Showing papers on "Terrane published in 1992"

Remnants of ≥3800 Ma crust in the Chinese part of the Sino-Korean craton

Abstract: Ion microprobe U-Pb analyses of zircons from the Sino-Korean craton have identified remnants of ≥3800 Ma crust at two localities—near Caozhuang east of Beijing, and near Anshan, northeast China. Near Caozhuang, the detrital zircons in an Archean metaquartzite are 3550 Ma or older; about one-fourth have ages between 3800 and 3850 Ma. The ages for detrital zircons with concordant U-Pb ages form a polymodal distribution, interpreted to show that the detritus that formed the quartzite was derived from a terrane containing early Archean rocks of several ages. Near Anshan, sheared gneiss is present in a complex containing ca. 3300 and 3000 Ma granites. Some of the zircons in this gneiss are concordant with a weighted mean 207 Pb/ 206 Pb age of 3804 ±5 Ma (2 σ), interpreted to be the age of the protolith of the gneiss.

Crustal growth in West Africa at 2.1 Ga

Abstract: Birimian (∼2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales; The sedimentary units are characterized by an abundance of clastic turbiditic sediments. Andesites and calc-alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc-alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U-Pb zircon age at 2186 ± 19 Ma, while Rb-Sr and Sm-Nd methods give ages of 2162 ± 19 Ma and 2141 ± 24 Ma, respectively. A Sm-Nd garnet-whole rock age of 2153 ± 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U-Pb and Sm-Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archean crust: eNd(2.1-Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U-Pb ages for detrital zircons from clastic sediments range from 2098 ±11 Ma to 2125 ± 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archean rocks from the Man craton. In accord with Abouchami et al.'s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc-alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archean craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some ∼60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archean processes but contrasts with 1.7 – 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archean crust.

Are crustal thickness variations in old mountain belts like the Appalachians a consequence of lithospheric delamination

Abstract: Comparison of Moho geometry and reflection character of active collision zones, like the Alps, with those of fossil collision zones, like the Appalachians, lends support for the view that lithospheric delamination is both a common and fundamentally important component of collisional orogeny. In particular, the relative crustal thinning commonly observed beneath the internides of old collisional orogens is an expected consequence of delamination. Neither precollisional crustal thickness variations nor an unrelated postcollision extensional event is required to produce this feature. Acceptance of the delamination hypothesis, in turn, fundamentally alters one's view of the tectonic evolution of old collisional orogens. In the case of the Appalachians, delamination has direct implications for the interpretation of Alleghanian granites, the direction of subduction leading to final closure between Laurentia and Gondwana, and the thermal state of the lithosphere at the onset of Atlantic rifting. A corollary of the delamination hypothesis is that lateral reflectivity variations cannot be used to define paleogeographic terranes in the deep crust of old orogens. Conversely, the observation that delamination leaves a distinctive geophysical imprint on the crust itself provides an important additional clue for deciphering the tectonic evolution of such regions.

Early Paleozoic orogenic belt of the Andes in southwestern South America: Result of Laurentia-Gondwana collision?

Abstract: The late Precambrian to early Paleozoic age rock units of the Pampean ranges, the Puna, and the North Patagonian massif of southwestern South America constitute the Famatinian orogenic belt. They are interpreted as an Ordovician collisional belt between the Occidentalia terrane and the Gondwana craton. They include mafic and ultramafic belts of Neoproterozoic to early Paleozoic age. An intense tectonothermal event resulted from the collision; syntectonic granitoids represent crustal melting. In that collision syntectonic to late-tectonic foreland basins developed. The recently proposed juxtaposition of Laurentia and East Antarctica-Australia in the Neoproterozoic raises the possibility that Laurentia and western South America were close together in the early Paleozoic, and therefore that the Famatinian belt resulted from Laurentia-Gondwana collision. Occidentalia, which is bordered by a Cambrian carbonate platform similar to that of eastern North America, may be a sliver detached from Laurentia during Late Ordovician time.

