Limiting Effects of UHP Metamorphism on Length Scales of Oxygen, Hydrogen, and Argon Isotope Exchange: An Example from the Qinglongshan UHP Eclogites, Sulu Terrain, China

Abstract

New and previously published stable isotope (oxygen and hydrogen) and 40Ar/39Ar data are compared from selected outcrops of the Sulu UHP terrain near Qinglongshan (Jiangsu Province, China). These rocks exhibit unusually low, heterogeneous δ18O and δD values acquired in a Neoproterozoic geothermal system despite undergoing Triassic (220-240 Ma) UHP metamorphism. Incremental heating 40Ar/39Ar analyses of muscovite, biotite, and K-feldspar from metagranite, quartzite, and gneiss (all metamorphosed to the coesite-eclogite facies) yield cooling ages between 190 and 204 Ma. In contrast, phengite 40Ar/39Ar data from eclogite, quartzite, and gneiss contain variable amounts of extraneous argon, consistent with inheritance from a Neoproterozoic protolith. Plots comparing phengite δ18O, δD, and 40Ar/39Ar total gas ages from different lithologies within individual Qinglongshan outcrops only meters apart highlight significant inter-outcrop isotopic heterogeneities (ΔDphengite = 26‰; Δ18Ophengite = 8.8‰; Δ40Ar/39Arphengite = 664 Ma); however maximum intra-outcrop isotopic variations between lithologies are limited (ΔDphengite = 5‰; Δ18Ophengite = 1.8‰). The oxygen and hydrogen isotopic variations are interpreted to reflect primary isotopic heterogeneities acquired during Neoproterozoic hydrothermal fluid circulation with cold-climate meteoric waters. The limited intra-outcrop isotopic variations suggest that extensive isotopic exchange occurred during UHP metamorphism within discrete outcrops, irrespective of lithology. Likewise, extraneous argon within the phengites reflects inheritance from a Neoproterozoic protolith, and the age variations between outcrops is probably due to differential argon loss during thermal and baric equilibration accompanying differential exhumation following UHP metamorphism. The retention of extraneous argon in phengite is partially controlled by the host rock lithology, inasmuch as as the armoring effects of basaltic eclogite are greater than quartzite or gneiss. Collectively, these data indicate that the length-scale of isotopic exchange is defined by contiguous blocks of coherent rock. Isotopic exchange during UHP metamorphism occurred between different lithologies restricted to discrete blocks within an accretionary prism, but did not communicate with other blocks. This study underscores the closed system (outcrop scale) behavior of isotopic exchange that can occur during continental subduction, collision, and uplift under UHP conditions.