METAMORPHISM AND ALTERATION OF MAFIC AND ULTRAMAFIC ROCKS AT BLACK MOUNTAIN, SOUTHERN MOJAVE DESERT, CA

Black Mountain (BM), near El Mirage, CA exposes massive serpentinite, strongly foliated amphibolite that grades into massive garnet(Grt)-diopside(Di) rock, and actinolite bodies that are cross cut by diorite, granite and veins of dolomite + quartz. Besides holding significant information about the metamorphic and tectonic history of the Mojave, carbonate veining in BM serpentinites provides an opportunity to study natural CO₂ sequestration. In this study, field mapping, phase-equilibria, whole rock geochemistry, U-Pb dating, and O isotope analyses have been used to evaluate the relationship among the different units at BM in order to evaluate their origin, metamorphism, and hydrothermal alteration. In the field, transitions between the different rock types are often gradual, suggesting that they were metamorphosed as a single package; the diorite and granite include xenoliths of the ultramafic lithologies. Zircon ²⁰⁶Pb/²³⁸U ages measured by LA-ICP-MS are 245.8 ± 5.3 and 239.5 ± 5.2 Ma (2SD) for the diorite and granite, respectively. The ages establish an approximate age of metamorphism at BM and expand westward the previously recognized limit of Triassic magmatism in the Mojave. Inherited Carboniferous to Permian zircons suggest Paleozoic detrital components. Because Grt-Di rocks are transitional from amphibolite we ascribe an origin at BM similar to that producing rodingites by metasomatic alteration of gabbro during serpentinization. High Ni and Cr in the Grt-Di rocks is consistent with this model, and values δ¹⁸O(Grt) of 5.8 to 7.2‰ overlap with those reported for rodingites (Wenner, 1979, GCA 43:603–614). Metamorphic mineral assemblages such as tremolite (pseudomorphed by retrograde serpentine) in serpentinites, and zoisite+titanite occurring in the Grt+Di rocks, are consistent with amphibolite facies metamorphism. Dolomite+quartz veining records late stage carbonation of serpentites at BM. Dolomite forms massive veins and occurs in drusy veins with quartz along faults, but also forms cross-cutting mesh networks in serpentinites adjacent. Carbonation reaction was limited to fault zones because of reduced permeability within the serpentinites. As a whole, rocks at BM provide new insight into Mojave geology and a useful example of paleo-uptake of CO₂ by ultramafic lithologies.