Jeffrey M. Amato

Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy

Abstract Estimation of burial rates and duration of prograde metamorphism of ultra-high-pressure (UHP) rocks (T=590–630°C, P=2.7–2.9 GPa) of the Zermatt–Saas ophiolite from Lago di Cignana, Italy, may be made through combined Lu–Hf and Sm–Nd garnet geochronology in conjunction with petrologic estimates of the prograde P–T path. We report a Lu–Hf garnet–omphacite–whole-rock isochron age of 48.8±2.1 Ma from the UHP locality at Lago di Cignana, which stands in contrast to the Sm–Nd age of 40.6±2.6 Ma [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438] obtained from the same sample and mineral material. The Sm–Nd and Lu–Hf ages, as well as other ages determined on metamorphic garnet, zircon and white mica [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438; Mayer et al., Eur. Union Geosci. 10 (1999) Abstr. 809; Rubatto et al., Contrib. Mineral. Petrol. 132 (1998) 269–287; Dal Piaz et al., Int. J. Earth Sci. 90 (2001) 668–684] from Lago di Cignana and elsewhere in the Zermatt–Saas ophiolite, lie within a range of ∼50–38 Ma, which we interpret to encompass the duration of prograde metamorphism, and possibly the duration of garnet growth. The difference in measured Sm–Nd and Lu–Hf ages from Cignana can be accounted for by expected core to rim variations in Lu, Hf, Sm, and Nd. The measured yttrium content in garnet, which may be a proxy for Lu, is highest in garnet core and lowest in the mineral rim, generally following a profile that is predicted by Rayleigh fractionation. Preferential enrichment of Lu in the core produces a Lu–Hf age that is weighted toward the older garnet core. Sm–Nd ages, as predicted by Rayleigh fractionation of Sm and Nd during garnet growth, however, reflect later grown garnet as compared to Lu–Hf ages. The difference in Sm–Nd and Lu–Hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as Sm–Nd and Lu–Hf blocking temperatures were not exceeded for a long period of time. Based on the 12 Myr duration of prograde garnet growth estimated in this study, burial rates for rocks at Lago di Cignana were on the order of 0.23–0.47 cm/yr. These values correlate with continuous shortening rates of 0.4–1.4 cm/yr between the European plate and the African–Adriatic promontory between 50 and 38 Ma, which is on the order of that calculated for plate velocities from plate reconstructions, suggesting that the Zermatt–Saas ophiolite may have remained well-coupled to the down-going slab to UHP conditions.

Paleozoic rocks of northern Chukotka Peninsula, Russian Far East: Implications for the tectonics of the Arctic region

Paleozoic rocks exposed across the northern flank of the mid-Cretaceous to Late Cretaceous Koolen metamorphic dome make up two structurally superimposed tectonic units: (1) weakly deformed Ordovician to Lower Devonian shallow marine carbonates of the Chegitun unit which formed on a stable shelf and (2) strongly deformed and metamorphosed Devonian to Lower Carboniferous phyllites, limestones, and andesite tuffs of the Tanatap unit. Trace element geochemistry, Nd isotopic data, and textural evidence suggest that the Tanatap tuffs are differentiated calc-alkaline volcanic rocks possibly derived from a magmatic arc. We interpret the associated sedimentary facies as indicative of deposition in a basinal setting, probably a back arc basin. Orthogneisses in the core of the Koolen dome yielded a Devonian (between ∼369 and ∼375 Ma) U-Pb zircon age which is similar to the ages of the Tanatap tuffs as well as granitic plutons formed within a Devonian active continental margin of northern Alaska. The stratigraphy of the Chegitun unit is similar to that of the Novosibirsk carbonate platform which overlies the Late Precambrian Bennett-Barrovia block. The basement of the block is exposed in Chukotka where ortogneiss in the Chegitun River valley yielded Late Proterozoic (∼650 to 550 Ma) U-Pb ages. These two tectonic units form the shelf of the Chukchi and East Siberian Seas and may continue into northern Alaska as the Hammond subterrane. The deep-water Tanatap unit can be traced along the southern boundary of the Bennett-Barrovia block from the Novosibirsk Islands to northern Alaska This basin was paired with a Devonian magmatic arc that existed farther to the south. The northern margin of the Bennett-Barrovia block collided with North America in the Late Silurian to Early Devonian. In Chukotka, during Middle to Late Carboniferous time the reconstructed Devonian arc-trench system at the southern edge of the Bennett-Barrovia block collided with an unknown continental object, fragments of which now occur to the south of the South Anyui suture. Triassic to Cretaceous deformation strongly modified the Paleozoic units. Our results provide new constraints on the geometry and Paleozoic history of the Chukotka-Arctic Alaska block, the essential element involved in the opening of the Canada basin.

Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska–Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska–Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700–540 Ma, with prominent peaks at 720–660 Ma, 620–590 Ma, 560–510 Ma, 485 Ma, and 440–400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic–Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska–Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0–1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska–Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska–Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin.

Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico

New U-Pb zircon ages, geochemistry, and Nd isotopic data are presented from three localities in the Paleoproterozoic Mazatzal province of southern New Mexico, United States. These data help in understanding the source regions and tectonic setting of magmatism from 1680 to 1620 Ma, the timing of the Mazatzal orogeny, the nature of postorogenic magmatism, Proterozoic plate tectonics, and provide a link between Mazatzal subblocks in Arizona and northern New Mexico. The data indicate a period from 1680 to 1650 Ma in which juvenile felsic granitoids were formed, and a later event between 1646 and 1633 Ma, when these rocks were deformed together with sedimentary rocks. No evidence of pre-1680 Ma rocks or inherited zircons was observed. The igneous rocks have e Nd (t) from –1.2 to +4.3 with most between +2 and +4, suggesting a mantle source or derivation from similar-aged crust. Nd isotope and trace element concentrations are consistent with models for typical arc magmatism. Detrital zircon ages from metasedimentary rocks indicate that sedimentation occurred until at least 1646 Ma. Both local and Yavapai province sources contributed to the detritus. All of the samples older than ca. 1650 Ma are deformed, whereas undeformed porphyroblasts were found in the contact aureole of a previously dated 1633 Ma gabbro. Regionally, the Mazatzal orogeny occurred mainly between 1654 and 1643 Ma, during fi nal accretion of a series of island arcs and intervening basins that may have amalgamated offshore. Rhyolite magmatism in the southern Mazatzal province was coeval with gabbro intrusions at 1633 Ma and this bimodal magmatism may have been related to extensional processes following arc accretion.

Detrital zircon ages from the Chugach terrane, southern Alaska, reveal multiple episodes of accretion and erosion in a subduction complex

Detrital zircon U-Pb ages from the McHugh Complex reveal the Mesozoic history of the Chugach terrane accretionary complex in southern Alaska (United States). The majority of the eight samples of graywacke and argillite from the melange have youngest peaks that are also the dominant peaks on probability distribution diagrams. The youngest groups of zircons are 157 Ma and 146 Ma in two samples and range from 91 to 84 Ma in six samples that also have Jurassic zircons. Minor Proterozoic zircons are present only in samples with Late Cretaceous zircons. A trondhjemite that intrudes the McHugh Complex has a U-Pb zircon age of 120 ± 3 Ma. We show that the McHugh Complex consists of two packages, a mesomelange of argillite and chert that accreted during the Late Jurassic–Early Cretaceous and a graywacke-conglomerate assemblage that accreted during the Late Cretaceous. We propose that either the Talkeetna arc or Chitina arc in southern Alaska was the principal sedimentary source for the older McHugh Complex, whereas volcanic rocks from the Coast orogen of British Columbia were a probable source for the Late Cretaceous McHugh Complex and the Valdez Group flysch. The age gap between the two packages corresponds to a ridge subduction event recorded by forearc trondhjemite magmatism and regional uplift at 125–120 Ma that likely triggered subduction erosion. Our data suggest that the youngest detrital zircon ages can approximate the time of accretion in subduction complexes because arc detritus is added to the downgoing plate near the trench and is subsequently rapidly accreted.

