Semail Ophiolite Research Articles

Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U–Pb zircon geochronology

The Semail (Oman–United Arab Emirates) and other Tethyan-type ophiolites are underlain by a sole consisting of greenschist- to granulite-facies metamorphic rocks. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Early models envisioned that the metamorphic sole of the Semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement; however, calculated pressures from granulite-facies mineral assemblages in the sole suggest the metamorphic rocks formed at >35 km depth, and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle (up to 15–20 km). We have used high-precision U–Pb zircon dating to study the formation and evolution of the metamorphic sole at two well-studied localities. Our previous research and new results show that the ophiolite crust formed from 96.12–95.50 Ma. Our new dates from the Sumeini and Wadi Tayin sole localities indicate peak metamorphism at 96.16 and 94.82 Ma (±0.022 to 0.035 Ma), respectively. The dates from the Sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust, and were later juxtaposed with the base of the ophiolite. These data, combined with existing geochemical constraints, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. The 1.3 Ma difference between the Wadi Tayin and Sumeini dates indicates that, in contrast to current models, the highest-grade rocks at different sole localities underwent metamorphism, and may have returned up the subduction channel, at different times.

CRETACEOUS – NEOGENE STRUCTURAL EVOLUTION OF SE ABU DHABI, UNITED ARAB EMIRATES

Regional 2D seismic profiles and 2D cross‐sections extracted from high‐resolution 3D seismic data were interpreted together with well data to reconstruct the sequence stratigraphy of the Late Cretaceous – Miocene sedimentary succession in SE Abu Dhabi. This region is a major hydrocarbon province with numerous giant oilfields producing from large‐scale anticlines. The paper focusses on the structural development of the Sahil, Asab and Shah fields.Six sequence boundaries within the Aptian to Miocene succession were identified on the basis of erosional truncations and onlap patterns. The deepest‐lying erosional surface is the top‐Albian sequence boundary. This was identified by angular truncation of the Nahr Umr Formation in the SE of the study area near an uplifted domal structure which hosts the Mender and nearby Lekhwair fields. The unconformity is interpreted to result from reactivation of the basement together with a eustatic fall in sea‐level which resulted in regional uplift and erosion. A second phase of uplift occurred during the mid‐Turonian and resulted in significant erosion of the Shilaif Formation along structural highs. The top‐ Shilaif Formation unconformity forms a sequence boundary and is observed at the Shah, Zararrah and Sahil anticlines. The unconformity is coeval with the onset of obduction of the Semail Ophiolite onto the Arabian margin, and the associated flexural bulge probably resulted in structural uplift and reactivation of basement structures. A global sea‐level fall is also reported during middle Turonian time. The third sequence boundary occurs in the lower Campanian as an angular unconformity surface between the Halul and Fiqa Formations. This event is most obvious along the Shah and Zararrah anticlines and is interpreted to have occurred as a result of flexural uplift related to a further episode of obduction of the Semail Ophiolite. The fourth sequence boundary occurs in the Campanian – Maastrichtian and was identified by angular truncations of the Fiqa Formation against the Simsima Formation, an event which caused widespread erosion of a significant portion of the Fiqa Formation. This tectonic activity was most likely controlled by the final emplacement of the Semail Ophiolite, and probable reactivation of basement faults. The Simsima Formation is bounded above by the fifth sequence boundary, which coincides with the Cretaceous/Tertiary boundary and which is associated with the final stages of Late Cretaceous tectonic activity. The final sequence boundary is the supra‐Dammam unconformity, which resulted in a peneplain surface associated with widespread erosion and an extended hiatus.Major trap formation in the study area occurred from the Cenomanian to the Maastrichtian, and coincided with obduction‐related emplacement of allochthonous thrust sheets onto the plate margin in Oman. However, the structural growth history of the Shah, Asab and Sahil anticlines suggest multiple episodes of uplift. Growth of the Shah anticline was initiated during deposition of the Shilaif Formation in Cenomanian – early Turonian times, but neither the Asab nor Sahil anticlines show significant growth or faulting older than Campanian. In all the anticlines, the most pronounced tectonic activity observed was in the Campanian during deposition of the Fiqa Formation. Significant growth of structures was completed by Paleocene time, suggesting that the Zagros orogeny had little structural influence in SE Abu Dhabi.

