ALONG-STRIKE VARIATION IN THE MAGMATIC DEVELOPMENT OF THE COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA

One hundred new zircon U-Pb ages from plutons in the southern Coast Mountains batholith are compiled with 423 existing ages to create a robust, whole-arc geochronologic database. This database is used to investigate magmatic patterns and to probe the causes of spatial and temporal changes in the growth of an arc developing above a long-lived subduction zone. Intrusive ages range from ~220 – 40 Ma along the 1000 km of the batholith exposed from Vancouver, British Columbia into SE Alaska. We focus here on the post-120 Ma record, by which time a single magmatic arc was clearly established. The timing of magmatism, the width of batholith at various time intervals, and arc migration rates all change markedly along the strike of the batholith. Based on the distribution of zircon U-Pb ages, the northern half of the batholith is dominated by young (~60 - 50 Ma) intrusions and to a lesser degree, by those with ages between ~95 – 80 Ma. In contrast, the southern half of the batholith is dominated by plutons with ages ranging from ~115 – 90 Ma. Further subdivision of the batholith into arc-perpendicular transects shows that the young magmatism so prevalent in the north decreases southward, both in terms of number of ages as well as areal extent of dated intrusions. Likewise, the number of 115 – 90 Ma ages and the width of the batholith across which those ages are determined increases southward. Newly recognized 80 – 70 Ma plutons are a significant component of the southern batholith, intruding during a period of relative magmatic inactivity to the north. In addition to these age variations, the rates and patterns of arc migration also vary along strike. The arc generally migrates eastward, but migration rates decrease from north to south along the arc. Moreover, the trailing (outboard) edge of magmatism stalls in the northernmost part of the arc at about 90 Ma and remains stationary through 50 Ma, a trend not observed to the south. Many of these magmatic trends vary spatially on the order of 100 km arc length. This scale suggests that the magmatic growth of the batholith is likely controlled by spatially complex changes in the upper plate rather than larger-scale variability of a presumably uniform subducting plate.