Protracted eclogite-facies metamorphism of the Dulan area, North Qaidam ultrahigh-pressure terrane: Insights on zircon growth during continental subduction and collision

Abstract

Continental subduction and collision are recorded by ultrahigh-pressure (UHP) terranes; UHP terranes that form at early stages of an orogeny tend to be small and experience short residence at eclogite-facies depths, whereas terranes that form at mature stages of an orogeny tend to be larger and experience longer residence at these depths, but accurately determining eclogite-facies residence time requires a large geochronologic dataset tied to metamorphic conditions (via trace elements and/or inclusions). In the Dulan area, North Qaidam UHP terrane, China, it remains unclear whether the terrane experienced a long residence at eclogite-facies depths, marking the mature stage of an orogeny or two distinct (ultra)high pressure ([U]HP) events (with short residence times), interpreted as the transition from oceanic subduction to continental collision, where one (U)HP event is related to the former and second (U)HP event to the latter. To address this issue, we report new zircon U–Pb ages and trace-element data from eclogite and host paragneiss from the Dulan area and show that this terrane records ~42 Myr of eclogite-facies metamorphism at (U)HP conditions, similar to other large UHP terranes. Zircon from 11 eclogite and 2 gneiss samples yields weighted mean ages of 463–425 Ma, flat heavy rare earth element (HREE) patterns without negative Eu anomalies, and eclogitic mineral inclusions, indicating eclogite-facies conditions. Paragneiss metamorphic ages overlap with ages from eclogite but are generally younger, suggesting that a lack of internally generated fluids may have inhibited zircon growth and/or recrystallization until early decompression and white mica consumption in felsic gneiss generated fluids; thus, we interpret that these felsic rocks record the later stages of continental collision. Dataset patterns from all new and previously published analyses for the Dulan area (34 eclogite and 14 gneiss) suggest that metamorphic zircon in eclogite records prograde, peak and possibly early retrograde conditions, in contrast to the prediction from mass balance models that metamorphic zircon should only grow during exhumation and cooling. We reconcile our observations with these model predictions by recognizing that differential solubility can lead to grain-scale zircon growth or recrystallization over a large segment of the pressure–temperature (P–T) path even where zircon abundance decreases at the whole-rock scale.