Romania: matter of interest in Apuseni Mountains

Fig. 1. Bended polygonal colums of basaltic andesite from Detunata Goală (photo by F. Forray) 

DETUNATA (APUSENI MTS., ROMANIA)

by Nicolae Har

Detunata is one of the most famous outcrop of basaltic andesites from Romania. It is located in the area of Bucium village (Alba county) in the Apuseni Mountains. The volcanics occur in two separate bodies: the Detunata Goală, with polygonal columns of rock partly covered by forest, and the Detunata Flocoasă - fully covered by forest (Fig. 1).

How to get there

County: Alba (Romania)
Locality: Bucium, 12 Km (8 mi) from Abrud
GPS location: Lat N 46.27691  / Long S 23.19730
Trail track: detunata.gpx (by F. Forray). If you don't have a program to read GPX files, download for free the EasyGPS  program.
	Map of the region (click for bigger map)

Additional data

Surface area: 280,000 m2 (40,000 m2 Detunata Goală and 24,000 m2 Detunata Flocoasă)
Elevation: 1,168 m Detunata Goală and 1,265 m Detunata Flocoasă 

Geological framework

The tectonic evolution of the Apuseni Mountains controlled the distribution of the Neogene volcanics in time and space. Five important areas of the Neogene volcanism are known in the Apuseni Mountains: Mureş Valley, Zarand, Brad – Săcărâmb, Zlatna – Stănija, and Bucium – Roşia Montană- Baia de Arieş. Volcanic rocks in the above mentioned areas include rhyolites, andesites, and basaltic andesites. The youngest rocks are olivine - bearing basaltic andesites from Detunata (7.6 ± 0.4 Ma age, acc. to Pécskay et al., 1995), which belong to the Bucium – Roşia Montană – Baia de Arieş volcanic area. These rocks are found in both areas, Detunata Goală and Detunata Flocoasä respectively.

Olivine - bearing basaltic andesites from the Detunata have porphyritic textures with intergranular – intersertal groundmass. The magmatic bodies intruded into the Santonian – Maastrichtian flysch-type sediments of the Bucium Unit. The composition of the rock regards both primary minerals of igneous origin, and secondary minerals, formed due to later reaction. The primary assemblage is dominated by plagioclase feldspar, pyroxene (diopside) and olivine.  Magnetite, apatite, opacitized hornblende and volcanic glass are also present in small quantities (Fig. 2). (Table 1). 

Table 1. Mineralogical composition of basaltic andesites from Detunata (Har, 2001)

The basaltic rock contains xenoliths of micaceous quartzitic sandstones originated from the surrounding flysch. Most of the xenoliths were partly fractured and assimilated by the basaltic melt, and consequently isolated xenocrysts of quartz may occur. At the contact between quartz xenocrysts and the basaltic rock, reaction coronas developed (Fig. 3). These coronas consist of pyroxene, glass, and a SiO2 polymorph represented by tridymite (Har, 2005).

Fig.2. Phenocrysts of pyroxene and olivine in basaltic andesite from Detunata. (photo by N. Har)	Fig.3. Glass, tridymite and pyroxene in the reaction zone between the basaltic rock and the quartz xenocrysts. (photo by N. Har)

The geochemistry of basaltic andesite from Detunata shows between 53.04 and 55.52% SiO2, most likely the initial basaltic geochemistry being changed due to the assimilation of the micaceous quartzitic sandstones xenoliths. The alkali content (Na2O K2O = 2.84 - 4.47 %) is also typical for a basaltic andesite. The relative high content of MgO (5.69 - 7.43%) is due to olivine of forsteritic composition (Fo75,0Fa 25,0). 

ZDROJ:http://bioge.ubbcluj.ro/geology/news/trip.htm