Name(s): XXXXXXXXX
Student Number(s): XXXXXXXXX
Petrographic Description Sample 1
Sample: Hypothetical Sample
Location: Nowhere, Canada
Name: Moderately fractured, moderately albite-epidote-
sericte-chlorite-hematite altered hornblende granite
Textures: Porphyritic with coarse-grained potassic
feldspar phenocrysts in a medium-grained, poorly
foliated grouodmass.
Field relations: Phase of a composite granitoid
batholith
Description: Rock is a pale pink and green,
moderately fractured intrusive igneous rock that has
been altered by hydrothermal fluids. Most fractures
are small and are infilled with a white-green colored
mineral assemblage, however, one third of the rock
(and thin section) comprises a large vein of matrix
MINERAL
plagioclase
MODAL %
32
Plate 1: Top - Hand sample, Bottom - representative
photomicrograph of sample 1, PPL. Field of view = 4 mm
supported breccia (discussed at end of report). Modal
quartz
36
orthoclase
12
chlorite (clinochlore)
15
clinozoisite (epidote)
8
mineralogy of primary mineralogy (igneous) suggests that this
was a hornblende granite.
Mineral Descriptions
plagioclase: Relict subhedral plagioclase (1.1 mm avg)
commonly found in complex clusters (glomerocrysts) that reach
6.5 mm in size. Glomerocrysts have irregular grain boundaries
and show albite and carlsbad twinning (Plate 2). Individual and
glomercysts have no primary inclusions, however, they contain
abundant secondary inclusions of clinozoisite-epidote, located
magnetite
<1
within the cores of the plagioclase grains, and very fine grained
hematite
<1
potassium feldspar (orthoclase) : Relict interstitial anhedral
hematite.
orthoclase (0.75 mm avg) with minor perthite exsolution
white mica (sericite)
<1
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titanite
GEOL 235: Lab Assignment #0 - Granites
<1
Name(s): XXXXXXXXX
Student Number(s): XXXXXXXXX
laminae. Minor primary inclusions of quartz and plagioclase, however, contain secondary inclusions of sericite
giving a ‘dusty’ appearance in ppl.
quartz : Anhedral ameboid quartz (4.1 mm avg) with undulose extinction and abundant 2-phase fluid inclusions (up
to 0.01 mm) (Plate 2).
chlorite : Partial pseudomorphic chlorite (1.5 mm avg) after amphibole (hornblende?). Titanite, albite, magnetite
and epidote inclusions along basal cleavage.
clinozoisite : Small (<0.1 mm) subhedral to anhedral inclusions within plagioclase and chlorite.
Summary
Primary : The primary crystallization sequence was plagioclase, titanite (?), hornblende, quartz, orthoclase.
Plagioclase glomerocrysts were likely formed during magmatic differentiation.
Secondary : Moderately altered plagioclase and potassium feldspar with up to 30% partial replacement to epidotewhite mica (sericite)-hematite. Quartz shows evidence of extensive recrystallization and fluid infiltration by the
undulose extinction and with numerous secondary fluid inclusions (up to 20%)(Plate 2).
Structure : Moderate fracture/veining is present with thin section at a frequency of 1.65 veins per cm with an
average vein aperture of 2.1 mm. One third of thin section has been extensive altered during brittle deformation
(Plate 2).
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GEOL 235: Lab Assignment #0 - Granites
Name(s): XXXXXXXXX
Student Number(s): XXXXXXXXX
Plate 2: Photomicrographs of sample 1. A) Hematite-clinozoisite (Czo) altered plagioclase (Plag) glomerocryst in a
quartz (Qtz) groundmass (plane polarized light, PPL-scale bar = 1 mm) l). B) cross polarized light, XPL. C) Chlorite
(Chl) after amphibole (hornblende?) with clinozoisite and titanite inclusions ( PPL-scale bar = 1 mm). D) cross
polarized light, XPL. E) Brittle fracture contact (dashed line) with hornblende granite. Fracture is infilled with a claysericite-epidote-hematite(?) matrix supporting clasts of plagioclase, quartz, calcite, and orthoclase (PPL-scale bar = 1
mm). F) same as E, XPL.
Page 3 of 3
GEOL 235: Lab Assignment #0 - Granites