u-pb and sm-nd isotopic results from the la ronge ......intermediate volcanism in the mullock lake...

Saskatchewan Geological Survey 1 Summary of Investigations 2013, Volume 2

U-Pb and Sm-Nd Isotopic Results from the La Ronge ‘Horseshoe’ Project Area, Southwestern Reindeer Zone: 1.84 Ga Felsic-Intermediate Volcanism in the Mullock Lake Assemblage

Ralf O. Maxeiner, Nicole M. Rayner 1, and Rob A. Creaser 2

Maxeiner, R.O., Rayner, N.M., and Creaser, R.A. (2013): U-Pb and Sm-Nd isotopic results from the La Ronge ‘Horseshoe’ project area, southwestern Reindeer Zone: 1.84 Ga felsic-intermediate volcanism in the Mullock Lake assemblage; in Summary of Investigations 2013, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of the Economy, Misc. Rep. 2013-4.2, Paper A-8, 10p.

Abstract In support of a bedrock mapping project north of La Ronge (La Ronge ‘Horseshoe’ project), this report adds to a collection of isotopic studies with new U-Pb zircon results from a felsic volcanic rock and three Sm/Nd results from volcanic and sedimentary rocks. Hornblende-bearing calcic psammite-psammopelite, garnetiferous pelite-psammopelite, as well as locally intercalated intermediate to mafic tuffs and possible rare oligomictic pebble conglomerate comprise the Hebden Lake assemblage. Feldspar porphyry dykes cutting a succession of polymictic conglomerate and psammite, here tentatively equated with a basal portion of the Hebden Lake assemblage, are juvenile (εNdT=+4.0). A calcic psammite, believed to be part of the same assemblage, but sampled 20 km west of Hebden Lake, is also juvenile (εNdT=+3.1). This suggests that the Hebden Lake assemblage was deposited in a setting proximal to an emerging arc complex, an interpretation consistent with its stratigraphic position atop the presumed 1.89 Ga mafic volcanoplutonic Freestone Lake assemblage.

The Mullock Lake assemblage, which is dominated by a fluvial-alluvial to shallow-marine succession of polymictic conglomerate, and potassic psammite to psammopelite, also contains a considerable component of intermediate to felsic volcanic and subvolcanic rocks, particularly near its base. Previous work indicated deposition between circa 1846 Ma and 1843 Ma. A U-Pb zircon crystallization age of a felsic volcaniclastic rock is 1846 ±3 Ma and contains inherited zircon ranging between 2.54 Ga and 1.88 Ga. An εNdT isotopic signature of +0.1 for the felsic volcanic breccia indicates some interaction with older crust, consistent with the inherited zircon. The Mullock Lake assemblage is interpreted to have been deposited in a rapidly rising, volcanically active environment, where subvolcanic intrusions were being unroofed at the same time as new intrusions were being emplaced into slightly older sedimentary rocks of the same assemblage. Inherited zircon and evolved Nd signatures in the 1846 Ma felsic volcanic rock suggests that the Flin Flon–Glennie complex had already begun to amalgamate with Sask Craton–aged crust at depth.

Keywords: geochronology, Glennie Domain, Reindeer Zone, Trans-Hudson Orogen, Mullock Lake assemblage, McLennan Group, Hebden Lake assemblage, Nd.

1. Introduction In 2010, the Saskatchewan Geological Survey commenced a 1:20 000-scale bedrock mapping project north of La Ronge (La Ronge ‘Horseshoe’ project), in order to update the geological framework for this lithologically and structurally complex area of the southwestern Reindeer Zone. One of the main goals of the project is to understand the depositional history of the area’s supracrustal components, to place them into distinct lithotectonic assemblages, and to assess their potential to host various mineral deposit types, an effort supported by geochemistry and geochronology. In addition, understanding the relative and absolute age constraints of the intrusive and thermotectonic history will further help to facilitate paleo-plate tectonic reconstructions and the timing of collisional and mineralizing events.

Most of the lithotectonic domains of the Reindeer Zone are represented in the La Ronge ‘Horseshoe’ project area and include, from west to east, the Wathaman, Rottenstone, La Ronge, Kisseynew, and Glennie domains. Rocks in the transect include elements of the Neoarchean Hearne Craton (Black Bear Island Inlier) and Neoarchean to

1 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8. 2 University of Alberta, Department of Earth and Atmospheric Sciences, 126ESB, Edmonton, AB T6G 2R3.


Siderian 3 Sask Craton (Nistowiak, Iskwatikan, Hunter Bay inliers), as well as the Orosirian Flin Flon–Glennie complex rocks and younger sedimentary and intrusive components.

A number of isotopic studies have already been conducted as part of the ‘Horseshoe’ project (Maxeiner and Kamber, 2011; Maxeiner et al., 2012). This publication adds to that collection with U-Pb zircon results from a felsic volcanic rock and three Sm/Nd results from volcanic and sedimentary rocks.

