Rincon Range Research

Metamorphic Petrology

2001 - 2002

Ben Hallett and Prof. Chris Daniel

Ben with the Rio Mora Range in the background.

 

Overview

Major growth of continental lithosphere from Wyoming southward to New Mexico occurred in the middle Proterozoic (~1800 to 1600 Ma). Proterozoic rocks of the southwestern U.S. (Fig. 1) record two major orogenic cycles: the Yavapai Orogeny (~1700 Ma) and the Mazatzal Orogeny (~1650 Ma) (Hoffman, 1988; Karlstrom and Bowring, 1988). “Anorogenic” plutonism and associated contact and regional metamorphism occurred across the southwestern U.S. at ~1400 Ma (Karlstrom et al., 1997; Pedrick et al., 1998; Read et al., 1999; Williams et al., 1999).

There is considerable controversy regarding whether the metamorphism and deformation in north central New Mexico were the result of one or two orogenic events. Grambling (1986), Bowring and Karlstrom (1990), and Nyman et al. (1994) suggest metamorphism and deformation in north-central New Mexico were caused by a single crustal shortening event at either ~1650 Ma or ~1400 Ma. In contrast, more recent studies have proposed regional metamorphism and deformation at both ~1650 Ma and ~1400 Ma (Pedrick et al., 1998; Read et al., 1999; Williams et al., 1999). If two events were responsible for the metamorphism and deformation of the rocks, it is still unclear which event produced which deformational fabrics and metamorphic minerals.

Figure 1: Proterozoic exposures across the southwestern U.S. Color bands corresond to crustal province boundaries. Arrow shows the study area location (After Bowring and Karlstrom, 1990).

 

Research Significance and Goals:

This work seeks to characterize the long-term development of continental crust by more accurately documenting the timing of high-grade metamorphism and deformation in north central New Mexico. Relative timing of metamorphism and fabric development is correlated with absolute timing from U-Th-Pb monazite geochronology to date metamorphic mineral growth and fabric development.

This study examines Proterozoic migmatites of the Rincon Range, in north central New Mexico (Fig. 2). The Rincon Range, near the village of Guadalupita, lies in an area between the El Oro gneiss dome in the south and the Black Lakes to the north. The Rincon Range is important as it exposes a transition in metamorphic grade from the characteristic Al2SiO5 “triple point” medium grade terrain in the north to the high-grade gneisses, migmatites, and sillimanite-K-feldspar rocks in the south. This transition is not exposed in other uplifts of the region. The area of highest-grade metamorphism is characterized by screens of migmatitic supracrustal rock within the granitic Guadalupita Pluton in the central part of the range.

This research has two main components: (1) a petrographic study of deformational fabrics and metamorphic reaction textures including chemical zoning analyses and estimates of metamorphic P-T conditions, and (2) the application of U-Th-Pb chemical age dating of monazite to constrain absolute timing of metamorphism and deformation.

 

Support:

This material is based upon work supported by the National Science Foundation under Grant No. 0196116 awarded to Christopher Daniel and a PriceWaterhouseCoopers fellowship awarded to Ben Hallett. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

 

Geologic Setting:

North-central New Mexico is characterized by exposures of Proterozoic metamorphic and igneous rocks uplifted by Cordilleran and Rio Grande Rift Zone tectonism. In the northern uplifts of the Taos, Tusas, and Santa Fe ranges, the oldest Proterozoic rocks, mafic metavolcanic sequences including the ~1760 to 1720 Ma Pecos, Gold Hill, and Moppin Complexes are exposed. Overlying these sequences, and exposed across the region, are the schists, gneisses, amphibolites, and felsic metavolcanics of the Vadito Group. The overlying Hondo Group consists of the km-thick Ortega Quartzite overlain by schists and phyllites of the Rinconada Formation Felsic plutons intruded these older supracrustal rocks between ~1.4 Ga and ~1.68 Ga.

The region generally shows a zone of higher metamorphic grade rocks in the northern Taos and Cimarron ranges, interpreted to be the deepest Proterozoic crustal section exposed in the area. The Ortega Fold and Thrust Belt (Fig. 3) is characterized by km-scale folding and steeply south-dipping ductile shear zones, as well as metamorphism near the Al2SiO5 triple point. The southern part of the region shows a significant increase in ~1.68 and ~1.4 plutonism. This area is characterized by older arc-related metavolcanics thrust over the Ortega Fold and Thrust Belt by the Pecos thrust.


Figure 3: Protoerozoic exposures across north-central New Mexico (after Williams et al., 1999).

 

The Rincon Range

The Precambrian rocks of the Rincon Range, in the Ortega Fold and Thrust Belt (Fig. 3), consist of supracrustal rocks of the Vadito and Hondo groups intruded by the ~1682 Ma Guadalupita Pluton in the south (Fig. 4). The pluton is characterized as a K-feldspar, plagioclase, and quartz rich gneiss. A mica-defined foliation (S2) is present throughout the pluton. The margins of the pluton show multiple screens of metasedimentary and metavolcanic rocks intruded by and included within the pluton (Read et al., 1999).

The Rincon Range exhibits a general structural transition from sub-horizontal dominant foliation (S2) in the south to sub-vertical in the north. The dominant foliation is folded by F3 folds. Mica-defined S3 fabrics are axial planar to these folds. Steeply-dipping Proterozoic ductile thrusts in the north repeat the overturned stratigraphy several times. The range is interpreted to be an overturned limb of a regional scale fold nappe (Read et al., 1999).

 

Figure 4: Geologic map of the Ricon Range (after Read et al., 1999).

 

Page created by Ben Hallett on 5/5/02.