Archean and Early Proterozoic Field Relationships in the Careen lake Area of the Western Granulite Domain 1

Transcription

1 Archean and Early Proterozoic Field Relationships in the Careen lake Area of the Western Granulite Domain 1 C.H. Crocker2 and K.D. Collerson 2 Crocker, C.H. a_nd Colle~so~, K.D. (1988): Archean and Early Proterozoic field relationships in the Careen Lake area of the w~stern ~ranul1te Domam;,n Summary of Investigations 1988, Saskatchewan Geological Survey Saskatchewan Energy and Mmes, Miscellaneous Report ' As part of a collaborative project between the Saskatchew~n D~partment of Energy and Mines, the University of California at Santa Cruz and the University of Kansas, a new phase of geological mapping at a scale of 1:20,000 was initiated during the summer of 1988 in the Western ~ranulite and Cree Lake Domains. The objective?f th1~ st~dy is to resolve crustal evolutionary relat1onsh1ps 1n the Western Granulite Domain and to develop an integrated tectonic history for the Virgin River Shear Zone in the context of crustal assembly of the North American Craton. This report on the geology of the Careen Lake area was undertaken to investigate the crustal evolutionary history of Archean gneisses in the Western Granulite Domain and to resolve the relationship between the Clearwater Anorthosite Intrusion and these gneisses. The report.contains details of lithological relationships, rock descriptions and a provisional lithostructural chronology. Approximately 40 percent of the area mapped during the summer was burned over recently. Much of the lithostructural interpretation is based on observations made in these areas. 1. Regional Geology Located in northwestern Saskatchewan, the Western Granulite Domain of the Canadian Shield has been interpreted by Lewry and Sibbald (1980) as 'deep' Archean continental crust only mildly affected by Hudsonian thermotectonism. The Western Granulite Domain forms part of the Rae Craton (Hoffman, 1988). It is separated from the Hearne Craton in the east by the Virgin River, Black Lake and Tulemalu Shear Zones, collectively termed the S~owbird _Line. Timing of movement along the Virgin River portion ~f the ~nowbird Line is constrained by the Junction Granite which yields a U-Pb age of 1820 Ma and contains xenoliths of blastomylonite from the Virgin R_iver Shear Zone (Bickford et al., 1986). The Snowbird Line has been interpreted as an intracontinental reactivation structure related to the Trans-Hudson Orogen (Le"".'ry and Collerson, in press; Hoffman 1988). Gneisses 1n the Careen Lake area west of the Virgin River Shear _Zone (F!gure_ 1) are derived from a variety of plutonic protohths, including granite, granodiorite diorite, tonalite, anorthosite, gabbro-anorthosite ;nd pyroxenite. Subordinate rock types include metapelites metapsammites and quartzites. Intrusive contacts be- ' tween P:otoliths of!he orth:>gneisses have been largely rotated into parallelism to yield a layered gneissic sequence. Metamorphic mineral parageneses are interpreted by Lewry and Sibbald (1980) as indicating prograde granulite facies overprinted by retrograde amphibolite facies. 2. Previous Work The upper Clearwater River area east of Lloyd Lake was mapped by Sproule (1941) at a scale of 1:253,440. A more detailed investigation of the upper Clearwater portion of NTS area 74F (Lloyd Lake) was undertaken at a scale of 1:100,000 by Scott (1985). Sibbald (1973) and Lewry {1974) mapped approximately 7000 km in the La Lo~he and Mudj~tik map areas and developed a regional synthesis of the terrain (Lewry and Sibbald, 1977). A reconnaissance platinum group element evaluation of mafic and ultramafic rocks from the Fournier-Careen -Camel Lakes area, including the Clearwater Anorthosite Intrusion, was undertaken by Hulbert (1987). 3. Lithostructural Chronology The western Careen Lake area is dominated by a compie~ seque~ce of mixed high-grade quartzofeldspathic gneisses (Figure 1; Table 1). Within this area, extensive burn has exposed many small-scale lithological features that have allowed age relationships to be deduced. The nature and relation of the protoliths are largely preserved in areas of low finite strain. M~ny of the mixe? gneisses contain angular agmatitic trains of anorthos1te, gabbro and pyroxenite engulfed in a leucosome of leucogranite and diorite. These relationships are interpreted as indicating that the oldest rocks in the region are part of a layered basic complex that has been intruded by multiple phases of granite tonalite and diorite. ' Finely layered mixed quartzofeldspathic gneisses were produced by transposition of agmatitic units and sheetlike igneous bodies during deformation. The gneisses are cut by semiconcordant to concordant sheets of wanite, diorite and possibly a second phase of anorthos1te. The sheets range in thickness from 10 to (1) F1eld Support provide<! under the Saskatchewan component of the Canada- Saskatchewan Subslellary Agreement on Mineral Development , In Joint venlure wtth the University of Callfomla, Santa Cruz (2) Earth Sciences Board University of California, Santa Cruz, California, Saskatchewan Geological Survey 97

