S. Yu. Skuzovatov

Evidence of neoproterosoic continental subduction in the Baikal-Muya fold belt

The results of isotope-geochemical studies of eclogits and host rocks of the North-Muya block are presented. The studies showed broad variations both in the character of distribution of incompatible elements and in the Nd and Sr isotope composition of eclogits from the North-Muya block. The Nd isotope composition of eclogits is characterized by broad variations, which is reflected in the value of ɛNd(T), which has both positive (from +0.3 to +6.9) and negative values (from −0.5 to −16.8). The isotope characteristics for the both samples of eclogits (Mu 12-11, 12-12) with the lowest values of ɛNd(T) clearly indicate protolith contamination by an ancient source of Meso- or Paleoarchean age. Consequently, the melts of the protoliths of the eclogites intruded into the continental crust, and the eclogite-gneiss complex of the North-Muya block may be considered as a paleozone of the continental subduction.

Sources of clastic material of the Neoproterozoic metasedimentary rocks of the Baikal–Patom belt, Northern Transbaikalia: Evidence from Sm-Nd isotope data

First systematic data on the variations of εNd(T) in the Neoproterozoic sedimentary sequence of the Baikal–Patom fold belt (Northern Transbaikalia) are reported. The range of obtained εNd(T) is–19.4…–2.0. It was established that the rocks of the Ballaganach and most part of the Dal’nyaya Taiga groups are characterized by εNd(T) from–19.4 to–16.3. Upsection, beginning from the Khomolkha Formation, the rocks show a sharp change in the initial Nd isotope composition (εNd(T)–8.3…–2.0). The results of Sm-Nd study of metasedimentary rocks of the BPB, as well as the comparison of their Sm-Nd characteristics with those of the inferred source areas suggest that the input of terrigenous material at the early stage of sedimentation in the Baikal–Patom belt was mainly related to the destruction of the Early Proterozoic crust of the Siberian craton. Owing to a change of sedimentation setting from passive continental margin to the “foreland” basin at the late stage, the terrigenous material of the Siberian craton was diluted by clastics from juvenile Neoproterozoic crust of the Baikal–Muya belt.

Geodynamic background for large-scale mineralization in the southern environs of the Siberian Craton in the Proterozoic

The geodynamic position of the productive Kevaktinskii and Dal’netaiginskii stratigraphic units with Au–U mineralization is considered within the Baikal–Patom highland. The Au–U specialization of rocks of the Kevaktinskaya Group results from its formation in the environments of the passive continental margin with introduction of the juvenile material related to riftogenesis and further collision, whereas the same specialization of the Dal’netaiginskaya Group is explained by exhalation activity in the back-arc spreading zone. The formation of most of the Au and U large-scale deposits within the southern environs of the Siberian Craton was controlled by Paleozoic oogenesis upon closure of the Paleoasian Ocean.

Isotopic (Sm–Nd, Pb–Pb, and δ34S) and Geochemical Characteristics of the Metasedimentary Rocks of the Baikal–Patom Belt (Northern Transbaikalia) and Evolution of the Sedimentary Basin in the Neoproterozoic

This paper reports the results of a detailed isotopic (Sm–Nd, Pb–Pb, and δ34S) and geochemical studies of Neoproterozoic metasedimentary rocks from the Patom and Bodaibo domains of the Baikal–Patom belt (northern Transbaikalia). It was shown that the metasedimentary rocks of these domains are strongly variable in their geochemical and isotope geochemical characteristics. Regular variations in these characteristics were observed, and their correlation with the main stages of the evolution of the sedimentary paleobasin in the Neoproterozoic was established.

Isotope–Geochemical Evidence for the Nature of Protolite Eclogite of the Kokchetav Massif (Kazakhstan)

In the present paper, the results of our isotope–geochemical studies on eclogites of the ultrahighpressure metamorphic complex of the Kokchetav massif are reported. The fact that the distribution of nonmobile elements in most of the samples was close to that of E-type MORB basalts is shown by using geochemical multielement diagrams normalized to N-MORB. Six samples were found to have a negative anomaly over niobium that may have resulted from contamination with crustal material. For eclogites of the Kokchetav massif, the 147Sm/144Nd ratio was found to range widely from 0.143 to 0.367. The εNd-values calculated for the age of the highly barometric stage of metamorphism (530 million years) varied from–10.3 to +8.1. Eclogites show a dispersion of model ages from 1.95 billion years to 670 million years. On the graphs in the εNd(T)–87Sr/86Sr and εNd(T)–T coordinates, eclogites were shown to form trends that can be interpreted as a result of contamination of the eclogite protolith by the host rocks. Based on the data obtained, it is proposed that the basalts of rift zones that may have geochemical characteristics of N-MORB basalts and at the same time may be contaminated by the continental crust may have served as proxies for eclogite protoliths of the Kokchetav massif.

GEOCHRONOLOGY AND SR-ND ISOTOPE GEOCHEMISTRY OF LATE PALEOZOIC COLLISIONAL GRANITOIDS OF UNDINSKY COMPLEX (EASTERN TRANSBAIKAL REGION)

There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [Şengor et al., 1993, Zorin, 1999; Parfenov et al., 1999, 2003; Willem et al., 2012; and others]. The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB. All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time. Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleocontinent.

GEOCHEMISTRY, ZIRCON U-PB GEOCHRONOLOGY, ND-HF ISOTOPIC CHARACTERISTICS AND TECTONIC IMPLICATIONS OF THE SOUTH MUYA BLOCK METASEDIMENTS (NORTHEASTERN CENTRAL ASIAN OROGENIC BELT)

The Neoproterozoic to Cenozoic collage of the Central Asian Orogenic Belt is well-known to include Precambrian continental blocks and microcontinents traditionally attributed to rifting of Siberia or Gondwana prior to CAOB assembly that significantly contributed into the geochemical and isotopic composition of younger subduction- and accretion-related crustal lithologies via processes of crust-mantle interaction and crustal recycling.

High-pressure mafic granulites of the South Muya Block (Central Asian Orogenic Belt)

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.