Ga Mafic Dykes Research Articles

Post-Taltson sedimentary and intrusive history of the southern Rae Province along the northern margin of the Athabasca Basin, Western Canadian Shield

The southern Rae Province of the western Canadian Shield underwent extensive reworking during Paleoproterozoic time. Widespread granitic plutonism and poorly defined metamorphism resulting from the ca. 2.5 to 2.3 Ga Arrowsmith Orogen was followed by more intense thermotectonic overprints attributed to the 1.99–1.92 Ga Taltson Orogen in the west and ca. 1.91–1.90 Ga tectonic activity associated with the Snowbird Tectonic Zone in the east. The Thluicho Lake Group is a fining-upwards, conglomerate–arkose–argillite succession deposited in an intermontane setting towards the end of the Taltson Orogen. Following deposition, the Thluicho Lake Group underwent two phases of regional folding and greenschist-facies metamorphism marking the latest Taltson and Snowbird overprints, respectively, prior to the emplacement of extensive 1.82 Ga mafic dykes. These depositional constraints support correlation of the Thluicho Lake Group with the lithologically similar Nonacho Group in the Northwest Territories, whereas petrological similarities and coeval emplacement ages indicate that the mafic dykes represent a southeastward extension of the Sparrow dykes. Mafic volcanic rocks geochemically linked to the dykes are intercalated within the Martin Group, a continental redbed succession deposited in trans-tensional basin settings, indicating that deposition was ongoing at 1.82 Ga. Similarities in age, lithological association and depositional setting suggest that the Martin Group is broadly correlative with the Baker Lake Group of the lower Dubawnt Supergroup in Nunavut. Widespread lamprophyre dykes of the type that fed minette volcanic rocks of the Christopher Island Formation in the Baker Lake Group are also spatially associated with the Martin Group. The broadly coeval Hudson granites, which extend over much of the same ground as the lamprophyres in Nunavut, also appear to have analogs in the southern Rae Province. The formation of widespread trans-tensional basins, together with extensive dyke emplacement, implies a period of intense brittle to brittle–ductile deformation throughout the western Churchill Province at about 1.83–1.82 Ga. This deformation is attributed to a broadly east-west stress regime imposed by accretion of the Nahanni-Fort Simpson Terrane to the west, combined with terminal collision in the Trans-Hudson Orogen to the east.

Age and tectonic implications of Paleoproterozoic mafic dyke swarms for the origin of 2.2 Ga enriched lithosphere beneath the Ungava Peninsula, Canada

Mafic dyke swarms represent short-lived magmatic events that carry important temporal and chemical constraints for the evolution of the lithospheric mantle. Five baddeleyite U–Pb isotopic analyses of Paleoproterozoic mafic dyke swarms intruding the Ungava Peninsula yielded ages of 2508 ± 6, 2220 ± 1, 2212 ± 3, 2199 ± 5 and 2149 ± 3 Ma. These ages fall within the 2.5–2.0 Ga period previously defined by five other swarms, which together comprise a magmatic record of more than 500 Ma. The similar age, composition and surface area of two 2.51 Ga swarms, along with the lack of coeval supracrustal rocks, suggest they correspond to failed rifts. An important magmatic hiatus of 280 Ma follows, but ends with the intrusion of numerous dykes between 2.23 and 2.20 Ga in the thinnest lithosphere of the northern Ungava Peninsula. These dykes exhibit chemical signatures that vary from enriched older 2229–2212 Ma dykes ( ɛNd ( t) < +0.4, La/Yb > 4) to relatively depleted 2209–2199 Ma voluminous younger dykes ( ɛNd ( t) > +1.4, La/Yb < 4). The depleted character, great volume, and occurrence of these younger dykes in a thin lithosphere may represent decompression melting of the asthenosphere, following delamination of the Archean lithospheric keel ca. 2.21–2.20 Ga. Later dykes emplaced ca. 2.17 Ga within a rejuvenated Paleoproterozoic lithosphere also have depleted chemical compositions, and map the extent of rifting coeval with the emplacement of early basalts in the Labrador Trough. Dykes emplaced in the 2.15–2.00 Ga period further to the South are proximal to coeval continental basalts, with which they share enriched isotopic compositions ( ɛNd ( t) = −1.5 to −7) and a positive correlation between La/Yb and Zr/Nb (La/Yb = 4–10; Zr/Nb = 7–20) that suggest assimilation of an Archean crustal component. A progressive decrease in the ɛNd ( t) values of these magmas with decreasing age further implies increasing contamination, until the Archean lithosphere failed ca. 2.0 Ga and permitted the eruption of isotopically depleted basalts. The ca. 2.2 Ga depleted northern dykes and the 2.1–1.9 Ga Povungnituk basalts of the Cape Smith foldbelt share a negative correlation between Zr/Nb and La/Yb that is expected for the incorporation of an enriched lithospheric component. This component is represented by the alkaline rocks comprised in the Circum-Ungava supracrustal belts (carbonatites, lamprophyric lavas, alkaline basalts) and by the Lac Aigneau lamprophyric dykes (1941 ± 3 Ma) intruding the Ungava Peninsula. These alkaline magmas collectively have 2.2–2.1 Ga depleted-mantle Nd model ages ( T DM) similar to a 2.15 Ga Sm–Nd isochron defined by the basaltic rocks of the supracrustal belts. Although these Nd T DM ages and isochron are 100–300 Ma older than magmatic ages, they are identical with the ca. 2.2 Ga mafic dykes of the Ungava Peninsula. This time correlation, along with the similar chemical signature of the northern dykes and younger Povungnituk basalts, suggest that the composition of younger lavas and alkaline magmas was influenced by an enriched component associated with the pervasive metasomatism of the lithosphere, coeval with the emplacement of the many ca. 2.2 Ga dykes.

