Preliminary Constraints on the Age, Origin, and History of a Highly Metamorphosed Tonalite, Parenteau Lake, Northwestern Flin Flon

Domain

TJ. Bohay 1•2

, K.M. Ansde/1 1, and K.E. Ashton

Bohay, T.J., Ansdell, K.M., and Ashton, K.E. (1994): Preliminary constraints on the age, origin, and history of a highly metamor· phosed tonalite, Parenteau Lake, northwestern Flin Flan Domain; in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 94-4.

The Parenteau Lake area is located along the boundary between the northwestern Flin Flon Domain (Attitti Block) and the Hanson Lake Block to the west. The boundary is defined as the Sturgeon-weir Shear Zone, which merges in this area with the Pelican Slide (Lewry et al., 1991), a broad mylonite zone along which Paleo­proterozoic rocks of the northern Hanson Lake Block, northwestern Flin Flan Domain, and Kisseynew Domain have been thrust southwestward over the Archean Sahli Granite, located 5 km to the west of Parenteau Lake (Lewry et al., 1989, 1990; Ashton et al., 1993) (Figure 1 ).

The northwestern Flin Flon Domain in the Parenteau Lake area is dominated by upper amphibolite facies (Digel et al., 1991; Ashton and Digel, 1992), gneissic to migmatitic granodiorite·tonalite and subordinate su­pracrustal rocks. Most of the granodioritic-tonalitic orthogneisses exhibit a grey, fine- to medium-grained bi­otite-quartz-feldspar±hornblende paleosome, and a white to pink, medium-grained K-feldspar-hornblende leucosome (Figure 2), the mineralogy of which suggests derivation by the melt reaction:

biotite+plagioclase+quartz=K-feldspar-rich melt+ hornblende±magnetite±titanite (Winkler, 1974).

Associated centimetre-scale hornblendite pods are thought to result in situations where the melt is being continuously removed from the reaction site, leaving a concentration of solid phase hornblende (Ashton et al., 1993; Figure 3).

The effects of deformation related to the Sturgeon-weir Shear Zone and Pelican Slide are first noticeable a few kilometres east of Parenteau Lake and increase in inten­sity westward toward a broad zone of ultramylonite on Mirond Lake. In the Parenteau Lake area, these effects are patchy. They include variable dismemberment of the leucosome to form feldspar±quartz porphyroclasts, widespread hornblende blastesis and pervasive recrys­tallization, which has annealed most shear-related fab­rics (Figure 4).

Within this variably mylonitized migmatitic terrane are thin units of uniform, medium-grained tonalite which do

not appear to exhibit an obvious leucosome. They con­tain the hornblendite pods and K-feldspar porphyro­clasts, thought to result from partial melting, along with lenticular mafic schlieren and angular mafic inclusions displaying an internal gneissosity oblique to that of the host rock (Figures 5 and 6). Textural fabrics indicate that they have been subjected to the same shearing as the adjacent migmatitic units.

Four possible theories can be postulated to explain the absence of leucosome in the Parenteau Lake tonalites:

1) They were emplaced after the peak of metamor­phism, thereby escaping the high-grade conditions that caused partial melting of the adjacent rocks.

2) They escaped partial melting due to their more ba­sic composition and correspondingly higher melting temperature.

3) They are diatexites, formed by near-complete melt­ing of precursors with essentially the same bulk com­position.

4) They underwent partial melting, but the effects have been masked or removed by shearing.

The object of this study was to provide preliminary petrological, chemical, barometric, and chronological constraints on the possible age and development of these unlayered tonalites from Parenteau Lake.

1 . Petrology

The Parenteau Lake tonalite is typically light grey and contains about 10 percent hornblende porphyroblasts, 8 percent biotite, 35 percent quartz, and 45 percent pla­gioclase, along with accessory K-feldspar, titanite, apa­tite, zircon, pyrite, and magnetite (Figure 7). K-feldspar occurs as ovoid grains up to more than 1 cm in size. They may be porphyroblasts or porphyroclasts derived by the near-complete dismemberment of leucosome. Bi­otite defines a weak foliation, which is similar in orienta­tion to that in adjacent migmatites. Magnetite is commonly rimmed by titanite. Feldspars are variably al-

(1) Department of Geological Sciences, University of Saskatchewan , Saskatoon, Saskatchewan, S7N OWO. (2) Present Address: Department of Geology, McMaster University, Hamijton, Ontario, L8S 4M1.