Provenance of Upper Cretaceous–Paleogene sandstones in the foreland basin system of the Sierra Madre Oriental, northeastern Mexico, and its bearing on fluvial dispersal systems of the Mexican Laramide Province

Sandstone petrography and detrital zircon U-Pb analysis of Upper Cretaceous–Paleogene sandstones in the foreland basin of the Sierra Madre Oriental in northeastern Mexico indicate long-distance sediment transport from arc, basement, and thrust-belt sources lying to the west, northwest, and south. The basin fill, termed the Difunta Group, consists of sublitharenites, litharenites, feldspathic litharenites, and lithic arkoses derived from mixed sources that included sedimentary rocks, magmatic arc rocks, and subordinate basement rocks. Six age populations comprise the detrital zircon content of the sandstones: Proterozoic (1900–900 Ma), early Paleozoic (500–400 Ma), late Paleozoic–Early Triassic (288–235 Ma), Jurassic (180–151 Ma), Early Cretaceous (150–111 Ma), and Late Cretaceous–Paleogene (110–54 Ma). These grains were derived from several arc terranes, ranging in age from Permian to Paleogene, in western Mexico and the southwestern United States, from sedimentary rocks and possibly interbedded tuffs of the Sierra Madre Oriental orogen and from basement sources or their derivative sandstones of the southwestern United States. The petrographic and geochronologic provenance data corroborate existing models for derivation of much foreland detritus from arc sources to the west, identify the Sierra Madre orogen itself as an important source for sediment, and these data modify the Late Cretaceous–Paleogene paleogeography of Mexico to include a long, orogen-parallel fluvial system with headwaters in the southwestern United States. The difference in average ages of the youngest grains in the sandstones and their inferred depositional ages is 10.5 m.y., indicating that the initial coarse fill of the foreland basin was derived from early Laramide uplift and eastward arc migration in northwestern Mexico and the southwestern United States.

Arc–continent collision and the formation of continental crust: a new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

Abstract: Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earths history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian–Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U–Pb zircon dating yields ages of 493 ± 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 ± 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc–continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian–Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc–continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.

Cretaceous to Recent extension in the Bering Strait region, Alaska

A key issue presented by the geology of northern Alaska concerns the demise of the Brooks Range going west toward the Bering Strait region. The main Brookian tectonic and stratigraphic elements continue into the Russian Far East, but the thick crustal root and high elevations that define the modern physiographic Brooks Range die out approaching the Bering and Chukchi shelves, which form an unusually broad area of submerged continental crust. Structural, geochronologic, and apatite fission-track data indicate that at least three episodes of extension may have affected the crust beneath the Bering Strait region, in the middle to Late Cretaceous, Eocene-early Oligocene, and Pliocene(?)-Recent. This extension may explain the present thinner crust of the region, the formation of extensive continental shelves, and the dismemberment and southward translation of tectonic elements as they are traced from the Brooks Range toward Russia. Evidence for these events is recorded within a gently tilted 10- to 15-km thick crustal section exposed on the western Seward Peninsula. The earliest episode is documented at high structural levels by the postcollision exhumation history of blueschists. Structural data indicate exhumation was accomplished in part by thinning of the crust during north-south extension bracketed between 120 and 90 Ma by 40Ar/39Ar and U-Pb ages. The Kigluaik Mountains gneiss dome rose through the crust during the later stages of this extension at 91 Ma. Similar gneiss domes occur within a broad, discontinuous belt of Cretaceous magmatism linking interior Alaska with northeast Russia; mantle-derived melts within this belt likely heated the crust and facilitated extension. Apatite fission-track ages indicate cooling below ≈120–85°C occurred sometime between 100 and 70 Ma, and the area subsequently resided at shallow crustal depths (<3–4 km) until the present. This suggests that denudation of deep levels of the crust by erosion and/or tectonism was mostly completed by the Late Cretaceous and thus that the present-day thinner crust of the Bering Strait region developed primarily in the Cretaceous. Regional tilting and at least several more kilometers of local erosion followed in Eocene-early Oligocene time as documented by fission-track ages from deeper levels of the crustal section exposed in the Kigluaik Mountains. This event is generally coeval with development of the offshore transtensional Hope and Norton Basins which flank the Seward Peninsula to the north and south. Modern seismicity, active normal faults, and basin-range topography document Pliocene(?) to Recent north–south extension across the region. Fission-track data indicate that exhumation during this period was quite limited, less than 2–3 km. This inferred history of protracted extension in the Bering Strait region stands in sharp contrast to well-documented Cretaceous and Tertiary north–south shortening in interior Alaska. This contrast in tectonic histories suggests a model in which contraction and westward extrusion of crustal fragments from interior Alaska by strike-slip faulting were accommodated by north–south extension in the Bering Strait region. This resulted in the counterclockwise rotation of extruded crustal blocks, the extensional thinning of the western part of the Brooks Range crustal root, and the formation of transtensional basins and unusually broad continental shelves between Alaska and Russia.

