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1 Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA
2 Department of Geology and Geography, West Virginia University, Morgantown, West Virginia 26506, USA
3 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA
4 Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
5 Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA
6 U. S. Geological Survey, Anchorage, Alaska 99508, USA
Correspondence: E-mail: amato{at}nmsu.edu
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FOOTNOTES |
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ABSTRACT |
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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.
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INTRODUCTION |
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Most of the previous work on Neoproterozoic paleogeography has focused on data from the current circum-Atlantic continents and the paleo-Pacific realm. Continental rocks in northern Alaska and northeastern Russia make up the Arctic Alaska–Chukotka terrane (e.g., Moore et al., 1994). This terrane, including continental shelves, has an area of approximately 3,000,000 km2, or 85% of the area of Greenland. The geology of this region is so poorly known that, despite its size, it does not even appear on most reconstructions (e.g., Li et al., 2008). The Arctic Alaska–Chukotka terrane may be an important paleogeographic link between other, larger continental masses such as Laurentia, Baltica, and Siberia.
We report new U-Pb ages from nine igneous rocks on Seward Peninsula, Alaska, ranging in age from Neoproterozoic to Devonian. Nd isotopic data from several of these samples are used to evaluate their origin and to assess the possibility that older, unexposed crust may be present in the Arctic Alaska–Chukotka terrane. Detrital zircon data from 11 metamorphosed siliciclastic rocks are presented. All of these data are used to evaluate the Neoproterozoic–Paleozoic history of the Arctic Alaska–Chukotka terrane and other constituents of Rodinia, and we discuss the implications of these new data for Mesozoic reconstructions of the Amerasian basin.
We confirm that the Arctic Alaska–Chukotka terrane is exotic to western Laurentia and test affinities to other areas with a history of Neoproterozoic magmatism and published Neoproterozoic detrital zircon populations in overlying sedimentary rocks. The Neoproterozoic magmatic ages of Seward Peninsula basement rocks 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. Of these options, we believe that in the Neoproterozoic, the Arctic Alaska–Chukotka terrane occupied a position near Baltica, northeast of Laurentia, in an arc-system along strike with the Avalonian-Cadomian arc.
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GEOLOGIC BACKGROUND |
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In the Brooks Range, the Lower Paleozoic carbonate platform was succeeded by lithologically diverse sequences of mixed carbonate, siliciclastic, and minor volcanic rocks that likely record foundering of the platform during the latter part of the Middle Devonian (Moore et al., 1997; Dumoulin et al., 2002). The felsic volcanic rocks in these sequences are coeval with large 390 Ma granitic orthogneisses common in the southern Brooks Range and to small bodies on the Seward Peninsula (Aleinikoff et al., 1993). Upper Devonian (380–375 Ma; McClelland, 2006) bimodal volcanic rocks of the southern Brooks Range may also record rifting of part of the platform. Devonian intrusive rocks of this age are also found in Chukotka (Cecile et al., 1991; Natal'in et al., 1999). Over a large area in the central and western Brooks Range, Upper Devonian–Lower Mississippian siliciclastic rocks unconformably overlie the mixed carbonate and siliciclastic strata.
The pre-Mississippian unconformity of the North Slope is overlain by a thick passive-margin succession, known as the Ellesmerian sequence, which is also present in the north-vergent thrust sheets of the Brooks Range. In Chukotka, thick and widespread Triassic turbidite deposits dominate the stratigraphic section but are underlain by Devonian clastic rocks and a Carboniferous to Permian platformal carbonate succession. Across the Arctic Alaska–Chukotka terrane, there was a transition in Late Jurassic to Early Cretaceous time to syntectonic deposition associated with the onset of subduction or the collision of island arcs against the southern margin of the Arctic Alaska–Chukotka terrane. Also, the Arctic basin began to open in the Early Cretaceous, separating the Arctic Alaska–Chukotka terrane from its original location. The exact configuration of the opening of the Arctic is controversial (e.g., Lawver et al., 2002; Miller et al., 2006), but there is strong evidence that the North Slope block rifted from the Canadian Arctic (Grantz et al., 1990; Mickey et al., 2002). Detrital zircon studies place Chukotka close to Siberia before opening of the Arctic basin (Miller et al., 2006).
