HIGH-GRADE CONTACT-METAMORPHISM OF CALCAREOUS ROCKS FROM THE OSLO RIFT, SOUTHERN NORWAY

Shallow-level plutons caused extensive contact metamorphism of Lower P aleozoic shale and carbonate sequences in the Permian Oslo Rift. A >50 0 m long and 100 m wide shale-limestone xenolith embedded within monzo nites belonging to the Skrim plutonic complex experienced high-grade c ontact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I ) assemblages in calcite-saturated rocks include wollastonite, melilit es, fassaitic pyroxenes, phlogopite, titanian grossular, kalsilite, ne pheline, perovskite, cuspidine, baghdadite, pyrrhotite, and occasional graphite. Mineral reactions involving detrital apatite produced a ser ies of silicate apatites, including the new mineral species Ca-3.5(Th, U)(1.5)Si3O12 (OH). This assemblage equilibrated at T = 820-870 degree s C with a C-rich, internally buffered pore-fluid (20-40 mol% CO2 + CH 4). During cooling the shale-limestone xenolith experienced infiltrati on of C-poor (< 0.1 mol% CO2) fluids, triggering the formation of retr ograde (Stage II) mineral assemblages comprising monticellite, tilleyi te, vesuvianite, grandite garnets, diopside, and occasional hillebrand ite. Rare potassium iron sulfides (rasvumite and djerfisherite) formed at the expense of primary pyrrhotite. These assemblages probably form ed near 700 degrees C. Formation of diffuse sodalite-bearing veinlets was associated with breakdown of nepheline and the replacement of kals ilite and wollastonite by potassium feldspar. The sodalite-bearing Sta ge III assemblage formed by the infiltration of saline brines at a max imum temperature of 550 degrees C. Low-temperature (Stage IV) retrogre ssion of the Stage I-m assemblage produced scawtite, giuseppettite, hy drogrossulars, phillipsite, thomsonite, and three hitherto undescribed mineral species.