Elena I. Demonterova

Formation history of the Tuva-Mongolian Massif (Western Hubsugul region, North Mongolia) based on U-Pb dating of detrital zircons from sandstone of the Darkhat group by the LA-ICP-MS method

1498 The Tuva–Mongolian Massif was defined in 1971 as a block with an Early Precambrian basement and Vendian–Cambrian carbonate cover of the platform type [1]. Subsequently, the contours of the massif were changed substantially and its dimensions were reduced [2]. The present day boundaries correspond to the contour outlining the distribution area of the Ven dian–Cambrian carbonate cover [3, 4] (Fig. 1). In the south, the Tuva–Mongolian Massif is separated by a fault from the Dzabkhan Massif. Formerly, these two massifs formed a single block, which is evident from the development of similar carbonate rocks in their covers [3]. In this connection, the validity of the Tuva–Mongolian Massif as a geological structure that developed against the background of surrounding younger Paleozoic complexes is beyond doubt [5]. The age of blocks constituting the basement of the massif, as well as the history of their amalgamation and rela tion with other blocks, remains insufficiently known despite the availability of many special publications dedicated to these aspects [2–4, 6–9]. In this commu nication, we present the data on ages of zircons from sandstone of the Darkhat Group underlying the Ven dian–Cambrian carbonate rocks on the eastern mar gin of the central Tuva–Mongolian Massif. It appeared that these detrital zircons adequately char acterize the provenance and reflect practically the entire pre Vendian formation history of the massif.

Tectonics of the Baikal Rift Deduced from Volcanism and Sedimentation: A Review Oriented to the Baikal and Hovsgol Lake Systems

As known from inland sedimentary records, boreholes, and geophysical data, the initiation of the Baikal rift basins began as early as the Eocene. Dating of volcanic rocks on the rift shoulders indicates that volcanism started later, in the Early Miocene or probably in the Late Oligocene. Prominent tectonic uplift took place at about 20 Ma, but information (from both sediments and volcanics) on the initial stage of the rifting is scarce and incomplete. A comprehensive record of sedimentation derived from two stacked boreholes drilled at the submerged Akademichesky ridge indicates that the deep freshwater Lake Baikal existed for at least 8.4 Ma, while the exact formation of the lake in its roughly present-day shape and volume is unknown. Four important events of tectonic/environmental changes at about approximately 7, approximately 5, approximately 2.5, and approximately 0.1 Ma are seen in that record. The first event probably corresponds to a stage of rift propagation from the historical center towards the wings of the rift system. Rifting in the Hovsgol area was initiated at about this time. The event of ~5 Ma is a likely candidate for the boundary between slow and fast stages of rifting. It is reflected in a drastic change of sedimentation rate due to isolation of the Akademichesky ridge from the central and northern Lake Baikal basins. The youngest event of 0.1 Ma is reflected by the (87)0Sr/ (86)Sr ratio increase in Lake Baikal waters and probably related to an increasing rate of mountain growth (and hence erosion) resulting from glacial rebounding. The latter is responsible for the reorganization of the outflow pattern with the termination of the paleo-Manzurka outlet and the formation of the Angara outlet. The event of approximately 2.5 Ma is reflected in the decrease of the (87)Sr/(86)Sr and Na/Al ratios in Lake Baikal waters. We suggest that it is associated with a decrease of the dust load due to a reorganization of the atmospheric circulations in Mainland Asia. All these tectonic and climatic events could (and actually did) influence the biota of Lake Baikal. The Hovsgol rift basin was shaped to its recent form between 5.5 and 0.4 Ma. However, freshwater Lake Hovsgol appeared only in the latest pre-Holocene time as a result of meltwater inflow and increase of atmospheric precipitations during the Bølling-Allerød warming. Prior to this, a significantly smaller, saline outflow-free precursor of Lake Hovsgol existed. It explains why two, now connected, lakes of similar water chemistry within similar climatic and tectonic conditions differ so much in their biodiversity.

Lithospheric Control on Late Cenozoic Magmatism at the Boundary of the Tuva-Mongolian Massif, Khubsugul Area, Northern Mongolia