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SEDIMENTARY ROCKS OF SEWARD PENINSULA |
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The Nome Group consists mainly of greenschist-facies and blueschist-facies marble, calcareous schists, metapelitic rocks, and metabasites that have been divided into map units (Fig. 3) based on their dominant lithology (Till et al., 1986). Several of the lithologic subunits have unknown depositional ages. Their inferred ages were based on field relationships with adjacent units with fossils, predominantly conodonts (Till et al., 1986), but the original depositional relationships have been obscured by intense deformation. The fossiliferous units of Till et al. (1986) include a dolostone inferred to be Cambrian based on the presence of lapworthellids. The "Mixed Unit" includes predominantly marble and graphitic quartzite and has Ordovician conodonts near the structural top of the unit. Dolostone units of Silurian and Ordovician age and an Ordovician impure chloritic marble are also present. A Devonian marble and dolostone unit is the youngest fossiliferous part of the Nome Group. The ages of unfossiliferous units are speculative. These include the Solomon schist, an unfossiliferous quartzose pelitic schist previously inferred to be Cambrian or Precambrian (Till et al., 1986) but which contains Devonian zircons (Till et al., 2006), and the Casadepaga schist, a chlorite-albite schist unit that is intruded by the Devonian Kiwalik orthogneiss and that was previously assumed to be Ordovician because it is generally found between carbonate units with Ordovician fossils (Till et al., 1986).
The third group of metasedimentary rocks is the Kigluaik Group, originally defined by Moffit (1913), which consists of upper-amphibolite-facies metasedimentary rocks with mafic and felsic orthogneisses and minor ultramafic rocks. The usage of the term "Group" does not comply with the recommendations of the International Commission on Stratigraphy, as the Kigluaik Group was never formally subdivided into mappable formations. Therefore, we propose the term "Kigluaik metamorphic complex" as a substitute and include all of the high-grade metasedimentary rocks of the Kigluaik, Bendeleben, and Darby Mountains within this category. The ages of the protoliths are poorly known because no fossils are present. The protolith compositions are similar to those in the structurally overlying Nome Group and likely represent a higher metamorphic grade equivalent of the Nome Group protoliths (Till and Dumoulin, 1994).
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METHODS AND DATA PRESENTATION |
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analytical uncertainties greater than 10% of the age are also omitted. All errors quoted in the text are at the 2 level. Data are presented both on conventional concordia diagrams and on relative probability diagrams (Ludwig, 2003). Significant peaks on the probability diagrams consist of three or more analyses with the same age. Igneous sample BM2 was also dated using LA-MC-ICP-MS, and detrital sample KM11 has both SHRIMP and LA-MC-ICP-MS analyses. Full details of the analytical techniques and concordia diagrams can be found in the GSA Data Repository.1 Table 1 is a summary of samples analyzed and their localities, and Tables DR1 and DR2 (see footnote 1) contain the complete analytical data set for igneous and sedimentary rocks. Sample names were simplified in the tables and figures, but both simplified and original sample numbers are given in Table 1.
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U-Pb AGES FROM PROTEROZOIC-PALEOZOIC IGNEOUS ROCKS |
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Neoproterozoic Metavolcanic Rocks
The oldest igneous rocks in this study were collected from a 50-m-thick unit of foliated, fine-grained igneous rocks in the Bendeleben Mountains. The foliation is conformable to the surrounding metasedimentary rocks. We interpret these rocks as metaigneous, based on their bulk composition, and more specifically as metavolcanic, based on the average grain size of quartz and plagioclase of 250 µm. The average grain size in the younger orthogneisses, which have a more clearly intrusive origin, is several millimeters to centimeters in length. As these rocks were metamorphosed to upper-amphibolite facies (sillimanite + K-feldspar), we attribute the presence of titanite in the rock to metamorphic processes. The zircon morphology indicates igneous growth with no evidence for rounding in a sedimentary environment.
