THE GEOLOGY OF JOSHUA TREE NATIONAL PARK, RE-DISCOVERED

Limited data has been published regarding the petrology, mineralogy, and geochemistry of the Mesozoic granitic rocks in Joshua Tree National Park in southeastern California. The available descriptions focus primarily on the extensive jointing and exfoliation that draw climbers and hikers.

Whole rock geochemistry reveals that the monzogranite units are peraluminous (one is strongly peraluminous) due in part to low concentrations of calcium. The geochemistry of the peraluminous granitoids in Joshua Tree is comparable to the peraluminous (but not the metaluminous) rocks of the Old Woman-Piute Mountains to the east.

The porphyritic quartz monzonite in Joshua Tree is characterized by orthoclase megacrysts (2 to 10 cm. in length), hornblende, and accessory sphene. The mineralogy of the Gold Park diorite varies considerably: a dark gray unit dominated by hornblende and biotite with either actinolite or clinopyroxene. The metamorphic rocks in Joshua Tree are known collectively as the Pinto gneiss (Precambrian?).

The monzogranites intruded into the Gold Park diorite and the Pinto gneiss and vary in whole-rock geochemistry, mineralogy, and grain size. Mapping suggests zonation of the granites outward from a core of the probable youngest unit, the Oasis monzogranite, which is depleted in calcium, iron, and magnesium along with barium and strontium. The Oasis monzogranite features muscovite with tiny red Mn-rich garnets and can be referred to as a leucogranite given its white appearance and total lack of dark minerals. The Queen Mountain monzogranite has a typical granite appearance with biotite plus or minus hornblende. The inner zone of the Queen Mountain surrounds the Oasis monzongranite and is coarse but equigranular whereas the outer zone has noticeably larger orthoclase crystals (generally +/- 2 cm.). The White Tank monzogranite contains biotite plus or minus minor amounts of muscovite. Its jointed boulders weather to a distinctive yellow and form the outermost granitic zones of Joshua Tree National Park. The texture can be described as either a medium-grained equigranular phase, a more felsic coarse-grained equigranular phase, or a more mafic seriate (mixed grain size) phase, each with slightly different geochemistry and modal mineralogy.