Zircon U-Pb ages, REE concentrations and Hf isotope compositions of granitic leucosome and pegmatite from the north Sulu UHP terrane in China:Constraints on the timing and nature of partial melting

摘要

Granitic leucosome and pegmatite are widely distributed within biotite-bearing orthogneiss in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, U-Pb SHRIMP dates, and in situ trace element and Lu-Hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks. Zircon grains separated from biotite-bearing orthogneiss typically have three distinct domains: (1) pre-metamorphic (magmatic) cores with Qtz + Kfs + Pl + Ap inclusions, which record a Neoproterozoic protolith age of -790 Ma, (2) mantles with Coe + Phe + Ap inclusions that record Triassic UHP age at 227 ±3 Ma, and (3) narrow rims with quartz inclusions that record HP granulite-facies retrograde metamorphism at -210 ± 3 Ma. In contrast, zircons separated from granitic leucosome have only two distinct domains: (1) the central UHP areas with Coe + Phe+Ap inclusions record Triassic UHP age of 227 ±3 Ma, and (2) outer magmatic areas with Qtz + Kfs + Ab + Ap inclusions that record partial melting time of 212 ±2 Ma. Zircons separated from pegmatite contain mineral inclusions of Qtz + Kfs + Ap and show regular magmatic zoning from centre to edge. The centres record partial melting time of 212 ± 2 Ma in line with the outer domains of granitic leucosome, whereas the edges give a younger age of 201 ±2 Ma related to Pb loss and partial recrystallization during late Triassic regional amphibolite-facies retrogression. These data indicate that partial melting in the north Sulu UHP gneissic rocks took place during post-UHP, retrograde HP granulite-facies metamorphism.Pre-metamorphic (magmatic) zircon cores from biotite-bearing orthogneiss give uniform ~(176)Hf/~(177)Hf of 0.28187 ±0.00003 (2 SD; standard deviation) corresponding to εHf_(790)) and Hf model ages (T_(DM2)) of about - 16.3 and 2.41 Ga, respectively. This is consistent with the generation of its protolith by reworking of Paleoproterozoic to late Archean crust In contrast, UHP zircon domains from biotite-bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low Lu/Hf (<0.006), low Th/ U (<0.1) and considerably higher, ~(176)Hf/~(177)Hf (0.28233 ±0.00002; 2 SD) than the pre-metamorphic cores. The uniform but significantly different Hf isotope composition between the UHP (εHf_(227) = -14.6 ±0.8; 2 SD) and pre-metamorphic (εHf(227)= - 27.7) domains indicates equilibration of the Lu-Hf isotope system only within the UHP metamorphic mineral assemblage. The disequilibrium between whole rock and UHP zircon suggests that about two thirds of the whole rock Hf retained in the pre-metamorphic zircon domains. Zircon domains crystallized during partial melting at 212 Ma in granitic leucosome and pegmatites have a Hf isotope composition indistinguishable from that of the UHP zircon domains. This suggests that only Hf (and Zr) equilibrated during UHP metamorphism was remobilized during partial melting while pre-metamorphic zircon remained stable or was not accessible. In contrast, the magmatic zircon edges from pegmatite have somewhat lower ~(176)Hf/~(177)Hf (-0.28216) and εHf(t) (-17.6±1.2; 2SD) indicating some release of less radiogenic Hf for instance by dissolution of pre-metamorphic zircon during late regional amphibolite-facies retrogression.