2 3km...J - Early pyroxenite Supracrustal lithologies I ;::.I Mixed quartzofeldspathic gneisses... >,:/t!j Anorthos ite sheets ~ Undifferentiated diorite, ~ an orth os ite, ton al ite sheets ~ Clearwater Intrusion fill Granite Figure 1 - Geofogicaf map showing principal lithological relations in the Careen Lake area of the Western Granulite Domain. 98 Summary of Investigations 1988

3 100 cm and commonly display layer-parallel foliation. They were emplaced late in the structural evolution of the terrain after the development of S-fabrics in the layered quartzofeldspathic gneisses. The mixed gneisses show evidence of incipient anatectic melting producing diffuse structureless patches, as well as net veins and pegmatite dykes which commonly obliterate earlier foliation. The mixed quartzofeldspathic gneisses are also intruded by sheets of massive posttectonic meladiorite. Supracrustal rocks are common throughout the area. At one locality, a contact of quartzite and calc-silicate rock against fractured pyroxenite may represent a fossil regolith surface. These supracrustal rocks may also record sedimentation prior to the intrusion of the component granitoids of the mixed quartzofeldspathic gneisses. Intrusive relationships between granite and supracrustal rocks were not, however, observed at this locality. Elsewhere, supracrustal units occur as isolated outcrops or as discrete 50 to 100 cm wide layers within the layered quartzofeldspathic gneisses. All rocks in the area are cut by pink granitic pegmatite dykes which were apparently emplaced in fractures that commonly offset the banding in the gneisses. These dykes are also faulted, producing small conjugate mylonitic shear zones. Chlorite-rich mylonites are commonly developed along the centres of some pegmatites, indicating movement and passage of fluids along these fractures following emplacement of the dykes. The Clearwater Anorthosite Intrusion crops out in the western part of the map area. As contacts with compositionally layered mixed quartzofeldspathic gneisses are obscured by sand deposits and muskeg, it is difficult to determine the relative age of the Clearwater Anorthosite. At some localities within the intrusion, primary igneous layering is truncated and angular anorthosite blocks are engulfed in porphyritic units of melagabbro, indicating multiple intrusions of anorthosite. The anorthosite body is cut by tabular sheets of posttectonic granite. 4. Major Rock Units Anorthosite, Leucogabbro and Pyroxenite Agmatite: Variably deformed blocks of anorthosite, leucogabbro and pyroxenite occur within the mixed quartzofeldspathic gneisses. At some localities these units are relatively continuous, forming up to 85 percent of individual outcrops, but more commonly they are a minor component of the gneisses. Supracrustal Rocks: Included amongst the supracrustal rocks is an assemblage of metapsammites, metapelites and quartzites. The metasediments are commonly interlayered with metavolcanics. Mixed Quartzofefdspathic Gneisses: These strongly flattened gneisses were derived from a variety of rocks by deformation of intrusive rocks, including granite, granodiorite, tonalite and diorite, which cut early agmatitic anorthosites and gabbros. Table 1 - Simplified Table of Geological Events YOUNGEST 9) Mylonitization (probably Proterozoic) and small-scale faulting related to the Virgin River Shear Zone 8) Intrusion of pegmatites 7) Intrusion of late granite and diorite 6) Possible emplacement of the Clearwater Anorthosite Intrusion 5) D2 folding and localized anatectic reworking 4) Homogeneous deformation (D,) of the agrnatitic unit forming the mixed quartzofeldspathic gneisses and syntectonic (D1) intrusion of granite, tonalite, diorite and more anorthosite 3) Intrusion of granitoids to produce units of agmatitic anorthosite and gabbro 2) Deposition of the Careen Lake Group (Scott, 1985) and other supracrustal material, including pelites, psammites, quartzites and metavolcanics (as inferred from their interpreted position overlying the regolith on the large pyroxenite body) 1) Fractionation of mafic magma to produce anorthosite, gabbro-anorthosite and pyroxenite which was emplaced in an unidentified crust OLDEST - ARCHEAN(?) Homogeneous Granites: Homogeneous granites occur as concordant sheets within the mixed quartzofeldspathic gneisses. These wider layers of granite, anorthosite to leucogabbro, and diorite probably represent later stage intrusions into the compositionally layered mixed quartzofeldspathic gneiss complex. Where volumes of individual rock types exceed 50 percent, they have been mapped as distinct units. Granite sheets within the Clearwater Anorthosite body may be of similar age. Clearwater Anorthosite Intrusion: This large body of medium- to coarse-grained blue-grey anorthosite to gabbro-anorthosite exhibits well-defined cumulus layering on scales ranging from a half to many metres in thickness. Primary layering is locally cut by melagabbro and mafic diorite. 