Geochemical diagnostics of metasedimentary dark enclaves: a case study from the Peninsular Ranges Batholith, southern California

Dark enclaves rich in amphibole and biotite are ubiquitous in granitoid rocks and typically represent fragments of mafic magmas, cumulates, restites, or country rocks. To develop criteria for identifying dark enclaves of non-magmatic origin, we investigated dark enclaves from a complete spectrum of light (carbonate- or feldspar-rich) to dark (amphibole-rich, biotite-rich, or composite) enclaves, reflecting progressive thermal and chemical equilibration with the host tonalite, the Domenigoni Valley pluton in the Peninsular Ranges Batholith, California. Metasedimentary dark enclaves have geochemical characteristics that overlap those of literature-compiled igneous dark enclaves. When compared with modelled igneous differentiation paths, metasedimentary enclaves can have anomalous CaO and K2O contents for a given SiO2, but other major-element systematics may not deviate noticeably from igneous differentiation trends. In addition, the fact that literature-compiled mafic enclaves trend towards high K2O+CaO suggests that not all mafic enclaves are of igneous origin. In this work, we provide criteria for identifying enclaves of possible metasedimentary origin.