U – Pb geochronological studies in the southwestern Core Zone

Fig. 4. Geology of the southwestern core zone in the area around the Smallwood Reservoir, Labrador southern part of NTS map area 23I. Geochronological data summarized from James et al. 1996zr, zircon; ti, titanite; ig, igneous crystallization age; mt, metamorphic age. Ellipses show sample locations this study. Most samples were collected from island outcrops in the Smallwood Reservoir. ARSZ, Ashuanipi River shear zone; LTSZ, Lac Tudor shear zone; GRSZ, George River shear zone.

4. U – Pb geochronological studies in the southwestern Core Zone

4 . 1 . McKenzie Ri6er domain The McKenzie River domain Fig. 4 is sepa- rated from the NQO and the Crossroads domain by the Paleoproterozoic Ashuanipi River and Lac Tudor shear zones, respectively. Both shear zones are inferred to be transpressive dextral mylonite zones having components of east-over-west re- verse displacement. Field relations demonstrate they were developed concomitant with amphibo- lite – facies metamorphism. The Ashuanipi River shear zone corresponds approximately to the infl- ection between a regionally persistent, paired Bouguer gravity anomaly; negative on the NQO side and positive on the Core Zone side see James et al., 1996, Fig. 2, page 218. The domain is dominated by the Flat Point gneiss, which has an emplacement age of 2776 9 5 Ma as determined by U – Pb dating of zircon James et al., 1996. o Nd values from the Flat Point gneiss from − 0.5 to − 5, at 2800 Ma Kerr et al., 1994, suggest the rocks incorporated a component of older crust. The rocks are meta- morphosed to upper-amphibolite facies and have a gneissosity which is folded into relatively open and flat-lying superposed, mainly dome-and- basin, folds. The age of the gneissosity and the folding are unknown, but are assumed to be Archean. The gneissosity and superposed folds are overprinted by a steep, north-striking and east-dipping foliation accompanied by a pervasive recrystallization that locally obliterates the gneis- sosity and preexisting structure. The north-strik- ing foliation is interpreted to be a Paleoproterozoic fabric for three reasons: 1 it has the same attitude as the foliation in ca. 1815 Ma tonalite see discussion of sample MR1 be- low, 2 it is defined by the peak-assemblage metamorphic minerals and amphibolite – facies metamorphism was attained by ca. 1805 Ma see discussion of sample MR2 below, and 3 it becomes progressively more intense in areas near the major Paleoproterozoic high-strain zones e.g. Lac Tudor shear zone, which define the domain boundaries. Metamorphosed supracrustal rocks of un- known depositional age occur as thin B 1 km tectonically bound units within the Flat Point gneiss. Informally named the Lobstick group, they include metasedimentary pelitic migmatite and lesser amounts of quartzite, marble, calcsili- cate derived from impure siliceous carbonate, and amphibolite of uncertain protolith. These supracrustal rocks may be correlative with litho- logically similar, Laporte group rocks, which oc- cur to the north of the study area. Geochronology samples were collected from an outcrop contain- ing migmatitic Lobstick group rocks, a tonalite dyke which cross cuts the metamorphic leuco- some, and meta-tonalite which is in tectonic con- tact with the Lobstick group rocks. The supracrustal rocks are steeply foliated and have steeply plunging folds of relict sedimentary bed- ding. They do not contain the superposed fold structures contained in the Flat Point