POST-ENTRAPMENT MODIFICATION OF FLUID INCLUSIONS DUE TO OVERPRESSURE - EVIDENCE FROM NATURAL SAMPLES

Effects of post-entrapment fluid-inclusion modification are examined with reference to retrogression-related quartz veins from the Caledonian, Ose Thrust, northern Norway. The inclusions occur in secondary trails, and contain high-density hypersaline aqueous fluids. On morphological characteristics, they are subdivided into, Type A: elongate, ellipsoidal and/or irregular inclusions, and Type B: more equant, regular, and/or negative crystal form. With reference to previous research on post-entrapment modification of inclusions in quartz it is proposed that Type A inclusions experienced little or no post-entrapment modification, whereas Type B inclusions show features characteristic of post-entrapment permanent inelastic stretching and/or leakage. This produces increased homogenization temperatures (T-h), associated with increased inclusion volume and lowering of density, whilst maintaining constant salinity. The similarity of data for degree of fill and salinity between Type A and Type B inclusions indicates that Type B inclusions have primarily modified by stretch rather than leakage. However, the spread towards slightly larger volume of vapour in Type B inclusions suggests that some leakage has also occurred. Because stretched and/or partially leaked inclusions have increased T-h, isochore projections significantly underestimate trapping pressure (P-t) relative to unmodified inclusions. Therefore, recognition of post-entrapment inclusion modification due to overpressure is crucial to avoid misinterpretation of data, but has considerable potential for constraining the detail of P-T trajectories of individual rocks. On this basis, rocks from the Ose Thrust zone, north Norway, are shown to have experienced rapid uplift on a 'clockwise' P-T-t path during the final stages of Caledonian (Scandian) orogenesis.