Geochemistry and petrology of the hornblende gabbro of Mount Gleason, San Gabriel Mountains, Los Angeles County, California

Located in the San Gabriel Mountains northeast of Los Angeles, the hornblende gabbro of Mount Gleason is a distinctive coarse-grained mafic unit composed of subequal amounts of hornblende and plagioclase. Five major and several minor closely spaced bodies totaling approximately 4.6 km2 in area intrude the eastern portion of the Precambrian San Gabriel Anorthosite-Syenite complex. In outcrop, the hornblende gabbro is extremely variable with discontinuous wavy layering similar in some respects to comb layering. Hornblende crystals are commonly oriented roughly perpendicular to layering and range in length from 1 mm to 10 cm. Primary mineral phases, in order of decreasing modal abundance, are calcic plagioclase (14 to 72%), hornblende (5 to 70%), epidote (1 to 35%), opaque oxides (predominantly magnetite-ilmenite, trace to 7% ), and apatite (0 to 5%). Secondary chlorite commonly occurs at low concentrations (average= 3.5% ). Hornblende ranges in composition from ferroan pargasite to tschermakitic hornblende. Total Al and Al(IV) are high, averaging -2.6 and -2.0 cations per 23 oxygens, respectively. Plagioclase grains are typically very calcic with somewhat more sodic rims. Values as high as Anoo for grain interiors and as low as ~ for rims were observed; however, examples of anomalously sodic cores and An-rich rims were also noted. Epidote, some of which forms euhedral crystals, has an average pistacite content of 19.8%. Apatite compositions are dominated by the hydroxide and fluorine species (OH:F:O = 53:40:3). Unusually low Si02 concentrations (38.0 to 44.4%) result in these rocks being classified as alkalic despite low ~O + N820 values (1.28 to 3.61 % ). The hornblende gabbro is further characterized by being low in MgO and high in CaO and Al20 3� Rare-earth patterns are convex upward and peak between Pr and Eu. A small, positive Eu anomaly is present. The light REE average -10 to 15x chondrite while the heavy REE are -4x chondrite. Strontium is abundant (872 to 1133 ppm) while Rb is exceptionally low (0.7 to 1.8 ppm), resulting in low Rb/Sr ratios ( 0.006). Initial 87Sr/36Sr values calculated for three whole-rock samples are discordant, varying from 0.7036 to 0.7041; apatite separated from one of the samples has a ratio of 0. 7034. Age dating by the 40 Ar/39 Ar method is poorly constrained due to excess argon in the samples. Previously published dates of 194 � 1 Ma and 186.6 � 1. 7 Ma for a hornblende crystal from the Mount Gleason pluton are not supported by subsequent age data from the Rabbit Peak exposure which yielded an 40 Ar/39 Ar date of 136.9 � 0.44 Ma. Based on textural, mineralogical and geochemical similarities, the various exposures of hornblende gabbro in question are presumed to be comagmatic and are interpreted to have crystallized as a cumulate from a water-saturated, mantlederived magma under P /f conditions near 6 to 7 kb and 85D�C. The pressure and temperature indicators used, such as the presence of euhedral epidote or high total Al and Al(IV) in hornblende, are not conclusive but yield results consistent with conditions inferred by previous workers for other nearby Mesozoic plutons. Cumulate processes are interpreted as the most likely explanation for the unusual chemistry of the hornblende gabbro (for example, the very low Si02 concentrations and their occurrence with low MgO) and do not require postulating a magma of the same composition. Similar hornblende-plagioclase cumulates, such as those found in a number of ocean-rim terranes from northeastern Japan to the Peninsular Ranges of southern California, have been theorized by some workers to constitute a significant deep-crustal component in island-arc settings or possibly to play an important role in the evolution of typical Sierran granodiorites.