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Chapter 15 Spring 2010 - Chapter 15 Continental Flood...

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Unformatted text preview: Chapter 15: Continental Flood Basalts Large Igneous Provinces (LIPs) q Oceanic plateaus q Some rifts q Continental flood basalts (CFBs) Table 15-1. Major Flood Basalt Provinces Name CRB Keeweenawan Deccan Parana Karroo Volume (1.7x10 km ) (4x10 km ) (10 km ) 6 2 6 3 5 3 5 3 Age Miocene Precambrian Cret.-Eocene Locality NW US Superior area India Brazil (area > 10 km ) early Cret. (2x10 km ?) 6 3 early Jurassic S. Africa Figure 15-1. Columbia River Basalts at Hat Point, Snake River area. Cover of Geol. Soc. Amer Special Paper 239. Photo courtesy Steve Reidel. Large Igneous Provinces (LIPs) Figure 15.1. Map of the major large igneous provinces (LIPs) on Earth, including continental flood basalt provinces, volcanic passive margins, oceanic plateaus, aseismic submarine ridges, ocean basin flood basalts, and seamount groups. After Saunders et al. (1992) and Saunders (pers. comm.). Figure 15.3. Flood basalt provinces of Gondwanaland prior to break-up and separation. After Cox (1978) Nature, 274, 47-49. Mantle Plumes q 'snapshot' view of a theoretical situation involving several plumes in varying stages of development distributed around the globe is shown in the figure (right), kindly provided by Paul Tackley of UCLA. This may represent a typical pattern of plume distribution at any period of geological time. Tectonic Setting of CFBs q Continental hot spots 3 3 Columbia River Plateau Yellowstone Deccan Traps q Continental rifting 3 CAMP (Central Atlantic Magmatic Province) Columbia River Basalts Figure 15.5 Setting of the Columbia River Basalt Group in the Northwestern United States. Pink star is the location proposed by Camp and Ross (2004) of the 16.6 Ma outbreak of the plume and plume-related basaltic volcanism. Yellow star is the location of the deep plume conduit proposed by Jordan et al. (2004). Blue areas are Quaternary basalts and pink areas are rhyolite centers. Heavy dashed curves represent the progressive younging of rhyolitic centers (with ages in Ma). Those on the east represent the proposed Yellowstone hotspot track (heavy arrow). Those on the west are the opposing westward track leading to Newberry Volcano (N) with ages reported by Jordan et al. (2004). After Camp and Ross (2004). Figure 15.5 (contined). The cross-section is diagrammatic, generally across southern Oregon and Idaho (south of the main CRBG) and illustrates the westward deflection of the plume head by the deep keel of the North American craton to beneath the thinner accreted terranes and the migration of the hotspot tracks both east and west. After Jordan et al. (2004) AGU with permission. Columbia River Basalts Figure 15.7 Variation in wt.% of selected major element oxides vs. Mg# for units of the Columbia River Basalt Group. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from BVTP (Table, Hooper (1988a), Hooper and Hawkesworth (1993). CFB Chemistry q CFB's are mostly tholeiitic and similar to OIB Incompatible trace elements and isotopes are enriched, like OIB BUT show much more variability toward more enriched compositions. Distinctive enrichments of the most highly incompatible elements (K, Ba, Rb, Th, Pb, and LREE) over typical OIB. Noticeable depletions in HFSE (Nb, Ta) compared to OIB. Sr and Nd isotope ratios overlap with OIB, but extend to more enriched compositions (higher 87Sr/86Sr and lower Nd). q q q CFB Chemistry Columbia River Basalts q No depletion in HREE indicating relatively shallow melting depths (<70 km) q Strong enrichments in highly incompatible trace elements (REE, LILE) indicating enriched mantle source(s) Distinctive ernirchments in Ba and Pb and depletions in HFSE (Nb, Ta) compared to OIB. Columbia River Basalts q CFBs: 87Sr/86Sr = 0.7035 to 0.713 and +5 to -10, enriched mantle source(s) Nd = Columbia River Basalts Melting within a heterogeneous plume head (initial stages of the Yellowstone hot spot). The plume head contains recycled stringers of recycled oceanic crust that melts before the peridotite, yielding a silica-rich basaltic magma equivalent to the main Grande Ronde basalts and leaves a garnet-clinopyroxene residue. The large plume head stalls and spreads out at the base of the resistant lithosphere and the basaltic magma ponds (underplates) at the base of the crust, where it melts some crust to create rhyolite. Basalt escapes along a northward trending rift system to feed the CRBG. CFB Petrogenesis q Basically OIB petrogenesis + added components from continental lithospheric mantle and crust Figure 15-14. Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM is the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or enriched OIB source reservoir. Winter (20010 An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Flood Basalt Provinces of the last 250 Myrs Province Age (Myr)Volume (10E6 Paleolatitud Duration Km^3) e (degrees) (Myr) Columbia River 16 1 0.25 45 N ~ 1 (for 90%) Ethiopia 31 1 ~ 1.0 10 N ~1 North Atlantic 57 1 >1.0 65 N ~1 Deccan 66 1 >2.0 20 S ~1 Madagascar 88 1 ? 45 S ~ 6? Rajmahal 116 1 ? 50 S ~2 Serra Geral/ Etendeka 132 1 >1.0 40 S ~ 1 or ~ 5? Antarctica 176 1 >0.5 50-60 S ~ 1? Karoo 183 1 >2.0 45 S 0.5 - 1 Newark CAMP 201 1 >1.0? 30 N ~ 0.6 Siberian 249 1 >2.0 45 N? ~1 Figure 15.6 Dewey and Burke model for the evolution of a continental rift by the concatenation of a series of 3-rift triple junctions, each centered on a hotspot. Two arms of each hotspot link up to adjacent hotspots, although generally not perfectly. The third arm fails and becomes a rift valley (aulacogen). The hotspots need not be coeval and different segments can form sequentially. From Dewey and Burke (1974) ...
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