Zi-jie Wu, Long-wei Qiu, Hi-peng Wng, Mi-shn Zhong, Pei-long Cui

a School of Geosciences, China University of Petroleum, Qingdao 266580, China

b Liaoning Provincial Institute of Geological Exploration Co., Ltd, Dalian 116100, China

c Liaoning Metallurgical Geological Explorationo Research Institute Co., Ltd, Anshan 114000, China

1.Objective

The Xing’an block (XB) is located in the eastern part of the Xingmeng orogenic belt, bounded by Erguna block (EB)to northwest and the Songnen block (SB) to the southeast(Fig.1a).The Xinghuadukou Group is generally considered to be the metamorphic basement of the major blocks in the Xingmeng orogenic belt, while the depositional age is still debated.It was previously generally believed to have formed in the Precambrian, and the youngest depositional age of mica schists in the Xinghuadukou Group obtained by Zhao YD et al.(2017) was 416 Ma, which is in the Early Devonian.The biotite quartz schist of Xinghuadukou Group from Naiman area, was carried out zircon LA-ICP-MS U-Pb dating in order to constrain the deposition timing of Xinghuadukou Group.

Fig.1.Generalized geologic map of the Naiman area (a); U-Pb concordia diagram and representative CL image for zircons from the Xinghuadukou Group in the Naiman area (b); age frequency histogram for zircons from the Xinghuadukou Group in the Naiman area (c).

2.Methods

U-Pb dating of the zircons was simultaneously conducted by LA-ICP-MS at Wuhan SampleSolution Analytical Technology Co.Ltd., Wuhan, China.The detailed experimental principles and technological processes adopted can be found by reference to Zong KQ et al.(2017).

3.Results

A total of 40 zircons were analyzed, and the U-Pb isotope dating results are presented in Table 1.CL images of these zircons show that the zircons in the sample take various forms, being euhedral and subhedral, and most have grain size ranging from 50 um to 100 um, and with length to width ratios of 1:1-2:1.Most zircons have highly variable U and Th concentrations (U=134-2253×10-6and Th=30.2-883×10-6),as well as Th/U ratios of 0.04-1.62 (Table 1).

Table 1.Zircon LA-ICP-MS U-Pb data for the Xinghuadukou Group in the Naiman area.

According to the U-Pb concordia diagram (Fig.1b) and age frequency histogram (Fig.1c), The206Pb/238U age is mainly divided into ten age ranges: 361-364 Ma (2 grains),380-409 Ma (8 grains), 417-464 Ma (9 grains), 476-522 Ma(4 grains), 690-769 Ma (7 grains), 835-911 Ma (3 grains),1035-1138 Ma (2 grains), 1526 Ma (1 grain), 1787 Ma (1 grain), 2420 Ma (1 grain).

These ages range from 361 Ma to 2420 Ma, at a large span; the zircon population reflects a complex provenance with a large space-time range involving multi-stage magmatic activity.It is worth noting that the youngest age in the sample is 361 Ma (spot No.XhdkTW2-13).The youngest U-Pb age in a detrital or shallow metamorphic clastic rock series can be defined as the formation age of the original rock and can be taken as the upper age limit of the deposition period of the rock (Nelson DR, 2001).We believe that the upper age limit of the Xinghuadukou Group in the Naiman area of the Xing’an block is 361 Ma (Late Devonian), which is 55 Ma later than the Early Devonian age previously reported (Zhao YD et al., 2017).Furthermore, the large number of Precambrian zircon ages obtained in this study indicates that some of the sedimentary sources of the Xinghuadukou Group are the products of the Precambrian volcanism and metamorphism,thus showing that Precambrian metamorphic basement in the Xing’an block is an older geological body than the Xinghuadukou Group, not the Xinghuadukou Group.

4.Conclusions

(i) The upper age limit of the Xinghuadukou Group in the Naiman area of the Xing’an block is 361 Ma, belong to the late Late Devonian (Famennian).

(ii) The metamorphic basement of the Xing’an block is not the Xinghuadukou Group, but the older Precambrian geological body.

Acknowledgement

The study is supported by the China Geological Survey Project (DD20160047-03).Thanks are given to referee for providing constructive comments and suggestions.