Detrital zircon geochronology of some Neoproterozoic to Triassic rocks in interior Alaska
Published:January 01, 2007
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Dwight C. Bradley, William C. McClelland, Joseph L. Wooden, Alison B. Till, Sarah M. Roeske, Marti L. Miller, Susan M. Karl, J. Grant Abbott, 2007. "Detrital zircon geochronology of some Neoproterozoic to Triassic rocks in interior Alaska", Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska, Kenneth D. Ridgway, Jeffrey M. Trop, Jonathan M.G. Glen, J. Michael O'Neill
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We report 777 U-Pb SHRIMP detrital zircon ages from thirteen sandstones and metasandstones in interior Alaska. About sixty grains per sample were analyzed; typically, half to three-fourths of these were concordant within ± 10%.
Farewell terrane. Two quartzites were collected from Ruby quadrangle and a third from Taylor Mountains quadrangle. All three are interpreted to represent a low stratigraphic level in the Nixon Fork platform succession; the samples from Ruby quadrangle are probably late Neoproterozoic, and the sample from Taylor Mountains quadrangle is probably Cambrian in age. The youngest detrital zircon in any of the three is 851 Ma. The two Ruby quadrangle samples area almost identical: one has a major age cluster at 1980–2087 and minor age clusters at 944–974 and 1366–1383 Ma; the other has a major age cluster at 1993–2095 Ma and minor age clusters at 912–946 and 1366–1395 Ma. The Taylor Mountains sample shows one dominant peak at 1914–2057 Ma. Notably absent are zircons in the range 1800–1900 Ma, which are typical of North American sources. The detrital zircon populations are consistent with paleontological evidence for a peri-Siberian position of the Farewell terrane during the early Paleozoic.
Mystic subterrane of the Farewell terrane. Three graywackes from flysch of the Mystic subterrane, Talkeetna quadrangle, were sampled with the expectation that all three were Pennsylvanian. Asample from Pingston Creek is Triassic (as revealed by an interbedded ash dated at ca. 223 Ma) and is dominated by age clusters of 341–359 and 1804–1866 Ma, both consistent with a sediment source in the Yukon-Tanana terrane. Minor age clusters at 848–869 and 1992–2018 Ma could have been sourced in the older part of the Farewell terrane. Still other minor age clusters at 432–461, 620–657, 1509–1536, and 1627–1653 Ma are not readily linked to sources that are now nearby. A sample from Surprise Glacier is mid-Mississippian or younger. A dominant age cluster at 1855–1883 and a minor one at 361–367 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 335–336, 457–472, 510–583, and 1902–1930 have no obvious nearby source. A sample from Ripsnorter Creek is Silurian or younger. The dominant age cluster at 937–981 Ma and a minor one at 2047–2077 Ma could have been sourced in the Farewell terrane. Minor age clusters at 1885–1900 and 2719–2770 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 429–490, 524–555, 644–652, 1023–1057, 1131–1185, and 1436–1445 Ma have no obvious nearby source. The so-called Mystic subterrane is structurally complex and would appear to include more than one Phanerozoic turbidite succession; more mapping and detrital zircon geochronology are needed.
Wickersham and Yukon-Tanana terranes. A grit from Wickersham terrane in Tanana quadrangle and a grit from Yukon-Tanana terrane in Talkeetna quadrangle have similar, exclusively Precambrian detrital zircon populations, supporting previous correlations. The Wickersham sample has major age clusters at 1776–1851 and 1930–1964 Ma, and the youngest grain is 1198 Ma. The Yukon-Tanana grit has a major age cluster at 1834–1867 Ma, and the youngest grain is 1789 Ma. A North American source has been previously proposed, and this seems likely based on detrital zircon data.
Ruby terrane and Minook Complex. Detrital zircons from quartzites in the Ruby terrane show two quite different age patterns. A sample from the Bear Creek area of Tanana quadrangle has detrital zircon ages that are similar to those from the Wickersham and Yukon-Tanana grits. The dominant age clusters are 1823–1856 and 1887–1931 Ma. In contrast, a quartzite from nearby Senatis Mountain (Tanana quadrangle) yielded a completely different detrital zircon age spectrum, featuring a broad peak with no significant gaps from 1024 to 1499 Ma and a minor age cluster at 1671–1695 Ma. The youngest concordant zircon is 1024 ± 6 Ma. A quartzite from the Minook Complex, a sliver along the Victoria Creek strike-slip fault in Tanana quadrangle, is similar to the Senatis Mountain sample. Its detrital zircon population is dominated by grains between 1103 and 1499 Ma, with peaks within that range at 1161–1234 and 1410–1490 Ma; minorolderage clusters are at 1643–1676, 1765–1781, and 1840–1874 Ma. The youngest concordant grain is 1103 ± 6 Ma. Finally, a quartzite from Illinois Creek (Nulato quadrangle) at the extreme west end of the Ruby geanticline, previously assigned to the Ruby terrane, also has a detrital zircon age spectrum like that at Senatis Mountain. Mesoproterozoic zircons are predominant, with main age groups at 1329–1391 and 1439–1493 Ma and lesser ones at 1058–1072, 1184–1193, 1681–1692, and 1852–1879 Ma. The youngest concordant grain is 1058 ± 33 Ma. These barcodes are dominated by Mesoproterozoic zircons that are strikingly similar in age to detrital zircons in Neoproterozoic Sequence B in northwestern Canada (and easternmost Alaska, where it equates to the lower Tindir Group). Among other rocks, the Ruby geanticline thus might include a shortened, metamorphosed, and offset continuation of this ancient North American basin, which was sourced in the Grenville orogen.
Rampart Group, Angayucham-Tozitna terrane. The Rampart Group is thought to have been deposited in an ocean basin that closed during the Brookian Orogeny. Detrital zircons from graywacke of the Rampart Group are dominated by an age cluster at 380–404 Ma, with lesser ones at 351–364, 426–440, 484–504, 909–920, 1001–1020, 1127–1128, 1211–1217, and 1912–1953 Ma. The youngest grain is 260 ± 1 Ma. The dominant 380–404 Ma age cluster can be reasonably linked to sources in Devonian plutons of the now-adjacent Brooks Range and Ruby terrane.
Sandstones and metasandstones of poorly known age and dubious tectonic affinity are common in Alaska. In reconnaissance 1:250,000-scale mapping, the demands of regional coverage allow time for little more than a basic description of such rocks while in the field—but ample time for follow-up studies in the lab. Detrital zircon geochronology has emerged as a particularly valuable tool for this kind of reconnaissance geology. Detrital zircons provide constraints on the depositional ages of sandstones; they provide a new basis for correlations; they can help with mapping decisions; and they can suggest provenance links. All of these, in turn, can shed new light on paleogeography and tectonic evolution.
Since 1994, we've been collecting sandstones from throughout interior Alaska (Fig. 1) for detrital zircon geochronology. The sample suite is somewhat random because collections were made as opportunities arose during the course of fieldwork that was funded for other reasons. The samples fall into two broad groups. An older group of 13 samples, the topic of this paper, are from sandstones that predate assembly of Alaska's terranes. A younger group of ∼25 samples are from strike-slip and foreland basins formed during Mesozoic juxtaposition of the terranes; these will be discussed elsewhere. These new data establish detrital zircon “barcodes” (characteristic suites of age clusters) for rocks that purportedly belong to the Wickersham, Yukon-Tanana, Ruby, and Angayucham-Tozitna terranes, the Nixon Fork and Mystic subterranes of the Farewell terrane, and to two fault slivers that were of questionable affinity. Published information on detrital zircons in Alaska is sparse; Table 1 102 summarizes previous detrital zircon results from Triassic and older sandstones and metasandstones.
