Examining the behaviour of Sr and 40Ar in white mica in response to deformation and fluid-mediated chemical exchange

A schist and a marble are used to investigate ⁴⁰Ar and Sr behaviour in white mica that show distinct records of deformation and fluid-mediated chemical exchange. The rocks were obtained from the upper level of the Middle Allochthon in the northern Scandinavian Caledonides. They underwent eclogite-facies metamorphism (2.4-2.6 GPa/590-660°C) at c. 486-481 Ma and were deformed during juxtaposition with the overlying Upper Allochthon in lower amphibolite/greenschist facies conditions at c. 430-420 Ma. Deformation of the schist is recorded by anastomosing shear bands that delimit polymineralic lenses of white mica, quartz, garnet, and apatite. White mica are only locally deformed along shear bands but show irregular zoning indicative of dissolution-reprecipitation, with high-celadonite zones (X_Cel: 0.22) generally enveloped by low-celadonite zones (X_Cel: 0.09). Fluid activity during rock deformation is evinced by the presence of chlorite in shear bands and surrounding partially retrogressed garnet, as well as dissolution-reprecipitation of plagioclase. Mobilization of trace elements is evident, with low-celadonite zones enriched in V, Sr, Nb, Ba and depleted in Li, Ti, Co, Zn relative to high-celadonite zones. The former also shows slight enrichment of average B (35.9 µg/g) compared to the latter (27.2 µg/g), but δ¹¹B values are the same for both zones (-13.5 and -13.6 ‰), suggesting locally-derived fluids. Deformation and foliation development of the marble is characterized by shape preferred orientation of calcite and white mica. The mica show variable grain size and are all deformed, highlighted by electron channeling contrast imaging that reveals abundant µm-scale kink bands within individual grains, a feature that is significantly less apparent in the schist mica. The marble mica show uniform high-celadonite content (X_Cel: 0.28), representing preservation of high-pressure mica chemistry during deformation. They show no trace element variations, except for a decrease B content with δ¹¹B values (36.3 µg/g and -18.1 ‰ to 15.5 µg/g and -22.0 ‰), which may be explained by heterogeneous devolatilization and preferential loss of ¹¹B during high-grade metamorphism. Thus, it is evident the mica remained closed to chemical exchange during deformation. The white mica ⁴⁰Ar/³⁹Ar dates from the schist are dispersed (491 ± 4 Ma to 427 ± 4 Ma), with the older dates typically provided by high-celadonite zones, and vice versa. The marble mica yielded a similar range of dates (486 ± 4 Ma to 428 ± 4 Ma), with finer-grains yielding younger dates. Weighted averages of single-spot Rb/Sr dates show a similar pattern for the schist (high-celadonite: 485 ± 8 Ma; low-celadonite: 427 ± 15 Ma). However, Rb/Sr dates from the marble mica only reproduce the older population of dates (481 ± 4 Ma) regardless of grain size. These results demonstrate that loss of ⁴⁰Ar from white mica can be facilitated by either fluid-mediated chemical exchange or deformation, but re-equilibration of Sr isotopes to reset Rb/Sr dates requires fluid-driven processes in lower amphibolite/upper greenschist facies conditions.

Research funded by NCN grant no. UMO-2021/40/C/ST10/00264 (C.J. Barnes), supported by “Juan de la Cierva” Fellowship JFJC2021-047505-I by MCIN/AEI/10.13039/501100011033 and CSIC (M. Bukała)