Extreme zircon O isotopic compositions from 3.8 to 2.5 Ga magmatic rocks from the Anshan area, North China Craton

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.