Elemental and Isotopic Evidence for Granitoid Genesis From Deep-Seated Sources in the Coast Mountains Batholith, British Columbia

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British Columbia, arc magmatism, batholiths, magmatic flux, garnet, pyroxene, amphibole

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Major element, trace element and Nd–Sr isotopic data are presented for 82 plutonic rocks from the southern Coast Mountains Batholith (CMB) in British Columbia, Canada, ranging in emplacement age from 210 to 50 Ma. The rocks are part of a large composite magmatic arc batholith, which the major element data show to be of calc-alkaline affinity. The majority of CMB samples lack the depletion in Eu that would be consistent with equilibration of magmas and plagioclase-bearing crystalline residues or fractionates, suggesting that equilibration took place deeper than the pressure limit of plagioclase stability at 35–40 km depth. The CMB samples show a wide variation in the slope of normalized rare earth element (REE) patterns, with chondrite-normalized La/Yb ratios above 10 being mostly confined to periods of high magmatic flux in the arc at 160–140, 120–80, and 60–50 Ma. The clearest relationships between major and trace elements are negative correlations between SiO2 and each of Sc, Y, and the heavier REE Gd to Lu. Nd and Sr isotopes mostly document juvenile origins for the granitoids, but show variations to higher 87Sr/86Sr and lower εNd during high-flux periods. The results are interpreted to indicate a deep origin for most CMB magmas, below ∼40 km where mafic to intermediate rock assemblages previously added to the arc crust by mantle melting were transformed to an (amphibole-bearing)-eclogite facies cumulate or restite, such that melting residues consisted mainly of two pyroxenes, garnet and variable proportions of amphibole. Thickened orogenic crust, for which there is clear geological evidence during the period 100–80 Ma, promoted this process. During high-flux periods, larger amounts of older rocks, mostly mafic rocks and some metasediments added to the base of the arc during orogenic shortening, became involved in magma genesis.

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Journal of Petrology, v. 53, no. 7, p. 1505-1536