Timing and characteristics of the Archean subaqueous Blake River Megacaldera Complex, Abitibi greenstone belt, Canada

Precambrian Research 214–215 (2012) 1–27

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Timing and characteristics of the Archean subaqueous Blake River Megacaldera Complex, Abitibi greenstone belt, Canada W.U. Mueller a , R. Friedman b , R. Daigneault a,∗ , L. Moore a , J. Mortensen b a

Centre d’études sur les ressources minérales (CERM)/Université du Québec à Chicoutimi, 555, Boul. de l’Université, Chicoutimi, Québec, Canada G7H 2B1 Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences (EOS), University of British Columbia. 6339 Stores Road, Vancouver, BC, Canada V6T 1Z4 b

a r t i c l e

i n f o

Article history: Received 6 October 2011 Received in revised form 4 February 2012 Accepted 7 February 2012 Available online 15 February 2012 Keywords: Caldera complex Blake River Group Archean tectonics Abitibi Greenstone Belt Geochronology

a b s t r a c t The Archean Blake River Group of the Abitibi greenstone belt represents a megacaldera complex which evolved over 8–11 M.y. from approximately 2704 to 2696 Ma. The early Misema Caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems. These remnants are found in the Jevis-Clericy, Montsabrais-Renault, Clifford-Tannahill and Colnet regions, where summit calderas are delineated by circular ring dyke structures. The secondary 330◦ trending New Senator Caldera formed within the envelope of the Misema Caldera and exhibits a box-work graben-type structure. Finally, the felsic-dominated, 070◦ -striking Noranda Caldera, well known for its VMS endowment, represents the final collapse of the megacaldera complex. Deformation has overprinted the calderas, but structural patterns can be used in order to reconstruct the pre-existing volcanic architecture. Structures such as mafic ring dyke complexes and rhyolitic domeflow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation. Precise U-Pb geochronological analyses were conducted in selected areas with specific emphasis on the Misema and New Senator calderas. Felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution. The Misema Caldera formed between 2704 and 2702 Ma via the amalgamation of shield volcanoes that were probably active prior to 2704 Ma. The New Senator Caldera was generated between 2702 and 2700 Ma during paroxysmal felsic volcanism, followed by the collapse of the Noranda Caldera culminating between 2700–2696 Ma. The Misema Caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture, whereas the SE-trending New Senator Caldera is more compatible with a SW-trending principal compression direction related to oblique convergence in the Abitibi belt. The Noranda Caldera is interpreted as a NE rift structure that formed in the final stages of megacaldera evolution. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Constraining the evolution of complex Archean caldera systems is possible with the advent of new chemical abrasion methods and thermal ion mass spectrometry (TIMS), that produce precise U-Pb zircon age determinations with errors of 1–2 Ma. Commonly, Archean volcanic edifices were characterized by a single age date, but as shown by the 2732–2724 Ma Hunter Mine Caldera, numerous ages are needed to give an approximation of edifice construction and duration (Mueller and Mortensen, 2002). Because of the enormous economic potential of modern (Iizasa et al., 1999)

∗ Corresponding author. Tel.: +1 418 545 5011x5634; fax: +1 418 545 5012. E-mail addresses: [email protected], [email protected] (R. Daigneault). 0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2012.02.003

and ancient subaqueous calderas (Mueller et al., 2008, 2009a), the timing of early Earth volcanic systems is critical. Large, 2–100 km in diameter explosive calderas such as the Valles or Long Valley calderas (Smith and Bailey, 1968; Bailey, 1989) are circular to ellipsoid in map view and represent volcano-tectonic collapse structures. Smaller 1–10 km diameter depressions at the summit of shield volcanoes, such as in Hawaii (Tilling and Dvorak, 1993) or on Easter Island (Vezzoli and Acocella, 2009) are primarily a result of magma withdrawal and migration along rifts. Explosive evacuation of high-level magma chambers in continental rift settings creates felsic ash-flow calderas (Lipman, 1997, 2000), but explosions may occur under deep water, as documented by Fiske et al. (2001) with the felsic Myojin Knoll Caldera in the Izu-Bonin Arc. Although magma drainage due to large explosion is effective, Mueller et al. (2009) argued that subaqueous Archean caldera

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subsidence may also develop due to constant effusion of large volumes of magma such as that observed at Hawaii (Walker, 1988). The paroxysmal phases of calderas generally chronicle the last stage of volcanic evolution, as indicated by the 9 km × 16 km Las ˜ Canadas Caldera on Tenerife with three caldera events occurring within 1 M.y. whereas, in contrast, pre-caldera construction required 8 Ma (Martí and Gudmundsson, 2000). On oceanic islands, the pre-caldera phase is characterized by mafic shield volcanoes; their amalgamation and subsequent stratovolcano construction may develop within 1 to 8 M.y. Oceanic arcs, such as the Kermadec Arc may feature an early mafic shield phase, which is followed by intermediate to felsic stratovolcanoes and simple caldera edifices (Smith et al., 2003). The timing of event construction is usually well constrained in the subaerial realm (e.g. Easter Island and Tenerife), but remains scarce in modern subaqueous settings due to limited access. Complex calderas with nested, overlapping and summit caldera clusters on a mafic shield volcano base suggest a protracted history. The 2704–2696 Ma Archean Blake River Group (BRG) of the Abitibi greenstone belt was interpreted as a subaqueous complex composed of three overlapping calderas, the 80 km in diameter, EW trending Misema Caldera, the 35 km × 14 km, NW-trending New Senator Caldera and the final 15 km × 20 km NE-trending Noranda Caldera (Pearson and Daigneault, 2009). The dimension of this nested caldera complex can be compared to the Olympus Mons Summit Caldera on Mars. This challenging interpretation will be constrained in this paper by analyzing (1) the volcanic architecture defined by the synvolcanic pattern of mafic dyke/sill complexes, (2) felsic centres and volcaniclastic deposits and (3) the structural pattern imposed during regional deformation. Furthermore, precise U-Pb zircon determinations will chronicle the evolution of a multiphase Archean caldera system.

Table 1 Volcanic cycles and sedimentary basins in the Northern and Southern Volcanic Zones (Mueller et al., 2009a). Sedimentary basins and age brackets (SB-4) NVZ-SVZ: strike-slip basins (phase I: 2680-2690 & phase II: 2670–2680 Ma) along major E-trending fault zones (SB-3) NVZ-SVZ: interarc and foreland basins (2686–2705 Ma) due to arc–arc collision-shortening (thrusting) (SB-2) NVZ: basins associated with volcanic arc unroofing and shoshonitic volcanism (2692–2720 Ma) and clastic deep-basin turbidite successions (SB-1) NVZ: intra-arc flysch basins with mafic-felsic volcanism (2692–2735 Ma) during incipient arc evolution Volcanic arc stages (VAS) and age brackets (VAS-4) NVZ-SVZ: calc-alkaline - alkaline volcanism in molasse basins (2670–2690 Ma): Arc fragmentation (VAS-3) SVZ: komatiitic – tholeiitic – calc-alkaline volcanism (2696–2706 Ma): Ocean floor volcanism and Val-d’Or arc adjacent to BLAKE RIVER MEGACALDERA COMPLEX (VAS-2) NVZ-SVZ: komatiitic - tholeiitic - calc-alkaline - shoshonitic volcanism (2705–2720 Ma): Arc formation and ocean floor volcanism (VAS-1) NVZ: komatiitic - tholeiitic - calc-alkaline volcanism (2720–2735 Ma): Arc formation with calderas and ocean floor volcanism with large subaqueous shield volcanoes

(1) volcanic successions, (2) porphyry stocks and (3) previous arc, inter-arc and fore-arc sedimentary basins (Mueller and Corcoran, 1998; Corcoran and Mueller, 2007). Major east-trending faults played an important role in the evolution of the Abitibi belt, as they separate volcanic zones (Fig. 1), and also host late-stage strike-slip basins with alkaline volcanism, syenite plutons and porphyritic plutonic suites. Significant Abitibi gold deposits are linked to these fault zones (Robert and Poulsen, 1997; Robert, 2001) that flank the studied caldera complex.

2. General Abitibi and Blake River Complex geology 2.2. Blake River Complex geology 2.1. Abitibi geology The 300 km × 700 km Abitibi greenstone belt (Fig. 1) is an extensive supracrustal succession composed of mafic and felsic volcanic rocks, various sedimentary basins of arc affinity, and plutonic suites. On a regional scale, the Abitibi greenstone belt has been subdivided into Northern (NVZ) and Southern Volcanic Zones (SVZ), which record various stages of arc formation, evolution, collision and fragmentation (Chown et al., 1992; Mueller et al., 1996). The age span of this volcano-sedimentary greenstone succession is roughly 65 M.y. from 2735 to 2670 Ma (Table 1) and is consistent with oceanic arc evolution and collision caused by oblique Archean subduction (Mueller et al., 2009a). Arc-forming events span ca. 15 M.y. and are referred to as volcanic cycles (Dimroth et al., 1982; Chown et al., 1992). The Abitibi sedimentary basins are associated with arc and forearc development, and exhibit the transition from Archean thrusting to strike-slip tectonics (Camiré and Burg, 1993; Daigneault et al., 2002, 2004). Detrital zircons vary greatly in these tectonic arc basins. Ages of up to 3.1 Ga (Gariépy et al., 1984; Davis, 2002) emphasize the age of the source basement rocks. Younger ages (David et al., 2006, 2007) can be interpreted as continuous sedimentation due to long-lived activity along major fault zones (Daigneault et al., 2004). The interstratification of sedimentary and volcanic rocks, as well as the presence of unconformities with the volcanic sequences, indicates a dynamic arc relationship (Sloss, 1963; Shanmugam, 1988; Mueller et al., 1989). Strike-slip basins contain the youngest detrital zircons (2672–2678 Ma) and porphyry stocks display overlapping ages (e.g. 2672 Ma, Davis, 2002). Sedimentary basins have marginal faults, but also rest unconformably on

The 2900 km2 Blake River Group (BRG) is a succession of mafic to felsic, tholeiitic to calc-alkaline volcanic units (Goodwin, 1982; Ludden et al., 1982) initially subdivided by Goodwin (1977) into the Misema and Noranda Subgroups (Fig. 2A). The two subgroups display volcanic interfingering, suggestive of a diachronous evolution. The region has undergone subgreenschist to greenschist facies metamorphism. Dimroth et al. (1982) considered the BRG a large basalt plain with a shield volcano system, upon which a central felsic edifice developed. This central felsic edifice was referred to as the Noranda Cauldron (de Rosen-Spence, 1976) or Caldera (Lichtblau and Dimroth, 1980) and is associated with the giant goldrich Horne massive sulfide deposit (Chartrand and Cattalani, 1990; Hannington et al., 1999). Pearson and Daigneault (2009) redefined the BRG as a megacaldera complex (BRMCC) based on (1) a radial and concentric organization of synvolcanic mafic to intermediate dykes, (2) an overall dome geometry defined by the volcanic strata and younging directions and (3) the distribution of felsic and intermediate to mafic subaqueous volcaniclastic units. The 40 km × 80 km Misema Caldera is composed mainly of tholeiitic basalt whereas the New Senator Caldera is composed of both tholeiitic and calc-alkaline units (Fig. 2B). The felsic-dominated, Noranda Caldera records the youngest event (de Rosen-Spence, 1976; Lichtblau and Dimroth, 1980) but the position of the Horne Mine rests outside this structure, as recognized by Kerr and Mason (1990). Within the context of the megacaldera, the Horne Mine could be readily explained as part of the New Senator Caldera (Mueller et al., 2008). It is also within this context that mafic dykes and sills, volcaniclastic deposits and felsic dome-flow complexes will be discussed in the next section.

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Fig. 1. Geology of the Archean Abitibi greenstone belt (Quebec segment) with the location of various calderas. Blake River Megacaldera Complex (Misema-New SenatorNoranda calderas) in the Southern volcanic zone (Fig. 2) is the focus of study. The Val-d’Or arc floored and interstratified with komatiites is located on strike to the east. Note the extensive east-trending faults bordering and dividing volcanic zones. Pie diagrams exhibit distribution of lithological units.

