HIGH-PRESSURE METAMORPHISM IN THE MUNCHBERG GNEISS COMPLEX AND THE ERZGEBIRGE CRYSTALLINE COMPLEX - THE INFLUENCE OF FLUID AND REACTION-KINETICS

Citation
R. Klemd et E. Schmadicke, HIGH-PRESSURE METAMORPHISM IN THE MUNCHBERG GNEISS COMPLEX AND THE ERZGEBIRGE CRYSTALLINE COMPLEX - THE INFLUENCE OF FLUID AND REACTION-KINETICS, Chemie der Erde, 54(4), 1994, pp. 241-261
Citations number
NO
Categorie Soggetti
Geosciences, Interdisciplinary
Journal title
ISSN journal
00092819
Volume
54
Issue
4
Year of publication
1994
Pages
241 - 261
Database
ISI
SICI code
0009-2819(1994)54:4<241:HMITMG>2.0.ZU;2-0
Abstract
The differences in the PT-estimates derived from various lithologies o f the same geologic unit in the Munchberg Gneiss Complex (MGC) and the Erzgebirge Crystalline Complex (ECC) do not necessarily imply contras ting thermochronologic histories and a sub-sequent tectonic juxtaposit ion, but may be the result of reaction kinetics. For instance, the dec omposition of plagioclase in response to pressure increase generally r equires higher pressures in acid rock compositions than in basic ones. In addition, an overstepping of discontinuous reactions by several ki lobar is not unusual, especially under fluid deficiency and at lower t emperatures. On the other hand, the obliteration of high-pressure (HP) assemblages during decompression can be easier in non-mafic rocks tha n in metabasites or calc-silicate rocks. Leucocratic, quartz-rich rock s can be deformed easier than the more competent metabasites, giving r ise to re-crystallisation and fluid low, which influences interface ki netics and diffusion processes. Moreover, in contrast to omphacite (st abilised in metabasites), the breakdown of jadeite, usually representi ng the equivalent HP-phase in acid rocks, produces no symplectites, wh ich provide compelling evidence of a former HP-assemblage. This behavi our limits the recognition of a HP-history in non-mafic rocks. The sta bilisation, preservation or obliteration of particular mineral assembl ages depends on the metamorphic history. This, however, does not only include the specific PT-trajectory followed by a rock but also the dur ation of the individual PT-stages as expressed in T-t (or P-t) paths. Rapid exhumation due to tectonic processes reduces the time for re-equ ilibration and the reaction progress can be too slow to keep pace with the fast changing PT-conditions. Equilibrium is not or only locally a chieved and, frequently, the reactions are only initiated by considera ble overstepping. Accordingly, the PT-conditions recorded by different bulk rock compositions may scatter in a wide range, and reaction kine tics have to be considered for the reconstruction of a PT-path. In the case of the MGC and the ECC, the different retrograde assemblages, in dicating various PT-conditions, seem to be related to different re-cry stallisation stages which represent the repeated attempt to respond to rapidly changing PT-conditions during a more or less continuous exhum ation process and are not ascribed to discrete thermal 'events'. Furth ermore, an in situ metamorphism is favoured for the metabasic eclogite s in both complexes.