Late Miocene to Quaternary volcanic rocks from the frontal arc to the back‐arc region of the Central Volcanic Zone in the Andes show a wide range of δ 11 B values (+4 to −7 ‰) and boron concentrations (6 to 60 ppm). Positive δ 11 B values of samples from the volcanic front indicate involvement of a 11 B‐enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust‐mantle origin for these lavas. The δ 11 B values and boron concentrations in the lavas decrease with increasing depth of the Wadati‐Benioff Zone. This across‐arc variation in δ 11 B values and decreasing B/Nb ratios from the arc to the back‐arc samples are attributed to the combined effects of boron‐isotope fractionation during progressive dehydration in the slab and a steady decrease in slab‐fluid flux toward the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Three‐component mixing calculations for slab‐derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab‐fluid component dominates the boron composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30% crustal material. Modeling of fluid‐mineral boron‐isotope fractionation as a function of temperature shows that dehydration reactions liberate continuously changing fluid compositions from the slab during progressive subduction. A combination of a boron‐isotope fractionation model and a temperature model for the Central Andean subduction zone fits the across‐arc variation in δ 11 B and we conclude that the boron‐isotope composition of arc volcanic rocks, especially in island arcs, is dominated by changing δ 11 B‐composition of boron‐rich slab‐fluids during progressive dehydration. Owing to the decrease in slab‐derived fluid flux crustal contamination becomes more important toward the back‐arc. Because of the boron‐isotope fractionation effect, across‐arc variations in δ 11 B need not necessarily reflect different mixing proportions between boron derived from the slab‐fluid and the mantle wedge.

Slab

Volcanic arc

10.1029/2002gc000438

Cite

Citations (125)

Upper-Crustal Structure of the Central Andes Inferred from Dip Curvature Analysis of Isostatic Residual Gravity

Andes (2006)

Ulrich Riller Hans‐Jürgen Götze Sabine Schmidt Robert B. Trumbull Fernando Hongn

The relationship between Bouguer gravity, isostatic residual gravity and its dip curvature, first-order structural elements and distribution of Neogene volcanic rocks was examined in the southern Altiplano and Puna Plateau. In the southern Altiplano, strong positive Bouguer gravity corresponds to areas affected by late Cenozoic faulting and large-scale folding of upper crustal rocks. Dip curvature analysis of isostatic residual gravity shows that elongate zones of maximum curvature correspond remarkably well with the structural grain defined by first-order folds and faults. Similarly, isostatic residual gravity in the Puna is largely controlled by prominent, upper-crustal structures and also by the distribution of Miocene and younger volcanic rocks. In particular, the Central Andean Gravity High, one of the most prominent features of the residual gravity field, corresponds with domains of low topography, i.e., internally- drained basins, which are surrounded by zones of Neogene faults and abundant felsic volcanic rocks. Dip curvature analysis of the isostatic residual gravity field shows that elongate zones of maximal curvature correlate with the strike of prominent Neogene faults. Our study suggests that such analysis constitutes an important tool for imaging upper-crustal structures, even those that are not readily apparent at surface. For example, upper-crustal faults in the Salar de Atacama area, the presence of which is suggested by the dip curvature of residual gravity, offers a plausible explanation for the pronounced angular departure of the volcanic belt from its overall meridional trend and its narrowing south of the salar. In contrast to previous interpretations, our study suggests that gravity anomalies of the Central Andes are largely controlled by the distribution of late Cenozoic volcanism and tectonism. Dip curvature analysis of gravity fields bear great potential for elucidating first-order structural elements of deformed, upper-crustal terrains such as the modern Andes.

Neogene

Lineation

10.1007/978-3-540-48684-8_15

Cite

Citations (13)