Contrasting responses of Rb-Sr systematics to regional and contact metamorphism, Laramie Mountains, Wyoming, USA

Archean supracrustal sequences of pelitic, quartzitic, calcareous and mafic compositions in the central Laramie Mountains, Wyoming, have been affected by two metamorphic events: a 1.78 Ga amphibolite-grade regional metamorphi sm, and a 1.43 Ga contact metamorphism resulting from the intrusion of the Laramie Anorthosite Complex (LAC). Rb-Sr whole-rock isotopic data from both outside and within the LAC contact aureole define a linear array that lies along a 1.78 Ga isochron. This date has been independently established as the time of amphibolite facies regional metamorphism associated with collis ion of the Archean Wyoming province and the Proterozoic Colorado province a long the Cheyenne belt. The Rb-Sr isotopic data require that Sr was redistr ibuted during regional metamorphism on a scale of at least tens of metres. Although within the 2 km-wide aureole of LAC the pelitic rocks were thermal ly metamorphosed at temperatures greater than 800 degrees C, none of the wh ole-rock Rb-Sr data from samples within the LAC aureole show evidence of re setting at 1.43 Ga. It is interpreted that the regional metamorphism involv ed fluid transport which facilitated Sr isotopic resetting, whereas the con tact metamorphism occurred in a relatively dry environment in which isotopi c mobility was restricted to centimetre-scale or less. Rb-Sr data for bioti te, feldspar and whole rock from a regional metamorphosed pelitic schist gi ve an isochron age of 1450 +/- 40 Ma, which is interpreted as a cooling age resulting from crustal uplift. Rb-Sr data for biotite, quartz + feldspar a nd whole rock from a pelitic schist affected by contact metamorphism give a n isochron age of 1420 +/- 43 Ma, the time of isotopic re-equilibration in response either to crustal uplift or to both contact metamorphism and crust al uplift. This study demonstrates that although the response of isotopic s ystems to metamorphism is complex, isotopic data provide insight into metam orphic processes that is difficult to obtain by other means.