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Abstract

ABSTRACT: We analyzed detrital zircons in seven samples of metasedimentary rock from north-central Idaho, U.S.A., to test the previous assignment of these rocks to the Mesoproterozoic Belt–Purcell Supergroup. Correlating these rocks with known sedimentary units through field observations is difficult if not impossible due to the high metamorphic grade (amphibolite facies) and intensity of deformation. Zircon analysis by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) reveals that five of the seven samples contain multiple zircon populations between 1700 and 1400 Ma and a scatter of Paleoproterozoic and Archean ages, similar to results reported from the Belt Supergroup to the north and east. These results indicate that the likely protoliths of most high-grade metamorphic rocks northwest of the Idaho batholith were upper strata of the Belt Supergroup. In contrast, a quartzite at Bertha Hill north of Pierce lacks grains younger than 1600 Ma and thus is distinctly unlike the Ravalli Group of the Belt Supergroup, with which it was previously correlated. Possible correlatives that contain similarly old populations of zircons and feldspar-poor quartzite include the Neihart Formation (lowermost Belt Supergroup in Montana), Neoproterozoic quartzite (Syringa metamorphic sequence), and Cambrian quartzite. A sample from the North Fork of the Clearwater River yielded a large number of zircons with concordant Neoproterozoic ages, all of which had low Th/U ratios that suggest either a Neoproterozoic metamorphic event or the transport and deposition of zircons that were metamorphosed in the Neoproterozoic. SHRIMP (sensitive high-resolution ion microprobe) dating of a granite (now augen gneiss) that intruded sedimentary rocks west of Pierce, Idaho, yields an age of 1379 ± 12 Ma based on seven of fourteen analyses; this provides a lower age limit for sediment deposition of some rocks mapped as metamorphosed Belt Supergroup, and which had detrital zircon populations in the 1700 to 1400 Ma range. Additional analyses of three zircon rims yield an age range of 87–82 Ma, which is similar to the youngest ages from the North Fork sample. We interpret these ages to reflect the time of zircon overgrowth synchronous with the emplacement of the Cretaceous Idaho Batholith. None of the metasedimentary rocks dated can be older than Mesoproterozoic, and, with the exception of the Bertha Hill quartzite, none can be older than the Belt–Purcell Supergroup.

Item Type:	Book Section
Authors/Creators:	Lewis, RS and Vervoort, JD and Burmester, RF and McClelland, WC and Chang, Z
Publisher:	SEPM Society for Sedimentary Geology
Related URLs:	http://www.sepm.org/
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