Detecting the Elevated Crust to Mantle Section in the Kohistan-Ladakh Arc, Himalaya, from GOCE Observations

Kukesi massif is located in the eastern part of the Mirdita Ophiolite (northern Albania), which marks suture after Neo-Thetyan ocean closure. It is formed of well-preserved mantle and crustal sections which exhibit Supra-Subduction Zone affinity (e.g. Dilek and Furnes 2009, Lithos). Lower part of the mantle section of the Kukesi massif consist mainly of harzburgites, whereas dunites are located close to Moho. Crustal section records transition from lower part formed by peridotites and pyroxenites (so called intermediate zone after Hoxha and Boullier 1995, Tectonophysics) to gabbros. In this study we focus on composition and origin of pyroxenites occurring in the mantle and lower crustal parts of the Kukesi massif.In this study we studied 9 samples. They have composition of olivine websterite, clinopyroxenite, orthopyroxenite, hornblende-clinopyroxenite and websterite. Five of the analyzed samples have mantle origin (M): we studied (M)-olivine websterites and (M)-clinopyroxenite from harzburgitic part, as well as two (M)-orthopyroxenitic veins (one with clinopyroxenitic central part - composite vein) with minor amphibole&#160;cross-cutting dunites from one locality. From intermediate zone in crustal (C) part we collected (C)-hornblende-clinopyroxenites and (C)-websterite.&#160;Clinopyroxene composition is homogeneous in (M)-olivine-websterites (Mg#=84.5-87 and 88.8-90.5; Al=0.07-0.1 and 0.05-0.07, respectively), (M)-clinopyroxenite (Mg#=84-86, Al=0.04-0.08), (C)-hornblende-clinopyroxenites (Mg#=88.5-91, Al=0.08-0.12a.p.f.u.) and (C)-websterite (Mg#=87-88; Al=0.13-0.16a.p.f.u.). It differs widely between (M)-orthopyroxenitic veins: from Mg#=85-94 and Al=0.02-0.08 a.p.f.u &#160;in clinopyroxenitic part of composite vein to Mg#=93.6-95 and Al=0.01-0.03 in the purely orthopyroxenitic one. Orthopyroxene from two samples of &#160;(M)-olivine websterites have either&#160;Mg#=83 and Al~0.07 a.p.f.u (Foolivine=81.5) or Mg#=87&#160;&#160;and Al~0.04 a.p.f.u (Foolivine=86). Orthopyroxene composition in composite(M)-vein varies in wide ranges (Mg#=83-89; Al=0.04-0.08 a.p.f.u.); the other vein is homogeneous (Mg#=90-91, Al=0.02-0.03 a.p.f.u, Foolivine=86.8-90); in (C)-websterite orthopyroxene has Mg#=82.4-84 and&#160;Al=0.12-0.14 a.p.f.u. Amphibole has composition of tremolite-actinolite. Spinel, where present, is highly chromian (Cr#=0.59-0.80).Clinopyroxene is LREE-depleted in most of the samples, the (La/Lu)N=0.03-0.08. It is also LREE-depleted in (M)-clinopyroxenite ((La/Lu)N=0.05-0.23), but the contents of trace elements are higher than in other samples (eg. LuN=0.79-2.75 vs. 0.40-0.85). In (M)-veins the LREE contents are approximately at primitive mantle level ((La/Lu)N=0.28-1.66). &#160;Clinopyroxene in all samples has positive Th-U, Pb and Sr anomalies and negative Ta and Zr anomalies, but concentrations of trace elements is significantly higher in (M) clinopyroxenite and veins.The presence of tremolite and actinolite points to a retrogressive metamorphism which affected the rocks. The LREE-depleted nature of clinopyroxene forming all the pyroxenites and presence of orthopyroxene &#160;point to crystallization of the rocks from tholeiitic melt, but variations in Mg# and REE content in clinopyroxene may reflect formation either from different generations of melts or from melts fractionated due to reactive percolation.&#160; Variations in composition of the parental melts is visible even in a scale of one outcrop, which is demonstrated by (M)-orthopyroxenite veins with various modal composition and mineral major and trace elements compositions.This study was financed from scientific funds for years 2018-2022 as a project within program &#8220;Diamond Grant&#8221; (DI 024748).

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New podiform chromitites Occurrence from the Masirah Ophiolite, Oman

10.5194/egusphere-egu21-72 ◽

2021 ◽

Author(s):

Sobhi Nasir

Keyword(s):

Tectonic Setting ◽

Shear Bands ◽

Mineral Inclusions ◽

Silicate Inclusions ◽

Mid Ocean Ridge ◽

Early Paleocene ◽

Podiform Chromitites ◽

Ocean Ridge ◽

Mantle Section

The Masirah ophiolite is one of the few true ocean ridge ophiolites that have been preserved (Rollinson, 2017) and lacks any indication that it formed in a subduction environment. The Masirah ophiolite in south-eastern Oman is a different and older ophiolite from the more famous northern Oman ophiolite. Chromite and copper ores comprise large deposits in the Samail ophiolite, northern Oman. In comparison, chromite and copper deposits have not been described in previous reports or previous exploration in Masirah ophiolite. Rollinson (2017) has proposed that the apparent absence of chromitites in the mantle section of Masirah ophiolite is an important discriminant between subduction related and ocean ridge ophiolites.&#160; However, during recent studies on the Batain ophiolite m&#233;lange, and Masirah ophiolite, several chromitite pods have been discovered. The chromitites occur as separated small concordant, lenticular pods (3&#8211;10 m in thickness), which have been extensively altered and deformed, with the host pyroxenite serpentinites serpentinized harzburgites and dunites. The largest chromitite pods found within the pyroxenite and dunite of Masirah are up to 10 m across. &#160;Unusual minerals and mineral inclusions (orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the chromitites from the&#160; Masirah ophiolites. &#160;The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. Both&#160; phlogopite and hornblende were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromite during the chromitite formation. High-T green hornblende and phlogopite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands. The chromitites parental magmas are rich in K, Na, LREE, B, Cs, Pb, Sr, Li, Rb and U relative to HREE, reflecting the alkalic fluids/melts that prevailed during the chromitites genesis.The mineral inclusions&#160; in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. It appears that this chromite has been formed through reaction between amid-ocean-ridge basalt-melt with depleted harzburgite in the uppermost mantle. &#160;The chromitite deposits have similar cr# (55-62% Al-chromitites), mg# Al2O3 and TiO2 contents to spinels found in MORB, and have been interpreted as having formed in amid-ocean ridge setting.&#160; This suggests that this chromitites is residual from lower degree, partial melting of peridotite, which produced low-Cr# chromitites at the Moho transition zone, possibly in a mid-ocean-ridge setting. The chemistry of both mineral inclusions and chromite&#160;&#160; suggests MORB-related tectonic setting for the chromitites that were crystallized at 1000 &#176;C&#8211;1300 &#176;C under pressures <3 GPa . The host peridotites were generated during the proto-Indian Ocean MORB extension and emplaced as a result of the obduction of the ophiolite over the Oman Continental margin during Late Cretaceous-Early Paleocene.Rollinson, H., 2017. Geoscience Frontiers, 8: 1253&#8211;1262.

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