gneiss. On the basis of orientation, the foliation in the supracrustal gneisses is correlated with the folia- tion in Paleoproterozoic tonalite see discussion of sample MR1 below. Sample MR1 is from a unit of strongly foliated and recrystallized tonalite that is in tectonic con- tact with Lobstick group supracrustal gneiss. MR1 tonalite differs significantly from the Flat Point gneiss in that it lacks metamorphic layering and the superposed fold structures that character- ize the latter. The sample yielded three fractions of concordant and nearly concordant zircons Fig. 5, Table 1, and the two overlapping concordant analyses give an age of 1815 9 3 Ma, interpreted to represent the igneous crystallization age. These data represent the first indication of Paleoprotero- zoic granitoid intrusive rocks in the McKenzie River domain, although the extent of ca. 1815 Ma tonalite is unknown. To determine the age of metamorphism, a sam- ple MR2 of K-feldspar + biotite + garnet-bear- ing leucosome contained in pelitic migmatite was collected. Two fractions of monazite Fig. 5 are concordant M2 and nearly concordant M1 and indicate that monazite crystallized in the leuco- some at 1805 9 3 Ma, and demonstrate that up- per-amphibolite facies metamorphism was attained by this time. The data only loosely con- Fig. 5. Schematic diagram of an outcrop in the McKenzie River domain showing field relationships and U – Pb concordia diagrams for samples MR1 to MR3. In the field, the distance between MR1 and MR3 sample locations is less than 50 m. D .T . James , G .R . Dunning Precambrian Research 103 2000 31 – 54 41 Table 1 U–Pb analytical results a Age Ma Corrected atomic ratios Concentration 206 Pb204Pb 208 Pb206 Pb 206 pb208 U 207 Pb235 U 207 Pb206 Pb 206 Pb238 U Pbc 207 Pb235 U Fraction description 207 Pb206 Pb Pbr U Weight mg pg ppm MR 1 meta-tonalite 366167 E, 60102040 N 37 Z1 3339 Large clear 0.2451 0.32534 124 4.9747 202 0.1109 14 1826 1815 1814 0.121 50 19.2 prisms 6490 0.2839 0.32295122 4.9282182 0.11067 20 1804 20.8 1807 54 1811 10 Z2 Clear, euhedral 0.06 Z3 12546 Large, euhedral 0.2439 0.32584 142 4.9908 208 0.11109 24 1818 1818 1803 0.083 86 32.8 12 MR 2 leucosome, msed migmatite 3861617 E, 601204 N 9633 39.246 0.32337 190 4.9287 296 0.11054 12 1806 1807 1808 M1 Large, clear 0.091 293 3317.3 56 10619 45.062 0.32294 132 4.9069 206 0.1102 14 1804 1803 1803 48 M2 Clear, yellow 0.1 252 3253.3 MR 3 tonalite dyke 386167 E, 6010204 N 8918 0.1183 0.32667 124 5.0751 200 0.11268 14 Z1 1822 Small, clear 1832 1843 0.021 512 177.1 24 3890 0.1461 0.32617 100 5.0618 168 0.11255 14 1820 38 1830 Z2 1841 215.6 610 0.012 Clear prisms 27 Clear, cracks 7645 0.1368 0.33024 110 5.2562 188 0.11544 12 1840 1862 1887 0.023 429 152.5 Z3 18072 0.1344 0.32499 112 5.0302 186 0.11226 12 1814 1824 1836 16 0.034 410 142.9 Clear prisms Z4 CR 1 tonalite orthogneiss 386602 E, 601415 N Large, cleaar, 41.4 7 17440 0.1387 0.47972 154 11.5395 400 0.17446 16 2526 2568 2601 0.054 Z1 76 euhedral 7 Z2 26173 Small, brown 0.1319 0.48777 150 11.9289 400 0.17737 16 2561 2599 2628 0.071 85 46.9 prisms 18 13576 0.1339 0.48608 164 11.8186 428 0.17634 16 2554 2590 82 2619 0.098 Large, clear Z3 45.1 euhedral CR 2 leucogranite 386602 E, 601451 N 2843 0.2202 0.31757 118 4.8755 188 Z1 0.11135 16 Brown prisms 1778 1798 1821 0.009 251 91.3 15 6 5907 0.2364 0.32289 162 4.9749 226 0.11174 30 1804 121.3 1815 Z2 1828 324 0.005 Small, brown prisms Small, brown 111 48 4209 0.2231 0.32588 94 5.1139 166 0.11381 10 1818 1838 1861 0.033 Z3 296 needles