The competition between rates of deformation and solidification in syn-kinematic granitic intrusions: Resolving the pegmatite paradox

Robert W.H. Butler (Corresponding Author), Taija Torvela

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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 languageEnglish
Pages (from-to)1-13
Number of pages13
JournalJournal of Structural Geology
Volume117
Early online date4 Sep 2018
DOIs
Publication statusPublished - 1 Dec 2018

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pegmatite
solidification
crystallization
kinematics
melt
shear zone
feldspar
tectonics
strain rate
continental crust
inhibitor
texture
magma
rate
crust
liquid
fluid

Keywords

  • Pegmatites
  • Melt-enhanced deformation
  • Continental deformation
  • Rheology

Cite this

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title = "The competition between rates of deformation and solidification in syn-kinematic granitic intrusions: Resolving the pegmatite paradox",
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.",
keywords = "Pegmatites, Melt-enhanced deformation, Continental deformation, Rheology",
author = "Butler, {Robert W.H.} and Taija Torvela",
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.",
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AU - Butler, Robert W.H.

AU - Torvela, Taija

N1 - 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.

PY - 2018/12/1

Y1 - 2018/12/1

N2 - 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.

AB - 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.

KW - Pegmatites

KW - Melt-enhanced deformation

KW - Continental deformation

KW - Rheology

U2 - 10.1016/j.jsg.2018.08.013

DO - 10.1016/j.jsg.2018.08.013

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VL - 117

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JO - Journal of Structural Geology

JF - Journal of Structural Geology

SN - 0191-8141

ER -