It is widely accepted that almost all of Alaska, including the entire area of Figure 1, is underlain by terranes that have been significantly displaced with respect to stable North America. We begin a brief tour of these terranes with Wrangellia (Fig. 1), which in Alaska includes a Pennsylvanian-Permian arc and a Triassic large igneous province and which collided with inboard rocks in the Cretaceous. The corresponding suture zone is marked by the informally named Kahiltna flysch basin of Late Jurassic to Late Cretaceous age. Wrangellia and the Kahiltna flysch basin(s) are beyond the scope of our study; new detrital zircon data are presented in this volume by Hampton et al. (2007) and Kalbas et al. (2007).
In the western Alaska Range, the next major terrane inboard is the Farewell, a microcontinental fragment of Siberian affinity. It includes the Nixon Fork and Dillinger subterranes, which are deposits of a passive margin platform and adjacent deep-water basin, respectively. Aproblematic package of Devonian to Jurassic rocks known as the Mystic subterrane (Bundtzen et al., 1997) has also been considered part of the Farewell terrane. We present new detrital zircon data for both the Nixon Fork and Mystic subterranes and question whether the Mystic subterrane (or terrane) as shown on various compilation maps represents a single tectonic entity.
In the eastern Alaska Range, the next major terrane inboard from the Kahiltna flysch is the metasedimentary and metaigneous Yukon-Tanana terrane. Its protoliths include a Neoproterozoic to lower Paleozoic siliciclastic-dominated continental margin assemblage and a Devonian-Carboniferous continental-margin igneous belt. To the northwest, the Wickersham terrane has been regarded as a lower-grade equivalent of the older protoliths of the Yukon-Tanana (Weber et al., 1985). Still farther inboard, the Ruby terrane includes similar protoliths but had a Late Jurassic to Early Cretaceous history of high-pressure metamorphism not present in the Yukon-Tanana. Our detrital zircon data bear on connections between the Wickersham, Yukon-Tanana, and Ruby terranes and reveal some unexpected complications in rocks previously assigned to the Ruby.
In the northern part of Figure 1, the Angayucham-Tozitna terrane is an oceanic tract related to closure of an ocean basin leading up to the Brookian orogeny. The southern (Tozitna) portion was emplaced over the Ruby terrane in the Late Jurassic to Early Cretaceous; our new detrital zircon data bear on the nature of the precollisional source of clastics.
Zircon separations were done at the U.S. Geological Survey in Anchorage, the University of Idaho, and by Apatite to Zircon, Inc., in Moscow, Idaho. Mineral separations done at the USGS and University of Idaho followed standard density and magnetic separation techniques. The separations by A to Z, Inc., were done using sodium polytungstate on the bulk sample, thus bypassing the Wilfley or Rogers table entirely. Zircon grains were hand picked with the aim of including all significant types based on size, color, and roundness.
Analytical techniques for detrital zircon geochronology have evolved significantly in the past twenty years, from multigrain analyses by thermal ionization mass-spectrometry (TIMS), to singlegrain TIMS analyses, and now to analyses of small spots on single grains using either SHRIMP (sensitive high-resolution ion microprobe) or LAICPMS (laser ablation inductively coupled mass spectrometry) technology. In this study we used the SHRIMP, which features the best spatial resolution of the three methods. Zircon U-Pb analyses were conducted on the SHRIMP-RG (reverse geometry) ion microprobe operated jointly by U.S. Geological Survey and Stanford University at Stanford, where the procedures outlined in Appendix 1 are used. Analyses were done during five sessions between 2000 and 2004.
Geochronological results are plotted on concordia diagrams, histograms, and probability density plots. The concordia diagrams show all data regardless of concordance; the histograms and probability plots show only those analyses that are 100 ± 10% concordant and that have 1s errors less than 4% of the age. For zircons older than 1400 Ma we quote the 207Pb/206Pb age, and for younger zircons we quote the 206Pb/238U age. Where one age is above and one age is below the 1400 Ma cutoff and the other requirements are met, we cite the 207Pb/206Pb age. We intended to analyze 60 detrital grains per sandstone sample. In a few cases we analyzed fewer than 60 because the yield was insufficient, and in a few other cases where discordance was a problem, we analyzed more than 60 grains. Results are summarized in Table 2 202, and analytical data are given in Table 3 302303304305306307308309310311312313314. The data are plotted as histograms and probability density curves in Figures 3–6. Both for brevity and to facilitate comparisons between the text and data tables, age clusters are identified without accounting for the corresponding uncertainties. Thus, ages of 350 ± 5, 355 ± 20, and 360 ± 5 would be referred to as an age cluster of 350–360 Ma, rather than 335–375 Ma.
DETRITAL ZIRCONS FROM THE NIXON FORK SUBTERRANE OF THE FAREWELL TERRANE
The Farewell terrane is a microcontinental fragment in the Alaskan interior that is about the size of Switzerland (Fig. 1). Its 850–980 Ma basement (McClelland et al., 1999; Bradley et al. 2003) is overlain by a passive-margin platform sequence (Nixon Fork subterrane) of late Neoproterozoic to Devonian age. Cambrian to Devonian fossils from the Nixon Fork are a mix of Siberian and North American forms that rule out the once popular view of the Farewell as a displaced piece of the passive margin of western Canada (Blodgett et al., 2002; Dumoulin et al., 2002).
Farewell Quartzite, Ruby Quadrangle—Samples TOA97-3-6-1b and 98-27
Two samples of quartzite were collected from the Farewell terrane in eastern Ruby quadrangle, a poorly exposed region where systematic geologic mapping has never been done. Our limited knowledge (Fig. 2A) 202 is based on a few brief helicopter-supported reconnaissance traverses and on follow-up geochronology on representative samples. Neoproterozoic basement rocks here include metarhyolite and orthogneiss. Metarhyolite from various areas of spotty outcrop yielded U-Pb TIMS zircon ages of 979, 980, and 921 Ma (McClelland et al., 1999). Orthogneiss bodies yielded U-Pb TIMS ages of 850 and 851 Ma (McClelland et al., 1999), which are presumably igneous ages and close to the age of the youngest, poorly dated rhyolite.
We interpret exposures of quartzite and metamorphosed carbonate rocks in the area of Figure 2A 202 to be part of an infolded cover sequence. Pebbles resembling a nearby orthogneiss are found in interlayered quartzite, quartz grit, and pebble conglomerate at outcrop TOA97-3-6-1 (Fig. 2A) 202, suggesting that the quartzite (1) is part of a cover sequence over orthogneiss basement, and (2) is younger than 850 Ma. By elimination, the quartzites must be sandwiched stratigraphically between the 850-Ma orthogneiss and the better known, Paleozoic part of the Nixon Fork platform not far to the south. Infolding of fossiliferous Paleozoic carbonates with older basement has been documented in the northern Medfra quadrangle (Patton et al., 1980; Bradley et al., 2003). We interpret the quartzites in the area of Figure 2A in the same manner.
The quartzites are not closely dated but are likely Neoproterozoic. They are probably older than the oldest Neoproterozoic strata at Lone Mountain, ∼200 km to the south in the McGrath quadrangle (Fig. 1). There, the Nixon Fork succession includes more than 600 m of section below the lowest fossiliferous Middle Cambrian beds (Babcock et al., 1994). The base of this succession is not exposed; the oldest exposed beds are regarded as upper Neoproterozoic (Babcock et al., 1994). The Lone Mountain section lacks white quartzites like those in Ruby quadrangle, and we therefore infer that the latter are likely older than anything exposed at Lone Mountain.
