Bol. Soc. Geol. Mexicana, Tomo XLIII, No. 1, 1982

Petrology of the metamorphic rocks of Zacatecas, Zac., Mexico

http://dx.doi.org/10.18268/BSGM1982v43n1a3

William A. Ranson¹, Louis A. Fernández¹ , Wm.B. Simmons Jr.² , Salvador Enciso de la Vega³

¹Department of Geology & Geography, Furman University, Greenville, South Carolina 29613
²Department of Earth Sciences, University of New Orleans, New Orleans, Louisiana 70122
³lnstituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México 20, D.F.

 Abstract

Triassic metasediments, a Cretaceous intrusive, and Tertiary intrusive rocks comprise the major rock types west of the city of Zacatecas. The metasediments are principally phyllites, containing conformable lenses of quartzite and marble, and fine-grained, ammonite-bearing quartzites. A detailed stratigraphic and structural study of the phyllites suggests that two episodes of deformation and accompanying metamorphism affected these rocks during Early Jurassic and Late Cretaceous time. K/Ar muscovite date for the phyllite yield ages of 74.8±1.5 and 73.2± .5 my for the last episode of metamorphism. The metasediments are intruded by a shallow diorite pluton. K/Ar whole-rock dates for the diorite set the age of intrusion at 73.8±2.1 and 75.1±1.9 my Rhyolite dikes intrude the metasediments and the diorite. Hypogene mineralization within the metasediment, the diorite, and the intrusive rhyolite appear to have occurred not long after the intrusion of the rhyolite. Subsequent erosion of metasediment and diorite resulted in the accumulation of a calcite- and hematite-cemented conglomerate. Magmatism continued into the Late Tertiary with the extrusion of rhyolite flows, tuffs, and volcanic breccias.

All metamorphic rocks exposed in the area of study are of the greenschist facies of regional metamorphism. Chemical and petrographic features of the metamorphic rocks indicate that a relatively dry system may not have allowed relic sedimentary minerals to enter into metamorphic reactions, thus resulting in disequilibrium assemblages. The clase association in time of the metasediments and the diorite intrusion suggests that the temperatures necessary for greenschist facies metamorphism were attained at shallow depths in the crust.