We dated three samples from this unit (Fig. 4). Sample BM1 is a leucocratic metarhyolite with major-element concentrations of SiO2 = 73% and Al2O3 = 14% (Werdon et al., 2005). Its mineralogy includes equant quartz, plagioclase, potassium feldspar, minor biotite, and minor Fe-Ti oxides. The zircons from sample BM1 are euhedral and 100–150 µm in length with an aspect ratio of 2:1. Zonation varies from sector zoning to homogeneous cores transitioning into weak oscillatory zonation. Some grains indicate minor resorption, and most cores are surrounded by a <10-µm-thick high-U rim. Eight zircons from sample BM1 yielded a weighted mean 238U/206Pb age of 870 ± 7 Ma and a weighted mean 207Pb/206Pb age of 873 ± 23 Ma. Sample BM2 is a similar felsic metavolcanic rock. Several of the 19 zircons analyzed were slightly discordant; the upper intercept on a Tera-Wasserburg concordia diagram yields an age of 880 ± 28 Ma, and the eight most concordant analyses yield a mean 238U/206Pb age of 868 ± 12 Ma; all of the analyses yield a weighted mean 207Pb/206Pb age of 875 ± 19 Ma. Sample BM3 is intermediate in composition and contains quartz, plagioclase, hornblende, and biotite. Of the ten zircons analyzed from sample BM3, the weighted mean 207Pb/206Pb age is 848 ± 24 Ma. The oldest grain has a 238U/206Pb age of 868 ± 7 Ma, and the intercept of all of the data from this sample on a Tera-Wasserburg concordia diagram is 878 ± 62 Ma, but the other nine grains ranged from 832 to 614 Ma, suggesting varying amounts of Pb loss. We interpret all three samples as forming in the same igneous event ca. 870 Ma.
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Neoproterozoic Thompson Creek Orthogneiss
The Thompson Creek orthogneiss in the Kigluaik Mountains is the largest volume orthogneiss known from Seward Peninsula. It is interpreted as having an intrusive protolith based on its coarse-grained augen gneissic texture. This unit is clearly deformed along with the surrounding sedimentary rocks, and map relationships suggest, but do not conclusively demonstrate, an original crosscutting relationship (Amato and Miller, 2004). This rock was previously dated using thermal ionization mass spectrometry (TIMS) with an upper concordia intercept of 555 Ma (Amato and Wright, 1998). Recent cathodoluminescence imaging of another sample from this unit (KM3) shows high-U tips growing on older cores, and SHRIMP dating of these zircons indicates that the three oldest cores have a weighted mean 238U/206Pb age of 565 ± 6 Ma (Fig. 5) and metamorphic tip ages averaging 92 ± 1 Ma. Sample KM4 is from a granitic orthogneiss in the northern Kigluaik Mountains. The zircons have 238U/206Pb ages younger than the 207Pb/206Pb ages, indicating Pb loss, but the zircon with the oldest 238U/206Pb age is 565 ± 4 Ma, coeval with the sample from the southern exposures of the Thompson Creek orthogneiss (Table DR1, see footnote 1).
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Devonian Metaplutonic Rocks
The youngest Paleozoic igneous rocks on the Seward Peninsula are Devonian, and together with coeval rocks in the Brooks Range, they represent the last significant episode of magmatic activity in the Arctic Alaska–Chukotka terrane until the mid-Cretaceous intrusive events. The Kiwalik orthogneiss and an adjacent felsic metavolcanic rock from northeast of the Darby Mountains both have an age of 391 Ma (Till et al., 2006). Two samples from this study have Devonian ages (Fig. 5): (1) a felsic orthogneiss from the Darby Mountains (sample DM1) yielded a weighted mean 238U/206Pb age of 390 ± 4 Ma; and (2) a foliated granite from the Bendeleben Mountains (sample BM4) has a range of 238U/206Pb ages between 403 and 378 Ma, indicating complexity but suggesting a Devonian age.
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Nd ISOTOPIC RESULTS |
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Nd values at 90 Ma to determine their influence on the petrogenesis of Cretaceous magmatism in the Kigluaik Mountains. We revisit these data to calculate their initial Nd(t) and model ages and report new data from five samples (Table 2). A discussion of the analytical techniques is presented in the GSA Data Repository (see footnote 1) and outlined in Farmer et al. (1991).
