The Harmancık “purple jade”: A unique blueschist-facies metatrachyte with significant geoarchaeological value

The so-called Harmancık “purple jade” from the Bursa region of northwestern Türkiye has attracted attention as a gemstone; however, its geological characteristics and provenance have long remained uncertain. This short communication presents new field observations along with mineralogical and petrological data concerning the provenance, metamorphic evolution, and geoarchaeological relevance of this unique rock type. The rock occurs as loose boulders scattered across cultivated fields in the villages of Akpınar, Kocapınar, and Gedikören, where it was previously interpreted either as part of Neogene sedimentary successions or as a high-pressure metamorphic rock affected by contact metamorphism. Our detailed mapping and field observations suggest that the present distribution of the blocks reflects Quaternary reworking, whereas their primary source lies within the Late Cretaceous ophiolitic mélange of the Tavşanlı Zone. An apparently in situ exposure demonstrates a direct association between blueschist and felsic jadeite-bearing metavolcanic rocks. Mineral assemblages dominated by jadeite (40–60 vol. %), K-feldspar, albite, lawsonite, and relic aegirine, together with locally preserved volcanic microtextures and whole-rock geochemical signatures, indicate that the rocks represent blueschist-facies metatrachyte, locally grading into minor metaphonolite rather than true jadeitite jade formed by metasomatic processes. The purple to lavender coloration is largely restricted to fluid pathways. These rocks constitute a rare example of alkaline felsic volcanic protoliths metamorphosed under blueschist-facies conditions, most plausibly as part of an accreted seamount. Moreover, the occurrence of Neolithic polished stone tools produced from this rock type demonstrates its prehistoric exploitation and underscores its geoarchaeological importance.