Cenozoic plutonism geochemistry in the extra-andean southern Patagonia as evidences of arc migration process
Alejandro Sánchez ValenzuelaMichel de Saint BlanquatFrancisco HervéMireille PolvéDiego MorataPhillipe De ParsevalMathieu Benoît
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Abstract:
In this work, we present the results of a petrographic and geochemical study that involves seven Cenozoic plutons located in a NS trending belt in western Patagonia, south of the present Chile Triple-Junction and to the east of the present magmatic arc. Four plutons were studied in the northern end, and three in the southern part of the belt. The petrographic and geochemical characteristics (major and trace element contents, eNdt, 87Sr/86Sr initial ratios), of these plutons are different enough to propose a further classification for them: Only the Monte Balmaceda intrusive complex in the southern end of the belt is alkaline (sensu stricto). The Río de Las Nieves and San Lorenzo plutons in the northern area, and the Torres del Paine intrusive complex, in the southern area have “intra-plate transitional” calc-alkaline affinity. Paso de Las Llaves and Cerro Negro del Ghío plutons in the northern area, and Cerro Donoso pluton in the southern area show “arc transitional” calc-alkaline character. The “transitional” character, together with the pluton’s location, and their regional geological context, can be explained by a Mio-Pliocene eastward arc migration coeval with the beginning of the Chile Rise subduction. A slab flattening hypothesis is favoured to explain the arc-migration, which together with the different length and time of arrival of the Chile Rise segments to the subduction zone, contributed to the heterogeneous geochemistry of the studied plutons.Keywords:
Back-arc basin
Island arc
Andesites
Island arc
Volcanic arc
Back-arc basin
Basement
Continental arc
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Back-arc basin
Island arc
Riphean
Volcanic arc
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The facial and geochemical peculiarities of the Late Emsian island arc complexes of the Main Urals Fault Zone (Southern Urals) are considered. The results of geodynamic reconstructions are given. It was shown that the rock composition of the complexes formed in three geodynamic settings: 1) on the island arc slope of the deep-water trench, 2) within the frontal island arc, and 3) in the junction zone of the northern edge of the frontal island arc and back-arc basin. The facies of the island arc slope include serpentinite-clastic edaphogenic breccias containing Co-Cu-massive sulfide mineralization and polymictic olistostrome. Volcanogenic strata and intrusive formations represent the frontal arc. Low-Ti magmatites belonging mainly to the tholeiitic and boninitic series and having a metallogenic specialization for massive sulfide mineralization dominate in the southern areas of the suture zone. Moderate-Ti magmatites of the tholeiitic composition, productive on Au-Cu-porphyry mineralization, are distributed in the northern regions. A gold-bearing volcano-intrusive association has formed in the junction zone of the northern edge of the frontal island arc and back-arc basin, where moderate-Ti rocks of tholeiitic and transitional from tholeiitic to calc-alkaline composition dominate. It is assumed that the differences in the geochemical characteristics and metallogenic specialization of the Late Emsian magmatites are associated with the changes in the geodynamic conditions of magma formation and ore genesis along and across the strike of the island arc system.
Island arc
Back-arc basin
Breccia
Forearc
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Continental arc
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Metasomatism
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Primary lateral connection of preaccretionary, particularly island-arc structures are commonly faulted by postaccretionary tectonics in the mosaic-block structure of the Central Asian fold belt. Tannu-Ola-Khamsara island-arc system is marked out as one of the largest island-arc structures of the Central Asian fold belt, this is a large fragment island arc-back-arc basin system that existed in Paleo-Asian Ocean at Vendian-beginning of Early Cambrian. Tannu-Ola-Khamsara island-arc system has a north-eastern strike, its length is about 550 km, its width is about 300 km, it contacts with turbidites of Western Sayan on the north-west, with metamorphites of Sangilen on the south-east. Obtained geochemical data indicate the fact of zone inclination of paleo-subduction in direction from Ondum to Tannu-Ola subzone of Tannu-Ola-Khamsara island-arc zone. The study of clust composition of magmatic and carbonate rocks from Low Cambrian conglomerates of Bayankol suite and their correlation with magmatic and carbonate rocks from island-arc complexes of Ondum subzone of Tannu-Ola-Khamsara zone was carried out by other free investigate methods for the purpose of confirmation of Ondum subzone frontal position. Bayankol suite was accumulating in the middle of Early Cambrian in the forarc basin, which apparently links to the geomorphological island-arc structure of Tannu-Ola-Khamsara island-arc system folded by volcanics of Late Vendian-Early Cambrian volcanic-plutonic series of significantly acidic composition. Island-arc zone uplift was fixed by the middle of Early Cambrian whereby plutonic rocks of noted volcanic-plutonic series lifted in an erosion level, disintegration of which has led to clust appearance in Bayankol suite conglomerates. It proves by correlation of geochemical and paleontological data on granitoid and carbonate rocks from conglomerate clusts of Bayankol suite and island-arc complexes of Ondum subzone Tannu-Ola-Khamsara island-arc zone. Ondum and Tannu-Ola subzones are one segment of island-arc area, thereby island-arc structures of Tannu-Ola subzone locate in backarc of Ondum subzone. Uplift process of island-arc zone is confirmed by stratigraphic break of Low Cambrian Ilchir suite formation and formational breaks of suite of analogue formation. A scheme of structural-formational zoning of caledonides of Tannu-Ola-Khamsara island-arc system is carried out.
