STRUCTURAL GEOLOGY AND FLUID INCLUSION STUDIES OF EXHUMED HIGH PRESSURE – LOW TEMPERATURE (HP-LT) METAMORPHIC TERRANES: AN EXAMPLE FROM SANTA CATALINA ISLAND, CALIFORNIA, USA

The Santa Catalina Island Schist (SCIS) has been the object of many studies which have focused on the petrology, geochemistry and geochronology. To derive the pressure-temperature-time-deformation (P-T-t-D) path under which the SCIS was exhumed, structural geologic and fluid inclusion analyses are being carried out.

The deformation structures have been studied by combined field mapping and microstructural thin section analysis. Specific attention has been paid to the large-scale faults and ductile shear zones that separate the different grade metamorphic rocks of the SCIS (blueschist, greenschist, and garnet-amphibolite facies metamorphosed sedimentary, volcanic and intrusive protoliths, as well as serpentinite bodies).

Initial results suggest that there are at least three major phases of ductile to semi-brittle deformation, as well as four groups of brittle faults. The early ductile stages of deformation are the result of a NW-SE and a NE-SW contraction. The brittle deformation features are characterized the result of similar NW-SE and NE-SW contractions followed by (younger) NE-SW and NW-SE extension, all in their present-day orientations (regional block rotations have not been removed from the above results).

The P-T constraints are provided by fluid inclusion (FI) studies on quartz veins occurring both within the fault zones and in the SCIS. By combining the FI data (P/T ratio values) with published P-T data derived from mineral thermobarometry, we have been able to add P-T constraints for the later stages of deformation.

Preliminary FI studies show that a low-salinity aqueous fluid was trapped during peak metamorphism and the retrograde P-T path of the SCIS. These results are interpreted to represent fluid trapping during the later stages of slab dewatering. Additionally, it seems likely that many fluid inclusions have been affected by tectonic events related to the exhumation of the SCIS.

We present an internally consistent, preliminary model for the exhumation of the SCIS - based on the structural geologic and fluid inclusion data that incorporates additional published P-T-t data.