Paleoproterozoic accretionary orogenesis in the North China Craton: A SHRIMP zircon study

The Inner Mongolia Suture Zone (IMSZ) and the Trans-North China Orogen (TNCO) incorporate the major Paleoproterozoic accretionary orogens in the North China Craton (NCC), with the Jiao-Liao-Ji Belt (JLJB) representing the third one. Here we investigate the Paleoproterozoic tectonics of the IMSZ and TNCO through zircon SHRIMP geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks. SHRIMP analysis of zircons with textures indicating extreme recrystallization under ultrahigh-temperature (UHT) conditions from the metapelites at Heling'er in the southern domain of the IMSZ reveals a single population with a weighted mean Pb-207/Pb-206 age of 1913 +/- 17 Ma. The zircons in another UHT granulite from this locality yield a weighted mean Pb-207/Pb-206 age of 1910 +/- 18 Ma. These data correlate with the ca. 1.92 Ga age reported from zircons in sapphirine-bearing UHT granulites further north and confirm the regional extent of the Paleoproterozoic UHT metamorphism within the IMSZ. Zircons in a charnockite from the southern margin of the Khondalite Belt fringing the UHT granulites in the IMSZ show two distinct age groups: an older population with a magmatic paragenesis and a weighted mean Pb-207/(206) Pb age of 1932 +/- 24 Ma, and a younger group of metamorphic zircons with an age of 1858 +/- 26 Ma. We also report zircon ages from charnockites in two localities around Xing'he in the Huangtuyao belt belonging to the Huai'an Complex within the westernmost domain of the TNCO at the junction with the IMSZ. The chamockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean Pb-207/Pb-206 age of 2477 +/- 2 Ma and the younger population showing an age of 1807 +/- 38 Ma. The internal structure as revealed from CL images and the overall high Th/U values (up to 2.42) of the older zircons suggest their magmatic affinity, whereas the younger group with extremely low Th/U (0.02-0.09) is of metamorphic origin. Zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older (magmatic) group having a weighted mean Pb-207/Pb-206 age of 2147 +/- 11 Ma and a minor younger group with an age of 1958 +/- 25 Ma. The range of ages from 2477 to 2147 Ma from magmatic zircons in the charnockites from the eastern periphery of the IMSZ, within the western margin of the TNCO, in combination with similar ages reported in recent studies from zircons in magmatic complexes within the IMSZ suggest a prolonged history of subduction-related arc magmatism and accretionary tectonics analogous to those in some of the Phanerozoic belts such as the Central Asian Orogenic Belt and the Western Pacific. Subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high-pressure (HP) and UHT metamorphic rocks. The available data from the IMSZ and TNCO suggest long-lived convergent margins associated with the southward subduction of the Yinshan Block and westward subduction of the Eastern Block in a double-sided subduction realm prior to the final amalgamation of the NCC and its incorporation within the Columbia supercontinent in the late Paleoproterozoic. (C) 2011 Elsevier B.V. All rights reserved.