Sub-seafloor epidosite alteration: Timing, depth and stratigraphic distribution in the Semail ophiolite, Oman

Pervasive epidotization of igneous rocks is a common feature in the ophiolite record of hydrothermally altered oceanic crust. Current genetic models view epidosites as markers of focussed upflow of hydrothermal fluid beneath oceanic spreading ridges. The epidosites are envisaged to form at the base of the sheeted dike complex (SDC) during active plate spreading. Our mapping of the Semail ophiolite in Oman has revealed abundant epidosites in the volcanic sequence, some exceeding 1km2 in extent. They are more frequent and far larger than the mineralogically identical epidosites in the SDC. We have also found epidosites that traverse the entire SDC from bottom to top. Thus, rather than being restricted to the base of the SDC, as implied by current models, epidosites in fact occur throughout the SDC and dominantly within the overlying volcanic pile. We report the occurrence of 19 epidosite bodies and their crosscutting relations with respect to host lava units, dikes, intrusive stocks and also seafloor umbers. The volcanostratigraphic affiliation of the dikes is identified by their whole-rock and clinopyroxene compositions. The relations set constraints on the timing of epidotization with respect to igneous activity in the ophiolite. At least one of the epidosites in the SDC formed during Lasail off-axis volcanism. Another epidosite in the SDC and many in the volcanic units formed later during post-spreading, Alley and Boninitic Alley supra-subduction zone volcanism. Only permissive, not compelling, evidence allows just two of the epidosites to have formed within the main-stage SDC during or shortly after its emplacement. We conclude that epidotization of the oceanic crust is not necessarily coupled to spreading ridges and that it can occur during fore-arc volcanism. This finding is consistent with evidence from the modern seafloor and it requires a different hydrothermal environment to that traditionally associated with alteration beneath spreading axes. The timing and stratigraphic constraints show that some of the epidosites formed within the top 1.4km of the crust, others between 1.2 and 1.7km depth, whereas the deepest formed between 2.6 and 3.8km below the paleo-seafloor. Finally, the timing constraints do not prescribe a strict temporal relationship between epidotization and the formation of the numerous seafloor massive-sulphide (VMS) deposits known in the ophiolite. However, a role for epidosites as source rocks for the metals in the deposits remains permissible.

Mineralogy, geochemistry and geotectonic of plagiogranites from Shahre-Babak ophiolite, Zagros zone, Iran

Shahre-Babak ophiolite is a part of the inner Zagros ophiolite belt in Iran. Major parts of intrusive masses of Share-Babak ophiolite are gabbro and plagiogranite. The SiO2 versus Na2O+K2O diagram shows that the palgiogranites are related to calk-alkaline series. Rare earth elements exhibit relatively similar pattern that indicates these rocks are syngenetic. Also, REE patterns display an enrichment of LREE compared to HREE, and are characterized by flat to slightly concaveup patterns from Gd to Yb. Such patterns contrast sharply with those of plagiogranites in more complete ophiolite sequences, such as the Semail ophiolite, Oman, or the Troodos ophiolite, Cyprus, and Neyriz, where patterns are much flatter and slightly LREE-depleted. The slightly LREE-enriched patterns of the Shahre-Babak plagiogranites support a partial melting origin for them. The low TiO2, Nb, Ta content and high LREE concentrations of the Shahre-Babak plagiogranites indicate that the rocks were likely derived from the anatexis of amphibolites, which were related to hydrothermal alteration of gabbros in intra-oceanic back-arc basin.

Mechanical anisotropy control on strain localization in upper mantle shear zones

AbstractMantle rocks at oceanic spreading centers reveal dramatic rheological changes from partially molten to solid‐state ductile to brittle deformation with progressive cooling. Using the crustal‐scale Wadi al Wasit mantle shear zone (SZ, Semail ophiolite, Oman), we monitor such changes based on quantitative field and microstructural investigations combined with petrological and geochemical analyses. The spatial distribution of magmatic dikes and high strain zones gives important information on the location of magmatic and tectonic activity. In the SZ, dikes derived from primitive melts (websterites) are distributed over the entire SZ but are more abundant in the center; dikes from more evolved, plagioclase saturated melts (gabbronorites) are restricted to the SZ center. Accordingly, harzburgite deformation fabrics show a transition from protomylonite (1100°C), mylonite (900–800°C) to ultramylonite (<700°C) and a serpentine foliation (<500°C) from the SZ rim to the center. The spatial correlation between solid‐state deformation fabrics and magmatic features indicates progressive strain localization in the SZ on the cooling path. Three stages can be discriminated: (i) Cycles of melt injection (dunite channels and websterite dikes) and solid‐state deformation (protomylonites‐mylonites; 1100–900°C), (ii) dominant solid‐state deformation in harzburgite mylonites (900–800°C) with some last melt injections (gabbronorites) and ultramylonites (<700°C), and (iii) infiltration of seawater inducing a serpentine foliation (<500°C) followed by cataclasis during obduction. The change of these processes in space and time indicates that early dike‐related ridge‐parallel deformation controls the onset of the entire strain localization history promoting nucleation sites for different strain weakening processes as a consequence of changing physicochemical conditions.

