Abstract

Nd, Sr, and Pb isotopic analyses are presented for plutonic and volcanic rocks from a northeast-southwest transect across the southern Coast Plutonic Complex (CPC). Plutonic and volcanic rocks have initial ϵNd of +4.2 to +8.9 and initial 87Sr/86Sr ratios of 0.702 44–0.704 83. Values of Pb isotopic ratios for the suite range from 18.23 to 19.03 for 206Pb/204Pb, from 15.43 to 15.67 for 207Pb/204Pb, and from 37.54 to 38.30 for 208Pb/204Pb. The isotopic data suggest that these rocks consist mainly of mantle-derived materials with little or no involvement of old continental material. There are no significant variations in Nd and Sr isotopic composition with rock type or age. Pb isotopic compositions are also independent of rock type and age except for 208Pb/204Pb, which, for a subset of volcanic rocks, rises slightly with decreasing age. Examined against position along the transect, the isotopic data are uniform except for a rise in initial 87Sr/86Sr toward the northeast defined by older metavolcanic rocks. Based on the Nd and Pb isotopic data, a number of igneous rocks from the southern CPC appear to have been derived strictly from partial melting of mantle material. Many intrusions, however, have isotopic compositions and tDM ages consistent with the combined effects of melting of mantle and subordinate anatexis of isotopically juvenile terranes (e.g., Wrangellia).

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