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The Uyandino-Yasachnaya magmatic arc is the largest volcanic-plutonic belt in the north-east of Russia. However, there is yet to be a consensus on the nature of the arc, despite a long study of the rocks composing it. Most researchers consider it to be an island arc formation, however, some researchers also believe that it of a riftogenic or heterogeneous nature. The arc is composed of volcanogenic-sedimentary strata of variegated composition and associated subvolcanic formations. The rocks were formed during the Oxfordian-Volgian stage; their formation either preceded that of the Late Jurassic-Early Cretaceous granitoid massifs, or was synchronous with it. The research focused on volcanogenic and subvolcanic formations of the southeastern part of the arc in the midstream of the Indigirka River. The purpose of the research was to determine their composition and geodynamic conditions of formation. For this purpose, the study of the structure of volcanogenic strata and sub-volcanoes, their petrographic and chemical compositions, relations with granitoids was carried out. The volcanogenic strata within the territory are represented by two formations of rhyolite and their clastolavas: low-alkaline tholeiitic Oxfordian-Kimmeridgian formation and calcalkaline Medium Kimmeridgian � Early Volgian formation. The lower formation developed in the island-arc setting the upper formation developed under transitional conditions of the island-arc to the marginal-continental regime. The subvolcanic massifs have a dacite-rhyodacite composition; they intrude volcanogenic strata of both these suites and are metamorphosed near the contacts of the Early Cretaceous massifs. Their parental melt was produced at the boundary of amphibolite and dacite-tonalite substrates at maximum temperature (up to 1050oC) and pressure (up to 11.1 kbar). They are of Middle or Late Volga age and formed in the initial stages of the development of the active margin of the continent.
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Rock samples have been dredged clming S0-35 and SEAPSO JV cruises (1984 and 1985) from cliJTemnl spreading ridges in lhe Lau Basin, soulh-weslern Pacific. They were analyzed for major and lrace elemenls and isotopic ralios (Sr, Nd). Nmlhem Lau Basin samples show N-MORB type characlerislics, bolh in major andlrace elemenls, whereas soulhern Lau Basin ones are differenlialed and show significanl Ta-Nb negalive anomalies in lheir extended REE pallerns, which is characlerislic of subcluclion zones.lsolopic ralios (Sr, Nd) are quile helerogeneous and inlermediale bel ween MORB and island arc values. The norlhern Lau Basin Ridge has geochemical characlerislics suggeslive ofmedio-oceanic lype spreading, whemas lhe soulhem LauBasin Ridge (Valu Pa Ridge) wilh ils geochemical characleristics influenced by subcluclecl slab material, could eilher be a nascenl island arc or corresponcllo an inilial slage of back-arc opening. A comparison of lhe LauBasin resulls vvilh lhose from the North Fiji Basin andlhe Okinawa Trough shows geochemicallrencls thalmflecllhe evolution from a rifling slags (island arc charac:Lerislics) loa malum slage (MORB characteristics) of back-arc basins.
Island arc
Back-arc basin
Trace element
Trough (economics)
Volcanic arc
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Island arc
Volcanic arc
Back-arc basin
Forearc
Peridotite
Obduction
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The case for separate Taltson and Thelon orogenies: Evidence from the Shield in western Saskatchewan
SUMMARY The Taltson magmatic zone is inferred to be the southern extension of the Thelon tectonic zone; together they are thought to have developed during a ca. 2.02-1.90 Ga orogeny that accreted the Rae and Slave cratons. The continuity between the Thelon tectonic and Taltson magmatic zones is predicated upon the similarity of the ages of igneous rocks and a linear aeromagnetic high that apparently links the two. The Taltson magmatic zone contains two suites of metaplutonic rocks: ca. 1.986-1.1959 Ga continental arc type plutons; and 1.955-1.910 Ga peraluminous plutons. They intruded a basement complex dominated by Mesoarchean to Paleoproterozoic orthogneisses and granitoid rocks. Peak metamorphic conditions were between 1.94 and 1.93 Ga. New mapping south of the western Athabasca Basin indicates that plutons of generally intermediate composition extend from the Virgin River shear zone beneath the western Athabasca Basin. The limited available geochronological data supports that interpretation as rocks with ages similar to the continental arc type plutons in the Taltson magmatic zone are found near the Virgin River shear zone, beneath the western Athabasca Basin and along basin‟s western margin. Plutons of similar age and composition to the peraluminous plutons of the Taltson magmatic zone and similar metamorphic ages are found in the basement to the western Athabasca Basin and the exposed Shield to the south. These data indicate that the Taltson magmatic zone extends into Saskatchewan, and is not spatially related to the Taltson aeromagnetic high, which represents the signature of the Taltson basement complex and not the igneous rocks of the Taltson magmatic zone. The wide distribution of rocks similar in age to those of the Taltson magmatic zone suggest that the Thelon orogeny is more complex than implied in most published models and likely included multiple magmatic arcs and accretionary events. This inference is supported by differences structural characteristics and metamorphic ages along the proposed orogenic belt.
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