Ultra-high pressure metamorphism in western Tianshan, China: Part II. Evidence from magnesite in eclogite

Zhang, Lifei; Ellis, David J.; Williams, Samantha; Jiang, Wenbo

doi:10.2138/am-2002-0708

Cited by 101 publications

(53 citation statements)

References 30 publications

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“…Qiangtang eclogite lies in central Qinghai-Tibet Plateau, and belongs to low-temperature (C-type) eclogite group according to P-T condition estimate, which is similar with those eclogites reported in Western Tianshan and Northern Qilian [25][26][27] and quite different from those of marginal plateau [1][2][3][4][5][6][7][8][9][10] .…”

Section: Geological Significancesupporting

confidence: 69%

Discovery of eclogite and its geological significance in Qiangtang area, central Tibet

et al. 2006

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Eclogite is first discovered from Qiangtang area, central Tibet, which occurs as a lens in the high-pressure metamorphic belt comprising glaucophane-muscovite schist and garnet-muscovite schist, along the south margin of Lungmu Co-Shuanghu Paleo-Tethys suture. A glaucophane 40 Ar/ 39 Ar age of 220 Ma and a phengite 40 Ar/ 39 Ar age of 221.9 Ma have been obtained from blueschists associated with eclogite. The rock consists of garnet, omphacite, amphibole, phengite, and rutile, etc, and the jadeite content in omphacite ranges from 32.7% to 38.95%. It is estimated that the temperature of the eclogite is <500℃ and pressure varies in the range of 1.56-2.35 GPa. The discovery of Qiangtang ecologite further supports that there is a suture zone of IndoChinese epoch in central Qiangtang area, and is very important for discssing the evolution of the paleo-ocean and paleo-plate tectonics in this area.

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Section: Geological Significancesupporting

confidence: 69%

Discovery of eclogite and its geological significance in Qiangtang area, central Tibet

et al. 2006

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“…In the UHP field, the occurrence of coesite inclusions in garnet is of significance because it identifies the possible episodic growth of garnet, which may, combined with garnet isopleth thermobarometry, decipher the P-T trajectory of a specific UHP rock. The first reports of UHP metamorphism in the southwestern Tianshan instigated debate that has continued until the recent discovery of coeiste in the Habutengsu Valley of this orogenic belt [3][4][5][6][7][8]. However, until now, coesite had not been optically identified in the rocks of the Atantayi Valley, where the eclogites involved in the "UHP versus HP" controversy were sampled [5,9].…”

mentioning

confidence: 99%

Coesite in the eclogite and schist of the Atantayi Valley, southwestern Tianshan, China

Lü

Zhang

2012

Chin. Sci. Bull.

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Coesite is an indicator mineral of ultra-high-pressure metamorphism. Since coesite was reported in the Habutengsu Valley, we have also found it in eclogite and schist from the Atantayi Valley in the southwestern Tianshan, China. Petrographic and micro-Raman analyses were carried out for the Atantayi metamorphic rocks and coesite was recognized in the predominant rock types, i.e. schist and eclogite, from three sections. The coesite-bearing schist consists mainly of garnet, Na-Ca amphibole, quartz, white mica and albite; the coesite-bearing eclogite is mainly composed of omphacite, garnet, glaucophane and zoisite. The coesite occurs as various mineral inclusions within porphyroblastic garnet. Findings of coesite in eclogite and associated schist indicate not only the regional in situ formation of the Atantayi ultra-high-pressure eclogite, but also the large areal extent of ultra-high-pressure metamorphism in southwestern Tianshan, extending up to 10 km north-south and 60-80 km east-west. Coesite is an indicator mineral of ultra-high-pressure (UHP) metamorphism [1]. In well-established UHP belts worldwide, coesite is usually preserved in relatively refractory minerals, such as garnet and zircon. An increasing number of studies show that garnet can preserve abundant information on the pressure-temperature (P-T) evolution of a metamorphic garnetiferous rock, as evidenced by the applicability and reliability of garnet isopleth geothermobarometry [2]. In the UHP field, the occurrence of coesite inclusions in garnet is of significance because it identifies the possible episodic growth of garnet, which may, combined with garnet isopleth thermobarometry, decipher the P-T trajectory of a specific UHP rock. The first reports of UHP metamorphism in the southwestern Tianshan instigated debate that has continued until the recent discovery of coeiste in the Habutengsu Valley of this orogenic belt [3][4][5][6][7][8]. However, until now, coesite had not been optically identified in the rocks of the Atantayi Valley, where the eclogites involved in the "UHP versus HP" controversy were sampled [5,9]. Detailed investigations of UHP minerals such as coesite are critical for elaborating on the spatial distribution and evolutionary characteristics of the UHP belt in the southwestern Tianshan. Recently, we collected a number of samples of different rock types along the Atantayi Valley and conducted detailed petrographic analyses on them. In order to find any evidence of ultra-high pressures in these rocks, we carefully examined tiny inclusions in garnets found in these samples employing Raman spectroscopy. This has provided fundamental information on the metamorphic evolution of the HP-UHP belt of the southwestern Tianshan.

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“…The peak metamorphic temperatures are usually located in the field of low temperature eclogite facies (~600℃) [23] , with the geothermal gradient as low as close to 5℃/km. For instance, the UHP eclogite from Southwest Tianshan, China, experienced lawsoniteblueschist facies, coesite eclogite facies with the peak conditions of 600℃ and 2.8-3.0 GPa [6,7] , and overprinted by epidote-blueschist facies during the retrograde stage. The peak pressure estimated for the eclogite facies of pelitic schist reaches up to 5.0 GPa, indicating the lowest geothermal gradients at about 4℃/km [8] , belonging to the typical cold subduction zone.…”

Section: Metamorphic Charactersmentioning

confidence: 99%

“…We first found the UHP mineral evidences, such as coesite pseudomorphs and relict metamorphic magnesite in the eclogite from Southwest Tianshan [6,7] , which gives rise to controversy [53,21] . Subsequently, we found reaction textures of meta-carbonate [8] and coesite exsolution needle in omphacite [9] , and recently well-preserved coesite inclusions in porphyroblastic garnet were found [54] confirming the UHP metamorphic experienced definitely in this region.…”

Section: Uhp Metamorphic Belt In Southwest Tianshan In Chinamentioning

confidence: 99%

The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

et al. 2008

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Ultra-high pressure metamorphism in western Tianshan, China: Part II. Evidence from magnesite in eclogite

Cited by 101 publications

References 30 publications

Discovery of eclogite and its geological significance in Qiangtang area, central Tibet

Discovery of eclogite and its geological significance in Qiangtang area, central Tibet

Coesite in the eclogite and schist of the Atantayi Valley, southwestern Tianshan, China

The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

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