Hydrocarbon generation and migration under a large overthrust: The carbonate platform under the Semail Ophiolite, Jebel Akhdar, Oman

The Natih B source rock of the Jebel Akhdar dome in the Oman Mountains hosts at least three different generations of solid bitumen. This study presents detailed analyses of thermal maturity based on solid bitumen reflectance, Rock-Eval pyrolysis, Fourier transform infrared spectroscopy and Raman spectroscopy of matrix and solid bitumen. The Natih B source rock was sampled in different valleys (Wadis) around the Jebel Akhdar dome. Detailed petrography of organic material was conducted by optical and scanning electron microscopy (SEM). The petrographic data was used to calibrate an integrated thermal basin model of the Wadi Nakhr area. Due to ophiolite obduction the Natih B source rock was buried to 7.4 to 8km and a maximum temperature of c. 240°C. Hydrocarbon generation and associated primary migration was burial induced and took place from 78 to 66Ma with solid bitumen precipitation in the source rock. A resulting high-reflective solid bitumen generation reflects temperatures of deepest burial in the order of 225°C to 240°C. Based on an elaborated paragenetic sequence two other solid bitumen generations were linked to two oil migration events taking place during uplift (55–50Ma and 48–45Ma), after deepest burial (around 65Ma).

Geochronological constraints on the metamorphic sole of the Semail ophiolite in the United Arab Emirates

The Semail ophiolite of Oman and the United Arab Emirates (UAE) provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and rutile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and rutile provide dominant single age populations that are 95–93 Ma, partially overlapping with the known age of oceanic crust formation (96.5–94.5 Ma), and partially overlapping with cooling ages of the metamorphic rocks (95–90 Ma). The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.

Structure and metamorphism beneath the obducting Oman ophiolite: Evidence from the Bani Hamid granulites, northern Oman mountains

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. 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. Peak metamorphic conditions of 770–900 °C and 11–15 kbar indicate metamorphism at depths far greater than can be accounted for by the preserved thickness of the ophiolite (∼15 km). 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. 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. 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. Metamorphic modeling using the program THERMOCALC in the system NCKFMASHTO (Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O) gives peak pressure-temperature conditions of 850 ± 60 °C and 6.3 ± 0.5 kbar, a pressure that is much lower than that of the metamorphic sole, suggesting a different origin. The 206 Pb/ 238 U zircon dates indicate that the gabbroic crust of the ophiolite formed by ridge magmatism from before 96.1 to 95.5 Ma. The 206 Pb/ 238 U 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 Bani Hamid granulites are younger; zircon and titanite U-Pb dates span ca. 94.5–89.8 Ma. 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. 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. These rocks may be representative of deep-level duplexes imaged on recent seismic sections across the mountains of northern Oman–United Arab Emirates.

Solid bitumen in calcite veins from the Natih Formation in the Oman Mountains: Multiple phases of petroleum migration in a changing stress field

Solid bitumen in calcite veins in Natih limestone on the southern flank of the Jebel Akhdar Anticline provide evidence for at least two previously unrecognized petroleum migration events in the Oman Mountains. We present field observation of bituminous calcite veins combined with microscopy using highly polished thin sections. This allows imaging of solid bitumen in reflected light and study of its microstructural context in transmitted light.Straight and en échelon, black impregnated, bedding-normal Natih A calcite veins strike 110°. They formed as one of the earliest structures in an extensional stress regime at the end of Cretaceous (Turonian–Santonian) either by flexure of the Arabian plate (Wasia–Aruma break) or by subsidence during subduction. The veins always contain planar arrays of small (<10μm) intracrystal solid bitumen inclusions and may contain angular mosaic type solid bitumen associated with white, solid bitumen-free calcite and/or round, homogeneous type solid bitumen. The solid bitumen formed in multiple events and were most likely derived from the Natih B source rocks.During the first migration event, microfracturing during obduction of the Semail ophiolite and emplacement of the Hawasina thrust sheets in Santonian–Campanian times formed pathways for oil into the veins. Mosaic type solid bitumen formed during further vein growth. It has a higher solid bitumen reflectance (BRr=3.40–3.76%) combined with a high optical anisotropy compared to the underlaying thermally overmature Natih B source rock (BRr=3.10–3.14%). The higher reflectance of mosaic type solid bitumen is interpreted to result from deformation and does not reflect maximum burial temperatures which are inferred to have been about 225°C.The second migration event is indicated by low-reflective homogeneous type solid bitumen with palaeotemperatures (BRr=0.86–0.92%; about 145°C) lower than maximum burial. It occurs along pressure solution seams which cross-cut the veins showing that this is the latest event in the paragenetic sequence, interpreted to have formed after lateral intraformational oil migration from the active Natih oil kitchen located approximately 40km SW of the study area during doming and uplift of the Oman Mountains in Oligocene to Pliocene times.