2. Local Geology The bedrock geology of Hebden Lake, located in the southwestern Glennie Domain, was mapped in 2011 (Maxeiner, 2011a; Maxeiner and Kamber, 2011). The area is underlain by the Freestone Lake assemblage, a mafic volcanoplutonic succession that comprises variably strained mafic volcanic rocks and locally imbricated gabbroic sheets, which is interpreted to be in part older than 1897 Ma (Corrigan et al., 2001). A locally hornblende-bearing pelite-psammite succession, informally named the Hebden Lake assemblage, is spatially associated with the mafic volcanoplutonic rocks. The Hebden Lake assemblage is presumed to be of similar age to the circa 1.87 to 1.86 Ga Crew Lake assemblage, which contains contaminated 4 (εNdT=-0.4) Neoarchean to Orosirian detritus (2.81 to 1.87 Ga; Maxeiner et al., 2012). By far the most extensive succession comprises circa 1.84 Ga (Maxeiner and Kamber, 2011) conglomerate, potassic psammite, and felsic to intermediate volcanic rocks, collectively referred to as the Mullock Lake assemblage (which includes the former McLennan Group). Distinctive trachytic diorite, quartz–phyric granite and feldspar porphyry dykes have also been included in this assemblage (Maxeiner, 2011a). Granitic to granodioritic rocks of the Bridgeman Lake pluton likely postdate the Freestone Lake and Hebden Lake assemblages, but predate the Mullock Lake assemblage. There is a metamorphic gradient across the Hebden Lake area, increasing from middle amphibolite facies conditions in the southwest to upper amphibolite facies in the northeast.

A preliminary U-Pb zircon age from a Mullock Lake assemblage boulder conglomerate (Figure 1) unconformably overlying a quartz-phyric granite had previously been acquired using a laser ablation inductively coupled plasma mass spectrometer at Laurentian University (Maxeiner and Kamber, 2011). A probability density diagram of results from 81 zircon grains revealed a dominant population at 1846 ±3 Ma and a secondary maximum at 1892 ±6 Ma. The data was interpreted to reflect derivation of detritus from the late Orosirian Reindeer Zone, and specifically the Flin Flon–Glennie complex. Lack of Archean detritus suggested that the rivers responsible for deposition of the conglomerate were sourcing a region that had not seen sufficient uplift and erosion to have unroofed older Archean (i.e., Sask Craton) components or that the complex had not yet been accreted to the Sask Craton or other unrelated Archean components. Based on their abundance, the younger population of 1846 Ma zircon detritus was thought to directly date both the quartz-phyric granite clasts dominating the conglomerate at the sample location and the subjacent in situ quartz-phyric granite source.

Recognition of felsic and intermediate volcanic rocks that appeared to be intercalated with polymictic conglomerate led Maxeiner and Kamber (2011) to believe that the volcanic rocks were likely part of the circa 1.84 Ga Mullock Lake assemblage, rather than representing older volcanic components of the Flin Flon–Glennie complex. To further test this hypothesis, a sample of felsic volcanic rock was collected from a mixed felsic-intermediate volcanic unit intercalated on map scale with polymictic conglomerate.

3. Results

a) Analytical Technique

U-Pb geochronology was carried out using the Sensitive High-Resolution Ion MicroProbe (SHRIMP) at the Geological Survey of Canada in Ottawa. SHRIMP analytical procedures followed those described by Stern (1997), with standards and U-Pb calibration methods following Stern and Amelin (2003). U-Pb results are presented in Table 1, with analytical details given in the footnotes.

Sm-Nd analytical work was carried out at the University of Alberta and is identical to those described in Rayner et al. (2009) for analyses carried out on rocks of the Pelican Narrows area.

3 This paper embraces the following proposed subdivision of the Paleoproterozoic Eon (after Gradstein et al., 2004) , from oldest to youngest: Siderian (2.50 to 2.30 Ga), Rhyacian (2.30 to 2.05 Ga), Orosirian (2.05 to 1.80 Ga), and Statherian (1.80 to 1.60 Ga). 4 ‘Contaminated’ suggests a mix of detrital ages including some much older than the estimated depositional age of the rock.


