Geochemical Evidence of A Near-Surface History for Source Rocks of the Central Coast Mountains Batholith, British Columbia

Major and trace elemental concentrations as well as Sr and Pb isotopic data, obtained for 41 plutonic samples from the Coast Mountains Batholith ranging in age from ∼108 to ∼50 Ma, indicate that the source regions for these rocks were relatively uniform and typical of Cordilleran arcs. The studied rocks are mineralogically and chemically metaluminous to weakly peraluminous and are mainly calc-alkaline. Initial whole-rock ⁸⁷ Sr/ ⁸⁶ Sr ratios range from 0.7035 up to 0.7053, whereas lead isotopic data range from 18.586 to 19.078 for ²⁰⁶ Pb/ ²⁰⁴ Pb, 15.545 to 15.634 for ²⁰⁷ Pb/ ²⁰⁴ Pb, and 37.115 to 38.661 for ²⁰⁸ Pb/ ²⁰⁴ Pb. In contrast to these relatively primitive isotopic data, δ ¹⁸ O values for quartz separates determined for 19 of the samples range from 6.8 up to 10.0‰. These δ ¹⁸ O values preclude the possibility that these melts were exclusively generated from the Mesozoic mantle wedge of this continental arc, just as the Sr and Pb data preclude significant involvement of an old (Precambrian) crustal/mantle lithospheric source. We interpret the high δ ¹⁸ O component to represent materials that had a multi-stage crustal evolution. They were originally mafic rocks derived from a circum-Pacific juvenile mantle wedge that experienced a period of near-surface residence after initial crystallization. During this interval, these primitive rocks interacted with meteoric waters at low temperatures, as indicated by the high δ ¹⁸ O values. Subsequently, these materials were buried to lower crustal depths where they remelted to form the high δ ¹⁸ O component of the Coast Mountains Batholith. This component makes up at least 40% (mass) of the Cretaceous through Eocene batholith in the studied area. The remainder of the source materials comprising the Coast Mountains Batholith had to be new additions from the mantle wedge. A prolonged period of contractional deformation beginning with the Early Cretaceous collisional accretion of the Insular superterrane is inferred to have been responsible for underthrusting the high δ ¹⁸ O component into the lower crust. We suggest that mafic rocks of the Insular superterrane (e.g. Alexander–Wrangellia) are of appropriate composition, and were accreted to and overthrust by what would become the Coast Mountains Batholith just prior to initiation of magmatism in the region.