The role of magma mixing in the petrogenesis of Eocene ultrapotassic lavas, Western Yunnan, SW China

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External Research Organisations

  • Johannes Gutenberg University Mainz
  • State Key Laboratory of Geological Process and Mineral Resources
  • Siberian Branch of the Russian Academy of Sciences
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Original languageEnglish
Article numberegac129
Number of pages27
JournalJournal of Petrology
Volume64
Issue number2
Early online date29 Dec 2022
Publication statusPublished - Feb 2023

Abstract

Magma mixing is a widespread magmagenic process. However, its significance in the formation of ultrapotassic magmas has been largely overlooked so far as they are commonly thought to originate directly from the mantle and ascend rapidly through the crust. The Hezhong ultrapotassic lavas in Western Yunnan (SW China) are (basaltic) trachy-andesitic in composition. These rocks display porphyritic textures with olivine, clinopyroxene (Cpx), spinel, and phlogopite occurring as both phenocryst and glomerocryst. Disequilibrium textures and complex zonation of crystals are commonly observed. Specifically, based on the textural and compositional characteristics, olivines can be classified into three different populations: two populations are characterized by highly to moderately magnesian olivines with normal chemical core-rim zonation (Fo~94-86 to Fo~89-79 and Fo~91-89 to Fo~86-84, respectively). The third population lacks obvious crystal zonation, but individual crystals exhibit some compositional variety at lower Fo contents (Fo83-76). Similarly, four populations of Cpx and two populations of spinel phenocrysts are recognized in terms of texture and composition. Notably, Cpx with reverse zoning contains a 'green-core' surrounded by a colourless mantle and rim. Hence, based on the variations of mineral assemblage, types of inclusions, and chemical compositions, all phenocryst/glomerocryst minerals can be divided into three groups. Mineral Group I (MG I) consists of high Fo cores of olivine, cores of the zoned spinel, and phlogopite. MG II only includes the green cores of reversed zoned Cpx (green-core Cpx), and MG III is composed of micro phenocrysts without obvious zoning and rims of large phenocrysts. Comparing these mineral groups with relevant minerals occurring in typical temporally and spatially associated igneous rocks, we suggest that the MG I and II could have been derived from magmas with compositions resembling an olivine lamproite and a trachyte, respectively. The overall bulk-rock geochemical and isotopic features of Hezhong lavas also agree with a mixing process between these two endmembers. Hence, we infer that mixing between these two magmas played a key role in the petrogenesis of the ultrapotassic Hezhong lavas and that the MG III crystallized from the mixed magmas. Our study highlights the complex formation of ultrapotassic magmas inferring that caution must be taken when using bulk chemical magma compositions are to deduce source signatures.

Keywords

    green-core clinopyroxene, magma mixing, magnesian olivine, mineral population, ultrapotassic magma

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Cite this

The role of magma mixing in the petrogenesis of Eocene ultrapotassic lavas, Western Yunnan, SW China. / Yang, Zongpeng; Hou, Tong; Wang, Dachuan et al.
In: Journal of Petrology, Vol. 64, No. 2, egac129, 02.2023.

