The metamorphic evolution and geochronology of blueschists and the immediate country rocks in the South Tianshan of Tajikistan
Palaeontology
Final Report Abstract
Peak metamorphic conditions obtained for the Garm rocks range between 600 and 660°C at pressures from 7 – 9 kbar below or close to the water-saturated ‘granite’ melting curve according to the here calculated P-T pseudosections. These P-T conditions are characteristic for medium P/T series (upper amphibolite- to granulite facies) metamorphic belts and are typical for continental regionally metamorphosed orogenic belts. The U-Pb zircon SHRIMP study of medium P/T series metamorphic rocks of the Garm Block in the Tajik South Tien Shan reveals an Early Permian metamorphic age. Ages of overgrown rim domains of the zircon grains and newly grown magmatic zircon grains from four migmatitic samples yield concordia ages in the range 299-290 Ma. Thus the Garm Block is the first Early Permian regional metamorphic terrane established in southern CAOB. A crosscutting granitoid intrusion from the Garm block yields a concordia age of 290 Ma identical to the age of metamorphism and similar to ages of post-collisional intrusions elsewhere in the Tien Shan. The crosscutting intrusions comprise a series of peraluminous I- type granites. Based on the compositional similarity to migmatitic leucosomes they are interpreted as examples of protolith-dependent in situ production of post-collisional granites that formed in deeper levels of the crust and supplied the melts that injected as migmatite veins in the amphibolite-facies paragneisses. The Garm detrital zircon ages yield major overlapping peaks at 700-550 Ma and 1150-700 Ma, and smaller peaks around 2700-2400 Ma and 3500-3300 Ma. The three youngest grains with ages between 592 and 574 Ma define an Ediacarian maximum depositional age of the sedimentary protoliths of the Garm rocks. The distribution of the detrital zircon ages in the Garm Block is remarkably similar to those of detrital zircon grains from the Kyzykum segment indicating that both areas of the South Tien Shan received clastic material from similar sources. The presence of Grenvillian ages 1150-800 Ma, characteristic for the Tarim Craton, and the lack of 1200 to 1600 Ma ages, characteristic for the Northern Tien Shan, points to a derivation of the Precambrian zircon grains from the southern continental blocks similar to the Tarim Craton. Six Garm samples of granitic compositions have strongly negative εNd values (-3.7 to -10,6) and Meso- to Paleoproterozoic Nd model ages (1.81-1.12 Ga) indicating a significant input of a Precambrian crustal component and confirming a formation of the South Tien Shan belt on a continental basement with Paleoproterozoic or older crust. The Garm Block formed as a consequence of lithospheric thickening during the Late Carboniferous collision of two continental blocks that created the South Tien Shan fold belt. The thickened crust was heated and weakened commencing crustal regional metamorphism, the production of granites and the formation of coeval alkaline melts as a result of the interaction of asthenospheric material with lithospheric keels of the Precambrian continents.
Publications
- Permian age of orogenic thickening and crustal melting in the Garm Block, South Tien Shan, Tajikistan. Journal Asian Earth Science, Volume 113, Part 2, 1 December 2015, Pages 711-727
Konopelko, D., Klemd, R., Mamadjanov, Y., Hegner, E., Knorsch, M., Fidaev, D., Kern, M., Sergeev, S.
(See online at https://doi.org/10.1016/j.jseaes.2015.09.004)
