Quantifying the P-T-t conditions of north-south Lhasa terrane accretion: new insight into the pre-Himalayan architecture of the Tibetan plateau
An 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...
Saved in:
| Published in | Journal of metamorphic geology Vol. 33; no. 1; pp. 91 - 113 |
|---|---|
| Main Authors | , , , , , , , |
| Format | Journal Article |
| Language | English |
| Published |
Oxford
Blackwell Publishing Ltd
01.01.2015
|
| Subjects | |
| Online Access | Get full text |
Cover
Loading…
| Summary: | An 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. |
|---|---|
| Bibliography: | Figure S1. Basong Tso complex petrography. All sections are parallel to the lineation and normal to the foliation. (a) Photomicrograph of sample W77 viewed under crossed polars, showing the main assemblage (biotite, garnet, plagioclase, quartz, tschermakite and zoisite). Zoisite laths align to form a lineation. (b) Back-scattered electron image of sample W77, which shows a ragged plagioclase grain. (c) Plane-polarised light (PPL) photomicrograph of sample W78, showing the main assemblage (biotite, garnet, muscovite, plagioclase and quartz) and the mylonitic C′-type shear band cleavage. Mica fish and the orientation of C/S planes indicate a dextral sense of shear. (d) Photomicrograph of sample W78 viewed under crossed polars, showing the boundary (dotted orange line) between the matrix and the quartzo-feldspathic segregations. Owing to their coarse grain size and textural associations, the latter regions are interpreted to represent a crystallized partial melt phase. (e) PPL photomicrograph of sample W76, showing the main assemblage (biotite, epidote, hornblende, plagioclase and quartz) and the protomylonitic C/S fabric. The orientation of the C/S planes indicates a dextral sense of shear. (f) Photomicrograph of sample W76 viewed under crossed polars, showing an epidote grain that displays euhedral grain boundaries against biotite, compared with embayed grain boundaries adjacent quartz, which indicates a magmatic origin for the epidote (Zen & Hammarstrom, ). Figure S2. H2O content (mol.%) of equilibrium assemblages as a function of pressure and temperature in samples W77 (a, b) and W78 (c, d). H2O content calculated by summation of the read bulk information matrix output from thermocalc, which gives the combined H2O as a molar percentage of the solid assemblage. This approach can only be applied to pseudosections calculated with H2O in excess, hence no supra-solidus information is given in S2b. Both samples show that H2O content increases with pressure, whereas it decreases with temperature. Therefore, without an external supply of fluid, equilibrium may not be maintained during rapid burial. Table S1. X-ray fluorescence (XRF) data (wt%) of study samples (red stars, Fig. b). No XRF data are available for sample W23 as it was too silicic to make good quality glass beads. Table S2. Representative mineral compositions for samples from the Zhala complex. The analyses are presented as the average of multiple grains. XMg = Mg/(Mg + Fe2+), XCa = Ca/(Ca + Na + K). Amphibole cation totals were recalculated following the scheme of Holland & Blundy (). All other cation totals were recalculated using AX (Holland, ). Table S3. Representative mineral compositions for samples from the Basong Tso complex. The analyses are presented as the average of multiple grains, except for garnet analyses, which are point analyses. XMg = Mg/(Mg + Fe2+), XCa = Ca/(Ca + Na + K), Sps = Mn/(Fe2+ + Mg + Ca + Mn), Prp = Mg/(Fe2+ + Mg + Ca + Mn), Grs = Ca/(Fe2+ + Mg + Ca + Mn), Alm = Fe2+/(Fe2+ + Mg + Ca + Mn). Amphibole cation totals were recalculated following the scheme of Holland & Blundy (). All other cation totals were recalculated using AX (Holland, ). Table S4. U-Pb zircon geochronology data for samples W20, W23, W27 and W76. Table S5. U-Pb monazite geochronology data for samples W24 and W78. istex:6A24F9CFB104170452367DACFF0EE374B44A4709 ArticleID:JMG12112 Natural Environment Research Council - No. NE/I528485/1 Natural Resources Canada - No. 20140099 ark:/67375/WNG-62026HFR-5 |
| ISSN: | 0263-4929 1525-1314 |
| DOI: | 10.1111/jmg.12112 |