Geochemical Diagnostics of Metasedimentary Dark Inclusions: a Case Study from the Peninsular Ranges Batholith, California
Lee, Cin-Ty A.
Master of Science
Dark enclaves rich in amphibole and biotite are ubiquitous in granitoid rocks and generally thought to represent fragments of mafic magmas, cumulates or restites. However, magmatic assimilation of metamorphic or sedimentary country rock can also form dark enclaves. 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 host tonalite from the Domenigoni Valley pluton in the Peninsular Ranges Batholith, California. Metasedimentary dark enclaves have a number of major and trace element characteristics that overlap those of literature-compiled igneous dark enclaves. Comparison to modeled igneous differentiation paths shows 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 there are literature-compiled mafic enclaves trending towards high K2O and high CaO suggests that not all mafic enclaves are of igneous origin. While the majority of dark enclaves may not be metasedimentary, this work provides some criteria for identifying enclaves should a case of metasedimentary origin arise.
Dark enclaves; Metasedimentary; Protolith; Wall rock assimilation; Metamorphism