Detrital zircon geochronology of some Neoproterozoic to Triassic rocks in interior Alaska

Dwight C Bradley; William C McClelland; Joseph L Wooden; Alison B Till; Sarah M Roeske; Marti L Miller; Susan M Karl; J. Grant Abbott

doi:10.1130/2007.2431(07)

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 + or -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. A sample 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+ or -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; minor older age clusters are at 1643-1676, 1765-1781, and 1840-1874 Ma. The youngest concordant grain is 1103+ or -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+ or -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, ii27-1128, 1211-1217, and 1912-1953 Ma. The youngest grain is 260+ or -1 Ma. THe dominant 380-404 Ma cluster can be reasonably linked to sources in Devonian plutons of the now-adjacent Brooks Range and Ruby terrane.

Detrital zircon geochronology of some Neoproterozoic to Triassic rocks in interior Alaska

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