Diamonds Discovered from High–Cr Podiform Chromitites of Bulqiza,Eastern Mirdita Ophiolite,Albania

Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by 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. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.

Kalaymyo Peridotite Massif in the Indo-Myanmar Ranges(Western Myanmar): Its Mineralogy and Petrology

Mesozoic ophiolites crop out discontinuously in the Indo-Myanmar Ranges in NE India and Myanmar,and represent the remnants of the Neotethyan oceanic lithosphere(Sengupta et al.,1990;Mitchell,1993).These ophiolites in the Indo-Myanmar Ranges are the southern continuation of the Neotethyan ophiolites occurring along the Yarlung Zangbo Suture Zone(YZSZ)in southern Tibet farther northwest(Mitchell,1993;Fareeduddin and Dilek,2015),as indicated by their coeval crystallization ages and geochemical compositions(Yang et al.,2012;Liu et al.,2016).The Kalaymyo ophiolite is located in the central part of the eastern Indo-Myanmar Ranges(Fig.1).composition of these ophiolites from the central Tibetan Plateau(CTP)is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs,which is inconsistent with most ophiolites worldwide(Robinson and Zhou,2008;Zhang et al.,2008).But the generation and tectonic nature of these ophiolites are still controversial.*The Kalaymyo peridotites consist mainly of harzburgites,which show typical porphyroclastic or coarse-grained equigranular textures.They are composed ofolivine(Fo=89.8–90.5),orthopyroxene(En86-91Wo1-4Fs8-10;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 of90.1–90.8 and Si O2(41.5–43.65 wt.%),Al2O3(1.66–2.66wt.%)and Ca O(1.45–2.67 wt.%)contents.TheydisplayLightRareEarthElement(LREE)-depleted chondrite-normalized 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 patterns with superchondritic(Pd/Ir)CN ratios(1.15–2.36).Their calculated oxygen fugacities range between 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-ocean-ridge(MOR)environment.The observed enrichment in LREE and Pd was a result of their reactions with enriched MORB-like melts,percolating through these already depleted,residual peridotites.The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges hence represent mid-ocean ridge(MOR)–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward migrating subduction–accretion system along the eastern margin of India.

Early Devonian Ultrapotassic Magmatism in the North China Craton: Geochemical and Isotopic Evidence for Subcontinental Lithospheric Mantle Metasomatism by Subducted Sediment–Derived Fluid

The North China Craton(NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism occurred in discrete pulses in the Early and Middle Devonian;whereas the next episode of alkaline magmatism took place in the early Mesozoic. The Gucheng pluton is exposed in the northern part of the NCC and forms a composite intrusion, consisting of K-feldspar–bearing clinopyroxenite, clinopyroxene–bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene(Wo43-48En19-35Fs18-38), sanidine(An0 Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in-situ oxygen isotopes(δ18O=5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the NCC. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. SHRIMP U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the NCC facing the Palaeo-Asian Ocean.