Role of sediment in generating contemporaneous, diverse "type" granitoid magmas

• Published in: Geology (Boulder) 2022-04, Vol.50 (4), p.427-431
• Main Authors: Li Shan, Li Shan, Miller, Calvin F, Tao Wang, Tao Wang, Xiao Wenjiao, Xiao Wenjiao, Chew, David M
• Format: Article
• Language: English
• Subjects: alkali metals; alkaline earth metals; Asia; B-11/B-10; boron; chemical composition; Composition; genesis; Geochemistry; Geology; granites; igneous and metamorphic rocks; igneous rocks; Isotope composition; Isotope ratios; Isotopes; Li-7/Li-6; lithium; magmas; magnesium; metals; Mg-26/Mg-24; Nd-144/Nd-143; neodymium; O-18/O-16; oxygen; Petrology; plutonic rocks; Radiogenic materials; rare earths; Rocks; Seawater; Sediment; sediments; Songpan-Ganzi Terrane; Sr-87/Sr-86; stable isotopes; strontium; Strontium 87; Strontium isotopes; Tectonics; Tibetan Plateau; Typology; Zircon
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G49509.1

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 ppm to +11.6 ppm) and low whole-rock Li-B-Mg isotopic ratios compared to mantle and/or seawater (δ7Li = +5.1 ppm to -3.2 ppm; δ11B = -10.7 ppm to -16.5 ppm; δ26Mg = -0.23 ppm to -0.59 ppm). 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.