Temporal variations in the influence of the subducting slab on Central Andean arc magmas: Evidence from boron isotope systematics

摘要

The Pampean flat-slab segment in the southern Central Andes represents an ideal setting at which to investigate how changes in the tectonic configuration of a subduction zone (convergence angles and rates, seamount subduction and shallowing slab angle) affects the recycling of subducted components to arc magmas. To constrain sources, particularly of slab-derived fluids and their contribution to arc magmatism, boron isotope and select major and trace element compositions were determined for pyroxene- and zircon-hosted melt inclusions obtained from a suite of Paleocene to Miocene arc magmatic rocks, from the southern Central Andes. Considerable changes in delta B-11 values and boron concentrations are observed with time. Significantly lower delta B-11 values (average = -1.9 +/- 2.2 parts per thousand (1 sigma)) and B/Nb ratios (average =3.3 +/- 1.3 (1 sigma)) were obtained for melt inclusions from Oligocene arc rocks similar to 24 Ma) compared to those from the Paleocene (similar to 61 Ma) (averages = +1.6 +/- 0.8 parts per thousand and 17.8 +/- 1.4 (1 sigma), respectively) and the Miocene (similar to 18 Ma) (averages = +4.7 +/- 1.9 parts per thousand and 11.9 +/- 5.5 (1 sigma), respectively).