Miocene Volcanism of the Baikal Rift across the Boundary of the Siberian Craton: Evidence for Lithospheric Mantle Melting

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Journal of Petrology Pub Date : 2023-08-22 DOI:10.1093/petrology/egad062
E. Demonterova, A. Ivanov, V. Savatenkov, M. Chu, S. V. Panteeva, Hao-Yang Lee, I. Bindeman
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Abstract

Continental rifting is usually viewed in terms of two contrasting models of active and passive extension. The origin of the Baikal Rift, adjacent to the southern part of the Siberian Craton, has been described by both models in the past. It is expected that basaltic magmatism in an active model scenario should be primarily sourced from a mantle plume or plume-fed asthenosphere, whereas melting of the lithospheric mantle is expected to be a predominant source for magmatism in the passive model. In this paper, we focus on the Miocene volcanic rocks sampled along two 60 km-long profiles that cross the boundary between the Neoproterozoic Tuva-Mongolian massif and the Archean-Paleoproterozoic Siberian Craton. Most of the samples studied are trachybasalts. In terms of trace element concentrations normalized to primitive mantle, the lavas mimic OIB-like patterns with troughs at Rb, Th–U, Pb, and Y, and peaks at Ba, Nb, Ta, K, and Sr. Moreover, similar trace-element patterns to the studied samples are also observed for Miocene and Quaternary lavas located in the southwestern of the Baikal Rift, and adjacent regions of non-rifted Mongolia. According to the ratio of CaO to MgO, and TiO2/Al2O3 to SiO2, 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. The Sr-Nd isotopic characteristics of the studied lavas are 87Sr/86Sr 0.70427–0.70469 and 143Nd/144Nd 0.51267–0.51284. They are identical to the coeval Miocene lavas of neighbouring volcanic fields, but they differ from the Quaternary lavas that extend to lower 87Sr/86Sr (0.7038–0.7044) with near identical 143Nd/144Nd. Isotopes of Hf for studied samples show values εHf = 6.0–7.7, except for the two samples taken within the boundary between two lithospheric blocks with εHf 4.6 and 4.8. The δ18O of olivine from lava samples is everywhere higher than that of the asthenospheric mantle and ranges from 5.5 to 6.4‰. Variations of δ18O versus Mg#, 87Sr/86Sr and εHf in the studied samples do not correlate, but do unequivocally rule out crustal assimilation. The isotopic variations are consistent with recycling of mafic crustal lithologies at mantle depths. Lavas from the Tuva-Mongolian massif 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. The equation for Δ8/4Pb=[208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100 is from (Hart, 1984). The correlation of lead isotopes with the mafic recycled component, 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. The predominant role of the 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.
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西伯利亚克拉通边界贝加尔湖裂谷的中新世火山作用:岩石圈地幔融化的证据
大陆裂谷作用通常分为主动伸展和被动伸展两种截然不同的模式。贝加尔湖裂谷的起源毗邻西伯利亚克拉通的南部,在过去已经被两种模型所描述。在活动模式下,玄武岩岩浆活动应主要来自地幔柱或由地幔柱供给的软流圈,而在被动模式下,岩石圈地幔的熔融预计是岩浆活动的主要来源。本文研究了横跨新元古代图瓦-蒙古地块与太古宙-古元古代西伯利亚克拉通边界的两条60 km长剖面的中新世火山岩样本。研究的大多数样品都是粗面玄武岩。在将微量元素浓度与原始地幔进行规定化的过程中,贝加尔湖裂谷西南部和蒙古非裂谷地区中新世和第四纪熔岩的微量元素模式与研究样品相似,在Rb、Th-U、Pb和Y处有低谷,在Ba、Nb、Ta、K和sr处有峰值。根据CaO / MgO和TiO2/Al2O3 / SiO2的比值,所研究熔岩的组成与基性岩性的实验熔体一致。样品的微量元素数据表明,石榴石是部分熔融过程中的残留相。火山岩的Sr-Nd同位素特征分别为87Sr/86Sr 0.70427 ~ 0.70469和143Nd/144Nd 0.51267 ~ 0.51284。它们与邻近火山场的中新世熔岩相同,但与第四纪熔岩不同,第四纪熔岩延伸至87Sr/86Sr(0.7038 ~ 0.7044)较低,143Nd/144Nd接近相同。研究样品的Hf同位素εHf值为6.0 ~ 7.7,除两个岩石圈块体边界内的样品εHf值分别为4.6和4.8外。岩浆样品中橄榄石的δ18O值普遍高于软流圈地幔的δ18O值,在5.5 ~ 6.4‰之间。研究样品中δ18O与mg#、87Sr/86Sr和εHf的变化不相关,但明确排除了地壳同化的可能性。同位素变化与地幔深部基性地壳岩性的再循环一致。图瓦-蒙古地块和西伯利亚克拉通火山岩的铅同位素差异表现为:非克拉通火山岩206Pb/204Pb值较低(< 17.785),Δ8/4Pb值较高(61 ~ 75);非克拉通火山岩火山岩(> 17.785和55 ~ 61)的铅同位素差异表现为相反关系。公式Δ8/4Pb=[208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100来自(Hart, 1984)。铅同位素与基性再循环组分的对比、克拉通边界处铅同位素值的急剧变化以及铅同位素比值与其他同位素比值的解耦表明,206Pb/204Pb和Δ8/4Pb的值与岩石圈地幔内的硫化物等古代辅助矿物相有关。岩石圈源在中新世火山岩形成中的主导作用表明,贝加尔湖裂谷的火山作用是被动构造作用,而不是主动裂陷作用。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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