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93 hits

  • Thumbnail for 51
    51

    A very poorly sorted rock with variably sized and angular quartz and feldspar pieces (both plagioclase and orthoclase; some fresh, others sericitized) set in a finer-grained matrix of chlorite, sericite, carbonate, silky quartz, leucoxene, magnetite and pyrite. Matrix chlorite corrodes quartz grains marginally. A few microfragments of rock (quartzite, granite, gneiss) also are present. Accessory detritals include muscovite, sphene, tourmaline and epidote. Most of the quartz pieces are markedly undulatory. A very low degree of incipient metamorphism is indicated by abundant secondary epidote in the matrix which also locally contains minute platelets of stilpnomelane.

  • Thumbnail for 671
    671

    This allotriomorphic granite shows evidence of weathering as most feldspars are partially obscured by a dusting of clay minerals. Tartan twinning of microcline is obvious throughout the sample. The curving of biotite cleavage and subhedral grain shapes indicates the biotite is not pristine. It is commonly found occurring with subhedral hornblende as well as large, clear to light brown, anhedral sphene crystals.

  • Thumbnail for 878
    878

    Grain shapes in this thin section are consistantly anhedral. Weathering of the feldspars give them a dusty appearance. Because the thin section is cut too thick, the birefringence of quartz and some k-feldspars is as high as second order blues. Mnay hornblende crystals are nearly opaque and they are frequently found surrounding and likely replacing inclusions of pyroxene (perhaps). The biotite and hornblende are rather skeletal. Euhedral sphene and apatite are occasionally scattered throughout.

  • Thumbnail for 874
    874

    This thin section is divided in half diagonally by texture and grain size. The fine-grained side consists of roughly equigranular quartz, microcline, albite, biotite, and riebeckite. More quartzofeldspathic-rich portions have sutured grain boundaries whereas in more mafic patches the grains are typically equant and subhedral. The mineral phases in the coarse-grained half of the thin section are the same though the grain size increases several fold and clear grain boundaries are rarer.

  • Thumbnail for 498
    498

    This sample is quite coarse-grained. Tartan twinning in the k-feldspar is overprinted by flame and blebby lamellae and both are riddled with inclusions of hornblende, micas, and monazite or zircon. Radiation halos are found around the zircons or monazite crystals found as inclusions in biotite grains.

  • Thumbnail for 876
    876

    The riebeckite grains in this sample are embayed in a skeletal fashion, with embayments strongly controlled by the amphibole cleavage. The feldspars have classic albite and tartan twins in addition to patchy lamellae.

  • Thumbnail for A-6-8B
    A-6-8B

    This sample is an almost monomineralic hornblende amphibolite with trace occurances of poikiloblastic orthoclase and biotite. Biotite growth concentrates along the margins of a high relief, clear mineral with fifth-order birefringence, extreme enough that the inclusions appear to not go extinct. Rarely are radiaiton halos observed in hornblende crystals.

  • Thumbnail for SS-3674-3
    SS-3674-3

    The largest crystals in this sample are augite, in which twinning is common, as is alteration to a mixture of chlorite and biotite, and the formation of glomeroporphyroclasts. Chlorite and biotite also are found rimming olivine grains, which are themselves clustered with augite and biotite crystals. The mafic phases are typically subhedral in shape, in contrast to the nephaline, k-feldspar, and zeolites which are anhedrally intergrown together in the interstices between augite crystals.

  • Thumbnail for SS-1
    SS-1

    The dominant mafic phase in this thin section is biotite, followed by augite. These two phases comprise the phenocryst population. Due to secondary alteration, the once euhedral grains of augite are now 'patchy' in appearance. The groundmass consists of a cloudy matrix of biotite microlites set amongst a indistinguishable mixture of sanidine and zeolites. Interstitial calcite is found in the groundmass, and can be seen infilling voids in the augite phenocrysts.

  • 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 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 502
    502

    This fine-grained granite has a mosaic texture. The quartz seems undisturbed, however, most other phases display evidence of weathering and disequilibrium. The dusting of clays and presence of sericite inclusions in the feldspars indicates they are chemically weathered. Biotite and muscovite are skeletal in appearance and biotite is frequently interfingered with chlorite.