Saskatchewan Geological Survey 141

~ Lo. te MediuM-gro.ineci to pegno. t;t;c gro.nito;ci rocks

0 Eo.rly Mediun-gro.ined to pegMC1 titic grC1nitoici rocks

~ Gneissic to riignotitic leucogronociiorite

GJ Seclinentory rocks

0 VolcC1nic rocks

0 'Q' Leucogronodiorite-tonolite

~ Archeo.n(') enderbitic ro c ks 0 Gneissic to dio texitic gronodiorite-tonolite

645000 650000 655000 660000

6110000

kr,

0 1

6105000

M!RDND

6100000

Figure 1 • Geological map of the Attitti·Mirond lakes area showing the location of Parent~au Lake (PL) and samples sites used for geobarometry and geochronology (modified after Ashton et al., 1993). The Sturgeon-weir Shear Zone marks the boundary be­tween the northwestern Flin Flon Domain (Attitti Block) to the east and Hanson Lake Block to the west.

tered to sericite and carbonate, and locally contain inclu­sions of apatite. Carbonate also rims K-feldspar. Chloritic alteration is present around mafic xenoliths.

2. Geochemistry Seven samples of variably metamorphosed granitoid rocks were submitted for X-ray fluorescence and neu­tron activation analysis of 11 major oxides and 15 trace elements. Six of the samples are from the Parenteau Lake area and were chosen because they contained dif· fering proportions of leucosome. The other sample was from the lower amphibolite facies Johnson Lake Pluton (Byers and Dahlstrom, 1954), located about 35 km southwest of Parenteau Lake, and was included to com­pare the Parenteau Lake rocks with an inferred lower

142

grade equivalent. If the migmatitic granodiorite-tonalites and the unlayered Parenteau Lake tonalites are com­plete in situ melts of equivalent lower grade plutonic rocks found in the Flin Flon Domain, their chemical sig­natures should be similar.

The results (Table 1) confirm that the samples range from granodiorite to tonalite in composition based on modal mineralogy (Figure 8). Both the Parenteau Lake tonalite and the Johnson Lake Pluton have Rb-Y+Nb contents typical of granitoids associated with island arcs, and trace element variation patterns (Figure 9) consistent with melt generation associated with subduc­tion zone processes (e.g. Pearce et al., 1984). From these trace element patterns, it is likely that the petro­genesis of these two plutons was similar both to that of syn-tectonic granitoids in the Pelican Window (Sun et

Summary of Investigations 1994

Figure 2 - Migmatitic granodiorite-tonalite from 4 km east of Parenteau Lake showing interlayering of fine-grained pa· leosome and medium-grained hornblende-K-feldspar leu­cosome. Note the coarse feldspar porphyroclastslblasts.

Figure 3 - Migmatitic granodiorite-tonalite from southwestern Parenteau Lake showing various stages in the development of homblendite pods and their apparent genetic link with K-feld­spar-rich melt.

Figure 4 - Mylonitized migmatitic granodiorite-tonalite from east­ern Mirond Lake showing relict layering, hornblendite pods, feld­spar porphyroclastslblasts, and pervasive hornblende blastesis.

Saskatchewan Geological Survey

Figure 5 - Parenteau Lake tonalite from east shore showing hornblendite pod at top, lenticular mafic sch/ieren, and feldspar porphyroclastslblasts, but no distinct migmatitic layering.

Figure 6 - Parenteau Lake tonalite from southwestern shore showing angular mafic inclusion with oblique internal foliation, mafic schlieren, and homblendite pods.

Figure 7 - Photomicrograph of Parenteau Lake tonalite from southwestern Parenteau Lake showing typical texture and min­eralogy. Field of view is about 4 mm wide.

143

Table 1 - Chemical compositions of Parenteau Lake tonalite and Johnson Lake granodiorite.

wt.% Parenteau Lake tonalite (0284)

Si02 63.3 Ti02 0.68 Al203 15.7 Fe203 5.36 MnO 0.09 MgO 2.71 cao 5.24 K20 1.63 Na20 4.50 P20s 0.22 LOI 0.80 Total 100.2

ppm Rb 40 Sr 531 Ba 541 Ta 21 Th bd u 3 Zr 147 Nb 11 y 13 Cr 232 Co 14 Ni 24 Cu 10 Zn 61 Ga 26 As 8 Sn 11 Pb 2 Bi 12

bd=below detection limits

II 47-93-0284y • 47-93-0167 • 47-9 3- 0 172 O 47-93-0171e • H-93-5204>< • 47-9 3-0522 T F0-86-832

