Characterization of metacarbonate and adjacent rocks in El Mezquite creek, La Paz plutonic complex, Baja California Sur, Mexico: succession of geologic events according to field relations and geochronology

Abstract

The La Paz Plutonic Complex is located in southern Baja California Peninsula. In this complex calc-silicate rocks are associated with other metamorphic and intrusive rocks. The focus of this research is on the calc-silicate rocks in the eastern part of the Los Cabos Block with marble, calc-silicates and amphibolitic gneiss which extend for more than two kilometers and grouped as an informal unit called El Mezquite calc-silicates. Field relationships, megascopic, and microscopic characteristics indicate that the protoliths of the metasedimentary rocks were siliciclastic rocks alternated with calcareous sediments and volcaniclastic rocks deposited in a marine environment and subsequently metamorphosed. In the Arroyo El Mezquite, outcrops of undeformed igneous rocks of tonalitic composition and amphibolitic gneisses and orthogneiss are observed, theses last. The metamorphic unit is in contact with the undeformed intrusives and the calc-silicate rocks. This lithology indicates two metamorphic events, the first of regional character, represented by orthogneiss and amphibolitic gneiss and the second as contact metamorphism, represented by migmatites. Geochronology of the tonalite and the amphibolite gneiss with the U-Pb method on zircons yielded ages of 85.44 ± 0.66 Ma and 116 ± 1.5 Ma, respectively, indicating to magmatic events. The first age indicates indirectly the maximum age for contact metamorphism, while the second allows us to infer the possible minimum age of deposition and potentially the maximum age of regional metamorphism. The data obtained from the zircons of tonalite and amphibolitic gneiss were used to estimate their emplacement temperature of 650 ± 33 °C and 701 ± 42 °C, respectively. The presence of minerals such as diopside, wollastonite and rutile traces indicate temperatures of at least >700 °C of the metamorphism. The regional metamorphism reached the amphibolite facies. The El Mezquite calc-silicate rocks were deposited in the zone between a continental platform and an oceanic basin, subsequently affected by regional metamorphism (<116 Ma) and a contact metamorphic overprint (>85 Ma).