Pb Crystallization Ages Research Articles

Paleomagnetism of the Quottoon plutonic complex in the Coast Mountains of British Columbia and southeastern Alaska: evidence for tilting during uplift

The Quottoon plutonic complex (part of the great tonalite sill) was emplaced into eastern parts of the Coast shear zone along the west flank of the Coast Mountains. U–Pb crystallization ages range from 72.3 to 55.5 Ma. A regional compilation of K–Ar hornblende dates from the tonalite sill indicates a west to east decrease from ~60 Ma to ~54 Ma across the intrusive suite. Paleomagnetic samples were collected at 160 sites ([Formula: see text] 8 samples per site) along six transects across the Quottoon complex between the Skeena River and Willard Inlet. Directions of characteristic remanent magnetization (ChRM) were successfully determined by principal component analysis of thermal demagnetization data for about half of the samples collected. Site-mean ChRM directions were determined for 56 sites from the Skeena River, Quottoon Inlet, Steamer Passage, and Wales Island transects, using criteria of [Formula: see text] 4 sample ChRM directions per site and 95% confidence limit (α95) [Formula: see text] 25°. For Filmore Island and Willard Inlet, few sites provided adequate determinations of site-mean ChRM directions, but many sample ChRM directions were well determined and consistent. The mean direction from the Skeena River – Quottoon Inlet collection is nearly concordant with the expected Eocene direction. Mean directions from other transects have clockwise-rotated declinations and inclinations that are shallower than the expected direction by up to 23°. Consistent with expectations for east-side-up tilting of crustal panels, sites from the western part of each transect have mean directions farther from the Eocene expected direction than do sites from the eastern part of the transect. The magnitude of east-side-up tilt varies along strike: the Skeena River to Quottoon Inlet segment experienced little or no tilt; Steamer Passage south of Portland Inlet records 15° tilt; north of Portland Inlet, tilts are 40° for Wales Island and 35° for Filmore Island; farther north, tilt decreases to 12° for Willard Inlet. Tilting was driven by Eocene extension of the Coast Mountains, with tilted crustal panels bounded by northwest-striking, east-side-down normal faults and northeast-striking transfer faults or shear zones. The ACCRETE seismic transect imaged east-dipping reflectors down to ~15 km depth, which likely correspond to the east-side-down normal faults. The most significant of the northeast-striking transfer structures must underlie Portland Inlet. Based on a tilting domino model, about 30% extension is required to produce the 40° tilt of crustal blocks at Wales and Filmore islands.

Late thermal evolution of Proterozoic rocks in the northeastern Llano Uplift, central Texas

The Llano Uplift of central Texas, USA contains large exposures of polydeformed Grenvillian metaigneous and metasedimentary rocks that were intruded by late- to post-orogenic granites. New U–Pb and 40 Ar/ 39 Ar single mineral ages and Rb–Sr mineral isochrons document the retrograde cooling history that followed Grenville-age metamorphism in the north-central and northeastern Llano Uplift, and explain discrepancies between previous U–Pb crystallization ages and younger determinations based on Rb–Sr and K–Ar isotopic systems. Dating has been carried out in the Valley Spring Gneiss (VSG) and the post-tectonic Lone Grove Pluton (LGP) in three areas of the Llano Uplift, from west to east: VSG rocks in the Babyhead area north of Llano, Texas, yielding hornblende, muscovite and biotite 40 Ar/ 39 Ar ages and Rb/Sr mineral isochron ages from ∼1098 to ∼1014 Ma; a sample from the LGP at Petrick Quarry, yielding U–Pb titanite and hornblende and biotite 40 Ar/ 39 Ar ages from ∼1096 to ∼1076 Ma; and VSG samples from the Inks Lake area in the eastern Llano Uplift, yielding U–Pb titanite, hornblende and biotite 40 Ar/ 39 Ar , and Rb-Sr mineral isochron ages from ∼1087 to ∼1029 Ma. Titanite from the Oatman granite to the southwest of these areas yields a U–Pb age of 1119±6 Ma. These ages, combined with closure-temperature estimates, document cooling in the Llano Uplift from ∼1100–1000 Ma at rates between 5 and 14°C Ma −1. Although rapid uplift following early high-pressure metamorphism has important regional thermal effects, the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity. The low-temperature history documented in this study is probably unrelated to uplift rates. This supports the suggestion that magmatic heating was an important factor in the late high-temperature/low-pressure metamorphism of the Llano region. It further suggests that the late pressure and temperature conditions recorded in the rocks were likely not synchronous across the region.

A multielement geochronologic study of the Great Dyke, Zimbabwe: significance of the robust and reset ages

New Sm–Nd, U–Pb, and Pb–Pb age determinations indicate that the Great Dyke of Zimbabwe, an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide, is over 100 Ma older than previously believed based on Rb–Sr ages. The intrusion was emplaced as a series of subchambers with similar stratigraphy, comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite, the top sections of which include Pt-enriched sulfide zones, and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite. The Sm–Nd method has yielded a combined mineral/whole-rock isochron of 2586±16 Ma and ε Nd( t) of +1.1 for samples from the Darwendale, Sebakwe, and Wedza Subchambers as well as the satellite East Dyke. This isochron age is in excellent agreement with the U–Pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the Selukwe Subchamber with an error-weighted mean at 2587±8 Ma. Two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant, and although not well constrained, suggest Pb loss from the zircons at ca. 830 Ma. This may be related to the onset of the widespread and diachronous Pan-African tectonothermal event in southern Africa. Whole–rock samples and clinopyroxene and plagioclase separates from a Darwendale Subchamber drill core yielded a 207Pb/ 204Pb vs. 206Pb/ 204Pb isochron age of 2596±14 Ma, which is in agreement with the Sm–Nd isochron and the rutile U–Pb crystallization age. This new age information shows that emplacement of the Great Dyke and its satellite dikes closely followed the amalgamation of the Kaapvaal and Zimbabwe Cratons, and was contemporaneous with emplacement of the youngest of the trondhjemite–tonalite–granodiorite granitoid suite in the Zimbabwe Craton. Assuming that amalgamation of the Kaapvaal and Zimbabwe Cratons was largely by NNW-directed convergence, it follows that the source of the Great Dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes. Isochrons of 206Pb/ 204Pb vs. 238U/ 204Pb and 207Pb/ 204Pb vs. 235U/ 204Pb yield ages with large errors, but well constrained initial Pb ratios ( 206Pb/ 204Pb = 14.15±0.30 and 207Pb/ 204Pb = 15.04±0.06). Assuming a two-stage model for common lead evolution, this result yields a μ value of 9.5. Along with the calculated initial Sr and Nd isotopic compositions, these data are consistent with derivation of the Great Dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction. While a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the Great Dyke, the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source.

Zircon UPb and hornblende 40Ar 39Ar ages for the Dufek layered mafic intrusion, Antarctica: Implications for the age of the Ferrar large igneous province

New high-precision zircon UPb ages for the Dufek intrusion establish the crystallization age for the only layered mafic intrusion (sensu stricto) known in the Ferrar igneous province, Antarctica, one of the largest igneous provinces in the world. Three concordant zircon fractions from the capping Lexington Granophyre yield an age of 183.9 ± 0.3 Ma (2σ), which we interpret as the crystallization age for the Dufek intrusion. Concordant zircon fractions from a silicic dike crosscutting the upper layered gabbroic sequence yield a slightly younger crystallization age of 182.7 ± 0.4 Ma (2σ) . For both samples, agreement within errors between the two UPb and 207pb∗/ 206Pb∗ ages for three fractions each with a unique set of physical and chemical characteristics demonstrates that inheritance of older zircon components from the country rocks was imperceptible. It also suggests that assimilation or anatexis of the lower Paleozoic country rocks was not the dominant mechanism in the formation of the granophyre. These new UPb dates agree, within analytical errors, with recent zircon and baddeleyite UPb ages for dolerite sills in both the Ferrar and Karoo igneous provinces, which demonstrates the vastness of the rift axis along which simultaneous magmatic activity occurred. Contemporaneity between the Dufek intrusion and other Ferrar Group rocks supports the possibility that this pluton acted as a conduit for magmas of the widely distributed Ferrar sills and flows. Hornblende 40Ar 39Ar ages for the Lexington Granophyre (175 ± 2 Ma) barely overlap with those for a silicic dike (179 ± 2 Ma) in the layered gabbro section just below the granophyre. The younger of the two hornblende ages may be explained by degassing of inclusions or by protracted hydrothermal alteration preventing the ArAr geochronometer form becoming a closed-system in the higher level granophyre until after it had done so in the more deeply positioned silicic dike. The 179 ± 2 Ma age of the Dufek silicic dike is interpreted to represent cooling through the Ar retention temperature for hornblende of about 500°C. Combining this cooling age with the UPb crystallization age of 182.7 ± 0.4 Ma yields a cooling rate for the dike of approximately 100°C/m.y. Inasmuch as this rate is not rapid, it can be inferred that the bulk of the Dufek intrusion was still fairly hot when the dated silicic dike was emplaced.

