Geology and geochemistry of the Redrock Granite and anorthosite xenoliths … 7 GEOLOGY AND GEOCHEMISTRY OF THE REDROCK GRANITE AND ANORTHOSITE XENOLITHS (PROTEROZOIC) IN THE NORTHERN BURRO MOUNTAINS, GRANT COUNTY, NEW MEXICO, USA VIRGINIA T. MCLEMORE, NELIA DUNBAR, PAULA J. KOSUNEN, O. TAPANI RÄMÖ, MATT HEIZLER AND ILMARI HAAPALA McLEMORE, VIRGINIA T., DUNBAR, NELIA, KOSUNEN, PAULA J., RÄMÖ, O. TAPANI, HEIZLER, MATT, and HAAPALA, ILMARI 2002. Ge- ology and geochemistry of the Redrock Granite and anorthosite xenoliths (Pro- terozoic) in the northern Burro Mountains, Grant County, New Mexico, USA. Bulletin of the Geological Society of Finland 74, Parts 1–2, 7–52. Mineral ages from the A-type granites and anorthosite xenoliths in the Red- rock area in the northwestern Burro Mountains in southwestern New Mexico cluster around ~1220–1225 Ma and provide yet another example of bimodal igneous activity during this time period in the southwestern United States. The metaluminous to peraluminous, marginally alkaline to subalkaline Redrock Gran- ite exhibits the textural, mineralogical, and geochemical features of A-type gran- ite that was emplaced at a relatively high crustal level. Field relationships, whole rock and mineral geochemical and isotopic trends suggest that the four phases of the Redrock Granite are genetically related, with the miarolitic biotite/alkali feldspar granite being the youngest phase. Spatial relationships and geochemi- cal data suggest that the anorthosite xenoliths were coeval with the Redrock Granite, which is consistent with the anorthosite being derived from the upper mantle, possibly due to deep mantle upwellings, and the Redrock Granite from the lower crust. The process involved melting in the upper mantle, emplace- ment of anorthosite in the crust resulting in partial crustal melting and thinning, and, finally, intrusion of shallow silicic plutons, the Redrock Granite. The Red- rock Granite and anorthosite were presumably derived from sources character- ized by subtle, long-term LREE depletion, with εNd (at 1220 Ma) values on the order of +1 to +2. Key words: granites, A-type granites, anorthosite, xenoliths, geochemistry, chem- ical composition, electron probe data, isotopes, magmatism, Proterozoic, Bur- ro Mountains, New Mexico, United States Virginia T. McLemore, Nelia Dunbar, and Matt Heizler: New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Tech- nology, Socorro, NM, 87801, USA E-mail: [email protected] Paula J. Kosunen, O. Tapani Rämö, and Ilmari Haapala: Department of Geol- ogy, P.O. Box 64, FIN-00014 University of Helsinki, Finland 8 Virginia T. McLemore, Nelia Dunbar, Paula J. Kosunen, O. Tapani Rämö, Matt Heizler and Ilmari Haapala INTRODUCTION Recent studies of Proterozoic rocks in the Burro ics can be divided into ten phases: (1) Mazatzal Mountains in southwestern New Mexico have un- orogeny, 1700–1600 Ma (Karlstrom & Bowring covered several interesting geologic relationships. 1988, 1993, Karlstrom et al. 1990), (2) Late Pro- In the Redrock area of the northwestern Burro terozoic granitic plutonism, 1500–1300 Ma (Sta- Mountains (Fig. 1), there is a spatial association cey & Hedlund 1983, Karlstrom & Bowring 1988, of anorthosites and A-type granites. The bimodal 1993, Adams & Keller 1996, Karlstrom et al. anorthosite-granite association supports current 1997, 2001, Karlstrom & Humphreys 1998), (3) theories involving mantle underplating and partial pre-Grenville extension and formation of continen- melting of the lower crust that resulted in intru- tal margin at 1300–1260 Ma (Pittenger et al. 1994, sion of both types of magmas (Anderson & Bender Adams & Keller 1994, 1996, Karlstrom et al. 1989, Emslie 1991, Haapala & Rämö 1990, Rämö 1997, Karlstrom & Humphreys 1998, Barnes et al. & Haapala 1995). Mineral ages from the Redrock 1999), (4) 1260–1000 Ma period of mafic and A- area cluster around ~1220–1225 Ma and it is one type granitic magmatism and volcanic activity in of the few areas of bimodal igneous activity dur- Texas and Arizona, coincident with the Grenville ing this time period in southwestern United States orogeny and perhaps extension (Adams & Keller (Bickford et al. 2000, McLemore et al. 2000a, b). 