Dolerite Dykes
Dolerite (Greek: doleros, meaning "deceptive") or diabase is a mafic, holocrystalline, hypabyssal rock equivalent to volcanic basalt or plutonic gabbro in composition. It displays distinctive bluish-black to dark grey and normally has a fine-grained texture (grain size and alignment of the minerals) of euhedral (crystals that are well-formed with sharp, easily recognised faces) lath-shaped plagioclase crystals (62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3% up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%).
Dolerite is typically found in dykes which are sheet intrusions where a mass of molten magma takes advantage of a pre-existing linear feature in a host rock, such as a long rupture or fault, and forces its way into these spaces. Dolerite dykes often exhibit fine-grained to aphanitic (individual mineral grains cannot be seen with the naked eye) chilled margins which may contain tachylite (dark mafic glass). Typical of any other dykes a dolerite dyke has a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions: length and depth. Thickness can vary from sub-centimetre scale to metres, whereas lateral dimensions can extend over kilometres. Dykes are also typically high angle to near vertical in orientation.
Dolerite dykes occur in regions of crustal extension (such as in an aborted rift) and being basaltic in composition manifest ascent of vast volumes of mantle-derived magmas through fractured lithosphere throughout Earth history (Wikipedia, 2012a & 2012b).
Aborted Rift
An aborted rift is rift that has not undergone complete process of rift formation. A rift is a major linear tectonic structure of the earth’s crust hundreds or thousands of kilometers long that forms when the crust is horizontally extended (crustal extension); this process usually occurs where there is a vast arching uplift (due to mantle plume). It is believed that the entire thickness or a significant part of the thickness of the earth's crust is being extended.
A rift consists mainly of a series of fractures, among which faults with inclined surfaces predominate. The crustal extension can be several kilometers, and in some cases, several tens of kilometers long (Belousov, V.V., 1979).
Dolerite dykes at Teluk Cempedak, Pahang
In the Kuantan area, there are a total of 65 dolerite dykes, both fresh and weathered. The individual length of the exposed dykes is usually less than 5 m and the average width less than 1 m. They always show sharp contacts with the host granite and are almost near vertical (Zainol, 1988). Haile et al. (1983) dated the mafic dykes of the Kuantan area using the K-Ar dating method. The ages of the main intrusions are in the range 79 to 129 Ma (Cretaceous).
Petrographic and geochemical studies show the dykes have tholeiitic affinities and this is also supported by the normative plot which establishes that the dykes range from olivine tholeiite to quartz tholeiite (Azman et al., 2002).
As noted in the last paragraph under the section on “Dolerite Dykes” above, dolerite dykes occur in regions of crustal extension. The exercise of this EarthCache is to determine the probable direction of fractures (that hosted the dolerite dykes at Teluk Cempedak) that are possibly due to the crustal extension of an aborted rift in the Kuantan.
Image 1: Dolerite dyke and orientation of compass – parallel.
(Source: GC3K5W6)
How to claim this EarthCache?
Field equipment required: A compass.
Instruction on how to use a compass is given in Wikipedia (2012c). Compass readings are affected by the presence of iron and steel objects. Be sure to look out for—and stay away from—pocket knives, belt buckles, railroad tracks, trucks, electrical lines, and any magnetic materials when using a compass in the field.
At the EarthCache and waypoints coordinates determine the strike directions of the dolerite dykes. Place the compass parallel to the strike of the dyke as shown in Image 1 and Image 2. There are 12 dykes; take the strike directions of all the dykes. List out the strike directions in acceding order. Strike out the first 2 and the last 2 readings (these readings are due to local variations of the host granite and not the general trend of the dyke strike direction). Average out the 8 readings in the middle. For example: 65°, 92°, 101°, 112°, 115°, 115°, 119°, 124°, 127°, 130°, 161°, 170°. The average strike direction of the 8 middle readings is 118°. Therefore, the probable direction of fractures (possibly due to crustal extension of the aborted rift in the Kuantan area) that hosted the dolerite dykes at Teluk Cempedak is 118° or ESE. The names of compass points are as shown in Image 3.
Image 2: Example of compass reading (shown here as 158.
Note: the orientation of compass is parallel to the dyke.
(Source: GC3K5W6)
Image 3: Compass points (Wikipedia, 2012d).
Send me the following;
1. The text "GC7CJG1 Teluk Cempedak - Dolerite Dykes" on the first line.
2. The answers to the following questions;
- All 12 readings of the strike direction of the dolerite dykes sorted in ascending order.
- The average strike direction of the 8 middle readings.
- The possible direction of fractures (possibly due to crustal extension of the aborted rift in the Kuantan area) that host the dolerite dykes.
3. Provide a photo of yourself or a personal item to prove you have visited the site.*
Glossary
1 Augite is a common rock forming single chain inosilicate mineral with formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees (Wikipedia, 2012e).
2 Basalt is a common extrusive igneous (volcanic) rock formed on the surface of the Earth. By definition, basalt must be an aphanitic (individual mineral grains cannot be distinguished with the naked eye) igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase (Wikipedia, 2012f).
