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

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.