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. msmit@eos.ubc.ca3St Olaf College, Northfield, MN. ghagenpe@gmail.com

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

sbriggs@umail.ucsb.educottle@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