The Mara Rosa Arch in the Tocantins Province: further evidence for Neoproterozoic crustal accretion in Central Brazil

The Mara Rosa volcano-sedimentary sequence consists of several NNE-trending belts of metavolcanic (metabasalts to metarhyolites) and metasedimentary rocks (micaschists, quartzites, cherts), exposed over large areas of the Tocantins Province, a Neoproterozoic (Brasiliano) orogenic region in the central part of Brazil. The supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post-orogenic granitic and dioritic bodies. These volcano-sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite-greenstone terrain of the Archaean basement. In this paper we present geochronological and isotopic data for rocks of the metavolcanic-metaplutonic rock association. Samples of a felsic metavolcanic rock from the Posse gold mine, and a metatonalite yielded UPb zircon ages of 862±8 Ma and 856 −7 +13 Ma, respectively. These are interpreted as crystallization ages of the igneous protoliths. Titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632±4 Ma. SmNd isotopes for these rocks indicate primitive compositions, with ϵ Nd(T) of +4.6 and +3.7 and T DM model ages of ∼1.0 Ga. A syn-tectonic dioritic intrusion has an UPb crystallization age of 630±6 Ma, and ϵ Nd(t) of +1.9 and TDM of ∼1.0 Ga. Additional SmNd analyses on metasedimentary rocks and post-orogenic granites of the Mara Rosa region yield T DM ages between ∼1.2 and 1.0 Ga. The isotopic data, combined with preliminary trace element results, suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system, off the coast of the São Francisco-Congo continent at around 860 Ma. Recrystallization and deformational ages of ∼630 Ma may represent the timing of final ocean closure and continental collision. The rock associations, structural pattern, geochronological and isotopic characteristics of the rocks from the Mara Rosa region are very similar to those of Neoproterozoic arc terrains exposed ∼ 300 km to the southwest, in the Arenspólis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.

Paleoproterozoic crustal history of the southwestern Grenville Province: evidence from Nd isotopic mapping

Nd isotopic mapping in the North Bay area of the Central Gneiss Belt, southwestern Grenville Province, has revealed the precise trend of a TDM model age line developed between the uplifted southern margin of the Archean Superior craton (TDM = 2.7 Ga) and a Paleoproterozoic allochthon (TDM = 1.9 Ga). Separating these two crustal blocks is a narrow zone of gneisses with intermediate TDM ages. These transitional gneisses are interpreted to reflect a remnant fault or ductile shear zone, of uncertain age, along which crustal material from both blocks mechanically mixed during their juxtaposition. Accordingly, the nature of the TDM line in the North Bay area is interpreted to be tectonic. In the Temiscaming area, widespread exposures of mature metasedimentary gneisses are shown by their TDM ages to be dominantly of Paleoproterozoic provenance. These results are consistent with the existing detrital zircon geochronology, inferring a maximum depositional age of ~1.7 Ga. The anorogenic chemistry of the North Bay orthogneiss and mixed calc-alkaline–alkaline chemistry of the Temiscaming gneisses suggest a connection between Paleoproterozoic anorogenic magmatism and synsedimentary quartzite deposition, which is a common association in 1.9–1.6 Ga accretionary orogens of southern Laurentia. The relatively close correspondence between widespread 1.9 Ga TDM ages and U–Pb crystallization ages as old as 1.74 Ga implies that rocks of the Central Gneiss Belt were originally the juvenile products of Paleoproterozoic orogenesis.

