Canadian Tectonic Group 27th Annual Meeting, Mattawin, Quebec, Oct 13-14, 2007 Field Trip Guidebook FROM CRUSTAL THICKENING TO EXHUMATION: THE TECTONIC ARCHITECTURE AND EVOLUTION OF THE GRENVILLE OROGEN IN THE PORTNEUF - ST.MAURICE REGION, QUEBEC by Leopold Nadeau and Pierre Brouillette, Geological Survey of Canada 490, rue de la Couronne, Quebec, QC, G1K 9A9 GEOLOGICAL OVERVIEW The Portneuf-Mauricie region, south-central Grenville Province, is located at the juncture between the geologically and tectonically contrasting northeast and southwest segments of the Grenville Province (Fig. 1). It is partly underlain by the allochthonous monocyclic and polycyclic belts. Basement rocks are in fault or in disconformable contact with the Paleozoic rocks of the St. Lawrence Platform. Major rock units include the Montauban Group, the La Bostonnais Complex, the Grenville Supergroup, granulites of the Mekinac-Taureau domain, and a variety of crosscutting plutons ranging from gabbro to porphyritic granite and monzonite (Figs. 2 and 3). The region comprises four contrasting lithotectonic domains (Fig. 1; Nadeau and Corrigan, 1991; Nadeau et al. 1992a) On the northwest, the structurally lowest Mekinac-Taureau domain makes up a broad crustal-scale dome composed mainly of transposed and granoblastic, intermediate and felsic granulitic orthogneisses. This dome is structurally overlain on its south and east flanks by the allochthonous monocyclic Morin Terrane, and to the east. by the allochthonous polycyclic Portneuf-Mauricie domain. These domains were intruded by a number of small gabbronorite intrusions at 1.07 Ga and by large masses of porphyritic granite and monzonite at 1.06 Ga; the latter also underlie most of the Pare des Laurentides domain farther east. These domains exhibit contrasting tectonic fabric (Fig. 4) and geophysical signatures (Fig. 5, 6 and 7) reflecting the physical properties of the rocks, their lithological makeup, metamorphic grade, and tectonic history. 1 he Portneuf-Mauricie domain is singled out by a strong, northerly elongated, positive anomaly on the Bouguer gravity map (Fie 5) This anomaly comprises three high points corresponding to gabbronorite intrusions, including the Laoevrere Gabbronorite. The Mekinac-Taureau domain exhibits a low and uniform magnetic relief, which differs from the uniformly high magnetic relief of the Morin Terrane (Fig. 6 and 7). In contrast, the magnetic signatures of the Portneuf-Mauricie and Pare des Laurentides domains are marked by pronounced oval-shaped magnetic high corresponding to granitic intrusions MEKINAC-TAUREAU DOMAIN This domain is dominated by intermediate and granitic, migmatitic gneisses deformed at granulite facies. Lower grade upper amphibolite facies gneisses are restricted to the Reservoir Taureau area. Although homogeneous at map scale 'these gneisses exhibit locally major compositional and textural changes. They include screens of quartzite, marble and sillimanite-gamet-biotite paragneiss, transposed and boudinaged mafic dykes and small masses ot metagabbro and meta-anorthosite, namely in the Matawin area. A U-Pb zircon igneous crystallisation age ca. 1.37 Ga and Sm/Nd model ages in the 1.44-1.62 Ga range were obtained from orthogneiss samples of the eastern part of Mekinac-Taureau domain (Nadeau and van Breemen, in prep.). Mekinac-Taureau domain forms a broad dome over 80 kilometres in diameter (Fig. 4). The planar structure, defined by the preferred orientation of the mineral foliation and ofstromatic granitic mobilisate, is well developed and dips moderately (Fig. 4). The foliation is conformable with the mafic and metasedimentary screens and the axial planes of metric isoclinal folds. The mineral and stretching lineations are generally poorly developed. These are dispersed along a partial girdle with a maximum point concentration to the SE. The divergent linear orientations of the core of the dome are replaced outward by southeast-directed penetrative lineations along the flanks. A second population of E-NE lineations is developed, locally, especially on the east side of the dome. MORIN TERRANE Morin Terrane is distinguished chiefly by the abundance of Grenville Supergroup