Late Paleozoic alkaline granitoids of the southwestern and Northern Mongolia: U–Pb ID TIMS zircon dating, petrogenesis and implications for post-accretion and anorogenic activity of the Central Asian Orogenic Belt

Abstract

In tectonic history of many orogenic belts, alkaline granitoids with A-type affinity mainly reveal lithosphere extension events. However, precise dating of these rocks is often hampered by several problems either due to absence of accessory zircon or due to high U and Th contents disturbing the U–Pb isotope system. This study reports results of chemical abrasion isotope dilution–thermal ionization mass spectrometry (CA ID TIMS) U–Pb dating of zircon from five plutons of aegirine- and/or alkaline amphibole-bearing alkali-feldspar granites and quartz syenites from the Trans-Altai Gobi (SW Mongolia) and Northern Mongolia in the Central Asian Orogenic Belt and reveals their close petrogenesis in different tectonic position. In the SW Mongolia the earliest Carboniferous alkaline granitoids (ca. 358–346 Ma) were formed during the post-accretion extensional event suggesting the Late Devonian accretion of the Trans-Alai Gobi–Eastern Junggar oceanic island arc to the Mongolian–Chinese Altai peri-Siberian continental margin. Alkaline granitoids of the Northern Mongolia were emplaced in the Early Permian during two independent geographically overlapping anorogenic near-coeval igneous pulses. The earliest pulse (ca. 285 Ma) related to evolution of the western half of the Northern Mongolia–Western Transbaikalia volcano-plutonic belt of alkaline granitoids with the bimodal volcanic sequences, is ascribed to the most significant Early Permian magmatic event throughout the Central Asian Orogenic Belt. The latest pluton (276 ± 1 Ma) identifies the initial emplacement stage of the Middle Permian–Early Triassic Khangai granitoid batholith.Alkaline granitoids from the Trans-Altai Gobi and Northern Mongolia possess similar ferroan, mainly peralkaline, A2-type granite geochemical characteristics, but differ in Nd isotopic signatures. For the Trans-Altai Gobi plutons εNd(t) = +6.7; for Northern Mongolian plutons εNd(t) varies between –1.7 to + 0.6. These values are consistent with the Nd isotopic evolution of the continental crust of the hosting terranes, ‘Hercynian’ isotope province for the Trans-Altai Gobi and ‘Caledonian’ province with a pre-Ediacaran microcontinent crust for the Northern Mongolia. Thereby studied alkaline granitoids were formed manly though anatexis of the host terrane crust during post-accretion and anorogenic extension events in different parts of the Central Asian Orogenic Belt.