253-2 A multimineral perspective on the provenance of the Mesoproterozoic Belt Supergroup and its long metamorphic history

Abstract:

The Mesoproterozoic Belt Supergroup is a > 15 km sequence of predominantly siliciclastic sediments in the northern U.S Cordillera and adjacent Canada.  These sediments were deposited in an extensional basin between western Laurentia and another continental mass during the breakup and/or reorganization of the supercontinent Nuna.  Here we review recent work on the provenance and later metamorphic history of the western part of the basin by combining more traditional detrital zircon U-Pb dating and Hf isotope analysis with detrital zircon trace element analysis, detrital monazite and rutile U-Pb dating and trace element analysis, and Pb isotope analysis of detrital feldspar. 

The Prichard Formation and Ravalli Group of the Belt have homogeneous detrital mineral age spectra, with a main zircon age peak at 1.61 Ga and monazite age peaks at 1.67, 1.60, and 1.50 Ga, whereas the overlying Missoula Group and Lemhi strata have a principal zircon age peak between 1.7 and 1.8 Ga, monazite age peaks at 2.43, 1.78, 1.73, and 1.68 Ga, and a rutile age peak around 1.63 Ga.  The Prichard/Ravalli zircon and monazite age spectra (along with zircon Hf isotopes and trace element compositions) are most consistent with sources in the Gawler craton in southern Australia, and Belt rocks may have direct correlatives in Tasmania, with both points suggesting a modified SWEAT configuration for Nuna.  Conversely, Missoula/Lemhi data are most consistent with provenance from the Mojave and Yavapai terranes in the southwestern US. 

Belt Supergroup rocks experienced an eventful life after lithification.  Several episodes of post-depositional metamorphic monazite growth occurred between ~1.38 and 1.00 Ga, generally aligning with published garnet Lu-Hf ages.  The tectonic significance of these events remains obscure but may be attributable to the consolidation of Rodinia after Nuna breakup.  Additional monazite growth episodes occurred locally in the southern parts of the basin at ~720 and ~513 Ma, which may reflect the thermal imprint of magmatism associated with Rodinia breakup and subsequent Early Paleozoic intraplate magmatism.  Mesozoic metamorphic disturbances are also recorded across several minerals, with discordant detrital zircons projecting to a ~100 Ma lower intercept, sparse Mesozoic monazite growth, and whole-scale re-equilibration of the Pb isotope composition of feldspars throughout the basin.  These Mesozoic events most likely reflect reactive fluid flow during Sevier and Laramide orogenesis.

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