Omphacite Melting and the Destruction of Early High‐Pressure Rock Records

Abstract

AbstractThe presence of eclogite marks high‐pressure metamorphism and deep, cold burial of crust, and is therefore a widely accepted indicator for the onset of modern‐style plate tectonics. However, eclogite is rarely preserved in many heavily granulite‐overprinted orogens, which are particularly common across ancient metamorphic terranes (i.e., Archean and Paleoproterozoic terranes). Here, we show that eclogite melting processes may be principally responsible for the poor preservation of high‐pressure records. We demonstrate eclogite melting via detailed petrological, geochronological, and geochemical analyses on eclogites and separated centimetric leucosomes from the central Himalaya. The central Himalayan eclogites were overprinted by strong granulite‐facies metamorphism, such that omphacite is only sparsely preserved. Thermodynamic modeling results indicate the eclogite experienced two types of anatectic reactions: phengite dehydration melting at high‐pressure in the presence of omphacite, and subsequent omphacite dominated melting during exhumation. Diagnostic features for these melting reactions include (a) the occurrence of Kfs‐Pl‐Qz multiphase solid inclusions and Cpx‐Pl‐Qz polymineralic inclusions; (b) the high Na2O/K2O melts and Na‐rich plagioclase in leucosomes; (c) the consistent trace elements patterns between omphacite and leucosomes; and (d) obvious Na2O zonation in clinopyroxene. Omphacite dominated melting is characterized by the jadeite breakdown, releasing Na and Al into the melts. This melting mechanism subsequently forms a less sodic clinopyroxene and high Na2O/K2O melts. These Himalayan findings appear relevant to early Earth explorations because of the high thermal gradients and intense granulite‐facies overprints, implying that partial melting of eclogite dominated by omphacite breakdown could have erased early high‐pressure records of modern‐style plate tectonics.