Summary

Near-primary, Middle-Late Triassic, volatile-rich picritic tuff gives insight into the trace element and isotopic composition of the mantle wedge beneath the Stikine terrane of the North American Cordillera. The most mobile isotopes (Pb) suggest 2% addition of molten subducted sediment to the depleted asthenospheric mantle wedge. The least mobile isotopes (Hf) are similar to modern E-MORB presently erupted at the southern end of the Explorer Ridge and suggest that enriched domains in the northeastern Pacific mantle may be long-lived (222

Unravelling arc mantle using trace element and isotopic “intelligence” from the Middle-Late Triassic Stikine arc

1 2 3D. Milidragovic, A. Zagorevski, D. Weis1 British Columbia Geological Survey, Victoria, British Columbia, V8W 9N3

2Geological Survey of Canada, Ottawa, Ontario, K1A 0E8

3University of British Columbia, Vancouver, British Columbia, V6B 5J3 British Columbia Geological Survey Geofile 2017-12

Background

The Stikine terrane of the North American Cordillera (Fig. 1) experienced a period of arc building and extensive mineralization during the Middle to Late Triassic. Volumetrically minor (<100 m-thick), olivine-phyric picritic tuff was deposited near the base of the Triassic volcanic arc at 222 Ma. The ultramafic composition of the tuff (MgO =21-33 wt.%) reflects, in part, accumulation of 20-65% olivine.Despite greenschist-facies metamorphic overprint, the tuff shows well-preserved primary textures, including euhedral pseudomorphed olivine, frothy devitrified lapilli groundmass, and cuspate glass shards with bubble-wall morphologies. The tuff was emplaced as result of violent exsolution of magmatic volatiles from oxidized (logfO ~FMQ+1) and relatively cool (1200 2oC) picritic (MgO ~16 wt.%) parent saturated in olivine

(Fo ) and chromite (Milidragovic et al., 2016). The 91

relatively low temperature of the picritic magma is attributed to its high water content (5-7 wt.% H O). 2

Stikin

e T

erra

ne

Study area

Explorer

Seattle

Location of Stikine arc picrites

2 mm

1 mm

0.5 mm

Tuff breccia

Finely-bedded ash-tuff

partially palagonitized cuspate glass

shards in ash-tuff

crystal (olivine) and scoria/glass-rich ash-tuff

olivine-rich scoriaceous lapillus

Juan de Fuca

Gorda

GVB

High Cascades

Trace element geochemistry

Picritic tuff of Stikine terrane has a calc-alkaline trace element character (relative Nb-Ta depletion, La/Yb =2-3), typical of arc magmas. The abundance of MREE/HREE in the calculated parental picrite (corrected for olivine accumulation), is lower than that of MORB and back-arc basin basalts (BABB) and requires an ambient asthenospheric mantle source that was depleted relative to Earth`s model depleted mantle (DM). The calculated ambient mantle composition requires prior extraction of a relatively low-

degree melt from DM (F 0.05). The relative enrichment in LREE and depletion in HFSE of the parental picrite magma requires addition of 1-2% sediment melt, generated at 2.2-3.5 GPa, to ambient mantle.

CsBa U

Nb La PrP

ZrSm

GdTb Y Er YbRb

ThKTa Ce

Sr NdHf Eu

Ti Dy Ho Tm LuPb

0.1

1

10

100

C/C i

PM

Full range of analyses (n =43) reported by Milidragovic et al. (2016)

PM-normalized trace element profiles of the picritic tuff samples (n=10; this study)

CsBa U

Nb La PrP

ZrSm

GdTb Y Er YbRb

ThKTa Ce

Sr NdHf Eu

Ti Dy Ho Tm LuPb

0.1

1

10

100

C/C i

PM

N-MORB

BABB

thParental picrite composition (median and 95 percentile envelope)

CsBa U

Nb La PrP

ZrSm

GdTb Y Er YbRb

ThKTa Ce

Sr NdHf Eu

Ti Dy Ho Tm LuPb

0.01

0.1

1

10

100

C/C i

PM

2 %

5 %

DM

ambient mantle

Calculated mantle source of Stikine picrites

PM-normalized composition of picrite mantle source (assuming aggregate fractional melting)

CsBa U

Nb La PrP

ZrSm

GdTb Y Er YbRb

ThKTa Ce

Sr NdHf Eu

Ti Dy Ho Tm LuPb

0.01

0.1

1

10

100

C/C i

PM

o2.2 GPa/ 650C

o2.2 GPa/ 750C

o3.5 GPa/ 900C

o4.5 GPa/ 1050C

o2.2 GPa/ 750C

2% hydrous melt

1% hydrous melt

1% hydrous fluidresidual mantle after melt extraction from DM

Parental picrite composition

Modelled addition of GLOSS-II to “baseline” mantle assuming sediment-melt/fluid partitioning of Spandler et al. (2007) and Hermann and Rubatto (2009)

NOTE: Hf is ~immobile during subduction (<4GPa) whereas Nd is not!

Isotope geochemistry

Contrasting mobility of isotopic systems during subduction (see previous panel) allows us to characterize both the “baseline” mantle and subducted crust component in the mantle source of Stikine terrane picrites. Hf is immobile and therefore representative of the “baseline” mantle, whereas Pb is highly mobile and largely inherited from the subducted

87 86ocean floor. The behaviour of Nd is intermediate between these two isotopic systems. Sr/ Sr ratios of the picritic tuff are highly radiogenic and reflect ~syn-depositional alteration by Middle-Late Triassic sea-water.