CR 3 diorite dyke 366002 E, 601415 N 12075 0.2525 0.32278 86 4.919 290 Best clear prisms 0.11053 10 0.034 1803 1806 1808 1833 694.2 Z1 105 471 174.3 23 18209 0.2262 0.32139 134 4.8995 214 0.11057 10 1797 1802 1809 Z2 0.045 Small, brown prisms 3867.3 218 4777 10.996 0.32161 152 4.8551 240 0.10949 10 1798 1795 1791 M1 0.045 Clear, pale yellow 1146 CR 4 pegmatite 386602 E, 6014151 N 24977 0.1889 0.33642 1556 5.4943 268 0.11845 10 0.192 1869 Z1 1900 1933 Large, pale pink 160 60.4 26 56.4 7 19898 0.1909 0.3244 96 4.9989 162 0.11176 12 1811 1819 1828 0.046 Z2 155 1 large prism 73 16946 39.429 0.32165 186 4.8867 286 0.10968 12 M1 1798 Large, dark 1797 1796 0.117 524 5926.6 yellow 18625 29.371 0.32222 108 4.873176 0.10968 12 1801 5786.6 1798 Clear, pale yellow 1794 57 M2 679 0.077 37 Clear, pale yellow 15446 21.575 0.32121 112 4.8485 180 0.10948 12 1796 1793 1791 0.048 595 3759.8 M3 D .T . James , G .R . Dunning Precambrian Research 103 2000 31 – 54 Table 1 Continued Age Ma Concentration Corrected atomic ratios Pbr Pbc 206 Pb204Pb 208 Pb206 Pb 206 pb208 U 207 Pb235 U 207 Pb206 Pb 206 Pb238 U 207 Pb235 U 207 Pb206 Pb Weight Fraction description U mg pg ppm 4640.4 167 5236 24.456 0.32083 138 4.8429 218 0.10948 10 1794 1792 1791 Clear M4 0.066 652 CR 5 spotted diorite duke 39138 E, 6014188 N 7363 0.2571 0.31986 120 4.8956 192 0.11101 12 0.038 1789 196 1802 1816 Clear, cracks Z1 73.9 20 8 Large, cracks 37935 0.3131 0.3206 112 4.9098 184 0.11107 12 1793 1804 1817 0.058 271 106.5 Z2 40810 0.3295 0.32262 110 4.9396 180 0.11105 10 1802 23 1809 277 1817 110.9 Z3 Prisms, cracked 0.168 12 Elongate prisms 8498 0.3297 0.32406 112 4.9618 180 0.11105 14 1810 1813 1817 0.023 224 90.2 Z4 3 Clear prisms 5190 0.2768 0.3253 130 4.9847 196 0.11114 20 1816 1817 1818 0.013 53 20.8 Z5 381 0.098 0.31651 104 4.7448 198 0.10872 20 1773 1191 1775 T1 1778 13.7 42 0.525 Clear brown 801 458 0.0951 0.31696 98 4.7592 184 0.1089 18 1775 1778 T2 1781 Clear, medium 0.349 51 16.8 brown 533 474 0.1191 0.31821 108 4.7822 200 0.109 22 57 1781 0.217 1782 1783 19 T3 Medium brown, prisms T4 341 15 large brown 355 0.0916 0.31717 104 4.7466 194 0.10854 22 1776 1776 1775 0.139 42 13.7 prisms CR 6 granite dyke 386167 E, 6010304 N 122.5 34 4420 0.0951 0.31967 116 4.8567 182 0.11019 16 1788 1795 1803 Z1 0.02 2 large grains 370 9459 0.1036 0.30827 118 4.6515 176 0.10944 18 1732 14 1759 Z2 1790 126.7 394 0.018 Small grain 4 32919 0.1246 0.32029 142 4.8659 222 0.11018 12 1791 1796 1802 Z3 Small, clear, 0.025 261 88.9 euhedral 28133 0.098 0.3165 136 4.8557 212 0.11017 14 1788 103.5 1795 4 1802 312 0.02 Clear, euhedral Z4 Z5 11193 3 large grains 0.2565 0.31813 186 4.8344 262 0.11021 30 1781 1791 1803 0.022 113 42.5 4 CR 7 De Pas monzogranite 389070 E, 6016195 N 15 Z1 7451 Sharp, elongate 0.1797 0.3225 114 4.9203 176 0.11065 18 1802 1806 1810 0.023 240 85.9 prisms 7724 0.1896 0.32281 156 4.925 228 0.11065 24 1803 87.3 1807 242 1810 11 Z2 Sharp, elongate 0.018 a The analytical methods followed by Dunning at Memorial University of Newfoundland are described in Dube´ et al. 1996 and references therein. UTM co-ordinates for each sample are shown in parentheses. All UTM co-ordinates are for Grid Zone 20, NAD 1927, and NTS map area 23I Woods Lake map sheet. Z-zircon. T-titanite. M-Monazite. Pbr is the total radiogenic lead after correction for blank, common lead and spike. Pbc represents the picograms of common lead in the analysis. Corrected atomic ratios are corrected for fractionation, spike, 4–10 pg laboratory blank, initial common