The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling: examples from the Ballachulish contact aureole, Scotland

Austrheim

Whitehouse

2012

American Mineralogist

We present an electron backscatter diffraction, cathodoluminescence, and radiogenic U-Pb dating study of large zircon grains (0.8-1.5 mm) that show evidence of intracrystalline deformation, fracturing, grain size reduction and a large spread in U-Pb ages. The samples are from an amphibolite facies deformation zone within granulite facies anorthositic rocks (Bergen Arc, Norway). Large zircon grains show three main lattice distortion types: (I) distortions with rotations around <001> and an orientation change of ∼0.3 °/µm subparallel to (100); (II) highly distorted, half circular shaped zones located at grain edges with at least 0.8-1°/µm distortions; and (III) low-angle boundary networks forming deformation zones up to 100 µm wide. Types II and III distortions exhibit significant disturbances of the otherwise homogeneous CL signature.Crystal plastic deformation with the slip system [010](100) resulted in type I distortions. Stress concentrations at grain contacts between rheologically hard grains caused localized crystal plastic deformation with minor amount of microfracturing forming type II distortions. Type III distortions formed by crystal plastic deformation often associated with inclusions using several slip systems. Distortions of types I and II show minor and moderate resetting of the original ca. 900 Ma zircon grains, respectively, due to enhanced pipe diffusion along dislocation walls. In type II distortions, accelerated lattice diffusion through the highly distorted crystal lattice, combined with exceptionally high boundary to volume ratio, caused significant chemical disturbance and age resetting to 410 Ma. Fine-grained aggregates contain grains with low internal deformation and an oscillatory zoned CL signature (Z-grains) or high internal deformation and a disturbed CL signature (D-grains). Z-and Dgrains are interpreted to have formed by heterogeneous nucleation and growth, and fracturing along strain-hardened low-angle boundaries present within types I and II, respectively. Z-grains show a clustered chemical signature with a 437 ± 11 Ma age interpreted to directly date the Caledonian amphibolite facies reworking.

Section: Brittle and Crystal Plastic Deformation Of Zircon At Mid-crumentioning

confidence: 87%

Brittle-ductile microfabrics in naturally deformed zircon: Deformation mechanisms and consequences for U-Pb dating

Austrheim

Whitehouse

2012

American Mineralogist

“…Tschermakite (Ts 3a&3b ) coarsens forming tabular to prismatic grains with decussate texture in the migmatite. Mobility of grain boundaries may be increased by the presence of an intergranular fluid or melt and/or increased temperature (Heilbronner & Tullis, , and references therein; Bergman & Piazolo, ). The fluxing of fluid or melt is likely to have been key during grain size coarsening, as expanded upon in the next section.…”

Section: Discussionmentioning

confidence: 99%

Local partial melting of the lower crust triggered by hydration through melt–rock interaction: an example from Fiordland, New Zealand

Stuart

Daczko

Journal Metamorphic Geology

2016

We investigate a low‐strain outcrop of the lower crust, the Pembroke Granulite, exposed in northern Fiordland, New Zealand, which exhibits localized partial melting. Migmatite and associated tschermakite–clinozoisite (TC) gneiss form irregular, elongate bodies that cut a two‐pyroxene–pargasite (PP) gneiss. Gradational boundaries between rock types, and the progressive nature of changes in mineral assemblage, microstructure and chemistry are consistent with the TC gneiss and migmatite representing modified versions of the PP gneiss. Modification is essentially isochemical, where partial modification involves hydration of the assemblage and mineral chemistry changes, and complete modification involves additional recrystallization and in situ partial melt production. Microstructures of quartz and plagioclase, including small dihedral angles, string of beads textures and films surrounding amphibole and garnet grains are consistent with the former presence of melt in modified rock types. The documented rock modification is attributed to melt–rock interaction occurring during porous melt flow of a dominantly externally derived, hydrous silicate melt. Microstructures indicate melt flow occurred along grain boundaries and field relationships show it was focused into channels tens of metres wide, with preference for following the pre‐existing foliation. Melt–rock interaction at the grain scale resulted in hydration and modification of the host PP gneiss, which resulted in localized partial melting. These relationships indicate prograde hydration during localized melt–rock interaction drove migmatization of the lower crust.

“…Inversely, grain size increase through annealing (temperature‐related coarsening and nucleation; e.g. Covey‐Crump, ; Heilbronner & Tullis, ; Bestmann et al ., ; Piazolo et al ., , ; Bergmann & Piazolo, ), namely grain boundary migration and grain growth (e.g. Urai et al ., ; Drury & Urai, ; Baker, ), will have the opposite rheological effect, thus strengthening the rock (see Jessell et al ., for further discussion).…”

Section: Introductionmentioning

confidence: 99%

The effect of pre‐tectonic reaction and annealing extent on behaviour during subsequent deformation: insights from paired shear zones in the lower crust of Fiordland, New Zealand

Smith

Journal Metamorphic Geology

Daczko

et al. 2015

Granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow (<20 mm) felsic dykes, in Fiordland, New Zealand, forming~10-80 mm wide garnet reaction zones. The metamorphic reaction changed the abundance of minerals, and their shape and grain size distribution. The extent of reaction and annealing (temperature-related coarsening and nucleation) is greatest close to the dykes, whereas further away the reaction is incomplete. As a consequence, grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres. The aspect ratios of clusters of S1 pyroxene and pargasite in the orthogneiss, which are variably reacted to post-S1 garnet, decrease from high in the host, to near equidimensional close to the dyke. Post-reaction deformation localized in the fine-grained partially reacted areas. This produced a pattern of 'paired' shear zones located at the outer parts of the garnet reaction zone. Our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction. The weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage.