metamorphic p-t studies in the jan lake area, hanson lake ......sure granulite facies metamorphism,...

Metamorphic P-T Studies in the Jan Lake Area, Hanson Lake Block 1

Syed Abbas-Hasanie2, John F. Lew,y2 and Dexter Perkins3

Abbas-Hasanie, S., Lewry, J.F. and Perkins, D. (1991 ): Metamorphic P·T studies in the Jan Lake area, Hanson Lake Block; in Summary of Investigations 1991, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 91-4.

Recent work (Lewry and Macdonald, 1988; Lewry et al. 1989, 1990; Lewry, 1990) suggests that highly strained gneisses comprising the 'Pelican Slide' , overlying and enveloping the Sahli monzocharnockite in the Hanson Lake Block, represent a fundamental, high-grade metamorphic, ductile mylonitic 'sole zone' separating Ar-chean basement and overlying Early Proterozoic alloch-thons. Better documentation of the kinematic, ther-mobaro-metric and isotopic history of rocks in this high-strain zone is critical to an overall understanding of tec-tonic evolution in the Reindeer Zone of the Trans-Hud· son Orogen. The objective of this study is to better es-tablish the metamorphic history of rocks in the Pelican Slide. Part of that history can be established by geother-mometric and geobarometric studies of critical metamor-phic mineral parageneses in suitable rocks. Results will be correlated with parallel geochemical and isotopic studies being conducted at the University of Sas-katchewan (Kerrich, this volume) and detailed fieldwork by Shi Rupan (Shi and Lewry, this volume).

During the first two weeks of August 1991 , the first author collected over 100 samples of both paragneisses and orthogneisses from lakeshore outcrops around Jan Lake and southwest Mirond Lake, with special emphasis on sampling pel itic metasediments. Appropriate calib ra-tions of standard cation exchange geothermometers and geobarometers will be applied to selected samples of pelitic rocks containing garnet, biotite, sill imanite, plagioclase, quartz, cord ierite and K-feldspar. Chemical analysis of minerals will be carried out by electron microprobe at the University o f North Dakota. Metamor-phic temperatures and pressures derived from applica· tion o f the garnet-biotite and garnet-cordierite ther-mometers of Perchuk and Lavrente'eva (1983) and gros-sular-anorthite-sill imanite-quartz barometer o f Newton and Haselton (198 1) will be presented after completion of microprobe mineral analysis.

This report gives details of sample collection and prelimi-nary observations on metamorphism in the Jan Jake area, based on fieldwork and initial th in-section examina-tion.

1. Regional Background

Recent models (Lewry et al. , 1990: Bickford et al., 1990) suggest that the mostly juvenile Reindeer Zone under-

went early thrust imbrication and nappe development with coeval development of localized, gently inclined, ductile high-strain zones, followed by polyphase refold-ing. In the Jan-Mirond Lakes area, the early event ap· pears to be represented by a compound progressive strain, S1 / S2 layer-parallel foliation and locally preserved intrafolial isoclinal folds probably of more than one generation (cf. Lewry et al. 1989). Over much of the region, this early planar fabric is essentially my!onitic in many rock types. Unequivocal evidence of the D1 event is only locally preserved. D2 deformation included exten-sive development of generally tight to isoclinal meso-scopic to microscopic folds in most rock types and a pervasive, regionally developed S2 axial plane schis-tosity which generally overprints and obliterates S1 ex-cept in hinge zones of some 02 folds.

This early progressive strain sequence evidently oc-curred under high- to very high-grade metamorphic con· ditions, as demonstrated by extensive anatexis of petitic to psammopelitic paragneisses, broadly coeval develop· ment of fibrolitic to prismatic sillimanite-K feldspar-garnet· bearing assemblages, and absence of primary mus-covite in pelitic to psammopelitic rocks. Migmatization of both metasedimentary and meta-igneous rocks of fel· sic to intermediate compositions during both 0 1 and D2 is evident throughout the area (cf. Lewry et al., 1989).

