Deformation-Enhanced Element Mobility in Feldspar: A Strain Speedometer? Naomi Barshi * , Christie Rowe, and Vincent van Hinsberg Department of Earth and Planetary Sciences, McGill University, Montréal, QC, Canada * [email protected]Hypotheses 1. Deformation enhances diffusion in minerals. 1 2. This enhancement varies systematically with strain for a variety of elements. 3. The differences in concentration contrasts measured in zoned plagioclase across a strain gradient can be compared to infer a duration of deformation and estimate strain rate. Trends toward edge of pluton • Grain size of all minerals decreases • Plagioclase and quartz in deformed samples have undulose extinction • Plagioclase phenocrysts lose their zonation and decrease in size and abundance • Biotite increasingly but incompletely overprints hornblende and associated magmatic fabric • Plagioclase develops a strong shape-preferred orientation Remaining Challenges • How do I translate this qualitative strain gradient into a quantitative strain metric for feldspar at the mineral scale? • What are the best strain markers to use? • Plagioclase dominates the San José tonalite, but how much bulk strain does it accomodate (represent)? 1. Describe and sample a strain gradient (done) 2. Establish a quantitative strain scale against which to measure element concentrations (current work) 3. Measure concentrations of the same elements in zoned feldspars across a strain gradient (next step) 4. Measure concentrations of different elements at each strain increment constrain the duration of enhanced diffusion, which we interpret as the duration of deformation (next step) Strain Gradient References 1. Demonstrated in metals, e.g. Cohen, M., Bulletin of the Japan Institute of Metals 9, 271–278 (1970); and in tourmaline by Büttner, S. H., Mineralogical Magazine 69, 471–490 (2005). 2. Johnson, S. E., Tate, M. C., & Fanning, C. M., Geology 27, 743–746 (1999). 3. Vernon, R. H., Johnson, S. E. & Melis, E. A., Journal of Structural Geology 26, 1867–1884 (2004). Acknowledgements Thanks to Field Rheology and FlexPet groups and field assistant Matt Paulson. This project is funded by NSERC (van Hinsberg and Rowe), FQRNT (Rowe), GREAT (Barshi), and a Robert Wares Fellowship (Rowe). *[email protected]FDA 238, 3450 University Street Montréal, QC, H3A 0E8, CANADA Geologic Setting Oriented samples were taken along two strain gradients in the outer and oldest unit of the Lower Cretaceous San José pluton, Peninsular Ranges batholith. This tonalite pluton experienced solid-state deformation during subsequent pluton emplacement 2 . Plagioclase-dominated rheology and simple deformation history make it an ideal location to investigate element mobility in feldspar. (Overview maps modified from Johnson et al. 2 and satellite image Digital Globe/GoogleEarth 2014.) Ensenada La Paz 115 o W 110 o W 29 o N 33 o N Map area Pacific Ocean San Diego USA México Gulf of California Baja California Country Rocks Outer Unit Central Unit Strain in- creases Strain gradient transects 1 km 115º40’W 31º05’N 30º55’N 115º50’W Other Intrusives San José tonalite Metasediments/ Metavolcanics I will measure element concentrations across zoning at each strain increment, illustrated here for undeformed (dark blue 3 ) and intermediate and high strain (light blue). When elements with different diffusion coefficients are compared (for example Ca and Ba), their diffusion paths can be modeled to calculate a duration of diffu- sion equivalent to the age of the crystal. If diffusion enhances element mobility, then deformed crystals will appear older. We interpret this difference as the duration of deformation. Northwest Northeast Approach 5mm Recrystallized quartz and matrix plagioclase in microshears Phenocrysts rare, mostly unzoned White-mica-filled fractures Foliation Quartz partially recrystallized Bent twins Undulose extinction Kink bands Phenocrysts with well-preserved zoning Microshears Biotites connect to form foliation (polarizers at 45º) Biotites not connected View of the north-eastern transect taken from the south-eastern transect illustrated in Geologic Setting. Cars in lower left-hand corner for scale. Field photos oriented to show strike and thin section and sample scans oriented to show dip of fabric and foliation (cuts are foliation-perpendicular, lineation-parallel when present). line scans line scans 5mm Static diffusion models applied to deformed plagioclase give anomalous ages Static diffusion models applied to undeformed plagioclase give true age Change in Concentration Constrasts Undeformed Most deformed Dotted lines indicate photos were taken on equivalent outcrops on parallel transect. Plg Plg Plg All red scale bars are 5mm. Plg Bt Plg Qtz Incipient foliation
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Deformation-Enhanced Element Mobility in Feldspar: A Strain Speedometer? Naomi Barshi*, Christie Rowe, and Vincent van HinsbergDepartment of Earth and Planetary Sciences, McGill University, Montréal, QC, Canada
2. This enhancement varies systematically with strain for a variety of elements.
3. The differences in concentration contrasts measured in zoned plagioclase across a strain gradient can be compared to infer a duration of deformation and estimate strain rate.
