HYDROGEN ISOTOPE EVIDENCE FOR GIANT METEORIC-HYDROTHERMAL SYSTEMS ASSOCIATED WITH EXTENSION AND MAGMATISM IN THE SOUTHERN CANADIAN CORDILLERA

Abstract:

Over 400 published mineral and fluid inclusion δD values from the southern Canadian Cordillera and our new data from the Eocene Penticton Group Volcanics and Coryell Intrusive Suite of the Southern Omineca Belt and the Western Metamorphic Belt of the Central Coast Orogen are compiled using GIS. δD_H2O is estimated using published D/H fractionation factors at 400°C; the error is ±20‰, small enough to distinguish deep magmatic/metamorphic fluids from meteoric-hydrothermal fluids. Histogram plots of δD_H2O values estimated from minerals reveal peaks at δD = –60‰ (deep fluid) and ­–110‰ (Early Cenozoic meteoric-hydrothermal fluid); this provides a clear distinction between the two kinds of fluid. Our analysis reveals that syn-extensional meteoric-hydrothermal systems (δD_H2O < –80‰) affected the eastern margin of the Coast Ranges Batholith between latitude 49° and 55° and the Omineca Belt between latitude 49° and 52°45’; both regions were affected by detachment faulting during late stages of magmatism in the Early Cenozoic (e.g., Parrish et al., 1988; Crawford et al., 2009). Zones that escaped the effects of meteoric-hydrothermal systems, preserving the D/H signature of deep fluids (δD > –80‰), include the Western Metamorphic Belt, the Western and Central Coast Ranges Batholith, the belt of Jurassic metamorphism that extends from the Cariboo Mountains to the Purcell Mountains, and the deepest structural levels of the Shuswap Metamorphic Core Complex; most of these samples have quartz-feldspar ¹⁸O/¹⁶O fractionations indicative of magmatic temperatures. High δD_H2O values (> –50‰) suggest seawater alteration of the plutons of Vancouver Island (Magaritz and Taylor, 1986). Histogram plots of vein quartz fluid inclusion δD values (Nesbitt and Muehlenbachs, 1995) reveal three peaks that include the two produced by the mineral δD values, but these data are dominated by a large peak at δD = –150, a value similar to modern meteoric waters in the region; this suggests either a young age for these veins or recent resetting of fluid inclusion δD values. This sensitivity to D/H exchange calls into question the reliability of fluid δD values derived from fluid inclusions. These meteoric-hydrothermal systems of the Canadian Cordillera may have been the largest on Earth.