Igneous and Metamorphic U-Pb Ages from Volcanic and ... · volcanism and plutonism, common in the eastern Flin Flon Domain. A sample of anatectic leucogranite contains inherited zircon

Igneous and Metamorphic U-Pb Ages from Volcanic and Anatectic Units in the Kakinagimak Lake Area, Northwestern Flin Flon

Domain (parts of NTS 63M/01)

N.M. Rayner 1 and R.O. Maxeiner

Rayner, N.M. and Maxeiner, R.O. (2008): Igneous and metamorphic U-Pb ages from volcanic and anatectic units in the Kakinagimak Lake area, northwestern Flin Flon Domain (parts of NTS 63M/01); in Summary of Investigations 2008, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2008-4.2, CD-ROM, Paper A-4, 10p.

Abstract New U-Pb geochronological results are presented for three samples from the Kakinagimak Lake area, east of Pelican Narrows. A sample of homogeneous dacitic feldspar porphyry, interpreted as a subvolcanic intrusion, yields a crystallization age of 1846 ±7 Ma and evidence for metamorphism between 1.81 and 1.82 Ga. A strongly lineated, plagioclase-phyric intermediate rock has a minimum crystallization age of 1831 ±2 Ma determined by isotope dilution, an imprecise ion probe crystallization age of 1841 ±15 Ma, and records metamorphic zircon growth at 1808 ±4 Ma. Crystallization ages from both rocks fall well within the range of known successor arc volcanism and plutonism, common in the eastern Flin Flon Domain.

A sample of anatectic leucogranite contains inherited zircon ranging from 1883 to 2039 Ma and giving a weighted mean of 1957 ±12 Ma. The anatectic rock also has two generations of zircon overgrowths dated at 1868 ±6 Ma and 1807 ±8 Ma. The older generation of zircon overgrowth possibly reflects an older episode of high grade metamorphism related to formation of the Flin Flon–Glennie Complex or, less preferred, may represent another inherited component, possibly representing the crystallization age of the host granodiorite gneisses. The youngest episode of zircon growth in the leucogranite is interpreted to be related to peak metamorphism, remelting, and emplacement of the anatectic melt sheet.

Keywords: U-Pb, geochronology, Paleoproterozoic, Flin Flon Domain, zircon.

1. Introduction Recent mapping (Maxeiner, 2007a) of the Kakinagimak Lake area has documented a lithotectonic assemblage of metamorphosed sedimentary, volcanic, and plutonic rocks inferred to be part of the upper amphibolite facies, western extension of the Flin Flon domain (Figure 1). Geochronological investigations at Kakinagimak Lake were undertaken as part of the federal government’s Targeted Geoscience Initiative 3 program (TGI-3) for the Flin Flon area, the objective of which is to sustain and enhance base metal exploration in established mining communities. The intent of this geochronological study is to test the inferred link with the lower grade Flin Flon Domain to the east. An excellent review of work on Flin Flon Domain geology is presented in a paper by Syme et al. (1998) which documents: 1) early arc and ocean-floor formation between 1.91 to 1.88 Ga; 2) amalgamation of an intra-oceanic collage at 1.88 to 1.87 Ga (Amisk collage); 3) successor arc plutonism, volcanism, and sedimentation between 1.87 and 1.84 Ga; 4) culmination of continental collision at circa 1.83 Ga; 5) peak metamorphism from 1.815 to 1.805 Ga; and 6) uplift, cooling, and continued deformation until about 1.77 Ga. Previous age determinations in the larger Pelican Narrows area (Ashton et al., 2005) identified 1.87 to 1.85 Ga igneous crystallization ages (Heaman et al., 1993; Heaman and Ashton, 1996), 1.81 Ga metamorphic zircon ages (Ashton et al., 1992; Heaman et al., 1992), and 1.79 Ga titanite cooling ages (Heaman et al., 1993).

2. Methods Heavy minerals were separated using standard crushing, grinding, and heavy liquid concentration techniques, followed by magnetic sorting of the heavy minerals with a Frantz isodynamic separator. Samples analyzed by ID-TIMS (isotope dilution–thermal ionization mass spectrometry) were mechanically abraded prior to analysis (Krogh, 1982). Dissolution in concentrated HF, extraction of U and Pb, and mass spectrometry followed methods described by Parrish et al. (1987). Mass spectrometric data reduction and numerical propagation of analytical uncertainties

1 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8.

Saskatchewan Geological Survey 1 Summary of Investigations 2008, Volume 2

Figure 1 - Location and detailed geology of the Kakinagimak Lake area showing sample sites. Inset abbreviations: FF, Flin Flon; and PN, Pelican Narrows.


Grindley

Lake

McWilliamsLake

FileBay

Dezort

LakeBentz Bay(Attitti Lake)

Galbraith

Lake

Schotts Lake

Keep

Lake

Scott

Lake

McCallLake

Gifford

Bay

Cornell Bay

Kakinagimak Lake

Cawsey

Lake

Schotts Lakedeposit

611

0000 m

N612

0000 m

N670000 m E 675000 m E

611

5000 m

N

0 21

kilometres

Legend

Altered felsic volcanic, volcaniclastic rocks

Heterogeneous calcic psammopelite, psammite,and intermediate volcanic tuff

Mafic calc-silicate rock

Gabbro, microgabbro

Gneissic to migmatitic granodiorite-tonalite

Homogeneous biotite granodiorite

Homogeneous hornblende granodiorite

Intermediate volcanic and volcaniclastic rocks

Leucogranodiorite, leucogranite

Impure marble, quartzite

Feldspathic psammite and derived diatexite

Mafic volcanic and volcaniclastic rocks

Granite pegmatite

Migmatitic calcic psammopelite

Migmatitic pelite/derived diatexite

Migmatitic psammite-(psammopelite)

Migmatitic psammopelite-(pelite)

Quartz diorite, diorite, and diorite gneiss

Quartz monzonite

Quartzite

Syn- to Post-tectonic Plutons

?Missi Group

Arc and ‘Successor’ Arc Plutons

?’Amisk Collage’

