Descripción preliminar de la geología y mecánica de emplazamiento del complejo ultrabásico del cretácico de Loma Baya, Guerrero, México

Abstract

In Papanoa, the coast of Guerrero cuts across a Mid-Cretaceous magmatic belt, within which four mafic-ultramafic complexes are emplaced. One of them is the Loma Baya complex. It covers an area of O. 75 km2 and intrudes volcanic and hypabysal rocks of mainly intermedia te composition. In order of abundance the ultramafic intrusive is formed by olivine clinopyroxenites, wehrlites, clinopyroxenites, dunites, minor hornblende pegmatites and chromitites. Olivine averages Fo 90 and clinopyroxenes are diopside and diopsidic augi.te. Orthopyroxenes are virtually absent in the whole sequence. Chemically, the ultramafic suite is richer in FeO than ophiolitic suites and calk-alkalic cordilleran intrusives from western Unites States, Canada and Alaska. Microprobe analyses revealed various oxides, sorne of which were identified as seudostratified primary magnetite and ferrochromite, as well as secondary magnetite, formed during the serpentinization process. The former ones clearly differ from the primary oxides because their Si02 and MgO contents are higher, between 8 to 20°/o and 14 to 24%, respectively. With respect to other localities, massive chromites show higher Mg content and lower Cr. Main structural features such as foliation and internal brecciation show a vertical tendency averaging 73%. The whole complex seems to be emplaced following a N-04 E plane, dipping 53°/o W. It is considered that the ultramafic emplacement took place during the Upper Cretaceous through a diapiric mechanism. The forced intrusión of the complex is explained as a result ofvolume increase (30%?) anda density loss approximate to one third of the ultramafic mass during the serpentinization.