3. Volcanic architecture 3.1. Blake River Complex mafic dyke system The Blake River mafic dyke and sill system (Figs. 2B and 3) displays four distinct geometric patterns that include (1) the dominant regional ring organization, (2) N-trending linear dykes (N010 and N350) traceable for 10 s of kilometres, (3) local circular to ellipsoid, 2–12 km diameter ring dykes and (4) a well-defined, triple junction dyke pattern 10 s of kilometres in size represented by the Horseshoe dyke arrangement (Fig. 3). These patterns together support the hypothesis of amalgamated mafic shield complexes. N-trending linear dykes in the northwest portion of the BRG are interpreted as rift-fracture features. One of these terminates at the large ring dyke complexes of the Montsabrais Volcanic Complex (MVC; Figs. 2B and 3). Ellipsoid inner and outer ring faults of the Misema Caldera discussed by Pearson and Daigneault (2009) are interpreted as zones of weakness where dyke injection and subsidence led to sector collapse (Fig. 2B). The MVC and the Reneault Volcanic Complex (RVC; Dimroth et al., 1982, 1985; Figs. 2B and 3) are two prominent ring dyke systems, from which abundant subaqueous mafic pyroclastic material originated (Dimroth et al., 1982, 1985). The Jevis South Tuff is believed to have originated from a similar but smaller ring dyke complex, referred to as the Jevis South Volcanic Complex (JSVC; Fig. 3). The km-scale, circular-ellipsoidal mafic dyke geometry is characteristic of the Misema shield and caldera event. Major triple junction structures such as the Horseshoe dyke arrangement (Fig. 3; Pearson and Daigneault, 2009) compares favourably with rifts observed on Hawaii (Fiske and Jackson, 1972) and Tenerife (Walter et al., 2005) shield volcanoes. Triple junction rifts are extensional structures controlled by the magma chamber and its pulses, as well as by regional and gravitational stresses (Fiske and Jackson, 1972; Gudmundsson, 2006). The parallel arrangement of the N010-trending, km-long dyke-filled linear rifts suggests the development of new shield volcanoes, as documented by Swanson

et al. (1976) with the overlapping Mauna Loa and Kilauea shield volcanoes and their parallel aligned rifts. Magma chamber withdrawal and migration contributes to mafic summit caldera formation, subsequent collapse and alignment as well as the clustering and overlapping of edifices. These processes are evident in modern subaqueous environments where the lateral collapse of overlapping, 1–2 km diameter summit calderas is recorded at Pacific seamounts such as the Vance (e.g. V-D and V-E volcanoes), President Jackson (PJ-G volcano) and Taney Seamounts (T-A volcano; Clague et al., 2000). Magma chamber migration and withdrawal into dykes is also common (Tilling and Dvorak, 1993; Geshi et al., 2002). This explains elegantly the shift observed in the BRG from linear rifts to overlapping ring dyke complexes. The oval geometry of the ring dyke complexes commonly mimics primary ellipsoidal caldera geometry (Fig. 3) and is consistent with geometries observed at modern mafic summit calderas (e.g. Axial Seamount and Mauna Loa). Within the BRG, dyke patterns, regional mafic-felsic volcanic rock distributions and overall geometry compare favourably to modern amalgamations of subaqueous shield volcanoes. As suggested by Swanson et al. (1976) and confirmed by Rubin (1992), faulting and subsidence of shield volcanoes can be caused by a forceful injection of magma into rift zones, such as in the Holie Pali and Hilina fault systems on Hawaii. The inner ring fault of the BRMCC (Pearson and Daigneault, 2009) is considered a zone of weakness that facilitated magma injection, allowing for subsidence following the assembly of the peripheral shield complexes. The dyke injections pave the way for graben-type subsidence, which would account for the collapse of the Misema Caldera and subsequent formation of the New Senator and Noranda calderas. The collapse of the Misema Caldera along the inner ring fault was a protracted event that commenced during shield amalgamation. Subsidence occurred as a result of piecemeal faulting, dyke emplacement and lateral extrusive events, rather than as a result of a short-lived explosive magma draining event (e.g. Valles Caldera; Smith and Bailey, 1968). Consequently, the

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Fig. 2. (A) General geology of the Blake River Group with the distribution of Misema and Noranda subgroups (Goodwin, 1977, 1982). Numerous volcanic centres are cored by plutonic rocks. The various Noranda felsic cycles I–V were used to designate specific volcanic centres (e.g. Duprat). Previous age determinations and their locations are indicated. (B) Outline of the three calderas in the Blake River Group: A-Misema, B-New Senator, and C-Noranda. Pyroclastic sequences are located at the margin of the megacaldera complex: 1-d’Alembert Tuff; 2-Kino North Tuff; 3-Jevis South Tuff; 4-Stadacona Tuff; 5-Montsabrais volcaniclastic sequence. TTG-tonalite-trondjemitegranodiorite; HoCF-Horne Creek fault; HuCF-Hunter Creek fault; BF-Beauchatel fault.; PF, Pink fault, ABCF, ABC fault, TS fault. Note the location of mines along the New Senator Caldera margin faults (modified from Pearson and Daigneault, 2009). Volcanic centres are identified in italic. See Refs. Ayer et al. (2005), Corfu and Noble (1992), Goutier and Melanc¸on (2007), and Vaillancourt (1996).

800–1250 m-thick dykes and sills associated with shield amalgamation should have a variable intrusive history and record numerous injection phases (e.g. Gudmundsson, 1984; Mueller and Donaldson, 1992). The dyke-sill intrusions are gabbroic to quartz dioritic with one ring dyke complex having a central pyroxenite heart (e.g. Reneault volcanic centre; Figs. 2B and 3) instead of a granitoid core (e.g. Montsabrais volcanic centre). Metre- to 10 s of m-thick fine grained basalt dykes are the higher level counterparts of gabbroic and dioritic intrusions and were highly irregular and

discontinuous when injected into unconsolidated volcaniclastic deposits. Cole et al. (2005) remarked that ring dykes may represent caldera chambers at depths generally >2 km. Similar to polyphase plutons, mafic intrusions (Piercey et al., 2008; Thanh et al., 2009), and thick mafic sills (Wingate, 2001; Peressini et al., 2007) differentiate and evolve with time. In gabbro sills, the exsolution of relatively felsic melt pockets develop as the last event. Thick sills may require several million years to develop and cool (Mortensen, 1993; Wingate, 2001); for mafic complexes, cooling periods of

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Fig. 3. General geology of the Blake River Group showing the distribution and architecture of dykes and sills. Note the parallel array of linear faults and dykes with ring dyke terminations (e.g. MVC). Overlapping ring dyke complexes are prominent in the north and may be connected by linear dyke-filled faults (modified from Pearson and Daigneault, 2009).

10 M.y. are not uncommon, with a similar time frame for injection periods (e.g. Peressini et al., 2007). 3.2. Volcaniclastic sequences The volcaniclastic deposits are located between the inner and outer ring faults and are aligned parallel to the ellipsoidal architecture of the Misema Caldera, suggesting that they were extruded during the early, mafic-dominated caldera event (Fig. 2B). The mafic to intermediate subaqueous pyroclastic rocks associated with volcanic centres were recognized by early mappers (e.g. Tassé et al., 1978; Dimroth et al., 1985) and were revaluated in detail by Ross et al. (2011). Some of the edifices, such as the Montsabrais volcanic centre (Figs. 2B and 3) indicate shoaling, as suggested by wave-generated cross-beds directly overlying pillowed flows. Initial volcanic facies analyses of the d’Alembert Tuff (Tassé et al., 1978), Dufresnoy pyroclastic sequence, termed the Clericy Fiskite (after Richard Fiske; Trudel, 1978), and Montsabrais pyroclastic breccia (Dimroth et al., 1985), showed deposition under sediment gravity flow conditions. Modern volcanic facies mapping of the Jevis South Tuff and Kino North Tuff have recently been conducted (Pilote et al., 2008; Mueller et al., 2008, 2009a), and the former (Fig. 4) is a 100 m-thick succession composed of numerous pyroclastic density current deposits interstratified with pillowed flow units and felsic flow breccia. Basaltic and gabbroic dykes and sills intrude both the pillowed flows and the subaqueous pyroclastic deposits. Because abundant pyroclastic debris is adjacent to ring dyke complexes, Dimroth et al. (1985) suggested an explosive centre. A subaqueous summit caldera setting is still a viable depositional setting, as is evidenced by recent explosive summit eruptions documented on Kiluea (Fiske et al., 2009).The Jevis South Tuff represents subaqueous, below storm-wave base deposition, whereby the succession is interstratified between pillowed flows, overlies subaqueous felsic flow breccia (Fig. 4) and is intruded by dykes forming peperite. The pyroclastic deposits are composed of 7 distinct eruption-fed density current units that are 5–45 m-thick and fine upward. A complete density current deposit has a tripartite division with local subdivisions due to water ingestion during flow (Fiske and Matsuda, 1964; Tassé et al., 1978; Mueller and White, 1992): (I) a 2–25 m-thick massive, matrix to clast supported lapilli tuff breccia

(Fig. 5A and B); (II) 1–10 m-thick lapilli tuff (Fig. 5C) with normal or inverse grading; (III-A) 0.5–15 m thick coarse- to fine-grained tuff with parallel to wavy stratification and low-angle cross-beds with erosive bases (Fig. 5D), or (III-B) 0.1–5 m-thick beds of parallel and wavy laminated, coarse-to fine-grained tuff. The transport process ranges from laminar debris flow type conditions (Division I), to high and low-density turbidity currents with highly variable flow and fallout rates (Division II and III-A), and finally to prominent suspension fallout from the water column (Division III-B). Deposits are well-bedded and stratified similar to the explosive deposits of the subaqueous Myojin Knoll Caldera (Fiske et al., 2001). The base of the pyroclastic density current deposits is composed of brecciato lapilli-sized pumice clasts (Fig. 5A and B), as well as clasts from underlying felsic flows. Contacts between pyroclastic density current deposits may be sharp (Fig. 5C) or diffuse. Commonly, only a change in pyroclast size enables the distinction between brecciasize bedded units. 3.3. Felsic volcanic centres Felsic volcanic edifices play a prominent role in BRG history, as ore deposits (e.g. Horne Mine) are commonly linked to felsic volcanism (Fig. 2B). The distribution of felsic rocks follows the general ellipsoidal trend of the Misema Caldera, but half of their total area in plan view is concentrated within the New Senator and Noranda Caldera limits (Fig. 2B). The shape of the New Senator Caldera in the NW sector is underlined by the distribution of the felsic centres. Circular to ellipsoidal geometry is also associated with local ring dyke complexes (e.g. Clifford volcanic centre; Fig. 3). Domes arranged in a circular pattern within a caldera are common to ashflow calderas and can evolve over several million years (e.g. 4 M.y., Valles Caldera; Cole et al., 2005). Principal characteristics of felsic units are best exposed in the New Senator and Noranda calderas. The Noranda Caldera is the most intensively studied because the felsic flows are traceable across the synvolcanic caldera faults (e.g. Gibson and Watkinson, 1990). The flows are massive through lobate to brecciated divisions of de Rosen-Spence et al. (1980). The Waite Rhyolite flow 8, Hére Creek Rhyolite flow 1 and Don Rhyolite flow 5 are examples of these types of flows, and range from 30 to 400 thick, are traceable

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Fig. 4. Volcanic facies map of the Jevis South Tuff with pyroclastic density current deposits interstratified with pillowed and brecciated flows and a felsic breccia unit. The dykes and sills with dyke-in-dyke injections intrude the pyroclastic debris. A medium-grained gabbro (MW-02-07) related to this sequence was sampled along strike.

for 10 km along strike and are thought to be derived from the high-level Flavrian and Powell plutons (de Rosen-Spence, 1976; de Rosen-Spence et al., 1980; Figs. 2B and 3). Using the ‘Rhyolite Zones’ of de Rosen-Spence (1976), the Noranda Caldera was divided into 5 (intra)caldera cycles (Gibson and Watkinson, 1990). Each volcanic cycle (Fig. 2B) is considered to be a differentiation trend from basalt-andesite to andesite-rhyolite. In contrast, the southern part of the New Senator Caldera exists outside of the Mine Series and is therefore poorly studied. The 800–1000 m thick Glenwood rhyolite (Figs. 6 and 7) is an area being remapped in detail (L. Moore, Ph.D. in progress; Genna et al., 2011). Facies mapping has revealed a subaqueous felsic dome-flow complex with (1) an early constructive dome-flow building stage composed of aphyric lavas, and (2) late, quartz-feldspar phyric endogenic lobes that inflate the existing complex. The highly viscous 100–400 m thick aphyric flows display a lateral change in volcanic facies architecture from lobate to brecciated flows (Fig. 7). These flows have 5–50 m-thick massive lobate interiors with flow-banding and local columnar joints (Fig. 5E). The quartz-feldspar phyric endogenous lobes contain 2–5% quartz and 20–40% feldspar and intrude the aphyric volcanic flows. As seen in Fig. 7, these lobes ballooned and then began to flow laterally. Mafic dykes that cut the complex are associated with more regional extension of the host shield complex and feed overlying mafic pillowed and brecciated lava flows. Larger mafic dykes delineate the margins of the Glenwood rhyolite (Fig. 6) and intrude the felsic dome-flow sequence (Fig. 7). The 0.20–5.0 m-thick dykes are massive, have flow-banded margins, and display vesiculation and brecciation characteristic of shallow level emplacement near the Archean seafloor. Key sample locations (Fig. 7) were chosen to constrain the age of the Glenwood felsic event.

