The Alexander terrane is an unusual tectonic fragment in the North American Cordillera in that it contains a long and very complete stratigraphic record, including sedimentary or volcanic rocks representing every period and nearly every epoch between Neoproterozoic and Late Triassic time. The terrane is also unusual in that the southern portion of the terrane experienced arc-type magmatism during Neoproterozoic–early Paleozoic time, whereas the northern portion of the terrane consists mainly of Paleozoic shelf-facies strata. This long and diverse history provides opportunities to reconstruct the evolution and displacement history of the terrane, and specifically test the prevailing interpretation that the terrane formed in the paleo-Arctic realm.
This study presents U-Pb geochronologic data and Hf isotopic information for detrital zircons from arc-type rocks in the southern portion of the terrane. Information has been generated from seven samples of Ordovician through Devonian age, complementing the information available from previous studies of Ordovician through Triassic strata. Together, these data sets yield a robust record of the magmatic history of the southern Alexander terrane, with dominant age groups of 640–550 Ma, 490–400 Ma, 380–340 Ma, and 310–275 Ma (dominant ages of 579, 441, 361, and 293 Ma). There are few pre–640 Ma grains in any of the samples. Hf isotope compositions of the detrital zircons are exceptionally juvenile, with most epsilon Hf(t) values between +15 and +5.
Collectively, the available geologic, U-Pb geochronologic, and Hf isotopic evidence suggests that the southern Alexander terrane formed within a juvenile Neoproterozoic–early Paleozoic arc system, with little continental influence, whereas the northern portion of the terrane formed in proximity to a continental landmass that experienced similar Neoproterozoic–early Paleozoic ages of continental-affinity magmatism. Our data are consistent with previous suggestions that the Alexander terrane resided in the paleo-Arctic realm during early Paleozoic time, with the northern portion of the terrane adjacent to Baltica and the Caledonides, and the southern portion of the terrane forming further offshore as a juvenile north-facing oceanic arc.
The Alexander terrane is a displaced crustal fragment that occurs along the western margin of the northern North American Cordillera (Fig. 1). In contrast to many Cordilleran terranes, there is little doubt that the Alexander terrane is far-traveled, and likely exotic to western North America. However, there has been considerable debate about where the terrane formed, with previous models including the Sierra-Klamath region of California (Jones et al., 1972), near its present position in the northern Cordillera (Churkin and Eberlein, 1977), along the western margin of the paleo-Pacific (Gehrels and Saleeby, 1987), within the paleo-Pacific (Samson et al., 1989), or as part of the Appalachian system (Harms et al., 2003; Wright and Wyld, 2006; Grove et al., 2008). In recent years, there has been a growing consensus that the terrane formed within the paleo-Arctic during Neoproterozoic–early Paleozoic time and moved westward into the paleo-Pacific during late Paleozoic–early Mesozoic time (Soja, 1994; Bazard et al., 1995; Gehrels et al., 1996; Butler et al., 1997; Soja and Krutikov, 2008; Colpron and Nelson, 2009, 2011; Miller et al., 2011; Beranek et al., 2012, 2013a, 2013b; Nelson et al., 2013). Support for this displacement history came originally from geologic comparisons, faunal affinities, and paleomagnetic data, and more recently from U-Pb ages and Lu-Hf isotopic data for detrital zircons from the northern portion of the terrane.
This study presents U-Pb ages and Hf isotopic data for seven samples of Cambrian through Devonian sandstone from the Alexander terrane of southern southeast (SE) Alaska (Figs. 1 and 2). We also report U-Pb ages from a tonalitic pluton that provide critical constraints on the depositional age of sampled strata. Characterization of this portion of the terrane is critical because it preserves evidence of Neoproterozoic–early Paleozoic arc activity that is missing to the north, and because it provides an exceptionally complete stratigraphic record from Neoproterozoic through Triassic time (Fig. 2). Our objective in conducting these analyses was to provide additional constraints on the magmatic and tectonic evolution of the Alexander terrane, and to use this new information specifically to evaluate the hypothesis that the Alexander terrane originated in the paleo-Arctic realm.
The Alexander terrane consists of three distinct components, which are exposed in SE Alaska, the Saint Elias Mountains region to the north, and the Banks Island region to the south (inset of Fig. 1). In southern SE Alaska, the terrane consists largely of plutonic, volcanic, and sedimentary rocks of Neoproterozoic through Triassic age (Gehrels and Saleeby, 1987). These rocks are interpreted to grade northward into a sequence of mainly shelf-facies strata that occur in the Saint Elias Mountains region of northwestern British Columbia and southwestern Yukon (Campbell and Dodds, 1983; Mihalynuk et al., 1993; Beranek et al., 2013a, 2013b). Similar shelf-facies strata (now metamorphosed to quartzite and marble) also occur to the south, along the coast of British Columbia (Fig. 1; Roddick, 1970), where they are referred to as the Banks Island assemblage (Tochilin et al., 2014).
In southern SE Alaska, the Alexander terrane consists mostly of Neoproterozoic–early Paleozoic plutonic rocks and Neoproterozoic–Upper Triassic marine sedimentary and volcanic rocks (Fig. 2). The oldest rocks are greenschist- to amphibolite-facies rocks of the Wales Group, which include metavolcanic rocks of mafic, intermediate, and felsic composition, metasedimentary rocks derived from volcanic-lithic sandstone-siltstone, phyllite and schist of pelitic composition, and marble layers that range in thickness from centimeter to kilometer scale. Meta-rhyolite from the Wales Group on southern Prince of Wales Island (Fig. 1) has yielded U-Pb (zircon) ages of ca. 595 Ma (Gehrels et al., 1996) and ca. 565 Ma (Oliver et al., 2011). These rocks are intruded by orthogneisses of mafic to felsic composition, which yield U-Pb (zircon) ages of 560–530 Ma (Gehrels and Saleeby, 1987). Rocks of the Wales Group are interpreted to have formed in a marine volcanic arc environment; deformation and metamorphism are interpreted to have occurred during the Late Cambrian–Early Ordovician Wales orogeny (Fig. 2; Gehrels and Saleeby, 1987).
Next youngest are Ordovician through latest Silurian rocks, which are also interpreted to have formed in a marine volcanic arc environment (Churkin and Eberlein, 1977; Gehrels and Saleeby, 1987). This assemblage includes Ordovician–Lower Silurian rocks of the Descon Formation, which consists mainly of volcanic rocks of basalt to rhyolite composition interlayered with volcanic-rich sedimentary rocks (Eberlein et al., 1983). The Descon Formation locally consists of conglomeratic sandstone that is interpreted to at least locally rest unconformably on the Wales Group (Gehrels and Saleeby, 1987). This critical depositional relationship was reexamined as part of this study. Rocks of the Descon Formation grade upward into Silurian turbidite, limestone, and conglomerate, which underlie much of the terrane from central Prince of Wales Island to northern Kuiu Island (Figs. 1 and 2).
Devonian strata record a major episode of tectonism that is manifested by accumulation of locally thick conglomeratic red beds (Ovenshine et al., 1969), formation of a regionally extensive unconformity at the base of the Lower Devonian Karheen Formation (Eberlein et al., 1983), and SW-vergent thrust faults that imbricate Silurian and older rocks (Gehrels and Saleeby, 1987). This phase of tectonism has been referred to as the Klakas orogeny (Gehrels and Saleeby, 1987).
Carboniferous through Triassic strata are exposed in restricted areas of central Prince of Wales Island and northern Kuiu Island (Fig. 1). Carboniferous strata consist mainly of limestone and chert, with subordinate sandstone of the Klawak Formation (Fig. 2). Permian strata include conglomeratic sandstones of the Halleck Formation (Fig. 2) that occur in the northern Kuiu Island area. These conglomerates are interpreted to have been shed from a deeper-water facies of the Alexander terrane to the east that was uplifted during Permian time (Muffler, 1967; Jones et al., 1981; Karl et al., 2010). Triassic strata, locally referred to as the Nehenta Formation (Fig. 2), overlie the Permian and older strata on a regional unconformity. The Triassic volcanic and sedimentary rocks are interpreted to have formed in a rift environment (Fig. 2; Gehrels and Saleeby, 1987).
As shown on Figure 2, the geology of the Alexander terrane changes dramatically northward. Neoproterozoic–early Paleozoic igneous rocks are present only locally north of Prince of Wales Island, and instead, the terrane is dominated by Lower Paleozoic turbidites and Middle-Upper Paleozoic limestone, dolomite, chert, and marine clastic strata. These marine strata are interpreted to continue northward from SE Alaska into the Saint Elias Mountains region (Fig. 1), where the terrane is dominated by Cambrian–Ordovician mafic volcanic rocks of extensional origin, Ordovician–Silurian limestone and marine clastic strata, Devonian clastic strata that may be the distal equivalent of the Karheen clastic wedge, and Carboniferous limestone, marine clastic strata, chert, and subordinate mafic to intermediate volcanic rocks (Campbell and Dodds, 1983; Mihalynuk et al., 1993; Beranek et al., 2012, 2013a, 2013b). The rocks in the Saint Elias Mountains region (north of Glacier Bay on Fig. 1) are interpreted to be the northward continuation of strata in SE Alaska, although detailed correlations and stratigraphic continuity have not been demonstrated.
PREVIOUS DETRITAL ZIRCON ANALYSES FROM THE ALEXANDER TERRANE
Initial detrital zircon data from the Alexander terrane consisted of isotope dilution–thermal ionization mass spectrometry (ID-TIMS) analyses of zircons from the Descon (Ordovician), Karheen (Devonian), Klawak (Permian), and Nehenta (Triassic) Formations in southern SE Alaska (Fig. 2; Gehrels et al., 1996). Between 10 and 30 grains were analyzed from each sample, with grains selected to represent each of the different color and morphology groups present. The resulting ages (included in Table DR11) are mainly early Paleozoic, except for small, pink, and rounded grains of Precambrian age in the Karheen Formation. Additional small, pink, and rounded grains from the Karheen Formation were analyzed by laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and reported by Grove et al. (2008; see also Table DR1 [see footnote 1]).
