This EarthCache will bring you to a quartz monzonite site at Pangkor Island.
Type of EarthCache: Mineralogy / Igneous (plutonic) feature.
General geology of Pangkor Island
Pangkor Island is entirely made up of medium and coarse-grained porphyritic biotite granite and quartz monzonite with associated minor intrusives of aplite and pegmatite dykes and quartz veins (collectively known as granites, granitic rocks, or intrusives). Igneous structures such as schlieren, xenoliths, and tourmaline clots could also be found in these intrusives (Abdul Rahman, 1980; Huzaidi, 1985; Wong, 1991). In between the granites are Quaternary deposits consisting of unconsolidated gravel, sand, silt, clay, and peat (Image 1).
Jointing is well developed in the granitic rocks and sheared in places. Weathering is most marked along these joints. The general direction of all the dykes and veins of the island are in the direction of 100-140o (Abdul Rahman, 1980).
Bignell (1972) dated the granites from Pangkor Island using K:Ar method, to be 209 +/- 8 Ma, which is Upper Triassic.
Geochemical analyses of these granites of Pangkor Island indicate that they are of the S-type (sedimentary protolith) granite (Abdul Rahman, 1980; Huzaidi, 1985), which was formed by melting of deeply-buried high grade sedimentary metamorphic rocks. Geochemical analyses also show that these granites are similar to that of the Main Range Granite (Abdul Rahman, 1980), which is the main granitic body of Peninsular Malaysia.
Image 1: Geological map of Pangkor Island (modified after Geological Survey Malaysia, 1990).
(Map also shows the location of Quartz Monzonite of Pangkor Island EarthCache).
Classification of igneous rocks
Based on mineralogical composition, igneous rocks are classified by using a double triangle QAPF diagram (Image 2). The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for classification in QAPF diagram. Q, A, P and F percentages are normalized (recalculated so that their sum is 100%).
QAPF diagrams were created by the International Union of Geological Sciences (IUGS): Subcommission on the Systematics of Igneous Rocks. The diagrams are accepted by geologists worldwide as a classification of igneous, especially plutonic rocks (Wikipedia a, 2012). Under this classification, the granites of Pangkor Island are in the granite (syeno-granite and monzo-granite) and quartz monzonite fields (Image 2).
Image 2: QAPF diagram showing the classification of granites in Pangkor Island – fall in the granite and quartz monzonite fields.
Quartz monzonite of Pangkor Island
Since exact mineral composition cannot be determined in the field, it is given that the igneous intrusive at the EarthCache site is a porphyritic (contains phenocrysts) quartz monzonite. A phenocryst is a relatively large and usually conspicuous crystal distinctly larger than the grains of the rock matrix or groundmass of a porphyritic igneous rock. The matrix or groundmass of a rock is the finer grained mass of material in which the phenocrysts are embedded (Image 3).
Quartz monzonite (or adamellite) is an intrusive igneous rock that has an approximately equal proportion of K-feldspar or potassium feldspar (typically orthoclase) and Na-feldspar or sodic plagioclase (typically oligoclase). Quartz is present in significant amount. Biotite and/or hornblende constitute the dark minerals (Wikipedia b, 2012). Because of its colouring, it is often confused with granite, but whereas granite contains more than 20% quartz, quartz monzonite is only 5-20% quartz (Image 2).
Identification of rock-forming minerals in quartz monzonite
Felsic minerals
The felsic (light coloured) rock-forming minerals of quartz monzonite are the K-feldspar or potassium feldspar (typically orthoclase), Na-feldspar or sodic plagioclase (typically oligoclase), and quartz.
K-feldspar occur as phenocrysts as well as in the matrix of the rock. It could be pink, white, or grey and it could be euhedral (crystals that are well formed with sharp, easily recognised faces) or anhedral (crystals that have no well-formed faces). The crystals are commonly elongated with a tabular appearance (Image 3).
Sodic plagioclase occur as irregular grains and rarely as tabular crystals in the matrix of the rock. In colour it is white to dark grey (Image 3).
Quartz are found as greyish (not white), glassy anhedral grains with no cleavage scattered among the K-feldspar and sodic plagioclase grains in the matrix (Image 3).
Mafic minerals
The mafic (dark coloured) rock-forming minerals of a quartz monzonite are biotite and/or hornblende. Biotite occur as irregular masses of black, brown, or dark green flakes (Image 3), whereas hornblende occur as dark green to black euhedral long, stubby, or irregular grains in the matrix of the rock (Image 4).
Image 3: Porphyritic biotite granite (Source: GC3K5YM). Note: this photo example defines what are phenocrysts and matrix. It is not the rock type of the EarthCache.