Research output: Contribution to journalArticleResearchpeer review

Yang, Z, Hou, T, Wang, D, Marxer, F, Wang, M, Chebotarev, D, Zhang, Z, Zhang, H, Botcharnikov, R & Holtz, F 2023, 'The role of magma mixing in the petrogenesis of Eocene ultrapotassic lavas, Western Yunnan, SW China', Journal of Petrology, vol. 64, no. 2, egac129. https://doi.org/10.1093/petrology/egac129
Yang, Z., Hou, T., Wang, D., Marxer, F., Wang, M., Chebotarev, D., Zhang, Z., Zhang, H., Botcharnikov, R., & Holtz, F. (2023). The role of magma mixing in the petrogenesis of Eocene ultrapotassic lavas, Western Yunnan, SW China. Journal of Petrology, 64(2), Article egac129. https://doi.org/10.1093/petrology/egac129
Yang Z, Hou T, Wang D, Marxer F, Wang M, Chebotarev D et al. The role of magma mixing in the petrogenesis of Eocene ultrapotassic lavas, Western Yunnan, SW China. Journal of Petrology. 2023 Feb;64(2):egac129. Epub 2022 Dec 29. doi: 10.1093/petrology/egac129
Yang, Zongpeng ; Hou, Tong ; Wang, Dachuan et al. / The role of magma mixing in the petrogenesis of Eocene ultrapotassic lavas, Western Yunnan, SW China. In: Journal of Petrology. 2023 ; Vol. 64, No. 2.
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abstract = "Magma mixing is a widespread magmagenic process. However, its significance in the formation of ultrapotassic magmas has been largely overlooked so far as they are commonly thought to originate directly from the mantle and ascend rapidly through the crust. The Hezhong ultrapotassic lavas in Western Yunnan (SW China) are (basaltic) trachy-andesitic in composition. These rocks display porphyritic textures with olivine, clinopyroxene (Cpx), spinel, and phlogopite occurring as both phenocryst and glomerocryst. Disequilibrium textures and complex zonation of crystals are commonly observed. Specifically, based on the textural and compositional characteristics, olivines can be classified into three different populations: two populations are characterized by highly to moderately magnesian olivines with normal chemical core-rim zonation (Fo~94-86 to Fo~89-79 and Fo~91-89 to Fo~86-84, respectively). The third population lacks obvious crystal zonation, but individual crystals exhibit some compositional variety at lower Fo contents (Fo83-76). Similarly, four populations of Cpx and two populations of spinel phenocrysts are recognized in terms of texture and composition. Notably, Cpx with reverse zoning contains a 'green-core' surrounded by a colourless mantle and rim. Hence, based on the variations of mineral assemblage, types of inclusions, and chemical compositions, all phenocryst/glomerocryst minerals can be divided into three groups. Mineral Group I (MG I) consists of high Fo cores of olivine, cores of the zoned spinel, and phlogopite. MG II only includes the green cores of reversed zoned Cpx (green-core Cpx), and MG III is composed of micro phenocrysts without obvious zoning and rims of large phenocrysts. Comparing these mineral groups with relevant minerals occurring in typical temporally and spatially associated igneous rocks, we suggest that the MG I and II could have been derived from magmas with compositions resembling an olivine lamproite and a trachyte, respectively. The overall bulk-rock geochemical and isotopic features of Hezhong lavas also agree with a mixing process between these two endmembers. Hence, we infer that mixing between these two magmas played a key role in the petrogenesis of the ultrapotassic Hezhong lavas and that the MG III crystallized from the mixed magmas. Our study highlights the complex formation of ultrapotassic magmas inferring that caution must be taken when using bulk chemical magma compositions are to deduce source signatures.",
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T1 - The role of magma mixing in the petrogenesis of Eocene ultrapotassic lavas, Western Yunnan, SW China

AU - Yang, Zongpeng

AU - Hou, Tong

AU - Wang, Dachuan

AU - Marxer, Felix

AU - Wang, Meng

AU - Chebotarev, Dmitry

AU - Zhang, Zhaochong

AU - Zhang, Hongluo

AU - Botcharnikov, Roman

AU - Holtz, François

N1 - Publisher Copyright: © The Author(s) 2022.

PY - 2023/2

Y1 - 2023/2

N2 - Magma mixing is a widespread magmagenic process. However, its significance in the formation of ultrapotassic magmas has been largely overlooked so far as they are commonly thought to originate directly from the mantle and ascend rapidly through the crust. The Hezhong ultrapotassic lavas in Western Yunnan (SW China) are (basaltic) trachy-andesitic in composition. These rocks display porphyritic textures with olivine, clinopyroxene (Cpx), spinel, and phlogopite occurring as both phenocryst and glomerocryst. Disequilibrium textures and complex zonation of crystals are commonly observed. Specifically, based on the textural and compositional characteristics, olivines can be classified into three different populations: two populations are characterized by highly to moderately magnesian olivines with normal chemical core-rim zonation (Fo~94-86 to Fo~89-79 and Fo~91-89 to Fo~86-84, respectively). The third population lacks obvious crystal zonation, but individual crystals exhibit some compositional variety at lower Fo contents (Fo83-76). Similarly, four populations of Cpx and two populations of spinel phenocrysts are recognized in terms of texture and composition. Notably, Cpx with reverse zoning contains a 'green-core' surrounded by a colourless mantle and rim. Hence, based on the variations of mineral assemblage, types of inclusions, and chemical compositions, all phenocryst/glomerocryst minerals can be divided into three groups. Mineral Group I (MG I) consists of high Fo cores of olivine, cores of the zoned spinel, and phlogopite. MG II only includes the green cores of reversed zoned Cpx (green-core Cpx), and MG III is composed of micro phenocrysts without obvious zoning and rims of large phenocrysts. Comparing these mineral groups with relevant minerals occurring in typical temporally and spatially associated igneous rocks, we suggest that the MG I and II could have been derived from magmas with compositions resembling an olivine lamproite and a trachyte, respectively. The overall bulk-rock geochemical and isotopic features of Hezhong lavas also agree with a mixing process between these two endmembers. Hence, we infer that mixing between these two magmas played a key role in the petrogenesis of the ultrapotassic Hezhong lavas and that the MG III crystallized from the mixed magmas. Our study highlights the complex formation of ultrapotassic magmas inferring that caution must be taken when using bulk chemical magma compositions are to deduce source signatures.

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