rocas Ígneas en sección delgada

Download Rocas Ígneas En Sección Delgada

Post on 03-Jan-2016

21 views

Category:

Documents

9 download

Embed Size (px)

TRANSCRIPT

  • 1

    Igneous rocks in thin section Contents Image scales (frame widths) Quartz

    Plagioclase Potassium feldspars

    Myrmeckite Micas

    Amphiboles Pyroxenes

    Other primary minerals and minor minerals

    Secondary (subsolidus) minerals

    20x = 6 mm 40x = 3 mm

    100x = 1 mm 200x = 0.5 mm

    400x = 0.25 mm 1000x = 0.1 mm

    Quartz

    Quartz crystal in alkali granite

    Plane polarized light, 20x

    NEIGC86-B2-7

  • 2

    Quartz crystal in alkali granite

    Cross polarized light, 20x

    NEIGC86-B2-7

  • 3

    Strained quartz crystal in a metaluminous granite. Strain has caused the quartz crystal to deform into domains with slightly different extinction angles.

    Cross polarized light, 20x

    DIG-D

  • 4

    Fluid inclusions in quartz in alkali granite. The inclusion in the center has an irregular outer boundary, inside of which is a layer of liquid water, a layer of liquid CO2, and a central bubble of vapor (mostly CO2). At the time of trapping of this secondary inclusion, the fluid was a binary H2O-CO2 fluid.

    Plane polarized light, 1000x

    NEIGC86-B2-7

  • 5

    Fluid inclusions in quartz in alkali granite. Several inclusions containing (probably) water and a central vapor bubble.

    Plane polarized light, 1000x

    NEIGC86-B2-7

  • 6

    Plagioclase

    Plagioclase, unzoned, in a hornblende diorite

    Plane polarized light, 40x

    NEIGC84-A5-5C

  • 7

    Plagioclase, unzoned, in a hornblende diorite. Note the strong, parallel sets of albite twins, and the less visible set of pericline twins inclined almost at right angles to the albite twins.

    Plane polarized light, 40x

    NEIGC84-A5-5C

  • 8

    Plagioclase, zoned, in a dacite porphyry. This plagioclase appears quite homogeneous in plane light.

    Plane polarized light, 100x

    Rhyolite-2

  • 9

    Plagioclase, zoned, in a dacite porphyry. This plagioclase has fine oscillatory zoning, in which the composition varies between more and less anorthite-rich compositions. Internal unconformities followed by euhedral overgrowths are also visible.

    Cross polarized light, 100x

    Rhyolite-2

  • 10

    Plagioclase, zoned, in a dacite porphyry. Notice the concentric layers (zones) of inclusions in this crystal.

    Plane polarized light, 40x

    Rhyolite-2

  • 11

    Plagioclase, zoned, in a dacite porphyry. Notice that the concentric inclusion zones also have different birefringence, indicating these zones have different anorthite content. The interior of this crystal has patchy zoning rather than concentric, indicating skeletal early growth. Albite, carlsbad, and pericline twins cut across the composition zones.

    Cross polarized light, 40x

    Rhyolite-2

  • 12

    Potassium feldspars

    Orthoclase in a dacite hypobyssal intrusive.

    Plane polarized light, 40x

    Py-28

  • 13

    Orthoclase in a dacite hypobyssal intrusive. Notice the two carlsbad twin domains, separated by the carlsbad composition plane.

    Cross polarized light, 40x

    Py-28

  • 14

    Microcline from a peraluminous granite.

    Plane polarized light, 20x

    Kinsman

  • 15

    Microcline from a peraluminous granite. Note the "grid" or "tartan plaid" twinning pattern that results from crossing albite and pericline twin domains that form during inversion of monoclinic orthoclase to triclinic microcline during cooling. This inversion and change in crystal system is associated with increasing ordering or aluminum in crystallographic sites.

    Cross polarized light, 20x.

    Kinsman

  • 16

    Microcline from a peraluminous granite. Closeup of the plaid twinning. Notice how the twin domains are spindly and somewhat wispy. This is in contrast to the straight and generally continuous twin domains in plagioclase.

    Cross polarized light, 100x.

    Kinsman

  • 17

    Perthite from a metaluminous biotite granite. Note the faint, irregular stripes that run from the upper right to lower left.

    Plane polarized light, 20x.

    4.7.84H

  • 18

    Perthite from a metaluminous biotite granite. Note the lighter irregular stripes and patches of albite that separate gray stripes and patches of microcline. The color difference is mostly due to the different optical orientations of the two different minerals. The microcline and albite both exsolved (unmixed) from an originally homogeneous high temperature of intermediate composition. The sample is oriented to obscure the twinning.