Geochronology and tectonometamorphic history of the Snowbird Lake area, Northwest Territories, Canada: New insights into the architecture and significance of the Snowbird tectonic zone

The results of mapping, geochronology, petrology, and geochemistry of rocks of the Snowbird Lake map sheet (NTS 65D), western Churchill Province, Canada provide new insight into the tectonometamorphic history of the southern Rae Province and significance of the Snowbird tectonic zone, western Churchill Province, Canada. U/Pb TIMS and SHRIMP data document a complex igneous and metamorphic history, with major contrasts between the Rae Province, Chipman domain, and Hearne Province. The Chipman batholith represents the oldest plutonic rock; crystallized at ∼3.4 Ga, metamorphosed at 2.85 Ga, and intruded at 11 kbar by ca. 1.9 Ga mafic dykes. Subsequent Neoarchean activity in the Rae Province involved plutonism at ca. 2.735, 2.67–2.66, and 2.55 Ga, all events that are documented in other portions of the Rae Province. In contrast, the Hearne Province shows only 2.72–2.66 igneous activity. Calcareous sediments are interpreted to have formed on the Rae Province passive margin, having been deposited on exhumed ca. 2.55 Ga basement between 2.07 Ga (youngest detrital zircon) and 1.92 Ga (metamorphic zircon). Burial to ∼7.6 kbar and heating to granulite facies at ca. 1.92 Ga document tectonic thickening in a compressional setting at ca. 1.92 Ga. This compressional setting may reflect terminal collision in the Taltson orogen or accretion of the Hearne Province during the earliest stage of the Hudsonian orogeny. Collectively, these data suggest that the Snowbird tectonic zone was not in an intracratonic setting prior to ca. 1.9 Ga orogenesis.

Nature of mantle source contributions and crystal differentiation in the petrogenesis of the 1.78 Ga mafic dykes in the central North China craton

Circa 1.78 Ga mafic dyke swarms and some coeval volcanic associations constitute a Large Igneous Province in the central North China craton. The 1st generation of dykes intruded at ca. 1780 Ma and is chemically delineated into 3 groups: the LT Group is gabbroic and has low-Fe–Ti contents, acting as parental magma; the NW Group is high in Fe–Ti-contents and doleritic with an iron-enriched trend; whereas the EW Group is doleritic to andesitic and crystallized from relict siliceous liquids with a silica-enriched trend. They have an EM-I type source and record integral magmatic processes. These include contamination of lithospheric material and assimilation of crustal melts with in-situ crystallization in a magma chamber (the LT Group) and fractional crystallization in magma channels (the NW Group) and even with additional alteration (the EW Group). The 2nd generation is slightly younger (ca. 1760 Ma) and scarcely distributed. It has high-Fe–Ti contents, originated from a mixing source of DM and EM-I types. The dykes could be associated with a palaeo-plume: the 1st generation represents lower mantle melts generated from the plume head, whereas the 2nd generation records extra melts from asthenosphere entrained by the plume tail.

