Large Igneous Provinces (LIPs) of Canada back to 2.5 Ga and Links with Supercontinent Breakup
Richard Ernst
(Ernst Geosciences; Richard.Ernst@ErnstGeosciences.com),
and
Wouter Bleeker
(Geological Survey of Canada, Ottawa, Canada; wbleeker@NRCan.gc.ca)
Extracted and modified from Ernst and Bleeker (2010)
Introduction
Herein we explore the use of large igneous provinces (LIPs) and interpreted LIP fragments as tools for determining the timing and location of breakup margins within Canada and selected adjacent regions in Greenland and the United States. Our premise is that LIPs can provide a more efficient method to identify Precambrian breakup or attempted breakup events than is possible using other types of geological piercing points (Bleeker 2003; Bleeker and Ernst 2006; Ernst and Bleeker 2010).
We work back in time from the Present with a special focus on the poorly understood Proterozoic breakup history, and leave the Archean breakup history of Canada for future analysis. We focus mainly on the age and distribution of the magmatic units, particularly on the diabase dyke swarms which are uniquely valued for preserving the record of magmatic events because they typically extend far into stable cratonic interiors (e.g., Halls 1982; Fahrig 1987; Bleeker 2004; Ernst and Buchan 2004). This contribution is extracted and modified from Ernst and Bleeker (2010).
Large Igneous Provinces
Large Igneous Provinces (LIPs) are high volume (>100,000 cu km), short duration (or consisting of short duration pulses) intraplate-type magmatic events consisting mainly of flood basalts and their associated plumbing systems, but also may include a silicic component and associated carbonatites and kimberlites (Fig. 1) (Coffin and Eldholm 1994, 2001, 2005; Courtillot and Renne 2003; Ernst et al. 2005; Bryan and Ernst 2008). A further distinction is LIP “fragments” (Ernst 2007); these are magmatic units of smaller size or areal extent that are thought to have originally belonged to a LIP-scale event but have been reduced in extent by erosion or tectonic fragmentation. LIP “fragments” have essential characteristics indicating that they were emplaced as part of a larger-size event. For instance, a swarm of dolerite dykes with an average width of 30 m are likely to belong to a LIP-scale event even if their preserved areal extent is sub-LIP scale.
Figure 1: Revised classification of Large Igneous Provinces; modified after Bryan and Ernst (2008) to include associated kimberlites and carbonatites. LIP classifications are based on the initial work of Coffin and Eldholm (1994, 2005).
LIP Record of Canada
In Figures 2 and 3 we schematically summarize the Large Igneous Province (LIP) record of Canada (and adjacent Greenland and the United States), and especially giant dyke swarms; these are the most prominently preserved expression of Proterozoic LIPs, and are particularly useful for recognizing mantle paleo-plumes and associated continental breakup events. The October 2010 LIP of the Month (Claoué-Long and Dean Hoatson, 2010) provided a similar overview of the LIP record for Australia.
Figure 2: Barcode of Canadian LIP and LIP fragment events discussed herein. Names which are underlined belong to LIPs sensu stricto; those which are not underlined are interpreted to be ”fragments” of LIPs.
Figure 3: LIPs and their association with specific breakup margins in a Laurentian context. LIP events are shown in red or orange, with broad lines marking dyke swarms, and generalized polygons marking other units (sills, volcanics, layered intrusions). Stars mark plume centres identified as the convergence point of radiating dyke swarms. Blue lines mark the inferred portion of a breakup margin (or attempted breakup margin) that is potentially linked to the given LIP. Background generalized geology modified after Hoffman (1989). Light grey areas marks cratons, dark grey areas mark orogenic belts. Labels for cratons: C = Core Zone, H = Hearne, M = North Atlantic, R = Rae, S = Sask, Sl = Slave, Sup = Superior, and W = Wyoming. Other labels: KSZ = Kapuskasing Structural Zone, T = Thompson promontory and U = Ungava promontory of the Superior craton. L = Lewisian portion of the North Atlantic craton. Modified after Ernst and Bleeker (2010), and LIP distributions are mostly from Ernst and Buchan (2001, 2004), and Buchan and Ernst (2004).
We recognize that we have been liberal (in Figs. 2 and 3) in interpreting intraplate events as LIP fragments, but our enthusiasm is based on the trend over the past 20 years of typical dramatic increases in the known extent of many intraplate events as the vast reservoir of undated dolerites and volcanic units are being dated, and as intraplate events are being traced from one block to another through improving paleocontinental reconstructions (www.supercontinent.org).
We consider the paleocontinental reconstruction history back to 2500 Ma by focussing on breakup margins, and attempt to match each breakup margin of the cratonic blocks that comprise Canada with an associated LIP; this approach is based on the well-established link in the Mesozoic-Cenozoic record between LIPs and continental breakup (e.g., Storey, 1995; Courtillot et al. 1999).
Summary of links with breakup margins
Many LIPs have an associated radiating dyke swarm, which typically converges on a cratonic margin, identifies a mantle plume centre and are linked to breakup or attempted breakup that formed that cratonic margin (Ernst and Bleeker 2010, and references therein).
With reference to Fig. 3, in Paleoproterozoic time the various margins of the Superior craton were linked with LIP events as follows: 2.50-2.45 Ga on the southeast side, 2.22-2.17 Ga on the northeast side, 2.12-2.08 Ga on the south side, and possibly 2.00- 1.97 Ga on the north side (Ungava promontory). An additional event (the Circum-Superior event) at ca. 1.88 Ga is focussed on the Thompson promontory during a time of ocean closure. The Slave craton has numerous Proterozoic LIP events, and those dyke swarm events at ca. 2.1 Ga and 2.2 Ga have weak fanning patterns focussed on the western and eastern sides, respectively. In addition, an event at 2.19 Ga is likely linked to breakup on the southwest to south side of the Slave craton. The North Atlantic craton (southern Greenland and Nain) has ca. 2.04 Ga magmatism linked to its western margin, and 2.24 Ga magmatism may be linked to breakup along the southeastern craton margin. The Wyoming craton has an event at 2.01 Ga linked to breakup along the southeast side. There are several other events at 2.17, 2.11 and 2.49 Ga that are not yet linked to a specific margin of the Wyoming craton. Events within the Hearne craton provide a link with the southern Superior craton in the Paleoproteozoic (Bleeker 2004). Insufficient geochronology is available to constrain the breakup history of the Rae craton.
Mesoproterozoic breakup events at 1.75 Ga, 1.38 Ga are linked with northern and northwestern Laurentia. Neoproterozoic breakout of Laurentia is linked to a ca. 0.72 Ga event on the northern side (possibly separation of Siberia). Ca. 0.7 Ga and ca. 0.62-0.55 Ga events on the eastern margin of Laurentia are linked to the formation of Iapetus ocean and separation of Baltica and possibly Amazonia. Ca. 0.78 and perhaps 0.69 Ga events on the western side of Laurentia are linked with breakup of Australia and/or South China. The youngest LIP events are linked with breakup of Pangea: 0.20 Ga on the east side (formation of the Atlantic Ocean), ca. 0.13 Ga on the north side (formation of the Arctic Ocean).
Discussion
There are a number of implications that follow from this work that can be further investigated:
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