Origin of the Cenozoic porphyry Cu deposits in southern Tibet

• Author / Creator

  Wang, Rui

• The Gangdese magmatic belt features extensive Cenozoic magmatic rocks that record geodynamic changes related to the India–Asia collision starting at ~55–50 Ma. Paleocene–Eocene magmas throughout the belt have similar continental arc features. Their intrusive rocks have intermediate [La/Yb]N and intermediate-to-low Sr/Y ratios, negative Eu anomalies, and consists mainly of pyroxene and plagioclase. These geochemical and mineralogical characteristics suggest that the Paleocene–Eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase. In addition, their magmas were also less oxidized (ΔFMQ -1.2 to +0.8). The relatively low water contents and oxidation state of these magmas may reflect final dehydration of the remnant Neo-Tethyan slab, and with the result that only three small porphyry Cu-Mo deposits associated to this suite.
  
  The Oligo-Miocene igneous rocks show a sharp longitudinal distinction of petrography, magmatic geochemistry, and association with porphyry-type mineralization. The eastern Gangdese group (east of ~89°E) is characterized by mainly intermediate–felsic calc-alkaline plutons related to porphyry Cu-Mo±Au deposits, and minor potassic rocks. Their intrusive igneous rocks have high [La/Yb]N and Sr/Y ratios, weak or absent Eu anomalies, and amphibole as common phenocrysts. Their magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement. In addition, their magmas were also more oxidized (ΔFMQ+0.8 to +2.9). In contrast, the western group is characterized by potassic volcanic rocks with relatively high Th and K2O contents, low Sr/Y ratios, and low εNdi values. There are only one small-sized porphyry Cu deposit to the west of ~89°E. We propose that the sharp longitudinal distinction between magmatism and metallogenic potential in the Oligo-Miocene Gangdese belt reflects the variable extent of underthrusting of the Indian plate continental lithosphere beneath Tibet. Where subduction-modified Tibetan lithosphere was partially melted by upwelling asthenosphere following slab breakoff in the east, hydrous, oxidized magmas with the result to generate several large porphyry deposits. In contrast, underthrusting of the Indian plate to the west limited the involvement of asthenospheric melts and limited the extent of lithospheric partial melting, with the result that few porphyry deposits are associated with these magmas.

• Subjects / Keywords

  • Porphyry deposit
  • Miocene
  • Paleocene-Eocene
  • Gangdese, Tibet

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3BK16X8N

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.