THE TONALITE-TRONDHJEMITE-GRANDIORITE (TTG) TO CALC-ALKALIC TRANSITION IN THE LATE ARCHEAN PLUTONIC ROCKS OF THE CENTRAL WYOMING PROVINCE

2.95-2.82 Ga quartzofeldspathic gneisses and granitoids are exposed in the Bighorn, western Owl Creek and northeastern Wind River uplifts in the central Wyoming province. Together with slightly younger granitoids in the Beartooth Mountains, these rocks make up the Beartooth-Bighorn magmatic domain (BBMD; Mogk et al., 1992). They occupy the center of the Wyoming province, an area that, except for the rocks in the northeastern Wind River Range, has been unaffected by subsequent Archean magmatism, sedimentation and deformation (Chamberlain et al., 2003). The granitoid rocks within the BBMD are of both typical Archean trondhjemite-tonalite-granodiorite (TTG) as well as modern calc-alkalic affinity. The TTG and calc-alkalic granitoids were intruded contemporaneously, although TTG are more abundant in the older gneisses.

Unradiogenic Nd and radiogenic 207Pb/204Pb isotopic compositions of the TTG suite relative to contemporaneous depleted mantle require the incorporation of pre-existing isotopically evolved crust into their sources. The calc-alkalic rocks include transitional compositions between Archean TTG and modern, continental margin calc-alkalic rocks. The Nd, Sr and Pb isotopic compositions of the calc-alkalic suite are slightly less variable but within the range of the TTG suite. We interpret the calc-alkalic rocks as having similar sources to those for the TTG suite, perhaps forming by partial melting of preexisting TTG.

Although TTG may form in modern plate tectonic environments, the spatial distribution of Wyoming Archean TTG intrusions suggests that alternative mechanisms to subduction-related processes, perhaps mechanisms peculiar to the Archean, should be considered. Models for modern TTG invoke partial melting of hydrous basalt that is subducted or underplated beneath a continental arc. The Wyoming Archean TTG are distributed across a broad region, and could be related to Archean flat-slab subduction or a plume-related mafic underplate. Geographical association of the Wyoming TTG with a layer of thick, fast lower crust supports this model. Regardless of the origin, the shift from TTG-dominated to calc-alkalic-dominated continental crust in the Wyoming province was not a sudden event, but instead involved a gradual transition taking place over several 100 million years.