II.4. Acidic (silicic) igneous rocks

GRANODIORITE

Appearance: Light coloured, medium- to coarse-grained intrusive igneous rocks with more than 20% of quartz. Within feldspars, the Na-rich plagioclase prevails over K-feldspar (orthoclase). The mafic minerals are represented by amphibole, pyroxene and biotite. This is the intrusive equivalent of dacite. It is formed mostly in subduction zones (active continental margins) and collisional settings (e.g., Alps along the Insubric-Periadriatic line). It forms also small volume shallow intrusive bodies of andesitic to dacitic volcanoes.

Mineral content:

Essential minerals: Na-rich plagioclase > K-feldspar (orthoclase), quartz, biotite, amphibole

Accessory minerals: apatite, magnetite, zircon

Secondary minerals: sericite, chlorite, epidot

Rock types:

Locations in the Carpathian-Pannonian region: Mórágy; Calimani, Stiavnica-Kremnica (Central Slovakian volcanic complex).

Figure II.199. – Granodiorite

Figure II.200. – Granodiorite

Figure II.201. – Granitoid (mostly granodiorite, tonalite) intrusive bodies along the Periadriatic tectonic line in the Alps

DACITE

Appearance: Ligth grey coloured, often crystal-rich volcanic rock. The most abundant phenocrysts are plagioclase, the mafic minerals are usually represented by amphibole and biotite, occasionally orthopyroxene. It contains much less alkali feldspar and quartz. It occur most commonly in subduction zone volcanoes. The known largest volume pyroclastic deposit in the Earth (Fish Canyon Tuff) is composed by dacite.

Mineral content:

Essential minerals: Na-rich plagioclase > K-feldspar (sanidine), biotite, amphibole, pyroxene

Accessory minerals: quartz, titanite, apatite, magnetite, zircon

Secondary minerals: sericite, chlorite, epidot

Locations in the Carpathian-Pannonian region: Visegrád Mts. (Holdvilág creek, Pilisszentlélek, Csódi hill), Börzsöny (Bajdázó), Central Slovakian volcanic complex, Tokaj Mts., Vihorlát, Calimani, Harghita, Ciomadul

Figure II.202. – Dcaitic lava dome of Mt. St. Helens (Washington, USA). Note that the dacitic magma erupts almost in solidified form (photo: USGS)

Figure II.203. – Crystal-rich dacite with plagioclase, amphibole and biotite phenocrysts from Csomád

Figure II.204. – Typical microscopic photo of the dacite with plagioclase, amphibole and biotite phenocrysts from Kis-Csomád, Ciomadul (photo with one nicol).

Figure II.205. – Typical microscopic photo of the dacite with plagioclase, amphibole and biotite phenocrysts from Kövesponk, Ciomadul (photo with one nicol).

Figure II.206. – Typical microscopic photos of the garnet-bearing rhyodacite from Pilisszentkereszt (Visegrád Mts.). Left with one nicol, right with crossed nicols.

Figure II.207. – Typical microscopic photos of the garnet-bearing rhyodacite from Pilisszentkereszt (Visegrád Mts.). Left with one nicol, right with crossed nicols.

Figure II.208. – Typical microscopic photos of the amphibole-biotite dacite from Királyrét (Börzsöny). Left with one nicol, right with crossed nicols.

Figure II.209. – Typical microscopic photos of the amphibole-biotite dacite from Királyrét (Börzsöny). Left with one nicol, right with crossed nicols.

Figure II.210. – Typical microscopic photos of the dacite from Bad’an, (Slovakia). Left with one nicol, right with crossed nicols.

Figure II.211. – Typical microscopic photos of the dacite from Bad’an, (Slovakia). Left with one nicol, right with crossed nicols.

Figure II.212. – Typical microscopic photos of the dacite from Kyslinki, (Slovakia). Left with one nicol, right with crossed nicols.

Figure II.213. – Typical microscopic photos of the dacite from Kyslinki, (Slovakia). Left with one nicol, right with crossed nicols.

Figure II.214. – The dacitic lava dome complex of Ciomadul from north (photo: Szabolcs Harangi).