Detrital zircon grains were analyzed by LA-ICP-MS (with ∼100 grains per sample) from the northern Alexander terrane (Saint Elias Mountains area on the inset of Fig. 1) by Beranek et al. (2013a, 2013b). The results from these studies differed from the SE Alaska data in that most samples were dominated by Precambrian rather than early Paleozoic grains (Table DR1 [see footnote 1]).
Finally, Tochilin et al. (2014) conducted LA-ICP-MS analyses (with ∼100 grains per sample) on the original SE Alaska samples, several additional samples from SE Alaska (shown on Fig. 2), and metasedimentary rocks in coastal British Columbia that were included in the Alexander terrane by Wheeler and McFeely (1991). The latter rocks are referred to as the Banks Island assemblage. Tochilin et al. (2014) confirmed that strata in SE Alaska are dominated by early Paleozoic grains, and they showed that ages from the Banks Island assemblage are mainly Precambrian and similar to ages from the Saint Elias Mountains (SEM) region of the northern Alexander terrane. On the basis of the similarity of lithologies, zircon ages, and Hf isotope signatures, Tochilin et al. (2014) proposed that the Banks Island assemblage formed in proximity to the northern portion of the terrane (SEM on inset of Fig. 1) and was displaced ∼1000 km southward relative to the SE Alaska portion of the terrane during Jurassic–Cretaceous time.
Following are descriptions of each of the samples analyzed, organized from oldest to youngest. Samples are keyed to locations on Figure 1 and to stratigraphic position on Figure 2.
12MP56: This sample is from pebbly sandstone that occurs ∼2 m above the base of the Descon Formation on southeastern Prince of Wales Island. The sandstone consists mainly of mafic and felsic metavolcanic lithic and marble grains, with subordinate plagioclase, quartz, and epidote. The sandstone occurs near the base of an ∼20-m-thick section of coarse sedimentary breccia that contains angular clasts of marble and mafic metavolcanic rocks that are up to several meters in length. This breccia rests unconformably on greenschist-facies metavolcanic and metasedimentary rocks and marble of the Wales Group (Gehrels and Saleeby, 1987; Gehrels and Berg, 1992). Figure 3 shows outcrop photographs of the sandstone analyzed (Fig. 3A) and the sedimentary breccia that occurs at the base of the Descon Formation (Fig. 3B).
12MP57: This sample is from a conglomeratic sequence within the lower part of the Descon Formation (Gehrels and Berg, 1992) on southeastern Prince of Wales Island. It consists of coarse-grained graywacke containing pebbles to cobbles of quartz diorite, diorite, marble, metagraywacke, and metavolcanic and metaplutonic rocks of variable composition. The sandstone consists mainly of mafic and felsic metavolcanic lithic and marble grains, plagioclase, quartz, and minor K-feldspar. The conglomeratic horizon sampled is shown on Figure 3C. This sample consists exclusively of sandstone matrix, with no clasts. In contrast, a sample from the same outcrop (“Moira”) reported by Tochilin et al. (2014) consists of both clasts and sandstone matrix.
12MP48: This sample is from the Silurian Bay of Pillars Formation (Muffler, 1967), collected from the north end of Kuiu Island. The rock consists of coarse- to medium-grained sandstone (Fig. 3D) with mostly quartz, plagioclase, K-feldspar, chert, and limestone grains.
12MP51: This sample is from the Silurian Bay of Pillars Formation (Brew et al., 1984), collected from northeastern Prince of Wales Island. The rock is fine- to coarse-grained sandstone that displays graded beds with pebbly basal horizons. The rock sampled is coarse-grained sandstone with abundant mafic volcanic lithic fragments, and subordinate quartz, plagioclase, and K-feldspar. Pebbles consist mainly of volcanic material.
12MP52: This sample is a coarse-grained sandstone layer within a conglomeratic sequence mapped as the Silurian Bay of Pillars Formation (Brew et al., 1984) on northeastern Prince of Wales Island. The sandstone sampled consists mainly of angular grains of quartz, plagioclase, and K-feldspar. Clasts in the conglomerate are mainly plutonic rock of quartz diorite and quartz alkali syenite composition (Fig. 3E). The sample analyzed for detrital zircon analysis consists exclusively of sandstone matrix (no clasts included).
12MP54: This sample was collected from a sandstone horizon within a conglomeratic sequence of probable Devonian age on eastern Prince of Wales Island (Buddington and Chapin, 1929; Eberlein et al., 1983). The lithology sampled is coarse-grained sandstone that consists mainly of quartz, plagioclase, and mafic volcanic lithic grains. Pebbly horizons contain mainly plutonic and mafic volcanic clasts. Only sandstone was sampled for detrital zircon analysis.
12MP55: This sample was collected from the sandstone matrix of a conglomeratic sequence of probable Devonian age on eastern Prince of Wales Island (Buddington and Chapin, 1929; Eberlein et al., 1983). The sandstone occurs within a sequence of mainly conglomerate that contains angular clasts of diorite, quartz diorite, marble, and metavolcanic rocks of mafic to felsic composition (Fig. 3F). The sandstone is dominated by grains of plagioclase, K-feldspar, and mafic metavolcanic lithic fragments.
