Edwin Gnos

Cretaceous volcanic rocks of the South Tethyan suture zone, Pakistan: implications for the Réunion hotspot and Deccan Traps

The location, ages, and geochemical characteristics of marine volcanic rocks preserved in the South Tethyan suture zone of Pakistan suggest that the Reunion hotspot was active off northwestern Greater India well before the emplacement, far to the south, of the Deccan flood basalts, the great bulk of which were erupted at 65–66 Ma and are widely believed to be associated with the hotspot’s plume-head phase. Most of the suture zone samples have Nd–Pb–Sr isotopic ratios (e.g. age-corrected ϵNd(t)=+3.0 to +4.6) close to those expected for modern-type Reunion source mantle in the Late Cretaceous, and their incompatible element patterns resemble those of recent Reunion shield lavas. 40Ar–39Ar incremental heating yields ages of 73.4–72.0 Ma. Nevertheless, unless even older ages are discovered among the suture zone rocks, a pre-Deccan marine phase of Reunion hotspot activity on the Tethyan side of Greater India can be accommodated within the framework of the plume-head model.

K-Ar ages of the metamorphic sole of the Semail Ophiolite: implications for ophiolite cooling history

K−Ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the Semail Ophiolite. The hornblende ages of 99±0.5 and 102±0.8 Ma and the 90 Ma ages of coexisting micas from the high-grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism (T 800–850° C, P 6.5–9 kbar). The new pressure estimates are based on findings of kyanite in garnet-amphibolite and cordierite in quartzitic rocks. These data indicate a cooling rate of 10–30° C/Ma. The oldest mica ages of 95±1 Ma are observed in the lowest-grade greenschists. These also largely represent cooling ages, but might in part also include formation ages. The pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low-grade metamorphic zone, through the high-grade rocks and into the base of the overlying ophiolite. Radiometric ages of hornblendes (92.3±0.5 and 94.8±0.6 Ma) indicate that the crustal gabbro sequence cooled below 500° C later than the base of the ophiolite sequence. Metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate. The peak of metamorphism must have been contemporaneous with the main magmatism in the Semail Ophiolite. One of the dated muscovites yields an age of 81.3±0.8 Ma, but this is related to discrete deformation zones that were active during late-stage emplacement of the ophiolite.

Rock-forming moissanite (natural α-silicon carbide)

Abstract We report the first occurrence of moissanite (SiC) as a rock-forming mineral (8.4 vol%) in one unique specimen of a terrestrial rock. The sample has a homogeneous, porphyritic texture, and was found as a beach pebble thought to be derived from a Tertiary volcanic province of the Aegean Sea region. The matrix is bluish-colored and consists of very fine-grained brucite, calcite, and magnesite, in which macrocrysts of quartz (25.3 vol%) and moissanite are found. Other accessory phases are phlogopite-3T, magnesiochromite, an Fe-rich phase, Cl-bearing brucite, Al-rich orthopyroxene, and unidentified MgFe-silicates (4 vol%). The bulk-rock composition shows a “kimberlitic” chemistry (55.8 wt% SiO2, 28.5 wt% MgO, 1.4 wt% CaO, 18.1 wt% LOI). Colorless gemmy, and blue or black moissanite crystals are subhedral and display characteristic hexagonal symmetry (6H polytype). Most moissanite grains contain metallic Si and Fe-silicide (Fe3Si7) inclusions, and more rarely, other Fe-silicides with varying amounts of Al (≤24.5 wt%), Ca (≤8.0 wt%), Mn (≤6.8 wt%), Ti (≤16.3 wt%), and Ni (≤2.6 wt%). The δ13C value of the moissanite is -28.1‰. According to available data, the fO2 stability field of SiC is five to six log units below the iron-wüstite (IW) buffer curve. Therefore, the observed Fe-bearing silicates cannot have been equilibrated with SiC under ambient pressure. Instead, our finding indicates that the rock most likely formed at the ultrahigh-pressure conditions of the upper mantle or transition zone.

