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TitleQuantifying the P-T-t conditions of north-south Lhasa terrane accretion: new insight into the pre-Himalayan architecture of the Tibetan plateau
AuthorWeller, O M; St-Onge, M R; Searle, M P; Waters, D J; Rayner, NORCID logo; Chen, S; Chung, S -L; Palin, R M
SourceJournal of Metamorphic Geology vol. 33, issue 1, 2015 p. 91-113,
Alt SeriesEarth Sciences Sector, Contribution Series 20140099
Mediapaper; digital; on-line
File formatpdf
AreaLhasa; Basong Tso; China
Lat/Long WENS 93.7500 94.0833 30.5833 30.2500
Subjectsgeochronology; geochemistry; structural geology; monazite; zircon dates; Lhasa terrane; Tibetan plateau
Illustrationsgeological sketch maps; location maps; photographs; tables; phase diagrams; Concordia diagrams; schematic diagrams
ProgramGEM: Geo-mapping for Energy and Minerals GEM Tri-Territorial Information Management & databases (Tri-Territorial Bedrock Framework)
Released2014 11 19
AbstractAn integrated field, petrological and geochronological study of the Basong Tso region of south-eastern Tibet has constrained the timing and P-T conditions of north-south Lhasa terrane accretion and provides new insight into the tectonothermal evolution of the Tibetan plateau. Two distinct high-grade metamorphic belts are recognized in the region: a southern belt (the Basong Tso complex) that consists of sheared schist and orthogneiss; and a northern belt (the Zhala complex) that comprises paragneiss and granite. Combined pseudosection modelling and U-Pb geochronology of monazite and zircon indicates that the Basong Tso complex records peak metamorphic conditions of 9 ± 0.5 kbar and 690 ± 25 °C at c. 204-201 Ma, whereas the Zhala complex experienced peak metamorphic conditions of 5.0 ± 1.0 kbar and 740 ± 40 °C at c. 198-192 Ma. Microstructural analysis suggests that the two belts share a common early prograde history, after which the Basong Tso complex attained peak conditions following rapid burial, and the Zhala complex approached peak conditions along an isobaric path. Overall it is inferred that the Basong Tso and Zhala complexes represent the lower and upper structural levels of an evolving orogen that underwent Barrovian-type metamorphism following collision (M1), followed by Buchan-style overprinting at higher structural levels due to heat advection by syn-tectonic granites (M2). Mylonitization (sensu lato) of the Basong Tso complex and juxtaposition of the two units occurred after attainment of peak conditions. The dominance of Mesozoic regional metamorphism across most of the Tibetan plateau indicates that Cenozoic crustal thickening processes, where present, are only manifested at depth.
Summary(Plain Language Summary, not published)
Metamorphic rocks hold the key to understanding crustal thickening in Tibet. The recent discovery of several new metamorphic localities in southern Tibet has revealed the existence of a suture zone that divides the Lhasa terrane into northern and southern parts. Two distinct high-temperature belts are recognised. Analysis of prograde histories indicates that one belt attained peak conditions following rapid burial, whereas the other belt approached peak conditions with little change in pressure. The two belts are proposed to represent distinct stages within an evolving orogeny. That nearly all of the metamorphism in the Lhasa terrane is Mesozoic in age suggests that the Tibetan plateau was uplifted by underthrusting of Indian lower crust and not by Cenozoic homogeneous crustal thickening. These results and the consequent tectonic model have direct application to the bedrock geology of the southern half of Baffin Island in Arctic Canada.

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