Gerald M. Ross

Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (northwestern United States): implications for age of deposition and pre-Panthalassa plate reconstructions

The Mesoproterozoic Belt-Purcell Supergroup is a thick succession of siliciclastic and carbonate sedimentary rocks in the northwestern United States and adjacent Canada. Recent stratigraphic studies provide strong evidence for an enclosed basin setting with a tectonically active western margin. The age of the western (present coordinates) landmass is unknown, but has been inferred to be different from North American basement that borders the Belt basin on the basis of whole rock Sm-Nd studies. We report U-Pb isotopic analyses of individual detrital zircon, monazite, and xenotime grains separated from westerly derived clastic units in the Belt Supergroup as well as Sm-Nd isotopic data for the same grains where appropriate. These data provide new constraints on the age and isotopic character of the western landmass. Most U-Pb data are concordant or slightly discordant; exceptions are noted below. The Revett Formation (Ravalli Group, lower Belt Supergroup) contains grains with concordant ages of 1590–1600 Ma and a grain with a discordant207Pb/206Pb age of 1780 Ma: the ca. 1600 Ma age is very unusual in North America and effectively dates “Belt Island”, long postulated to be the Revett source in what are now terranes of southeast Washington and northeast Oregon that were accreted in the Mesozoic. Detrital zircons from the Missoula Group (Bonner and Mount Shields formations; upper Belt Supergroup) form a distinct group at 1670–1859 Ma with two ca. 2.6 Ga grains. U-Pb ages from monazite range from 1642 to 1786 Ma, and together with Nd isotopic data (TDM model ages of 2.05–3.36 Ga) suggest variable interaction with previously differentiated crust. The Buffalo Hump Formation in northeast Washington may correlate with the Bonner Formation or may lie unconformably on the Belt Supergroup; it contains detrital zircons of ca. 1840 Ma and, surprisingly young, 1070–1244 Ma (several grains). Monazites have U-Pb ages of 1753–1774 Ma and Nd signatures similar to the Bonner Formation data. Several major conclusions can be drawn from these data. The ages of 1590–1600 Ma have no known possible source within western North America (although 1576 Ma augen gneiss is present in the Priest River Complex northwest of the Belt basin), and the 1642–1786 Ma ages are uncommon to rare in Precambrian rocks to the immediate south and east; together these data support previous Sm-Nd whole rock and stratigraphic conclusions of a dominantly western source for the Belt basin. The ages in the Buffalo Hump Formation can be interpreted in two ways. Either the Buffalo Hump correlates with the Bonner Formation and therefore the top of the Belt Supergroup is substantially younger (< 1070 Ma) than previously inferred (1250 Ma) and may overlap with Grenville-age tectonic activity. Alternatively, the Buffalo Hump Formation may represent a new unit that is younger than the Belt. Irrespective of this issue, these data help to constrain proposed pre-Paleozoic plate reconstructions; from the 1070–1244, 1590–1600 and 1642–1786 Ma source ages and related Nd data, we suggest that the basement blocks of south-central Australia (Gawler, Musgrave, Willyama, Arunta and possibly Mount Isa) were joined to the western side of Laurentia, adjacent to the Belt basin, prior to Neoproterozoic formation of the Panthalassa Ocean.

Detrital zircon reference for Cambrian to Triassic miogeoclinal strata of western North America

U-Pb analyses of 656 single zircon grains from Cambrian to Triassic miogeoclinal strata provide a latitudinal and temporal reference for the ages of grains that accumulated along the western margin of North America. Comparisons between this detrital zircon reference and the ages of grains in potentially displaced terranes outboard (west) of the miogeocline should help establish when the terranes first arrived in sedimentary proximity to western North America. North-south variations in the ages of grains in Cambrian and Devonian to Triassic strata, which reflect the north-south changes in the age of cratonal rocks near the margin, should also help place constraints on a terranes paleolatitude during these time periods. The technique cannot be used to determine paleolatitude during Ordovician time, however, because miogeoclinal strata from northern Canada to northern Mexico are dominated by grains shed from the Peace River arch (northwestern Canada).

Provenance of the Mesoproterozoic (1.45 Ga) Belt basin (western North America): Another piece in the pre-Rodinia paleogeographic puzzle

The Mesoproterozoic (1.47-1.40 Ga) Belt basin in western North America contains a thick succession of siliciclastic strata dominated by sediment derived from source areas that lay to the west and southwest of the present basin. The age and identity of this source area is important for reconstruction of the Mesoproterozoic history of western North America. Over 400 new SHRIMP U-Pb ages for detrital zircons, supplemented by Ar-Ar ages of detrital muscovite from sandstones within the Belt basin, document distinctive provenances of the basin fill; the predominant sediment influx came from a western craton, whereas subordinate input came from Laurentian sources to the east and south of the basin. During deposition of the lower two-thirds of the Belt basin, the western source was composed predominantly of Proterozoic rocks with 1920-1460 Ma ages; abundant grains have 1610-1500 Ma ages. These latter ages are unrepresentative of Laurentian sources, falling within the North American magmatic gap (1610-1490 Ma), but are widespread in central and eastern Australia and Baltica, the former being the most likely candidate for a former fluvial connection to western North America. The presence of detrital-zircon grains (1480-1440 Ma) with ages contemporary with sedimentation in the Belt basin suggests that the western source area hosted active magmatism during Belt sedimentation. The Missoula Group in the upper Belt records the sedimentologic effects of basin reorganization, which introduced sediment from a new southwestern source that, in contrast to earlier Belt history, resembles crust of the southwestern United States and lacks grains within the North American magmatic gap. Evidence for extension is widespread in the Belt basin in the form of mafic volcanic rocks, rapid rates of sedimentation, and intrabasinal syndepositional faults. The predominant contribution of detrital material from the western source area throughout Belt deposition, in concert with the large-scale facies patterns and evidence for rapid rates of subsidence, is taken to suggest that the Belt basin represents an extensional basin with a tectonically active western side. Although a wholly extensional environment has been suggested by previous workers, we suggest that an alternative model, wherein the Belt basin developed within an extensional domain along a collisional/convergent plate margin, analogous to Tethyan sedimentary basins such as the Black and Caspian Seas, is equally viable and paints a different picture of western North America at ca. 1450 Ma.

Tectonic setting of the Windermere Supergroup revisited

Neo-Proterozoic (< 780 Ma) rocks in western North America compose a regionally persistent stratigraphic succession that contains basal sedimentary and volcanic rocks that accumulated during active faulting. In the Canadian Cordillera, the synrift component is overlain by a thick succession that includes shelf, shelf-edge, and basinal strata and implies substantial postrift subsidence. Although fragmentary in nature due to the effects of sub-cambrian erosion, when reconstructed the Canadian stratigraphic record is similar in thickness, facies, and lateral persistence to the overlying Cambrian-Ordovician passive margin. The neo-Proterozoic record of western North America is thus interpreted as a passive-margin succession, rather than a simple rift, which predates a younger rift that resulted in widespread early Paleozoic passive-margin sedimentation. If correct, this may imply that break-up of western Laurentia was a neo-Proterozoic phenomenon rather than early Paleozoic.

Tectonic entrapment and its role in the evolution of continental lithosphere: An example from the Precambrian of western Canada

New geophysical data acquired over the buried crystalline basement of western Canada provide constraints on the history of tectonic assembly of the western Canadian Shield in the interval 1.75–1.85 Ga. Specifically, these data provide new perspectives on the evolution of an Archean continental fragment (Heame province) that was trapped in a tectonic “vise” between coeval orogenic belts that dipped beneath the Hearne province. The Trans-Hudson orogen developed along the southeastern margin of the Hearne province as a series of ocean floor, oceanic arc, and arc marginal basins were telescoped and thrust obliquely beneath the Hearne. Along the northwest edge of the Hearne, collapse and subduction of a narrow marginal basin, now marked by the subsurface extension of the Snowbird Tectonic Zone, led to formation of magmatic arc and collision of older Proterozoic terranes. The Hearne province itself is characterized by regional granulite-grade metamorphism and evidence of extensive and pervasive partial melting of the crust. The internal character of the Hearne province seen on crustal seismic reflection profiles is that of a crustal-scale structural fan with reflection fabrics that verge toward the bounding orogens. The deformation of the Hearne is predominantly of Paleoproterozoic age and constitutes a thorough reworking of this formerly Archean crustal domain over a distance of more than 600 km across strike. Entrapment and thermal weakening of the Hearne resulted from mechanical coupling of inferred buoyant subduction-collision zones and removal of or modification of Archean lithospheric mantle that may have originally formed the keel to the Hearne. Long-period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day Hearne province, which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination/convective removal of thickened lithosphere in the Proterozoic. Tectonic entrapment, as illustrated by the Proterozoic structural and thermal evolution of the Hearne province crust and subcontinental mantle, may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents.

Paleoproterozoic collisional orogen beneath the western Canada sedimentary basin imaged by Lithoprobe crustal seismic-reflection data

Exceptionally clear images of crustal structure of the Canadian Shield that underlies the western Canada sedimentary basin beneath 3.5–2.2 km of Phanerozoic sedimentary strata have been obtained on a seismic-reflection profile acquired by Lithoprobe. The profile crosses tectonic domains of central Alberta and delineates a major buried orogenic belt of Paleoproterozoic (∼1.8 Ga) age associated with crustal scale thrust imbrication and deflections in the crust-mantle boundary. Available geochronologic data suggest that crustal imbrication observed in the Alberta basement was coeval with that documented in the Trans-Hudson orogen to the east (1.80–1.83 Ga) and implies that a large region of continental crust, extending >1000 km from the western Superior province to the Snowbird tectonic zone, underwent considerable shortening during assembly of this part of the Canadian Shield.

Sedimentary Cover of the Canadian Shield through Mesozoic Time Reflected by Nd Isotopic and Geochemical Results for the Sverdrup Basin, Arctic Canada

The Sverdrup Basin of the Canadian Arctic Islands contains a sedimentary record, with only short breaks, from Early Carboniferous to Late Cretaceous time and can be used to document the nature of sediments delivered from northern Canada and Greenland. Sm‐Nd isotopic analysis of 72 sedimentary rock samples from the Sverdrup Basin, coupled with trace element characterization, shows that for most of Carboniferous to Late Cretaceous time, the sediment supply in the northern part of North America was dominated by a single broad provenance; 56 of the 72 samples lie squarely within the Nd isotopic evolution of a clastic sedimentary cover delivered to the region following 450–350 Ma Caledonian and Franklinian mountain building in Greenland and the Canadian Arctic Islands. Cratonic Shield sources in Greenland and Canada are hardly evident in the record, and significant contributions to the sediment budget from any source other than the post‐mid‐Paleozoic orogenic cover occurred only during four relatively short periods. First, during Carboniferous time, pre–Late Ordovician rocks of the Franklinian orogen contributed to alluvial clastic rocks in small rift basins in northern Ellesmere Island. Second, during Early Cretaceous time, Shield basement contributed to more widespread deltaic deposits in central and southern Ellesmere Island. Third, minor volcanic contributions to much of the basin occurred during Late Triassic–earliest Jurassic time and also, fourth, during Late Cretaceous time. Sedimentary materials from Caledonian and Franklinian mountains dominated the provenance of the continental and continent‐margin sedimentary system for at least 370 m.yr., a period of time extending far beyond the existence of the mountains themselves. This dominance was achieved by recycling of widespread Middle and Upper Devonian strata into Mesozoic units in the Canadian Arctic and Cordillera. We assess the extent to which the results call for cover of the Greenland‐Canadian Shield from 450 to 80 Ma and conclude that while much of the Shield was probably covered by Ordovician to Middle Devonian carbonate units, the northerly derived Upper Devonian clastic sedimentary rocks probably covered about one‐half of the Shield in its western and northern portions. This cover was progressively removed through Mesozoic time.

Nd Isotopes and the Source of Sediments in the Miogeocline of the Canadian Cordillera

Nd isotopes in clastic 600-75 Ma sedimentary rocks from the miogeocline of Alberta and British Columbia are used to (1) constrain the importance of proximal and distal North-American basement and juvenile Cordilleran sedimentary sources; (2) document large-scale changes in provenance; and (3) develop a North American miogeoclinal reference for Nd isotopes as a baseline for continental input to Cordilleran terranes. Initial

Nd isotopes, geochemistry, and constraints on sources of sediments in the Franklinian mobile belt, Arctic Canada

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