lead calculated using the model of Stacey and Kramers 1975 for the age of the sample, and 1 pg uranium blank. Numbers in parentheses after the corrected atomic ratios refer to uncertainties of 2 s on the final digits of the isotope ratios. The uncertainties were calculated using an unpublished error propagation program, as reported in Dube´ et al. 1996. Fig. 6. Lobstick group supracrustal rocks including pelitic migmatite right, calc – silicate gneiss centre and quartzite left. The metasedimentary rocks are cut by a late syn-tectonic tonalite dyke folded. Sample MR3 was collected from a similar dyke. 45 probability of fit which yields a lower inter- cept age of 1802 + 9 − 14 Ma, interpreted to represent the igneous crystallization age of the dyke. The upper intercept is 2850 Ma with a large uncertainty. This brackets formation of the steeply east-dipping foliation in the host rocks to be between 1815 Ma, the emplacement age of the foliated tonalite MR1 and 1802 Ma. The field and geochronological data demon- strate that Archean Flat Point gneiss and \ 1815 Ma Lobstick group rocks were intruded by tonalite, metamorphosed to upper-amphibolite – facies and deformed in the interval between 1815 and 1802 Ma. The data indicate that the gneissos- ity and superposed folds in the Flat Point gneiss are older than 1815 Ma. It is tacitly assumed that these features are Archean, although this has not been confirmed by a geochronological test. 4 . 2 . Crossroads domain Field relations demonstrate that the oldest rocks in Crossroads domain are high-grade supracrustal gneisses, informally named the Overflow group, consisting principally of metasedimentary pelitic migmatite Fig. 7, mi- nor amounts of mafic and felsic metavolcanic rocks and associated chert – magnetite iron forma- tion. The precise age of the supracrustal rocks is undetermined, although they are constrained to be \ 2704 Ma on the basis of U – Pb geochrono- logical data from younger intrusive units de- scribed below. o Nd values for two samples of the metasedimentary rocks are approximately + 1 and + 2 at 2700 Ma, and they have depleted- mantle model ages T dm ages of approximately 2800 Ma Kerr et al., 1994. The Nd data indicate the rocks were probably not derived from erosion of Middle or Early Archean crust, and suggest that their depositional ages are between 2700 and 2800 Ma. One possibility is that they could have been derived from erosion of coeval Overflow group volcanic rocks. These supracrustal rocks are provisionally correlated with similar Archean high-grade metasedimentary and metavolcanic rocks in the Orma domain see Nunn and Noel, 1982; Nunn, 1993. Fig. 7. Archean pelitic migmatite Overflow group containing superposed folds of Archean age and a deformed Archean amphibolite dyke, Crossroads domain. strain the metasedimentary rocks to be older than 1805 Ma. They are probably older than 1815 Ma, the age of the MR1 tonalite, although field rela- tions do not unequivocally prove this. A sample of paleosome from the pelitic migmatite has a depleted-mantle Nd-model age of 2310 Ma Kerr et al., 1994. To place minimum constraints on the age of the foliation and leucosome formation in the Lobstick group rocks, sample MR3 was collected from a weakly deformed, late syn-tectonic tonalite dyke Fig. 6 which cross-cuts the supracrustal rocks, their foliation and included leucosomes. Four fractions of zircons Fig. 5 define a line having a Overflow group rocks are intruded by variably deformed and metamorphosed plutonic units