A modern structural regime in the Paleoarchean (∼3.64 Ga); Isua Greenstone Belt, southern West Greenland

The footwall gneisses beneath the western part of the Paleoarchean (∼3.8 Ga) Isua Greenstone Belt, southern West Greenland, are interpreted here in terms of a ∼3.64 Ga stack of mylonitic crystalline thrust-nappes, the oldest example known on Earth. In present coordinates, the kinematic history of the thrust-nappe stack is couched in terms of initial longitudinal (strike-parallel) thrusting towards the southwest, followed by transverse thrusting to the northwest, and subsequent extensional collapse of the thickened crust toward the southeast. Diorite and tonalite that form the western margin of granitoids, structurally overlying the western part of the Isua Greenstone Belt and its footwall, contain ∼3.5 Ga mafic dykes, some of which are deformed and/or truncated at fault contacts within the granitoids. Accordingly, a component of the deformation structurally above the Isua Greenstone Belt occurred after ∼3.5 Ga, significantly later than the formation of the underlying mylonitic nappes, probably during the Neoarchean. The structural regime of mylonitic thrust-nappe stacking is very similar to that in modern mountain belts. It would appear that the deformational behaviour, rheological constitution and overall strength of Paleoarchean and modern continental crust were similar.

Paleomagnetism of 2.44 Ga mafic dykes in Russian Karelia, eastern Fennoscandian Shield — implications for continental reconstructions

Results of a paleomagnetic study of Paleoproterozoic dykes and Archean host rocks in the eastern Fennoscandian shield are reported here. Most of the dykes are related to ca 2440 Ma layered intrusions and have isotopic ages between 2349 and 2476 Ma. The dykes form an extensive NW–SE and E–W trending swarm. The majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature. The high temperature remanence component (D), which is thought to represent the primary 2.44 Ga magnetization of the dykes, has a paleomagnetic pole at 20°S, 278.8°E (A95=6.0°, N=11 sites). Another D-like component (D′) with shallower inclination (VGP 9.6°N, 256.2°E, eight samples), at one site from baked Archean host rock may also represent the primary remanence of the dykes. The commonest remanence component (A) records an extensive Svecofennian partial overprint and is interpreted to be ca 1.84 Ga old. It yields a paleomagnetic pole at 52.6°N, 226.8°E (A95=2.7°, N=23). The fourth component, B, has a paleopole at 53.9°N, 169.6°E (A95=6.3°, N=16) and is inferred to be ca 1.75 Ga on the basis of isotopic dating, although ages of 2.1, 1.3 and 0.3 Ga are also possible. Baked contact tests for the 2.44 Ga dykes are negative, as shown by the occurrence of component D in Archean host rocks beyond one dyke width. It is proposed that the negative baked contact test indicates regional reheating and remagnetization of the Archean basement at ca 2.44 Ga ago due to the spatially vast occurrence of coeval 2.44 Ga mafic dykes, layered intrusions, volcanics and granites in the area. Based on this hypothesis, it is suggested that component D in the dykes may be primary and acquired at ca 2.44 Ga ago. The similarity of pole D with poles from coeval dykes and layered intrusions elsewhere in the Karelian Province and Kola Peninsula suggest that, within the Archean domain of the Fennoscandian shield, no large scale block movements have taken place since 2.44 Ga. At 2.44 Ga the Fennoscandian Shield was located at the latitude of ca 30°S which is different to that at 1.84 and 1.75 Ga, when Fennoscandia was located at the latitudes of ca 30°N. A continental reconstruction based on pole D and the pole from the 2.45 Ga Matachewan dykes of the Superior craton in Laurentia, places the Kola Peninsula adjacent to southern Greenland. Reconstructions based on recent geological interpretations and a reconstruction yielding pole D′, are also discussed.