Late Palaeozoic to Neogene geodynamic evolution of the northeastern Oman margin

When the highlands of Arabia were still covered with an ice shield in the latest Carboniferous/Early Permian period, separation of Gondwana started. This led to the creation of the Batain basin (part of the early Indian Ocean), off the northeastern margin of Oman. The rifting reactivated an Infra-Cambrian rift shoulder along the northeastern Oman margin and detritus from this high was shed into the interior Oman basin. Whereas carbonate platform deposits became widespread along the margin of the Neo-Tethys (northern rim of Oman), drifting and oceanization of the Batain basin started only in Late Jurassic/Early Cretaceous time. Extensional tectonics was followed in the Late Cretaceous by contraction caused by the northward drift of Greater India and Afro-Arabia. This resulted in the collision of Afro-Arabia with an intra-oceanic trench and obduction of the Semail ophiolite and the Hawasina nappes south to southwestward onto the northern Oman margin ~ 80 m.y. ago. During the middle Cretaceous, the oceanic lithosphere (including the future eastern ophiolites of Oman) drifted northwards as part of the Indian plate. At the Cretaceous–Palaeogene transition (~ 65 Ma), oblique convergence between Greater India and Afro-Arabia caused fragments of the early Indian Ocean to be thrust onto the Batain basin. Subsequently, the Lower Permian to uppermost Maastrichtian sediments and volcanic rocks of the Batain basin, along with fragments of Indian Ocean floor (eastern ophiolites), were obducted northwestward onto the northeastern margin of Oman. Palaeogene neo-autochtonous sedimentary rocks subsequently covered the nappe pile. Tertiary extensional tectonics related to Red Sea rifting in the Late Eocene was followed by Miocene shortening, associated with the collision of Arabia and Eurasia and the formation of the Oman Mountains.

Sapphirine-quartz and sapphirine-corundum assemblages in metamorphic rocks associated with the Semail Ophiolite (United Arab Emirates)

Granulite facies metasedimentary and metavolcanic rocks occur imbricated with the Cretaceous Semail Ophiolite in the United Arab Emirates and Oman. Peak metamorphic assemblages in highly oxidized lithologies of these rocks include sapphirine (7 : 9 : 3)-hercynitic spinel-magnetite-ilmeno-hematite-plagioclase-enstatite-quartz and sapphirine (2 : 2 : 1)-corundum-ilmeno-hematite-phlogopite-plagioclase. The observed mineral assemblages in quartzitic rocks are an example of overlapping stability of sapphirine-quartz and spinel-quartz and are therefore not diagnostic for high pressures. Both types of sapphirine occur within the stability field of Mg-rich cordierite. The common association of sapphirine and spinel with magnetite and ilmeno-hematite in quartzitic rocks suggests that sapphirine was formed by a reaction including spinel, hematite and quartz on the educt side. Metamorphic conditions estimated from associated parageneses point to temperatures of 800–850°C and pressures of 6.5–9 kbar.

The Source of the Great Dyke, Zimbabwe, and Its Tectonic Significance: Evidence from Re‐Os Isotopes

