Volcanic Rocks of the Early Proterozoic Flin Flon Domain in Saskatchewan 1

2 3 B.R. Watters , J. Dostal , W.L. Slimmon, D.J. Thomas, and B.A. Reilly

Watters, B.R., Dostal, J ., Slimmon, W.L., Thomas, D.J., and Reilly, BA (1994): Volcanic rocks of the Early Proterozoic Flin Ron Domain in Saskatchewan; in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 94-4.

A significant portion of the Early Proterozoic Flin Flan and Manitoba is formed by the Amisk Group. The lower Domain in adjoining parts of northern Saskatchewan part of the group consists mainly of pillowed basalt and subordinate basaltic pyroclastic rocks whereas the upper part in­ cludes subaerial units with more volcaniclastic material and minor in­ termediate and felsic lithologies. In 5 4' 45' the Amisk Lake area of Saskatche­ wan (Figure 1), the Amisk Group is composed of three contrasting suites: 1) island arc tholeiite (IAT), 2) mid-ocean ridge basalt (MORB)­ like rocks and/or oceanic Island ba­ salt (018)-like rocks, and 3) calc-alkaline rocks.

The island arc tholeiitic suite com­ prises volcanic rocks from the Flin Flon area (Flin Flon Assemblage) and the Amisk Lake area (Birch Lake Assemblage). Collectively, 54'35' these rocks range from basalt Amisk through andesite to rhyolite without apparent discontinuities although La.ke most rocks have Si02<58 percent (Figure 2). The relative proportions of major rock types are similar to those of modern island arc tholeii­ tic suites (Ewart, 1992) where there is a strong predominance of ~ Ordivician Dolomite t= · Flin Flon Assemblage mafic members. The chondrite­ 3 normalized rare earth element fo""'cl ls,·6 s.l Missi Group E_=:_:::.:~ Athapapuskow Assemblage (REE) patterns of the basaltic rocks (Figure 3) are flat or only EJ Welsh Lake Assemblage L~ , : l Mystic Lake Assemblage slightly enriched or depleted in the light rare earth elements (LREE). Felsic Intrusions ~ Birch Lake Assemblage The mantle-normalized trace ele­ ment patterns of these rocks are Mafic Intrusions ~ Sandy Bay Assemblage depleted in high field strength ele­ ments (HFSE: Nb, Zr, Hf, and Ti} O West Amisk Assemblage relative to REE and have notice­ able negative Nb and Ti anomalies § Muskeg Bay Assemblage (Figure 4). The felsic rocks have REE patterns similar to those of Figure 1 • Simplified geological map and regional setting of the Amisk Lake-Flin Flon area. Missi Island in Amisk Lake is left blank because revision mapping has not been mafic rocks, although the rhyolites completed. display negative Eu anomalies

(1) Project funded by an NSERC LITHOPROBE operating grant to B.R. Watters and J. Dostal. Rock samples for this wor1< were provided by Slimmon, Thomas, and Reilly. (2) Department of Geology, University of Regina, Regina, Saskatchewan, S4S OA2 . (3) Department of Geology. St. Mary's University, Halifax. Nova Scotia, B3H 3C3.

Saskatchewan Geological Survey 93 80 (Figure 3). Their mantle-normalized spider diagrams O Flin ffo11 and Birch l ake Assemblages (Figure 4) resemble those of the basaltic rocks. • Sandy Bay Assemblage 75 • West Amisk Assemblage • • • The MORB- or OIB-like basalts are represented by vol­ canic rocks of the Sandy Bay Assemblage (from the 70 • ·, • 0 area immediately east of Amisk Lake}. Compared to 0~ .~. modern IAT as well as to the IAT basalts and basaltic Cale-alkaline • # ••• • • ... 65 •• andesites from the Flin Flon Domain, these basalts .! •• • have higher Ti, higher TiN, fractionated heavy rare .•... ~.·' • 0 60 ·~ •• ct O 0 earth elements (HREE}, and for a given Mg#, a higher 0" 0 ii) •• 41' •• O O o 0 0 content of transition elements, particularly of Cr and Ni. 0 0 0 0 55 0 Bo Their REE and incompatible trace element patterns re­ •• 0 0 .~00~ semble tholeiitic OIB and E-type MOAB (Figures 3 and °.8 0 0 0 Tholeiitic 4). According to the ratios of incompatible trace ele­ 50 • ments, the basaltic rocks can be subdivided into two dis­ • I ,~·· • tinct groups. Group 1 has lower ZrN, TiN, and LaNb 45 • ratios as well as lower abundances of Zr, Ti, and Nb. 0 2 3 4 5 The Group 2 basalts are more enriched in LREE with FeO*/ MgO the (LaNb}n ratio varying between 1 and 2 in Group 1 and between 1.7 and 4.2 in Group 2. The distribution of Figure 2 - Variations of FeO•/MgO versus SiC>e (weight per­ incompatible trace elements in basalts of Group 2 is cent) in the island arc tholeiitic (open circle), MORB-like (filled similar to that of plume-related 018 (Figures 3 and 4). circle), and ca/c-a/kaline (diamond) suites from the Saskatche­ wan segment of the Flin Flon Domain. The line separating the calc-alkaline and tholeiitic fields is after Miyashiro (1974). The The calc-alkaline suite of the West Amisk Assemblage analyses are plotted on a LOI-free basis. in the Amisk Lake (west) area includes rocks mostly with Si02 ranging from 57 to 68 percent. Compared to IAT rocks of equivalent Si02 con­ tent, the calc-alkaline rocks are A. MORB-lilc:e Basalts D_ CA Andes ites higher in some incompatible trace ,oo • 9 1-30-0041 • 1 197 elements such as Th and LREE. 0 91-30-00114 a 1194 Their trace element patterns (Fig­ ~ BHV0-1 ure 4) are characterized by nega­ tive Ti and Nb anomalies, typical of subduction-related rocks. The suite ,o ~ resembles modern calc-alkaline an­ desitic suites from ensimatic arcs or arcs developed at thin sialic mar­ gins. B. IAT Basalts E. C A Oac i te s 100 • 89-12-0099 . 1219 0 99-12-0138 0 121~ GI 1 . Discussion ... 91-30-0144 :§ . i:: Individual rock types of the IAT 0 .J::. (.J suite have a trace element compo­ sition similar to their modern ..,. 10 0 oceanic IAT correlatives. The IAT a:0 basalts were derived by relatively large degrees of partial melting at shallow depths. The isotopic as c _ IAT Andes1te, Dac:ile, Rh)l'olite 89-12-0:2'26 well as trace element data indicate 100 . 0 8 9 - 12010S that Archean continental crust was "' 89-12-0224 not involved in their genesis and . 6'9-12 0016 suggest an intraoceanic arc setting for the Flin Flon Island arc tholeiitic suite. 10 The close spatial and temporal as­ sociation between felsic and mafic units of the IAT suite as well as their Nd isotopic ratios imply that Figure 3 - Chondrite-norrnalized REE abundances in volcanic rocks of the Saskatchewan the rocks are genetically related. segment of the Flin Flan Domain: A, MORB-like basalts; B, /AT basalts; C, /AT andesites The felsic magmas were probably and rhyolites; D, Cale-alkaline andesites; E, Cale-alkaline dacites; and F, Cale-alkaline derived by extensive differentiation rhyolites. Normalizing values after Sun (1982). Average of E-type MORB (Sun and of the volumetrically more signifi­ McDonough, 1989) and USGS standard rock BHV0-1 (tholeiitic basalt from Hawaii) are cant tholeiitic basalt magma. How- shown for comparison.

Summary of Investigations 1994 94 Flin Flon IAT are comparable to A. M ORB-1,~e Basalts 8 IAT Ba solts modem primitive island arc suites. . 91 J0-0041 • 89·12-0Dti8 0 9 1 ·30·00 44 0 89·1 2-0099 . E-M DAB . 6 9 11-0 348 The Nd isotopic ratios for the IAT 10 rocks of the Flin Flon Domain, as well as their trace element charac­ teristics, support a derivation in an ., intraoceanic arc environment from -;: c a depleted mantle source with little, ti! if any, contamination by signifi­ ~ 100 cantly older continental crust. The C. CA Amk'$•1es O IA T Andu11e, Oacrtt, Rh yolite ~ presence of island arc calc-alkaline (J . 9 2- 23· 13ti4 • 89-1 2- 10 9 0 a: O 92 23·020 9 0 89·1 2-001 6 volcanic rocks in the Flin FJon Domain suggests that the island

10 arc evolved to a relatively ad­ vanced stage and indicates an in­ creasing depth of melting with progressive evolution of the arc.

