A Metamorphic P-T Study in the Maclean Lake Belt. T.L. Jungwirth 1 and T.K. Kyser 1. Plutons. Plutonic complex. Undifferentiated

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1 A Metamorphic P-T Study in the Maclean Lake Belt T.L. Jungwirth 1 and T.K. Kyser 1 Jungwirth, T.L. and Kyser, T.K. (1992): A metamorphic P-T study in the Maclean Lake Belt; in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep The Maclean Lake Belt is a lithostructural component of the La Ronge Domain of the Trans-Hudson Orogen (Figure 1). It is divided into two major lithostratigraphic units: the McLennan Group and the Maclean Lake Gneisses. The Mclennan Group is dominated by red-pink to buffgrey, feldspar-rich meta-arkoses which were derived from a dominantly granitic source area and deposited under shallow marine to fluviatile conditions (Lewry, 1983). The Maclean Lake Gneisses are predominantly psammitic to psammopelitic greywackes (Lewry, 1984) with minor polymictic conglomerates, calc-silicate units, and km hornblende gneisses. The psammitic to psammopelitic greywackes primarily consist of quartz and plagioclase with 10 to 25 percent biotite and minor amounts of hornblende and accessory minerals. The greywackes are volcanogenic deposits, derived from the Central Metavolcanic Belt lying to the northwest, and have been interpreted as proximal turbidites intercalated with units of conglomeratic fan deposits (Lewry, 1981). Zircons from a sample of a greywacke indicate an age of deposition younger than 1827 :!:6 Ma (Kyser et al., this volume). The contact between the Mclennan Group and Maclean Lake Gneisses has been interpreted as a conformable depositional transition (Coombe et al., 1986). The Mclennan Group unconformably overlies both the Central Metavolcanic Belt and the Maclean Lake Gneisses (Thomas, 1990), but structurally underlies the Central Metavolcanic Belt (Lewry et al., 1991). The Maclean Lake Belt is separated from the Central Metavolcanic Belt by the northwesterly dipping Mclennan Lake Tectonic Zone (Lewry, 1983; Thomas, 1984). Predominantly metavolcanic rocks Metasedimentary rocks Plutons Plutonic complex Undifferentiated Thrust or tectonic zone Major fault 104 oo Sample area Dominant foliation trend Figure 1 - Map of the La Ronge Domain showing area of study. Modified from Thomas and Harper (1989). Rocks in the Maclean Lake Belt are intensely strained in comparison to those in the Central Metavolcanic Belt and according to Lewry (1984) have been affected by at least two foliation and fold forming events. The outcrop pattern is dominated by doubly-plunging tight to isoclinal 02 flexural folds (Coombe et al., 1986). Lewry (1984) describes the belt as a series of fold/ thrust packages that have undergone extreme crustal shortening. 1. Sampling During the summer of 1991, representative samples were collected from the Maclean lake Belt, between Dickens lake and Colin Lake, east of Highway 102 (Figure 2). Samples from the Maclean (1) Department of Geological Sciences, University of sas1<atchewan, Saskaloon. Saskatchewan, S7 N owo. Saskatchewan Geological Survey 189

2 l'f::u 73PIID 731\'t KM 104'30' Figure 2 - Sample location map with the samples used for mineral geothermobarometry shown by boxes. Lake Gneisses have been divided into three general categories based on mineralogy and textures: a) Weakly foliated to gneissic psammites, the most common and diverse rock type, are composed primarily of quartz, feldspar, 5 to 15 percent biotite, and generally also contain garnet and sillimanite porphyroblasts. These are equivalent to the psammites to psammopelitic greywackes of Lewry (1984). b) Cale-silicates are a minor group. The samples are green to white, medium grained, and granoblastic in