|
Hongyan Li, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Kai Ye, Institute of Geology and Geophysics, Chinese Academy of Sciences (China)
Jinbo Liu, Institute of Geology and Geophysics, Chinese Academy of Sciences (China)
Zhiwei Tian, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
|
|
Cycle processes and roles of metamorphic fluids during the subducted oceanic crust have been well understood. It is widely accepted that large volumes of aqueous fluids are liberated from hydrated oceanic crust, and rise into the overlying mantle wedge, and cause partial melting of the mantle wedge to generate arc magmas. Contrasting to the oceanic crusts, the continental crusts are rather dry and buoyant. Thus, it was deduced that no significant dehydration occurs during subduction of continental crust, and syn-collisional magmatism is generally absent. However, some field observations, together with U-Pb dating, REE and Hf isotope analysis results of zircons, will be presented here, to disclose obvious fluid/melt activities and a partial melting process during the early stage of exhumation of the subducted continental crust in the Sulu UHP terrane.
In the northern Sulu UHP terrane, widespread migmatization domains can be observed. In the field, dark gneiss and pale gneiss show structures similar to magma mingling. Leucogranite veins locally occur in the UHP unit. The leucogranite veins are centimeters to meters in length, and are strongly deformed and occasionally show foliation in concordance with the surrounding gneisses. (1) In the Weihai area, zircons from a pegmatite vein within gneiss, with clear oscillatory zonings, gave two ages of 221±7Ma and 199±3Ma. Zircons from the country gneiss show a core-mantle-rim structure. Oscillatory zonings are well preserved in the cores and the rims, and irregular patch structure is observed in mantle domains. The cores and rims show magmatic zircon REE characteristics, with some difference in LREE, and yield ages of 772±7 Ma and 195±4 Ma, respectively. However, the zircon mantles show a large age span. Uniform zircon Hf isotopic values are observed in the pegmatite and the rims of the country gneiss. Thus, we interpret the age of 221 Ma as the time of pegmatite crystallization and 199 Ma as the time for pegmatite to intrude the granitic gneiss. (2) In the Rongcheng area, both dark gneiss and pale gneiss in a migmatite domain have protolith ages of ca 780Ma. Core-mantle-rim structure is well preserved in the zircons of the pale gneiss, and the mantles and rims show CL features of metamorphism and partial melting origin, respectively. Their ages of 242±9 Ma and 220±4 Ma are regarded as the time of peak metamorphism and partial melting of the pale gneiss.
In the southern Sulu UHP terrane, some zoisite-kyanite-paragonite quartz noddles occur in epidote-bearing eclogite in the Donghai area. Petrographic studies suggest they are products of dehydration of former lawsonite in eclogite. This is also supported by the similarity of the Nd isotope between the eclogite and zoisite in the noddles. Zircons from a noddle give a SHRIMP U-Pb age of 219±9Ma, which indicates an aqueous fluid activity during the early stage of exhumation of the subducted continental crust.
|
