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Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57-73% and highly variable SiO 2 contents of 0.32-25.85% that are positively correlated with BaO and SO 3 contents. The occurrence of MS inclusions provides petrographic evidence for partial melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO 2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local dehydration melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during ÔhotÕ exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction-zone metamorphism.

The ultrahigh-pressure (UHP) eclogite in the Dabie orogen preserves petrological evidence for the existence of hydrous silicate melts that formed during continental subduction-zone metamorphism. This is indicated by occurrence of multiphase solid (MS) inclusions in garnet that primarily consist of K-feldspar + quartz AE epidote/allanite. All the MS inclusions are euhedral to subhedral in morphology and surrounded with radial cracks in the host garnet. Their trace element compositions were analysed by two different approaches of laser sampling. The mass budget method was used to estimate the trace element abundances of MS inclusions from their mixtures with the host garnet. The results are compared with the direct sampling of MS inclusions, providing a first-order approximation to the trace element composition of MS inclusions. The MS inclusions exhibit consistent enrichment of LILE, Sr and Pb, but depletion of HFSE in the primitive mantle-normalized spidergram. Such arc-like patterns of trace element distribution are common for continental crustal rocks. The melts have variably high K, Rb and Sr abundances, suggesting that breakdown of phengite is a basic cause for partial melting of the UHP eclogite. These MS inclusions also exhibit consistently low HFSE and Y contents, suggesting partial melting of the eclogite in the stability fields of rutile and garnet. Consequently, the trace element composition of MS inclusions provides a proxy for that of hydrous silicate melts derived from dehydration melting of the UHP eclogite during continental collision.

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Laser ablation inductively coupled plasma mass spectrometry analyses of U-Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh-pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction-zone metamorphism. Zircon U-Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U-Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti-in-zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8°C and 742 ± 19 to 778 ± 34°C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction-zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti-in-zircon temperatures for the younger zircon at lower pressures indicate the initial ÔhotÕ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE-HREE patterns and no Eu anomalies, and yields Zr-in-titanite temperatures of 551 to 605°C at 0.5 GPa, and the other shows LREE depletion and flat MREE-HREE patterns, and gives Zr-in-titanite temperatures of 782-788°C at 2.0 GPa. The former is amenable for U-Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ÔhotÕ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction-zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P-T-t path of eclogites during the continental collision. It appears that the mid-T ⁄ UHP eclogite facies zone would not only form by subduction of the continental crust in a P-T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.

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Magnetotelluric and seismological studies suggested the presence of partial melts in the middle to lower Himalaya-Tibetan crust. However, the melt fractions inferred by previous work were based on presumed electrical conductivity of melts. We performed measurements on the electrical conductivity of peraluminous granitic melts with 0.16-8.4 wt % H 2 O (the expected compositions in the Tibetan crust) at 600-1,300°C and 0.5-1.0 GPa. Peraluminous melt exhibits lower electrical conductivity than peralkaline melt at dry condition, but this difference diminishes at H 2 O > 2 wt %. With our data, the observed electrical anomalies in the Tibetan crust could be explained by 2-33 vol % of peraluminous granitic melts with H 2 O > 6 wt %. Possible reasons for our inferred melt fractions being higher than seismological constraints include the following: (1) The real melts are more Na and H 2 O rich, (2) the effect of melt reducing seismic velocities was overestimated, and (3) the anomalies at some locations are due to fluids. Plain Language SummaryWhether there are molten zones present in the Tibetan crust is a focus of geophysical and petrological research. Previous interpretations of MT data to infer melt fractions are often based on presumed electrical conductivity values of partial melts. These felsic melts are believed to derive from metapelites and have peraluminous composition, but previous experimental data of electrical conductivity are only for metaluminous and peralkaline melts. In this contribution, we carry out electrical conductivity measurements on anhydrous and hydrous peraluminous granitic melts with 0.16-8.4 wt % of H 2 O at 600-1,300°C and 0.5-1.0 GPa. We find that the electrical conductivity of peraluminous melt is lower than that of peralkaline melt under dry condition, but their difference quickly diminishes at H 2 O greater than 2 wt %. Based on our experimental results, the melt fractions for various regions in the Himalaya-Tibetan crust are inferred and compared with seismological constraints.GUO ET AL. 3906