USING K-FELDSPAR MEGACRYSTS AS RECORDERS OF MAGMA PROCESSES IN THE TWENTYNINE PALMS AND QUEEN MOUNTAIN PLUTONS IN JOSHUA TREE NATIONAL PARK

It is well established that magmas spend the majority of their time as magma mushes (crystals + melt) before they entirely crystalize to plutons. Over time these magma mushes may undergo differentiation, magma mixing and assimilation to create the compositional variations that are preserved in plutons. What magma processes occur in individual plutons to create their compositional characteristics over time is often masked by multiple phases of magmatism that are often not preserved in the final rock composition. Crystals growing in the magma mush, however, witness the processes that the magma experiences and record the chemical signatures in individual crystal growth zones that can be tapped for single mineral geochemistry.

The 325 Ma megacrystic Twentynine Palms pluton (TPP) is located in the northern part of Joshua Tree National Park, southern California. It is composed of megacrystic quartz monzonite with 1-20 cm blocky K-feldspar phenocrysts embedded in a medium grained, equigranular matrix of plagioclase, hornblende, and minor quartz and biotite, with local cumulate textures. It surrounds the contemporaneous equigranular, compositionally highly variable Queen Mountain monzogranite (QMP) composed of plagioclase, K-feldspar, quartz, biotite, and locally hornblende or muscovite. The QMP contains sparse K-feldspar phenocrysts up to 4 cm similar to the ones in the TPP that increase in size and abundance toward the TPP contact. Geochemical modeling of major oxides and trace elements suggest that the QMP magma likely fractionated from an early TPP magma and the TPP rocks now represent crystal cumulates.

K-feldspar megacrysts of TPP are used to a) test the hypothesis that the QMP was indeed fractionated from the TPP, and b) to determine if other magma processes not detectable through whole rock geochemistry were involved in the petrogenesis of these two plutons. 7-20 cm K-feldspar megacrysts were cut in half and powders milled from the core, interior and rims of the crystals for XRF major oxide and trace element geochemistry to interpret the processes that caused the compositional variations in the two plutons. The TPP and QMP might represent a cross section of a horizontally and vertically extensive magma mush zone that underwent open system differentiation and efficient crystal-melt separation in the middle crust.