MANTLE DRIP FROM THE RISING LEMHI ARCH: 500 MA PLUTONS AND DETRITAL ZIRCONS IN UPPER CAMBRIAN SANDSTONES, EASTERN IDAHO

The Lemhi/Salmon River Arch in east-central Idaho, where Neoproterozoic and Cambrian strata are absent between Mesoproterozoic Belt Supergroup below and middle Ordovician quartzite above, and where lower Paleozoic strata are generally thin, was first recognized by Larry Sloss 50 years ago. Yet its definition and origin have remained elusive. We here propose that the topographic uplift of the Lemhi Arch was punctuated in Late Cambrian time by convective removal of dense lithospheric mantle and associated emplacement of rising alkalic plutons of the Beaverhead-Big Creek belt (K. Lund et al., GSA Bull. 2009).

Upper Cambrian feldspathic sandstone of the Worm Creek Member St. Charles Formation in SE Idaho has a large ca 500 Ma zircon population (90% of zircons) of 497.4 +/- 3.0 Ma (SHRIMP age). Alkalic plutons of the Deep Creek Suite in the Salmon River Mountains west of Salmon, Idaho are of the same age (497 +/- 6 Ma; Lund et al, 2009). Biostratigraphy constrains the Worm Creek to the ca. 499 Ma Dunderbergia and Elvinia trilobite zones, overlying the Sauk II to Sauk III unconformity. Thus the depositional age of the Worm Creek sandstone is nearly the same as the zircons it contains. This suggests that immediately after crystallization, the plutons were exhumed, eroded and redeposited as feldspathic sandstone.

This sandstone contains rounded quartz and magmatic zircon grains and up to 30% fine-grained weathered oligoclase and microcline, suggesting a plutonic source (Haynie, USU M.S. thesis, 1957). There is no sanidine or altered glass, as would be expected if the zircons were transported as volcanic ash. Facies patterns show ten-fold northwestward thickening of the sandstone from the Logan area toward the Lemhi arch.

This required rapid uplift could be effected by convective removal of dense lithosphere in the Salmon River/Lemhi Arch, in analogous fashion to the late Cretaceous Sierran La Posta plutons, (Kimbrough et al., 2001). The size of the uplifted region is on the order of 450 by 250km, somewhat larger but generally comparable to uplift in well-documented mantle drips in younger rocks including the 200 by 390 km Wallowa uplift and circular 200 km southern Sierran drip. Dense mafic material beneath the western edge of the Belt basin may have served as an anchor and been removed during this event.