Late Paleozoic to Jurassic silicic magmatism at the Gondwana margin: Analogy to the Middle Proterozoic in North America?

Suzanne Mahlburg Kay Institute for the Study of the Continents, Cornel l University, Ithaca, New York 14853

Victor A. Ramos Universidad de Buenos Aires, Buenos Aires, Argent ina

Constantino Mpodozis Servicio Nacional de Geologa y Minera, Santiago, Chi le

Patricia Sruoga Consejo Nacional de Invest igaciones Cientf icas y Tecnolg icas, Buenos Aires, Argent ina

ABSTRACT A vast region of upper Paleozoic to Middle Jurassic (300-ISO

Ma) silicic magmatic rocks that erupted inboard of the Gondwana margin is a possible Phanerozoic analogue to the extensive Middle Proterozoic (1500-1350 Ma) silicic magmatic province that underlies much of the southern mid-continent of North America. Like the North American rocks, the Gondwana silicic magmas appear to be melts of crust that formed about 200-300 m.y. earlier. In the North American case, this older crust formed and was accreted to the continent during a major period of crustal formation (1700-1900 Ma), whereas in the Gondwana case, the crust that melted consisted mainly of magmatic arc terranes accreted to the continental margin during the Paleozoic. In both cases, basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older (and previously melted) crust. The similarities between the North American and Gondwana silicic rocks suggest that both suites formed in exten-sional settings where basaltic magmas, ponded at the base of the preheated crust, caused extensive crustal melting that inhibited up-ward passage of the basalts. In both cases, silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes, and before breakup of the supercontinent. By analogy with the polar wander curves for Gondwana, the granite-rhyolite provinces may have formed during a period of very slow motion of the super-continents relative to the poles.

INTRODUCTION Most of the south-central region of the North American continent is

underlain by an extensive Middle Proterozoic (1.3-1.5 Ga) granite-rhyolite province (Van Schmus et al., 1987) that was derived by melting crust created at 1.7-1.9 Ga (Nelson and DePaolo, 1985). Recent seismic profiling by the Consortium for Continental Reflection Profiling (CO-CORP) has revealed extensive layered reflections in the mid-crust in this region (Pratt et al., 1988). Understanding the origin of this layering and the general tectonic setting of the province is difficult because the region is largely concealed by Phanerozoic strata (Anderson, 1983). In this paper, these Proterozoic granite-rhyolite provinces are compared to a possible Phanerozoic analogue: a vast region of upper Paleozoic to Middle Jurassic (290-160 Ma) silicic magmatic rocks that lie inboard of the Gondwana margin.

As in the North America case, the Gondwana granite-rhyolite prov-ince occurs on the outer margins of a supercontinent and represents remelting and reworking of crust that was accreted several hundred mil-lion years earlier. In both cases the rhyolites appear to be related to extensional tectonic regimes underlain by relatively hot mantle (e.g., Gur-nis, 1988) which yielded basalt that underplated and melted the preheated, preexisting crust.

Figure 1. Map of south-ern Gondwana, modified from Lawver and Scotese (1987), showing Gond-wana granite-rhyolite prov-inces. Boundary of ac-creted terranes separates old cratonic Gondwana from younger accreted terranes. Named areas north of boundary are regions of Jurassic and Cretaceous basalts (see Dalziel et al., 1987).

324 GEOLOGY, v. 17, p. 324-328, April 1989

DISTRIBUTION AND CHARACTER OF THE GONDWANA GRANITE-RHYOLITE PROVINCES

The distribution of major late Paleozoic to Middle Jurassic granite-rhyolite provinces in the Southern Hemisphere is shown in Figure 1 on a Gondwana reconstruction from Lawver and Scotese (1987). These prov-inces lie along the margin of the Gondwana supercontinent, to the south of and overlapping a discontinuous zone of sutures which forms the bound-ary between Gondwana and the fragments sutured to it during the middle to late Paleozoic (e.g., Ramos,1986). The suture zone approximately fol-lows the Samfrau geosyncline of Du Toit (1937). These rhyolites postdate the sutures but are older than Gondwana break-up in each sector.

The Gondwana granite-rhyolite provinces are particularly prominent in southern South America, occur in Antarctica, and are also important in eastern Australia. The principal provinces are (1) late Carboniferous-Early Permian to Triassic granites and rhyolites of northern and central Chile and Argentina, here termed the Choiyoi province (Groeber, 1946; Caminos in Turner, 1980; Zeil, 1981; Nasi et al., 1985); (2) Upper Triassic granites to Middle Jurassic rhyolites of Patagonia and their offshore exten-sions (Lesta et al. and DeGiusto et al. in Turner, 1980; Gust et al., 1985), here called the Chon Aike group (called Tobifera in Gust et al., 1985); (3) Antarctic Peninsula Middle Jurassic granites and rhyolites (Dalziel et

al., 1987); and (4) late Carboniferous granite-rhyolites of the New England fold belt in Australia (i.e., Shaw and Flood, 1981; McPhie, 1987).