Sedimentary Rocks

Volcanic and Associated Rocks

Felsic volcanic, volcaniclastic, subvolcanic rocks

Legend cont’d

Pp

Psp

Ps

Mbl

Qz

Pc

As

Ar

Fv

Iv

Cm

Mv

Gdh

Gdb

Gd

Qdi

Ga

Qmz

P

Lgd

Ms

Geochronology sample

Fv

Iv

Mv

Gd

Pc

Mbl

Lgd

Iv

Psp

Gdb

Qz

Ar

Ar As

Qmz

Ms

GdGd

Gd

Ms

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Gdh

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Psp

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Ar Ar

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Psp

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Gd

Gd

HEARNE

ATHABASCABASIN

HEARNE

RAE

REINDEERZONE

Flin Flon Domain

Kakinagimak Lake area

SASK CRATON

RM0701-151

RM0701-068(300 m south of map boundary)

RM0701-064

RM0701-151

PN

FF

follow Roddick (1987). ID-TIMS isotopic data are presented in Table 1. Only the plagioclase-phyric intermediate rock was analyzed.

Tabl

e 1

- TIM

S U

-Pb

geoc

hron

olog

ical

resu

lts.

Prior to SHRIMP (sensitive high resolution ion microprobe) analysis, the internal features of the zircon grains (zoning, structures, alteration, etc.) were characterized with backscattered electrons (BSE) utilizing a Zeiss Evo scanning electron microscope. SHRIMP analytical procedure and U-Pb calibration details are given by Stern (1997) and Stern and Amelin (2003). The analytical work presented here was collected over three sessions on three separate ion probe epoxy mounts under varying instrumental conditions. Specific analytical details for each sample are given in the footnotes of the data table. In all analytical sessions, an O-primary beam was used with strength ranging from 3 to 7 nA. The count rates for ten isotopes of Zr+, U+, Th+, and Pb+ in zircon were sequentially measured over six scans with a single electron multiplier. The 1σ external errors of 206Pb/238U ratios reported in the data table (Table 2) incorporate an error of 1.0% in calibrating the standard zircon (Stern and Amelin, 2003). No fractionation correction was applied to the Pb-isotope data; common Pb correction utilized the Pb composition of the surface blank (Stern, 1997). Isoplot v. 3.66 (Ludwig, 2003) was used to generate concordia plots and calculate weighted means. All ages quoted in the text are given at the 95% confidence level. Isotopic ratios in Tables 1 and 2 (both ID-TIMS and SHRIMP) are given at 1σ uncertainty, as are SHRIMP ages. However, ID-TIMS ages are reported in the table with 2σ uncertainties.

3. Results

a) Dacitic Feldspar Porphyry, Sample RM0701-068 (Geological Survey of Canada (GSC) lab #z9364)

A sample of fine- to medium-grained, homogeneous dacitic feldspar porphyry (Figure 2a) was collected from a massive, 2 km-long, 1 km-wide unit from File Bay, located just south of the area mapped in 2008 on the map sheet of Ashton and Leclair (1991). The rock is interpreted as a subvolcanic intrusive into the surrounding volcanic rocks (unit Fv, Maxeiner, 2007a). A similar lithology from the same unit, interpreted as a synvolcanic tonalite and sampled less than one kilometre away (Ashton and Leclair, 1991), provided two multigrain TIMS fractions yielding discordant 207Pb/206Pb ages of 1835 Ma (3.4% discordant) and 1864 Ma (4.4% discordant) (Heaman and Ashton, 1996). The older of these two ages was interpreted to be the minimum age of tonalite crystallization (ibid.).

Zircons recovered from the dacitic feldspar porphyry are predominantly large, elongate to stubby prisms, with a subordinate number of slightly faceted to rounded more equigranular grains. Roughly half are dark brown in colour with the remainder being either pale brown or clear (Figure 3A inset). Inclusions, as


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Table 2 - SHRIMP U-Pb geochronological results. U Th Th Pb* 204Pb 204Pb ± 204Pb 208*Pb ± 208Pb 207*Pb ±207Pb 206*Pb ±206Pb Corr. 207*Pb ±207Pb 206Pb ±206Pb 207Pb ±207Pb Disc.

Spot name (ppm) (ppm) U (ppm) (ppb) 206Pb 206Pb f(206)204 206*Pb 206Pb 235U 235U 238U 238U Coeff. 206*Pb 206Pb 238U 238U 206Pb 206Pb (%)

