EGU2017-1568
Sophie Briggs1, John Cottle1, Matthijs Smit2, Nicole Arnush 12Pacific Center for Isotope and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences,
University of British Columbia, Vancouver, Canada. [email protected] Olaf College, Northfield, MN. [email protected]
Linking zircon age to metamorphic stage using U-Pb/REE depth-profiling of zircon combined with Lu-Hf garnet geochronology
Motivation
1Department of Earth Science, University of California Santa Barbara
[email protected]@geol.ucsb.edu
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400 500 600238U/206Pb Age (Ma)
Th/U
detr
ital z
ircon
Zircon is capable of (re)crystallizing at any stage during metamorphism. Recrystallization events are often preserved in thin (<10 um) rims on inherited zircon grains in metamorphic rocks. Laser ablation split-stream (LASS) depth-pro�ling of unpolished zircon surfaces has enabled U-Pb isotope and trace element analysis of these thin zircon domains.
The potential value of this high spatial resolution method as a petrochronologic tool hinges on the question:
Can zircon depth-pro�le data be linked to speci�c metamorphic conditions and tectonothermal events?
50 μm
pitspot 10
50 μm
pitspot 10 - BSE
50 μm
laserbeam
CL
20 μm
Continental arc
Accretionary wedge complex(Alpine Schist)
Locus of rifting 83 - 52 Ma
Hikurangi Plateau (LIP)
105 - 90 Ma
4 0.280
0.290
0.310
0.300
12 168
MSWD = 1.08176Hf/177Hf = 0.28343 ± 44
176Lu/177Hf17
6 Hf/
177 H
f
79.9 ± 2.3 Ma
Results
Geologic setting
78.0 ± 1.7 Ma
010
2030
4050
6070
0 50
100
010
2030
4050
6070
-0.05 0
0.05
Th/U
WC14-31-14
010
2030
4050
6070
-10 0 10 20
Th (ppm))
0 20 40 60 80
Yb (ppm))
010
2030
4050
6070
-0.2 0
0.2
0.4
Ce (ppm)
0 5 10
Lu/Ho
238U/206Pb Age (Ma)
pulse
lase
r pul
se
45.1 ± 2.6 Ma
010
2030
4050
6070
0
200
400
600
800
010
2030
4050
6070
-0.2 0
0.2
0.4
Th/U
010
2030
4050
6070
-50 0 50
100
Th (ppm))
0 500
1000
Yb (ppm))
010
2030
4050
6070
-2 0 2 4 6
Ce (ppm)
0 100
200
300
Lu/Ho
238U/206Pb Age (Ma)
WC14-31-10
lase
r pul
se
LASS zircon depth-pro�les:
Lu-Hf garnet geochronology:
0.001
0.01
0.1
1
10
100
1000
10000
zirc
on/c
hond
rite
La Ce Pr Sm Eu Gd Tb Dy Ho Er Tm Yb Lu0.001
0.01
0.1
1
10
100
1000
10000
100000
zirc
on/c
hond
rite
inherited cores
REE zircon rim
45.1 ± 2.6 Ma
78.0 ± 1.7 Ma
72.2 ± 1.7 Ma
50 μm
BSE laserbeam
50 μm
CL laser beam
50 μm
BSE laserbeam
0.001
0.01
0.1
1
10
100
1000
10000
DRE
E (zirc
on/g
arne
t)
La Ce Pr Sm Eu Gd Tb Dy Ho Er Tm Yb Lu0.001
0.01
0.1
1
10
100
1000
10000
100000
zirc
on/c
hond
rite published
DREE (zircon/garnet) <900° C
45.1 ± 2.6 Ma
78.0 ± 1.7 Ma
72.2 ± 1.7 Ma
garnet unstable
garnet stable
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu0.001
0.01
0. 1
1
10
100
1000
10000
100000 REE garnet rim
garn
et/c
hond
rite
rim
towardscore
Volume % of Crystal (Core to Rim)
0
20
40
60
80
100
20 40 60 80 100Volume % of crystal (core to rim)
Cum
ulat
ive
% L
u
75 Ma
78 M
a
97 M
a
80 M
a
85 Ma
92 Ma
DREE Summary:
Garnet chemistry: DREE (zircon/garnet):
An amphibolite facies schist (WC14-31 ) yields 3 distinct zircon rim ages, two of which are close in age to garnet from the same sample. Calculated zircon/garnet REE partition coe�cients overlap with published values for zircon that formed during garnet-stable conditions.
