AGE AND PROVENANCE OF PRECAMBRIAN METAMORPHIC ROCKS IN THE PIONEER METAMORPHIC CORE COMPLEX, SOUTH-CENTRAL IDAHO: THE SOUTHERN EDGE OF THE LOWER BELT SUPERGROUP, THE GROUSE CREEK BLOCK, AND HINTS OF A PROXIMAL GRENVILLE SOURCE IN THE NORTHERN CORDILLERA

The Pioneer Mountains of south-central Idaho comprise a core complex that is exposed on several structural levels. The Precambrian metamorphic rocks there have all been mapped as Paleoproterozoic, based on whole-rock Rb-Sr geochronology. In fact, none of the rocks are Paleoproterozoic. SHRIMP U-Pb ages zircons for several metamorphic units indicate that the lower structural plate contains Archean and Mesoproterozoic protoliths. A late Archean felsic orthogneiss (2608 and 2674 Ma Wetherill regression ages) is structurally overlain by quartzite, calc-silicate, and pelitic paragneiss containing Paleoproterozoic zircons with ages between 1450 and 1800 Ma. This complex is intruded by Neoproterozoic orthogneiss with concordia ages ca. 696 Ma. A large Eocene alkalic stock forms the bulk of this lowest structural plate, with ages on syenite of 49.0 +/- .4 Ma

In the middle structural plate of the core complex, the sedimentary rocks are all post-1050 Ma. Zircons from gneissose quartzite of the Mesoproterozoic (“Ygq”) unit have complex internal structure, comprising 50 to 150 micron round cores of magmatic zircon with metamorphic rims up to 30 microns thick. The cores have ages between 1050 and 1330 Ma, whilst the rims are up to 50 m.y. younger, but still of “Grenville-age”. The rocks may be Neoproterozoic, or more likely Ordovician based on identical detrital zircon assemblages in two samples of the Clayton Mine Quartzite. More significantly it is highly unlikely that the medium-sand sized zircon grains with well-preserved rims were transported thousands of km across Laurentia from the Grenville orogenic belt. Rather the morphology and structure indicate a more proximal, western 1050 to 1330 M Grenville-aged magmatic source area. Similar ages have been reported in northern Idaho by J. Vervoort.

The Pioneer Mountains basement represents the northern extension of the Grouse Creek block south of the Snake River Plain. We suggest that much of the heterogeneous paragneiss (“mafic and calc-silicate gneiss”), is in fact metamorphosed lower Belt and Piegan Group at the southern edge of the Belt Basin. The 696 Ma orthogneiss was likely related to alkalic plutons of the Big Creek-Beaverhead belt, and to Cryogenian rifting of Laurentia.