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Description
Isla Cerralvo near the mouth of the Gulf of California is composed primarily of Cretaceous granodiorite plutons intruded into quartzofeldspathic schist. Metamorphic grade of wallrock schist is predominately greenschist facies but increases to amphibolite facies (andalusite+sillimanite) in close proximity to intrusive contacts. Mineral assemblages in the schist indicate shallow emplacement of plutons in the crust ( ~4-8 km depth), a conclusions supported by concordant zircon U/Pb ages and 40 Ar/39 Ar ages(hornblende and biotite) from plutons that indicate rapid crystallization and cooling to below ~300°C between 90 to 100 million years ago (Montrella, 2004). Hornblende-biotite granodiorite outcropping on the western coast of the Isla Cerralvo is characterized by abundant widely distributed mafic enclaves as well as younger cross-cutting greenschist facies mafic dikes. Coastal outcrops of this granodiorite map unit are broadly homogeneous with the exception of a distinctive zone of a strongly heterogeneous intermingled mafic, felsic, and hybrid rocks along a~ 100 meter section of coast in the west-central part of the island. The occurrence of mixed/mingled mafic and felsic outcrops in this area poses questions relating to magma emplacement mechanisms, the origin of mafic enclaves, and processes of granitoid hybridization. The Cerralvo enclaves are fine-grained altered hornblende+plagioclase rock with igneous textures. Mafic-felsic contacts are variable; in some places mafic rocks are sharply chilled against felsic rocks; in other places contacts are gradational through hybrids to granodiorite. Whole rock major and trace element XRF data documents basaltic compositions for the enclaves. These observations indicate the enclaves represent basaltic magma injected into the granodiorite host before it was completely solidified. Convective stirring disaggregates the mafic injections and promotes hybridization of the granodiorite host as reflected by lower silica contents here relative to the "main phase" granodiorite. Mafic synplutonic injections on Cerralvo are similar in compositon to the ubiquitous fine-grained dikes that cut the granodiorite host. Based on this and age relationships (Montrella, 2004) I speculate that the dikes and mafic synplutonic injections may be derived from a common source.
