Melting and melt transport processes in mantle conditions revealed from Voykar Ophiolite, Polar Urals.
Belousov I.A.*, Batanova V.G.*, Sobolev A.V.*,** and Savelieva G.N.***
* Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia;
** Max-Planck Institute fur Chemie, Mainz, Germany; *** Geological Institute RAS, Moscow, Russia.
Voykar Ophiolite is located in the northern part of Uralian Ophiolite Belt. Mantle section rocks comprise most of the ophiolitic sequence. It is up to 6-8 km thick and consists mostly of spinel harzburgites with multiple dunite bodies and pyroxenite veins. Dunites form large bodies (up to several hundred meters thick) marking channels of focused porous flow of olivine-saturated melts (Savelieva et al., 2008). Chromitites are associated with dunites. Age determination of zircons from the chromitites revealed that formation of dunite bodies was associated with magmatic event in the mantle 585±6 mln years ago (Savelieva et al., 2007). Pyroxenites form veins crosscutting dunite bodies and residual harzburgites. Mineral composition of pyroxenite veins vary from clinopyroxenites to orthopyroxenites. Morphology and thickness of pyroxenitic veins are rather diverse (Belousov et al., 2009). Earliest generation of pyroxenite veins crosscutting dunites have diopside–enstatite composition from 1–2 to 50 cm thick. Thin (2–5 cm) diopsidite veins are the most abundant in dunite and sometimes may replace thin dunite veins along the strike. Dark green zoned websterite veins are the latest. Swarms of thick (0.5–2.5 m) pegmatoid pyroxenite veins were traced for about 3 km from large dunite body located in Hoila riverhead. Most of pyroxenites have coarse-grained and pegmatitic structures and have reactional contacts with residual harzburgites. Together with clino- and orthopyroxenes (Cpx, Оpx) most pyroxenites contain magmatic high-Al amphibole (Amf). Some clinopyroxenites and websterites contain olivine.
Mineral compositions reflect conditions and peculiarities of formation of different rock types. Temperatures calculated from two pyroxene thermometer (Wells, 1977) are 830-1000ºС (on average 900ºС) for harzburgites and pyroxenites and reflect process of mineral re-equilibration during cooling. Real temperature and pressure estimates could be made based on stability field of magmatic amphibole during hydrous peridotite melting (Grove et al., 2006): 900-1010 ºС and 0.7-1.7 GPa. Chromian spinels from harzburgites have #Cr (Cr/Cr+Al) 0.25-0.5. Chromian spinels from dunites have higher #Cr (0.34-0.82). Redox conditions, calculated based on compositions of coexisting olivines and spinels (Ballhaus et al., 1991) for temperatures of 1000ºС and pressures of 1.4 GPa, are typical for supra-subduction zone (SSZ) environment – 0-1.5 log units above FMQ buffer for harzburgites and 0.6-2.9 log units above FMQ for dunites.
Heavy REE contents in clinopyroxenes from harzburgites from Voykar massif refer to depleted part of abyssal peridotite field. However, clinopyroxenes from almost all samples are enriched in LREE and Sr due to refertilization of harzburgites by migrating SSZ melts. Highest degrees of refertilization of harzburgites are near contacts with pyroxenite veins. Samples of harzburgites located far from contacts with pyroxenite veins have low degrees of refertilization. Composition of Cpx from the least refertilized sample of harzburgite could be reproduced my modeling. REE contents in these Cpx are best fitted by the model of fractional melting of MORB source. Melting started in garnet stability field (8%) and proceeded in spinel stability field (8-10%) (Batanova et al., in press). Compositions of clinopyroxenes from the rest of harzburgites could be reproduced by additional modeling of chromatographic effect during porous melt percolation (Navon & Stolper, 1987), or open-system melting (1-3%) with melt inflow (Ozawa & Shimizu, 1995). During these processes composition of residual clinopyroxene was enriched in LREE and depleted in HREE relative to primary composition. Chondrite-normalized REE contents in Cpx from dunites are in general more enriched relative to Cpx from harzburgites. Clinopyroxenes from pyroxenites have sub parallel trace element patterns, which are similar to those of Cpx phenocryst from sample of high-Ca boninite from Troodos upper pillow lavas. Across harzburgite-pyroxenite vein contact clinopyroxenes display decrease in Mg-number, HREE and Zr content and increase in LREE and Sr content.
Magnesium hornblendes from pyroxenites are in equilibrium with clinopyroxenes. They have pronounced enrichments of such fluid mobile elements as Rb, Ba, Sr and Pb, which are testifying their formation in SSZ environment (Belousov et al., 2009). Their compositions reflect range of compositions from relatively enriched in REE (MORB level) to highly depleted in REE and enriched in LILE at the same time, reflecting change from melt-like to fluid-like composition of agent.
Therefore, mantle section rocks of Voykar Ophiolite reflect influence of melting and melt/fluid migration processes. Clinopyroxene compositions from harzburgites reflect processes of ancient melting and processes of melt infiltration and enrichment in SSZ setting. Dunites are produced by interaction with olivine-saturated melts. Large dunite bodies are representing channels of focused migration of such melts. Pyroxenite veins were formed as the result of reactional interaction of high-SiO2 fluid-saturated SSZ melts with residual peridotites. Migration of such melts (fluids) could be in cracks with pyroxenite edge formation due to interaction with residual peridotite.
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