GP21A-0121
Magnetic Fabric Techniques are used to Characterize Deformation of Deep Crustal Granulites of the Arunta Block Central Australia Lake Superior State University
2
University of Wisconsin Madison
Sampling Traverse
N
Granulites of the Capricorn Ridge shear zone of the Arunta Inlier of central Australia record deformation at 776 ± 38 °C and 800 MPa (~ 30 km depth). The planarity and continuity of compositional bands that parallel the mesoscopic foliation vary spatially, recording variations in finite strain magnitude within the lithologically heterogeneous shear zone. As is typical of granulite facies terranes, strain markers are absent. We have investigated the degree to which magnetic foliation and lineation track strain in different rock types by comparing magnetic anisotropy to field fabrics, with the goal of quantifying strain across this deep crustal shear zone.
• • • • •
Increase our understanding of deep crustal deformation Compare the AMS and ARM fabrics of different rock types at the same locality Quantify magnetic anisotropy by site and rock type Determine magnetic mineralogy and magnetic grain size Compare magnetic anisotropy with nonmagnetic strain analysis
Samples were collected at seven sites along a 700 meter transect of the Capricorn Ridge shear zone of central Australia. One inch cores from two to four rock types were sampled at each site for a total of over 160 samples. The anisotropy of magnetic susceptibility and anisotropy of remnant magnetization was analyzed in the magnetics research lab at Lake Superior State University. The principle ellipsoid direction from the two methods are plotted on stereonets by rock type. Shape parameter (T) versus anisotropy magnitude (P) plots 120˚ 140˚ quantify the ellipsoid shape and amount of anisotropy at each site. At the University of Minnesota’s Institute -20˚ -20˚ for Rock Magnetism the magnetic susceptibility was measured for a wide range of temperatures allowing for the determination of ordering and transition temperatures for each rock type. Hysteresis data -40˚ -40˚ collected at Minnesota provides information on the 120˚ 140˚ grain size distribution for each rock type.
Acknowledgements • •
National Science Foundation EAR-0441562 LG, BT, PK PK received a fellowship from the National Science Foundation supported Institute for Rock Magnetism where temperature dependent and field dependent magnetic measurements were collected.
(AMS)
Mt. Chapple
31
Scale
85
0m 150m 300m 84
7406950
74 64
72 56 80
Mt. Hay 120˚
50
75
Site 1
52
79 63
77
52
Site 2 Site 3 Site 4 Site 5
69
81
Site 6 Site 7
140˚
79
64 80 68
7406500 UTM Coordinates
-20˚
Mafic Granulite
Maximum
Maximum
Intermediate
Intermediate
Charnockite
Minimum Pole to Field Foliation Field Lineation
N
Mafic Granulite
-20˚
-40˚
Figure: 7 General location map showing location of survey within central Australia as well as within the context of local geology. Locations and spacing of sample sites are ploted on an expanded UTM grid.
-40˚
Charnockite
Minimum Pole to Field Foliation Field Lineation
N
N
120˚
140˚
N
Ordering and Transition Temperatures 6 Mafic Granulite
1.0
Mafic Granulite Charnockite Porphyroclastic
0.5
Shape 0 Parameter T -0.5
-1.0
Quartzofeldspathic
Oblate
2
4
6
Prolate
Degree of Anisotropy P
8
CR06-09 CR06-10 CR06-11 CR06-12 CR06-13 CR06-14 CR06-15 CR06-16 CR06-17 CR06-18 CR6-19.1 CR06-19.2 CR06-20 CR06-21 CR06-22 CR06-23 CR06-24 CR06-25
Quartzofeldspathic
AMS P vs. T Site 1
1.0
Mafic Granulite Charnockite Porphyroclastic
Site 2
Site 3 Site 4 Site 5
0.5
Shape 0 Parameter T -0.5
Quartzofeldspathic
Oblate
2
4
6
Prolate
Site 6
-1.0 Site 7
Figure: 3 Shape parameter (T) versus anisotropy magnitude (P) plot of ARM data for site averages of the four rock types. Charnockite has the greatest variability and maximum value of anisotropy, while its shape ranges from slightly oblate to prolate. The mafic granulite AMS shape ranges from triaxial to highly oblate and have similar anisotropy magnitudes. The porphyroclastic and quartzofeldspathic rock types have small variation in both the magnitude of anisotropy and its shape.
Degree of Anisotropy P
8
CR06-09 CR06-10 CR06-11 CR06-12 CR06-13 CR06-14 CR06-15 CR06-16 CR06-17 CR06-18 CR6-19.1 CR06-19.2 CR06-20 CR06-21 CR06-22 CR06-23 CR06-24 CR06-25
Figure: 5 Ordering and transition temperature histogram of site averages for the four rock types. The dominate temperatures for all four rock types are -175 to -150o C and 550 to 575oC are consistent with the Verwey transition and Curie temperature for magnetite, suggesting that magnetite is the dominant magnetic mineral in each samples. Three of the mafic granulite samples have transition temperatures at 325 to 350oC possibly due to phyrrotite. Four mafic samples had temperatures at -250 to -225oC which is likely due to the mineral ilmenite. One quartzofeldspathic sample has a transition temperature at -10oC likely due to the presences of hematite.
0.60 0.55
Site 1
Site 2
Single Domain
Single Domain = Fine Grain
0.50
Pseudo Single Domain = Medium Grain
0.45
Multi Domain = Coarse Grain
0.40 0.35
Site 3
0.30
Site 5 Site 6 Site 7
Figure: 4 Shape parameter (T) versus anisotropy magnitude (P) plot of ARM data for site averages of the four rock types. Charnockite represents the greatest variability and maximum value of anisotropy, while its shape ranges from oblate to triaxial. The mafic granulite AMS shape that ranges from slightly prolate to highly oblate. The porphyroclastic and quartzofeldspathic rock’s AMS is oblate for all sites.