Figure II.215. – The dacitic lava dome of Vár-tető in Ciomadul (left) and a closer view of the dacite at right (photo: Szabolcs Harangi).

Figure II.216. – Dacitic pumice fall tephra layer at Kézdivásárhely, about 20 km from Ciomadul and a closer view of the well-sorted pumiceous tephra (photos: Szabolcs Harangi).

Figure II.217. – The Ciomadul dacite: Typical microscopic photos with amphibole and plagioclase phenocrysts and antecrysts (left) and clinopyroxene xenocrysts surrounded by amphiboles (right). Photos with one nicol.

Figure II.218. – The Ciomadul dacite: Typical microscopic photos with large plagioclase antecryst including an amphibole Left with one nicol, right with crossed nicols.

Figure II.219. – The Ciomadul dacite: Typical microscopic photos of the dacite from Nagy-Haram with large amphibole phenocryst (left) and a rounded quartz antecryst at right. Left with one nicol, right with crossed nicols.

Figure II.220. – The Ciomadul dacite: microscopic photos of the dacite from Kövesponk with typical crystal clot. This dioritic-granodioritic crystal clot with plagioclase, amphibole, titanite, zircon and biotite could represent a pre-existingcrystal mush and remobilized. Left with one nicol, right with crossed nicols.

Figure II.221. – The Ciomadul dacite: microscopic photos of the dacitic pumice with amphibole and biotite phenocrysts from Tusnad. Photo with one nicol.

Figure II.222. – The Ciomadul dacite: microscopic photos of the dacitic pumice with amphibole phenocrysts from Bixad. Photo with one nicol.

GRANITE

Appearance: Light coloured, medium- to coarse-grained intrusive igneous rock containing more than 20% quartz and the amount of alkali feldspars (orthoclase) exceeds that of the plagioclase. The mafic minerals are usually biotite, but amphibole and pyroxene could also occur. Certain granite varieties muscovite, garnet and cordierite could be also found. Granites usually occur where thick continental crust developed, i.e. at subduction zones along active continental margin (e.g. Andes, Western USA) and at collision zones (e.g., Alps, Himalaya).

Mineral content:

Essential minerals: K-feldspar (sanidine) > Na-rich plagioclase, quartz, biotite, amphibole

Accessory minerals: titanite, apatite, magnetite, zircon, muscovite, pyroxene, andalusite, cordierite, garnet, turmalin, topas

Secondary minerals: sericite, chlorite, epidot

Rock types:

The characteristic accessory minerals of peraluminiuous (S-type; the silicic magma is formed by melting of metasedimentary rocks) granites are Al-rich minerals, such as garnet, cordierite, andalusite and muscovite. In the metaluminuous (I-type; the silicic magma is formed by melting of metaigneous rocks) granites, amphibole and pyroxene represent the mafic minerals in addition to biotite. The peraluminuous granites are characterized by the appearance of alkali mafic minerals such as alkali pyroxene and alkali amphibole. The feldspar content of the granite is also indicative on the origin of the silicic magma. If only one alkali feldpar type is found what often shows perthitic feature, it implies crystallization from relatively dry magma hipersolvus granites). Two different feldspar types (K-feldspar and Na-rich plagioclase) suggest that the crystallization could take place from water-saturated magma (subsolvus granites).

Figure II.223. – Granite with orthoclase megacryst (Erdősmecske).

Figure II.224. – Formation of granitic magma in collision zones: beneath the collision front, shallow detachment of the oceanic crust could induce upwelling of hot asthenosphere. This could result in partial melting in the lower lithosphere followed by crustal melting due to the basaltic underplating beneath the thick crust. Intermittent uprise of crustal or hybrid melts build silicic magma reservoir in the crust and granites are formed by crystallization of this magma.

Figure II.225. – Medium-grained granite

Figure II.226. – Coarse-grained granite (orthoclase, plagioclase, quartz and biotite)

Figure II.227. – Aplite vein in granite (Erdősmecske)

Figure II.228. – Typical microscopic picture of granite from Mórágy. Left with one nicol, right with crossed nicols.