Trends toward edge of pluton • Grain size of all minerals decreases• Plagioclase and quartz in deformed samples have undulose extinction• Plagioclase phenocrysts lose their zonation and decrease in size and
abundance• Biotite increasingly but incompletely overprints hornblende and
associated magmatic fabric• Plagioclase develops a strong shape-preferred orientation
Remaining Challenges• How do I translate this qualitative strain
gradient into a quantitative strain metric for feldspar at the mineral scale?
• What are the best strain markers to use?• Plagioclase dominates the San José tonalite,
but how much bulk strain does it accomodate (represent)?
1. Describe and sample a strain gradient (done)
2. Establish a quantitative strain scale against which to measure element concentrations (current work)
3. Measure concentrations of the same elements in zoned feldspars across a strain gradient (next step)
4. Measure concentrations of different elements at each strain increment constrain the duration of enhanced diffusion, which we interpret as the duration of deformation (next step)
Strain Gradient
References1. Demonstrated in metals, e.g. Cohen, M., Bulletin of the Japan Institute of Metals 9, 271–278 (1970); and in tourmaline by Büttner, S. H., Mineralogical Magazine 69, 471–490 (2005).2. Johnson, S. E., Tate, M. C., & Fanning, C. M., Geology 27, 743–746 (1999). 3. Vernon, R. H., Johnson, S. E. & Melis, E. A., Journal of Structural Geology 26, 1867–1884 (2004).
AcknowledgementsThanks to Field Rheology and FlexPet groups and field assistant Matt Paulson. This project is funded by NSERC (van Hinsberg and Rowe), FQRNT (Rowe), GREAT (Barshi), and a Robert Wares Fellowship (Rowe).
*[email protected] 238, 3450 University StreetMontréal, QC, H3A 0E8, CANADA
Geologic Setting
Oriented samples were taken along two strain gradients in the outer and oldest unit of the Lower Cretaceous San José pluton, Peninsular Ranges batholith. This tonalite pluton experienced solid-state deformation during subsequent pluton emplacement2. Plagioclase-dominated rheology and simple deformation history make it an ideal location to investigate element mobility in feldspar. (Overview maps modified from Johnson et al.2 and satellite image Digital Globe/GoogleEarth 2014.)
745
Variable gabbro,diorite and tonalite;some hypabyssal
Hornblende-biotite tonalite
Gabbro
Probable magmaticring complexes
Mafic cone-sheetassemblages
Trend lines of bedding inAlisitos Fm., and bed-ding or foliation east ofAlisitos Fm.
K-feldspar-bearinggranitoids
Alisitos Fm.; volcanic andvolcaniclastic rocks
Alisitos Fm.; reef limestones,volcaniclastic, volcanic, calc-silicate, and argillitic rocks
Main Mártir Thrust separ-ating western and easternprebatholithic rocks
Contractional fault
Crestal traces ofselected folds
A'
? ? ? ?
Tonalitessourced atHigher P
Oceanic crust
MainMártir Thrust North American
continental ortransitional crust
????????????
?
116.2 ± 0.9 Ma
114.5 ± 0.9 Ma
Figure 1. Preliminary geologic map and accompanying cross section of 1200 km2 in Sierra San Pedro Mártir area, northern Baja Cali-fornia, Mexico. U-Pb zircon ages were determined by using SHRIMP II machine at Research School of Earth Sciences, Australian NationalUniversity, Australia. Regional map shows location ofstudy area and extent of exposed Peninsular Rangesbatholith (colored).Thermal ionization mass spectrom-etry (TIMS) age for Sierra San Pedro Mártir pluton isfrom Ortega-Rivera et al. (1997). Moho in cross section
~115-97 Ma
Some typical western-belt tonalites occur eastof suture, indicating an eastward dip
Sierra San Pedro Mártirpluton is derived from bothwestern oceanic andeastern continental sources B
~140-110 Ma
Continental-margin arcinitiated at least 140 Ma.
Gabbros and metaluminousgranitoids, some of which fedring complexes and calderas
A
E
?
Type Aptian-Albian AlisitosFm. deposited here
North America
North America
W
Figure 2. Possible convergence history between westernisland arc and North American continental margin to formgeologic relationships in Figure 1.A:Development of Alisitosarc, possibly on older arc rocks; final phase of AlisitosFormation entered marine basin between converging islandand continental arcs. B: Arc collision, development ofsuture, waning of deformation, and final emplacement of
Country Rocks
Outer Unit
Central Unit
Strain in-creasesStrain
gradient transects
1 km
115º40’W31º05’N
30º55’N
115º50’W
Other Intrusives
San José tonalite
Metasediments/Metavolcanics
I will measure element concentrations across zoning at each strain increment, illustrated here for undeformed (dark blue3) and intermediate and high strain (light blue). When elements with different diffusion coefficients are compared (for example Ca and Ba), their diffusion paths can be modeled to calculate a duration of diffu-sion equivalent to the age of the crystal. If diffusion enhances element mobility, then deformed crystals will appear older. We interpret this difference as the duration of deformation.