    Cross polarized light, 20x.

    4.7.84H

  • 19

    Perthite from a metaluminous biotite granite. The somewhat less altered and narrower albite exsolution lamellae are in sharp contact with a much larger microcline domains. You can see a lamella closeup here, and then see a Becke line test from focused to a lowered stage position at the lamella-microcline contact. The Becke line goes into the higher index phase, albite.

    Plane polarized light, 100x.

    4.7.84H

  • 20

    Perthite from a metaluminous biotite granite. Same as the image above in cross polarized light.

    Cross polarized light, 100x.

    4.7.84H

  • 21

    Perthite from a metaluminous biotite granite. Closeup showing the characteristic grid twinning in the microcline host (upper center and upper left) and albite twinning in the lamellae (lower center to center right).

    Cross polarized light, 200x.

    4.7.84H

  • 22

    Myrmeckite

    Myrmekite patch that appears to be replacing microcline.

    Plane polarized light, 40x.

    Kinsman

  • 23

    Myrmekite patch that appears to be replacing microcline. Faint twins in the myrmekite clearly shows that the probably quartz "worms" are in a plagioclase matrix.

    Cross polarized light, 40x.

    Kinsman

  • 24

    Micas

    Muscovite, peraluminous granite. Igneous muscovite is generally colorless with good cleavage. Radiation halos can be visible around radioactive inclusions (but not visible here).

    Plane polarized light, 40x.

    NEIGC84-A5-6

  • 25

    Muscovite, peraluminous granite.

    Cross polarized light, 40x.

    NEIGC84-A5-6

  • 26

    Biotite, metaluminous granite, showing several grains in different orientations. Grain on the far right is oriented with the cleavages N-S, and so is almost opaque. The largest grain is inclined and is lighter in color. Grains with the cleavages E-W have the least absorption (not shown in this image).

    Plane polarized light, 40x.

    4.7.84G

  • 27

    Biotite, metaluminous granite. This is the same area as the image above in cross polarized light. The birefringent colors of the biotite are muted because of the color of the biotite itself.

    Cross polarized light, 40x.

    4.7.84G

  • 28

    Biotite, metaluminous granite, showing a closeup of one of the same biotite crystals above at extinction, occupying the entire center of the image. Damage produced to this soft mineral during thin section grinding causes speckles of light on the biotite, where the crystal lattice has been deformed. This means that biotite in standard thin sections rarely goes completely extinct. This is called "incomplete extinction" or sometimes "birds eye maple extinction".

    Cross polarized light, 200x.

    4.7.84G

  • 29

    Biotite, peraluminous granite. This graphite- and garnet-bearing granite has high-Ti, low-Fe3+ red-brown biotite. This patch of biotite rimming garnet (mineral occupying the lower part of the image) shows the range of pleochroic colors because of different crystals being in different orientations.

    Plane polarized light, 20x.

    Kinsman

  • 30

    Amphiboles

    Green hornblende in a diorite.

    Plane polarized light, 40x.

    NEIGC84-A5-5C

  • 31

    Green hornblende in a diorite.

    Cross polarized light, 40x.

    NEIGC84-A5-5C

  • 32

    Green hornblende in a diorite. The ~120 and ~60 cleavage intersections are clearly visible in this end section of a crystal.

    Plane polarized light, 100x.

    4.8.84Q

  • 33

    Brown hornblende, hornblende gabbro. The extensive dark and light brown areas are several hornblende crystals. Note the numerous inclusions of opaques and plagioclase.

    Plane polarized light, 20x.

    4.8.84Q

  • 34

    Brown hornblende, hornblende gabbro, in the image above. The two hornblende orientations do not happen to yield the maximum interference colors, which happen to be second order blue in this section.. Note how plagioclase inclusions have more closely spaced twins than does the hornblende.

    Cross polarized light, 20x.

    4.8.84Q

  • 35

    Pyroxenes

    Enstatite (orthopyroxene, OPX) in norite. The large OPX in the center is oriented with its c crystallographic axis oriented N-S.

    Plane polarized light, 100x

    NEIGC83-C1-14

  • 36

    Enstatite (orthopyroxene, OPX) in norite. The large N-S oriented enstatite grain near the center of the image (see image above) is extinct, in keeping with the orthorhombic symmetry of this mineral. Birefringence ranges to upper first order.

    Cross polarized light, 100x

    NEIGC83-C1-14

  • 37

    Enstatite (orthopyroxene, OPX) in norite. Here the section has been rotated clockwise ~45 to show the birefringence of the large grain.

    Cross polarized light, 100x

    N