CONSTRUCTION AND MAGMATIC EVOLUTION OF THE COMPOSITE PALMS GRANITE PLUTON, JOSHUA TREE NATIONAL PARK, CALIFORNIA

The Palms pluton is representative of upper crustal granites found at Joshua Tree National Park. Our recent geologic mapping and geochronology work has shown this pluton to be composite, and we now are able to divide it into representative subunits. These units have varying textures (porphyritic to equigranular), mineral assemblages (± muscovite, garnet, relict hornblende), and a range of U-Pb zircon ages from 75-81 Ma. Unlike intrusive bodies beneath the Palms, the compositional range within each unit is restricted. There is only very minor evidence of host rock contamination at its external margins, preserved as schlieren, biotite clots, and rare host rock blocks. One unit (Keys Ranch) shows evidence of magma mingling, represented by rounded microdiorite enclaves.

Collectively, the mineral fabrics of the units are weakly developed and predominantly magmatic. Foliations are defined by biotite alignment and/or enclave orientation and are generally steep and northwest to north-striking. In map view, we observe more circular, blobby units in the center of the pluton, while outlying units are generally more linear. Units on the west and northwest sides become sheeted toward the external margins (with sheets both ~2 m and ~10 m in thickness), while those on the south to east margins form sharp contacts with the host rocks. Internal contacts vary locally, but most units exhibit sharp but mushy contacts. The contact between the Lost Horse and Keys Ranch units is gradational, and the Lost Horse unit lacks the characteristic enclaves. These internal contacts are often subvertical, although become obscured with the sheeting to the west. There appears to be a very weak fabric overprinting the contact between the Indian Cove and Lost Horse units, which likely occurred after significant cooling.

The Palms may have grown in at least two stages: one pulse at ~80-79 Ma and another at ~77-74 Ma. At 79 Ma, a blobby granite pluton (Lost Horse) intruded coevally with nearby diorites and hornblende gabbros, producing some enclaves in the central portions (Keys Ranch). At ~77 Ma, an elongate granite pluton (Indian Cove) intruded at the eastern side. At ~75 Ma, there may have been a great heat pulse at depth, with the possible remobilization of the southern Keys Ranch unit and small subunits showing evidence of host rock assimilation (Wall Street).