I Al b ite

D

Johnson Lake granodiorite (B32)

64.4 0.52

15.7 4.19 0.08 1.88 4.02 2.82 4.40 0.20 0.60

98.8

49 902 802

23 7 7

171 9

14 88

8 bd bd 79 29

6 11 9

bd

Jr.n orthite

E

al., 1991), and to less deformed and less metamor­phosed, 1860 to 1840 Ma granitoids in the western Flin Flon Domain (Ansdell and Kyser, 1992). The trace ele­ment patterns exhibited by the two plutons are compara­ble (Figure 9), suggesting either that both plutonic bodies crystallized from magmas derived from a similar source by similar processes, or that the Parenteau Lake tonalite does represent a complete in situ melt of a plu­ton similar to the Johnson Lake Pluton. However, in the latter case, none of the melt could have been removed from the system during on-going in situ melting be­cause no fractionation of elements has occurred. A more detailed geochemical study of unmelted grano­diorites and tonalites, migmatitic equivalents, and possi­ble complete melts or diatexites is required to fully solve this problem.

3. Geobarometry In the late 1980s, a method was proposed to determine the depth of emplacement of granodioritic and tonalitic plutons containing the igneous assemblage hornblende­biotite-titanite-magnetite-quartz-plagioclase-K-feldspar using the partitioning of aluminum in hornblendes (Ham­marstrom and Zen, 1986). Hollister et al. (1987) later adapted the technique for use on metamorphosed plu­tons, calibrating it to an accuracy of ±1 kbar by using plutons for which the pressure was independently known. Since the Parenteau Lake tonalites have the re­quired assemblage, the technique was used in an at­tempt to determine regional metamorphic pressures.

Plagioclase-hornblende pairs in two polished thin­sections were analyzed by wavelength-dispersive x-ray spectrometry using the JEOL JXA-8600 electron mlcro­probe at the University of Saskatchewan. The amphi­bole in both samples was edenitic hornblende according to the calcic amphibole classification of

Leake (1978). The results obtained from this geobarometer were as fol­

IJ.. To nali t e E: Gra.nocliori te C Ad .... elhte D Trandhj eu te E: Gra.ni te

Ort hocl('l)s ~.

lows: 5.0 kbar for sample 0284 and 5.7 kbar for sample 0171a (Figure 1 ).

Regional P-T estimates indicate an increasing metamorphic gradient from about 5 kbar near the Mani­toba border (Digel et al., 1991) to 1 O kbar at the Sahli Granite (Craig, 1989). Peak pressures derived from garnetiferous amphibolites and garnet-clinopyroxene-plagio­clase rocks in the Wildnest-Attitti lakes area, located 15 to 20 km east of Parenteau Lake, were 6.6

Figure 8 - Normative anorthite-orthoc/ase-albite diagram for Parenteau Lake area grano­diorites and tonalites. Norm compositions in Bohay (1994).

to 7.9 kbar (Ashton and Digel, 1992). Pe1ites from the same area yielded inconsistently low values of 2.5 to 4.1 kbar due to demonstra­ble widespread retrogression. Meta­morphic assemblages and the higher proportion of partial melt in the Parenteau Lake area suggest that pressures of about 8 kbar or more were probably attained.

144 Summary of Investigations 1994

IOO ~ ----,-S- Parenteau Lake ton.alit~- 1

F \ ·"7 Johnson Lake granodiorite r- ··· --· - ··-·- ··- -·-· - ··-

10 b- ·"' ,f"d I -~ ·.?'---'();/ r ·\, I F LJ. ./ / \ : ~ ~j I l ' ~ I

~ \/\ i f ~ ! . L_ . I

0 1 I __ .1- __L _.-1.. _ _J __ : _ . i _ _ , .. 1 _. J_. -1 . Sr K Rb Ba Nb Zr Ti Y

Figure 9 - Element variation diagrams nonnalized to ocean­ridge granite (Pearce et al., 1984) for the Parenteau Lake tonalite and Johnson Lake Pluton. Data from Table 1.

As the pressures obtained in this study are intermediate between these estimates and the low values obtained from retrogressed pelitic rocks in the Wildnest-Attitti lakes area, it is possible that they also reflect the ef­fects of retrogradation. Retrograde textures observed in rocks of the Parenteau Lake area include the replace­ment of garnet by cordierite in pelitic rocks, and the breakdown of garnet to cummingtonite±plagioclase in mafic dykes. These retrograde effects could be due to re-activation along the Sturgeon-weir Shear Zone in much the same way that Digel et al. (1991) cited re-acti­vation of the Spruce Rapids Shear Zone to explain the observed low P-T data collected for wackes adjacent to it, but a more likely explanation is that the results from the Parenteau Lake tonalite reflect conditions during post-tectonic recrystallization and hornblende blastesis.

4. Geochronology

Zircons from a sample of Parenteau Lake tonalite (0284) were analyzed using both the single-grain Pb­evaporation technique and conventional U-Pb methods to determine its age of crystallization and subsequent metamorphism. The age of peak metamorphism in the northwestern Flin Flon Domain is taken as 1807 +3/ -2 Ma based on metamorphic zircons from a felsic gneiss located on Attitti Lake, about 15 km east of Par­enteau Lake (Heaman et al., 1992). A sheared 1806 ±2 Ma pegmatite in the Sturgeon-weir Shear Zone rep­resents a minimum age for the mylonitization (Ashton et al., 1992) and suggests that deformation took place at about the peak of metamorphism.

The sample was crushed and zircons separated using standard Wilfley Table, magnetic separation, and heavy liquid techniques at the University of Saskatchewan. Two zircon populations were identified: one consisted of

Saskatchewan Geological Survey

flat, stubby, euhedral crystals with slightly rounded termi­n~tions; whereas the other consisted of elongate, cylin­drical, euhedral crystals. The zircons in both populations were clear and generally inclusion-free.

a) Single-Grain Pb-Evaporation Technique

Single zircons from both populations were analyzed us­ing the procedures outlined by Kober (1987), Ansdell and Kyser (1990), and Ansdell (1992). Only two zir­cons, both from the population of stubby crystals, were successfully analyzed, yielding 207PbJ206Pb ages of 1839 ±16 Ma and 1848 ±18 Ma. Since this data was ob­tained from only one evaporation step, the best interpre­tation is that these ages represent the minimum crystallization age of the zircons. The clarity and colour of all the zircons, and the lack of success with the Pb­evaporation technique in this study, may suggest that the zircons are low in uranium and thus low in radio­genic lead.

b) Conventional U-Pb Dating

Selected zircons from both populations were then abraded and divided into four multigrain fractions by handpicking, prior to spiking with a mixed 205pb-233U spike and dissolution in Teflon microcapsules. U and Pb were then separated using ion exchange columns and analyzed by peak jumping using the secondary electron multiplier of the Finnigan-MAT 261 thermal ioni­zation mass spectrometer at the University of Saskatch­ewan. Procedural blanks for Pb are approximately 150 to 200 pg. Each fraction yielded surprisingly old 207PbJ206Pb ages. Two fractions from the population of elongate zircons yielded 207PbJ206Pb ages of 1885 and 1893 Ma, whereas the two fractions of stubby zircons yielded 207PbJ206Pb ages of 1961 and 2055 Ma (Figure 10). Ashton et al. (1993) suggested that this rock may be the metamorphosed equivalent of the 1845 to 1860 Ma (Ansdell and Kyser, 1991) granodiorite-tonalite suite in the southern Flin Flon Domain. This inferred cor­relation is supported by the similarity of major and trace element geochemistry of the Parenteau Lake tonalite and the lower metamorphic grade Johnson Lake Pluton. If this scenario is correct, then the zircon population con­sists dominantly of inherited grains. However, the Paren­teau Lake area is on the eastern margin of the Pelican Window, and there is a possibility that the tonalite is similar to some of the granodioritic to tonalitic or­thogneisses present within the Pelican Window (Lewry et al., 1989). Sun et al. (1992) obtained 207Pbf206Pb ages of 1879, 2234, and 2723 Ma, using the Pb-evapo­ration technique, for some of the orthogneisses in the Pelican Slide, and brings into question the age of crys­tallization of orthogneisses in this region. The interpreta­tion of the zircon data from Parenteau Lake is presently equivocal, but does warrant further U-Pb analyses of zir­con and titanite to constrain the extent of inheritance, age of igneous crystallization, and high-grade metamor­phism.

