JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 11
PAGES 1541–1564
1997
Chemical Stratigraphy and Petrogenesis of the Early Proterozoic Amisk Lake Volcanic Sequence, Flin Flon–Snow Lake Greenstone Belt, Canada M. I. LEYBOURNE1∗, N. A. VAN WAGONER1 AND L. D. AYRES2 1
DEPARTMENT OF GEOLOGY, ACADIA UNIVERSITY, WOLFVILLE, N.S., CANADA B0P 1X0
2
DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF MANITOBA, WINNIPEG, MAN., CANADA R3T 2N2
RECEIVED JUNE 1, 1996; REVISED TYPESCRIPT ACCEPTED JULY 1, 1997
The Amisk Lake area contains an 8·5 km thick sequence of subaerial and subaqueous flows and volcaniclastic rocks, including shallow water turbidites. The lower stratigraphy is dominated by transitional and tholeiitic basalt and basaltic andesite flows and volcaniclastic rocks. The transitional rocks are similar in many respects to modern boninites [concave-up rare earth element (REE) patterns, high mgnumber, Cr and Ni, and low Ti/V] and the parental magmas were produced by partial melting of a refractory harzburgitic mantle, previously metasomatized by H2O±large ion lithophile element (LILE)-rich fluids from subducting Early Proterozoic oceanic crust. Mixing with an ascending tholeiitic partial melt or mid-ocean ridge basalt (MORB)-source diapir produced the variable light REE (LREE) enrichment in these rocks. Tholeiitic and olivine-phyric picritic rocks range from LREE-depleted to moderately LREEenriched and are transitional from T-type MORB to E-type MORB, with mild addition of a slab-derived fluid. The lower part of the Amisk stratigraphy we interpret to have formed as a large volcano or volcanic complex within a backarc basin that was characterized by rapid volcanism and subsidence. The intercalation of subaerial and subaqueous lavas indicates focused volcanism consistent with a tholeiitic seamount or ocean island. The upper stratigraphy is more silicic and is dominated by calc-alkalic basaltic to andesitic volcanic rocks with trace element and LREE patterns similar to high-K calc-alkalic and shoshonitic rocks from modern island arcs, including strong negative Nb anomalies and LILE enrichment. These rocks represent the transition to island arc magmatism in the Amisk Lake sequence.
KEY WORDS:
backarc; arc; calc-alkalic; tholeiitic; Proterozoic
∗Corresponding author. Present address: Ottawa–Carleton Geoscience Centre, Department of Geology, University of Ottawa, Ottawa, Ont., Canada K1N 6N5. Telephone: 613-562-5773 or 613-947-0351. Fax: 613-562-5192. e-mail:
[email protected]
INTRODUCTION Many ancient and modern volcanoes and volcanic successions are complex, characterized by rapid lateral and vertical changes in facies and composition. Petrogenetic studies of volcanoes and volcanic successions are thus influenced by a large number of variables, and are strongly dependent on three-dimensional stratigraphic control. Most previous studies of ancient volcanic belts have relied on chemical discrimination diagrams to deduce the tectonic setting of the volcanic successions, with little emphasis on petrogenetic processes or stratigraphic relationships. For example, on the basis of previous geochemical studies, the Early Proterozoic Amisk Lake succession has been interpreted as either an island-arc volcano (Fox, 1976; Watters & Armstrong, 1985; Watters, 1991) or a backarc volcanic sequence (Gaskarth & Parslow, 1987). Volcanic rocks of the Flin Flon area, 20 km to the east, have been interpreted to have been erupted in a combination of arc and backarc environments (Syme, 1990), or in an oceanic island arc (Stauffer et al., 1975; Thom et al., 1990). More recently, models of Early Proterozoic volcanism in the Flin Flon Belt have been refined by regional and detailed mapping and lithogeochemical studies (e.g. Heaman et al., 1993; Reilly, 1993; Watters et al., 1994; Stern et al., 1995a, 1995b; Lucas et al., 1996). These studies have shown that the Flin Flon Belt represents a collage of different tectono-stratigraphic domains which were assembled during arc accretion events at around
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1·88–1·87 Ga (Lucas et al., 1996). Volcanic rocks that make up the different tectono-stratigraphic assemblages are interpreted to have formed in backarc basins (Watters et al., 1994; Stern et al., 1995a) and oceanic arcs (Stern et al., 1995b) with lesser volumes of rocks with ocean island and oceanic plateau affinities (Stern et al., 1995a, 1995b; Lucas et al., 1996). Detailed studies of the physical volcanology and stratigraphy of the Amisk Lake volcanic succession are now available (Ferreira, 1984; Ayers et al., 1991). These detailed studies from a portion of the Flin Flon Belt provide a framework that constrains interpretations based on the chemistry of the volcanic rocks within a stratigraphy that is more or less continuous. In this paper we present detailed major, trace and rare earth element data on samples collected during stratigraphic mapping of the Amisk Lake volcanic succession, and, using these geochemical data in combination with stratigraphic and volcanological data, we draw analogies with, and, more importantly, also critically examine differences from, modern volcanic settings.
