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Information about dahl poster

Published on August 26, 2007

Author: Alohomora

Source: authorstream.com

Slide1:  Integrating geochronologic and Earthscope investigations to constrain the role of the Archean Wyoming craton in Precambrian supercontinent cycles Peter S. Dahl, Department of Geology, Kent State University, Kent, OH 44242 Abstract Joint application of geochronologic and geophysical techniques represents a potentially powerful approach for deciphering the role of the Archean Wyoming craton in the assembly and dispersal of Precambrian supercontinents (e.g., Kenorland, Laurentia, and Rodinia). Recent geochronologic investigations focused along the northern and eastern margins of the Wyoming craton have constrained the formational ages of subsurface geophysical trends, the nature of which in turn illuminate the tectonic significance of the ages themselves, regarding the ~1900-1700 Ma assembly of SW Laurentia. A. Published gravity and magnetic anomalies associated with the NE-trending Great Falls tectonic zone (GFtz) are truncated to the east by apparently younger anomalies defining the Dakota segment of the southern Trans-Hudson orogen (THO; eastern cratonic margin). This apparent truncation is supported by new U/Pb ages of magmatic monazite in late-syn- to post-orogenic pegmatites, which constrain terminal Wyoming-Medicine Hat collision at ~1765-1750 Ma in the Big Sky orogen (Ruby Range, MT) and terminal Wyoming-Superior collision at ~1715-1690 Ma in the southern THO (Black Hills, SD). B. Published geophysical data from within the GFtz have been interpreted as defining a NW-dipping suture between the Wyoming craton and Medicine Hat block, which probably formed at ~1865 Ma according to published U/Pb ages of magmatic zircon and associated metamorphic monazite in the intervening, arc-related rocks (Little Belt Mtns.). In other cases, the Earthscope facility, if appropriately arrayed and deployed, might constrain untested tectonic hypotheses that are suggested largely from the geochronology. C. Cratonward and southwest of the GFtz arc terrane, in the Big Sky orogen, both the U/Pb and K/Ar mineral systems in metamorphic rocks preserve a minor but widespread component of ~1855-1800 Ma (mixed?) dates, suggesting that the magmatic arc and metasedimentary foreland were in close proximity even prior to the published ~1770 Ma age of peak metamorphism. D. A pre-Laurentia, post-Kenorland configuration in which the eastern Wyoming craton was rifting away from the southern Superior craton (present coordinates) is permitted by published sedimentological and paleomagnetic data. This configuration is newly supported by the recent discovery that a thick mafic sill was intruded in the Black Hills at ~2480 Ma (U/Pb, magmatic sphene; Blue Draw gabbro) and thereby correlates with known rift-related rocks of the same age and lithology in southern Ontario. E. Published ages in rocks from the NE-trending, NW-dipping Snowy shear zone (SSZ; NW Beartooth Mountains), which lies cratonward of and parallel to the GFtz and Big Sky orogen, have been interpreted in terms of ~2550 Ma juxtaposition of the Montana metasedimentary terrane (MMT) and the Bighorn-Beartooth magmatic terrane (BBMT). This interpretation is supported by ~2550 Ma U/Pb ages of metamorphic monazite newly obtained in deformed rocks of the Madison mylonite zone (MMZ), which is collinear with the SSZ and may represent its southwestern extension. Further, these metamorphic ages, along with recently published ~2550-2600 Ma ages of syn-collisional granite magmatism in the Black Hills, may record the final assembly of ~2900-2600 Ma Kenorland. F. In map view, the proposed ~2550 Ma SSZ-MMZ suture appears to be truncated by the SW margin of Laurentia, suggesting that whatever landmass may have rifted away from it during the ~2450 Ma breakup of Kenorland or ~800 Ma breakup of Rodinia should exhibit the continuation of this older suture. This poster draws heavily from 1999-2005 research of graduate students Carson Jones, Clayton Loehn, Jim McCombs, andamp; Hazel Roberts. They worked with me in the instrumental lab facilities of research colleagues Mike Hamilton, Simon Kelley, Frank Mazdab, Bob Tracy, andamp; Joe Wooden, whose assistance in obtaining the data is also acknowledged. This research was funded by National Science Foundation grant EAR-9909433 (P.S.D.). Acknowledgments ARCHEAN WYOMING PROVINCE DNAG 1993 Archean Wyoming Craton andamp; Laramide Uplifts D Dahl et al. (2005, in review) Rifting in the Black Hills (SD) at ~2480 Ma (Blue Draw metagabbro, BDM) and thermotectonism in southwestern Montana (SWM) at ~2450-2490 Ma may reflect the incipient breakup of the ~2900-2600 Ma supercontinent, Kenorland. Following this scenario, the BDM and East Bull Lake mafic intrusive suite (Ontario) may represent a ~2480 Ma rift axis (Dahl et al. 2003, and in review). At the same time, SWM may have been the site of a failed rift or of collision with another landmass as the Wyoming province separated from Kenorland (see monazite ages in panel A). Neoarchean basement in the Black Hills is comprised of ~2560-2600 Ma syntectonic granites and older