The oldest (~ 1.9 Ga) metadolerites of the southern Siberian craton: age,petrogenesis, and tectonic setting

Geological, geochronological, and isotope-geochemical studies of the metadolerites of the Angaul complex, widespread in the Urik-Iya graben of the southern Siberian craton, were carried out. The metadolerites forming separate conformal bodies (sills) among the metasandstones of the Ingash Formation were studied in detail. U-Pb zircon (SHRIMP) dating of metadolerites yielded an age of 1913 ± 24 Ma, and U-Pb baddeleyite (ID-TIMS) dating of these rocks yielded an age of 1914.0 ± 1.7 Ma. Thus, the date of 1914 ± 2 Ma can be taken as the most precise age estimate for the studied rocks. The metadolerites of the Angaul complex correspond in chemical composition to the normal-alkaline tholeiitic basalts. Metadolerites are differentiated rocks with mg# of 36 to 58. They show fractionated REE patterns: (La/Yb)_n = 1.2-3.5. All metadolerites, independently of their mg# value, have low contents of Nb (1.6-10.2 ppm) and show well-pronounced negative Nb-Ta anomalies in multielement patterns (Nb/Nb* = 0.19-0.54). The metadolerites are characterized by positive ε_Nd(T) values ranging from 0.4 to 5.2, which correlate well with their SiO₂ content and mg# value. The isotope-geochemical parameters of the metadolerites of the Angaul complex indicate that fractional crystallization, along with the assimilation of the host rocks (AFC), might have been the main process during the formation of the most differentiated metadolerites. The geochemical characteristics of metadolerites with the maximum mg# values of 57-58 and ε_Nd(T) = 5.2 suggest that the parental mantle source of the metadolerites resulted from mixing of predominant depleted mantle material with the subcontinental-lithosphere material. Intrusion of the dolerites of the Angaul complex, as well as the deposition of the sedimentary strata of the Ingash Group, took place at the Paleoproterozoic stage of intracontinental extension caused by the collapse of the orogen resulted from the collision of the Biryusa block with the Tunguska superterrane in the southern Siberian craton.     

Low-Ti melts from the southeastern Siberian Traps Large Igneous Province: Evidence for a water-rich mantle source?

Siberian Traps Large Igneous Province (STLIP) is one of the most voluminous volcanic provinces on Earth. The dominant erupted rocks are low-Ti basalts, which make up 80% by volume of the classical Noril’sk lava sequence. In the west Siberian basin and Maymecha-Kotuy area, the low-Ti basalts make up about 99% and 50% by volume, respectively. Dolerite sills in the Angara-Taseevskaya Syncline at the southeastern STLIP exhibit trace element patterns and Sr isotope ratios typical of the low-Ti basalts of the Noril’sk sequence. The most Mg-rich (MgO 9.5–11 wt%) and hence least differentiated dolerites are characterized by trace element patterns with Ta-Nb depletion, low Ce/Pb and high Sr/Pr. These trace element features are similar to water-saturated, mantle wedge-derived island arc basalts. These imply an important role of subduction fluid-derived trace elements in the source of melting beneath the Angara-Taseevskaya Syncline and other regions of the STLIP. Less magnesium rocks (MgO 3.8–6.1 wt%) with less prominent Ta-Nb depletion, higher Ce/Pb and lower Sr/Pr could be produced via olivine-plagioclase fractionation of primary high-magnesium melts.     

Sr and Nd isotope composition of deformed peridotite xenoliths from Udachnaya kimberlite pipe

New results of Rb–Sr and Sm–Nd isotope analyses have been obtained on samples of deformed peridotite xenoliths collected from the Udachnaya kimberlite pipe (Yakutia). The data obtained imply two main stages of metasomatic alteration of the lithospheric mantle base matter in the central part of the Siberian Craton. Elevated ratios of Sr isotopes may be considered as evidence of an ancient stage of metasomatic enrichment by a carbonatite melt. The acquired Nd isotope composition together with the geochemistry of the deformed peridotite xenoliths suggests that the second stage of metasomatic alteration took place shortly before formation of the kimberlite melt. The metasomatic agent of this stage had a silicate character and arrived from an asthenosphere source, common for the normal OIB type (PREMA) and the Group-I kimberlite.