Figure 1 – Detailed geological map of the Hebden Lake area (after Maxeiner and Kamber, 2011), showing locations of samples RM1101-2004 (felsic volcanic breccia), RM1101-2002 (feldspar porphyry dyke), and Mullock Lake boulder conglomerate (results in Maxeiner and Kamber, 2011); index map abbreviations: FFD – Flin Flon Domain, GD – Glennie Domain, KD – Kisseynew Domain, LRD – La Ronge Domain, MD – Mudjatik Domain, PLD – Peter Lake Domain, RD – Rottenstone Domain, WB – Wathaman Batholith, and WD – Wollaston Domain.

b) Felsic Volcanic Breccia, Kisseynew Domain, Southwest Reindeer Zone Sample RM1101-2004; GSC lab number: z10743 (UTM: 512400 m E, 6143309 m N 5)

A unit of felsic volcanic rock (Maxeiner, 2011a) occurs on the west and north sides of Lovell Bay on Hebden Lake (Figure 1), where it thickens to about 300 m (surface extent) in the nose of a southwest-plunging F2 antiform. The rocks are relatively well preserved and reveal a number of primary textures in the low-strain area of the fold nose. On the limbs of the fold, the unit thins considerably and is much more strained, generally resulting in very fine-grained, non-descript siliceous rocks that are locally mylonitized. Units of conglomerate occur on either side of the felsic volcanic rocks, but contacts between the two rock types have not been observed.

Where primary textures are preserved in the volcanic rocks, locally flow-banded rhyolites (Figure 2A) are interlayered with tuff breccias (Figures 2B and 2C) and felsic to intermediate tuffs (Figure 2D). Many of these individual facies are not laterally continuous over large distances, likely due to rapidly changing eruptive styles as well as faulting and later disruption by an assortment of felsic to mafic dykes.

5 All UTM coordinates are in NAD 83, zone 13.

MPs

Ps

G

P

Gdg

Pcs

Dip

Gdg

MPs

Fv

Gq

Fv Gam

P

Gq

Ps

Mvc

Gd

Pcs

Ps

Ga

Mv

MPc

MCg

Mv

Ga MPsp

MCg

PcsMCg

MCg

MPs

Pcs

Ga

MPs

Fv

Ga

Mv

Pr

MCg

Mv

Gf

Pp

MCg

Mv

MPs

Ga

Gf

MCg

Fv

Pp

Di

Fp

MCgMv

Fp

MCg

G

Iv

MPs

Dip

Mvc

Fv

MCg

PcsPp

Gf

Iv

Gq

Gq

Pp

MPs

Fp

MCg

Dip

MCg

MPc

Fp

FpMPc

Ifs

Gf

Lg

Fv

GfFp

MPs

Lg

MCg

Ps

Mvc

GdPs

Ga

Lg

Fv

Gq

MCg

Pr

Pr

Gq

GqMPs

MPc

P

P

Fp

0 21 km3

HebdenLake

Hebden Lake fo

ld

RamslandLake

Mullock\Lake

FreestoneLake

Haugen L

ake

syn

form

NorthLake

LovellBay

Mullock Lake antifo

rm

Trace of F axial surface4

Symbol Legend

Trace of F axial surface3

Trace of F axial surface2GlacialDrift

GlacialDrift

GlacialDrift

Gam Magnetic gabbro

Gf Aplite and leucogranite

MPc Calcic potassic psammopelite

Pr Paleosaprolite

Dip Trachytic dioriteIfs Iron formation

Gq Quartz-phyric granitePcs Calcic psammopelite

Fp Feldspar porphyry

MPsp Potassic psammopelite

MPs Potassic psammite

MCg Conglomerate

Lg Syenogranite, leucogranite

Pp Pelite (includes Ppc)P Pegmatite

G Monzogranite

Gd Granodiorite

Gdg Hornblende granodiorite

Di Sheared diorite, microdioritePs Psammite

Fv Felsic volcanic rock

Mv Mafic volcanic rock, mafic calc-silicate rocks, and altered Mv

Ga Gabbro

Iv Intermediate volcanic rock

Undivided Intrusive Rocks

Mullock Lake Assemblage

Freestone Lake assemblage

Legend Hebden Lake assemblage

Bridgeman Lake pluton (circa 1.85 Ga)

61

44

00

0 m

N6

14

80

00

mN

510000 m E 514000 m E 518000 m E

55

°28

’ N

104°50' W 104°45' W

102

RM1101-2002

RM1101-2004

Mullock Lake boulder conglomerate(<1846 Ma)

Sample locations

HEARNE

ATHABASCABASIN

HEARNE

RAE

REINDEER

ZONE

GD

Hebden Lake area

SASK CRATON

KD

FFD

RD

LRDWB

PLD

MD

WD

Bob Lake area

Saskatchewan G

eological Survey 4

Summ

ary of Investigations 2013, Volum

e 2

Table 1 – SHRIMP U-Pb data.

U Th Th Yb Hf 204Pb 206Pb* f(206)204 208*Pb 207*Pb 206*Pb Corr 207*Pb 206Pb ±206Pb 207Pb ±207Pb Disc.

