Magma contamination in Himalayan leucogranite by metapelite host rocks: insights from chemical and boron isotopic compositions of tourmaline

Contamination is a common scenario of intracrustal magmatic processes that may significantly change the compositions of involved anatectic melts. Here, we present major element and B isotope analyses on tourmalines from the Guowu leucogranite and their host rocks, which allow us to unravel the potential contaminations of the Himalayan leucogranites by metapelite host rocks. Three types of tourmaline have been identified in our samples. Tur-M in the host schist has the highest Mg/(Mg+Fe) ratio (0.52–0.62) and low Al content, suggesting its metamorphic character. Tourmalines in the leucogranite (Tur-L) and in the contact zone between leucogranite and schist (Tur-C) have an identical core-rim texture, and their cores are characterized by the lowest Mg/(Mg+Fe) ratio (0.13–0.18) and high Al content, which are consistent with the composition of magmatic tourmaline in the Himalayan leucogranites. The rims of Tur-L show higher Mg/(Mg + Fe) ratio (0.32–0.45) and moderate Ca, Ti, F contents, reflecting the contribution of the host schist via contamination. The composition of Tur-C rims is similar to that of Tur-M, suggesting a more significant contribution from contaminated schist than that for Tur-L. The different types of tourmaline share consistent B isotope compositions with δ¹¹B ranging from −14 to −12‰. The compositional characteristics of the contaminated tourmaline and the mineral assemblage in the contaminated zone suggest that the involved components derived from the host schist, probably produced by the decomposition of tourmaline, biotite, plagioclase, primarily include Ca, K, Mg, Ti, B and F. In addition, contamination by tourmaline-bearing metapelite may be an alternative interpretation responsible for the peraluminous character of the Himalayan leucogranites. The occurrence of contaminated tourmaline in leucogranites suggests that contamination of host rocks is a possible way promoting the formation of tourmaline in the Himalayan leucogranites, and tourmaline is useful for deciphering the related contamination processes.

围岩混染是壳内岩浆作用的常见场景，可显著改变所涉及深熔熔体（anatectic melt）的成分。本文针对果沃（Guowu）淡色花岗岩（leucogranite）及其寄主岩（host rock）中的电气石（tourmaline）开展主量元素与硼同位素分析，借此揭示喜马拉雅淡色花岗岩被泥质片岩（metapelite）寄主岩混染的潜在过程。本次样品中共识别出三类电气石：寄主片岩中的Tur-M具有最高的Mg/(Mg+Fe)比值（0.52~0.62）与较低的铝含量，指示其变质成因；淡色花岗岩内的Tur-L以及淡色花岗岩与片岩接触带内的Tur-C均具有一致的核-环结构（core-rim texture），二者的核部均表现出最低的Mg/(Mg+Fe)比值（0.13~0.18）与较高的铝含量，这与喜马拉雅淡色花岗岩中岩浆成因电气石的成分特征相符。Tur-L的环部具有更高的Mg/(Mg+Fe)比值（0.32~0.45）与中等水平的钙、钛、氟含量，反映了寄主片岩通过混染作用带来的组分贡献；而Tur-C的环部成分与Tur-M相近，表明其受混染片岩的贡献程度较Tur-L更为显著。三类电气石均具有一致的硼同位素组成，δ¹¹B值介于-14‰至-12‰之间。结合混染成因电气石的成分特征与混染带内的矿物组合（mineral assemblage）可知，源自寄主片岩的组分主要包括钙、钾、镁、钛、硼与氟，这些组分大概率由电气石、黑云母（biotite）、斜长石（plagioclase）的分解所产生。此外，含电气石泥质片岩的混染作用，或可解释喜马拉雅淡色花岗岩的过铝质（peraluminous）特征。淡色花岗岩中混染成因电气石的产出表明，寄主岩混染是促进喜马拉雅淡色花岗岩中电气石形成的潜在途径，而电气石可用于解析相关的混染作用过程。