Metabasites from the Central East Kunlun Orogenic Belt Inform a New Suture Model for Subduction and Collision in the Early Paleozoic Proto-Tethys Ocean

Abstract

The discovery of eclogite outcrops in the East Kunlun Orogen Belt (EKOB) has confirmed the existence of an Early Paleozoic HP-UHP metamorphic belt. However, the protoliths and metamorphic histories of widespread metabasites remain poorly constrained. We collected three types of metabasites from the central part of EKOB. We present an integrated study of petrography, whole-rock geochemistry, Sr-Nd isotopes, estimated P–T conditions, and zircon U-Pb isotope ages. The results show that amphibolites and retrograde eclogites have clockwise P–T paths with peak conditions of, respectively, 11–12 kbar and 675–695 °C, and 21.5–22.2 kbar and 715–750 °C. Zircon dating of metabasites from Dagele yields Late Ordovician (~449 Ma) to Early Silurian (~440 Ma) protolith ages and Early Devonian (~414 Ma) amphibolite facies metamorphic ages. Retrograde eclogites from east Nuomuhong have a protolith age of ~902 Ma and metamorphic ages of ~418 Ma, consistent with other eclogites from East Kunlun. Our data suggest that the protoliths of Dagele metabasites represent arc-type magmatism during the subduction of a small back-arc oceanic basin. Instead, the protoliths of retrograde eclogites are Neoproterozoic tholeiitic basalts emplaced into continental crust and subsequently deeply subducted. We develop a new model for Early Paleozoic subduction and collision in the East Kunlun region, emphasizing the role of ‘dominant’ and ‘secondary’ suture boundaries. This model helps explain the ages and metamorphic histories of the metabasites studied here and offers new perspectives on the evolution of the Proto-Tethys Ocean.

The Discovery of the New UHP Eclogite from the East Kunlun, Northwestern China, and Its Tectonic Significance

“…It generally believed that the external fluids associated with retrograde metamorphism will not alter the significant element migration of eclogites [31,67], especially for high-field-strength elements (HFSE) and some rare earth elements [67,68]. As discussed above, the low loss on ignition (LOI) and relatively uniform distribution patterns of the trace and rare earth elements in the eclogites suggest that the vast majority of elements in the eclogites only underwent a slight degree of retrogression, which is associated with the petrological characteristics of the preservation of abundant omphacite.…”

“…Recently, another HP/UHP metamorphic belt was identified along the East Kunlun Orogenic Belt [20,[24][25][26][27][28][29][30][31], in particular the UHP metamorphic records identified from not only the eclogite, but also the gneiss from Kehete area. Despite the East Kunlun metamorphic belt being a response to the final closure of the branch ocean of the Proto-Tethys Ocean in East Kunlun areas, different geologists have different views on scientific issues of eclogite, such as whether the protoliths of eclogite are continental or oceanic basalt, which orogeic stage eclogite produces, and whether the peak metamorphism is high pressure or ultra-high pressure.…”