Abstract
Abstract. Four Cenozoic, rhönite-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 rhönite. In three samples, rhönite occurs
either as a reaction product surrounding coarse-grained corundum, spinel and
phlogopite or along cleavage planes in phlogopite. In one sample
rhönite forms disseminated crystals in a mantle xenolith, possibly
formed by a reaction of coarse-grained orthopyroxene or spinel with a melt.
Rhönite 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) SiIV + NaVII = (Al, Fe3+)IV + CaVII, (2) MgVI = (Fe2+, Mn2+)VI and (3) TiVI + (Mg + Fe2+ + Mn2+)VI = 2Fe3+VI. Rare-earth-element (REE) patterns of euhedral rhönite 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 rhönite 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 rhönite show
positive Nb, Ti and V anomalies in spidergrams normalized to primitive
mantle. Mineral assemblages of the studied samples indicate that rhönite
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
rhönite 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 rhönite. Similar to metasomatic mantle amphibole, the compositions of Changle
rhönites 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.