GEOLOGIC FRAMEWORK The study area is the west end of the Flin Flon greenstone belt of the Reindeer Zone of the Trans-Hudson Orogen (formerly Churchill Province) of the Canadian Shield (Lewry et al., 1990). Like other greenstone belts of both Proterozoic and Archaean age, the Flin Flon Belt comprises folded metavolcanic and metasedimentary rocks intruded by ultramafic to granitoid plutons (e.g. Bailes, 1971; Watters et al., 1994; Lucas et al., 1996). Volcanic rocks are assigned primarily to the Amisk Group, which has been dated by U–Pb zircon techniques at 1886±2 Ma (Gordon et al., 1990) on the basis of samples from Flin Flon, 20 km east of the study area. Recent U–Pb studies on the West Mainland of Amisk Lake (Neagle Lake) suggest that felsic volcanic rocks are of similar age at 1888±3 Ma (Heaman et al., 1993), consistent with more regional compilations of U–Pb and Nd ages (Stern et al., 1995a, 1995b). The samples used in this study come from detailed mapping of volcanic and volcaniclastic rocks on islands within Amisk Lake (Fig. 1). The islands provide excellent exposure of the lithologies, although the lithological and structural relationships between islands can only be inferred. The volcanic succession within this study comprises a lower mafic subgroup at least 7 km thick and an upper intermediate to felsic subgroup at least 3 km thick (Ayres et al., 1991). The volcanic stratigraphy records alternating periods of submergence and emergence (Figs 1 and 2) of a volcanic succession, reflecting fluctuating rates of volcano construction and more constant, but relatively rapid
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subsidence of the volcano as a result of loading of Early Proterozoic crust. The volcanic sequence is isoclinally folded, faulted (Fig. 1), and metamorphosed to greenschist and locally amphibolite facies. Rock units are well exposed on the shores of numerous islands, providing cross-sectional views of the volcanic stratigraphy. A major fold, the Winterton Island anticline, provides three-dimensional control for some of the stratigraphy (Fig. 1). In the study area, the west limb sequence is 8·5 km thick and comprises 7 km of interlayered mafic flows and volcaniclastic rocks overlain by 1·5 km of intermediate flows and volcaniclastic rocks. Small-scale folds and faults locally complicate the west limb stratigraphy, but the continuity of mappable formations across the study area indicates that most faults have not markedly disrupted the stratigraphy. The exception is the fault northeast of Crater Island (Fig. 1), which has probably removed part of the upper mafic pillowed and sheet flow formation. As described in the next section, however, the stratigraphy of the upper mafic pillowed and sheet flow formation on the east limb of the anticline is controversial. We have mapped rock units on the east limb as a continuous sequence (Figs 1 and 2, East Channel section) comprising 4·4 km of mafic flows and volcaniclastic rocks. Reilly (1994), however, has mapped a major fault within the lower part of our upper mafic pillowed and sheet flow formation (Fig. 1) and has subdivided our formation into two formations. The stratigraphy of the intermediate to felsic sequence is poorly documented because of faults, intrusions, and poor inland exposure. Our examination was restricted to the lowermost 1·5 km, a subaerial sequence contiguous with the mafic units on the west limb of the Winterton anticline, and two isolated, fault-bounded sequences, one of which is subaerial, and the other subaqueous (Fig. 2). Subaqueous mafic units are composed of pillowed and sheet flows with up to 50% intercalated tuff and fine lapilli tuff produced by the downslope reworking and transport, in part by turbidity currents, of subaerially erupted pyroclastic ash and lapilli (Ayres et al., 1991). The intervening subaerial units are dominantly tuff and fine lapilli tuff of airfall and surge origin, with 10–50% intercalated pahoehoe flows. Subaqueous and subaerial mafic units are invariably separated by coarse flow-foot breccia deposits produced by the flow of subaerial lava into the ocean. These surf zone deposits (Figs 1 and 2) thus represent ancient shorelines. In the overlying felsic to intermediate succession both subaerial and subaqueous units comprise intercalated block lava flows, volcaniclastic debris flow conglomerate, and minor airfall tuff, and, in the subaqueous formation, turbiditic volcaniclastic sandstone. An important constraint on the interpretation of the volcanism is the occurrence of the lowermost subaerial
