Ann L. Heatherington

Paleoproterozoic crust within the Great Falls tectonic zone: Implications for the assembly of southern Laurentia

The Great Falls tectonic zone and the Vulcan structure both have been proposed as the site of a Paleoproterozoic suture between the Archean Hearne and Wyoming provinces. Both hypotheses remain viable because all Precambrian rocks composing the Vulcan structure and much of the Great Falls tectonic zone are buried beneath Phanerozoic cover. The primary exceptions to this are the mafic to felsic igneous and metamorphic rocks of the Little Belt Mountains (Montana), previously considered the northernmost exposures of the Wyoming province. New U-Pb zircon ages from the late kinematic Pinto diorite (207Pb/206Pb age: 1864 ± 5 Ma) and a gneissic unit intruded by the Pinto (207Pb/206Pb age: 1867 ± 6 Ma), however, confirm their Paleoproterozoic age. These rocks exhibit an overall calc-alkaline affinity and the depletion in high field strength elements typical of convergent margin environments. Whole-rock Sm-Nd data (initial epsilon of −1 to +4) and a lack of premagmatic zircons indicate that the magmas were principally derived from a depleted mantle source, not from older crust. These data suggest that at least some rocks within the Great Falls tectonic zone originated at a convergent margin that developed during the closure of an ocean basin along the northwestern margin of the Wyoming craton ca. 1.9 Ga.

Precambrian zircons from the Florida basement: A Gondwanan connection

A Gondwanan origin for the pre-Cretaceous basement of Florida is suggested by U- Pb ages of 515 to 2860 Ma for single zircons separated from subsurface samples of lower Paleozoic sandstone of the Suwannee basin (Alachua County, Florida) and Neoproterozoic Osceola granite (Osceola County, Florida). Forty individual grains analyzed by SHRIMP ion microprobe yielded ages from 515 to 2860 Ma; ages >1000 Ma were predominantly concordant. Two principal populations are evident: (1) 515 to 637 Ma (avg. = 574 Ma; 206 Pb/ 238 U ages) and (2) 1967 to 2282 Ma (avg. = 2130 Ma; 207 Pb/ 206 Pb ages). Only six zircons were recovered from the granite; four are Pan-African and two are Archean. For the sandstone, the similarity between the Sm-Nd model age (1245 Ma) and the average of the single zircon ages (1326 Ma) suggests that these zircons are chronologically representative of the aggregate provenance of the sandstone. The two dominant zircon age groupings correspond chronologically to the Pan-African and Birimian or Eburnian (Africa) and to the Brasiliano and Trans-Amazonian (South America) orogenic cycles. The presence of detritus from rocks of these two orogenic cycles clearly places the basement of Florida in Gondwanaland proximal to the West African and/or Trans-Amazonian-San Luis cratons in the early Paleozoic, a location it probably shared with other circum-Atlantic exotic terranes (Avalonian, Cadomian, and/or Carolina).

Crustal Evolution in the Southern Appalachian Orogen: Evidence from Hf Isotopes in Detrital Zircons

Paired U-Pb and Lu-Hf isotopic compositions of individual detrital zircons reveal that Mesoproterozoic lithosphere in the southernmost Appalachian orogen evolved almost continuously from ∼1200 to ∼900 Ma, with a noticeable concentration only at ∼1050 Ma. The evolution of the Lu-Hf system in this crust is characterized by a systematic, linear evolution pattern of 176Hf/177Hf versus age over this interval, which suggests a limited range of mixing ratios between juvenile and older crustal components to create a new Mesoproterozoic lithospheric reservoir. Extrapolation of the Lu-Hf systematics of this reservoir from the Mesoproterozoic to the Paleozoic using typical crustal values shows general coincidence with the lower part of the Hf isotopic array defined by Paleozoic detrital zircons. Because the Paleozoic array extends to both much higher and much lower ϵHf values than are likely to develop from extrapolations of the Mesoproterozoic array, the Paleozoic array is most compatible with an orogen formed largely from the accretion of numerous discrete crustal blocks that may or may not have derived partly from the southern Appalachian Mesoproterozoic lithosphere. These data are most compatible with existing models for Appalachian orogenesis that invoke accretion of exotic terranes, few, if any, of which were true juvenile terranes.

Source of the Northeastern Idaho Batholith: Isotopic Evidence for a Paleoproterozoic Terrane in the Northwestern U.S.

The northeastern portion of the Idaho batholith (NIB) intruded Proterozoic rocks of the Belt-Purcell supergroup between 50 and 90 Ma. Whole-rock Sm-Nd isotopic analyses of batholithic rocks yield depleted mantle model ages (

Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

T_{DM}

Mesozoic Igneous Activity in the Suwannee Terrane, Southeastern USA: Petrogenesis and Gondwanan Affinities

) between 1.72 and 1.93 Ga and values of

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

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