in- cluding tonalite and granite orthogneisses that contain several phases of metamorphic leuco- some, granitic rocks belonging to the ca. 1840 – 1810 Ma De Pas batholith, deformed granitic plutons, some of which are probably related to the De Pas batholith, and several ages of variably deformed and metamorphosed mafic dykes. To better understand age relationships between the various units, samples from one outcrop contain- ing unequivocal contact relationships Fig. 8 were collected for U – Pb geochronological studies. Sample CR1 is from the paleosome of a tonalite orthogneiss which intrudes Overflow group supracrustal gneisses. Three zircon fractions from the sample define a discordia line Fig. 8, Table 1 with an upper intercept of 2704 9 15 Ma, inter- preted to represent the igneous crystallization age of the rock. This age is significantly older than the ca. 2620 Ma emplacement age determined for a monzogranite orthogneiss from the southeastern Crossroads domain James et al., 1996, although it is consistent with 2682 – 2675 Ma emplacement ages of tonalite intrusions in the Orma domain see Nunn et al., 1990. The lower intercept of the discordia line is 1815 Ma, and is thought to represent incipient Pb-loss during a ca. 1815 Ma metamorphic event. Crossroads domain or- thogneisses have o Nd values of between 0 and + 2 at 2650 Ma, and T dm model ages between 2800 and 2650 Ma Kerr et al., 1994. The Nd and U – Pb data indicate that these intrusions are juve- nile, Late Archean additions to the crust. Nd data from Orma domain tonalite orthogneiss are simi- lar; rocks have o Nd values of + 1 at 2675 Ma and T dm ages of approximately 2770 Ma Kerr et al., 1994. The gneissosity in CR1 orthogneiss is cut by a pink, recrystallized leucogranitic dyke, which is deformed by a locally intense foliation that also overprints the host orthogneiss. A discordia defined by two fractions of zircon collected from the dyke CR2, Fig. 8 suggest an igneous crystal- lization age of 1836 9 10 Ma. This age is coeval, within error, of the 1831 9 5 Ma age James et al., 1996 determined from a sample of De Pas batholith granite collected from the southern Crossroads domain. On this basis, the dyke is interpreted to be related to De Pas magmatism. Fig. 8. Sketch map of an outcrop in Crossroads domain showing field relationships and U – Pb concordia diagrams for samples CR1 to CR4. The fresh, ENE-striking mafic dyke Mesoproterozoic? is undeformed and does not contain minerals suitable for U – Pb dating. The diagram represents an area of approximately 600 m 2 . Fig. 9. Metamorphosed diorite dyke CR3 sample location; left side of photograph cutting CR1 tonalite orthogneiss. The data constrain the age of the gneissosity in the host rocks i.e. in sample CR1 to be \ 1836 Ma, and as Archean rocks in the domain do not contain any isotopic evidence of thermal events between ca. 2620 and 1836 Ma, we interpret the gneissosity to be an Archean feature. The tonalite orthogneiss, leucogranite dyke and it’s contained foliation are cross-cut by a grey- weathering, recrystallized and weakly deformed diorite dyke CR3, Figs. 8 and 9, which is in turn cut by an undeformed granitic pegmatite CR4. The igneous crystallization age of the diorite dyke is interpreted to be 1809 9 2 Ma based on two fractions of concordant and nearly concordant zircons. A single monazite fraction from CR3 was dated at 1795 9 3 Ma and interpreted to represent the time of metamorphism. The pegmatite dyke CR4, Fig. 8 is interpreted to have a crystalliza- tion