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Fig. 1. Simplified stratigraphic map of the central Amisk Lake area showing major lithologies and structures. The lower and upper pillowed and sheet flow formations are subaqueous; the mafic sheet flow, mafic tuff, and lower intermediate formations are subaerial; the three tuff–breccia formations (surf zone deposits) are transitional between subaerial and subaqueous conditions and represent palaeoshorelines; and the upper intermediate to felsic formation has both subaerial and subaqueous units. Locations of stratigraphic sections (Fig. 2) are shown by heavy lines: 1, west mainland; 2, West Channel; 3, Crater Island (in two segments); 4, Ing Island; 5, east Missi Island; 6, Chamney Island; 7, East Channel; 8, Iskwasoo Island (northwest side); 9, Iskwasoo South (southeast side of Iskwasoo Island). The inferred assemblage boundary after Reilly (1994) and Lucas et al. (1996) separates the Sandy Bay tectonic assemblage on the east from the Crater Island lithotectonic assemblage on the west. Inset shows location of the study area; FFSN, Flin Flon–Snow Lake greenstone belt.
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Fig. 2. Generalized stratigraphic sections and correlations of the Amisk Lake area showing the major lithologies, depositional environments and locations for samples used in this study. Stratigraphic section numbers refer to Fig. 1. ISZ, Iskwasoo Fracture Zone (Reilly, 1994).
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units near the core of the Winterton Island anticline, close to the base of the preserved stratigraphic sequence (Figs 1 and 2). This suggests that much of the lower subaqueous base of the volcanic succession, and any underlying oceanic crust, is missing.
Lithotectonic assemblages In the Flin Flon area to the east, Bailes & Syme (1989) recognized a number of major faults that separate lithologically and chemically distinct metavolcanic and related metasedimentary sequences. Subsequent work, both near Flin Flon and elsewhere in the Flin Flon–Snow Lake belt, including the Amisk Lake area, has resulted in the identification of a number of fault-bounded lithotectonic assemblages (Reilly, 1994; Stern et al., 1995a, 1995b; Lucas et al., 1996). Each of these assemblages has distinct lithologic and chemical characteristics, and the assemblages are interpreted to represent ocean floor, ocean plateau, and juvenile ocean arc sequences that were tectonically juxtaposed to form an accretionary collage at about 1·88–1·87 Ga, possibly by arc–arc collision (Lucas et al., 1996). In our study area, Reilly (1994) and Lucas et al. (1996) have proposed that an assemblage boundary exists on the eastern limb of the Winterton Island anticline (Fig. 1) within our upper mafic pillowed and sheet flow formation; this boundary separates their Sandy Bay ocean floor– plateau assemblage on the east from their Crater Island assemblage on the west. This boundary, termed the Iskwasoo shear zone, is poorly exposed, and, from field observation, we are not convinced that it is a major structure. However, the presence and/or nature of this boundary is immaterial to the present study because, although our East Channel section (Figs 1 and 2, no. 7) extends across this boundary, all of our chemically analysed samples are from the Crater Island assemblage of Reilly (1994) and Lucas et al. (1996).
ANALYTICAL METHODS As an outgrowth of detailed stratigraphic and physical volcanological studies, more than 2000 samples of volcanic flows and fragmental rocks were collected for lithological study (Ferreira, 1984; Ayres et al., 1991). We selected 80 samples, mostly from flows and a few from subvolcanic intrusions, for this geochemical study. To minimize contamination and alteration problems, samples were selected on the basis of well-preserved primary flow textures, absence of metamorphic and tectonic fabrics, and low incidence of secondary veins, amygdules, and visible alteration. Gaps in the sample suite (Fig. 2) represent areas where flows are more recrystallized
and samples were not considered suitable for analysis, or flows are absent. Samples were crushed to