4. Blake River Megacaldera complex The distribution of volcanic rocks and mafic dyke and sill complexes, combined with fault geometry, are the striking features of the Blake River Megacaldera Complex. Based on critical observations, a nested, graben-type caldera complex within a megacaldera was defined by Pearson and Daigneault (2009). The Misema shield and caldera system is the early mafic volcanic expression of the BRG (Fig. 2B). Pearson and Daigneault (2009) argued for major volcanic subsidence due to the weight of the overlying large-scale shield volcano amalgamation, as indicated by inner and outer ring faults. The presence of a giant mafic magma chamber underlying the Misema Caldera is inferred by subhorizontal reflectors imaged in seismic profiles (Green et al., 1990) and is a prerequisite for generating the observed widespread mafic plumbing system. The mafic intrusive system makes up 20% of the BRG, further supporting a large mafic magma chamber. The inner and outer ring ellipsoidal fault system of the Misema structure with associated ring dykes, indicates zones of magma drainage and migration. The Dufresnoy fault, a segment of the inner fault system, facilitated the emplacement of the D’Alembert pluton (samples MW-01-07 and MW-03-07) and displays a reverse sense of movement inherent to caldera collapses (Acocella, 2007; Mueller et al., 2008). The New Senator Caldera formed within the envelope of the Misema Caldera and terminates at the inner ring fault zone (Fig. 2B). The faults of this event are superposed on and affected the Misema Caldera. The NSC is transected by 070◦ faults of the Noranda Caldera, giving a relative timing of events and further supporting the inference of an earlier caldera-forming event. The New Senator Caldera exhibits a box-work, graben-type structure as expressed by the bordering faults. The prominent McDougall–Despina fault (e.g. Setterfield, 1987) represents the NE boundary, whereas the

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Fig. 5. Characteristics of the Jevis South Tuff (A–D) and Glenwood rhyolite (E). Arrows indicate top. (A) A pyroclastic lapilli tuff breccia intruded at the interface (yellow arrows) by another pyroclastic unit and by an irregular basalt dyke. Scale: field book = 20 cm. (B) Pumice clast in lapilli tuff breccia with quartz-filled elongated vesicles. Scale: coin = 1.8 cm. (C) Sharp contact (arrows) between two distinct pyroclastic facies. Scale: pen = 13 cm. (D) Upper part of a pyroclastic density current deposit: a graded, stratified and cross-bedded volcanic facies. Scale: pen = 14 cm. (E) Felsic aphanitic endogenous lobe with well-defined columnar joints. Black arrow points to a columnar joint. Scale: pen = 13 cm.

ABC, TS and Pink faults delineate the southwest margin (Fig. 2B). The significant role of the McDougall-Despina fault in the history of the New Senator Caldera is substantiated by a dip-slip collapse of >750 m (Setterfield et al., 1995), compatible with a subsidence structure. The felsic-dominated Noranda Caldera is also restricted to the confines of the inner ring of the Misema Caldera. As with the NSC, structures associated with the NC have overprinted the rim zones of the MC between the inner and outer ring faults. Because the

NC is the youngest event, it was readily identified, but it hindered recognition of previous calderas by masking major bounding faults and older dome systems. The heart of the NC contains numerous synvolcanic plutons (e.g. Flavrian and Powell) that are related to both the NSC and the NC (Pearson and Daigneault, 2009), with the Dufault and D’Alembert plutons occurring late in caldera history. The NC has a graben-type geometry bounded by the Hunter Creek Fault (HuCF) to the northwest and the Horne Creek Fault (HoCF) to the south and is terminated by the inner ring Dufresnoy Fault (DF)

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640 000

650 000

645 000

Quémont Horne 5 345 000

McPhee Dyke Fault

2699 ± 1 13

2702 ±1.6 11 2702.9 ±1.4 9

Kiwanis Pluton

2696.6 ±.712

Glenwood synvolcanic Fault

2702.2 ±3 10

Glenwood Rhyolite

2698.2 ±.8 14

2700.6 ± 1.6 Ma*

5 340 000

McPhee dyke Fault

Cadilla

er L c Lard

ake Fa

ult Zon

e

0

1

2

kilometres

Sample site in this study with reference to Table 2 *Age of Lafrance et al. 2005 Fault Mine 12

Sedimentary rocks

Volcaniclastic rocks

Gabbro-diorite

Plutonic rocks

Felsic volcanic rocks

Mafic volcanic rocks

Fig. 6. Geology of the Rouyn-Noranda area, representing the lower part of the New Senator caldera (Fig. 2b). Sample locations are indicated. 9 = LM-08-03; Glenwood rhyolite, endogenous dyke-lobate intrusion (see Fig. 7 for detailed volcanology); 10 = LM-08-04, Glenwood rhyolite, quartz-phyric endogenous brecciated lobe; 11 = LM-03-06, Kiwanis tonalitic pluton; 12 = LM-02-06, quartz-diorite phase of the mafic dyke sequence; 13 = LM-07-02A, felsic dyke; 14 = WR-06-07, McPhee dyke. Fishroe rhyolite south of the New Senator caldera has an age of approximately 2701 Ma.

Volcanic facies of Glenwood Rhyolite (Cap d'Ours)

Mafic breccia

2702.2 ± 3 Ma

2702.9 ± 1.4 Ma Aphyric felsic flow phase: dome construction

N

Quartz- and feldsparphyric phase: dome inflation

Aphyric phase: dome construction

0

200 m

Sites for age determinations

Legend Mafic dyke Endogenous lobe facies (quartz- and feldspar-phyric) Flow breccia facies Massive lobate to in-situ brecciated facies Inferred geometry of quartz-feldspar porphyry Inferred geometry of felsic lobes

Fig. 7. Volcanic facies map of the Glenwood rhyolite with the various flow facies. Generally there is a major dome building phase, followed by a quartz-phyric dyke and an endogenous dome stage causing inflation. Moore, PhD thesis.

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

to the northeast (Fig. 2B). The original NC stratigraphy suggests pre-, syn-, and post-caldera emplacement of felsic to intermediate flows, whereby caldera subsidence varied from 500 m at the northern margin to 1200 m at the southern extremity (de Rosen-Spence, 1976; Gibson and Watkinson, 1990). Based on this asymmetric subsidence, Pearson and Daigneault (2009) proposed a trapdoor caldera, although a piecemeal caldera is a viable alternative because the numerous volcanic blocks (e.g. Powell and Flavrian Blocks) display different vertical displacements (Péloquin et al., 1990).

9

has exerted a control on fold nucleation. Primary volcanic structures such as synvolcanic depressions, dome-flow complexes or plutons (Fig. 10A–C) affected by horizontal shortening will produce the superposition of anticlines and synclines. An original ring dyke pattern associated with a summit caldera will conserve its initial geometry despite the horizontal shortening because of the dykes’ original subvertical attitude (Fig. 10A and B). Peripheral local ring dyke structures distributed along the Misema Caldera are considered primary structures locally accentuated by regional deformation. Examples include Montsabrais, D’Alembert, Clifford, Tannahill, Colnet, and Jevis South volcanic centres (Fig. 3).

5. Deformation pattern of the BRG The BRG experienced regional deformation attributed to NS horizontal shortening (compression; Dimroth et al., 1982, Daigneault et al., 2002) which produced folds, faults and shear zones. Although deformation overprints volcanic construction, several structural patterns are considered to be inherited from the pre-existing volcanic architecture. The BRG is characterized by a heterogeneous deformation pattern with a central domain of relatively low-strain, in which subhorizontal strata lie. In contrast, the marginal inner-outer ring zones of the Blake River Megacaldera complex represent a domain of higher strain, expressed by subvertical volcanic units and the presence of subvertical ductile planar fabrics. Variations in younging directions in the volcanic rocks indicate folding. Anticlinal and synclinal fold axial traces (Fig. 8), traceable for 10 s of kilometres, are recognized at the BRG margin and are generally east-trending. The fold axial traces in the marginal zone (10 s of km long) follow the ellipsoidal geometry of the Blake River Megacaldera Complex, whereas the volcanic strata in the central BRG display a more variable attitude. Fold axial traces are generally discontinuous and only a few kilometres long. Locally, mafic ring dyke complexes without a central plutonic core exhibit doubly-plunging axes (e.g. the Renault Volcanic Complex; Fig. 8). The main planar ductile fabric is a variably developed slaty cleavage. Generally, the E-trending cleavage displays a subvertical dip, except close to the southern Cadillac-Larder-Lake Fault Zone (Fig. 8) where moderately dipping fabrics are prevalent. As for the fold axial traces, the development of the cleavage is prominent at the BRG margin and reflects strain partitioning. A large magma chamber beneath the Misema Megacaldera (Pearson and Daigneault, 2009) can explain elegantly the relative low-strain domain with moderate to shallow dipping strata in the central BRG segment (Fig. 9a). The pluton acted as a buttress to deformation, helping to preserve the overlying volcanic rocks from significant shortening and tilting. In contrast, the caldera periphery, interpreted as the margin of a shallow magma chamber, displays significant faulting, folding and possible tilting of blocks (Fig. 9a). The inner to outer ring zone of the Blake River Megacaldera Complex represents the principal area of BRG deformation. Anastomosing faults and fractures are readily documented in analogue experiments and field observations (Fig. 9b). The faulted and tilted blocks of the inner - outer ring fault system are favourably disposed to folding during regional deformation. A significant relationship is observed between the fold axial traces and the principal cleavage trajectories. Major E-W trending cleavage trajectories are locally axial-planar with fold axial traces in the northern and southern parts of the BRG, but transect fold axial traces in the eastern portion of the BRG with angular relationships of 60–70◦ (Fig. 8). The discontinuity of the fold axial traces, the cross-cutting relationship between fold axial traces and the main cleavage, as well as the locally doubly plunging fold axes in mafic ring dykes are features more compatible with development over a previous volcanic inheritance. The relationship between fold axial traces and mafic ring dykes and rhyolitic dome-flow complexes can be integrated into a primary volcanic architecture that

5.1. Folds in the southern New Senator Caldera The complexity of intracaldera volcanic sequences is readily expressed in the southern portion of the NSC (Fig. 11) where the main geological elements of synvolcanic titling and collapse with supersposed deformation are well exposed. Several synvolcanic fractures can be recognized or interpreted. The southeast NSC is limited on the west by the multiphase McPhee dyke that was injected into a NNW-trending synvolcanic fault. The Glenwood Rhyolite is limited on the west by a N-trending mafic dyke and on the east by a swarm of thinner NE-trending dykes that manifest the synvolcanic Glenwood Fault. Mafic dykes in the Glenwood fault are interpreted as feeders to the pillowed and brecciated mafic volcanic sequences located directly to the northeast. E-trending fold axial traces are located south of the Glenwood felsic complex (Fig. 11). The volcanic strata (S0 ) exhibit a global Z asymmetry, which is progressively accentuated from west to east. A pre-folding S0 reconstruction suggests a N-S attitude to the volcanic strata (Fig. 11), consistent with the attitude adjacent to the McPhee fault. This segment of NNW-trending mafic volcanic rocks is in abrupt contact with the Glenwood Rhyolite and the limit between them is faulted. A synvolcanic fault separating different tilted volcanic blocks is inferred to form during caldera margin collapse (e.g. Geshi et al., 2002; Acocella, 2007). This reconstruction demonstrates how fold patterns can be used to delineate synvolcanic architecture. Folds in this sector represent the manifestations of ductile deformation near the Cadillac Larder Lake fault, which limits the Misema Caldera on the south (Fig. 11). 6. U-Pb sampling and methodology Precise U-Pb geochronological analyses were performed on samples from selected areas in order to help explain the relative timing of volcanic events (Table 2 and Appendix A). All areas were mapped to verify whether units are intrusive or extrusive, as extrusive events account for volcanic construction and intrusive events document edifice inflation. Mafic deposits dominate the BRG and best chronicle the Misema event and the timing of the BRG synvolcanic fault system; whereas the felsic sequences better document the evolution of the New Senator and Noranda events. Only the coarsest-grained, least altered and least deformed rocks were chosen, but all rocks have undergone subgreenschist to greenschist facies metamorphism. Because the zircon content of mafic and felsic rocks may be minimal, 30–40 kg samples were collected. Felsic exsolution pods within mafic gabbroic and dioritic dykes were sampled separately using a rock saw equipped with a diamond-blade. All mineral treatment protocols and analyses were carried out at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, Vancouver, Canada. The detailed analytical procedures for U-Pb geochronology, including the chemical abrasion isotope dilution-thermal ionization mass spectrometry (ID-TIMS) technique applied to single grains of zircon, following the method of Scoates and Friedman (2008).

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W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

Fig. 8. Structural pattern of the Blake River Group with dip trajectories. Note the presence of circular to ellipsoid trajectories that mimic felsic domes or mafic ring dyke structures. The linearly arranged synclines and anticlines are prominent between the inner and outer ring faults. A late Archean NE to ENE-E-trending schistosity cuts the folds. DPMFZ, Destor Porcupine Manneville fault zone; CLLFZ, Cadillac Larder Lake fault zone; PFZ, Parfouru fault zone. See Refs. Henry et al. (1997), Kennedy et al. (2004), and Roche et al. (2000).

