tonalite Min.
(ˈtɒnəlaɪt)
[ad. G. tonalit (G. vom Rath 1864, in Zeitschr. der deutsch. geol. Ges. XVI. 249).]
Orig., a particular variety of quartz-diorite; later used for any quartz-diorite. Now spec. a rock in which quartz represents 20–60 per cent of quartz plus feldspars (a higher proportion than in quartz-diorite) and alkali feldspar is less than 10 per cent of total feldspars.
| 1879 Rutley Stud. Rocks xii. 244 The rock termed tonalite by Vom Rath, which occurs in the Tonale Pass in the Tyrol,..formerly regarded as a variety of granite, is a micaceous quartz-diorite. 1885 Lyell Elem. Geol. (ed. 4) 571 Tonalite..consists of quartz, oligoclase, and hornblende. 1913 Jrnl. Geol. XXI. 213 Lindgren has defined tonalite (or quartz diorite), as containing less than 8 per cent of alkali feldspar, granodiorite as containing 8–20 per cent of alkali feldspar. 1932 A. Johannsen Descr. Petrogr. Igneous Rocks II. 379 Diorites with less than 5 per cent of quartz may be called quartz-bearing-diorites. With more than 5 per cent, they become quartz-diorites (tonalites). 1962 W. T. Huang Petrology iv. 91 From granite, granodiorite, tonalite to diorite, the proportional amounts of quartz and alkalic feldspar decrease, while that of sodic plagioclase increases. 1976 Earth-Sci. Rev. XII. 14 For field 5 the term tonalite is recommended, whether hornblende is present or not, in agreement with Johannsen..; whereas quartz diorite, frequently used for this field, is restricted to field 10*. Ibid. 27, 20–60% of light-colored minerals... Plag 90–100% of total feldspar: (5) tonalite. 1977 Sci. Amer. Mar. 94/1 The average chemical composition [of gneisses] resembles that of the common igneous rocks diorite and tonalite. |
Hence tonaˈlitic a.
| 1963 Revista de la Asociación Geológica Argentina XVIII. 97 In the Pampean Ranges, tungsten ore deposits are related to tonalitic-granitic intrusives of the pre-Cambrian tectomagmatic cycle. 1978 Nature 16 Mar. 241/1 Although the gneiss belts show great structural complexity..granodioritic and tonalitic compositions greatly predominate. |