Re‐Os data for chromite separates from 10 massive chromitite seams sampled along the 550‐km length of the 2.58‐Ga Great Dyke layered igneous complex, Zimbabwe, record initial 187Os/188Os ratios in the relatively narrow range between 0.1106 and 0.1126. This range of initial 187Os/188Os values is only slightly higher than the value for the coeval primitive upper mantle (0.1107) as modeled from the Re‐Os evolution of chondrites and data of modern mantle melts and mantle derived xenoliths. Analyses of Archean granitoid and gneiss samples from the Zimbabwe Craton show extremely low Os concentrations (3–9 ppt) with surprisingly unradiogenic present‐day 187Os/188Os signatures between 0.167 and 0.297. Only one sample yields an elevated 187Os/188Os ratio of 1.008. Using these data, the range of crustal contamination of the Great Dyke magma would be minimally 0%–33% if the magma source was the primitive upper mantle, whereas the range estimated from Nd and Pb isotope systematics is 5%–25%. If it is assumed that the primary Great Dyke magma derived from an enriched deep mantle reservoir (via a plume), a better agreement can be obtained. A significant contribution from a long‐lived subcontinental lithospheric mantle (SCLM) reservoir with subchondritic Re/Os to the Great Dyke melts cannot be reconciled with the Os isotope results at all. However, Os isotope data on pre‐Great Dyke ultramafic complexes of the Zimbabwe Craton and thermal modeling show that such an SCLM existed below the Zimbabwe Craton at the time of the Great Dyke intrusion. It is therefore concluded that large melt volumes such as that giving rise to the Great Dyke were able to pass lithospheric mantle keels without significant contamination in the late Archean. Because the ultramafic‐mafic melts forming the Great Dyke must have originated below the SCLM (which extends to at least a 200‐km depth), the absence of an SCLM signature precludes a subduction‐related magma‐generation process.

P4/n and P4nc long-range ordering in low-temperature vesuvianites

Abstract Low-temperature vesuvianites crystallize in the two space groups P4/n or P4nc due to different arrangements of Ca-dodecahedra and M2+,3+ square pyramids that form strings along the fourfold axes. Long-range ordered vesuvianites of acentric P4nc symmetry may have the same diffraction symmetry as centrosymmetric disordered P4/nnc vesuvianites. In contrast, P4/n long-range ordered vesuvianites exhibit glide plane violating reflections and can easily be identified. We report the first successful Xray single-crystal structure refinement of a P4nc vesuvianite, a = 15.487(2), c = 11.764(2) Å from the N’chwaning II mine of the Kalahari manganese fields (RSA). This untwinned crystal has Cu2+ and Mn2+, Mn3+ forming the square pyramid and exhibits an acentric ordering pattern with 85% string A and 15% string B. This is compared to a reinvestigation of the structure of a P4/n vesuvianite from Asbestos Quebec (Canada), a = 15.531(2), c = 11.817(2) Å. The crystal is composed of a merohedral (110) twin with a close to 1:1 twin ratio and has mainly Fe3+ in square pyramidal coordination. In this centric structure, string A is 84% and string B is 16% occupied. Criteria to determine the symmetry of low-temperature vesuvianites, studied by diffraction experiments, are discussed.

Tetrahedral vacancies and cation ordering in low-temperature Mn-bearing vesuvianites: Indication of a hydrogarnet-like substitution

Abstract Strongly zoned Mn-rich vesuvianites with MnO concentrations up to 14.3 wt% from the N’chwaning II mine of the Kalahari manganese field (South Africa) crystallized at hydrothermal conditions below 450 °C. These vesuvianites are by far the most Mn-rich samples hitherto described and have either space group P4nc or P4/n due to partial long-range ordering. Most crystals are assembled of P4nc and P4/n domains yielding P4 average symmetry. The crystal structure of one Mn-rich crystal of average composition Ca19Mn3.5Al9.5Si17.4(O,OH)78 was refined from single-crystal X-ray data (R1 = 3.85%) in space group P4/n (a = 15.571(2), c = 11.789(2) Å). Mn2+ and Mn3+ are concentrated on the fivefold-coordinated square pyramidal Y′ site. Additional Mn3+ was located on the octahedral sites Y2a (35%), Y1a (22%), Y2b (13%) and Y1b (8%). Electron microprobe analyses and crystal-structure refinements indicated tetrahedral vacancies in the orthosilicate tetrahedra (Z1 and Z2) but not in the disilicate units (Z3). Z1 tetrahedra with up to 17% vacancies have strongly increased Z1-O distances of 1.67 Å. Structural and chemical evidence combined with the similarity of the structures of vesuvianite and garnet suggest a partial hydrogarnet-like substitution of SiO4 tetrahedra by H4O4.

