Post-collisional felsic rocks produced by re-melting of syn-collisional rocks in southern Tibet
Abstract
Post-collisional felsic rocks (PCFRs) provide a crucial window to investigate the mechanisms responsible for the refinement of continental crust in collision zones. The well-known PCFRs from the Gangdese magmatic belt in southern Tibet is accepted to be generated by the partial melting of a thickened lower crust. However, there is only little direct evidence to show which kind of rocks from the thickened lower crust have been involved in the generation of PCFRs. This study reports field, zircon U-Pb age, zircon Hf isotope, and whole-rock major and trace element data on a large set of PCFRs and syn-collisional dioritic gneisses from eastern Gangdese. The syn-collisional dioritic gneisses occur as either country rocks of or xenoliths within the post-collisional granitoids. Both dioritic gneissic country rocks and xenoliths contain disseminated leucosomes or leucosome veins. The presence of garnets in the leucosomes or leucosome veins provides direct lines of evidence for the re-melting of the syn-collisional rocks. The PCFRs occur as plutons or veins and in some cases contain dioritic gneiss xenoliths. These PCFRs contain zircons with clear core-rim textures, in which the inherited cores yield abundant syn-collisional ages of 66-47 Ma, with a few or without Mesozoic ages and the youngest rims are dated at 27-21 Ma. These inherited cores and zircons from the dioritic gneisses share identical zircon morphologies and zircon ɛHf(t). The field relations, zircon textures, zircon U-Pb ages, and zircon ɛHf(t) strongly support the interpretation that the PCFRs are produced by re-melting of the syn-collisional rocks. Our work not only provides direct evidence for the generation of PCFRs by re-melting the metamorphosed rocks in the middle-lower crust, but also the evidence for the re-melting of syn-collisional rocks, instead of the previously-considered Mesozoic subduction-related rocks, to produce the PCFRs in southern Tibet. This re-melting process is a refining process of continental crust towards more felsic composition and is likely common in other orogenic belts [1, 2], showing the importance of re-melting of pre-existing lower crust in the formation of continental crust in collision zones.
Refs.: [1] Andersen et al. (2014) JGS, London 166, 233-247. [2] Xiong et al. (2014) JGS, London 171, 847-863.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V31D0152Y
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 1037 Magma genesis and partial melting;
- GEOCHEMISTRY;
- 1115 Radioisotope geochronology;
- GEOCHRONOLOGY;
- 8104 Continental margins: convergent;
- TECTONOPHYSICS