Using k-feldspar megacrysts as recorders of magma processes in the Twenty-nine palms pluton in Joshua Tree National Park

Abstract

Plutons spend the majority of their active time in the deep crust as magma mushes (crystals+ melt) before they entirely crystalize to rock. Crystals growing in the magma mush record the processes that the magma experiences, much like tree rings do when they record the environment they were exposed to during their growth. Any changes in the magma chemistry are thus recorded by the cores and rims of the minerals in the mush. To better understand the magma processes and their significance, crystal-scale geochemistry of different types of minerals becomes useful. For my project I am using K-feldspar megacrysts from the porphyritic Twentynine Palms pluton to unravel its magmatic history. The megacrystic Twentynine Palms quartz monzonite is located in the northern part of Joshua Tree National Park, which is a constituent of the Triassic magmatic arc of the Transverse Ranges, emplaced at 325 Ma (Barth and Wooden, 2006). The Twentynine Palms pluton is composed of a megacrystic quartz monzonite with 1-20 cm blocky K-feldspar phenocrysts in a medium grained, equigranular matrix of largely plagioclase, hornblende, and minor quartz, biotite and accessories. It is surrounded by the equigranular Queen Mountain monzogranite. The highly alkali composition of these two plutons is unique in the area in contrast to the lighter colored, mostly equigranular, quartz- and feldspar-rich Cretaceous plutons of Joshua Tree National Park (Brand, 1985). My project will test the hypothesis that the Queen Mountain pluton was fractionated from the Twentynine Palms by analyzing the elements of growth rings from core to rim of K-feldspar megacrysts to identify the processes in the magma reservoir. The study will first determine the changes in the abundance and sizes of the K-feldspar megacrysts in the field to determine if any potential regional patterns indicate cumulate textures. The nice euhedral K-feldspar megacrysts weather out of the rock easily and are collected from the gruss. In the lab, the megacrysts are cut in half and for thin sections, and mineral zones from core to rim will be microdrilled with a Dremel tool for XRF element analyses and analyzed with geochemical software. Petrography will determine any consistent inclusion patterns. The elment data will be interpreted in light of the processes that may have occurred in the Twentynine Palms magma chamber to test the hypothesis that is underwent fractional crystallization. This will be evident in the decrease of compatible and decrease of incompatible elements from core to rim.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016