Age and origin of the tourmaline-rich hydraulic breccias in the Tatra Granite, Western Carpathians

In the crystalline basement of the Tatra Mountains (Poland/Slovakia) two types of tourmaline-rich breccia zones are described, both having originated from magmatic fluids and showing a 350 +/- 1 Ma Re-Os model age. Both T1 brecciated pegmatites and T2 vein-breccias are cemented by a tourmaline-quartz matrix. In T1 breccias, the metasomatic replacement of feldspars by tourmaline and overgrowths of small Tur(2) tourmalines on the primary pegmatitic Tur(1) tourmaline crystals suggest an important role for metasomatic processes. The microcrystalline nature of the quartz-tourmaline matrix and high vacancy ratios in the X-site in tourmaline from T2 breccias reflect relatively rapid crystallization from an oxidized fluid. The formation of both types of breccias is related to the internal structure of the Tatra granitoid intrusion. The layered character of the granitoid body probably stimulated the formation of pegmatite-like pockets, with local overpressure causing metasomatic alteration, rock brecciation and cementation by tourmaline (T1 breccias), the final hydrofracturing of overburden rocks and fluid escape. The resulting fracture zones (T2) are interpreted as the paths for fluid migration and rapid crystallization as the pressure and temperature dropped from 7.5-6 kbar and 630-570 degrees C in the case of the T1 brecciated pegmatites to 2.5-1.5 kbar and 465-430 degrees C in the vein T2 breccias.