CONTEMPORANEOUS GRANULITES AND ECLOGITES IN THE SONGSHUGOU COMPLEX (QINLING BELT, EAST-CENTRAL CHINA): RELICS OF AN EARLY PALEOZOIC PAIRED METAMORPHIC BELT?

Granulites, eclogites and mantle peridotites provide a telling record of oceanic and continental interaction. In east-central China's multi-stage Qinling Belt, they are juxtaposed in the ~25 × 5 km Songshugou Complex. In this communication, we combine phase equilibria modeling with conventional thermobarometry and U/Pb geochronology to illuminate their P-T-t evolution and draw conclusions for the orogen's evolution.

Thrust on Devonian migmatites (700-770 °C, 0.5-0.7 GPa), the Songshugou Complex comprises a core of ultramafic rocks mantled by amphibolite mylonite with garnet-amphibolite and high-pressure granulite lenses. At its southern margin, mafic and felsic granulites occur.

Dunites from the complex's core show crustal reequilibration at ~700 °C. Five partly clinopyroxene-bearing garnet amphibolites taken from lenses in the metamafic mantle were metamorphosed at 494-542 °C, 2.13-2.54 GPa and overprinted at 660-700 °C, 0.9-1.2 GPa; zircons from the least retrograded sample yielded a ²⁰⁶U/²³⁸Pb age of 506.1 ± 9.1 Ma. Two mafic and two felsic granulites from Songshugou's southern margin underwent peak metamorphism at 760-915 °C, 0.93-1.30 GPa; their early exhumation was nearly isothermal. ²⁰⁶U/²³⁸Pb zircon ages obtained from the same samples are 516 ± 12, 488.5 ± 3.1, and 497.1 ± 1.8 Ma.

The data illuminate contemporaneous formation of eclogites and granulites in the Early Palaeozoic, both along clockwise PT-paths. We interpret this rock association as remnant of a paired metamorphic belt, with the ultramafics representing the overriding plate's upper mantle. Arc magmatism affecting the area from about 510 Ma to 400 Ma shows two climaxes at ~485 and ~430 Ma; the earlier is genetically related to formation of the investigated eclogites and granulites. Continuous high heat flow and magmatic activity, especially the strong thermal pulse during the second climax, which might have been triggered by ridge-subduction, obscured the petrologic record of the investigated paired HT – HP event.