Late Cenozoic lavas from the western wall of the Khubsugul rift trough were erupted within the Tuva-Mongolian Massif with a pre-Vendian basement, and the lavas in the eastern wall of the trough were erupted within Early Caledonian terranes. The composition of the lavas was determined to vary across the strike of the boundary of the Tuva-Mongolian Massif. The western wall of the trough is dominated by hawaiites and contains subordinate volumes of basanites and much lower amounts of olivine tholeiites and basaltic trachyandesites. The eastern wall contains, in addition to hawaiites, widespread olivine tholeiites and basaltic andesites with subordinate amounts of basaltic trachyandesites. The boundary zone contains practically all rock types (except basaltic andesites) in roughly equal proportions. The trace-element simulations of the partial melting processes demonstrates that the basaltic magmas were produced mainly by 0.5–5% partial melting of garnet lherzolite, with the probable mixing with partial melts derived from spinel lherzolite. The main factor controlling the compositional variations of the lavas was likely the variable depths of their derivation due to variations in the lithosphere thickness at the boundary of the Tuva-Mongolian Massif. Based on the assumption that the source of the magmas was relatively homogeneous and on the results of simulations with the use of experimental data on peridotite melting, we concluded that the asthenospheric sources of the basaltic magmas occurred at depths of 75 ± 10 km (24.6 ± 3.2 kbar) beneath the Tuva-Mongolian Massif and at 60 ± 12 km (20.1 ± 3.8 kbar) beneath the Early Caledonian terranes.

Catastrophic outburst and tsunami flooding of Lake Baikal: U–Pb detrital zircon provenance study of the Palaeo-Manzurka megaflood sediments

ABSTRACT Lake Baikal, the largest freshwater reservoir on Earth (~600 × 30 km in size and up to 1.6 km in depth), has more than 300 contributing rivers but only one N-trending outflow – River Angara. In the Pliocene or Pleistocene, another N-trending outflow operated through the Palaeo-Manzurka to Lena. Provenance analysis using U–Pb dating of detrital zircons from the Palaeo-Manzurka sediments demonstrates that the dominant source of the zircons was the lake deposits, while the contribution of zircons from local bedrocks was limited to about 8% only. Looking for an explanation of this, we propose a hypothesis that formation of the Palaeo-Manzurka sediments took place in association with a catastrophic mega-landslide (~15 × 3 km) into the lake and the resulting mega-tsunami flooding.

New carbonatite complex in the western Baikal area, southern Siberian craton: Mineralogy, age, geochemistry, and petrogenesis

A dike–vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite–ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable 40Ar–39Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO2 concentrations and is rich in K2O (K2O/Na2O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite–ankerite carbonatite), TiO2, P2O5 (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)CN ratios of the metapicrite (4.5–11) and carbonatite (4.5–17) testify that their source contained residual garnet, and the high K2O/Na2O ratios of the metapicrite (9–15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.

Extension in the Baikal rift and the depth of basalt magma generation

It is considered that the opening of the Baikal riftsystem occurred as a result of counterclockwise rotation of the Amur microplate relative to stable SiberianCraton [1]. The pole of rotation is located on thenortheastern termination of the rift system in the areaof the Chara depression [1, 2] (Fig. 1). According togravimetrical data, it was established that the value ofthe extension increases linearly from the pole of rotation towards the South Baikal depression [2]. Therecent data of the seismic profiling across the SouthBaikal depression show that under this depression therise of the Moho boundary is absent. However, in thebottom parts of the Earth’s crust, a highvelocityanomaly, interpreted as a series of deep mafic intrusions, is established [3]. The Crustal extension is compensated by magma intrusion, and not its thinning [3].At the same time, within the largest Baikal depressions, volcanism manifestations [4] do not occur (Fig. 1).Thus, the problem of interrelation of volcanism andrifting still remains open. In this work, the depth offormation of primary basalt magmas under differentvolcanic fields of the Baikal Rift System is determinedon the basis of the chemical composition of volcanicproducts. It is shown that between the depth of magmageneration and the crustal extension value a reversecorrelation exists; i.e., the stronger the extension, thelower the depth that testifies that rifting processes control volcanism. The role of the inflow of deep mantlematter in generation of basalt magmas is also considered.In the northeastern Baikal rift system, there are tworelatively small Late Cenozoic volcanic fields (~100 kmin diameter), the Udokan and the Vitim (Fig. 1). Theformer is situated on the southern slope of the Charadepression; the latter one, in the area of small depressions outside the axial zone of the rift system. In thesouthwestern rift system, the Late Cenozoic volcanismwas manifested on the vast territory (more 450 km indiameter) within the depressions (Tunka and Khubsugul), on their slopes (as an example, on the KhamarDaban Ridge), as well as in areas without visible extension structures (the East Sayan Ridge).The calculation of the pressure of generation of primary magma was carried out using the proceduredescribed in [5] with insufficient modifications, whichinclude the following: (1) the assumption was usedthat the magma source is similar in chemical composition to enriched KLB–1 lherzolite, but not to the intermediate composition between KLB–1 and HK–66lherzolites; (2) the correction for olivine and clinopyroxene fractionation was not made. The first calculation modification was made on account of the comparison of calculated pressure values obtained formantle xenoliths and the host basalts of the Oka highland [6]. The second modification is connected withnonessential change in the calculated values for compositions of real basalts and compositions, corrected forolivine and clinopyroxene fractionation [5]; i.e., it simplifies the calculation procedure. Finally, the equationwas used based on experimental data for melting ofKLB–1 lherzolite [7]:

