due to the presence of a magmatic chamber below it, and that the mafic magmas rising from deeper levels were unable to erupt without interaction with more evolved melts. intermediate hybrid magmas produced as a consequence of magma mixing and mingling between basaltic and trachytic magmas, are present in the early and mid-history of payún matrú volcano. we present here new information about the quaternary payún matrú volcanic field derived from field work, petrography, and major element geochemi
Forming age and tectono-petrogenises of the Jiugequan ophiolite in the North Qilian Mountain, NW China
XiaoHong Xia,ShuGuang Song
Chinese Science Bulletin , 2010, DOI: 10.1007/s11434-010-3207-3
Abstract: The Jiugequan ophiolite is one of the representative ophiolite fragments in the Early Paleozoic orogenic belt of the North Qilian Mountain. It has been drawn much attention and extensively studied in recent years. In this study, ion microprobe (SHRIMP) U-Pb dating was carried out for zircons from isotropic gabbro from the Jiugequan ophiolite. Eighteen analyses yield a relatively consistent apparent 206Pb/238U ages from 480 to 508 Ma with a weighted mean age of 490±5 Ma (MSWD=1.06), which is believed to be the crystallization age of the gabbro and thus the forming age of the Jiugequan ophiolite. Major and trace element geochemical study indicates that the diabase-basalts from the Jiugequan ophiolite have N-MORB and E-MORB characteristics with some subduction-related signatures. The petrological, geochemical and chronological data enable us to conclude that the Jiugequan ophiolite is most likely to be formed at a spreading center of back-arc basin during the early Ordovician, while the ancient Qilian oceanic plate subducted northwards. The acquisition of forming age and determination of tectonic setting for Jiugequan ophiolite provide significant constraints on the evolution of intra-oceanic subduction system in the North Qilian orogenic belt during the Early Paleozoic era.
The blueschist-associated perovskite-andradite-bearing serpentinized harzburgite from Dob iná (the Meliata Unit), Slovakia
Puti? M,Koppa M,Snárska B,Koller F
Journal of Geosciences , 2013, DOI: 10.3190/jgeosci.128
Abstract: Unique, perovskite(Prv)-andradite(Adr)-bearing serpentinized harzburgite fragments occur in Late Jurassic-Early Cretaceous subduction-accretionary wedge complex of the Meliata tectonic Unit (Slovakia, Western Carpathians). These rocks reflect magmatic (E1) and partial melting (E2) mantle events followed by metamorphic blueschist-facies (E3) and fluid-rock interaction (E4) events accompanied by crystallization of Prv and/or Adr. The incompletely solidified slightly depleted (residual) harzburgite composed of early-magmatic assemblage of Ol, Opx1, Cr-Spl, ± Cpx1 (E1 event) underwent partial melting (E2 event) under subsolidus mantle conditions. The late-magmatic aggregates of Cpx2, Cr-Spl, scarce Opx2, due to this batch melting, occur around the Opx1 (±Cpx1) porphyroclasts. The fractures and cleavage planes of the kinked Opx1 porphyroclasts are filled by Cpx3 as a result of subsolidus recrystallization of the Opx1. Cloudy lower Al Cpx4 associated with pargasite in reaction rims between the Opx1 and Cpx2, or antigorite replacing Ol, could represent the E3 high-pressure metamorphic event (dated at 160 to 150 Ma, according to published 40Ar-39Ar data for “phengitic” white mica from blueschists) in serpentinites. Serpentinized zones of harzburgite contain Prv, which occurs in serpentinized Opx1 porphyroclasts. Orthopyroxene (Opx1) breakdown and formation of Prv, (Ti-rich) Adr and chrysotile appears to be postdating the closure of the Meliata back-arc basin (c. 200 to 160 Ma) and HP metamorphism. Perovskite genesis can therefore be bound to the exhumation-related serpentinization in subduction channel and emplacement of serpentinites into subduction-accretionary wedge (E4 event at 300-500 °C/5-15 kbar, at 150 to 130 Ma?). The bulk-rock trace-element patterns suggest enrichment in LREE, Ba, Cs, U, Pb, Sb and Li in comparison with HREE, HFSE, Rb and Sr. Such element mobility indicates fluid-rock interaction. Pervading aqueous fluids rich in CO2 were transporting Ca, Ti, Fe, LREE partitioned into Prv grown along the Cpx exsolution lamellae in Opx1. The sources of such fluids might have been the blueschist- to greenschist-facies rocks from the Meliata mélange type subduction-accretionary wedge. The late-metamorphic H2O-CO2/Si-Fe-Mn-rich fluids caused partial replacement of Prv by pyrophanite and crystallization of (Ti-rich) Adr at the expense of Cpx and Prv.
Sm-Nd dating and Nd-Sr isotopic characteristics of the Shimian ophiolite suite, Sichuan Province
Weizhou Shen,Shijin Xu,Jianfeng Gao,Zhusheng Yang,Qiwen Yang
Chinese Science Bulletin , 2002, DOI: 10.1360/02tb9415
Abstract: By measuring the Sm-Nd and Rb-Sr isotopic compositions of harzburgite and gabbro from Shimian ophiolite suite, we got the whole rock Sm-Nd isochron age of (938±30) Ma (2σ), and the εNd of 7.6±0.8 (2σ), which shows that the ophiolite was formed at the Early Neoproterozoic. The obvious change (0.70209–0.70708) of ISr values of the ophiolite is caused by the meteoric hydrothermal alteration. The high εNd values indicate that the primitive magma was derived from the intense depleted mantle reservoir. It is suggested that this area was in a back-arc basin environment during the Early Neoproterozic.
Palaeoproterozoic mafic and intermediate metavolcanic rocks in the Turku area, SW Finland
Markku V?is?nen,Gustav Westerlund
Bulletin of the Geological Society of Finland , 2007,
Abstract: Twenty two samples were analysed from mafic and intermediate volcanic rocks from the Turku area along ~60 km long north-south geotraverse. Geographically and lithologically the samples were divided into the Pargas, Turku and Vahto groups. The Pargas and Turku groups consist of mafic units and intercalations within migmatitic gneisses. Their geochemical compositions are very similar. In average, they show quite flat REE curves and the multielement diagrams show moderate LILE enrichments, minor negative Ta-Nb anomalies and straight HFSE curves. We infer these as transitional between MORB and VAB. The Vahtogroup consists of thicker volcanic sequences in connection with synorogenic plutonic rocks. The data show enriched LREEs, LILEs, Th and P combined with negative Ta-Nb anomalies, i.e. the Vahto group shows subduction component. We interpret that the Vahto volcanicrocks belong to the H me belt. The geochemical data indicate that the Pargas and Turku groups were formed in extensional, possibly back-arc or intra-arc setting while the Vahto group was formed at continental volcanic arc.
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