The genetic relationship between andesites and dacites at Tungurahua volcano, Ecuador (original) (raw)
Related papers
Lithos, 2005
In order to provide mantle and crustal constraints during the evolution of the Colombian Andes, Sr and Nd isotopic studies were performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in the Granatifera Tuff, a deposit of Cenozoic age, in which mantle-and crustal-derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions. Garnet-bearing xenoliths are the most abundant mantle-derived rocks, but websterites (garnet-free xenoliths) and spinel-bearing peridotites are also present in minor amounts. Amphibolites, pyroxenites, granulites, and gneisses represent the lower crustal xenolith assemblage. Isotopic signatures for the mantle xenoliths, together with field, petrographic, mineral, and whole-rock chemistry and pressure-temperature estimates, suggest three main sources for these mantle xenoliths: garnet-free websterite xenoliths derived from a source region with low P and T (16 kbar, 1065 8C) and MORB isotopic signature, 87 Sr/ 86 Sr ratio of 0.7030, and 143 Nd/ 144 Nd ratio of 0.5129. Garnet-bearing peridotite and websterite xenoliths derived from two different sources in the mantle: i) a source with intermediate P and T (29-35 kbar, 1250-1295 8C) conditions, similar to that of sub-oceanic geotherm, with an OIB isotopic signature ( 87 Sr/ 86 Sr ratio of 0.7043 and 143 Nd/ 144 Nd ratio of 0.5129); and ii) another source with P and T conditions similar to those of a sub-continental geotherm (N38 kbar, 1140-1175 8C) and OIB isotopic characteristics ( 87 Sr/ 86 Sr ratio=0.7041 and 143 Nd/ 144 Nd ratio=0.5135).
New δ 18 O data from magmatic quartz, plagioclase and zircon crystals in Neogene large-volume, rhyodacitic ignimbrites from the Central Andean Ignimbrite Province reveal uniformly high-δ 18 O values (δ 18 O (Qtz) from + 8.1 to + 9.6‰ -43 analyses from 15 ignimbrites; δ 18 O (Plag) from + 7.4 to + 8.3‰ -10 analyses from 6 ignimbrites; δ 18 O (Zrc) from + 6.7 to +7.8‰ -5 analyses from 4 ignimbrites). These data, combined with crustal radiogenic isotopic signatures of Sr, Nd and Pb, imply progressive contamination of basaltic magmas with up to 50 vol.% upper crust in these large volume silicic systems. The narrow range of δ 18 O values also demonstrate that surprising homogeneity was achieved through space (100's km) and time (~10 Ma to recent) in these large-volume magmas, via residence in their parental middle to upper crustal bodies. Low-δ 18 O values of many large volume (>10 km 3 ) silicic magmas in North America and Kamchatka, discussed here for comparison, reflect the influence of meteoric-hydrothermal events and glaciations in lowering these δ 18 O values via the assimilation of hydrothermally-altered crustal material. Conversely, there is a scarcity of a low-δ 18 O signature in the Central Andes and subduction-related or influenced systems in North America, such as the Oligocene Great Basin of Nevada and Utah, the Southern Rocky Mountain Volcanic Field of Colorado, and the SW Nevada volcanic field system. In these regions, the generally heavy-δ 18 O magmatic signature is interpreted as a reflection of how a broadly compressional regime, high elevation, aridity and evaporation rates limit availability and infiltration of large amounts of surface meteoric water and hydrothermal alteration of the shallow crust. This leads us to speculate that the δ 18 O values of large volume silicic magmas in these areas record a paleoelevation and paleoclimate signal. If this is the case, δ 18 O values of ignimbrites can potentially be used to track the effects of a meteoric-hydrothermal derived δ 18 O signature from upper crustal rocks that are subsequently assimilated to produce these magma types.
