Charles F. Gower

Birthdate for the lapetus Ocean? A precise U-Pb zircon and baddeleyite age for the Long Range dikes, southeast Labrador

Mafic dike swarms represent direct evidence of early tensional environments that may have eventually led to ocean formation. The first precise U-Pb zircon and baddeleyite age of 615 {plus minus} 2 Ma for a Long Range dike, southeastern Labrador, could mark the beginning of Iapetus Ocean formation. Baddeleyite (ZrO{sub 2}) is more widespread than previously recognized. It is ideal for use with the U-Pb system because it has low initial common lead and sufficient uranium levels, and it does not lose significant radiogenic lead with time. Baddeleyite therefore provides a unique opportunity for dating dikes associated with rift-related events with unprecedented accuracy.

Eastern Laurentia in Rodinia: constraints from whole-rock Pb and U/Pb geochronology

Abstract Whole-rock Pb isotopic signatures and U/Pb geochronology refute a Rodinian correlation of northeastern Laurentia and proto-Andean Amazonia. According to this previously proposed model, the Labrador–Scotland–Greenland Promontory (LSGP) of northeastern Laurentia collided with the proto-Andean margin of Amazonia, at the Arica Embayment, during the Grenville/Sunsas Orogeny (ca. 1.0 Ga). Links between the two margins were based upon the correlation of the LSGP with Arequipa-Antofalla Basement (AAB), a Proterozoic block along the proto-Andean margin of Amazonia adjacent to the Arica Embayment. Specifically, similarities in 1.8–1.0 Ga basement rocks in both regions suggested that the AAB was originally a piece of the LSGP. Furthermore, similarities in unique, post-collisional, but pre-rift, glacial sedimentary sequences also supported a link between the AAB and LSGP. Tests of these apparent similarities fail to support correlation of the AAB and the LSGP and, thus, eliminate a direct link between northeastern Laurentia and southwestern Amazonia in Rodinia. However, Pb isotopic compositions and U/Pb geochronology provide the basis for two new correlations, namely, (1) the ca. 1.3–1.0 Ga basement in the central and southern Appalachians may be an allochthonous block that was transferred to Laurentia from Amazonia at ca. 1.0 Ga, and (2) an allochthonous AAB may be a piece of the Kalahari Craton that was transferred to Amazonia at ca. 1.0 Ga. Based on these new correlations and a previously proposed Grenvillian connection between southern Laurentia (Llano) and Kalahari, we propose that Amazonia may have collided with a contiguous southeastern Laurentia/Kalahari margin at ca. 1.0 Ga.

The evolution of the Grenville Province in eastern Labrador, Canada

Abstract Rifting at c. 1.71 Ga separated the Mealy Mountains terrane from pre-Labradorian Laurentia and resulted in a short-lived (backarc?) basin, followed by southward subduction giving rise to 1.68-1.66 Ga Labradorian calc-alkaline arcs, partly built on older crust in the Mealy Mountains terrane. The arcs and subduction were also short lived, terminating when accretion against the Makkovikian and Eastern Churchill provinces occurred at 1.65 Ga. The Trans-Labrador batholith developed as a result of crustal thickening during suturing of the 1.68-1.66 Ga arc-related rocks to pre-Labradorian Laurentia. Further bimodal magmatism, in response to crustal thickening, continued until c. 1.62 Ga, followed by waning granitic magmatism until 1.60 Ga. The Labrador Orogen and older rocks in the Mealy Mountains terrane are flanked to the south by Pinwarian (1.51-1.45 Ga) granitoid rocks, possibly the inboard manifestation of a northward-subducting continental-margin arc at the southern margin of Laurentia. Extension, mostly coincident with the 1.71 Ga zone of weakness, started at c. 1.45 Ga. Although rifting was successful further west, it is only represented by basaltic magmatism (Michael and Shabogamo gabbros) along a linear zone in Labrador. Similar tectonism characterized the same zone periodically for at least the next 150 million years during which time the 1.27 Ga Harp-Nutak dykes were emplaced and slightly younger rocks of the Seal Lake Group formed north of the Grenville Province. After c. 1.23 Ga tectonic conditions changed, partly in response to Elzevirian accretion of arc terranes preserved in the southwest Grenville Province, that are speculated to have once also existed south of the exposed eastern Grenville Province. Alternating compressional and extensional conditions terminated at c. 1.0 Ga during collisional orogenesis associated with the final stages of development of the Grenville Orogen. Grenvillian deformation in Labrador (1.08-0.97 Ga) resulted in northwesterly thrusting, the effects of which are focused in the Exterior Thrust Belt, and widespread plutonism in the Interior Magmatic Belt. Thrusting in the Exterior Thrust Belt is sited along the zone of 1.71 and 1.43 Ga rifting. Uplift and cooling continued until 0.90 Ga as crustal stability was gradually achieved.

