The crust‐mantle interaction in continental subduction channels: Zircon evidence from orogenic peridotite in the Sulu orogen

Abstract

A combined secondary ion mass spectrometer and laser ablation‐(multicollector)‐inductively coupled plasma mass spectrometer study of zircon U‐Pb ages, trace elements, and O and Hf isotopes was carried out for orogenic peridotite and its host gneiss in the Sulu orogen. Newly grown zircon domains exhibit weak zoning or no zoning, relatively low Th/U ratios (<0.1), low heavy rare earth element (HREE) contents, steep middle rare earth element‐HREE patterns, negative Eu anomalies, and negative to low δ18O values of −11.3 to 0.9‰ and U‐Pb ages of 220 ± 2 to 231 ± 4 Ma. Thus, these zircons would have grown from metasomatic fluids during the early exhumation of deeply subducted continental crust. The infiltration of metasomatic fluids into the peridotite is also indicated by the occurrence of hydrous minerals such as amphibole, serpentine, and chlorite. In contrast, relict zircon domains exhibit magmatic zircon characteristics. Their U‐Pb ages and trace element and Hf‐O isotope compositions are similar to those for protolith zircons from ultrahigh‐pressure metamorphic rocks in the Dabie‐Sulu orogenic belt. Thus, these relict magmatic zircons would be physically transported into the peridotite by metasomatic fluids originated from the deeply subducted continental crust. Therefore, the peridotite underwent metasomatism by aqueous solutions derived from dehydration of the deeply subducted continental crust during the early exhumation. It is these crustally derived fluids that would have brought not only such chemical components as Zr and Si but also tiny zircon grains from the deeply subducted crustal rocks into the peridotite at the slab‐mantle interface in continental subduction channels. As such, the orogenic peridotite records the crust‐mantle interaction at the deep continental subduction zone.

Crustal Metasomatism at the Slab‐Mantle Interface in a Continental Subduction Channel: Geochemical Evidence From Orogenic Peridotite in the Sulu Orogen

“…These observations suggest that some spinels underwent fluid metasomatism related to the formation of chlorite and amphibole (e.g., Endo et al, ; Gervilla et al, ; McInnes et al, ; Mellini et al, ; Prabhakar & Bhattacharya, ; Shaw et al, ; Shaw & Dingwell, ; Xu et al, , ). Rare zircon grains were recovered from the Tengjia dunites with amphibole‐rich veinlets (Li, Chen, Zheng, & Hu, ), indicating that the growth of metasomatic zircons was contemporaneous with the formation of amphiboles. Therefore, the third stage of crustal metasomatism (Stage 5 in Figure ) is mainly recorded in amphiboles, spinels and zircon.…”

“…The amphibole rims are primarily tremolite, which forms at pressures lower than ~2.0 GPa (Jenkins, ). Metasomatic zircons have U‐Pb ages of 220–231 Ma and contain quartz rather than coesite inclusions, indicating that this episode of crustal metasomatism occurred in the quartz stability field during the exhumation of deeply subducted continental crust (Li, Chen, Zheng, & Hu, ). The Ti‐in‐zircon thermometry of metasomatic zircons yields temperatures of <700 °C (Li, Chen, Zheng, & Hu, ).…”

“…Metasomatic zircons have U‐Pb ages of 220–231 Ma and contain quartz rather than coesite inclusions, indicating that this episode of crustal metasomatism occurred in the quartz stability field during the exhumation of deeply subducted continental crust (Li, Chen, Zheng, & Hu, ). The Ti‐in‐zircon thermometry of metasomatic zircons yields temperatures of <700 °C (Li, Chen, Zheng, & Hu, ). In combination with amphibole stability, it appears that this episode of crustal metasomatism would have occurred during the late stage of exhumation of the deeply subducted continental crust (Stage 5 in Figure ).…”

“…Parent/daughter ratios are adopted from whole‐rock trace elements. Initial Sr‐Nd isotope ratios were calculated at t 1 = 780 Ma and t 2 = 220 Ma, respectively, based on protolith and metamorphic U‐Pb ages for zircons from the Tengjia peridotites and host gneisses (Li, Chen, Zheng, & Hu, ).…”

Zircon Macrocrysts from the Drybones Bay Kimberlite Pipe (Northwest Territories, Canada): A High-Resolution Trace Element and Geochronological Study

“…Neither Eu nor Y anomalies are observed, and the average Y/Ho ratio is essentially chondritic (25 ± 2). The REE characteristics of zircon (i.e., the shape of chondrite-normalized profiles, Ce, Eu, and Y anomalies) are sensitive to its interaction with fluids and melts [33,35,36]. Because the Y/Ho ratio shows the least variation across the dataset, we used it as a "common denominator" to gauge the extent of Pb loss (or gain).…”