The two quartzites have remarkably similar detrital zircon populations (Figs. 3A–3D). Sample TOA97-3-6-1b has a major peak at 1980–2087 Ma, and subsidiary peaks at 944–974, 1366–1383, 1957–1968, and 3123–3138 Ma. Similarly, sample 98-27 has a major peak at 2010–2095 Ma and lesser peaks at 912–946, 1366–1395, and 1937–1967 Ma. The youngest concordant detrital zircons in the quartzites are 888 ± 40 and 851 ± 22 Ma, respectively.
Farewell Quartzite, Taylor Mountains Quadrangle—Sample 04SK241c
The quartzite from the Nixon Fork subterrane in the Taylor Mountains quadrangle (Fig. 2B) is from ∼300 km south of the other two samples. Here, the oldest known rocks are late Neoproterozoic quartzites that are conformably overlain by Cambrian limestones. The Cambrian section has minor interbedded quartzite, including our detrital zircon sample. The Ordovician section includes carbonates and graptolitic shale, and the Silurian is algal boundstone. Younger Paleozoic strata appear to be absent. The rest of the section includes Triassic limestone, Jurassic chert, and Upper Cretaceous flysch of the Kuskokwim Group. All of these rocks were imbricated in a Late Cretaceous or early Tertiary thrust belt (Fig. 2B).
The detrital zircon population is dominated by a broad peak from 1935 to 2057 Ma; lesser peaks are at 1877–1883, 1914–1918, and 2713–2751 Ma (Figs. 3E and 3F). The youngest concordant grain is 1169 ± 15 Ma, far older than the suspected Cambrian depositional age.
Discussion of Farewell Terrane Samples
The three Farewell quartzites have similar detrital zircon populations, dominated by zircons with age ranges of 1957–2087, 1937–2095, and 1914–2057 Ma, respectively. Sample 04SK241c differs in that ca. 900-Ma and ca. 1300-Ma zircons are lacking. Although it is unlikely that the three quartzites correlate precisely, all are from low in the Nixon Fork platform succession.
What is now referred to as the Farewell terrane was long regarded as a displaced piece of the Paleozoic passive margin of western Canada (e.g., Blodgett and Clough, 1985). This notion—a foundation of subsequent reconstructions of the assembly of Alaska (e.g., Plafker and Berg, 1994)—is no longer tenable because conodonts, trilobites, gastropods, and other fossils all support a non–North American origin near the Siberian craton (Dumoulin et al., 2002; Blodgett et al., 2002). In light of this faunal evidence, a search for plausible Siberian zircon sources might also be fruitful. At present, we know of two: (1) the Uchur Group of southeastern Siberia (depositional age ca. 1650 to ca. 1350 Ma), which contains abundant ca. 2050 Ma detrital zircons (Khudoley et al., 2001), and (2) the Goloustnaja migmatite along the southern margin of the Siberian craton, which yielded a U-Pb zircon SHRIMP age of 2018 ± 28 Ma (Poller et al., 2005). The detrital zircons of the Farewell terrane do not appear to be of North American provenance because the familiar peak at ca. 1.8 Ga (a hallmark of North American detrital zircon populations) (Gehrels et al., 1995) is not present in any of the three Farewell terrane samples.
The abundant detrital zircons around 2050 Ma are a remarkably close match for U-Pb zircon ages from two areas of Paleoproterozoic rocks in western Alaska. Granitoids from the Kilbuck terrane yielded igneous crystallization ages of 2052 and 2070 Ma (Moll-Stalcup et al., 1996); granitoids from the Kilbuck's alongstrike correlative, the Idono Complex, yielded igneous crystallization ages of 2062 and 2066 Ma (Miller et al., 1991). This suggests a possible connection with the Farewell terrane that had not previously been entertained. It is possible that the Idono and Kilbuck are merely older basement tracts of a Farewell-Kilbuck microcontinent.
The dominance of ca. 2050 Ma detrital zircons in the Farewell quartzites also suggests an intriguing new link to the Brooks Range. Moore et al. (1997a) reported U-Pb TIMS ages of detrital zircons from arkose of the Carboniferous Nuka Formation, an enigmatic unit high in the Brooks Range thrust stack. The Nuka analyses do not meet modern standards in that they were done on multigrain fractions and are extremely discordant. Nonetheless, 10 out of 11 multigrain zircon fractions yielded upper intercept (207Pb/206Pb) ages in the range of 2013–2078 Ma, suggesting derivation from a granitic source of this age range. A provenance link between rocks of the Brooks Range and Farewell terrane is consistent with recently documented faunal similarities among their lower Paleozoic successions. The Ordovician carbonates of the western Brooks Range and Nixon Fork subterrane share strikingly similar conodont faunas, which are a mix of North American and Siberian forms (Dumoulin et al., 2002).
DETRITAL ZIRCONS OF THE MYSTIC SUBTERRANE
The Mystic subterrane is a problematic tract of middle Paleozoic, late Paleozoic, and early Mesozoic rocks in the western Alaska Range (Fig. 2C). It was once treated as a separate terrane (e.g., Jones et al., 1982) but later grouped with the Nixon Fork and Dillinger into the Farewell terrane (Decker et al., 1994). The basis for this linkage is a widespread Devonian limestone—a minor but conspicuous unit in the complex Mystic tract—that depositionally overlies both Nixon Fork and Dillinger lithologies. The Mystic outcrop area includes Silurian, Devonian, Mississippian, and Pennsylvanian carbonates, Devonian and Triassic black shale and phosphorite, Devonian barite, Permian conglomerate, Triassic gabbroic sills, undated pillow lavas, undated mélange, and siliciclastic turbidites of Silurian(?), Carboniferous(?) and Triassic ages. In their reconnaissance geologic mapping of the Talkeetna quadrangle, Reed and Nelson (1980) lumped most of this assemblage into a catchall map unit (their unit Pzu) and recognized structural complications including large recumbent folds and mélange. Here we discuss the Mystic subterrane separately from better known and more tractable parts of the Farewell terrane.
Mystic Subterrane at Pingston Creek, Talkeetna Quadrangle—Sample 03ADw415c
Reed and Nelson (1980) thought that the abundant flysch-like rocks of unit Pzus were Pennsylvanian in age, based on their interpretation that the nonmarine Mt. Dall conglomerate (now known to be Permian; Bradley et al., 2003) grades downward into flysch. While this may turn out to be true for some of the Mystic flysch, it is far from the complete story. Outcrops on bluffs overlooking the headwaters of Pingston Creek (Fig. 2C) consist mostly of turbidites made up of interbedded slate, granule conglomerate, and sandstone, plus rare micritic limestone (barren of conodonts) and rare felsic ashfall tuff. This section is slightly overturned but not intensely deformed. A Late Triassic (Carnian, ca. 223 Ma) depositional age for this section is suggested by zircon geochronology. Azircon separate from a conspicuous 1-m-thick ashfall tuff (sample 03ADw415g) is dominated by rounded detrital grains having a range of ages (see next paragraph) but including a few fresh, euhedral grains. We interpret the euhedral grains to be magmatic. The youngest grains, each concordant at 1s, are 221.8 ± 2.3, 225.0 ± 2.6 Ma, 228.9 ± 2.3, and 235.2 ± 1.2 Ma (SHRIMPages) (Fig. 4C). Another zircon yielded a concordant TIMS age of 234 Ma (R. Friedman, written commun., 2004). Our preferred interpretation is that the youngest zircons approximate the eruptive age of the tuff at ca. 223 Ma and that the slightly older zircons are either xenocrysts or detrital grains from slightly older parts of the same igneous system. Alternatively, the tuff might have an eruptive age of 234–235 Ma, in which case the younger zircons would be interpreted as exhibiting lead loss. Either way, the ash-fall tuff, and associated sandstones, are Triassic.
