Dongyang Lian

Deep mantle origin and ultra-reducing conditions in podiform chromitite: Diamond, moissanite, and other unusual minerals in podiform chromitites from the Pozanti-Karsanti ophiolite, southern Turkey

Abstract The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chromitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.

Tectonic Evolution of the Western Yarlung Zangbo Ophiolitic Belt, Tibet: Implications from the Petrology, Mineralogy, and Geochemistry of the Peridotites

The northern subbelt in the western segment of the Yarlung Zangbo suture zone, Tibet, China, includes the Dajiweng, Kazhan, Baer, Cuobuzha, Jianabeng, and Zhalai ophiolitic massifs. These ophiolites are strongly dismembered, typically 1–2 km wide and 10–20 km long, and composed chiefly of peridotites with minor volcanic and siliceous sedimentary rocks. No cumulates have been observed in the northern ophiolitic belt. Harzburgites of the Dajiweng and Zhalai ophiolites have prominent light rare earth element (LREE)–enriched (U-shaped or spoon-shaped) chondrite-normalized rare earth element (REE) patterns. Such patterns have generally been interpreted as the result of modification by suprasubduction zone (SSZ) melts/fluids. However, the abundance of peridotites sampled from mid-ocean ridge with similar LREE-enriched REE patterns suggest that this feature is not unique to SSZ peridotites. The U-shaped REE patterns of the Dajiweng harzburgites, combined with their low heavy rare earth element (HREE) contents and their mineral chemistry, indicate that these rocks most likely have been modified by SSZ melts (e.g., boninitic melts) in a forearc setting. In contrast, the Zhalai harzburgites, which also have U-shaped REE patterns but are characterized by high HREE contents, high Al2O3/SiO2 ratios, low MgO/SiO2 ratios, and relatively fertile mineral compositions, most likely have been refertilized in a mid-ocean ridge setting. The Zhalai, Kazhan, Baer, and Cuobuzha peridotites are similar to abyssal and back-arc peridotites in mineral chemistry and whole-rock geochemistry. Combining the mafic intrusions from Jianabeng, Baer, and Cuobuzha massifs, we propose that the ophiolites in the northern belt of the western segments have been trapped in an intraoceanic forearc–arc–back-arc system. According to the zircon U-Pb age of mafic intrusions, the geochemical characteristics of both mafic and ultramafic rocks, a detrital zircon study of Zhongba terrane, and the klippen structure of ophiolitic massif in the southern belt, we conclude that the northern and southern ophiolitic belts were developed in the same intraoceanic subduction system.

Geochemical, Geochronological, and Sr-Nd Isotopic Constraints on the Origin of the Mafic Dikes from the Pozanti-Karsanti Ophiolite: Implications for Tectonic Evolution

The Pozanti-Karsanti ophiolite is situated in the eastern segment of the Tauride belt. This ophiolite is composed of mantle peridotites, ultramafic to mafic layered cumulates, hornblende gabbros, plagiogranite with minor sheeted dikes and pillow lavas. Metamorphic sole and the Aladag mélange were accreted beneath the base of the Pozanti-Karsanti ophiolite. Gabbroic and doleritic dikes intrude the Pozanti-Karsanti ophiolite and the metamorphic sole around Late Cretaceous, as determined by the zircon U-Pb ages (86.9 ± 3.1 Ma) of the dikes, but no dikes have been observed in the underlying Aladag mélange. The Pozanti-Karsanti dikes show light rare earth element–depleted chondrite-normalized rare earth element patterns and high field strength element–depleted (Nb, Ta, and Ti) but large ion lithophile element–enriched (Rb, Ba, and U) primitive mantle–normalized trace element patterns. These dikes have Th/Nb ratios and oxygen fugacity higher than those of mid-ocean ridge basalt (MORB) but lower than island arc basalt. The Pozanti-Karsanti mafic dikes have (87Sr/88Sr)t ratios of 0.70433–0.70489 and εNd(t) of +1.8 to +2.4, indicating a depleted mantle source and mixing of crustal or sedimentary components. Combining the characteristics of both trace elements and radiogenic isotopes, we conclude that these dikes are derived from Sp or Sp + Grt facies (Sp > Grt) depleted MORB mantle, with some addition of a subduction component in a forearc setting. On the basis of previous studies and our new work on the Pozanti-Karsanti ophiolite, we conclude that this ophiolite formed during the subduction initiation of an intraoceanic subduction zone in the Late Cretaceous.

