Detrital zircon U-Pb-Hf isotopes and whole-rock geochemistry of Ediacaran - Silurian clastic sediments of the Uzbek Tienshan: sources and tectonic implications

ABSTRACT The paper presents first high-precision data, U-Pb detrital zircon ages, whole-rock geochemistry, Hf-in-zircon and whole-rock Nd isotopes, from pre-Devonian (Ediacaran and Silurian) clastic sediments (sandstones) of the Tamdytau, Bukantau and Nuratau mountainous ranges of the Kyzylkum Desert and Nuratau Range in the western Uzbek Tienshan. The sediments form a turbidite-type complex associated with ocean plate stratigraphy units (oceanic pillow basalt, chert, siliceous mudstone and siltstone) and arc volcanic rocks. Four sandstone samples from the Tamdytau and Northern Nuratau Mts. (Besapan and Kaltadavan formations, respectively) yielded maximum depositional ages in the range of 570–540 Ma. These ages indicate the formation of pre-Devonian sedimentary units during a relatively narrow time interval from the latest Neoproterozoic (Ediacaran) to the earliest Cambrian, i.e. ca. 50–70 Myr. Five samples of turbidites from the Bukantau Mts. (Baimen Fm.) yielded a maximal depositional age of ca. 440 Ma, i.e. early Silurian (Llandovery). The petrographic, major and trace element compositions, as well as εNd(t) values ranging from -16 to -9 of those sandstones suggest their generally mature character and derivation from recycled orogens with a limited contribution of juvenile crust material. All sandstone samples yielded similar detrital zircon U-Pb age patterns characterized by major peaks at 650–570, 870–730, 1050–900 and 2400 Ma and by a smaller peak at ca. 1800 Ma. These patterns are similar to the U-Pb age spectra from the basement of the Tarim Craton. However, the Kyzylkum basement may contain a larger proportion of late Archaean rocks and igneous formations related to Ediacaran – earliest Cambrian orogenic events. All samples showing U-Pb detrital zircon age spectra with peaks at 650–570 Ma carry relatively large amounts of zircon grains with juvenile Hf isotope characteristics, i.e. positive εHf(t) (up to +10) suggesting their derivation from continental or Island arcs. The presence of exotic tectonic blocks composed of Ediacaran arc-type rocks hosted by the accretionary complex of the South Tienshan suggests that an extended arc system once existed at the southern convergent margin of the Turkestan Ocean. That arc system provided clastic material for the Palaeozoic sediments of the South Tienshan, but was probably destroyed by tectonic erosion during early Palaeozoic oceanic subduction.