Sample 03ADw415c, a granule conglomerate a few tens of meters upsection from the ashfall tuff, yielded abundant detrital zircons. The zircon population is dominated by age clusters of 341–359 and 1804–1866 Ma; subsidiary groups include 432–461, 620–657, 848–869, 1509–1536, 1627–1653, 1992–2018, and 2685–2694 Ma (Fig. 4B). The youngest detrital grain is 293 ± 2 Ma. Detrital grains in sample 03ADw415g are shown with those from sample 03ADw415c in Figures 4A and 4B. The youngest clearly detrital grain in the ashfall tuff is 275 ± 2 Ma.
Mystic Subterrane near Surprise Glacier, Talkeetna Quadrangle—Sample 03ADw407d
This sample is from calcareous siliciclastic turbidites near Surprise Glacier, Talkeetna quadrangle (Fig. 2C). The sample location is a few kilometers from Mystic Pass, where the Mystic subterrane got its name. The strata are subvertical and quite disrupted. The zircon population is dominated by grains of 1855–1930 Ma; lesser groups include 335–336, 361–367, 457–472, and 510–538 Ma (Fig. 4D). The youngest concordant grain is 335 ± 5 Ma, indicating that the depositional age is Visean (mid-Mississippian) or younger.
Mystic Subterrane at Ripsnorter Creek, Talkeetna Quadrangle—Sample 03AM4b
This sample is from a fault sliver of turbiditic granule conglomerate that is interbedded with calcareous sandstone and black slate along Ripsnorter Creek, a few hundred meters south of the Denali strike-slip fault in Talkeetna quadrangle (Fig. 2C). This outcrop was not visited by Reed and Nelson (1980), but it is shown on their geologic map as either belonging to (1) their calcareous unit Pzsl (“Paleozoic shale and limestone”), which later came to be called the Pingston terrane (Jones et al., 1982), or (2) their unit Pzsv, which was later assigned to the Yukon-Tanana terrane (see next major heading). Neither unit assignment is appropriate. Yukon-Tanana rocks have been metamorphosed, whereas those at Ripsnorter Creek have not. The Pingston terrane consists of Triassic fine-grained limestone and calcareous black shale pervaded by Triassic mafic sills; granule conglomerate is totally lacking. At Ripsnorter Creek, the granule conglomerate contains conspicuous orange-weathered, fine-grained sedimentary clasts in a dark gray shaly matrix and strongly resembles turbidites seen throughout the Mystic outcrop belt. The main peaks in the detrital zircon population (Fig. 4F) are at 429–490, 937–981, and 1131–1185 Ma. Lesser peaks are at 524–555, 644–652, 1023–1057, 1436–1455, 1885–1900, 2047–2077, and 2719–2770 Ma. The age of the youngest zircon is somewhat problematic. The most reliable of the youngest grains is 429 ± 3 Ma and only 1% discordant. Another grain has a 206Pb/238U age of 378 ± 2 Ma but is 16% discordant and has a 207Pb/206Pb age of 440 ± 38 Ma. On this basis, we suggest a depositional age of a mid-Silurian or younger.
Discussion of Mystic Subterrane Samples
The three samples from the Mystic subterrane are probably not even the same stratigraphic unit or the same depositional age. Our detrital zircon results suggest the existence of a Pingston Creek1 flysch unit of Triassic age, a Surprise Glacier flysch unit of Carboniferous(?) age, and a Ripsnorter Creek flysch unit of Silurian(?) age.
The Triassic sample from Pingston Creek represents a previously unrecognized rock sequence. Detrital zircons from the Pingston Creek sample show that it was derived from an extremely varied source. The age clusters at 341–359, 1804–1866, and 2685–2694 Ma suggest a Yukon-Tanana source (Table 1) 102. The minor age clusters at 848–869 and 1992–2018 Ma could have come from the Farewell terrane. However, age clusters at 432–461, 620–657, 1509–1536, and 1627–1653 Ma cannot be readily related to rocks that now are close to the Mystic belt. The 1509–1536 Ma cluster falls within the “North American magmatic gap” (1490–1610 Ma; Ross and Villeneuve, 2003). One possible correlative unit—the turbiditic Middle and Upper Triassic Perseverance Group of the Taku terrane—crops out some 1000 km along strike in southeast Alaska (Gehrels, 2002). A detrital zircon population from the Perseverance has a dominant age cluster of 349–364 Ma (Gehrels, 2002), showing a striking overlap with the Pingston Creek zircons. This is only a partial match, however, because the Perseverance contains only a few Precambrian grains.
The sample from Surprise Glacier is mid-Mississippian or younger; we assign it a Carboniferous(?) age because it is clearly something different from the Triassic unit, and because most flysch sequences of the world include detrital zircons that are not much older than the depositional age. The age clusters at 361–367 and 1855–1883 are good matches to igneous (Dusel-Bacon et al., 2004) and detrital zircon ages (Table 1) 102 from the Yukon-Tanana terrane. Potential sources of the lesser age clusters at 335–336, 457–472, and 510–538 Ma are not obvious.
The sample from Ripsnorter Creek is mid-Silurian or younger; we assign it a Silurian(?) age because it is clearly different from the Triassic and Carboniferous(?) units, and because most flysch sequences include detrital zircons that are not much older than the depositional age. It is most likely part of the Silurian Terra Cotta Mountains Sandstone of the Dillinger subterrane, which outcrops in McGrath quadrangle (Bundtzen et al., 1997). Detrital zircon barcodes are not yet available for the Terra Cotta Mountains Sandstone. The Ripsnorter Creek sample was derived from a varied source, but one that was quite different from the Pingston Creek or Surprise Glacier sandstones. The major age cluster at 937–981 and the minor one at 2047–2077 Ma could have come from the Farewell terrane. Zircons with age ranges of 1885–1900 and 2719–2770 Ma are consistent with a Yukon-Tanana source (cf. Table 1 102). The other age clusters (Fig. 4F and Table 2 202) cannot be readily related to rocks that now are close to the Mystic belt. However, those at 429–441, 471–490, 524–555, 1023–1057, and 1131–1185 are comparable to igneous and (or) detrital zircon ages from the Alexander terrane of southeastern Alaska (Gehrels et al., 1996; Karl et al., 2006).
DETRITAL ZIRCONS FROM THE WICKERSHAM AND YUKON-TANANA TERRANES
The Wickersham terrane, which consists of low-grade “quartz-eye grit,” red and green mudstone, and minor limestone of inferred late Neoproterozoic to Cambrian age, is mapped in Livengood and Tanana quadrangles (Weber et al., 1992; Chapman et al., 1982). The Wickersham is bounded to the southeast by higher-grade metasedimentary rocks of comparable protolith, which are assigned to the vast Yukon-Tanana terrane. The Yukon-Tanana underlies much of east-central Alaska and has been traced as far southeast as the Alaskan panhandle and as far southwest as the Talkeetna quadrangle. In addition to protoliths that seem to correlate with the Wickersham grit (Weber et al., 1985), the Yukon-Tanana terrane also includes a metamorphosed Devonian-Mississippian continental-margin magmatic belt (Dusel-Bacon et al., 2004, and references therein).
