GEOCHEMICAL AND PETROLOGICAL STUDIES OF PERALKALINE ROCKS IN CENTRAL CHIHUAHUA, MEXICO: EVIDENCE OF CONTINENTAL RIFTING?

The ratio between the total aluminum and total alkalis contents indicates the alkalinity level of the igneous rocks. A peralkaline rock is characterized by having lower total aluminum than total alkalis. The presence of peralkaline rocks may indicate the tectonic setting at where the magmas formed, which can be continental rift or hot spot.

The Chihuahua State is located in northern Mexico, just to the south of the Rio Grande Rift, in New Mexico. Most of the volcanic activity in Chihuahua State is related to Farallon plate subduction, which created the Sierra Madre Occidental (SMO).

The latest volcanic activity occurred 35 to 27.5 M.y. ago approximately, to the north and southwest of Chihuahua City, in the Calera-Del Nido block and the towns of Laborcita de San Javier and Cusihuiriachic, respectively. The volcanic rocks consist mainly of rhyolitic and andesitic tuffs. The youngest rocks are peralkaline tuffs (27.5 M.a.), capping older rhyolites and andesites of the Large Silicic Province (SMO).

The study area, Laborcita de San Javier, exposes a volcanic section of more than a 1,000 m thick, which contains calc-alkaline felsic ash-flow tuffs with mafic to intermediate interbedded lavas. The peralkaline tuffs extend at the top of this sequence.

Mauger and Dayvault (1983), report peralkaline rhyolitic tuffs in Calera-Del Nido block, with the peralkalinity index ranging from 0.94 to 1.20. This study focuses in the thin layer of peralkaline rocks at Laborcita de San Javier, which present a similar index value as those in Calera-Del Nido block, contrasting the 0.60 value of the subjacent rhyolites. The presence of peralkaline rocks in the Chihuahua State indicates a change in the tectonic regime from compression (Farallon plate subduction) to distension (Basin and Range and/or Rio Grande Rift), about 27 M.a. ago.