75 10 0 12 5 15 0 17 5 20 0 22 5 25 0 30 0 32 5 35 0 37 5 40 0 42 5 45 0 47 5 50 0 52 5 55 0 57 5 60 0
Degrees C
Day et al. Plot
Site 4
50
25
0
5 -2
0 -5
5 -7
00
-1
25
-1
50
-1
75
-1
-1
Figure: 2 Anisotropy of magnetic susceptibility (AMS) stereonet plots for site averages of each rock type showing their (filled square), intermediate (filled triangle), and minimum (filled circle) magnetic directions. The charnockite, porphyroclastic and quartzofeldspathic granulite plot in tight groups for the three principle directions whereas the mafic granulite show a nearly random distribution. The field measured pole to foliation (open circle) and lineation (open square) directions are similar to the AMS minimum and maximum respectively for all rock types accept the mafic granulite.
00
0
25
Quartzofeldspathic
-1
Figure: 1 Anisotropy of remnant magnetization (ARM) stereonet plots for site averages of each rock type showing their maximum (filled square), intermediate (filled triangle), and minimum (filled circle) magnetic directions. The charnockite, porphyroclastic and quartzofeldspathic granulite plot in relatively tight groups for the three principle directions whereas the mafic granulite show grouping of the maximum directions only. The field measured pole to foliation (open circle) and lineation (open square) directions are similar to the ARM minimum and maximum respectively for all rock types accept the mafic granulite.
Porphyroclastic
50
Quartzofeldspathic
-1
Quartzofeldspathic
1
75
Porphyroclastic
Porphyroclastic
2
-1
Porphyroclastic
3
00
Charnockite
Quartzofeldspathic
4
-2
Charnockite
Porphyroclastic
25
Mafic Granulite
F r e q u e n c y
Charnockite
5
50
Mafic Granulite
ARM P vs. T
Methods
N
-2
Objective
N
7407400
-2
Samples collected along a 700m transect include mafic (most common), porphyroclastic, charnockitic, and quartzofeldspathic granulites. The field foliation generally strikes ESE and dips steeply to the SW and lineation plunges steeply to the SSE in all rock types. We measured anisotropy of magnetic susceptibility (AMS) and anisotropy of remnant magnetization (ARM) to determine the shape and orientation of the magnetic anisotropy ellipsoids of these deep crustal rocks. The mafic granulites’ ARM principle directions show a tighter grouping than do the same samples’ AMS principle directions. The other rock types have a tighter grouping of data points with AMS; however, the difference between the ARM and AMS ellipsoids is not as substantial as that exhibited by the mafic granulite samples. The maximum AMS axes of the porphyroclastic, charnockitic, and quartzofeldspathic granulites plunge steeply to the SSE, similar to the observed field lineation. In contrast, the mafic granulites generally show a shallowly plunging, NW-trending maximum ARM direction. The mafic granulite ARM ellipsoids have a relatively consistent anisotropy orientation, but shapes range from highly oblate to triaxial to slightly prolate. The charnockite AMS ellipsoid exhibits a consistent triaxial shape. The porphyroclastic and quartzofeldspathic granulite AMS ellipsoid shape varies across the transect. The charnockites generally show the highest degree of anisotropy whereas the mafic granulites show the lowest. Field observations and magnetic fabrics are consistent with lower crustal strain localization within the charnockites relative to the mafic granulites. These observations suggest lithology-specific deformation processes, fundamentally reflecting rheology, are responsible for the magnetic fabrics in the shear zone.
N
(ARM)
01
Abstract
Anisotropy of Magnetic Suceptibility
31
Anisotropy of Remnant Magnetization
05
00
Russell J. White , Paul R. Kelso , Laurel B. Goodwin and Basil. Tikoff
2 1
00
2
31 10
1
50
1
Mrs/Ms
0.25
Pseudo Single Domain
0.20 0.15 0.10 0.05 0.00
Multi Domain
CR06-09 CR06-10 CR06-11 Site 1 CR06-12 CR06-13 CR06-14 Site 2 CR06-15 CR06-16 CR06-17 Site 3 CR06-18 CR6-19.1 Site 4 CR06-19.2 CR06-20 Site 5 CR06-21 CR06-22 Site 6 CR06-23 CR06-24 Site 7 CR06-25
1 2 3 4 5 6 7 8 9 10 11 12
Bcr/Bc Figure: 6 Day et al. plot indicating the magnetic grain size for site averages of all four rock types. Charnockites show the most uniform grain size with all data points plotting within the multidomain. The mafic granulites plot primarily within the pseudo-single-domain region with a few data points in the multidomain region. Quartzofeldspathic and porphyroclastic units display a wider range in magnetic grain size distribution.
Conclusions • ARM, AMS and field directions correlate well for the charnockite, porphyroclastic, and quartzofeldspathic units • Mafic granulites deform differently than other rock types under deep crustal conditions, their magnetic anisotropy directions are scattered • Ordering and transition temperatures suggest magnetite as the dominant magnetic mineral plus minor phyrrotite and ilmenite in some samples • Day et al.diagram indicates magnetic grains are multidomain to large pseudo-single-domain grain size • Charnockites have the highest anisotropy values (P) consistent with greater amounts of deformation • Mafic granulites have the lowest average anisotropy values (P) consistent with lower amounts of deformation • Shape and magnitude of magnetic fabric is independent of the thickness of the compositional banding observed in the field