Table 2 Distribution of U-Pb age determinations in the Blake River megacaldera (TIMS and chemical abrasion method; location in UTM NAD 83). No

Rock sample

Protolith

Age Ma ± 2

Interpretation

Locus and description

1

WUM-07-07

Diorite

2701.1 ± 1.2

2 pt wtd 76 avg

2

MTS-08-04

Pegmatoidal gabbro

2698.7 ± 1.1

3 pt wtd 76 avg

3

WR-06-03

Tonalite

2695.9 ± 2.8

3 pt regression

4

MW-02-07

Gabbro

2700.5 ± 1.4

3 pt wtd 76 avg

5

BMR-13-11-05

Gabbro

2704.0 ± 3.4

4 pt regression

6

WUM-35A-07

Gabbro

2704.2 ± 1.4

4 pt regression

7

MW-01-07

Tonalite

2692.0 ± 1.2

2 pt wtd 76 avg

8

MW-03-07

Tonalite

2692.1 ± 1.1

5 pt wtd 76 avg

9

LM-08-03

Rhyolite

2702.9 ± 1.4

3 pt wtd 76 avg

10

LM-08-04

Rhyolite

2702.2 ± 3.0

1 76 date

11

LM-03-06

Tonalite

2702.0 ± 1.6

4 pt regression

12

LM-02-06

Diorite

2696.6 ± 0.7

7 pt wtd 76 avg

13

LM-07-02A

Rhyolite

2699.0 ± 1.0

6 pt wtd 76 avg

14

WR-06-07

Gabbro

2698.2 ± 0.8

5 pt wtd 76 avg

15

WR-04-07

Pegmatoidal gabbro

2696.4 ± 1.5

4 pt wtd 76 avg

16

WUM-19-07

Tonalite

2680.7 ± 1.1

2 pt wtd 76 avg

Misema caldera: Montsabrais ring dyke UTM17–614637/5371326 Misema caldera: Montsabrais ring dyke UTM17–614632/5371319 Misema caldera: core of Montsabrais volcanic centre; UTM17-616188/5370823 Misema caldera: gabbro sill - Jevis, UTM17–651021/5358334 Misema caldera: Clericy gabbro I intruded by Clericy tonalite; UTM17–659091/5355241 Misema caldera: gabbro II intruded by Clericy tonalite 2696 Ma; UTM17–659514/5355061 Interface Misema-Noranda caldera: marginal d’Alembert pluton; UTM17–649377/5358280 Interface Misema-Noranda caldera: central d’Alembert pluton; UTM17–648869/5358290 New Senator caldera: irregular to lobate QFP-dyke, Glenwood rhyolite; UTM-17–648177/5344256 New Senator caldera, Glenwood rhyolite; UTM17 N–648375/5344279 New Senator caldera: Kiwanis pluton east, melano-tonalite; UTM17–646118/5344532 New Senator caldera: Diorite-quartz diorite dyke system; UTM17–645963/5344521 New Senator caldera: felsic dyke in ponded flows; UTM17–644509/5345183 New Senator caldera: border zone, McPhee dyke, quartz-rich gabbro; UTM17–642691/5343393 Misema caldera: east arm Horseshoe dyke; UTM17–627599/5345001 BRG-Bousquet-La Ronde mine: tonalite dyke in Kewagama Formation; UTM17–689567/5349216

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

11

Fig. 9. Formation of fold patterns, synclines and anticlines due to shortening. (A) Reverse and normal faults with block rotation formed during caldera formation. Tectonic shortening developed fold patterns and accentuated faulting at the margin where the influence of the subjacent magma chamber was minimal. In effect, the folds were buttressed against the magma chamber. (B) Experimental studies of fractures and faults developed between inner-outer rings prior to and following shortening. See Ref. Chadwick and Howard (1991).

Reported ages are based on weighted averages of 207 Pb/206 Pb dates for samples with concordant zircon data and upper intercepts for discordant data. 7. Precise U-Pb age determinations, results and sample descriptions 7.1. Montsabrais area Three samples were collected from the Montsabrais area (Fig. 12, Table 2); two from a diorite dyke belonging to the ring dyke system that represents the deep-seated roots of the Montsabrais volcanic complex, and one from the central tonalitic Montsabrais pluton which cores the ring dyke complex. Sample WUM-07-07 (no. 1) is a medium- to fine-grained diorite, giving an age of 2701.1 ± 1.2 Ma (Fig. 12-1a-c). The diorite has less than 5% visible interstitial quartz, and intrudes pillowed flows and shallow water volcaniclastic deposits. Sample MTS-08-04 (no. 2) was collected approximately 100 m from WUM-07-07 and is from a late felsic (pegmatoidal) pocket of quartz-amphibole-plagioclase

in the massive medium-to fine-grained diorite (Fig. 12-2a-c). An age of 2698.7 ± 1.1 Ma was obtained. Sample WR-06-03 (no. 3) is the leucotonalitic phase of a tonalite-granodiorite plutonic suite of the Montsabrais pluton (Fig. 3). This plutonic complex intruded the core of the Montsabrais volcanic centre. The sample is a fine-to medium-grained, equigranular leucotonalite with locally abundant synvolcanic fractures and hydrothermal alteration (Fig. 12-3a-c). An age of 2695.9 ± 2.8 Ma was obtained. 7.2. Jevis-Clericy area Three samples from a mafic dyke system and two from the d’Alembert Pluton were collected in the Jevis-Clericy area (Figs. 3 and 13, Table 2). Sample MW-02-07 (no. 4) is from a linear gabbro dyke-sill, several 100 m thick and traceable for several kilometres along strike. The dyke intruded high into the volcanic sequence as it cuts the subaqueous pyroclastic rocks and massive to pillowed flows near the Jevis South Volcanic Complex. It gave an age of 2700.5 ± 1.4 Ma (Fig. 13-4a-c). The gabbro displays a subophitic

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Fig. 10. (A) Deformation of a mafic ring dyke complex showing a complex anticline-syncline-anticline pattern. (B) Development of an anticline by the intrusion of a felsic dome or resurgent synvolcanic granite pluton. (C) Plan view of a mafic ring dyke with a central resurgent dome. (D) Non-deformed Wolf summit caldera (Galapagos) with concentric and anastomosing faults-fractures-fissures that at depth are linked to dykes (adapted from Munro and Rowland, 1995).

texture in outcrop that is commonly referred to as leopard texture, and was later subjected to hydrothermal alteration. The polyphase Clericy gabbro was sampled at two sites and is medium- to coarsegrained and up to 1 km thick. BMR-13-11-5 (no. 5) represents the central massive segment, which is intruded by the 2696 Ma Clericy tonalite, and gave an age of 2704.0 ± 3.4 Ma (Fig. 13-5a and c). A N60 to N70, hydrothermally altered (chlorite-epidote) fracture set is omnipresent in the gabbro (Fig. 13-5c). Sample WUM-35A07 (no 6) produced an age of 2704.2 ± 1.4 Ma, (Fig. 13-6a) and is the most evolved phase, as indicated by the well-developed felsic (pegmatoidal) pockets of quartz-amphibole-feldspar (Fig. 13-6c).

The d’Alembert pluton is composed of an early leucotonalite phase and a later tonalite phase. It intrudes the inner ring fault, which was referred to in this segment as the Dufresnoy fault. The pluton was sampled from two sites. Sample MW-01-07 (no. 7) is the marginal tonalite phase and gave an age of 2692.0 ± 1.2 Ma (Fig. 13-7a-c). The abundance of mafic to intermediate volcanic xenoliths is consistent with a high level emplacement, probably near the roof of the intrusion. Sample MW-03-07 (no. 8) yielded an age of 2692.1 ± 1.1 Ma (Fig. 13-8a and b) and corresponds to the central tonalite phase with far fewer xenoliths. Grain size increases towards the centre of the pluton, where it

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

640 000

13

650 000

645 000

ult reek Fa

C Horne

5 345 000

Glenwood Rhyolite

Glenwood synvolcanic Fault

80

McPhee Dyke Fault 80

80

70 70 75 70

70 80 50 70

80

70 80

70

60

70 60

60

65

55

5 340 000

60 50 50

Cadillac Larder Lake Fault Zone

Volcaniclastic rocks

Sedimentary rocks Plutonic rocks

Felsic volcanic rocks

Gabbro-diorite 0

Mafic volcanic rocks 1

kilometres

2

Mine

Fault Syncline Anticline Bedding plane trajectories with top

50°

Prefolding recontruction

Fig. 11. Southern segment of the New Senator caldera (NSC) with folds, synclines and anticlines. Note the dyke-sill system bounding different volcanic dome-flow complexes that locally formed in a volcano-tectonic collapse structure (e.g. Glenwood rhyolite). The initial stratal arrangement below the Glenwood Rhyolite is N-S.

is medium to coarse-grained with local, 1 cm long plagioclase (Fig. 13-8c).

7.3. Rouyn-Noranda area Six samples were collected from the Rouyn-Noranda area, which is referred to as the southern portion of the New Senator Caldera (Figs. 2b, 3, 6 and 14, Table 2). Two samples were collected from the Glenwood felsic centre (Fig. 7). Sample LM-08-03 (no. 9) yielded an age of 2702.9 ± 1.4 Ma (Fig. 14-9a and b). This sample is a quartzfeldspar phyric rock containing 2-5%, 0.5–2 mm quartz phenocrysts and was derived from an endogenic lobe (Figs. 7 and 14-9c). Sample LM-08-04 (no. 10) was sampled from the margin of a quartz-phyric endogenous brecciated lobe. It is fine grained, containing up to 1% quartz phenocrysts ranging in size from 0.5 to 10 mm (Fig. 14-10c). The age yielded is 2702.2 ± 3 Ma (Fig. 14-9a). Sample LM-03-06 (no. 11) is from the melano-tonalitic Kiwanis pluton, an E-trending 200 m thick sill-like body that intrudes the mafic and felsic aphanitic flows of the BRG. The sample was collected from the western portion of the sill (Fig. 6) and yielded an age of 2702.0 ± 1.6 Ma (Fig. 14-11a and c). Sample LM-02-06 (no. 12) is from a 300 m thick NE-trending quartz diorite dyke and yielded an age of 2696.6 ± 0.7 Ma (Fig. 14-12a-c). This dyke intrudes both the felsic volcanic rocks and the tonalitic Kiwanis pluton (sample LM-03-06). The quartz-diorite contains locally coarse-grained, metre-scale, quartz-amphibole-plagioclase pods (Fig. 14-12c). This complex dyke-sill system was injected into a series of synvolcanic

E-W and NS-NNE trending fracture zones (see Fig. 6). Sample LM07-02A (no. 13) is from a quartz-feldspar dyke that cuts a ponded mafic flow. The dyke contains 5–15%, 1–3 mm long quartz grains (Fig. 14-12c). The contact with the mafic flow is sharp and the dyke margins are chilled. The dyke yielded an age of 2699.0 ± 1 Ma (Fig. 14-13a, b). Sample WR-06-07 (no. 14) was collected from the McPhee dyke (Fig. 6), a 500 m-thick dyke of composite gabbro to quartz-rich gabbro. The sample is from a more evolved portion of the dyke consisting of a quartz-rich phase. An age of 2698.2 ± 0.8 Ma was obtained (Fig. 14-14a and b). 7.4. Horseshoe dyke Sample WR-04-07 (no. 15) from the Horseshoe dyke yielded an age of 2696.4 ± 1.5 Ma (Fig. 15-a-c, Table 2). The Horseshoe dyke arrangement represents a volcanic rift zone composed of a kmthick, multiphase dyke system. All arms of the rift were sampled, but only the most evolved phase with large pockets and 5–20 cmthick quartz-amphibole-plagioclase veins yielded results. 7.5. Bousquet area A late, fine-to medium-grained tonalite dyke intruded the Kewagama Group sedimentary rocks near the Bousquet-LaRonde Mine (Table 2). The dyke contains fragments of the host rock and displays a chilled margin contact (Fig. 15-16c). The purpose of sampling WUM-19-07 was to constrain the age of sedimentation

14

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

Fig. 12. Geochronological results for the Montsabrais area. Sample 1 - WUM-07-07: (A) Concordia diagram of 2 chemically abraded zircons from a diorite of the Montsabrais volcanic centre. (B) Weighted average of 207 Pb/206 Pb dates. (C) Massive medium-to fine-grained diorite with <5% quartz. Scale: coin = 1.8 cm in diameter. Sample 2 - MTS08-04: (A) Concordia diagram of 3 chemically abraded zircons from a diorite in the ring dyke complex. (B) Weighted average of 207 Pb/206 Pb dates. (C) Small felsic pocket of quartz-amphibole in medium- to fine-grained diorite. These pockets commonly have abundant zircons, and represent the last magmatic differentiation event in dyke-sill development. Scale: coin = 1.8 cm in diameter. Sample 3 - WR-06-03: (A) Concordia diagram of the leucotonalitic phase of the Montsabrais pluton. The 4 point regression line has an upper intercept of 2696.3 ± 1.3, which is considered to be the age of crystallization. (B) Weighted average of 207 Pb/206 Pb dates. (C) Massive equigranular leucotonalite with local sericite and chlorite fractures. Scale: pen = 14 cm.

around the Blake River Caldera. Sample WUM-19-07 (no. 16) from this dyke yielded an age of 2680.7 ± 1.1 Ma (Fig. 15-16a and b). 8. Discussion The Blake River Megacaldera Complex is represented by several calderas that are superposed to form a nested caldera arrangement. Edifice architecture is not only dependent on volcanic processes, such as flows and pyroclastic deposits, but is also correlative with magma evolution and composition. Intrusive and extrusive systems within the same volcano may vary greatly in age, and the stratigraphic correlations are challenging. Volcaniclastic deposits and thick, extensive lava flows may be used as marker horizons. With the inherent complexity of Archean volcanic systems, and volcanoes as point sources, basic stratigraphic concepts that allow

sequence predictability are difficult to use (e.g. Walther’s law; e.g. Catuneanu, 2006). In order to determine an evolutionary history of a large-scale volcanic complex such as the BRMCC, several criteria must be met. Sound regional (e.g. Bailey, 1989; Naumann and Geist, 2000; Vezzoli and Acocella, 2009) and detailed mapping must be conducted to determine the significance of volcaniclastic (Fritz et al., 1990; Mueller, 1991; Fiske et al., 2009), intrusive and extrusive volcanic facies (de Rosen-Spence et al., 1980). Detailed structural analyses (Riller et al., 2001; Pérez-López et al., 2007) and the distribution of mafic dyke patterns must be examined (e.g. Lambert et al., 1992; Gudmundsson, 2006; Pearson and Daigneault, 2009) in order to aid in evaluation of large-scale tectonics (Riller et al., 2001; Wright et al., 2006). In conjunction with precise U-Pb geochronological analyses, these parameters help to unravel the complex and protracted evolution of the BRG.