a) Dacitic feldspar porphyry, RM0701-068 (GSC lab number z9364; UTM 669833 m E, 6106737 m N, NAD 83, Zone 13)9364-7.1 186 46 0.26 62 4 0.000084 0.000045 0.00146 0.0740 0.0025 5.103 0.103 0.3278 0.0049 0.8085 0.1129 0.0014 1827.48 23.58 1847 22 1.19364-12.1 87 15 0.18 27 4 0.000166 0.000212 0.00288 0.0467 0.0091 4.984 0.202 0.3173 0.0058 0.5568 0.1139 0.0039 1776.59 28.47 1863 62 4.69364-13.1 221 41 0.19 74 6 0.000093 0.000053 0.00161 0.0537 0.0025 5.137 0.100 0.3360 0.0043 0.7319 0.1109 0.0015 1867.29 20.52 1814 25 -2.99364-20.1 334 73 0.23 116 5 0.000046 0.000025 0.00080 0.0619 0.0017 5.319 0.859 0.3430 0.0525 0.9757 0.1125 0.0040 1900.99 257.12 1840 66 -3.39364-21.1 280 42 0.15 90 20 0.000253 0.000089 0.00438 0.0460 0.0042 4.981 0.109 0.3242 0.0043 0.6944 0.1114 0.0018 1810.27 20.94 1823 29 0.79364-22.1 2747 84 0.03 895 2 0.000002 0.000004 0.00004 0.0096 0.0002 5.136 0.064 0.3386 0.0038 0.9449 0.1100 0.0005 1879.95 18.28 1800 7 -4.59364-22.1.2 2849 88 0.03 867 2 0.000003 0.000007 0.00005 0.0091 0.0003 4.816 0.063 0.3164 0.0036 0.9252 0.1104 0.0006 1772.13 17.76 1806 9 1.99364-25.1 264 51 0.20 87 1 0.000013 0.000041 0.00023 0.0584 0.0022 5.122 0.101 0.3275 0.0049 0.8354 0.1134 0.0012 1826.16 23.99 1855 20 1.69364-26.1 254 50 0.20 84 8 0.000118 0.000084 0.00204 0.0575 0.0034 4.955 0.106 0.3303 0.0044 0.7097 0.1088 0.0017 1839.82 21.23 1779 28 -3.49364-31.1 414 94 0.23 137 2 0.000020 0.000016 0.00035 0.0651 0.0014 5.088 0.079 0.3263 0.0042 0.8853 0.1131 0.0008 1820.19 20.48 1850 13 1.69364-34.1 416 88 0.22 138 3 0.000025 0.000016 0.00043 0.0643 0.0014 5.085 0.063 0.3278 0.0036 0.9288 0.1125 0.0005 1827.84 17.32 1840 8 0.79364-39.1 2961 104 0.04 936 8 0.000009 0.000003 0.00016 0.0103 0.0002 5.000 0.065 0.3279 0.0037 0.9146 0.1106 0.0006 1828.26 17.8 1809 10 -1.19364-41.1 657 166 0.26 220 9 0.000047 0.000020 0.00081 0.0763 0.0017 5.098 0.065 0.3279 0.0034 0.8814 0.1128 0.0007 1828.12 16.7 1845 11 0.99364-45.1 339 62 0.19 113 8 0.000082 0.000032 0.00142 0.0555 0.0016 5.163 0.077 0.3313 0.0036 0.8143 0.1130 0.0010 1844.65 17.67 1849 16 0.29364-47.1 74 25 0.35 25 1 0.000055 0.000078 0.00095 0.1056 0.0043 5.115 0.111 0.3235 0.0046 0.7341 0.1147 0.0017 1806.68 22.24 1875 27 3.69364-48.1 217 37 0.17 70 3 0.000054 0.000072 0.00094 0.0499 0.0030 5.056 0.090 0.3197 0.0038 0.7525 0.1147 0.0014 1788.21 18.63 1875 21 4.69364-52.1 278 57 0.21 92 7 0.000092 0.000041 0.00160 0.0594 0.0021 5.112 0.086 0.3273 0.0042 0.8364 0.1133 0.0011 1825.24 20.53 1853 17 1.59364-53.1 218 41 0.19 70 3 0.000056 0.000074 0.00097 0.0534 0.0035 5.002 0.102 0.3214 0.0044 0.7544 0.1129 0.0015 1796.31 21.59 1847 25 2.79364-56.1 209 38 0.19 68 8 0.000132 0.000089 0.00229 0.0538 0.0045 4.977 0.105 0.3247 0.0041 0.6883 0.1112 0.0017 1812.66 19.89 1818 28 0.39364-59.1 338 76 0.23 112 1 0.000008 0.000025 0.00014 0.0671 0.0016 5.152 0.153 0.3248 0.0091 0.9739 0.1151 0.0008 1812.94 44.42 1881 12 3.69364-59.4 297 67 0.23 90 1 0.000014 0.000018 0.00025 0.0688 0.0033 4.710 0.132 0.2992 0.0076 0.9423 0.1142 0.0011 1687.56 37.69 1866 17 9.69364-60.1 2509 78 0.03 780 7 0.000010 0.000004 0.00018 0.0102 0.0002 4.796 0.066 0.3237 0.0043 0.9828 0.1074 0.0003 1807.95 20.92 1756 5 -2.99364-64.1 115 24 0.21 38 4 0.000114 0.000107 0.00198 0.0583 0.0045 5.001 0.115 0.3288 0.0042 0.6535 0.1103 0.0019 1832.61 20.42 1804 32 -1.69364-65.1 2569 27 0.01 818 9 0.000013 0.000007 0.00022 0.0028 0.0003 5.093 0.063 0.3327 0.0036 0.9104 0.1110 0.0006 1851.49 17.18 1816 9 -1.99364-65.2 3316 23 0.01 1159 25 0.000024 0.000005 0.00042 0.0024 0.0003 5.331 0.146 0.3670 0.0097 0.987 0.1054 0.0005 2015.2 45.93 1720 8 -17.19364-67.1 227 60 0.27 76 5 0.000077 0.000046 0.00134 0.0768 0.0026 5.100 0.093 0.3273 0.0043 0.7938 0.1130 0.0013 1825.23 20.83 1848 20 1.39364-72.1 1940 13 0.01 621 6 0.000012 0.000005 0.00020 0.0019 0.0002 5.131 0.059 0.3346 0.0036 0.9577 0.1112 0.0004 1860.73 17.16 1819 6 -2.39364-77.1 149 47 0.33 51 3 0.000062 0.000044 0.00107 0.0956 0.0034 5.149 0.104 0.3254 0.0037 0.6557 0.1148 0.0018 1815.85 17.98 1876 28 3.29364-78.1 197 37 0.19 65 4 0.000078 0.000040 0.00136 0.0573 0.0022 5.013 0.094 0.3284 0.0041 0.7476 0.1107 0.0014 1830.42 19.84 1811 23 -1.19364-80.1 212 43 0.21 69 5 0.000090 0.000039 0.00156 0.0638 0.0022 4.970 0.083 0.3196 0.0040 0.8172 0.1128 0.0011 1787.91 19.43 1845 18 3.19364-82.1 2718 65 0.02 898 43 0.000054 0.000007 0.00094 0.0063 0.0003 5.294 0.060 0.3437 0.0037 0.9719 0.1117 0.0003 1904.58 17.66 1827 5 -4.29364-82.1.2 2699 62 0.02 874 42 0.000055 0.000008 0.00094 0.0076 0.0004 5.202 0.061 0.3364 0.0036 0.9452 0.1122 0.0004 1869.28 17.28 1835 7 -1.99364-86.1 2672 17 0.01 864 19 0.000024 0.000005 0.00042 0.0018 0.0002 5.245 0.059 0.3377 0.0036 0.9695 0.1126 0.0003 1875.72 17.29 1842 5 -1.89364-86.1.2 2624 17 0.01 813 13 0.000017 0.000007 0.00030 0.0019 0.0003 4.958 0.062 0.3237 0.0037 0.9442 0.1111 0.0005 1807.8 17.93 1817 8 0.59364-88.1 331 67 0.21 109 6 0.000063 0.000051 0.00109 0.0617 0.0022 5.065 0.083 0.3251 0.0040 0.8269 0.1130 0.0011 1814.44 19.54 1848 17 1.89364-95.1 193 37 0.20 63 1 0.000012 0.000057 0.00020 0.0616 0.0040 5.148 0.123 0.3243 0.0055 0.7869 0.1152 0.0017 1810.44 26.7 1882 27 3.89364-95.2 1822 37 0.02 581 7 0.000014 0.000010 0.00023 0.0057 0.0004 5.076 0.084 0.3322 0.0044 0.8549 0.1108 0.0010 1848.87 21.07 1813 16 -29364-95.3 186 35 0.19 63 2 0.000040 0.000046 0.00069 0.0552 0.0024 5.206 0.116 0.3340 0.0054 0.8026 0.1130 0.0015 1857.9 26.22 1849 24 -0.59364-95.3.2 156 28 0.19 50 1 0.000026 0.000035 0.00045 0.0528 0.0028 5.027 0.127 0.3239 0.0061 0.8184 0.1126 0.0017 1808.65 29.88 1841 27 1.89364-95.4 321 54 0.17 104 6 0.000072 0.000046 0.00124 0.0501 0.0021 4.995 0.093 0.3249 0.0039 0.7302 0.1115 0.0014 1813.35 18.96 1824 23 0.69364-97.1 221 41 0.19 72 7 0.000116 0.000070 0.00201 0.0529 0.0031 5.025 0.109 0.3258 0.0045 0.7289 0.1118 0.0017 1818.15 22.03 1830 27 0.69364-114.1 237 44 0.19 77 5 0.000073 0.000073 0.00126 0.0538 0.0031 5.074 0.100 0.3240 0.0042 0.7392 0.1136 0.0015 1809.25 20.32 1857 24 2.69364-115.1 249 50 0.21 83 0 0.000000 0.000026 0.00001 0.0663 0.0033 5.071 0.074 0.3276 0.0037 0.846 0.1123 0.0009 1826.76 18.01 1836 14 0.5Analytical session November 2007, mount IP452, 23 µm spot size