paragneisses which are imbricated with metric to sub-kilometric amphibolite and charnockitic gneisses of unknown age and origin and. secondly, by the occurrence of anorthosite, gabbro and porphyritic and/or pyroxene-bearing granite and monzonite masses of the Morin, Lac Croche, Saint-Didace and Lejeune complexes. Supracrustal rocks of the Grenville Supergroup were formed and regionally metamorphosed ca. 1250 Ma and 1180 Ma respectively (Friedman and Martignole, submitted). In addition, it may also comprise a younger metasedimentary sequence possibly deposited after ca 118 Ga (Corrigan and van Breemen, 1997). Grenville Supergroup paragneisses are cut by the Morin Complex anorthosite - mangerite - chamockite - granite (AMCG) suite emplaced in the 1150-1135 Ma range (Doig, 1991), and by the 1080-1070 Ma intrusive rocks of the Saint-Didace Complex (Nadeau and van Breemen, in prep.). Northeast of Shawinigan, Morin Terrane is tectonically imbricated between Mekinac-Taureau and Portneuf-Mauricie domains, via a thick ductile thrust zone. This zone is reactivated locally into late-Grenvillian oblique-slip senestral extensional faults. In the extreme north-east of the region, the boundary between the Portneuf-Mauricie and Pare des Laurentides domains is obliterated by a large granitic intrusion of the Riviere-a-Pierre Suite (Hebert and Nadeau, 1995). Gneisses of the Morin Terrane are commonly at granulite facies, except in the Shawinigan area where upper amphibolite facies rocks are also present. They exhibit a granoblastic texture and a variably developed compositional layering, locally enhanced by stromatic granitic mobilisate. Four structural subdomams have been distinguished (Fig. 4). Subdomains M 1 and M2 exhibit a moderately dipping, N-NO striking planar fabric with sub-horizontal lineations Ml subdomain owns its regular structural pattern to the eastward spreading of the external anorthosite nappe of the Morin Complex (Martignole and Schrijver, 1970). Subdomain M2 has a less regular planar fabric attributed to the emplacement of the ovoid intrusive masses of the Lac Croche Complex. These subdomams are also marked by the presence of folds with sub-horizontal axes parallel to the lineation (Fig. 4). These structures are viewed as the crests of recumbent folds overturned to the north-east following the tectonic transport of the external anorthosite nappe of the Morin Complex (Martignole and Schrijver, 1970). Martignole (1992) also pointed out that dextral strike-slip ductile shear zones (Morin shear zone) conformable to the axial plane of these folds were active during the overthrusting of the Morin Terrane. To the east the Saint-Gabriel anticlinorium (subdomain M3) marks the deflection to the NE-striking undulating planar structures of subdomain M4 (Fig. 4). Early recumbent folds overturned to the NO have been recognised m the eastern part of subdomain M3 (Goulet, 1978) and in the Saint-Paulin area (Hocq 1969), part of subdomain M4. The latter also includes a number of late, open upright folds with axes shallowly plunging to the S-SE. The lineations are distributed about two poles: S-SE lineations are parallel to those in subdomains Ml and M2 while SE plunging lineations correspond to one of the characteristic orientations of the Portneuf-Mauricie domain. PORTNEUF-MAURICIE DOMAIN The ca 1 45 Ga volcano-sedimentary paragneiss sequence of the Montauban Group and the ca. 1.41-1.38 Ga calc-alkalic metaplutonic rocks of the La Bostonnais Complex constitute the distinctive lithological assemblages of the Portneuf-Mauricie domain (Figs. 2 and 3; Nadeau et al. 1992b). The Montauban area, long known for its Au, Pb, and Zn volcanogenic massive sulphide mineralizations, corresponds to a regional metamorphic low. This area is underlain by middle to upper amphibolite facies paragneiss in which primary structures are locally preserved. The Montauban Group (Rondot, 1978) is made up of a sequence of well-layered, intermediate to felsic gneisses with intercalated subordinate amphibolite, locally pillowed metabasalt, and minor quartzite, which set the region apart from adjacent high-grade Grenvillian terrains. Proximal lapilli tuff and vesicular basaltic pillow lava