Abstract

The ca. 100 Ma Cassiar batholith is the single largest plutonic body in the hinterland of the Canadian Cordillera and is part of widespread middle Cretaceous to Eocene magmatism that occurred in the Omineca crystalline belt. The Cassiar batholith is dominated by muscovite-biotite granite and biotite ± muscovite granodiorite along with subordinate biotite ± hornblende granodiorite, quartz monzodiorite, and quartz monzonite. The batholith has a wide range of strontium, neodymium, and oxygen isotope compositions [(87Sr/86Sr)i = 0.706 to 0.734; ϵNd = −2.7 to −17.1, TDM = 2000 to 980 Ma; δ18O = +7.6‰ to +11.6‰], but the batholith's lead isotope compositions are limited (206Pb/204Pb = 19.14 to 19.24; 207Pb/204Pb = 15.70 to 15.72; 208Pb/204Pb = 39.14 to 39.26). These data require that the Cassiar magmas have been generated from at least two distinct sources—an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition. We propose that the felsic source comprised Mesoproterozoic to Neoproterozoic metasedimentary rocks and Paleoproterozoic basement rocks of the North American cratonic margin. Determining the mafic source is more problematic, but most likely it was Proterozoic basement rocks of mafic to intermediate composition, such as have been reported from various parts of the Omineca crystalline belt.

The Cassiar batholith is lithologically, geochemically, and isotopically similar to other major batholiths in the Cordilleran Interior, including the batholiths of southeastern British Columbia and the Idaho batholith. We interpret these similarities to reflect melting of common crustal source materials, namely Proterozoic basement and cover rocks of western Laurentia. The Cassiar batholith and other Cordilleran Interior granitoids are distinct from granitoids generated in known subduction-zone environments in terms of their mineralogical, geochemical, and isotopic characteristics. These differences reflect a predominantly or exclusively crustal source region for the Cordilleran Interior granitoids. In contrast, subduction-zone magmatism contains a significant mantle input, even in cases where subduction takes place beneath thick continental crust. These results strongly suggest that Cordilleran Interior granitoids did not form directly from a subduction setting. Rather, an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension.

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