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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
The IUGS diagram for plutonic rocks has two triangular parts. The top triangle in the diagram is for rocks that contain quartz, and the bottom (upside down) triangle is for rocks that contain feldspathoids (generally leucite or nepheline). Rocks that contain quartz never contain feldspathoids and vice versa, so it is generally clear which half (top or bottom) of the diagram to use. Due to coarse grain size of plutonic rocks, it is often possible to make rough determinations of the QAPF proportions while examining outcrops or hand specimens— something generally not possible for volcanic rocks. More accurate determinations can be made by examining a rock in thin section with a petrographic microscope. As modifiers to rock names, the IUGS suggests using the prefixes mela- (meaning dark colored) and leuco- (meaning light-colored) when appropriate. A melagabbro, for example is a gabbro that has a darker color than most gabbros, and a leucogranite is a granite that has a lighter color than most granites. These prefixes, and other commonly used adjectives for igneous rocks, are listed in Table 6.2.
The Zr/Hf ratio as an indicator of granite magma evolution of rare metal deposits related to post-orogenic granites
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
G.P. Zaraisky, A.M. Aksyuk, A.V. Fedkin, R. Seltmann
Low values of mafic components in the original magma and, as a result, low quantity of mafic minerals favored the concentration of rare metals in the residual melts. Change of the fluid-magmatic differentiation type leads to a change of the dominating mineralization type from W-Mo-Sn specialization typical for leucogranites with Zr/Hf of about 25-15 (Fig. 5) and Li-F granites with Zr/Hf of 15-5 to Ta-Nb specialization typical for the most evolved varieties of Li-F granites with Zr/Hf of 5 and below (Fig. 6). A mechanism of such a regular and widely common Zr/Hf decrease during granite crystallization differentiation can be proposed. Zircon is the main Zr- and Hf-bearing mineral in granite rocks of the calciferous-alkaline series. According to experiments by Linnen (1998) Hf has a higher affinity with the granite melt than Zr; increasing Li content in the granite melt makes this difference even stronger. Therefore, while the granite magma crystallizes and accessory zircon fractionates the Zr content in the melt decreases and the Hf content increases. Thus, the bulk Zr/Hf ratio must constantly decrease during the evolution of the granite magma from biotite granite to leucogranite and further to Li-F granite.
Geochemical characteristics and structural setting of lithium–caesium–tantalum pegmatites of the Dorchap Dyke Swarm, northeast Victoria, Australia
Published in Australian Journal of Earth Sciences, 2023
B. R. Hines, D. Turnbull, L. Ashworth, S. McKnight
The Anglers Rest Granite is an Early to Middle Devonian I-type granite, forming a large, northeast-trending pluton south of Glen Wills (Figure 2; Morand et al., 2005). The Anglers Rest Granite is a homogeneous, equigranular medium to coarse-grained leucocratic granite. It is composed of pink, medium- to coarse-grained equigranular biotite leucogranite with minor muscovite and hornblende locally present (Figure 4j). Accessory minerals include common allanite, rare sphene, apatite, zircon, magnetite and ilmenite. Three biotite K–Ar dates obtained from the Anglers Rest Granite indicate an age of 400 ± 16 Ma (Richards & Singleton, 1981).
Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides
Published in GFF, 2021
Yuan Li, David G. Gee, Anna Ladenberger, Håkan Sjöström
This leucogranite is composed mainly of quartz, K-feldspar (microcline with typical twinning), plagioclase and minor biotite and muscovite. There is a clear foliation defined by parallel layers of both the micas and a grain shape fabric of quartz and feldspar (Fig. 5A).