Drilling at Soultz-sous-Forêts, France, conducted as part of the European Hot Dry Rock (HDR) Project, penetrated granitic basement down to 5,080 m and revealed a porphyritic monzogranite that has no equivalent in the granitic outcrops of adjacent basement aeras. The EPS-1 borehole core (from 980 to 2,230 m) shows a monzogranite that has a high-K calcalkaline composition and is enriched in K, Th, U , and REE. Petrological and mineralogical characteristics indicate that the monzogranite formed through two crystallization stages: a first, deep-seated, stage was characterized by the crystallization of liquidus phases in the 755-790^{\circ}C range, at 3.5 kb (11-12 km depth). This was followed, during ascent and at emplacement level, by consolidation at 665-715^{\circ}C, 1.5-2.0 kb (4.5-5.5 km depth) associated with the development of K-feldspar layering. Published Rb/Sr ages (325 Ma), new 40Ar/39Ar cooling ages (319 Ma), a U/Pb on zircon emplacement age of 331 Ma, and petrological arguments such as the presence of monzogabbrodiorite to quartz-syenite enclaves, indicate that the Soultz monzogranite had a mainly crustal origin with a possible contribution of mantle-derived components represented by the enclaves. Emplacement was related to collision tectonics during the transition between Early and Late Carboniferous.