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Nd(t) value of the 870 Ma metavolcanic rock from the Bendeleben Mountains (sample BM1) is +0.14 (Fig. 6), which is slightly above the "bulk earth" or chondritic uniform reservoir (CHUR) value (e.g., DePaolo and Wasserburg, 1976). Three of the 680 Ma orthogneisses were dated: two from the Kigluaik Mountains (samples KM1b at 687 Ma and KM2b at 663 Ma) and one from the Cape Nome orthogneiss (sample CN1 at 680 Ma) near Nome. These three have Nd(t) values that range from ~ –4 to –2.5. The Thompson Creek orthogneiss was sampled from both the main body to the south and from probable equivalent units to the north, only one of which (sample KM4 at 565 Ma) has been dated. These rocks have a range of Nd(t) values: the highest is +1.4, the lowest –2.7, but most are between +0.3 and –1. These variations could reflect variable contributions from older, more evolved crust during its evolution. The Devonian foliated granite from the Bendeleben Mountains (sample BM4 at 390 Ma) has an Nd(t) value of –0.4. High-grade metapelitic rocks in the Kigluaik Mountains similar to samples from this study yielded Nd(t) values between –9 and –11, calculated at an inferred depositional age of Early Ordovician (485 Ma).
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DETRITAL ZIRCON U-Pb GEOCHRONOLOGY RESULTS |
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The data from the 11 samples fall into three distinct groups of detrital zircon ages, plotted as relative distribution plots (Fig. 7A): Group 1 samples have youngest grains of Cambrian or older age and have abundant zircons that are Neoproterozoic (late Cryogenian–Ediacaran) from 720 to 540 Ma; group 2 includes samples with youngest grains that are Ordovician in age, including a peak at 484 Ma; and group 3 samples have youngest grains that are Devonian or Carboniferous and have abundant Silurian and/or Devonian grains. Groups 1 and 2 have very similar patterns for the detrital ages of 520 Ma and older, but the group 2 samples do not have the prominent peak at 680 Ma that most of the group 1 samples have. The group 3 samples have a strong Silurian peak but have very few zircons older than Ordovician.
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DISCUSSION |
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Age of Arctic Alaska Terrane Basement
A compilation of published ages from pre-Mesozoic igneous rocks in northern Alaska and northeastern Russia plotted on a circum-Arctic map serves as a starting point for the discussion of the original relationship of Arctic Alaska–Chukotka to other circum-Arctic terranes (Table DR3 [see footnote 1]; Fig. 8). Evidence for Neoproterozoic crust comes from U-Pb dates on Neoproterozoic igneous rocks from this and other studies. The oldest exposed orthogneiss in the Arctic Alaska–Chukotka terrane is 971 Ma in the southern Brooks Range (McClelland, 2006). The oldest rocks directly dated on Seward Peninsula are the group of three 870 Ma fine-grained metavolcanic rocks in the Bendeleben Mountains (samples BM1, BM2, BM3). Extensive mapping in the Kigluaik Mountains (Amato and Miller, 2004) has not revealed any similar unit, and the overall volume of this unit is fairly small, being limited to several outcrops in the southwestern Bendeleben Mountains. The field relationships indicate that this unit is concordant with the regional foliation in the metasedimentary rocks. We interpret this unit as a fragment of an older volcano-sedimentary sequence that was folded into the metasedimentary rocks prior to metamorphism and deformation in the Cretaceous. The ca. 680 Ma orthogneisses on Seward Peninsula (samples KM1, KM2) and ca. 700–630 Ma orthogneisses elsewhere in the Arctic Alaska–Chukotka terrane (Table DR3, see footnote 1) are similarly interpreted as forming part of the basement. The next youngest basement rocks include the granitic Thompson Creek orthogneiss at 565 Ma (samples KM3, KM4) and the York Mountains gabbro (sample YM1) at 539 Ma. The uncertainties on each age determination allow for a gap as short as 14 m.y., suggesting they may be part of the same magmatic event. The presence of both granite and gabbro during the same episode of magmatism suggests bimodal magmatism possibly associated with crustal extension. The Devonian igneous rocks (samples BM4, DM1) are younger than many of the fossiliferous Nome Group units and therefore are not considered part of the basement assemblage (Fig. 3).
