Oxygen isotope geochemistry of ultrahigh-pressure metamorphic rocks from 200–4000 m core samples of the Chinese Continental Scientific Drilling

Abstract

Oxygen isotope studies were carried out for ultrahigh-pressure metamorphic rocks from the main hole of the Chinese Continental Scientific Drilling in the Sulu orogen. The samples of interest include various types of lithology (mainly eclogite and gneiss) in a depth of 200 to 4000 m, containing five continuous core segments between contrasting lithologies. The results show a large variation in δ 18O value from − 10.41 to 9.63‰ for constituent minerals. Distinct 18O depletions are observed in the frequently alternated layers of eclogite and gneiss, in which the variations of δ 18O values are gradual, regardless of lithologies. Both equilibrium and disequilibrium O isotope fractionations are observed between quartz and the other minerals. Special attention was paid to the relationship between distance, petrography and δ 18O value of adjacent samples. The results show O isotope heterogeneities between the different and same lithologies on scales of about 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental collision. Amphibolite-facies retrograde metamorphism during exhumation caused mineral reactions and O isotope disequilibria between some of the minerals. Considerable changes occur in δ 18O and petrography at the contact between eclogite and gneiss, suggesting that the contact between different lithologies is the most favorable place for fluid activity. Despite the widespread retrogression, retrograde fluid was internally buffered in the stable isotope compositions. The retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl. Although locally external fluids became available along fault zones and lithological layers, it still acted within the exhumed slabs with internal origins. Fluid flow also took place after the amphibolite-facies retrogression, but it only affects the O isotope compositions of feldspar and mica. Premetamorphic protoliths are deduced to have heterogeneous δ 18O values due to varying degrees of meteoric water–rock interaction before continental collision. Minimum depth of the 18O depletion is up to 3300 m. Together with areal 18O depletion in surface outcropped rocks along the Dabie–Sulu orogenic belt, at least 66,000 km 3 of supracrustal rocks were interacted with meteoric water along the northern margin of the South China Block during the Neoproterozoic.