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- Mohammad, Y O, 7602, et al.
(författare)
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Geochronological and mineral chemical constraints on the age and formation conditions of the leucogranite in the Mawat ophiolite, Northeastern of Iraq: insight to sync-subduction zone granite
- 2016
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Ingår i: Arabian Journal of Geosciences. - : Springer Science and Business Media LLC. - 1866-7511 .- 1866-7538. ; 9:12
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Tidskriftsartikel (refereegranskat)abstract
- Daraban leucogranite occurs as cactus-like dykes hosted in serpentinized harzburgite in the mantle section of the Mawat ophiolite, Kurdistan region, northeastern Iraq. It has a primary mineral assemblage of quartz+orthoclase+albite+tourmaline+muscovite, with Mg-rich biotite, phlogopite, zircon, ilmenite–titanohematite exsolution, xenotime, and monazite as the most abundant accessory minerals. New laser ablation inductively coupled plasma mass spectrometry U–(Th)–Pb dating of zircon, monazite, and xenotime reveal a single episode of leucogranite magmatism in the Mawat ophiolite at 92.6±1.2Ma. These data indicate that the intrusion ages of leucogranite rocks postdate the 105±5Ma formation age of Mawat ophiolite obtained by K–Ar hornblende method. The leucogranite magma originated by anatexis of pelagic sediments during the late Cretaceous subduction in the Neo-Tethys Ocean, leading to the formation of the Mawat ophiolite as part of the main Zagros ophiolite belt in Iraq and Iran. Tourmaline and biotite from leucogranite dykes were examined regarding their microchemistry and formation environment. Electron microprobe studies show that the tourmalines form mainly dravite–schorl solid solutions with a tendency to schorl compositions. Biotites in the leucogranite have bimodal composition represents by phlogopite and Mg-rich biotite. The tourmaline and biotite compositions, as well as field observations, appear to exclude a late-stage magmatic differentiation origin for the leucogranite. A probable source is S-type granitic magma rich in boron that resulted from the anatexis of silica-rich Ca-poor subduction wedge sediments like those of the Qulqula group. This intruded the ophiolites during the subduction stage. Calculated biotite and Fe–Ti oxide equilibria indicate that the parent magma formed along the subduction zone and solidified in the mantle wedge at a pressure 3.8–4.2kbar, equivalent to 12.5–13.8km depth. © 2016, Saudi Society for Geosciences.
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| 2. |
- Mohammad, Y O, 7602, et al.
(författare)
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Mg-staurolite and kyanite inclusions in metabasic garnet amphibolite from the Swedish Eastern Segment: evidence for a Mesoproterozoic subduction event.
- 2011
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Ingår i: European Journal of Mineralogy. - 0935-1221. ; 23:4, s. 609-631
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Tidskriftsartikel (refereegranskat)abstract
- We present evidence that the 1674 Ma Bora°s Mafic Intrusion of the Swedish Eastern Segment experienced high-pressure metamorphism related to a Sveconorwegian subduction-exhumation cycle. Mg-rich staurolite is found as inclusions in garnet in metaluminous amphibolites. The inclusion assemblages include staurolite (XMg 0.34–0.40), kyanite, euhedral anorthite, clinozoisite and quartz. The thermodynamic packages winTWQ and Theriak-Domino were used to investigate the P-T conditions of the matrix and inclusion mineral parageneses. The bulk composition of the rock does not have a stability field for staurolite-bearing parageneses. In our samples minerals of an eclogite-facies paragenesis became isolated from the whole rock in the first stage as inclusions in garnet. High Zn levels in the staurolite (0.6–1 wt% ZnO) show that it must have formed as either chloritoid or staurolite, both of which concentrate Zn. Euhedral anorthite inclusions have trace-element compositions including high Sr and insignificant Eu anomalies, which support their interpretation as pseudomorphs after lawsonite in plagioclase-out conditions. Rutile lamellae in the garnet are also indicative of a high-pressure origin. Calculated phase diagrams show that the most likely original paragenesis was garnet þ clinopyroxene þ Mg-rich chloritoid þ lawsonite þ kyanite þ quartz, which has a stability field for the whole-rock composition at 600 °C and 2.23–2.45 GPa. These conditions correspond to depths greater than 75 km, thus the Bora°s Mafic Intrusion was situated in the mantle at that time, implying subduction of the crustal block in which it was situated. The minerals now observed in the inclusions and in the rock matrix formed under amphibolite-facies conditions at lower pressures of 0.6–0.9 GPa and slightly increased temperatures around 650 °C, reflecting rapid exhumation from the mantle. Sm-Nd dating of garnet gives 957.1 ° 9.4 Ma, consistent with less precise Lu-Hf data, and represents either garnet growth during subduction or resetting during exhumation. Our investigations of staurolite in amphibolites documented in the literature show that staurolite cannot form in equilibrium with amphibolitefacies parageneses in normal metabasic rocks, which always have metaluminous compositions. A two-stage process is required in which a peraluminous assemblage with kyanite and possibly chloritoid first forms, due to plagioclase-out reactions in eclogite-facies conditions. Staurolite forms in the second stage during exhumation as pressure decreases, in domains which are not in contact with the common amphibolite-facies assemblage, for example by hydration reactions involving kyanite and garnet or by breakdown of chloritoid at higher temperatures. The pressures estimated for garnet growth and the development of inclusions correspond to minimum depths of 75–83 km (for basaltic or granitic overburden) and the Bora°s Mafic Intrusion is an integral part of the Eastern Segment in which retrograde eclogite metabasic bodies occur within orthogneisses in at least four other localities. This implies that a major part of the Eastern Segment experienced a high-pressure metamorphic event and the entire block of continental crust was involved in a subduction-exhumation cycle during the Sveconorwegian orogeny.
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