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, Øse 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 Øse 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.