b) Plagioclase-phyric intermediate rock, RM0701-064 (GSC lab number z9432; UTM 672457 m E, 6109635 m N, NAD 83, Zone 13)9432-1.1 41 0 0.01 13 0 0.000010 0.000010 0.00017 0.0055 0.0016 4.908 0.153 0.3240 0.0046 0.5625 0.1099 0.0029 1809 23 1797 48 -0.79432-2.1 48 0 0.01 15 0 0.000010 0.000010 0.00017 0.0029 0.0011 4.898 0.101 0.3174 0.0041 0.7061 0.1119 0.0017 1777 20 1831 27 2.99432-3.1 49 5 0.11 16 0 0.000010 0.000010 0.00017 0.0353 0.0033 4.973 0.125 0.3229 0.0044 0.6428 0.1117 0.0022 1804 22 1827 36 1.39432-4.1 966 49 0.05 300 1 0.000004 0.000006 0.00006 0.0136 0.0005 4.897 0.065 0.3215 0.0034 0.8633 0.1105 0.0007 1797 17 1807 12 0.69432-6.1 1035 4 0.00 315 3 0.000010 0.000010 0.00017 0.0004 0.0004 4.859 0.057 0.3188 0.0034 0.9447 0.1106 0.0004 1784 17 1808 7 1.49432-8.1 826 58 0.07 257 2 0.000010 0.000010 0.00017 0.0205 0.0007 4.847 0.057 0.3204 0.0034 0.9425 0.1097 0.0004 1792 17 1795 7 0.29432-10.1 516 9 0.02 153 12 0.000088 0.000024 0.00153 0.0056 0.0014 4.718 0.062 0.3101 0.0034 0.8947 0.1103 0.0007 1741 17 1805 11 3.59432-14.1 28 1 0.03 9 0 0.000010 0.000010 0.00017 0.0142 0.0046 4.962 0.191 0.3219 0.0070 0.6584 0.1118 0.0033 1799 34 1829 54 1.79432-15.1 1304 12 0.01 401 1 0.000004 0.000006 0.00007 0.0030 0.0004 4.890 0.060 0.3215 0.0035 0.9295 0.1103 0.0005 1797 17 1805 8 0.49432-15.1 36 3 0.08 11 0 0.000010 0.000010 0.00017 0.0315 0.0036 5.013 0.111 0.3178 0.0049 0.7775 0.1144 0.0016 1779 24 1871 26 4.99432-17.1 45 0 0.01 14 1 0.000095 0.000173 0.00164 0.0036 0.0070 5.023 0.170 0.3198 0.0054 0.5965 0.1139 0.0031 1789 26 1863 50 49432-19.1 1156 8 0.01 357 0 0.000001 0.000007 0.00001 0.0021 0.0003 4.918 0.060 0.3231 0.0036 0.9471 0.1104 0.0004 1805 18 1806 7 0.19432-19.1 48 4 0.08 14 2 0.000121 0.000164 0.00210 0.0236 0.0068 4.660 0.146 0.3052 0.0042 0.5421 0.1107 0.0029 1717 21 1811 49 5.29432-21.1 39 0 0.01 12 0 0.000010 0.000010 0.00017 0.0052 0.0016 5.052 0.160 0.3224 0.0054 0.6293 0.1137 0.0028 1801 26 1859 45 3.19432-25.1 33 4 0.12 10 0 0.000022 0.000128 0.00039 0.0438 0.0095 4.950 0.148 0.3201 0.0053 0.6477 0.1122 0.0026 1790 26 1835 42 2.49432-26.1 37 0 0.01 11 0 0.000010 0.000010 0.00017 0.0042 0.0015 4.874 0.189 0.3222 0.0054 0.5391 0.1097 0.0036 1800 26 1795 61 -0.39432-28.1 943 152 0.17 301 1 0.000003 0.000008 0.00006 0.0476 0.0019 4.866 0.063 0.3200 0.0037 0.9301 0.1103 0.0005 1790 18 1804 9 0.89432-29.1 150 7 0.05 47 0 0.000010 0.000010 0.00017 0.0147 0.0012 5.000 0.085 0.3220 0.0036 0.7425 0.1126 0.0013 1799 18 1842 21 2.39432-30.1 538 16 0.03 161 1 0.000006 0.000009 0.00010 0.0090 0.0006 4.752 0.066 0.3120 0.0039 0.9434 0.1105 0.0005 1750 19 1807 8 3.29432-30.1 54 0 0.01 16 5 0.000321 0.000100 0.00556 -0.0089 0.0040 4.768 0.143 0.3229 0.0045 0.5669 0.1071 0.0027 1804 22 1751 46 -3.1