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Depositional Setting of Seward Peninsula Metasedimentary Rocks
The depositional setting for at least some of the Paleozoic sedimentary and metasedimentary rocks on Seward Peninsula is known from relatively unmetamorphosed exposures in the York Mountains and, based on these rocks, can be inferred from some of the metamorphosed rocks in the Nome Group and Kigluaik metamorphic complex. The York Slate (sample YM2) is an immature, lithic-rich, turbiditic sandstone likely associated with an arc sequence. The inferred Cambrian-age strata of the Nome Group were deposited in shallow water (Till et al., 1986; Dumoulin et al., 2002), and the Ordovician strata were deposited in inner- and middle-shelf environments. Ordovician rifting is inferred from undated tholeiitic metagabbros associated with Ordovician metasedimentary rocks on central and eastern Seward Peninsula (Till and Dumoulin, 1994). Devonian magmatism also suggests another possible rifting event synchronous with deposition.
The four samples from the Casadepaga schist (KM5–KM8) have minor calcite, suggesting a carbonate contribution, but they also contain abundant albite and chlorite, which provide evidence for mafic volcanic detritus in these samples and in sample KM9 from the Mixed Unit of the Nome Group. Higher-grade equivalent rocks such as sample BM5 from the Bendeleben Mountains contain plagioclase and actinolite, also suggesting either mafic volcanic input or erosion of a mafic source. High-grade schists (samples KM10, KM11, BM6, BM7) with garnet and sillimanite probably had an exclusively pelitic protolith. The sample with the youngest, Pennsylvanian zircons (sample KM12) is a graphitic quartzite, suggesting abundant organic material was present. Meter-thick pure graphite layers on the north flank of the Kigluaik Mountains (Harrington, 1919) and the nearly ubiquitous presence of graphite in metapelitic schists on Seward Peninsula, including our sample with the youngest zircons (Pennsylvanian, sample KM12), indicate that organic material was abundant, although some of this graphite could be have been formed during younger hydrothermal events.
The presence of both carbonate and siliciclastic rocks of Paleozoic age in the Nome Group can be explained in two ways. First, the two lithologic types could have been deposited in the same basin, and the variations in lithology could have formed in response to changes in sea level, tectonic setting, or source regions over time. Alternatively, the two lithologies could have been coeval but deposited in different depocenters, where the carbonates were associated with a shallow platform and the siliciclastic rocks represent a deeper basinal siliciclastic facies. These two lithologies could have been tectonically intercalated during Mesozoic deformation. We prefer the latter interpretation, as it is a simpler way to explain the stratigraphic relationships, but this is an inference, as no specific constraints can be determined from these polydeformed and transposed rock sequences.
Depositional Ages of Metasedimentary Rocks
The ranges of depositional ages of parts of the Nome Group and the carbonate rocks in the York Mountains (Fig. 3) are known from fossils and conodonts (Till et al., 1986), but large parts of the Nome Group, the Kigluaik metamorphic complex, and the York Slate remain undated (Table 1). The detrital zircon data provide the only constraints on the maximum depositional age for these units. Deposition of all of these samples must have occurred prior to the Late Jurassic or Early Cretaceous, the assumed age of the regional high-pressure metamorphic event.
Six samples in this study have youngest age peaks that are Cambrian or older (Fig. 7, group 1). Four of these samples (KM5–KM8) are from the previously undated Casadepaga schist in the Nome Group, formerly presumed to be Ordovician, and a sample of Kigluaik metamorphic complex schist from the Bendeleben Mountains (sample BM5), also previously undated (Fig. 7; Table 1). The sample from the York Slate (YM2) has exclusively Precambrian zircons. These six samples must have been deposited after the age of the youngest peak, but none contains the abundant Ordovician zircons found in the other samples. Youngest significant peaks in group 1 are 558 Ma, 524 Ma, and 517 Ma.
