Ediacaran, Early Ordovician and early Silurian arcs in the South Tianshan orogen of Kyrgyzstan

Abstract

The oldest magmatic rocks from the South Tianshan orogen of Kyrgyzstan (STS) are important for better understanding of the Neoproterozoic and early Palaeozoic evolution of the southwestern Central Asian Orogenic Belt. Bulk rock major and trace element and Sm-Nd isotopic composition and zircon U-Pb ages of granitoids from mélange blocks reveal two previously unknown episodes of arc magmatism in the STS orogen of Kyrgyzstan, namely, the Ediacaran (ca. 624 Ma) and Early Ordovician (ca. 472 Ma) episodes. Moderately positive ɛNd(t) value of + 5.8 of the Ediacaran granodiorite indicates a mainly juvenile source for this rock. Negative ɛNd(t) value of −9.1 for the Early Ordovician granodiorite indicates melting of the predominantly Precambrian crustal material. The latter is also supported by the inherited zircon cores with an age range of 1.04–2.7 Ga. The geochemical signature of the Silurian (late Llandovery and Wenlock) basalts suggests formation in arc settings. Arc-related origin of the Ediacaran granodiorite suggests that mature oceanic basin with subduction system existed in the western STS since the late Neoproterozoic. The arc also evolved in the Ordovician and Silurian and was inactive in the middle Cambrian to the earliest Ordovician. Ediacaran granodiorite, old zircon cores and negative ɛNd(t) values of the Ordovician granodiorite suggest the existence of Precambrian microcontinents in the Kyrgyz STS. These crustal fragments were likely rifted off the Tarim and / or Karakum-Tajik microcontinents, where the rocks with similar ages occur. In the Palaeozoic, they traveled north and were incorporated in the STS orogen in the late Carboniferous.