Petrology and geochemistry of ultramafic and mafic rocks in the late Silurian-early Devonian Darbut ophiolitic mélange of west Junggar (NORTHWESTERN CHINA): implications for petrogenesis and tectonic evolution
5
Citation
106
Reference
10
Related Paper
Citation Trend
Abstract:
New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite.Keywords:
Peridotite
Ultramafic rock
Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe2+) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21–1.24 and Al/Si < 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (<300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.
Ultramafic rock
Brucite
Peridotite
Cite
Citations (1)
The Wuwamen ophiolite is located on the southern margin of Middle Tianshan Mountains, Xinjiang. In this paper,petrology, mineralogy and geochemistry of the ultramafic rocks from Wuwamen ophiolite were studied to constrain their origin and tectonic setting. The ultramafic rocks from Wuwamen ophiolite are serpentinized lherzolites composed of olivine(Fo=89.1- 90.6),orthopyroxene(Wo0.4- 2.4En87.2~90.7Fs8.9- 10.9; Mg#=89.0-91.0), clinopyroxene(Wo49.1- 51.3En16.0~48.4Fs0.9- 4.3; Mg#=90.2-92.1) and spinel(Mg#=71.8- 77.5; Cr#=9.3- 13.4). They are characterized by relatively lower content of Mg O(37.74%- 41.34%), and higher content of Al2O3(2.58%-3.39%), Ca O(2.23%-3.68%) and Ti O2(0.05%-0.11%). The ultramafic rocks are also depleted in rare earth elements(REE; total REE = 1.73×10-6-4.63×10-6) and incompatible elements(e.g., Rb=0.4×10-6-1.39×10-6, Zr=0.73×10-6-3.28×10-6, Hf=0.04×10-6-0.11×10-6), and enriched in compatible elements(e.g., Cr=2516×10-6-2793×10-6, Co=84.6×10-6-119×10-6, Ni=1641×10-6-2261×10-6). These data indicate that the ultramafic rocks from Wuwamen ophiolite are residues of mantle rocks after low degrees(ca. 5%-10%) of partial melting in a Mid-Ocean-Ridge(MOR) environment.
Ultramafic rock
Cite
Citations (2)
Dike
Cite
Citations (29)
Ultramafic rock
Cite
Citations (6)
Peridotite
Platinum group
Chromitite
Cite
Citations (0)
<p>Figure S1: Discrimination diagrams for the granite dikes in the Monhhan ophiolite. Table S1: Major and trace element compositions of the Monhhan ophiolite and intruding granite. Table S2: LA-ICP-MS zircon U-Pb analytical data for the gabbro, intruding granite, and associated sandstone in the Monhhan ophiolite. Table S3: Lu–Hf data for zircons from the gabbro and intruding granite in the Monhhan ophiolite.</p>
Dike
Table (database)
Trace element
Cite
Citations (0)
Ultramafic rock
Cite
Citations (50)
The northeast Indian ophiolite belt is exposed in parts of Nagaland and Manipur states. We present the results of an investigation into the petrology and constituent mineral chemistry of peridotite from the ophiolite belt of Phek District, Nagaland. Based on relict primary mineral compositions, the studied rocks are identified as abyssal peridotite. Spinel composition shows that the rocks have undergone low degree of partial melting (5 11%). Equilibration of the ophiolite mantle sequence peridotite was calculated at a temperature range of 850–1165oC, pressure ranging between 19 to 25 kbar, and an oxygen fugacity between 0.437 to 0.657 log units above the FMQ buffer, whereas for the xenolith peridotite the temperature was estimated to be between 850 to 1100 oC, pressure ranging from 12 to 17 kbar and oxygen fugacity between 0.577 to 0.706 log units above the FMQ buffer. Equilibrium condition suggests that the rocks were equilibrated in an upper mantle environment.
Peridotite
Mineral redox buffer
Fugacity
Cite
Citations (0)
In the Alpine nappe stack, the Antrona Ophiolite (Italian side of the Western Central Alps) is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic domain) and the underlying Camughera-Moncucco continental Unit (middle Penninic).The ophiolite sequence includes ultramafic rocks, metagabbros and mafic rocks covered by calcschists.Ultramafites constitute a huge body of serpentised peridotites including interbedded layers of gabbros, clinopyroxene-rich and amphibole-rich rocks, and chloriteschist.In spite of the Alpine tectonic and metamorphic reworking, the ultramafic portion of the Antrona Ophiolite still preserves relict textures and minerals that can be referred to the pre-Alpine or early Alpine evolution.A detailed microstructural analysis performed at polarised microscope and SEM on less serpentinised, olivine-rich samples is here presented.It is integrated with a quantitative textural analysis of Lattice Preferred Orientation (LPO) by neutron diffraction acquired on selected samples of olivine-rich samples.The results allow to infer a mantle origin for the ultramafic rocks, suggesting T conditions > 800°C for the activation of slip systems in olivine.
Ultramafic rock
Cite
Citations (9)
<p>Figure S1: Discrimination diagrams for the granite dikes in the Monhhan ophiolite. Table S1: Major and trace element compositions of the Monhhan ophiolite and intruding granite. Table S2: LA-ICP-MS zircon U-Pb analytical data for the gabbro, intruding granite, and associated sandstone in the Monhhan ophiolite. Table S3: Lu–Hf data for zircons from the gabbro and intruding granite in the Monhhan ophiolite.</p>
Dike
Table (database)
Cite
Citations (0)