Apparent Ages (Ma)


Table 2 - SHRIMP U-Pb geochronological results (cont'd). U Th Th Pb* 204Pb 204Pb ± 204Pb 208*Pb ± 208Pb 207*Pb ±207Pb 206*Pb ±206Pb Corr. 207*Pb ±207Pb 206Pb ±206Pb 207Pb ±207Pb Disc.

Spot name (ppm) (ppm) U (ppm) (ppb) 206Pb 206Pb f(206)204 206*Pb 206Pb 235U 235U 238U 238U Coeff. 206*Pb 206Pb 238U 238U 206Pb 206Pb (%)

b) Plagioclase-phyric intermediate rock, RM0701-064 (GSC lab number z9432; UTM 672457 m E, 6109635 m N, NAD 83, Zone 13) (cont'd)9432-32.1 355 6 0.02 107 1 0.000010 0.000010 0.00017 0.0039 0.0006 4.867 0.073 0.3153 0.0035 0.8183 0.1120 0.0010 1767 17 1831 16 3.59432-33.1 44 1 0.03 13 1 0.000115 0.000191 0.00200 0.0048 0.0073 4.964 0.201 0.3174 0.0053 0.5198 0.1134 0.0040 1777 26 1855 65 4.29432-34.1 59 4 0.07 19 0 0.000010 0.000010 0.00017 0.0252 0.0028 5.077 0.086 0.3210 0.0039 0.7925 0.1147 0.0012 1795 19 1875 19 4.39432-39.1 992 61 0.06 307 2 0.000006 0.000014 0.00011 0.0173 0.0012 4.860 0.060 0.3189 0.0036 0.9504 0.1105 0.0004 1784 18 1808 7 1.39432-42.1 433 8 0.02 129 1 0.000008 0.000025 0.00014 0.0053 0.0011 4.754 0.066 0.3093 0.0037 0.913 0.1115 0.0006 1737 18 1823 10 4.79432-42.1 49 1 0.01 15 0 0.000010 0.000010 0.00017 0.0060 0.0015 4.944 0.108 0.3214 0.0051 0.8045 0.1116 0.0015 1796 25 1825 24 1.69432-46.1 48 0 0.01 15 0 0.000010 0.000010 0.00017 0.0047 0.0019 4.975 0.093 0.3219 0.0041 0.7601 0.1121 0.0014 1799 20 1834 22 1.99432-47.1 947 81 0.09 296 3 0.000010 0.000010 0.00017 0.0240 0.0007 4.871 0.062 0.3203 0.0035 0.9172 0.1103 0.0006 1791 17 1804 9 0.79432-49.1 684 82 0.12 217 2 0.000010 0.000010 0.00017 0.0360 0.0010 4.896 0.062 0.3220 0.0036 0.9234 0.1103 0.0005 1800 18 1804 9 0.39432-51.1 1206 190 0.16 379 2 0.000006 0.000006 0.00010 0.0465 0.0009 4.828 0.061 0.3154 0.0037 0.9668 0.1110 0.0004 1767 18 1816 6 2.79432-57.1 836 23 0.03 253 7 0.000029 0.000010 0.00051 0.0073 0.0006 4.819 0.061 0.3150 0.0033 0.8886 0.1110 0.0007 1765 16 1815 11 2.79432-61.1 992 28 0.03 308 3 0.000010 0.000010 0.00017 0.0071 0.0005 4.932 0.064 0.3233 0.0034 0.8686 0.1106 0.0007 1806 17 1810 12 0.2Analytical session March 2008, mount IP467, 17 µm spot size

