Distribution and U-Pb isotope ages of some lineated plutons, northwestern Mexico

Abstract

Characteristically lineated and foliated rocks of middle to late Mesozoic(?) age crop out in ranges in north-central Sonora throughout an area of 15,000 km2, between lat 31° 307′N and 30° 30′N. The terrane consists predominantly of layered sedimentary, volcanic, and volcaniclastic units, commonly metamorphosed to greenschist facies. Associated plutons, which are unambiguously intrusive, consist mainly of biotite- or biotite-muscovite–bearing granite. The layered suite and intrusive rocks are distinguished by subhorizontal, penetrative lineation, commonly defined by smeared mineral grains, which consistently trend northeast. Foliation is also predominantly low dipping. South of lat 30° 30′N, sporadic occurrences of lineated granitic and metamorphic rocks suggest the existence of this deformational fabric at least to Sierra Mazatan, which lies east of Hermosillo near lat 29°00′N. Cogenetic suites of zircon from one undeformed and three deformed plutons yield U-Pb ages from 75 to 55 m.y. These intrusive bodies are elements of a widespread, time-transgressive, late Mesozoic magmatic suite. They are not known to be affected by folds and faults commonly related to the Laramide orogeny. The apparent ages of the plutons are interpreted to be crystallization ages and therefore indicate that lineation and foliation formed, at least locally, later than 55 m.y. ago. Outcrops of distinctively deformed rocks appear to crudely define a north-trending belt. Rocks outside of this general zone are composed of sedimentary, volcanic, and volcaniclastic rocks of Precambrian and Mesozoic age which locally have been strongly folded and metamorphosed to greenschist or higher facies. However, postdeformational pegmatites and intrusive rocks indicate pre-Tertiary minimum ages for related episodes of deformation and metamorphism.

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