JGR paper: The role of melt-rock interaction in the formation of Quaternary high-MgO potassic basalt
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Melt-rock interaction between ascending melt and peridotite results in mantle metasomatism and also leads to compositional modi fi cation of the primary melt. While this process is known to occur, it is less well understood how the reactions and the composition of the resulting magma temporally evolve. Here whole-rock major and trace element, Sr-Nd-Pb-Hf isotopes, and olivine major element composition of Quaternary Nuominhe basalts in the Greater Khingan Range of northeast China are presented to unravel how melt-rock interaction modified the composition of the high-MgO potassic basalts as time progressed. The Nuominhe basalts are predominantly basanite with high MgO (8.1 – 16.8 wt %) and high total alkali content (K2O + Na2O = 6.0 – 9.2 wt %). They have high K2O/Na2O ratios (K2O/Na2O = 0.77 – 1.24) and low SiO2 and Al2O3 content (SiO2 = 44.4 – 48.7 wt %, Al2O3 = 10.5 – 13.2 wt %). They are characterized by enrichment in strongly incompatible elements, positive Ba, K, and Sr and negative Th, U, Zr, Hf, and Ti anomalies, similar to the composition of enriched mantle (EM1)-type oceanic island basalts (OIBs). Their isotopic composition also compares to that of EM1-type OIBs (i.e., with 87Sr/86Sr = 0.70467 – 0.70483, ε Nd = - 4.1 to - 1.5, ε Hf = - 0.3 to 2.3, 206Pb/204Pb = 17.03 – 17.36). These elemental and isotopic characteristics are consistent with the interpretation that the potassium-rich melts were derived from recycled crustal materials with EM1 signature.

Phlogopite-bearing mantle xenoliths and zoned olivine xenocrysts with high Fo 89 – 92 and low CaO ( < 0.1 wt %) core and low Fo 75 – 86 and high CaO ( > 0.1 wt %) rim composition record interaction between the ascending melt and mantle peridotite. Basalts erupted during late stages (Late Pleistocene and Holocene) of activity at the Nuominhe volcanic field show notably higher SiO2 content, Rb/Nb, Ba/Nb, K/La, and Ba/La, and lower MgO content than early-stage basalts (Early and Middle Pleistocene), which we infer to re fl ect a temporally decreasing extent of melt-rock interaction. During early stages of melt ascent, a reaction zone between melt channels and unreacted peridotite formed; at later stages this reaction zone effectively sealed the ascending melt from further reaction,resultinginincreasingRb/Nb,Ba/Nb,K/La,andBa/Lasignaturesoftheerupted lavas.

Variations of the Nuominhe high-MgO potassic basaltic rocks with their eruptive episodes.