Late Paleozoic to Triassic evolution of the Gondwana margin: Evidence from Chilean Frontal Cordilleran batholiths (28°S to 31°S)

Abstract: Upper Paleozoic to Triassic Chilean granitoids in the Andean Frontal Cordillera between 28°S and 31°S record crustal and mantle conditions at the Gondwana margin during the final assembly and initial breakup of the Pangea supercontinent. This period overlaps the end of Paleozoic terrane accretion and precedes Andean subduction. Integration of new trace-element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the Gondwana margin. In the model, the Carboniferous to Early Permian is a period of oblique convergence. Associated Elqui complex granitoids are diverse. Those in the Guanta and Montosa units are predominantly related to subduction processes, whereas those in the Cochiguas and El Volcan units are dominated by melting of the subduction complex and older crust. Progressive oblique collision of the last pre-Pangea terrane (Equis) along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the Elqui complex. Subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt, which intruded and melted the crust, producing the post-collisional Ingaguas complex. The Los Carricitos granitoids formed in thickened crust, whereas the Chollay, El Colorado, and El Leon units formed in thinner crust. The Ingaguas complex is part of the Choiyoi granite-rhyolite province, whose formation, similar to that of other Gondwana silicic provinces, was probably accentuated by anomalously hot upper mantle associated with the Pangea supercontinent.

Pre‐Mid‐Mesozoic tectonic evolution of the Yukon‐Tanana Terrane, Yukon and Alaska

Abstract: Yukon-Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid-Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian-Early Mississippian, mid-Permian, and Late Triassic-Early Jurassic time. From Late Devonian to mid-Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid-Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid-Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent-continent collision. Although the Triassic-Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other “pericratonic” terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed by transcurrent faulting.

Upper Jurassic-Lower Cretaceous basinal strata along the Cordilleran Margin: Implications for the accretionary history of the Alexander-Wrangellia-Peninsular Terrane

Abstract: Upper Jurassic and Lower Cretaceous basinal strata are preserved in a discontinuous belt along the inboard margin of the Alexander-Wrangellia-Peninsular terrane (AWP) in Alaska and western Canada, on the outboard margin of terranes in the Canadian Cordillera accreted to North America prior to Late Jurassic time, and along the Cordilleran margin from southern Oregon to southern California. Nearly all of the basinal assemblages contain turbiditic strata deposited between Oxfordian and Albian time. Arc-type volcanic rocks and abundant volcanic detritus in many of the assemblages suggest deposition within or adjacent to a coeval arc complex. On the basis of the general similarities between the basinal sequences, we propose that they record involvement of the AWP in the Late Jurassic-Early Cretaceous evolution of the Cordilleran margin. A geologically reasonable scenario for the accretion of the AWP includes (1) Middle Jurassic accretion to the Cordilleran margin, in particular the Stikine and Yukon-Tanana terranes, in a dextral transpressional regime, (2) Late Jurassic-Early Cretaceous overall northward translation of the AWP and evolution of a series of transtensional basins within a complex dextral strike-slip system along the Cordilleran margin, and (3) mid-Cretaceous structural imbrication of the AWP and inboard terranes that either terminated or resulted in a change in the character of deposition in the marginal basins. Mid-Cretaceous deformation along the inboard margin of the AWP was broadly synchronous with contractional deformation throughout the Cordillera and most likely due to changes in subduction zone parameters along the Cordilleran margin, outboard of the AWP, rather than collision of the AWP.