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

The Jurassic 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 fossil Tethyan oceanic domain. We investigated the upper mantle peridotites of the Skenderbeu massif, situated at this transition within the ophiolite, to document the geochemical fingerprint 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. LITHOSPHERE; v. 10; no. 1; p. 54–78 | Published online 14 July 2017 doi:10.1130/L606.1

Geochemistry and tectonic significance of the Gongzhu peridotites in the northern branch of the western Yarlung Zangbo ophiolitic belt, western Tibet

The Gongzhu ophiolite is situated in the northern branch of the western Yarlung Zangbo ophiolitic belt. This massif consists of a strongly dismembered ophiolitic sequence dominated by mantle peridotites. The peridotites comprise lherzolite with low- to moderately-depleted mineral and bulk rock compositions. The degree of partial melting deduced from Cr# values of the Gongzhu peridotites varies between 7% and 10%. The mineral and whole rock compositions of the Gongzhu peridotites are comparable to those of abyssal peridotites. The chondrite normalized REE compositions of the peridotites typically display U-shaped or spoon-shaped patterns, and primitive mantle-normalized PGEs patterns show Ir depletion relative to Os and Ru, and Pt enrichment relative to Rh and Pd. On the basis of the petrological, mineralogical and geochemical data, we concluded that the Gongzhu peridotites either formed in the back-arc setting of an intra-oceanic subduction system or the Gongzhu and Dajiweng peridotites both formed in the in the same incipient forearc/proto-forearc environment of an intra-oceanic subduction zone.

Ocean-continent Transition to Suprasubduction Zone Origin of the Western Yarlung Zangbo Ophiolites in SW Tibet, China: Multi-stage, Transient Evolution of the Neotethyan Oceanic Lithosphere

The ophiolites that crop out discontinuously along the ~2000 km Yarlung Zangbo Suture zone (YZSZ) between the Nanga Parbat and Namche Barwa syntaxes in southern Tibet represent the remnants of Neotethyan oceanic lithosphere (Fig. 1a). We have investigated the internal structure and the geochemical makeup of mafic-ultramafic rock assemblages that are exposed in the westernmost segment of the YZSZ where the suture zone architecture displays two distinct sub-belts of ophiolitic and mélange units separated by a continental Zhongba terrane (Fig. 1b). These two sub-belts include the Daba – Xiugugabu in the south (Southern sub-belt, SSB) and the Dajiweng – Saga in the north (Northern sub-belt, NSB). We present new structural, geochemical, geochronological data from upper mantle peridotites and mafic dike intrusions occurring in these two sub-belts and discuss their tectonomagmatic origin. In-situ analysis of zircon grains obtained from mafic dikes within the Baer, Cuobuzha and Jianabeng massifs in the NSB, and within the Dongbo, Purang, Xiugugabu, Zhaga and Zhongba in the SSB have yielded crystallization ages ranging between130 and 122 Ma. Dike rocks in both sub-belts show N-MORB REE patterns and negative Nb, Ta and Ti anomalies, reminiscent of those documented from SSZ ophiolites. Harzburgitic host rocks of the mafic dike intrusions mainly display geochemical compositions of abyssal peridotites (Fig. 2), with the exception of the Dajiweng harzburgites, which show the geochemical signatures of forearc peridotites (Lian et al., 2016). Extrusive rocks that are spatially associated with these peridotite massifs in both sub-belts also have varying compositional and geochemical features. Tithonian to Valanginian (150 – 135 Ma) basaltic rocks in the Dongbo massif have OIB-like geochemistry and 138 Ma basaltic lavas in the Purang massif have EMORB-like geochemistry (Liu et al., 2015). Tuffaceous rocks in the Dajiweng massif are 140 Ma in age and show OIB-like geochemistry. We interpret these age and geochemical data to reflect a rifted continental margin origin of the extrusive rock units in both sub-belts. These data and structural observations show that the western Yarluang Zangbo ophiolites represent fragments of an Ocean-Continent Transition (OCT) peridotites altered by fluids in an initial supersubduction setting. We infer that mafic-ultramafic rock assemblages exposed in the SSB and NSB initially formed in an ocean – continent transition zone (OCTZ) during the late Jurassic, and that they were subsequently emplaced in the forearc setting of an intraoceanic subduction zone within a Neotethyan seaway during 130 to 122 Ma. The NSB and SSB are hence part of a single, S-directed nappe sheet derived from a Neotethyan seaway located north of the Zhongba terrane. *

