Mineral chemistry from the Late Cretaceous peralkaline rhyolite at Hadjer el Khamis, Chad: constraints on magmatic conditions

J. Gregory Shellnutt*, Yoshiyuki Iizuka, Tung Yi Lee, Wei Yu Chen

*此作品的通信作者

研究成果: 雜誌貢獻期刊論文同行評審

摘要

The Late Cretaceous (74.4 ± 1.3 Ma) peralkaline rhyolite from Hadjer el Khamis erupted during a period of rifting within the Bornu and Termit basins of the Lake Chad region. The rhyolites are representative of a residual silicic magma that was derived by fractional crystallization of mantle derived basaltic parental magma. The primary phenocrysts of the rocks are quartz, alkali feldspar, arfvedsonite, aegirine, hedenbergite, and ilmenite. From the whole rock and mineral compositions, we conclude that the pre-eruptive temperatures were between 800 °C and 900 °C, but probably closer to 850 °C to 900 °C. The Fe-rich mafic mineral compositions and absence of magnetite indicate that the relative oxidation state of the melt was below the fayalite-magnetite-quartz buffer. The estimated relative oxidation state is calculated to range from ∆FMQ –1.1 to ∆FMQ – 2.8. The water content of the magma was likely close to or at saturation (i.e. H2O ≈ 3.2 wt%) whereas the fluorine content was likely less than saturation (i.e. Fmeltwt% < 0.6 wt%) due to the absence of magmatic fluo-rite. The pre-eruptive magma was likely volatile-rich, but underwent a period of significant degassing prior to or during eruption that led to a drier lava with higher viscosity, lower effusivity that allowed for slow cooling, the development of columnar joints, and the crystallization of hypersolvus feldspar.

原文英語
頁(從 - 到)25-44
頁數20
期刊Neues Jahrbuch fur Mineralogie, Abhandlungen
198
發行號1
DOIs
出版狀態已發佈 - 2022 9月

ASJC Scopus subject areas

  • 地球化學與岩石學

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