Erroneous contact between the two-pyroxene diorite through granodiorite and hornblende-biotite granodiorite units of the emigrant gap composite pluton corrected through detailed mapping, petrography, and geochemistry
The Middle Jurassic Emigrant Gap composite pluton is subdivided into peridotite, gabbro, two-pyroxene diorite through granodiorite, and hornblende-biotite granodiorite units. In a recent map accepted for publication by the U.S. Geological Survey, R. A. Schweickert and colleagues show a contact separating the two latter units in the northwesternmost part of the pluton. However, previous reconissance work by Kevin Bryan (San Diego State University), indicated that this contact may have been erroneously located. In order to assess this possibility I remapped and sam pied the area of the contact proposed by R. A. Schweickert and colleagues. The results of my work indicate that the area is completely underlain by hornblende-biotite granodiorite which is geochemically and petrologically indistinguishable from other samples analyzed from the hornblende-biotite granodiorite unit. Thus, my work indicates that the contact proposed by Schweickert and co-workers is erroneously placed, and in fact should be located where previously mapped by Kevin Bryan as part of his senior thesis project at San Diego State University.
