Grain shapes in this thin section tend to be subhedral to anhedral with a great variety in grain size. Lath-shaped plagioclase grains have both albite and Carlsbad twins, while larger, more equant grains show concentric zoning. Other phases in this sample include strongly pleochroic biotite, clinopyroxene, nephaline, orthoclase, and opaques. The clinopyroxene, biotite, and opaques tend to cluster together. Fairly large, euhedral apatite crystals are found throughout.
A foliation defined by aligned hornblende is cross-cut by a seam running from the upper-left to lower-right of the thin section. The seam is coarse-grained and unfoliated with a quartzofeldspathic margin and grains of resorbed, subhedral clinopyroxene in the middle.
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.
Clinopyroxene and plagioclase crystals with a hypidiomorphic granular texture. Clinopyroxene is cloudy and patchy in appearance due to alteration by a fine-grained material. Alteration by randomly-oriented, feathery chlorite pervades grain boundaries and fractures in the thin section.
The salt-n-pepper groundmass of opaques, pyroxenes, and feldspars appears almost intserstitial due to the high abundance of microphenocrysts. The phenocrystic population is dominated by subhedral plagioclase and, to a much lesser degree, clinopyroxene. Rounded, phenocryst-sized pockets of granular quartz are found throughout the groundmass.
The plagioclase laths in this microporphyritic basalt are separated by size into seperate groundmass and phenocryst populations. Glass and opaques are the other groundmass phases. Clinopyroxene and its periodic pseudomorph calcite are additional phenocrysts.
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.
Plagioclase, the most abundant mineral in this thin section, grades smoothly in size from groundmass mircrolites to subhedral microphenocrysts. Tiny, equant opaques join the plag microlites in forming the groundmass. Clinopyroxene, the second most abundant mineral, is found as relatively euhedral microphenocrysts. Plucking of the pyroxenes is severe. A fiberous phase is frequently found infilling or perhaps pseudomorphing a rectangular phenocrystic phase.
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 cryptocrystalline basalt consists of a groundmass of plagioclase microlites, equant opaques, glass, and interstitial, anhedral phlogopite. Microphenocrysts of subhedral clinopyroxene comprise the remainder of the sample.
Plagioclase laths in this thin section have a continuous gradation in size from groundmass to phenocrysts. Most are euhedral in shape and this phase makes up roughly 70-80% of the minerals in this sample. Subhedral clinopyroxene makes up the remaining 20-30% of the minerals. The larger, blocky plagioclase is locally zoned and a sieve texture is observable in some grains.
Amidst the groundmass of glass, randomly oriented plagioclase laths and equant to anhedral patches of secondary serpentine, are strewn subhedral phenocrysts of plagioclase and twinned clinopyroxene.
Several glomeroporphyroblasts of clinopyroxene and opaques with hornblende, biotite, and chlorite rims are scattered throughout this syenite. The remainder consists of altered k-feldspars and rare quartz grains with rather sutured grain boundaries.
Sieve textured plagioclase blocks and laths dominate the phenocrystic population in this microporphyritic vesicular basalt. Clinopyroxene phenocrysts are less abundant. Both phases are found with glass as constituents of the groundmass.
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 range in grain size for the plagioclase laths in this thin section is continual from the groundmass euhedral microlites to the subhedral phenocrysts. Concentric zoning and sieve textures are common in the plagioclase phenocrysts. Clinopyroxene phenocrysts are also subhedral, commonly twinned, and infrequently clustered in glomeroporhpyroclasts. In addition to plagioclase, the groundmass contains clinopyroxene, opaques, and serpentine.
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.
Subhedral phenocrysts of plagioclase, clinopyroxene, and olivine are scattered randomly throughout this basalt. Plagioclase has a sieve texture and is frequently zoned. The corners of most grains are rounded. Both clinopyroxene and olivine are generally equant in shape and olivine is more abundant than the pyroxene. All three phases show some degree of resorption. The groundmass contains moderately-sized crystals of all three phases. This sample is very fresh and unaltered.
The largest crystals in this sample, visible in handsample, are clinopyroxene, much of which has strong exsolution lamellae. Much smaller are the olivine crystals, which have subsequently been broken into a serpentine-framed mosaic of optically-continuous fragments.
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.
The least altered phase in this thin section is plagioclase, which forms roughly aligned, variably sized laths that make up both the groundmass, which is dominantly glass, and the phenocryst population. A second phenocryst, clinopyroxene, remains only as inclusions in the calcite pseudomorphs that replaced it. Vesicles, clays, and hematite pseudomorphs are abundant in this thin section.
The olivine that is so conspicuous in the JPN-3 handsample is conspicuously absent in thin section and represented by a mere 1 or 2 grains. Rather, phenocrysts of plagioclase and pyroxenes dominate the phaneritic phases. Both phases are subhedral and the plagioclase in particular indicates several different generations of growth. Some plagioclase is concentrically zoned, though most grains contain alternating inclusion-rich and poor zones. Inclusions are of pyroxenes and glass blebs. The groundmass contains glass, plagioclase microlites, and tiny pyroxenes.
The minerals in this sample all appear to intergrow with one another. Most grain boundaries are lobate, especially between feldspar phases and flame lamellae and perthitic intergrowths abound. Although a few hornblende crystals are euhdral in shape, the majority are found as rims around possible clinopyroxene or olivine grains and they tend to group with the other mafic phases. Euhedral apatite appear throughout.