c) Anatectic leucogranite, RM0701-151 (GSC lab number z9433; UTM 673017 m E, 6116120 m N, NAD 83, Zone 13)9433-1.1 651 31 0.05 206 2 0.000013 0.000011 0.00023 0.0142 0.0006 5.181 0.065 0.3264 0.0035 0.9193 0.1151 0.0006 1821 17 1882 9 3.29433-2.1 186 84 0.46 72 6 0.000111 0.000030 0.00192 0.1279 0.0052 5.934 0.091 0.3573 0.0044 0.8603 0.1204 0.0010 1970 21 1963 14 -0.39433-4.1 139 45 0.34 50 1 0.000012 0.000036 0.00021 0.0971 0.0026 5.705 0.075 0.3426 0.0037 0.8759 0.1208 0.0008 1899 18 1968 12 3.59433-4.2 480 21 0.05 151 4 0.000031 0.000011 0.00054 0.0126 0.0009 5.086 0.065 0.3250 0.0037 0.9403 0.1135 0.0005 1814 18 1856 8 2.39433-5.1 22 3 0.15 8 5 0.000772 0.000223 0.01337 0.0322 0.0094 5.779 0.252 0.3574 0.0073 0.5705 0.1173 0.0042 1970 35 1915 66 -2.89433-6.1 27 3 0.12 9 0 0.000044 0.000356 0.00076 0.0445 0.0155 5.883 0.317 0.3394 0.0068 0.4825 0.1257 0.0060 1884 33 2039 87 7.69433-7.1 217 13 0.06 69 0 0.000004 0.000030 0.00008 0.0208 0.0015 5.134 0.142 0.3261 0.0065 0.7976 0.1142 0.0019 1819 32 1867 31 2.69433-8.1 271 31 0.12 93 2 0.000019 0.000020 0.00034 0.0303 0.0017 5.683 0.080 0.3477 0.0037 0.8366 0.1186 0.0009 1923 18 1934 14 0.69433-9.1 118 53 0.46 43 6 0.000180 0.000081 0.00312 0.1271 0.0051 5.534 0.106 0.3421 0.0043 0.745 0.1173 0.0015 1897 21 1916 23 19433-13.1 93 35 0.39 33 3 0.000123 0.000105 0.00214 0.1101 0.0089 5.211 0.128 0.3342 0.0051 0.7151 0.1131 0.0020 1859 25 1850 32 -0.59433-14.1 77 20 0.26 28 0 0.000010 0.000010 0.00017 0.0857 0.0029 5.760 0.097 0.3450 0.0046 0.8532 0.1211 0.0011 1910 22 1972 16 3.19433-21.1 1047 93 0.09 345 5 0.000017 0.000007 0.00029 0.0251 0.0008 5.306 0.059 0.3364 0.0035 0.9719 0.1144 0.0003 1869 17 1871 5 0.19433-21.2 47 21 0.46 17 5 0.000339 0.000106 0.00587 0.1288 0.0063 5.534 0.133 0.3408 0.0051 0.7109 0.1178 0.0020 1890 25 1923 31 1.79433-21.3 414 20 0.05 128 4 0.000039 0.000026 0.00067 0.0129 0.0011 5.013 0.079 0.3198 0.0043 0.9056 0.1137 0.0008 1789 21 1859 12 3.89433-23.1 1001 55 0.06 324 10 0.000035 0.000008 0.00060 0.0152 0.0005 5.256 0.061 0.3333 0.0036 0.9563 0.1144 0.0004 1854 17 1870 6 0.99433-23.2 140 12 0.09 46 1 0.000024 0.000022 0.00042 0.0258 0.0018 5.609 0.073 0.3354 0.0037 0.8986 0.1213 0.0007 1865 18 1975 10 5.69433-25.1 1280 64 0.05 402 41 0.000115 0.000020 0.00199 0.0142 0.0010 4.945 0.059 0.3247 0.0034 0.9117 0.1105 0.0006 1813 16 1807 9 -0.39433-27.1 94 9 0.10 32 3 0.000116 0.000106 0.00202 0.0250 0.0049 5.516 0.126 0.3422 0.0051 0.7356 0.1169 0.0018 1897 25 1909 28 0.69433-30.1 1118 60 0.06 354 2 0.000007 0.000004 0.00012 0.0157 0.0004 4.982 0.059 0.3273 0.0034 0.9179 0.1104 0.0005 1825 16 1806 9 -1.19433-35.1 87 21 0.25 31 3 0.000132 0.000088 0.00228 0.0727 0.0041 5.763 0.119 0.3487 0.0046 0.724 0.1199 0.0017 1928 22 1954 26 1.39433-39.1 928 55 0.06 300 0 0.000000 0.000009 0.00001 0.0174 0.0007 5.214 0.063 0.3320 0.0036 0.9458 0.1139 0.0005 1848 18 1863 7 0.89433-45.1 27 7 0.29 10 1 0.000147 0.000227 0.00255 0.0850 0.0106 6.023 0.222 0.3529 0.0053 0.5182 0.1238 0.0039 1948 26 2012 58 3.19433-46.1 1349 67 0.05 421 2 0.000006 0.000003 0.00011 0.0143 0.0003 4.911 0.053 0.3226 0.0033 0.9794 0.1104 0.0002 1802 16 1806 4 0.29433-49.1 96 27 0.29 34 2 0.000068 0.000068 0.00118 0.0784 0.0060 5.781 0.104 0.3479 0.0046 0.8049 0.1205 0.0013 1925 22 1964 19 29433-51.1 1444 28 0.02 454 2 0.000004 0.000004 0.00007 0.0053 0.0004 5.047 0.068 0.3276 0.0038 0.9188 0.1117 0.0006 1827 19 1828 10 0.19433-51.2 1051 53 0.05 341 4 0.000013 0.000009 0.00022 0.0149 0.0005 5.081 0.062 0.3353 0.0037 0.9542 0.1099 0.0004 1864 18 1798 7 -3.79433-53.1 18 4 0.24 7 1 0.000219 0.000337 0.00380 0.0747 0.0146 6.109 0.314 0.3525 0.0068 0.4869 0.1257 0.0057 1947 33 2038 82 4.59433-55.1 49 7 0.15 17 2 0.000109 0.000089 0.00188 0.0465 0.0072 5.686 0.121 0.3431 0.0051 0.7697 0.1202 0.0017 1902 24 1959 25 2.99433-58.1 58 4 0.06 19 5 0.000297 0.000109 0.00514 0.0144 0.0052 5.221 0.119 0.3287 0.0043 0.6721 0.1152 0.0020 1832 21 1883 31 2.79433-59.1 24 3 0.13 8 3 0.000357 0.000212 0.00618 0.0357 0.0104 5.593 0.218 0.3486 0.0066 0.5905 0.1164 0.0037 1928 32 1901 58 -1.49433-60.1 46 3 0.07 16 1 0.000078 0.000180 0.00135 0.0281 0.0072 5.741 0.197 0.3409 0.0056 0.5789 0.1221 0.0035 1891 27 1988 51 4.99433-80.1 926 60 0.07 292 5 0.000018 0.000005 0.00031 0.0185 0.0004 4.930 0.068 0.3247 0.0034 0.8337 0.1101 0.0009 1813 17 1801 14 -0.69433-91.1 38 3 0.07 13 2 0.000149 0.000170 0.00259 0.0215 0.0067 5.605 0.163 0.3396 0.0047 0.5772 0.1197 0.0029 1885 23 1952 43 3.49433-96.1 26 5 0.19 9 5 0.000611 0.000218 0.01059 0.0509 0.0105 5.235 0.214 0.3387 0.0064 0.5697 0.1121 0.0038 1880 31 1834 63 -2.5Analytical session February 2008, mount IP462, 23 µm spot size

Notes (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.zUncertainties reported at 1σ (absolute) and are calculated by numerical propagation of all known sources of error.f206204 refers to mole fraction 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).