Depth pro�le samples thick, chemically homogeneous rim. Convolute zoning and inclusions/porosity at core-rim interface indicate earlier modi�cation by a dissolution-reprecipitation process.
Depth pro�le samples thin convolute rim followed by an older, dark un-zoned domain. Porous texture of convolute rim indicates modi�cation by dissolution-reprecipitation.
EPMA garnet compositional maps.
Lu-Hf isochron comprising four multi-grain garnet aliquots provides the ‘mean age’ of garnet growth.
Lu distribution of WC14-31 garnet (bold) indicates age may be biased toward core.
Garnet rim REE abundance (LA-ICP-MS). Pro�les colored by distance of the analysis from the garnet rim.
REE partition coe�cients (REEppm zircon/REEppm garnet) using zircon rim average coupled with 8 garnet rim analyses.
Method
Alpine Schist garnet and zircon (re)crystallization is contemporaneous with the rifting of Zealandia from Gondwana during 83 – 52 Ma.
The newly proposed geologic continent Zealandia is 94% submerged below sea level. Uplift along the active Paci�c-Australia plate boundary exposes the Alpine Schist accretionary wedge complex in the South Island.
Zircon was mounted on �at crystal faces in epoxy. Unpolished surfaces were analysed by LASS-ICP-MS using 35 μm spots for 40 seconds at 2 Hz = 80 shots.
Data selection criteria:
500
300
100
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0 20 40 60 80 100 120 140
Th/U
0
0.9
206 P
b/20
7 Pb
238U/206Pb
63 Marim
190 Macore
mixing
Anatectic pegmatites:
Zircon rims summary:
Mn Ca
1 mm
1) ≥10 data point ‘plateau’ age2) no mixing or incomplete re-equilibration3) Th/U correlates with age4) REE is reset, not inherited from core
Implications
References
monazite 208Pb/232Th age (Ma)40 50 60 70 80 90 100
0
14
28
42
57
(n=453)
rela
tive
prob
abili
ty
0
5
10
15
20
25
40 50 60 70 80 90 100
Lu/H
o
238U/206Pb Age (Ma)
Alpine Schist intruded by pegmatites
garnet-stable0
0.004
0.008
0.012
0.016
0.02
40 50 60 70 80 90 100
Th/U
238U/206Pb Age (Ma)
WC14-09WC14-31
MR13-28
WC14-27
MR13-09
173°0'0"E
172°0'0"E
171°0'0"E
170°0'0"E
170°0'0"E
169°0'0"E
169°0'0"E
43°0
'0"S
43°0
'0"S
44°0
'0"S
0
90
45
Kilometers
±
S O U T H E R N A L P S
43°0'0"S
44°0
'0"S
169°
0'0"
E
171°0'0"E 172°0'0"E170°0'0"E 44°0'0"S
0 5025K ilometers
0 5025K ilometers
2 3
4
0 5025K ilometers
97.3 ± 0.3 Ma
91.8 ± 0.3 Ma78.4 ± 0.5 Ma 79.9 ± 2.3 Ma 85.1 ± 0.8 Ma
75.4 ± 1.3 Ma
anatectic pegmatite intrusion: 80 - 51 Ma
40
50
60
70
80
90
100
40
50
60
70
80
90
100
Age (M
a)Age
(M
a)
NESW
Metamorphic Zones of the
171°0'0"E170°0'0"E
43°0
'0"S
43°0
'0"S
44°0
'0"S
44°0
'0"S
0 5025K ilometers
Datum: NZG D_2000P rojection: New Zealand Trans vers e Merc ator
± A lpine S c his t,Wes tland, New Zealand
Legend
K-feldspar amphibolite
Oligoclase amphibolite
Garnet amphibolite
Garnet gs - amph
Garnet greenschist
Biotite greenschist
Chlorite greenschist
Pumpellyite-actinolite
Prehnite-pumpellyite
S O
U
T
H
E R
N
A
L P
S
Garnet Lu-Hf sample location
Alpine Fault
Published dates: Metamorphic Zones
100 ± 12 (Mortimer and Cooper, 2004)
98 ± 8 (core)
71 - 64 (Cooper and Ireland, 2013a)
86 ± 0.2 (rim) (Vry et al., 2004)
Western Province
