Along-Strike Variation in the Magmatic Tempo of the Coast Mountains Batholith, British Columbia, and Implications for Processes Controlling Episodicity in Arcs

The growth of the Coast Mountains batholith has been documented as episodic through time, and it has become a type example of a continental arc system that developed through non-steady-state magmatism. The magmatic record, however, is not well known along the length of the arc, hindering evaluation of the processes controlling the tempo and patterns of batholith growth. A new, robust geochronologic database (485 U-Pb zircon and titanite ages, 120 of which are newly presented herein) covering nearly 1,000km of arc length reveals significant along-strike variation in the tempo of batholith emplacement, the timing of arc cessation, and the arc cooling history. Zircon ages range from 180 to 40Ma along the length of the arc and overlap with titanite ages, with the exception of parts of the central batholith where Eocene extension and exhumation of lower crustal rocks led to a more complex history. New analysis of zircon ages reveals significant along-strike differences in the timing of high flux magmatic events. Small-scale (<150km) intra-arc variations in magmatic tempo suggest that small-scale processes, likely operating within the arc system, appear to have driven the episodic growth of the Coast Mountains batholith. In contrast, rates of Cretaceous-Paleogene eastward arc migration are consistently 2.5km/Myr along the length of the arc. These rates are similar to those documented in North American arc systems, which suggests that arc migration has an external, plate-scale driver and/or is an intrinsic, self-modulating feature of most continental arcs. Plain Language Summary Earth's exterior is composed of tectonic plates that move relative to one another. When two plates converge, the denser of the two is forced down below the other, lighter one. That process causes rocks in the upper of the two plates to partially melt, forming an arcuate welt of new igneous rocks (a batholith) resulting from cooling and crystallization of the melts. Although plate convergence can occur continuously for tens to hundreds of millions of years, melt production in the upper plate appears to happen during punctuated, episodic events. The processes that control such episodicity are not understood. The Coast Mountains of British Columbia formed the upper plate of a large, ancient convergent system where >1,700km of continuous batholith is presently exposed. This study shows that the timing of episodic melting events varied in the Coast Mountains batholith and that variability occurred at relatively short (150km) spatial scales, happening at different times, even with a single plate convergent system. We interpret our results as indicating that episodic melt production is controlled by heterogeneous processes modifying the upper plate and not by a larger-scale factor, such as the rates or relative angles of plate convergence.