Abstract

Extension behind a Late Jurassic continental margin volcanic arc in southern Chile caused rifting and the development of a narrow marginal basin floored by oceanic crust. Extension ceased and the basin was closed and uplifted in mid-Cretaceous time, so the basin floor is now exposed as the upper part of an autochthonous ophiolite complex composed of gabbros, sheeted dikes, and pillow lavas, with minor plagiogranite and associated siliceous dikes. Many of the rocks are altered. The metamorphic grade increases from zeolite or greenschist facies in the pillow lavas to amphibolite facies in the gabbros, but the maximum intensity of recrystallization occurs in the sheeted dike unit and is associated with loss of Rb and K and increasing K/Rb ratio, contrasting with the effects produced by low-temperature alteration of basalts by sea water. Metamorphic effects seem to be related to hydrothermal convective systems operating at the spreading axis at the time of basin formation.

Geochemically, the rocks have affinities with mid-oceanic ridge basalts, but K, Rb, and Ba contents and Ba/Sr and Ce/Yb ratios are higher and K/Rb ratios are lower in the least altered rocks than in mid-oceanic ridge basalts. Similar features are apparent in some other marginal basin basalts. Fractionation trends are tholeiitic, the mafic rocks displaying a wide range of Fe/Mg ratios (0.9 to 5.2) but without any concomitant silica enrichment. Rare-earth elements, TiO2, and Zr correlate positively and Cr and Ni negatively with Fe/Mg, while the gabbros have lower contents of some incompatible elements as a result of their cumulate nature.

The leucocratic rocks within the mafic complex have been derived from two distinct sources. Some trondhjemites and granophyres have compositions indicating derivation by refusion of continental material bordering the mafic complex. The plagiogranites, however, have a distinctive geochemistry, consistent with an origin by high-level differentiation of the mafic magmas. Such rocks, normally lying in or just below the sheeted dike unit, may be a common if minor component of oceanic crust.

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