The close association of IAT with MORB-like tholeiites in the East Amisk Lake area suggests that the Figure 4 • Mantle-normalized patterns for volcanic rocks from the Flin F/on Domain: suite was formed in a back-arc set­ A. MORB-like basalts; B, IA T basalts; C, Cale-alkaline andesites; and D, /AT andesite, ting and the compositional vari­ dacite and rhyo/ite. Normalizing values after Sun and McDonough (1989). Average of E· ations of the MORB-like basalts type MORB (Sun and McDonough, 1989) and USGS standard rock BHV0-1 (tholeiitic can be attributed to partial melting basalt from Hawaii) are shown for comparison. of a rising mantle diapir during back-arc spreading. The composi­ ever, a simple process of fractional crystallization can­ tional characteristics of the Group not account for the small differences in the abundances 2 MOAB-like tholeiites imply partial melting of a garnet of incompatible trace elements between the basalts and peridotite source. These lavas were tapped from the rhyolites. diapir at relatively deep levels (at a depth of 100 to 60 km; McKenzie and O'Nions, 1991). Magmas gener­ The MORB-like basalts are not directly related to the ated by melting of the diapir at shallower depth (in the IAT suite and were derived from a different source. The spinel stability field) gave rise to the Group 1 basalts. Nd isotopic data for the MOAB-like basalts point to a de­ pleted mantle source. The two groups of these basalts could have been generated by variable degrees of par­ 3. References tial melting of a similar or common source. The Group 2 Condie, K.C. (1989): Geochemical changes in basalts and an­ tholeiites were formed by a lower degree of melting desites across the Archean-Proterozoic boundary: ldentifi· than the Group 1 tholeiites. Assuming the mantle cation and significance; Lithos, v23, p1-18. source had the composition of a primitive mantle, the differences between the two basalt groups suggest that Ewart, A. (1992): The mlneralogy and petrology of Tertiary­ the partial melting involved a garnet peridotite source. Recent orogenic volcanic rocks: With special reference to The basalts of Group 2 require a garnet-bearing source the andesitic-basaltic compositional range; in Thorpe, R.S. (ed.), Andesites: Orogenic Andesltes and Related Rocks, while some of the basalts of Group 1 may have been John Wiley & Sons Lid., p25-98. generated by the melting of spine! peridotite. Close simi­ larities of the Group 2 basalts to the Hawaii 018 (Fig­ McKenzie, D. and O 'Nions, AK (1991): Partial melt distribu­ ures 3 and 4) suggest that the rocks were derived from tions from inversion of rare earth element concentrations; a mantle plume. J. Petrol., v32, p1021-1091 . Miyashiro, A. (1974 ): Volcanic rock series in island arcs and ac­ 2. Conclusions tive continental margins; Amer. J. Sci., v274, p321-357. In the Flin Flon Domain as a whole, the IAT suite is Sun, S.S. (1982): Chemical composition and origin of the volumetrically far more important than the MOAB-like Earth's primitive mantle; Geochim. Cosmochim. Acta, v46, p179-192. and calc-alkaline rocks. Compared to rocks of typical Proterozoic greenstone belts (Condie, 1989), the IAT Sun, S.S. and McDonoogh, W.F. (1989): Chemical and iso­ suite is lower in abundances of incompatible trace ele­ topic systematics of oceanic basalts: Implications for man· ments suggesting a more immature arc system and tie composition and processes; ir> Saunders, A.O. and probably more depleted mantle sources. The low abun­ Norry, M.J. (eds.), Magmatism in the Ocean Basins, Geol. dances of strongly incompatible trace elements in the Soc. Lon., Spec. Publ. 42, p313·345.

Saskatchewan Geological Survey 95