EXAMINING THE GEOCHEMICAL RELATIONSHIPS BETWEEN THE TWENTYNINE PALMS AND QUEEN MOUNTAIN PLUTONS IN JOSHUA TREE NATIONAL PARK
A place where these questions can be investigated is in the northern part of Joshua Tree National Park, California, which contains outcrops of a Triassic arc magma plumbing system consistent of two contemporaneous plutons: the ca. 235 Ma Twentynine Palms pluton (TPP) and the Queen Mountain pluton (QMP). The TPP is a megacrystic quartz monzonite composed of 1-20 cm blocky K-feldspar phenocrysts in a medium grained, equigranular matrix of largely plagioclase, hornblende, and minor quartz, biotite and accessories. The size of the K-feldspar varies substantially between different pluton domains. The QMP is a medium grained granodiorite with plagioclase, K-feldspar, quartz, biotite, hornblende and accessories. In addition, both plutons contain small pendants and blocks (TPP) and smaller, cm-dm pieces and biotite clots (QMP) of Pinto gneiss, the host rock into which the magmas intruded. Together, the plutons form a roughly bull’s-eye shape map pattern with the megacrystic TPP forming the outer rim and the QMP, the interior of the complex. The contact between the two plutons is steep and sharp to gradational where exposed and dikes of the QMP intruded the TPP. The QMP contains sparse Alkali-feldspar phenocrysts up to 4 cm large, which resemble smaller phenocrysts in the TPP. Both plutons exhibit local solid-state deformation with the strongest fabric along the TPP margin.
Given these field relationships and concurrent crystallization of both plutons, we are investigating through petrography and element and isotope geochemistry the hypothesis that the two units may be related to one parent magma and are potentially representing a fractionate (QMP) and cumulate (TPP) pair. If our hypothesis is confirmed, this would suggest that we may be looking at a cross section of a horizontally and vertically extensive magma mush zone that underwent extensive open system differentiation and efficient crystal-melt separation.