Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

J.E. OTAMENDI A,E,*, G.I. VUJOVICH B,E, J.D. DE LA ROSA C, A.M. TIBALDI A,E, A. CASTRO C, R.D. MARTINO D,E, L.P. PINOTTI A,E

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: LONDRES; Año: 2009 p. 258 - 279

0895-9811

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crustthat formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina.Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form thelower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearingcumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. Themafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, correspondingto pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxedinto the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mgnumber(ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al2O3 contents.This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al2O3 content crystallized from mantle-derived melts before mafic magmas reached thecrustal levels currently exhumed. However, some gabbronorites have incompatible trace element signaturestypical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those ofthe mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneoussuite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites