HETEROGENEOUS GENERATION AND EVOLUTION OF DIAGENETIC QUARTZARENITES IN THE SILURIAN-DEVONIAN FURNAS FORMATION OF THE PARANA BASIN, SOUTHERNBRAZIL

Silurian-Devonian sandstones of the Furnas Formation in the intracrato nic Parana Basin, southern Brazil, display strongly heterogeneous dist ribution of quartz, kaolinite and illite authigenesis. The sandstones were affected by extensive dissolution/kaolinization of detrital felds par, mica and clay under an eodiagenetic meteoric regime. Circulation of hot fluids through fractures related to the uplift and magmatism al ong the regional Ponta Grossa Arch promoted heterogeneous kaolinite an d feldspar illitization and quartz cementation. This is supported by i ntense illitization of kaolinite at shallow depths, by K/Ar dating of the illite, whose age coincides with the magmatism, and by high fluid inclusion homogenization temperatures in quartz overgrowths from outcr op-samples. The progressive destruction of feldspar generated diagenet ic quartzarenites with poorly connected pore systems and low reservoir potential. Evidence yielded by this study suggests that the applicati on of simplified 'isochemical' models of diagenetic processes within t he system SiO2-Al2O3-K2O-H2O may have very limited validity, applying basically to the deep-burial mesodiagenesis of some sandstones in rapi dly subsiding basins. (C) 1998 Elsevier Science B.V. All rights reserv ed.