University Of Guyana
Department of Mining and Geological Engineering
Faculty of Technology
GEM1205- Igneous and Metamorphic Petrology
Lecturer: [Andrew Chater]
Term Paper
Intrusive Batholiths
Niossi Alsopp (1027609)
30th April, 2018
Introduction
On a cloudy, breezy afternoon in April, the Gem 1205 students (about 40) embarked on an
adventurous field trip with aim to identify, classify and describe rock types found among
boulders along the Kingston, seawalls. Students were closely accompanied by their lecturer, Dr.
Andrew Chater. The journey started on the seawalls close to UG road and ended in the vicinity
of Atlantic gardens along the same road. Dr. Chater spoke extensively on rocks that were seen.
The discussions were greatly appreciated and provided much needed information for this report.
From that, I will be describing mostly pegmatite and some of its occurrence and also migmatite. I
found these set of rocks very interesting.
Pegmatite
This sample is a course grained, leucocratic felsic igneous rock. When crystals are this large, it’s
called a pegmatite. It forms usually from water in late stage magma, the water carrying dissolved
ions keeps the crystal well supplied with minerals for its growth.
The most dominant minerals is alkali feldspar (>35%), Vitreous quartz (>25%) and Muscovite
(<10%).
The muscovite is smallest in proportion and has a vitreous luster with a sheet like appearance.
This sample contains euhedral crystals and which varies in size. The white colored orthoclase
stands out clearly and the quartz is also obvious. These three minerals visibly makes up the
ground mass of this rock. K-feldspar shows blocky texture in this sample. The grains also show
intergrowth textures.
Quartz-mica Porphyritic Granitic Pegmatite
This sample is a large grained (1-21mm) felsic igneous rock, with the largest phenocrysts being
of a pink color. Typically this mineral is most obviously orthoclase (pink, but also white or
greenish.)
The black mineral is the smallest in grain size (1-5mm), and has a vitreous luster. It seems to
have one perfect cleavage (basal) and when scratched, comes off as flakes. This is therefore
muscovite mica. Some of the black minerals also seems to be amphiboles since its vitreous in
luster and also flaky when scratched. It being amphibole would fit with the other minerals since
they all appear in felsic rocks.
One of the colorless mineral is medium grained size (2-12mm), it is vitreous with cleavages on
two planes. It have a massive crystal habit with no distinct shape. This is plagioclase feldspar and
they are quite a few in this mineral sample.
The phenocrysts of this sample are another grey colored mineral of 6-20mm in diameter. It
seems to have a massive crystal habit but it doesn’t show any evidence of a formation or crystal
shapes or cleavages. This is clearly quartz. Quartz (>20%) is essential.
This is a coursed grained felsic rock with phenocrysts meaning it’s a Quartz-mica porphyritic
granitic pegmatite. This suggests that it was intrusively cooled within a pluton, sill or dike, where
the slow cooling allowed the large minerals to be formed.
Samples Displaying Pegmatitic veins/cracks
Quartz Diorite with Pegmatitic Crack.
This sample is mesocratic, displays granitic
veins and has a gneissic foliation which
suggests it was part of regional metamorphismthe lower granulite facies. (Higher grade than
green schist).
Light colored Pegmatitic crack is filled with
quartz and feldspars and occurs at much lower
temperatures.
Greenstone with Pegmatitic Crack
This rock occurs from mafic lava flow and is
most likely an amphibole (amphibolite grade).
The apple green color is epidote.
Light colour is late stage movement of water in
cracks (pegmatite)
Granodiorite with Pegmatitic Vein
This sample is medium grain, mesocratic, with
dominant quartz and feldspar with minor mafic
minerals.
It is cut by a Pegmatitic vein (lighter colored
area) of quartz and feldspars with no visible
micas.
Migmatite (Agmatite)
This sample of migmatite is referred to as an agmatite. It is a fine-grained metamorphic rock that
contains light coloured bands showing evidence of partial melting (metamorphosed basalt). The
rock contains alternating layers of light-colored bands, known as leucosome and dark-colored
bands, known as melanosome The Leucosome is made up of light-colored minerals, such
as muscovite, feldspar and quartz and the melanosome is made of dark-colored minerals, such as
biotite, pyroxene and amphibole. Melanosomes (<20 %) is dominant in this sample as compared
to the leucosome (10%)
Petrogenesis
The name Agmatite applies to the group of migmatites which present an appearance that has
suggested the terms “eruptive breccia” and”intrusion breccia”, but has a different genesis. As
these migmatites consisting of fragments of older rocks cemented by granite are genetically and
petrologically very different from many of the rocks that have been called eruptive breccias
many of which are volcanic rocks, this group was proposed as agmatites.
Other forms of Migmatite
This sample (agmatite) shows veins and segregations of light-colored granitic composition
(leucosome), within dark-colored amphibole and biotite rich material (melanosome).
This sample consists of darker bands rich in mafic minerals and lighter quartzofeldspathic parts.
This sample consists of darker bands rich in mafic minerals and lighter parts of quartz and
feldspars. .
Summary
The general scenario from this report, suggested that most of the rocks were large intrusive
batholiths which were quarried from a mine. Pegmatites form as a late-stage magmatic fluid
which starts to crystallize. This fluid is rich in water, other volatiles, and chemical elements
incompatible in main magmatic minerals. This is the reason why pegmatites are so coarse
grained and contain so many unusual minerals. Migmatites form under extreme temperature
conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks.
This is the reason why it never crystallizes from molten magma.
Recommendations
For further fieldtrip, I would suggest smaller groups of students at one time since this makes the
session more interactive and even more interesting since more emphasis will be placed on what
is important and more attention will be paid.
This trip was very useful to my GEM 1205 course since it give a practical, hands-on experience
with intrusive batholiths and provided many useful information as it relates to metamorphism.
One negative remark is that because of the size of the class, many students were not showing
interest in what was really occurring at the time and was blatantly showing this by not getting
involved or by doing their own thing.
Reference
Press, C. U. (2009, 5 1st). On the Occurrence of Agmatite in the Rogart Migmatite Area, Sutherland: A
Study in Granitization. Retrieved from Geological Science:
https://www.cambridge.org/core/journals/geological-magazine/article/div-classtitleon-the-occurrenceof-agmatite-in-the-rogart-migmatite-area-sutherland-a-study-ingranitizationdiv/198BA803064B35CEB0218D3805ED408A
Sepp, S. (n.d.). Sandatlas. Retrieved from Pegmatite: http://www.sandatlas.org/pegmatite/
Conclusion
It can be concluded that most of the rocks would have gone through regional metamorphism and
some form of contact metamorphism to incorporate pegmatite.