Deciphering the (U)HP conditions in quartzofeldspatic rocks: mineral composition and phase preservation.

Quartzofeldspatic rocks constitute a major component of the continental crust. Although their subduction to (U)HP conditions has been documented in multiple cases, the preservation of unequivocal evidence for (U)HP metamorphism in these lithologies remains rare.

The direct proof is the presence of the UHP index minerals, namely diamond and coesite. Where preserved, coesite typically occurs as inclusions in refractory phases as garnet, making its identification challenging, especially in the case of very small inclusions and/or the absence of the diagnostic back-reaction textures to quartz. Various methods can be used for the unequivocal identification of coesite. In addition to Raman spectroscopy and the EBSD, we propose the use of the cathodoluminescence (CL) spectrometry. Considering the distinct wavelengths emitted by coesite (c. 550 nm) and quartz (c. 660 nm), application of the CL spectrometry coupled with SEM-EDS enables rapid and unambiguous detection of coesite inclusions, even at submicron scale.

In the absence of UHP index minerals, quartzofeldspathic rocks commonly retain stable mineral assemblages over a wide P–T range, limiting their usefulness for peak pressure estimates. In such cases, other UHP indicators need to be searched, including trace-element substitutions in major minerals. We have investigated garnet with coesite inclusions from subducted metagranites of the Eger Crystalline Complex, Bohemian Massif, where garnet shows chemically distinct concentric domains with minor amounts of P, Na, and Li, that systematically coincide with coesite locations. From the correlation of these elements, we infer (Na,Li)1P1M2+−1Si−1 substitution. This coupled substitution is clearly connected to UHP conditions in natural samples and can be therefore considered as a tool indicating UHP conditions.

A further peculiarity of (U)HP quartzofeldspathic rocks is the frequent absence of jadeite-rich clinopyroxene, despite its predicted stabilityby experiments and thermodynamic modelling. This absence has been attributed either to complete retrograde decomposition or to its non-participation in the peak assemblage. Here we describe a quartzofeldspathic gneiss from Erzgebirge that is composed mainly of quartz, garnet, plagioclase, K-feldspar, muscovite, and kyanite and that contains relics of jadeite included in kyanite and garnet (the latter also contains coesite). Observed domains up to 3 mm in size of fine-grained plagioclase-muscovite symplectite, surrounded by plagioclase-muscovite mosaic and occasionally associated with small garnet grains are interpreted as pseudomorphs after jadeite. Peak P-T conditions estimated to >28 kbar and 600-800 °C are consistent with the coexistence of jadeite, coesite and Ca-poor garnet. The subsequent decompression led to jadeite breakdown into plagioclase-muscovite symplectite and Ca redistribution, reflected by increasing Ca content in newly formed garnet. Based on our observations, we propose that jadeite should be considered as a part of the HP mineral assemblage in quartzofeldspatic rocks, and the presence of muscovite-plagioclase assemblage associated with Ca-poor garnet may indicate its former presence.

This work was funded by the Czech Science Foundation grant GACR 24-12845S.