This gabbro has a subophitic texture. Alteration to chlorite and iddingsite occurs mostly between grain boundaries.
The dominant texture in this gabbro is an ophitic intergrowth of equant plagioclase laths within larger, subhedral, clinopyroxene grains. Alteration of some phases to chlorite, talc and clays is localized and occurs more in non-ophitic sections of the rock.
Texturally, this thin section still resembles a gabbro, with the exception of several talc and serpentine filled veins cross cutting the otherwise equigranular mosaic of clinopyroxene and olivine.
This fine-grained, hypidiomorphic, inequigranular plutonic rock is intermediate in composition, with plagioclase as the dominant feldspar. K-feldspar is present in much lower abundance, and quartz is scarce. Mafic phases include anhedral biotite and hornblende, the latter of which is strongly chloritized.
There is no glass in this fine-grained thin section. Rather, allotriomorphic, randomly-oriented plagioclase is peppered with fine-grained granules of clinopyroxene and opaques. Few clinopyroxene phenocrysts cluster together to form glomeroporphyroclasts. The feldspars show signs of kaolinitization and sericitization.
Plagioclase is roughly twice as abundant than clinopyroxene in this sample, though where present, clinopyroxene grains are large and encompass multiple inclusions of plagioclase giving a spotty ophitic texture to the gabbro. Iddingsite replacement is extensive and plagioclase is altering to clays. Apatite is found in short stubby prisms, hexagonal cross sections, and long needles.
Plagioclase and clinopyroxene abundances are lower in this gabbro compared to others of Birds River. Radiating splays of zeolites with serpentinized margins, as well as iddingsite and chlorite replacement of an equant mineral phase contributes nearly half the bulk of the thin section.
Though clinopyroxene and plagioclase were undoubtedly once more abundant in this gabbro, subsequent alteration has left radiating zeolites, a cloudy film of clays, and veinlets and pseudomorphs of iddingsite the dominant minerals in this thin section.
This inequigranular mosaic of pyroxenes and olivine is highly fractured. Subparallel swarms of high-density fractures are visible throughout and contain the highest concentrations of serpentine. The infilled minerals of one conspicuous vein have a nice comb texture.
This thin section has a diabasic texture. While the plagioclase has remained relatively unaltered, clinopyroxene has been almost entirely pseudomorphed by serpentine. Anhedral clusters of calcite is found throughout.
The dominant texture in this gabbro is an ophitic intergrowth of equant plagioclase laths within larger, subhedral, clinopyroxene grains. Alteration of some phases to chlorite, talc and iddingsite is localized and occurs more in non-ophitic sections of the rock.
The clinopyroxene abundance in this thin section is lower than other gabbros from Birds River, lending more of a subophitic texture to the rock. Plagioclase is the most dominant mineral. Iddingsite is less abundant.
As is characteristic of a harzburgite, this sample is greater than 90% olivine, with a few large clinopyroxene grains, fracture-infilled serpentine, and occasional, moderately-sized rutile grains making up the remaining constituents of the rock. Serpentinization is minimal.