Low-Grade Metamorphism of Permian Mafic Rocks From the Gorzów Wielkopolski Block (Fore Sudetic Monocline, Nw Poland): Age and Mechanism

Pawet Bylina

doi:10.2478/v10002-007-0004-y

Abstract

Low-Grade Metamorphism of Permian Mafic Rocks From the Gorzów Wielkopolski Block (Fore Sudetic Monocline, Nw Poland): Age and MechanismThe metavolcanic rocks in the Gorzów Wielkopolski area (NW Poland) are andesite-basalts and andesites derived from contaminated within-plate subalkaline basalt melts. K-Ar dating of primary K-feldspar yields an extrusion age of 285±5 Ma. This new date suggests that these rocks may be the youngest dated Permo-Carboniferous volcanic rocks within the eastern margin of the North German Basin.Petrological and micro-thermometric data indicate that the volcanic rocks were altered by a sub-greenschist facies, metamorphic process that proceeded within temperatures and pressures ranging from 195-290°C and 63-96 MPa, respectively. The major metamorphic phases, corrensite, pumpellyite, laumontite and prehnite, formed due to interaction between the original volcanic rock and low-saline Na2SO4-rich hydrothermal fluids. Numerical modelling indicates fluid flow in fractures as the most probable explanation for the low-grade hydrothermal metamorphism. Model calculations suggest that the process was of short duration, ca 140-250 years. K-Ar dating reveals three Mesozoic episodes at 121±2 Ma, 188-190 Ma and 149±4 Ma that can be ascribed to the metamorphism. These ages are similar to previously determined Mesozoic ages from altered volcanic- and clastic rocks in the North German Basin and in

Highlights

Mafic igneous rocks are useful as indicators of geodynamic setting
The focus of this paper is threefold. It investigates the age of Rotliegend metavolcanic rocks from boreholes located in the vicinity of Gorzów Wielkopolski, NW Poland, the P-T conditions of their metamorphism and the reasons for the unusual alteration process they record
The rocks are andesite-basalts and andesites derived from a within-plate, subalkaline basalt melt

Summary

Introduction

Mafic igneous rocks are useful as indicators of geodynamic setting. They are extremely sensitive to chemical changes during metamorphism. Low-grade metamorphism associated with influx of large volumes of hydrothermal fluids will modify chemical and mineral compositions. Under low-grade metamorphic conditions, equilibrium may not be attained and metastable phases may form and persist. Rough estimates only of metamorphic environment may be determined from. The focus of this paper is threefold. It investigates the age of Rotliegend metavolcanic rocks from boreholes located in the vicinity of Gorzów Wielkopolski, NW Poland, the P-T conditions of their metamorphism and the reasons for the unusual alteration process they record

Methods

Findings

Discussion

Conclusion