Oxygen 18/oxygen 16 and D/H studies of plutonic granitic and metamorphic rocks across the Cordilleran Batholiths of southern British Columbia

Abstract

Hydrogen and oxygen isotope ratios of 500 samples, mainly from granitic plutons, were measured along a 700-km, E-W traverse across the “accreted terranes” of southern British Columbia (latitudes 49°–52°N). Despite the geological complexity and range of intrusive ages (Late Triassic to Tertiary) and although there are “steps” in the isotopic values at some geologic boundaries (e.g., across the Strait of Georgia), two clear patterns emerge: (1) The ^(18)O/^(16)O and D/H ratios of the waters involved in hydrothermal interactions with the granitic rocks show a regular eastward trend of depletion in D and ^(18)O. Enormous areas were affected by the hydrothermal processes, but the most intense alteration is localized along major north trending lineaments (e.g., Okanagan Lake). (2) Independent of the hydrothermal effects, the primary δ^(18)O values of the granitic rocks also change systematically eastward, from +7.0 to +8.5 in Vancouver Island, reaching a minimum of +5.5 to +7.0 in the western and central Coast Plutonic Complex, then increasing progressively from the eastern Coast Batholith to the Okanagan Batholith, and attaining a maximum of +10.0 to +12.0 in the Nelson Batholith. Two groups of samples are unique in their high δD values. The first group is represented by two geographically isolated batholiths (Guichon and Thuya) that were not affected by the Tertiary meteoric-hydrothermal systems and that have therefore preserved their Early Jurassic to Triassic K/Ar ages. The second group is represented by the Jurassic plutons of Vancouver Island; there, the hydrothermal fluids were both D-rich and ^(18)O-rich (δ^(18)O > 0), as evidenced by the fact that feldspars in the altered granites are enriched in ^(18)O relative to coexisting quartz. Both “anomalies” can be explained if these terranes were located closer to the equator and/or in a maritime environment at the time of intrusive and hydrothermal activity, in agreement with available paleomagnetic data. Excluding these anomalous areas, two distinct ages of meteoric-hydrothermal activity can be identified, Cretaceous in the west and early to mid-Tertiary in the east. The isotopic trends in the rocks are similar to the present-day patterns of meteoric waters in the region, with one primary difference: the paleowaters are enriched in D by about 20%, compatible with a northward translation of these terranes, a climatic change, or both. The similarities of the patterns suggest a topography similar to that of the present day (a mountain chain along the coast) during the early Tertiary. The whole-rock δ^(18)O values of the granitic rocks (determined by extrapolation of the trends of δ^(18)O quartz versus δ^(18)O feldspar, to correct for hydrothermal alteration effects) suggest that the main part of the Coast Plutonic Complex formed from magmas similar to those in oceanic island arcs, derived from basaltic crust or from an upper mantle source. Toward the east, an increasingly larger component of sedimentary rocks must be present in the source regions of the granitic magmas. It may be significant that right at the edge of the North American craton there is a drop in the δ^(18)O of the plutons from +11 down to +9.5, suggesting involvement of old granitic crust instead of sedimentary rocks. These new data from southern British Columbia confirm and amplify previous studies that have documented: (1) the existence of widespread and pervasive hydrothermal effects throughout the North American Cordillera, and (2) regular west-to-east asymmetries in the amounts of different kinds of parent rocks in the source regions from which these Mesozoic and Cenozoic granitic magmas were derived.