Image 4: Hornblende granite (modified from Jones, 2012). Note: this photo example shows how hornblende grains look like in the matrix of the rock. It is not the rock type of the EarthCache.
How to claim this EarthCache?
Field equipment required:
1. A 10x or 20x hand lens.
2. A millimetre measure for measuring grain sizes.
At the listed coordinates, observe and note the mineral composition, the texture (grain size and alignment of the minerals) in the rock, and the colour of each of the rock-forming mineral. Later, you are to provide the full name of the rock type at the EarthCache site; for now, it is porphyritic quartz monzonite.
Send me the following;
1. The text "GC7CJG8 Pangkor Island - Quartz Monzonite" on the first line.
2. The answers to the following questions;
- The phenocrysts are K-feldspar.
(1) What is the colour of the phenocrysts?
(2) Are the phenocrysts aligned in a definite pattern? Describe the pattern.
(3) Measure the length (in millimetres) of 10 phenocrysts. Report the result as acceding order of data, the range of their lengths, and their average length. For example: 32, 33, 40, 40, 41, 42, 42, 43, 45, 45 mm; range = 32-45 mm; average = 40.3 mm.
- Describe the matrix in term of mineral (K-feldspar, sodic plagioclase, quartz, biotite, hornblende) distribution. Also, note:
(1) What is the colour of each of the rock-forming mineral?
(2) Which mafic mineral is more abundant, biotite or hornblende?
(3) What is the overall colour of the rock mass?
- Measure the diameters of 10 minerals found in the matrix. If the mineral is elongated, measure the longest length. Report the result as acceding order of data, the range of their diameters, and their average diameter. For example: 7, 9, 10, 10, 12, 13, 14, 16, 17, 17 mm; range = 7-17 mm; average = 12.5 mm.
- From the results in task 4, what is the overall grain size of the rock? Use the following classification to determine the grain size:
Very coarse-grained: > 10 mm
Coarse-grained: 2-10 mm
Medium-grained: 1-2 mm
Fine-grained: < 2 mm
In the above example the rock is very coarse-grained.
- Provide the full name of the rock as follow: [fine-grained or medium-grained or coarse-grained or very coarse-grained] porhyritic [name the most abundant mafic mineral]-[name the second most abundant mafic mineral] quartz monzonite. For example, the full name of the rock is fine-grained porhyritic pyroxene-olivine quartz monzonite :). To answer this question you have to answer Question #2 and Question #4 first.
3. Provide a photo of yourself or a personal item to prove you have visited the site.*
References
1 Abdul Rahman, M.E., 1980. Granite geology of Pulau Pangkor. Unpublished B.Sc. (Hons) thesis, Department of Geology, University of Malaya, 88 pp. 2 Bignell, J.D., 1972. The geochronology of the Malayan granites. Unpublished Ph.D. thesis, St. Catherine’s College and the Department of Geology and Mineralogy, Oxford. 3 Geological Survey Malaysia, 1990. Geological map of Lumut-Teluk Intan. 4 Google Earth, 2012. U.S. Department of State Geographer. 5 Huzaidi, H., 1985. The Geology and Petrology of the Pulau Pangkor Granite. Unpublished B.Sc. (Hons) thesis, Department of Geology, University of Malaya, 26 pp. 6 Jones, C.E., 2012. Granite: Coarse-Grained Felsic Rock. Department of Geology & Planetary Science [online]. Available from http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/6Granite.html [accessed on 27 May 2012]. 7 Wikipedia a, 2012. QAPF diagram [online]. Available from http://en.wikipedia.org/wiki/QAPF_diagram [accessed on 4 Aug 2012]. 8 Wikipedia b, 2012. Quartz monzonite [online]. Available from http://en.wikipedia.org/wiki/Quartz_monzonite [accessed on 4 Aug 2012]. 9 Wong, T.W., 1991. Geology and Mineral Resources of the Lumut-Teluk Intan area Perak Darul Ridzuan. Geological Survey Malaysia, Map Report 3, 96 pp.
* Effective immediately from 10 June 2019, photo requirements are permitted on EarthCaches. This task is not optional, it is an addition to existing logging tasks! Logs that do not meet all requirements posed will no longer be accepted.
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Finding the answers to an EarthCache can often be challenging, and many people tend to shy away from these caches because of this. However, it is my opinion that geocaching is also meant to be a fun family experience that simply aims to introduce interesting and unique locations such as this one. Flexibility on logging requirements, however, can only be applied if it can be established that you have actually taken the time to visit the site. For this reason, a proper log describing your adventure accompanied by a good number of photos would be much appreciated.
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