Eastern Province
Otago Schist
Alpine Schist
C a p l e s
R a k a i a
ALPINE FAULT
WesternProvince
EasternProvince
72 ± 2(Cooper and
Ireland, 2013b)
71 ± 2 (Mortimer and Cooper, 2004)
Garnet Sm-Nd
Garnet Lu-Hf
Monazite
Zircon rim
97 ± 0.392 ± 0.3
107 ± 0.2
80 ± 2
26?re-xl
85 ± 0.8
75 ± 1
78 ± 0.5
Garnet Lu-Hf age(this study)
sea�oor spreading
pegmatite intrusion
lithospheric thinning
Alpine Schist intruded by anatectic pegmatites yields zircon rims with higher sumREE and enriched HREE (high Lu/Ho ratio).
These zircon rims likely formed either during �uid/melt in�ux associated with pegmatite intrusion, or during garnet-unstable conditions.
ZealandiaZealandia was assembled at the paleo Paci�c-Gondwana margin during Mesozoic subduction. The Alpine Schist represents the metamorphosed sediments of the accretionary wedge complex, long thought to have been metamorphosed during Jurassic – Cretaceous subduction. New metamorphic dates show that the Alpine Schist records metamorphism occurring concurrently with rifting and the �nal stages of break-up of Gondwana.
We address this question by applying the depth-pro�ling method to zircon from amphibolite facies Alpine Schist (New Zealand) dated previously by Lu-Hf garnet geochronology and assessing:1) rare earth element (REE) partitioning between zircon and garnet2) independent constraints on the timing of garnet growth, and generation of anatectic pegmatites.
Cottle et al. 2009
Depth-pro�ling is capable of resolving multiple zircon growth/modi�cation events spanning a signi�cant portion of a rock’s metamorphic history.
Zircon rims formed during garnet-stable conditions are distinctly depleted
in HREE, and yield D(REE) zircon/garnet values consistent with published values.
Zircon rims formed during �uid/melt in�ux are characterized by enriched
HREE and convoluted internal zoning.
Limitations: the presence/absence, thickness and continuity of zircon rim domains is not predictable and may be governed by location of zircon in the rock and/or access to �uids/melts.
Regional implications: Both garnet and zircon (re)crystallization in the Alpine Schist are contemporaneous with the rifting of the Zealandia microcontinent from East Gondwana during 83 – 52 Ma, suggesting that compressional and extensional tectonic regimes existed in close proximity during the formation of the Zealandia microcontinent.
Rubatto, D., & Hermann, J. (2007). Experimental zircon/melt and zircon/garnet trace element partitioning and implications for the geochronology of crustal rocks. Chemical Geology, 241(1), 38-61.Hermann, J., & Rubatto, D. (2003). Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust. Journal of Metamorphic Geology, 21(9), 833-852.[14] Rubatto, D., Hermann, J., & Buick, I. S. (2006). Temperature and bulk composition control on the growth of
1973-1996.Rubatto, D. (2002). Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and metamorphism. Chemical Geology, 184(1), 123-138.
Metamorphic zircon overgrowths are easily distinguished from igneous cores based on Th/U (<<1)
Concordia plots show mixing between modi�ed zircon rims and unmodi�ed, inherited igneous cores
