Discovery of the Paleoproterozoic metamorphic basement in the Tuomoerrite area of North Qaidam: Constraint on the location of Early Paleozoic suture

The Paleoproterozoic gneiss and amphibolite were recognized in the Tuomoerrite area, eastern part of the North Qaidam ultra-high pressure metamorphic belt.Their tectonic affinities have significance for the understanding of regional tectonic framework.In the Tuomoerrite area, there are abundant early Paleozoic volcano-sedimentary rocks, ophiolitic rocks and intrusives, with minor gneiss and amphibolite lens discovered in the northeast part.The protoliths of gneiss and amphibolite are granidiorite and basic rock respectively.LA-ICP-MS U-Pb dating of zircon from gneiss and amphibolite yields ages of 2413±28 Ma(upper intercept age, n=29, MSWD=2.3), 1966±46 Ma(weighted mean ²⁰⁷Pb/²⁰⁶Pb age, n=4, MSWD=1.7), and 1922±22 Ma(upper intercept age, n=29, MSWD=1.3) respectively.Combined with cathodoluminescence images of zircon, 2413±28 Ma represents the magmatic crystallization age of the protolith of the gneiss.1966±46 Ma and 1922±22 Ma represent the metamorphic ages of the gneiss and amphibolite respectively.The gneiss shows typical adakitic geochemical feature, with high Sr, low Y, MgO, Cr, and Ni.The magmatic zircons have ε_Hf(t) values between -0.8 and +4.0 and yield two-stage Hf model ages ranging from 2704 Ma to 2965 Ma.They also have positive ε_Nd(t) ranging from +0.8 to +1.6.These geochemical and isotopic characteristics indicate that the gneisses were likely formed by partial melting of thickened mafic lower-crust.The rock assemblage, geochemical features and ages of these metamorphic rocks are consistent with those of the Delingha Complex to the north, indicating that they belong to the northern Oulongbuluke block rather than the North Qaidam belt.This understanding further constrains that the suture of the North Qaidam belt lies between the Saibagou ophiolite and the newly recognized Paleoproterozoic metamorphic rocks.