Spot Name (ppm) (ppm) U (ppm) (ppm)206Pb % ± (ppm) %

206*Pb % ±235U % ±

238U % ± Coeff206*Pb % ±

238U 238U 206Pb 206Pb (%)

10743-26.1 280 123 0.454 278 10,069 3.3E-6 85 78 0.006 0.136 1.7 5.04 1.12 0.3257 1.07 0.957 0.1123 0.3 1817 17 1837 6 1.2

10743-1.1 157 48 0.318 163 8,556 -6.6E-6 564 44 -0.011 0.098 2.7 5.07 1.36 0.3274 1.21 0.889 0.1124 0.6 1826 19 1839 11 0.8

10743-5.1 38 73 1.998 180 7,400 1.7E-4 71 11 0.291 0.588 2.3 5.19 2.11 0.3345 1.26 0.594 0.1124 1.7 1860 20 1839 31 -1.3

10743-21.1 608 146 0.248 254 10,345 2.1E-5 49 173 0.036 0.076 1.5 5.13 1.04 0.3304 1.01 0.971 0.1125 0.3 1841 16 1841 5 0.0

10743-13.1 424 173 0.421 480 8,778 1.5E-5 78 121 0.026 0.127 1.4 5.15 1.06 0.3319 1.02 0.960 0.1126 0.3 1848 16 1841 5 -0.4

10743-27.1 370 168 0.468 309 10,791 -1.9E-6 1074 105 -0.003 0.138 1.5 5.12 1.09 0.3293 1.02 0.938 0.1127 0.4 1835 16 1844 7 0.6

10743-28.1 149 49 0.338 170 8,627 -3.9E-5 80 42 -0.067 0.104 2.6 5.07 1.19 0.3259 1.05 0.879 0.1128 0.6 1819 17 1845 10 1.6

10743-17.1 228 58 0.265 271 7,891 1.3E-5 246 65 0.022 0.082 2.5 5.18 1.16 0.3333 1.03 0.893 0.1128 0.5 1854 17 1845 9 -0.6

10743-14.1 445 156 0.361 194 10,632 1.1E-6 692 128 0.002 0.107 1.5 5.23 1.06 0.3358 1.03 0.968 0.1129 0.3 1866 17 1847 5 -1.2

10743-9.1 150 56 0.385 156 9,713 -8.7E-6 201 43 -0.015 0.117 2.5 5.14 1.16 0.3301 1.05 0.906 0.1130 0.5 1839 17 1848 9 0.6

10743-33.1 218 80 0.381 220 9,110 2.4E-6 699 61 0.004 0.113 2.1 5.06 1.12 0.3247 1.03 0.926 0.1130 0.4 1813 16 1848 8 2.2

10743-24.1 360 123 0.352 190 10,382 1.2E-4 18 100 0.208 0.107 1.7 5.03 1.09 0.3230 1.02 0.934 0.1130 0.4 1804 16 1849 7 2.7

10743-32.1 218 51 0.239 168 10,711 -1.3E-5 38 62 -0.022 0.070 2.7 5.16 1.10 0.3308 1.03 0.941 0.1131 0.4 1842 17 1849 7 0.4

10743-18.1 416 144 0.358 200 10,313 -3.3E-6 81 117 -0.006 0.110 1.6 5.10 1.05 0.3263 1.02 0.967 0.1133 0.3 1821 16 1853 5 2.0

10743-15.1 155 53 0.352 160 9,170 -5.2E-6 140 44 -0.009 0.109 2.5 5.16 1.15 0.3300 1.06 0.924 0.1133 0.4 1838 17 1854 8 1.0

10743-12.1 140 48 0.351 154 9,236 -5.4E-5 66 40 -0.094 0.104 2.8 5.16 1.22 0.3300 1.05 0.860 0.1134 0.6 1838 17 1854 11 1.0

10743-19.1 227 72 0.325 353 9,812 3.6E-5 31 64 0.063 0.096 2.2 5.16 1.10 0.3295 1.03 0.939 0.1135 0.4 1836 16 1857 7 1.3

10743-22.2 142 69 0.501 297 8,493 1.8E-5 28 39 0.031 0.149 2.3 5.11 1.14 0.3223 1.05 0.918 0.1151 0.5 1801 16 1881 8 4.9

10743-22.1 172 34 0.203 134 10,718 7.2E-6 548 49 0.012 0.057 3.3 5.32 1.21 0.3340 1.04 0.863 0.1155 0.6 1858 17 1888 11 1.9

10743-3.1 618 388 0.649 336 14,178 3.6E-6 82 189 0.006 0.193 0.9 5.94 1.03 0.3555 1.01 0.980 0.1211 0.2 1961 17 1973 4 0.7

10743-3.2 596 382 0.663 314 14,434 -5.4E-6 39 176 -0.009 0.194 1.0 5.77 1.04 0.3435 1.01 0.976 0.1219 0.2 1903 17 1984 4 4.7

10743-10.2 803 70 0.090 155 10,224 5.1E-6 91 232 0.009 0.028 2.1 5.67 1.02 0.3362 1.01 0.984 0.1222 0.2 1869 16 1989 3 7.0