Some isotopic constraints on the evolution of the granulite and upper amphibolite facies terranes in the eastern Musgrave Block, central Australia

The central to eastern Musgrave Block, central Australia, consists of the Meso- to Neoproterozoic Fregon and Mulga Park subdomains that are separated by a zone of mylonites and pseudotachylytes known as the Woodroffe Thrust. The subdomains were previously considered to have undergone different geological histories, but the differences relate mainly to metamorphic grade. The Fregon Subdomain attained at least transitional granulite facies metamorphic conditions and the Mulga Park Subdomain reached upper amphibolite facies. Felsic gneisses from both the Fregon and Mulga Park subdomains contain zircons that are structured with euhedral centres overgrown by round elongate rims. Ion-microprobe UPb analyses of the cores yield ages of 1557±24 and 1554±28 Ma, respectively, and are interpreted to represent the time of igneous crystallization. These ages place a maximum age constraint on the deposition or emplacement of the gneiss precursor. High-grade metamorphism, grading from upper amphibolite to granulite facies, took place between 1200 and 1150 Ma and prior to the intrusion of several granitoid suites at around 1150 Ma. The Kulgera Adamellite and Ayers Ranges Adamellite granitoid suites of the Fregon Subdomain yield ion-microprobe UPb crystallization ages of 1150±12 and 1152±11 Ma, respectively. A concordant UPb isotope dilution analysis on the Ayers Ranges Adamellite yields a more precise age of 1152±3 Ma. The porphyritic suite intruding the Mulga Park Subdomain yield an ion-microprobe UPb magmatic age of 1159±20 Ma. These emplacement ages indicate extensive Grenvillian felsic magmatism not previously recognized in Australia. The Woodroffe Thrust formed during a period of substantial crustal shortening, the Petermann Orogeny (∼550 Ma), when the Fregon Subdomain was thrust onto the Mulga Park Subdomain. Synkinematic muscovite and biotite record KAr and RbSr ages between 560 and 520 Ma. This Petermann event is not recorded in the Fregon Subdomain due to confinement of the strain and associated mineral recrystallization to the underlying Mulga Park Subdomain. We suggest that the granulites were not exposed prior to the Petermann Orogeny, as the basal quartzites of the Amadeus Basin occur only on the amphibolite facies Mulga Park Subdomain.

Metamorphic terranes, isotopic provinces, and implications for crustal growth of the western United States

Simplified maps of the western Cordillera, indicating distribution of metamorphic facies and times of principal recrystallization have been compiled. Despite rearrangement by transcurrent shuffling, general trends are recognizable. Narrow belts of Phanerozoic high‐P blue schist, and serpentinized peridotite lie outboard from broad, penecontemporaneous high‐T metamorphosed calcalkaline magma‐intruded belts bordering the Pacific Ocean, and both are disposed approximately parallel to the present‐day North American margin; they represent recrystallized products of mid‐Paleozoic and younger subduction zones, Andean‐type margins, and exotic island arcs. In contrast, dominant metamorphic mineral parageneses within the continental interior are spatially associated with chiefly mesozonal granitic plutons. Glaucophane schists, eclogites, and large tracts of ophiolitic ultramafics are absent, in part due to selective overprinting of old accretionary margin lithotectonic assemblages by subsequent orogenies, high‐T recrystallization, and anatexis. For example, at depth within the California Coast Ranges, high‐P phases are currently being destroyed; farther inland, a general paucity of oceanic petrotectonic assemblages is evident, possibly reflecting the differential sinking of dense basaltic/peridotitic units during thermal softening. Preserved metamorphic field gradients reflect the magnitude of subsequent uplift and erosion as well as original PT conditions. Late Mesozoic to mid‐Cenozoic recrystallization accompanied earlier crustal thickening + ductile deformation, succeeded by later extension + brittle faulting + mylonitization; Mesozoic‐Cenozoic metamorphism involved cratonal rocks in the Mojave‐Sonoran Desert, but in Nevada and to the north, eugeoclinal sections were recrystallized. Metamorphic intensity decreases eastward toward the Phanerozoic hinge line. Basement rocks in the Rockies display the effects of shallow level crustal deformation but much less widespread evidence of post‐Proterozoic plutonism and regional metamorphism. Initial Pb, Nd, and Sr isotopic compositions and crystallization ages of igneous rocks, in comparison with times of intrusion, recrystallization, and metamorphic facies distributions are compatible with a geologic scenario involving gradual, proximal growth of the ancient continent chiefly southward from the Archean Wyoming craton during early and mid‐Proterozoic time, followed by late Proterozoic‐Phanerozoic westward development. Accretion involved the incorporation of some exotic, mostly oceanic, outboard terranes. However, continental growth in the western U.S. Cordillera during both Proterozoic and Phanerozoic time periods appears to have been due chiefly to partial fusion and magma ascent above subducting paleo‐Pacific lithospheric plates, combined with metamorphic (+ sedimentary) reworking in the forearc + trench + back arc setting, rather than resulting from the amalgamation of preexisting ancient continental fragments.