Co-mingling of acid and basic magma with implications for the origin of mafic I-type xenoliths: Field and petrochemical relations of an unusual dike complex at eagle lake, Sequoia National Park, California, U.S.A.

Open-system magmatic processes result in a continuum of disequilibrium features, from hybrid magmas to lamprophyre dikes. Through application of recent theoretical work on magma chamber convection we quantify thermal and mechanical parameters for a single, somewhat unique, plutonic system and propose a general scheme for interpreting magma mixing based on petrographic, dynamical and heat-transfer considerations. At Eagle Lake, intrusion of nearly aphyric basaltic melt into mostly crystallized acidic host has created a 2-4-m-wide dike-like train of ellipsoidal mafic inclusions set in a matrix of unfoliated leucocratic material. The host magma (∼65% SiO₂) is estimated to have been 70-90% crystallized at the time of mafic melt injection; mafic magma (∼51% SiO₂) was intruded near its liquidus, at 1050-1150°C. Formation of a hybrid compositional product is not observed. Upon intrusion, the mafic magma formed ellipsoidal pillows of average areal size 50 cm², many are encased by fine-grained margins interpreted as remnant quench rims. The host acted initially as a brittle solid; the absence of evidence for creep or brittle-ductile transitional flow implies strain rates around 10^-2 s^-1 within the zone of mafic melt injection. A thermal model predicts that the heat released during thermal equilibration was sufficient to raise the temperature of the host to above its rheological locking point in the region adjacent to the mafic conduit; lack of foliation in the leuco-cratic material in this region supports the model. Schlieren zones developed along dike complex margins, where strain gradients were maximized. The schlieren are chemically linked to the mafic melt; mineral deformation features indicate strain rates ∼10^-5 s^-1 at 700-1000°C. A general model of mixing is proposed in which the relative volumes, temperatures and viscosities of the distinct magmas play a pivotal role in determining the spatial scale of the resultant compositional heterogeneity. For example, evidence for intrusion of mafic melt 10³ years after silicic pluton emplacement will not be visible on outcrop scale; vigorous convection will have eradicated all but xenolithic and xenocrystic traces. We believe that many I-type mafic "xenoliths" in Sierran plutons are the result of this style of magma mixing. Alternatively, if intrusion occurs 10⁶ years following pluton emplacement, map-scale features such as lamprophyre dikes and large inclusion swarms are evident. In the context of a general model, the Eagle Lake quartz monzodiorite pluton preserves the earliest stages of formation of individual mafic inclusions.