Modelling anatexis in intra-cratonic rift basins: an example from the Neoproterozoic rocks of the Scottish Highlands

The Neoproterozoic metasediments of northwestern Scotland were deformed dur ing the 470 Ma Grampian orogeny. Their pre-Ordovician history has proved di fficult to elucidate, due to conflicting evidence. While the stratigraphic record indicates deposition in intracontinental rift basins associated with the break-up of Rodinia, isotopic dates in the range 870 780 Ma from grani te gneiss, early pegmatites and metamorphic garnets have been attributed to a Neoproterozoic 'Knoydartian' orogeny. Stratigraphic evidence for this or ogeny is lacking, and it is not represented elsewhere on the Laurentian mar gin. An alternative interpretation is that much of: the Knoydartian history can be related to extensional, not collisional processes. Specifically, it has been proposed that the 870 Ma West Highland granite gneiss that is int ruded into the Moine rocks of northwestern Scotland is not the product of s ynorogenic anatexis but represents a suite of granite sheets that were gene rated during extensional rifting and were subsequently deformed and gneissi fied during the Grampian orogeny. This contribution presents numerical mode ls of extension-related anatexis to test this hypothesis. We first develop a methodology to estimate stretch values and the duration of extension and thermal subsidence for the Moine rift basins. A thermal mo del is then constructed for these basins using transient finite element tec hniques. This model shows that lithospheric extension sufficient to produce major rift basins, even if they are filled with feldspathic sediment with Neoproterozoic heat production characteristics, will not lead to crustal an atexis. However, a regional suite of malic dykes in the more easterly (Loch Ell) Moine suggests that stretching led to decompression melting of the ma ntle. We model the effect of advecting heat into the extending lithosphere by the introduction of a modest volume of basaltic magma, and show that sub stantial granitic melt can be generated in the basement beneath the basin. The amount of anatexis varies with the locus of basalt intrusion. Some 30% more granite is generated by dykes emplaced along basin-bounding faults tha n by either dykes emplaced beneath the centre of the basin, or by underplat ing sills. The spatial distributions of the West Highland gneiss and of the mafic suite are compatible with this finding. There is clear field evidenc e that the protolith of the West Highland gneiss consisted of a suite of pr e-tectonic granite sheets, and our modelling demonstrates that they could h ave been generated during the later stages of extensional rifting and Moine sedimentation.