Tourmaline chemistry, boron, and strontium isotope systematics trace multiple melt–fluid–rock interaction stages in deeply subducted continental crust

Abstract

The generation, transport, and recrystallization of slab-derived melts/fluids play a critical role in the deep recycling of elements in subduction zones. While boron (B) isotope systematics have been invoked as an important tracer of these processes, its behavior during metamorphic dehydration and partial melting of deeply subducted continental slabs, and the partitioning of B isotopes between minerals and melts/fluids is not fully understood. Here, we investigate these processes through an in situ study of the major, trace-element, and B-Sr isotope variations in different occurrences of tourmaline in migmatite from the Yuka terrane, North Qaidam orogen (China), which resulted from partial melting of a continental slab at different stages during subduction and exhumation. Based on textural and detailed high-resolution X-ray mapping studies, tourmaline was classified into four paragenetic generations (Tur-I, Tur-II, Tur-III, and Tur-IV). Dravitic Tur-I occurs in melanosomes and shows increasing Fe, Ca, and Ti contents from the core to the outer rim. In addition, it has relatively homogeneous Sr isotope values (0.7407–0.7416) and decreasing δ ¹¹B values (-3.8 to −8.6 ‰) and X _Mg (Mg/(Mg + Fe)) ratios, indicating formation in a rock buffered by an aqueous fluid during the prograde to peak metamorphism. Schorlitic Tur-II occurs within selvage zones between melanosomes and leucosomes, and yields high-Fe values and low δ ¹¹B (-13.5 to −10.9 ‰) and more variable ⁸⁷Sr/ ⁸⁶Sr (0.7343–0.7418) values, indicating crystallization in the presence of a hydrous melt external derived from breakdown of Fe-rich mineral(s) during partial melting of the subducted slab. Dravitic Tur-III formed in the matrix and also enveloped Tur-II. It shows homogeneous ⁸⁷Sr/ ⁸⁶Sr values (0.7411–0.7420) and decreasing δ ¹¹B values (-6.8 to −9.9 ‰) and X _Mg ratios as well as increasing Fe and Ti contents from core to outer rim. Formation of Tur-III reflects a transitional stage from hydrous melt to aqueous fluid during exhumation. Tur-IV in the leucosomes is essentially a schorl-dravite solid solution with small amounts of Ca. Its ⁸⁷Sr/ ⁸⁶Sr values (0.7402–0.7416) and δ ¹¹B values (-11.4 to −8.5 ‰) are intermediate between the respective values of Tur-II and Tur-III. The formation of Tur-IV likely results from interaction between melt and fluid and, based on its chronological sequence, and is interpreted to have formed during the exhumation stage of the Yuka terrane. Overall, the variable X _Mg ratios and δ ¹¹B values in the different generations of tourmalines are a consequence of the evolution of the melt/fluid at different depths within the deeply subducted slab. Decreasing δ ¹¹B values from Tur-I to Tur-II and Tur-III are controlled by the breakdown of different minerals during partial melting or metamorphic dehydration of the subducted slab, while the co-variations of the elemental geochemistry and B isotopic compositions of tourmaline reflect different depths of formation during subduction and exhumation of the lithosphere. These observations suggest tourmaline may serve as a useful tracer of multiple melt/fluid–rock interactions and of boron cycling in continental subduction zones. The heterogeneity of δ ¹¹B in melts/fluids at different depth levels of the continental subducted slab may also result in locally variable B isotope values in syn- and post-collisional magmas.

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