238U230Th226Ra disequilibria in young Mount St. Helens rocks: time constraint for magma formation and crystallization
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Isochron dating
Andesites
Magma chamber
Fractional crystallization (geology)
Pyroxene
Melt inclusions
The Miocene Karamağara volcanics (KMV) crop out in the Saraykent region (Yozgat) of Central Anatolia. The KMV include four principal magmatic components based on their petrography and compositional features: basaltic andesites (KMB); enclaves (KME); andesites (KMA); and dacites (KMD). Rounded and ellipsoidal enclaves occur in the andesites, ranging in diameter from a few millimetres to ten centimetres. A non-cognate origin for the enclaves is suggested due to their mineralogical dissimilarity to the enclosing andesites. The enclaves range in composition from basaltic andesite to andesite. Major and trace element data and primitive mantle-normalized rare-earth element (REE) patterns of the KMV exhibit the effects of fractional crystallization on the evolution of the KME which are the product of mantle-derived magma. The KMA contain a wide variety of phenocrysts, including plagioclase, clinopyroxene, orthopyroxene, hornblende and opaque minerals. Comparison of textures indicates that many of the hornblende phenocrysts within the KMA were derived from basaltic andesites (KMB) and are not primary crystallization products of the KMA. Evidence of disequilibrium in the hybrid andesite includes the presence of reacted hornblendes, clinopyroxene mantled by orthopyroxene and vice versa, and sieve-texture and inclusion zones within plagioclase. The KMV exhibit a complex history, including fractional crystallization, magma mixing and mingling processes between mantle and crust-derived melts. Textural and geochemical characteristics of the enclaves and their hosts require that mantle-derived basic magma intruded the deep continental crust followed by fractional crystallization and generation of silicic melts from the continental material. Hybridization between basic and silicic melts subsequently occurred in a shallow magma chamber. Modelling of major element geochemistry suggests that the hybrid andesite represents a 62:38 mix of dacite and basaltic andesite. The implication of this process is that calc-alkaline intermediate volcanic rocks in the Saraykent region represent hybrids resulting from mixing between basic magma derived from the mantle and silicic magma derived from the continental crust. Copyright © 2005 John Wiley & Sons, Ltd.
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Andesitic rocks, of the Pliocene Myojin-iwa volcanic field, in the northern part of Niigata Prefecture, Northeast Japan, show transitional chemical characteristics between typical calc-alkaline and tholeiitic series. The most primitive basaltic andesite among these andesitic rocks (the Myojin-iwa Formation) has geochemical characteristics similar to those of other Pliocene-Quaternary basaltic rocks from the back-arc side of the NE Japan, and has initial Sr isotope ratio slightly higher than those of the latter basaltic rocks.Andesites from the Myojin-iwa area show SiO2 contents ranging from 53.3% to 62.8, and initial Sr and Nd isotope ratios vary from 0.70324 to 0.70378 and 0.512791 to 0.512926, respectively. The initial Sr isotope ratios display positive correlation with SiO2, and their initial Nd isotope ratios gradually decrease with increasing SiO2. This suggests that the genesis of andesites from the area can not be attributed to simple fractional crystallization of the primary basaltic magma, but to an assimilation and fractional crystallization (AFC) process. Based on petrographical evidence, major and trace element compositions and Sr-Nd isotope systematics, the Paleogene granitoid rocks constituting the upper crust beneath the Myojin-iwa and surrounding areas are the possible candidates for assimilants. An AFC model using the granitoid rocks as the assimilant can successfully reproduce the chemical variations of the andesites from the Myojin-iwa Formation by assuming the r values (ratio of the rate of assimilation to the rate of fractional crystallization) of less than 0.2.
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The Austral Volcanic Zone of the Andes (AVZ, 49–55°S) consists of six Pleistocene to Recent volcanic centres resulting from the subduction of the Antarctic plate beneath southernmost South America. Andesites from the southernmost volcano in the AVZ, the Cook Island volcanic complex, have very distinctive geochemical characteristics, including high MgO, CaO, Sr, K/Rb and LREE/HREE, low FeO/MgO, K, Rb, Ba and HREE, and Sr, Nd, O and Pb isotope compositions similar to mid-ocean ridge basalts. These high-MgO andesites are interpreted to have formed by small (<5%) degrees of partial melting of subducted oceanic lithosphere (MORB), followed by rapid upward migration of these partial melts so that near-surface fractional crystallization and crustal contamination were minimal. Andesites and dacites from Mt. Burney, the next volcanic centre to the north, and from Aguilera, Nunatak and Lautaro, the three most northerly volcanoes in the AVZ, have progressively higher SiO2, K, Rb, Ba, 87Sr/86Sr and δ18O, and lower MgO, CaO, Sr, Ni, Cr, K/Rb and 143Nd/144Nd. These rocks are interpreted to have formed by increasing degrees of near-surface fractional crystallization, combined with small amounts of crustal contamination of a parental composition similar to the high-MgO andesites from Cook Island, the most primitive magma type identified in the AVZ. These south-to-north changes in the geochemistry and petrogenesis of the volcanic centres of the AVZ may result from variations in the stress pattern within the continental lithosphere due to the change from direct convergence to strike-slip plate motion which occurs along the southernmost margin of South America.
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There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt—andesite-dacite-rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt-trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of ‘within-plate’ character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust.
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A set of andesites closely related to the Au mineralization was developed from Jintan Gold Deposit. Element concentration of the andesites and geochemical correlation analysis were made. According to the results, both the zircons structure of the andesites and ω(Th)/ω(U) (0.30-1.02, ω is element mass fraction) indicate that the zircons are the origin of magma, and the zircon U-Pb age is (303.65±4.8) Ma, which stands for the formation age of andesites. The ω(SiO2) of andesites is from 54.29% to 69.05%, ω(Al2O3) is from 11.27% to 19.46% and ω(K2O+Na2O) is from 2.46% to 6.81%, ω(MgO) is from 3.60% to 10.45% and Mg index changes from 46 to 66. The light rare earth elements (LREE) of andesites is enriched, and the differentiation between LREE and HREE is obvious. Eu of the vast majority of the sample shows moderate negative anomaly (δω(Eu)=0.44-0.77), while Ce shows slight negative anomalies (δω(Ce)=0.93~0.98). It is enriched in large ion lithophile elements (such as K, Rb, Ba, Th and U), and depleted in high field strength elements (such as Nb and Ti). The magma undergones the fractional crystallization and partial melting. The andesites are probably formed by partial melting of mantle wedge, which is metasomatized by subducting fluids when the Kanggurtag ocean southward subducts, oceanic crust dehydrates and subducts into the mantle wedge. Meanwhile the magma undergones fractional crystallization, and suffers significant continental crust material contamination. Based on comprehensive analysis, subduction of the Kangguer ocean is still underway in the East Tianshan mountain region at the time of Late Carboniferous.
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