New Mexico Geological Society Annual Spring Meeting — Abstracts

The Precambrian rocks in the Jawbone Mountain area can be grouped into three packages of supracrustal rocks, one of which is intruded by plutonic rocks. The oldest package comprises the Moppin Metavolcanic Series which contains a continuous suite of calc-alkaline volcanic and volcaniclastic rocks. Moppin lithologies include biotite schist, chlorite schist, and hornblende schist. This suite is interpreted as having accumulated in a volcanic arc setting. The Moppin Metavolcanic Series is intruded by the Maquinita granodiorite (ca. 1755 Ma) and the granite of Hopewell Lake.

The next younger package of supracrustal rocks is the Burned Mountain metarhyolite (ca. 1700 Ma). This metarhyolite, with well-preserved eutaxitic textures and clasts resembling flattened pumice fragments, is interpreted to be a sub-aerial ash-flow tuff. Trace element geochemistry compares favorably with continental rhyolites associated with crustal extension.

A thick sequence of siliciclastic sediments is the youngest package of Precambrian rocks in the study area. The base of this package, containing metaconglomerate, arkose, and micaceous quartzite, is a transition from rhyolite accumulation to clean sandstone deposition. The sandstone, now a 1 km thick vitreous quartzite, was deposited on an inner shelf and exhibits bedforms produced by tidal and storm processes.

Regional metamorphism from greenschist to lower amphibolite facies has occurred in all the Precambrian rocks in the study area.

Foliations of two main types occur. A bedding~parallel cleavage is related to early deformation events which produced tight to isoclinal folding. An axial-planar cleavage associated with a later, broad, upright folding is seen on a more limited basis.