PETROGENETIC STUDY FROM THE ANOROGENIC MAFIC-ULTRAMAFIC MAGMAS CLOSE TO THE CAMBRIAN-PRECAMBRIAN BOUNDARY IN NW SONORA REGION

The Caborca region located NW of Mexico, within the Sonora desert area, contains a very well represented succession of Precambrian and Cambrian rocks. A very detailed study of the Precambrian sequences have been made by authors such as Stewart et at. (1984), among others, mainly from a stratigraphic thematic. This work is one of the first studies of this sequence from a petrological view on the igneous rocks in the region.

A particular interest is taken on the sequences close to the Cambrian-Precambrian boundary. The exact position of the igneous rocks located on the lower unit of the Puerto Blanco Formation and La Ciénega Formation, is still under debate. Our studies on the igneous rocks recognize two main groups. The first is represented by the ultramafic sills and lava flows on the units 2 and 3 of La Ciénega Formation. They are characterized by an absence of primary plagioclase (<5%), pyroxene and olivine phenocrystals have been completely replaced. The second group is related to the Cambrian Puerto Blanco Formation, and is represented by mafic lava flows that evolve from ultramafic picrobasalts to trachybasalts.

The geochemical studies evidence an ultramafic magmatism with values of Mgv >60, Cr >950 ppm and Ni >400 ppm obtained from the Precambrian rocks, and Mgv >60, Cr<670 ppm, Ni<250 ppm for the Cambrian lava flows. The magmatism is related to the Alkaline magma series (norm Ne <5%), although the Cambrian sequence present a Transitional magma series affinity (absence of norm Ne and Qz). The REE diagrams display an enrichment on LREE and an absence of negative Eu anomaly. The spider diagrams present a typical OIB hump specter characterized by a positive anomaly on Nb-Ta. The anorogenic association of these rocks is supported by several tectonic setting discrimination diagrams based on highly incompatible elements (Hf, La, Nb, Ta, Th, Y, Yb, Zr). Virtually all rocks present some degree of alteration or metamorphism. On the older and ultramafic rocks, all the primary mineralogy seems to be erased and is replaced mainly by carbonate minerals. On the younger Cambrian flows a green schist facies is represented by epidote, chlorite and actinolite association.

The radiometric dating has been an issue due to the metamorphism present, and the absence of zircons as phenocrystals, although more precision techniques are been explored.