145

TB 0284 I representing the minimum age

0 .38 I 20~0 of crystallization. Preliminary U-Parenteau Lake metatonalite ; Pb analysis of the same stubby

-.. ·-··· ···- -·· -· ·-- - .. - · -·-· .J 1000 zircon population yields Q . 16

l9SO 2D7PbJ206pb ages of 1961 and 2055 Ma whereas a second

1900 population of elongate zircons => yields 207pbJ206Pb ages of 0 . 34

::: lS50 1885 and 1893 Ma. Together,

....._ these ages suggest that the

.0 0. 32 Parenteau lake tonalite may be Cl. 1750

older (approximately 1890 Ma) "' 0

1700 than the 1860 to 1840 Ma gra-... 0.30 nodiorite-tonalite suite of the

1650 southern Flin Ron Domain. Fur-1600 ther U-Pb analyses are re-

0. 28 quired to clarify age relation-J !,SO

ships in the Parenteau Lake ?SOO area, but these preliminary data

0. 26 - ·-'- ·-· '·- -- .._ __ · - - -- · · ---·· - - ---.1.- ·-- ... , L-.. confirm that the tonalite was

3 . 0 ... ' . 0 6.0 1.0 emplaced prior to peak meta-207 235 morphism and partial melting.

Pb / u Figure 10 - U·Pb concordia plot of zircon fractions from the Parenteau Lake tonalite. Fractions B and C are from the population of stubby euhedral grains, whereas fractions D and E are from the population of elongate zircons.

5. Details of the geological history of the Parenteau Lake tonalite are still unclear but the geo-

5. Conclusions

1. Modal and normative mineralogy indicate that the rocks sampled at Parenteau Lake are tonalitic. The major and trace element composition of the Paren­teau Lake tonalite and the Johnson Lake Pluton of the southern Flin Flon Domain are comparable, and indicate that they formed in volcanic arc environ­ments by similar processes associated with subduc­tion zones. Assuming no element fractionation as a result of loss of melt during ongoing partial melting, the trace element signatures are consistent with an origin by complete in situ melting and crystallization.

2. The presence of hornblendite pods in the Parenteau Lake tonalite suggests that a melt-generating reac­tion took place. This is supported by the presence of feldspar porphyroclasts/blasts which most likely result from dismemberment of coarse-grained leu­cosome. The recrystallized texture of the tonalite, to­gether with its foliation parallel to the main shear fo­liation associated with the Sturgeon-weir Shear Zone and Pelican Slide, Indicates further that it was emplaced prior to the main shearing event.

3. Aluminum-in-hornblende geobarometry yields equili­bration pressures of 5.0 and 5.7 kbar, about 2 to 3 kbar lower than estimates for the 181 O to 1805 Ma peak metamorphic conditions based on other thermobarometric studies in the area. They most likely record retrograde conditions during re­crystallization following the main shearing event, but could alternatively be related to reactivation along the Sturgeon-weir Shear Zone.

4. A population of stubby zircons from the Parenteau Lake tonalite yield Pb-evaporation ages of about 1839 and 1848 Ma. These are best interpreted as

146

chemical and geochronological results suggest that it originated as an arc-related or syn-tectonic magma well before the time of peak metamorphism and the main shearing event. Since complete in situ melting of a tonalite is unlikely in a dominantly migmatitic terrane, the absence of dis­tinct layering in the tonalite is tentatively attributed to a number of factors which come together in the Par­enteau Lake area. Firstly, the proportion of melt was probably small because these rocks are tonalitic, rather than granodioritic, as are many of the more migmatitic granitoids in the area. Secondly, much of the melt that did form may have been simultane­ously expelled due to coeval shearing related to the Pelican Slide and Sturgeon-weir Shear Zone. The absence of melt material around the hornblendite pods is consistent with this idea. Thirdly, the effects of widespread recrystallization may have acted to mask minor amounts of melt, leaving a more or less homogeneous rock exhibiting only vague leucoso­mal layering.

6. Acknowledgments

Technical support was provided by: Blaine Novakovski (thin sections); Tom Bonli (microprobe); Angie Bilanski, Michelle Innes, and Melodi Kujawa (crushing and zircon separation); and Gerard Zaluski (mass spectrometry). The geochemical facilities at the University of Sask­atchewan are funded by NSERC Operating grants to Kurt Kyser and Rob Kerrich and use of these facilities is gratefully acknowledged. Andre Lalonde is thanked for a helpful review of this manuscript.

Summary of Investigations 1994

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Ashton, K.E., Drake, A.J., and Lewry, J.F. (1993): The Wildnest·Tabbernor Transect: Attitti·Mirond lakes area (parts of NTS 63M-1 and ·2); in Summary of Investigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 93·4, pS0-66.

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147