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

15

Fig. 13. Geochronological results for the Jevis-Clericy area. Sample 4 - MW-02-07: (A) Concordia diagram of 3 chemically abraded zircons from a gabbro dyke-sill. (B) Weighted average of 207Pb/206Pb dates. (C) Medium-grained gabbro with a patchy leopard texture (subophitic texture). Scale: coin = 1.8 cm in diameter. Sample 5 - BMR13-11-05: (A) Concordia diagram with a well-defined regression line. The regression line has an upper intercept of 2704.0 ± 3.4, considered to be the age of crystallization. (C) Medium-grained quartz gabbro with chlorite-epidote hydrothermal alteration. Scale: pen = 4 cm. Sample 6 - WUM-35A-07: (A) Concordia diagram with a well-defined 4 point regression line of the Clericy gabbro (II) with the most evolved quartz-rich gabbro phase. The regression line has an upper intercept of 2704.2 ± 1.4 Ma. C) A mediumgrained quartz-rich gabbro with cm-scale quartz-amphibole pods. WUM-35a-2007 was sampled adjacent to this observed pod. Scale: coin = 1.9 cm in diameter. Sample 7 - MW-01-07: (A) Concordia diagram of a tonalite from the margin of the d’Alembert pluton. (B) Weighted average of 207 Pb/206 Pb dates. (C) Massive, medium-grained, equigranular tonalite with mafic xenoliths. Scale: field book = 20 cm. Sample 8 - MW-03-07: (A) Concordia diagram of a tonalite from the centre of the d’Alembert pluton. (B) Weighted average of 207 Pb/206 Pb dates. (C) Close-up of the sampled tonalite. Scale: pen = 8 cm.

16

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Fig. 14. Geochronological results for the Rouyn-Noranda area. Sample 9 - LM-08-03: (A) Concordia diagram of an irregular to lobate QFP of the Glenwood rhyolite. (B) Weighted average of 207Pb/206Pb dates. (C) Sampled feldspar phyric rock with quartz phenocrysts. Sample 10 - LM-08-04: (A) Concordia diagram of the Glenwood rhyolite. (B) Weighted average of 207 Pb/206 Pb dates. Sample 11 - LM-03-06: (A) Concordia diagram of the Kiwanis melano-tonalite Clericy gabbro with a well-defined regression line that has an upper intercept of 2702.1 ± 2.0 Ma. (C) Massive equigranular Kiwanis tonalite with extensive hydrothermal epidote alteration in fractures and as patches. Scale, marker = 12 cm. Sample 12 - LM-02-06: (A) Concordia diagram of a quartz diorite. (B) Weighted average of 207 Pb/206 Pb dates. (C) Sampled medium-grained quartz gabbro with interstitial and phenocrystic quartz. Scale, coin = 1.9 cm in diameter. Sample 13 - LM-07-02A: (A) Concordia diagram of 2 chemically abraded zircons from a gabbro dyke-sill. (B) Weighted average of 207 Pb/206 Pb dates. (C) Close-up of the quartz-feldspar-phyric dyke in contact with mafic flow. Pen (4 cm) points to quartz phenocrysts. Sample 14 - WR-06-07: (A) Concordia diagram of the McPhee gabbroic dyke. (B) Weighted average of 207 Pb/206 Pb dates.

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

17

Fig. 15. Geochronological results for the Horseshoe and Bousquet dykes. Sample 15 - WR-04-07: (A) Concordia diagram for the eastern arm of the Horseshoe rift dyke complex. (B) Weighted average of 207 Pb/206 Pb dates. (C) A sampled 10cm-thick vein of quartz-amphibole-plagioclase between quartz-rich gabbro pulses. Scale: blue pen = 17 cm. Sample 16 - WUM-19-07: (A) Concordia diagram of the LaRonde tonalite dyke cutting the Kewagama Group. (B) Weighted average of 207 Pb/206 Pb dates. (C) Contact between the tonalite sill and the turbiditic deposits of the Kewagama Formation.

8.1. Blake River Megacaldera U-Pb age determinations The Archean Blake River Megacaldera Complex is similar in size to the Toba Caldera event and is also comparable to Olympus Mons on Mars. As such, a protracted life span is anticipated, and the new BRG U-Pb age determinations (Table 2) show that shield volcano construction, amalgamation, and the development of various caldera-forming phases span 8–11 M.y. from 2704 to 2696 Ma, with late plutons emplaced between 2690 and 2692 Ma (Mortensen, 1993; this study; Table 3). Magma withdrawal and migration is chronicled by the mafic dyke pattern.

8.1.1. Mafic Misema Caldera event The development of the Misema Caldera is characterized by a complex series of volcanic and structural events (Pearson and Daigneault, 2009; Mueller et al., 2009a,b; this study). In considering the Misema Subgroup as an early evolutionary event with numerous volcanic centres, a complexity is substantiated by the 2900 km2 dyke-sill geometry. These mafic dykes and sills represent the skeleton of the Blake River megacaldera, and yield important information concerning synvolcanic fractures, magma and hydrothermal fluid pathways, and felsic centres. The dyke framework (Figs. 2 and 3) enabled Mueller et al. (2009a,b; this study) to identify numerous shield volcanoes, including Jevis-Clericy, Montsabrais-Renault, Clifford-Tannahill, and Colnet (Fig. 3), upon which summit calderas formed. The observed ring dyke structures are interpreted as the roots of the summit calderas (Mueller et al., 2009b). The amalgamation of the Misema shield volcanoes served as the foundation for the subsequent bimodal volcanism of the New Senator and Noranda calderas. The timing of formation of the volcanic base has been a problematic issue, as historically, volcanism

in the Noranda Volcanic Complex (NVC) was simply defined as volcanic cycles I-V. Previous age determinations of plutonic and felsic rocks in the NVC and environs were used to support this concept, but the presence of overlapping calderas opens up new avenues for interpretation.

8.1.1.1. Jevis-Clericy shield volcano. The dyke complex skeleton is revealing because plumbing systems within volcano-tectonic depressions require time to develop. The bimodal cycle V of the Mobrun-Clericy area, with the former 20 Mt Bouchard-Hebert VMS deposit, was considered the youngest NVC event (Gibson and Watkinson, 1990), based on age determinations of 2696–2698 Ma from felsic volcanic rocks near the Mine and an age of 2696 Ma for the Clericy tonalitic pluton (see Fig. 2a; Table 3). In contrast, samples of the Clericy gabbro collected for this study, yielded the oldest age of the BRG thus far at 2704.2 ± 1.4 Ma (sample WUM-35A-2007; Table 2) and 2704.0 ± 3.4 Ma (sample BMR-13-11-05). The elongate gabbroic intrusive body trends NW-SE and is parallel to the inner and outer ring faults. The gabbro shows pervasive N60-N70 fractures and alteration (Fig. 13-5c) that coincide with the younger NC fracture pattern. These ages constrain the volcanic base of the Misema Caldera event >2704 Ma and possibly up to 2706–2707 Ma. An additional gabbro sill that parallels the inner ring fault (Dufresnoy fault) in the same region yielded an age of 2700.5 ± 1.4 Ma (sample MW-02-07) confirming multiphase volcanism in the same area. This NW-SE trending dyke-sill system is related to gabbros that intrude the Jevis South tuff (Fig. 4). The young ages indicate volcanic rifting correlative with the NC event, but using MC fracture patterns. Associated pyroclastic deposits of the Jevis South tuff and Kino North tuff were sampled but geochronological results were unobtainable.

18

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

Table 3 Selective U-Pb age determinations in the Blake River and Malartic & Louvicourt Groups, Val-d’Or. Sample description

Age (Ma ± 2␴ error)

Reference

Malartic-Louvicourt Groups Bourlamaque pluton, quartz diorite phase Sullivan Héva Fm felsic volcaniclastic deposits Jacola Fm komatiites and volcaniclastic deposits Dunraine quartz diorite sill, Val-d’Or Val-d’Or Fm rhyodacite; mine Louvicourt Val-d’Or Fm rhyolite, Colombière village Vicour quartz diorite sill, Sigma II Mine

2700.0 ± 1.0 2702.0 ± 1.0 2703.8 ± 1.3 2704.0 + 3.0/−2.0 2704.0 ± 2.0 2705.0 ± 1.0 2706.0 ± 1.0

Wong, 1991 Pilote et al., 1999 Scott et al., 2002 Pilote et al., 1999 Pilote et al., 1999 Wong, 1991 Pilote et al., 1999

Blake River Group Massive Dufault granodiorite pluton D’Alembert rhyolite Clericy quartz-feldspar-phyric rhyolite Montsabrais pluton in core of MVC centre Massive Clericy leucotonalite pluton Quartz-feldspar-phyric Cyprus rhyolite Granophyric-quartz dyke phase Flavrian pluton Volcaniclastic deposit, Bouchard-Hébert Mine Massive tonalitic Powell pluton phase Héré Rhyolite Massive leucotonalite Flavrian pluton Evain rhyolite, quartz-phyric Spherulitic Fishroe rhyolite, BRG, Granada Massive leucotonalite Flavrian pluton Four Corners massive quartz-phyric rhyolite Clarice felsic centre north, quartz-feldspar rhyolite Lac Maron rhyolite Duprat rhyolite, quartz-phyric rhyolite Quemont rhyolite, quartz-phyric rhyolite Horne Mine, massive quartz-phyric rhyolite Clarice felsic centre south, quartz-feldspar rhyolite Hebertcourt rhyolite, quartz-feldspar phyric flow

2690.0 + 2.2/−2.0 2694.5 ± 0.9 2696.0 ± 1.1 2696.2 ± 0.9 2696.3 +1.7/−1.4 2696.4 ± 1.0 2697.0 ± 2.0 2697.9 +1.3/−0.7 2700.1 ± 1.0 2700.3 ± 1.2 2700.0 + 3/−2 2700.6 ± 0.9 2700.6 ± 1.6 2700.8 + 2.6/−1.0 2700.9 + 1.4/−1.1 2701.0 ± 2.0 2701.4 ± 1.0 2701.4 ± 1.0 2701.9 ± 1.1 2702.0 ± 1.0 2703.0 ± 2.0 2703.0 ± 0.9

Mortensen, 1993 McNicoll in Ross et al., 2011 Lafrance et al., 2005 McNicoll in Ross et al., 2008 Mortensen, 1993 McNicoll et al., 2009 Galley and van Breeman, 2002 Mortensen, 1993 McNicoll et al., 2009 McNicoll et al., 2009 Galley and van Breeman, 2002 McNicoll et al., 2009 Lafrance et al., 2005 Mortensen, 1993 Mortensen, 1993 Corfu et al., 1989 McNicoll in Ross et al., 2011 McNicoll et al., 2009 McNicoll et al., 2009 McNicoll et al., 2009 Nunes and Jensen, 1980 McNicoll et al., 2009