  • Thumbnail for 868
    868

    The coarseness of this granite obscures in thin section the graphic texture so evident in handsample. The feldspars contain inclusions of muscovite as well as zircon (or another like mineral) and untwinned grains are readily distinguished from quartz by the comprehensive dusting of clay minerals due to weathering.

  • Thumbnail for 484
    484

    Perthitic intergrowths of k-feldspar and albite are prevalent throughout this coarse-grained, nearly allotriomorphic granite. Riebeckite and biotite are found intergrown together in anhedral masses. The rare tiny, equant olivine crystal is observable.

  • Thumbnail for SC-21
    SC-21

    This thin section is comprised entirely of glass and aligned plagioclase microlites. A few hematite nodules are found throughout. One weathered biotite phenocrysts can be seen.

  • Thumbnail for BR-5
    BR-5

    Without a handsample it is difficult to know if BR-05 is a phyllite or a schist, though the composition and texture are clearly metamorphic. Moderately-aligned muscovite makes up the bulk of this sample. Interspresed throughout are aligned prisms of biotite and kyanite.

  • Thumbnail for 990
    990

    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.

  • Thumbnail for ERB-02
    ERB-02

    This sample is a strongly mylonitized pelitic schist with a continuous schistoscity defined by strongly alligned biotite, fibrous sillimanite, and opaques and interspersed with discontinuous quartz and k-feldspar ribbons. K-feldspar is strongly sericitized. The sense of shear is ambiguous.

  • Thumbnail for 679
    679

    This allotriomorphic granite is dominated by feldspar phases which are altering to clays. The mica phases are anhedral, emabyed, and filled with unaligned opaque inclusions.

  • Thumbnail for ERB-01
    ERB-01

    Anastamosing spaced schistocity defined by aligned elongate minerals including biotite, hornblende, garnet, and opaques. Foliation wraps quartz and k-feldpsar porphyroclasts with core-mantle textures and subgrain development in the old grains. K-feldspar is strongly sericitized. The sense of shear is ambiguous.

  • Thumbnail for 23-5A
    23-5A

    A continuous foliation defined by aligned euhedral biotite characterizes this fine-grained sample. Granoblastic quartz and feldspars fill the voids between biotite crystals. Small, equant opaqes are distributed evenly throughout.

  • Thumbnail for ERB-0X3
    ERB-0X3

    In this sample, a spaced, anastamosing schistoscity defined by biotite, garnet, hornblende and opaques wraps around deformed feldspar lenses which display undulose extinction and bent growth as well as deformation twins. Polyganized quartz and feldspar mantles the feldspar porphyroclasts. Quartz ribbons are prevalent throughout this sample. The sense of shear is ambiguous.

  • Thumbnail for ERB-03
    ERB-03

    In this mylonitized sample, biotite, sillimanite, and the opaques define a spaced schistoscity overprinting a background matrix of seriate-interlobate quartz and feldspar. The foliation wraps around polymineralic porphyroclasts with fish-like shapes. The sense of shear is ambiguous. A lineation is defined by aligned sillimanite and biotite.

  • Thumbnail for 865
    865

    A noteworthy characteristic of this allotriomorphic granite is the presence of feldspar laths with albite twinning in the core and anhedral k-feldspar rims. The remainder of the rock is comprised of equigranular and subhedral biotite, hornblende, and k-feldspar grains. Randomly oriented prismatic apatite grains are abundant in this sample.

  • Thumbnail for ERB-07a
    ERB-07a

    This coarse-grained sample is dominated by subhedral to anhedral k-feldpsar and subhedral garnet, with pockets of quartz aggregates clustering near the feldspar grains and less abundant biotite and orthopyroxene grouped with the garnet phase. Symplectic growth concentrates along boundaries between garnet and k-feldspar. Contacts between garnet are typically host to a mixture of chlorite and biotite. The orthopyroxene grains are strongly fractured and embayed.