M. E. Bickford

Episodic zircon ages, Hf isotopic composition, and the preservation rate of continental crust

U/Pb detrital zircon ages from global modern river sediments define eight peak clusters centered at 2700, 2500, 2010, 1840, 1600, 1150, 600, and 300 Ma. These clusters extend vertically into both positive and negative e Hf (T) space and are similar to those in orogenic granitoids that correlate well with supercontinent formation. We suggest that the clusters are preservation peaks that reflect juvenile and reworked continental crust selectively preserved during continental collisions. The greatest contribution of juvenile continental crust is associated with the 1600 and 1150 Ma clusters and may reflect a change in the style of collisional orogens in the Paleoproterozoic involving thicker and stronger lithosphere. Age gaps at 2400–2200, 1400–1300, 900–650, and 185–120 Ma represent times when crustal production and recycling rates were about the same. Although some new continental growth may occur during continental collisions, supercontinent assembly does not require an increase in production rate of continental crust. Rather, we suggest that the preservation rate increases by an increased probability of capture of both new and reworked continental crust in collisional orogens.

Transcontinental Proterozoic provinces

Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

Proterozoic collisional tectonism in the Trans-Hudson orogen, Saskatchewan

Isotopic and structural data from the juvenile Reindeer zone of the Trans-Hudson orogen, northern Saskatchewan, indicate a pre-1.85 Ga thermotectonic event, possibly reflecting arc-continent collision, followed by a more extensive, nappe-forming, ca. 1.83-1.80 Ga thermotectonism during terminal continent-continent collision. Preliminary data from the adjacent, ensialic Cree Lake zone suggest high-grade reworking of Archean crust by the pre-1.85 Ga event. In the Rae province to the west, high-grade metamorphism and reworking of Archean crust occurred about 2.0 Ga and may be related to the formation of the coeval Taltson magmatic zone.

Evolution of the Early Proterozoic Colorado province: Constraints from U-Pb geochronology

The Colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the Archean Wyoming craton during the Early Proterozoic, between about 1790 and 1660 Ma. Correspondence in ages between metamorphism, deformation, and plutonism; association of volcanic rocks with comagmatic calc-alkalic plutons; and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton.

The Wathaman batholith: An Early Proterozoic continental arc in the Trans-Hudson orogenic belt, Canada

The Wathaman batholith, a major element of the Trans-Hudson orogenic belt in northern Saskatchewan and Manitoba, has an exposed strike length of 900 km. This immense body separates dominantly Archean (Rae-Hearne) provinces to the northwest from Early Proterozoic, arc-related terranes of the orogen to the southeast. Major- and trace-element data for 42 samples from the Saskatchewan part of the batholith are similar to those of Phanerozoic arc batboliths (for example, Sierra Nevada and Andean batholiths). Data plotted on trace-element discrimination diagrams show that the batholith has continental-arc affinities. U-Pb zircon geochronology of five main-phase granite samples suggests that most of the batholith formed between 1865 and 1850 Ma and is coeval with smaller plutons of the La Rouge arc. The main deformation of the batholith is limited by the 1830 Ma age of a late granite sheet that crosscuts folds and foliation of main-phase granites. The 15- to 25-m.y. crystallization span of the batholith is similar to the histories of many more recent continental magmatic arcs.

The formation of continental crust: Part 1. A review of some principles; Part 2. An application to the Proterozoic evolution of southern North America

Understanding of the evolution of continental crust has been dominated by work in tectonics, experimental petrology and geochemistry, and geochronology and isotope geochemistry. In part 1, these approaches are discussed in the light of keynote articles, most published by the Geological Society of America, in an attempt to show how new data and new thinking have influenced understanding of the formation and modification of continental crust. In part 2, the evolution of continental crust in southern North America from 1.8 Ga until 1.3 Ga is discussed as an application of ideas generated during the past 30 years.