8.1.1.2. Montsabrais-Renault shield volcano system. The Montsabrais-Renault shield volcano system has overlapping mafic ring dyke systems and from this geometry, two separate shield volcanoes are inferred (Fig. 3). A connection with the eastern Jevis-Clericy and western Clifford-Tannahill shield volcano fields are shown by a 10 s of km-scale connected linear dyke-rift system. In contrast to the Renault shield volcano, referred to as the Reneault volcanic centre (Dimroth et al., 1982; this study), samples from the Montsabrais volcanic centre yielded zircons for age determinations. The ring dyke complex, the root of the Montsabrais summit caldera, yielded two tenable ages. The central mafic ring dyke composed of massive diorite WUM-07-07 formed at 2701.1 ± 1.2 Ma (sample WUM-07-07), whereas the late pegmatoidal melt pockets revealed an age of 2698.8 ± 1.1 Ma (sample MTS-08-04). The felsic pockets, a late magmatic event, suggest that the ring dyke complex had a prolonged magmatic history so that older mafic phases of the Montsabrais ring dyke system not previously detected should be expected. The explosive volcanic debris originating from these centres must have been contemporaneous in age with the ring dykes. The polyphase Montsabrais pluton, a tonalitic to granodioritic body, intruded the core of the MVC. An age of 2695.9 ± 2.8 Ma (sample WR-06-03) was obtained from the leucotonalitic phase. A similar pluton age of 2696.2 ± 0.9 Ma was obtained from an outcrop zone further east along the logging road (Ross et al., 2008). The Hébécourt rhyolite straddling the outer ring fault and adjacent to the MVC has been dated at 2703 ± 0.9 (McNicoll et al., 2009; Table 3) and lends additional support for an early summit caldera system. Because the rhyolite is part of the older Misema Caldera event, is adjacent to the MVC, and overlies older mafic volcanic rocks, a contemporaneous MVC is inferred. Mafic complexes and ring dykes can evolve over a period of 5–13 M.y. (Lameyre et al., 1976; Peressini et al., 2007; Murgulov et al., 2009) and the MVC, with its associated felsic volcanic rocks, was volcanically active for 7 M.y. between 2703 and 2696 Ma. Peripheral BRG volcanism

straddled the three caldera-forming events between 2704 and 2696 Ma. The mafic roots of the larger 10–12 km summit calderas were possibly active for 2–5 M.y. or even longer. It is therefore possible to infer that mafic shield amalgamation and Misema collapse are estimated to have occurred at about 2704 Ma. 8.1.2. The Bimodal New Senator Caldera event The NSC is a graben-type structure characterized in the NW sector by a series of crescent-shaped faults that are interpreted to indicate a successive southeastwardly magma chamber migration (Fig. 3; Pearson and Daigneault, 2009), similar to that observed at ˜ Caldera, Tenerife (Martí and Gudmundsson, 2000). the Las Canadas The central 2701 Ma Flavrian leucotonalite (Mortensen, 1993) is the core intrusion of the NSC, as well as the hydrothermal motor for VMS deposits (Hannington et al., 2003). Along the migrating crescent-shaped faults are fringe volcanic centres such as the 2701 Ma Four Corners rhyolite and the 2701.4 Ma Duprat rhyolite centre (Table 3) that developed contemporaneously. The NSC and the NC are characterized by a greater number of felsic centres with respect to the entire Blake River Complex on surface. These centres are located inside the New Senator Caldera limits (e.g. the Duprat and Inmont centres) or delineate its margins (Fig. 2b). The modal age of the felsic centre in the NSC area appears to be 2701 Ma despite the relatively few numbers of determinations. However, rather constant felsic activity is indicated throughout BRG evolution, as indicated by the age distribution from 2704 to 2694 Ma (Fig. 16). The southern NSC in Rouyn-Noranda proper shows a temporal evolution that commenced with an early felsic dome-flow complex at 2703–2702 Ma (samples LM-08-03 and LM-08-04; Fig. 6) that formed on a mafic, massive, brecciated and pillowed substrate. The high-level, sill-like Kiwanis pluton subjacent to the Glenwood rhyolite intruded felsic aphanitic flows that yielded an age of 2702.0 ± 1.6 Ma (sample LM-03-06). This further supports an older NSC event. The Horne Mine and Quemont rhyolites are

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

19

Plutons Montsabrais and Clericy Plutons

Kiwanis Flavrian Powell Pluton Pluton Pluton 6

n=17

D'Alembert Pluton

4 2 0

2704

2703

2702

2701

2700

2699

2698

2697

2696

Felsic volcanism

2694

2693

2692

2691

2690

2689

2688

2694

2693

2692

2691

2690

2689

2688

2694

2693

2692

2691

2690

2689

2688

2687

2695

12 10 8 6 4 2 0

n=49 2704

2703

2702

2701

2700

2699

2698

2697

2696

2695

2687

Mafic-Intermediate volcanism Montsabrais Late phase Late phase ring dyke of Montsabrais of McPhee dyke ring dyke diorite

Clericy gabbro

Late phase of Horseshoe dyke

2 0

n=7 2704

2703

MISEMA

2702

2701

NEW SENATOR

2700

2699

2698

2697

2696

2695

2687

NORANDA

Fig. 16. Age histogram for felsic volcanism, mafic-intermediate volcanism and plutonic activity within the Blake River Group (Ages are from Tables 2 and 3), and position of the Misema, New Senator and Noranda caldera events (n = number of ages).

dated at 2702 Ma, respectively (Table 3), and are synchronous with Glenwood rhyolite formation. The Kiwanis pluton is intruded by a NS-EW network of gabbros, diorites and quartz-diorites that yield an age of 2696.6 ± 0.7 Ma (sample LM-02-06). The McPhee gabbro dyke system is interpreted as dyke reinjection along older NSC structures, as indicated by an age of 2698.2 ± 0.8 Ma (sample WR-06-07). Only a late quartz-rich gabbro phase of the McPhee dyke could be dated, but older phases can be expected because the dyke is interpreted to be cut by the 2700 Ma Powell Pluton. Next to the McPhee dyke, a series of ponded flows are cut by felsic QFP-dykes yielding an age of 2699 ± 1 Ma (sample LM07-02A). Felsic effusive rocks to the south of the NSC, represented by the Fishroe rhyolite, yield an age of 2700.6 ± 1.6 Ma (Lafrance et al., 2005) and are part of a north facing volcanic succession overlain by the Stadacona volcaniclastic rocks (Ross et al., 2011; Fig. 6). Older ages of the Glenwood, Horne and Quémont rhyolites directly to the north challenge the stratigraphic reconstruction. However, collectively the ages of the NSC based on felsic volcanic and plutonic rocks, support a caldera-forming event between 2702 and 2700 Ma (Fig. 16). 8.1.3. Felsic-dominated Noranda Caldera event The Noranda caldera has been studied in detail because of numerous occurrences of VMS deposits. Recent age determinations from the Geological Survey of Canada and the Quebec provincial survey documented a range of felsic activity between 2699 and 2696 Ma within the central mining camp. The Cyprus rhyolite, with an age of 2696.4 ± 1 Ma is the youngest felsic rock (McNicoll et al., 2009). With the ages of the synvolcanic Flavrian pluton at 2700 +3/−2 Ma and the granophyric-quartz phyric phase at 2697 ± 2 Ma

(Galley and van Breemen, 2002), it is more probable that Noranda Caldera formation took place from 2700 to 2696 Ma. Fig. 16 displays the age spectrum of felsic volcanism for the Blake River Group (ages from Tables 2 and 3). Two main phases of felsic volcanism can be distinguished. The first phase occured at approximately 2701 Ma and is interpreted to be associated with evolution of the New Senator Caldera, whereas the second phase is considered to be related to Noranda Caldera evolution. Given that the Horne Creek synvolcanic fault associated with the NC cuts the late 2698 Ma phase of the McPhee dyke, a similar age for the collapse of the NC Caldera can be inferred (Fig. 16). 8.2. Blake River megacaldera tectonics The formation of the BRG occurred during arc-related subduction along the limit occupied by the Cadillac Larder Lake Fault. Despite a horizontal shortening component related to the regional deformation, it is possible to establish the prevalent stress field present during the different stages of caldera evolution. The radial and mafic ring dykes within the BRG represent the fracture pattern associated with the shield-building phase. Such a fracture pattern is typical of a gravitational stress regime with a vertical principal stress axes ␴1 (Fig. 17). The gravitational stress regime was also responsible for the formation of different mafic summit calderas expressed by ring dyke complexes. However, the SEtrending fractures related to the graben-type NSC are explained by an external regional stress regime with a SE-trending principal stress ␴1 (Fig. 17b). The interpreted oblique convergent setting during terrain accretion in the Abitibi belt (Mueller et al., 1996; Daigneault et al., 2002, 2004) would have generated a NW-SE

20

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

Misema event >2704 - 2702 Ma DPMFZ

Gravitational stress regime

Inner Ring Fault

Local summit caldera

Pa

Outer Ring Fault

rfo

σ1

ur

u

amalgamated shield volcanoes

Fa

ul

t Inner Ring Fault

Incipient NSC fractures

Incipient rift structure

CLLFZ

1

Outer Ring Fault

Subduction zone

New Senator event 2702 - 2700 Ma DPMFZ New Senator Caldera

Influence of regional stress regime

Pa

rfo

ur

u

SE migration of the Flavrian Pluton

Fa

ul

t

Incipient rift structure

CLLFZ

Noranda event 2700 - 2696 Ma

Subduction zone

1

1

DPMFZ

NE trending rift stress regime Noranda Caldera

Pa

rfo

ur

u

Fa

ul

t

Horseshoe intrusion

CLLFZ

Subduction zone Horseshoe intrusion

Fig. 17. Evolution of the Blake River Megacaldera Complex with varying stress regimes due to gravitational and tectonic influence. (A) The presence of summit calderas and rift zones support the existence of numerous shield volcanoes in an oceanic island setting. (B) Migration of the magma chamber is indicated by the crescent-shaped faults. (B and C) A plate tectonic setting with constraints developed during oblique convergence elegantly explains the formation of graben calderas. DPMFZ, Destor-Porcupine-Manneville fault zone; CLLFZ, Cadillac-Larder Lake Fault zone.

trending principal stress regime compatible with the New Senator fracture system (Fig. 17b). It is noteworthy that the attitude of these faults is parallel to the SE-trending Parfouru Fault which delimits the BRG on the east. The Parfouru Fault was interpreted as a possible candidate for an original transform fault (Daigneault et al., 2002), compatible with SE-trending plate movement and subsequent SEtrending principal stress axes ␴1 . The proposed migration of the Flavrian pluton from NW to SE is consistent with NW-SE directed horizontal plate motion. The 070 fractures and synvolcanic faults associated with the youngest Noranda Caldera are the expressions of a major extensional rift system (Fig. 17c). A key element supporting the rift is the presence of the NE-trending Horseshoe rift dyke system located directly in the prolongation of the NC. Detailed mapping in the central camp reveals a NE-trending lava ridge located directly in the heart of the graben (Gibson, 1989). All volcanic flow units thin outward from this lava ridge. This fissure system also supports a NE-trending rift. The general geometry of the Blake River Megacaldera Complex can be explained by the shift from a vertical tectonic regime dominated by gravitational stresses to a horizontal tectonic regime

related to large-scale Archean plate tectonics and oblique subduction, as suggested for the Abitibi greenstone belt (Daigneault et al., 2002, 2004; Mueller et al., 2009b). 9. Conclusions The 2704–2696 Ma Archean Blake River Group of the Abitibi greenstone belt is a subaqueous complex of three overlapping calderas based on the reconstruction of volcanic architecture, taking into account: (1) the synvolcanic fracture pattern underlined by mafic dyke/sill complexes, (2) the spatial distribution of felsic centres and volcaniclastic deposits and (3) the structural pattern imposed during regional deformation. The 80 km in diameter, dominantly mafic Misema Caldera is underlain by a concentric and radial pattern of mafic dykes and sills that represent the roots of the volcanic system. They correspond to a synvolcanic fracture pattern favourable for multiphase mafic and felsic magma introduction. The numerous local ring dyke structures, 10 km in diameter, are considered the expressions of remnant summit calderas that developed on amalgamated shield volcano complexes (Jevis-Clericy,

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

Montsabrais-Renault, Clifford-Tannahill and Colnet volcanic complexes). The bimodal, 35 km × 14 km, New Senator Caldera formed within the envelope of the Misema Caldera and exhibits a box-work graben-type structure. The distribution of the felsic centres delineates both the inside and the outside of the caldera boundaries. Finally, the 15 km × 20 km, felsic-dominated, Noranda Caldera, well known for its VMS endowment, represents the final collapse of the megacaldera complex. Deformation has overprinted the volcanic features, but structural patterns can be used to reconstruct the pre-existing volcanic architecture. Structures such as mafic ring dyke complexes and rhyolitic dome-flow complexes have nucleated fold geometries. Folds in the southern portion of the NSC display evidence of ductile deformation near the Cadillac Larder Lake fault which cuts the Misema Caldera to the south. Precise U-Pb geochronological analyses were conducted in selected areas with specific emphasis on the Misema and New Senator Calderas. The formation of the Misema Caldera occurred between 2704 and 2702 Ma via the amalgamation of shield volcanoes. Ages from the Clericy gabbro at 2704.2 ± 1.4 Ma indicate volcanism that may have been active prior to 2704 Ma. The age of the Montsabrais shield volcano can be constrained from a ring dyke complex diorite dated at 2701.1 ± 1.2 Ma. A pegmatoidal pocket of the same diorite yielded an age slightly younger at 2698.8 ± 1.1 Ma and the younger Montsabrais pluton yielded an age of 2695.9 ± 2.8 Ma. The New Senator Caldera was generated between 2702 and 2700 Ma during paroxysmal felsic volcanism. The Glenwood felsic centre in the southern sector of the New Senator Caldera yielded an age of 2702.9 ± 1.4 Ma and the

21

Kiwanis pluton subjacent to the Glenwood rhyolite yielded an age of 2702.0 ± 1.6 Ma. In the same area, a felsic QFP-dyke yielded an age of 2699 ± 1 Ma. A late pegmatoidal phase of the McPhee gabbro dyke intruded along the synvolcanic McPhee fault, which represents the western limit of the NSC, gave an age of 2698.2 ± 0.8 Ma. The collapse of the Noranda Caldera culminated between 2700 and 2696 Ma with abundant felsic volcanism. The late phase of the NE-trending Horseshoe rift dyke arrangement yielded an age of 2696.4 ± 1.5 Ma. The Misema Caldera was generated by a gravitational stress field consistent with the formation of a ring and radial dyke architecture, whereas the New Senator Caldera is more compatible with a SW-trending principal compression direction related to oblique convergence in the Abitibi belt. The Noranda Caldera is interpreted as a NE rift structure that formed in the final stage of megacaldera evolution. Acknowledgements This research was part of a 5 year project concerning the Blake River Megacaldera Complex, and received financial support from the CONSOREM (Mineral Exploration Research Consortium), DIVEX (Diversification of mineral exploration in Quebec), FUQAC (Foundation of the University of Quebec at Chicoutimi) and NSERC (Natural Sciences and Engineering Research Council of Canada). Prof. D. Gaboury (UQAC) is thanked for the review of a preliminary version of the manuscript. Judit Ozoray revised the English syntax. Finally, the manuscript was greatly improved by the comments of Patricia Corcoran and Desmond Moser.

