This hypocrystalline thin section contains a trachytic groundmass of plagioclase laths, biotite needles, elongate strings of quartz, and serpentine pseudomorphs. The phenocrystic population consists of rounded quartz grains, often with a calcite rim, and concentrically-zoned, euhedral to subhedral plagioclase blocks. The plagioclase is being replaced in places by calcite, which is also found in fractures within the sample.
The groundmass of this thin section is predominantly glass with a roughly-aligned mesh-work of plagioclase, much of which has been replaced by calcite. Pseudomorphism of euhedral plagioclase phenocrysts by calcite and serpentine is pervasive; calcite grains of variable crystallographic orientations preserve the Carlsbad and albite twins characteristic of plagioclase. Throughout the sample, ellipsoids of chalcedony-rimmed serpentine masses are common; the smallest and most spherical of these have only chalcedony. The largest, amoeboid vugs of this basalt are mantled first by a rim of chalcedony, and inwards of that, a rim of serpentine. The core of the vugs contain radiating masses of an unidentified zeolite.
Tectonized harzburgite from fault slice along Lionshead Fault. No harzburgite exposed in continuous section. This sample is a strongly serpentinized harzburgite. Serpentine and opaques are the dominant phases in this thin section. Fractured, resorbed, and inclusion-rich orthopyroxenes are also present.
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.
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.
Composed mainly of a variably textured aggregate of antigorite and talc. Some parts are mainly fine interlaces of serpentine; others consist of both serpentine and talc completely intertwined and still other parts are mainly talcose. Veinlets and bands of these minerals, together with some carbonate are widespread. There is also considerable carbonate which appears in two types of two distinct ages 1) corroded patches of an older turbid carbonate and 2) veinlets of a younger, clear magnesite. Streaks and patches of fine-grained magnetite are common throughout.
Allotriomorphic plagioclase and highly-fractured olivine are the dominant minerals in this thin section. Clinopyroxene, when present, is found adjacent to olivine. Fractured zones in this rock follow olivine clusters and frequently cross-cut plagioclase crystals in swarms between nodes of granular olivine.
Granular, inequigranular gabbro containing occasional olivine crystals. Serpentine is localized to fractures. Grain shapes are very rounded and amorphous.
Most of this thin section is serpentine. Scattered throughout are optically-continuous, serpentine-framed fragments of olivine and somewhat larger, texturally-similar orthopyroxene fragments.
Interpenetrating zones of optically-continuous, pebbly-textured olivine, fibrous serpentine, and radiating talc. This thin section has interfingered scaly and fibrous textures.
The four phenocrystic phases in this thin section all display disequilibrium textures. Tabular plagioclase pheoncrysts are oscillatorily-zoned at the core and surrounded by as many as three distinct, cloudy, inclusion-rich rims. Quartz phenocrysts are rounded by resorbtion; thick coronas of calcite and hematite pepper, though do not entirely replace, their rims. Relict biotite phenocrysts, pseudomorphed by an opaque phase, are recognized more readily by the consistant rectancular and hexagonal shape than by the rare inclusion of the residual 'host'. These grains, frequently clustered together, have thick black rims and are sometimes infilled with a calcite-like mineral. This calcite-like mineral, frequently found replacing the biotite and as part of the quartz phenocryst coronas, is also observed pseudomorphing a tabular-shaped mineral found both as a phenocryst and part of the groundmass. The calcite seems to be replacing rectangular serpentine phenocrysts, presumably, pyroxene pseudomorphs themselves. The trachytic groundmass consists predominantly of microlites of plagioclase, prismatic opaques, glass, and the periodic rectangular calcite pseudomorph. Granular hematite veins cross-cut the thin section.
This medium-grained phaneritic rock contains an equal ratio of plagioclase to clinopyroxene grains. Much of the plagioclase is strongly sericitized and several grains are concentrically-zoned. The clinopyroxenes are frequently twinned, heavily-fractured, riddled with inclusions, and show low degrees of alteration to chlorite and serpentine. The pyroxene cleavage is prominent in this sample. Chlorite, serpentine, iddingsite, biotite, and opaques tend to cluster in randomly oriented mats, where chlorite and serpentine growth are often at the expense of biotite.
Serpentine comprises the bulk of this thin section, with skeletal masses of clinopyroxene and isolated granules of olivine dotting the characteristic serpentine mesh-work. The pyroxenes are larger and more complete than the olivine crystals.
Hypidiomorphic, inequigranular clinopyroxene with serpentine-filled fractures and interstices.
This thin section is a jumbled mess of serpentine, chlorite, calcite, quartz, and other unidentifiable minerals. Texturally, it looks reminiscent of a basalt.
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.
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.
The predominant mineral in this thin section is opaque red chert which is cross-cut by veins infilled with chlorite and serpentine. Towards the margins of the thin section is a metamorphosed basalt with a groundmass plagioclase mircolites and quartz, calcite, and chlorite in-filled vesicles and veins. According to collector David Sawyer, the interpillow red chert is an important indicator of deep sea origins.
Strongly serpentinized dunite. Only 5-10% olivine remains. The characteristic serpentine mesh-work texture is highly evident in this thin section.
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 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.