The geology of the hyperstheme Gabbro of Ardnamurchan point and implications for its evolution as an upper crustal basic magma chamber

Abstract

The Hypersthene Gabbro of Ardnamurchan Point is formed by an outer Marginal Border Group ( MBG ) and a younger Inner Series (IS). The MBG is a single large intrusion which corresponds to a high - melt - percentage magma chamber. The IS is dominated by numerous gabbronorite sheet intrusions which formed a large low - melt - percentage magma body. The country rocks around the MBG show polyphase metamorphism. An early (Ml) phase of high-grade metamorphiam was followed by sudden cooling and then by hydrothermal metamorphism ( M2 ), related in part to the emplacement of the IS. The sudden cooling was caused by self - propagation of tensile fracture networks containing vigorously convecting hydrothermal fluids. The fracture networks were initiated by tectonic fracturing. The fractures networks also propagated into the MBG and partly preserved the magma chamber boundary layer formed during Ml. The contact of the MBG was approximately stationary during Ml. Wall - rock melting occurred in an episodic process triggered by movement on concentric inward - dipping normal faults due to fluctuations in magma pressure. The heat flux Q(_m) in the boundary layer was approximately equal to the heat flux Qc in the adjacent wall rocks. The preservation of the end – M1 instantaneous metamorphic thermal gradient in the country rocks by the subsequent sudden cooling allows direct measurement of Q(_c) and hence of Q(_m) ( 8 - 40Wm(^-2) and other parameters of the boundary layer of the MBG magma chamber. The interior of the MBG magma chamber was probably just stably stratified but cooling at the chamber walls produced density currents and slow mixing between the layers. The chamber was not well - mixed: variations in previous crustal contamination of the magmas have been preserved. The IS shows evidence for interstitial melt expulsion related to the formation of igneous lamination. Hydrothermal circulation in the IS, at up to 1000ºC, produced oxidation of the rocks and may have led to the formation of hydrous melts.