Northwest Northeast
Approach
5mm
Recrystallized quartz and matrix plagioclase in microshears
Phenocrysts rare, mostly unzoned
White-mica-filled fractures
Foliation
Quartz partially recrystallized
Bent twins
Undulose extinction Kink bands
Phenocrysts with well-preserved zoning
Microshears Biotites connect to form foliation
(polarizers at 45º)
Biotites not connected
View of the north-eastern transect taken from the south-eastern transect illustrated in Geologic Setting. Cars in lower left-hand corner for scale.
Field photos oriented to show strike and thin section and sample scans oriented to show dip of fabric and foliation (cuts are foliation-perpendicular, lineation-parallel when present).
line
scan
s
separate nucleation and growth of the crystals followed byimpingement can be inferred. Two alternative explanationsfor the truncated zoning are: (1) fortuitous impingement andconsequent cessation of growth at the contact, and (2)contact melting. A third possible alternative interpretation isthat large fragments of plagioclase, with zoning truncatedby fracture surfaces, impinge and grow together in a kind ofsintering process. However, in the San Jose´ rocks, inferredplagioclase fragments are typically much smaller than theoriginal crystals, and occur along original grain boundariesand in microshear zones. Large fragments, similar in size tothe primary grainsize of the rock, as described for melt-filledinterboudin zones, are uncommon.
5.4. Deformation history from a spatial deformationgradient?
An important question is whether the deformation andmicrostructural history of a high-strain rock near the marginof the San Josepluton can be inferred by evaluating theprogressive microstructural evolution along transects fromthe margin inwards. Means (1995) discussed two end-member shear-zone types, one of which (Type 1) widenswith time as deformation ceases in the most highly strainedrocks, owing to work hardening, and the other (Type 2)maintains constant width, but the most actively deformingzone narrows with time.Means (1995) noted that Type 2shear zones are much more e�ective recorders of rockhistory, because a large part of the history is preserved in atraverse from low to high strain.
One way to evaluate the San Jose´ pluton is to consider themarginal northern unit as a deforming shell undergoingsome component of outward stretching. In this instance, thewidth of rock a�ected by solid-state deformation would
increase with time as the solid-state deformation frontmigrates inward from the plutonmargins (similar to a Type 1shear zone). Additional widening of this zone of solid-statedeformation may (or may not) occur owing to (a) crystal-lization as the rocks cool, and (b) expulsion of melt owingto partitioning of deformation, with zones of highly non-coaxial deformation forming linked pathways for meltmigration (Vigneresse and Tiko�, 1999; Rosenberg andHandy, 2000; Brun and Vigneresse, 2002).
Such a deforming shell with a component of outwardstretching forms an unusual shear zone in that deformationcannot easily stop in one part of the stretching zone whilecontinuing in another, unless the part that has stoppeddeforming is isolated by boudinage. We have found nocompelling evidence for large-scale boudinage in themarginal northern unit. Thus, even though the zone ofsolid-state deformation widened with time, the progressiveevolution of microstructure seen from low-strain to high-strain rocks across the zone may reflect the microstructuralevolution experienced by the most highly strained rocks.This would allow us to infer progressive histories ofindividual rocks from a microstructural progression.
Although the stretching shell hypothesis described aboveis consistent with the data that we have collected in andaround the San Jose´ pluton, as well as the boundaryconditions that we have inferred for the deformation of themarginal northern unit (Johnson et al., 2003), extractingdeformation histories fromfinite microstructures is anotoriously difficult problem, and further work on thepluton may well lead to the favoring of a di�erenthypothesis.
6. Conclusions
The most general inferences on deformation history thatwe can make from our microstructural observations are: (1)the deformation was superimposed on an existing magmaticfoliation; (2) some submagmatic deformation occurred; (3)abundant brittle deformation occurred, at least some ofwhich may have accompanied submagmatic flow, asindicated by melt-filled fracture-controlled openings inplagioclase; (4) much of the solid-state deformationoccurred at relatively high-temperatures, as indicated bylocal c-slip in quartz, grain-boundary movement (recrys-tallization) in plagioclase and probably hornblende, andformation and recrystallization of myrmekite; (5) crystal–plastic deformation is suggested by apparently recrystal-lized aggregates of plagioclase, quartz and biotite, and byapparent recovery structures in quartz, although we areunable to determine whether or not brittle deformation wasinvolved in the formation of these aggregates; and (6) brittledeformation continued at subsolidus conditions, as indi-cated by fracture-controlled openings in plagioclase filledby deuteric minerals.
Crystal–plastic strain may have followed initially rapid,
Fig. 30. Two zoned plagioclase grains with an irregular contact that mayhave resulted from contact melting. The cores of both grains are separated,and are surrounded by some oscillatory zones, suggesting initial separationof the two crystals, rather than nucleation and growth of one on the other.Crossed polars.
R.H. Vernon et al. / Journal of Structural Geology 26 (2004) 1867–1884 1881
line
scan
s
5mm
Static diffusion models applied to deformed plagioclase give anomalous ages
Static diffusion models applied to undeformed plagioclase give true age
Cha
nge
in C
once
ntra
tion
Con
stra
sts
UndeformedMost deformed
Dotted lines indicate photos were taken on equivalent outcrops on parallel transect.
