P–T–t path for the lower plate of the Eocene Tatla Lake metamorphic core complex, southwestern Intermontane Belt, British Columbia

Abstract

The Tatla Lake metamorphic complex (TLMC) is a metamorphic core complex located along the western edge of the Intermontane Belt in southwestern interior British Columbia. Low- to moderate-angle normal faults separate lower plate greenschist- and amphibolite-grade, highly strained, commonly mylonitic rocks from unstrained to weakly deformed strata of the upper plate. The lower plate is divided into a core of granoblastic gneiss and migmatitic tonalite and an overlying, 1–2.5+ km thick mylonitic package called the ductilely sheared assemblage (DSA). Amphibolite-grade metamorphism of the gneissic core (Mc) largely accompanied the development and folding of gneissic layering (ca. 107–79 Ma). Eocene (ca. 55–47 Ma) fabric and mineral assemblages in the DSA (Ms) obscure any earlier history. Three metamorphic zones are observed within southern DSA metapelites with increasing structural depth: chlorite–biotite, garnet–staurolite, and garnet–staurolite–kyanite–sillimanite. The middle zone is about 300 m thick; the latter zone is now about 4 km below low-grade upper plate rocks, indicating late- or post-Ds metamorphic omission. DSA P–T conditions are calculated with the garnet–biotite thermometer and garnet–Al2SiO5–quartz–plagioclase (GASP) and total Al in hornblende barometers. Southern DSA metapelites record Eocene Ms conditions of 480–619 °C (± 50 °C), generally increasing with depth. One sample gave a calculated P–T of 0.72 ± 0.15 GPa and 500 ± 50 °C. P–T data from this area suggest that up to 10 km of structural section may be missing. Zoned garnet (pre-Ds) core to rim GASP pressures of 0.70–0.36 ± 0.15 GPa, for an outcrop-sized pelitic xenolith within a Late Cretaceous tonalitic body (U–Pb: 71 Ma) in the northwestern DSA, record its ascent during pluton emplacement and subsequent Eocene tectonic uplift. A total Al in hornblende crystallization pressure of 0.54 ± 0.1 GPa was calculated for the surrounding body. Biotite and hornblende K–Ar dates of 53.4–45.6 Ma for DSA and gneissic core rocks record cooling of the lower plate through the 530–280 °C (± 40 °C) interval. Mc metamorphism in the gneissic core is thought to have developed in response to crustal thickening and compression, beneath a regional mid-Cretaceous thrust belt. Characteristics of Eocene Ms metamorphism in the DSA, such as truncated and thinned metamorphic zones, are consistent with development during extensional tectonic exhumation of the lower plate.