12MP58: This is a tonalite on southeastern Prince of Wales Island that intrudes the conglomeratic strata from which samples 12MP56 and 12MP57 were collected. The age of the pluton is of interest to determine whether the conglomeratic strata are Ordovician–Silurian (Gehrels and Berg, 1992) or Devonian (Eberlein et al., 1983) in age. The rock is nondeformed and consists of quartz, plagioclase, and hornblende (commonly altered to chlorite and epidote), with a color index of 30.
Samples were processed using standard methods of zircon separation (Gehrels, 2000; Gehrels et al., 2008; Gehrels and Pecha, 2014). Zircon grains were mounted with fragments of Sri Lanka primary zircon standard and R33 secondary standard for U-Pb, as well as Mud Tank, Temora2, FC-1, 91500, R33, and Plesovice standards for Hf analysis (Gehrels et al., 2008; Woodhead and Hergt, 2005; Sláma et al., 2008; Cecil et al., 2011; Gehrels and Pecha, 2014). For U-Pb analysis, individual grains were selected at random and analyzed by LA-ICP-MS. Analysis locations were determined with the use of cathodoluminescence (CL) images, generated with a Hitachi 3400N scanning electron microscope (SEM) equipped with a Gatan Chroma CL system (www.geoarizonasem.org). U-Pb analytical data can be found in Table DR1 (see footnote 1). The results are reported on age-distribution plots, which sum all of the measured ages (and uncertainties) from each sample and then normalize all curves by the number of constituent analyses (Figs. 4 and 5).
One of the challenges in interpreting the resulting ages is that many samples contain several age groups that overlap in age. In some cases, there is little overlap in ages, and the different age groups are easy to characterize using the maxima in age probability plots. In others, however, the age groups have significant overlap, and resolving the different age components is challenging. In an effort to recognize and describe these different age components objectively, we used the Unmix routine of Isoplot (Ludwig, 2008), which identifies the number of age groups present, as well as the age and uncertainty of each component. This routine uses a maximum likelihood analysis (from Sambridge and Compston, 1994) to determine the Gaussian distributions that best fit the set of measured ages and uncertainties. Fortunately, in most cases, these maximum likelihood ages (MLAs) are similar to the maxima in age probability plots. Table DR1 reports ages determined from the maxima in age probability plots as well as from the maximum likelihood method for every sample and every set of samples reported herein (see footnote 1).
Hf analyses were conducted utilizing the analytical methods described by Cecil et al. (2011) and Gehrels and Pecha (2014). The average uncertainty for analyses is 1.9 epsilon units (2σ), as shown on Figure 6. External precision, based on analysis of standards from the same mounts as unknowns, is 2–3 epsilon units (2σ). Full Hf analytical data can be found in Table DR2 (see footnote 1). Because changes in epsilon Hf(t) through time provide useful information about magmatic and tectonic processes, Figure 6 provides curves that show the sliding window average of epsilon Hf(t) values through time. The average for each point in time factors in the ages and epsilon Hf(t) values of the eight preceding and eight succeeding analyses.
RESULTS OF U-Pb ANALYSES
Our two samples of Ordovician strata yield very similar age distributions, with dominant age groups that yield MLAs of 572 Ma (12MP56) and 569 Ma (12MP57). Both samples also contain subordinate populations of slightly older grains (591 Ma for 12MP56 and 598 Ma for 12MP57). Sample 12MP57 also contains several younger grains with a MLA of ca. 487 Ma, and two older grains with ages of ca. 1152 Ma and ca. 1423 Ma (Fig. 4; Table DR1 [see footnote 1]).
The ages from these samples are generally similar to ages reported by Tochilin et al. (2014) from Ordovician strata of the Descon Formation on Prince of Wales Island. A conglomeratic sandstone (“Moira”), collected from locality 12MP57, yields abundant 600–550 Ma grains, whereas a sample of more typical Descon sandstone (“Descon”) yields mainly 500–450 Ma grains. When ages are combined from all three samples from the lower Descon Formation (samples 12MP56, 12MP57, and Moira), the 640–500 Ma ages define a bimodal age distribution with MLA of 597 and 568 Ma (Table DR1 [see footnote 1]).
The Neoproterozoic grains in these samples were most likely derived from igneous rocks of the Wales Group (Fig. 2), which has yielded U-Pb ages of ca. 595 Ma (Gehrels et al., 1996) and ca. 565 Ma (Oliver et al., 2011) from meta-rhyolites on southeastern Prince of Wales Island (Fig. 1). The coincidence of these igneous and detrital age groups suggests that Wales Group magmatism, at least on southern Prince of Wales Island, occurred in two main phases at ca. 597 Ma and ca. 568 Ma.
Paleozoic grains in these samples were likely shed from Ordovician plutonic and volcanic rocks that are widespread on Prince of Wales Island and nearby smaller islands (Fig. 1). The younger igneous rocks are part of an igneous complex that ranges in age from Early Ordovician through latest Silurian (Gehrels and Saleeby, 1987). Plutons in the 500–450 Ma range within this complex are mainly dioritic to quartz dioritic in composition. With results from all four Ordovician samples combined (Fig. 5), the MLAs are 597, 568, and 469 Ma.
