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GEODYNAMICS & TECTONOPHYSICS
PUBLISHED BY THE INSTITUTE OF THE EARTH'S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES
2017 VOLUME 8 ISSUE 3 PAGES 465-469 ISSN 2078-502X
https://doi.org/10.5800/GT-2017-8-3-0263
Proceedings of the Second Russia-China International Meeting on the Central Asian Orogenic Belt (September 6-12, 2017, Irkutsk, Russia)
Petrography and mineralogy of retrograde metaperidotites from Alag Khadny accretionary wedge (SW Mongolia):
FLUID MODIFICATION IN SUPRASUBDUCTION ZONE
M. A. Gornova1, D. Enkhbat2, A. A. Karimov1, V. A. Belyaev1, 3, O. Gerel4, O. Javkhlan4
1 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of RAS, Irkutsk, Russia
2 Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia
3 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
4 Mongolian University of Science and Technology, Ulaanbaatar, Mongolia
For citation: Gornova M.A., Enkhbat D., Karimov A.A., Belyaev V.A., Gerel O., Javkhlan O., 2017. Petrography and mineralogy of retrograde metaperidotites from Alag Khadny accretionary wedge (SW Mongolia): fluid modification in suprasubduction zone. Geodynamics & Tectonophysics 8 (3), 465-469. doi:10.5800/GT-2017-8-3-0263.
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The Main Mongolian Lineament (MML) separates northern "Caledonian" tectonic province from southern "Hercynian" in SW part of Mongolia of the Central Asian Orogenic Belt (CAOB). The position of Eastern part of MML is widely discussed at recent time, since, this is an important for reconstruction of geodynamic evolution of this region. Some researchers suggest that ophiolite from the Erdene Uul and Maykhan Tsakhir Uul mountain ranges are Eastern part of an ophiolitic nappe system thrust northwards over the Dzabkhan-Baydrag continent, namely the Khantaishir and the Dariv ophio-lites [Stipskâ et al., 2010; Buriânek et al., 2017]. Others have a different view, they suggest that investigated ophiolites refers to Gobi-Altai ophiolite system (523±5
- 518±6 Ma), which likely formed in front of the Gobi Altai microcontinent by initiation of a new south-dipping subduction zone following arc-microcontinent collision in Northwest Mongolia [Jian et al., 2014]. However, ophiolites of this critical region of Mongolian the CAOB have not been investigated in detail.
Geological setting and petrography. The studied metaperidotites are situated in the area of the Maykhan Tsakhir Uul mountain range. After recent mapping by Czech geologists, the serpentinies were included into the Alag Khadny crystalline complex interpreted as an accretionary wedge [Hanzl, Aichler, 2007]. The Alag Khadny crystalline complex overlies Neoproterozoic crystalline basement rocks of the Zamtyn Nuruu Crys-
talline Complex. The Alag Khadny accretionary wedge forms 10 km long and up to 4 km wide E-W oriented belt. The northern part of the belt is built of orthogneiss bodies alternating with layers of mica schists and paragneisses. Retrogressed eclogites, marbles and serpentinites form lenses and boudins enclosed in mica schists and orthogneisses. The southern part of the belt contains large body of serpentinized peridotite rimmed by lenticular bodies of ophicarbonates. The matrix of the mélange is fragment of Neoproterozoic to Early Cambrian carbonate platform scrapped off the subducting plate during the mid-Cambrian accretion [Buriánek et al., 2017]. The eclogites geochemically correspond to T-MORB modified by fluid circulation and were metamorphosed at peak pressure-temperature (PT) conditions of 20-22.5 kbar and 590-610 °C [Stípská et al., 2010]. The subduction event is dated at ca. 538±20 Ma on zircon from orthogneisses [Buriánek et al., 2017].
