Journal ArticleDOI
Crustal contributions to arc magmatism in the Andes of Central Chile
Wes Hildreth,Stephen Moorbath +1 more
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In this article, 15 andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis.Abstract:
Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280±20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO2) K2O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward — opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. 87Sr/86Sr values rise northward from 0.7036 to 0.7057, and 143Nd/144Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from −95 to −295 mgal, interpreted to indicate thickening of the crust from 30–35 km to 50–60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30–40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust.read more
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Journal ArticleDOI
Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust
TL;DR: Two geochemical proxies are particularly important for the identification and classification of oceanic basalts: the Th-Nb proxy for crustal input and hence for demonstrating an oceanic, non-subduction setting; and the Ti-Yb proxy, for melting depth and hence indicating mantle temperature and thickness of the conductive lithosphere as mentioned in this paper.
Journal ArticleDOI
Chemical composition and evolution of the upper continental crust: Contrasting results from surface samples and shales
TL;DR: The average chemical composition of the upper continental crust (UC) as a function of age is estimated from chemical analyses, geologic maps, stratigraphic sections and isotopic ages as discussed by the authors.
Journal ArticleDOI
The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones
TL;DR: In this article, a model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modeling, which is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones.
Journal ArticleDOI
A model for Trondhjemite-Tonalite-Dacite Genesis and crustal growth via slab melting: Archean to modern comparisons
Mark S. Drummond,Marc J. Defant +1 more
TL;DR: In this paper, the authors introduced the importance of subducted oceanic crustal age on arc petrogenesis and demonstrated that Archean TTD crustal generation processes are also present in selected high-Al Phanerozoic TTD terranes.
Journal ArticleDOI
Element transport from slab to volcanic front at the Mariana arc
TL;DR: Agarwal et al. as mentioned in this paper presented a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab.
References
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The continental crust: Its composition and evolution
TL;DR: In this paper, the authors describe the composition of the present upper crust and deal with possible compositions for the total crust and the inferred composition of lower crust, and the question of the uniformity of crustal composition throughout geological time is discussed.
Journal ArticleDOI
Approximation of terrestrial lead isotope evolution by a two-stage model
J.S. Stacey,Jan Kramers +1 more
TL;DR: In this paper, a two-stage model for terrestrial lead isotope evolution is proposed, which permits the age of the earth to be that of the meteorite system and also yields good model ages for samples of all ages.
Journal ArticleDOI
A Guide to the Chemical Classification of the Common Volcanic Rocks
T. N. Irvine,W. R. A. Baragar +1 more
TL;DR: In this paper, a system was presented whereby volcanic rocks may be classified chemically as follows: Subalkaline Rocks:A.B. Tholeiitic basalt series:Tholeitic picrite-basalt; tholeiite, tholeitic andesite; dacite; rhyolite.
Journal ArticleDOI
Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types
E. Bruce Watson,T. Mark Harrison +1 more
TL;DR: In this article, the saturation behavior of zircon in crustal anatectic melts as a function of both temperature and composition has been studied and a model of Zr solubility given by: In D Zr Zircon/melt = −3.80−[0.85(M−1)]+12900/T where T is the absolute temperature, and M is the cation ratio (Na + K + 2Ca)/(Al · Si).
Book
Orogenic Andesites and Plate Tectonics
TL;DR: In this article, the authors define Orogenic Andesite and discuss its properties and properties, including the following: 1.1 Topography, gravity, heat flow, and conductivity.
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