Origin of Peridotites and Chromitites in the Pozanti-kar Santi Ophiolite, Turkey

The Pozanti-Karsanti ophiolite (PKO) in Turkey’s eastern Tauride belt comprises mantle peridotites, ultramafic to mafic cumulates, isotropic gabbros, sheeted dikes and pillow lavas. The mantle peridotites are dominated by spinel harzburgites with minor dunites. The harzburgites and dunites have quite depleted mineral and whole-rock chemical composition, suggesting high degrees of partial melting. Their PGEs vary from Pd-depleted to distinct Pd-enriched patterns, implying the crystallization of interstitial sulphides from sulphur-saturated melts (e.g. MORB-like forearc basalt). U-shaped or spoon-shaped REE patterns indicate that the PKO peridotites may have also been metasomatized by the LREE-enriched fluids released from a subducting slab in a suprasubduction zone. Based on the mineral and whole-rock chemical compositions, the PKO peridotites show affinities to forearc peridotites. Chromitites occur both in the mantle peridotites and the mantle-crust transition zone horizon (MTZ). Chromitites from the two different horizons have different textures but similar mineral compositions, consistent with typical high-Cr chromitites. Chromitites hosted by mantle harzburgites generally have higher total platinum-group element (PGE) contents than those of the MTZ chromitites. However, both chromitites show similar chondritenormalized PGE patterns characterized by clear IPGEs, Rh-enrichments relative to Pt and Pd. Such PGE patterns indicate no or only minor crystallization of Ptand Pd enriched sulphides during formation of chromitites from a sulphur-undersaturated melt (e.g. boninitic or island arc tholeiitic melt). Dunite enveloping chromitite lenses in the hosting harzburgite resulted from melt-rock reaction. We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites. So far, more than 200 grains of microdiamond and more than 100 grains of moissanite (SiC) have been separated from podiform chromitites. These minerals have been identified by EDX and Laser Raman analyses. The diamonds and moissanite are accompanied by large amounts of rutile. Additionally, zircon, monazite and sulphides are also common phases within the heavy mineral separates. Both diamond and moissanite have been analyzed for carbon and nitrogen isotopic composition using the CARMECA 1280-HR large geometry Secondary Ion Mass Spectrometer at the Helmholtz Zentrum Potsdam. In total, 61 δCPDB results for diamond were acquired, exhibiting a range from −28.4 ‰ to −18.8 ‰. 31 δCPDB results for Moissanite vary between -30.5 ‰ to -27.2 ‰, with a mean value of -29.0 ‰. Diamond has relatively large variation in nitrogen isotopic composition with 40 δNAIR results ranging from -19.1 ‰to 16.6 ‰. The discovery of diamond, moissanite and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions. Considering the unusual minerals, the high Mg# silicate inclusions, and the needle-shaped exsolutions in the PKO chromitites, the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment. We suggest melt-rock reactions, magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PKO chromitites.