GEOCHEMICAL JOURNAL, 2004
This study reports new geochemical and radiogenic isotope data for Pliocene to Holocene (<2.8 Ma) alkaline (trachybasalt, basaltic trachyandesite, and trachyandesite) as well as subalkaline (basalt to dacite) volcanic rocks from the Michoacán-Guanajuato volcanic field (MGVF), located in the west-central part of the Mexican Volcanic Belt (MVB). There is no clear correlation of most geochemical parameters with differentiation indicators such as SiO 2 . The rare-earth elements show light-REE enrichment, flat heavy-REE pattern, and absence of Eu anomaly. Depletion of Nb as compared to large ion lithophile elements such as Ba is observed for most rocks, probably suggesting involvement of subducted Cocos or Rivera plate. However, other HFSE such as Zr and Ti and key trace elements such as B and Be, and isotopic data do not support this conclusion. In most binary plots, the MGVF data lie at mantle compositions toward the lower end of subduction-input parameters, as is the case of magmas from well-known rifts; this contrasts with the data for subductionrelated Central American Volcanic Arc (CAVA) rocks that clearly show high values of subduction input. The average isotopic ratios of the MGVF rocks show the following ranges: .66. There are apparently no significant differences between the isotopic ratios for alkaline and subalkaline rocks, although the Sr and Pb isotopic ratios for alkaline rocks are somewhat higher and Nd isotopic ratios lower than those for subalkaline rocks. The available geochemical and isotopic evidence does not support the origin and evolution of the MGVF magmas by a simple model such as simple fractional crystallization (FC), nor by direct (slab melting) or indirect (fluid transport to the mantle) participation of the subducted Cocos plate. Instead, it appears that the MGVF basic magmas were generated in a heterogeneously veinedmantle source enriched in LILE, HFSE, and REE, but the intermediate and acid magmas could also contain a crustal component.
Geology, 1975
Low-silica latite highly enriched in largeion-lithophile elements and moderately potassic low-Si andesite were erupted in central Peru during late Cenozoic time. FeO*/MgO ratios of 0.93 to 1.25 at 53 wt percent Si0 2 indicate a definitely calcalkalic character. The combination of low FeO*/MgO ratios, low Si0 2 , and high Cr, Co, and Ni with large-ion-lithophile and light rare-earth elements makes it very unlikely that the parent magmas were produced by high-pressure partial melting of subducted ocean-floor basalt (eclogite). The data are more compatible with small degrees of partial melting of ultramafic material or mixtures of basalt and ultramafic mantle. The initial melts probably contained 52 to 53 percent SiO z and had a content of large-ion-lithophile elements nearly as high as that of the rocks. Sr 87 /Sr 86 ratios of from 0.7042 to 0.7051 and low to very low Rb/Sr ratios indicate an isotopically variable source region that, at least in part, had earlier been depleted in Rb relative to Sr.
Journal of Geophysical Research, 1989
Mexico, southwestern New Mexico, and Arizona are commonly capped by basaltic andesites, most from 29-20 Ma. We refer to these mafic lavas as the Southern Cordilleran Basaltic Andesite (SCORBA) suite, and they may constitute the most extensive Cenozoic basaltic suite in North America. The SCORBA suite has trace element and isotopic characteristics of orogenic (arc) rocks (e.g., Ba/Nb > 40), and silica content (53-56% SiO2) like the Grande Ronde Basalt, which represents about 80% of the volume of the Columbia River Group. Geochemical and isotopic data are presented on SCORBA lavas and rare mafic lavas (PRE-SCORBA) interlayered with older ignimbrites from a 700-km-long NE-SW transect of southern Chihuahua, Mexico. SCORBA and PRE-SCORBA lavas with relatively low K/P (<7) and differing Ba/Nb (50 versus 18) have similar isotopic compositions, arguing against their isotopic signatures being controlled by crustal assimilation. Along the entire length of the transect, the basaltic rocks have ENd and 87Sr/86Sr near bulk Earth and 2ø6pb/2ø4pb and 2ø7pb/2ø4pb ratios that lie along a 1.7 Ga pseudoisochron. The Pb isotopic variation is geographically controlled, becoming more radiogenic from east to west, reflecting mixing in mantle source regions. The eastern mantle source has low 2ø6pb/2ø4pb and is a mixture of an enriched, enriched-mantle-like (EMI) component with one or more depleted components, which could include an intraplate component with relatively high Nb/Y (>0.8). The western mantle source contains less of the intraplate component and is more oceanic in character. Overprinting both the eastern and western sources is a Cenozoic subduction component that is responsible for the western radiogenic Pb, and this component fades out inland to the east. This transect crosses the inferred position of the Mojave-Sonora megashear, previously proposed to be a major lithospheric boundary, separating Proterozoic basement to the east from Phanerozoic basement to the west at the latitude of the transect. Most chemical changes near the inferred position of the megashear are subtle, and they may be gradational rather than abrupt. The uniformity of Sr and Nd isotopic compositions across the inferred position of the megashear indicates that one or more of the following statements is true: (1) Phanerozoic and Proterozoic subcontinental lithospheres are essentially indistinguishable in Sr and Nd compositions in southern Chihuahua, (2) the megashear is not a lithospheric boundary separating Phanerozoic and Proterozoic crust in the vicinity of the transect, or (3) the isotopic signatures were acquired in the asthenosphere rather than in subcontinental lithosphere.
