AGE AND ORIGIN OF LATE JURASSIC AND PALEOCENE GRANITOIDS, NELSON BATHOLITH, SOUTHERN BRITISH-COLUMBIA

In the Middle Jurassic Nelson Batholith, southern British Columbia, yo ung Ar-40/Ar-39 ages (i.e., 50-60 Ma) and distorted isobaric surfaces in the batholith suggest the possibility of Paleocene granitic plutoni sm. We present the results of a study undertaken to evaluate this poss ibility. Geochemical criteria successfully distinguish a suite of gran itoids within the Nelson Batholith that differ from Nelson granites of similar SiO2 content. The granitoid suite is composed of 71.6-75.7 wt .% SiO2 leucocratic biotite granite and quartz monzonite with strong e nrichments in alkaline, alkaline earth, and rare earth elements. Nd an d Pb isotopic compositions suggest that biotite granite and quartz mon zonite are not related. Biotite granite yields a U-Pb age of 158.9 +/- 0.6 Ma (concordant zircons). Quartz monzonite crystallized at 61 +/- 1 Ma, based on interpretation of titanite and zircon analyses. Zircons from this sample lie along a line from 61 to 160 Ma and demonstrate t he presence of Middle Jurassic inheritance. Based on its petrographic and isotopic similarity to other Middle Jurassic plutons in the Nelson Batholith - Valhalla Complex area, we include the 159 Ma biotite gran ite with the Jurassic plutonic suite. This result demonstrates that ma gmatism in southern British Columbia was active at least until the ear ly Late Jurassic (Oxfordian). The Paleocene (61 Ma) quartz monzonite t hat intrudes the southern Nelson Batholith is the structurally highest occurrence of ''Ladybird'' granite yet documented in southern British Columbia. Comparison of new and published geochemical and isotopic da ta for Paleocene granitoids throughout the southern Omineca Belt, Brit ish Columbia, suggests that these granitoids were not derived from a s ingle, old crustal source.