* refers to radiogenic Pb (corrected for common Pb).Discordance relative to origin = 100 * (1-(207Pb/206Pb age-206Pb/238U age)/(207Pb/206Pb age)).Calibration standard 6266; U=910 ppm; Age= 559 Ma; 206Pb/238U=0.09059.Error in 206Pb/238U calibration 1.0%.Th/U calibration: F= 0.03900*UO+ 0.85600.

Apparent Ages (Ma)


well as zoning, fractures, and some alteration features, are common in both prismatic and equigranular grains. Evidence for core/overgrowth relationships is common. Forty-three SHRIMP analyses were carried out on thirty-three separate zircon grains; analytical results are displayed in Table 2 and in Figure 3A. The determined 207Pb/206Pb ages range from 1720 Ma to 1882 Ma, and the dataset can be subdivided into two groups based on U concentrations and Th/U ratios (Table 2). The unfilled ellipses on the concordia diagram (Figure 3A) represent low-U zircon grains ranging from U concentrations of 74 ppm to 657 ppm and with Th/U ratios of 0.15 to 0.35. The weighted mean of the 207Pb/206Pb ages for this group is 1846 ±7 Ma (n=31, MSWD=1.08) and is interpreted as the time of crystallization of the porphyry. The TIMS results of Heaman and Ashton (1996) indicate a component of zircon with a minimum age of 1864 Ma; but such inheritance is not documented in this new sample. High-U (1822 to 3316 ppm), low-Th/U (0.01 to 0.04) zircon, plotted in grey in Figure 3A, range in age from 1842 to 1720, with the most prominent mode between 1810 and 1820 Ma. This subset of zircon, typically observed as overgrowths, does not yield a mean age that is consistent with any single age population (MSWD=29) and, in some cases, replicate analyses of the same zone of an individual zircon do not yield reproducible results. These zircon grains likely underwent significant isotopic disturbance as a result of interaction with metamorphic fluids and/or radioactive decay damage, the timing of which is unconstrained. The low-Th/U ratios of these zircon grains are consistent with a metamorphic origin (Rubatto, 2002; Williams and Claesson, 1987).

Figure 2 - Outcrop photographs of geochronological sample locations: A) sample RM0701-068 (GSC lab #z9364), dacitic feldspar porphyry, File Bay; B) sample RM0701-064 (GSC lab #z9432), plagioclase-phyric intermediate rock, Gifford Bay; and C) sample RM0701-151 (GSC lab #z9433), leucogranite, central Kakinagimak Lake.

b) Plagioclase-phyric Intermediate Rock, Sample RM0701-064 (GSC lab #z9432)

In the Gifford Bay area, a strongly transposed, layered succession of intermediate to felsic volcanic rocks contains localized synvolcanic alteration and accumulation of sulphides (Maxeiner, 2007b). Other than compositional layering, primary volcanic features are scarce within the Gifford Bay succession. Granodiorites, interpreted to be part of a suite of 1.86 Ga successor arc plutonic rocks (Ashton et al., 2005), cut the volcanic succession north of Gifford Bay. The outcrop targeted for sample collection is from a unit of fine-grained intermediate rocks (Figure 1, unit Iv; Maxeiner, 2007a), which is interpreted as volcanic in origin and therefore speculated to be >1.86 Ga. The sample is from a 20 to 30 cm-thick, conformable feldspar-phyric layer (Figure 2B). The rock is characterized by strongly lineated, 1 to 2 cm long, recrystallized plagioclase phenocrysts in a fine-grained matrix and contains abundant plagioclase, about 20 to 30% hornblende, minor carbonate, and sulphide.

Zircon grains recovered from this sample belong to two very high quality morphological groups: clear, colourless, simple, well-faceted prisms (Z1) and subordinate clear, colourless, multi-faceted to resorbed ovoid grains (Z2). Three fractions of each morphology were submitted for ID-TIMS analyses (Table 1) and


isotope ratios are plotted in Figure 3B, inset. The determined 207Pb/206Pb ages range from 1821 to 1832 Ma and are all slightly discordant (0.5 to 0.7%). Although there is no clear relationship between age and morphology, the two analyses yielding the oldest ages belong to the ovoid Z2 group and are analytically indistinguishable. An 1831 Ma weighted mean 207Pb/206Pb age for these two fractions is interpreted to represent a minimum age constraint for this porphyritic intermediate rock.

Subsequent ion probe investigations were undertaken to constrain the significance of the younger 207Pb/206Pb TIMS dates. SEM imaging indicates more complex zircon morphologies than that observed under transmitted light. High-U overgrowths surround unzoned, low-U cores on most grains. The rims are as thin as 1 to 2 µm, but can be up to 30 µm wide. In addition, high-U material developed as distinct grains. Both high-U and low-U zircon grains are characterized by broadly similar, low-Th/U ratios. The weighted mean 207Pb/206Pb age of sixteen analyses of the high-U zircon (grey ellipses of Figure 3B) is 1808 ±4 Ma, which is interpreted to represent new zircon growth during a period of high-grade metamorphism. SHRIMP results from the low-U cores (unfilled ellipses of Figure 3B) yielded a relatively imprecise age of 1841 ±15 Ma (MSWD=0.76). This result agrees (within error) with the minimum age determined by TIMS and likely represents the timing of crystallization. As a result of obtaining these relatively young U/Pb zircon crystallization ages, the feldspar-phyric layer more likely represents a younger sill within the older Gifford Bay volcanic succession.

c) Anatectic Leucogranite, Sample RM0701-151 (GSC lab #z9433)