Petrographic-geochemical types of Triassic alkaline ultramafic rocks in the Northern Anabar province, Yakutia, Russia

A classification suggested for alkaline ultramafic rocks of the Ary-Mastakh and Staraya Rechka fields, Northern Anabar Shield, is based on the modal mineralogical composition of the rocks and the chemical compositions of their rock-forming and accessory minerals. Within the framework of this classification, the rocks are indentified as orangeite and alkaline ultramafic lamprophyres: aillikite and damtjernite. To estimate how much contamination with the host rocks has modified their composition when the diatremes were formed, the pyroclastic rocks were studied that abound in xenogenic material (which is rich in SiO2, Al2O3, K2O, Rb, Pb, and occasionally also Ba) at relatively low (La/Yb)PM, (La/Sm)PM, and not as much also (Sm/Zr)PM and (La/Nb)PM ratios. The isotopic composition of the rocks suggests that the very first melt portions were of asthenospheric nature. The distribution of trace elements and REE indicates that one of the leading factors that controlled the diversity of the mineralogical composition of the rocks and the broad variations in their isotopic–geochemical and geochemical characteristics was asthenosphere–lithosphere interaction when the melts of the alkaline ultramafic rocks were derived. The melting processes involved metasomatic vein-hosted assemblages of carbonate and potassic hydrous composition (of the MARID type). The alkaline ultramafic rocks whose geochemistry reflects the contributions of enriched vein assemblages to the lithospheric source material, occur in the northern Anabar Shield closer to the boundary between the Khapchan and Daldyn terranes. The evolution of the aillikite melts during their ascent through the lithospheric mantle could give rise to damtjernite generation and was associated with the separation of a C–H–O fluid phase. Our data allowed us to distinguish the evolutionary episodes of the magma-generating zone during the origin of the Triassic alkaline ultramafic rocks in the northern Anabar Shield.

The first mineralogical, geochemical, and isotope-geochronological data on neogene alkaline basaltic volcanism of the Heven Zalu Uriin Sar’dag Plateau (Northern Mongolia)

The petrological and geochemical investigations of volcanism of inland rift areas are aimed at determina� tion of the sources of magmatic melts and conditions of magma formation, as well as to provide additional information for development of the models of state and interaction of the lithospheric mantle and sub� lithospheric mantle on the various stages of their geo� logical evolution. In this paper we report the first iso� tope geochronological, mineralogical, and geochemi� cal data on the volcanic Heven Zalu Uriin Sar’dag Plateau in Northern Mongolia composing the eastern sector of the Khubsugul Neogene volcanic area. This volcanic area is of significant interest in the study of the Early Neogene stage of the tectonomagmatic evo� lution of the Baikal rift zone.

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

Neogene (N12-N21?) 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 (LaN/YbN = 10.8−12.6, DyN/YbN = 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.

40Ar/39Ar age of the onset of high-Ti phase of the Emeishan volcanism strengthens the link with the end-Guadalupian mass extinction

ABSTRACT Precise time constraints of the main extrusive phase of the Emeishan large igneous province (ELIP) remain unresolved because basalts commonly do not contain suitable minerals for U–Pb dating, whereas previous 40Ar/39Ar studies on basalts yielded tectonothermal overprint ages. The timing for the ELIP was deduced from indirect dating of minor intrusions of ultramafic/mafic and felsic compositions by geochronological methods and geological correlations. The extrusive part of the ELIP consists of an older low-Ti and younger high-Ti basalt phases. We have found fresh samples of plagioclase-phyric rocks at the lower Qiaojia extrusive section (the Yunnan province of China), which belong to the ELIP unit of the high-Ti basalt series. 40Ar/39Ar dating on plagioclase from two samples conducted at two different laboratories using different age standards yielded statistically indistinguishable results with the weighted mean age of 260.1 ± 1.2 Ma for five individual measurements. This provides the direct constraints on the onset of the ELIP high-Ti basalt extrusive phase. The obtained age is within the error or slightly older than the age of the Guadalupian–Lopingian boundary and felsic ignimbrite capping the ELIP lava succession (both dated at 259.1 ± 0.5 Ma). Our new data are strengthening the short duration of the, at least, high-Ti phase of the ELIP volcanism and its temporal link with the end-Guadalupian mass extinction. Estimation of the total duration of the ELIP volcanism awaits finding of suitable for dating low-Ti basalts.