Occurrence of abiotic methane in the eastern United Arab Emirates ophiolite aquifer

Methane with carbon and hydrogen isotope composition diagnostic of abiotic gas related to serpentinization of peridotites has been detected for the first time in the ophiolitic aquifer in the United Arab Emirates (UAE), along the Wadi Ham fault. This methane is isotopically similar to that previously reported in serpentinization-related springs in Oman, in the same Semail ophiolite nappe. Abiotic gas may be widespread in these ophiolitic rocks. Conventional thermogenic gas fields in the sedimentary basin overthrusted by the ophiolite could be “contaminated” to some extent by this abiotic gas. This may confound the accurate characterization of these thermogenic gases (origin, type of source rock type and maturity) and thus the associated petroleum system model.

Mobilization and Redistribution of Elements in Laterites of Semail Ophiolite, Oman: A Mass Balance Study

Several samples of laterites were collected from four paleosol profiles, Ibra, East Ibra, Al-Russayl, and Tiwi representing the vertical lithological variation within each profile. The mineralogical and geochemical composition of laterites in every section revealed differences in thickness and redistribution of elements reflecting different conditions of weathering processes. Elemental mass balance was calculated for every profile relative to the parent rock. The results indicated redistribution of elements from the surface to deeper zones with an enrichment of elements in the saprolite and oxide zones. Among the different sections, the profile of East Ibra composite 1 and 2 is characterized by high concentration of all elements compared to the other profiles. Sc/Fe ratio in different zones indicates low values for the profile of Tiwi profile 1, Ibra profile and Al-Russayl composite 2 and 3 profile due to the significant enrichment of Fe in these zones independently of redox conditions. Large fluctuations characterize Th/U ratios and reflect redox condition more reduced in Tiwi area than in East Ibra and Al-Russayl areas.

Element scavenging by recently formed travertine deposits in the alkaline springs from the Oman Semail Ophiolite

Ultramafic rocks, such as the Semail Ophiolite in the Sultanate of Oman, are considered to be a potential storage site for CO2. This type of rock is rich in divalent cations that can react with dissolved CO2 and form carbonate minerals, which remain stable over geological periods of time. Dissolution of the ophiolite mobilizes heavy metals, which can threaten the safety of surface and groundwater supplies but secondary phases, such as iron oxides, clays and carbonate minerals, can take up significant quantities of trace elements both in their structure and adsorbed on their surfaces.Hyperalkaline spring waters issuing from the Semail Ophiolites can have pH as high as 12. This water absorbs CO2 from air, forming carbonate mineral precipitates either as thin crusts on the surface of placid water pools or bottom precipitates in turbulent waters. We investigated the composition of the spring water and the precipitates to determine the extent of trace element uptake. We collected water and travertine samples from two alkaline springs of the Semail Ophiolite. Twenty seven elements were detected in the spring waters. The bulk of the precipitate was CaCO3 in aragonite, as needles, and rhombohedral calcite crystals. Traces of dypingite (Mg5(CO3)4(OH)2·5H2O) and antigorite ((Mg,Fe)3Si2O5(OH)4) were also detected. The bulk precipitate contained rare earth elements and toxic metals, such as As, Ba, Cd, Sr and Pb, which indicated scavenging by the carbonate minerals. Boron and mercury were detected in the spring water but not in the carbonate phases. The results provide confidence that many of the toxic metals released by ophiolite dissolution in an engineered CO2 injection project would be taken up by secondary phases, minimizing risk to water quality.