Three samples of known Silurian age were analyzed as part of this study, and an additional sample (“Saginaw”) was analyzed and reported by Tochilin et al. (2014; see also Fig. 2 herein). All four yield mainly 470–410 Ma ages, with variable proportions of two main age groups (Fig. 4). An older age group is dominant in 12MP51 (450 Ma) and 12MP52 (452 Ma) and subordinate in 12MP48 (453 Ma) and Saginaw (450 Ma). A younger age group is dominant in 12MP48 (428 Ma) and Saginaw (428 Ma) and subordinate in 12MP51 (429 Ma) and 12MP52 (427 Ma). With results from all four Silurian samples combined, the MLAs are 450 and 425 Ma (Table DR1 [see footnote 1]).
Two new samples of probable Devonian age were analyzed from southern Prince of Wales Island (Fig. 1). These samples have MLA of 436 Ma (12MP54) and 449 Ma (12MP55) and subordinate age groups of 483, 451, and 418 Ma (12MP54) and 420 Ma (12MP55; Fig. 4; Table DR1 [see footnote 1]).
Tochilin et al. (2014) reported U-Pb ages from four samples of the Lower Devonian Karheen Formation on Prince of Wales Island. These samples yield dominant MLAs of 447–430 Ma (Karheen 1), 433 Ma (Karheen 2), 430 Ma (Karheen 3), and 423 Ma (Tah), as well as subordinate ages that are slightly older (Fig. 4; Table DR1 [see footnote 1]).
The MLAs are 450 and 426 Ma when all six Silurian samples and all 10 Silurian–Devonian samples are combined (Fig. 5; Table DR1 [see footnote 1]).
Results from Upper Paleozoic and Triassic strata were reported by Tochilin et al. (2014) and are summarized on Figure 4. These samples yield dominant MLAs of 435 and 368 Ma (Klawak), 453, 426, 366, and 291 Ma (Halleck 1), 430 and 291 Ma (Halleck 2), and 439 and 414 Ma (Nehenta). With results from all four samples combined (Fig. 5), the MLAs are 433, 362, and 294 Ma (Table DR1 [see footnote 1]).
Thirty-five zircon grains were analyzed from this pluton. All grains apparently belong to a single age group with a weighted mean age of 429 ± 3.8 Ma (2σ). This demonstrates that the conglomeratic strata intruded by this pluton on southeastern Prince of Wales Island are not Devonian in age (as reported by Eberlein et al., 1983), but are more likely Ordovician–Silurian in age (as shown by Gehrels and Berg, 1992).
RESULTS OF Hf ISOTOPE ANALYSES
The Hf isotopic compositions of zircon grains in our samples are exceptionally juvenile, with most epsilon Hf(t) values in the range of +15 to +5 (Fig. 6; Table DR2 [see footnote 1]). These values are quite similar to the juvenile compositions reported from both detrital zircons (Tochilin et al., 2014) and igneous rocks (Cecil et al., 2011) of the southern Alexander terrane (Fig. 6, inset; Table DR2 [see footnote 1]).
COMPARISON OF U-Pb AND Hf DATA AMONG THE THREE PORTIONS OF THE ALEXANDER TERRANE
Figures 5 and 6 provide comparisons of the U-Pb and Hf isotope data available from strata belonging to the Alexander terrane in the Saint Elias Mountains, coastal British Columbia (Banks Island assemblage), and southern SE Alaska. Several important patterns emerge from these comparisons:
(1) Early Paleozoic (500–400 Ma) detrital zircons are common in all three portions of the terrane, with very similar records of magmatism in the Banks Island and Saint Elias assemblages and a somewhat different history in SE Alaska. As shown in Figure 6, magmatism in SE Alaska commenced at ca. 470 Ma and extended until ca. 400 Ma, with an MLA of ca. 441 Ma. In contrast, magmatism recorded in detrital zircons from the northern part of the terrane commenced at ca. 520 Ma and continued through ca. 390 Ma, with very similar MLAs of 476–442–415 Ma for the Banks Island assemblage and 478–447–417 Ma for the Saint Elias Mountains (Fig. 6).
(2) Neoproterozoic magmatism is also recorded in all three regions of the Alexander terrane, but with somewhat different records in each area (Fig. 6). In SE Alaska, magmatism was continuous from ca. 630 to ca. 530 Ma, with an age maximum at 579 Ma. These ages are primarily from basal strata of the Ordovician Descon Formation that rest unconformably on igneous rocks of the Wales Group of similar age (Figs. 4–6). Grains of this age are uncommon in younger strata of SE Alaska, suggesting that rocks of the Wales Group are relatively restricted in distribution, and/or were rarely exhumed during Paleozoic time. The magmatic record in the Banks Island and Saint Elias Mountains assemblages begins earlier (ca. 800 Ma) and continues longer, ending at ca. 550 Ma, with MLAs of 779 and 565 Ma (Banks Island) and 631 Ma (Saint Elias; Fig. 6).
(3) Upper Paleozoic and Triassic strata of southern SE Alaska also contain younger than 400 Ma detrital zircons, with MLAs of 361 and 293 Ma (Fig. 6). The ca. 361 Ma grains were probably shed from volcanic rocks of this age, which occur on Prince of Wales and adjacent islands, whereas the ca. 293 Ma grains may have been sourced from Pennsylvanian–Permian intrusive bodies that occur locally on Prince of Wales Island (Eberlein et al., 1983).