Relationship among metamorphic grade, vesuvianite "rod polytypism," and vesuvianite composition

Abstract Single-crystal X-ray study of different vesuvianite samples of known origin shows that different metamorphic grade results in different arrangements of structural rods oriented parallel to the vesuvianite c axis, interpreted as .rod polytypism.. There is a systematic dependence of space-group symmetry and rod arrangement on crystallization temperature: P4nc-dominant < 300 °C, P4/n-dominant ~300.500 °C, and P4/nnc > 500 °C. Partial occupancy of the T sites (B, Al, Fe3+) and increased F-content seem to stabilize rod disorder causing P4/nnc space-group symmetry. All studied vesuvianites in calcsilicate rocks and marbles from regional- and contact-metamorphic upper amphibolite facies have disordered rods (P4/nnc symmetry). Electron-microprobe analyses of metamorphic vesuvianites from alpine and non-alpine occurrences, supported by structural investigation, showed that in addition to homo- and heterovalent substitution, partial occupancy of the commonly vacant T sites by B, Al, or Fe3+, and the (O4H4)4. → SiO44- (hydrogarnet-type) substitutions are significant in nature. With few exceptions, T-site occupancy seems to be restricted to high-grade metamorphic rocks whereas the .hydrovesuvianite. substitution is only found in vesuvianites formed at very low metamorphic grade. The cell parameters of vesuvianite with empty T sites increase with increasing Ti + Mg → 2 Al substitution, and this increase is even more pronounced with increasing .hydrovesuvianite. component. An increase in boron on T sites leads to a decrease of c but an increase in a. Fluorine incorporation and T-site substitution (B, Al, Fe3+) in vesuvianite are coupled with a decrease in hydroxyl groups. This causes vesuvianites to be stable under higher XCO2 conditions, and in an assemblage with quartz at conditions above the experimentally determined upper stability of quartz + T site vacant, F-free vesuvianite. Optically anomalous vesuvianites have ordered rods and are generally characterized by an intergrowth of P4/n and P4nc domains. In addition to B-rich vesuvianite and wiluite, P4nc-dominant vesuvianites are also commonly optically positive.

Kinoshitalite, Ba(Mg)3(Al2Si2)O10(OH,F)2, a brittle mica from a manganese deposit in Oman: Paragenesis and crystal chemistry

Abstract Kinoshitalite, ideally Ba(Mg)3(Al2Si2)O10(OH,F)2, was studied in a granulite-facies manganese ore associated with calcium-bearing magnesian tephroite, hausmannite, calcite and manganoan clinochlore, and in a second assemblage together with hausmannite, altered tephroite, manganoan diopside, and calcite. The metamorphic rocks occur enclosed in peridotites of the Semail Ophiolite, Sultanate of Oman and were metamorphosed during ophiolite obduction. Single-crystal X-ray data, collected on an inclusion-free kinoshitalite of Ba0.99K0.06Na0.01 (Mg2.64Mn0.31Al2.01Si2.03)O10(OH1.61F-Cl0.02) composition, yielded a C-centered lattice of monoclinic symmetry with a = 5.316(1), b = 9.230(2), c =10.197(2) Å, β = 100.06(1)°, V = 492.6 Å3 , and Z = 1 characteristic of the 1 M polytype. The structure was refined in two models assuming complete Si, Al ordering in the space groups C2 and C1 - allowing for additional twinning. The data clearly suggest that in spite of the Si/Al ratio of 1 assumption of complete Si, Al ordering can be rejected. Two models, both in agreement with space group C2/m, causing Si, Al disorder, are discussed. Three-dimensional Si, Al disorder would lead to violation of the Al avoidance rule and substantial Ba displacement. The rather well defined Ba position capped by two six-membered rings of Si3Al3O18 composition suggests one dimensional disorder where completely Si, Al ordered layers exist parallel to (001). The disorder occurs perpendicular to (001) and may be interpreted as random stacking faults.