Multiple partial melting events in the Sulu UHP terrane: Zircon U-Pb dating of granitic leucosomes within amphibolite and gneiss

摘要

Thin layers and lenses of granitic leucosome are widely distributed within amphibolites, paragneisses and orthogneisses of the Sulu UHP terrane. They are parallel to, or cross-cut, foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre- to decimetre-scale segregations. Variously migmatized rocks extend at least 350km from SW Sulu (Maobei) to NE Sulu (Weihai), in a band at least 50km wide. A combined study of mineral inclusions, cathoduluminescence (CL) images, U-Pb LA-ICP-MS dates, and in-situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these UHP rocks. Most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited (igneous or metamorphic) cores or as new, euhedral crystals. The overgrowths and new crystals commonly show perfectly euhedral shapes, have pronounced oscillatory zoning and contain felsic mineral inclusions, such as Kfs + Pl + Qtz ± Ilm ± monazite (Mon). In contrast, the inherited igneous or metamorphic cores are rounded or irregular, contain low-P or UHP mineral inclusions and show clear dissolution textures. These data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the UHP rocks. The REE patterns of the anatectic zircon show steep slopes from the HREE to LREE with strongly to moderately negative Eu anomalies (Eu/Eu*=0.31-0.72) and pronounced positive Ce anomalies (Ce/Ce*=6.8-26.5). Abundant U-Pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the Sulu UHP terrane. Anatectic zircon from 12 granitic leucosomes within amphibolites, paragneisses, and orthogneisses from Sulu UHP slices II and III yields consistent mean U-Pb ages of 219.0±1.2 to 218.3±1.6Ma, 218.8±2.0 to 217.3±1.7Ma and 218.2±1.4 to 215.0±1.5Ma, respectively. In contrast, anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from Sulu UHP slice III records younger mean U-Pb ages of 151.9±1.3 to 151.1±1.8Ma and 155.9±1.8 to 153.7±1.7Ma, respectively. These data imply that the Sulu UHP terrane experienced two Mesozoic partial melting events. The first partial melting event (219-215Ma) was probably associated with a Late Triassic granulite facies stage of 'hot' exhumation, whereas the second (156-151Ma) is interpreted as the result of Middle-Late Jurassic extension and thinning of the previously thickened crust of the Sulu UHP terrane. Both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks.