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Minerals, rocks, discontinuities and rock mass
Published in Ömer Aydan, Rock Mechanics and Rock Engineering, 2019
Gabbro is a dark, coarse-grained, intrusive igneous rock and contains varied percentages of pyroxene, plagioclase, amphibole and olivine. The amount of quartz is less than 5%. Gabbro is generally coarse grained, with crystal sizes in the range of 1 mm or greater. Gabbro is found in the ocean crust, underneath the basalt layer (0.5–2.5 km), from 2.5 to 6.3 km deep. The lunar highlands have many gabbros.
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
In continental regions, when mafic magmas make it to the surface, basaltic volcanism occurs. Sometimes, however, magma stalls, producing large volumes of gabbro that crystallize in deep magma chambers that later may be uplifted and exposed by erosion at Earth’s surface. The connection between mafic magma chambers—perhaps later to become plutons—and mafic volcanism is well established. Figure 6.28, for example, shows gabbro outcrops on Meall Meadhonach, northernmost Scotland, that were uncovered after uplift and erosion of Earth’s crust. Nearby basalt occurrences of about the same age suggest that the same mafic magma may have produced both plutonic and volcanic rocks.
Zircon U–Pb age, whole-rock geochemistry and Nd–Sr–Pb isotope constraints on petrogenesis of the Eocene Zajkan gabbro–monzogranite intrusion, Tarom-Hashtjin magmatic belt, NW Iran
Published in Australian Journal of Earth Sciences, 2022
M. A. A. Mokhtari, H. Kouhestani, K. Z. Qin
Gabbro, which has fine- to coarse-grained granular texture and dark grey colour in hand specimens, is generally characterised by granular, ophitic and sub-ophitic textures, with plagioclase (60–65%) and clinopyroxene (30–35%) as major minerals along with minor K-feldspar, quartz, apatite, and opaque minerals (Figure 3a). Plagioclase present as large, euhedral to subhedral crystals, which are weakly altered to sericite. Some plagioclase has inclusions of clinopyroxene, apatite and opaque minerals, and others show zoning. Clinopyroxene occurs as large euhedral crystals that are altered to actinolite and contain inclusions of plagioclase with ophitic textures (Figure 3b). Quartz and K-feldspar (<3%) occur as small xenomorphic crystals between plagioclase and clinopyroxene crystals.
Geology of New Zealand’s Sub-Antarctic Islands
Published in New Zealand Journal of Geology and Geophysics, 2019
James M. Scott, Ian M. Turnbull
A gabbroic intrusion centred on Menhir (Figure 6) was described by Marshall and Browne (1909), Oliver et al. (1950), and Morris (1984). Much of this gabbro is covered by peat except on the northern coast. It may extend at least as far as Middle Bay in the sub-surface, based on the occurrence of a gabbro xenolith in a flow at Middle Bay (Oliver et al. 1950). The gabbro is variably but mostly coarse-grained, grey with white-weathering feldspars, and composed of labradorite, olivine, clinopyroxene, and opaque oxides. Adams et al. (1979) noted that the whole-rock K-Ar age of 16.0 ± 1.0 Ma may be compromised by very low radiogenic Ar and may be part of the younger volcanic rocks because an outcrop of hornfelsed Complex Point Group adjacent to Menhir Gabbro yielded a K-Ar age of 9 Ma.
Age and structure of the Permian Brook Street Terrane, Takitimu Mountains, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2023
Matthew J. Campbell, Nick Mortimer, Gideon Rosenbaum, Charlotte M. Allen, Paulo M. Vasconcelos, Hamish J. Campbell
Zircon was obtained from one White Hill Intrusives rock. Sample 010-TK is a greenish, fine-medium grained gabbro collected near McLean Peaks (Table 1, Figure 2). This is the easternmost White Hill Intrusives sill mapped by Houghton (1986) and intrudes relatively high in the Takitimu Subgroup stratigraphic pile - into MacLean Peaks Formation. The gabbro consists of <4 mm crystals of plagioclase, clinopyroxene, with accessory quartz, alkali feldspar and magnetite.