Detrital zircon results from Neoproterozoic metamorphic and Cambrian(?) sedimentary rocks, western Brooks Range, Alaska: early signatures of the Arctic Alaska - Chukotka terrane
Abstract
In the western Brooks Range, Neoproterozoic metamorphic rocks and overlying Neoproterozoic(?) to Mississippian carbonate platform rocks are exposed within the Nanielik antiform. The carbonate platform sequence is one of the best exposed in the Brooks Range, and is considered characteristic of the Arctic-Alaska Chukotka terrane (AAC). New detrital zircon data were collected from the metamorphic rocks and the base of the sedimentary section. Amphibolite-facies mafic, calcareous, quartzitic, and pelitic rocks at the Nanielik antiform are among the oldest known rocks in the western Brooks Range and the oldest known metamorphic rocks in the AAC, having yielded 40Ar/39Ar ages of 720×10 Ma (metamorphic hornblende) and 680×20 Ma (metamorphic white mica). The protoliths of these metasedimentary and metaigneous rocks were intruded by granitic rocks at 750 Ma (Karl and others, 1989). Two metasandstone samples from this succession contained dominant detrital zircon populations between 1.5-1.65 Ga, minor peaks at 1.0-1.15 and 1.3-1.45 Ga, and 2-6% Archean grains. Forty percent of the zircon grains in each sample yielded ages between 1.5 and 1.65 Ga; the youngest grain populations in each sample were between 0.9 and 1.1 Ga. The protolith of these rocks was deposited between 750 and 900 Ma. Two samples of carbonate-clast conglomerate were collected from the base of the carbonate platform sequence. They are likely Cambrian or older on the basis of on their stratigraphic position. In these samples, 36-43% of the detrital zircons yielded ages between 0.95 and 1.15 Ga; smaller populations range in age between 1.3 and 1.7 Ga. A correlative rock with a more complex deformational history, collected outside the Nanielik antiform, contained detrital zircons that define prominent peaks between 1.2 and 1.4 Ga and smaller peaks in the range 1.0-1.2 Ga. Each of the three samples contained 2-5% Archean zircons. Detrital zircon samples from the Nome Complex, Seward Peninsula, also part of the AAC, contain similar detrital zircon populations to the Cambrian(?) carbonate-clast conglomerates. However, the depositional age of the Nome Complex samples is known to be Middle Devonian or younger; the zircons in those samples were recycled from older rocks. Samples from the Ruby terrane, central Alaska, contain dominantly 1.0-1.4 Ga detrital zircons, with larger populations in the range 1.3-1.5 Ga, similar to the Cambrian(?) sample collected outside the Nanielik antiform. The protolith age of these rocks is unknown.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T13B2530T
- Keywords:
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- 9315 GEOGRAPHIC LOCATION Arctic region;
- 1100 GEOCHRONOLOGY