Zircons in the Shenglikou ultrahigh-pressure garnet peridotite massif and its country rocks from the North Qaidam terrane (western China): Meso-Neoproterozoic crust–mantle coupling and early Paleozoic convergent plate-margin processes

Abstract

The North Qaidam terrane lies between the Qaidam and Qilian blocks of western China, and contains the Shenglikou ultrahigh-pressure (UHP) garnet peridotite massif. Simultaneously analyzed U–Pb ages, trace element patterns and Hf-isotope compositions of zircons from the peridotite massif and its country rocks (migmatite and gneiss) reveal the complex evolution of the terrane. Zircons in peridotite and pyroxenite from the massif document metamorphism at ca. 429 ± 2 Ma (2 σ, n = 30, MSWD = 1.03). The peridotitic and pyroxenitic zircons have similar trace-element patterns and ranges in ɛ Hf( t) (+3.5 to −7.0), suggesting that these zircons crystallized from a metamorphic fluid, sourced mainly from the surrounding garnet-bearing ultramafic rocks. The protolith of the peridotite massif is interpreted as a fragment of the metasomatized Archean subcontinental lithospheric mantle (SCLM) from the Qilian block. Hf depleted-mantle model ages ( T DM) mainly cluster in the interval 1.1–0.9 Ga, implying that the protolith of the massif probably underwent mantle metasomatism in Meso-Neoproterozoic time. Zircons from the migmatite yield concordant 206Pb/ 238U ages, with a major peak at 468 ± 4 Ma (2 σ, n = 7, MSWD = 2.2) and smaller populations of 1003 ± 6 Ma, 951 ± 8 Ma, 738 ± 4 Ma and 577 ± 14 Ma. Due to the systematically lower Th/U ratios and trace element concentrations in the early Paleozoic zircons, compared to the Proterozoic grains, we suggest the early Paleozoic (∼468 Ma) zircons were formed by metamorphic recrystallization of their Proterozoic precursors. Their variable Hf isotopic compositions, i.e., ɛ Hf( t) from +14.2 to −15.9, T DM from 1.7 Ga to 0.6 Ga and T crust (average crustal model ages) from 2.4 Ga to 0.6 Ga, imply complex origins, including the introduction of juvenile components during events at ∼1.1–0.9 Ga. Zircons from the gneiss show a major concordant population with 206Pb/ 238U ages of 430 ± 5 Ma (2 σ, n = 10, MSWD = 2.3) and minor populations with ages of 806 ± 13 Ma and 613 ± 24 Ma, indicating that metamorphic recrystallization of the protolith occurred in the early Paleozoic (∼430 Ma). The original protolith of the gneiss was ancient (ca. T crust = 2.8–1.8 Ga), with some Meso-Neoproterozoic (ca. 1.1 Ga) juvenile crustal components. These data together with published results suggest that the North Qaidam terrane is a collisional orogen with Archean crustal components, and the Qaidam and Qilian blocks underwent crust–mantle coupling events in Meso-Neoproterozoic time (∼1.1–0.9 Ga) and convergent plate-margin processes including the UHP metamorphism in early Paleozoic (∼468–430 Ma), i.e., the beginning of continental subduction at ∼468 Ma, the subsequent continental deep subduction and final continent–continent collision at ∼430 Ma.