Elsevier

Science Bulletin

Volume 60, Issue 19, October 2015, Pages 1698-1707
Science Bulletin

Article
Earth Sciences
Metamorphism and zircon U–Pb dating of garnet amphibolite in the Baoyintu Group, Inner Mongolia

https://doi.org/10.1007/s11434-015-0890-0Get rights and content

Abstract

Garnet amphibolite in the Baoyintu Group, Inner Mongolia, is mainly composed of garnet, hornblende, plagioclase, quartz and minor rutile/ilmenite. Garnet occurs as porphyroblasts surrounded by plagioclase, forming typical “white-eye socket” texture. Garnet shows grossular content of 0.26–0.28 and pyrope of 0.11–0.13, without significant zoning. Plagioclase is generally zoned with anorthite (An) increasing from core to rim. The P–T pseudosection calculated using THERMOCALC in the system MnNCKFMASHTO shows that the garnet amphibolite experienced a clockwise P–T path with a peak at ~1.3 GPa/725 °C defined from the minimum An content in plagioclase and maximum pyrope content in garnet, followed by an isothermal decompression. LA-ICP-MS zircon dating for the garnet amphibolite indicates metamorphic ages of 399 ± 6 Ma. The peak P–T condition corresponds to a thermal gradient of ~18 °C/km, indicating typical medium-pressure type that commonly occurs in orogenic process with crustal thickening. We prefer to interpret this orogenic event to be a result of collision, following the closure of the Paleo-Asian Ocean before the early Devonian although there is a popular view that the Paleo-Asian Ocean may have lasted to the early Mesozoic.

Introduction

The Central Asian Orogenic Belt (CAOB) is a giant accretionary orogen among the Siberian Craton, the North China Craton (NCC) and Tarim Craton, which is characterized by a series of island arcs, forearc or backarc basins, ophiolitic belts and microcontinents from the Neoproterozoic to Mesozoic 1., 2.. The tectonic evolution of CAOB has been highly disputed. Some geologists propose that the Paleo-Asian Ocean was ultimately closed in Late Permian–Early Mesozoic 2., 3., 4., 5., 6., but others emphasize the closure of the Paleo-Asian Ocean happened in Devonian which was followed by an extensional tectonic environment in the late Paleozoic 7., 8., 9., 10., 11., 12., 13., 14., 15.. In general, a collision-related orogeny resulted from an ocean closure may cause medium-pressure or kyanite-type metamorphism 16., 17.. However, the metamorphism related to the tectonic evolution of the Paleo-Asian Ocean has been rarely investigated. The Baoyintu Group is located in an area which was controversially considered as the basement of the CAOB 18., 19., 20. or the northern part of NCC [21] (Fig. 1a). It develops typical medium-pressure progressive garnet, staurolite and kyanite zones with numerous garnet amphibolite blocks or lenses [18]; however, its metamorphic P–T paths, metamorphic ages and related tectonic settings are not well constrained. The paper is aimed to reveal the metamorphic P–T conditions, paths and ages through investigating garnet amphibolite in the Baoyintu Group to constrain the tectonic evolution of CAOB.

Section snippets

Geological setting

The Baoyintu Group is mainly distributed in Huogeqi, Baoyintu and Tugurige areas, the middle west Inner Mongolia (Fig. 1b). It is comprised of (garnet) amphibolites, (garnet, kyanite, staurolite) mica schists, quartzites and marbles. This metamorphic sequence was divided to the Inner Mongolia prairie stratigraphic region and proposed to be formed in Paleo-Proterozoic, forming the basement of CAOB [19]. This was supported by a Sm–Nd whole-rock isochron of 2,485 ± 128 Ma from amphibolite of the

Petrography and mineral compositions

The mineral compositions were determined using a JXA-8100 microprobe at Peking University. Operation conditions involved a 15-kV acceleration voltage, a 10-nA beam current, a 1–2 μm beam diameter, 15–20 s counting time and 5 s background counting times for each element (Kα for all measured elements). The K, Ca and Ti were measured by PETJ crystal, Na, Si, Mg and Al were analyzed by TAP crystal(s) (two separate channels), and Cr, Mn, Fe and Ni elements were analyzed by LIFH crystal. At final stage,

Phase equilibria modeling and P–T path

For the mineral assemblages and compositions presented above, the model system MnNCKFMASHTO (Fe2O3) is chosen here to calculate P–T pseudosections for sample 13LS08. Quartz is considered to be in excess. A fluid phase is assumed to be pure H2O and in excess for the subsolidus conditions, and a melt phase assumed to be in excess for the suprasolidus conditions.

The bulk-rock compositions obtained by ICP-OES analysis at China University of Geosciences (Beijing) for sample 13LS08 show SiO2 = 48.59

Geochronology

Sample 13LS02 was chosen for zircon U–Pb dating. Zircon grains were separated by conventional heavy liquid and magnetic separation followed by hand-picking under a binocular microscope. Selected grains were mounted in an epoxy resin, polished down to expose the grain center, photographed in transmitted and reflected light and imaged using cathodoluminescence (CL). The zircon LA-ICP-MS U–Pb isotopic analyses were performed at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry

Metamorphic P–T path and mineral evolution

Petrographic observation and phase modeling presented above suggest a decompression clockwise P–T path where the peak pressure condition can be constrained using the minimum An content in plagioclase and the Al(M2) content in hb-1 amphibole to be 1.2–1.3 GPa for the temperature conditions of 710–720 °C defined by garnet compositions. The other mineral compositions involving pl-1 and pl-2 plagioclase with higher An contents, hb-2 amphibole and garnet are modeled to indicate decompressional P–T

Acknowledgments

This work was supported by the National Basic Research Program of China (“973” Program) (2013CB429801) and the China Survey of Geology (1212011121077). We sincerely thank Jiahui Qian, Renbiao Tao, Zhuang Li and Zhanzhan Duan for their valuable suggestions for preparing the manuscript. Hong Qin and Fang Ma are appreciated for their help in analyses of the bulk-rock compositions and LA-ICP-MS zircon U–Pb dating.

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