Metamorphic Geology of Leornian

Units: A12 and A13

image MS-03

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The prominent light-coloured bands consist of felsic-rich gneiss (unit A12). The darker bands superficially appear to be mafic-rich bands which might have segregated from the felsic material through a process of metamorphic differentiation. However, note that the marked foliation present in the felsic-rich bands is absent from the mafic-rich bands. Furthermore, the margins of the mafic-rich bands are locally discordant to the foliation in the felsic-rich bands. The mafic bands therefore post-date the structural event responsible for imparting foliation to the felsic-rich layers. The mafic layers probably originated in basic magmatic sheets intruded into originally homogeneous "acid" gneiss. Subsequently, the basic sheets intruded into an originally homogeneous "acid" gneiss. Subsequently, the basic sheets have themselves been metamorphosed to form amphibolites. Note that a thin, light-coloured highly discordant pegmatite sheet post-dates both gneiss and amphibolite.

Scale: matchbox is 100 mm long.

Grid reference: 968 231

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This page is maintained by Roger Suthren

Last Modified: 21 October, 2008