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Geochemistry and protoliths of the Hatta metamorphic sole marbles, northern Oman Mountains, United Arab Emirates.
Abstract and Figures
Banded marbles constitute a distinct unit of the Hatta metamorphic sole in the Northern Oman Mountains. The pure and impure marbles were studied petrographic ally and geochemically to determine their geochemical characterstics and decipher the metamorphic history and variations in their protoliths
mineralogy. The marbles are made up mainly of calcite beside pelitic and feldspathic phases. Authigenic
albite, with the peculiar "Roe Tourne twinning", is recorded in the marble samples. Crystal and twin
morphologies of the calcite define a spectrnm that corresponds to increasing temperature from thin twins
at very low temperatures to straight thick twins and finally to recrystallization at grain boundaries at high
temperatures. All the components necessary for albite authigenesis during metamorphism are supplied by the host rocks. Microstylolites with filling materials are recorded in the marbles; the latter are the product
of metamorphic reactions between the detritus and the host rock component (calcite).
The metamorphic rocks in the study area show polyphase deformation, mylonitic fabrics, foliation,
stretching lineation and folding. The sub-ophiolite metamorphic sole and the upper sedimentary melange
(Hawasina Complex) comprise a distinct thrnst slice in the Hatta area. The absence or rarity exposures of
amphibolites in the Hatta zone may be attributed to the higher level of detachment of the basement blocks
during the initial separation from the oceanic crust.
Trace elements and REE contents are lower in the pure marbles as compared to in the impure
(siliceous) marbles. This is attributed mostly to the concentration of REE in the clay fractions and the
authigenic Fe-Mn coats of sediment grains rather than concentration in heavy minerals such as zircon.
The marbles show a relatively flat REE normalized pattern with no to fair negative Ce anomaly and
positive Eu anomaly. The presence of a positive Eu anomaly is consistent with the authigenic plagioclase
enrichment. The (La/Yb), values of the studied marbles are similar to the average values recorded for the
Arabian Sea limestone and the Indian ocean carbonates. Evidently, the banded marbles are
metamorphosed "Oman Exotic" limestone which crop out within imbricated thrnst sheets of the Hatta
metamorphic sole beneath the Semail ophiolite .
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The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet+diopside amphibolites, enstatite+cordierite+sillimanite+spinel±sapphirine quartzites, alkaline mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain garnet+diopside+scapolite+plagioclase±wollastonite. P860°C and 10.5±1.1 kbar to 14.7±2.8 kbar. Garnet+clinopyroxene+hornblende+plagioclase amphibolites from the metamorphic sole record peak PT mode) and 84011.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 5793 Ma, synchronous with formation of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic sequence and deposition of CenomanianJurassic and early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites which were at or around 1100–1500°C. All data from the sub-ophiolite metamorphic sole in Oman and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting and that obduction occurred along a NE-dipping high-temperature subduction zone during Late Cretaceous times.
Geochemical and isotopic approaches to constraining provenance of sedimentary rocks complement the information inferred from petrography. Geochemical approaches have several advantages, including applicability to both matrix-rich sandstones and shales and ability to constrain provenance age and geochemical history. Five provenance components, or terrane types, have been defined on the basis of whole-rock chemical and Nd-isotopic composition, including Old Upper Continental Crust, Recycled Sedimentary Rocks, Young Undifferentiated Arc, Young Differentiated Arc, and various Exotic Components, such as ophiolites. Among the most important geochemical characteristics that define these provenance types are Nd isotopic composition (reflecting average provenance age), europium anomalies (reflecting intracrustal igneous differentiation processes), large-ion lithophile element enrichments (provenance composition), alkali and alkaline earth depletions (weathering and alteration), Zr and Hf enrichments (heavy mineral enrichments), and high Cr abundances (ultramafic sources). Pb isotopic compositions of whole rocks and framework (quartz, feldspar) and accessory (e.g., zircon, monazite) grains constrain the age and crustal history of sediment sources. The U-Pb system may be used to date the time of crystallization of small populations and, for favorable circumstances, individual sand-sized grains of detrital quartz. The Early Proterozoic Pokegama Quartzite contains detrital quartz populations that give a Pb-Pb age of 2647 ± 1 6 Ma, consistent with detrital zircon ages and with the age of the Archean Superior Province, from which this formation is mainly derived. The initial Pb isotopic composition may be approximated by the Pb isotopic composition of leached feldspars, due to their low U/Pb and Th/Pb ratios. Pb isotopic compositions of detrital feldspars may also provide information about sedimentary provenance. Itfeldspars from the Pokegama Quartzite and Early Proterozoic Chelmsford Formation fall within the field of Archean Superior Province igneous K-feldspars. They are distinct from K-feldspars found in other potential provenance terranes, including the Penokean Orogen or, in the case of the Pokegama, the Minnesota River Valley gneisses.
