A subduction-dismembered Neoproterozoic large igneous province in the Qinling Orogenic Belt, China: Implications for 850 Ma–initiated mantle plume activity in the greater Yangtze Block

Abstract

Large igneous provinces (LIP) provide invaluable clues for recognizing the growth, breakup, and cycles of supercontinents and constraining the activity of ancient mantle plumes (or superplumes). The widespread intraplate magmatism is considered as a possible fingerprint of ancient plume activities and LIPs. Here, we present the results of an integrated study on protoliths of eclogites and amphibolite-facies bimodal volcanic rocks from North Qinling Ultrahigh-pressure Metamorphic (UHPM) Terrane, Qinling Orogen, and identify a dismembered Neoproterozoic LIP. These rocks share similar protolith ages of ca. 850–800 Ma and experienced high- to ultrahigh-pressure metamorphism at ca. 500–480 Ma. The mafic rocks are mainly tholeiitic and display enrichment of LILEs and HFSEs, similar to the E-MORB/OIB characters. They also have typical features of continental flood basalts (CFBs) with low CaO/Al2O3 (0.68–0.87), Lu/Yb (0.14–0.15), Zr/Nb (8–13), high Nb/La (0.87–1.24, mean 1.07), La/Nb (0.80–1.15), La/Ta (11–20), Nb/Yb (3.0–6.2) ratios, and positive εNd(t) (+4.3 to + 5.4). Their occurrence, geochemical features, age data, and high mantle potential temperature (1500–1550 °C) suggest that they are remnants of Neoproterozoic CFBs with mantle plume origin. Based on the lateral distribution length (ca. 300 km) of meta-basaltic rocks and the maximum subduction depths (ca. 100–200 km) recorded by UHP eclogites, the identified ca. 850 Ma-initiated CFBs in the North Qinling terrane should represent an LIP, covering more than 100,000 km2. This Neoproterozoic LIP, together with the coeval volcanic sequences in the South Qinling terrane and the northern Yangtze, would have occupied a maximum landmass of 300,000 km2, which lends support for the onset of a mantle plume at ca. 850 Ma within the Rodinia supercontinent. We conclude that the Qinling Block is one of the fragments of the Rodinia supercontinent with a passive margin covered by CFBs, which was subducted to mantle depths in the Early Paleozoic.