1996, Smith et al. 1997, Shannon et al. 1997, The main goals of our research of the Proterozo- Mosher 1998, Barnes et al. 1999, Reese et al. ic rocks in the Burro Mountains are (1) petrology 2000, McLemore et al. 2000b, Bickford et al. (protolith history and magmatic evolution) of the 2000), (5) Paleozoic period of alkaline and car- granitic plutons, (2) interplay of tholeiitic and po- bonatite magmatism and extension at ~500 Ma tassic mafic magmatism in the petrogenesis of the (McLemore & McKee 1988b, McLemore et al. granitic rocks, (3) Proterozoic geochronology of 1999a, McMillan et al. 2000), (6) Paleozoic peri- the Burro Mountains region, and (4) tectonic ev- od of basin formation and uplift as part of the olution of southern Laurentia. Ancestral Rocky Mountains (Florida uplift, The purpose of this paper is to describe the li- Pedregosa Basin; Ross & Ross 1986), (7) Creta- thology and chemistry of the ~1220 Ma Redrock ceous continental arc, shallow marine deposition Granite and spatially associated anorthosites in the (Drewes 1991), (8) Laramide compressional de- Redrock area of the northern Burro Mountains formation and magmatic arc volcanism and plu- (Fig. 1). The work reported here builds on results tonism (Late Cretaceous to early Tertiary, Drewes summarized by McLemore et al. (2000b) that 1991), (9) mid-Tertiary calc-alkaline volcanism to briefly described the various rock types and re- bimodal volcanism with caldera formation gional geologic setting, in terms of geochemical, (Schoolhouse Mountain caldera related to the Da- isotopic, and geochronological data. til–Mogollon field, McIntosh et al. 1991), and (10) late Tertiary–Quaternary Basin and Range exten- sional deformation (Coney 1978). Each of these GEOLOGIC SETTING tectonic periods left remnant structural trends in the Burro Mountains that were either reactivated The regional structural evolution of the southwest- or crosscut by younger tectonic events and togeth- ern United States and northern Mexico, specifical- er have resulted in a structurally complex, rela- ly the Burro Mountains, has been dominated by a tively thin, brittle, and anisotropic crust in south- succession of dynamic and sometimes rapidly western United States. changing plate tectonic settings from the Proter- The Burro Mountains in southwestern New ozoic to the Recent (Coney 1978, Karlstrom & Mexico comprise a complex Proterozoic terrain Bowring 1988, Karlstrom et al. 1990, 2001). This that spanned at least from 1633 to ~1000 Ma (Ta- prolonged history of complex continental tecton- ble 1). The Proterozoic rocks include metamorphic Geology and geochemistry of the Redrock Granite and anorthosite xenoliths … 9 Fig. 1. Generalized geologic map of the Proterozoic rocks in the Burro Mountains. Geology complied from Drewes et al. (1985). 10 Virginia T. McLemore, Nelia Dunbar, Paula J. Kosunen, O. Tapani Rämö, Matt Heizler and Ilmari Haapala TABLE 1. Age relationships of the Proterozoic and younger rocks in the northern Burro Mountains (youngest to oldest). Best age estimates are in parenthesis (from Stacey & Hedlund 1983, McLemore et al. 2000b, unpub- lished 40Ar/39Ar and U/Pb isotopic data). Units Tertiary rhyolite and quartz monzonite dikes and plugs Tertiary–Cretaceous andesite sills and dikes Cretaceous Beartooth Quartzite Cambrian–Ordovician to Proterozoic syenite bodies Proterozoic rocks Gabbro/diabase/diorite dikes Pegmatite dikes Serpentinite veins Fine-grained alkali feldspar and biotite granite dikes (~1200–1220 Ma) Rhyodacite-dacite porphyry dikes (~1200–1220 Ma) Redrock Granite (~1220 Ma) Miarolitic biotite granite/alkali feldspar granite Hornblende granite Biotite-hornblende granite Anorthosite/leucogabbro xenoliths (~1225 Ma) Gneissic granite/granodiorite (Ygd of Hedlund 1980a, b) (~1440–1450 Ma) Jack Creek Rapakivi Granite (~1465 Ma) Porphyritic granite (~1440–1460 Ma) Minette (~1465 Ma) Burro Mountain Granite (Yg of Hedlund 1980a, b) (~1440–1460 Ma) Bullard Peak and Ash Creek metamorphic rocks (>1633, 1550–1570 Ma) Quartzo-feldspathic gneiss (granulite of Hedlund 1980a) Gabbro/diabase/diorite intrusions (~1633 Ma) rocks (Bullard Peak and Ash Creek Group, >1633, wide) within the Redrock Granite, and (4) a 1550–1570 Ma) that were intruded by granitic and younger group of gabbro/diabase/diorite intrusions mafic rocks (Fig. 1; Hewitt 1959, Hedlund 1980a, (<1200 Ma). These age relationships are summa- b, Drewes et al. 1985, McLemore & McKee rized in Table 1. 1988a, McLemore et al. 2000b). Chemically and During the latest Proterozoic and Cambrian, the petrologically distinct granitic rocks in the Burro Burro Mountains were uplifted and subsequently Mountains include (1) Burro Mountain Granite eroded during the Paleozoic and again during the (~1440–1460 Ma), (2) gneissic granite/granodior- Laramide and mid-Tertiary. The Burro Mountains ite (~1440–1450 Ma), (3) Jack Creek Rapakivi were either a highland during much of Phanero- Granite (~1465 Ma, McLemore et al. 2000b), (4) zoic time, or older sedimentary rocks were erod- Redrock Granite (~1220 Ma, McLemore et al. ed before deposition of Cretaceous rocks when 2000b), (5) rhyodacite–dacite porphyry dikes, (6) seas partially covered the mountain range. Lara- fine-grained alkali feldspar and biotite granite mide compressional tectonics and mid-Tertiary dikes, and