Lithium isotopic compositions of the New England Batholith: correlations with inferred source rock compositions

Abstract

A strong correlation exists between the Li isotopic compositions of CarboniferousTriassic granites from the New England Batholith, and the previously inferred involvement of sedimentary and mantle/infracrustal source components. Isotopically (Nd and Sr) juvenile, low-K, Cordilleran I-type granites of the Clarence River supersuite have δ7 Li= +2·2 to +8‰ similar to those of arc magmas, the inferred source of these granites (Bryant et al. 1997). Isotopic variability within this supersuite probably arises from heterogeneity within primary mantle-derived magmas, combined with subsequent modifications through interactions with crustal materials. Oxidised, high-K granites of the Moonbi Supersuite have more homogenous and slightly lighter Li isotopic compositions (δ7 Li= +1·9 to +4·2‰). The observed range of values lies within the range of arc magmas, and is consistent with partial melting of arc shoshonites within the crust (cf. Chappell 1978) or the involvement of high-K mantle-derived magmas (cf. Shaw & Flood 1981; Landenberger & Collins 1998). S-type granites of the Bundarra (δ7 Li= −0·1 to +2·1‰; average= +1˙3‰; n=6) and Hillgrove supersuites (δ7 Li= +0·4 to +1·7‰; average= +0·8‰) define a narrow range of isotopic compositions which are, overall, lower than those observed in NEB I-type granites or generally observed in primary arc magmas. Their isotopic compositions are equivalent to those typically observed in shales (primarily δ7 Li= −3·2 to +2·0‰; Moriguti & Nakamura 1998; Teng et al. 2004). No difference is evident in the isotopic compositions of the two S-type supersuites despite inferred differences in the degree of weathering experienced by the sedimentary protolith, or differences in mineralogy of the granites. Granites of the Uralla Supersuite, which have been have formed from mixtures of local meta-igneous and meta-sedimentary components, span a broad range of values (δ7 Li= −1·3 to +3·9‰) which overlap with both the sediment-poor New England Batholith I-type intrusions of the Clarence River and Moonbi supersuites, and the S-type granites of the Bundarra and Hillgrove supersuites. Lower δ7 Li values primarily occur in lower-K plutons from the northern portion of the Uralla Supersuite.Overall, anatexis and magma differentiation do not appear to contribute to significant fractionation of Li isotopes relative to the inferred source components. However, subtly lower δ7 Li values, evident in the three leucogranites analysed herein, imply that subtle Li isotopic fractionation may occur in association with the exsolution of an aqueous fluid. Like most isotopic systems, the Li isotopic composition of rocks is not a definitive guide to source rock compositions, but given the results herein, the present authors suggest that it may play a very useful role in understanding crustal processes.

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

“…Most of the cores (up to 20% in that granite) returned ages only slightly older than the magmatic age. Overlapping ranges of Hf values from mainly +3 to +16 ) suggest formation from either highly depleted magmas (e.g., Clarence River Suite, Bryant et al 2004) or mixed depleted and evolved magmas (e.g., Moonbi Suite, . Jeon et al (2014) suggested that Moonbi Supersuite was derived mainly from juvenile sources mixed with minor amounts of the Devonian volcanogenic accretionary complex and oceanic crust.…”

Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids

“…The upper mantle represents the compositions of fresh peridotites and fresh N-MORB (d 7 Li = 3.8 ± 1.5%; Chan et al 1992;Elliott et al 2006;Jeffcoate et al 2007;Magna et al 2006;Seitz et al 2004;Tomascak et al 2008). The range of unaltered continental rocks includes magmatic, metamorphic, and sedimentary rocks (Bryant et al 2004;Magna et al 2010;Marks et al 2007;Teng et al 2004Teng et al , 2006aTeng et al , b, 2008Teng et al , 2009. Weathered continental rocks are from Huh et al (2001), Kisakürek et al (2004), Moriguti and Nakamura (1998), Rudnick et al (2004), and Teng et al (2004).…”

Tectonic significance and geodynamic processes of large‐scale Early Cretaceous granitoid magmatic events in the southern Great Xing'an Range, North China

“…Zircon oxygen isotopes are sensitive to the contribution from hydrated components (supracrustal materials or subducted oceanic slabs, also, hydrothermally altered crustal materials) that underwent high‐ or low‐temperature water‐rock interaction, whereas its Hf isotopes can be used to identify juvenile components by direct mantle input or partial melting of subducted oceanic slabs or ancient crustal input. In addition, whole‐rock lithium isotopes have many advantages as geochemical tracers for a wide range of geological processes, covering fluid/melt‐mineral interactions from the Earth's surface to the mantle [e.g., Bryant et al ., ; Teng et al ., ; Genske et al ., ; Sauzéat et al ., ]. Thus, combined Hf and O isotope data in individual zircon crystals and whole‐rock lithium isotopes can give further detailed genetic information about parent magmas and therefore provide important constraints on fluid circulation between subducted oceanic slabs and the lithosphere (mantle and crust) [ Fukao et al ., ; Grove et al ., ; Roberts and Spencer, ; X.‐C.…”