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Petrology and geochemistry of an unusual tridymite-hercynite xenolith in tholeiite from southeastern China

Petrologie und Geochemie eines ungewöhnlichen Tridymit-Hercynit Xenolithen in Tholeiit aus Südost-China

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Summary

A rare variety of Fe-Al-rich xenolith, tridymite-hercynite rock, was entrained in the Cenozoic basalt from Niutoushan, southeastern China. The tridymite-hercynite rock consists of hercynite (40–50%), tridymite (35–45%), ilmenite (≈2%), and glass (10–20%). Mineral grains are smaller than 0.05 mm with a granular texture. Compared to “normal” igneous rocks, this tridymite-hercynite rock has higher FeO, Al2O3, and lower SiO2 CaO, MgO, and alkalis (Na2O + K2O).

The origin of this type of xenolith is problematical. Based upon the available mineralogical and chemical data, three possible origins have been evaluated:

  1. (1)

    The product of an extreme tholeiitic fractionation under low oxidation conditions; but the major element geochemistry appears to negate this model.

  2. (2)

    The Fe-rich (Si-poor portion of silicate-immiscible liquids from an extremely fractionated residual tholeiite magma; however, the corresponding Si-rich end-member fraction is missing, and the composition of the tridymite-hercynite rock is different from that of any Si-poor melt phase of known immiscible silicate liquids.

  3. (3)

    The product of metamorphism, including partial to complete melting, of a sedimentary protolith. An Fe-Al-rich shale (quartz 5%, kaolinite 75%, hematite 20%) has the approximate composition of the tridymitehercynite rock. The trace-element abundances of the xenoliths suggest that the source rock for the Fe-Al-rich sedimentary protolith was an undifferentiated volcanic rock at the active continental margin.

Zusammenfassung

Fe-Al-reiche Tridymit-Hercynit-Gesteine kommen als seltene Einschlüsse in känozoi-schen Basalten aus Niutoushan, Südost-China vor. Das Tridymit-Hercynit-Gestein besteht aus Hercynit, (40–50%), Tridymit (35–45%), Ilmenit (2%) und Glas (10–20%). Korndurchmesser sind kleiner als 0.05 mm; die Textur ist granular. Verglichen mit “normalen” magmatischen Gesteinen hat dieses Tridymit-Hercynit-Gestein höhere FeO, Al2O3 und niedrigere SiO2, CaO, MgO und Alkali (Na2O + K2O) Gehalte.

Die Herkunft dieser Xenolithe ist problematisch. Auf der Basis der verfügbaren mineralogischen und chemischen Daten sind folgende Möglichkeiten überprüft worden:

  1. 1.

    Als Produkt extremer tholeiitischer Fraktionierung unter niedrigen Oxydationsbedingungen; die Hauptelement-Geochemie spricht jedoch gegen dieses Modell.

  2. 2.

    Als Fe-reicher (Si-armer) Anteil von Silikat-unmischbaren Schmelzen aus einem extrem fraktionierten tholeiitischen Restmagma; die entsprechende Si-reiche Endglied-Fraktion fehlt jedoch, und die Zusammensetzung des Tridymit-Hercynit-Gesteines ist verschieden von der Si-armer Schmelzphasen bekannter unmischbarer Silikatschmelzen.

  3. 3.

    Als Produkt der Metamorphose mit teilweiser bis kompletter Aufschmelzung, eines sedimentären Protolithen. Ein Fe-Al-reicher Schiefer (Quarz 5%, Kaolinit 75%, Hämatit 20%) hat die ungefähre Zusammensetzung des Tridymit-Hercynit-Gesteins. Die Spurenelement-Gehalte des Xenolithen weisen darauf hin, daß das Ursprungsgestein des Fe-Al-reichen sedimentären Protolithen ein nicht differenziertes vulkanisches Gestein an einem activen kontinentalen Rand war.

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Authors and Affiliations

  1. Department of Geological Sciences, University of Tennessee, 37996, Knoxville, TN, USA

    Q. Qi & L. A. Taylor

  2. Department of Earth Sciences, Nanjing University, Nanjing, China

    X. Zhou

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  1. Q. Qi
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  2. L. A. Taylor
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Qi, Q., Taylor, L.A. & Zhou, X. Petrology and geochemistry of an unusual tridymite-hercynite xenolith in tholeiite from southeastern China. Mineralogy and Petrology 50, 195–207 (1994). https://doi.org/10.1007/BF01164605

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