The Bandurrias gabbro: Late Oligocene alkaline magmatism in the Patagonian Cordillera

Abstract

The authors present petrological and geochemical data for the Late Oligocene (ca. 27 Ma) Bandurrias gabbro, a small (approximate to 4.5 x 3 km), isolated intrusion near Coyhaique (45degrees35' Lat. S), southern Chile, in the present-day backarc region of the Patagonian Cordillera. Its mineral chemistry (olivine, clinopyroxene, feldspars, amphibole, biotite and analcime) is typical of gabbros with alkaline affinity. The development of ophitic and subophitic textures, the extremely elongated acicular apatite crystals, and the low CaO content of olivines (0.17-0.46%), as well as the low Al-VI/A(IV) ratio (0.00-0.12) of clinopyroxenes, indicate low-pressure (subvolcanic) crystallization conditions. The major, trace, and rare earth element geochemistry also indicate an alkaline affinity (Sigma(alkali) = 4.25-7.20%, high TiO2 and P2O5 contents, presence of normative nepheline, Ti/V > 50). Geochemical patterns are very similar to those of an ocean island-type alkaline basalt. (La/Lu)N ratios range from 6.99 to 10.60, and the YbN and LUN values (< 10) suggest the presence of garnet as residual phase in the mantle source. Low Th/La ratios (0.11-0.15) are similar to those of a primitive mantle, and the Zr/Nb (6.3-7.6), La/Nb (0.76-1.03), Ba/Nb (7.6-17.4), Ba/Th (81-120), Th/Nb (0.09-0.15), K/Nb (370-467), Th/La (0.11-0.15), and Ba/La (10.0-16.9) ratios are typical of an enriched mantle-type reservoir. The magma composition suggests generation by low degree partial melting in an extensional or transtensional geodynamic setting. The emplacement of the Bandurrias gabbro took place during a major reconfiguration in the Nazca-South American plate convergence during 2826 Ma, in either the latest phase of slow, oblique subduction or the subsequent faster, less oblique period of subduction. The data do not allow the authors to rule out either alternative.