Abstract
In the Wudalianchi volcanic field, eruptions started with low-Mg potassic lava flows 2.5–2.0 Ma ago and later changed to both low- and moderate-Mg potassic compositions. Volcanic rocks from the Molabushan and Longmenshan volcanoes record an unusually wide range of Pb abundances (from 3.7 ppm to 21 ppm relative to predominant range of 10–15 ppm). To determine the cause of these, we performed a comparative trace-element and Pb isotope study of rocks from these volcanoes and older lava flows. On a uranogenic lead diagram, older low-Mg lavas from lithospheric mantle sources plot on a secondary isochron with a slope corresponding to an age of 1.88 Ga. This contrasts with moderate-Mg volcanic rocks from the Molabushan cone, interpreted to have been derived from a recent convective mantle source, which define a flat linear pattern. Low-Mg rocks from the Molabushan flow have lead isotopic compositions that indicate mixed Gelaqiu and Molabu sources. Relative to rocks from the Molabushan cone, moderate-Mg lavas and slags from the East Longmenshan volcano have modified compositions characterized by Pb, S, and Ni abundances, Ni/Co, Ni/MgO ratios as well as 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, Ce/Pb, Th/Pb, and U/Pb ratios. We infer that the older Wudalianchi magmas were likely derived from a Paleoproterozoic lithospheric fragment, related to the evolved primordial mantle, and that later magmas were generated in the convecting mantle. These were influenced by segregation of small amounts of sulfides.
Highlights
In the northern part of the Songliao Basin, potassic volcanic rocks define a chain of volcanic fields known as “the WEK zone” [1,2] and “theWudalianchi zone” [3]
The aim of this paper was to determine how sulfides were extracted from magmas beneath the Molabushan and Longmenshan volcanoes using trace elements and Pb isotopic compositions of volcanic rocks
The younger Molabushan and Longmenshan volcanoes had low-Mg units derived through melting of the Gelaqiu-like source material
Summary
In the northern part of the Songliao Basin, potassic volcanic rocks define a chain of volcanic fields (from south to north: Erkeshan, Wudalianchi, Keluo) known as “the WEK zone” [1,2] and “theWudalianchi zone” [3]. The potassic rocks have been related to an enriched post-Archean continental lithospheric mantle comparable to the OIB (ocean island basalt) end-member EM1 (Enriched Mantle 1). New data on rocks from the Molabushan and East Longmenshan volcanoes of the Wudalianchi area reveal an unusually wide range of Pb abundances from 3.7 ppm to 21 ppm with respect to a predominant range of 10–15 ppm (Table S1)
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