occur with thinly bedded intermediate to felsic rocks interpreted as distal pyroclastic subaqueous fallout deposits, and laterally intercalated epiclastic sediments. Such volcanic rocks are consistent with deposition of the Montauban Group in a shallow submarine environment at the late-stage of an andesitic to felsic volcanic cycle, a common occurrence in mature island-arc or back-arc settings where Au-rich volcanogenic sulfide deposits are formed. Igneous zircons from a lapilli tuff have yielded an age of extrusion ofca. 1.45 Ga for the volcanic rocks of the Montauban Group (Nadeau and van Breemen, 1994). The La Bostonnais Complex is a typical arc-related calc-alkalic suite dominated by rocks ranging in composition from two pyroxene-hornblende diorite to homblende-biotite granodiorite, and includes subordinate monzogranite and minor ultramafic rock and gabbro. This plutonic suite intruded the Montauban Group within a few tens of millions of years after its deposition. Four samples from regionally extensive plutonic bodies ranging in composition from diorite to granodiorite yielded U-Pb zircon igneous crystallization ages of 1380-1410 Ma (Nadeau and van Breemen, 1994). The Lac Nadeau intrusion is a zoned, Ni-Cu-PGE-mineralized mafic-ultramafic body dated at 1.40 Ga (Sappin et al., 2004). Rock textures and structures in the La Bostonnais Complex range from massive with relict plutonic textures and mineralogy, to well foliated and in part gneissic and migmatitic. The trace element geochemical signature of the complex differs from that typical of an Andean-type magmatic arc but shares many similarities with evolved intra-oceanic plutonic arcs (Gautier, 1993). The La Bostonnais Complex plutonic suite may perhaps correspond to the deeper and more-evolved product of a more-mature island arc, signalled by the Montauban Group volcanics. Although generally gently to moderately dipping to the east, the planar fabric of the Portneuf- Mauricie domain is locally steep, especially north-east of Montauban where it wraps around plutonic masses. Mineral and stretching lineations define two sub-orthogonal poles plunging gently SE and N-NE (Fig.4). The SE pole corresponds to early lineations associated with northwest-directed thrusting and peak metamorphism (Nadeau and Corrigan 1991). Conversely, N-NE trending lineations locally mark late-Grenvillian oblique-senestral extensional ductile shear zones. These are possibly a consequence of the exhumation of the Mekinac-Taureau domain dome, responsible for the preservation of the Montauban metamorphic low. EPILOGUE The deformation pattern of the Trois-Rivieres region has been described in terms of fold interference (e.g. Hocq 1969- Goulet 1978; Martignole, 1992). The first mention of thrust faults is attributed to Philpotts (1967; p. 40). These faults occur in the Larose and Barnard lakes area (SNRC 311/11; 46°47'N, 73°02'W) where they may form part of the boundary zone between the Morin Terrane and Mekinac-Taureau domain. Regional studies in the Grenville Province during the last three decades have highlighted the erogenic role of ductile thrust, extensional, and strike-slip shear zones (e.g. Davidson, 1984; Hanmer, 1988; Rivers et al., 1989; Nadeau and Hanmer, 1992; Martignole and Pouget, 1994; Corrigan, 1995; Corrigan et van Breemen, 1997). The Taureau and Morin shear zones, as well as the thick shear zone that marks the imbrication of the Morin Terrane between the Mekinac-Taureau domain and the Portneuf-Mauricie domain, are among those recently recognized and currently under study. There may remain a number of important structures whose recognition will be essential to a full understanding of the nature and the structural position of the geological entities composing the regional mosaic. REFERENCES Corrigan D 1995 Mezoproterozoic evolution of the south-central Grenville orogen: structural, metamorphic, and geochronologic constraints from the Mauricie transect. Ph.D. thesis, Carleton University , Ottawa, Ont. Corrigan D. and van Breemen, 0., 1997, U-Pb age constraints for the lithotectonic evolution of the Grenville Province along the Mauricie transect, Quebec; Canadian Journal of Earth Sciences, v. 34, p. 