Geologic controls of hydrogen isotope ratios of structural water in serpentinites from San Pedro Limon-Palmar Chico, Tierra Caliente Terrane, Mexico

Abstract

The area of San Pedro Limon-Palmar Chico is geologically located in the Tierra Caliente Complex. The complex is composed of an Albian-Cenomanian volcano sedimentary sequence of island arc type. A series of volcanic edifices is interpreted from the features of volcanic and sedimentary facies. The sedimentary sequences (mainly clastic) were deposited in small basins. structurally controlled by the volcanoes. In San Pedro Limon, a 15 km2 dioritic to peridotitic stock intrudes mainly the sedimentary sequence, whereas small wedges of serpentinites not larger than I km2 are emplaced along N-NW structural trends. The San Pedro Limon stock is interpreted as having been emplaced by a torsion mechanism during a Neogene transpressive disturbance. The stock shows a transitional petrographic variation from hornblende-rich diorites to clinopyroxene-rich peridotites with intermediate hornblende pyroxenite (orthopyroxene<1 0%) and pyroxene hornblendite facies. Nine ultramafic samples. ranging from 10% to 100% serpentinization, were analyzed by infrared spectroscopy. Only low-temperature serpentines such as lizardite and chrysotile were identified. Hydrogen isotopic ratios were determined on structural water from nine serpentinites and one sample of chrysotile. 00 ranges from -85 to -116 per mil. Which unequivocally indicates a meteoric origin. Furthermore, the 00 values decrease with increasing degree of serpentinization.The serpentinization must have occurred at low temperature under kinetic conditions. The main serpentinization phase is associated with a tectonic transpressive regime, IWld occurred simultaneously with the mechanical emplacement. of intrusives which developed local hydrothermal conditions.