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May 2017 LIP of the Month

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Do the Mesoproterozoic “Granite-Rhyolite” Provinces of the midcontinent, and related A-Type granite plutons, constitute a Silicic Large Igneous Province (SLIP)?

By M. E. (Pat) Bickford, Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070 USA (


Richard Ernst (Carleton University, Ottawa) recently gave a talk on LIPs at the Central New York Earth Science Student Symposium at Syracuse University. Following Richard’s talk, I spoke with him, asking whether the ca. 1.45-1.35 Ga “Granite-Rhyolite Provinces” of the midcontinent USA, and their extensions as “A-type” granite plutons across the western US, might be considered a Silicic LIP (SLIP). Richard suggested posting a description of these interesting, and somewhat enigmatic, rocks on his website so that comments might be received. Much of the following is slightly modified from a recent paper, “Mesoproterozoic-trans-Laurentian magmatism: A synthesis of continent-wide age distributions, new SIMS U-Pb ages, zircon saturation temperatures, and Hf and Nd isotopic compositions:“ by M. E. Bickford, W. R. Van Schmus, K. E. Karlstrom, P. A. Mueller, and G. D. Kamenov, that was published in Precambrian Research (2015,v. 265, p. 286-312.). Readers of this post are referred to this paper for more details.

Statement of the Problem

One of the most interesting and enigmatic problems in understanding the growth of Laurentia during the Mesoproterozoic is the origin of the 1.50–1.34 Ga “granite–rhyolite” provinces of the North American midcontinent region and the coeval and related “A-type” plutons that are known from Labrador through the midcontinent to California (Fig. 1).

Figure 1: Map of United States showing Mesoproterozoic terranes. Blue line is the “Nd line” of Van Schmus et al. (1996) Eastern Granite-Rhyolite Province (EGR) rocks are ca. 1480 Ma whereas Southern Granite-Rhyolite Province SGR rocks are ca. 1370 Ma. Rocks of the ca. 1100 Ma Mid-continent Rift System are shown in green. A-type plutons of these ages are known from Labrador to California. Inner and Outer Tectonic Zones represent essentially the older Yavapai and younger Mazatzal Provinces, both of which are Paleoproterozoic. 

The existence and approximate extent of these rocks in the midcontinent has been known since the early studies of Muehlberger et al. (1966, 1967), Goldich et al. (1966) and Lidiak et al. (1966) and they were recognized as an important trans-Laurentian assemblage by Silver et al. (1977). Major advances in understanding these rocks in the midcontinent came over the next three decades, and were summarized in major sections of

Chapter 4 in the Precambrian Volume (C2) of the GSA Decade of North American Geology (DNAG) publications (Van Schmus et al., 1993a and related subsections: Lidiak et al., 1993; Van Schmus et al., 1993b; Bickford and Anderson, 1993; Van Schmus and Hinze, 1993; Gilbert and Denison, 1993). Since then several other papers have presented additional data on the midcontinent basement, including Van Schmus et al. (1996), Barnes et al. (1999), Barnes et al. (2002), Rohs (2001), Goodge and Vervoort (2006), Menuge et al. (2002), Vigneresse (2005), Rohs and Van Schmus (2007), Van Schmus et al. (2007), Dewane and Van Schmus (2007) and Fisher et al. (2010).

Many workers have also studied these co-called “A-type” plutons in general (e.g., Anderson, 1983; Anderson and Cullers, 1999; Anderson and Morrison, 2005), and new insight came from the Hf isotopic study of Goodge and Vervoort (2006), which showed that most of the plutons they studied were derived from older crust of the craton, in accordance with earlier Sm–Nd work, although their sampling did not include many samples from areas interpreted as “juvenile” (i.e., 1.5–1.4 Ga crust) by Van Schmus et al. (1996). The tectonic setting and processes that resulted in the formation of this vast terrane of uniformly high-silica rocks have been of keen interest for decades. In many papers A-type plutons were considered as “anorogenic” (Anderson, 1989) because they are generally less foliated than rocks they intrude. Many authors concluded that extensional environments played a major role (e.g., Anderson and Cullers, 1999; Frost and Frost, 2008). However, numerous workers began to document “orogenic” fabrics associated with these plutons in the Rocky Mountains (Kirby et al., 1995; Duebendorfer and Christensen, 1995; Karlstrom and Humphreys, 1998) and regional metamorphism (Karlstrom et al., 1997; Williams et al., 1999; Shaw et al., 2005). More recently Menuge et al. (2002) and Slagstad et al. (2009) suggested that rocks of the granite–rhyolite provinces were formed in a back-arc setting related to early phases of the Grenville orogeny, some models have invoked plume-related crustal extension (e.g. Hoffman, 1989; Anderson and Morrison, 2005), and other models have viewed 1.4 Ga Laurentian magmatism to have involved inboard continental arc and back arc magmatism linked to accretion of 1.5–1.4 Ga Laurentian terranes (Karlstrom et al., 2001; Whitmeyer and Karlstrom, 2007) during outboard subduction- related and transcurrent tectonism (Nyman et al., 1994; Rivers, 1997).