0.702 0.703 0.704 0.705 0.706-4

-2

0

2

4

6

8

10

12

ODP888

ODP1027

87 86Sr/ Sr (222 Ma)

(

22

2 M

a)

Nd

87 86 vs. Sr/ SrNd

Mantle array

sea-water alteration

3 4 5 6 7 8 9 10 11 12 13 (222 Ma)Nd

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

e(2

22

Ma

)H

f

Av. uncertainty

Terrest

rial a

rray

1% Cascade sed. meltin Explorer Ridge source

High Cascades segment - Cascade arc

Juan De Fuca/Gorda MORB

Explorer E-MORB

Stuhini Gr. picrite

Garibaldi volcanic belt (GVB)alkaline basalt - northern GVB

Legend vs. Hf Nd

Baseline mantle143 144

Radiogenic initial Nd/ Nd ratios indicate LREE enrichment of the source was short-lived.

176 177 Initial Hf/ Hf of picrites N-MORB/DM,

but similar to alkaline GVB basalts and Explorer Ridge E-MORB.

Isotopic composition of “baseline” mantle was below “Terrestrial array” and was similar to alkaline GVB basalts /Explorer Ridge E-MORB.

17.0 17.5 18.0 18.5 19.0 19.5206 204Pb/ Pb(222 Ma)

15.4

15.5

15.6

15.7

207

204

Pb

/P

b(2

22

Ma

)

NHRL

ODP888

ODP1027

Av. uncertainty

NA SEDEX

YTT SEDEX

17.0 17.5 18.0 18.5 19.0 19.5206 204Pb/ Pb(222 Ma)

37.0

37.2

37.4

37.6

37.8

38.0

38.2

38.4

38.6

38.8

39.0

208

204

Pb

/P

b(2

22

Ma

)

NH

RL

ODP888

ODP1027

Av. uncertainty

Source of sediment Pb isotopic compositions are

identical to those of modern High Cascades basalts and project towards sediment of northern Cascadia basin above NHRL.

Pb isotopes suggest a predominantly juvenille source of detritus (e.g. Stikinia, Quesnellia) with lesser evolved Precambrian detritus (e.g. Mullen et al., 2017).

207 204 206 204Pb/ Pb vs. Pb/ Pb 208 204 206 204Pb/ Pb vs. Pb/ Pb

ReferencesHermann, J., Rubatto, D., 2009. Accessory phase control on the trace element signature of sediment melts in subduction zones. Chemical Geology 265, 512–526. Milidragovic, D., Chapman, J.B., Bichlmaier, S., Canil, D., Zagorevski, A., 2016. H O-driven generation of picritic melts in the Middle-Late Triassic Stuhini arc of the Stikine terrane, British Columbia, Canada. Earth and Planetary Science Letters 454, 65-77.2

Mullen, E.K., Weis, D., Marsh, N.B., Martindale, M., 2017. Primitive arc magma diversity: New geochemical insights in the Cascade Arc. Chemical Geology 448. 43-70.Spandler, C., Mavrogenes, J., Hermann, J., 2007. Experimental constraints on element mobility from subducted sediments using high-P synthetic fluid/melt inclusions. Chemical Geology 239, 228-249.ISOTOPIC DATA SOURCES:Carpentier, M., Weis, D., Chauvel, C., 2013. Large U loss during weathering of upper continental crust: The sedimentary record. Chem. Geol. 340, 91–104. Carpentier, M., Weis, D., Chauvel, C., 2014. Fractionation of Sr and Hf isotopes by mineral sorting in Cascadia Basin terrigenous sediments. Chem. Geol. 382, 67–82. Cousens, B.L., 1988. Isotopically depleted alkali lavas from Bowie Seamount, northeast Pacific Ocean. Can. J. Earth Sci. 25, 1708-1716.Cousens, B.L., Chase, R.L, Schilling, J.-G., 1985. Geochemistry and origin of volcanic rocks from Tuzo Wilson and Bowie Seamounts, northeast Pacific Ocean. Can. J. Earth Sci. 22, 1609- 1617.Cousens, B.L., Allan, J.F., Leybourne, M.I., Chase, R.L., van Wagoner, N., 1995. Mixing of magmas from enriched and depleted mantle sources in the northeast Pacific: West Valley segment, Juan de Fuca Ridge. Contrib. Mineral. Petrol. 120, 337-357.Cousens, B., Weis, D., Constantin, M., Scott, S., 2017. Radiogenic isotopes in enriched mid-ocean ridge basalts from Explorer Ridge, northeast Pacific Ocean. Geochim. Cosmochim. Acta 213, 63-90.Davis, A.S., Clague, D.A., Cousens, B.L., Keaten, R., Paduan, J.B., 2008. Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply. Geochem. Geophys. Geosyst. 9, 25 p. Salters, V.J.M., Mallick, S., Hart, S.R., Langmuir, C.E., Stracke, A., 2011. Domains of depleted mantle: New evidence from hafnium and neodymium isotopes. Geochem. Geophys. Geosyst. 12, Q08001.

AcknowledgmentsThis project is funded by Government of Canada’s GEM-2 program. Teck Resources provided logistical support during fieldwork. J. Chapman, S. Bichlmaier, and S. Caroll assisted in the field and provided valuable discussion. B. Kieffer, K. Gordon, and V. Lai (PCIGR) performed sample dissolutions and elemental and isotopic analysis.