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J. L. Wooden

Publications and source records attributed to J. L. Wooden.

79 records · Page 5Linked to original sources
USGS1992

The nature of Archean terrane boundaries: An example from the northern Wyoming Province

The Archean northern Wyoming Province can be subdivided into two geologically distinct terranes, the Beartooth-Bighorn magmatic terrane (BBMT) and the Montana metasedimentary terrane (MMT). The BBMT is characterized by voluminous Late Archean (2.90-2.74 Ga) magmatic rocks (primarily tonalite, trondhjemite, and granite); metasedimentary rocks are preserved only as small, rare enclaves in this magmatic terrane. The magmatic rocks typically have geochemical and isotopic signatures that suggest petrogenesis in a continental magmatic arc environment. The MMT, as exposed in the northern Gallatin and Madison Ranges, is dominated by Middle Archean trondhjemitic gneisses (3.2-3.0 Ga); metasedimentary rocks, however, are significantly more abundant than in the BBMT. Each terrane has experienced a separate and distinct geologic history since at least 3.6 Ga ago based on differences in metamorphic and structural styles, composition of magmatic and metasupracrustal rocks, and isotopic ages; consequently, these may be described as discrete terranes in the Cordilleran sense. Nonetheless, highly radiogenic and distinctive Pb-Pb isotopic signatures in rocks of all ages in both terranes indicate that the two terranes share a significant aspect of their history. This suggests that these two Early to Middle Archean crustal blocks, that initially evolved as part of a larger crustal province, experienced different geologic histories from at least 3.6 Ga until their juxtaposition in the Late Archean (between 2.75 to 2.55 Ga ago). Consequently, the boundary between the BBMT and MMT appears to separate terranes that are not likely to be exotic in the sense of their Phanerozoic counterparts. Other Archean provinces do appear to contain crustal blocks with different isotopic signatures (e.g. West Greenland, India, South Africa). The use of the term exotic, therefore, must be cautious in situations where geographic indicators such as paleontologic and/or paleomagnetic data are not available. In these cases, isotopic signatures are one of the most useful features for assessing overall genetic relations amongst geologically distinct terranes.

Idaho, Montana, Wyoming↗
USGS1992

Pre-Elsonian mafic magmatism in the Nain Igneous Complex, Labrador: The bridges layered intrusion

Decades of work on the pristine, unmetamorphosed, and well exposed anorthositic, mafic and granitic rocks of the Nain igneous complex, Labrador, have led to the conclusion that all plutonic rocks in that area were emplaced in a short time intercal at about 1300 ± 10 Ma). We report here new isotopic data for mafic intrusive rocks that appear to have crystallized several hundred Ma earlier than the bulk of the plutonic activity in the Nain complex. The Bridges layered intrusion (BLI) is a small (15–20 km 2 ) lens of layered mafic rocks about 1.5 km thick, surrounded and intruded by anorthositic, leuconoritic and leucotroctolitic plutons in the middle of the coastal section of the Nain igneous complex. BLI shows very well developed magmatic structures, including channel scours, slump structures, and ubiquitous modally graded layering. Most rocks, however, show granular textures indicative of recrystallization, presumably caused by emplacement of younger anorthositic rocks. BLI contains cumulate rocks with slightly more primitive mineral compositions (An 60–83 , Fo 66–71 ) than those of other mafic intrusions in the Nain igneous complex, including Kiglapait. Sm-Nd isotopic data for 7 BLI whole-rocks ranging in composition between olivine melagabbro and olivine leucogabbro yield an age of 1667 ± 75 Ma, which we interpret as the time of primary crystallization. The internal isotopic systematics of the BLI have been reset, probably by intrusion of adjacent anorthositic plutons. A SmNd mineral isochron (plag, whole-rock, mafics) for a BLI olivine melagabbro gives an age of 1283 ± 22 Ma, equivalent within error of a mineral array (plag, whole-rock, opx, cpx) for an adjacent, igneous-textured, leuconorite vein (1266 ± 152 Ma). The initial Nd ratio for BLI corresponds to ϵ Nd = −3.18 ± 0.44. Other whole-rock samples, however, some with vein-like alteration (Chlorite, serpentine, amphiboles), show ϵ Nd values as low as −9.1, suggesting variable contamination by direct assimilation of early Archean crustal rocks and/or by fluids that have interacted with such crust. Adjacent anorthositic rocks also show variable ϵ Nd some as low as −14.7, implying larger degrees if crustal assimilation, perhaps by parental magmas during lower crustal ponding prior to emplacement. These contamination effects preclude straightforward determination of the isotopic character of mantle sources for both BLI and the anorthositic rocks.