299-316. Gautier, E., 1993, Geochimie et petrologie du complexe de La Bostonnais et du gabbro du Lac Lapeyrere. M.Sc. thesis, Universite Laval, Quebec, Que, 129pp. Nadeau, L. and Brouillette. P., 1994, Structural map of the La Tuque area (NTS 31P), Grenville Province, Quebec; Geological Survey of Canada. Open File 2938 (scale 1:250 000). Nadeau, L. and Brouillette, P., 1995, Structural map, Trois-Rivieres Region. Grenville Province, Quebec; Geological Survey of Canada, Open File 3012, scale 1 :250 000. Nadeau L and Corrigan. D., 1991, Preliminary notes on the geology of the St.Maurice tectonic zone, Grenville orogen Quebec; m Current Research, Part E, Geological Survey of Canada, Paper 91-1E, p. 245-255. Nadeau L Brouillette, P., and Hebert, C., 1992a, Geology and structural relationships along the east margin ot the St.Maurice tectonic zone, north of Montauban, Grenville orogen, Quebec; in Current Research, Part C, Geological Survey of Canada, Paper 92-1C, p. 139-146. Nadeau L Brouillette, P.. and Hebert, C., 1999, New observations on relict volcanic features in medium-grade gneiss of the Montauban group, Grenville Province, Quebec; in current Research 1999-E; Geological Survey of Canada, p. 149-160. Nadeau L., and van Breemen, 0., 1994, Do the 1.45-1.39 Ga Montauban group and La Bostonnais complex constitute a Grenvillian accreated terrane? In Waterloo '94. Geological Association of Canada - Mmeralogical Association of Canada, program with Abstracts, v. 19, p. A81 Nadeau L and van Breemen, 0., 2001, U-Pb zircon age and regional setting of the Lapeyrere gabbronorite Portneuf-Mauricie region, south-central Grenville Province, Quebec; Radiogenic Age and Isotopic Studies: Report 14; Geological survey of Canada, current research 2001 -F6 Nadeau L van Breemen, 0., and Hebert, C., 1992b, Geologic, age et extension geographique du groupe de Montauban et du complexe de La Bostonnais; dans Resume de conferences, Ministere de 1'Energie et des Ressourcesdu Quebec, DV 92-03, p. 35-39. Rondot J., 1978, Region du Saint-Maurice; Ministere des Richesses naturelles du Quebec, Rapport geologique, DPV-594, 91 p., cartes 1927 et 1928 (echelle 1:10 000). Sappin A.-A.. Constantin, M., Cark, T., and van Breemen, 0., 2004, Geologic des indices de Ni-Cu-EGP des lacs Nadeau et Long, region de Portneuf. Ministere des Ressources naturelles. de la Faune et des Pares, Quebec, KP 2004-03, 11 p. FIELD TRIP ROAD LOG The field trip stops are shown on Figures 3 and 8. The UTM coordinates in the field trip log refer to the NAD 27 map projection. Stop1 Pelitic rocks of the Saint Boniface paragneiss sequence 670200E ; 5157200N Sillimanite-gamet porphyroblastic pelitic gneiss. Paragneiss is severely sheared and migmatitic. The rocks are cross-cutted by a number of ductile and ductile-brittle shear zones, and by minor normal faults. Note the presence of a few thin diabase sheets along post-Grenvillian faults. Stop 2. Quartzite layers of the Saint-Boniface paragneiss sequence 669800E ; 5157175N Typical impure quartzite layer of the Saint-Boniface paragneiss sequence. Such quartzite layers are commonly decimetre to metre thick; they are discontinuous and rarely outcrop as mappable rock unit. Stop 3. Anatexis of the Saint-Boniface paragneiss sequence 667250E ; 5153550N Highly sheared calc-silicate gneiss and migmatitic metapelite hosting slightly discordant sheets oftourmaline-gamet bearing leucogranite and variably boudinaged and desaggregated pegmatite dykes. Stop 4. Intrusive and structural relationships between the Saint-Didace Monzonite and the Shawinigan Gabbronorite 661050E ; 5156250N The Saint-Didace Monzonite is the largest late-Grenvillian granitoid intrusive in the region. This is a composite body dominated by porphyritic monzonite and granite. Although largely massive, the rocks commonly grade to gneissic, and more locally to mylonitic namely at proximity to lithological contacts. A massive porphyritic monzonite sample has yielded a zircon crystallization age at 1072 +/- 2 Ma (unpublished). The granitoid rocks of the Saint-Didace Monzonite are associated with small gabbroic bodies including the Shawinigan Gabbronorite. The latter has yielded a zircon igneous crystallisation