The Rum layered intrusion testifies to modification by injection of hot magma and remobilization of pre-existing cumulate rocks.
The Isle of Rum in the Inner Herbrides of Scotland is a classic and much-studied example of an igneous layered intrusion. Dated at 60 Ma, it’s emplacement was related to the development of the proto-Icelandic plume.
New work by Leuthold et al. focusses on a particular layer within the intrusion, ‘Unit 9’, which shows a progression from peridotite (olivine-rich) through troctolite (olivine + plagioclase) to gabbro (plagioclase + clinopyroxene).
By integrating field and geochemical observations, this study challenges the idea that Unit 9 was formed through progressive fractional crystallization of a single parental liquid. Instead, the authors hypothesise that multiple generations of rimmed clinopyroxenes with sharp boundaries in Cr2O3 and REE indicate that Unit 9 underwent two separate episodes of partial melting in response to the intrusion of hot picritic magma.
This upward and lateral migration of melts and the reactive remobilisation of a cumulate pile may be an important process in all layered intrusions and open magma chambers.
Leuthold J, Blundy JD, Holness MB, & Sides R (2014) ‘Successive episodes of reactive liquid flow through a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland)’. Contributions to Mineralogy and Petrology, 168(1), 1-27. http://dx.doi.org/10.1007/s00410-014-1021-7