Journal of Volcanology and Geothermal Research, 2017
Subduction-related magmas that erupted in the Central Andes during the past 10 Ma are strongly affected by crustal assimilation as revealed by an increase in 87 Sr/ 86 Sr isotope ratios with time that in turn are correlated with increased crustal thickening during the Andean orogeny. However, contamination is not uniform and can be strongly influenced locally by crustal composition, structure and thermal condition. This appears to be the case along the NW-SE San Pedro-Linzor volcanic chain (SPLVC) in northern Chile, which straddles the boundary of a major zone of partial melt, the Altiplano_Puna Magma Body (APMB). Herein we report 40 Ar/ 39 Ar ages, compositional and isotope data on lavas from the SPLVC that track the influence of this zone of partial melting on erupted lavas with geochronological and geochemical data. Ages reported here indicate that SPLVC has evolved in the last 2 M.y., similar to other volcanoes of the Western Cordillera (e.g. Lascar, Uturuncu, Putana). 87 Sr/ 86 Sr ratios increase systematically along the chain from a minimum value of 0.7057 in San Pedro dacites to a maximum of 0.7093-0.7095 for the Toconce and Cerro de Leon dacites in the SE. These changes are interpreted to reflect the increasing interaction of SPLVC parental magmas with partial melt within the APMB eastwards across the chain. The 87 Sr/ 86 Sr ratio and an antithetic trend in 143 Nd/ 144 Nd is therefore a proxy for the contribution of melt from the APMB beneath this volcanic chain. Similar 87 Sr/ 86 Sr increases and 143 Nd/ 144 Nd decreases are observed in other transects crossing the boundary of the APMB. Such trends can be recognized from NW to SE between Aucanquilcha, Ollagüe, and Uturuncu volcanoes, and from Lascar volcano to the N-S-trending Putana-Sairecabur-Licancabur volcanic chain to the north. We interpret these isotopic trends as reflecting different degrees of interaction of mafic parental melts with the APMB. High 87 Sr/ 86 Sr, and low 143 Nd/ 144 Nd reveal zones where the APMB is thicker (~20 km) and more melt-dominated (~25% vol. partial melt) while lower 87 Sr/ 86 Sr, and higher 143 Nd/ 144 Nd reveal thinner marginal zones of the APMB where lower contents of partial melt (b10% vol) involves reduced interactions. The lowest Srisotope ratios, and higher Nd-isotope ratios (where available) occur in magmas erupted outside the APMB (e.g. San Pedro, Lascar and Aucanquilcha volcanoes), indicating a diminished influence of crustal partial melts on parental mafic magmas. These geochemical parameters provide a useful tracer for the extent and significance of crustal partial melt bodies in magma genesis in the Central Andes.
Revista Geologica de Chile
Miocene to Holocene volcanic rocks ot Chimborazo and associated volcanos in the Westem C:>rdillera ot Ecuador range in composition trom basal tic andesite to rhyolite. They were erupted through Cretaceous accretionary matic basement. Three calk-alkaline magmatic trends can be distinguished in the Pleistocene and Holocene volcanic rocks. They are interpreted to reflect magma evolution within the immature continental crust. Early Plaistocene and Late Pleistocene volcanic rocks ranga in composition trom basaltic andesite to dacite and are characterized by a moderate increase in alkalinity and Iittle variation in the KlRb, Baila, Thrra, and SrlY ratios. They show distinct geochemical differences that are attributed to variable proportions ot lowercrustal melts in the mafic end members. Volcanic rocks have lower 143Nd/ 1 4-4Nd ratios (0.5128 to 0.5129) than the basement, suggesting an origin ot this isotopic characteristics trom subduction modified manlle. Chemical modelling suggests that Late Pleistocene basalts may contain about 12% more of a lower crustal melt than Early Pleistocene parental melts. The development within both magmatic suites is explained by shallow level magma evolution. Other medium-K to high-K Pleistocene andesites may have been produced by mixing ot a low-K basaltic melt generated trom subduction-modified mantle and a high silica partial melt trom the lower crust containing rutile and gamet. Partial melting ot lower crust is supported by high SrlY (-200), Thrr a (-42), and La/Yb ratios (-60) in the high-K andesites. 143Nd/ 1 4-4Nd ratios (0.51292) and Ó 18 0 values (-+9%0) in the high-K andesites are indistinguishable trom those in Cretaceous accretionary basement and support a genetic relationship.
