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Open AccessArticle

Metamorphic Evolution of Garnet-Bearing Ultramafic Rocks in the Hujialin Area, Sulu Ultrahigh-Pressure Orogenic Belt, Eastern China

1
Shandong Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China
2
College of Mining Engineering, North China University of Science and Technology, Tangshan 063210, China
3
Shandong Institute of Geological Sciences, Jinan 250013, China
4
College of Earth Sciences, Jilin University, Changchun 130061, China
5
State Key Laboratory of Geological Processes, China University of Geosciences, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(3), 225; https://doi.org/10.3390/min10030225
Received: 26 January 2020 / Revised: 25 February 2020 / Accepted: 26 February 2020 / Published: 29 February 2020
(This article belongs to the Special Issue High‐and Ultrahigh‐Pressure Rocks)
The Rizhao Hujialin area is located in the central Sulu ultrahigh-pressure orogenic belt, where garnet clinopyroxenite is exposed in the upper part of an ultramafic rock complex and serpentinized dunite is exposed in its lower part. Based on textural criteria, the garnet clinopyroxenites were divided into three types: Equigranular garnet, porphyroclastic garnet, and megacrystic garnet pyroxenites. The garnet clinopyroxenites have convex-upward chondrite-normalized rare earth element patterns, large positive Pb anomalies, and depletion of high-field-strength elements (e.g., Nb, Zr, and Ti), suggesting a mantle source protolith overprinted by fluid metasomatism. Petrographic, mineral chemistry, phase equilibrium modeling, and zircon U–Pb geochronology data show that the evolutionary stages of the Hujialin garnet clinopyroxenites were as follows: Stage I: formation of the magmatic protoliths; stage II: formation of megacrystic garnet pyroxenite accompanying subduction; stage III: formation of porphyroclastic or equigranular garnet clinopyroxenite with a mineral assemblage of garnet + clinopyroxene + ilmenite + humite accompanying initial exhumation at ~215.0 ± 5.7 Ma; stage IV = progressive cooling and decompression associated with the crystallization of water-bearing minerals such as clinochlore and pargasite at 206 Ma; and Stage V = late epidote amphibolite-facies retrograde metamorphism producing a mineral assemblage of garnet + clinopyroxene + amphibole + chlorite + epidote + ilmenite at ~180–174 Ma associated with fluid activity in shear–tensional fractures and/or pores. The P-T conditions of the peak metamorphism were estimated at 4.5 ± 0.5 GPa and 800 ± 50 °C. Retrograde metamorphism recorded conditions of 1.0 GPa and 710 ± 30 °C during the exhumation and cooling process. The mineral transformation from early high-Al clinopyroxene to garnet and to late diopside records the general metamorphic evolution during subduction and exhumation, respectively. One zircon U–Pb analysis presents the Palaeoproterozoic age of 1817 ± 40 Ma, which is coeval with widespread magmatic and metamorphic events in the North China Craton. View Full-Text
Keywords: Sulu orogenic belt; ultrahigh-pressure; garnet clinopyroxenite; metamorphic evolution; zircon U–Pb geochronology Sulu orogenic belt; ultrahigh-pressure; garnet clinopyroxenite; metamorphic evolution; zircon U–Pb geochronology
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Wang, Z.; Zhang, S.; Li, X.-P.; Wang, S.; Wang, D.; Kong, F.-M.; Liu, J.; Li, Z.; Li, D.; Wang, J.; Yang, Y. Metamorphic Evolution of Garnet-Bearing Ultramafic Rocks in the Hujialin Area, Sulu Ultrahigh-Pressure Orogenic Belt, Eastern China. Minerals 2020, 10, 225.

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