Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south‐eastern San Gabriel Mountains, southern California

Abstract

ABSTRACTMid‐Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south‐eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc‐silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic‐intermediate magmatism and high‐grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near‐isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo‐chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma‐1. The geotectonic evolution of the Cucamonga granulites is similar to mid‐Cretaceous high‐P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.