MAKING SENSE OF MULTIPLY DEFORMED SHEAR ZONE ROCKS USING INTEGRATED EBSD, CVA, AND PETROCHRONOLOGIC ANALYSES (Invited Presentation)

We present a new method of linking microstructures, EBSD-derived crystallographic vorticity axis (CVA) analysis, and petrochronology to attribute fabric development to specific deformation events in shear zone rocks with complex histories. Specifically, we compare single phase CVA patterns of fabric-forming minerals with those of synkinematic petrochronometers (e.g., titanite) to associate the timing of fabric development with deformational events. This approach is particularly useful in complexly deformed rocks where overprinting is incomplete, such that it is unknown which fabric is being dated by the petrochronometer. Here we present a case study to understand shear zone fabric development in the George Sound Shear Zone (GSSZ), in Fiordland, New Zealand. The GSSZ is significant in that it is one of the largest, Cretaceous intra-arc shear zones in Fiordland, and it developed along a pre-existing crustal-scale discontinuity that was a locus of pluton emplacement for two high Magma Addition Rate (MAR) events.

The GSSZ forms the contact between the Carboniferous Large Granite (1^st MAR event) and the Cretaceous Misty Pluton (2^nd MAR event). Here, the GSSZ is ~3-5 km wide, and mylonitic fabrics are developed within both the Misty pluton and the Large Granite; mylonitic fabrics are also observed within the Large Granite outside of the GSSZ. CVA analyses from the GSSZ mylonites reveal distinctive patterns in the Large Granite and Misty diorite that we attribute to Carboniferous (transtension) and Cretaceous (transpression) periods of fabric development, respectively. Single phase CVA analysis of the Large Granite shows that quartz and plagioclase preserve the transtensional CVA pattern, whereas biotite and titanite preserve a transpressional CVA pattern. Importantly, U-Pb titanite petrochronology of the Large Granite shows only Cretaceous titanite growth at 122 +/- 2 Ma. Therefore, titanite dates are linked to the transpressional fabric and cannot be used to infer the timing of the quartz and feldspar fabric development (presumably Carboniferous). These results demonstrate the necessity of combining EBSD and CVA analysis with petrochronology to demonstrate that synkinematic accessory phases show the same CVA pattern and kinematic deformation geometry as the fabric being dated.