Water as a Petrologic Catalyst Driving ~(18)O/~(16)O Homogenization and Anatexis of the Middle Crust in the Metamorphic Core Complexes of British Columbia

摘要

In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral #delta#~(18)O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz #delta#~(18)O = 12.3 +- 0.5 per thousand (1 #sigma# S.D.) for metapelite (N = 11), 12.0 +- 0.1 per thousand for quartzite (N = 2), 12.6 +-0.6 per thousand (N = 4) for < 1 m thick amphibolite layers, and 12.1 +- 0.4 per thousand (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable ~(18)O/~(16)O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of ~(18)O/~(16)O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to l.5 per thousand lower in #delta#~(18)O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in ~(18)O/~(16)O must be a result of exchange with magmatic or metamorphic H_2O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz #delta#~(18)O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphic-hydrothermal system.LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H_2O, aided by internal muscovite dehydration at ≈8 kbar and ≈750 deg C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (≈30 percent) partial melting of the metapelites produced magmas near H_2O saturation (10 to l4 wt percent), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt percent). As these H_2O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H_2O. The released H_2O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H_2O was released and again re-used for a final episode of ~(18)O/~(16)O exchange with the unmelted metamorphic lithologies. In addition to the direct ~(18)O/~(16)O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H_2O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30 percent partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes