Rhönite in Cenozoic alkali basalt from Changle, Shandong Province, China, and its significance

Fan-Mei Kong,Xiao-Han Liu,Ling-Quan Zhao,Hans-Peter Schertl,Xu-Ping Li

doi:10.5194/ejm-32-325-2020

Fan-Mei Kong, Xiao-Han Liu + Show 3 more

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https://doi.org/10.5194/ejm-32-325-2020

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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.

Highlights

High-Ti minerals discovered in the Cenozoic alkali basalts of the Changle area (Shandong Province, China) were shown to be rhönite by Johnston and Stout (1985)
The official name according to the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) is rhönite and refers to the Rhön, a low mountain range in Germany found along the border region between Bavaria, Hesse and Thuringia
Later studies showed that rhönite is an aluminosilicate that is isostructural with aenigmatite (Bonaccorsi et al, 1990; Nédli and Tóth, 2003; Grew et al, 2008; Anan’ev et al, 2011), Grew et al (2008) considered it to be a member of the sapphirine supergroup

Summary

Introduction

High-Ti minerals discovered in the Cenozoic alkali basalts of the Changle area (Shandong Province, China) were shown to be rhönite by Johnston and Stout (1985). The mineral name is spelled in different ways, such as rhoenite (Johnston and Stout, 1985; Grützner et al, 2013), rhonite (Anan’ev and Selyangin, 2011) or rhönite (Kunzmann, 1989, 1999; Grapes et al, 2003; Treiman, 2008; Grapes and Keller, 2010; Sharygin et al, 2011). Brögger (1890) suggested that aenigmatite (later shown to be isostructural with rhönite) could be a triclinic member of the amphibole group; a crystallographic study later indicated that this was not the case (Palache, 1933). Later studies showed that rhönite is an aluminosilicate that is isostructural with aenigmatite (Bonaccorsi et al, 1990; Nédli and Tóth, 2003; Grew et al, 2008; Anan’ev et al, 2011), Grew et al (2008) considered it to be a member of the sapphirine supergroup. Later studies showed that rhönite is an aluminosilicate that is isostructural with aenigmatite (Bonaccorsi et al, 1990; Nédli and Tóth, 2003; Grew et al, 2008; Anan’ev et al, 2011), Grew et al (2008) considered it to be a member of the sapphirine supergroup. Sharygin et al (2011) showed that rhönite has a broad range in XMg (Mg / (Mg + Fe2+))

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