Continental lower crust

Abstract: Preface. List of Contributors. Symbols. 1. The Seismic Velocity Structure of the Deep Continental Crust (W.S. Holbrook, W.D. Mooney and N.I. Christensen). Wide-angle seismic data. Middle- and lower-crustal velocities. Seismic velocities: constraints on composition. Fluids and anisotropy. Discussion. 2. Multi-Genetic Origin of Crustal Reflectivity: A Review of Seismic Reflection Profiling of the Continental Lower Crust and Moho. (W.D. Mooney and R. Meissner). The seismic reflection method. Observations of the continental lower crust and Moho. Multi-genetic origin of lower crustal reflections. Reflectivity and viscosity in the lower crust. 3. Electrical Properties of the Lower Continental Crust (A. Jones). Review of methods. Summary of results. Causes of enhanced electrical conductivity in the continental lower crust. 4. Magnetic Properties of the Lower Continental Crust (P.N. Shive et al.). Long-wavelength magnetic anomalies. Magnetic properties of lower crustal rocks. Nature of magnetic bodies in the lower crust. 5. Thermal State of the Continental Crust (D.S. Chapman and K.P. Furlong). Geotherm models. Thermophysical parameters. Framework geotherms. Processes and conditions that affect or modify the thermal state of the continental lower crust. Crustal heterogeneities. 6. Rheology of the Lower Crust (E.H. Rutter and K.H. Brodie). Deformation mechanisms. Data from experimental rock mechanics. Localization of strain into shear zones. The size of recrystallized grains. Structures and microstructures of naturally deformed rocks. Flexure of the continental lithosphere. 7. Xenoliths - Samples of the Lower Continental Crust (R.L. Rudnick). The samples. Effects of transport. Mineralogy. P-T estimates. Physical properties. Chemical composition. Isotopic studies. Case studies. A lower crustal composition from xenoliths. Representativeness of xenoliths as lower crust. 8. Exposed Crustal Cross Sections as Windows on the Lower Crust (J.A. Percival, D.M. Fountain and M.H. Salisbury). Compressional uplifts. Wide, oblique transitions. Impactogenic uplifts. Transpressional uplifts. Characteristics of lower crust in cross sections. Implications for the nature of the lower crust. 9. Magmas as Tracers of Lower Crustal Composition: An Isotopic Approach (G.L. Farmer). General technique. Case example - Western U.S. Other examples. 10. Fluids in the Deep Crust - Petrological and Isotopic Evidence (S.M. Wickham). Equilibrium thermodynamic calculations. Fluid inclusions. Stable isotope constraints on deep crustal fluids. Melting and fluid transport. 11. Magma Genesis and Crustal Processing (R.W. Kay, S. Mahlburg Kay and R.J. Arculus). Magmas in the crust: processes and models. Magmatic underplanting. Anatexis and granitoid melt generation. Continental growth, recycling and the lower crust. 12. Temporal Evolution of Regional Granulite Terranes: Implications for the Formation of Lowermost Continental Crust (K. Mezger). Characteristics of regional granulite terranes.

First discovery of monotremes in South America

Abstract: UNTIL now, the egg-laying monotremes were only known from the Australian continent, where they have lived since the early Cretaceous period to the present1. Here we report the first monotreme from outside the Australian continent, an ornithorhyn-chid, from sediments of late early Palaeocene age in Patagonia, southern Argentina. This discovery demonstrates the Gondwanan nature of monotremes and supports the hypothesis that the Patagonian Terrane of southern South America had a biotic history distinct from that of the rest of the continent.

Geochemical evidence for the position of the Caples-Torlesse boundary in the Otago Schist, New Zealand

Abstract: Comparison of the whole rock geochemical compositions of 142 samples of Otago Schist with greywackes and argillites from the Caples and Torlesse terranes confirms the essentially isochemical nature of Otago Schist metamorphism and permits correlation of individual schist samples to each terrane. These correlations, along with lithological, structural and petrographical information, have been used to establish a preliminary position for the long-sought-after Caples-Torlesse terrane boundary in the Otago Schist. The boundary has been involved in Late Mesozoic and Cenozoic deformation, follows a sinuous course near the schist axis for some 350 km between Dunedin and the Barrier Range and divides the Aspiring lithologic association into Caples and Torlesse parts. A number of pods of ultramafic schist are exposed in the vicinity of the terrane boundary. The methods used in this paper may have application to the location of terrane boundaries in major schist belts elsewhere.

Early Mesozoic tectonic evolution of the western U.S. Cordillera

Abstract: The early Mesozoic evolution of the U. S. Cordillera differs greatly from its previous history of mainly miogeoclinal sedimentation with outboard marginal-basin-island-arc mobile zones. The Early Mississippian and Permian-Triassic thrust emplacement of eugeoclinal strata across the miogeocline signaled the initial propagation of subduction-related tectonism onto the sialic edge. Following these events, the sialic edge and the resulting accreted terranes became an active continental margin. The active margin history records not only eastward subduction of oceanic crust beneath North America, but also the formation, migration, and accretion of marginal basin and fringing island-arc systems along the continental margin. At the close of the Jurassic, the fringing arc-marginal basin system collapsed, resulting in a more direct interaction of major Pacific basin plates with hte Cordilleran margin. Such interactions are manifested by Andean and San Andreas types of marginal regimes which characterized the Cretaceous and Cenozoic. In this chapter we will discuss the tectonic evolution of the U. S. Cordilleran margin during the early phases of its active margin history (Triassic through Jurassic).

Oblique subduction, collision of microcontinents and subduction of oceanic ridge: Their implications on the Cretaceous tectonics of Japan

Abstract: The Izanagi plate subducted rapidly and obliquely under the accretionary terrane of Japan in the Cretaceous before 85 Ma. A chain of microcontinents collided with it at about 140 Ma. In southwest Japan the major part of it subducted thereafter, but in northeast Japan it accreted and the trench jumped oceanward, resulting in a curved volcanic front. The oblique subduction and the underplated microcon-tinent caused uplifting of high-pressure (high-P) metamorphic rocks and large scale crustal shortening in southwest Japan. The oblique subduction caused left-lateral faulting and ductile shearing in northeast Japan. The arc sliver crossed over the high-temperature (high-T) zone of arc magmatism, resulting in a wide high-T metamorphosed belt. At about 85 Ma, the subduction mode changed from oblique to normal and the tectonic mode changed drastically. Just after this the Kula/Pacific ridge subducted and the subduction rate of the Pacific plate decreased gradually, causing the intrusion of huge amounts of granite magma and the eruption of acidic volcanics from large cauldrons. The oblique subduction of the Pacific plate resumed at 53 Ma and the left-lateral faults were reactivated.

Tectonic setting and U-Pb geochronology of the Early Tertiary Ladybird Leucogranite Suite, Thor-Odin - Pinnacles Area, Southern Omineca Belt, British Columbia

Abstract: The Thor-Odin - Pinnacles area is a structural culmination in the Shuswap complex of the southern Omineca Belt of the Canadian Cordillera It comprises amphibolite-facies rocks that were deformed during Mesozoic-Paleocene compression and were exhumed in the footwalls of Eocene normal faults during crustal extension The Ladybird leucogranite suite coincides with the extended terrane in the southern Omineca Belt It is generally restricted to a midcrustal level which lies in the hanging walls of deep-seated thrust faults and the footwalls of extensional faults Field relationships of the leucogranites and U-Pb geochronology place timing constraints on compressional and extensional shear zones The last thrust motion on the Monashee decollement occurred in the latest Paleocene, and the shear zone had stopped by 58 Ma Crustal-scale normal faults were active in the early Eocene, indicating that crustal extension closely followed the compressional regime Geological and geochronological data are consistent with an anatectic crustal origin for the Ladybird granite The granites apparently postdate the thermal peak of metamorphism (Carr, 1990) and were generated during the final stages of thrusting, perhaps due to decompression melting as the midcrustal rocks were carried up a thrust ramp and unroofed and/or due to the introduction of hydrous fluids into the system In situ magma and hot intrusions probably played an important role in the nucleation of extensional shear zones The extensional regime then facilitated the intrusion of vast late-synkinematic to posttectonic plutons U-Pb systematics reveal that zircons in high-temperature shear zones may have suffered high-temperature Pb loss, perhaps due to deformation- or fluid-enhanced diffusion, and that monazite systematics from samples from high-grade terranes are complex Magmatic monazite populations contain crystals of different ages that do not coincide with zircon ages and apparently represent neither a crystallization age nor a cooling age

Neodymium isotopic evidence for the tectonic assembly of Late Archean crust in the Slave Province, northwest Canada

Abstract: The ca. 2.7–2.5 Ga Slave Province is a “granitegreenstone” terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks. The Nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km, east-west, transect at 65°N across the structural grain of the province. Initial ɛ Nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province, as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust. Supracrustal sequences (ca. 2.71–2.65 Ga) from the central and eastern parts of the province have positive ɛ Nd(1) values (+0.3 to +3.6), consistent with juvenile sources and formation remote from significantly older crust. Syn to late-deformation (ca. 2.63–2.60 Ga), mantle-derived diorites and related tonalites (type I) from the central and eastern parts of the province have similar initial ɛ Nd values (-0.1 to +2.7). In contrast, samples from the westernmost plutons, which intrude exposed pre-3.1 Ga crust, have much lower ɛ Nd(1) values (-1.0 to4.6) suggesting contamination of these magmas by older crust. The ɛ Nd(1) values of post-deformation granites (s.s.) (type II) also vary systematically across the province: values for granites west of longitude 110°30′W range from-0.2 to -5.3; those to the east range from +0.6 to +3.7. These data suggest mixed crustal sources dominated by Mid to Early Archean material (ɛ Nd-2.6 to- 17 at 2.6 Ga) for the western granitoid rocks and juvenile sources for the eastern granites. The Nd isotopic data are consistent with the geology of the province in that exposures of Mid to Early Archean crustal rocks, predating the principal 2.7–2.5 Ga orogenic event are restricted to the western part of the province. The asymmetric pattern defined by the Nd isotopic data indicates the presence of distinct crustal rocks beneath the Slave Province. Similar isotopic variations observed across Phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin. This process may also have been an important mechanism in the cratonization of the Slave Province.