age of 1800 9 2 Ma based on data from zircon and three concordant monazite analyses. To provide additional constraints on the age of the supracrustal rocks, their included metamor- phic and structural features, and ages of intru- sions, samples of a diorite dyke CR5 and a granite dyke CR6 were also dated. The diorite dyke Fig. 10, which has a similar mineralogy and texture to the CR3 dyke, cross-cuts relict primary layering, gneissosity and foliation in host Overflow group mafic metavolcanic rocks. How- ever, the diorite dyke is itself metamorphosed, it contains a distinctive ‘spotted’ hornblende – por- phyroblastic texture, and has a weak foliation. Five fractions of zircon Fig. 11 define a discor- dia line with an upper intercept of 1817 9 2 Ma, interpreted to represent the igneous crystallization age of CR5. Four fractions of titanite from the same rock define an age of 1775 Ma. The titanite data may represent a metamorphic cooling age, or they may indicate renewed thermal activity and crystallization of new titanite at 1775 Ma. Sample CR6 is from an undeformed, white-weathering granite dyke that is discordant to gneissosity in host Overflow group metasedimentary migmatite. Five fractions of zircon define a discordia line 85 probability of fit with an upper intercept age of 1806 9 5 Ma Fig. 12 and interpreted to be the crystallization age of the rock. The Crossroads domain contains intrusions of variably foliated and recrystallized K-feldspar Fig. 10. Metamorphosed ‘spotted’ diorite dyke CR5 sample location, lower left cutting Archean mafic and felsic volcanic rocks left side of photo of the Overflow group. Fig. 11. U – Pb concordia diagram for sample CR5. megacrystic granite, granodiorite and charnockite belonging to the main De Pas batholith, sensu stricto, and presumed satellite intrusions of the De Pas batholith, which occur to the east of the main batholith. The satellite intrusions are corre- lated with the batholith on the basis of lithology. Two samples of De Pas batholith megacrystic granite, from the main part of the batholith in the southern Crossroads domain, have emplacement ages of 1831 9 5 Ma James et al., 1996 and 1811 9 3 Ma Krogh, 1986, as determined by U – Pb dating of zircon. One of the presumed satellite intrusions, consisting of strongly foliated megacrystic granite, is dated at 1823 9 5 Ma James et al., 1996. From farther north in the batholith, Dunphy and Skulski 1996 have deter- mined that a foliated De Pas batholith tonalite has an emplacement age of 1840 Ma on the basis of preliminary U – Pb dating of zircon. In an attempt to obtain minimum ages of em- placement for De Pas K-feldspar megacrystic granite, and to constrain the timing of deforma- tion that overprints these rocks, a unit of isotropic to very weakly foliated, pink, biotite monzogran- ite containing xenoliths of strongly foliated K- feldspar megacrystic granite was sampled. On the basis of lithology and structure, the xenoliths are correlated with foliated De Pas K-feldspar megacrystic granite. Field relations suggest that the biotite monzogranite CR7 is late syn-tec- tonic with respect to the deformation in the in- cluded megacrystic granite. Two fractions of concordant zircons from sample CR7 Fig. 13 yield an age of 1810 9 3 Ma, interpreted to repre- sent the crystallization age of the rock. This age is within error of the youngest emplacement ages from the De Pas batholith. The strong foliation in the batholith is inferred to have formed between 1810 and 1823 Ma.

5. Discussion