Evidence for partial melting and alkali-rich fluids in the crust from a 3-D electrical resistivity model in the vicinity of the Coqen region, western Lhasa terrane, Tibetan Plateau

• Published in: Earth and planetary science letters 2023-10, Vol.619, p.118316, Article 118316
• Main Authors: Sheng, Yue, Jin, Sheng, Comeau, Matthew J., Hou, Zengqian, Becken, Michael, Dong, Hao, Zhang, Letian, Wei, Wenbo, Ye, Gaofeng
• Format: Article
• Language: English
• Subjects: alkali-rich fluid; electrical conductivity; Indian Plate; Lhasa terrane; melt fraction
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2023.118316

Both low resistivity zones and low velocity zones are distributed in the middle-lower crust of the western Lhasa terrane, Tibetan Plateau, China. Some estimates from electrical resistivity data suggest large volume fractions of silicate melts that are difficult to reconcile with seismic velocity data that prefer lower volumes. A second conductive phase, such as saline fluids, that drastically reduces the conductivity but does not significantly affect the seismic velocity because of its low volume may be able to explain these differences. In this study, a 3-D model of the electrical resistivity structure is generated on a profile along longitude 85°E from a latitude of 29°N to 32.5°N. Based on experimental measurement of melts and alkali-rich fluids (e.g., H2O-NaCl), we estimate the volume fraction of each phase that is required to explain the conductive anomalies observed in the geophysical model. The model reveals that the maximum bulk conductivity of the mid-lower crust in the south (1.52 S/m) is much higher than the conductivity of the mid-lower crust in the north (0.18 S/m) when taking 31°N as a rough boundary, near Coqen region. We hypothesize that the conductive zones in the south of the Coqen region may result from a silicate melt and alkali-rich fluid (multicomponent) system. In contrast, partial melting alone can explain the conductive zones in the north. The hypothesis can reconcile the predictions from electrical resistivity data and seismic data, and it corresponds well with zircon Hf isotope data. For example, a combination such as the presence of