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Description
This thesis provides a framework for the evolution of pre-batholithic wall rock and intrusions in part of the Los Cabos Block at the southern tip of the Baja California peninsula, Mexico. The La Gata Metamorphic Complex (LGMC) is comprised of highgrade psammitic-pelitic schist intruded by granitic rocks and dikes, and occupies a position in the footwall of the San Jose del Cabo fault, a major down-to-the-east late Cenozoic normal fault related to the opening of the Gulf of California. The most noteworthy mineralogic feature of the LGMC is andalusite porphyroblasts in pelitic schists partially to completely pseudomorphed by coarse-bladed sillimanite. Peak metamorphic conditions in the La Gata Metamorphic Complex coastal section occur in a broad zone of upper amphibolite facies migmatitic schist adjacent to granitic plutonic intrusives that make up the northern end of the La Gata range. Sillimanite + K-feldspar assemblages in pelitic rocks indicate peak metamorphic T-P conditions of 640-675 °C at 2.5-3.0 Kbar. There is a Buchan-style high TIP zonation of regional metamorphic assemblages in schist along the La Gata coast over a distance of ~8 km from the highest grade assemblages down to lower-grade rocks with T-P conditions estimated at ~520-560°C and 2.5-3.0 Kbar. The coastal section thus records a temperature gradient in peak metamorphic conditions of ~100°C. Fine-grained basaltic andesite dikes post-date regional amphibolite facies metamorphism of the La Gata Metamorphic Complex. These widespread dikes were involved in extensional deformation expressed as numerous and widely distributed small-displacement normal faults and ductile shears in sea cliff exposures. Hornblende and biotite 40Ar/39Ar data suggest rap id cooling of the La Gata Metamorphic Complex between ~90 and 98 Ma. A similar age for hornblende from one of the fine-grained basaltic andesite dikes indicates that extension-related ductile deformation in the dikes is also Cretaceous in age. This is important because the array of normal faults here might otherwise be confused as Neogene structures related to Gulf of California rifting. The Las Cruces granite is a large body of K-feldspar megacrystic granodiorite located on the northern part of the Los Cabos Blocks on the west side of the LGMC. Whole rock geochemical characteristics together with a U/Pb zircon age of ~95 Ma support a possible correlation of the Las Cruces granite with the voluminous La Pasta-type intrusions of the eastern Peninsular Ranges batholith.
