Suprasubduction forearc gabbro of the Duushkunnug massif (Tuva): unusual geochemical composition and the problem of geodynamic interpretation

This article presents evidence in favor of the autochthonous (no accretion) nature of the Duushkunnug gabbro massif, occurring in the forearc Tuvan segment Vendian - Early Cambrian island arc system of the Altai-Sayan folded area. Also shown is an attempt to justify the geodynamic nature of these gabbros. The Duushkunnug gabbro massif represents one of the bodies of the basement highs Khemchik-Systyghem collisional deflection, formed in place forearc structure. The size of massif is 300x130 m, it is oriented in the latitudinal direction along the axis of the anticline. A massif is protrusive body among the Silurian shales, as evidenced by the crumpled in microsludge slates in his contacts. The content of amphibole up to 10%, magmatic hornblende forms rims around grains of clinopy-roxene. Most of the mafic minerals are usually replaced by actinolite and chlorite. Plagicolase of basic composition is half replaced by saussurite. The age of gabbro according to Ar-Ar dating of hornblendite is 537,5±4,9 Ma. Gabbros are characterized by moderate contents TiO₂ - 1,2-1,7 wt. %, Al₂O₃ - 14 wt. %, Na₂O 2,4-2,7 wt. %, K₂O - 0,1-0,2 wt. %, P₂O₅ 0,1 wt. % moderately fractionated character of distribution of rare elements: (Th/Yb)_n = 2,5 content of REE at the level of the chondrite 30-11 units, a slight predominance of the LREE over the HREE: (La/Yb)n = 1,9-2,1 a positive Pb anomaly and slight negative Nb anomaly on spidergram. Slight negative Nb anomaly, a positive Pb anomaly, a small predominance of LREE over HREE - all of these symptoms collectively indicate the certain number of subduction component in the mantle source of the gabbro. Along with that, by the discrimination diagrams the compositions of the gabbros fall in the field of both island arc tholeiites and E-MORB, or are intermediate between basalts of island arcs and oceanic plateaus. From gabbroids Irbiteu massif in the Tannu-Ola island arc subzone (their age 539±6 Ma years; s_Nd(T) = +7,8) [Mongush et all, 20116] they are relatively high titanium and low-alumina content, weakly pronounced negative Nb anomaly, a lower value of s_Nd(T) = +5,8. Our data on the geologic position, age, and, in part, the geochemical composition of the gabbro indicate that Duushkun-nug gabbro massif on its geodynamic position can be considered as supra-subduction forearc intrusive. Enriched relative to island-arc gabbroids chemical composition of gabbro of the Duushkunnug massif was probably due to the fact that at the turn of the Vendian and Cambrian were still active, albeit in small scale, mantle source or sources of the initial basalts. Under the initial basalts refers to the first igneous rocks, formed at the stage of initiation of subduction in the result of decompression melting of the mantle. The results of our research can be considered as a development of hypotheses about the initial stages of subduction, described in the articles [Reagan et al., 2010; Dilek, Furnes, 2011; Whattam, Stern, 2011].

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