Figure II.229. – Typical microscopic picture of granite from Rigó hill (Velence Mts.). Left with one nicol, right with crossed nicols.

Figure II.230. – Typical microscopic picture of granite aplite from Erdősmecske. Left with one nicol, right with crossed nicols.

Locations in the Carpathian-Pannonian region: Mórágy, Velence Mts., Zala basin and along the Balaton line, Veporids (Slovakia)

Velence Mts. (ca. 280-300 Ma; peraluminuous S-type biotite monzogranite)

Figure II.231. – Characteristic appearance of the granites in the Velence Mts. The Rigó hill quarry at Sukoró exposes granites, porphyritic granite and aplite as well as mafic xenoliths (photos: Szabolcs Harangi)

Figure II.232. – Characteristic appearance of the granites in the Velence Mts. The Rigó hill quarry at Sukoró exposes granites, porphyritic granite and aplite as well as mafic xenoliths (photos: Szabolcs Harangi)

Figure II.233. – Typical microscopic picture of granite from Rigó hill (Velence Mts.). Left with one nicol, right with crossed nicols.

Figure II.234. – Typical microscopic picture of granite from Rigó hill (Velence Mts.). Left with one nicol, right with crossed nicols.

Figure II.235. – Typical microscopic picture of granite aplite from Rigó hill (Velence Mts.). Left with one nicol, right with crossed nicols.

Mecsek (ca.320-340 Ma; metaluminuous I-type microcline megacryst-bearing biotite monzogranite)

Figure II.236. – Typical appearance of the granite in Kismórágy. Note the dark mafic xenolith in the monzogranite and the coarse-grained, microcline-rich granite at right (Photo: Balázs Koroknai)

Figure II.237. –Typical appearance of the granite in Erdősmecske.

Figure II.238. – Aplite dyke and aplite vein in the granite of Erdősmecske.

Figure II.239. – Typical microscopic picture of granite from Erdősmecske. Left with one nicol, right with crossed nicols.

Figure II.240. – Typical microscopic picture of granite aplite from Erdősmecske. Left with one nicol, right with crossed nicols.

Figure II.241. – Mafic mineral accumulation interpreted previously as restites in the grantite of Erdősmecske. Left with one nicol, right with crossed nicols.

Figure II.242. – Mafic mineral accumulation interpreted previously as restites in the grantite of Erdősmecske. Left with one nicol, right with crossed nicols.

Figure II.243. – Typical microscopic picture of granite from Mórágy. Left with one nicol, right with crossed nicols.

Figure II.244. – Typical microscopic picture of granite from Kismórágy. Left with one nicol, right with crossed nicols.

Figure II.245. – Typical microscopic picture of granite from Kismórágy. Left with one nicol, right with crossed nicols.

Figure II.245. – Typical microscopic picture of granite aplite from Kismórágy. Left with one nicol, right with crossed nicols.

Figure II.246. – Typical microscopic picture of granite aplite from Kismórágy. Left with one nicol, right with crossed nicols.

RHYOLITE

Appearance: Light coloured or red, often banded (the banding is related to the difference in volatile, i.e. vesicles and crystallites in the glassy groundmass), aphyric-glassy (obsidian-like) or variously porphyritic volcanic rock, the volcanic equivalent of granite. It contains more than 20% quartz and the amount of alkali feldspars (orthoclase) exceeds that of the plagioclase. The mafic minerals are usually biotite. Rhyolite occurs mostly at areas where thick crust developed, i.e. mostly along subduction zones (active continental margins; e.g. Andes), but can be found also in extensional regions such as in Taupo, New Zealand. In Large Magmatic Provinces they are associating with flood basalts. Rhyolites are the dominant volcanic products along the Snake River Plain – Yellowstone area, where they represent large volume caldera-forming volcanic products. The so-called supervolcanic eruptions are fed either dacitic or rhyolitic magmas. Rhyolites occur also in Iceland, where the Hekla produced violent explosive eruption of rhyolitic magma many times. Here, the rhyolitic magma is formed by melting of basaltic crustal material.

