EVIDENCE AND IMPLICATIONS OF SYNKINEMATIC CONTACT METAMORPHISM AROUND THE WAXHAW PLUTON, SOUTH-CENTRAL NORTH CAROLINA

The heterogeneously deformed Waxhaw granite (WG) intrudes low-grade volcaniclastic rocks in the Carolina zone (CZ) near Waxhaw, NC. Geologic mapping in the vicinity of Waxhaw, NC indicates the WG lies in the footwall of the Gold Hill fault (GHf), which in this area is a steep, NW-dipping, dextral reverse-oblique fault that separates CZ volcaniclastic rocks of differing age and composition. Mapping along the NW-boundary of the WG has revealed that volcanic and semi-pelitic rocks in that area have been thermally metamorphosed by the pluton. The resultant metamorphic aureole extends approximately one km into the country rock and progresses from a sillimanite-in isograd near the pluton to an andalusite-in isograd around the perimeter of the aureole.

Metamorphic minerals in semi-pelitic and volcanic rocks in the contact aureole can be divided into a primary assemblage and a secondary assemblage. The primary assemblage consists of And + Sil + Bio + Chl + Ms + Qtz +/- Str within the sillimanite-isograd and And + Chl + Bio + Ms + Qtz within the andalusite-isograd. These minerals are partially to completely replaced by a secondary assemblage consisting of Ctd + Grt + Ms + Hem + Mgn + Wm + Fe-Chl within the sillimanite-isograd and Fe-Chl + Wm + Ms + Qtz within the andalusite-isograd.

Microstructural and metamorphic textures suggest that contact metamorphism attributed to the WG was synkinematic with respect to deformation along the GHf. Metamorphic conditions during the growth of the primary assemblage are estimated at 2.0-2.5 kbars and 520 to >600 degrees C. The secondary assemblage is interpreted to record an increase in pressure to 3.5-4.0 kbars during contact metamorphism followed by a period of isobaric cooling within the aureole at approximately 4.0 kbars. The increase in pressure that facilitated growth of the secondary assemblage is attributed to NW over SE thrusting along the GHf. An Rb-Sr whole rock age obtained from the WG is considered tenuous at this time and a new high resolution U-Pb zircon age is pending. In the meantime, circumstantial evidence, including recently reported Ar/Ar cooling ages, suggest the WG is a Carboniferous pluton, thus deformation along this portion of the GHf appears to be late Paleozoic.