Top-down structures of mafic enclaves within the Valle Fértil magmatic complex (Early Ordovician, San Juan, Argentina)

Abstract

Magmatic structures related to the mechanical interaction between mafic magmas and granitoids have been studied in the Valle Fértil calc-alkaline igneous complex, Argentina. Excepcional outcrops with vertical walls of more than 300 m high allow us the study of three-dimensional geometries of individual blobs of mafic magma as well as the geometry of pipe-like structures in which mafic microgranular enclaves are concentrated in more than 50 times the normal abundance in the granodiorite mass. The shape of enclaves and pipe-like structures are interpreted as the ressult of top-to-down intrusions of a mafic magma into a granodiorite-tonalite mass. These sinking structures are the result of a reverselly stratified magma chamber with gabbros and diorites at the top and granodiorite-tonalite at the bottom. They may account for most of the structures found in microgranular enclaves and magma mingling zones that characterize calc-alkaline batholiths. Synplutonic intrusions from the top is the only plausible mechanism to account for the observed structures. The model may be of general application to calc-alkaline batholiths characterized by the presence of mafic microgranular enclaves. An implication of these reverselly stratified magma chambers is the presence of a petrological inversion which may be the consequence of cold diapirs emplaced below the mantle wedge in a suprasubduction setting.