A FIELD AND PETROGRAPHIC STUDY OF MAGMA MIXING/MINGLING TEXTURES IN THE DIORITE OF FIRST LAKE, CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA

The diorite of First Lake in the east-central Sierra Nevada batholith contains spectacularly preserved mixing and mingling textures. In this area, we define three distinctive types of mixing/mingling based on relative outcrop-scale homogeneity – 1) mingling-, 2) inhomogeneously mixed- and 3) homogeneously mixed zones. We use field and petrographic observations combined with mathematical approximations of magmatic properties to discuss probable origins for distinct magmatic interactions.

Within the diorite of First Lake, the mingling zone has macroscopic mingling textures that can be identified from a distance. Closer inspection reveals two types of interaction: lighter colored diorite with distinct zones of enclaves or, alternatively, more mafic diorite intruded by pegmatite tubes. In thin section, very few microscopic mixing/mingling textures are present. The inhomogeneously mixed zone makes up the majority of the exposed diorite and appears from a distance to be homogeneous in composition. However, outcrop-scale (2-10 cm) clusters of anhedral plagioclase characterize the inhomogeneously mixed diorite and suggest a state between hybrid magma and mingled magma. Thin section-scale features in this zone include complexly zoned plagioclase, undulose quartz (indicating reheating), and reaction rims. The homogeneously mixed zone has no outcrop-scale mixing textures; however, mineral-scale textures abound in this zone. In thin section, quartz with hornblende rims (also visible with the naked eye), sphene rims on oxides or biotite and complexly zoned plagioclase are present and suggest mixing to generate a hybrid magma.

Using these observations, we apply previous work on physical properties of mixing magmas to discuss the probable origin of these distinct zones within a mixed/mingled pluton. We interpret greater inhomogeneity to represent increased crystallinity and/or contrasts in melting temperature, viscosity and density between interacting magmas. Presence of a variety of textures suggests that there may have been multiple episodes of magmatic interaction. This conclusion is consistent with other studies in the Sierra Nevada (e.g., Sisson et al., 1996) that suggest multiple intrusions of diorite and/or mixing during batholith formation.