PETROGENESIS OF LATE CRETACEOUS ADAKITE INTRUSIVES IN PUERTO LIBERTAD, SONORA, MÉXICO

Paleozoic and Mesozoic sequences in the Puerto Libertad region (NW Sonora, Mexico) are commonly intruded by Laramidic Granitoids. Petrogenetic study of such bodies across a north to south transect at Sierra Picú, El Destierro, Sierra Tordilla and Sierra Los Mochos, spans up to 35 km of NW-SE-oriented extension in the region. Intrusive bodies display abundant concentrations of spheroidal ferromagnesian minerals and are intruded by aplitic and pegmatitic dike swarms. U/Pb zircon dates yield ages between 71.9 and 74.9 Ma. Granodioritic and monzogranitic rocks dominate the north and center region, while sienogranitic varieties dominate the south sector. A subtle propylitic alteration of chlorite+calcite+epidote appears to selectively affect the ferromagnesian minerals. In addition, they show a notable structural affectation, marked by normal and listric faults, evidence of this include cataclastic rocks with macro and micro faulting. Geochemically, this magmatism shows a relatively high silica content (56-75 wt%), values of Sr/Y= 29-50, La/Yb =26-39, Sr up to 600ppm and Ba up to 1400 ppm. Collectively these traits accentuate the adakitic signature of these rocks. Geothermobarometry data calculated from amphiboles and plagioclase suggest temperatures for the intrusives reached 680 – 701 °C and pressures reached 1.83 -2.31 Kb, suggesting that the intrusives were emplaced at depths between 4.9 and 6.2 km. These petro-tectonic relationships suggest that a crustal fusion process was involved in generation of laramide adakitic intrusives.