Rhönite in Cenozoic alkali basalt from Changle, Shandong Province, China, and its significance
2020
Abstract. Four Cenozoic, rhonite-bearing alkali-olivine basalt samples from the
Changle area (Shandong Province, China) show an intracontinental character
and were generated in an extensional setting. Petrographic studies document
different generations of rhonite. In three samples, rhonite occurs
either as a reaction product surrounding coarse-grained corundum, spinel and
phlogopite or along cleavage planes in phlogopite. In one sample
rhonite forms disseminated crystals in a mantle xenolith, possibly
formed by a reaction of coarse-grained orthopyroxene or spinel with a melt.
Rhonite exhibits a wide range of compositions: 22.9 wt %–33.0 wt %
SiO2 , 13.3 wt %–19.0 wt % Al2O3 , 9.4 wt %–19.9 wt % MgO and
10.210.2 wt %–24.5 wt % FeO. The derived primary substitutions include (1) Si IV + Na VII = (Al, Fe3+ ) IV + CaVII , (2) Mg VI = ( Fe2+ , Mn2+ ) VI and (3) Ti VI + (Mg + Fe2+ + Mn2+ ) VI = 2 Fe3+ VI . Rare-earth-element (REE) patterns of euhedral rhonite crystals from the mantle xenolith
(sample SS17) and those surrounding spinel (sample CL04) have a concave-upward shape
for the heavy rare-earth elements (HREEs) and are slightly enriched in the light rare-earth elements (LREEs). These patterns resemble
those of kaersutitic amphibole and kaersutite reported from basanite,
olivine nephelinite, transitional alkali-olivine basalt and hawaiite. In
contrast, REE patterns of the other two samples containing fine-grained,
anhedral and acicular rhonite crystals (samples CL01 and EGS03) are
relatively steep, with lower HREE and higher LREE abundances, similar to
those of ocean island basalts (OIBs). All types of Changle rhonite show
positive Nb, Ti and V anomalies in spidergrams normalized to primitive
mantle. Mineral assemblages of the studied samples indicate that rhonite
crystallized at different stages within a temperature range from about
950 to 1180 ∘ C and at pressures below 0.5 kbar, with
fO2 below the NNO buffer. The chemical composition of Changle
rhonite is interpreted to depend on the composition of the initial
silicate melt, the redox conditions during crystallization and the
composition of the minerals involved in reactions to form rhonite. Similar to metasomatic mantle amphibole, the compositions of Changle
rhonites cover the I-Amph (I-amphibole) and S-Amph (S-amphibole) fields, indicating that they may
have formed due to an intraplate metasomatic event, overprinting an older
metasomatic subduction episode.
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