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24 hits

  • Thumbnail for BB18
    BB18

    This fine-grained granite contains abundant feldspars, many of which display concentric zoning from plagioclase cores to k-feldspar rims. Biotite grains are altering to and interfingered with chlorite. Rounded hornblende crystals are rare in this sample.

  • Thumbnail for BB3
    BB3

    Large, euhedral to subhedral plagioclase blocks are in far greater abundance than either k-feldspar or quartz, both of which are anhedral in shape. All the ferromagnesian phases are anhedral in shape and tend to cluster together. Hornblende growth is at the expense of clinopyroxene. Opaques cluster with the ferromagnesian phases. Quartz displays first-order yellows in this slightly thickened sample.

  • Thumbnail for BB21
    BB21

    A groundmass of glass, clay, calcite, indiscernible quartzofeldspathic crystals, and small, spherical aggregates of chalcedony dominate this sample. Phenocrysts of biotite and calcite pseudomorphs after plagioclase (presumably) comprise the phaneritic crystal population.

  • Thumbnail for BB27
    BB27

    A notable feature of this thin section are the large, equant and tabular, concentrically-zoned plagioclase phenocrysts with accumulations of k-feldspar around the margins. The quartzofeldspathic phases within the groundmass of the sample are subhedral in shape, often exhibiting rather cuspate-lobate grain boundaries. Subhedral biotite is scattered throughout the sample, though it frequently clusters with chlorite, calcite, and opaques in greenish aggregates, which appear in handsample as green phenocrysts.

  • Thumbnail for BB32
    BB32

    This is a hypidiomorphic, generally equigranular syenite. Some grains of both feldspar phases have a sieve-like texture. Biotite is abundant, chlorite and muscovite much less so.

  • Thumbnail for BB33
    BB33

    This sample has a classic, medium-grained, granitic texture. Grains are generally comparable in size and shape is typically subhedral to anhedral. Biotite is anhedral and weakly pleochroic.

  • Thumbnail for BB30
    BB30

    The phenocrysts in this porphyritic granite are inclusion-rich, clay-altered, subhedral plagioclase blocks that do not appear to be in equilibrium with the surrounding groundmass. The groundmass is a mottled smattering of quartzofeldspathic minerals that have no discernable crystal habit. Amidst the background blur of first-order whites and grays are petrographically messy clusters of chlorite, hornblende, and iddingsite, with growth of chlorite and iddingsite at the expense of hornblende. Large, prismatic and hexagonal apatite crystals are readily apparent in this thin section. A few prismatic crystals of monazite, zircon, or xenotime are present (though difficult to definitively identify).

  • Thumbnail for BB6
    BB6

    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.

  • Thumbnail for BB14
    BB14

    Phenocrysts of resorbed plagioclase and k-feldspar give this otherwise glassy rock its porphyritic texture. A couple of resorbed pyroxene phenocrysts are present. Aligned needles of an opaque phase give the glassy groundmass a trachytic texture.

  • Thumbnail for BB5
    BB5

    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.

  • Thumbnail for BB12
    BB12

    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.

  • Thumbnail for BB31
    BB31

    This sample has a granitic texture and contains roughly equal amounts of both feldspar phases. The k-feldspars are more sericitized than the plagioclase grains, many of which are concentrically zoned. The hornblende grains are anhedral and a times, form almost granular aggregates of equant grains. Hornblende is altering to chlorite, which also interfingers with the relatively subhedral biotite phase. Apatite and epidote are present in trace amounts.

  • Thumbnail for BB13
    BB13

    In thin section, this flow-banded aphanitic rock displays a few fractured and embayed plagioclase and clinopyroxene phenocrysts in a groundmass dominated by aligned microlites of the same two phases. One or two phenocrysts of orthoclase are also present. The microlites define a trachytic texture, which is observable in handsample as flow bands.

  • Thumbnail for BB9
    BB9

    This medium-grained allotriomorphic diorite is dominated by plagioclase, much of which is concentrically zoned and larger in size than most other phases in the sample. Clusters of the ferromagnesian phases appear to be pseudomorphing a previous, rectangular, phenocrystic phase.

  • Thumbnail for BB28
    BB28

    The most noteworthy feature in this thin section is the micrographic intergrowth between quartz and k-feldspar that pervades the rock. The k-feldspar is dirty in appearance due to alteration to clay. Biotite, which is interfingered with chlorite, is subhedral to anhedral and riddled with inclusions.

  • Thumbnail for BB35
    BB35

    Grain boundaries between the anhedral quartz and k-feldspar grains in this sample are framed with myrmekitic intergrowths. These intergrowths even appear in plane polarized light due to the high degree of feldspar alteration to clay. The mica phases are sparsely distributed and small in size.

  • Thumbnail for BB8
    BB8

    Large k-feldspar and quartz crystals provide a backdrop for the smaller, less well-preserved mafic phases in this sample. Decomposing biotite is frequently found interfingered with chlorite. Hornblende grains are rather skeletal. Plagioclase crystals have relatively rounded corners and tend to cluster together. Large anhedral aggregates of sphene are interspersed in the interstices between k-feldspar grains.

  • Thumbnail for BB10
    BB10

    The groundmass of this thin section contains a combination of glass and indiscernible crystals. It is densely populated by phenocrysts of a wide range of sizes and composition, which due to their angularity, lend a very fragmental texture to the rock. The quartzofeldspathic phases span a broad range of sizes, though the largest phenocrysts are all brittley-fractured and may be strongly embayed. No reaction rims are present in this sample. Biotite grains do not get as large as the quartzofeldspathic phases and display varying degrees of 'freshness.'

  • Thumbnail for BB7
    BB7

    The groundmass of this thin section contains a combination of glass and indiscernible crystals. It is densely populated by phenocrysts of a wide range of sizes and composition, which due to their angularity, lend a very fragmental texture to the rock. The quartzofeldspathic phases span a broad range of sizes, though the largest phenocrysts are all brittley-fractured and may be strongly embayed. No reaction rims are present in this sample. Biotite grains do not get as large as the quartzofeldspathic phases and display varying degrees of 'freshness.'

  • Thumbnail for BB19
    BB19

    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.

  • Thumbnail for BB1
    BB1

    The two feldspar phases in this medium-grained, hypidiomorphic syenite are found in roughly equal abundance, though the plagioclase grains are subhedral to the k-feldspar's anhedral shape, and typically displays concentric zoning. Minor myrmekitic intergrowths between quartz and plagioclase are found between feldspar contacts. Both biotite and hornblende have highly irregular grain shapes.

  • Thumbnail for BB2
    BB2

    This hypidiomorphic, equigranular monazite has a classic granitic texture with a mosaic of grains all crystallizing simultaneously and impinging on one another's growth. Hornblende is particularly abundant and generally in contact with some combination of biotite, chlorite, and opaques. Some plagioclase grains are concentrically zoned.

  • Thumbnail for BB22
    BB22

    The micas in this hypocrystalline rhyolite form euhedral needles and are generally aligned. Phenocrysts of plagioclase have been replaced by calcite and chalcedony. The groundmass of glass contains indiscernible crystals. This thin section is strongly altered and due to the fine grain size, difficult to identify minerals in.

  • Thumbnail for BB29
    BB29

    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.