Two orogenic cycles recorded by eclogites in the Yuka–Luofengpo terrane: Implications for the Mesoproterozoic to early Paleozoic tectonic evolution of the North Qaidam orogenic belt, NW China

Abstract

Eclogites documenting two orogenic cycles not only contain essential information on subduction and exhumation mechanisms during the most recent orogenic cycle, but can also provide insights into the earlier cycle. Such rocks are ideal for deciphering the tectonic evolution of orogenic belts with a multi-orogenic history. This is the case for the North Qaidam orogenic belt (NQOB) in northwest China where, for the first time, we have discovered eclogites that preserve a record of two orogenic cycles. The first orogenic event occurred at ca. 929–916 Ma and the second at ca. 439–430 Ma. The metamorphic P–T conditions of these two events were 5.0–7.0 kbar/616–669 °C and 29.0–32.5 kbar/620–670 °C, respectively. Zircon U–Pb dating yielded protolith ages of 1280–1070 Ma for these eclogites, which are much older than the protolith ages for other eclogites in the NQOB. The eclogites exhibit chondrite-normalized rare earth element patterns that are flat or depleted in light rare earth elements, similar to present-day N- and E-MORB. Magmatic zircon cores from the eclogites have high εHf(t) values of 8.4 ± 4.0 to 18.1 ± 1.1, comparable to depleted mantle within uncertainty, and indicating a juvenile crustal origin. Our data show that the eclogite protoliths were probably pre-Rodinia Mesoproterozoic oceanic crust that was emplaced in an active continental margin during oceanic subduction, and experienced Neoproterozoic metamorphism related to the amalgamation of Rodinia and early Paleozoic continental deep subduction. Using published data for all eclogites and their host gneisses, we reconstructed the Mesoproterozoic to early Paleozoic tectonic evolution of the NQOB. Given the similar Meso–Neoproterozoic evolution of the NQOB and adjacent Tarim Block, we suggest that the Qaidam Block was the eastward extension of the Tarim Block during the Neoproterozoic.