Continental rifting is usually viewed in terms of two contrasting models of active and passive extension (McKenzie, 1978; Turcotte & Emerman, 1983; Schmid et al., 2022). In the active model lithospheric extension is caused by mantle processes (mantle convection, rising buoyant plumes, etc.) whereas in the passive model it is related to the reorganization of lithospheric plates or smaller block boundaries by remote tectonic stress. It is expected that the basaltic magmatism produced by the active model is primarily sourced from plume or plume-fed asthenosphere. Such magma could be contaminated by lithospheric components on its passage to the surface. In the passive model, mantle upwelling is related to lithospheric extension or upper mantle flow reorganization, for example, by shallow mantle upwelling from under thick lithospheric structures (Lebedev et al., 2006). Lithospheric sources for the magmatism are expected in the passive model. Thus, the composition of the basaltic magma may be key to resolving the active vs. passive model conundrum for rifts. In the past, both models have been applied to the Baikal Rift in the middle of the Eurasian continent (Figure 1).
Figure 1: Distribution of late Cenozoic (Oligocene to Recent) volcanic rocks in Central Asia and East Asia (Yarmolyuk et al., 2011). The Siberian Craton is after Donskaya (2020). The Tarim block & North China Craton are after Zheng et al. (2013). Major faults and displacements are after Petit & Déverchère (2006) and Arzhannikova et al. (2018). s
Figure 2: (a) Digital elevation model (DEM) of the Lake Baikal region, (b) DEM of the studied area between the Urik and Bolshaya Belaya rivers with late Cenozoic lava flows shown by green fields. The sampling sites are marked by yellow squares and circles in the Tuva-Mongolian massif and purple squares in the Siberian Craton. (c) Cross-section (A-B) intersecting the boundary between the Siberian Craton and the Tuva-Mongolian massif. The ages of the lavas are from Ivanov et al. (2015).
We focus on the Miocene volcanic rocks of the western Baikal Rift sampled along two 60-km-long profiles crossing the boundary between the Neoproterozoic Tuva-Mongolian massif and the Archean-Paleoproterozoic Siberian Craton (Figure 2a, b). There are two reasons for choosing this particular period and region. First, coeval magmas erupted through two lithospheric blocks with different histories and thus the role of the lithosphere can be inferred through analysis of time-integrated radiogenic isotope components. Second, the Miocene volcanism in the Baikal Rift was the most voluminous (Ivanov et al., 2015). The starting hypothesis was that lavas within the different lithospheric blocks might provide information on the heterogeneous lithosphere with different ages through which the magma passed to the surface. We used a set of coeval lava samples analysed for major elements, trace elements, and Sr–Nd–Hf–Pb–O isotopes.
Most of the samples studied are trachybasalts. In terms of trace element concentrations normalized to primitive mantle, the lavas mimic OIB-like patterns. According to the ratio of CaO to MgO, and TiO2/Al2O3 to SiO2 (Figure 3a, b), the compositions of the studied lavas coincide with experimental melts derived from mafic lithologies. Trace element data of samples suggest that garnet was a residual phase during partial melting.
Figure 3: (a) CaO vs. MgO, and (b) TiO2/Al2O3 vs. SiO2. Symbols as for Figure 2a. Grey circles: compositions for partial peridotite experimental melts, grey tringles: compositions for experimental eclogite-derived melts (Demonterova et al., 2023, Supplementary data, Table S3_Exper). (c) δ18O for olivine data vs. 87Sr/86Sr, and (d) εHf (b) of rock data. The isotopic mixing curves between the mantle source (M) with δ18O +5.8, εHf +15, 87Sr/86Sr 0.703, and crust (S) with δ18O +20, εHf -5, 87Sr/86Sr 0.71, modelling after Shcherbakov et al. (2022). Symbols as in Figure 2 .
Assimilation and recycling at mantle depths of crustal rocks are remarkably different with respect to fractionation of oxygen isotopes (Figure 3). Thus, the lack of co-variations of δ18O with 87Sr/86Sr (Figure 3c) and εHf (Figure 3d) unequivocally rule out crustal assimilation and are in agreement with the recycling of crustal lithologies at mantle depths.
Lavas from the Tuva-Mongolian massif (off-craton lavas) and the Siberian Craton differ in lead isotopes by lower values of 206Pb/204Pb (< 17.785) and higher values of Δ8/4Pb (61–75) for on-cratonic samples, and the reverse relationship for off-cratonic lava (> 17.785 and 55–61) respectively (Figure 4a). The correlation of lead isotopes with the mafic recycled component (Figure 4b), the sharp change of lead isotopic values at the cratonic boundary and decoupling of lead isotope ratios from other isotopic ratios lead us to suggest that the values of 206Pb/204Pb and Δ8/4Pb are associated with an ancient accessory mineral phase such as sulphide confined within the lithospheric mantle.
Figure 4. Variations of (a) Δ8/4Pb, (b), TiO2/Al2O3 for the lava samples in the area between the Urik and Bolshaya Belaya rivers relative to distance from the Main Sayan Fault in kilometres. The line of Δ8/4Pb equal to +60 is taken as a formal divider for DUPAL and non-DUPAL components. The equation Δ8/4Pb=[208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100 is from Hart (1984). Symbols for the studied lava are the same as in Figure 2.
In conclusion, Miocene magmas of the western Baikal Rift were derived from mafic lithologies located within lithospheric mantle. We do not see any contribution of plume or plume-fed asthenospheric sources. The predominant role of lithospheric sources in the formation of the Miocene volcanic rocks indicate that the volcanism of the Baikal Rift was caused by a passive tectonic process rather than active rifting.
References
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Ivanov, A.V., Demonterova, E.I., He, H., Perepelov, A.B., Travin, A.V. & Lebedev, V.A. (2015). Volcanism in the Baikal rift: 40 years of active-versus-passive model discussion. Earth-Science Reviews 148, 18–43.
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