PETROLOGY, GEOCHEMISTRY, AND STRUCTURE OF THE CHUGWATER ANORTHOSITE, LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING

The Chugwater Anorthosite is one of several 1.43 Ga intrusions that make up the Laramie Anorthosite Complex in the Laramie Mountains, Wyoming, USA. The southwestern portion of the Chugwater Anorthosite has a mappable magmatic stratigraphy totaling at least 8000 meters, and probably 10,000 m or more. It consists of three major units, each of which comprises a lower, dominantly anorthosite portion (>90 vol.% plagioclase) and an upper, dominantly gabbroic anorthosite section (80–90 vol.% plagioclase). Each anorthosite portion contains layers of gabbroic anorthosite and vice versa , on a variety of scales ranging down to centimeters. Plagioclase is dominantly An 50–55 , although higher and lower values are also found. This “main series” of the Chugwater Anorthosite mainly lacks modal and normative olivine. Oxygen fugacity ranged from FMQ to FMQ + 0.5. We interpret that the “main series” was produced by at least three major injections of mildly hyperfeldspathic magma containing approximately 40% entrained plagioclase megacrysts (tabular crystals at least 5 cm across). The Ti contents of the megacrysts (0.3–0.4 wt% TiO 2 ) suggest that these had crystallized at pressures near 10 kbar, consistent with initial formation in a magma chamber at or near the base of the crust. Emplacement pressure is poorly constrained, but appears to have been near 3.5–4 kbar. However, before the main series had completely solidified, it was repeatedly intruded by at least two (and probably more) leucotroctolitic magmas. Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse against gabbroic anorthosite, suggesting that sufficient residual melt remained in the latter to permit local mixing with leucotroctolite. We estimate that 80–85% of the Chugwater Anorthosite is main-series, 10–15% is mixed rock, and 5% is leucotroctolite. Prior to final solidification, the entire Chugwater Anorthosite was domed, probably as a result of gravitational instability of the relatively buoyant plagioclase-rich material. The emplacement of the Chugwater Anorthosite as a series of crystal-rich magmas, followed by continued fractionation in a magma chamber at a mid-crustal level and subsequent doming, are characteristics that make the Chugwater Anorthosite intermediate between the Poe Mountain Anorthosite to the north ( in situ fractionation in a magma chamber) and the classic diapiric emplacement of a crystal-rich mush.

[1]  S. Swapp,et al.  CHEMICAL AND ISOTOPIC EVOLUTION OF THE ANORTHOSITIC PLUTONS OF THE LARAMIE ANORTHOSITE COMPLEX: EXPLANATIONS FOR VARIATIONS IN SILICA ACTIVITY AND OXYGEN FUGACITY OF MASSIF ANORTHOSITES , 2010 .

[2]  D. Lindsley,et al.  MAGMATIC AND STRUCTURAL EVOLUTION OF AN ANORTHOSITIC MAGMA CHAMBER: THE POE MOUNTAIN INTRUSION, LARAMIE ANORTHOSITE COMPLEX, WYOMING , 2010 .

[3]  M. ño,et al.  Phase equilibria of dikes associated with Proterozoic anorthosite complexes , 2007 .

[4]  J. Scoates,et al.  Geochronologic, geochemical and isotopic constraints on the origin of monzonitic and related rocks in the Laramie anorthosite complex, Wyoming, USA , 2003 .

[5]  J. Scoates,et al.  The evolution of troctolitic and high Al basaltic magmas in Proterozoic anorthosite plutonic suites and implications for the Voisey's Bay massive Ni-Cu sulfide deposit , 2000 .

[6]  J. Scoates.  The plagioclase-magma density paradox re-examined and the crystallization of Proterozoic anorthosites , 2000 .

[7]  B. Frost,et al.  The 1.76-Ga Horse Creek anorthosite complex, Wyoming A massif anorthosite emplaced late in the Medicine Bow orogeny , 2000 .

[8]  A. Boudreau,et al.  Metal redistribution during fluid exsolution and migration in the Middle Banded series of the Stillwater Complex, Montana , 1999 .

[9]  H. Havenith,et al.  The deformation of the Egersund–Ogna anorthosite massif, south Norway: finite-element modelling of diapirism , 1999 .

[10]  M. Fram,et al.  Some Phase Equilibrium Constraints on the Origin of Proterozoic (Massif) Anorthosites and Related Rocks , 1999 .

[11]  J. Scoates,et al.  Orogenic to Post‐Orogenic Origin For the 1.76 Ga Horse Creek Anorthosite Complex, Wyoming, Usa , 1997, The Journal of Geology.

[12]  D. Lindsley,et al.  Residual-liquid origin for a monzonitic intrusion in a mid-Proterozoic anorthosite complex: The Sybille intrusion, Laramie anorthosite complex, Wyoming , 1996 .

[13]  J. Scoates,et al.  The Geochemical Evolution of Anorthosite Residual Magmas in the Laramie Anorthosite Complex, Wyoming , 1996 .

[14]  D. Xirouchakis.  Contact Metamorphism Adjacent to the Laramie Anorthosite Complex, Albany County, Wyoming , 1996 .

[15]  B. Marsh.  Solidification fronts and magmatic evolution , 1996, Mineralogical Magazine.

[16]  B. Lafrance,et al.  Syn-emplacement recrystallization and deformation microstructures in the Poe Mountain anorthosite, Wyoming , 1996 .