(4) Strata of southern SE Alaska yield few pre–640 Ma grains, in contrast to Paleozoic strata from the Banks Island assemblage and Saint Elias Mountains (Figs. 5 and 6). The ages of these older grains in all three areas have significant overlap, however, with dominant age groups of 2050–1630 Ma, 1500–1310 Ma, and 1260–990 Ma (Fig. 6). In SE Alaska, the old grains are most abundant in conglomeratic sandstones that record Silurian–Early Devonian and Permian tectonism.
(5) Hf isotope signatures from SE Alaska are very different from the signatures of strata to the north, both in value and in pattern through time. In terms of values for Neoproterozoic–early Paleozoic grains, there is little overlap, with zircons in southern SE Alaska consistently yielding juvenile epsilon Hf(t) values, and grains in strata to the north yielding much more evolved epsilon Hf(t) values (Fig. 6). The patterns through time for 500–380 Ma grains are also very different, with SE Alaska showing the most juvenile values just as the northern assemblages show more negative values. The mismatches in magmatic ages, epsilon Hf(t) values, and epsilon Hf(t) trends collectively demonstrate that the juvenile igneous rocks in SE Alaska were not the source of more-evolved grains in northern assemblages.
RESTORATION OF THE EARLY PALEOZOIC CONFIGURATION OF THE ALEXANDER TERRANE
The available U-Pb and Hf isotopic data yield important constraints on the origin of the Alexander terrane, especially when combined with geologic relations and paleomagnetic data. As summarized by Gehrels and Saleeby (1987), patterns of early Paleozoic magmatism, sediment transport, and deformation in southern SE Alaska suggest that: (1) the southern portion of the terrane evolved in a marine magmatic arc that faced southwest (in present coordinates) during Ordovician–Silurian time, (2) arc magmatism ceased during latest Silurian–earliest Devonian time, when SW-vergent thrust faults formed and a NW-SE–trending orogenic highland was created in the southern portion of the terrane, and (3) Lower Devonian conglomeratic strata (Karheen Formation and equivalents) accumulated in a clastic wedge that was shed northeastward (in present coordinates) from this orogenic highland. Rocks of the Saint Elias Mountains region were interpreted by Gehrels and Saleeby (1987) to have resided in a back-arc position relative to the magmatic arc.
Figure 7 is an attempt to reconstruct the early Paleozoic configuration of the Alexander terrane. The first feature to be accounted for is the Chatham Strait fault, which cuts diagonally across the Alexander terrane and has ∼180 km of Tertiary dextral offset (Hudson et al., 1981; Karl et al., 2010). Second is the interpretation that rocks of the Banks Island assemblage were adjacent to the Saint Elias Mountains portion of the terrane prior to ∼1000 km of left-lateral displacement on the Kitkatla shear zone and related Early Cretaceous structures (Monger et al., 1994; Tochilin et al., 2014). This provides a palinspastic restoration of the Early Devonian orogen and clastic wedge (Fig. 7D) and Ordovician-Silurian arc system (Fig. 7E) that are interpreted to trend at an oblique angle across the terrane (Fig. 7). Also shown on panel D of this diagram is the direction toward the geomagnetic north pole interpreted from paleomagnetic analysis of Lower Devonian red beds of the Karheen Formation, assuming that the terrane was in the Northern Hemisphere (Bazard et al., 1995; Butler et al., 1997).
The configuration of early Paleozoic tectonic elements portrayed in Figure 7 suggests that the Alexander terrane consists of an oblique slice through an Ordovician–Silurian convergent margin system and latest Silurian–Early Devonian collisional orogen and clastic wedge. The Ordovician–Silurian arc apparently faced southwest, and the Lower Devonian clastic wedge was shed from a source area to the southwest (both in present coordinates). Using the paleomagnetic declination observed in Lower Devonian strata, and assuming that the terrane was located in the Northern Hemisphere, this suggests that the long axis of the terrane was oriented east-west during Early Devonian time, with the Ordovician–Silurian arc system facing northward and the succeeding latest Silurian–Early Devonian orogen located to the north.
ORIGIN OF THE ALEXANDER TERRANE IN THE PALEO-ARCTIC?
The U-Pb and Hf isotope data summarized here, combined with the reconstructed early Paleozoic tectonic setting, orientation, and paleolatitude of the Alexander terrane, can be used to test the prevailing view that the Alexander terrane formed within the paleo-Arctic ocean basin (Soja, 1994; Bazard et al., 1995; Gehrels et al., 1996; Butler et al., 1997; Soja and Krutikov, 2008; Colpron and Nelson, 2009, 2011; Miller et al., 2011; Beranek et al., 2012, 2013a, 2013b; Nelson et al., 2013).