Geologic setting

The midcontinent region within Laurentia

In the context of Precambrian continental history, the “mid- continent region” within Laurentia encompasses the area between exposed basement of the Rocky Mountains in the west and the Appalachian Mountains in the east (Fig. 1). Buried extensions of midcontinent Precambrian rocks, that are variably overprinted by 1.3–1.0 Ga Grenville tectonism, extend into the Appalachians and into the plains of eastern Canada (Whitmeyer and Karlstrom, 2007). The Laurentian midcontinent also extends from southern Texas northward to exposed basement in the Great Lakes area, with buried extensions into the plains of southern Saskatchewan and Manitoba in Canada. Within the midcontinent, the buried Precambrian basement, including several small regional uplifts, ranges in age from Archean to Neoproterozoic, although most of it is Paleoproterozoic and Mesoproterozoic (ca. 2500–1000 Ma; Van Schmus et al., 1993b).

Constraints on the geology of the buried midcontinent basement are provided by (a) peripheral geology; (b) scattered uplifts in SE Missouri, NE Oklahoma, SE Oklahoma, the Llano Province in Texas, and the Van Horn-El Paso region of southwestern Texas; (c) regional geophysical data, particularly gravity and aeromagnetic compilations; and (d) variably distributed drill holes to basement that have provided spot control for regional interpretation of features on the geophysical maps or correlation with the exposed rocks on the periphery or in the scattered uplifts.

The western and northern parts of the midcontinent basement (Fig. 1) are dominated by Paleoproterozoic rocks that presumably represent northeastward extensions of the Yavapai and Mazatzal provinces in the southwestern U.S. (cf. Whitmeyer and Karlstrom, 2007), whereas the eastern and southern parts are dominated by ca. 1500–1350 Ma Mesoproterozoic terranes underlain mainly by granites, rhyolites, and sparse associated sedimentary rocks (cf. Van Schmus et al., 1993b, 1996).

Mesoproterozoic geology of the midcontinent region

The midcontinent granite–rhyolite provinces are known from northwestern Ohio to the Texas Panhandle and make up most of the basement of western Ohio, Indiana, Illinois, Kentucky, Tennessee, Missouri, Kansas, Oklahoma, and northern Texas (Fig. 2). Extensive U–Pb zircon dating has shown that the eastern mid- continent is underlain by granite and rhyolite formed about 1500–1440 Ma, whereas the southern mid-continent is underlain by a very similar suite of rocks formed about 1400–1340 Ma. Van Schmus et al. (1993b) referred to these two provinces as the Eastern Granite–Rhyolite Province (EGRP) and the Southern Granite–Rhyolite Province (SGRP), respectively. Plutons of both suites were emplaced into Paleoproterozoic basement in north- eastern Kansas and northwestern Missouri, and the older suite may underlie parts of the younger suite in southern Oklahoma. Spatial overlap of the younger suite within the older suite may be seen in the mostly 1480 Ma St. Francois Mountains of southeastern Missouri, where ca. 1360 Ma plutons intrude the older, ca. 1480 Ma rocks.

Rhyolites and dacites of the EGRP, which are primarily exposed in the St. Francois Mountains of SE Missouri, commonly preserve features such as eutaxitic textures, ?amme, and abundant quartz and alkali feldspar phenocrysts that identify them as ash- flow tuffs (Fig. 2). The related granites (Fig. 3) commonly display granophyric textures, indicating epizonal emplacement, although some have well-developed rapakivi textures. Upper crustal rocks of the extensive midcontinent occurrences are generally undeformed and unmetamorphosed. In exposed areas, such as the St. Francois Mountains, a few small gabbroic bodies and diabasic dikes are known, but these are rarely encountered in drill holes, suggesting that they are not common throughout the region. Similarly, there are a few drill-hole penetrations of somewhat recrystallized quartzose sedimentary rocks of unknown age, but regionally these are rare. The association of plutonic rocks of this age and type with anorthosite is known in the Wolf River batholith of Wisconsin (cf. Van Schmus et al., 1975b) and in rocks in Canada (cf. Rivers, 1997), but on the scale of the U.S. mid-continent this association appears rare.