5. Field Characteristics of the Major Rock Units a) Leucogabbro and Anorthosite Agmatite The oldest anorthosites occur as 10 to 20 cm diameter angular blocks of coarse-grained grey plagioclase with up to 10 percent intercumulus orthopyroxene. Hornblende commonly replaces the orthopyroxene. The angular anorthosite blocks contain plagioclase crystals, ranging between 5 and 15 cm in length, that show evidence of recrystallization to fine-grained (5 and Saskatchewan Geological Survey 99

4 10 mm diameter) equigranular aggregates. Granitoid rock which intrudes these anorthosites, producing agmatitic units, may be the oldest granite in the area. Anorthosite also occurs as flattened-foliated xenoliths up to 50 cm in size within the mixed gneisses. b) Leucogabbro, Tonalite and Anorthosite Bands North of Careen Lake, medium- to coarse-grained units of anorthosite and leucogabbro containing extremely dark plagioclase occur as subconcordant to concordant layers up to 2 min width interlayered with granites and mixed quartzofeldspathic gneisses. Anorthosite layers in leucogabbro are commonly boudinaged and exhibit leucosomes of partial melt in pressure shadows between the boudins. These units are locally associated with quartz-bearing anorthosites and diorites that appear to grade into tonalitic rock. These tonalites are distinguished from components in the mixed quartzofeldspathic gneisses by the presence of darker plagioclase and clear quartz. These may reflect the effect of assimilation between gabbroic magma and crustal melts. Sheets of leucogabbroic and anorthositic material north of Careen Lake appear mineralogically and texturally similar to the more mafic anorthosites and leucogabbros in the main mass of the Clearwater Intrusion. Hulbert (1987) has suggested that these sills were emplaced at the same time as the main Clearwater Anorthosite Intrusion. c) Clearwater Anorthosite Intrusion In areas mapped during the summer of 1988, the Clearwater Anorthosite Intrusion is dominated by blue-grey plagioclase with less than 1 O percent orthopyroxene, clinopyroxene, hornblende and magnetite. Gabbronorite layers in the anorthosite range from 1 to 30 cm in width and grade into pure anorthosite over a few centimetres. This primary layering is commonly oblique to the regional layering in the gneisses. It is difficult to establish stratigraphic relationships within the intrusion due to the extreme degree of minor faulting, as well as the effect of multiple intrusion within the anorthosite and late-stage dyking. Intrusive relationships are displayed at two localities along the eastern margin of the anorthosite body. At one of these localities, angular blocks of coarse-grained light grey anorthosite, ranging in diameter from tens of centimetres to many metres, are included in a matrix of coarse-grained melagabbro. Individual anorthosite blocks are veined by this phenocrystic pyroxene-bearing unit and are commonly exploded into small angular fragments of anorthosite. At the other locality, primary layering is cut by veins of massive anorthosite. Further to the west the anorthosite is much more homogeneous, displaying primary layering that consistently strikes northsouth and dips approximately 40 east. Mafic diorite dykes, usually less than 1 min width, intrude the anorthosite body and are in turn cut by much thinner pegmatitic granite dykes. However, these relationships are commonly obscured by minor faults. d) Pyroxenite The most significant exposure of the pyroxenite unit is a coarse-grained, 300 by 500 m ovoid body composed of orthopyroxenite and clinopyroxene-bearing orthopyroxenite with variable amounts of phlogopite. The unit displays a possible regolith surface defined by brown angular blocks of pyroxenite separated and penetrated by 2 to 5 cm wide pallid zones consisting of fine-grained plagioclase. Similar pyroxenite also occurs in the mixed quartzofeldspathic gneisses as deformed layers and angular to lenticular pods up to 1 m in diameter. e) Supracrustal Rocks Isolated outcrops of supracrustal rocks occur east of the