Lithosphere 2018
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PERIODICAL: Lithosphere, 2018, v.10; no. 1.

THEMED ISSUE: Ophiolites, Diamonds, and UHP Minerals: New Discoveries and Concepts on Upper Mantle Petrogenesis.        


                     


1. Yildirim Dilek, Jingsui Yang. 2018. Ophiolites, diamonds, and ultrahigh-pressure minerals: New discoveries and concepts on upper mantle petrogenesis. Lithosphere, 10(1): 1-13. [PDF

ABSTRACT

Ophiolitic peridotites represent variously depleted residues of the primitive mantle after multiple episodes of partial melting, melt extrac- tion, and melt-rock interactions. They display a wide range of compositional and geochemical heterogeneities at different scales, and their incompatible bulk-rock compositions and mineral chemistries are commonly inconsistent with their evolution through simple partial melting processes at shallow mantle depths. Approaching these issues from different perspectives, the papers in this volume concentrate on (1) melt evolution and magmatic construction of ophiolites in various tectonic settings, and (2) the occurrence of microdiamonds, ultrahigh-pressure (UHP) minerals, and crustal material as inclusions in ophiolitic chromitites and peridotites. Crustal and mantle rock units exposed in different ophiolites show that the mantle melt sources of ophiolitic magmas undergo progressive melting, depletion, and enrichment events, con- stantly modifying the melt compositions and the mineralogical and chemical makeup of residual peridotites. Formation and incorporation of microdiamonds and UHP minerals into chromite grains occurs at depths of 350–660 km in highly reducing conditions of the mantle transition zone. Carbon for microdiamonds and crustal minerals are derived from subduction-driven recycling of surface material. Host peridotites with their UHP mineral and diamond inclusions are transported into shallow mantle depths by asthenospheric upwelling, associated with either slab rollback–induced channel ow or superplumes. Decompression melting of transported mantle rocks beneath oceanic spreading centers and their subsequent ux melting in mantle wedges result in late-stage formation of podiform chromitites during the upper mantle petrogenesis of ophiolites. Future studies should demonstrate whether diamonds and UHP minerals also occur in peridotites and chromitites of nonsubduction-related ophiolites. 


2. Xiong, F., Yang, J., Dilek, Y., Xu, X., and Zhang, Z. 2018. Origin and significance of diamonds and other exotic minerals in the dingqing ophiolite peridotites, eastern bangong-nujiang suture zone, tibet. Lithosphere, 9(1). [PDF]

ABSTRACT

We have recovered unusual mineral assemblages of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kya- nite, and andalusite from the peridotites of the LateTriassic Dingqing ophiolite along the Bangong-Nujiang suture zone in south-centralTibet. Diamond grains are 100–300 μm, pale yellow and reddish-orange to colorless, commonly anhedral in habit, and range in morphology from elongated to octahedral and subhedral crystals. They display a characteristic shift in the Raman spectra between 1325 cm-1 and 1333 cm-1, mostly at 1331.51 cm−1 or 1326.96 cm−1. Chromian spinels in the Dingqing harzburgites and dunites are high-Cr varieties with Cr# = 40.1–49.1 and Cr# = 65.3–69.6, respectively. Recovered diamonds, ultrahigh-pressure, and highly reduced minerals appear to have crystallized and then been encapsulated in chromian spinel grains over a wide range of depth in the diamond stability eld near the mantle transition zone. These chromian spinels and their host peridotites were then brought up to shallow mantle levels by the convecting asthenosphere, and were subsequently trapped in a mantle wedge of an intraoceanic subduction zone. The peridotites underwent further partial melting processes and reactions with island arc tholeiitic and boninitic melts, signi cantly modifying their whole-rock and mineral compositions.The preexist- ing chromian spinel grains with the inclusions of diamonds and other exotic minerals coalesced to form podiform chromitites during this stage in a suprasubduction zone setting. 


