New Mexico Geological Society Annual Spring Meeting — Abstracts

Early Proterozoic successions of bimodal volcanic rocks and associated vocaniclastic sediments with polyphase deformation occur in the Pedernal Hills and in the Manzano Mouritains, central New Mexico. The Manzano succession (1680 m.y.) is in fault contact with a somewhat younger Proterozoic succession (1650 m.y.) and the Pedernal succession appears to overlie a group of massive quartzites and mica schists.

Mafic volcanics are minor in abundance and occur as tuffs and breccias with some dikes and sills. Both calc-alkaline(CAB) and tholeiites (TH) are present. CAB are light-REE enriched (La/Yb = 6-10) and TH have nearly flat REE patterns (La/Yb= 13). The CAB exhibit negative Ta-Nb anomalies ona NMORB normalized graph, and in thi s respect are similar to modern arcrelated basalts. Two mantle sources are needed to explain the compositional differences between CAB and TH groups. Felsic volcanics occur in both successions and include ignimbrites and hypabyssal intrusives. Felsic unitsrarigefrom rhyodacite td high-silica rhyolite and have similar trace element distributions to modern, calc-alkaline, continental-arc felsic volcanics. They exhibit light-REEenriched patterns with negative and positive Eu anomalies and show notable depletions in Sr, P, Ta, and Nb relative to other incompatible elements. Felsic magmas can be produced by a two-stage fractional crystallization model using a basaltic parent in which andesites are not frequently erupted.

Geochemical and lithologic data are consistent with a continental-margin arc setting for formation of the two successions. Central New Mexico appears to be part of the early Proterozoic arc association which underlie. Colorado and northern New Mexico .