Saturday, May 24, 2014

Evidence of Crustal Growth and Reworking in the EoArchean and PaleoArchean

Zircon U-Pb-Lu-Hf-O isotopic evidence for ≥ 3.5 Ga crustal growth, reworking and differentiation in the northern Tarim Craton


Ge et al


Continental crust was largely generated before 2.5 Ga through mafic-ultramafic and TTG (tonalite-trondhjemite-granodiorite) magmatism, but it is contentious when did such primitive crust evolve into mature granodioritic to granitic composition similar to modern upper crust. Here we present zircon U-Pb-Lu-Hf-O isotopic data for late Paleoproterozoic metasedimentary rocks (Xingditag Group) in the Kuruktag area, northern Tarim Craton, NW China. CL-imaging reveals core-rim structures for most zircons from a garnet-bearing paragneiss and a semi-pelitic schist, whereas two quartzites are dominated by metamorphic zircons. SHRIMP and/or LA-ICP-MS U-Pb dating yielded a range of detrital ages from ca. 2.0 – 3.5 Ga for the zircon cores and a consistent metamorphic age of ca. 1.93 Ga for the rims for the paragneiss and schist. However, zircons from the two quartzites mainly record a ca. 1.85 Ga metamorphic event; detrital zircons are rare or absent. These data confirm that the Xingditag Group was deposited after ca. 2.0 Ga and was metamorphosed at ca. 1.93 and/or 1.85 Ga. Importantly, the ca. 2.0 – 3.5 Ga concordant detrital zircons exhibit low initial 176Hf/177Hf ratios (as low as 0.28045) and high δ18O values (6.6 – 11.4‰). These values are interpreted as recording primary magmatic features of the basement rocks in the northern Tarim Craton, because: 1) the dominantly prismatic or fragmentary morphology, oscillatory zoning and moderate Th/U ratios of the detrital zircons indicate a local provenance dominated by igneous rocks; and 2) the within-grain and overall heterogeneities argue against Hf and O isotopic resetting during metamorphism. Linear regressions of the initial 176Hf/177Hf values of these detrital zircons yield a remarkably consistent 176Lu/177Hf ratio of 0.01 for the oldest (TDM2 = 3.9 and 3.7 Ga) and youngest (TDM2 = 2.8 Ga) crustal components. These observations suggest that significant amounts of felsic continental crust may have been formed, altered and reworked as early as ca. 3.5 Ga, marking crustal differentiation and maturation during the Paleoarchean. Hafnium crustal model ages reveal that the oldest crustal component in the northern Tarim Craton may have been generated before ca. 3.9 Ga, much earlier than previously thought.

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