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Alison Till

Publications and source records attributed to Alison Till.

7 recordsLinked to original sources
USGS2014

Preface

The tectonic evolution of the Arctic realm, particularly in the Mesozoic, remains a subject of considerable uncertainty. The nature of the crustal entities involved, their origins, and the nature, location, and age of major tectonic boundaries are incompletely studied and understood ( Pease, 2011 ). The largest piece of continental crust of uncertain origin that plays a role in Arctic tectonics is the Arctic Alaska–Chukotka terrane or microplate. The terrane includes northern Alaska, northeasternmost Russia, and the adjacent continental shelves ( Fig. 1 ; Natal'in et al., 1999 ; Miller et al., 2006 ). Because of its size, the origin of this microplate and its movements during the Paleozoic and Mesozoic are critical components of tectonic and paleogeographic models ( Pease, 2011 ).

Alaska↗
USGS2014

U-Pb detrital zircon geochronology as evidence for the origin of the Nome Complex, northern Alaska, and implications for regional and trans-Arctic correlations

Detrital zircons from the Nome Complex, a metamorphic terrane in northern Alaska, reveal important constraints on the early Paleozoic history of the Arctic Alaska–Chukotka terrane, a microcontinental block with an origin exotic to Laurentia. Twenty-two samples (17 in this study, five previously published) produce three detrital zircon population patterns (called themes), indicating that at least three distinguishable source areas contributed to the metamorphic protolith. Detrital zircon populations from metamorphosed rift-related mafic volcaniclastic rocks, a lithologic subunit of the Nome Complex, contain a dominant population of 740–550 Ma zircons. Samples from three other lithologic units yielded populations dominated by early Paleozoic zircons and characterized by a large population of 450–420 Ma zircons. A few samples, taken from two different lithologic units, yielded populations dominated by Mesoproterozoic zircons (most around 1.25–0.9 Ga) and lacked zircons younger than 900 Ma. None of the 22 samples contained more than a few Archean zircons. The ages of the youngest detrital zircon populations indicate that little of the protolith for the Nome Complex can be as old as Proterozoic, as previously thought. Further, a significant part of the protolith sequence is Devonian or younger; these rocks are likely correlative with Devonian or Mississippian units in the Brooks Range, specifically marine parts of the Endicott or Lisburne Groups. Based on detrital zircon data, limiting factors can be placed on the paleogeographic history of the Nome Complex and associated parts of the Arctic Alaska–Chukotka terrane: (1) 740–550 Ma zircons were deposited in a rift-related basin formed on a continental margin in the early Paleozoic; at least some of those zircons may have been sourced from local basement; (2) a transition to new sediment sources is reflected in Devonian or younger protoliths with the appearance of 450–420 Ma and 1.25–0.9 Ga detrital zircons; and (3) 450–420 Ma and 1.25–0.9 Ga zircons may have been supplied from sources outside the Arctic Alaska–Chukotka terrane.

Alaska↗
USGS2014

Geochemical and Nd-Sr-Pb isotopic evolution of metabasites from rifting of continental lithosphere, Seward Peninsula, Alaska, and implications for paleogeographic reconstruction

The chemical character of mafic rocks from the Arctic Alaska–Chukotka terrane records rifting of continental crust during the early Paleozoic, possibly during the Ordovician. The mafic rocks are part of a metamorphosed Neoproterozoic to Devonian continental margin sequence preserved in a Mesozoic metamorphic terrane, the Nome Complex, of Seward Peninsula, Alaska. Protoliths of the mafic rocks include basalt and mafic clastic rocks, which were interlayered with calcareous, pelitic, and feldspathic sediments, and gabbro and diabase, likely feeder dikes and sills to the basalt. Major-element, trace-element, and rare-earth element (REE) analyses of these mafic rocks, together with analyses of Nd, Pb, and Sr isotopes, form two compositional groups. The two groups differ in Nb/Y (one plots as basalt, the other as alkali to subalkali basalt), TiO 2 , P 2 O 5 , and Nb (and other elements). The high-Ti group is characterized by enrichment of light REE; the low-Ti group lacks such enrichment. The trace-element and isotopic characteristics of the two groups resemble typical non-arc magmas derived from the mantle: the low-Ti group has compositions between normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB), while those of the high-Ti group are between E-MORB and ocean-island basalt (OIB). The two groups have overlapping positive values of ε Nd (+0.34 to +7.40). TiO 2 /Yb ratios suggest the high-Ti group formed from melts generated under normal thickness of continental crust, while the low-Ti group formed from melts generated at shallower conditions, presumably after rift-related crustal thinning had progressed. Geologic, paleontologic, and geochronologic characteristics of the Nome Complex support an origin along the NE margin of Baltica. The rift-related magmatism in the Nome Complex likely occurred during the opening of the Uralian ocean along that margin; by implication, related parts of the Arctic Alaska–Chukotka terrane may have experienced a similar origin.

Alaska↗
USGS2014

Late Devonian–Mississippian(?) Zn-Pb(-Ag-Au-Ba-F) deposits and related aluminous alteration zones in the Nome Complex, Seward Peninsula, Alaska

Stratabound base-metal sulfide deposits and occurrences are present in metasedimentary rocks of the Neoproterozoic and Paleozoic Nome Complex on south-central Seward Peninsula, Alaska. Stratabound and locally stratiform deposits including Aurora Creek (Zn-Au-Ba-F), Wheeler North (Pb-Zn-Ag-Au-F), and Nelson (Zn-Pb- Cu-Ag), consist of lenses typically 0.5–2.0 m thick containing disseminated to semimassive sulfides. Host strata of the Aurora Creek and Wheeler North deposits are variably calcareous and graphitic siliciclastic metasedimentary rocks of Middle Devonian or younger age based on detrital zircon geochronology; the Nelson deposit is within Ordovician–Devonian marble (Till et al., this volume, Chapter 4). Deformed veins such as Quarry (Zn-Pb-Ag-Ba-F) and Galena (Pb-Zn-Ag-F) occur in a unit composed mainly of marble and schist; fossil and detrital zircon data indicate that this unit contains rocks of Ordovician, Silurian, and Devonian age. None of these Zn- and Pbrich deposits or occurrences has spatially associated metavolcanic or intrusive rocks. All were deformed and metamorphosed to blueschist facies and then retrograded to greenschist facies during the Jurassic and Early Cretaceous Brookian orogeny. Disseminated Cu-rich deposits including Copper King (Cu-Bi-Sb-Pb-Ag-Au) and Wheeler South (Cu-Ag-Au) occur in silicified carbonate rocks and have textures that indicate a pre- to syn-metamorphic origin. The Zn- and Pb-rich sulfide deposits and occurrences consist mainly of pyrite, sphalerite, and/or galena in a gangue of quartz and carbonate. Minor minerals include arsenopyrite, chalcopyrite, magnetite, pyrrhotite, tetrahedrite, barite, fluorite, and chlorite; gold and electrum are trace to minor constituents locally. Sphalerite is uniformly unzoned and commonly aligned in the dominant foliation. These textures, together with the presence of folded layers of barite at Aurora Creek and folded sulfi de layers at Wheeler North, indicate that mineralization in the stratabound deposits predated deformation and metamorphism. Electron microprobe (EMP) analyses of the carbonate gangue show three major compositions comprising siderite, ankerite, and lesser dolomite. The Cu-rich deposits differ in containing chalcopyrite and bornite in a quartzose matrix. Altered wall rocks surrounding the Zn- and Pb-rich deposits and occurrences have aluminous assemblages composed of muscovite + chloritoid + siderite + chlorite + quartz ± tourmaline ± ilmenite ± apatite ± monazite. Muscovite within these assemblages and in sulfide-rich samples is phengitic and locally enriched in barium; chloritoid at Aurora Creek is enriched in zinc. Minor minerals including pyrite, sphalerite, galena, chalcopyrite, barite, and hyalophane occur as fine-grained disseminations. These altered rocks vary from small lenses a few meters thick to large zones tens of meters in thickness that extend along strike, discontinuously, for 4 km or more. Whole-rock geochemical analyses of the altered rocks from deposit-proximal and deposit-distal settings reveal generally lower SiO 2 /Al 2 O 3 ratios and higher Fe 2 O 3 T /MgO ratios compared to those of unaltered clastic metasedimentary rocks of the Nome Complex and of average shale or graywacke. The deposit-proximal samples are also characterized by anomalously high Zn, Pb, Hg, and Sb, relative to the unaltered metasediments. These data, together with mass change calculations, suggest that the aluminous rocks formed as replacements of permeable graywacke in semi-conformable alteration zones, beneath the seafloor contemporaneously with Zn-and/or Pb-rich sulfide mineralization. Exposures of all three stratabound Zn-Pb deposits show evidence of deformation and recrystallization that occurred in a largely brittle deformational regime. This evidence includes small faults and veins that cut foliation and localized zones of breccia. Sulfide minerals, fluorite, quartz, chlorite, and carbonate minerals crystallized within these structures, which probably formed during Cretaceous deformation of the Nome Complex. Previous studies of the Zn-Pb(-Ag-Au-Ba-F) deposits and occurrences have invoked models of epigenetic veins, volcanogenic massive sulfides (VMS), or carbonate- replacement deposits (CRD). In contrast, our field and laboratory data (including sulfur isotopes; Shanks et al., this volume) suggest that these Zn- and/or Pb-rich deposits represent different levels of sediment-hosted, seafloor-hydrothermal systems, with stratabound and locally stratiform deposits such as Aurora Creek and Wheeler North having formed on the seafloor and/or in the shallow subsurface like many sedimentary-exhalative (SEDEX) deposits worldwide. The deformed veins such as Quarry and Galena are interpreted to have formed deep in the subsurface, possibly as feeders to overlying SEDEX deposits such as Aurora Creek. Formation of all of the Zn- and Pb-rich deposits and occurrences took place during episodic rifting of the continental margin between the Ordovician and Mississippian(?). Regional relationships are consistent with at least some of the deposits having formed in Late Devonian–Mississippian(?) time.