Figure 2: Rhyolitic ash-flow tuff, Wildcat Mtn., St. Francois Mtns, SE Missouri.

Figure 3: Knoblick granite intruding rhyolite, Knoblick Mtn., St. Francois Mtns, SE Missouri.

More or less continuous rocks of the SGRP occur in the sub- crop of southern and southwestern Missouri, northern Arkansas, most of Oklahoma, the Texas Panhandle, and southern Kansas. Outcrop representatives of this province occur in NE Oklahoma (Spavinaw Granite), SE Oklahoma (Arbuckle Mountains), and the Van Horn region of Texas (Carrizo Mountains). The subcrop in Kansas is only known from drill holes and aeromagnetic interpretations, and it is presumed to be discontinuous along its northern margin (erosional windows and outliers); this continues eastward into Missouri and Arkansas. Extension of the SGRP east of the Mississippi River is assumed but poorly controlled. Finally, isolated plutons coeval with the SGRP occur in SE Missouri and the basement of northern Missouri and Kansas with a few outliers farther north and west. The most detailed section of the SGRP is known from the Panhandle of Texas, where Barnes et al. (2002) compiled a comprehensive cross section that was based on numerous drill holes. Fig. 4 is a map showing the extent of the drill-hole and surface outcrop control in the mid-continent region.

Figure 4: Map of the south-central mid-continent region showing samples used to control con?guration of the Proterozoic basement (from Van Schmus et al., 1993b, 1996; Rohs and Van Schmus, 2007). Dashed line is the “Nd line” of Van Schmus et al. (1996).

Major advances in understanding the origin of the granite–rhyolite provinces began with the Sm–Nd studies of Nelson and DePaolo (1985), who showed that most of the rocks of this age formed from older (Paleoproterozoic) crust, and Bowring et al. (1991) who discovered that rocks of the St. Francois Mountains in SE Missouri had positive epsilon values for Nd and model ages that were essentially the same as the crystallization ages. Van Schmus et al. (1993b) expanded the Sm–Nd data set for the mid-continent, showing a possible boundary between units with old TDM and units with younger TDM (their Fig. 3), but did not comment extensively on the signi?cance of this line. Subsequently Van Schmus et al. (1996), Barnes et al. (1999), Rohs (2001), and Rohs and Van Schmus (2007) expanded the Nd studies to include much more of the granite–rhyolite outcrop and subcrop in the midcontinent region. Van Schmus et al. and Rohs and Van Schmus found that they could specify a “line”, the so-called “Nd line” (shown in blue on Fig. 1 and as a dashed line on Fig. 4) that separates Mesoproterozoic rocks with TDM model ages in the 1600-1800 Ma range on the north and west from similar rocks to the south and east whose TDM model ages are about 1500 Ma. These differences are interpreted to indicate that the rocks with older TDM were derived from, or have incorporated, older, presumably Paleoproterozoic, crustal material whereas those with the younger TDM model ages are more juvenile.

Deeper crustal manifestations of the 1.47 and 1.37 Ga magmatic events are exposed to the west in uplifts of the Rocky Mountains, in the Arizona Transition Zone, and across the Mojave Desert region. It is generally understood that these coeval (1.50–1.34 Ga) A-type granites represent emplacement at greater crustal depth, and that they likely once had a similar cover of rhyolite–dacite volcanic rocks. Thus the “granite–rhyolite provinces” may have covered most of southern Laurentia during the Mesoproterozoic!

There are also well-documented occurrences of ca. 1.50–1.32 Ga plutons in Eastern Canada. These have been summarized in papers by Gower and Krogh (2002), Rivers (1997), and more recently by Whitmeyer and Karlstrom (2007), and will be only briefly described here. There are plutons of this age in the SW Grenville Front region, mostly in southeastern Ontario, in the Pinware terrane in southern Labrador and eastern Quebec, in Labrador and southern Quebec, where they constitute parts of the early Elsonian orogeny, and in the Long Range Inlier of Newfoundland. As will be discussed later, although some of these plutons are as young as 1.32 Ga, most are in the 1.47–1.50 Ga range.