Genesis and mineralogical classification of Ni-laterites, Oman Mountains

The laterite profiles investigated in the present study developed after the emplacement of a slab of oceanic crust and upper mantle sequence (the Semail ophiolite) onto the East Arabian margin during the Late Cretaceous. The laterites formed as a result of prolonged weathering of the ophiolitic assemblage under tropical to subtropical conditions.Nine laterite profiles have been examined for their Ni potential along a NW–SE segment of the Oman Mountains. The profiles show a vertical succession from bedrock protolith through saprolite, oxide laterite, to clay laterite.A significant enrichment in Ni and Co occurred as a result of the lateritization process. Ni concentrations average 0.63% (Ibra), 0.72% (3 profiles, East Ibra), 0.67% (3 profiles, Al-Russayl), and 0.33% (2 profiles, Tiwi). Electron microprobe analyses showed that goethite, chlorite, kaolinite, serpentine and talc are the main Ni-bearing minerals. This study showed that the Ibra, East Ibra and Tiwi laterites are similar to the other hydrous Mg-silicates (type A) Ni deposits, where nickel-bearing talc (willemseite) or chlorite, and serpentine (nepouite, chrysotile, and antigorite) are the main constituents. Al-Russayl laterite is however, similar to the oxide type (Type C), where most likely the nickel is included in the oxyhydroxides (mainly goethite) minerals.

Volcanostratigraphic Controls on the Occurrence of Massive Sulfide Deposits in the Semail Ophiolite, Oman

Research Article| September 01, 2014 Volcanostratigraphic Controls on the Occurrence of Massive Sulfide Deposits in the Semail Ophiolite, Oman Samuel A. Gilgen; Samuel A. Gilgen † 1Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland †Corresponding author: e-mail, samuel.gilgen@geo.unibe.ch Search for other works by this author on: GSW Google Scholar Larryn W. Diamond; Larryn W. Diamond 1Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Ivan Mercolli; Ivan Mercolli 1Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Khalid Al-Tobi; Khalid Al-Tobi 2Earth Secrets, P.O. Box 1242, PC 130 Athaibah, Muscat, Sultanate of Oman Search for other works by this author on: GSW Google Scholar David W. Maidment; David W. Maidment 3Mawarid Mining, P.O. Box 476, PC 321 Sohar, Sultanate of Oman Search for other works by this author on: GSW Google Scholar Robert Close; Robert Close 4Gentor Resources, P.O. Box 1053, PC 130 Athaibah, Sultanate of Oman Search for other works by this author on: GSW Google Scholar Ahmed Al-Towaya Ahmed Al-Towaya 5Directorate General of Minerals, Muscat, Sultanate of Oman Search for other works by this author on: GSW Google Scholar Economic Geology (2014) 109 (6): 1585–1610. https://doi.org/10.2113/econgeo.109.6.1585 Article history received: 23 Apr 2013 accepted: 11 Nov 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Samuel A. Gilgen, Larryn W. Diamond, Ivan Mercolli, Khalid Al-Tobi, David W. Maidment, Robert Close, Ahmed Al-Towaya; Volcanostratigraphic Controls on the Occurrence of Massive Sulfide Deposits in the Semail Ophiolite, Oman. Economic Geology 2014;; 109 (6): 1585–1610. doi: https://doi.org/10.2113/econgeo.109.6.1585 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Semail ophiolite in Oman is capped by up to 2 km of basaltic-andesitic lavas that host copper-dominant, Cyprus-type, volcanogenic massive sulfide (VMS) deposits. This study identifies multiple volcanostratigraphic horizons on which the deposits are situated, based on characterization of footwall and hanging-wall lavas from 16 deposits or deposit clusters. Comparison of field and petrographic features, compositions of igneous clinopyroxenes, and whole-rock geochemical signatures permits classification of the lavas within a modified version of the established regional volcanostratigraphy. Four extrusive units host deposits: Geotimes (earliest), Lasail, Alley, and Boninitic Alley (latest). The latter was previously known only at few localities, but this study reveals its regional extent and significance as a host for VMS deposits. The Geotimes and Lasail units represent Late Cretaceous, ocean spreading ridge and related off-axis volcanic environments, respectively. The Alley and Boninitic Alley units represent younger, subduction-related volcanism prior to Coniacian-Santonian obduction of the ophiolite. Our results show that VMS deposits occur on or near the Geotimes/Lasail and Geotimes/Alley contacts as well as entirely within the Geotimes, Lasail, Alley, and Boninitic Alley units. Highest Cu grades tend to occur in deposits lying on or within the Geotimes, whereas highest Au grades occur in deposits within the Boninitic Alley. In contrast to earlier studies, we conclude that essentially every horizon marking a hiatus in lava deposition in the Semail ophiolite, i.e., contacts between the four major eruptive units, and umbers and sedimentary chert layers within the units, has exploration potential for Cu-Au VMS deposits. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Hydrothermal altered serpentinized zone and a study of Ni-magnesioferrite–magnetite–awaruite occurrences in Wadi Hibi, Northern Oman Mountain: Discrimination through ASTER mapping

