Zircon U-Pb and oxygen isotope evidence for a large-scale 18O depletion event in igneous rocks during the Neoproterozoic

Abstract

Oxygen isotope analysis and U-Pb dating were carried out on zircons from granite, granitic gneiss and eclogite in the Dabie-Sulu orogenic belt of east-central China. The results show a wide variation in zircon δ 18O values from −10.9 to 8.5‰. Most of the values are lower than normal mantle zircon and almost half have prominently negative values. The low δ 18O zircons have protolith ages of 700 to 800 Ma and metamorphic ages of 205 to 250 Ma, respectively. Fluid availability within the metaigneous rocks dictates the extent of metamorphic recrystallization and overgrowth. The igneous zircons have preserved their magmatic zoning and middle Neoproterozoic U-Pb age during the Triassic metamorphism, indicating low fluid availability. Widespread low δ 18O values are identified in the magmatic zircons of middle Neoproterozoic age, within an outcrop area of over 20,000 km 2 along the northern edge of the South China Block. The low δ 18O zircons record the presence of large volumes of low δ 18O igneous rocks that were derived from remelting of meteoric-hydrothermally altered rocks at some time between 700 and 800 Ma. The U-Pb ages for metaigneous protoliths and granites are correlated not only with the timing of rifting accompanying the breakup of Rodinia, but also with contemporary glacial deposits in the South China Block at paleolatitudes of 30 to 40°N. Melting of glacial ice and snow is suggested, instead of the direct involvement of meteoric water, to produce the low δ 18O fluid with oxygen isotopic signatures like the cold-climate meteoric water. The rifting created conditions favorable to anatexis of meteoric-hydrothermally altered rocks. Glaciated regions supplied copious water for the water-rock interaction during magma emplacement along rifting zones. Both rifting and glaciation favored the generation of the low δ 18O magmas in the region. The low δ 18O zircons are thus interpreted to have crystallized from the low δ 18O magmas of middle Neoproterozoic age, and onset of the generation of low δ 18O magmas in the northern margin of the South China Block is estimated to occur at about 758 ± 15 Ma. The large-scale remelting of hydrothermally altered crust not only results from repetitive emplacement of mafic magmas along the rifting zones with protracted episodes of water-rock interaction, but also involves rift systems that rapidly introduce large volumes of fluid through confined pathways and traps in a short space of time in response to tectonic triggers. Occurrence of the large-scale 18O depletion during the middle Neoproterozoic may be a manifestation of the cold paleoclimate related to the snowball Earth event.