Wickersham Grit, Sample 03RSR3b, Wickersham Terrane, Tanana Quadrangle
The main belt of Wickersham grit crops out in Livengood quadrangle, but lithologically similar rocks have been traced as far west as Tanana quadrangle. We sampled the grit in a fault slice just south of the Victoria Creek strike-slip fault, in Tanana B1 quadrangle (Fig. 2D). In their description of the grit member of the Wickersham in this area, Reifenstuhl et al. (1997) characterized the rocks as plagioclase-bearing quartzite that is typically bimodal, having outsized monocrystalline quartz grains to several mm diameter, in a finer-grained matrix. The main age clusters are at 1776–1851 and 1930–1964; lesser peaks are at 2088–2125, 2310–2318, 2357–2380, and 2539–2571 Ma (Fig. 5A and B) 502. The youngest grain is 1789 Ma, presumably quite a bit older than the putative late Neoproterozoic to Cambrian depositional age.
Unit Pzsv, Sample 03AM7f, Yukon-Tanana Terrane, Talkeetna Quadrangle
Rocks assigned to the Yukon-Tanana terrane occur as far west as the western edge of Talkeetna quadrangle in a narrow belt just north of the Denali fault (Fig. 2C). Reed and Nelson (1980) described this metamorphic belt as an isoclinally folded, polydeformed assemblage of quartzite, quartz semischist, quartz grit, metavolcanic rocks, limestone, green and maroon phyllite, and schist. The sample is a quartz-rich grit from between Pingston Creek and Tonzona River; the grit is several tens of meters thick. The dominant detrital zircon population is in the range 1789–1893 Ma; minor peaks are at 1924–1974, 2668–2684, and 2733–2769 Ma (Figs. 5C and 5D). Many grains were found to be discordant. The youngest concordant grain is 1789 ± 18 Ma. Arrays of discordant grains (Fig. 5D) point to a lead-loss event, perhaps related to metamorphism, at ca. 200 Ma (near the Triassic-Jurassic boundary).
Discussion of Wickersham and Yukon-Tanana Terrane Samples
Our new detrital zircon data are entirely consistent with older interpretations. Grits of unit Pzsv in the Talkeetna quadrangle were correlated with the now abandoned Birch Creek Schist by Reed and Nelson (1980), a unit that later came to be assigned to the Yukon-Tanana terrane (e.g., Jones et al., 1982; Wilson et al., 1998). A comparison between detrital zircon populations in unit Pzsv and a Yukon-Tanana quartzite (Jarvis belt) in Healy quadrangle (which has peaks in the probability curve at 1.75–1.95, 2.20, and 2.57–2.69 Ga; I.S. Williams and C. Dusel-Bacon, personal communication, 2005) supports this correlation. Our Wickersham grit sample likewise supports Weber et al.'s (1985) conjecture that the Wickersham is a correlative of the Yukon-Tanana at lower metamorphic grade. Given the abundance of grains at 1800–1900 and ca. 2600 Ma in these various samples, a North American provenance (Gehrels et al., 1995) seems likely for all.
DETRITAL ZIRCONS FROM THE RUBY TERRANE AND MINOOK FAULT BLOCK
The Ruby terrane is a belt of Neoproterozoic(?) and Paleozoic metasedimentary rocks of presumed continental-margin affinity, intruded by Devonian orthogneiss and Early Cretaceous granites (Patton et al., 1994). Rocks of the Ruby terrane occupy and define the core of the Ruby geanticline, a regional structure that is flanked on either side by rocks of the oceanic Angayucham-Tozitna terrane. The deformation history of the Ruby terrane closely parallels that of the southern Brooks Range: both involved Late Jurassic thrust emplacement of oceanic rocks over continental-margin rocks, and concomitant high-pressure metamorphism (e.g., Roeske et al., 1995). We present results for four detrital zircon samples. One is from the most southwesterly exposures of Ruby terrane, in Nulato quadrangle. Two other samples are from a problematic belt in Tanana quadrangle that has been identified as Ruby terrane by some but not all recent workers. The fourth sample is from the Minook fault block in Tanana quadrangle, which, based on detrital zircon data, appears to correlate with two of the other samples.
Quartzite, Illinois Creek, Sample 95-89, Nulato Quadrangle
Illinois Creek (Fig. 2E) is at the extreme southwest end of the Ruby terrane. Exposures are sparse but supplemented by trenches and drill holes connected with gold exploration. Our quartzite sample, from surface exposures, is from a meter-thick, fine-grained quartzite interlayered with dolomitic marble and calcschist. The detrital zircon population is dominated by Mesoproterozoic zircons, with main peaks at 1329–1391 and 1439–1493 Ma and lesser peaks at 1058–1072, 1184–1193, 1681–1692, and 1852–1879 Ma (Figs. 5K and 5L). The youngest concordant grain is 1058 ± 33 Ma.
The blueschist-facies metasedimentary rocks at Illinois Creek are strongly deformed, and original stratigraphic relations are dubious. A drill hole near the sample location intersected ∼80 m of quartz-mica schist and 20 m of quartzite structurally overlying ∼550 m of dolostone. Ordovician conodonts were recovered from the dolostone (late Llanvirnearly Llandeilo, Pygodus serra zone; Harris, 1984), but it is not assured that this is also the age of the quartzite.
Quartzite, Senatis Mountain, Sample 02ADw510a, Tanana Quadrangle
This quartzite is from the southeastern flank of the Ruby geanticline in Tanana B2 quadrangle (Fig. 2D). Most terrane maps (e.g., Figure 1, from Silberling et al., 1994) include this area as part of the Ruby terrane. Dover (1994), who deemphasized terranes in his Decade of North American Geology chapter, mapped it as his “Devonian metaclastic sequence.” The sample is from a relatively small area in which a localized Paleocene (ca. 61 Ma) metamorphic event overprinted an older (presumably Jurassic) metamorphic event (Till et al., 2003). Detrital zircons from the quartzite show a broad peak with no significant gaps from 1024 to 1499 ma, and a minor age cluster at 1671–1695 Ma (Figs. 5G and 5H). The youngest concordant zircon is 1024 ± 6 Ma.
Quartzite, Bear Creek Headwaters, Sample 94RQ, Tanana B3 Quadrangle
The detrital zircon sample is from a ridgeline in the head-waters of Bear Creek, north of the Yukon River in Tanana B3 quadrangle (Fig. 2D). It also lies within the belt of rocks mapped as the “Devonian metaclastic sequence” by Dover (1994). The sampled quartzite is ∼5 m thick and occurs as an isoclinally folded layer within muscovite-biotite-chlorite schist. The detrital zircon population is dominated by a broad peak at 1823–1931 Ma. Minor age clusters include 1148–1177, 1408–1410, 1776–1784, 2081–2092, 2288–2332, 2553–2565, 2595–2627, 2720–2795, and 2830–2831 (Figs. 5E and 5F). The youngest grain is 674 ± 26 Ma (206Pb/238U age) but −9.9% reversely discordant (the 207Pb/206Pb age of this grain is 612 ± 36 Ma). The next youngest grain is 1148 ± 46 Ma.
Quartzite, Minook Fault Block, Sample 02ATi23, Tanana Quadrangle
This quartzite is from unit pTaq (of Reifenstuhl et al., 1997) from the Minook fault block, a sliver along the Victoria Creek fault zone (Fig. 2D). The Minook block is noteworthy because its Paleocene metamorphic ages (Reifenstuhl et al., 1997; Till et al., 2003) contrast with Late Jurassic to Early Cretaceous metamorphic ages in the Ruby terrane not far to the northwest (Roeske et al., 1995). The detrital zircon population is dominated by grains between 1103 and 1499 Ma, with peaks within that range at 1161–1234 and 1410–1490 Ma. Minor older peaks are at 1643–1676, 1765–1781, and 1840–1874 Ma (Figs. 5I and 5J). The youngest concordant grain is 1103 ± 6 Ma.
Discussion of Ruby Terrane and Minook Fault Block Samples
Our four detrital zircon samples reveal new complexities in the geology of the Ruby geanticline. The zircon barcodes fall into two basic types. One is reminiscent of the Wickersham and Yukon-Tanana terranes, whereas the other is reminiscent of Neoproterozoic Sequence B of northwestern Canada.
The Bear Creek sample suggests that Ruby geanticline includes rocks of similar age (Late Neoproterozoic to Cambrian) and source (probably Laurentian) as the Wickersham grit.
Quartzites from Illinois Creek, Senatis Mountain, and the Minook fault block have detrital zircon populations dominated by Mesoproterozoic grains (Figs. 5G–5K). None of the flanking terranes provides an obvious source. Similar detrital zircon populations have been reported from Neoproterozoic Sequence B in northwestern Canada; this unit has a depositional age <1070 Ma (Rainbird et al., 1997), and its mainly Mesoproterozoic detrital zircons are regarded as evidence for transport by a huge river system sourced thousands of kilometers away in the Mesoproterozoic Grenville orogen of eastern North America. Whereas the Canadian detrital zircon sample was collected ∼800 km east of the Minook outcrops, Neoproterozoic Sequence B has been correlated with the lower Tindir Group that straddles the Alaska-Yukon border and is only ∼400 km east of Minook. Thus, it seems conceivable that the three Alaskan quartzites represent displaced correlatives of Neoproterozoic Sequence B. A less likely possibility is that the Alaskan quartzites were formed by erosion of Sequence B—but if so, why are no other zircons mixed in from other sources being simultaneously eroded? Finally, it is worth noting that although detrital zircon populations dominated by Mesoproterozoic grains are new to Alaska, detrital zircon barcodes dominated by Mesoproterozoic grains have also been reported from Siberia, Sweden, Scotland, and East Greenland (Watt and Thrane, 2001). Thus, although a North American Grenville source is certainly plausible for the Ruby samples, other possibilities should not be discounted.
These findings shed new and different light on a problematic belt: Dover's (1994) “Devonian metaclastic sequence.” Dover (1994) believed that this belt could be traced several hundred kilometers along the southeast limb of the Ruby geanticline and along the southern flank of the Brooks Range, where the rocks are called the Slate Creek subterrane or variants on that name. Moore et al. (1994) interpreted the Slate Creek as a subterrane of the Arctic Alaska terrane, whereas Patton et al. (1994) regarded the Slate Creek as a thrust panel of the oceanic Angayucham-Tozitna terrane. In the Tanana quadrangle, Dover (1994) did not emphasize terrane assignments but correlated his Devonian metaclastic sequence with other Devonian siliciclastic rocks of Alaska and northwestern Canada, including the Kanayut Conglomerate of the Arctic Alaska terrane in the Brooks Range and the Nation River Formation of parautochthonous North America in east-central Alaska. As summarized in Table 1 102, the main detrital zircon age clusters in the Nation River are 424–434, 1815–1838, and 2653–2771 Ma (Gehrels et al., 1999), and detrital zircons in the Kanayut Conglomerate are 400–430 Ma (Moore et al., 2004). The Mesoproterozoic detrital zircons from Senatis Mountain do not support these correlations.
Before our study, the Bear Creek and Senatis quartzites were regarded as Devonian (Dover, 1994), and the Illinois Creek quartzites as Ordovician (Harris, 1984), based on fossils from nearby carbonate rocks. In each case, these age assignments rest on the assumption that that quartzites and carbonates are interbedded. Given the structural complexity of the Ruby terrane (e.g., Roeske et al., 1995), a safer statement is that quartzite and carbonate are interlayered. This allows the possibility that the quartzites could be as old as Neoproterozoic.
DETRITAL ZIRCONS FROM RAMPART GROUP (ANGAYUCHAM-TOZITNA TERRANE), TANANA QUADRANGLE
The Tozitna terrane encompasses oceanic rocks on the south side of the Yukon-Koyukuk basin; more or less equivalent rocks on the north side of the basin are called the Angayucham terrane, and the two together are the Angayucham-Tozitna terrane. In Tanana and Livengood quadrangles, the Angayucham-Tozitna terrane includes the Rampart Group of Mertie (1937), which corresponds to unit JMms of Chapman et al. (1982): mafic volcanics and gabbroic sills intercalated with a metasedimentary assemblage of argillite, phyllite, chert, slate, tuff, sandstone, and limestone. Late Mississippian and Early Pennsylvanian radiolarians have been recovered from chert; a limestone has yielded pelecypod prisms and bryozoans that suggest a Permian age (Chapman et al., 1982, p. 6). A Rampart gabbroic intrusive yielded a preliminary U-Pb TIMS zircon age of 230 Ma (R. Friedman, written commun., 2004).
Rampart Group Sandstone, Sample 03ATi47b, Angayucham-Tozitna Terrane, Tanana Quadrangle
This sample is from a broad belt of unit JMms of Chapman et al. (1982), northwest of the Yukon River in Tanana C2 quadrangle (Fig. 2D). The detrital zircon population is dominated by age clusters at 380–404 Ma, with lesser peaks at 351–364, 426–440, 484–504, 909–920, 1001–1020, 1127–1128, 1211–1217, and 1912–1953 Ma (Figs. 6A and 6B). The youngest concordant grain is 260 ± 1 Ma (Late Permian).
Discussion of the Angayucham-Tozitna Terrane Sample
Detrital zircons require a depositional age of Late Permian or younger, in support of the rather sketchy paleontological evidence cited above. The detrital zircons show that our Rampart sandstone sample was derived from a varied continental source, in contrast with what might be anticipated from the purported oceanic nature of the Angayucham-Tozitna terrane. The dominant detrital zircon age cluster, at 380–404 Ma, matches dated orthogneiss in the Ruby terrane, the Seward Peninsula, and the Brooks Range (Rubin et al., 1990 and references therein). The various minor age clusters at 351–364, 426–440, 909–920, 1001–1020, 1127–1128, 1211–1217, and 1912–1953 Ma are each reminiscent of particular age clusters of detrital zircons from other rocks in Alaska (Tables 1 102 and 2 202), but there are no obvious correlations between whole populations.
SUMMARY OF TECTONIC IMPLICATIONS
We report detrital zircon barcodes for the Neoproterozoic to Cambrian of the Farewell terrane, the Silurian(?), Carboniferous(?), and Triassic of the Mystic subterrane, the Neoproterozoic to Cambrian of the Wickersham and Yukon-Tanana terranes, a newly recognized Neoproterozoic(?) succession in the Ruby geanticline, and the Permian-Triassic of the Angayucham-Tozitna terrane. As already discussed, the detrital zircon populations bear in various ways on depositional ages, correlations, and map unit assignments—as well as providing baseline data for future comparisons.
One of several unexpected results is that quartzites from the Ruby geanticline and Minook fault block have predominantly Mesoproterozoic detrital zircons, which are reminiscent of detrital zircons from Neoproterozoic Sequence B of northwestern Canada (Rainbird et al., 1997). We speculate that some rocks in the Ruby terrane are displaced fragments of this North American basin. Follow-up is clearly needed. Important targets for detrital zircon geochronology include metasedimentary rocks intruded by Devonian orthogneiss in the Ruby terrane, and the Slate Creek subterrane to the north.
