TY - JOUR

T1 - Mobility of elements in a continental subduction zone

T2 - Evidence from the UHP metamorphic complex of the Kokchetav massif

AU - Shatsky, V. S.

AU - Skuzovatov, S. Yu

AU - Ragozin, A. L.

AU - Sobolev, N. V.

PY - 2015/7/1

Y1 - 2015/7/1

N2 - We studied clastics of high-alumina garnet-kyanite-mica schists and garnet-kyanite-quartz granofelses, including diamond-bearing ones, found in the eluvial sediments near Lake Barchi. In contents of major elements the studied rocks correspond to argillaceous shales. The garnet-kyanite-quartz granofelses are poorer in K (0.49-1.35wt.% K2O) than the garnet-kyanite-mica schists (4.9-2.2wt.% K2O) but have the same contents of other major components. The REE patterns of most of the garnet-kyanite-phengite schists are similar to those of the Post-Archean Australian Shale (PAAS) (xLa/Yb=13). All garnet-kyanite-quartz rocks are much stronger depleted in LREE (xLa/Yb=1.4) and other incompatible elements. Our studies show that allanite and monazite are the main concentrators of LREE and Th in the garnet-kyanite-phengite rocks of the Barchi site. Monazite, occurring as inclusions in garnet, contains not only LREE but also Th, U, and Pb. Rutile of the nondepleted rocks is enriched in Fe and Nb impurities only. The garnet-kyanite-quartz granofelses bear rutile, apatite, and xenotime as accessory phases. Rutile of the depleted rocks shows wide variations in contents of Nb, Ta, and V impurities. In places, the contents of Nb and Ta reach 10.5 and 2.3wt.%, respectively. The rutile decomposes into rutile with Nb (1.4wt.%) and Fe (0.87wt.%) impurities and titanium oxide rich in Fe (6.61wt.%), Nb (up to 20.8wt.%), and Ta (up to 2.81%) impurities. Based on the measured contents of incompatible elements in differently depleted high-alumina rocks, the following series of element mobility during UHP metamorphism has been established: Th>Ce>La>Pr>Nd>K>Ba>Rb>Cs>Sm>Eu. The contents of U, P, and Zr in the depleted rocks are similar to those in the nondepleted rocks. The studies have shown that metapelites subducted to the depths with diamond stability conditions can be depleted to different degrees. This might be either due to their exhumation from different depths of the subduction zone or to the presence of an external source of water controlling the temperature of dissolution of phengite and the formation of supercritical fluid/melt.

AB - We studied clastics of high-alumina garnet-kyanite-mica schists and garnet-kyanite-quartz granofelses, including diamond-bearing ones, found in the eluvial sediments near Lake Barchi. In contents of major elements the studied rocks correspond to argillaceous shales. The garnet-kyanite-quartz granofelses are poorer in K (0.49-1.35wt.% K2O) than the garnet-kyanite-mica schists (4.9-2.2wt.% K2O) but have the same contents of other major components. The REE patterns of most of the garnet-kyanite-phengite schists are similar to those of the Post-Archean Australian Shale (PAAS) (xLa/Yb=13). All garnet-kyanite-quartz rocks are much stronger depleted in LREE (xLa/Yb=1.4) and other incompatible elements. Our studies show that allanite and monazite are the main concentrators of LREE and Th in the garnet-kyanite-phengite rocks of the Barchi site. Monazite, occurring as inclusions in garnet, contains not only LREE but also Th, U, and Pb. Rutile of the nondepleted rocks is enriched in Fe and Nb impurities only. The garnet-kyanite-quartz granofelses bear rutile, apatite, and xenotime as accessory phases. Rutile of the depleted rocks shows wide variations in contents of Nb, Ta, and V impurities. In places, the contents of Nb and Ta reach 10.5 and 2.3wt.%, respectively. The rutile decomposes into rutile with Nb (1.4wt.%) and Fe (0.87wt.%) impurities and titanium oxide rich in Fe (6.61wt.%), Nb (up to 20.8wt.%), and Ta (up to 2.81%) impurities. Based on the measured contents of incompatible elements in differently depleted high-alumina rocks, the following series of element mobility during UHP metamorphism has been established: Th>Ce>La>Pr>Nd>K>Ba>Rb>Cs>Sm>Eu. The contents of U, P, and Zr in the depleted rocks are similar to those in the nondepleted rocks. The studies have shown that metapelites subducted to the depths with diamond stability conditions can be depleted to different degrees. This might be either due to their exhumation from different depths of the subduction zone or to the presence of an external source of water controlling the temperature of dissolution of phengite and the formation of supercritical fluid/melt.

KW - Continental subduction

KW - Mobility of elements

KW - UHP metamorphism

UR - http://www.scopus.com/inward/record.url?scp=84936881915&partnerID=8YFLogxK

U2 - 10.1016/j.rgg.2015.06.004

DO - 10.1016/j.rgg.2015.06.004

M3 - Article

AN - SCOPUS:84936881915

VL - 56

SP - 1016

EP - 1034

JO - Russian Geology and Geophysics

JF - Russian Geology and Geophysics

SN - 1068-7971

IS - 7

ER -