Economic important minerals and ore deposits are common in hydrothermal altered serpentinized zone. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite sensor is capable of discrimination of such hydrothermal mineralized zone and detection of hydrothermal altered minerals. In the present study, the hydrothermal altered serpentinized harzburgites of Wadi Hibi area of Northern Oman Mountains have been discriminated by using ASTER VNIR–SWIR spectral bands by image processing methods and the occurrences of Ni-magnesioferrite–magnetite–awaruite in the rocks are studied. The color composite RGB image developed using ASTER spectral bands 8, 4 and 1, mapped well the occurrence of weathered peridotites by pale green to dark blue in colors and discriminated the hydrothermally altered serpentinized rocks by pale brown to dark blue colors due to the strong absorption of OH and Mg–OH molecules that occurred in the serpentine minerals of the rocks in the study area. The ASTER band ratios 4/7, 4/1, and 2/3×4/3 RGB images studied are capable of discrimination of hydrothermal mineralized areas more clear by pale blue to purple colors due to the strong absorption of such hydroxyl bearing serpentine minerals. The studied image processing methods are evaluated by applying to the region of Wadi Sarami situated in the Semail ophiolite (Oman). In addition to that, the occurrence of serpentine minerals namely, lizardite and antigorite in the hydrothermally altered serpentinized region are detected qualitatively and quantitatively using Spectral Angle Mapper (SAM) supervised classification image processing method and studied.The interpreted images are verified in the field and checked for the occurrences of minerals including Ni-magnesioferrite, magnetite, pentlandite and awaruite and are confirmed through laboratory studies. Petrographic study of serpentinized harzburgites shows that the rocks consist predominantly of antigorite and lizardite serpentines, olivine and have the opaque minerals assemblage of Ni-magnesioferrite+magnetite+awaruite+pentlandite developed during serpentinization of the rock. The occurrences of such minerals are confirmed by XRD, electron microprobe analyses and spectral measurements in the laboratory.ASTER sensor proved its capability in discriminating the hydrothermal altered serpentinized zone and detecting the mineral occurrences and thus the study recommends the technique to the exploration geologists, scientists and mining geologists for mapping of such rocks and minerals in the similar arid region.

Tectonics of the Musandam Peninsula and northern Oman Mountains: From ophiolite obduction to continental collision

ABSTRACT The tectonics of the Musandam Peninsula in northern Oman shows a transition between the Late Cretaceous ophiolite emplacement related tectonics recorded along the Oman Mountains and Dibba Zone to the SE and the Late Cenozoic continent-continent collision tectonics along the Zagros Mountains in Iran to the northwest. Three stages in the continental collision process have been recognized. Stage one involves the emplacement of the Semail Ophiolite from NE to SW onto the Mid-Permian–Mesozoic passive continental margin of Arabia. The Semail Ophiolite shows a lower ocean ridge axis suite of gabbros, tonalites, trondhjemites and lavas (Geotimes V1 unit) dated by U-Pb zircon between 96.4–95.4 Ma overlain by a post-ridge suite including island-arc related volcanics including boninites formed between 95.4–94.7 Ma (Lasail, V2 unit). The ophiolite obduction process began at 96 Ma with subduction of Triassic–Jurassic oceanic crust to depths of &amp;gt; 40 km to form the amphibolite/granulite facies metamorphic sole along an ENE-dipping subduction zone. U-Pb ages of partial melts in the sole amphibolites (95.6– 94.5 Ma) overlap precisely in age with the ophiolite crustal sequence, implying that subduction was occurring at the same time as the ophiolite was forming. The ophiolite, together with the underlying Haybi and Hawasina thrust sheets, were thrust southwest on top of the Permian–Mesozoic shelf carbonate sequence during the Late Cenomanian–Campanian. Subduction ended as unsubductable cherts and limestones (Oman Exotics) jammed at depths of 25–30 km. The Bani Hamid quartzites and calc-silicates associated with amphibolites derived from alkali basalt show high-temperature granulite facies mineral assemblages and represent lower crust material exhumed by late-stage out-of-sequence thrusting. Ophiolite obduction ended at ca. 70 Ma (Maastrichtian) with deposition of shallow-marine limestones transgressing all underlying thrust sheets. Stable shallow-marine conditions followed for at least 30 million years (from 65–35 Ma) along the WSW and ENE flanks of the mountain belt. Stage two occurred during the Late Oligocene–Early Miocene when a second phase of compression occurred in Musandam as the Arabian Plate began to collide with the Iran-western Makran continental margin. The Middle Permian to Cenomanian shelf carbonates, up to 4 km thick, together with pre-Permian basement rocks were thrust westwards along the Hagab Thrust for a minimum of 15 km. Early Miocene out-of-sequence thrusts cut through the shelf carbonates and overlying Pabdeh foreland basin in the subsurface offshore Ras al Khaimah and Musandam. This phase of crustal compression followed deposition of the Eocene Dammam and Oligocene Asmari formations in the United Arab Emirates (UAE), but ended by the mid-Miocene as thrust tip lines are all truncated along a regional unconformity at the base of the Upper Miocene Mishan Formation. The Oligocene–Early Miocene culmination of Musandam and late Cenozoic folding along the UAE foreland marks the initiation of the collision of Arabia with Central Iran in the Strait of Hormuz region. Stage three involved collision of Arabia and the Central Iran Plate during the Pliocene, with ca. 50 km of NE-SW shortening across the Zagros Fold Belt. Related deformation in the Musandam Peninsula is largely limited to north and eastward tilting of the peninsula to create a deeply indented coastline of drowned valleys (rias).

