PACROFI VI - Electronic Program


Metamorphic Fluid Inclusions in Quartz from the Zinkgruvan Basin, South-Central Sweden - Some New Ideas

Beverly Chomiak1 and Sten Lindblom2

  1. Department of Earth & Environmental Sciences, New Mexico Institute of Mining and Technology, Socorro, NM, USA 87801
    bchomiak@mailhost.nmt.edu.us

  2. Department of Geology & Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden


Many generations of re-equilibrated, dominantly aqueous, secondary fluid inclusions have been preserved in quartz phenocrysts (now porphyroclasts) in the metaporphyry of the Zinkgruvan Basin (Chomiak & Lindblom, 1995). All the phenocrysts were deformed by crystal-plastic processes. The extent and style of recrystallization, however, varies between locations and reflects different peak temperatures attained during amphibolite grade metamorphism. Differences in the microstructural setting of the inclusions allows us to establish relative ages for the different generations and to subdivide the multimodal Th histogram into meaningful subsets of fluid inclusions.

Pretectonic (type I) fluid inclusions are located in relatively unrecrystallized phenocrysts from the lower amphibolite grades. The inclusions form planar arrays of linear clusters (~40 microns long) whose orientation is coincident with intersecting sets of sub-basal deformation bands in the original host quartz. A cluster typically consists of a larger (2-4 microns) central inclusion and many smaller monophase satellite inclusions. The inclusions are removed from new grains which exhibit c-axis parallel deformation lamellae indicative of higher temperature (>600oC) prism slip (Okudaira et al., 1995). These relations suggest to us that the fluid inclusions predate peak metamorphism, and were stretched and necked down during pre-peak (T<600oC) deformation. The inclusions do not homogenize below 600oC, and we deduce that they were re-equilibrated at temperatures higher than their entrapment.

Synrecrystallization (type III) fluid inclusions are present in all metaporphyries. The inclusions are small (<10 microns) and display a few negative crystal faces. Inclusion planes may be crosscut, or truncated by, new grain boundaries within original phenocrysts. Many phenocrysts exhibit primary and secondary recrystallization (ie. newer grains inside new grains). In these phenocrysts three fluid inclusion generations with different Th distributions can be recognized: type IIIB (Th~320oC); IIIC (Th~240oC); and IIID (Th~180oC). Th increases with inclusion age. The inclusion planes have preferred orientations only on the scale of a single phenocryst. Where three or more sets of planes hosting same generation inclusions are present they usually intersect in a common axis. We no longer interpret these directions to be relict principal stress directions, because real angles between related planes are not random (as expected for flattening), nor do they converge on a single intersection geometry (as expected for plane strain). Instead, they resemble the intersection geometry of known slip planes in quartz with hexagonal symmetry. This circumstantial evidence suggests to us that at least some of the fluid inclusion planes originated as microfractures in beta-quartz, at near peak metamorphic conditions. Deformation structures (eg. prism lamellae) in the primary recrystallized grains support this idea. Inclusion morphology suggests re-equilibration with internal overpressures (Sterner & Bodnar, 1989). However, calculated isochores plot on the higher density (lower molar volume) side of peak metamorphic PT conditions constrained by mineral equilibria (Rickard, 1988). This suggets re-equilibration with internal underpressures if the fluid inclusions originated during or after peak metamorphism. This discrepancy will be addressed in future.

Retrograde (type V) fluid inclusions also are present in all the metaporphyries. The inclusions are larger (up to 20 microns) and assume irregular shapes. The fluid inclusion planes crosscut or are deflected into new grain boundaries, but straight transgranular segments exhibit preferred orientation on the scale of a thin section. Average Th are lower than those for the last synrecrystallization inclusions, but Th overlap in part. Morphological features indicative of re-equilibration are lacking.

Features on the Th histogram take on new meaning. Homogenization temperatures decrease in younger fluid inclusions. The younger fluid inclusions which exhibit low Th (<200oC) also display bimodal Th distributions which are attributed to different amphibolite subfacies. Mean Th for same generation fluid inclusions in the upper and lower grade subfacies may differ by 30o. The older fluid inclusions which exhibit higher Th generally display a greater range of Th (by subfacies) but unimodal distributions. The significance of these features, however, cannot be addressed until the PT history is better constrained.

References



Fig. 1 Th (oC) for all fluid inclusions studied. In all cases homogenization was to the liquid state.


Table 1. Statistical parameters on Th distributions for fluid inclusions of specific generations.
PretectonicSyntectonicSyntectonicSyntectonicSyntectonicRetrograde
1IIIAIIIBIIICIIIDV
mean Th600+455335239162149
modal Th435335240186/153171/146
N1324566840
range14010477102103
s3530192225
skew+1.51+0.30+0.11-0.43-0.09