Geochemical evidence of a near-surface history for source rocks of the central Coast Mountains Batholith, British Columbia

Abstract

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, d 18 O values for quartz separates determined for 19 of the samples range from 6.8 up to 10.0‰. These d 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 d 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 d 18 O values. Subsequently, these materials were buried to lower crustal depths where they remelted to form the high d 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 d 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.

The Architecture, Chemistry, and Evolution of Continental Magmatic Arcs

“…Similarly, the Salinian arc was emplaced into the Cordilleran Paleozoic miogeoclinal sequence and its Mesozoic cover, leading to the formation of magmas enriched in radiogenic isotopes with high δ 18 O (Kistler & Champion 2001, Chapman et al 2014 in the early stages of magmatism. A similar trend was observed in the earlier stages of magmatism in the Coast Mountains batholith (Wetmore & Ducea 2011), with high-δ 18 O magmas initially and plutons that are either S-type or crossover I-to S-type. Over time, the supracrustal input derived from partial melting of these sedimentary sequences decreases after a few tens of millions of years, and plutons become I-type.…”

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“…What stands out for the northern Sierra Nevada transects is the high apparent intrusive fl ux during the earlier, Jurassic stage of magmatism. This is similar to the Coast Mountains batholith, where the initial stages of magmatism were characterized by high apparent fl uxes, intermediate to felsic compositions, and high δ 18 O (Wetmore and Ducea, 2011), suggesting in that case, that the fertile framework represented by continental miogeoclinal rocks provided an important mass contribution to the magmatic budget.…”

“…does not preclude the generation of signifi cant volumes of fractionated granitoids. This has been documented in other geologic settings and is perhaps best exemplifi ed by the generation of the great Coast Mountains batholith of British Columbia and southeast Alaska (e.g., Mahoney et al, 2009;Wetmore and Ducea, 2011). The non-dependence of source lithosphere type on granitoid melt production can be seen in plots showing no relationship between Sr/Y and La/Yb, proxies for depth of melting, and initial 87 Sr/ 86 Sr, a proxy for type and/or maturity of preexisting lithosphere (Fig.…”

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

“…The continental crust has a bulk andesitic composition (Rudnick & Gao, ) even though mantle‐derived magmas produced in continental magmatic arcs, the main tectonic setting where new continental crust is produced, have a mafic composition (Kelemen et al, ). Both geological observations, such as crustal signatures in arc magmas (e.g., Behn et al, ; Buys et al, ; Chapman et al, ; Lackey et al, ; Plank & Langmuir, ; Wetmore & Ducea, ) and numerical modeling (e.g., Castro et al, ; Currie et al, ; Scholl & von Huene, ), indicate that subducted continental material interacts with the mantle wedge and/or the overlying active arc system. Some subducted continental material is recycled into the mantle (e.g., Hilde, ; Scholl & von Huene, ; von Huene & Scholl, ), but other material is also likely added to the active continental magmatic arc.…”