Zircon SHRIMP U-Pb and in-situ Lu-Hf isotope analyses of a tuff from Western Beijing: Evidence for missing Late Paleozoic arc volcano eruptions at the northern margin of the North China block

摘要

Zircon SHRIMP U-Pb and in-situ Lu-Hf isotopic analyses via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of a tuff within the Upper Paleozoic from Western Beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the North China block (NCB). SHRIMP U-Pb zircon dating of the tuff yielded a Pb-206/U-238 weighted mean age of 296 +/- 4 Ma (95% confidence, MSWD=3.3), which is very similar to the emplacement age of the newly discovered Carboniferous calc-alkaline, I-type continental arc granitoid plutons in the Inner Mongolia Paleo-uplift (IMPU) on the northern margin of the NCB. In-situ Lu-Hf analysis results of most zircons from the tuff yielded initial Hf-176/Hf-177 ratios from 0.282142 to 0.282284 and epsilon(Hf(t)) values from -15.9 to -10.7. These Lu-Hf isotopic compositions are very similar to those of the Late Carboniferous granitoids in the IMPU, but are very different to those of the Central Asian Orogenic Belt (CAOB). Together with the sedimentary and tectonic analyses results, we inferred that the source area of the tuffs within the NCB is the IMPU instead of the CAOB. Therefore, some are volcanoes once existed in the IMPU on northern margin of the NCB during the Late Carboniferous, but they were entirely eroded due to strong exhumation and erosion of the IMPU during the Late Carboniferous to Early Jurassic. (c) 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.