Abstract
While fully-crystallized granites, rich in feldspar, generally serve to strengthen the continental crust, their precursor melts are assumed to be important agents of crustal weakening. Many syn-tectonic granitic pegmatites are deformed within shear zones but ubiquitously preserve undeformed primary magmatic textures, implying that they were largely molten during shearing. Yet the shapes of pegmatite bodies indicate that they deformed with a greater competence than their surroundings. This co-located pair of material behaviours is paradoxical. We interpret field relationships in a typical pegmatite/shear zone association (Torrisdale, NW Scotland) and propose a mechanism by which syn-tectonic granitic melts may, in effect, act as competent bodies while not yet fully crystallized. Competence was rapidly increased by preferential crystallization on intrusion margins that served to encapsulate residual melt inside stiff rinds. Further crystallization may have been pulsed as the concentrations of crystallization-inhibitors (fluxes) increased in residual fluids. Postulating the existence of initial stiff rinds also consistent with modern estimates for rates of feldspar crystallization (cms/yr) from undercooled hydrous silicic magma to form pegmatites. These greatly outpace strain-rate estimates for shear zones. Thus, fully liquid granitic melts may only be present fleetingly and have little opportunity to weaken deforming crust before crystallization begins.
Original language | English |
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Pages (from-to) | 1-13 |
Number of pages | 13 |
Journal | Journal of Structural Geology |
Volume | 117 |
Early online date | 4 Sept 2018 |
DOIs | |
Publication status | Published - 1 Dec 2018 |
Bibliographical note
We thank Ian Alsop and Rob Strachan for discussions on the deformation at Torrisdale together with Alan Whittington and Mona-Liza Sirbescu for discussions and sharing manuscripts on melt rheologies and pegmatite crystallization. We also thank Elena Druguet for comments on a draft of these ideas, Luca Menegon and Denis Gapais for vigorous reviews of this paper, Bill Dunne for his editorial sweep-through, and participants at DRT2017 in Inverness for comments, in and out of the field. However, the views expressed here are exclusively those of the authors.Keywords
- Pegmatites
- Melt-enhanced deformation
- Continental deformation
- Rheology