Magmatically induced metamorphism and deformation in the Kigluaik gneiss dome, Seward Peninsula, Alaska

Field observations and U-Pb isotopic data from plutonic and high-grade metamorphic rocks within the Kigluaik gneiss dome on the Seward Peninsula, Alaska, document a Late Cretaceous age of peak metamorphism and shed light on the relationship between fundamentally mantle-derived magmatism and the development of the gneiss dome. The dome consists of upper-amphibolite-facies to granulite-facies metasedimentary rocks mantling the granitoid Kigluaik pluton. The main deformational fabric in the dome is a well-defined, moderately dipping foliation and compositional layering that contains a pervasive, east-west trending mineral-elongation lineation defined by sillimanite and hornblende. Leucosomal segregations in migmatite are boudinaged and isoclinally folded, and late-stage tension gashes are filled with melt, demonstrating that most deformation occurred at peak metamorphic temperatures. The large pluton in the core of the dome consists of a granitic cap overlying a mafic to intermediate root. Mafic pillows with crenulate margins and spectacular magma mingling textures indicate that the two magmas were coeval. The Kigluaik pluton is largely undeformed and discordant, although some dikes possess a weak deformational fabric. The lack of quench textures at the margins of the pluton and very limited alteration in adjacent wall rock suggest that the pluton was emplaced while country rocks were still at high temperatures. U-Pb analyses of three fractions of zircon from a highly strained and concordant garnet-bearing granite orthogneiss yield an intrusive age of 105 Ma; therefore significant deformation in the dome must have occurred after 105 Ma. Several U-Pb analyses of monazite from metapelite and pegmatite (derived from partial melting of metapelite) yield an age of 91 ± 1 Ma for high-temperature metamorphism and deformation. U-Pb analysis of eight fractions of zircon from the Kigluaik pluton shows that it crystallized at 92 ± 2 Ma. Thus there is both field and geochronologic evidence for coeval magmatism, metamorphism, and deformation at about 92 Ma, which may represent the latest stages of a more protracted tectonic event. We present a model for gneiss dome development wherein a large, silicic to intermediate magmatic diapir with its mantling gneisses ascended from ∼35 km to ∼15 km during an event associated with crustal extension in northern Alaska. We emphasize the close link between plutonism and gneiss dome development and the fundamentally mafic character of this plutonism.

Tectonic evolution of the Mesozoic South Anyui suture zone, eastern Russia: A critical component of paleogeographic reconstructions of the Arctic region

The South Anyui suture zone consists of late Paleozoic–Jurassic ultramafic rocks and Jurassic–Cretaceous pre-, syn-, and postcollisional sedimentary rocks. It represents the closure of a Mesozoic ocean basin that separated two microcontinents in northeastern Russia, the Kolyma-Omolon block and the Chukotka block. In order to understand the geologic history and improve our understanding of Mesozoic paleogeography of the Arctic region, we obtained U-Pb ages on pre- and postcollisional igneous rocks and detrital zircons from sandstone in the suture zone. We identified four groups of sedimentary rocks: (1) Triassic sandstone deposited on the southern margin of Chukotka; (2) Middle Jurassic volcanogenic sandstone that was derived from the Oloy arc, a continental margin arc, along the Kolyma-Omolon block, south of the Anyui Ocean, a sample of which yielded no pre-Jurassic zircons and a single peak at 164 Ma; (3) suture zone sandstone that yielded Late Jurassic maximum depositional ages and likely predated the collision; and (4) a Mid-Cretaceous syncollisional sandstone that had a maximum depositional age of 125 Ma. These rocks were intruded by postkinematic plutons and dikes with ages of 109 Ma and 101 Ma that postdate the collision. We present a seismic-reflection line through the South Anyui suture zone that indicates south-vergence of thrusting of the Chukotka block over the Kolyma-Omolon block, opposite of most existing models and opposite of the vergence in the Angayucham suture zone, the postulated along-strike equivalent in Alaska. This suggests that Chukotka and Arctic Alaska may have different pre-Cretaceous histories, which could solve space problems with existing reconstructions of the Arctic region. We combine our detrital zircon data and interpretations of the seismic line to construct a new GPlates model for the Mesozoic evolution of the region that decouples Chukotka and Arctic Alaska to solve space problems with previous Arctic reconstructions.