The clear contrast between detrital zircon populations from upper Neoproterozoic to Cambrian strata of the Farewell terrane and broadly coeval strata of the Wickersham and Yukon-Tanana terranes attests to the truly exotic nature of the Farewell. Zircon data are consistent with the Farewell-Siberia connection suggested by fossils (e.g., Blodgett et al., 2002; Dumoulin et al., 2002). Perhaps more surprising is the revelation that the Paleoproterozoic Kilbuck terrane and (or) Idono complex of western Alaska were likely the source of ca. 2050 Ma detrital zircons in the Farewell. Our new data also strengthen the link between the Farewell terrane and the Arctic Alaska terrane of the Brooks Range, which share similar lower Paleozoic stratigraphies, similar fossils of Siberian affinity (Dumoulin et al., 2002), and now, similar detrital zircons. The facts at hand are consistent with the idea that the Farewell, Kilbuck, and Arctic Alaska terranes represent dismembered parts of Şengör and Natal'in's (1996) postulated Bennett-Barrowia microcontinent.
The Farewell terrane's inboard suture is a northeast-striking thrust belt in Livengood and Tanana quadrangles (Fig. 1). As mapped by Weber et al. (1992), Wickersham grits are structurally interleaved with carbonate rocks now regarded as belonging to the Farewell terrane (Blodgett et al., 2002). The nature and timing of this key suturing event are poorly understood but would likely be clarified with detrital zircon studies because erosion of the Wickersham and Farewell terranes should have yielded quite different suites of detrital zircons. Specific units to target are the Devonian Quail, Cascaden Ridge, and Beaver Bend units, the Mississippian Globe quartzite, unnamed Permian and Triassic sedimentary units Trs and Ps, and Cretaceous flysch of the Wilber Creek unit (Weber et al., 1992).
The Farewell terrane's outboard suture is even more problematic because it ties in with the so-called Mystic subterrane. How do rocks with the outcrop area of the Mystic outcrop belt relate to bona fide parts of the Farewell terrane? Of the three Mystic subterrane samples, only the Silurian(?) sandstone shows an obvious detrital zircon linkage to the Farewell terrane. The Triassic sandstone yielded only four (out of 38) zircons of likely Farewell origin, whereas the Carboniferous(?) sandstone yielded not a single zircon (out of 30) that can be readily traced to the Farewell. Hence, although the Mystic belt is generally considered to be part of the Farewell terrane, parts of this tract appear to have other affinities. The Mystic is really more of a mapping catch-all than a legitimate tectonic element and is badly in need of a modern, multidisciplinary study. Among rocks currently assigned to the Mystic subterrane in Talkeetna quadrangle, detrital zircon studies are needed for unit Pzp, the Mt. Dall conglomerate, and additional sandstones in unit Pzus (of Reed and Nelson, 1980). Other important targets include the Terra Cotta Mountains Sandstone in the Dillinger subterrane, McGrath quadrangle (Bundtzen et al., 1997) and the Triassic Pingston terrane (Reed and Nelson, 1980).
The detrital zircon populations described in this paper, together with other published results, lay only a fraction of the necessary groundwork for detrital zircon studies in interior Alaska. Many more samples will need to be analyzed before the full significance of these initial results can be known. The interpretations presented here are deliberately sparse because in our experience most samples have yielded surprises and very few have matched our preconceptions. Even the most generally accepted aspects of Alaskan tectonic evolution must be treated with some skepticism.
Owing to factors such as geologic complexity, scale, remoteness, lack of roads, rugged terrain, impossible river crossings, and vast tracts with little outcrop, only a fraction of Alaska can be efficiently mapped without helicopter support. This increases the value of strategic sampling. Among the many laboratory-based tools not available to earlier generations of Alaskan geologists, detrital zircon geochronology is certainly the most powerful for the study of siliciclastic rocks. Once detrital zircon barcodes have been established for the better-known rock units, we expect this approach to lead to major advances in defining the ages, boundaries, and relationships between Alaska's exotic and pericratonic terranes.
Not part of, and not to be confused with, the Pingston terrane. There are no other geographic names in the area.
APPENDIX 1: U-Pb ANALYTICAL TECHNIQUES
Zircons are handpicked for final purity, mounted on double-stick tape on glass slides in 1 × 6 mm rows, cast in epoxy, ground and polished to a 1 micron finish on a 25 mm diameter by 4 mm thick disc. All grains were imaged with transmitted light and reflected light (and incident light if needed) on a petrographic microscope, and with cathodoluminescence and back scattered electrons as needed on a JEOL 5600 SEM to identify internal structure, inclusions, and physical defects. The mounted grains were washed with 1N HCl or EDTA solution (if acid soluble) and distilled water, dried in a vacuum oven, and coated with Au. Mounts typically sit in a loading chamber at high pressure (10–7 torr) for several hours before being moved into the source chamber of the SHRIMP-RG. Secondary ions are generated from the target spot with an O2− primary ion beam varying from 4 to 6 nA. The primary ion beam typically produces a spot with a diameter of 20–40 microns and a depth of 1–2 microns for an analysis time of 9–12 min. Nine peaks are measured sequentially for zircons (the SHRIMP-RG is limited to a single collector, usually an EDP electron multiplier): 90 Zr2 16O, 204Pb, Bgd (0.050 mass units above 204Pb), 206Pb, 207Pb, 208Pb, 238U, 248Th16O, 254U16O. Autocentering on selected peaks and guide peaks for low or variable abundance peaks (i.e., 96Zr216O 0.165 mass unit below 204Pb) are used to improve the reliability of locating peak centers. The number of scans through the mass sequence and counting times on each peak are varied according to sample age and U and Th concentrations to improve counting statistics and age precision. Measurements are made at mass resolutions of 6000–8000 (10% peak height), which eliminates all interfering atomic species. The SHRIMP-RG was designed to provide higher-mass resolution than the standard forward geometry of the SHRIMP I and II (Clement and Compston, 1994). This design also provides very clean backgrounds and—combined with the high-mass resolution, the acid washing of the mount, and rastering the primary beam for 90–120 seconds over the area to be analyzed before data are collected—assures that any counts found at mass of 204Pb are actually Pb from the zircon and not surface contamination. In practice greater than 95% of the spots analyzed have no common Pb. Concentration data for zircons are standardized against zircon standard SL-13 (238 ppm U) or CZ3 (550 ppm U) and age data against AS3 and AS57 zircons (1098 Ma) from the Duluth Gabbro (Paces and Miller, 1993), RG-6 (1440 Ma, granite of Oak Creek stock, Bickford et al., 1989), or R33 (419 Ma, quartz diorite of Braintree complex, Vermont, John Aleinikoff, personal commun.), which are analyzed repeatedly throughout the duration of the analytical session. Data reduction follows the methods described by Williams (1997) and Ireland and Williams (2003) and use the Squid and Isoplot programs of Ludwig (2001, 2003).
APPENDIX 2: SAMPLE DESCRIPTIONS
Sample TOA97-3-6-1b. Quartzite, Farewell terrane, Ruby A2 quadrangle, T16S R26E, at elevation ∼2300′ on northwest side of low summit in northern part of Section 7, 64°7′0″N, 153°48′10″W. Discontinuous exposures along tundra-covered ridge. White orthoquartzite (processed for detrital zircons) is interbedded with quartz grit and pebble conglomerate. The latter contains pebbles including blue quartz, feldspar, and pink altered granitoid like the nearby orthogneiss body dated at 852 Ma (location in Fig. 2A). Quartzite structurally overlies schist, which may represent depositional basement. Collected by Grant Abbott, 1997.
Sample 98-27. Quartzite, Farewell terrane, Ruby A2 quadrangle, T17S R26E, Section 2, minor summit along ridge at 2210 feet, 64°2′43″N, 153°43′53″W. Discontinuous exposures along tundra covered ridge. The detrital zircon sample is a massive white quartzite. Nearby is rubble of felsic tuff. Collected by Bill McClelland, 1998.