Given the abundance of 500–400 Ma magmatism within the Alexander terrane, an obvious possibility is that it formed in proximity to the Caledonian orogen, which is characterized by igneous rocks of this age (see overview of Corfu et al., 2014). Figure 8 compares the magmatic history of the Alexander terrane with U-Pb ages reported from eastern Greenland (Rohr et al., 2008; Kalsbeek et al., 2008; Rehnström, 2010; Corfu and Hartz, 2011; Andersen et al., 2011; Slama et al., 2011; Augland et al., 2012a; Andersen, 2013) and Baltica (Andersen et al., 2002, 2007; Bingen and Solli, 2009; Roberts et al., 2010; Kuznetsov et al., 2010; Corfu et al., 2011; Andersen et al., 2011; Augland et al., 2012b, 2014a, 2014b; Gee et al., 2014; Kristoffersen et al., 2014; Andresen et al., 2014; Slama and Pedersen, 2015). Shown separately for the Alexander terrane are ages from the arc-type (SE Alaska), and back-arc (Saint Elias and Banks Island) portions of the terrane (Gehrels et al., 1996; Grove et al., 2008; Beranek et al., 2013a, 2013b; Tochilin et al., 2014; this study).
The dominant age groups in all three regions are generally similar, with MLA of 441 Ma for SE Alaska, 480–443–412 Ma for the Saint Elias and Banks Island assemblages, 476–433 Ma for Baltica, and 458–422 Ma for Greenland. Three of the four regions also have subordinate Neoproterozoic age groups, with MLA of 579 Ma for SE Alaska, 640–571 Ma for Saint Elias–Banks Island strata, and 565 Ma for Baltica (derived mainly from the Timanide orogen of eastern Baltica). Paleoproterozoic and Mesoproterozoic ages are also generally similar, with the exception of abundant 1000–900 Ma ages in Baltica and Greenland that are of low abundance in the Alexander terrane (Fig. 8).
Figure 8 also compares Hf isotope data from the Alexander terrane with results from a variety of assemblages in and adjacent to Greenland (Rohr et al., 2008; Rehnström, 2010; Slama et al., 2011; Andersen, 2013), and Baltica (Andersen et al., 2002, 2007, 2011; Kuznetsov et al., 2010; Augland et al., 2012b, 2014b; Kristoffersen et al., 2014; Andresen et al., 2014). General fields that encompass most results, as well as sliding window averages, are shown for both regions (Fig. 8). This compilation shows that the Banks Island–Saint Elias Mountains assemblages of the Alexander terrane have considerable overlap in epsilon Hf(t) values with both Baltica and Greenland. The evolutionary patterns of Neoproterozoic–early Paleozoic magmatism are also quite similar with Baltica, but less so with Greenland, suggesting that Baltica is a reasonable source for the detritus in the Banks Island–Saint Elias assemblages. Missing components in both Greenland and Baltica are 500–400 Ma igneous rocks with juvenile Hf isotope signatures, which is not surprising given the widespread older crust in these areas.
Figure 9 attempts to use the available isotopic and paleomagnetic data to evaluate the possibility that the Alexander terrane was located in the paleo-Arctic during Late Silurian and Early Devonian time (Soja, 1994; Bazard et al.; 1995; Gehrels et al., 1996; Butler et al., 1997; Colpron and Nelson, 2009, 2011; Miller et al., 2011; Beranek et al., 2012, 2013a, 2013b; Nelson et al., 2013). Base maps for these reconstructions are from Cocks and Torsvik (2011) and Torsvik et al. (2012). The paleolatitude and orientation of the Alexander terrane are constrained during Early Devonian time by the paleomagnetic data of Bazard et al. (1995) and Butler et al. (1997), which indicate a paleolatitude of 14° ± 4° and an east-west orientation of the terrane (from restoration of ∼127° of counterclockwise rotation, assuming that the terrane was located in the Northern Hemisphere). Unfortunately, there are no reliable paleomagnetic data for Silurian rocks of the Alexander terrane (Butler et al., 1997), so Figure 9B assumes that the terrane occupied the same orientation relative to Laurentia and Baltica as during Early Devonian time.
This restoration shows that the existing information is consistent with the Alexander terrane residing in close proximity to Baltica, along the northern Caledonides, during Late Silurian time (Fig. 9B). Strata of the Saint Elias Mountains–Banks Island assemblages would have received continental detritus from Baltica, the Timanide orogen, and Ordovician–Silurian continental-margin arcs and collision-related magmatism in the emerging Caledonian orogen, whereas juvenile arc–type rocks in southern SE Alaska would have formed in a north-facing oceanic arc that extended offshore into the paleo-Arctic ocean basin. Similar magmatic histories in the Pearya (Hadlari et al., 2014; Malone et al., 2014), Farewell (Malkowski and Hampton, 2014), and Arctic Alaska–Chukotka (Miller et al., 2011) terranes suggest that they may have been associated with this Ordovician–Silurian arc.
The available constraints for Early Devonian time are less consistent with a paleoposition along the northern Caledonides given the apparent latitudinal separation (Fig. 9A), and derivation of the Karheen clastic wedge from a source area to the north rather than south. Perhaps the Karheen clastic wedge was shed from an orogen along the northern (Siberian) margin of the paleo-Arctic, rather than the Caledonides. The next available constraint on paleoposition of the Alexander terrane is provided by Lower Permian volcanic rocks from central SE Alaska, which are known to have formed at ∼25° latitude in the Northern Hemisphere (Butler et al., 1997). By this time, the terrane appears to have rotated counterclockwise by ∼20° relative to the Early Devonian orientation of the terrane, although other Permian and Triassic rocks from the Alexander terrane show different senses and amounts of rotation (Butler et al., 1997).
