Proto-Avalonia: A 1.2–1.0 Ga tectonothermal event and constraints for the evolution of Rodinia

INTRODUCTION

Neoproterozoic crustal evolution was profoundly influenced by the amalgamation of the supercontinent Rodinia, its subsequent breakup, and Pan-African (650–550 Ma) collisional events that resulted in the formation of Gondwana (e.g., Powell et al., 1993; Dalziel, 1997). However, the timing remains controversial, as does the configuration of Rodinia (e.g., Karlstrom et al., 1999; Wingate and Giddins, 2000).

Most tectonic interpretations for the Neoproterozoic are based on the near-field effects of these events. However, global-scale geodynamic links imply that far-field effects should also be preserved. In this paper we use new Sm-Nd isotopic data that define the age of Avalonian basement to propose that the Neoproterozoic tectonothermal evolution of peri-Gondwanan terranes was a far-field response to these global-scale events in the same way that the Mesozoic and Cenozoic evolution of western North America was a far-field response to the breakup of Pangea. Hence, we suggest that major changes in the evolution of the peri-Gondwanan terranes can be explained in terms of Rodinian amalgamation, its subsequent breakup, and the Pan-African collisions that followed. Given the inherent difficulties in dating these events in the near-field, their far-field effects may provide hitherto overlooked constraints on their timing.

GENERAL GEOLOGY

Avalonia is the largest suspect terrane in the Appalachian orogen, occupying much of its southeastern flank (Fig. 1A). Pre-Mesozoic reconstructions suggest former continuity with similar sequences in the British Isles. Avalonia was one of several terranes, referred to as peri-Gondwanan terranes, that evolved during discrete tectonomagmatic episodes between 760 Ma and 550 Ma along an active continental margin of Gondwana (Fig. 1B; e.g., Keppie et al., 1991; Linnemann et al., 2000). These terranes were detached from Gondwana in the early Paleozoic and were subsequently involved in Appalachian-Caledonide orogenesis (Keppie et al., 1991). In Atlantic Canada, Avalonia is characterized by voluminous late Neoproterozoic (ca. 630–570 Ma) arc-related volcanic and cogenetic plutonic rocks, by coeval sedimentary successions deposited in a variety of intraarc and interarc basins (e.g., O'Brien et al., 1996; Murphy et al., 1999), and by latest Neoproterozoic (ca. 570–550 Ma) intra-continental wrench-related bimodal volcanic and sedimentary rocks. The transition from arc-related to wrench-related tectonic activity was diachronous, occurring at 595 Ma in New England, in the interval 610–560 Ma in southern New Brunswick, at 605 Ma in mainland Nova Scotia, between 575 Ma and 560 Ma in Cape Breton Island, at 575 Ma in Newfoundland, and in the intervals 570–560 Ma in Britain and 570–540 Ma in France (Murphy et al., 1999). Lower Cambrian platformal successions contain Acado-Baltic (Avalonian) fauna and overlie these Neoproterozoic successions (Keppie et al., 1991). Murphy and Nance (1991) linked late Neoproterozoic arc magmatism to subduction along the Gondwanan margin and its termination to the development of a transform system associated with ca. 550–540 Ma rift-drift transition of the Iapetus cycle.

The geologic record of Avalonia provides important constraints for the tectonothermal evolution of the Mesoproterozoic margin of Gondwana. Avalonian arc magmatism and the nature of its termination imply a peripheral position along the Gondwanan margin in the Neoproterozoic and Cambrian, and most reconstructions consequently show Avalonia facing an open ocean (Fig. 1B; e.g., Nance and Murphy, 1996; Dalziel, 1997).

The age and composition of the Avalonian basement are keys to understanding the configuration of this Gondwanan margin and the early tectonothermal events that occurred along it. However, nowhere is Avalonian basement exposed. Sm-Nd isotopic analyses of crustally derived late Neoproterozoic (ca. 630 Ma) to Early Silurian igneous rocks characterize the nature of the crustal source (e.g., Nance and Murphy, 1996). These rocks have similar ϵ_Nd values (t = 610 Ma) that range between −1.0 and +5.0 and Sm/Nd ratios typical of intracrustal melts. The ϵ_Nd growth lines consistently yield overlapping T_DM model ages of 0.8–1.1 Ga in Atlantic Canada and 1.0 to 1.3 Ga T_DM model ages in the British Midlands (Thorogood, 1990).