3. Feng Guangying, Yang Jingsui, Yildirim Dilek, Liu Fei, and Xiong Fahui. 2018. Petrological and Re-Os isotopic constraints on the origin and tectonic setting of the Cuobuzha peridotite,Yarlung Zangbo suture zone, southwest Tibet, China.  Lithosphere, 9(1). [PDF]

ABSTRACT

The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of theYarlung Zangbo suture zone in southwest Tibet comprises mainly clinopyroxene (cpx)-rich and depleted harzburgites. Spinels in the cpx-harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites.The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx-harzburgites have supra- chondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-oceanic ridge–type upper mantle units, their melt evolution trends re ect different mantle processes.The cpx-harzburgites formed from low-degree partial melting of a primitive mantle source, and they were subsequently modi ed by melt-rock interactions in a mid-oceanic ridge environment.The depleted harzburgites, however, were produced by remelting of the cpx-harzburgites, which later interacted with mid-oceanic ridge basalt– or island-arc tholeiite–like melts, pos- sibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites con rm that the Neo-Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its sea oor spreading evolution. 


4. Xu Xiangzhen, Cartigny Pierre, Yang Jingsui, Dilek Yildirim, Xiong Fahui, and Guo Guolin. 2018. Fourier transform infrared spectroscopy data and carbon isotope characteristics of the ophiolite-hosted diamonds from the Luobusa ophiolite,Tibet,and Ray-Iz ophiolite, Polar Urals.  Lithosphere, 9(1). [PDF]

ABSTRACT

We report new δ13C data and N content and aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray-Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain signi cant nitrogen contents (from 108 to 589 atomic ppm ± 20%) with a consistently low aggregation state and show identical infrared spectra domi- nated by strong absorption between 1130 cm–1 and 1344 cm–1, and therefore characterize type Ib diamond. Microdiamonds from the Luobusa peridotites have δ13C (PDB) values ranging from −28.7‰ to −16.9‰, and N contents from 151 to 589 atomic ppm.The δ13C and N content val- ues for diamonds from the Luobusa chromitites are −29‰ to −15.5‰ and 152–428 atomic ppm, respectively. Microdiamonds from the Ray-Iz chromitites show δ13C values varying from −27.6‰ to −21.6‰ and N contents from 108 to 499 atomic ppm.The carbon isotopes values have features similar to previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds we analyzed, in that they show enhanced δ13C variability and less advanced aggregation state than synthetic dia- monds. Our newly obtained N aggregation state and N content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e., <950 °C), and in a short residence time (i.e., within several million years) at high temperatures in the deep mantle. 


5. Niu Xiaolu, Liu Fei, Yang Jingsui, Dilek Yildirim, Xu Zhiqin, and Sein Kyaing. 2018. Mineralogy, geochemistry, and melt evolution of the Kalaymyo peridotite massif in the Indo-Myanmar Ranges (western Myanmar), and tectonic implications. Lithosphere, 9(1). [PDF] 

ABSTRACT

We present new whole-rock major, trace, and platinum group element (PGE) and mineral chemistry data from the Kalaymyo peridotite massif in the central part of the Indo-Myanmar Ranges (western Myanmar) and discuss its mantle melt evolution.The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse-grained equigranular textures.They are composed of olivine (forsterite, Fo = 89.8–90.5), orthopyroxene (enstatite, En86–91, wollastonite, Wo1–4, ferrosilite, Fs8–10; Mg# = 89.6–91.9), clinopyroxene (En46–49Wo47–50Fs3–5; Mg# = 90.9–93.6), and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole-rock compositions with Mg#s of 90.1–90.8 and SiO2 (41.5–43.65 wt%), Al2O3 (1.66–2.66 wt%), and CaO (1.45–2.67 wt%) contents.They display light rare earth element (LREE)– depleted chondrite-normalized (CN) REE patterns with (La/Yb)CN = 0.04–0.21 and (Gd/Yb)CN = 0.40–0.84, and show a slight enrichment from Pr to La with (La/Pr)CN in the range of 0.98–2.36. The Kalaymyo peridotites are characterized by Pd-enriched chondrite-normalized PGE pat- terns with superchondritic (Pd/Ir)CN ratios (1.15–2.36).Their calculated oxygen fugacities range between the quartz-fayalite-magnetite (QFM) oxygen buffers, QFM–0.57 and QFM+0.90.These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5%–15%) of partial melting at a mid-oceanic ridge environment.The observed enrichment in LREE and Pd was a result of their reactions with enriched mid-oceanic ridge basalt–like melts percolating through these already depleted residual peridotites.The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges therefore represent mid-oceanic ridge–typeTethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward-migrating subduction-accretion sys- tem along the eastern margin of India. 


