ZIRCON INCLUSIONS CONSTRAIN DEEP CRUSTAL P-T-T PATHS: AN EXAMPLE FROM ULTRAHIGH-PRESSURE ECLOGITES AND HIGH PRESSURE GRANULITES FROM THE NORTH QAIDAM TERRANE, WESTERN CHINA

Zircon inclusions record the P-T conditions during the time they are trapped; pairing the inclusion data with U-Pb dates constrains the P-T-t path for each sample. Ultrahigh-pressure eclogites and high pressure granulites from the North Qaidam terrane, near the Dulan region of Western China, contain zircons with both single phase and multiphase solid inclusions. These eclogites represent crust that has been subducted up to 100 km during continental collision, and the adjacent granulites represent burial to 50 km. For this study, zircons were mounted in epoxy, and imaged with SEM. CL, BSE, and EDS maps were used to pinpoint where inclusions are located in the zoned zircons. 1060 inclusion phases were identified by their EDS spectra from 18 rock samples from the North Qaidam terrane. The inclusions in these zircons ranged in size from about 5 to 25 μm.

Two eclogite samples, out of 12 total, included inherited cores from their protolith rocks which were dated to ~900 Ma. The cores primarily included feldspars, apatites, and micas, consistent with a magmatic protolith. The rims include the eclogite facies minerals garnet, omphacite, phengite, and rutile. The eclogite facies inclusion phases occur in zones of the zircons, visible in CL images, that match dates of 426-437 Ma, indicating these ages record eclogite facies metamorphism. Many included phases present in both prograde and retrograde conditions, like feldspars, pyroxenes, amphiboles, epidote, and most micas.

The three granulites have few inclusions, and they are predominately felsic mineral phases, with some large, multiphase solid inclusions. EDS mapping has shown these to be mixed sodic feldspar and potassium feldspar, suggesting that these are trapped crystallized melt inclusions. Multiphase solid inclusions have also been found in a nearby tonalite. Zircons from these recording the timing of high pressure partial melting. The granulites are approximately 400-420 Ma.