texture, with diopside, quartz and feldspar as the dominant minerals. c) Hornblende gneisses composed of up to 50 percent hornblende porphyroblasts averaging 5 by 20 mm in size. The matrix is fine grained, light grey, and consists of subequal amounts of quartz and feldspar. The only sample collected from the Mcl ennan Group is a weakly foliated, buff-pink meta-arkose composed primarily of potassium feldspar with 5 percent muscovite and biotite. Nineteen samples, exhibiting a variety of textures and mineralogical compositions, were chosen for detailed petrographic examination and whole rock geochemical analysis (Table 1). 2. Mineralogical Features Garnet occurring in the samples is generally subeuhedral. Poikiloblastic garnet commonly contains silicate inclusions, but biotite was not recognized. Matrix biotite is aligned parallel to the main regional foliation. Prograde muscovite and K-feldspar were found only in the meta-arkose sample. Retrograde muscovite occurs in quartz-sillimanite-muscovite knots (sample 06). Chlorite was found in only two samples as the product of biotite breakdown (samples 24 and 40). Sillimanite occurs in fibrolite or prismatic forms, and in quartz-sillimanite-muscovite knots. Samples 01 and 29 do not contain sillimanite in thin section, although it was observed in the field. Minute staurolite inclusions in plagioclase occur in samples 01, 13, 14, and 32. large cordierite porphyroblasts in samples 13 and 14 contain garnet, sillimanite, and biotite inclusions. 3. Protoliths ACF /AKF plots (Figure 3) reveal that most psammites (delineated by the dashed line) have a greywacke protolith. The meta-arkose (sample 36) and the sillimanite-muscovite-bearing psammite (sample 06) are chemically distinct from the analyzed psammites, but together with the amphibolites (samples 04 and 39) and the amphibole-bearing psammite, still plot in the metasedimentary field. The meta-arkose (36) corresponds to an arenaceous protolith. 190 Summary of Investigations 1992

3 Table 1 Major mineralogy, textures, and whole rock analysis of the samples. QTZ =quartz, PLG =plagioclase, BT =biotite, GT =garnet, STAUR=staurolite, AMPH=amphibole,.,, Sl =sillimanite, MU =muscovite, CO =cordierite, OP =opaques, KF=microcline, 0/ =diopside, TOUR=tourmaline, and CA=carbonate. QJ [) Cl) SAM'IJI JtOCIC TYPII 1IXroRII MDllltALOOY COMMil'ITS ON MINl!llALOGY sm All03 c.o MaO 1laX) JOO xn MaO Ti02 Pl05 LOI SUM :, G') Cl) 0 () IQ OI PS- l'ouatii> IT.(11' MJN\lt1! ST AtJll atyst AL5 62.l , ' ,,.. 04 AMl't!IIOl..111! QTULO,AMftl. ri'!!!. IT MlNOll OPAQ\JBS,u ,.,, 3.: '9.0 C/J c Cll PSAMMITB l'ouatbd QTULO-IT MIHOll OPAQUIIS 71l IU ' ' O.OI I.Jt '9.7 OI PSAMMITI! Rlt.lATBD IT-SI SI AND MU PIUISIINT AS A KNOT ' : o.m 0.33 Ml 1.23,.., a, PSAJ,,CM'J11 l'ouatbd QTULO,IT-ar.o l ).S ' 1.m ' o.cn 0.70:l 0.1, 0.7,.., '" 13 PSAMMml QHBISSIC QTULO,IT-OT MlN\1ra ST AUil CJlYST AL !I I lli 0.' SJ.al 14 PSAMMlll! GNIIISSIC Q12-PLCUT af MINUl1! STAUllCIYSTALS ' 1.34 l ' O.OI O.eMS 0.% " ' SI-CD 22 PSAMMllll l'ouatbd QTULO,IT-OT MINOROPAQUBS $ J.37 1.lf S.!16 0.'11 0$ !IU :14 PSAMMm! ONl!lSSIC QTLPLO-IT-OT BT BREAKING DOWN TO!'ORM Q. 6U U l 6.3 O.OI ) a. l6 PSAMMllll Cl!illlSSIC QlZ-PLO-IIT SI- PRISMATIC SJ ' I o.cn 0.72 Cl ,,.. OP :a l'sammitii POUATl!D QTULO,IT-OT-Sl 6'! , 1.13 ' !19.3 lf l'sammml WIIAICL Y POL QYZ-PLO-IT af ' ,.,, ' PSAMMml GNBISSIC Ql7,fl.O-BT -OT MINUl1! ST AUit atyst AL O.OI 0.7' a.a, MBT A-AJU(OS8 WIIAICL Y POL (11"Z-ICl' I T-MU D I.ti O.c:t '.l ll CALC.SIJCAT11 <llanoilastic QTLfLO-PI MIHORCA S, G O.OP 0-' u AMPHIIIOl..111! Q12-ft,O-AMPH MINOR OPAQtll!S I 1.61 O.ll m 0.3 I.OI,,.. " 40 PSAMMffll l'oua11!d Ql'Z-l'l.0-IT-CJ,. BT IRBAXIN'O DOWN TO POllM a U7 Ul I ,,.. TOUll 41 l'sammitll POUATl!D QTZ-ILG-IT O.O!I 0.6! PS.uocnt! l'olia 11!1) QTZ,ILG-..uG'ti-OP,.., n c:t ! O.OI ,,.. <o