Sample 04SK241c. Quartzite, Farewell terrane, Taylor Mountains D2 quadrangle, T10N R42W, northern edge of Section 2, elev. 750 feet at south end of north-south ridge, 60°59.464′N, 156°40.540′W. Quartzite occurs in a band a few meters wide between limestone and chert. On a nearby ridge along strike, Cambrian trilobites were found in what we map as the same limestone band. The chert is black, recrystallized, and lacks radiolarians. In thin section, the quartzite is a well-sorted, very mature, very fine-grained sandstone. Grains are 95% monocrystalline quartz having sutured boundaries. Minor constituents include chert, siliceous mudstone, carbonate mudstone, zircon, and quartzose siltstone. Collected by Sue Karl, 2004.
Sample 03AM415c. Sandstone in Mystic subterrane from Pingston Creek, Talkeetna C6 quadrangle, T29N R19W, elev. ∼3700 feet in area of small knob directly under the label for Section 11, 62°37′6.8″N, 152°46′50.5″W. Scattered outcrops on bluffs overlooking Pingston Creek expose turbiditic sandstone and granule conglomerate, dark gray slate, minor limestone (barren of conodonts), and minor tuff (sample 03ADw415g, U-Pb age of ca. 223 Ma). In thin section, the sample is a coarse to very coarse sandstone containing subangular to rounded clasts that include schist, sandstone, metasandstone, radiolarian chert, tuffaceous rocks, porphyry, and carbonate. The sandstone is somewhat sheared, nearly a semischist. Collected by Marti Miller, 2003.
Sample 03ADw407d. Sandstone in “Mystic subterrane” near Surprise Glacier, Talkeetna C5 quadrangle, T27N R17W, 2700′ in gorge in southeastern corner of Section 22, 62°42′50″N, 152°23′35″W. Outcrops on north wall of steep gorge of deformed, mainly thin-bedded turbidites. The detrital zircon sample is from a weakly calcareous, 50 cm thick bed. In thin section, the sample is a medium-grained semischist. Recrystallized and flattened clasts include felsic intrusive rocks, fine-grained metasedimentary rock, and mica schist in a groundmass of flattened carbonaceous schist clasts. Secondary calcite is present. Collected by Dwight Bradley, 2003.
Sample 03AM04b. Conglomeratic sandstone in Mystic subterrane from Ripsnorter Creek, Talkeetna D5 quadrangle, T31N R16W, elev. 2300 feet along in Section 21, due east of peak 3970, 62°45′59″N, 152°16′37.8″W. Cutbank exposures along the southern fork of Ripsnorter Creek. Calcareous sandstone, granule conglomerate, and calcareous black slate. The detrital zircon sample is a semischistose granule conglomerate. The larger clasts include limestone, black carbonaceous schist, fine-grained probably tuffaceous rock, and possible felsic prophyry. The medium-grained, semischistose groundmass includes the above components plus quartz, feldspar, and white mica. Collected by Marti Miller, 2003.
Sample 03RSR3b. Wickersham grit, Wickersham terrane, Tanana B1 quadrangle. T6N R13W, elev. 2500 feet, on ridge west of headwaters of Chicken Creek, 65°22′56″N, 150°10′27″W. Collected by Sarah Roeske, 2003.
Sample 03AM07f. Grit in unit Pzsv, Yukon-Tanana terrane, Talkeetna D6 quadrangle, T31N R18W, elev. 4150 feet, 62°45′03″N, 152°46′30″W. Unit is an interleaved mix of maroon and green phyllite, gray phyllite, tan phyllite (metatuff?), limestone, and particularly, quartztose sandstone to granule conglomerate. The sandstone contains outsize quartz grains and is referred to in the field as grit. The detrital zircon sample is from a prominent, resistant grit unit estimated to be a few tens of meters thick. In thin section, the detrital zircon sample is a semischist containing coarse clasts of quartz and feldspar in a finer, foliated groundmass of quartz, feldspar, white mica, and opaque minerals. Collected by Marti Miller, 2003.
Sample 94RQ. Quartzite, Ruby terrane near Bear Creek, Tanana B3 quadrangle, T7N R19W, elev. 3150 feet on east-west ridgeline in Section 27, 64°24.383′N, 151°26.694′W. The detrital zircon sample is from an isoclinally folded quartzite unit having a structural thickness of ∼5 m, within quartz-muscovite schist. Collected by Bill McClelland, 1994.
Sample 02ADw510a. Quartzite, Ruby terrane at Senatis Mountain, Tanana B3 quadrangle, T6N R17W, elev. 2400′ at south end of broad saddle in Section 7, 65°21′ 18″ N, 151°07′35″W. Rubble field of boulders of white quartzite (some foliated, some massive) and strongly crenulated metapelite. Collected by Dwight Bradley, 2002.
Sample 02ATi23. Quartzite in Minook fault block along Victoria Creek fault zone, Tanana B1 quadrangle, T7N R13W, elev. 2050 feet, in saddle near eastern edge of Section 32, 65°23.647′N, 150°15.408′W. In thin section, the detrital zircon sample is a medium-grained meta-quartzite with less than 10% disseminated white mica and biotite that define a foliation. Quartz grains have straight to slightly sutured grain boundaries. Accessory phases include zircon, rutile, tourmaline, and apatite. Collected by Alison Till, 2002.
Sample 95-89. Quartzite, Ruby terrane at Illinois Creek, Nulato A4 quadrangle, T17S R5E, elev. 950 feet due east of summit of low knob in Section 5, 64°02.877′N, 157°55.596′W. Detrital zircon sample was taken from a meter-thick, fine-grained quartzite interlayered with dolomitic marble and calcschist. Collected by Bill McClelland, 1995.
Sample 03ATi47b. Graywacke in Rampart Group, Angayucham-Tozitna terrane, Tanana C1 quadrangle, T9N R13W, elev. 2050′, on low knob near southwest corner of Section 8, 65°37.156′N, 150°19.448′W. In thin section, the detrital zircon sample is a fine- to medium-grained sandstone composed of angular to subrounded lithic clasts and subordinate grains of quartz and calcite. Lithic clasts include siltstone, chert, limestone, dolostone, volcanic rocks, and polycrystalline quartz. A weak fabric is imparted by the flattening of weaker lithic grains. Collected by Alison Till, 2003.
This study was supported by the Minerals Program of the U.S. Geological Survey and by NSF grants EAR 9406404, 9423534, and 0208162. Tom Bundtzen, Tim Kusky, Jim Baichtal, and Robert Blodgett were all involved in sampling the Farewell terrane. We thank Frank Mazdab and Brad Ito for SHRIMP technical support, Jeff Trop for late-night time-sharing on the SHRIMP, and Dan Grunwald for GIS support. Tom Moore, Cynthia Dusel-Bacon, and Gerry Ross kindly shared as yet unpublished information for Table 1 102. Rich Friedman provided preliminary TIMS results that, likewise, will be published elsewhere. Discussions with Julie Dumoulin and reviews by Cynthia Dusel-Bacon, George Gehrels, and Jeff Trop substantially improved the paper.
Figures & Tables
Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska
- absolute age
- clastic rocks
- ion probe data
- mass spectra
- North America
- Rampart Group
- sedimentary rocks
- SHRIMP data
- United States
- upper Precambrian
- Yukon-Tanana Terrane
- zircon group
- Ruby Terrane
- Wickersham Terrane
- Angayucham-Tozitna Terrane
- Farewell Terrane
- Minook Complex