The serpentinized peridotites are dominantly massive. Samples with LOI less than 9 wt. % show a diagnostic mineral assemblage of olivine + orthopyroxene + + tremolite + chlorite + spinel. Olivine (Ol1) grains are large and cut by serpentine veinlets. Ol2 is formed after Ol1 in veinlets. The rocks contain both large (up to 7 mm) and small (up to 300 |im) pseudomorphs after orthopyroxene, composed of tremolite + olivine3 + cli-nopyroxene2 + serpentine ± Cr-magnetite. In the centers of pseudomorphs there are relics of orthopyroxene with clinpyroxene lamellae. Olivine 3 from pseudomorphs forms small crystals 20-60 |im in size. There are three generations of clinopyroxene. Cpx1 composes irregular grains up to first hundred |im in size; it is located close to former orthopyroxene and does not have orthopyroxene lamellae. Cpx2 represents fine needlelike crystals in association with tremolite, formed after orthopyroxene. Cpx3 is irregular-shaped, up to 0.5 mm in size, and contains small olivine grains (up to 20 |im) and spinel; lamellae of orthopyroxene lack. Cpx3 develops after olivine and orthopyroxene pseudomorphs, as well as in cracks within olivine. Spinel grains are rounded and irregular, size is rather small with rare grains reaching 500-600 |im. Homogeneous central parts of grains are surrounded by thin ferrite-chromite rims, rim's thikness is wider in more serpentinized rocks. Chlorite occurs as coronas around metamorphosed chromian spinel. Flakes of chlorite are sometimes associated with tremolite and serpentine. Tremolite occurs as fine needle-like crystals after orthopyroxene and as prismatic crystals between olivine grains. Serpentine occurs either as needles or blades after both orthopyroxene and tremolite or as flame-like interpenetrating laths after olivine.
The rocks are tremolite-chlorite peridotites formed mainly after harzburgite with a porphyroclastic texture.
Fig. 1. Microanalysis profiles across spinel in Alag Khadny
metaperidotites.
Bulk-rock and mineral chemistry. Loss on ignition varies 5 to 13 wt. % in the rocks. Oxide contents varies: SiO2 44 to 46 wt. %., MgO 43 to 47 %, AI2O3 0.65 to 1.6 wt. %, CaO 0.26 to 3.01 wt. %, FeO 6 to 7.7 wt. %, TiO2 0.01 to 0.06 wt. %. Na2O and K2O abundances
Geodynamics & Tectonophysics 2017 Volume 8 Issue 3 Pages 465-469
0,9 -i
0,8 -
0,7 -
0,6 -
Q. CO
0,5-
° 0,4 J
0,3 -
0,2 -
0,1 -
m
O :->,, TS....,-
SSZ peridotites [Parkinson, Pearce, 1998]
o
Altered rims of spinel of metaperidotites low-amphibolite facies [Sântti et al., 2006]
Relict cores of spinel of metaperidotites low-amphibolite facies [Sântti et al., 2006]
Abyssal peridotites [Dick, Bullen, 1984]
• Core
O Rim
+ Small grain
Sample №
• 29
• 12
• 20
O 48
• 45
• 11
• 08
-1-1-1-1-1-r
0,8 0,6 0,4 0,2
Mg# Sp
I Fig. 2. Cr/(Cr+Al) vs. Mg/(Mg+Fe2+) of spinel from Alag
Khadny metaperidotites.
are below 0.1 and 0.01 wt. %, respectively. In the Alag Khadny metaperidotites MgO exceeds and AhO3, CaO, TiO2, Na2O and K2O are below primitive mantle abundances. The rocks show decrease in AhO3, CaO, TiO2 with increase in MgO, as would be expected in residues after varying melting degree. At the same time, CaO -AhO3 positive correlation lacks, and samples with similar AhO3 contents show large scatter in CaO. This indicates apparent gain of CaO during metamorphism. Based on MgO, AhO3 and TiO2 abundances, the proto-liths of the Alag Khadny metaperidotites were harz-burgites with intermediate depletion degree.
Central parts of spinel grains are homogeneous (Fig. 1) with Cr# 0.3-0.5, Mg# 0.65-0.50 and low Fe2O3 (1.6-4.6 wt. %), TiO2 (<0.1 wt. %), MnO (<0.3 wt. %)
and ZnO (<0.4 wt. %). Such composition is common for primary spinels of mantle residual peridotites. On Mg# - Cr# plot (Fig. 2), the studied spinels correspond to abyssal peridotites. The ferrite-chromite rims are more Cr# (0.80-0.99) with lower Mg# (<0.3), AhO3, MgO, and higher Fe2O3 (38-42 wt. %) and MnO (1-2 wt. %). During prograde metamorphism of peridotites in the greenschist facies, their spinels attain FeO, MnO, ZnO zoning due to release of these oxides from olivine and pyroxenes serpentinization. In the epidote-amphibolite facies, ferrite-chromite forms around the zonal spinel core because of AhO3, MgO loss and FeO, Fe2O3 gain. In the Alag Khadny metaperidotites the spinel cores are not zoned and thus ferrite-chromite rims formation is not related to prograde regional epidote-facies metamorphism, but is caused by the retrograde metamor-phic event.