D 1 / D2 structures are refolded about broad open to tight semi-upright folds during 03 and 04 to produce the dominant major structures in the region, including the complex, elongate, F3/F4 interference dome, cored by the Archean Macmillan Point and Sahli basement 'windows' (Lewry, et al., 1990). F3 folds in the study area are north· to northwest-trending open to tight folds, locally refolded by generally upright, variably plunging, open h folds about northeast-trending axial surfaces. The interpretative geological section through the Han· son Lake Block and adjacent domains presented by Lewry et al. (1990) indicates that basement rocks may underlie much o f the adjacent Kyaska nappe sheet (Kis-seynew Domain) and Flin Flan Domain and are ex-posed in the core of the Hanson Lake Block solely as the result of F3/ F4 refolding and uplift.

(1) S:iskalchowan rroJecl A. 125. from wl1ict1 lllis report derives, was funded in 1991 under the Canacla-Saskalchewan Partnership Agreemen! 011 Mineral Ocvelopmen! 1990-95

(:?) Dcpartrnc nl or Geology, Urnvcrsily or Rc gim1, Reoina. Saskalchewan. (3) Dep:irtmenl of Gco1ouy nnd Geological Engineenng. Univer~ity o f North Dakota. Grand ,orks. North Oakola.

Saskatchewan Geological Survey 69

LEGEND

~ GRANODI ORITE - GRANITIC ROCKS v~ v v -~ ~~~ HANSEN LAKE VOLCANICS WITH MINOR PLUTONS

~ LOW -GRADE KISSEYNEW METASEDIMENTS

~ HIGH - GRADE KISSEYNEW GNEISSES

D PORPHYROCLASTIC ORTHOGNEISSES D OUAR TZO - FELDSPATHIC GNEISSES ~ PELIT IC TO SEMIPELITIC GNEISSES ~ ARCHEAN GRANITIC BASEMENT ~ ( SG - SAHLI GRANITE; MPG -MACMILL AN POINT GRANITE)

FAULT

-r-T THRUST

e SAMPLE LOCATION

0 10 k m

Figure 1 - General geology of the Hanson Lake Block and environs (after Lewry et al., 1989) showing general sample locations for this study. PN= Pelican Narrows.

2. Metamorphic History

Locations of samples collected this summer are shown in Figure 1. To date, only a few of these samples have been examined in thin section. Mineral assemblages so far documented in pelitic to psammopelitic rocks in-clude:

Quartz-plagioclase-biotite-garnet-sillimanite-K feldspar-cordierite

Quartz-plagioclase-biotite-garnet-sillimanite-K feldspar

Quartz-plagioclase-biotite-garnet-sillimanite

Quartz-plagioclase-biotite-garnet-cordierite

Ouartz-plagioclase-biotite-garnet

Primary muscovite was not seen. Such assemblages, in conjunction with extensive partial melting, indicate general upper amphibolite facies metamorphism. How-ever, a possible earlier attainment of higher pressure

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granulite facies metamorphism is implied by previously documented preservation of relict spine! in cores of some cordierit e crystals in pelites and the occurrence of orthopyroxene in a metabasite sill east of MacMillan Point (Craig, 1989). Trace amounts of green spinel have been observed in only one thin section related to the present study. Preservation of a spinel-sillimanite-garnet-quartz assemblage in silica saturated rocks suggests the attainment o f relatively high pressure granulite facies metamorphic conditions (e.g. Annerson and Siefert, 1981) . Sparse documentation of kyanite in high-grade pelitic gneisses in the Jan Lake area (Perkins, in prepara-tion) may also reflect such an early high pressure event.

Observed textural relations indicate two phases of cor-dierite growth in the pelite assemblages. In the first, cor-d ierite appears to have developed coevally with garnet via the prograde reaction:

sillimanite + biotite + quartz K-feldspar + garnet + cordierite + H20

Summary of Investigations 199 1

P-T conditions of metamorphism for this reaction are constrained by the experimental work of Holdaway and Lee (1977). Depending on fluid pressure, possible temperatures and confining pressures for the reaction range from 650° to 800°C and 3 to 7 Kb. Since these pelitic rocks underwent anatexis, PH20 was probably less than PTota1 during prograde metamorphism. Moreover, the presence of graphite, noted in some pelitic assemblages, implies dilution of the fluid phase by addition o f C02.

