TIME AND GEOCHEMISTRY OF PSEUDOTACHYLYTE ASSOCIATED WITH THE LATE CRETACEOUS SAN JUAN DE LOS PLANES FAULT ZONE NEAR LA PAZ, BAJA CALIFORNIA SUR, MEXICO

Frictional melts could result as a consequence of transforming kinetic energy—due to rapid fault slip—to sufficient heat (T>>1000°C) to trigger melting of host rock mineralogy. The resulting melt cools very rapidly, instantaneously, to ambient temperature of the host and solidifies a dark glass called pseudotachylyte.

The Late Cretaceous San Juan de Los Planes fault (SJLPF) zone near La Paz, Baja California Sur, is one of the main fault systems—possibly a compressional fault originally—affecting the Mesozoic Los Cabos granitic and metamorphic block. This is a pseudotachylyte-bearing fault zone that was later exhumed from a deeper structural level in the upper crust into the surface. The main lithology hosting the dark glass veinlets is the Las Cruces granitoid, occasionally a tonalite based on its mineralogy. Common pseudotachylyte textures include dark glass matrix with minor inclusions of minerals or rock fragments, fluidal textures and branching injection veins, as well as contractional fractures in the glass due to rapid cooling.

Geochemical modelling using major and trace element compositions of dark glass (melt) and tonalite (host) permits examining a non-model equilibrium partial melting model in order to determine if there is a possible petrogenetic relationship between the pair. Modal compositions of glass and host were determined by least squares regression analysis and the end member mineral compositions were established. The melt (glass) composition is dominated by 70% plag, 17% k-spar and 13% bio, whereas the tonalite host has 75% plag, 9% bio, 8% k-spar and 8% qtz. With the exception of Eu, a melt composition, with similar trace element abundances to the actual glass (best fit), is calculated for a model assuming ~50% partial melting of the host.

The timing of compressional(?) deformation has been constrained dating the host rock tonalite with U-Pb zircon geochronology at 98.2 ± 1.2 Ma and the actual pseudotachylyte glass with Ar-Ar at 85.4 ± 0.5 Ma. The dated dark glass veinlet is made up of mostly plagioclase (the dominant phase) and minor amounts of biotite and k-spar. Therefore, the earliest deformation associated with the SJLPF occurred in Late Cretaceous time with the presence of upper crustal level deformational fabrics such mylonites, cataclasites and pseudotachylytes.

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