Multistage Magmatism in Ophiolites and Associated Metavolcanites of the Ulan-Sar’dag Mélange (East Sayan, Russia)

Abstract

We present new whole-rock major and trace element, mineral chemistry, and U-Pb isotope data for the Ulan-Sar’dag mélange, including different lithostratigraphic units: Ophiolitic, mafic rocks and metavolcanites. The Ulan-Sar’dag mélange comprises of a seafloor and island-arc system of remnants of the Paleo-Asian ocean. Detailed studies on the magmatic rocks led to the discovery of a rock association that possesses differing geochemical signatures within the studied area. The Ulan-Sar’dag mélange includes blocks of mantle peridotite, podiform chromitite, cumulate rocks, deep-water siliceous chert, and metavolcanic rocks of the Ilchir suite. The ophiolitic unit shows overturned pseudostratigraphy. The nappe of mantle tectonites is thrusted over the volcanic-sedimentary sequence of the Ilchir suite. The metavolcanic series consist of basic, intermediate, and alkaline rocks. The mantle peridotite and cumulate rocks formed in a supra-subduction zone environment. The mafic and metavolcanic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc calc-alkaline andesitic basalts, andesites, and dacites; (3) tholeiitic basalts of mid-ocean ridges; and (4) oceanic island basalts. U–Pb dating of zircons from the trachyandesite, belonging to the second geochemical type, yielded a date of 833 ± 4 Ma which is interpreted as the crystallization age during mature island-arc and intra-arc rifting stages. The possible influence of later plume magmatic-hydrothermal activities led to the appearance of moderately alkaline igneous rocks (monzogabbro, trachybasalt, trachyandesite, subalkaline gabbro, and metasomatized peridotites) with a significant subduction geochemical fingerprint.

Nd-Sr Isotopic Study of Magmatic Rocks and 40Ar/39Ar Dating of the Mafic Dike of the Proterozoic Ulan-Sar’dag Ophiolite Mélange (Southern Siberia, East Sayan, Middle Belt, Russia)

“…Major geological units of eastern Tuva-Sayan, modified from [37] with new information from the authors [26]. Captions of the ophiolite massifs: 1-Dunzhugur, 2-Holbin-Hairhan, 3anorthern branch of Ospa-Kitoi, 3b-southern branch of Ospa-Kitoi, 4-Ulan-Sar'dag, 5-Ehe-Shigna Shishkhid branch.…”

“…Based on the geochemical data, we recently discussed the Ulan-Sar'dag massif composed of magmatic rocks with different geochemical signatures [26], which formed due to multistage magmatism, according to the evolution of the subduction zone in the studied segment of CAOB.…”

“…The units are tectonically obducted on the Gargan block. The age of the trachyandesite from the volcanogenic sequence of the USM determined by U-Pb dating of the zircon is 833 ± 4 Ma [26]. The Ulan-Sar'dag massif is a lenticular body elongated in an eastwest direction with dimensions of 2 × 5 km 2 .…”

“…It occurs as an ophiolitic mélange, composed of various structurally mixed different ages, origins, lithostratigraphic units. These units are represented by ophiolitic altered ultramafic-mafic rocks with podiform chromitites, foliated serpentinites, deep sea sedimentary cherts, shaled volcanic-sedimentary (Ilchir suite) rocks with MORB, island-arc and OIB-like geochemical affinities, mafic dikes, and limestones (Irkut suite) (Figures 3 and 4) [24][25][26]. The volcanic-sedimentary slice is composed of two units: lower-metavolcanic unit and upper-black, black-quartz-carbonate schists and marbles.…”

“…Generally, geochemical signatures for the magmatic rocks associated with Eastern Sayan ophiolites reveal a wide compositional series: mid-ocean ridge basalt (MORB)-type basalts (tholeiites), boninites, island-arc andesites and dacites, and ocean island basalt (OIB)-type basite rocks. There are numerous isotopic data of the island arc assemblage and ophiolites in a different segment of CAOB, but isotope geochemical data of Proterozoic Eastern Sayan ophiolites are limited [4,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. [35,36].…”