The U-Pb geochronologic and Hf isotopic data presented herein, combined with previously published geologic, U-Pb geochronologic, Hf isotopic, biogeographic, and paleomagnetic data, provide new insights into the magmatic and tectonic evolution of the Alexander terrane. First-order conclusions are as follows:
(1) The southern portion of the Alexander terrane is dominated by volcanic and plutonic rocks, and their sedimentary derivatives, that record evolution in a convergent margin setting from Neoproterozoic (ca. 630 Ma) through Early Devonian (ca. 400 Ma) time. The available detrital zircon ages record dominant ages of 579 Ma for Neoproterozoic magmatism and 441 Ma for early Paleozoic magmatism (Fig. 6).
(2) The scarcity of pre–630 Ma detrital zircons in Neoproterozoic–Lower Paleozoic strata, combined with the highly juvenile Hf isotopic signature of 630–400 Ma zircons, suggests that there was little continental influence in the southern Alexander terrane during Neoproterozoic through early Paleozoic time.
(3) The evolution of the southern Alexander terrane changed markedly during Early Devonian time, with the cessation of arc-type magmatism, the onset of thrusting, uplift, and regional metamorphism, and the accumulation of conglomeratic red beds that contain Paleoproterozoic and Mesoproterozoic detrital zircon grains. This event, referred to as the Klakas orogeny, is interpreted to record the collision of the southern Alexander terrane with a continental landmass (Gehrels and Saleeby, 1987).
(5) Regional patterns of early Paleozoic magmatism, deformation, and uplift suggest that the Alexander terrane consists of an oblique slice through an Ordovician–Devonian convergent margin/collisional system.
(6) The Neoproterozoic–early Paleozoic history of the northern portion of the terrane (Beranek et al., 2012, 2013a, 2013b; Tochilin et al., 2014) records a very different history, with Lower Paleozoic strata yielding abundant Precambrian detrital zircons and Neoproterozoic and Ordovician–Silurian grains with highly evolved Hf isotopic signatures. This suggests that the northern portion of the terrane evolved in proximity to, and received abundant detritus from, a continental region that experienced very similar ages of Neoproterozoic and Ordovician–Silurian magmatism as the southern portion of the terrane.
(7) The detailed patterns of magmatism recorded in the arc-affinity and continental-margin–affinity portions of the Alexander terrane are somewhat different (Fig. 6). Magmatism in the southern portion of the terrane is more restricted in time, with main phases from 630 to 530 Ma (maximum at 579 Ma) and 480 to 400 Ma (maximum at 441 Ma). In contrast, magmatism in the continental margin arcs (as recorded in sandstones of the Saint Elias–Banks Island assemblages) started and ended earlier during Neoproterozoic time (duration from 760 to 550 Ma, with maxima at 640–571 Ma) and started earlier and continued later during Ordovician–Early Devonian time (duration from 520 to 390 Ma, with maxima at 480, 443, and 412 Ma).
(8) The Hf isotopic compositions of southern and northern portions of the terrane are also very different (Fig. 6). In terms of value, epsilon Hf(t) values are highly juvenile for both Neoproterozoic and early Paleozoic magmatism in southern SE Alaska, whereas the Saint Elias–Banks Island assemblages yield grains with intermediate to highly negative epsilon Hf(t) compositions. The patterns through time are also different for the main phase of Ordovician–Silurian magmatism, with continental-affinity zircons pulling down to more negative values at the same time that juvenile zircons pull up to higher values (Fig. 6).
(9) The similarity of the detrital zircon U-Pb and Hf isotope data from the Alexander terrane with results from Baltica and Greenland is consistent with the popular view that the Alexander terrane formed in proximity to Baltica, the Timanide orogen, and the northern Caledonides, as suggested by Soja (1994), Bazard et al. (1995), Gehrels et al. (1996), Butler et al. (1997), Soja and Krutikov (2008), Colpron and Nelson (2009, 2011), Miller et al. (2011), Beranek et al. (2012, 2013a, 2013b), Nelson et al. (2013), and many others. Integration of the detrital zircon data with the paleomagnetic results of Bazard et al. (1995) and Butler et al. (1997) and faunal data of Soja (1994) and Soja and Krutikov (2008) supports a more specific conclusion that Neoproterozoic–Lower Paleozoic strata in the northern portion of the terrane (Banks Island–Saint Elias assemblages) consist largely of detritus that was shed from Baltica, the Timanide orogen, and Caledonian magmatic arcs and collisional magmatism, whereas the SE Alaska portion of the terrane evolved in a north-facing oceanic arc farther out in the paleo-Arctic. During Early Devonian time, the terrane was blanketed by the Karheen clastic wedge, which was shed from a northern source that may have been part of the Caledonides or an unrelated orogenic system farther north within the paleo-Arctic.
This work was funded by National Science Foundation EAR-0947094 and EAR-0948359 (for support of the field and analytical work) and EAR-1338583 (for support of the Arizona LaserChron Center). Ken Kanipe, Chen Li, Clayton Loehn, and Gayland Simpson provided invaluable assistance with sample preparation, imaging, and analysis. Thanks go to Don Willson and James Gehrels for logistical support in the field. This paper was reviewed by Maurice Colpron and Brian Mahoney.