Литосфера (Oct 2017)
Metabasites of Middle Transangaria, Yenisei Ridge: E-MORB relicts of Neoproterozoic lithosphere
Abstract
This work presents first results on40Ar/39Ar dating by stepwise heating method, as well as geochemistry of rare trace elements by ICP-MS and isotopic (Nd, Sr) composition of metabasites from tributaries basins of Velmo and Bolshoi Pit rivers (Transangaria, Yenisei ridge). The studied rocks form small areas of concordant boudined plate-shape amphibolite bodies among Late Archaean marbles, calciphyres, schists, and are derived from a metapicrite-basalt complex. They have schistose and banded structure and consist of hornblende with andesine, biotite, zoisite, carbonate, quartz, as well as accessory аpatite, sphene, and ilmenite.Mineral paragenesis corresponds to the conditions of low-temperature metamorphism of amphibolite facies.Chemical composition of rocks (wt %): 44-49 SiO2, 2-4 Na2O + K2O, 1.1-1.8 TiO2, 12-17 Fe2O3, 8-11 CaO, 7-11 MgO; FeO(t)/MgO 1-2 correspond to basalts and trachybasalts of the tholeiitic series from the ocean floor. Rock-forming age of amphibole is Late Precambrian (≈700 Ma) that matches early stages of Paleo Asian Ocean development. According to distribution of LILE (ppm) (≈20-1000 Ba,≈100-635 Sr) and HFSE (ppm) (46-83 REE, 4-10 Nb, 0.3-0.7 Ta, 30-90 Zr, 0.6-1.1 Th, ≈0.2 U), studied rocks coincide with tholeiitic E-MORB, which formed in the setting of back-arc spreading and had enriched asthenospheric source. Isotopic specifics: εNd(t)from +3.6 to -5.2; TNdDM ≈ 1.4-2.2 Ga; 0.7046-0.715487Sr/86Sr(t)) indicate that mantle diapirism could have been accompanied by mixing of plume, subduction and crust (DMM + PREMA + EM) material. High concentrations of HREE (LaN /YbN 1-3, LREE/HREE - 2.2-3.2) and Y allow us to assume absence of restite garnet and extraction of primary tholeiitic E-MORB magma under setting/conditions of ≈ 4-20 % equilibrium melting of spinel lherzolite from upper mantle.
