18O/16O and D/H studies along a 500 km traverse across the Coast Range batholith and its country rocks, central British Columbia

Abstract

Early Tertiary and Mesozoic ground waters in British Columbia were low in δ18O and δD, making it easy to document interactions of meteoric-hydrothermal H2O with the Coast Range plutons. The isotopic data on the igneous rocks can be grouped as follows:[Formula: see text]Zone I represents the gneissic migmatite core of the batholith, west of the quartz-diorite line. Zone II is the granodioritic eastern part of the batholith, and Zone III is a broad zone that includes the Jurassic Topley intrusions and extends from the eastern edge of the batholith to the Pinchi fault zone. Only in Zone I are isotopically 'normal' plutonic rocks found, and even there small amounts of meteoric water were apparently responsible for the late sericitization. All of the rocks in Zones II and III underwent widespread interaction with hot meteoric H2O, including the volcanic and sedimentary country rocks which have δD = −113 to −167 and δ18O = 1.9 to 10.8. Values of δD biotite < −140 are found in essentially all of the low-18O rocks, as well as in those that have 18O-zoned quartz or high Δ18Oqz–feld. Dike rocks have δD similar to their host rocks, but are typically lower in 18O (has the finer grain size facilitated exchange?). The batholithic intrusions apparently created gigantic meteoric-H2O circulation systems, larger than has heretofore been documented. The calculated δD of the H2O is −120 ± 20 assuming T = 500 to 200 °C, indicating that about 45–55 m.y. ago, the meteoric H2O had a uniform δD throughout the area. However, samples from the 140 m.y. old Topley intrusions suggest more D-rich waters, perhaps indicating a warmer climate in the Jurassic. These higher-temperature meteoric-hydrothermal effects are not found east of Pinchi Lake, a terrane that includes the Permian Cache Creek formation and various sediments, volcanic rocks, and blueschists that have δD = −65 to −117 and δ18O = 13.5 to 28.4. This area lacks igneous intrusions, so the meteoric H2O interactions in this area occurred at much lower temperatures than in Zones II and III (≈ 100 °C). Only the finest-grained rocks underwent partial D/H exchange with the meteoric waters responsible for local silicification, serpentinization, and vein carbonate deposition.