Mineral content:

Essential minerals: K-feldspar (sanidine) > Na-rich plagioclase, quartz, biotite

Accessory minerals: zircon, apatite, magnetite, ilmenite, pyroxene, amphibole

Secondary minerals: sericite, chlorite, epidot

Rock types:

Figure II.247. – Rhyolite lava rock.

Figure II.248 – Rhyolitic obsidian

Figure II.249. – Rhyolitic perlite.

Figure II.250. – Perlite with obsidian clasts (Pálháza, Tokaj Mts)

Figure II.251. – Typical microscopic picture of rhyolitic perlite from Tállya, Tokaj Mts. Left with one nicol, right with crossed nicols.

Figure II.252. – Typical microscopic picture of rhyolite from Stara Kremnica. Left with one nicol, right with crossed nicols.

Figure II.252. – Typical microscopic picture of perlitic rhyolite from Stara Kremnica. Left with one nicol, right with crossed nicols.

Figure II.253. – Theoretical structure of a rhyolitic lava flow

Figure II.254. – Rhyolitic obsidian lava flow at Newberry, Oregon, USA. It is 1300 old, 1.8 km long and 20 m thick at the flow front. (photo: Willie Scott, USGS)

Figure II.255. – Rhyodacitic lava dome at Novarupta, Alaska, USA formed after the cataclysmic Katmei-Novarupta eruption in 1912. It is 75 m thick and its diameter is 275 m (Fotó: Lilly Clairborne)

Figure II.256. – A closer view of the Novarupta rhyodacitic lava dome. It consists of large fractured blocks (Fotó: Lilly Clairborne)

Locations in the Carpathian-Pannonian region: Mecsek (Permian), Staivnica-Kremnica area (Central Slovakian Volcanic complex: e.g., Stara Kremnica, Skalka, Hlinik, Szabó cliff), Mátra (Gyöngyössolymos), Tokaj Mts. (e.g., Pálháza, Cser-hegy, Tállya), Vihorlát (Barabás-Kaszonyi-hill), Calimani (Dragoiasa)

Miocene rhyolitic lava rocks

Figure II.257. – Typical microscopic picture of rhyolite from Stara Kremnica. Left with one nicol, right with crossed nicols.

Figure II.258. – Typical microscopic picture of banded rhyolite from Stara Kremnica. Left with one nicol, right with crossed nicols.

Figure II.259. – Typical microscopic picture of rhyolitic obsidian from Pálháza (Tokaj Mts.). Left with one nicol, right with crossed nicols.

Figure II.260. – Typical microscopic picture of rhyolitic obsidian from Pálháza (Tokaj Mts.). Left with one nicol, right with crossed nicols.

Miocene rhyolitic pyroclastic rocks

Figure II.261. – The Miocene rhyolitic ignimbrites in the Bükk foreland.

Figure II.262. – Non-welded ignimbrite in the Bükk foreland (Szomolya; photo: Szabolcs Harangi).

Figure II.263. – Welded, fiamme-bearing ignimbrite in the Bükk foreland (Tibolddaróc, Bükkalja; photo: Szabolcs Harangi).

Figure II.264. – Characteristic microscopic picture of non-welded ignimbrite (left; Mocsolyástelep) and welded, fiamme-bearing ignimbrite (Pünkösdhegy, Demjén). Photos with one nicol.

Figure II.265. – Characteristic microscopic picture of non-welded ignimbrite from Tar. Note the cuspate shaped glass shards. Photos with one nicol.

Figure II.265. – Characteristic microscopic picture of non-welded ignimbrite with pumice clasts from the borehole of Nyékládháza-1, depth of 185 m. Photos with one nicol.

Figure II.266. – Characteristic microscopic picture of welded fiamme-bearing ignimbrite from the borehole of Nyékládháza-1, depth of 223 m. Photos with one nicol.

Figure II.267. – Characteristic microscopic picture of welded fiamme-bearing ignimbrite from the Tarizsa valley. Photos with one nicol.

Figure II.268. – Characteristic microscopic picture of welded fiamme-bearing ignimbrite from Bogács. Photos with one nicol.