4 c Figure 3 - Whole rock geochemical data plotted on ACF / AKF diagram. Dashed line indicates psammites of this study. 4. Constraints on P-T History Lewry et al. (1978), Lewry (1981), Coombe et al. (1986), and Macdonald (1987) reported that the metamorphic grade of the Maclean Lake Belt is generally upper amph ibolite facies although locally it has attained granulite facies. a) Petrogenetic Grids Mineral assemblages observed in the psammites and meta-arkose sample were used to constrain the P-T of metamorphism (Figure 4). The typical psammite mineral assemblage consists of quartz, plagioclase, biotite, and garnet. Sillimanite is the sole aluminosilicate polymorph (samples 06, 13, 14, 26, 28). Staurolite (samples 01, 13, 14, 32) and cordierite (samples 13, 14) are present in some psammite samples. The meta-arkose sample contains quartz, microcline, muscovite, and biotite. These mineral assemblages correspond to metamorphic grades of amphibolite to upper amphibolite facies using Br F mineral stability fields on a petrogenetic grid for pelitic schists compiled by Spear and Cheney (1989). b) Geothermobarometry Figure 4 - P-T constraints on metamorphism based on mineralogy of four analyzed psammite and meta-arkose samples using mineral stability fields from the petrogenetic grid for pelitic schists of Spear and Cheney (1989). Mineral geothermobarometry data from Table 2 have been plotted showing P-T conditions from the compositions of the cores (solid squares) and rims (dots) of garnet and biotite. The possible P-T path is represented by the dashed arrow. Four psammite samples were selected for mineral geothermobarometry to further constrain the P-T conditions of metamorphism in the area and to determine a possible P-T path (Table 2). The chemical compositions of biotite, garnet and plagioclase in mutual contact were determined using a JEOL 7300 electron microprobe at the University of Saskatchewan. Biotite-garnet equilibria were used with the geothermometer of Ferry and Spear (1978), which is based on experimental calibration of the Fe/ Mg exchange between garnet and biotite. Using data from the cores of both the garnet and biotite, temperature estimates of 628 to 666 C were calculated. Data from the rims of biotite and garnet yield temperature estimates of 545 to 605 C. The temperatures of the cores most possibly reflect peak metamorphic temperatures, whereas the rim temperatures represent later cooling to the garnet-biotite closure temperature which is approximately 525 to 580 C (Spear, 1989). The quartz-plagioclase-garnet-siflimanite geobarometer of Ghent (1976) was used to calculate metamorphic pressures. The geobarometer uses the exchange of Ca between the grossular component of garnet and the anorthite component of plagioclase. Pressures calculated using data from the cores of plagioclase and garnet are 4.4 to 4.8 and 7.4 kbars, whereas the pressures from the composition of the rims are 3.9 to 4.8 and 7.1 kbars. Differences in pressures between core and rim are slight and well within the margin of error. The metamorphic P-T path for the psammites of the Maclean Lake Belt as indicated by cation geothermobarometry is limited. The mineral parageneses record a single metamorphic grade with little evidence of prograde or retrograde metamorphism. Mineral stability fields based on the psammite and meta-arkose mineralogy indicate amphibolite to upper amphibolite facies metamorphism. A possible P-T path for the four psammite samples, as reflected by the mineral geothermobarometry data from Table 2, indicate that sample 29 may reflect peak high pressure metamorphic conditions, although kyanite was not recognized during this study. The other samples indicate nearly isothermal uplift after peak metamorphism. Although the mineral composition of sample 29 is similar to the other psammites, the rock has a weakly foliated granoblastic texture in contrast to the other psammites which are foliated, much finer grained and thus, more susceptible to recording retrograde conditions. Another possible explanation for the high pressure calculated for sample 29, is 192 Summa,y of Investigations 1992

