Cordierite and its retrograde breakdown products as monitors of fluid-rock interaction during retrograde path metamorphism : case studies in the Schwarzwald and the Bayerische Wald (Variscan belt, Germany)

Abstract

Retrograde breakdown products of cordierite are commonly termed pinite, but their modes, compositions and formation conditions are only poorly known. A systematic study on pinitised cordierite from high-temperature metamorphic pelites of the Schwarzwald and the Bayerische Wald using electron microprobe (EM), transmission electron microscope (TEM), scanning electron microscope (SEM), x-ray diffraction, petrographic microscope and Fourier Transformation Infrared Spectroscopy (FTIR) investigations was carried out. On the basis of composition, phase assemblage, textural position and grain size four pinite types (border, mat, fissure, isotropic type) were distinguished that formed by distinct allochemical processes under different pressure-temperature conditions at different times. They probably represent general features of cordierite breakdown. Border-type (b-type) pinite consists of muscovite and biotite, formed at 350 - 550 °C from a K+-bearing fluid, most likely derived from the breakdown of K-feldspar to muscovite and quartz. B-type pinitisation may be related to granite intrusion in the Carboniferous. Mat-type (m-type) pinite encompasses chlorite-muscovite pinite and complex m-type pinite, bearing clay minerals, and is in general considered to represent the alteration of cordierite by an alkli-bearing fluid. Petrogenetic grids define an upper T limit for chlorite-muscovite of around 500 - 550 °C. Complex clay m-type pinite of this study has the principal constituents chlorite, berthierine, I/S R1 and I/S R0, Na/K-illite and random chlorite/berthierine mixed-layers. The observed features are similar to those of non-equilibrium assemblages from the early and the late diagenetic zones as complex m-type pinite lacks stable equilibrium and should have been formed much below 200 °C. In this case, its genesis is a complex two-stage stage process, with chlorite and I/S R1 representing the primary product of m-type pinitisation. Secondary berthierine and I/S R0 are formed by the replacement of chlorite and I/S R1 driven by the infiltration of an additional very low-grade fluid. M-type pinitisation could be related to low-temperature meteoric alteration of granites. F-type pinites form alteration veins penetrating intact cordierite. They are filled with tiny ( K). A dramatic fall in Si counting rates occurred during electron beam exposure at standard conditions, suggesting that i-type pinites may represent amorphous gel-like highly hydrated material. This assumption is confirmed by the occurrence of fissures penetrating i-type pinite, which are strongly suspected to represent dehydration shrinkage fissures. Unfortunately, TEM could not give further evidence. Therefore, the phase inventory and structural state of i-type pinite remain unclear. I-type pinitisation is probably a leaching process at very low temperatures (weathering) with network-modifying species being preferentially dissolved. H2O and CO2 contents of partly pinitised cordierite were determined by in situ FTIR measurements in order to detect late-stage modifications of the cordierite channel compositions, possibly related to pinitisation. The volatile contents of all investigated samples were found to be consistent with equilibration with a H2O undersaturated melt, during high-grade metamorphism. Therefore, a systematic change of cordierite channel volatile contents as a key step within the process of pinitisation is not indicated. Nevertheless, the crystallographic control of f-type pinitisation implies that the channel structure determines the c axis to be the preferred direction of cordierite dissolution/leaching.