10743-10.1 373 127 0.352 234 10,744 1.8E-4 12 140 0.304 0.104 1.4 9.31 1.06 0.4360 1.02 0.966 0.1548 0.3 2333 20 2400 5 3.4

10743-20.1 156 71 0.470 157 9,838 1.5E-5 45 63 0.026 0.131 1.9 10.71 1.09 0.4697 1.05 0.956 0.1653 0.3 2482 22 2511 5 1.4

10743-11.1 320 81 0.261 134 10,977 8.0E-6 68 132 0.014 0.077 1.8 11.14 1.06 0.4796 1.04 0.978 0.1684 0.2 2525 22 2542 4 0.8

10743-25.1 172 67 0.399 149 8,340 1.9E-5 82 71 0.033 0.113 1.9 11.30 1.08 0.4808 1.04 0.958 0.1704 0.3 2531 22 2562 5 1.5

Analytical Details: IP656, 16µm spot, 7nA primary intensity, five scans

Error in 206Pb/238U calibration 1.0% (included)

No mass fractionation correction applied to 207/206 resultsNotes (see Stern, 1997):

Spot name follows the convention x-y.z; where x = sample number, y = grain number and z = spot number. Multiple analyses in an individual spot are labelled as x-y.z.z

Uncertainties reported at 1 sigma and are calculated by using SQUID 2.22.08.04.30, revised 30 Apr 2008

* refers to radiogenic Pb (corrected for common Pb)

Disc.: Discordance given as difference between measured 206*Pb/238U ratio and the expected 206*Pb/238U ratio at t=207*/206* age, in percent.

Calibration standard 6266; U = 910 ppm; Age = 559 Ma; 206Pb/238U = 0.09059

Apparent Ages (Ma)

f206204 refers to mole percent of total 206Pb that is due to common Pb, calculated using the 204Pb-method; common Pb composition used is the surface blank (4/6: 0.05770; 7/6: 0.89500; 8/6: 2.13840)


Figure 2 – Outcrop photographs taken in close proximity of sample location of RM1101-2004: A) partly flow-banded (yellow arrow) felsic volcanic breccia; station RM11-36-ST04; UTM 512408 m E, 6143313 m N; B) close-up of the matrix of the felsic volcanic breccia, illustrating the fragmental nature of the rock; also note quartz veins and infillings due to later silicification event; same station as A; C) close-up of blocky felsic volcanic breccia with irregular mafic and felsic volcanic fragments; station RM11-36-ST06; UTM 512342 m E, 6143394 m N; D) interlayered intermediate and felsic volcanic lapilli and ash tuffs; station RM11-36-ST05; UTM 512444 m E, 6143326 m N; E) quartz-filled amygdaloidal intermediate volcanic flow; station RM11-36-ST03; UTM 512338 m E, 6143279 m N; and F) polymictic cobble conglomerate with predominant felsic volcanic, quartz-eye granite and amphibolite clasts; station RM11-36-ST15; UTM 513039 m E, 6143644 m N.

D

A B

C

FE


Figure 3 – Zircon yield of sample RM1101-2004, felsic volcanic breccia: A) displaying the variety of analyzed zircon; B) back-scatter electron (BSE) microscope image of zircon grains 10 to 13; and C) BSE image of zircon grains 14 to 17; for further explanation see text.

In close proximity to the sample site, the felsic volcanic rocks are interlayered with intermediate volcanic rocks on a decimetre to metre scale, and distinct units of intermediate volcanic rocks, tens of metres thick, are also found. Thin tuffaceous layers are recognized (Figure 2D) as are amygdaloidal (Figure 2E) and porphyritic varieties. The amygdaloidal varieties are likely derived from flows and/or high-level intrusions.

The outcrop where the felsic volcanic breccia sample was collected consists of metre-scale layers containing decimetre-scale, flow-banded blocks (Figure 2A) contained in a more micaceous gritty felsic volcanic matrix (Figure 2B). A nearby outcrop (Figure 2E) also features amygdaloidal intermediate volcanic rocks, locally containing potential relict pillows, with which the felsic volcanic breccia appears interlayered. Quartz-phyric and feldspar-phyric rocks are locally present. An extensive unit of polymictic conglomerate, located just a few hundred metres to the east of the sampled felsic volcanic outcrop, is dominated by rounded pebble to cobble-sized clasts of felsic volcanic, feldspar porphyry and mafic volcanic clasts (Figure 2F). The sampled felsic volcanic rock is intruded by a mafic dyke that contains xenoliths of the felsic volcanic rocks.