     

Platinum‐group elements and rhenium in mantle xenoliths from the East Sayan volcanic field (Siberia,Russia): evaluation of melt extraction and refertilization processes in lithospheric mantle of the Tuva‐Mongolian massif

We determined the concentrations of platinum‐group elements (PGE) and rhenium in granular spinel lherzolites entrained as xenoliths in the Late Cenozoic volcanic rocks of the East Sayan within the Sarkhoi palaeo‐arc block of the Tuva‐Mongolian massif. Major element, PGE and rhenium variations in the East Sayan xenoliths can be explained by impregnation of up to 15% of arc‐type melt into initially depleted mantle harzburgite. Such a refertilization process probably took place in the Middle Neoproterozoic, when the Tuva‐Mongolian massif was in a subduction environment. East Sayan xenoliths show close similarities to Vitim xenoliths, whose host basalts erupted within an off‐cratonic crustal block. Both East Sayan and Vitim xenoliths are different from cratonic and circum‐cratonic peridotite xenoliths of worldwide localities.     

Segmental closure of the Mongol-Okhotsk Ocean: Insight from detrital geochronology in the East Transbaikalia Basin

The Late Paleozoic–Early Mesozoic Mongol-Okhotsk Ocean extended between the Siberian and Amur–North China continents.The timing and modalities of the oceanic closure are widely discussed.It is largely accepted that the ocean closed in a scissor-like manner from southwest to northeast(in modern coordinates),though the timing of this process remains uncertain.Recent studies have shown that both western(West Transbaikalia)and eastern(Dzhagda)parts of the ocean closed almost simultaneously at the Early–Middle Jurassic boundary.However,little information on the key central part of the oceanic suture zone is available.We performed U-Pb(LA-ICP-MS)dating of detrital zircon from wellcharacterized stratigraphic sections of the central part of the Mongol-Okhotsk suture zone.These include the initial marine and final continental sequences of the East Transbaikalia Basin,deposited on the northern Argun-Idemeg terrane basement.We provide new stratigraphic ages for the marine and continental deposits.This revised chronostratigraphy allows assigning an age of~165–155 Ma,to the collisionrelated flexure of the northern Argun-Idemeg terrane and the development of a peripheral foreland basin.This collisional process took place 5 to10 million years later than in the western and eastern parts of the ocean.We demonstrate that the northern Argun-Idemeg terrane was the last block to collide with the Siberian continent,challenging the widely supported scissor-like model of closure of the MongolOkhotsk Ocean.Different segments of the ocean closed independently,depending on the initial shape of the paleo continental margins.     

Neogene basanites in western Kamchatka: Mineralogy,geochemistry, and geodynamic setting

Neogene (N ₁ ² -N ₂ ¹ ?) K-Na alkaline rocks were found in western Kamchatka as a subvolcanic basanite body at Mount Khukhch. The basanites have a microphyric texture with olivine phenocrysts in a fine-grained doleritic groundmass. The olivine contains inclusions of Al-Cr spinel. The microlites consist of clinopyroxene, plagioclase, magnetite, and apatite, and the interstitial phases are leucite, nepheline, and analcime. The Mount Khukhch basanites are characterized by elevated concentrations of MgO, TiO₂, Na₂O, and K₂O, high concentrations of Co, Ni, Cr, Nb, Ta, Th, U, LREE (La_N/Yb_N = 10.8?12.6, Dy_N/Yb_N = 1.4?1.6) at moderate concentrations of Zr, Hf, Rb, Ba, Sr, Pb, and Cu. The values of indicator trace-element ratios suggest that basanites in western Kamchatka affiliate with the group of basaltoids of the within-plate geochemical type: Ba/Nb = 10?12, Sr/Nb = 17?18, Ta/Yb = 1.3?1.6. The basanites of western Kamchatka show many compositional similarities with the Miocene basanites of eastern Kamchatka, basanites of some continental rifts, and basalts of oceanic islands (OIB). The geochemistry of these rocks suggests that the basanite magma was derived via the ～6% partial melting of garnet-bearing peridotite source material. The crystallization temperatures of the first liquidus phases (olivine and spinel) in the parental basanite melt (1372–1369°C) and pressures determined for the conditions of the “mantle” equilibrium of the melt (25–26 kbar) are consistent with the model for the derivation of basanite magma at the garnet depth facies in the mantle. The geodynamic environment in which Neogene alkaline basaltic magmas occur in western Kamchatka was controlled by the termination of the Oligocene—Early Miocene subduction of the Kula oceanic plate beneath the continental margin of Kamchatka and the development of rifting processes in its rear zone. The deep faulting of the lithosphere and decompression-induced magma generation simultaneous with mantle heating at that time could be favorable for the derivation of mantle basite magmas.     

Age of Granulites of the Cheremshanskaya Unit (Sharyzhalgai Uplift,Siberian Craton,Russia): New Data

Doklady Earth Sciences - The present paper presents new U–Pb and Sm–Nd age data obtained for granulites of the Cheremshanskaya Unit enclosing metamorphosed volcano-sedimentary sulfide...