Origin of sapphirine-plagioclase symplectites in metabasites from Mitterbachgraben, Dunkelsteinerwald granulite complex, Lower Austria
Carsweli, Dennis A.; Möller, Charlotte; O'brien, Patrick J.
European Journal of Mineralogy Volume 1 Number 3 (1989), p. 455 - 466
published: Jul 27, 1989
manuscript accepted: Mar 7, 1989
manuscript received: Mar 3, 1988
ArtNo. ESP147050103014, Price: 29.00 €
Abstract Sapphirine-plagioclase symplectites in Mg, Al-rich metabasites (pyriclasites) associated with peridotite bodies in the Moldanubian zone of Lower Austria are shown to have formed from kyanites which were part of an early, higher pressure assemblage. The complex sapphirine-forming reaction also involved the substantial loss of grossular component from original garnets and of Ca-Tschermaks and jadeite components from original clinopyroxenes. Subsequent reactions produced overgrowths of pleonaste spinel on sapphirine and finer-grained symplectite of orthopyroxene-spinel-plagioclase around garnet where in contact with clinopyroxene. The original kyanite eclogite assemblage is deduced to have formed at ca. 1100-1150 °C/30 kbar with sapphirine formation at ca. 1000 °C and P ≤ 20 kbar and final garnet breakdown at T ≤ 850 °C and P ≤ 8 kbar. These silica undersaturated and normative feldspar rich rocks are thought to correspond to cumulates from basaltic mantle melts generating during a lithospheric extension episode which preceded the crustal thickening responsible for the stabilisation of the early, high-pressure, metamorphic assemblage. The deduced retrograde P-T path implies fairly fast uplift, with rapid unloading relative to the rate of heat loss.