Three samples have youngest zircon age peaks in the Ordovician or Early Silurian, coeval with abundant conodont ages in Nome Group carbonate rocks (Fig. 7, group 2). All of these samples are from the Kigluaik metamorphic complex: two (KM9, KM12) are from the Kigluaik Mountains, and the other (BM6) is from the Bendeleben Mountains. Each has a peak at ca. 485 Ma, an age that also shows up as a peak in the samples with younger maximum depositional ages. The remaining three samples (Fig. 7, group 3) have youngest peaks in the Early Devonian (KM9: 399 Ma; BM7: 362 Ma), and Pennsylvanian (KM12: 321 Ma).
Paleogeography of the Arctic Alaska Terrane
We used the combined data sets of Nd isotopic composition and model ages from basement rocks, the U-Pb zircon ages of the pre-Mesozoic igneous rocks, and the detrital zircon ages from overlying metasedimentary rocks to evaluate the possibilities for the origin and evolution of the Arctic Alaska terrane based primarily on the data from Seward Peninsula. We suggest that similarities in Neoproterozoic basement ages and Paleozoic faunal assemblages between Seward Peninsula, the Brooks Range, Chukotka Peninsula, and Wrangel Island argue for a shared pre-Mesozoic history.
Relationships with Western Laurentia
Although it has long been recognized that only a small part of east-central Alaska near the Canadian border is an in situ piece of western Laurentia (e.g., Plafker and Berg, 1994), some of the other terranes in southern Alaska also have Laurentian affinities (Fig. 1). A detrital zircon study from the Lower Cambrian Adams Argillite from this region is notable for its paucity of Neoproterozoic or Cambrian zircons relative to the samples from this study (Gehrels et al., 1999). Although redating of this unit did yield a few 700 Ma grains, post–1.05 Ga zircons are rare. In Cambrian Cordilleran miogeoclinal strata from British Columbia, Nevada, and Sonora, post–1.0 Ga zircons are rare to nonexistent (Gehrels et al., 1995; Stewart et al., 2001). Vendian-age sandstones (650–540 Ma) from southeastern Siberia also lack Cryogenian-age detrital zircons (Khudoley et al., 2001). These observations rule out western Laurentia and Siberia as sources for detrital zircons in the Arctic Alaska–Chukotka strata and support models that depict the Arctic Alaska–Chukotka terrane as exotic to western Laurentia.
By contrast, Neoproterozoic ages are abundant in both the basement rocks and in the detrital zircon populations of cover rocks on the Seward Peninsula. In the samples from this study, detrital zircons in the age range 720–660 Ma and 620–590 Ma are ubiquitous (Fig. 7A). When all of the data are combined, the most prominent peaks on a relative probability diagram are between 700 and 540 Ma (Fig. 7B). In the sample of York Slate (YM2), a peak at 718 Ma is the second most significant peak, and a minor peak at 656 Ma is also present. In the Casadepaga schist (samples KM5–8), the major peaks are around 680 Ma, 640 Ma, and 600 Ma. The Kigluaik metamorphic complex samples also have strong contributions from Late Neoproterozoic zircons (BM5–7, KM10–KM11). The two samples with the youngest detrital age peaks (KM9, KM12) have relatively minor amounts of Neoproterozoic zircons and have prominent Paleozoic age peaks. Therefore, the Paleozoic metasedimentary rocks of Seward Peninsula have source regions very different from those in coeval North American miogeoclinal rocks. Neoproterozoic basement ages and Neoproterozoic detrital zircons in Paleozoic cover rocks link the Arctic Alaska–Chukotka terrane to other areas with Neoproterozoic crust formation.
Mesoproterozoic Relationships
Using the detrital zircon ages and Nd model ages, we infer that crust in the range of 2.0–1.1 Ga is present in the Arctic Alaska–Chukotka terrane. Rocks with these ages are found on several cratons, including Laurentia. The 1.8 Ga ages are common in the rocks of the Trans-Hudson–Penokean orogeny (Bickford and Hill, 2006), and 1.75–1.70 Ga ages are common in rocks from the Yavapai Province (Karlstrom et al., 1987). The Mazatzal Province in southwest Laurentia consists of rocks ranging in age from 1.68 to 1.63 Ga (Karlstrom and Bowring, 1993; Amato et al., 2008), and the Grenville Province has rocks in the 1.3–1.1 Ga age range (Mosher, 1998; Tollo et al., 2004). Note that Laurentia formed the core of the Rodinia supercontinent until at least 800 Ma, and similarly aged rocks are found on adjacent cratons such as Australia and Baltica (e.g., Karlstrom et al., 2001; Li et al., 2008; Pease et al., 2008). The Mesoproterozoic ages demonstrate that the Arctic Alaska–Chukotka terrane consists partly of pre-Neoproterozoic continental crust and was not a newly formed juvenile Neoproterozoic arc terrane.