Origin of giant Martian polygons

Abstract: Extensive areas of the Martian northern plains in Utopia and Acidalia planitiae are characterized by 'polygonal terrane'. Polygonal terrane consists of material cut by complex troughs defining a pattern resembling mudcracks, columnar joints, or frost-wedge polygons on earth. However, the Martian polygons are orders of magnitude larger than these potential earth analogues, leading to severe mechanical difficulties for genetic models based on simple analogy arguments. Plate-bending and finite element models indicate that shrinkage of desiccating sediment or cooling volcanics accompanied by differential compaction over buried topography can account for the stresses responsible for polygon troughs as well as the large size of the polygons. Although trough widths and depths relate primarily to shrinkage, the large scale of the polygonl pattern relates to the spacing between topographic elevations on the surface buried beneath polygonal terrane material. Geological relationships favor a sedimentary origin for polygonal terrane material, but our model is not dependent on the specific genesis. Our analysis also suggests that the polygons must have formed at a geologically rapid rate.

Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand

Abstract: The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.

Giant granulite terranes of northeastern Superior Province: the Ashuanipi complex and Minto block

Abstract: The Ashuanipi complex and Minto block of the Superior Province are large regions that have been classified as "high-grade gneiss" terranes on the basis of the presence of orthopyroxene-bearing units. Like the granite-greenstone and metasedimentary subprovinces of the southern Superior Province, the two terranes consist predominantly of intrusive rocks, but are distinguished by their primary magmatic orthopyroxene. Both "high-grade" and "gneiss" are misnomers because granulite-facies gneisses are only sparingly present and the regions consist dominantly of massive, unmetamorphosed plutonic rock. The Ashuanipi complex consists of a deformed, metamorphosed package of metasedimentary rocks and primitive, early tonalite cut by widespread orthopyroxene + garnet granodiorite (diatexite), as well as plutons of tonalite, granite, and syenite. Based on its lithological and chronological similarity and on-strike position, the complex appears to be the continuation of metasedimentary subprovinces such as the Quetico. Its evolution involved deposition of immature greywacke in an accretionary prism, early arc (tonalitic) magmatism and deformation, followed by widespread intracrustal magmatism in the period 2700-2670 Ma. Both metamorphic and igneous rocks record equilibration under granulite-facies conditions (700-835°C; 0.35-0.65 GPa; a,,,, -0.3) and indicate exposure levels of -20 km. The Minto block at the latitude of Leaf River consists of several north-northwest-trending domains of similar scale and diversity to the east-trending subprovinces of the southern Superior Province. The central Goudalie domain is dominantly amphibolite-facies tonalitic rocks including some with ages > 3 Ga, with small belts of volcanic and sedimentary origin. Lake Minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite, clinopyroxene + orthopyroxene granodiorite, orthopyroxene-biotite diatexite, and granite. The hornblende granodiorite suite constitutes most of the Utsalik and Tikkerutuk domains and is thought to represent continental arc magmatism. On the basis of their distinct aeromagnetic and lithological character, two additional domains are evident north of the Leaf River area, the Inukjuak domain in the west and the Douglas Harbour domain in the east. The northerly grain of the Minto block appears to have been established in situ with respect to the easterly belts of the southern Superior Province (i.e., no large-scale block rotation) during the same interval of time (3.0-2.7 Ga). Modification of the tectonic framework for the Superior Province is required to explain Minto arc magmatism. In the interval -2730-2690 Ma ago, a continental magmatic arc built the Berens River and Bienville subprovinces and Minto block on the southern and eastern edges, respectively, of a northern protocratonic foundation. In the same period, primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime.