The Andes - Magmatism
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Despite growing acceptance that several mantle, crustal, and subducted reservoirs contribute to arc magmas along continental margins, there is still no real consensus concerning the proportions of these contributions nor concerning the loci and mechanisms of mixing among source components or among variably evolved magma batches.
In order to elucidate this question Hildreth and Moorbath (1988) completed a geochemical study of 15 Quaternary andean volcanic centers. These volcanoes are equidistant from the Chile trench along the volcanic front of a single arc segment, and beneath which the nature and age of the down-going plate and the rate and geometry of subduction all appear to be nearly constant. In contrast to this, the thickness and the average age of the continental crust markedly increases northward along the segment (Figure 1).

Figure 1. Major late Quaternary volcanic centers of South-Central Andes. JFR schematically indicates the zone of impingement of the Juan Fernandez Ridge on the Chile Trench. Paleozoic (Pz) and Mesozoic (Mz) basement rocks crop out widely in the Coastal Cordillera of Chile and in the Cordillera Frontal of Argentina.  River-mouth sediment samples are indicated: (A): Aconcagua river; (Mp): Maipo river; (R): Rapel river; (Mq): Mataquito river; (H): Huenchullami river; (Ml): Maule river. From Hildreth and Moorbath (1988).

The results obtained from this study showed that from South to North  along the volcanic front (at 57.5 % SiO2 ) K2O rises from 1.1 to 2.4 wt% (Figure 2 and 3), Ba from 300 to 600 ppm , and Ce from 25 to 50 ppm, whereas FeO/MgO declines from >2.5 to 1.4 (Figure 4).

Figure 2. Elevations relative to sea-level and K 2O contents at 57.5 % SiO2 of centers on the volcanic front between latitudes 33 oS and 37.5o S. Bouguer gravity-anomaly profile along the volcanic front is also shown. Northward increase in K2Ocorrelates with apparent crustal thickness. Shaded field encloses the range of (K 2O)57.5 values for rock suites from volcanic front centers farther south (38 o- 41.5o S). >From Hildreth and Moorbath (1988).

Figure 3. K2O - SiO 2 variation (in wt. %) in suites of volcanic rocks from centers along the volcanic front, 33o S and 37o S. Separate panels are used to reduce clutter and have no other significance. From Hildreth and Moorbath (1988).

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 South to North (Figure 4), but K/Rb drops steeply and scatters greatly within many  (biotite free) andesitic suites (Figure 5). 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 (Figure 6).

Figure 4. Least-squares regression lines for  K 2O vs. SiO2 and FeO*/MgO vs. SiO2 for volcanic front centers in central Chile. TH: tholeiitic; CA: calcalkaline. From Hildreth and Moorbath (1988).

Figure 5. Sr and Nd isotopic ratios vs. latitude for central Chile volcanic centers. Inset shows distribution of these data relative to MORB and bulk-earth (BE) values. Upright crosses are rhyolites. X's in upper panel represent Sr-isotopic values of river-mouth sediments from five major river systems that drains parts of the Andes indicated by the latitudinal lenghts of the horzontal bars. From Hildreth and Moorbath (1988).

Pb isotopes data define a limited range that can be on both sides of the Stacey-Kramer line, is bracketed by values  of local  basement rocks, in part plots above the field  of Nazca plate sediments, and shows no indication of a steep (mantle + sedimentary) Pb mixing trend (Figure 7). 87Sr/86Sr values rise northward from 0.7036 to 0.7057, and 143Nd/144Nd values drop from 0.5129 to 0.5125 (Figure 8).

Figure 6. Sr content (ppm) vs 87 Sr/86Sr for volcanic-front centers 33o- 37o S. Only the Marmolejo suite (CM) shows an extended correlation, interpreted as a result of mid- to upper- crustal AFC and illustrated by Curves A-E, all of which employ 700 ppm Sr and 0.7046 as deep-crustally established base-level values of ascending magma. Curve A represents simple mixing with a contaminant that averages 100 ppm Sr at 0.710. AFC Curves B-E reflect the following parameters: Sr content and  87Sr/ 86Sr of average contaminant, bulk D Sr, and r = mass assimilation rate/crystallization rate. Curve B (100, 0.750, 2.0, 0.2) models assimilation of an upper-crustal Permian granite. Curve C (200, 0.712, 1.5, 0.5), Curve D (200, 0.720, 1.5, 0.2), and Curve E (200, 0.710, 1.0, 0.5) model assimilation of Paleozoic mid-crustal intermediate rocks, illustrating effects of various values of D and r. Ticks on curves are in 10% increments of wt% contaminant for A, and of fraction of the initial magma remaining for B-E. Tupungato (CT), Alto (CA), Planchon (PPA), and the high-Sr part of the Azul (AZ) suite may also reflect less pronounced AFC arrays. From Hildreth and Moorbath (1988).

Figure 7. Pb-isotope values for volcanic-front centers between 33 oS and 36o S. Data from individual centers are connected by tie lines except for Descabezado Grande, Cerro Azul and Planchon-Peteroa-Azufre to reduce clutter. Cerro Marmorejo shows wide range. VP-11 represents a Tertiay rhyolitic intrusion near Volcan Palomo. Field for 37o-41 o S include data from Antuco, Llaima, Villarica, Puyehue and Osorno. Field for Rhyolites include data from Laguna del Maule, Rio Puelche, and Loma Seca Tuff rhyolites, all behind the front. S-K is the average crustal Pb-evolution curve. From Hildreth and Moorbath (1988) and references here within.

Figure 8. 208Pb/ 204Pb versus 207 Pb/ 204Pb for volcanic-front centers between 33o and 41 o S. Pm: Permian Granite; Pz: Paleozoic granitic xenoliths. >From Hildreth and Moorbath (1988).

A northward climb in basal elevation of volcanic front  edifices from 1350 m to 4500 m elevation coincides  with a Bouger anomaly gradient from –95 to –295 mgal (Figure 2), interpreted to indicate a 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.
The thick northern crust contains an abundance of Paleozoic and Triassic rocks whereas the proportion of younger arc intrusive increases southward. 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 intracrustral AFC can subsequently modify ascending magmas, but the base level geochemical signature at each center reflects the depth of its zones of melting, assimilation, storage and homogenization and age, composition and proportional contribution of the lowermost crust.

In general we can concluded that there is an important contribution of slab components to magma compositions and that sediments have been subducted in enormous volumes beneath the entire arc segment. Chemical and isotopic changes in the magmas cannot be attributed only to the mantle wedge but are also caused by significant contributions derived from the continental crust.

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Authors: Fernando Barra, Robert Fromm, Victor Valencia