4/29/2023 0 Comments Jmicrovision 1.2.7 windows![]() The sole records an inverted and highly condensed P-T gradient and shows intense mylonitic fabrics ( Searle and Malpas, 1980, 1982 Ghent and Stout, 1981 Hacker and Mosenfelder, 1996 Gnos, 1998 Searle and Cox, 2002 Searle and Ali, 2009). The metamorphic sole to the ophiolite consists of a narrow thrust slice of hornblende + plagioclase ± garnet amphibolites with small enclaves of garnet + clinopyroxene granulite, epidote amphibolite, and a variety of greenschist facies metasedimentary rocks including piemontite (Mn epidote) quartzites, marbles, and rare metavolcanics. Late gabbros, trondhjemites, and tonalites have U-Pb zircon dates of 95.3–95.1 Ma ( Rioux et al., 2013), likely related to subduction below the ophiolite. ![]() 96.1–95.5 Ma, thus dating formation of the ophiolite crustal sequence ( Rioux et al., 2012a, 2013). New high-precision 206Pb/ 238U zircon dates suggest that the early magmatism associated with the lower Geotimes or V1 volcanism in the Oman part of the ophiolite occurred during the narrow time span of ca. The crustal section is underlain by an ∼12–15 km thickness of upper mantle harzburgites and dunites ( Fig. The ophiolite preserves a complete oceanic crustal sequence as much as 7–8 km thick, including layered peridotites and gabbros, isotropic gabbros, late crosscutting wehrlites, sheeted dikes, and pillow lavas with interleaved radiolarian cherts. 1 and 2) is thought to represent a slice of oceanic crust and upper mantle formed at a fast-spreading ridge ( Glennie et al., 1973 Lippard et al., 1986 Nicolas et al., 1989 Searle and Cox, 1999 Goodenough et al., 2010) during the Cenomanian ( Tilton et al., 1981 Warren et al., 2005 Rioux et al., 2012a, 2013). The Semail ophiolite of northern Oman and the United Arab Emirates (UAE) ( Figs. These rocks may be representative of deep-level duplexes imaged on recent seismic sections across the mountains of northern Oman–United Arab Emirates. A minimum of 130 km shortening is recorded by restoration of the major folds within the Bani Hamid thrust sheet, and more than 30 km offset has occurred along the west-directed breaching out-of-sequence Bani Hamid thrust. Peraluminous granitic dikes intruding the mantle sequence peridotites are as young as 91.4 Ma and likely reflect localized partial melting of crustal material during the late stage of the obduction process. The Bani Hamid granulites are younger zircon and titanite U-Pb dates span ca. The 206Pb/ 238U zircon dates from the metamorphic sole range from 95.7 to 94.5 Ma, and suggest that metamorphism and melting was either synchronous with or slightly postdated ridge magmatism. The 206Pb/ 238U zircon dates indicate that the gabbroic crust of the ophiolite formed by ridge magmatism from before 96.1 to 95.5 Ma. Metamorphic modeling using the program THERMOCALC in the system NCKFMASHTO (Na 2O-CaO-K 2O-FeO-MgO-Al 2O 3-SiO 2-H 2O-TiO 2-O) gives peak pressure-temperature conditions of 850 ± 60 ☌ and 6.3 ± 0.5 kbar, a pressure that is much lower than that of the metamorphic sole, suggesting a different origin. The Bani Hamid granulites represent metamorphosed cherts and calcareous turbidites probably derived from the distal Haybi Complex and Oman Exotic limestones, which have an alkali basaltic substrate. The Bani Hamid thrust slice includes two-pyroxene quartzites (± hornblende, cordierite, sapphirine), diopside + andradite garnet + wollastonite + scapolite marbles and calc-silicates and amphibolites (hornblende + plagioclase ± clinopyroxene ± biotite) with localized partial melting, intruded by hornblende pegmatites. In the mountains of northern Oman, the 1.2-km-thick Bani Hamid thrust sheet is composed of intensely folded granulite and amphibolite facies rocks within mantle sequence peridotites, exhumed by late-stage out-of-sequence thrusting along the Bani Hamid thrust. Peak metamorphic conditions of 770–900 ☌ and 11–15 kbar indicate metamorphism at depths far greater than can be accounted for by the preserved thickness of the ophiolite (∼15 km). The normal metamorphic sole of the Semail ophiolite comprises a highly condensed sequence of hornblende + plagioclase ± garnet amphibolites with small enclaves of garnet + clinopyroxene granulites immediately beneath the mantle sequence peridotites, tectonically underlain by a series of epidote amphibolite and greenschist facies lithologies in a highly deformed ductile shear zone. ![]() ![]() The Cretaceous Semail ophiolite (northern Oman and the United Arab Emirates) includes an intact thrust slice of Tethyan oceanic crust and upper mantle formed above a northeast-dipping subduction zone that was the site of initiation of obduction. ![]()
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