GARNET GROWTH, RESORPTION, AND COMPOSITIONAL ZONING AS RECORDS OF COLLISIONAL EVENTS: SALMON RIVER SUTURE ZONE, IDAHO

Western North America includes classic examples of accreted terranes; however, some aspects of the duration and nature of continental growth by accretion are still poorly understood. Metamorphic rocks found along terrane boundaries provide a record of accretion; yet interpretations can be difficult. This study evaluates possible processes and events causing garnet zoning in the Salmon River suture zone (SRSZ) in west-central Idaho.

Regional metamorphism may result from crustal thickening during collisional events. Minerals, such as garnet, record conditions and timing of collisional events as they grow, often resulting in compositional zoning and inclusion density changes. In the SRSZ garnets contain two distinct stages that preserve abrupt inclusion density and garnet compositional zoning pattern changes across a sharp core-rim boundary. These abrupt changes may be attributed to: 1) multiple terrane accretions, 2) thrust faulting and/or plutonic events, or 3) mineralogical reactions during a single accretion.

Samples with 2-stage garnet from the SRSZ were selected for pseudosection modeling using THERIAK-DOMINO. P-T conditions were determined using major element garnet zoning and observed mineral assemblages. Inclusions in garnet core and rim were used to define prograde mineral assemblages that equilibrated with garnet. P-T path predictions indicate an increase in P from 7 to 9 kbar at 625°C during garnet core growth. The garnet rim crystallized over a range starting at 9 kbar 625°C and ending at 11 kbar 700°C. Predicted garnet modes indicate growth of garnet punctuated by resorption during staurolite growth along this P-T path. Therefore, staurolite stability from 610 to 625°C provides an explanation for the 2-stage garnets. This P-T path of heating then loading likely occurred during a single accretionary event. Garnet Sm-Nd ages and trace element zoning, in progress, will be used to further support this hypothesis.