The fate of calcareous pelites in collisional orogens

Abstract

Although calcareous pelites are important constituents of sediments involved in orogenic processes, their prograde metamorphism is significantly less studied than that of pure pelites with negligible amounts of modal calcite. This paper presents mineral equilibria modelling in the system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–H2O–CO2, with the aim of constraining the prograde evolution of calcareous pelites in collisional orogenic settings. A suite of model bulk‐rock compositions is used to investigate the influence of different proportions of calcite in the protolith on (a) the equilibrium assemblages at different pressure, temperature and fluid composition (P–T–X(CO2)) conditions; (b) the melt fertility and (c) the fluid evolution and the main decarbonation reactions occurring during prograde metamorphism of calcareous pelites. In spite of being purely theoretical, the reliability of the modelling is tested by comparing the predicted assemblages with those observed in a wide set of natural samples from the Himalayan metamorphic core. Comparison between the predicted and the modelled assemblages demonstrates that even a small amount of calcite in the calcareous pelitic protoliths has a strong influence on the final mineral assemblages and compositions, with potential effects on their melt productivity. Specifically, it appears that up to ~800°C, the melt productivity of calcic metapelites remains low, and melt production occurs gradationally because it is mostly controlled by continuous biotite dehydration melting reactions, rather than by muscovite breakdown. Moreover, the study demonstrates that calcareous pelites could be non‐negligible CO2‐source rocks in orogenic settings, and that in such contexts, an internal buffered behaviour is likely for most of them.

Magnesium Isotopes Archive the Initial Carbonate Abundances of Metasedimentary Rocks Prior to Thermal Decarbonation

“…Results from different indexes display good consistency, and the ICAs of Himalayan carbonate-bearing metasedimentary rocks are highly variable, ranging from 0.81 wt.% to 83.57 wt.% (Figure S2 in Supporting Information S1). We also calculate the ICA of each sample following the method used by Groppo et al (2021), which is in good agreement with the result from this study (Table S5 in Supporting Information S1). S6 of the Supporting Information S1.…”

“…These necessitate further investigations on the relationship between bulk Mg isotopic compositions of carbonate-bearing metasedimentary rocks and their ICAs. In the large and hot Himalayan orogenic belt, metamorphic rocks derived from carbonate-bearing protoliths are widespread (Groppo et al, 2021;Rolfo et al, 2017). These lithologies (referred to below as "carbonate-bearing metasedimentary rocks," although some are totally decarbonated) were derived from protoliths consisting of Proterozoic sediments with varying carbonate contents deposited on the northern Indian passive continental margin, and metamorphosed in an internally buffered system (Groppo et al, 2017(Groppo et al, , 2021Rapa et al, 2017), making them appropriate targets to investigate this correlation (Text S1 in Supporting Information S1).…”

Fluid Redox Fingerprint of the CaCO3+Antigorite Dehydration Reaction in Subducted Metacarbonate Sediments

“…The Alpine polyphasic P-T-t evolution of this unit has been determined through 40 years of multiple petrologic studies on several lithologies (see [20,21] for a review). ([26]; Table 1) and other subducted carbonate and carbonate-bearing metasediments from the Western Alps [27][28][29][30], Himalaya [31], modern trench sediments and GLOSS II [32], and other UHP localities (i.e., Sulu, Thiashan, Kokchetav; [33][34][35]). In addition, modern marine limestones and chalk are shown (see compilation in [36,37]).…”

“…Its bulk composition (Table 1) is located along a chemical trend that joins the (hydrated) abyssal peridotites (i.e., the (C)MFS−H2O sub-system) to the marine limestones and chalks (i.e., the CM(F)(S)-COHS sub-system), and it is compatible with some carbonate-rich ophicarbonates (Figure 1a). ([26]; Table 1) and other subducted carbonate and carbonate-bearing metasediments from the Western Alps [27][28][29][30], Himalaya [31], modern trench sediments and GLOSS II [32], and other UHP localities (i.e., Sulu, Thiashan, Kokchetav; [33][34][35]). In addition, modern marine limestones and chalk are shown (see compilation in [36,37]).…”

“…Figure 1. (a) Comparison between DM675 bulk composition([26]; Table1) and other subducted carbonate and carbonate-bearing metasediments from the Western Alps[27][28][29][30], Himalaya[31], modern trench sediments and GLOSS II[32], and other UHP localities (i.e., Sulu, Thiashan, Kokchetav;[33][34][35]). In addition, modern marine limestones and chalk are shown (see compilation in[36,37]).…”