The sample yielded abundant, stubby, prismatic, sharply terminated zircon (Figure 3A). This morphology is commonly observed in zircon recovered from volcanic or hypabyssal rocks. Most grains are fractured and some exhibit orange iron-oxide staining. A few grains are turbid and rounded to subrounded, whereas many are euhedral. In back-scatter electron images, euhedral to rounded grains (Figures 3B and 3C) commonly exhibit faint oscillatory zoning and rare core/overgrowth relationships (Figure 3B,; upper left zircon).

Twenty six analyses were carried out on 23 separate zircon grains, which yielded 207Pb/206Pb dates between 1837 and 2562 Ma (Table 1). The majority of the zircon grains form a single population with a mean 207Pb/206Pb age of 1846 ±3 Ma (Figure 4; n=17, MSWD=0.72, probability=0.77). Six zircon grains (about 30% of the analyzed grains) were identified to be older, yielding 207Pb/206Pb ages of 1.88, 1.98, 2.40, 2.51, 2.54, and 2.56 Ga. These older grains are either xenocrysts that were entrained in the magma as it interacted with older Neoarchean- to Orosirian-aged rocks or alternatively the felsic volcanic breccia contains an older detrital component. Ages between 1.98 and1.88 Ga are consistent with derivation from oceanic and arc components of the Reindeer Zone, whereas ages between 2.54 and 2.40 Ga are characteristic of the largely buried Sask Craton (e.g., Rayner et al., 2005).

c) Sm-Nd Results

Sm/Nd analyses were obtained for three volcanic, subvolcanic, and sedimentary samples of various lithotectonic assemblages in the La Ronge ‘Horseshoe’ project area. This was done to build a provincial Sm/Nd database to characterize and fingerprint the various lithotectonic assemblages of the Reindeer Zone, to augment existing geochronological information, and to better understand the provenance of

30 µm

10

B

11

12

13

30 µm

C

1415

16 17

12

300 µm

A


Figure 4 – Concordia plots for sample RM1101-2004 (GSC lab number z10743): A) displaying all analyzed zircon grains and B) displaying only the young zircon population; for further explanation see text.

Table 2 – Sm-Nd isotopic data for selected samples from the Hebden Lake area.

sedimentary rock sequences and the paleo-plate tectonic environments of volcanic assemblages.

The εNdT value for the felsic volcanic breccia described above (sample RM1101-2004) is +0.1 (Table 2) and yields a model age of 2.36 Ga (Goldstein et al., 1984). This suggests that the magma from which the rock crystallized was not juvenile, but has interacted with older crust, consistent with the results of the SHRIMP U-Pb geochronology.

A second sample (RM1101-2002, Figure 1) was collected on the northeast end of Freestone Lake (Maxeiner and Kamber, 2011). The sampled rock is a feldspar porphyry (Figure 5), which is part of a more extensive felsic subvolcanic intrusive suite that occurs as dykes cutting a conglomerate-psammite succession of uncertain origin. The feldspar porphyry is typically light grey to buff, containing irregular and recrystallized feldspar phenocrysts up to 5 mm long, in a fine- to medium-grained seriate matrix. The rock is generally massive to moderately foliated and metre-scale dykes have been isoclinally folded. Typical samples contain about 20% quartz, 10% biotite, and 5% hornblende, with the remainder made up of a mixture of K-feldspar and plagioclase. The porphyry cuts an oligomictic conglomerate (Figure 5), which is dominated by pebble to cobble-sized, vein-quartz and felsic volcanic clasts; the conglomerate and interbedded psammite were tentatively correlated with the Mullock Lake assemblage (Maxeiner, 2011a), although not physically connected with the main part of this succession.

2500

2300

2100

1900

1700

0.28

0.32

0.36

0.40

0.44

0.48

0.52

3 5 7 9 11 13

0.30

0.32

0.34

0.36

0.38

4.6 5.0 5.4 5.8 6.2

207Pb/235U

207Pb/235U

20

6P

b/2

38U

20

6P

b/2

38U

RM1101-2004 (felsic volcanic breccia)Crystallization age1846 ±3 Ma (95% conf.)n=17, red ellipses, MSWD = 0.72, probability of fit 0.77

RM1101-2004 (felsic volcanic breccia)(GSC lab number z10743)All dataBlue ellipses interpreted as inherited

17801780

18201820

18601860

19001900

19401940

19801980

20202020

analysis 10.2, possibly nicking 2.4 Ga core

B

A error ellipses plotted at 2ó

error ellipses plotted at 2ó

Sample Sm (ppm) Nd ± 2 SE 147Sm/144Nd 143Nd/144Nd uncert. TDM ~TMa ?NdT

1101-2002 3.59 20.4 0.1065 0.511716 0.000012 2.05 1900 41101-2004 5.37 28.5 0.1137 0.511604 0.000009 2.36 1900 0.11101-6001 2.51 12.5 0.1213 0.511855 0.000012 2.15 1900 3.1Notes:TDM is the depleted mantle model age calculated using the linear model of Goldstein et al. (1984).