Alaska↗
USGS2014

Pb isotope geochemistry of stratabound Zn-Pb(-Ag-Au-Ba-F) deposits and occurrences in the Nome Complex, Seward Peninsula, Alaska: Metal sources and regional comparisons

A detailed study of the Pb isotope geochemistry of Zn-Pb(-Ag-Au-Ba-F) stratabound sulfide deposits within metasedimentary rocks of the Neoproterozoic to Mississippian(?) Nome Complex provides key information for understanding deposit genesis and crustal evolution. A total of 106 new analyses of galena (and other sulfi des) and metasedimentary rocks hosting the deposits shows that (1) Pb isotope signatures of the deposits are heterogeneous when considered as a group; (2) the stratabound Nelson deposit, and deformed veins at Quarry and Galena, are isotopically similar; (3) stratabound and locally stratiform lenses such as Wheeler North and Aurora Creek had different isotopic evolutions; and (4) the occurrence at Bluff and the postmetamorphic, undeformed Pb-Zn-Ag veins and replacements at Hannum, Independence, Foster, and Omilak show the highest values of 206 Pb/ 204 Pb in the region. Pb isotope data for the stratabound Zn-Pb deposits and occurrences do not lie along similar secondary or anomalous lead evolution lines, and there is no shared, two-stage lead line that would provide intersections with a primary or single-stage lead isotope growth curve. Lead isotopic characteristics of the Nelson stratabound deposit and the deformed veins at Quarry and Galena indicate that they largely shared metal and fluid sources. Quarry and Galena also display sufficient Pb isotopic contrast compared to Aurora Creek and Wheeler North to eliminate such veins as subsurface “feeders” for these stratabound deposits, if the deformed veins and deposits formed as closed isotopic systems (without a contribution from externally derived lead). The Pb isotope composition of galena from Aurora Creek formed by a multistage process. It is thus possible that the Aurora Creek deposit originally contained Pb isotope compositions that resembled those from Quarry and Galena. That early-formed Pb was probably remobilized and mixed with radiogenic lead contributed by Mesozoic hydrothermal fluids similar to those associated with the gold-quartz veins in the region. Values of 207 Pb/ 204 Pb and 206 Pb/ 204 Pb from each of the deposits and occurrences plot within the Pb isotope fields of the host metasedimentary rocks and Mesoproterozoic basement rocks of Seward Peninsula; Pb isotope compositions in the deposits thus reflect a local source control. The processes that generated the premetamorphic Zn-Pb(-Ag-Au-Ba-F) sulfide deposits in the Nome Complex differed from those that generated Zn-Pb-Ag deposits in the western Brooks Range, such as the giant Red Dog ore body. Taken as a group, the stratabound lenses and deformed veins in the Nome Complex did not form in a single, widespread, homogeneous hydrothermal system. The Brooks Range deposits, which consist of a range of host rock types and styles of mineralization distributed over a large area, have a high degree of regional Pb isotope homogeneity. The Wheeler North deposit is isotopically similar to Red Dog and related deposits and may have formed in a related hydrothermal system. A preliminary comparison of the Pb isotope compositions of sedimentary-exhalative (SEDEX)–type deposits within the Arctic Alaska–Chukotka terrane and deposits in crustal blocks of Laurussia shows: (1) noteworthy Pb isotopic overlap exists between some of Zn-Pb-Ag deposits in Ireland and the deposits in Arctic Alaska ; but (2) no exact isotopic match exists between any of the deposits in Arctic Alaska and any deposit in crustal blocks involved in the Paleozoic evolution of Laurussia.

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USGS2009

Detrital zircon geochronology of Cretaceous and Paleogene strata across the south-central Alaskan convergent margin

Ages of detrital zircons are reported from ten samples of Lower Cretaceous to Paleogene metasandstones and sandstones from the Chugach Mountains, Talkeetna Mountains, and western Alaska Range of south-central Alaska. Zircon ages are also reported from three igneous clasts from two conglomerates. The results bear on the regional geology, stratigraphy, tectonics, and mineral resource potential of the southern Alaska convergent margin. Chugach Mountains - The first detrital zircon data are reported here from the two main components of the Chugach accretionary complex - the inboard McHugh Complex and the outboard Valdez Group. Detrital zircons from sandstone and two conglomerate clasts of diorite were dated from the McHugh Complex near Anchorage. This now stands as the youngest known part of the McHugh Complex, with an inferred Turonian (Late Cretaceous) depositional age no older than 91-93 Ma. The zircon population has probability density peaks at 93 and 104 Ma and a smattering of Early Cretaceous and Jurassic grains, with nothing older than 191 Ma. The two diorite clasts yielded Jurassic U-Pb zircon ages of 179 and 181 Ma. Together, these findings suggest a Mesozoic arc as primary zircon source, the closest and most likely candidate being the Wrangellia composite terrane. The detrital zircon sample from the Valdez Group contains zircons as young as 69 and 77 Ma, consistent with the previously assigned Maastrichtian to Campanian (Late Cretaceous) depositional age. The zircon population has peaks at 78, 91, 148, and 163 Ma, minor peaks at 129, 177, 330, and 352 Ma, and no concordant zircons older than Devonian. A granite clast from a Valdez Group conglomerate yielded a Triassic U-Pb zircon age of 221 Ma. Like the McHugh Complex, the Valdez Group appears to have been derived almost entirely from Mesozoic arc sources, but a few Precambrian zircons are also present. Talkeetna Mountains - Detrital zircons ages were obtained from southernmost metasedimentary rocks of the Talkeetna Mountains (schist of Hatcher Pass) and, immediately to the south, the northernmost sedimentary sequence of the Matanuska forearc basin (Arkose Ridge Formation). Detrital zircons from the Paleogene Arkose Ridge Formation are as young as 61 and 70 Ma; the population is dominated by a single Late Cretaceous peak at 76 Ma; the oldest zircon is 181 Ma. Sedimentological evidence clearly shows that the conglomeratic Arkose Ridge Formation was derived from the Talkeetna Mountains; our detrital zircon data support this inference. Zircons dated at ca. 90 Ma in the Arkose Ridge sample suggest that buried or unmapped plutons of this age may exist in the Talkeetnas. This is a particularly interesting age as it corresponds to the age of the supergiant Pebble gold-molybdenum-copper porphyry prospect near Iliamna and suggests a new area of prospectivity for Pebble-type deposits. The schist of Hatcher Pass, which was previously assigned a Jurassic depositional age, yielded surprisingly young Late Cretaceous detrital zircons, the youngest at 75 Ma. The probability density curve has four Cretaceous peaks from 76 to 102 Ma, a pair of Late Jurassic peaks at 155 and 166 Ma, three Early Jurassic to Late Triassic peaks at 186, 197, and 213 Ma, minor Carboniferous peaks at 303 and 346 Ma, and a minor Paleoproterozoic peak at 1828 Ma. The schist of Hatcher Pass was largely derived from Mesozoic arc sources, most likely the Wrangellia composite terrane, with some contribution from one or more older, inboard sources, probably including the Yukon-Tanana terrane. We postulate that the schist of Hatcher Pass represents metamorphosed rocks of the Valdez Group that were subducted and then exhumed along the Chugach terrane's 'backstop' during Paleogene transtension. Western Alaska Range - Six detrital zircon samples were collected from a little studied belt of turbidites in Tyonek quadrangle on strike with the Kahiltna assemblage of the central Alaska Range.

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