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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 northern-most exposures of the Wyoming province. New U-Pb zircon ages from the late kinematic Pinto diorite ([sup.207]Pb/[sup.206]Pb age: 1864 [+ or -] 5 Ma) and a gneissic unit intruded by the Pinto ([sup.207]Pb/[sup.206]Pb age: 1867 [+ or -] 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. Keywords: Precambrian orogeny, Wyoming province, Laurentia, geochronology, isotopes, tectonics.

Heating a mixture containing (Cr(en)3)Cl3 and excess of (S)-tryptophanatoH produces the complexes Lambda-fac-Cr(S-trp)3 and Delta-fac-Cr(S-trp)3. Extraction with acetone helps separate these diastereomeric species. Spectroscopic studies help characterize these complexes. Irradiation exerts no influence on the Delta-fac-Cr(S-trp)3, while Delta-fac-Cr(S-trp)3 forms Delta-fac-Cr(S-trp)3 with a quantum yield of 0.03. Asymmetric C of (S)-tryptophan plays a vital stereochemical role as revealed by the photoinversion.

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; 206Pb/238U ages) and (2) 1967 to 2282 Ma (avg. = 2130 Ma; 207Pb/206Pb 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).