Abstract

The Gardner Lake area is one of the key areas being studied in detail in the Beartooth Mountains, Montana and Wyoming. Foliation, compositional banding, and rock units indicate open southward-plunging folds traceable through metasediments and migmatites into granitic gneisses. The attitude of axial-plane foliation in highly contorted metasediments is found also in foliation trends of the gneisses. Complete gradations from metasediments to pink leucocratic gneiss are indicated by distribution of mappable units, abundance and composition of feldspars and ferromagnesian minerals, percentage and character of rounded and overgrown zircons, length-width ratios of zircons, and variations in chemical analyses. Replacement in situ by hot alkaline solutions seems the most likely origin for the granitic gneisses. Boudinage and rotation of amphibolite blocks within highly contorted feldspathized strata denote regional metamorphism prior to granitization. Mafic intrusive bodies are pregranitization, penegrantization, and postgranitization in age, and their major trends, northwest and northeast, suggest that the present topographic expression is controlled by structural elements developed during feldspathization. Seven K-A and Rb-Sr ages on widely dispersed micas and feldspars indicate this event as 2.7 billion years ago.

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