TMa is the age at which the value for epsilon Nd (εNdT) is calculated. It can be calculated at any age required, please request.Uncertainty in Nd isotopic composition is equivalent to 2 Standard Errors.


Figure 5 – Outcrop photographs of feldspar porphyry cutting well-foliated oligomictic conglomerate; RM11-33-ST10; UTM 518819 m E, 6146893 m N; for further explanation see text.

The εNdT for this porphyry sample (sample RM1101-2002) is +4.0 (Table 2) and yields a calculated model age of 2.05 Ga (Goldstein et al., 1984). This suggests that it was derived from a relatively juvenile magma source that had little interaction with older crust.

The third sample (RM1101-6001) is a calcic psammite associated with mafic volcanic rocks of the Freestone Lake assemblage. The psammite comes from the Bob Lake area (see Figures 1 and 2 in Maxeiner, 2011b), approximately 20 km to the west of Hebden Lake, where units of calcic psammite, psammopelite, and pelite structurally, and possibly stratigraphically, overlie the mafic volcanoplutonic rocks of the Freestone Lake assemblage. At the sample location, the rock is grey to brownish grey, fine

grained, compositionally layered on centimetre to decimetre scale, and contains minor, layer-parallel, centimetre-scale injections of white leucosome. Typical samples contain variable amounts of biotite and hornblende, defining the compositional layering and generally accounting for 10 to 15% of the rock.

The εNdT for the Bob Lake calcic psammite (RM1101-6001), is +3.1 (Table 2) and yields a model age of 2.15 Ga (Goldstein et al., 1984). This suggests that the sedimentary detritus in the calcic psammite was most likely derived from a relatively juvenile arc complex, possibly with a minor older detrital component, that was undergoing erosion.

4. Discussion and Conclusions The isotopic data presented in this paper can be used to refine some of the components and interpretations surrounding the lithotectonic assemblages that have been proposed for the southwestern Reindeer Zone area.

The Hebden Lake assemblage represents the main sedimentary succession that is interpreted to stratigraphically overlie the Freestone Lake assemblage at Freestone Lake (Maxeiner and Kamber, 2011) and at Bob Lake (Maxeiner, 2011b). It comprises a hornblende-bearing psammite-psammopelite and more aluminous garnetiferous pelite-psammopelite succession, as well as locally intercalated intermediate to mafic tuffs and possibly rare oligomictic pebble conglomerate near its base. The assemblage was also recognized in the Otter Lake area (Maxeiner et al., this volume). Feldspar porphyry dykes (sample RM1101-2002) cutting a clastic sedimentary succession, here tentatively equated with the Hebden Lake assemblage, are juvenile (εNdT=+4.0). A calcic psammite, believed to be part of the same assemblage, but sampled 20 km west of Hebden Lake, is also juvenile (εNdT=+3.1). This suggests that the Hebden Lake assemblage was deposited in a setting proximal to an emerging arc complex. The fact that the assemblage is always closely associated with, albeit commonly structurally separated from, the mafic volcanoplutonic Freestone Lake assemblage, suggests that it formed in a forearc or backarc environment. Although the age of the crosscutting feldspar porphyry dykes is unknown, their juvenile character suggests that the magma responsible for dyke formation remained uncontaminated during emplacement, which in turn infers that collision with Archean cratonic material had not yet occurred.

The Mullock Lake assemblage, which is dominated by a fluvial-alluvial to shallow-marine succession of polymictic conglomerate, and potassic psammite to psammopelite, also contains a considerable component of intermediate to felsic volcanic and subvolcanic rocks particularly near its base. Deposition was previously bracketed between circa 1846 Ma, the interpreted age of quartz-phyric granite stratigraphically underlying the polymictic conglomerate (Maxeiner and Kamber, 2011; see earlier description of results), and >1843 Ma, the age of the North Lake aplite cutting feldspathic psammite (Kyser et al., 1992). The age of the felsic volcanic rock dated in this paper is identical to that of underlying the quartz-phyric granite, which would, at first glance, seem inconsistent. However, we believe this temporal overlap suggests that the Mullock Lake assemblage was deposited in a rapidly rising,


volcanically active environment, where subvolcanic intrusions were being unroofed at the same time as new intrusions were being emplaced into conglomerate a few hundred thousand to a million years older. Map evidence supports this interpretation, with quartz-eye granite and porphyritic diorite boulders contained in a polymictic conglomerate that unconformably overlies these rocks. A similar polymictic conglomerate, in locations a few hundred metres away and presumed to belong to the same stratigraphic unit, is seen to be intruded by the same quartz-phyric granite intrusion. These rocks likely formed in a dynamic forearc environment in which a mostly Orosirian-aged juvenile volcanic to subvolcanic terrane was being eroded, but in which there continued to be synsedimentary and syntectonic volcanism. However, the fact that the felsic volcanic rocks of the Mullock Lake assemblage contain inherited zircon and have contaminated εNd concentrations, also suggests that the Flin Flon–Glennie complex had already begun to amalgamate with Sask Craton–aged crust at depth.