[17]  J. Scoates,et al.  A strontium and neodymium isotopic investigation of the Laramie anorthosites, Wyoming, USA: Implications for magma chamber processes and the evolution of magma conduits in Proterozoic anorthosites , 1996 .

[18]  J. Scoates,et al.  Baddeleyite (ZrO2) and zircon (ZrSiO4) from anorthositic rocks of the Laramie anorthosite complex, Wyoming: Petrologic consequences and U-Pb ages , 1995 .

[19]  J. Scoates,et al.  High-Al gabbros in the Laramie Anorthosite Complex, Wyoming: implications for the composition of melts parental to Proterozoic anorthosite , 1995 .

[20]  M. Hamilton,et al.  Petrogenesis of a Mid-Proterozoic Anorthosite-Mangerite-Charnockite-Granite (AMCG) Complex: Isotopic and Chemical Evidence from the Nain Plutonic Suite , 1994, The Journal of Geology.

[21]  David Anderson,et al.  Quilf: A pascal program to assess equilibria among Fe-Mg-Mn-Ti oxides , 1993 .

[22]  M. Fram,et al.  Pressure effects, kinetics, and rheology of anorthositic and related magmas , 1993 .

[23]  Donald H. Lindsley,et al.  Equilibria among Fe-Ti oxides, pyroxenes, olivine, and quartz; Part I, Theory , 1992 .

[24]  R. Wiebe.  Chapter 6 Proterozoic Anorthosite Complexes , 1992 .

[25]  D. Lindsley.  Experimental studies of oxide minerals , 1991 .

[26]  Donald H. Lindsley,et al.  Oxide minerals : petrologic and magnetic significance , 1991 .

[27]  J. A. Grant,et al.  Contact metamorphism and partial melting of pelitic rocks in the aureole of the Laramie anorthosite complex, Morton Pass, Wyoming , 1990 .

[28]  R. Wiebe.  Evidence for unusually feldspathic liquids in the Nain Complex, Labrador , 1990 .

[29]  B. Frost,et al.  Magmatic CO2 and saline melts from the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming , 1989 .

[30]  D. Lindsley,et al.  Geochemical evolution of the Maloin Ranch Pluton, Laramie anorthosite complex, Wyoming; petrology and mixing relations , 1989 .

[31]  Paula M. Davidson,et al.  Thermodynamic analysis of pyroxene-olivine-quartz equilibria in the system CaO-MgO-FeO-SiO 2 , 1989 .

[32]  Donald H. Lindsley,et al.  Fe-Ti oxide-silicate equilibria; assemblages with fayalitic olivine , 1988 .

[33]  M. Fuhrman,et al.  Crystallization Conditions of the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming , 1988 .

[34]  T. Irvine.  Layering and Related Structures in the Duke Island and Skaergaard Intrusions: Similarities, Differences, and Origins , 1987 .

[35]  J. Longhi,et al.  Two-stage models for lunar and terrestrial anorthosites Petrogenesis without a magma ocean , 1985 .

[36]  R. F. Emslie.  Proterozoic Anorthosite Massifs , 1985 .

[37]  S. Goldberg.  Geochemical relationships between anorthosite and associated iron-rich rocks, Laramie Range, Wyoming , 1984 .

[38]  W. Boynton.  Geochemistry of the rare earth elements : meteorite studies , 1984 .

[39]  R. Allmendinger,et al.  COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 2: Precambrian basement structure and its influence on Laramide deformation , 1982 .

[40]  R. Allmendinger,et al.  COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 1: Laramide structure , 1982 .

[41]  J. Fountain,et al.  Geochemistry and petrogenesis of the Laramie anorthosite complex, Wyoming , 1981 .

[42]  A. Streckeisen.  To each plutonic rock its proper name , 1976 .

[43]  George W. De Vore.  The Role of Partial Melting of Metasediments in the Formation of the Anorthosite-Norite-Syenite Complex, Laramie Range, Wyoming , 1975, The Journal of Geology.

[44]  L. B. Owen,et al.  Gravity Interpretation of the Laramie Anorthosite Complex, Wyoming , 1973 .

[45]  S. Smithson,et al.  Geology and geophysics of syenites associated with Laramie anorthosite, Wyoming , 1970 .

[46]  S. Smithson,et al.  Quantitative geophysical study of petrogenesis of syenites related to laramie anorthosite, Wyoming, U.S.A. , 1970 .

[47]  R. F. Emslie.  The geology of the Michikamau Intrusion, Labrador , 1970 .

[48]  L Streckeisen Albert,et al.  Classification and nomenclature of igneous rocks (final report of an inquiry) , 1967 .

[49]  A. T. Anderson.  Mineralogy of the labrieville anorthosite, quebec , 1966 .

[50]  M. Klugman.  Resume of the Geology of the Laramie Anorthosite Mass , 1966 .

[51]  A. F. Hagner,et al.  Geologic map of anorthosite areas, southern part of Laramie Range, Wyoming , 1957 .

[52]  A. Buddington.  Adirondack igneous rocks and their metamorphism , 1939 .

[53]  R. Fuller.  Deuteric alteration controlled by the jointing of lavas , 1938 .

[54]  M. A. Peacock,et al.  A Physiographic Study in the Basin Range Province@@@The Geomorphology and Volcanic Sequence of Steens Mountain in Southeastern Oregon , 1933 .

[55]  K. Fowler.  The anorthosite area of the Laramie Mountains, Wyoming , 1930 .

[56]  N. L. Bowen.  The Problem of the Anorthosites , 1917, The Journal of Geology.