Abstract
Abstract Granite typology categorizes granitoid rocks based upon distinguishing characteristics that are interpreted to indicate sources, conditions of generation, and, by implication, tectonic setting. Complexities of elemental and isotopic geochemistry, however, commonly preclude simple typological interpretation and suggest more complex petrogenetic histories. Granitoids from the Songpan-Ganzi terrane in the eastern Tibetan Plateau were emplaced within a short interval (~15 m.y.). They display mineralogical and geochemical characteristics that are consistent with a wide range of proposed typologies (I-, S-, and A-type; high Ba-Sr and adakitic variants). Despite their close spatial and temporal association, these granitoids exhibit diversity in geochemical characteristics that indicates a broad spectrum of contributing sources. Radiogenic isotope data reveal a continuum from primitive to evolved crustal compositions; i.e., 87Sr/86Sr(t) = 0.704–0.715 and εNd(t) = +2 to −11. All granitoid “types” have variable but commonly high zircon δ18O (+4.1‰ to +11.6‰) and low whole-rock Li-B-Mg isotopic ratios compared to mantle and/or seawater (δ7Li = +5.1‰ to −3.2‰; δ11B = −10.7‰ to −16.5‰; δ26Mg = −0.23‰ to −0.59‰). These stable isotopic compositions suggest that the Songpan-Ganzi granitic magmas of all “types” had contributions from sediment, ranging from minor to dominant. The highly variable isotopic compositions of the granitoids rule out a single homogeneous source for these diverse yet contemporaneous granitoids. Their compositional variability may have been significantly influenced by sedimentary contributions, and these results demonstrate the difficulty of straightforward assignment and interpretation of granitoids using conventional typology.
Highlights
INTRODUCTIONGranitoids (sensu lato, intermediate to felsic quartz-bearing intrusive rocks) are the predominant constituents of Earth’s continental crust (Campbell and Taylor, 1983; Kemp and Hawkesworth, 2003)
Granitoids are the predominant constituents of Earth’s continental crust (Campbell and Taylor, 1983; Kemp and Hawkesworth, 2003)
The widely used paradigm of I- and S-type granites divides granitic rocks into groups interpreted to be derived from metaigneous (I) and metasedimentary (S) sources, respectively (Chappell and White, 1974)
Summary
Granitoids (sensu lato, intermediate to felsic quartz-bearing intrusive rocks) are the predominant constituents of Earth’s continental crust (Campbell and Taylor, 1983; Kemp and Hawkesworth, 2003). We applied O-Li-B-Mg isotope analysis to a prime example of diverse granitic rocks in the Songpan-Ganzi terrane (Tibetan Plateau). These intrusions, which were formed and emplaced during a geologically short time interval, span much of the global compositional range of granitoids, including examples of the most widely cited typologies (Fig. 1). A-type granitoids are dominated by granite, Stype granitoids are dominated by two-mica granite, and high Ba-Sr granitoids are dominated by monzonite and quartz monzonite These diverse granitoids were emplaced during a brief interval of ∼15 m.y. between 213 Ma and 198 Ma (Table S3)
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