Depositional History of the Chhattisgarh Basin, Central India: Constraints from New SHRIMP Zircon Ages

The Indian Shield includes the Singhbhum, Bastar, and East and West Dharwar cratons. Proterozoic sedimentary basins formed on these cratons have preserved rocks with a range of degrees of metamorphism and deformation. In the Chhattisgarh Basin, within the Bastar Craton, the ca. 2200–2500-m-thick Chhattisgarh Supergroup has been preserved in nearly pristine condition. Previous work has shown that the Sukhda Tuff, located about 2200 m from the base of the section, was formed ca. 1007 Ma. New U-Pb SHRIMP age determinations show that the Singhora Tuff, located about 100 m above the base of the basin, is not older than Ma. Thus, most of the Chhattisgarh Supergroup was deposited between 1400 and 1000 Ma. Age data for detrital zircons from sandstones show that, regardless of their stratigraphic position, there is a unimodal age peak near 2500 Ma, the typical age of adjacent granitic and rhyolitic basement rocks, indicating that these constituted the principal provenance of the sediments in the Chhattisgarh Basin. However, near the top of the succession, the Sarnadih Sandstone and a volcaniclastic sandstone near Sukhda Village, show a wide range of ages with peaks from ca. 1000 through 2680 Ma. The ca. 1000-Ma detrital zircons were probably derived from igneous sources similar to the Sukhda Tuff, but the zircons with other ages indicate a different source. Age data from rock units in the Central Indian Tectonic Zone to the north of the basin match this age spectrum better than any to the south of the basin and are consistent with a change in provenance direction to a northerly source late in the basin-filling cycle.

Crustal history of the Rae and Hearne provinces, southwestern Canadian Shield, Saskatchewan: constraints from geochronologic and isotopic data

Abstract The Cree Lake Zone (Hearne Province) and Western Granulite domain (Rae Province), in northern Saskatchewan, are contrasting parts of the variably reworked, (?) upper plate western hinterland of the Paleoproterozoic Trans-Hudson Orogen. Their junction is defined by the Virgin River Shear Zone, a part of the more extensive Snowbird “tectonic zone”. Subsurface extension of the ∼ 2.0 Ga Thelon Orogen lies west of the Western Granulite domain. We present UPb zircon ages, SmNd and RbSr isotopic data from the Cree Lake Zone and Western Granulite domain, and from the Junction Granite which intrudes the Virgin River Shear Zone. Data from the Western Granulite domain indicate that prevalent granulite facies metamorphism occurred between ∼ 2.3 and 2.0 Ga, but is imposed on Archean protoliths ∼ 2.8−3.0 Ga old. Low amphibolite facies retrogression, which increases toward the Virgin River Shear Zone, probably postdates 2.1 Ga. Data from the Cree Lake Zone also yield Archean protolith ages as old as 3.0 Ga, and some rocks yield interpreted high-grade metamorphic overprint ages of ∼ 2.3−2.0 Ga . However, two samples yield data indicating “Hudsonian” high-grade metamorphic overprint ∼ 1.87 to 1.81 Ga . The Junction Granite has a UPb zircon age of 1.82 ± 0.30 Ga , but relict zircons and SmNd data show that it was derived by melting of Archean crust. High-grade mid-crustal thermal resetting in the Western Granulite domain may be coeval with the 1.9–2.0 Ga Thelon Orogen, although the data do not preclude an earlier event or events. Similar, ∼ 2.3−2.1 Ga reworking extends into the Cree Lake Zone, but high-grade “Hudsonian” (1.87-1.84 Ga) reworking is also evident in parts of this zone well to the west of the Wollaston domain, and may extend entirely across the zone to the Virgin River Shear Zone. Relative importance of the “Thelon” and “Hudsonian” overprints in Cree Lake Zone is unknown. Data from the Junction Granite and Western Granulite domain suggest that all recorded high-strain events in the Virgin River Shear Zone postdate ∼ 2.3 Ga and are thus unrelated to deep-crustal Archean granulite facies mylonites in the Tantato-Black Lake segment of the Snowbird tectonic zone, north of the Athabasca basin.

U-Pb Studies of Zircon Cores and Overgrowths, and Monazite: Implications for Age and Petrogenesis of the Northeastern Idaho Batholith

U-Pb isotopic studies of zircons, many containing xenocrystic cores with euhedral overgrowths, and monazite from igneous rocks and metasedimentary inclusions of the northeastern Idaho batholith yield linear arrays on concordia diagrams. We interpret these as mixing lines between an old component (cores) and a young component (overgrowths and zircons without cores). The lower intercept of such arrays with concordia may yield the minimum age of the rocks if the overgrowths and zircons without cores are discordant, or the crystallization age if they are concordant. Monazites yield apparently concordant ages either equal or less than the lower intercept zircon ages. The samples studied yield lower intercept ages ranging from