Labrador↗
USGS1989

River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

The River Valley pluton is a ca . 100 km 2 body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An 60–70 ) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo 70–80 . A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. Ten samples, including both igneous and deformed lithologies give a Pb-Pb whole-rock isochron of 2560±155Ma, which is our best estimate of the time of primary crystallization. The River Valley pluton is thus the oldest anorthositic intrusive yet reported from the Grenville Province, but is more calcic and augitic than typical massifs, and lacks their characteristic Fe-Ti oxide ore deposits. The River Valley body may be more akin to similar gabbro-anorthosite bodies situated at the boundary between the Archean Superior Province and Huronian supracrustal belt of the Southern Province west of the Grenville Front. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2377 ± 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2185 ± 105 Ma, which is similar to internal Pb-Pb isochron ages of 2165 ± 130 Ma and 2100 ± 35 Ma for two igneous-textured rocks. It is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the Rb-Sr and Pb-Pb systems from a younger event such as the Grenvillian orogeny of ca . 1.0 Ga. None of the isotopic systems we investigated, however, gives an age near 1.0 Ga, suggesting that neither the River Valley pluton, nor the immediately surrounding gneisses were strongly affected by metamorphism associated with the Grenvillian orogeny. Initial isotopic ratios for the River Valley pluton correspond to single-stage model parameters of μ = 8.06, ϵ Nd = 0 to −3, and I Sr = 0.7015 to 0.7021. Collectively, these suggest either an enriched mantle source or crustal contamination of a mantle-derived magma. The crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby Superior Province.

Geochimica et Cosmochimica Acta↗
USGS1988

Pb, Sr, and Nd isotopic compositions of a suite of Late Archean, igneous rocks, eastern Beartooth Mountains: Implications for crust-mantle evolution

A series of compositionally diverse, Late Archean rocks (2.74–2.79 Ga old) from the eastern Beartooth Mountains, Montana and Wyoming, U.S.A., have the same initial Pb, Sr, and Nd isotopic ratios. Lead and Sr initial ratios are higher and Nd initial ratios lower than would be expected for rocks derived from model mantle sources and strongly indicate the involvement of an older crustal reservoir in the genesis of these rocks. Crustal contamination during emplacement can be ruled out for a variety of reasons. Instead a model involving subduction of continental detritus and contamination of the overlying mantle as is often proposed for modern subduction environments is preferred. This contaminated mantle would have all the isotopic characteristics of mantle enriched by internal mantle metasomatism but would require no long-term growth or changes in parent to daughter element ratios. This contaminated mantle would make a good source for some of the Cenozoic mafic volcanics of the Columbia River, Snake River Plain, and Yellowstone volcanic fields that are proposed to come from ancient, enriched lithospheric mantle. The isotopic characteristics of the 2.70 Ga old Stillwater Complex are a perfect match for the proposed contaminated mantle which provides an alternative to crustal contamination during emplacement. The Pb isotopic characteristics of the Late Archean rocks of the eastern Beartooth Mountains are similar to those of other Late Archean rocks of the Wyoming Province and suggest that Early Archean, upper crustal rocks were common in this terrane. The isotopic signatures of Late Archean rocks in the Wyoming Province are distinctive from those of other Archean cratons in North America which are dominated by a MORB-like, Archean mantle source (Superior Province) and/or a long-term depleted crustal source (Greenland).

Earth and Planetary Science Letters↗
USGS1986

Sr, Nd and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador: Implications for crustal contamination and basement mapping

We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have I Sr = 0.7025−0.7033, ε Nd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have I Sr = 0.7032−0.7066, ε Nd = −0.3 to −4.4 and variable, but generally unradiogenic 207 Pb/ 204 Pb and 206 Pb/ 204 Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture.

Geochimica et Cosmochimica Acta↗
USGS1983

Isotopic evidence from the eastern Canadian shield for geochemical discontinuity in the proterozoic mantle

Most workers agree that Proterozoic anorthosite massifs represent the crystallization products of mantle-derived magmas1,2, although the composition of the parental melts is a major unsolved petrological problem 3. As mantle-derived rocks, the massifs can be used as geochemical probes of their late Precambrian upper mantle sources. We report here Nd and Sr isotopic compositions of anorthosites and related rocks from the Grenville and Nain Provinces of the eastern Canadian shield. Here 75% of the Earth's known anorthosite is found in a 1,600-km belt from the Adirondack Mountains of northern New York State to the eastern coast of Labrador4 (Fig. 1). The results indicate that the massifs were derived from at least two distinct mantle source regions which were established before 1,650 Myr ago, and were episodically involved in magmatism over ???500 Myr. One reservoir, below the Grenville Province, and probably below much of the eastern Superior Province, was isotopically similar to the depleted, modern-day mid-ocean ridge basalt (MORB) source. The other reservoir was chondritic to moderately enriched, and is most easily identified in the Nain Province, but may have occurred scattered throughout the Superior Province. ?? 1983 Nature Publishing Group.

Nature↗