Clearwater Anorthosite Intrusion and appear to be intruded by sheets of massive garnetiferous granite pegmatite, forming a supracrustal gneiss complex. Supracrustal rock types include quartzites, metapelites and metavolcanics. The metamorphic mineralogy of the fine-grained highly foliated metavolcanics includes biotite, orthopyroxene and plagioclase, and shows no signs of retrogression from granulite facies. The pegmatite sheets, which exhibit pinch-and-swell structure and have sharp irregular contacts with the metavolcanics, are made up of 1 to 10 cm diameter porphyroblasts of garnet, plagioclase and quartz with minor cordierite and graphite. The pelitic gneisses exhibit a strong foliation defined by biotite, and sillimanite that shows retrogression to fibrolite. Garnet porphyroblasts in the pelitic gneisses are commonly larger than 10 cm in diameter. Quartzite layers, which range from 5 to 20 cm in thickness, are rare in the supracrustal gneisses. Garnetiferous quartz diorite to granite was observed at two localities in small layers ranging in width from 10 to 50 cm. These migmatitic layers are interpreted as partial melts of pelitic components within the supracrustal assemblage. f) Mixed Quartzofeldspathic Gneisses This composite unit is dominated by compositionally layered granodiorite ( < 60 percent) with subordinate amounts of granite ( < 20 percent), diorite ( < 15 percent) and agmatized anorthosite, leucogabbro and pyroxenite. All lithological components in the gneisses show variable development of composite layering (generally due to variation in modal proportions of hornblende, orthopyroxene, plagioclase, blue and white quartz, and potassium feldspar), as well as a strong layer-parallel foliation. The gneisses exhibit variable degrees of anatectic melting, shown by net veining, homogenization of rock types and a loss of banding. With increasing anatexis, banding is diffuse and net veins of white feldspar, quartz, orthopyroxene, clinopyroxene and hornblende are developed. After extensive homogenization, the only evidence of banding and folding is diffuse foliation defined by mafic minerals and slightly folded net veins. 100 Summary of Investigations 1988

5 Following the development of these partial melts, the gneisses were intruded by dioritic and pegmatitic granite dykes. These diorite dykes are distinguished in the field from the diorite bands in the mixed quartzofeldspathic gneisses by a greater percentage of fine-grained hornblende and a darker feldspar. The granite pegmatites are pink to orange in color, virtually free of mafic minerals, seldom folded and never exceed 50 cm in width. g) Granites Concordant sheets of medium-grained granite, 1 to 25 m thick and locally much thicker, intrude the finely layered mixed quartzofeldspathic gneisses. Metamorphic minerals include blue quartz, white plagioclase, potassium feldspar, minor orthopyroxene and hornblende. Granite north of Careen Lake is interlayered with anorthosite and tonalite. Where granite constitutes as much as 50 percent of these outcrops, it has been mapped as a separate unit. The granite occurring west of the Clearwater River commonly contains porphyroclastic augen of dark antiperthitic feldspar similar to augen found in finely banded and highly foliated layers of quartz monzonite-tonalite commonly interlayered with this granite. The granite layers have an average width of 30 cm and commonly display pinch-and-swell structure. h) Structure The general trend of the foliation and compositional banding in the gneisses from the Careen Lake area ranges from 350 to 020. Banding in the gneisses generally ranges from 1 to 5 cm in width, although concordant sheets of granite, anorthosite and diorite commonly exceed 3 m in width. lntrafolial folds (F1) and local rotated structures indicate rotational shear during flattening. Layering in the mixed quartzofeldspathic gneisses commonly displays extensive small-scale isoclinal folding (F2). A large synform (F3), which plunges at shallow angles to the north through the centre of the field area, has an amplitude greater than the length of the field area. There are three dominant foliations in the Careen Lake area. S1 is defined by the alignment of ferromagnesian minerals and quartz lenticles parallel to compositional layering in the mixed quartzofeldspathic gneisses. S2 fabrics are developed in the hinges of F2 mesoscopic folds. Regionally developed fabrics oblique to S1 are believed to have formed during F2 deformation and are therefore considered to be an S2 foliation surface. Foliated sheets of finely layered, porphyroclastic tonalite are interlayered with the slightly foliated homogeneous granite. These sheets cut the mixed quartzofeldspathic gneisses