Origin of gypsiferous intrusions in the Hawasina Window, Oman Mountains: Implications from structural and gravity investigations

ABSTRACT Gypsiferous intrusions are exposed in road-cuts in the south-central Hawasina Window in the central Oman Mountains. They are located at lower structural levels in the allochthonous Hawasina Complex and lie along faults that cut Upper Cretaceous structures related to the obduction of the Semail Ophiolite and Hawasina Complex deep-water sediments onto the Arabian Plate. The intrusions form gypsiferous pods that are up to 200 m long, in which the gypsum occurs as a dark, fine-grained matrix that contains a pervasive network of anastomosing veins of gypsum and anhydrite. The intrusions contain abundant sub-angular to sub-rounded litharenites, and less common fragments of chert and fine-grained limestone. Although these clast types are undated, their petrographic characteristics suggest they originate from the local Hawasina (Hamrat Duru Group) country rock. Very well-rounded pebbles and cobbles of feldspathic litharenites, some of which show a well-developed cleavage, and rarer cobbles of well-rounded vein quartz appear to have come from the basement. Gravity investigations indicate salt diapirs are not present beneath the Hawasina Window. Instead, the gypsiferous intrusions are interpreted as having been brought up from depth during compression to form disconnected pods along deep-rooted faults, bringing with them small amounts of the basement country rock. Strontium isotope analysis and regional considerations, in particular the distribution, age and nature of other evaporite units on the eastern Arabian Plate, suggest the gypsum may have its origins in the Neoproterozoic (Ediacaran) to lower Cambrian Ara Group evaporites, perhaps from a previously unknown extension of the Fahud Salt Basin beneath the Hawasina thrust sheets.

Structure of the northern Oman Mountains from the Semail Ophiolite to the Foreland Basin

Abstract The northern Oman Mountains record the Late Cretaceous emplacement of the Semail Ophiolite and associated subduction trench and deeper-water sediments into a foredeep on the downwarped Arabian continental margin. In-sequence piggy-back thrusting was modified by later thrust sheets of Permian–Mesozoic continental slope sediments that breached the overlying allochthonous stack. Sections through these culminations show asymmetries linked to deep-seated faults perpendicular to the margin edge which created offsets (promontories) that influenced detachment. Seismic profiles across the foredeep indicate an unconformity at the top of the Mesozoic carbonate platform, the product of early flexural uplift, onlapped by sediments. A second onlap phase is linked with rapid subsidence during final emplacement of the allochthon. Passive margin conditions resumed during Maastrichtian time, conformable along the axis of the foredeep. A Paleocene–Eocene (Pabdeh) foredeep was initiated in the northern mountains, recorded on seismic profiles with another onlapping sequence. Subsidence and infill persisted into the Oligocene when compression, linked with the Zagros collision, resulted in uplift and en echelon folding, probably due to the reactivation of deep-seated faults. Post-Miocene erosion of one culmination exposed the cap of a gypsum/anhydrite intrusion. The origin remains uncertain, but derivation from the Neoproterozoic to Cambrian Ara Group salt is preferred.

