New Mexico Geological Society Annual Spring Meeting — Abstracts

Miocene silicic volcanic rocks in the Socorro-Magdalena area have been dated by the high precision ⁴⁰Ar/³⁹Ar dating method to detennine their eruptive history. The silicic iavas are located on the Morenci lineament, an accommodation zone associated with the Rio Grande rift. The distribution of the silicic lavas is arbitrarily divided into the Socorro Peak, the Pound Ranch, the Magdalena Peak, and the Squaw Peak eruptive centers. Previous studies by Bobrow et aL (1983) identified sporadic volcanism from 18 to 7 Ma and discussed the petrographic, geochemical, and petrologic of evolution the lavas.

⁴⁰Ar/³⁹Ar dates on mineral separates have been obtained by single crystal laser fusion and incremental furnace heating procedures. Many of the silicic lavas contain sanidine which is ideal for dating. Plagioclase, biotite, and hornblende separates allow for the comparison of ages on two or more minerals from a single rock sample.

The ages on biotite pairs from the same samples are problematic. All of the biotite separates analyzed by ⁴⁰Ar/³⁹Ar and the previously reported K/Ar dates are older than the more precise sanidine dates. Excess argon, chloritization, and xenolithic contamination are possible explanations for the older apparent ages of the biotites. Additional ⁴⁰Ar/³⁹Ar sanidine and biotite dates from all of the eruptive centers will be reported later.

The Miocene silicic rocks of the Socorro-Magdalena area are younger than previously detennined. An episode of rhyolitic volcanism from 7.0 to 8.6 Ma is preserved in the Socorro Peak area. The other silicic centers progressively increase in age to the southwest with conventional K/Ar from 10 to 18 Ma.