Elsewhere, textural relations suggest that later cordierite has partially replaced garnet and sillimanite. In this case, it may have formed by the following decompression reaction (Barber and Yardley, 1985):

garnet + sillimanite + quartz + H20 = cordierite

It is clear that peak metamorphic conditions in the Han-son Lake Block may have, at least locally, attained high pressure granulite facies, with subsequent decline to lower pressure upper amphibolite facies conditions. The early high pressure, high temperature metamorphism in-d icated by the presence o f kyanite and spine! in pelitic rocks may be related to crustal thickening due to nappe emplacement (Lewry, et al . 1990). It should be possible to relate changes in P-T conditions of metamorphism to deformational events by textural evidence. At present, it is tentatively suggested that Early Proterozoic high-pres-sure granulite facies metamorphism, including that of the Sahli basement (Craig, 1989), was initiated during nappe development in the Re indeer Zone and persisted throughout the 02 event. It has been suggested that high-grade regional metamorphism in the Reindeer Zone outlasted the 0 3 deformation (Lewry, et a/. 1990). a view supported by field observations made during the present study. It is possible that decompression began during D:i reg ional fold ing, contemporaneous uplift and erosion.

3. References.

Annerson, H. and Siefert, F. (1981): Stability of the as-semblage orthopyroxene-sillimanite-quartz in the system Mg O-FeO-Fe203-Al203-Si02-H20 ; Contrib. Mineral. Petrol., v77, p158-165.

Saskatchewan Geological Survey

Barber, J.P. and Yardley, W.D. (1985): Conditions of high-grade metamorphism in the Oalradian of Connemara, Ireland; J. Geo!. Soc., v142, Pt1, p87-96.

Bickford, M.E., Coflerson, K.D., Lewry, J.F., Van Schmus, W.R. and Chiarenzelli , J.R. (1990): Proterozoic collisional tec-ton ism in the Trans-Hudson Orogen, Saskatchewan; Geol., v18, p14-18.

Craig, L.G. (1989): Geology of the Pelican Narrows area of east-central Saskatchewan; unpubl. Ph.D. thesis, Univ., of Sask., 254p.

Holdaway, M.J. and Lee, S.M. (1977): Fe-Mg cordierite stability in high-grade pelitic rocks based on experimen-tal, theoretical and natural observations; Contrib. Mineral. Petrol., v63, p 175-198.

Lewry, J.F. {1990): Bedrock Geology, Tulabi-Church Lakes area: derivation and significance of porphyroclastic gneis-ses in the Pelican Window; in Summary of Investigations 1990, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 90-4, p36-43.

Lewry, J.F. and Macdonald, R. (1988}: Observations on defor-mation in the Glennie Domain and Hanson Lake Block; in Summary of Investigations 1988, Saskatchewan Geologi-cal Survey, Sask. Energy Mines, Misc. Rep. 88-4, p35-41.

Lewry, J.F., Macdonald, R. and Stauffer, M.R. (1989): The development of highly strained rocks in the Pelican Win-dow during high-grade metamorphism and pervasive anatexis; in Summary of Investigations 1989, Sas-katchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 89-4, p58-65.

Lewry, J.F., Thomas, D.J., Macdonald, R. and Chiarenzelli, J. (1990): Structural re lations in accreted terranes of the Trans-Hudson orogen, Saskatchewan: telescoping in a col-lisional regime?; in Lewry, J F. and Stauffer, M.R. (eds.), The Early Proterozoic Trans-Hudson Orogen of North America; Geol. Assoc. Can., Spec. Pap. 37, p76-94.

Newton, R.C. and Haselton, M.T. (1981): Thermodynamics of garnet-plagioclase-Al2SiOs-quartz geobarometer; in New-ton, R.C. and others, (eds.), Thermodynamics of Minerals and Melts; Springer-Verlag, New York.

Perchuk, L.L. and Lavrent'eva, 1.V. (1 983): Experimental inves-tigation of exchange equilibria in the system cordierite-gar-net-biotite; in Saxena, S.K., (ed .), Kinetics and Equilibrium in Mineral Reactions, Springer-Verlag, New York, p 199-239.

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