The absence of low-thermal gradients in old metamorphic rocks (<350 °C GPa-1) has been used to argue for a fundamental change in the style of plate tectonics during the Neoproterozoic Era. Here, we report data from an eclogite xenolith in Paleoproterozoic carbonatite in the North China craton that argues for cold subduction as early as 1.8 Ga. The carbonatite has a sediment-derived C isotope signature and enriched initial Sr-Nd isotope composition, indicative of ocean-crust components in the source. The eclogite records peak metamorphic pressures of 2.5-2.8 GPa at 650-670 °C, indicating a cold thermal gradient, 250(±15) °C GPa-1. Our data, combined with old low-temperature events in the West African and North American cratons, reveal a global pattern that modern-style subduction may have been established during the Paleoproterozoic Era. Paleoproterozoic carbonatites are closely associated with granulites and eclogites in orogens worldwide, playing a critical role in the Columbia supercontinent amalgamation and deep carbon cycle through time.
Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.
Research Article| February 09, 2018 Genesis of the world’s largest rare earth element deposit, Bayan Obo, China: Protracted mineralization evolution over ∼1 b.y. Wenlei Song; Wenlei Song 1Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China Search for other works by this author on: GSW Google Scholar Cheng Xu; Cheng Xu * 1Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China *E-mail: xucheng1999@pku.edu.cn Search for other works by this author on: GSW Google Scholar Martin P. Smith; Martin P. Smith 2School of Environment and Technology, University of Brighton, Brighton BN24GJ, UK Search for other works by this author on: GSW Google Scholar Anton R. Chakhmouradian; Anton R. Chakhmouradian 3Department of Geological Sciences, University of Manitoba, Manitoba R3T2N2, Canada Search for other works by this author on: GSW Google Scholar Marco Brenna; Marco Brenna 4Department of Geology, University of Otago, Dunedin 9054, New Zealand Search for other works by this author on: GSW Google Scholar Jindřich Kynický; Jindřich Kynický 5Department of Geology and Pedology, Mendel University, Brno 361300, Czech Republic Search for other works by this author on: GSW Google Scholar Wei Chen; Wei Chen 6State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China Search for other works by this author on: GSW Google Scholar Yueheng Yang; Yueheng Yang 7Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Search for other works by this author on: GSW Google Scholar Miao Deng; Miao Deng 1Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China Search for other works by this author on: GSW Google Scholar Haiyan Tang Haiyan Tang 8College of Earth Science, Jilin University, Changchun 130012, China Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (4): 323–326. https://doi.org/10.1130/G39801.1 Article history received: 12 Oct 2017 rev-recd: 11 Jan 2018 accepted: 11 Jan 2018 first online: 09 Feb 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Wenlei Song, Cheng Xu, Martin P. Smith, Anton R. Chakhmouradian, Marco Brenna, Jindřich Kynický, Wei Chen, Yueheng Yang, Miao Deng, Haiyan Tang; Genesis of the world’s largest rare earth element deposit, Bayan Obo, China: Protracted mineralization evolution over ∼1 b.y.. Geology 2018;; 46 (4): 323–326. doi: https://doi.org/10.1130/G39801.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The unique, giant, rare earth element (REE) deposit at Bayan Obo, northern China, is the world’s largest REE deposit. It is geologically complex, and its genesis is still debated. Here, we report in situ Th-Pb dating and Nd isotope ratios for monazite and Sr isotope ratios for dolomite and apatite from fresh drill cores. The measured monazite ages (361–913 Ma) and previously reported whole-rock Sm-Nd data show a linear relationship with the initial Nd isotope ratio, suggesting a single-stage evolution from a Sm-Nd source that was formed before 913 Ma. All monazites show consistent εNd(1.3Ga) values (0.3 ± 0.6) close to those of the adjacent 1.3 Ga carbonatite and mafic dikes. The primary dolomite and apatite show lower 87Sr/86Sr ratios (0.7024–0.7030) than the recrystallized dolomite (0.7038–0.7097). The REE ores at Bayan Obo are interpreted to have originally formed as products of ca. 1.3 Ga carbonatitic magmatism and to have undergone subsequent thermal perturbations induced by Sr-rich, but REE-poor, metamorphic fluids derived from nearby sedimentary rocks. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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