THE EVOLUTION OF MAGMA AT AMBOY CRATER, MOJAVE DESERT, CALIFORNIA

Six eruptive events have been identified at Amboy Crater, a Quaternary cinder cone in the Mojave Desert, California (1). Although the exact timing is not well known, events one through six are considered to be in approximate chronological order. Mineral compositions and detailed plagioclase zoning patterns are presented here in order to reconstruct the petrogenetic processes involved in the evolution of Amboy Crater magmas. This work may also be useful in better constraining the age relationships between the eruptive events. Basalts collected from the six flow units were analyzed by XRF, and yielded results ranging from 5.40 to 8.63 MgO and 8.45 to 9.22 CaO, which is comparable to previous work (2). Electron microprobe data for plagioclase zoning indicates that average An compositions for individual crystals range from 61 to 70 and for individual flows range from 57 to 71. Cores tend to have higher An contents (63 to 71, average 68) while the rims tend to have lower An contents (51 to 77, average 64). Later events tend to have more complex oscillatory zoning, with more zones on each individual crystal. There is an emerging picture of a complex magmatic history at Amboy Crater, consistent with previous research citing complex magmatic systems of cinder cones related to Cascades volcanism (3). (1) Parker (1962). (2) Glazner (1991). (3) Strong (2001).