However, the origin of these Nd isotopic signatures is unclear. If the felsic rocks are relatively pure crustal melts, they may record mantle extraction ages for Avalonian basement. Alternatively, if the felsic rocks represent variable mixtures of ancient recycled crust and new mantle material, the depleted mantle model age is a hybrid value with no geological meaning (Arndt and Goldstein, 1987). Central to the resolution of this uncertainty is the isotopic signature of tectonothermal events that occurred prior to the main cycle of Avalonian magmatism, because model ages produced by the mixing of juvenile melts and recycled crust ca. 630–570 Ma should differ from those of the older igneous suites. Hence, the isotopic signature of the early arc phase (760–660 Ma) can be used to test the extent of isotopic inheritance in the main phase (630–540 Ma), in addition to characterizing the isotopic composition of the basement.

EARLY ARC MAGMATISM

Recent age dating (⁴⁰Ar/³⁹Ar, hornblende; U-Pb, zircon) suggests that vestiges of ca. 760–660 Ma tectonothermal activity are preserved in Avalonia and its European correlatives. Representatives occur in mainland Nova Scotia (the 734 Ma calc-alkalic Economy River Gneiss, Doig et al., 1993), in Cape Breton Island (the ca. 676 Ma arc-related Stirling belt, Bevier et al., 1993, and the 700–630 Ma backarc volcanism in the Creignish Hills, Keppie et al., 1998), in southern Newfoundland (the 763 Ma rift ophiolites of the Burin Group volcanics, the Connaigre Bay Group calc-alkalic rhyolite, and the 680 Ma calc alkalic Grey River–Cinq Cerf gneiss belt, O'Brien et al., 1996), and possibly in Massa-chussetts (the ca. 700 Ma arc-related Fishbrook Gneiss, Olzewski, 1980). Similar evidence for early arc-related activity is found in correlative Avalonian rocks in southeastern Ireland (the ca. 700 Ma calc-alkalic orthogneiss of the Ross-lare Complex, Gibbons and Horak, 1996), in Britain (the ca. 670 Ma calc-alkalic Malverns Plutonic Complex, Tucker and Pharoah, 1991; Strachan et al., 1996), and in the Cadomian rocks of the Armorican massif of France (the ca. 745 Ma orthogneiss of the Pentevrian Complex, Egal et al., 1996). Despite the tectonic significance of these rocks, little is known about their evolution. Here we attempt to define their development by presenting Sm-Nd isotopic data from two representatives; the ca. 734 Ma Economy River Gneiss and the ca. 670 Ma Malverns Plutonic Complex.

The Economy River orthogneiss crops out in the Cobequid Highlands (Fig. 1A) of mainland Nova Scotia, where it is bounded to the south by the late Paleozoic Cobequid fault and to the north by a ca. 600 Ma granite gneiss. The orthogneiss contains quartz, perthite, plagioclase, and hornblende and a variably developed mylonitic fabric that imparts a foliation consisting of alternations of quartzo-feldspathic and mafic layers. A strong subhorizontal to moderately inclined mineral lineation is defined by elongate quartz grains. U-Pb dating on zircons has yielded an upper intercept age of 734 ± 2 Ma that is interpreted to date the age of intrusion of its granite protolith (Doig et al., 1993). Geochemical analyses (Doig et al., 1993) show that the gneiss is intermediate to felsic in composition and characterized by moderate enrichment in large ion lithophile elements such as K, Rb, Ba, and Th, as well as Ce and Sm, relative to high field strength elements such as Nb, Y, and Zr. This signature is typical of arc environments, and the gneiss is interpreted as a local representative of a regionally extensive early (ca. 750–670 Ma) subduction event.

The Malverns Plutonic Complex of the Malvern Hills in the English Midlands (Fig. 1A) is a variably metamorphosed and deformed inlier dominated by calc-alkalic dioritic, tonalitic, and granitic rocks with minor ultramafic, mafic, and hybrid lithologies and small areas of quartzo-feldspathic schist and gneiss that may represent fragments of host rock. A U-Pb zircon age of 677 ± 2 Ma obtained from a granodiorite is considered to date closely the emplacement age (Tucker and Pharaoh, 1991) and ⁴⁰Ar/³⁹Ar mineral ages of ca. 650 Ma are interpreted to date cooling following metamorphism at upper green-schist to amphibolite facies (Strachan et al., 1996). The complex is overlain unconformably by a Cambrian-Ordovician overstep sequence that contains Avalonian fauna.

ISOTOPIC DATA

Representative samples of the Economy River Gneiss and the Malvern Plutonic Complex were analyzed for their Sm-Nd isotopic compositions at the Atlantic Universities Regional Isotopic Facility (AURIF) at Memorial University, Newfoundland (Table 1). The methods are described in Longerich et al. (1990). For the Economy River Gneiss (ϵ_Nd)_t values (calculated for t = 734 Ma) are positive, ranging from +1.29 to +4.09, with Nd model ages (T_DM) ranging from 998 Ma to 1194 Ma (Table 1). For the Malverns Plutonic Complex (ϵ_Nd)_t values (calculated for t = 677 Ma) are slightly negative, ranging from −0.11 to −2.03, and T_DM model ages range from 1043 Ma to 1147 Ma. The model ages for both suites compare favorably with those of both the main (630–570 Ma) Avalonian arc phase and subsequent Paleozoic tectonothermal events in their respective areas (Fig. 2). Differences in the (ϵ_Nd)_t values between the two suites are in part related to their difference in ages, but also reflect differences in the Sm/Nd ratios of their respective crustal sources.

