Timing of formation and tectonic nature of the purportedly Neoproterozoic Jiageda Formation of the Erguna Massif, NE China: Constraints from field geology and U-Pb geochronology of detrital and magmatic zircons

摘要

This paper reports new secondary ion mass spectrometry and laser ablation-inductively coupled plasma mass spectrometry U-Pb age data for detrital and magmatic zircons from metasedimentary and igneous rocks from the Jiageda Formation (JF) of the Erguna Massif, NE China. These data constrain the timing of formation and tectonic nature of the JF, as well as the tectonic affinity of the Erguna Massif. The JF within the Erguna Massif is dominated by metasedimentary rocks and volcanics that have a gneissic or schistose structure. The majority of zircons from metasedimentary, volcanic, and intrusive rocks in this area show striped absorption and fine-scale oscillatory growth zone patterns, and relatively high Th/U ratios (0.11-11.39) that are indicative of a magmatic origin. Four samples from the base of the type section of the JF in the village of Jiageda yield detrital zircons with similar age populations that peak at similar to 790, similar to 850, similar to 890, and similar to 960 Ma, in addition to a few Mesoproterozoic to Paleoproterozoic and Neoarchean zircons, whereas a single sample from the upper part of the JF yields zircons that include a youngest age group of similar to 764 Ma. Zircons from granites and mafic dikes that intruded the type section of the JF yield weighted mean ages of 261, 238, 183, and 160 Ma. In comparison, zircons from four volcanic samples collected 15 km northwest of the village of Jiageda and one metasedimentary sample from the JF in the Badaguan area all yield late Mesozoic crystallization ages (158, 132, 126, and 128 Ma for the four volcanic rocks, and a youngest age of 178 2 Ma for the metasedimentary rock). Combining these results with knowledge of the field relationships of the rocks in this area and previously published ages, we infer that (1) the originally determined Neoproterozoic JF does not represent a standard stratigraphic sequence but instead consists of Neoproterozoic to Mesozoic metasedimentary and volcanic rocks, while the true Neoproterozoic JF is exposed only in the village of Jiageda; and (2) nearly all of the sediments within the Neoproterozoic JF were sourced directly from units that were exposed within the study area and adjacent regions. Our new data also provide precise geochronological evidence for the existence of a Precambrian terrane within the Erguna Massif. The ages of detrital zircons from Neoproterozoic units within the Erguna Massif and Precambrian geochronological data from adjacent cratons and massifs near the southern Siberian Craton indicate that the Erguna Massif and the massifs of the southern Siberian Craton (e.g., the Tuva-Mongolian and Central Mongolian massifs) are not linked to the Siberian and the North China cratons, but instead most likely represent a fragmented Precambrian terrane derived from the Tarim Craton. (C) 2016 Elsevier B.V. All rights reserved.