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Abstract: Most zircon from Archean (3.8–2.5Ga) trondhjemitic rocks, meta-gabbro, meta-diorite and monzogranite from the Anshan area, North China Craton, has δ18O values in the range of 4.6–7.5‰, but some has extreme compositions (0.02–11.0‰, with one value as low as −11.3‰) as a result of pre-, syn- and post-magmatic processes. Nearly all zircon grains with very low δ18O (<4.0‰) are U-rich and show evidence of radiogenic Pb loss, irrespective of their origin (magmatic, metamorphic, recrystallized, inherited or trapped). The low δ18O is not a primary feature of the zircon or the magma from which it crystallized, but is a consequence of high-temperature hydrothermal alteration, probably involving meteoric water. Radiogenic Pb loss is an indicator that the zircon O isotopic composition is likely to have been modified, even if the measured δ18O falls within, or is above, the “normal” igneous range (4.6–7.5‰). Excluding those showing strong Pb loss, most magmatic zircon grains have δ18O values in the “normal” range or only slightly above the previously reported highest δ18O value for Archean igneous zircon (7.5‰), with many, including some analyzed from Eoarchean rocks, being higher than 6.5‰, evidence for crustal reworking in the Anshan area 3.8Ga ago. The 2.5Ga Qidashan syenogranite has the highest zircon δ18O (5.8–11.0‰), consistent with its derivation by melting of sedimentary materials as a result of long-term crustal evolution in the region, as indicated by its chemical and Nd isotopic compositions. [Copyright &y& Elsevier]

Abstract: The Chilungshan andesite belongs to the Chilung Volcano Group in northern Taiwan and constitutes the western end of the island chains of the Ryukyu arc. Chemically it is a normal, medium-K calc-alkaline andesite. SHRIMP dating of magmatic zircons yielded an emplacement 206Pb/238U age of 1.04±0.06Ma with εHf(t) of 6.1 to 13.4 and δ18O of 5.02 to 6.28‰. The zircons contain inherited cores and felsic to andesitic silicate-melt (glass) inclusions besides apatite, quartz, K-feldspar, biotite, garnet, Fe-Mg minerals (pyroxene or hornblende) and plagioclase with variable An values. Plagioclase phenocrysts also contain quartz, K-feldspar and felsic melt inclusions. This study confirms the presence of micro-scale heterogeneity in the andesite as a result of mixing between mantle-derived and continental material. Contamination of continental crust occurs either within the magma chamber at a medium to deep crustal level and/or during dehydration and melting of a subducted oceanic slab and overlying sediments in an island-arc environment. [Copyright &y& Elsevier]

Abstract: We report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from Lushan in the far south of the Trans-North China Orogen of the North China Craton that reveal the oldest rocks yet dated from the orogen. A detailed investigation of the internal structures of zircons, combined with targeted SIMS U–Pb and oxygen isotope analyses and ICP-MS Hf zircon analyses, has resulted in the discovery of previously unknown geological events in the North China Craton. Magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829±18 Ma to 2832±11 Ma in the tonalites and from 2838±35 Ma to 2845±23 Ma in amphibolite. Hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination. Oxygen isotope ratios in zircons range from 5.1 to 7.3‰ VSMOW, with high δ18O results recorded only from metamorphic domains. The earliest stage of metamorphic zircon growth, present in all four samples analyzed in this study, occurred between 2772±17/22 in the tonalites and 2776±20 and 2792±12 Ma in the amphibolites. A younger metamorphic event at 2638±61 in the tonalite and between 2671±13 and 2651±13 Ma in the amphibolites is recognized in zircons from three of the four samples investigated. This is the first time that metamorphic events at either of these times have been recorded in the North China Craton. Importantly, there is no evidence for the 1.8 Ga high-grade metamorphic event that is manifest in the northern and central parts of the Trans-North China Orogen. [Copyright &y& Elsevier]