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In-situ elemental and boron isotopic variations of tourmaline from the Sanfang granite, South China: insights into magmatic-hydrothermal evolution

In-situ elemental and boron isotopic variations of tourmaline from the Sanfang granite, South China: insights into magmatic-hydrothermal evolution
In-situ elemental and boron isotopic variations of tourmaline from the Sanfang granite, South China: insights into magmatic-hydrothermal evolution
Tourmaline often occurs in boron-rich granites and its genesis is still in dispute either formed from residual boron-rich silicate melt or from magmatic hydrothermal fluids with or without external fluid involvement. Here, we present a systematic investigation of in-situ major, trace elemental and boron isotopic variations of tourmaline from the Sanfang granite in Guangxi Province of South China, which demonstrate both the magmatic and hydrothermal origin for the formation of the tourmalines. The tourmaline occurs mainly in quartz-tourmaline nodules and tourmaline pegmatitic segregations within the Sanfang granite. Four types of tourmaline are identified: (1) isolated and disseminated tourmaline (Tur-D type) distributed in granite; (2) euhedral and subhedral tourmaline in quartz-tourmaline nodules (Tur-N type); (3) the earlier stage tourmaline (Tur-PE type) in pegmatitic segregations and (4) the later stage tourmaline (Tur-PL type) which replaced the Tur-PE type tourmaline in pegmatitic segregations. All the tourmalines belong to the alkali group representing dravite-schorl solid solution series with the former three types being schorl and the Tur-PL type being dravite. Petrography, chemical discrimination diagrams and Al occupations in the Y-site suggest that the Tur-D, Tur-N and Tur-PE type tourmalines are of magmatic origin and the Tur-PL type tourmalines are of hydrothermal fluid origin. Elemental and boron isotopic composition variations of tourmalines reflect the compositional and environmental evolution from late boron-rich magma to exsolved hydrothermal fluids. Chemical variations from the Tur-D to the Tur-N tourmalines are controlled by magma differentiation. The increase of Mg/(Mg + Fe) and Ca/(Ca + Na) ratios from the Tur-N to the Tur-PE tourmalines suggests the contamination of surrounding strata (e.g. the Sibao Group). Hydrothermal tourmalines have higher Mg/(Mg + Fe) and lower Na/(Na + Ca) ratios than the magmatic tourmalines. There is clear correlation at least in some trace elements (e.g. Sr, Pb) with major elements for all tourmalines, indicating the potential crystal chemical effects on their incorporation. Other trace element incorporation in tourmaline is largely controlled by melt and fluid composition. Hydrothermal tourmalines have high and stable V, Co and Ni contents, indicating relatively constant partition coefficient between hydrothermal tourmalines and the fluid. Hydrothermal tourmalines exhibit lower total REE contents but more pronounced positive Eu anomalies than the magmatic tourmalines. The δ11B values of all tourmalines from the Sanfang granite vary from −14.4‰ to −9.3‰, which indicate the boron in the Sanfang granite was mainly derived from partial melting of the metasedimentary source rocks. Boron isotopic variations of tourmaline are controlled by fractionation between melt-fluid and Rayleigh fractionation.
Boron isotope, Geochemistry, Magmatic-hydrothermal evolution, South China, Tourmaline
0009-2541
190-204
Zhao, He Dong
80aeb4b8-8ed7-4b86-82db-0ac697e3e2c5
Zhao, Kui Dong
bd5bd864-d7d0-40b4-ae43-37cdc7632a37
Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Jiang, Shao Yong
dc929006-69ac-4366-bf2b-dc97527602d3
Zhao, He Dong
80aeb4b8-8ed7-4b86-82db-0ac697e3e2c5
Zhao, Kui Dong
bd5bd864-d7d0-40b4-ae43-37cdc7632a37
Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Jiang, Shao Yong
dc929006-69ac-4366-bf2b-dc97527602d3

Zhao, He Dong, Zhao, Kui Dong, Palmer, Martin R. and Jiang, Shao Yong (2019) In-situ elemental and boron isotopic variations of tourmaline from the Sanfang granite, South China: insights into magmatic-hydrothermal evolution. Chemical Geology, 504, 190-204. (doi:10.1016/j.chemgeo.2018.11.013).

Record type: Article

Abstract

Tourmaline often occurs in boron-rich granites and its genesis is still in dispute either formed from residual boron-rich silicate melt or from magmatic hydrothermal fluids with or without external fluid involvement. Here, we present a systematic investigation of in-situ major, trace elemental and boron isotopic variations of tourmaline from the Sanfang granite in Guangxi Province of South China, which demonstrate both the magmatic and hydrothermal origin for the formation of the tourmalines. The tourmaline occurs mainly in quartz-tourmaline nodules and tourmaline pegmatitic segregations within the Sanfang granite. Four types of tourmaline are identified: (1) isolated and disseminated tourmaline (Tur-D type) distributed in granite; (2) euhedral and subhedral tourmaline in quartz-tourmaline nodules (Tur-N type); (3) the earlier stage tourmaline (Tur-PE type) in pegmatitic segregations and (4) the later stage tourmaline (Tur-PL type) which replaced the Tur-PE type tourmaline in pegmatitic segregations. All the tourmalines belong to the alkali group representing dravite-schorl solid solution series with the former three types being schorl and the Tur-PL type being dravite. Petrography, chemical discrimination diagrams and Al occupations in the Y-site suggest that the Tur-D, Tur-N and Tur-PE type tourmalines are of magmatic origin and the Tur-PL type tourmalines are of hydrothermal fluid origin. Elemental and boron isotopic composition variations of tourmalines reflect the compositional and environmental evolution from late boron-rich magma to exsolved hydrothermal fluids. Chemical variations from the Tur-D to the Tur-N tourmalines are controlled by magma differentiation. The increase of Mg/(Mg + Fe) and Ca/(Ca + Na) ratios from the Tur-N to the Tur-PE tourmalines suggests the contamination of surrounding strata (e.g. the Sibao Group). Hydrothermal tourmalines have higher Mg/(Mg + Fe) and lower Na/(Na + Ca) ratios than the magmatic tourmalines. There is clear correlation at least in some trace elements (e.g. Sr, Pb) with major elements for all tourmalines, indicating the potential crystal chemical effects on their incorporation. Other trace element incorporation in tourmaline is largely controlled by melt and fluid composition. Hydrothermal tourmalines have high and stable V, Co and Ni contents, indicating relatively constant partition coefficient between hydrothermal tourmalines and the fluid. Hydrothermal tourmalines exhibit lower total REE contents but more pronounced positive Eu anomalies than the magmatic tourmalines. The δ11B values of all tourmalines from the Sanfang granite vary from −14.4‰ to −9.3‰, which indicate the boron in the Sanfang granite was mainly derived from partial melting of the metasedimentary source rocks. Boron isotopic variations of tourmaline are controlled by fractionation between melt-fluid and Rayleigh fractionation.

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More information

Accepted/In Press date: 17 November 2018
e-pub ahead of print date: 22 November 2018
Published date: 20 January 2019
Keywords: Boron isotope, Geochemistry, Magmatic-hydrothermal evolution, South China, Tourmaline

Identifiers

Local EPrints ID: 428422
URI: http://eprints.soton.ac.uk/id/eprint/428422
ISSN: 0009-2541
PURE UUID: de9caa03-ddc9-4af7-93db-a82a538f053d

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Date deposited: 22 Feb 2019 17:31
Last modified: 06 Oct 2020 21:02

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Contributors

Author: He Dong Zhao
Author: Kui Dong Zhao
Author: Shao Yong Jiang

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