Ultra-high pressure minerals in the Luobusa Ophiolite, Tibet, and their tectonic implications

Abstract

Numerous ultra-high-pressure minerals have been recovered from podiform chromities in the Luobusa ophiolite, Tibet. Recovered minerals include diamond, moissanite, Fe-silicides, wüstite, Ni-Fe-Cr-C alloys, PGE alloys and octahedral Mg-Fe silicates. These are accompanied by a variety of native elements, including Si, Fe, Ni, Cr and graphite. All of the minerals were hand-picked from heavy-mineral separates of the chromitites and care was taken to prevent natural or anthropogenic contamination of the samples. Many of the minerals and alloys are either enclosed in, or attached to, chromite grains, leaving no doubt as to their provenance. The ophiolite formed originally at a mid-ocean ridge (MOR) spreading centre at 177±33 Ma, and was later modified by suprasubduction zone magmatism at about 126 Ma. The chromitites were formed in the suprasubduction zone environment from boninitic melts reacting with the host peridotites. The UHP minerals are believed to have been transported from the lower mantle by a plume and incorporated in the ophiolite during seafloor spreading at 176 Ma. Blocks of the mantle containing the UHP minerals were presumably picked up by the later boninitic melts, transported to shallow depth and incorporated in the chromitites during crystallization.

Possible recycled origin for ultrahigh-pressure chromitites in ophiolites

“…The podiform chromitite and dunite envelope have been interpreted as a shallowseated (uppermost mantle) product of harzburgite/melt reaction and related melt mixing (e.g., Arai and Yurimoto, 1994;Zhou et al, 1994;Arai and Abe, 1995). However, this has been seriously questioned and re -examined by discovery of diamond and other ultrahigh -pressure (UHP) minerals from podiform chromitites (e.g., Robinson et al, 2004;Trumbull et al, 2009). Coesite exsolved from chromian spinel in a Tibetan chromitite decisively indicates that chromian spinel had experienced a pressure condition higher than the uppermost part of the mantle where the chromitite is now located (Yamamoto et al, 2009).…”

Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes

“…The medium grey phase (Fig. 3c) is hexagonal Fe 5 Si 3 xifengite with minor P. Various Fe silicides including xifengite are reported from chromite-rich lithologies of Luobusa (Robinson et al, 2004) and Ray-Iz . A minor phase is cubic SiC beta-moissanite (Fig.…”

“…The identification of ultra-high pressure (UHP) phases in chromitites and harzburgites of ophiolite complexes (Robinson et al, 2004;Yang et al, 2014Yang et al, , 2015 appears to be changing that view. It is now being argued that mantle sections of ophiolites either originate in, or were processed within, the Earth's mantle at depths as great as 600 km Xu et al, 2015;Griffin et al, 2016).…”

Foreword

“…A wide variety of unusual minerals, including ultra-high pressure phases such as diamond and coesite, highly reduced minerals such as moissanite, and native elements, carbides, alloys, oxides, sulphides (arsenides) and silicates were found from the podiform chromitites of the Luobusa ophiolite in the Yarlung-Zangbo suture (Yang et al, 2007;Robinson et al, 2004;Bai et al, 2000). Xu et al (this issue) reported thousands of diamond along with over 40 kinds of minerals discovered from the Kangjinla chromitite deposit in the Luobusa ophiolite, providing new key evidence for discussing the origin of the diamond and the hosted chromitite and ophiolite.…”

“…These are not only contributed to the knowledge of HPM zones in the Qinghai-Tibet plateau, but also to other places in China and the rest of the world. In turn, these discoveries will have an important impact on the further understanding of the tectonic structure and the characteristic configuration of the Qinghai-Tibet plateau, and the converging dynamic activities between the plates.A wide variety of unusual minerals, including ultra-high pressure phases such as diamond and coesite, highly reduced minerals such as moissanite, and native elements, carbides, alloys, oxides, sulphides (arsenides) and silicates were found from the podiform chromitites of the Luobusa ophiolite in the Yarlung-Zangbo suture (Yang et al, 2007;Robinson et al, 2004;Bai et al, 2000). Xu et al (this issue) reported thousands of diamond along with over 40 kinds of minerals discovered from the Kangjinla chromitite deposit in the Luobusa ophiolite, providing new key evidence for discussing the origin of the diamond and the hosted chromitite and ophiolite.…”