Trace Element Variations as Indicators of Fluid Sources and Infiltration History During UHP Metamorphism of Continental Crust

Abstract:

Concentrations of incompatible trace elements (Li, Be, B, F, Rb, Sr, Cs, and Ba) in white mica can be used to constrain the sources and physical conditions accompanying fluid-mediated mass transfer in high-pressure convergent margin settings. The Luliang Shan terrane hosts the oldest, coolest, and most hydrated UHP lithologies within the North Qaidam UHP belt of NW China. Previous work documented extensive fluid flow forming thick phengite+garnet-bearing lithologies (selvage) at the interface of eclogite and quartzofeldspathic gneiss at around 600°C and 75 km depths. We performed in-situ LA-ICP-MS and ion microprobe measurements of trace elements in white micas to determine the sources of infiltrated fluids (i.e. local vs. extra-regional sources of infiltrating fluids). High-Ti and Si phengites within the selvage contain concentrations of Li, Rb, and Cs up to an order of magnitude higher than values predicted from simple mixing of gneiss and eclogite needed to reproduce the major element chemistry recorded in the selvage. Combined with d¹⁸O and ⁴⁰Ar_E data, the observed enrichments would require extensive fluid mobility to form the UHP selvages. Alternatively, retrograde Ti- and Si- poor muscovites from the host gneiss and shear zones have lower Li, Cs, Rb, and Sr but possess remarkably high B concentrations (up to 3000 ppm), and Be (up to 60ppm) best explained by interaction with fluids derived from devolatilization of altered sediments occurring in more shallow, cool regions of a subduction zone. We conclude that the contrast in the trace element chemistry exhibited by high-pressure phengite vs. low pressure muscovite indicate that the Luliang Shane terrane experiences at least two distinct types of fluid infiltration a high-temperature fluid that infiltrated at near peak UHP conditions to trigger selvage formation and a subsequent event which involved fluid from the shallower regions of a subduction channel and infiltrated the host gneiss as it was exhumed.