and display layer-parallel $3 foliation parallel to S1. Small-scale dextral and sinistral conjugate faults, typically with less than 1 O m offset, occur throughout the area. These randomly oriented faults are commonly displaced by minor ductile my1onite zones. Most mylonites trend at 040 to 070 and dip both to the east and west. Al- though these ductile zones decrease significantly to the west away from the Virgin River Shear Zone, they are still present in the Clearwater Anorthosite Intrusion. No direct field evidence for the Clearwater Fault (Scott, 1985) was found. Rock types north of this proposed fault, however, commonly lack blue quartz and contain much more anorthosite than rocks to the south. Rock types south of this line are dominated by blue quartzbearing mixed quartzofeldspathic gneisses which contain virtually no anorthosite bands. 6. Conclusions 1) At least two types of anorthosite are present in the map area. An older type consists of fragments of anorthosite in an agmatitic unit. This agmatite unit is considered to be a major protolith of the mixed quartzofeldspathic gneisses. A younger type, which may be synchronous with the large Clearwater Anorthosite Intrusion, occurs as sheets within the gneisses. 2) The Clearwater Anorthosite Intrusion, a large cumulate body covering an area of about 1 O by 20 km, shows evidence for multiple intrusion of anorthosite, leucogabbro and melagabbro. 3) The mixed quartzofeldspathic gneisses of the Western Craton near the Virgin River Shear Zone are considered to have developed through transposition of agmatized basic rock types that are cut by a variety of plutons. 4) The pelitic, psammopelitic and metavolcanic rocks contain metamorphic mineral assemblages that will allow the thermobarometric evolution of the terrain to be resolved. 5) Late granite sheets may reflect the influence of partial melting related to emplacement of the Clearwater Anorthosite Intrusion. 6) After the early folding episode (F1). the terrain experienced static granulite facies metamorphism. 7) Extensive anatectic melting obscures and apparently postdates the F2 folding. 8) The deformational effects of the Virgin River Shear Zone can be seen to the west in the Western Granulite Domain in a zone as much as 20 km wide. Proposed Pb-Pb, Rb-Sr and Sm-Nd isotope geochemical research on the supracrustal rocks, anorthosite bodies and mixed quartzofeldspathic gneisses will be used to develop a crustal evolutionary history for this portion of the Western Granulite Domain. 7. Acknowledgements We are grateful to Saskatchewan Energy and Mines for financial support. R. Macdonald and C.T. Harper coordinated, expedited and provided logistics for field work. Saskatchewan Geological Survey 101

6 J.F. Lewry and M.E. Bickford provided stimulating discussions on the interpretations of field relationships. Lisa Campbell contributed many ideas and excellent field assistance. This research was also supported by a National Science Foundation Grant (EAR ) to Collerson and Bickford. 8. References Bickford, M.E., Van Schmus, W.R., Macdonald, R., Lewry, J.F. and Pearson, J.G. (1986): U-Pb i:ircon geochronology project for the Trans-Hudson Orogen: current sampling and recent results; in Summary of Investigations 1986, Sask. Geol. Surv., Misc. Rep. 86-4, p Hoffman, P.F. (1988): United plates of America, the birth of a craton: Early Proteroi:oic assembly and growth of Laurentia; Annu. Rev. Earth Planet. Sci., v16, p Hulbert, L. (1987): Investigation of mafic and ultramafic rocks, and platinum group element mineralization in northern Saskatchewan: preliminary findings; in Summary of Investigations 1987, Sask. Geol. Surv., Misc. Rep. 87-4, p Lewry, J.F. (1974): Structural relationships in the La Loche (north) map sheet and adjacent areas; in Summary Report of Field Investigations by the Saskatchewan Geological Survey, 1974; Sask. Dep. Miner. Resour., p Lewry, J.F. and Collerson, K.D. (in press): The Trans-Hudson Orogen: extent, subdivision and problems. Lewry, J.F. and Sibbald, T.1.1. (1977): Variation in lithology and tectonometamorphic relationships in the Precambrian basement of northern Saskatchewan; Can. J. Earth Sci., v14, p ~~- (1980): Thermotectonic evolution of the Churchill Province in northern Saskatchewan; Tectonophysics, v68, p Scott, B.P. (1985): Geology of the Upper Clearwater River area; Sask. Energy Mines, Open File Rep. 85-2, 26p. Sibbald, T.1.1. (1973): 74-8-NW: Mudjatik (nw), in Summary Report of Geological Investigations Conducted in the Precambrian Area of Saskatchewan, 1973; Sask. Dep. Miner. Resour., p Sproule, J.C. (1941): Weitzel Lake, northern Saskatchewan; Geol. Surv. Can., Map 576A (with marginal notes), scale 1:253, Summary of Investigations 1988

7