Diamonds and other unusual minerals from peridotites of the Myitkyina ophiolite, Myanmar
Graphical abstract
Introduction
Ophiolites are remnants of oceanic lithosphere that were accreted to the continental margins or intra-oceanic arcs during closure of former ocean basins. Because they commonly record the full range of tectonic and magmatic processes from oceanic spreading to closure, they play a critical role in preserving ancient plate tectonic history and providing important information for understanding modern mid-oceanic ridge (MOR) and supra-subduction zone (SSZ) environments (Dilek and Furnes, 2011). In general, ophiolites are thought to originate under high-temperature and low-pressure conditions (Gass, 1968, Coleman, 1977), however, discovery of ultrahigh-pressure (UHP) and super-reduced (SuR) minerals in many ophiolites over the last 20 years (e.g., Bai et al., 1993, Bai et al., 2000a, Bai et al., 2000b, Robinson et al., 2004, Yang et al., 2007, Yang et al., 2014a, Yang et al., 2015a, Dobrzhinetskaya et al., 2009, Yamamoto et al., 2009, Xu et al., 2009, Xu et al., 2011, Howell et al., 2015, Huang et al., 2015, Tian et al., 2015, Xiong et al., 2016, Lian et al., 2017, Wu et al., 2017), has challenged this interpretation. Although these exotic minerals have been recovered from both podiform chromitites and their host peridotites, they are most abundant in chromitites. This may reflect the different modes of formation of chromitites and peridotites, in which peridotites have a greater susceptibility to partial melting, melt-rock interaction, fluid infiltration and other processes that could affect preservation of these exotic minerals.
Here, we report the first occurrence of diamond and other exotic minerals in ophiolitic peridotites of the Myitkyina ophiolite. Previous studies on this body have shown that it experienced a complicated geological evolution involving different tectonic settings (Yang et al., 2012, Liu et al., 2016b, Htay et al., 2017). The processes by which exotic minerals are incorporated into ophiolites and how they survive in the upper mantle are still poorly understood. Thus, in this paper, we present new mineral and whole-rock chemistry, including rare earth element (REE) and platinum group element (PGE) data, for peridotites of the Myitkyina ophiolite in order to better understand their processes of formation and their geodynamic setting. We also present preliminary data on some of the exotic minerals recovered from the peridotites to encourage further studies of their origin and preservation.
Section snippets
Geological background
Myanmar is divided into two major tectonic units, the Myanmar microplate to the west and the Shan Plateau (part of Sibumasu Terrane) to the east of the Sagaing Fault (Fig. 1) (Mitchell et al., 2007). The Indian plate obliquely collided with the Myanmar microplate and formed the Indo-Myanmar Range (IMR) in the western part of the microplate (Maurin and Rangin, 2009, Khin et al., 2017, Barber et al., 2017), which marks the presently active Indian plate boundary (Searle et al., 2007). The eastern
Analytical techniques
Mineral compositions of peridotites of the Myitkyina ophiolite were determined using a JEOL JXA-8100 electron microprobe (EMP) at the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences (CAGS), Beijing. An accelerating voltage of 15 kV, a current of 20nA, and a beam diameter of 5 μm were employed. The analytical errors are generally less than 2%.
All the samples for whole-rock geochemical analysis were carefully
Mineral compositions
Peridotites of the Myitkyina ophiolite are mainly composed of olivine, orthopyroxene, clinopyroxene and spinel. All of these minerals have homogeneous compositions in the same lithology and their major element compositions are listed in Table 1, Table 2, Table 3, Table 4 respectively.
Major oxide geochemistry
Whole-rock major and trace element compositions of peridotites from the Myitkyina ophiolite are listed in Table 5. The peridotites are variably altered with loss on ignition (LOI) values ranging from 0.34 to 8.92 wt%. In order to eliminate the effects of alteration, all the major oxides were normalized to 100% volatile free. Generally, the lherzolites have lower contents of MgO (37.32–42.12 wt%) and higher contents of SiO2 (42.38–45.25 wt%), Al2O3 (1.84–3.07 wt%) and CaO (1.70–2.75 wt%) than
Exotic minerals recovered from peridotites of the Myitkyina ophiolite
A variety range of exotic minerals have been discovered from peridotites of the Myitkyina ophiolite, including native elements (diamond, native Si, native Cr), carbides (moissanite), oxides (rutile), alloys (FeCrMn alloy), sulfides (pyrite, pentlandite, molybdenite, galena) and silicates (zircon). Minerals with important environmental significance are described below.
Depletion and refertilization history of peridotites from the Myitkyina ophiolite
Generally, the clinopyroxene contents of peridotites roughly reflect the degree of depletion. Based on their contents of clinopyroxene, peridotites of the Myitkyina ophiolite underwent different degrees of partial melting. These observations are consistent with their chemical compositions.
Silica, Al2O3 and CaO contents of the peridotites correlate negatively with MgO contents (Fig. 7a–c). Harzburgites commonly have lower SiO2, Al2O3 and CaO contents than the lherzolites. All the peridotites
Conclusions
This study presents systematic geochemical data on exotic mineral-bearing lherzolites and harzburgites of the Myitkyina ophiolite, Myanmar. The results show that both the lherzolites and harzburgites have been subjected to various degrees of partial melting, and that they probably represent mantle residues that originated in a MOR setting and then were refertilized by later fluid/melt metasomatism in either a MOR or SSZ setting. Diamond, moissanite, native elements, rutile, zircon and other
Acknowledgments
The authors thank Xiangdong Duan, Jing Li, Fahui Xiong, Zhao Liu, Zhihui Cai and Huaqi Li for assistance with the field work. We also thank the Key Laboratory of Nuclear Resources and Environment (East China Institute of Technology) for microprobe analyses and the China National Research Center for the geochemical analyses. This study was supported by Project IGCP-649, the National Nature Science Foundation of China (No. 40930313, 41541017, 41720104009) and the China Geological Survey (No.
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