The North American and Gondwana granite-rhyolite provinces show several important similarities, including areal extent and duration. To illus-trate their size equivalence, these provinces are shown at the same scale in Figures 2a and 2b. The North American rocks have been divided into two provinces on the basis of age (Van Schmus et al., 1987): an eastern province ranging from 1420 to 1500 Ma and a western province ranging from 1340 to 1400 Ma. The durations of activity in the late Carboniferous to Triassic (290-200 Ma) Choiyoi and the Late Triassic to Jurassic (200-155 Ma) Chon Aike provinces are like those of the North American provinces.

The Gondwana Choiyoi and Proterozoic North American provinces have other similarities. First, rocks of andesitic and basaltic composition are rare in both regions. Second, the Choiyoi rhyolites and granites are concentrated near or at the margins of the Gondwana continent, become sporadic toward the Precambrian continental interior, and are absent in the cratonic interior. Similarly, North American Middle Proterozoic gran-ites and rhyolites are inferred to be most abundant near the northeast-southwest-trending Grenville margin, but they become scattered and disappear to the northwest toward the Archean continental interior. Third,

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NORTH AMERICA

Figure 2. a: Map of south-ern South America show-ing distribution of granites and rhyolites in Choiyoi (dotted) and Chon Aike (double-dash pattern) prov-inces. Black areasmod-ern outcrop; small c i rc les -wells; large circlescal-deras. b: Eastern and western Middle Protero-zoic North American gran-ite-rhyolite provinces from Van Schmus et al. (1987). Scale same as in a, to em-phasize similarity of areal extent. K = Keeweenawan rift. P = Penokean orog-eny. See text for further explanation.

70W Serie T o b i f e r a

GEOLOGY, April 1989 325

as discussed below, the chemistry of the Gondwana silicic rocks that are nearest the margin is similar in many respects to that of the rhyolites and granites in the St. Francois Mountains in Missouri (Fig. 2b), which occupy a technically analogous position in the North American Proterozoic province.

CHOIYOI PROVINCE The Choiyoi province is shown in Figure 2a. The black areas show

the present outcrop of Carboniferous and Upper Permian-Triassic grani-toids and rhyolites; and the small circles represent wells where the Choiyoi rhyolites have been intersected in the Neuquen basin. The extent of the province is based on work by Digregorio and Uliana {in Turner, 1980), who suggested that a continuous blanket of rhyolites covers the floor of the Neuquen basin, and by Llambias and Leveratto (1975), who suggested that isolated outcrops in the eastern part of the region are rem-nants of a rhyolitic plateau. However, it seems equally likely that the easternmost part of the region was never continuously covered. In the west, deeper levels of the Choiyoi Province have been exposed along the Chilean-Argentine border by the younger Andean orogeny.

Upper Carboniferous rocks in the region are typified by the older rocks of the composite Elqui-Limari batholith (Elqui Granite, Fig. 2a), which are tonalites, granodiorites, and granites that have magmatic-arc affinities (Nasi et al., 1985). Arc-type plutons along the south-central Chilean coast (striped areas in Fig. 2a) are also Carboniferous age (see Parada, 1988). These older granitoids were deformed prior to the intrusion of the Choiyoi province granites.

Most Choiyoi province igneous rocks are silicic volcanic rocks and shallow-level plutons of similar composition (Caminos in Turner, 1980). The volcanic rocks are predominantly rhyolitic to high-silica rhyolitic ignimbrites. In regions of unaltered rocks, cooling units can be discerned, and several calderas have been recognized (see Fig. 2a) (Davidson et al., 1985). The volcanic sequences are commonly 2 km thick, but they can be as thick as 4 km (Corts, 1985). Well-studied volcanic sequences with associated porphyries occur in the Cordillera del Tigre (lat 32S) (see Corts, 1985) and in the La Tabla formation in the southern Atacama Desert (Jesinkey et al., 1987). The plutons are commonly fine grained, pink to red, and characterized by granophyric textures consistent with intrusion at shallow levels. They are typified by the El Leon and El

Colorado units from the young part of the composite Elqui-Limari batho-lith (Fig. 2a; Nasi et al., 1985). Compared to the older plutons, the Choiyoi plutons are more silicic and are composed of syenogranite and monzogran-ite rather than granodiorite.

