SUBDUCTION AND EXHUMATION OF THE SAXON GRANULITE COMPLEX: CONSTRAINTS FROM THE BOROSILICATE PRISMATINE, (VAC,MG,FE)(AL,MG,FE)9(SI,AL,B)5O21(OH,F), AT WALDHEIM, GERMANY

The classic area for the granulite facies, the Saxon Granulite Complex (SGC), constitutes an NW-vergent antiformal oval area 15 km× 44 km in a belt of Variscan metamorphic rocks that was subducted along the Saxothuringian suture. With the discovery of ultrahigh-pressure Variscan rocks in the Erzgebirge, ~40 km from the SGC, similar evidence was sought in the SGC, resulting in a 2021 report of coesite (Coe) as inclusions in prismatine (Prm) from its type locality near the Waldheim Railway Station. Previous studies had given peak pressures of 2.2–2.3 GPa at 1000°C for the Saxon deep-crustal granulites, whereas the assemblage Prm + Coe would require 3.0-3.4 GPa at 1000°C. Using microRaman spectroscopy we were unable to confirm the presence of Coe, although several of the other phases reported in 2021 were found as constituents of nanogranitoids included in Prm and Grt in samples from the type locality. Phase equilibria in the Prm granulite were modelled using Theriak-Domino software. The granulite is a migmatite consisting of ~75 vol% sodic Pl and ~25 vol% Grt, Crn, Ky, Sil, sapphirine and Bt. The model for aH₂O = 0.3 gave 2.4 GPa for the maximum Prp content of 66 mole% measured in cores of Grt porphyroblasts, consistent with the absence of omphacite, which the model predicts would occur in the Prm granulite at P > 2.3 GPa and T ~ 1000°C. Thus, our re-examination of the Prm granulite did not reveal any evidence for its subduction to depths sufficient to stabilize Coe.

Prm granulite has also been found 740 m ESE of the type locality, while a similar granulite with Tur but not Prm occurs 1.7 km SE. Our whole-rock analysis of Prm granulite from the type locality gave δ¹¹B = –8.93 ± 0.02‰, which together with δ¹⁸O = +7.67‰ previously reported, is consistent with the protolith of Prm and Tur granulites being a magmatic body similar to those of other felsic granulites in the SGC. More likely B was introduced prior to burial rather than during burial or exhumation. Nanogranitoids included in Grt and Prm suggest two episodes of partial melting of the Prm granulite; the second likely coincided with the onset of partial hydration following near-isothermal decompression. Conditions during the second episode favored crystallization of Prm as the main carrier of B, in contrast to most granitic systems in which Tur is the more widespread B carrier.