Mafic–felsic magma mixing limited by reactive processes: A case study of biotite-rich rinds on mafic enclaves

Abstract

Mafic enclaves in felsic plutons are often used to argue that intermediate magmas are formed by mafic–felsic magma mixing, but the extent and nature of mixing remains unclear. Here, we examine biotite-rich rinds on mafic enclaves from the Cretaceous Bernasconi Hills Pluton in the Peninsular Ranges Batholith of southern California to gain insight into magma mixing processes. Rinds differ from the enclave interior and the host monzogranite in being more fine-grained and more mafic and potassic. Rinds are also 2–5 times more enriched in rare earth elements than the host monzogranite and up to 3 times more enriched than enclave interiors. These observations indicate that the rinds were not generated by isochemical quenching, binary mixing between enclave and host monzogranite, or in-situ magmatic differentiation. Instead, rinds appear to have been formed by chemical reaction between the solidified enclave and a hydrous K-rich residual melt or fluid formed after progressive crystallization and cooling of the host magma body, transforming amphibole in the enclave into biotite-rich rinds. Field observations show snapshots of biotite-rich rinds being eroded away and new rinds simultaneously forming on freshly eroded surfaces of enclaves, consistent with rinds being formed by chemical reaction instead of as quenching products. Deformation of enclaves is accommodated primarily by ductile attenuation of the thin rind while the enclave as a whole tends to rotate as a rigid body with minimal internal deformation other than localized brittle failure. A comparison of the aspect ratios and cross-sectional areas of mafic bodies in the pluton shows that those with high aspect ratios (indicating greater accumulated strain) are systematically more biotite-rich and have smaller cross-sectional areas than those with lower aspect ratios, which are amphibole-rich. These relationships not only confirm that biotite-rich lithologies are more deformable but also indicate that the high aspect ratio biotite-rich bodies (also known as schlieren) derive from small parent bodies, consistent with a derivation from eroding enclave rinds rather than from the enclave itself. Finally, geochemical and thermodynamic modeling indicates that the biotite-rich rinds formed when the host felsic magma had cooled to a low melt fraction state (F=0.15–0.3; 700–760 °C), suggesting that such reactions occur late in the lifespan of a magma body. Thus, mafic–felsic mixing may not be an efficient process for making intermediate magmas unless the magma body can reside at this low temperature range long enough to permit rind formation and subsequent deformation.