Abstract

We address on a regional scale the controversial problem of determining the source materials involved in the formation of the granites of southwestern North America. Compositional data on plutonic biotites, whole-rock chemistry, and mineral assemblages in rocks collected as part of sampling traverses across die batholiths of central and southern California are assessed for this purpose. We conclude that two distinct types of granites occur in California. One type, largely overlooked in the past and which we term I-SCR (strongly contaminated and reduced I type), is confined to narrow northwest-striking belts in the western Sierra Nevada and Peninsular Ranges batholiths in prebatholithic continental margin terrenes containing reduced pelitic roof pendants. We interpret the I-SCR magmas to have arisen from local-scale contamination of I-type magmas that intruded reducing pelitic material in sedimentary roof pendant terrenes. We conclude that cordierite-bearing S types are absent from the California batholiths, although sedimentary materials were required to produce the I-SCR plutons. The second type of granite, which we refer to as I-SC (strongly contaminated but not reduced I type), occurs on the eastern side of the California batholiths as part of a well-known regional west-to-east progression from quartz diorites to granites. These granites are the eastern variant of a sequence that is, from west to east, I-WC (weakly contaminated), I-MC (moderately contaminated), and I-SC This eastward progression is not controlled by pendant occurrence and probably reflects increasing contamination on a regional scale of mantle I-type magmas with F-rich igneous and metamorphic rocks, or their sedimentary derivatives, of the Precambrian continental craton. We demonstrate that the nature of prebatholithic terranes can strongly influence the composition and regional distribution of granitic rocks developed in an active continental margin.

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