New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/69 U-Pb geochronology of Proterozoic igneous and metasedimentary rocks in southern New Mexico: Post-collisional S-type granite magmatism J.M. Amato, C.F. Ottenfeld, and C.R. Howland, 2018, pp. 137-145 Supplemental data available: http://nmgs.nmt.edu/repository/index.cfm?rid=2018003 in: Las Cruces Country III, Mack, Greg H.; Hampton, Brian A.; Ramos, Frank C.; Witcher, James C.; Ulmer-Scholle, Dana S., New Mexico Geological Society 69th Annual Fall Field Conference Guidebook, 218 p. This is one of many related papers that were included in the 2018 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. 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OTTENFELD, AND COLBY R. HOWLAND Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA, [email protected] ABSTRACT— U-Pb zircon ages, major element geochemistry, and Nd isotopic data are presented from several localities in the southern part of the Proterozoic Mazatzal province of southern New Mexico, excluding the Burro and San Andres Mountains. These data indicate that the bulk of granitic magmatism in the study area occurred at 1655 Ma, and that the granites are largely undeformed or only locally deformed. This group of igneous rocks is statistically younger than ~1675 Ma orthogneisses, indicating that a regional deformational event occurred between 1675–1655 Ma. The 1655 Ma granites are leucocratic with high SiO2 and Al2O3 and low MgO, have both biotite and muscovite, and thus are S-type granites. Nd isotopic compositions of three samples ranges from εNd = 1.1 to –2.0, but no useful mantle model ages were obtained owing to Sm/Nd modification after crystallization. We present a model for their generation as crustal melts following a collision. Our data also show that 1460 Ma plutons are present in the Fra Cristobal Range, Organ Mts., Antelope Hill in the southern San Andres Mountains, and likely at San Diego Mountain. This even was accompanied by metamorphism as indicated by younger zircon growth in granites at Cookes Peak and by metamorphic titanite growth at Mud Mountain. These new ages provide additional information regarding the development of the southern Mazatzal province. INTRODUCTION ology of the Burro Mountains including both ~1.6 Ga (Amato et al., 2008) and ~1.4 Ga events (Amato et al., 2011). The key Proterozoic rocks of southern New Mexico are part of the questions we are trying to address include: (1) What is the age Mazatzal province, long considered a type example of conti- of the Mazatzal province crust in southern New Mexico; (2) nental growth by accretion of arc rocks (e.g., Karlstrom and Does the Mazatzal province in southern New Mexico repre- Bowring, 1988). The Mazatzal province of Laurentia (Fig. 1) sent juvenile volcanic arc crust? (3) Is the Mazatzal province includes rocks ranging from about 1680 Ma to approximately 1630 Ma in New Mexico, southwestern Arizona, and northern Sonora, Mexico (Karlstrom et al., 2004; Amato et al., 2008). Trans-Hudson Two long-accepted aspects of the Mazatzal province have been Phanerozoic orogen more recently called into question. The first is that the Maza- Wyoming craton 1.1 Ga > 2.5 Ga tzal province represents a juvenile arc system; this model is 1.5–1.3 Ga lt R problematic in light of Hf isotopic studies (e.g., Grambling e be o Pre-1.5 Ga Cheyenn c k et al., 2016) and general petrographic and lithologic consid- y M erations, some of which will be highlighted here. The second n t o io s 40 N t . i t s l n hypothesis is that it accreted to the Yavapai province during the e a b 2.0–1.8 tGar t i Mazatzal orogeny that started around 1.65 Ga (e.g., Karlstrom s a on u p iti r a s h n t v ra and Bowring, 1988). The Mazatzal orogeny was considered r a t e Y v - Mojave e o v part of the progressive accretion of several crustal blocks that r ja ie v o resulted in the growth of Laurentia between 1.7 and 1.1 Ga e M 1.80–1.70 Ga Mazatzal S (Karlstrom et al., 2001). This is also controversial in light of the suggestion that ~1.4 Ga tectonism may have played a role Yavapai Yavapai - southern in the deformation along the suture between the Yavapai and Mazatzal graniteprovince - rhyolite Mazatzal provinces (Daniel et al., 2013), and if it was a con- Fig. 2 1.42–1.35 Ga tinental arc, there is no need to invoke a collision, as the arc would have been formed on Laurentia. Abilene gravity low The Amato Research Group at New Mexico State Univer- Caborca 0 100 sity has attempted to address these questions across a broad 30oN Grenville 1.3–1.0 Ga km orogen transect stretching from the Burro Mountains in southwestern 115 oW 110 oW 105oW 100o W New Mexico to the extensive exposures of the San Andres FIGURE 1. Subdivisions of southwestern Laurentia into Precambrian provinc- Mountains along the Rio Grande rift of central New Mexico es, showing exposures of Proterozoic rocks and tectonic provinceFIGURE boundaries. 1 (Fig. 2). In previous contributions, we have focused on the ge- Modified from Karlstrom et al. (2004). 138 AMATO, OTTENFELD, AND HOWLAND al., 1999). The Paleoproterozoic igneous rocks of the Maza- Socorro 12 tzal province in southern New Mexico can be divided into two 18 New Mexico broad age groups based on Amato et al. (2008) and a compila- tion in Karlstrom et al. (2004): 1680–1650 Ma and 1630–1620 13 19 Fig. 2 Ma. In general, the older group consists of rocks that are per- area vasively deformed, and the younger group includes rocks that 14 23 T or C are undeformed or only locally deformed. This study evaluates 8 20 this prior assessment. 15 Alamogordo 1 6 7 Mapping of Precambrian rocks in southern New Mexico 9 22 (Fig. 2) is available from various publications (e.g., Wood- 16 ward, 1970; Condie and Budding, 1979). U-Pb ages from the 10 San Andres, Florida, and Burro Mountains were reported by 2 Deming Las Cruces 21 Amato et al. (2008) showing a range from 1674–1617 Ma in 5 11 these areas. Additional details on the geology of individual re- 3 17 4 gions can be found in Ottenfeld (2015). In this paper, we focus on rocks from the ~1.65 Ga episode of magmatism and associ- 100 km ated sedimentary rocks. METHODS FIGURE 2. Proterozoic localities in southern New Mexico. Localities out- lined in gray are not part of this study. Localities outlined in black are part of this study: 8-Kingston; 9-Cookes Peak; 10-Fluorite Ridge; 11-Florida Mtns.; Major elements were analyzed by XRF and Nd isotopes 13-Fra Cristobal Mtns.; 14-Mud Mountain; 15-Caballo Mtns.; 16-San Diego were analyzed by TIMS, both at New Mexico State Univer- Mtn.; 21-Organ Mtns./Antelope Hill. Map compiled from Condie and Budding sity. U-Pb geochronology was carried out using laser ablation (1979), Condie (1981), Woodward (1970), and Bauer and Pollock (1993). See Data Repository for the full list. inductively coupled plasma mass spectrometry (LA-ICPMS) at the U. of Arizona and the U. of California at Santa Barba- ra. SHRIMP (sensitive high-resolution ion microprobe) dating lithologically and isotopically homogeneous, or can it be sub- was conducted for two samples at the Stanford–US Geological divided into smaller blocks that originated in different tectonic Survey Facility. The analytical techniques and data reduction settings? and (4) What is the age of initiation and duration of procedures are discussed in Amato et al.