22

Appendix A. U–Th–Pb isotopic data Compositional Parameters Wt. Sample (a)

mg (b)

U

Th U

Pb ppm

206

ppm (c)

(d)

(c)

Pb* x10-13 mol (e)

Radiogenic Isotope Ratios Pbc (pg)

Pb 204 Pb

208

Pb 206 Pb

207

Pb 206 Pb

207

Pb* (e)

Pbc (e)

(e)

(f)

(g)

(g)

% err (h)

206

Pb*

206

mol %

Pb 235 U

Isotopic Ages

206

(g)

% err (h)

Pb 238 U (g)

% err (h)

corr. coef.

207

207

Pb U

206

Pb 206 Pb

±

(i)

(h)

(i)

(h)

(i)

(h)

235

±

Pb U

238

±

% disc

WUM-07-07 0,001

119

0,454

70,2

3,3471

99,76%

133

0,68

7191

0,126

0,185393

0,103

13,307818

0,224

0,520609

0,147

0,929

2701,72

1,70

2701,74

2,12

2701,76

3,25

0,00

CA3

0,002

47

0,413

28,0

1,5439

99,53%

67

0,62

3702

0,114

0,185238

0,109

13,310921

0,328

0,521166

0,279

0,948

2700,34

1,79

2701,96

3,09

2704,12

6,17

-0,14

MTS-08-04 A

0,002

29

0,808

21,6

1,2581

94,54%

5

6,66

269

0,223

0,184220

0,387

13,188198

0,608

0,519215

0,415

0,777

2691,23

6,40

2693,21

5,74

2695,85

9,13

-0,17

B

0,002

40

0,334

23,9

1,7677

99,19%

40

1,17

2334

0,092

0,189264

0,117

13,792883

0,258

0,528550

0,194

0,906

2735,78

1,92

2735,59

2,45

2735,34

4,33

0,02

C

0,002

92

0,551

55,3

3,9701

99,71%

116

0,94

6449

0,152

0,185090

0,098

13,265632

0,262

0,519808

0,206

0,942

2699,02

1,61

2698,74

2,48

2698,36

4,53

0,02

D

0,002

103

0,906

66,7

4,4575

99,65%

105

1,26

5437

0,251

0,185057

0,099

13,269424

0,217

0,520049

0,142

0,933

2698,72

1,63

2699,01

2,05

2699,39

3,14

-0,02

E

0,001

65

0,639

41,2

1,4181

98,81%

29

1,38

1583

0,177

0,185015

0,197

13,280542

0,366

0,520604

0,271

0,848

2698,35

3,26

2699,80

3,45

2701,74

5,99

-0,13

MW-02-07 CA3

0,001

150

1,034

101,8

3,2496

98,92%

32

3,07

1534

0,286

0,185323

0,123

13,302224

0,276

0,520588

0,207

0,910

2701,09

2,04

2701,34

2,61

2701,67

4,56

-0,02

CA4

0,001

118

0,804

76,3

2,5564

99,03%

36

2,05

1898

0,223

0,185180

0,149

13,283393

0,546

0,520251

0,525

0,962

2699,82

2,47

2700,00

5,15

2700,25

11,59

-0,02

CA5

0,001

86

0,539

53,0

1,8608

98,80%

28

1,80

1549

0,149

0,185203

0,226

13,282023

0,437

0,520134

0,425

0,863

2700,02

3,73

2699,91

4,13

2699,75

9,37

0,01

BMR-13-11-5 A

0,016

119

0,915

80,9

40,8453

97,66%

15

81,03

778

0,255

0,184474

0,507

13,037841

0,690

0,512588

0,339

0,714

2693,51

8,37

2682,39

6,51

2667,67

7,40

0,96

B

0,012

106

1,001

73,9

27,4630

97,51%

14

58,11

730

0,277

0,184383

0,537

13,208966

0,731

0,519573

0,362

0,713

2692,69

8,87

2694,70

6,90

2697,37

7,99

-0,17

C

0,009

59

0,626

36,1

11,5540

99,91%

359

0,90

18770

0,174

0,185393

0,096

13,273286

0,238

0,519258

0,175

0,938

2701,72

1,58

2699,28

2,25

2696,03

3,85

0,21

D

0,009

91

0,973

58,9

17,5969

99,94%

559

0,95

27095

0,270

0,185018

0,094

13,188044

0,263

0,516970

0,208

0,947

2698,37

1,54

2693,20

2,48

2686,32

4,58

0,45

E

0,011

48

0,753

30,6

11,4338

99,25%

41

8,08

1888

0,209

0,185255

0,262

13,276767

0,575

0,519782

0,469

0,893

2700,49

4,33

2699,53

5,43

2698,26

10,34

0,08

WUM-35A-2007 A

0,006

66

0,612

40,2

8,4228

99,76%

132

1,78

6717

0,169

0,185740

0,097

13,346479

0,280

0,521147

0,228

0,948

2704,80

1,60

2704,48

2,65

2704,04

5,03

0,03

B

0,008

294

0,980

192,3

47,8780

99,95%

616

2,34

28397

0,271

0,185494

0,093

13,296878

0,242

0,519898

0,182

0,943

2702,62

1,53

2700,96

2,29

2698,75

4,02

0,14

C

0,006

130

0,794

81,6

17,0820

99,85%

225

2,20

11651

0,221

0,185190

0,097

13,177640

0,288

0,516083

0,238

0,950

2699,90

1,60

2692,46

2,72

2682,55

5,23

0,64

D

0,006

158

0,791

98,7

19,3216

99,89%

299

1,87

14551

0,220

0,185194

0,099

13,157748

0,269

0,515291

0,212

0,942

2699,95

1,64

2691,03

2,54

2679,18

4,65

0,77

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

CA1

LM-08-03 CA1

0,008

28

0,454

16,7

4,9804

99,11%

37

3,67

2083

0,126

0,185570

0,123

13,335971

0,328

0,521212

0,274

0,932

2703,30

2,03

2703,73

3,10

2704,32

6,04

-0,04

CA2

0,004

62

0,449

38,1

5,0946

98,54%

20

7,04

975

0,125

0,185298

0,189

13,194284

0,328

0,516432

0,211

0,841

2700,87

3,12

2693,65

3,10

2684,03

4,63

0,62

CA3

0,007

47

0,573

29,1

6,5732

98,89%

27

6,72

1344

0,159

0,185260

0,136

13,201736

0,323

0,516829

0,259

0,913

2700,54

2,25

2694,18

3,04

2685,72

5,70

0,55

CA4

0,005

33

0,485

20,6

3,7320

98,19%

18

5,77

980

0,134

0,185359

0,180

13,302072

0,380

0,520479

0,307

0,885

2701,42

2,96

2701,33

3,59

2701,21

6,77

0,01

CA5

0,002

77

0,487

48,4

4,0306

98,15%

18

6,27

989

0,135

0,185575

0,166

13,320688

0,399

0,520602

0,342

0,911

2703,34

2,74

2702,65

3,77

2701,73

7,55

0,06

LM-08-04 0,002

43

0,454

26,9

1,8279

97,42%

12

4,30

609

0,126

0,184516

0,188

13,102253

0,382

0,515003

0,297

0,875

2693,89

3,11

2687,04

3,60

2677,95

6,51

0,59

CA5

0,001

306

0,347

182,8

3,9821

98,67%

24

4,41

1393

0,096

0,185446

0,183

13,314195

0,374

0,520710

0,293

0,877

2702,19

3,02

2702,19

3,54

2702,19

6,46

0,00

LM-02-06 A

0,004

113

0,628

69,7

8,6208

99,66%

98

2,47

5171

0,173

0,184776

0,099

13,261553

0,294

0,520531

0,243

0,949

2696,22

1,64

2698,45

2,78

2701,43

5,37

-0,19

B

0,002

194

0,638

120,2

8,3820

99,35%

48

4,92

2371

0,177

0,184787

0,106

13,178281

0,264

0,517234

0,200

0,933

2696,31

1,75

2692,50

2,49

2687,44

4,40

0,33

C

0,003

425

1,272

291,7

27,1329

99,53%

72

11,95

3038

0,356

0,184250

0,101

12,947795

0,266

0,509667

0,206

0,939

2691,51

1,68

2675,86

2,51

2655,21

4,48

1,35

D

0,003

239

0,688

147,1

15,4506

99,80%

167

2,62

8633

0,191

0,184865

0,095

13,175569

0,304

0,516909

0,257

0,957

2697,00

1,56

2692,31

2,87

2686,06

5,65

0,41

E

0,002

97

0,705

60,8

4,2057

99,50%

68

1,75

3674

0,195

0,185003

0,119

13,252514

0,381

0,519538

0,337

0,952

2698,24

1,97

2697,81

3,59

2697,22

7,42

0,04

CA1

0,005

66

0,715

41,2

7,2055

99,96%

863

0,24

45995

0,198

0,184756

0,107

13,237170

0,206

0,519631

0,123

0,910

2696,03

1,77

2696,71

1,95

2697,62

2,71

-0,06

CA2

0,005

168

0,719

103,6

18,1354

99,99%

2958

0,18

####

0,199

0,184722

0,093

13,217086

0,221

0,518939

0,153

0,940

2695,73

1,54

2695,28

2,09

2694,68

3,38

0,04

CA3

0,005

69

0,805

43,2

7,3951

99,81%

185

1,16

9690

0,224

0,184938

0,117

13,175261

0,326

0,516691

0,276

0,937

2697,66

1,94

2692,29

3,07

2685,13

6,07

0,46

-0,17

LM-07-2A A

0,009

63

0,429

36,9

12,7031

99,86%

235

1,45

13318

0,119

0,185265

0,098

13,319822

0,326

0,521438

0,282

0,959

2700,58

1,62

2702,59

3,08

2705,28

6,22

B

0,005

49

0,685

30,0

5,4280

99,62%

90

1,72

4721

0,191

0,184154

0,098

13,017168

0,249

0,512664

0,187

0,938

2690,64

1,63

2680,90

2,35

2667,99

4,07

0,84

C

0,004

41

0,660

25,4

3,3807

99,64%

95

1,00

5138

0,182

0,185222

0,115

13,311095

0,260

0,521219

0,188

0,917

2700,19

1,90

2701,97

2,45

2704,35

4,16

-0,15

D

0,003

74

0,678

45,4

4,7819

99,78%

160

0,85

8590

0,188

0,184742

0,095

13,191642

0,222

0,517883

0,151

0,939

2695,91

1,57

2693,46

2,09

2690,20

3,33

0,21

E

0,003

68

0,702

42,5

4,2844

99,61%

89

1,37

4761

0,195

0,184828

0,092

13,200848

0,297

0,518003

0,251

0,957

2696,68

1,52

2694,12

2,80

2690,71

5,52

0,22

CA2

0,004

62

0,653

38,1

5,3709

99,76%

142

1,07

7659

0,181

0,185074

0,096

13,273154

0,231

0,520147

0,164

0,938

2698,88

1,58

2699,27

2,18

2699,81

3,61

-0,03

CA4

0,006

108

0,664

66,2

14,0104

99,89%

303

1,31

16319

0,184

0,185035

0,093

13,270480

0,285

0,520153

0,236

0,953

2698,53

1,54

2699,08

2,69

2699,83

5,20

-0,05

CA5

0,004

69

0,706

42,9

5,9449

99,58%

79

2,15

4015

0,195

0,184996

0,103

13,258411

0,253

0,519790

0,189

0,932

2698,18

1,70

2698,23

2,39

2698,29

4,18

0,00

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

CA1

23

24

WR-06-07 A

0,006

293

2,457

245,4

36,7036

99,80%

214

6,62

7731

0,682

0,184795

0,095

13,181232

0,238

0,517328

0,175

0,941

2696,38

1,56

2692,71

2,25

2687,84

3,84

0,32

B

0,004

143

2,408

119,7

12,1071

99,67%

138

3,37

5385

0,666

0,185067

0,099

13,276372

0,235

0,520294

0,168

0,934

2698,81

1,63

2699,50

2,22

2700,43

3,70

-0,06

CA1

0,001

119

2,330

98,5

2,5734

99,44%

82

1,18

3323

0,644

0,184949

0,168

13,269682

0,275

0,520363

0,185

0,800

2697,76

2,78

2699,03

2,59

2700,72

4,08

-0,11

CA3

0,001

592

1,223

418,1

12,8377

98,68%

28

14,19

1391

0,338

0,184887

0,142

13,246293

0,286

0,519621

0,209

0,880

2697,20

2,35

2697,36

2,70

2697,57

4,61

-0,01

CA4

0,001

987

2,030

775,6

21,4413

99,90%

443

1,75

18675

0,561

0,184908

0,093

13,272530

0,226

0,520591

0,159

0,941

2697,39

1,53

2699,23

2,13

2701,69

3,52

-0,16

CA5

0,001

282

1,239

194,4

6,0985

99,61%

99

1,95

4770

0,343

0,185093

0,100

13,255702

