Geochronology of the transition of eclogite to amphibolite facies metamorphism in the North Qinling orogen of central China

Abstract

Quartz-rich veins, together with their host high-pressure (HP) to ultra-high-pressure (UHP) rocks, are ideal for deciphering the fluid–rock interaction history in subduction zones. Previous studies of samples from Dabie–Sulu have shown that most quartz-rich veins within UHP/HP eclogite were formed during the prograde or early exhumation stage of host rocks. In contrast, a combined study of petrology and U–Pb, Sm–Nd and Lu–Hf geochronologies for a quartz-rich vein and its host retrogressed eclogite from North Qinling, which is associated with a granite intrusion and obliterated with amphibolite-facies overprint, reveals that the amphibolite-facies overprint is likely due to the late regional magmatism and the quartz-rich vein might derive from the nearby granite. A pool of U–Pb analyses of zircon grains from the retrogressed eclogite reveals zircons grew over an extended period of time but with two caveats at ca. 490 Ma and ca. 473 Ma. Analyses of zircon grains from the quartz vein yield a weighted mean 207Pb/ 206Pb ages of 428 ± 5 Ma (1σ), which is indistinguishable from that of formation of the adjacent granite. Combined elemental zoning patterns in the garnet with the spherical geometry effect, the Lu–Hf age of 416 ± 5 Ma (2σ) of the retrogressed eclogite is likely a mix age of eclogite-facies and amphibolite-facies metamorphism while the garnet–amphibole Sm–Nd age of 400 ± 8 Ma (2σ) approximates amphibolite-facies retrogression metamorphism. The latter is consistent with the garnet–biotite Sm–Nd age of 393 ± 4 Ma (2σ) of the adjacent granite, suggesting contact metamorphism. The > 70 Myr gap between the eclogite-facies metamorphism and the amphibole-facies overprint and the consistency of the timing the amphibolite-facies overprint for the eclogite and the cooling of the magma intrusion in this area indicate that the eclogite-facies metamorphism and amphibolite-facies metamorphism might be distinct metamorphic events, and the latter might be trigged by the granite intrusion.