age ca. 1077 +10/-2 Ma (Corrigan and van Breemen, 1997). This set of outcrops to be visited features intrusive and structural relationships between granitoid rocks and gabbronorite suggesting magma mixing and syntectonic emplacement. Stop 5. Tectonic contact, via a mylonitic granite sheet, between the Shawinigan Gabbronorite and the Saint-Boniface paragneiss 664050E ; 5155350N The contact zone corresponds commonly to a mylonitic granite sheet. Note that the structural overprint in the paragneiss is indicative of intense, although heterogeneous, ductile deformation. Is the anatexis of the paragneiss contemporaneous with the syntectonic emplacement of the gabbronorite? Stop 6. 'Old grey tonalitic orthogneiss' of the Shawinigan region 671400E ; 5158400N Such polydeformed migmatitic orthogneisses of the Jesuite complex are fairly common in the Shawinigan region. Are these rocks correlative with tonalitic orthogneisses of La Bostonnais Complex, of the Adirondacks Highlands,...? Stop 7. Late-Grenvillian ductile-brittle extensional shear zones 672375E ; 5159750N Gneisses of the eastern strand ofMorin Terrane are cut by late-Grenvillian ductile-brittle shear zones commonly hosting sheared and boudinaged pegmatite. These shear zones occur in conjugate sets, each shear zone with modest displacement. This shearing is attributed to penetrative gravitational collapse following crustal thickening and peak grenvillian regional metamorphism. Stop 8. Recumbent folds developed in mylonitic augen granite, lower part of the Jesuite complex 675025E ; 5163050N llustrates the most common type of folds present in the Jesuite complex. Folds are developed in mylonitic, initially K-feldspar porphyritic monzogranitic orthogneiss. Stop 9. Distributed conjugate extensional shears in retrogressed granulite. Stop 10. Interleafed metagabbro and K-feldspar porphyritic monzogranitic orthogneiss of the structurally lower part of the Jesuite complex 675150E ; 5166500N Gabbroic rocks are lithologically varied, including coarse-grained anorthositic-gabbro. Strain varies widely across the outcrop with marked gradients along lithological contacts. Are the contact relationships intrusive or structural? Stop 11. Transposed and boudinaged mafic dykes in Mekinac-Taureau charnockitic orthogneiss Stop 12. Marble tectonic injections in Mekinac-Taureau charnockitic orthogneiss. Stop; 13. Ductile shear zones and marble tectonic breccia; Mekinac-Taureau domain 671450E;5176000N Mekinac-Taureau domain orthogneisses are interrupted by dispersed paragneiss screens including structurally concordant sheets of marble tectonic breccia. These rocks contain abundant features resulting from intense ductile flow including sheetfolds. Marble tectonic breccia typically outcrops as metre-thick sheets spread over ten s to a few hundreds of meters along strike. They commonly intervene between contrasting rock types. Such tectonic marble breccia sheets are viewed as marking ductile fault zones, although the magnitude of tectonic transport is unknown. Stop 14. Metaplutonic make up of Mekinac-Taureau domain 674900E ; 5172625N This outcrop area allows for a close examination of the various granulite facies metaplutonic rock types which make up much of the Mekinac-Taureau domain. These are generally of monotonous appearance, although displaying a number of distinctive lithological attributes indicatives of a complex intrusive and metamorphic heritage Are all these rocks resulting from a single plutonic event? Which textural / structural overprinting features can be attributed a plutonic vs. metamorphic origin? What are the diagnostic evidences for poly-phased regional deformation and metamorphism? Stop 15. Tectonic boundary between Mekinac -Taureau domain and Morin terrane 677775E ; 5177000N The high-grade orthogneiss at this locality is part of the Mekinac-Taureau domain footwall beneath Monn Terrane Deformation and metamorphic structures are indicative of intense ductile deformation at or near peak metamorphic conditions. Boudinaged and dispersed mafic boudins provide a qualitative stain marker. Note the abundance of granitic mobilisate in the felsic rocks and the presence of orthopyroxene in amphibolite.