HIGH-PRECISION GEOCHRONOLOGY OF PLUTONS IN THE SOUTHERN COAST PLUTONIC COMPLEX: INSIGHTS INTO MAGMA RESIDENCE, MAGMA LOADING AND MECHANISMS OF ARC MAGMATISM

The Coast Plutonic Complex was a long-lived magmatic arc situated along the Mesozoic convergent margin of NW North America. The complex interplay between arc magmatism, terrane accretion and attendant deformation and metamorphism is evident in the Harrison Lake area of southern British Columbia, where Mesozoic plutons have been instructive regarding mechanisms of pluton emplacement in a convergent arc setting. Brown and McClelland (GSAB 2000) interpreted the plutons to have formed by horizontal sheeting and vertical inflation based on: 1) most are shallowly floored, zoned batholiths with sheeted margins, 2) metamorphic aureoles record a history of increasing pressure from <0.3 GPa to >0.6 GPa, and 3) U-Pb geochronology indicated crystallization periods of as much as 8 Ma for individual batholiths, a consequence of construction by episodic pulses of magma.

A critical test for the magma loading hypothesis is to very accurately and precisely date the plutons and their contact aureoles using the latest geochronology methods to assess if a direct correlation can be made between pluton construction and aureole crystallization. Presumably, older portions of a pluton should correlate with the age of the earlier, lower pressure aureole assemblages, whereas younger pluton ages should approach the age of the higher pressure assemblages.

We dated 10 samples from plutons and two from smaller granitic bodies in the Harrison Lake area using the U-Pb zircon CA-IDTIMS method pioneered by James Mattinson. Prior to dating, CL images of zircon interiors were obtained to aid in selection of crystals or parts of crystals devoid of complexities such as inherited cores and metamorphic overgrowths. The highly precise dates are concordant with internal errors on individual single grain analysis of ± 0.02-0.03%, which for most samples equates to ± 20-30 ka. The results allow us to significantly refine the ages of the plutons, some by 1-2 Ma and others by up to 50-100 Ma. Most plutons exhibit varying degrees of magma residence, with many having a range of zircon ages of 1 Ma. Some samples also indicate a prolonged period of emplacement of individual plutons between 1 to 12 Ma. These data point to a prolonged construction of the plutons, consistent with the magma loading hypothesis. Additional work will focus on the age of metamorphism in contact aureoles.