Multiple stages of influx and exsolution of H_2O from anatectic melts during adiabatic decompression, a petrologic catalyst responsible for ^(18)O/^(16)O homogenization of metamorphic core complexes?

Abstract

In the Thor-Odin complex, Holk and Taylor (1993, 1994) showed that mineral δ^(18)O values are uniform in a 5-km-thick section of the middle crust above the Monashee Decollement (MD), just below a 3-km-thick zone of isotopically similar early Tertiary Ladybird leucogranites. This pervasive homogenization of ^(18)O/^(16)O is due to massive influx of metamorphic H_2O; the same H_2O was locally responsible for leucogranite anatexis. Non-uniform quartz δ^(18)O (+8 to + 16) within the MD indicates that this fault was impermeable to aqueous fluids during thrusting, thus isolating the basement from the mid-crustal flow system. LITHOPROBE seismic profiles establish the MD as a west-dipping ramp with 20 km of vertical relief (Cook et al., 1992). Following Carr (1992), we propose an anatectic origin for the leucogranites during decompression melting as the middle crustal section moved up this thrust ramp. Partial melting of pelites and feldspathic grits from late Precambrian Windermere metasediments occurred as muscovite dehydrated at ≈8 kbar and ≈750°C at the base of the Monashee ramp. Controls on production of granitic melt in such rocks are supply of volatiles and amount of feldspar. Metapelites are volatile-rich but feldspar-poor, the opposite of the grit lithologies. Thus, at the base of the ramp large-scale (> 30%) melting of metapelite produced H_2O-satursted magmas (10-11 wt.%). Feldspathic grits underwent less partial melting because of limited H_2O. The pelite-derived melts became mobile and migrated upward, cooling adiabatically and exsolving H_2O. The evolved H_2O exchanged oxygen with lithologies infertile to melting (quartzites, marbles, etc.), concurrently acting as a catalyst driving continued hydrothermal melting of the feldspathic grit layers. Eventually, melting of grit units also exceeded the critical melt fraction. Continued decompression melting and exsolution, both by tectonic shortening and buoyant uprise of magma, went on until final crystallization of the leucogranites hip in the crust, where H_2O was released for a final episode of exchange with unmelted metamorphic lithologies. Thus, repeated episodes of H_2O exsolution and partial melting in the banging wall of the MD are the most likely processes responsible for both the pervasive oxygen isotopic homogenization and the production of the Ladybird leucogranites. It is estimated that 25-30% partial melting of a typical section of Windermere Supergroup occurred as a result of this process. Such large-scale hydrothermal anatexis of thick sequences of clastic rocks during crustal-scale thrust ramping may be a general phenomenon, and may determine the susceptibility of an orogen to subsequent wholesale extensional collapse along detachment faults.

Additional details