6. Wu Weiwei, Yang Jingsui, Dilek Yildirim, Milushi Ibrahim, and Lian Dongyang. 2018. Multiple episodes of melting, depletion, and enrichment of the Tethyan mantle: Petrogenesis of the peridotites and chromitites in the Jurassic Skenderbeu massif, Mirdita ophiolite,Albania. Lithosphere, 9(1). [PDF] 

ABSTRACT

The Mirdita ophiolite in Albania displays a structural-geochemical transition from a mid-ocean ridge–type (MOR) oceanic lithosphere in the west to a suprasubduction zone (SSZ) type in the east across an ~30-km-wide fossilTethyan oceanic domain. We investigated the upper mantle peridotites of the Skenderbeu massif, situated at this transition within the ophiolite, to document the geochemical ngerprint of the inferred tectonic switch. The peridotites comprise harzburgites and dunites with podiform chromitite deposits. We present new whole-rock major element, trace element, rare earth element (REE), and platinum group element chemistry to evaluate their mantle melt evolution and petrogenesis. Harzburgites have high average CaO, Al2O3, and REE contents, and contain Al-rich pyroxene and spinel with lower Cr contents. Dunites have low average CaO, Al2O3, and REE values, and contain Al-poor clinopyroxene and high-Cr spinel. Modeling of trace element compositions of the harzburgites suggests as much as ~10%–15% melting, whereas the trace element compositions of the dunites indicate ~20%–25% melting.The harzburgites and dunites and chromitites represent, respectively, the products of low-degree partial melting in a MOR setting, and the products of high-degree partial melting and refertilization in a forearc mantle.The harzburgites resulted from rock-melt interactions between ascending melts and residual peridotites beneath a MOR, whereas the dunites and the high-Cr chromitites formed as a result of interactions between boninitic melts and mantle peridotites in a mantle wedge.The Skenderbeu mantle units thus constitute a geochemical-petrological archive of a transition from MOR to SSZ melt evolution in space and time within the same ocean basin. 


7. Liu Fei, Dilek Yildirim, Xie Yanxue, Yang Jingsui and Lian Dongyang. 2018. Melt evolution of upper mantle peridotites and ma c dikes in the northern ophiolite belt of the western Yarlung Zangbo suture zone (southern Tibet). Lithosphere, 9(1). [PDF] 

ABSTRACT

The Yarlung Zangbo suture zone (YZSZ) in southern Tibet is divided by the Zhongba terrane into two subparallel belts in its western end. The northern belt (NB) is tectonically juxtaposed against an accretionary prism complex and the Gangdese magmatic arc of Eurasia along dextral oblique-slip faults. Peridotite massifs in this belt are intruded by ma c dikes, providing critical geochemical, geochronological, and isotopic information about the melting-melt extraction history of the Tethyan mantle. Peridotites consist of harzburgite and clinopyroxene harzbur- gite with minor Iherzolite and dunite-chromitite. Dolerite and microgabbro dikes crosscutting these peridotites display U-Pb zircon ages of 128–122 Ma, and show normal mid-oceanic ridge basalt (N-MORB) like rare earth element patterns with negative Nb, Ta, and Ti anomalies, high εNd(t) values (+8.39 to +9.28), and (143Nd /144Nd)t = 128 Ma ratios of 0.51290–0.51295. They display high (87Sr/86Sr)t ratios of 0.70433–0.70489, and 206Pb/204Pbof17.546–17.670,207Pb/204Pbof15.432–15.581,and208Pb/204Pbof37.724–37.845,suggestingthattheirN-MORB–likemantlesourcewas modi ed by island arc melts. Slab rollback–induced extension in an arc-trench system along the Eurasian continental margin led to ~7%–12% partial melting of subduction-in uenced, spinel lherzolite peridotites, producing dike magmas. The NB peridotite massifs and ophiolites thus represent a suprasubduction zone oceanic lithosphere formed in close proximity to the late Mesozoic active continental margin of Eurasia. 