Neoproterozoic Relationships
Our discovery of magmatism on Seward Peninsula at 870 Ma provides a distinctive paleogeographic marker for comparisons with other areas. Magmatism at 870 Ma occurred around the time of the initiation of breakup of the Rodinian supercontinent, and coeval magmatism is known from Scotland, the Scandinavian Caledonides, and the Taimyr Peninsula (Fig. 8) of Russia (Paulsson and Andreasson, 2002). These correlations are conjectural but warrant further consideration as more areas of magmatism of this age become known.
The Neoproterozoic basement rocks of 680 Ma, together with abundant detrital zircons of this age in the cover sequence, form a distinctive signature that allow for correlations between the Arctic Alaska–Chukotka terrane and other areas that were tectonically active at this time (Fig. 9). Siberia may have been connected to Laurentia in the Proterozoic (Pisarevsky et al., 2008), but the basement ages of Arctic Alaska are sufficiently different from the Siberian craton to indicate that the Arctic Alaska–Chukotka terrane did not form as part of Siberia. In addition, paleomagnetic data indicate that northern Laurentia and Siberia were not close at this time (Li et al., 2008). Several areas with magmatism during this time period, such as the rocks in the Spitsbergen Caledonides and the Franklin and Gunbarrel mafic rocks in North America, were unlikely to have supplied significant amounts of zircon into sedimentary rocks because the magmatism was either small-volume, predominantly mafic, or was associated with small-volume melting during high-pressure metamorphism (e.g., Peucat et al., 1989; Heaman et al., 1992; Harlan et al., 2003; Lund et al., 2003). Neoproterozoic magmatism occurred in Eastern Svalbard between Norway and Greenland (Fig. 9), an area also known as the Barentsian microcontinent. Barentsia has magmatism dated by Sm-Nd at ca. 700 Ma (Johansson et al., 2004), but most of the magmatism in this area is late Grenvillian (960–940 Ma) or Caledonian (450–410 Ma) (Johansson et al., 2005), and the 560 Ma magmatism there appears to be mainly rift-related (Gee and Teben'kov, 2004; Nystuen et al., 2008). The foreland of the Barentsian orogen includes Novaya Zemlya and the Taimyr Peninsula of northern Siberia (Fig. 8). Taimyr Peninsula arc magmatism at 755 Ma and 730 Ma was followed by rift-related magmatism beginning at 630 Ma (Vernikovsky et al., 2004). However, the relationships among the Taimyr region, Siberia, and the other plates are not well constrained.
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We propose the following Neoproterozoic paleogeographic associations for the Arctic Alaska–Chukotka terrane (Fig. 10): (1) The Arctic Alaska–Chukotka terrane was not part of western Laurentia or eastern Siberia at the time of its formation; (2) pre-Neoproterozoic basement ages are suggested by the Nd model ages and supported by the presence of Paleoproterozoic and Mesoproterozoic detrital zircons (Archean detrital zircons are extremely rare); (3) Proterozoic basement and detrital zircon ages between 2.0 and 1.1 Ga are consistent with derivation from a number of continents, including Laurentia, Baltica, or Siberia; (4) magmatism at 870 Ma is consistent with rifting of Rodinia in eastern Laurentia but is older than rifting in western Laurentia; and (5) detrital zircon ages and basement ages in the late Neoproterozoic (720–550 Ma) indicate strong affinities with Baltica subduction prior to the Timanian orogeny that culminated around 600 Ma and to the Avalonian-Cadomian arc system active during this period.
