Precambrian metamorphism in the Placitas-Juan Tabo area, northwestern Sandia Mountains, New Mexico
— John L. Berkeley and J. F. Callender

Precambrian rocks of the northwestern Sandia Mountains consist of a thick metasedimentary sequence which has been intruded by plutonic rocks of "granitic" composition. Metamorphic recrystallization occurred in two major episodes, a regional greenschist event and subsequent hornblende-hornfels contact metamorphism associated with regional plutonism. This paper summarizes the geology of a part of this Precambrian sequence and outlines the environment of metamorphism and metamorphic history of the area. The lack of modern, detailed studies on Precambrian rocks of the Sandia Mountains, coupled with extensive Phanerozoic cover and complex Cenozoic structures, prevented regional correlation with other nearby Precambrian rocks. However, recent work by Grambling, Robertson and Moench, and Condie (all, this guidebook), Condie and Budding (1979), Callender and others (1976), among others, suggests that the Precambrian events described here may be related to a broad zone of regional metamorphism, volcanism and plutonism about 1500 million years old.