3 Clinopyroxene is a member of the pyroxene group of minerals having a monoclinic crystal structure, such as augite, diopside, or jadeite.
4 Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass (Wikipedia g).
5 Holocrystalline is a term use to describe an igneous rock having only crystalline components and no glass.
6 Hypabyssal is the term use for igneous rocks that are formed near the surface of the Earth.
7 Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide (FeTiO3) (Wikipedia, 2012h).
8 K–Ar dating or potassium–argon dating or is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar) (Wikipedia, 2012i).
9 Ma is an abbreviation for megaannum which mean million years.
10 Mafic is a term used to describe an igneous rock that is dark in color.
11 Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron(II,III) oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO•Fe2O3, which is one part wüstite (FeO) and one part hematite (Fe2O3). This refers to the different oxidation states of the iron in one structure, not a solid solution (Wikipedia, 2012j).
12 A mantle plume is a posited thermal abnormality where hot rock nucleates at the core-mantle boundary and rises through the Earth's mantle becoming a diapir in the Earth's crust (Wikipedia, 2012k).
13 The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.
14 Normative means relating to an ideal standard or model.
15 The mineral olivine (when of gem-quality, it is also called peridot and chrysolite), is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface (Wikipedia, 2012l).
16 Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series (Wikipedia, 2012m).
17 Plutonic is a term used for igneous rocks formed from magma that has cooled and solidified below the surface of the earth.
18 Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level (Wikipedia, 2012n).
19 Tholeiite or tholeiitic basalt is relatively rich in silica and poor in sodium. Included in this category are most basalt of the ocean floor, most large oceanic islands, and continental flood basalts (Wikipedia, 2012f).
20 Tholeiitic is an adjective for tholeiite.
References
1 Azman A.G., Khoo, T.T., and Grapes, R., 2002. Geochemistry of mafic dykes from the Perhentian and Redang Islands: an example of the younger (dolerite) dykes petrogenesis from the Eastern Belt of Peninsular Malaysia. Geological Society of Malaysia Bulletin, 45, 235-241. 2 Belousov, V.V., 1979. Rift. In: The Great Soviet Encyclopedia, 3rd Edition [online]. Available at http://encyclopedia2.thefreedictionary.com/Extension+(geology) [accessed on 25 Aug 2012]. 3 Google Earth, 2012. U.S. Department of State Geographer. 4 Haile, N.S., Beckinsale, R.D., Chakraborty, K.R., Abdul Hanif Hussein, and Hardjono, T., 1983. Palaeomagnetism, geochronology and petrology of the dolerite dykes and basaltic lavas from Kuantan, West Malaysia. Geological Society of Malaysia Bulleting, 16, 71-85. 5 Wikipedia, 2012a. Diabase [online]. Available from http://en.wikipedia.org/wiki/Diabase [accessed on 25 Aug 2012]. 6 Wikipedia, 2012b. Dike [online]. Available from http://en.wikipedia.org/wiki/Dike_ (geology) [accessed on 25 Aug 2012]. 7 Wikipedia, 2012c. Compass Basics [online]. Available from http://www.ussartf.org/compass_basics.htm [accessed on 25 Aug 2012]. 8 Wikipedia, 2012d. Cardinal direction [online]. Available from http://en.wikipedia.org/wiki/Cardinal_direction [accessed on 25 Aug 2012]. 9 Wikipedia, 2012e. Augite [online]. Available from http://en.wikipedia.org/wiki/Augite [accessed on 26 Aug 2012]. 10 Wikipedia, 2012f. Basalt [online]. Available from http://en.wikipedia.org/wiki/Basalt [accessed on 24 Aug 2012]. 11 Wikipedia, 2012g. Gabbro [online]. Available from http://en.wikipedia.org/wiki/Gabbro [accessed on 24 Aug 2012]. 12 Wikipedia, 2012h. Ilmenite [online]. Available from http://en.wikipedia.org/wiki/Ilmenite [accessed on 26 Aug 2012]. 13 Wikipedia, 2012i. K-Ar dating [online]. Available from http://en.wikipedia.org/wiki/K-Ar_dating [accessed on 24 Aug 2012]. 14 Wikipedia, 2012j. Magnetite [online]. Available from http://en.wikipedia.org/wiki/Magnetite [accessed on 26 Aug 2012]. 15 Wikipedia, 2012k. Mantle plume [online]. Available from http://en.wikipedia.org/wiki/Mantle_plume [accessed on 25 Aug 2012]. 16 Wikipedia, 2012l. Olivine [online]. Available from http://en.wikipedia.org/wiki/Olivine [accessed on 26 Aug 2012]. 17 Wikipedia, 2012m. Plagioclase [online]. Available from http://en.wikipedia.org/wiki/Plagioclase [accessed on 25 Aug 2012]. 18 Wikipedia, 2012n. Texture (geology) [online]. Available from http://en.wikipedia.org/wiki/Texture_(geology) [accessed on 20 Aug 2012]. 19 Zainol Affendi, A.B., 1988. General Geology and dolerite dykes from Kuantan, Pahang. Unpubl. B.Sc. (Hons) thesis, Dept. of Geology, University of Malaya.
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