In our model, subduction beneath the Arctic Alaska–Chukotka terrane occurred from 700 to 600 Ma and was coeval with pre-Timanide magmatism in Baltica and the main episode of Avalonian-Cadomian arc magmatism. Although it is unclear whether these three areas shared a subduction zone, the similarity in ages among the Neoproterozoic rocks of the Arctic Alaska–Chukotka terrane, Baltica, and the peri-Gondwanan arcs suggests that either one or several subduction zones may have existed at the same time along strike adjacent to the Laurentian and Gondwanan continents (Fig. 10). Rifting of this arc terrane may have resulted in the bimodal magmatism on Seward Peninsula around 565–540 Ma and initiated its displacement away from the north Atlantic region and toward its Mesozoic position further west. This model requires the least amount of transport to the position of the Arctic Alaska–Chukotka terrane in the pre-Mesozoic configuration of the circum-Arctic terranes. Similar-aged magmatism on the Taimyr Peninsula should be investigated for further correlations.
Paleozoic Relationships
Uplift and erosion of most of the pre–680 Ma volcanic and sedimentary cover occurred prior to deposition of post-Neoproterozoic sedimentary rocks in the York Slate and the post–Early Cambrian Casadepaga schist of the Nome Group. The basement-cover unconformity thus occurs on 565 Ma granite locally and on ca. 680 Ma granite regionally (Fig. 3).
The stratigraphy and faunal assemblages in the overlying Paleozoic sections have been used to evaluate paleogeographic connections between various parts of the Arctic region. Natal'in et al. (1999) suggested that similarities in the Lower Paleozoic stratigraphy among the Chukotka Peninsula, Seward Peninsula, and the Hammond subterrane in the Arctic Alaska terrane indicate deposition on a unified, continuous basement block called the Bennett–Barrovia block. Similarities in macrofossils link the Arctic Alaska–Chukotka terrane to parts of northern Siberia in the mid-Paleozoic (Blodgett et al., 2002), leading to the hypothesis that the Alaska terranes either rifted from Siberia in the Devonian or, alternatively, that they had ties to northeastern Baltica in Cambrian to Devonian time (Blodgett et al., 2002). The faunal links suggest that Late Neoproterozoic to Ordovician translation must have occurred to bring Arctic Alaska within the Siberian faunal realm by Early Ordovician time (Dumoulin and Harris, 1994; Dumoulin et al., 2002).
Position of the Arctic Alaska–Chukotka Terrane in Mesozoic Reconstructions of the Arctic Region
In our model, the Arctic Alaska–Chukotka terrane was adjacent to northeasternmost Laurentia near the northern Arctic margin of Baltica prior to opening of the present Arctic Ocean basins. The most likely evolution involves a path through the Arctic region (present coordinates), from its origin near the northern Atlantic, to a position adjacent to the Canadian Arctic Islands by late Paleozoic time.
The exact position of the Arctic Alaska– Chukotka terrane in the Arctic region prior to the opening of the Arctic Ocean basins is controversial, and multiple models have been put forward in recent years, particularly for the origin of the Amerasian Basin of the Arctic (Lawver et al., 2002; Miller et al., 2006). In each model, however, all of the circum-Arctic crustal fragments were closer to each other in Jurassic time. In Russian models (Zonenshain and Natapov, 1990; Borisova et al., 2002; Kuznetsov, 2006), Arctic Alaska was part of a larger Precambrian block called "Arctida," which also included parts of the Taimyr Peninsula, Severnya Zemlya, the Novosibirsky block, and parts of Ellesmere Island, the Lomonosov Ridge, and Barentsia. In our Mesozoic reconstruction (Fig. 11), Seward Peninsula is adjacent to the Brooks Range and North Slope (Arctic Alaska), near the Canadian Arctic Islands. This location of Arctic Alaska from the Mississippian to Jurassic period is required by specific stratigraphic links between the North Slope and the Canadian Arctic Islands (Toro et al., 2004), but it opens a gap adjacent to the Barents shelf that may be a pre-Mississippian (Devonian?) oceanic basin created during left-lateral strike slip of Arctic Alaska. In the Early Cretaceous, Arctic Alaska reached its current location by counterclockwise rotation (e.g., Grantz et al., 1990).
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ACKNOWLEDGMENTS |
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RECEIVED FOR PUBLICATION July 9, 2008
REVISED MANUSCRIPT RECEIVED October 20, 2008
MANUSCRIPT ACCEPTED October 22, 2008
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