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
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.