A sample of anatectic leucogranite (unit Lgd of Maxeiner, 2007a) was collected from the east shore of central Kakinagimak Lake (Figure 1). This medium-grained, homogeneous, biotite-bearing rock (Figure 2C) forms part of a unit of leucocratic rocks compositionally variable from leucogranite to leucogranodiorite. Both on outcrop and map scale these rocks are seen to be associated with migmatitic granodiorite-tonalite gneisses, which were interpreted as the high-grade equivalents of 1.86 to 1.85 Ga successor arc plutons (e.g., Ashton et al., 2005; Maxeiner, 2007b). The leucogranite-leucogranodiorite was interpreted as variably mobilized sheets of partial melts derived predominantly from the granodiorite gneisses (Maxeiner, 2007b). Although generally massive, the leucogranite carries a weakly developed

Figure 3 - Concordia diagram illustrating U-Pb geochronological results. All error ellipses represent 2σ uncertainty. See text for discussion. A) inset: transmitted light image of representative zircon grains from sample RM0701-068; B) inset: TIMS results from RM0701-064; C) inset: BSE images of complex zircon with high-U overgrowths and low U cores. Numbers in white indicate the grain number, numbers in black correspond to the analysis spot number, see Table 2 for corresponding results.


northeast-dipping planar fabric from which it was inferred that the sheets were emplaced late during the D2 deformational event.

Zircon grains recovered from this sample are highly variable in quality and morphology. Most are prismatic and range from clear and colourless to brown and turbid, many with orange iron staining. Overgrowths are commonly observed under transmitted light. SEM images confirm the complex nature of these zircon grains. Cores typically have poor BSE response (low U) and may be zoned or unzoned, rounded and resorbed, or prismatic (Figure 3C, inset). Rims are bright in BSE images indicating high-U content; they are typically unzoned to faintly zoned (Figure 3C, inset). U-Pb results from the ion probe indicate a minimum of three age populations. One group of five overgrowths with U concentrations between 925 to 1450 ppm and Th/U ratios between 0.02 to 0.07 yield a weighted mean 207Pb/206Pb age of 1807 ±8 Ma (n=6, includes one replicate analysis MSWD=1.3). These high-U zircon grains are plotted as dark grey ellipses in Figure 3C. This age is consistent with the timing of metamorphism, as determined from sample RM0701-064 and in previous studies (Ashton et al., 1992; Heaman et al., 1992), and is interpreted as the time of emplacement of the anatectic melt sheets. A second generation of six overgrowths, plotted as light grey ellipses in Figure 3C, is indistinguishable in appearance from the first, also has elevated U concentrations (410 to 1050 ppm), low Th/U (0.05 to 0.09), and yields a weighted mean 207Pb/206Pb age of 1868 ±6 Ma (n=7, MSWD=1.01). Both types of overgrowths have not been documented in a single zircon grain; core/overgrowth relationships only record one population of high-U zircon or the other. The 1868 Ma dates may represent an inherited component, possibly sourced from the host granodiorite gneisses or may reflect an earlier metamorphic event related to formation of the Flin Flon–Glennie Complex. Considering both age groups (1807 and 1868 Ma) only occur as overgrowths and are morphologically and chemically indistinguishable it seems likely that both were formed by a similar process. Consequently an early metamorphic origin for the 1868 Ma zircon rims is the preferred interpretation. Nineteen analyses of low-U, high-Th/U cores (unfilled ellipses of Figure 3C) give ages that range from 1883 to 2039 Ma. The low-U content and consequent low precision of these analyses results in a single statistical population with a weighted mean 207Pb/206Pb age of 1957 ±12 Ma (MSWD=1.4). Additional, more precise geochronology of single low-U zircon cores would be necessary to confirm the existence of a unique ca. 1960 Ma inherited component. At this stage, the origin remains uncertain; the low-U zircon indicate the presence of older previously unrecognized volcanoplutonic rocks or a sedimentary source with a diverse older Paleoproterozoic detrital population.

4. Conclusions Two subvolcanic units from the Kakinagimak Lake area, northwest of the Flin Flon Domain, yield crystallization ages of 1841 ±15 Ma and 1846 ±7 Ma, consistent with known successor arc volcanic ages (Stern et al. 1999). Older (ca. 1.9 Ga) crystallization ages, typical of the eastern Flin Flon Domain (ibid.) have not been documented. The identification of exclusively younger crystallization ages might be an artefact of a sampling bias towards younger, less deformed rocks. Assuming that these subvolcanic plutons belong to a successor arc suite, the older host rocks may have experienced an earlier episode of high-grade metamorphism, resulting in more intense recrystallization and transposition, making them less attractive as sampling targets.

Metamorphic U-Pb zircon ages from an intermediate intrusive sill, together with generation of an anatectic leucogranodiorite sheet, indicate a strong thermal overprint at about 1807 ±8 Ma and 1808 ±4 Ma consistent with previously reported metamorphic ages for the region (Syme et al., 1998). Older ages, ranging from 1883 to 2039 Ma and giving a weighted mean of 1.96 Ga, are interpreted as inherited zircon in the leucogranodiorite and are of uncertain origin. An age of 1868 ±6 Ma, recorded by high-U overgrowths on these inherited cores, might record an earlier metamorphic event related to formation of the Flin Flon–Glennie Complex at ca. 1.87 Ga (ibid.) or alternatively and less preferred it may reflect crystallization of the widespread granodiorite host.

5. Acknowledgements The staff of the geochronology laboratories at the GSC are thanked for their superb efforts in the chemistry and mass spectrometry labs. Deborah Yang (University of Waterloo co-op student) is thanked for her efforts in the picking lab and assistance with drafting of the figures. The manuscript benefited from thoughtful reviews by Ryan Morelli, Otto van Breemen, and Ken Ashton. This is Natural Resources Canada/Earth Science Sector contribution number 20080441.

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Igneous and Metamorphic U-Pb Ages from Volcanic and ... · volcanism and plutonism, common in the eastern Flin Flon Domain. A sample of anatectic leucogranite contains inherited zircon

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