New Mexico Geological Society Annual Spring Meeting — Abstracts

Proterozoic rocks are well exposed in a fault bounded block within the core of the Joyita Hills (approximately 35 km NE of Socorro). This block is largely composed of two granitic gneisses: a massive, coarse grained quartz-feldspar-biotite ± hornblende gneiss, consisting predominantly of large crystals of K-feldspar (up to 30 mm in size, and comprising greater than 50% of the rock), in a fine-grained matrix of quartz, K-feldspar, plagioclase, biotite. and hornblende; and a megacrystic gneiss, consisting of megacrysts of K-feldspar (up to 20 mm in size) in a matrix of quartz-feldspar-biotite. The megacrystic gneiss is associated with strongly deformed aplite dykes and both show intrusive relations into the massive gneiss. Pods and dykes of ampbibolites crosscut both gneisses, and commonly having a NE-E trend. Pegmatites (consisting of quartz + K-feldspar ± biotite) occur throughout the area, are commonly from 5 cm to 50 cm in width, and can occasionally be traced for up to a few hundred meters. Pegmatites show three distinct orientations. In the southern half of the area, they are either shallowly dipping (30°) to the N or NW, or are vertical and NW trending, Most pegmatites in the northern domain are approximately vertical and approximately north-trending.

A strong stretching lineation, plunging 50° towards 150° is the dominant structural feature of the Proterozoic rocks of the Joyita Hills. Foliations are locally developed in the south of the area, but are more pervasively developed in the northern half of the area. Foliations show a range in orientations, from NE-to SE-trending, and generally east-dipping. However poles to all foliations lie on a single great circle, nonnal to the stretching lineation. In the southern pan of the area foliations show the whole range of orientations, and are best developed where there is a strong rheological contrast between gneiss and some planar structure (e.g., veins and aplite dykes). The northern part of the area is characterized by west-dipping, NNE-trending foliations, which are locally developed in the massive gneiss. K-feldspar porphyroclasts in the megacrystic gneiss commonly have well developed asymmetric tails, indicating a southeast over northwest sense of shear.

The relative timing of emplacement of the granitic protoliths to the two gneisses with respect to deformation can be determined. The massive gneiss is the oldest rock type within the Joyita Hins and was intruded by the megacrystic gneiss/aplite dykes. Both gneisses and the deformed aplite dykes exhibit stretching lineations with similar orientations. Locally a crosscutting relation is seen between the lineated massive gneiss and both the contact with, and foliation within, dykes of the megacrystic gneiss. This relation may indicate that intrusion of the granitic protolith to the megacrystic gneiss occurred during the defonnation, with later deformation being partitioned into the megacrystic gneiss.

The nature of the structures (e.g., very strong mineral lineation, asymmetric tails around K-feldspar porphyroclasts in the megacrystic gneiss) indicates a non-coaxial strain history, with kinematic indicators suggesting a southeast over northwest movement sense their present orientations). If the effects of Phanerozoic defonnations are removed, foliations become nearly vertical, with a nearly vertical stretching lineation, with the south side up relative to the north side. The distribution of structures indicates that in the Joyita Hills there is a transition between dominantly L-tectonites in the south and L>S tectonites the north. Both the nature and distribution of the structures suggests the presence of a wide shear zone, with a minimum thickness of 2-3 km. The zone of L-tectonites, which shows evidence of the most intense deformation, has a thickness of at least 1.5 km. The regional significance of such a shear zone is currently unknown.