gneisses (McCombs et al. 2004). Neoarchean deformational fabrics in these granites may represent E.Wyoming-S.Superior collision associated with terminal Kenorland assembly (cf., ~2600 Ma suturing of Minnesota River and Superior cratons; Southwick and Chandler 1996). Ages from Dahl et al. (in review) and McCombs et al. (2004) Map from DeWitt et al. (1989) Black Hills: Magmatism at 2560 Ma (granite) andamp; 2480 Ma (gabbro) A Terminal Tectonic Closure A post-tectonic tourmaline pegmatite cross-cuts the NE-SW-trending fabric of amphibole schist (Montana metasedimentary terrane). Magmatic monazite yields a 2σ upper-intercept age of 1762 ± 6 Ma, which constrains the timing of terminal cratonic collision in the Ruby Range (Great Falls tectonic zone, GFtz). The GFtz is truncated by the Trans-Hudson orogen (THO), along which terminal cratonic collision is similarly constrained at 1715 Ma (Black Hills; THO, Dakota segment). Magmatic Monazite Pegmatite CJ-9 Amphibole schist ≥ 1762 ± 6 Ma post-tectonic tourmaline pegmatite 1762 ± 6 Ma Ruby Range (CJ-9 locality), looking southwest 1760 ± 4 1767 ± 6 Jones et al. (2004) after Mueller et al. (2002) 1760 Ma 1715 Ma Ages of pegmatitic monazite from Jones et al. (Ruby Range, GFtz; 2004) and Redden et al. (Black Hills, THO; 1990). Base map modified after Mueller et al. (2002). 2465 ± 4 1760 ± 6 2466 ± 4 100 µm Paleoproterozoic Metamorphism Metamorphic Monazite Dillon quartzofeldsoathic gneiss S-27 Ruby Range (Montana Metased. Terrane) This monazite grain preserves evidence of regional metamorphism in the Ruby Range at ~2450 and ~1760 Ma (Jones et al. 2004). Protolith age for the host gneiss is believed to be ~3.3 Ga (Mogk et al. 2004). ≤1850 Ma Ages in the MMT? C ~1720 Ma ~1850 Ma? ~2450 Ma ~2790 Ma Tobacco Root Range Orthogneiss TRMR-2 Monazite ellipses are 95% conf. Dahl and Hamilton (2002) The Highland, Tobacco Root, and Ruby Ranges (MMT) preserve a minor component of ~1850-1800 Ma dates (see also Roberts et al. 2002), suggesting that the 1865 Ma GFtz arc and MMT foreland were in close proximity even prior to the ~1770 Ma peak of the Big Sky orogeny inferred by others (Brady et al. 2004, Cheney et al. 2004, Mueller et al. 2004) . E 7-68 8-104 HL - 3B HL - 4B 2691 ± 9 Ma 2574 ± 5 2580 ± 27 2650 ± 11 2627 ± 5 2559 ± 3 2489 ± 4 Madison Mylonite Zone andamp; Environs Madison River S. Madison Range (SW Montana) S. Madison Range (SW Montana) 10 mm 2574 ± 4 Ma 2691 ± 9 Ma 30 mm ~2690 Ma ~2480 Ma Y Th 8-104 Monazite 7-68 Monazite S. Madison Range, ID (Henry’s Lake) S. Madison Range, MT (Madison Mylonite Zone, MMZ) Beartooth Range, MT, South Snowy Block, (Yankee Jim Canyon) Beartooth Range, MT (North Snowy Block) Y NSB-2 Monazite Madison Mylonite Zone (MMZ), Snowy Block Shear Zones, andamp; Environs Age Comparisons, Madison Mylonite Zone F The table in the adjacent panel compares 40Ar/39Ar (mixed?) dates of micas and amphiboles (Erslev andamp; Sutter 1990; Roberts 1999) with U-Th-Pb isotopic and total-Pb spot dates of monazite (Loehn et al. 2004), all obtained from within the MMZ and environs. All apparent ages are listed with 2σ error. Erslev andamp; Sutter inferred that the MMZ was developed during SE-directed thrusting of Pre-Cherry Creek gneisses over younger Cherry Creek rocks at ~1800-1900 Ma. They further showed that original SE dips of the country rocks were locally reoriented to NW dips within the MMZ proper. Loehn et al. (2004) suggested that the MMZ was formed, instead, at ~2550 Ma and that the fabric reoriented thereby was originally formed during ~2670 Ma thermotectonism. They further suggested that the MMZ was reactivated at ~1700-1800 Ma as a local, cratonward manifestation of the ~1770-1720 Ma Big Sky orogeny. The preponderance of ~2550 Ma U/Pb monazite dates obtained in collinear shear zones extending from the western Beartooth Mountains (North and South Snowy blocks) southwest to the South Madison Range and environs (MMZ; and Henry’s Lake, ID) suggests that these zones constitute a single shear zone of ~2550 Ma age (see adjacent panel). Mogk et al. (1992) inferred the existence of a ~2550 Ma suture between the MMT and older Beartooth-Bighorn Magmatic Terrane (BBMT) in the North Snowy block. The new U/Pb monazite ages of Loehn et al. (2004) suggest that the MMT-BBMT suture extends southwest to the MMZ and beyond. It would seem that exploring its potential truncation at the (2450 Ma?, 800 Ma?) rifted margin of SW Laurentia is a worthy goal of the Earthscope facility. Preliminary Reinterpretation of the MMZ B In the Little Belt Mountains (GFtz), the Pinto diorite intruded older metasedimentary rocks in an island-arc setting. The arc magmatism is dated at 1865 ± 7 Ma (U/Pb zircon, Mueller et al. 2002), and synchronous metamorphism of the country rocks is independently dated at 1870 ± 7 and 1862 ± 5 Ma (U/Pb monazite, Dahl et al. 2000). These ages are considered as dating the NW-dipping subduction zone (Gorman et al. 2002) that separates the Wyoming craton and Medicine Hat block (see also Mueller et al. 2004). 1865 Ma Events in the GFtz Map modified from Erslev andamp; Sutter (1990) Total-Pb monazite ages from Loehn et al. (2004)

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