Olivine 1 has MgO (0.90-0.92) and NiO (0.31-0.45) abundances similar to residual mantle olivines testifying their primary nature (Fig. 3). Olivine 2 and olivine 3 contain 0.81-0.87 and 0.85-0.89 wt. % MgO, 0.02-0.44 and 0.33-0.48 wt. % NiO, respectively, and have a me-tamorphic genesis. In the Mg#Ol - Cr#Sp space (Fig. 4), compositions of Ol1 and Al-Cr Sp of the Alan-Hadny metaperidotites are located within the olivine-spinel mantle array and abyssal peridotite field, arguing for residual origin.
Orthopyroxenes are enstatite with high Mg#s (0.910.92), Al2O3 and Cr2O3 contents are 1.7-2.6 wt. % and 0.33-0.59 wt. %, respectively. Such compositions are common for orthopyroxenes in alpine-type peridotites. Presence of clinopyroxene lamellae within orthopyro-xene also points at magmatic genesis of the orthopy-roxene.
Clinopyroxenes of all the three generations are di-opsides of similar composition with a narrow range of Mg#, 0.94-0.97. They have contents of AhO3 (1.0-2.4 wt. %) and Cr2O3 (0.35-0.77 wt. %) similar to those in orthopyroxenes and tremolites. Cpx 1 is considered as a retrogressive product of clinopyroxene lamellae in orthopyroxenes. Cpx 2 and Cpx 3 could have formed during metamorphism, and Cpx 3 in course of Ca addition.
Tremolites have high Mg# (0.98), appreciable amounts of AhO3 (1.5-2.1 wt. %), Na2O (1.1-2.4 wt. %) and Cr2O3 (0.31-0.52). High AhO3 and Cr2O3 are probably derived from precursor orthopyroxene. They are similar to retrograde tremolites in terms of Na-Si relationship [Khedr etal., 2010].
P-T conditions of the Alag Khadny peridotite and geodynamic implications. The Alag Khadny peri-dotite preserved both primary magmatic paragenesis (olivine1 + orthopyroxene + Cr-Al spinel) and meta-morphic associations (chlorite + tremolite + olivine 2 + + olivine 3 + clinopyroxene 2 + clinopyroxene 3 + serpentine + ferrite-chromite rims of spinels).
0,5 — 0,4 —
oN <p
5 0,3 —
O iz
0,2 —
0,95
| Fig. 3. Mg/(Mg+Fe2+) vs. NiO (wt. %) content for olivine of the Alag Khadny metaperidotites.
According to primary mineral compositions, the Alag Khadny primary harzburgites are residual after 13-23 % of melting of depleted mantle (DM) during basalt formation in mid-ocean ridges, forearc or back-arc basins.
Petrographic observations and mineral compositions argue for the retrograde metamorphism of the rocks. Water addition to anhydrous harzburgites could trigger secondary mineral formation as a result of reactions:
Ol1 + En + Sp ^ Chl, En + Di + H2 O ^ Tr + Fo (Ol3), Tr + Fo + H2 O +Ca ^ Di (Cpx2, Cpx3) + Atg. Association olivine + orthopyroxene + tremolite + chlorite without anthophilite and talc is stable at 640750 °C and 16-20 kbar [Khedr et al., 2010, Fig. 12; Padron-Navarta et al., 2010]. The pressure range corresponds to spinel facies of the mantle, but temperature estimates are lower compared to the ocean geotherm. Lower temperature can be caused by water addition. Middle-temperature (400-700 °C) metamorphism of abyssal peridotites as result downward seawater flow alteration refers to retrograde type. But it happens at lower part of oceanic earth crust (P<5 kbar) during the mantle rise up and characterized by Ca elution [Bazylev et al., 1990]. The Alag Khadny metaperidotites are associated with eclogites and have P-T conditions of met-amorphism similar to the latter. Therefore, it is more probable that Alag Khadny peridotites (olivine1 + orthopyroxene + Cr-Al spinel) were in mantle wedge
above the subduction zone. The subducted metamorphosed oceanic crust experienced dehydration could be the source of water. The water penetrated mantle
1
0,9 0,8 0,7
0,6
Q.
(/)
0,5
o
0,4
0,3 0,2 0,1
0,96 0,94 0,92 0,9 0,88 0,86 0,84
Mg#OI
Fig. 4. Olivine-spinel mantle array (OSMA) of the Alag Khadny metaperidotites.
Geodynamlcs & Tectonophyslcs 2017 Volume S Issue 3 Pages 465-469
wedge peridotites leading to formation of metaperidotites containing water-bearing minerals. Finally, the Alag Khadny metaperidotites together with eclogites were exhumed into the crust.
Acknowledgements. The study was supported by the Russian Foundation for Basic Research, Project 1605-44038 Mong_a.
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