Tectonic setting and the origin of “A-type” granites

Most of the plutons in the midcontinent Granite–Rhyolite Provinces, and their more deep-seated western extensions, may be termed “A-type” (e.g., Collins et al., 1982; Eby, 1990), or “ferroan” (Frost and Frost, 1997, 2008, 2011, 2013). The petrogenesis and tectonic setting of A-type magmatism has been long debated and remains enigmatic. Proposed tectonic settings have varied from intracratonic–anorogenic (Anderson and Morrison, 1992), to plumes (Hoffman, 1989), to an intracratonic (continental back arc) expression of outboard arc magmatism (Karlstrom et al., 2001; Ahaal et al., 2000; Slagstad et al., 2009). The deeper crustal rocks are variably deformed (Shaw et al., 2005). Syntectonic emplacement has been documented for an increasing number of 1.45–1.34 Ga pluton aureoles (Duebendorfer and Christensen, 1995; Nyman et al., 1994; Kirby et al., 1995; Gonzales et al., 1996; Nyman and Karlstrom, 1997; McCoy et al., 2005; Shaw et al., 2001; Siddoway et al., 2000) and in some regions rocks of this age have been penetratively deformed and metamorphosed to upper amphibolite faces over wide regions (Williams et al., 1999; Daniel et al., 2013).

Petrogenetic models favor crustal extensional regimes (Frost and Frost, 2008, 2011, 2013), but differing strain ?elds in different areas variably record shortening and extension both parallel and perpendicular to the Laurentian margin. Variations in strain-?elds recorded in different pluton aureoles were attributed by Karlstrom and Humphreys (1998) to inboard tectonism during transform- type tectonics, where complex and evolving strain ?elds involved paired extension and contraction in opposite quadrants. Different degrees of deformation between plutons was attributed to rheological zonation, for example due to regional melt-fluid mid- crustal layers that separated zones of penetrative 1.4 Ga middle crustal deformation from an upper crustal zone where thermal softening around shallow level plutons led to weak aureole deformation (Shaw et al., 2005). New models suggest that accretion of the Mazatzal Province to the Yavapai Province may have occurred in the Mesoproterozoic (Picuris orogeny), well inboard of the Granite–Rhyolite Provinces (Daniel et al., 2013).

Further Discussion of the Problem and Consideration of “Silicic LIP”

Mesoproterozoic rocks of the “Granite-Rhyolite” Provinces, as well as their coeval A-type plutonic equivalents that are known across the continent, certainly constitute an almost unique large terrane of high-silica magmatic rocks. In this sense they are certainly a large silicic magmatic province. But, were they formed in a similar way as LIPs (Ernst, 2014), such as the intraplate context for Deccan traps, the Ontong-Java Plateau, or the Central Atlantic Magmatic Province (CAMP)? How are they similar to other Silicic LIPs such as the Mexican Sierra Madre rhyolitic province (e.g. Bryan and Ferrari, 2013)? As noted earlier, proposed tectonic settings for the origin of A-type granites have varied from intracratonic–anorogenic (Anderson and Morrison, 1992), to plumes (Hoffman, 1989), to an intracratonic (continental back arc) expression of outboard arc magmatism (Karlstrom et al., 2001; Ahaal et al., 2000; Slagstad et al., 2009). Bickford et al. (2015) made the following observations: 1) A-type plutons occur across a greater-than-3000 km long and greater-than-100 km wide belt in southern Laurentia; (2) Magmatism across the continent was diachronous, beginning ca. 1500 Ma in eastern Canada and culminating ca. 1440 Ma in the western US.; (3) The “Nd line” separates Mesoproterozoic magmatic rocks formed from, or within, older Paleoproterozoic crust from magmatic rocks that are essentially juvenile; (4), Calcalkaline, arc-like magmas of the same ages as the high-silica, A-type magmas, are known in eastern Canada and in isolated spots within the midcontinent (e.g. Hawn Park gneiss, Missouri; Blue River gneiss, Oklahoma). Bickford et al. (2015) concluded that these observations were consistent with widespread Mesoptoterozoic magmatism related to a broad intracratonic extensional setting along a long-lived dominantly convergent or transform margin, an active continental margin that persisted from ca. 1500 Ma until 1370 Ma, and perhaps beyond into the Grenville orogenic activity.

Given these observations, do these unique rocks constitute a Silicic Large Igneous Province? -- perhaps yes, and no, depending upon their interpreted tectonic environment (i.e. role of intraplate/ plume processes) and whether there can be role for an active continental margin setting in SLIPs.

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