8. Kyaw Soe Moe, Yang Jingsui, Paul Johonson, Xu Xiangzhen, and Wang Wuyi. 2017. Spectroscopic analysis of microdiamonds in ophiolitic chromitite and peridotite.  Lithosphere, 10(1), L603.1. [PDF] 

ABSTRACT

Microdiamonds ~200 μm in size, occurring in ophiolitic chromitites and peridotites, have been reported in recent years. Owing to their unusual geological formation, there are several debates about their origin. We studied 30 microdiamonds from 3 sources: (1) chromitite ore in Luobusa,Tibet; (2) peridotite in Luobusa,Tibet; and (3) chromitite ore in Ray-Iz, polar Ural Mountains, Russia.They are translucent, yellow to greenish-yellow diamonds with a cubo-octahedral polycrystalline or single crystal with partial cubo-octahedral form. Infrared (IR) spectra revealed that these diamonds are type Ib (i.e., diamonds containing neutrally charged single substitutional nitrogen atoms, N0s, known as the C center) with unknown broad bands observed in the one-phonon region.They contain uid inclusions, such as water, carbonates, silicates, hydrocarbons, and solid CO2. We also identi ed additional microinclusions, such as chromite, magnetite, feldspar (albite), moissanite, hema- tite, and magnesiochromite, using a Raman microscope. Photoluminescence (PL) spectra measured at liquid nitrogen temperature suggest that these diamonds contain nitrogen-vacancy, nickel, and H2 center defects. We compare them with high-pressure–high-temperature (HPHT) synthetic industrial diamond grits. Although there are similarities between microdiamonds and HPHT synthetic diamonds, major differences in the IR, Raman, and PL spectra con rm that these microdiamonds are of natural origin. Spectral characteristics suggest that their geological formation is different but unique compared to that of natural gem-quality diamonds. Although these microdiamonds are not commercially important, they are geologically important in that they provide an understanding of a new diamond genesis. 


9. Závada, P., Schulmann, K., Racek, M., Hasalová, P., Jeřábek, P., & Weinberg, R. F., et al. 2018. Role of strain localization and melt flow on exhumation of deeply subducted continental crust. Lithosphere, 9(1). [PDF]

ABSTRACT

A section of anatectic felsic rocks from a high-pressure (>13 kbar) continental crust (Variscan Bohemian Massif) preserves unique evidence for coupled melt ow and heterogeneous deformation during continental subduction. The section reveals layers of migmatitic granofels interlayered with anatectic banded orthogneiss and other rock types within a single deformation fabric related to the prograde metamorphism. Granofels layers represent high strain zones and have traces of localized porous melt ow that in ltrated the host banded orthogneiss and crystallized granitic melt in the grain interstices. This process is inferred from: (1) gradational contacts between orthogneiss and granofels layers; (2) grain size decrease and crystallographic preferred orientation of major phases, compatible with oriented growth of crystals from interstitial melt during granular ow, accommodated by melt-assisted grain boundary diffusion creep mechanisms; and (3) pressure- temperature equilibria modeling showing that the melts were not generated in situ. We further argue that this porous melt ow, focused along the deformation layering, signi cantly decreases the strength of the crustal section of the subducting continental lithosphere. As a result, detachment folds develop that decouple the shallower parts of the layered anatectic sequence from the underlying and continuously subducting continental plate, which triggers exhumation of this anatectic sequence.