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Minerals, rocks, discontinuities and rock mass
Published in Ömer Aydan, Rock Mechanics and Rock Engineering, 2019
Igneous rocks are created by melting and crystallization of magma. When the magma reaches the surface, the rocks are said to be extrusive (Fig. 2.4). Volcanic lava flows are examples of extrusive igneous rocks. If the magma cools within the Earth, it forms large bodies of crystalline rock called plutons or batholiths. These rocks are called intrusive igneous rocks. Igneous rocks are generally classified on the basis of three factors: (1) grain size and texture, (2) intrusive or extrusive, (3) silica content and mineral composition (Fig. 2.5). However, the classifications based on factors 2 and 3 are commonly used to describe igneous rocks as described next.
Plutonic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
We call large plutons batholiths (from the Greek bathos meaning depth and lithos meaning rock) if they are larger than 100 square kilometers in area. Figures 6.1 through 6.5 show views of small parts of the Sierra Nevada Batholith. Batholiths are the largest plutonic bodies but are generally compound (made of many individual plutons). All of the world’s batholiths have an overall felsic (granitic) or, less commonly, intermediate composition. Lopoliths, also seen in Figure 6.6, ae dish-shaped or funnel-shaped plutons. They can be huge and are typically layered and formed, like batholiths, from multiple magmatic intrusions. Most lopoliths have an overall mafic or ultramafic composition. Figure 6.6 also includes a laccolith, dikes, stocks, and a volcanic neck.
Minerals and rocks
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
The second type of major plutonic intrusion is a great body of granodiorite and granite called a batholith (Fig. 2.26). Batholiths are always formed from acid magma, and are characteristic of late igneous activity in mobile belts (see Section 4.6.1), where mountain building is taking place. They extend for tens or hundreds of kilometres along the mountain belt, are tens of kilometres wide, and extend downwards for about 10 km. For example, the Coast Range batholith of the American Rockies, which consists of many intrusions emplaced at the same time, is 2000 km long, 100 km wide and can be seen vertically through a range of 5 km. At no place, however, is the base of a batholith visible. The roof of a batholith may lie below the ground surface, and in that case the only granite outcrops are those of smaller intrusions (commonly between 5 and 10 km across), which rise above the main batholith as stocks (Fig. 2.26). They appear to be separate bodies of granite, but are joined at depth. The margins of a batholith usually dip outwards at a steep angle. Close to them the granite often contains blocks of country rock (xenoliths), which have been broken off during the rise of the acid magma. Xenoliths vary from a few centimetres to tens of metres across. In this marginal zone, the minerals may be arranged parallel to the edge of the intrusion to give foliation to the granite. This is caused by flow while crystallisation proceeds, before the magma solidifies completely.
Dating initial crystallisation of some Devonian plutons in central Victoria and geological implications
Published in Australian Journal of Earth Sciences, 2023
J. D. Clemens, G. Stevens, L. M. Coetzer
The geology and petrogenesis of this cordierite-rich, subvolcanic, S-type batholith were most recently described in Phillips and Clemens (2013) and Clemens and Phillips (2014). Phillips et al. (2022) demonstrated that the batholith is a very thin sheet-like body that was emplaced in multiple magma pulses. Several plutons were identified and delineated by Phillips and Clemens (2013), but the main mass is known as the Mount Wombat pluton. In the northeast, this body intrudes and contact metamorphoses the rhyolitic to rhyodacitic ignimbrites of the Violet Town Volcanics. Previous zircon U–Pb SHRIMP dating of the Mount Wombat pluton was carried out by Bierlein et al. (2001) and established an age of 374 ± 2 Ma. That sample was taken from near the town of Euroa, in the main part of the Mount Wombat pluton. For this part of the pluton, a more precise CA-TIMS date of 373.5 ± 0.3 Ma was determined by Tulloch et al. (2012, 2017).
Mafic intrusions in southwestern Australia related to supercontinent assembly or breakup?
Published in Australian Journal of Earth Sciences, 2022
H. K. H. Olierook, F. Jourdan, C. L. Kirkland, C. Elders, N. J. Evans, N. E. Timms, J. Cunneen, B. J. McDonald, C. Mayers, R. A. Frew, Q. Jiang, L. J. Olden, K. McClay
Sample LB1 was collected from a ∼2.8 m-wide, steeply dipping and west-northwest-trending, ophitic basaltic sheeted dyke (100°/75°S) at Lowlands Beach, between Albany and Denmark (Figures 1, 2 and 3; Harris & Li, 1995). The dykes crosscut a quartz monzonite host rock (Harris & Li, 1995) that probably belongs to the ca 1180 Ma Burnside Batholith (part of the Esperance Supersuite; Pidgeon, 1990), and are locally divided into ∼3 to 5 individual dyke sheets (Figure 3). Sample LB1 mainly comprises partly sericitised plagioclase and clinopyroxene (partly replaced by chlorite, hornblende and actinolite) and Fe–Ti-oxides. Titanite and allanite are also present (Figure 4 and Figure A; Table 2). Based on replacement minerals of clinopyroxene, metamorphic facies is similar to that at Whaling Cove at greenschist to amphibolite facies (Spear, 1995).
The geological history and hazards of a long-lived stratovolcano, Mt. Taranaki, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2021
Shane J. Cronin, Anke V. Zernack, Ingrid A. Ukstins, Michael B. Turner, Rafael Torres-Orozco, Robert B. Stewart, Ian E. M. Smith, Jonathan N. Procter, Richard Price, Thomas Platz, Michael Petterson, Vince E. Neall, Garry S. McDonald, Geoffrey A. Lerner, Magret Damaschcke, Mark S. Bebbington
Mt. Taranaki is located within the Taranaki Basin, a sedimentary basin containing up to 6 km of unconsolidated water-saturated Cretaceous-Cenozoic sediments (King and Thrasher 1996). Near Mt. Taranaki, the western margin of the basin is marked by the Cape Egmont Fault Zone, while the eastern boundary is marked by the Taranaki Fault Zone. Below these, Mortimer et al. (1997) identified a basement of diorite, gabbro, granitoid and metamorphic rocks of the Permian-Cretaceous Median Batholith. This abuts the Brook St. Terrane (a Permian arc) ∼10 km east of Mt. Taranaki. Around 30 km east of the volcano, a suture zone between the Brook St. Terrane and the volcaniclastic sandstones of Murihiku Terrane is marked by the N-S striking, Taranaki Fault (Figure 1). The fault is seismically detected to the base of the crust (Sherburn et al. 2006) and represents a thrust zone ∼8–10 km wide, that has accommodated over 15 km of dip-slip displacement over the last ∼80 Ma (Giba et al. 2010).