The Choiyoi rocks of the Elqui region are comparable to those of the North American Middle Proterozoic province. Undeformed shallow-level Choiyou plutons, typified by the Ingaguas super unit of Nasi et al. (1985), trend with time toward more alkaline and silicic compositions; i.e., mon-zogranite and syenogranite. Many Proterozoic North American granitoids (summary in Anderson, 1983), including most of those from the St. Fran-cois Mountains, are also monzogranite and syenogranite. Proterozoic gran-ites farther away from the margin toward the continental core, like those in the Wolf River and Montello batholiths in Wisconsin, are alkali granites. No equivalent has yet been found in the Choiyoi province, but most of the studied granites are closer to the continental margin.

CHON AIKE PROVINCE The boundary between the Choiyoi province and the younger Chon

Aike province to the south is transitional. Upper Triassic rhyolites and Lower Jurassic acidic ignimbrites are widely exposed in the North Patago-nian massif (Llambias et al., 1984). As in the Choiyoi province, rhyolitic volcanism is closely associated with leucogranitic stocks, ranging in age from 198 to 169 Ma (Cortes, 1981). The extent of Middle Jurassic (165-155 Ma) rhyolitic volcanism in the Chon Aike province to the south is shown in Figure 2a. The black areas indicate outcrop regions, and the circles indicate wells (both onshore and offshore) in the San Jorge and Magallenes basins where rhyolite has been intersected (Lesta et al. in Turner, 1980). Younger Middle Jurassic (150-145 Ma) arc-related vol-canic rocks crop out to the west and south (striped areas) and include part of the Tobifera proper, discussed by many authors.

Most Chon Aike province rocks are thick pyroclastic breccia flows and ignimbrites interfingered with clastic deposits (Sruoga and Palma, 1984). Compositions are mainly rhyolite to high-silica rhyolite (72%-78% Si02; Fig. 3a) (Llambias et al., 1984; Sruoga, 1988), although rare dacite also occurs. The ignimbrites are associated with intrusive rhyolitic domes (Sruoga, 1988). In northern Patagonia (lat 42S), the ignimbrites com-monly contain fluorite veins (Corbella, 1973), as do Proterozoic magmatic rocks in the St. Francois Mountains.

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Figure 3. a: Plot of Ce vs. SI0 2 comparing North American and South American granlte-rhyolite provinces. Ranges of A-type and I- and S-type granites shown for comparison (Whalen et al., 1987). Data for pre-Choiyoi (triangles) and Choiyoi (circles) rocks are from Nasi et al. (1985) and Mpodozis et al. (unpub.); data for Chon Aike rocks are from Sruoga (1988); data for St. Francois rocks are from Bickford et al. (1981) and Cullers et al. (1981). b: Same samples as in a, plotted on Rb vs. (Yb + Ta) granite discriminant diagram (Pearce et al., 1984). Fields are VAvolcanic arc, ORocean ridge, WPwithin plate, and SYN-COLsyncollisional.

326 GEOLOGY, April 1989 325

GEOCHEMICAL COMPARISON OF THE PROTEROZOIC AND GONDWANA RHYOLITES

The geochemical similarity of the North American Proterozoic and the South American Gondwana granite-rhyolite suites is a first-order ob-servation that has become apparent in reconnaissance-level geochemical studies. The comparisons made here are to show these similarities and to place constraints on tectonic setting.

The initial 87Sr/86Sr ratios of the two granite-rhyolite groups suggest that if they are crustal melts, most of that crust was extracted from the mantle no more than 200-300 m.y. earlier (if the crust had relatively high Rb/Si ratios). As summarized by Anderson (1983), most of the North American granites and rhyolites have initial 87Sr/86Sr ratios of 0.7051 0.0035; a few higher ratios suggest melting of older crust. Most, if not all, Choiyoi province granites in the central Chilean Andes have initial ratios between 0.703 and 0.707 (summary in Parada, 1988). The few Sr isotopic ratios from the Chon Aike rhyolites are between 0.703 and 0.704 and are consistent with melting of relatively young crust (Llambias and Leveratto, 1975). These data suggest that both the South American and North Amer-ican silicic rock suites resulted from melting of crust in terranes accreted to the continent several million years earlier (Nelson and DePaolo, 1985).

Many Middle Proterozoic North American granites have been termed anorogenic granites (broadly A-type; see review in Anderson, 1983). In general, these granites have high alkali contents, are transitional between peraluminous and metaluminous, and have high FeO/MgO ra-tios and incompatible-element abundances. Few analyzed Choiyoi and Chon Aike province rocks have characteristics as extreme as the Protero-zoic samples discussed by Anderson. However, if only granites and rhyolites from near the margin of the North American province are con-sidered, i.e., the St. Francois Mountain samples of Cullers et al. (1981), there is significant geochemical overlap between the North American and Gondwana samples.

