New Mexico Geological Society Annual Spring Meeting — Abstracts

Early Proterozoic supracrustal and plutonic rocks are exposed along the upper Rio Brazos in the Tusas Mountains of northern New Mexico. The supracrustal rocks can be subdivided into three lithostratigraphic units called, in order of decreasing age: the Moppin Metavolcanic Series (MMS); a metasedimentary and metarhyolite package (MSMR); and the Ortega Group. The lower exposed portion of the MMS is intruded by the Rio Brazos trondhjemite, dated at ca. 1650 Ma (Barker at al., 1974).

The MMS consists of amphibolites and mafic to felsic schists. Relict bedding, pillows, vesicles, and flow-bands are present locally. Volcanics and volcaniclastics in this units are tholeiitic.

The contact between the MMS and the MSMR is an unconformity that has been subsequently tectonized. The MSMR grades upsection from a phyllitic metaconglomerate to a micaceous quartzite with phyllite pebble interbeds. A distinctive member of this package is the, Burned Mountain metarhyolite, which is exposed near the base of the package and has a reported U/Pb zircon age of 1700+\-25 Ma (Silver,
in Reed et al., 1987).

The Ortega Group conformably overlies the MSMR package. It is an upward-maturing sequence of cross-bedding quartzites.

Proterozoic rocks in the map area show evidence of at least two of three regionally developed fabric-forming events that include an early phase of isoclinal folding and north-directed shearing, followed by a later nearly co=axial phase of upright large-scale open folding. Metamorphic conditions reached upper greenschist facies.

The MMS is interpreted to represent volcanics and volcaniclastics accumulated in an island arc or continental margin arc. The MSMR represents rhyolites and immature sediments that accumulated during crustal rifting or extension in a continental margin back-arc setting. Primary sedimentary structures in the Ortega Group indicate it was deposited on an inner tidal shelf (Seogaard and Ericksson, 1985).