5 Table 2 - Pressure (kbars) and temperature ( C)estimates from the psammite samples. Sample Tcore Trim Pcor Pnm that the high-pressure rocks represented by sample 29 were faulted into place next to the lower-pressure rocks (sample 01, 13, and 14}. Geological evidence for faulting has yet to be described. 5. Acknowledgments Sample collection was carried out in the summer with the help and cooperation of the Cameco Corporation. 6. References Coombe, W., Lewry, J.F., and Macdonald, R. (1986): Regional geological setting of gold in the La Range Domain; in Clark, L.A. (ed.), Gold in the Western Shield, Can. Inst. Min. Metal., Spec. Vol. 38, p Ferry, J.M. and Spear, F.S. (1978): Experimental calibration of the partitioning of Fe and Mg between biotite and garnet; Contrib. Mineral. Petrol., v66, pl Ghent, E.D. (1976): Plagioclase-garnet Al2SiOs-quartz : A potential geobarometer-geothermometer; Arner. Mineral., v61, p Lewry J.F. (1981): La Ronge Project: II, Geology of the Stanley Shear Zone; in Summary of Investigations 1981, Saskatchewan Geological Survey, Sask. Miner. Resour., Misc. Rep. 81-4, p (1983): Character and structural relations of the Mclennan Group' meta-arkoses, Mclennan-Jaysmith Lake area; in Summary of Investigations 1983, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 83-4, p (1984): Bedrock compilation, Lac la Ronge and Wapawekka areas; in Summary of Investigations 1984, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 84-4, p Lewry, J.F., Sibbald, T.1.1., and Rees, C.J. (1978): Metamorphic patterns and their relation to tectonism and plutonism in the Churchill Province in northern Saskatchewan; in Metamorphism in the Canadian Shield, Geol. Surv. Can., Paper 78-10, p Lewry, J.F., Thomas, O.J., Macdonald, R., and Chiarenzelli, J. (1991): Structural relations in accreted terranes of the Trans-Hudson Orogen, Saskatchewan: Telescoping in a collisional regime?; in Lewry, J.F. and Stauffer, M.R. {eds.), The Early Proterozoic Trans-Hudson Orogen of North America, Geo!. Assoc. Can., Spec. Pap. 37, p Macdonald, R. (1987): Update on the Precambrian geology and dornainal classification of northern Saskatchewan; in Summary of Investigations 1987, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 87-4, p Spear, F.S. (1989): Petrologic determination of metamorphic pressure-temperature-time paths; in Metamorphic Pressure-time Paths, Short Course in Geology, v7., Arner. Geophys. Union. Spear, F.S. and Cheney, J.T. (1989): A petrogenetic grid for pelitic schists in the system KMASH; Contrib. Miner. Petrol., v101, p Thomas, D.J. (1984): Geological mapping, Star Lake area (part of NTS 73P-16 and 74A 1); in Summary of Investigation 1984, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 84-4, p (1990): North Lake gold deposit: A model for arenite-hosted gold; in Summary of Investigations 1990, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 90-4, p Thomas, D.J. and Harper, C.T., (1989): Regional and depositscale characteristics of structurally-controlled lode gold mineralization in the La Range Gold Belt; in Beck, LS. and Harper, C.T. (eds.), Modern Exploration Techniques, Sask. Geol. Soc., Spec. Publ. 10, p Saskatchewan Geological Survey 193

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