Interaction between the Pinchal Granodiorite and the Kilometric Xenolite of Cerro Negro, Sierra de Moreno, Antofagasta Region, Chile.

Abstract

The processes associated with the interaction between the Pinchal Granodiorite (PG) and the kilometric Cerro Negro Xenolith (KCNX) are studied. The PG was cooled during the Late Carboniferous, whereas the age of the KCNX remain until now unknown. The KCNX is immersed in the top of the PG. It is formed by a 600 m thick stratified succession of hornfels including sandstones, pelites and conglomerates. The sandstones are observed in the hornfels showing granoblastic fabric with recrystallized plagioclase, quartz, biotite, actinolite, hornblende and garnet. These minerals were formed at temperatures comprised between 320 and 550°C. The PG is mainly granodioritic and, in minor proportion, monzdioritic. In the western flank of the KCNX a porphyritic facies is present in the PG, whereas in its eastern contact the pluton mainly exhibits equigranular facies. In the western edge, the contact of the KCNX with the PG corresponds to a ductile shear zone. In the eastern edge, the contact is characterized by assimilation, presence of metasedimentary small xenoliths and injections of granitic material. In the latter, magmatic foliation and oblate enclaves are observed, whose deformation is related with the flow associated to the descendent movement of the KCNX. The KCNX display megascopic structures such as synclinal folding and deflection of layers in its northern part, which are interpreted as due to the gravitational fall of the KCNX in the magma of the PG by a stoping mechanism. The homoclinal attitude of the KCNX in its southern part, with increase of the dip of the layers between the western edge and the eastern edge, is interpreted as stretching that accompanied the tilting towards the southeast during its fall. Foliation and lineations within the KCNX represent ductile penetrative fabrics during this deformation. In addition ptigmatic veins, centimetric folds and boudinage are observed. Also structures of fragile-ductile character as tension gashes are present. On the basis of the analysis of strain in deformed conglomerates a coaxial deformation with a prolate ellipsoid developed. Tectonites are classified as LS. The history of the process of stoping of the KCNX is reconstructed, which started with its loosening of the country rock, metamorphism, deformation and, finally, collapse and halting within the mass of the PG. The present work demonstrates that magmatic stoping can occur on a great scale in the upper part of a pluton.