Ultra-High Pressure Metamorphism and Geochronology of Garnet Clinopyroxenite in the Paleozoic Dunhuang Orogenic Belt, Northwestern China

Jia-Hui Liu,Meng-Yan Shi,Jun-Sheng Lu,Hao Wang,Qian Zhang,Zhen Li,Chun-Ming Wu

doi:10.3390/min11020117

Jia-Hui Liu, Meng-Yan Shi + Show 5 more

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https://doi.org/10.3390/min11020117

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Abstract

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.

Highlights

It is well known that the ultra-high pressure (UHP) metamorphism is anticipated to be formed during subduction at very low thermal gradient, usually less than 10 ◦C/km, or even as low as ~5 ◦C/km, which refers to metamorphic pressure high enough to stabilize coesite [1,2]
We present a comprehensive study of field investigation, petrology, geothermobarometric computation, and secondary ion mass spectrometry (SIMS) U-Pb dating of titanite for garnet clinopyroxenite in the Daquan area, northeastern Dunhuang orogenic belt, to obtain their P-T paths and certify UHP metamorphism in this Paleozoic orogenic belt
Three to four stages of metamorphic mineral assemblages are preserved in the garnet clinopyroxenite enclaves within a granitic pluton in the northeastern Paleozoic Dunhuang orogenic belt, northwestern China

Summary

Introduction

It is well known that the ultra-high pressure (UHP) metamorphism is anticipated to be formed during subduction at very low thermal gradient, usually less than 10 ◦C/km, or even as low as ~5 ◦C/km, which refers to metamorphic pressure high enough to stabilize coesite [1,2]. Clockwise metamorphic pressure–temperature (P-T) paths of eclogite, mafic granulite, amphibolite, and metapelite, typical metamorphic products of subduction setting, were retrieved elsewhere in this region [9,10,11,12,13,14,15,16,17,18,19], and the metamorphic event was dated to occur from Silurian to Devonian periods [10,11,14,15,16,17,18,19] These enabled people to believe that this area was a Paleozoic orogenic belt [13,15,16,17,18,19]. The recognition of UHP metamorphism is challenging because the presence of minerals formed under such conditions in some orogenic belts is possible but requires appropriate physical–chemical conditions for their formation and preservation and a mechanism(s) for their exhumation

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