0,233

0,519411

0,164

0,932

2699,04

1,65

2698,03

2,20

2696,68

3,62

0,09

C

0,007

439

1,613

319,9

61,4000

99,86%

289

7,18

13025

0,448

0,184779

0,094

13,144836

0,210

0,515941

0,135

0,943

2696,24

1,55

2690,10

1,98

2681,94

2,96

0,53

D

0,004

130

2,519

110,1

11,7715

99,59%

115

3,99

4502

0,698

0,184686

0,100

13,190122

0,227

0,517981

0,155

0,931

2695,40

1,66

2693,35

2,14

2690,61

3,41

0,18

E

0,005

173

1,601

127,9

19,0712

99,32%

60

10,67

2736

0,443

0,184796

0,113

13,217141

0,277

0,518734

0,214

0,925

2696,39

1,86

2695,28

2,61

2693,81

4,72

0,10

CA1

0,005

61

1,295

42,5

6,6044

99,68%

121

1,73

5814

0,358

0,184940

0,098

13,253225

0,242

0,519745

0,178

0,937

2697,67

1,61

2697,86

2,28

2698,10

3,92

-0,02

CA3

0,005

136

0,318

77,4

14,6968

99,89%

290

1,33

16839

0,088

0,184777

0,093

13,228859

0,201

0,519247

0,121

0,952

2696,22

1,54

2696,12

1,90

2695,99

2,67

0,01

WR-06-03 A

0,011

40

0,366

23,3

9,5890

99,76%

124

2,04

6577

0,101

0,184643

0,100

13,224425

0,271

0,519448

0,216

0,941

2695,02

1,64

2695,80

2,56

2696,84

4,75

-0,07

B

0,008

59

0,482

35,1

10,2815

99,84%

204

1,37

10932

0,133

0,184707

0,094

13,198871

0,211

0,518266

0,137

0,944

2695,59

1,55

2693,98

1,99

2691,82

3,00

0,14

C

0,006

81

0,325

46,5

10,5425

99,82%

165

1,68

8938

0,090

0,184894

0,100

13,250914

0,227

0,519783

0,155

0,932

2697,26

1,65

2697,69

2,14

2698,26

3,42

-0,04

D

0,007

195

0,469

108,9

27,8576

99,72%

105

7,19

5156

0,134

0,180623

0,098

12,169580

0,374

0,488652

0,336

0,968

2658,60

1,62

2617,57

3,51

2564,84

7,10

3,53

MW-01-07 A

0,002

241

0,537

135,4

8,3126

99,79%

153

1,50

7997

0,153

0,179948

0,095

12,066385

0,307

0,486329

0,262

0,957

2652,39

1,57

2609,58

2,88

2554,77

5,52

3,68

B

0,003

329

0,565

179,1

16,7520

99,89%

298

1,56

15449

0,164

0,178584

0,093

11,562794

0,237

0,469589

0,175

0,942

2639,77

1,55

2569,67

2,22

2481,76

3,60

5,99

C

0,002

180

0,597

105,9

6,4196

99,79%

160

1,12

8609

0,168

0,182608

0,094

12,663229

0,285

0,502949

0,235

0,953

2676,70

1,56

2654,93

2,69

2626,46

5,07

1,88

D

0,001

321

0,719

194,3

8,8031

99,89%

308

0,82

16379

0,202

0,183913

0,093

12,808412

0,285

0,505106

0,235

0,954

2688,47

1,53

2665,66

2,68

2635,70

5,09

1,96

E

0,002

123

0,607

71,0

5,0247

99,68%

102

1,38

5305

0,174

0,182749

0,097

12,293767

0,222

0,487897

0,149

0,938

2677,98

1,60

2627,10

2,08

2561,57

3,16

4,35

CA1

0,001

229

0,702

142,4

4,9622

99,72%

119

1,18

6222

0,194

0,184307

0,111

13,185566

0,209

0,518868

0,121

0,908

2692,01

1,84

2693,02

1,97

2694,38

2,66

-0,09

CA3

0,001

150

0,665

92,7

3,2455

99,62%

89

1,03

4812

0,184

0,184302

0,095

13,170383

0,208

0,518283

0,128

0,951

2691,97

1,57

2691,94

1,96

2691,89

2,83

0,00

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

WR-04-07

MW-03-07 0,008

113

0,639

67,7

17,8332

99,60%

85

5,91

4625

0,179

0,183978

0,104

12,844881

0,253

0,506364

0,189

0,930

2689,06

1,72

2668,34

2,39

2641,09

4,09

1,78

B

0,005

122

0,770

74,5

13,8523

99,85%

240

1,67

12682

0,217

0,183998

0,094

12,770970

0,225

0,503395

0,157

0,941

2689,24

1,55

2662,91

2,12

2628,37

3,40

2,26

C

0,005

107

0,693

63,8

10,0445

99,81%

186

1,53

10018

0,195

0,182488

0,095

12,582922

0,227

0,500088

0,159

0,939

2675,61

1,57

2648,94

2,14

2614,17

3,42

2,30

D

0,003

122

0,682

79,5

8,0821

97,45%

13

17,27

733

0,190

0,183887

0,223

13,021762

0,479

0,513592

0,385

0,889

2688,24

3,68

2681,23

4,51

2671,94

8,42

0,61

E

0,003

142

0,766

88,3

7,9217

99,81%

182

1,25

9669

0,213

0,183551

0,096

13,010360

0,236

0,514081

0,171

0,939

2685,22

1,59

2680,41

2,23

2674,03

3,74

0,42

CA1

0,001

512

0,754

354,3

11,0926

96,25%

9

35,38

496

0,208

0,184221

0,302

13,191120

0,654

0,519329

0,542

0,889

2691,24

4,99

2693,42

6,18

2696,33

11,94

-0,19

CA2

0,001

850

0,688

550,7

18,3763

98,08%

18

29,36

974

0,190

0,184211

0,179

13,160230

0,398

0,518139

0,317

0,899

2691,16

2,96

2691,21

3,76

2691,28

6,98

0,00

CA3

0,001

257

0,714

170,7

5,5467

97,32%

13

12,49

697

0,198

0,184456

0,234

13,161628

0,490

0,517507

0,393

0,883

2693,35

3,86

2691,31

4,63

2688,60

8,63

0,18

CA4

0,001

442

0,774

278,6

9,5356

99,58%

85

3,25

4503

0,215

0,184282

0,101

13,138059

0,238

0,517066

0,169

0,932

2691,79

1,67

2689,62

2,25

2686,73

3,72

0,19

CA5

0,001

220

0,695

136,8

4,7407

99,47%

65

2,06

3528

0,193

0,184360

0,107

13,128276

0,234

0,516465

0,158

0,924

2692,49

1,76

2688,92

2,21

2684,17

3,46

0,31

WUM-19-07 A

0,004

108

0,677

65,6

7,9869

99,61%

89

2,56

4772

0,190

0,186222

0,102

13,047712

0,266

0,508162

0,208

0,938

2709,08

1,68

2683,11

2,51

2648,78

4,51

2,23

B

0,004

70

0,612

41,5

5,8708

99,75%

134

1,23

7269

0,173

0,185593

0,096

12,873230

0,214

0,503066

0,139

0,940

2703,49

1,58

2670,42

2,01

2626,96

2,99

2,83

C

0,003

42

0,507

24,0

2,2417

99,58%

78

0,79

4357

0,145

0,184256

0,102

12,461778

0,255

0,490521

0,193

0,933

2691,55

1,69

2639,85

2,40

2572,93

4,09

4,41

D

0,004

235

0,345

127,7

19,0763

99,60%

81

6,22

4681

0,099

0,183191

0,106

12,315825

0,258

0,487593

0,195

0,929

2681,97

1,75

2628,78

2,43

2560,26

4,11

4,54

E

0,001

103

0,289

58,2

1,7372

99,12%

36

1,27

2101

0,081

0,183137

0,201

12,723163

0,484

0,503869

0,416

0,911

2681,48

3,33

2659,37

4,55

2630,40

8,99

1,90

CA1

0,005

127

0,415

74,9

13,8685

99,81%

169

2,20

9605

0,115

0,186295

0,095

13,426788

0,284

0,522720

0,233

0,951

2709,73

1,57

2710,15

2,68

2710,70

5,15

-0,04

CA2

0,005

18

0,400

11,0

1,9674

98,40%

20

2,63

1158

0,111

0,185367

0,175

13,223846

0,381

0,517396

0,311

0,891

2701,49

2,89

2695,76

3,59

2688,13

6,83

0,49

CA3

0,005

61

0,220

34,3

6,5789

99,52%

64

2,62

3840

0,061

0,183630

0,104

13,083401

0,217

0,516744

0,136

0,927

2685,93

1,72

2685,69

2,04

2685,36

2,98

0,02

CA4

0,005

40

0,433

23,7

4,3256

99,44%

58

2,02

3278

0,119

0,183885

0,106

13,187681

0,245

0,520141

0,176

0,924

2688,22

1,76

2693,18

2,31

2699,78

3,88

-0,43

CA5

0,003

122

0,197

67,1

7,8455

99,81%

164

1,22

9768

0,055

0,182976

0,099

12,992206

0,227

0,514976

0,158

0,931

2680,03

1,63

2679,09

2,14

2677,84

3,47

0,08

CA6

0,003

109

0,218

60,1

7,0149

99,93%

441

0,41

26148

0,060

0,183126

0,093

13,021060

0,227

0,515698

0,162

0,941

2681,39

1,54

2681,18

2,14

2680,91

3,54

0,02

W.U. Mueller et al. / Precambrian Research 214–215 (2012) 1–27

A

LM-03-06 A

0,005

384

0,655

234,9

41,6086

99,97%

966

1,21

50607

0,181

0,185244

0,092

13,268158

0,216

0,519475

0,146

0,944

2700,39

1,52

2698,92

2,04

2696,95

3,21

0,13

B

0,007

285

0,665

172,2

42,5948

99,96%

745

1,62

37633

0,186

0,184087

0,096

12,984173

0,341

0,511551

0,300

0,963

2690,04

1,59

2678,51

3,21

2663,25

6,54

1,00

C

0,006

227

0,696

141,3

29,2546

99,44%

53

15,62

2470

0,193

0,184648

0,102

13,133163

0,269

0,515850

0,209

0,939

2695,07

1,69

2689,27

2,54

2681,55

4,59

0,50

D

0,005

101

0,933

65,5

10,9303

99,83%

199

1,64

9542

0,258

0,185438

0,095

13,309033

0,303

0,520532

0,256

0,956

2702,12

1,57

2701,82

2,86

2701,43

5,65

0,03

(a) All single grains except for BMR-13-11-5. A, B etc. air abraded; CA1, CA2, etc. chemical abrasion, the latter after Scoates and Friedman (2008), Mattinson (2005). (b) Grain masses determined on Sartorious SE2 ultramicrobalance; some masses of CA grain estimated from photomicrographic grain dim., adjusted for partial diss. during chem. abrasion. (c) Nominal U and total Pb concentrations subject to balance uncertainty or photomicrographic estimation of weight and partial dissolution during chemical abrasion. (d) Model Th/U ratio calculated from radiogenic 208 Pb/206 Pb ratio and 207 Pb/235 U age. (e) Pb* and Pbc represent radiogenic and common Pb, respectively; mol % 206 Pb* with respect to radiogenic, blank and initial common Pb. (f) Measured ratio corrected for spike and fractionation only. Daly analyses, based on analysis of NBS-982. (g) Corrected for fractionation, spike, and common Pb; up to 35 pg of common Pb was assumed to be procedural blank: 206 Pb/204 Pb = 18.50 ± 1.0%; 207 Pb/204 Pb = 15.50 ± 1.0%; 208 Pb/204 Pb = 38.40 ± 1.0% (all uncertainties 1-sigma). Excess over blank was assigned to initial common Pb with S-K composition at 2.7 Ga. (h) Errors are 2-sigma, propagated using the algorithms of Schmitz and Schoene (2007) and Crowley et al. (2007). (i) Calculations are based on the decay constants of Jaffey et al. (1971). 206 Pb/238 U and 207 Pb/206 Pb ages corrected for initial disequilibrium in 230 Th/238 U using Th/U [magma] = 3. (j) Corrected for fractionation, spike, and blank Pb only.

25

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