January 2014
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100 Reads
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12 Citations
Acta Petrologica Sinica
In this paper, we present the U-Pb age of detrial and magmatic zircons from the sedimentary and volcanic rocks within the "Neoproterozoic" Yimianpo Group in Heilongjiang Province, NE China, combined with the age of the overlying rhyolite from the Taiantun Formation, to constrain the formation time and tectonic nature of the Yimianpo Group. The Yimianpo Group, from bottom to top, is subdivided into the Tangjiatun and Yangmugang formations. The Tangjiatun Formation, outcropped in Shangzhi region, is composed mainly of the meta-intermediate-acidic volcanic rocks (including rhyolite, intermediate-acidic tuff, and minor andesite) in the lower part and feldspar quartz sandstone as well as minor phyllite in the upper part. The Yangmugang Formation consists mainly of sandstone, siltstone, and slate in the lower part and thin layer of sandstone, slate, carbonaceous slate, and silty mudstone in the upper part. The Taiantun Formation, unconformably covering the Yangmugang Formation, is composed mainly of rhyolite and intermediate-acidic tuff. The majorities of the zircons from three samples collected from the Yimianpo Group are euhedral-subhedral in shape and display oscillatory growth zoning or striped absorption in cathodoluminescence (CL) images. Combined with their Th/U ratios (0. 30 ∼ 2. 57), it is suggested that these zircons crystallised from a magma. The dating results indicate: (1) that the 206Pb/238 U ages of 16 analytical spots of zircons from a rhyolite (sample 11HSZ4-1) in the lower part of the Tangjiatun Formation range from 289 Ma to 302Ma, yielding two groups of Concordia ages: 295 ± 2 Ma (mean square weighted deviation (MSWD) =1.08, n = 15) and 428 ±4Ma (n = 1), the former represents the formation time of the rhyolite, the latter is the age of a captured zircon; (2) that the 206Pb/238 U ages from 72 analytical spots on zircons from sample HSZ5-1 (a feldspar quartz sandstone) yield six groups of concordant ages: 234 ± 3Ma (MSWD = 1.2, n = 12), 260 ±2Ma (MSWD =0. 2, n = 12), 273 ±3Ma (MSWD =0. 03, n =6), 291 ± IMa (MSWD =0.49, n=35), 309 ±8Ma (MSWD =0.01, n=2) and 327 ±4Ma (MSWD=0.37, n=5), suggesting that the deposition of its protolith began after ∼234Ma; (3) that the 206 Pb/238U ages from 23 analytical spots on zircons from sample 11HSZ2-1 (a silty mudstone) in the Yangmugang Formation yield age populations of 226 ±4 Ma (n = 1), 267 ± 3 Ma (MSWD = 1.7, n = 15), 362 ± 7 Ma (MSWD = 1. 05, n =3), 411 ±6Ma (n = 1), 485 ± 12Ma (MSWD =0. 01, n =2), and 783Ma (n = 1), implying that it was deposited after 226Ma; and (4) that the 206Pb/238 U ages from 23 analytical spots on zircons from a overlying rhyolite (sample 11HSZ3-4) in the Taiantun Formation range from 185Ma to 193Ma, yielding a weighted mean 206Pb/238 U age of 189 ± IMa (MSWD = 1.5, n = 23), i. e., Early Jurassic, which is interpreted as the formation age of the rhyolite. Based on the dating results of magmatic zircons and detrital zircons and field covering relationship, we conclude that the rhyolites from the Tangjiatun Formation in the Yimianpo Group formed in the Early Permian, rather than Neoproterozoic as previously believed, while the feldspar quartz sandstone from the upper part of the Tangjiatun Formation should be classified to the Yangmugang Formation in age, and that the silty mudstone from the Yangmugang Formation, together with the feldspar quartz sandstone from the upper part of the Tangjiatun Formation, deposited between 226 ∼ 232Ma and 189 ±1Ma, i. e., Late Triassic-Early Jurassic, rather than Neoproterozoic as previously believed. Based on the similarity of age populations of detrital zircons with ones of the outcropped terranes in the study area, it is suggested that the sediments of the Yangmugang Formation in the Yimianpo Group mainly sourced from the Late Paleozoic terranes and minor Early Mesozoic, Early Paleozoic and Neoproterozoic terranes in the adjacent regions. Additionally, the Early Permian rhyolites in the Tangjiatun Formation, together with the coeval basaltic rocks and gabbros in the adjacent regions, constitute a typical bimodal volcanic rock association, implying an extensional environment.