Tracing the evolution of the pegmatite system and its interaction with the country rocks by chemical and boron isotope compositions of tourmaline in the Qinghe pegmatite from the Chinese Altay orogen

Zheng BeiQi1,2; Chen Bin1; Sun Yang1

2023

10.18654/1000-0569/2023.01.13

ACTA PETROLOGICA SINICA   Impact Factor & Quartile

1000-0569

2095-8927

Tourmaline crystalized in pegmatites and its surrounding altered wallrocks record the evolution, fluid exsolution and fluid-rock interactions of the pegmatite system. We conducted systematic elemental and boron isotope studies on tourmalines from a pegmatite dyke and its surrounding country rocks in Qinghe (the Altay orogeny) to investigate the origin and evolution of the pegmatite-forming magma and the scale of its fluid exsolution. The pegmatite dyke can be divided into the border, the intermediate and the core zones according to its textures and mineralogy assembalge. Outwards from the contact, banded altered rocks, fine-grained altered rocks to coarse-grained altered rocks appear in sequence. The B and P in the pegmatite-derived fluids, together with Fe and Mg in the country rocks, provided necessary sources for the formation of the tourmaline. All tourmalines belong to the alkali group, which can be further classified into schorl to dravite solid-solution series according to the analytical results. Most tourmalines follow the exchange vectors of (R1+Al)(Na+R2)(-1), (Al+O)(R+OH)(-1), (Na+Mg)(Al+X-vac)-1, (XvacAl)(NaR2+)-1, FeMg-1 and MnMg-1, which highlight the evolution trends of the increase of the Fe, Mn and Al contents and the decrease of the Na, Mg and Ca contents. Combined with the content of V and the correlation between Fe3+ and Al, we believe that the primary pegmatite magma was characterized by low oxygen fugacity, and that the high oxygen fugacity at the edge of the pegmatite dike could be related to the interaction between the pegmatite-forming magma and the surrounding rocks. In addition, tourmaline from pegmatite has much lower Sigma REE and (La/Yb)N than that from the granite, which suggests that the pegmatite was not a product after extreme fractionation of a parental granite. Instead, it originated from the direct melting of mica schist under amphibolite-faces conditions. The slight difference of boron isotope compositions between the altered and the unaltered wallrocks manifests a limited scale of fluid exsolution from the late-stage barren pegmatite. Tourmalines from the Qinghe pegmatite exhibit significantly lower d11B values than tourmalines from the mica schist, implying that the mica schist was not the only source for the Qinghe pegmatite, and instead, there might be a certain amount of claystones.

Pegmatite Tourmaline Boron isotope Altay orogenic belt

SCI

English

First ; Corresponding

Geology

Geology

WOS:000952014200013

SCIENCE PRESS

GEOSCIENCES

Web of Science

1.Southern Univ Sci & Technol, Dept Earth & Space Sci, Shenzhen 518055, Peoples R China
2.Shaoguan Univ, Sch Tourism & Geog, Shaoguan 512005, Peoples R China

Zheng BeiQi,Chen Bin,Sun Yang. Tracing the evolution of the pegmatite system and its interaction with the country rocks by chemical and boron isotope compositions of tourmaline in the Qinghe pegmatite from the Chinese Altay orogen[J]. ACTA PETROLOGICA SINICA,2023,39(1).

Zheng BeiQi,Chen Bin,&Sun Yang.(2023).Tracing the evolution of the pegmatite system and its interaction with the country rocks by chemical and boron isotope compositions of tourmaline in the Qinghe pegmatite from the Chinese Altay orogen.ACTA PETROLOGICA SINICA,39(1).

Zheng BeiQi,et al."Tracing the evolution of the pegmatite system and its interaction with the country rocks by chemical and boron isotope compositions of tourmaline in the Qinghe pegmatite from the Chinese Altay orogen".ACTA PETROLOGICA SINICA 39.1(2023).