Taken together, the Nd isotopic data indicate a significant degree of inheritance in the isotopic signature of Neoproterozoic and Paleozoic felsic igneous rocks, implying that the felsic magmatism that characterized main arc activity in Avalonia was predominantly generated by recycling preexisting crust. The data imply that this signature was established early in the history of Avalonia, and that this is a characteristic feature of the Avalonian basement and is unlikely to represent mixing of ancient and juvenile crust. There is no record of tectonothermal activity in Avalonia between the oldest T_DM age of ca. 1.2 Ga and ca. 760 Ma. This suggests that the isotopic signature of the Economy River Gneiss and Malverns Plutonic Complex represents a relatively simple evolution involving recycling of ca. 1.0–1.2 Ga crust. If so, the depleted-mantle model ages represent a genuine tectonothermal event that produced a basement that was recycled by subsequent Neoproterozoic and Paleozoic tectonothermal activity. The 1.0 Ga model ages calculated for younger felsic suites are therefore likely to represent an inherent characteristic of Avalonian basement.

DISCUSSION

A 1.0–1.2 Ga tectonothermal event in Avalonia would be coeval with convergence and continental collision between Amazonia-Baltica and Laurentia, a key event in the amalgamation of Rodinia (Åhäll and Gower, 1997). However, the primitive isotopic signature of Avalonian basement (proto-Avalonia) compared to the Grenville province (Fig. 2A) suggests that it formed as a series of primitive oceanic island arcs and, therefore, is not directly related to collisional orogeny. If so, the arcs presumably formed within a Panthalassa-type ocean that must have surrounded Rodinia following Grenvillian orogenesis (Fig. 3). Remnants of these primitive arcs may be preserved in the ca. 950–900 Ma metavolcano-sedimentary sequences of the Tocantins province of central Brazil (see Fig. 1B; Pimental and Fuck, 1992).

The breakup of Rodinia was complete by ca. 755 Ma (Wingate and Giddins, 2000) and the drift between Laurentia and east Gondwana may have generated a far-field effect along the west Gondwanan margin. This drift is coeval with renewed subduction in the peri-Rodinian ocean, which may account for the early phase (ca. 760–660 Ma) of arc magmatism in Avalonia and the Tocantins province, and for the eventual accretion of Avalonian and related terranes to the Gondwanan margin. If so, the change from sub-duction of western Pacific type to eastern Pacific type was related to the breakup and dispersal of Rodinia. Accretion of outboard Avalonian terranes is thought to coincide with ca. 650–630 Ma high-grade metamorphic events recorded in the Malverns Plutonic Suite (Strachan et al., 1996), and possibly in southern Newfoundland (O'Brien et al., 1996). Following the accretion and metamorphism, renewed subduction along the Gondwanan margin outboard of the accreted terranes recycled this juvenile crust during the main phase (630–570 Ma) of Avalonian arc activity. This main phase is coeval with the first phase of rifting between east Laurentia and Amazonia–Rio de la Plata (Cawood et al., 2001).

The tectonothermal evolution of Avalonia is thought to be geodynamically linked to the amalgamation and dispersal of Rodinia in a manner analogous to the relationship between the Mesozoic dispersal of Pangea and the evolution of western North America. Following the breakup of Pangea, the predominantly westward motion of North America resulted in subduction along its western margin, the piecemeal accretion of juvenile arc terranes, and renewed subduction outboard of these terranes. This is precisely the sequence of events recorded in Avalonia and its proto-Avalonian basement. The cessation of Avalonian arc magmatism and the development of an intracontinental wrench regime could be further linked to eventual ridge-trench collision in a manner similar to that of the modern San Andreas system. These geodynamic links indicate that careful evaluation of the evolution of peripheral terranes such as Avalonia may shed light on major aspects of Rodinian tectonics.

More generally, by comparing the isotopic characteristics of different generations of magmatic activity in a terrane, the relative contributions of juvenile and ancient crust may be evaluated. Similar Sm-Nd isotopic signatures for Neoproterozoic and early Paleozoic tectono-thermal events within Avalonia suggest that 1.2–1.0 Ga depleted mantle model ages represent a tectonothermal event rather than mixing between older crust and more juvenile magma and that subsequent events recycled this crust.