Tectonic evolution of the NE section of the Pamir Plateau: New evidence from field observations and zircon U-Pb geochronology

Abstract

The Pamir Plateau at the western end of the India–Asia collision zone underwent long-term terrane drifting, accretion and collision between early Paleozoic and Mesozoic. However, the detailed evolution of this plateau, in particular, the timing of the Proto- and Palaeo-Tethys ocean subduction and closure, remains enigmatic. Here we report new field observations and zircon U-Pb ages and Hf isotopic compositions of the representative rocks from the so-called Precambrian basement in the northeastern Pamir, i.e., the Bulunkuole Group. The rock associations of the Bulunkuole Group indicate volcano-sedimentary sequences with arc affinities. Geochronological data demonstrate that the deposition age of the Bulunkuole Group in the NE section of the Pamir was Middle to Late Cambrian (530–508Ma) rather than Paleoproterozoic. The deposition age became progressively younger from south to north. The amphibolite- to granulite facies metamorphism of the Bulunkuole Group took place at ca. 200–180Ma. Unlike the scenario in the Southern Kunlun terrane (SKT) in the eastern section of the West Kunlun Orogenic Belt (WKOB), early Paleozoic metamorphism (ca. 440Ma) was absent in this area. Two phases of magmatic intrusions, composed of granites and minor gabbros with arc geochemical signatures, emplaced at 510–480Ma and 240–200Ma. The amphibolite (meta mafic sheet? 519Ma) and the meta-rhyolite (508Ma) have zircon εHf(t) values of 1.6 to 5.9 and −1.5 to 1.4, respectively. The 511Ma gneissic granite sheet and the 486Ma gabbro have zircon εHf(t) values of −0.1 to 2.4 and 1.3 to 3.6, respectively. Zircon εHf(t) of the 245Ma augen gneissic granite sheet varies from −2.2 to 2.0 whereas the metamorphic zircons from the amphibolite (193Ma) and high-pressure mafic granulite sample (187Ma) have negative εHf(t) values of −5.3 to −2 and −15 to −12, respectively. In line with rock association and the deposition age of the Bulunkuole Group and the Saitula Group in the eastern section of WKOB, we propose that both of them were accretionary wedge between the Tarim and Tianshuihai terrane formed during the Proto-Tethys ocean south- southwestward subduction (present orientation). The timing of deposition and metamorphism documents two distinct phases of arc magmatism and sedimentary basin evolution. This indicates that the Proto-Tethys ocean closed at ca. 440Ma in the eastern section of WKOB whereas in the western section, a remnant of the Proto-Tethys ocean between the Tarim and the NE Pamir did not close till Late Triassic. This remnant ocean was filled by the Ordovician-Triassic sequences. The accretion between NE Pamir and the Central Pamir was completed by ca. 180Ma as demonstrated by the metamorphic zircon U-Pb age of the high pressure mafic granulite in the NE Pamir.