Composition, Metamorphism And Deforamation Of Subduction-accretionary Complex In The North Qilian Mountain

The north Qilian high-pressure (HP)/low-temperature (LT) metamorphic belt, which is composed mainly of greenschists, blueschists, eclogitic rocks, metasediments with minor slice of ultramafic rocks, is located between Qingshuigou and Baijingsi at the middle part of the north Qilian Orogenic belts. It is believed to be a deep unit of the Early Palaeozoic accretionary wedge associated with the subduction of oceanic crust.Eclogite occurs as lenses within blueschists and phengite-schists. It consists of garnet, omphacite, glaucophane, phengite and epidote, lawsonite could be observed in parts samples. The eclogite facies rocks could be divided into the following five types: eclogite, omphacite-bearing garnet blueschist, lawsonite eclogite, omphacitite, retrograded eclogite.Detailed mineralogy and microstructure imply that the eclogite facies rocks underwent multiple metamorphism including lawsonite blueschist/epidote blueschist facies, eclogite facies, post-peak epidote blueschist facies and greenschist facies . Peak pressure conditions estimated from omphacite- phengite- garnet assemblages were 2.1-2.6 GPa at temperature of 430-570 C. SHRIMP U-Pb dating of zircon yields ages of 463-489 which reflect an eclogite facies metamorphism..Based on REE and trace element analyses as well as Sr-Nd isotopic data, we divided the eclogites into two types. Type I eclogite is characterized by nearly flat REE normalized patterns and a distinctly positive Eu anomaly, suggesting cumulate protoliths, while type II eclogite has sligthly Eu anomalies and an enriched LREE pattern. The value ofεNd(t)from type I eclogite group change between 2.5 and 6.9, indicating depleted mantle sources; the value ofεNd(t)from type II eclogite changes between -3.3 and 1.4. These data indicate that the protoliths of type I eclogites originated in an oceanic environment; and type II eclogites show a possible mixing with a continental crust contaminant during emplacement of the protoliths, and probably derived from in continental/oceanic transitional zone or continental margin.