Ages, geochemistry and tectonic settings of the Paleozoic-Mesozoic rocks along the Hegenshan-Heihe suture zone in the eastern segment of the Central Asian orogenic belt

Abstract

The Hegenshan-Heihe belt (HHB) represents a suture zone resulting from the collision of the Xing’an and Songliao blocks located in the eastern segment of the Central Asian Orogenic Belt (CAOB). The formation of the HHB marks the destruction of the oceanic lithosphere and the closure of the Paleo-Asian Ocean (PAO). It has been a long-time debate regarding the tectonic evolution of the Xing’an and Songliao blocks and the intervening ocean during Paleozoic time, particularly the final closure timing of the PAO along the HHB. This project is aimed to address these issues by integrating the geochronology and geochemistry of the Paleozoic volcanic rocks and their tectonic setting with the detrital zircon data.

The geochronological and geochemical data for two suites of the Paleozoic volcanic and sedimentary rocks suggest two long-lived island-arc systems along the southern margin of the Xing’an block and the northern margin of the Songliao Block. LA-ICP-MS zircon U-Pb dating results indicate that the Ordovician volcanic rocks developing at the southern margin of the Xing’an block erupted at ca. 447Ma and geochemically resemble the arc-related volcanism with strongly enriched LREE and LILE. The zircon ages also revealed the basement information of the Xing’an block by yielding age populations similar to 2.5, 1.8, 1.2, and 0.8 Ga from the inherited zircons, most likely shed from the deep continental crust during the magma migration and contamination. Similar features were also discovered in the locally deposited Silurian-Devonian sedimentary strata. This study suggests an Andean-type continental margin developing at the southern margin of the Xing’an block, associated with the northward subduction of the PAO from Ordovician to Devonian (ca.465-392Ma). The Carboniferous volcanic rocks developed at the northern margin of the Songliao block, associated with the coevally deposited sedimentary strata, showing a close affinity with oceanic island-arc, and the sedimentary strata formed in a back-arc/forearc basin. This study also suggests a Western Pacific-type subduction and arc-basin system developing at the northern margin of the Songliao block during Ordovician to Carboniferous time (ca. 452-315 Ma).

The detrital zircon analysis suggests that a provenance transfer for the sedimentary strata developing in the HHB took place between the Late Carboniferous and Permian, with the Carboniferous strata sourcing Paleozoic detritus from the isolated island-arcs and the Permian strata sourcing Precambrian detritus from the approximate continent. Integrated with the previously reported data, this study suggests that the final closure of the PAO along the HHB occurred at some time between 303-312 Ma.

This study also reports a suite of Mesozoic bimodal volcanic rocks in Zhalantun area, which was previously thought to be Permian in age, namely the Dashizhai formation. This suite of volcanic rocks formed in a regional extensional environment, which was mostly likely induced by the post-collisional extension of the eastern CAOB. On the basis of new data presented in this thesis combined with previous studies, I present a new tectonic model for the Paleozoic subduction and collision followed by a Mesozoic extension for the Hegenshan-Heihe suture zone.