THE LINKAGE OF UPPER CRUSTAL PLUTONS TO MID-CRUSTAL SHEETED COMPLEXES IN JOSHUA TREE NATIONAL PARK, CALIFORNIA

Tilted crustal sections allow us to visualize the 3D architecture of an arc and its magmatic plumbing system. Our current work focuses on a segment of the Late Cretaceous Cordilleran arc in Joshua Tree National Park (southern California, USA), where we observe a 15-25º tilted section exposing crust from 0-30 km depth. Magmatism in the arc formed granite-granodiorite plutons in the upper crust, a mid-crustal (10-20 km depth) sheeted complex with meter-thick layers ranging from hornblende gabbro to two mica garnet granite, and lower crustal diorite to tonalite plutons.

The Palms pluton is representative of upper crustal granites found at Joshua Tree N.P. Our recent geologic mapping and geochronology work indicates this pluton was constructed of at least four distinctive 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 in the mid-crustal sheeted complex, the compositional range within each unit is restricted. There is only minor field 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. The majority of Palms units record an initial ⁸⁷Sr/⁸⁶Sr ratio of ~0.710, in contrast to a wider range of values (~0.708-0.716) in the sheeted complex.

The Palms grew in at least two stages: one pulse at ~80-79 Ma and another at ~77-74 Ma. At 79 Ma, a granite of Lost Horse intruded coevally with nearby diorites and hornblende gabbros, producing enclaves (granite of Keys Ranch). At 77 Ma, the granite of Indian Cove intruded the eastern margin. At 75 Ma, a possible great heat pulse at depth created a marginal subunit (Wall Street) and possibly remobilized the southern Keys Ranch unit.

The Palms pluton may represent a reservoir of homogenized magmas generated during voluminous magmatism, whereas the sheeted complex may preserve the record of lower volume events that have not coalesced into a large pluton. During this time, the sheeted complex may act as a filtering and differentiation zone for rising magmas, segregating a granitic upper crust from an intermediate mid-lower crust.