PETROLOGY OF THE PRECAMBRIAN MAFIC DYKES FROM NOVILLO GNEISS (OAXAQUIA), NE MEXICO

At the front of the Sierra Madre Oriental (SMOr) lies a laramidic structure known as the Huizachal-Peregrina Anticlinorium (HPA) which is important in NE Mexico because of its eroded core displaying the basement of the SMOr. The basement has been divided into: 1) Novillo Gneiss (Mesoproterozoic), 2) Granjeno Schist (late Paleozoic), 3) a tonalitic body (Mississippian) and, 4) a deformed Sedimentary Sequence (Silurian, and Carboniferous-Permian). The present study focused on the petrogenesis of a series of dykes intruded in the Novillo Gneiss at Novillo, Peregrina, and Caballeros Canyons located at the HPA. These dykes may be separated into two different generations: 1) pre-metamorphic and, 2) pos-metamorphic.

The pre-metamorphic dykes (from Novillo Canyon) intruded the protoliths of the Novillo Gneiss. The dykes were classified as amphibolites, with granoblastic and cataclastic textures and a mineralogical assembly of Grt + Qtz + Afs + Op + Cpx + Am + Bt + Ap. The chemical composition indicates that these rocks are tholeiitic basalts related to an OIB magma type, whose high REE concentrations are due to the magmatic source, to a certain degree of differentiation of the magma, and to a REE migration during granulite metamorphism. The tectonic setting for these dykes was identified as intraplate.

The pos-metamorphic dykes (from all canyons) were classified as gabbroic porphyrites with a paragenesis consisting of Pl + Cpx + Chl/Ser + Op + Bt in intergranular and porphyritic textures. Geochemically these dykes represent tholeiitic basalts, which are linked to a hot spot magmatism of an OIB source. The pos-metamorphic dykes from Novillo Canyon have a REE enrichment, and the dykes from Peregrina and Caballeros Canyons show minor degrees of evolution associated to the lowest REE concentrations.

Two different processes for the generation of the dykes are proposed. The first process comprises the pre and pos-metamorphic dykes from Novillo Canyon, which corresponds to an OIB source which generated magmas that lightly evolved during their ascent, in addition to metamorphism. The second one concerns the pos-metamorphic dykes from Peregrina and Caballeros Canyons, where these magmas were produced from an OIB source as well, and were hardly differentiated, thus displaying the closest characteristics to the original OIB source.