5. References Corrigan, D., Maxeiner, R.O., and Harper, C.T. (2001): Preliminary U-Pb results from the La Ronge–Lynn Lake

Bridge project; in Summary of Investigations 2001, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.2, CD-ROM, p111-115.

Goldstein, S.L., O’Nions, R.K., and Hamilton, P.J. (1984): A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems; Earth Planet. Sci. Lett., v70, p221-236.

Gradstein, F.M., Cooper, R.A., Sadler, P.M., Hinnov. L.A., Smith, A.G., Ogg. J.G., Villeneuve, M., McArthur, J.M., Howarth, R.J., Agterberg, F.P., Robb, L.J., Knoll, A.H., Plumb, K.A., Shields, G.A., Strauss, H., Veizer, J., Bleeker, W., Shergold, J.H., Melchin, M.J., House, M.R., Davydov, V., Wardlaw, B.R., Luterbacher, H.P., Ali, J.R., Brinkhuis, H., Hooker, J.J., Monechi, S., Powell, J. Röhl, U., Sanfilippo, A., Schmitz, B., Lourens, L., Hilgen, F., Shackelton, N.J., Lasker, J., Wilson, D., Gibbard, P., and van Kolfschoten, T. (2004): Geologic Time Scale 2004; Cambridge University Press, Cambridge, 589p.

Kyser, T.K., Fayek, M., and Sibbald, T.I.I. (1992): Geochronological studies in the La Ronge and Glennie domains; in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92-4, p130-134.

Maxeiner, R.O. (2011a): Bedrock geology of the Hebden Lake area, transition zone between Glennie, Kisseynew and La Ronge domains (part of NTS 73P07); 1:20 000-scale prelim. map with Summary of Investigations 2011, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2011-4.2-(4.2), URL <http://economy.gov.sk.ca/SOI2011V2_M4.2>.

__________ (2011b): La Ronge ‘Horseshoe’ project: preliminary bedrock geology of the Bob-Miles lakes area at the transition between the western Glennie Domain and southern Rottenstone Domain (parts of NTS 73P/06); in Summary of Investigations 2011, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2011-4.2, Paper A-8, 14p, URL <http://economy.gov.sk.ca/SOI2011V2_A8>.

Maxeiner, R.O. and Kamber, B.S. (2011): La Ronge ‘Horseshoe’ project: bedrock geology of the Hebden Lake area at the transition between the western Glennie Domain and southern Kisseynew and La Ronge domains (parts of 73P/07); in Summary of Investigations 2011, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2011-4.2, Paper A-7, 20p, URL <http://economy.gov.sk.ca/SOI2011V2_ A7>.

Maxeiner, R.O., Rayner, N.M., and Eglington, B.M. (2012): U-Pb and Sm-Nd isotopic results from the La Ronge ‘Horseshoe’ project area, western Glennie Domain and southern Rottenstone Domain: evidence for 2.22 to 2.52 Ga detritus; in Summary of Investigations 2012, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of the Economy, Misc. Rep. 2012-4.2, Paper A-10, 16p, URL <http://economy.gov.sk.ca/ SOI2012V2_A10>.

Rayner, N.M., Maxeiner, R.O., and Creaser, R.A. (2009): New U-Pb and Sm-Nd results from the Pelican Narrows area: 1865-1857 Ma successor arc sedimentation from juvenile sources, 1857 Ma juvenile successor arc plutonism, and 1837 Ma Missi Group sedimentation; in Summary of Investigations 2009, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2009-4.2, Paper A-9, 15p, URL <http://economy.gov.sk.ca/SOI2009V2_A9>.

Rayner, N.M., Stern, R.A., and Bickford, M.E. (2005): Tectonic implications of new SHRIMP and TIMS U-Pb geochronology of rocks from the Sask Craton, Peter Lake Domain, and Hearne margin, Trans-Hudson Orogen, Saskatchewan; Can. J. Earth Sci., v42, p635-657.


Stern, R.A. (1997): The GSC Sensitive High Resolution Ion Microprobe (SHRIMP): analytical techniques of zircon U-Th-Pb age determinations and performance evaluation; in Radiogenic Age and Isotopic Studies: Report 10, Geol. Surv. Can., Current Research 1997-F, p1-31.

Stern, R.A. and Amelin, Y. (2003): Assessment of errors in SIMS zircon U-Pb geochronology using a natural zircon standard and NIST SRM 610 glass; Chem. Geol., v197, p111-146.

u-pb and sm-nd isotopic results from the la ronge ......intermediate volcanism in the mullock lake...

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