PETROLOGY AND GEOCHEMISTRY OF HORNBLENDE CUMULATES IN JOSHUA TREE NATIONAL PARK, CALIFORNIA

Joshua Tree National Park contains several large, exposed granitic batholiths including the White Tank monzogranite and the Twentynine Palms porphyritic quartz monzonite. In close proximity to these batholiths are an exposed sequence of hornblende cumulate layers that are located north of the park. The objective of this project is to study the hornblende cumulates in this location to determine their relationship to the granite batholiths; either as cumulate layers, restite or as unrelated igneous outcrops. We will use petrography, major and trace element data on the hornblende crystals and whole rock samples, as well as 87Sr/86Sr and 143Nd/144Nd isotope data for the whole rocks and cumulates to determine if the hornblende crystals could be related to either the the White Tank or Twentynine Palms granite batholiths. Numerical modeling of major and trace elements will also be used to determine the degree of fractional crystallization that was required to produce the cumulate from the granitic magma.

There are two textures of hornblende cumulate in the study area. One is medium to coarse-grained with euhedral to subhedral hornblende crystals up to to 4 cm in length and includes plagioclase and olivine. The other cumulate texture consists of small (1-2 mm) hornblende crystals with interstitial plagioclase and accessory sphene. The small crystal cumulate is on the periphery of the outcrop and locally layered on the interior with the other cumulate texture.

Similar hornblende gabbros have been described throughout the Transverse Ranges and Mojave desert. Previous research has suggested that the hornblende-bearing rocks could be related to crustal melting, however the euhedral texture of the crystals is indicative of their formation as early-formed cumulates from granitic magma. We hope to show that these cumulates are related to the granite through fractional crystallization processes.