Journal of Petrology, 2012
Fonualei is unusual amongst subaerial volcanoes in theTonga arc because it has erupted dacitic vesicular lavas, tuffs and phreomagmatic deposits for the last 165 years. The total volume of dacite may approach 5 km 3 and overlies basal basaltic andesite and andesite lavas that are constrained to be less than a few millennia in age. All of the products are crystal-poor and formed from relatively low-viscosity magmas inferred to have had temperatures of 11001 0008C, 2^4 wt % H 2 O and oxygen fugacities 1^2 log units above the quartz^fayalite^magnetite buffer. Major and trace element data, along with Sr^Nd^Pb and U^Th^Ra isotope data, are used to assess competing models for the origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution of the within-dacite compositional array by closed-system crystal fractionation of a single magma batch. An origin by partial melting of lower crustal amphibolites cannot reproduce these data trends or, arguably, any of the dacites either. Instead, we develop a model in which the dacites reflect mixing between two dacitic magmas, each the product of fractional crystallization of basaltic andesite magmas formed by different degrees of partial melting. Mixing was efficient because the two magmas had similar temperatures and viscosities. This is inferred to have occurred at shallow (2^6 km) depths beneath the volcano. U^Th^Ra disequilibria in the basaltic andesite and andesite indicate that the parental magmas had fluids added to their mantle source regions less than 8 kyr ago and that fractionation to the dacitic compositions took less than a few millennia.
Journal of Petrology, 2008
Establishing the petrogenesis of volcanic and plutonic rocks is a key issue in unraveling the evolution of distinct subduction-related tectonic phases occurring along the South American margin. This is particularly true for Cenozoic times when large volumes of magma were produced in the Andean belt. In this study we have focused on Oligo-Miocene magmatism in central Chile at 338S. Our data include field and petrographic observations, whole-rock major and trace element analyses, U^Pb zircon dating, and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene, and zircon mineral separates. Combined with earlier dating results the new zircon ages define a 28Á8^5Á2 Ma period of plutonic and volcanic activity that ceased as a consequence of flattening subduction of the NazcaF arallon plate. Rare earth elements patterns are variable, with up to 92 times chondrite concentrations for light rare earth elements yielding (La/Yb) N between 3Á6 and 7Á0, and an absence of Eu anomalies. Initial Pb isotope signatures are in the range of 18Á358^19Á023 for 206 Pb/ 204 Pb, 15Á567^15Á700 for 207 Pb/ 204 Pb and 38Á249^39Á084 for 208 Pb/ 204 Pb. Initial 87 Sr/ 86 Sr are mostly in the range of 0Á70369^0Á70505, with two more radiogenic values at 0Á7066. Initial Hf isotopic compositions of zircons yield exclusively positive eHf i ranging between þ 6Á9 and þ 9Á6. The newly determined initial isotope characteristics of the Oligo-Miocene magmas suggest that the mantle source lithologies are different from both those of Pacific mid-ocean ridge basalt and ocean island basalt, plotting in the field of reference values for subcontinental lithospheric mantle, characterized by moderate large ion lithophile element^high field strengh element depletion and high 238 U/ 204 Pb. A Hf model age of 2 Ga is estimated for the formation of the subcontinental mantle^continental crust assemblage in the region, suggesting that the initial Sr and Pb isotope ratios inferred for the source of the Oligo-Miocene parental magmas are the result of later Rb and U enrichment caused by mantle metasomatism. A time-integrated model Rb/Sr of %0Á039 and m % 16 are estimated for the source of the parental magmas, consistent with ratios measured in peridotite xenoliths from continental areas. Evolution from predominant (490%) basaltic^gabbroic to andesitic^dioritic magmas seems to involve a combination of (1) original trace element differences in the metasomatized subcontinental mantle, (2) different degrees of partial melting and (3) fractional crystallization in the garnet-and spinel-peridotite stability fields.The genesis of more differentiated magmas reaching rhyolitic^granitic compositions most probably also includes additional crystal fractionation at both shallow mantle depths and within the crust, possibly leading to some very minor assimilation of crustal material.