Preserving Oman's geological heritage: proposal for establishment of World Heritage Sites, National GeoParks and Sites of Special Scientific Interest (SSSI)

Abstract The Oman Mountains contain a number of unique and very special geological sites of World Heritage importance. Large-scale developments in Oman and UAE have already affected some of these sites and it is now crucial to preserve these sites for posterity in law before urban development destroys them forever. This paper outlines the geological importance of 50 sites as National GeoParks and Sites of Special Scientific Interest (SSSI). Three geological sites in Oman are so unique and so special that they warrant World Heritage status: the Semail Ophiolite, the world's largest, best exposed and most complete ophiolite; Jebel al-Akhdar; and the Musandam peninsula. Other world-class sites include the Sifah eclogites, the Oman Exotics, Majlis al-Jinn, sites of ‘Snowball Earth’ significance and many others. All these sites are of World Heritage quality and fundamental geological importance, not only for Oman but also for the world. The total acreage of all these GeoPark sites is &lt;10% of Oman and the areas designated have little or no commercial value. The GeoParks will have long-lasting economic benefits as the post-oil and gas economy of Oman will have to rely on tourism and geo-tourism.

Tectonic evolution of the Oman Mountains: an introduction

It is now more than 20 years since the GeologicalSociety published The Geology and Tectonics ofthe Oman Region (Robertson et al. 1990) as oneof its early special publications. That volume, the‘blue book’ as it became fondly known to multiplegenerations of undergraduates, has been a majorreference on the geology of Oman for many whowork in the region. This present volume, moremodest in its size compared to its illustrious prede-cessor, is the fruit of a conference on The Geologyof the Arabian Plate and the Oman Mountainsheld in January 2012 at Sultan Qaboos Universityin Muscat, organized by Professor Sobhi Nasirwith colleagues from Sultan Qaboos University,the Oman Government and the Geological Societyof Oman. Since the first comprehensive study ofthe geology of the Oman Mountains during thelate 1960s and early 1970s by Ken Glennie and histeam from Shell, the Oman Mountains have beenknown to contain some of the most spectacularand best-exposed geology not only in the MiddleEast but across the world. A summary of the geo-logy and the lithostratigraphy is given in Figure 1.The publications of Glennie et al. (1973, 1974),building on many earlier Shell geologists’ internalreports (notably D. M. Morton, R. H. Tschopp,H. H. Wilson, B. M. Reinhardt and M. W. Hughes-Clarke) set out the major stratigraphic frameworkfor the whole area, defined the major structuresand critically interpreted the Semail Ophiolite as athrust sheet of oceanic crust and upper mantleemplaced onto the previously passive continentalmargin of Arabia. Since the Glennie et al. (1974)memoir was published, oil companies have carriedout increasingly more intensive work comparingwell sections in the interior with the Permian–Mesozoic shelf carbonates so beautifully exposedalong numerous wadis cutting through the JebelAl-Akhdar and Saih Hatat massifs.During the late 1970s and 1980s two majorresearchteamsfromtheUSGS(ledbyBobColemanand Cliff Hopson) and the Open University, UK(led by Ian Gass, John Smewing and Steve Lip-pard) conducted detailed geological surveys acrossthe ophiolite in the Muscat–Ibra transect (USGS)and northern ophiolite (OU), respectively. Theirresults were published in a Special Issue of theJournal of Geophysical Research (volume 86,editors Coleman & Hopson 1981) and the Geologi-cal Society of London Memoir no. 11 (Lippard etal.1986). Major systematic mapping of the OmanMountains as well as the interior, Batain coast andDhofar region has been carried out by the Bureaude Recherches Ge´ologiques et Minieres (BRGM),France, the Geological Survey of Japan and theUniversity of Berne, Switzerland (see for examplePeters et al. (1991)) and the whole UAE part of thenorthern Oman Mountains was recently mapped bythe British Geological Survey (UAE Ministry ofEnergy Petroleum & Mineral Resources 2012).During the last 20 years major research groupsfrom France, Japan, USA and the UK have con-ducted research projects in the country. Detailedfield studies of the Oman Ophiolite have beencombined with geochemical and isotopic studiesto determine the tectonic setting and evolution ofthe Late Cretaceous oceanic crust and upper mantle(Boudier & Juteau 2000). Field-based studies havealso been linked to ocean drilling sites, in particularthe East Pacific rise, to compare the ophiolite toregions of active fast spreading. Key to the obduc-tion–emplacement story are the amphibolite andgreenschist facies rocks in the metamorphic solethat record an inverted metamorphic field gradientalong the base of the ophiolite. The ophiolite hasbeen the source of recent detailed studies involvingthe possible sequestration of CO