Lithos: Petrological evidence for magma recharge and mixing......in NE China(Jian-qiang Liu)

2021-05-07
26

      Geochemical compositions of rocks from monogenetic volcanoes in intraplate settings are commonly used toconstrain their mantle source and melting processes; however, the subsequent magmatic evolution of suchsmall-volume volcanic systems is commonly ignored.

     

      Here we present petrographic observations, mineral chemistry, whole-rock major and trace element data, and thermodynamic modeling for Quaternary ultrapotassic rocks from the Ma'anshan monogenetic volcano of Xiaogulihe in Northeast China. We use these data to evaluate the crystallization conditions and recharge and mixing processes that the magmas have experienced within thecrust. The Ma'anshan volcanic products are predominantly phonotephrite and tephriphonolite. These rocks have small variations in geochemical composition with moderate SiO2 (47.29–49.4 wt%), high total alkali contents (K2O + Na2O = 9.85–11.87 wt%), and high K2O/Na2O ratios (3.18–4.21). Two types of assemblages were identified in these ultrapotassic rocks: Type-1 antecrysts (olivine and leucite) and hybrid crystals (clinopyroxene, formed by incongruent replacement of antecrysts) and type-2 autocrysts (olivine and leucite). For the type-2, normally zoned olivines, Ni decreases and Mn increases more rapidly with decreasing Fo% than for oceanic basalts, which can be explained by abundant olivine and leucite fractionation in strongly alkaline melts at shallow crustal levels (<200 MPa). The type-1 olivines are reversely zoned and their cores have lower Fo, CaO, and Ni, and higher Mn, than their rims. The type-1 leucites are sieve-textured, partially resorbed, and rimmed by type-1 olivines. The type-1 clinopyroxenes have diopsidic compositions, forming monomineralic crystal clusters. Although the type-1 antecrystic and hybrid crystals are rare, their presence provides robust evidence for magma recharge and mixing beneath the Ma'anshan monogenetic volcano.


      This study shows that magmas erupted inmonogenetic volcanic fields may experience complex processes within the crust despite their small volumes and relatively uniform compositions, which is important to consider in the interpretation of the construction, evolution, and longevity of their magma plumbing systems.

   

Fig1:Phenocrysts from the Xiaogulihe ultrapotassic rocks


Fig2:Schematic diagrams showing our magma recharge and mixing model beneath the Ma'anshan monogenetic volcano of Xiaogulihe