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摘要
The role of fluids during Archaean intra-crustal magmatism has been investigated via integrated SHRIMP U-Pb, delta O-18 and LA-MC-ICPMS Hf-176 isotopic zircon analysis. Six rock samples studied are all from the Nuuk region (southern West Greenland) including two similar to 3.69 Ga granitic and trondhjemitic gneisses, a 3.64 Ga granitic augen gneiss, a 2.82 Ga granodioritic Ikkattoq gneiss, a migmatite with late Neoarchaean neosome and a homogeneous granite of the 2.56 Ga Qrqut Granite Complex (QGC). All zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains. Within the zircon analysed, there is no evidence for metamictization. Initial epsilon(Hf) zircon values (n = 63) are largely sub-chondritic, indicating the granitic host magmas were generated by the remelting of older, un-radiogenic crustal components. Zircon from some granite samples displays more than one Pb-207/Pb-206 age, and correlated with Hf-176/Hf-177 compositions can trace multiple phases of remelting or recrystallization during the Archaean. Model ages calculated using Lu/Hf arrays for each sample indicate that the crustal parental rocks to the granites, granodiorites and trondhjemites segregated from a chondrite-like reservoir at an earlier time during the Archaean, corresponding to known formation periods of more primitive tonalite-trondhjemite-granodiorite (TTG) gneisses. Zircon from the similar to 3.69 Ga granite, the migmatite and QGC granite contains Eoarchaean cores with chondritic Hf-176/Hf-177 and mantle-like delta O-18 compositions. The age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses, further suggesting genesis of these granites by remelting of broadly tonalitic protoliths. Zircon oxygen isotopic compositions (n = 62) over nine age populations (six igneous and three inherited) have weighted mean or mean delta O-18 values ranging from 5.8 +/- A 0.6 to 3.7 +/- A 0.5aEuro degrees. The 3.64 Ga granitic augen gneiss sample displays the highest delta O-18 with a mildly supra-mantle composition of 5.8 +/- A 0.6aEuro degrees. Inherited Eoarchaean TTG-derived zircon shows mantle-like values. Igneous zircon from all other samples, spanning more than a billion years of Archaean time, record low delta O-18 sub-mantle compositions. These are the first low delta O-18 signatures reported from Archaean zircon and represent low delta O-18 magmas formed by the remelting and metamorphism of older crustal rocks following high-temperature hydrothermal alteration by meteoric water. Meteoric fluid ingress coupled with crustal extension, associated high heat flow and intra-crustal melting are a viable mechanism for the production of the low delta O-18 granites, granodiorites and trondhjemites reported here. Both high and low delta O-18 magmas may have been generated in extensional environments and are distinct in composition from Phanerozoic I-type granitic plutonic systems, which are typified by increasing delta O-18 during intra-crustal reworking. This suggests that Archaean magmatic processes studied here were subtly different from those operating on the modern Earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement.
- 出版日期2011-6