The similarity between the North American Proterozoic and the South American Gondwana rocks is illustrated in a plot of Si02 vs. Ce content (Fig. 3a). Here, Ce is used as an index of incompatible element concentration; ranges for arc-affinity I- and S-type granites and incompatible-element-rich anorogenic (A-type) granites are indicated (see Whalen et al., 1987). Note that the pre-Choiyoi granitoids plot in the I- and S-type range, whereas the Choiyoi samples extend into the A range, as do the St. Francois Mountain samples. The Chon Aike samples partially overlap both the Choiyoi and St. Francois fields.

Data from the same samples are plotted on the Rb vs. (Yb + Ta) discrimination diagram of Pearce et al. (1984) in Figure 3b. Although this diagram has some serious limitations for determining tectonic setting, sev-eral points can be made. First, the pre-Choiyoi rocks plot in the arc field. Second, the Choiyoi and St. Francois Mountain rocks both plot in the within-plate field, which is consistent with the extensional setting proposed for these rocks. The Chon Aike rocks overlap several fields. Many of these rocks are highly siliceous and have fractionated minor-element-bearing phases, which lowers Yb and Ta contents.

A tectonic environment similar to that of the extensive silicic unde-formed ignimbrites of the modern Andean Puna-Altiplano plateau (lat 18-27S), which have erupted through thick crust above a relatively flat subduction zone (30), has been proposed for the Permian rhyolites in Australia by McPhie (1987) and for Choiyoi province plutons in the Andes by Brook et al. (1987). Chemical differences between the Puna and the Choiyoi and Chon Aike provinces argue against this model. For ex-ample, samples from the Choiyoi, Chon Aike, and St. Francois provinces generally have higher Si02 contents (Choiyoi67%-78% Si02; Chon Aikemost 72%-78% Si02; St. Francois65%-78% Si02; Punamost Si02

Gurnis (1988) (among others) has proposed that during stationary periods for supercontinents, heat is trapped in and beneath the continents, leading to higher geothermal gradients and mantle melting. Furthermore, hotspots rising from beneath relatively stationary continents can also cause the mantle to melt. Basalt formed in the mantle by these processes rises and becomes trapped in the lower crust, resulting in substantial crustal melting. As a result, the crust becomes too ductile to allow the basalts to escape by a fracturing mechanism. The relatively fusible, undepleted (high in U, Th, H2O, granitic components), recently accreted terranes melt preferentially to the colder Precambrian cores of the supercontinent.

The initial history of the Chon Aike is similar to the other granite-rhyolite provinces, but regional extension continued (see Bruhn et al., 1978) and culminated in the opening of the South Atlantic 30 m.y. later. The Chon Aike province occurs in a region of accreted terranes south of the Gondwana suture line. To the north of this line is a band of younger Mesozoic basalts (Serra Geral to Ferrar in Fig. 1; see Dalziel et al., 1987), in some cases associated with rhyolites. Basalt, which rarely passed through the hot ductile crust in the granite-rhyolite provinces, erupted through this older and more refractory Precambrian crust. It is interesting that this band of Mesozoic basalts appears to parallel the Gondwana suture line, and not the ocean coasts where breakup occurred. Perhaps it is no coincidence that the North American Proterozoic anorthosites, which are thought to be cumulates of basaltic magmas, are found in a band in the more refractory Archean craton bordering the extensive North American granite-rhyolite provinces.

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Valencio, D.A., Vilas, J.F., and Pacca, I.G., 1983, The significance of paleomagnet-ism of Jurassic-Cretaceous rocks from South America: Pre-drift movements, hairpins and magnetostratigraphy: Royal Astronomical Society Geophysical Journal, v. 73, p. 135-151.

Van Schmus, W.R., Bickford, M.E., and Zietz, I., 1987, Early and Middle Protero-zoic provinces in the central United States, in Kroner, A., ed., Proterozoic lithospheric evolution: American Geophysical Union Geodynamics Series, v. 17, p. 43-68.

Whalen, J.B., Currie, K.L., and Chappell, B.W., 1987, A-type granites: Geochemi-cal characteristics, discrimination and petrogenesis: Contributions to Mineral-ogy and Petrology, v. 95, p. 407-419.

Zeil, W., 1981, Volcanism and geodynamics at the turn of the Paleozoic to the Mesozoic in the central and southern Andes (Chile-Argentina): Zentralblatt fr Geologie und Palaontologie, Teil 1, p. 298-318.

ACKNOWLEDGMENTS Supported by National Science Foundation Grant EAR-86-18993 and INT-

8703199. We thank R. W. Kay, the Cornell Andes Group, and K. D. Nelson and others in COCORP for helpful comments.

Manuscript received August 19, 1988 Revised manuscript received December 12, 1988 Manuscript accepted December 19, 1988

328 Printed in U.S.A. GEOLOGY, April 1989