The metamorphic rocks associated with the Semail ophiolite in the Oman Mountains represent two different facies series. Amphibolites and greenshists at the base of the Semail ophiolite constitute a metamorphic sole formed in a high-temperature, lowpressure environment. Eclogites, blueschists, lawsonite schists and other metasediments occurring at different structural levels in the basement and shelf deposits of the continental margin formed at high pressures and low temperatures. The amphibolites of the metamorphic sole formed as a result of heat transfer from the hot overthrust ophiolitic slab to the underlying oceanic sediments and volcanics shortly following intraoceanic detachment at a collapsed spreading centre about 100-90 Ma. The underlying greenschists represent sediments and volcanics that were metamorphosed farther away from the spreading centre as the ophiolitic slab became cooler and moved closer to the continental margin. Eclogites, blueschists and crossite epidote schists formed in the Saih Hatat window as a result of A-type subduction (crustal thickening) of the basement and shelf units of the Oman continental margin in response to a change in plate motion between Africa and Eurasia and transpression or collision with a fragment of Gondwanaland about 131 Ma. A-type subduction along an east-dipping zone resulted in the burial of the eastern edge of the continental margin to a minimum depth of 30 km. High P/T metamorphic rocks formed by this process followed 'clockwise' P-T paths. The Late Cretaceous emplacement of the ophiolite onto the continental margin of Oman resulted in the imbrication of the continental shelf deposits, the development of low-grade, high-P/T metamorphic rocks in the 'internal' thrust piles due to tectonic overloading and the juxtaposition of the metamorphic rocks representing different facies series. Extensional deformation associated with culmination collapse in the Late Cretaceous and Tertiary led to the exhumation of the various high P/T metamorphic rocks and the distortion of the metamorphic zones.
Differences in rare earth element (REE) relative fractionations and total abundances in marine cherts and shales reflect the depositional location of the sediments. Based on cerium anomaly (Ce/Ce*) and total REE abundance (SigmaREE) variations preserved in interbedded chert and shale sequences of the Franciscan assemblage, the Claremont Formation, and the Monterey Formation of coastal California, we can resolve three depositional regimes: spreading ridge proximal, ocean-basin floor, and continental margin. This resolution of depositional environment, previously unattainable in otherwise physically indistinct marine sedimentary rocks, provides a potentially powerful new tool for tectonic and stratigraphic reconstructions. Chert and shale deposited near the spreading ridge (within ˜400 km) under the influence of metalliferous activity are characterized by extremely low Ce anomalies (Ce/Ce* ˜0.29), those from an ocean-basin floor setting by well-developed yet less extreme Ce anomalies (Ce/Ce* ˜0.55), and those from continental margin regimes by no or slight anomalies (Ce/Ce* ˜0.90 to 1.30). SigmaREE in chert and shale are extremely low in regimes greatly influenced by continental input, owing to the high sedimentation rate, which minimizes exposure time to seawater and, thus, the adsorption of REE. Preliminary results indicate that the effect of diagenesis does not mask the primary REE depositional signature.