Crustal evolution in eastern labrador: Constraints from precise U-Pb ages

Abstract New U-Pb ages for rocks of eastern Labrador complement previous U-Pb dates for the Grenville and Makkovik provinces and substantiate the following conclusions. Eastern Labrador consists of two distinct crustal blocks that are different in age as well as magmatic, metamorphic and structural overprint. The northern block includes Archaean to Lower Proterozoic basement rocks (> 1900 Ma) and two distinct episodes of rhyolitic volcanism at 1860 and 1807 Ma. High-grade metamorphism, deformation and anatexis affected this block between 1860 and 1760 Ma ago (Makkovikian Orogeny). The southern block comprises the Trans-Labrador Batholith and the various terranes of the Grenville Province. In contrast to the northern region, the southern block shows no proof for the existence of continental crust older than 1710 Ma. This suggests that large amounts of juvenile material were added between 1710 and 1630 Ma ago, as dated by a large series of precise rock ages. At least towards its end, this crust-forming event was associated with high-grade metamorphism, deformation and anatexis (Labradorian Orogeny). The 1710-1630 Ma period of activity was followed by three distinct phases of anorogenic plutonism causing the emplacement of pegmatites (1500 Ma), a widespread suite of cotonitic metagabbros (Michael Gabbro, 1426 Ma) and granitic-syenitic bodies (1296 Ma). During the Grenvillian Orogeny, the region achieved its present structural configuration including the emplacement of granitoid plutons. The structurally highest regions, the Mealy Mountains and the Hawke River terranes were thrust over the Lake Melville terrane and part of the Groswater Bay terrane and they were only marginally affected by Grenvillian metamorphism. The more deeply buried Lake Melville and Groswater Bay terranes, however, were significantly modified during tectonothermal events at 1030 Ma and 980-960 Ma respectively.

Distribution of pre-1400 Ma crust in the Grenville province: Implications for rifting in Laurentia-Baltica during geon 14

Recent geochronological data show that geon 14 and older rocks are found at least 700 km south of the Grenville front in the eastern Grenville province. In the southwestern Grenville province, comparable rocks extend only 200 km south; farther south, in the Central metasedimentary belt and the Adirondack Mountains, the crust mostly postdates 1350 Ma. We explain this contrast by postulating a major rifting event, commencing at ∼1430 Ma, during which large sections of older crust were detached from that part of southern Laurentia now occupied by the Central metasedimentary belt. Crust was also removed from the area between northwest Britain and Scandinavia. Failed arms extended (1) from Missouri to where the Belt Supergroup accumulated, (2) from the Central metasedimentary belt eastward into Labrador, and (3) between Baltica and east of Greenland. The geologic history following late geon 14 rifting was dominated by a southward-subducting magmatic arc that eventually collided with southern Laurentia-Baltica, resulting in the Elzevirian orogeny at ∼1200 Ma.

Grenvillian magmatism in the eastern Grenville Province, Canada

Abstract U-Pb age determinations on six newly identified examples of Grenvillian plutonism from eastern Labrador have yielded the following ages: Gilbert Bay granite 1132−6+7 Ma; Second Choice Lake pegmatite 1003±6 Ma; Southwest Pond granite 963±6 Ma; Chateau Pond granite 964±2 Ma; Riviere Bujeault headwaters quartz syenite 964±5 Ma; Upper St Lewis River (west) granite 956±1 Ma. In addition, eight new K-Ar and Rb-Sr hornblende and biotite ages ranging from 953 Ma to 811 Ma are reported for three of these plutons. In conjunction with three previous U-Pb determinations, it is concluded that the U-Pb dates reflect two periods of Grenvillian plutonism that occurred in separate areas of eastern Labrador, and which were also characterized by distinct emplacement styles. North of the Mealy Mountains terrane boundary (Lake Melville terrane and its border regions with the Mealy Mountains and Hawke River terranes) plutonism occurred between ∼1130 and 1080 Ma and consisted of sporadic, minor granitic intrusions. These intrusions had little structural effect on the host rocks and are inferred to have been emplaced at a high structural level, a model consistent with previous suggestions for tectonic stacking in the region. South of the Mealy Mountains terrane boundary (Mealy Mountains and Pinware terranes) plutonism was brief (966 to 956 Ma), but widespread. Typical magmatic products were circular (in plan) plutons, up to 20 km in diameter, having monzonite, quartz syenite and granite compositions. These plutons exerted marked structural influence on their host rocks and are interpreted to have been emplaced at intermediate structural levels. In a broader regional context, by utilizing a previously demonstrated correlation between positive magnetic anomalies and Grenvillian plutons together with reconnaissance geological mapping, a belt of Grenvillian plutons is inferred to exist across the southern half of the eastern Grenville Province. This zone of plutonism serves to emphasize a distinct difference between an exterior (northern) thrust belt and an interior (southern) magmatic belt. A similar bipartite division is apparent in other parts of the Grenville-Sveconorwegian Orogen. The mineral geochronological data probably do not reflect the cooling histories of individual plutons; alternative explanations include either a slow regional cooling event or the distal effects of younger plutonism/metamorphism to the south, as yet unidentified.

A U-Pb Geochronological Framework for the Pinware Terrane, Grenville Province, Southeast Labrador

The Pinware terrane in the eastern Grenville Province of Labrador is characterized by abundant granitoid rocks but a lack of metasedimentary gneiss. On the basis of newly obtained U-Pb zircon ages, the following history is outlined. Supracrustal rocks, including felsic volcanics, were formed at

Roots of the Labradorian orogen in the Grenville Province in southeast Labrador: Evidence from marine, deep‐seismic reflection data

1640 \pm 7 Ma