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Minerals 2018, 8(12), 556; https://doi.org/10.3390/min8120556

Zircon Xenocrysts from Cenozoic Alkaline Basalts of the Ratanakiri Volcanic Province (Cambodia), Southeast Asia—Trace Element Geochemistry, O-Hf Isotopic Composition, U-Pb and (U-Th)/He Geochronology—Revelations into the Underlying Lithospheric Mantle

1
Mineralogy Section, Beaty Centre for Species Discovery, Research & Collections, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada
2
Geoscience, Australian Museum, 1 William Street, Sydney NSW 2010, Australia
3
John de Laeter Centre, The Institute for Geoscience Research (TIGeR), Curtin University, Perth WA 6845, Australia
4
Wisconsin Secondary Ion Mass Spectrometer Laboratory (WiscSIMS), Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA
*
Author to whom correspondence should be addressed.
Received: 26 October 2018 / Revised: 16 November 2018 / Accepted: 19 November 2018 / Published: 30 November 2018
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
Full-Text   |   PDF [6976 KB, uploaded 30 November 2018]   |  

Abstract

Zircon xenocrysts from alkali basalts in Ratanakiri Province, Cambodia represent a unique low-Hf zircon within a 12,000 km long Indo-Pacific megacryst zone. Colorless, yellow, brown, and red crystals ({100}, {101}, subordinate {211}, {1103}), with hopper growth and corrosion features range up to 20 cm in size. Zircon chemistry indicates juvenile, Zr-saturated, mantle-derived alkaline melt (Hf 0.6–0.7 wt %, Y <0.2 wt %, U + Th + REE (Rare-Earth Elements) < 600 ppm, Zr/Hf 66–92, Eu/Eu*N ~1, positive Ce/Ce*N, HREE (Heavy REE) enrichment). Incompatible element depletion with increasing Yb/SmN from core to rim at ~ constant Hf suggests single stage growth. Ti-in-zircon temperatures (~570–740 °C) are lower than predicted by crystal morphology (800–900 °C) and decrease from core to rim (ΔT = 10–50 °C). The δ18O values (4.88 to 5.01‰ VSMOW (Vienna Standard Mean Ocean Water)) are relatively low for xenocrysts from the zircon Indo-Pacific zone (ZIP). The 176Hf/177Hf values (+ εHf 4.5–10.2) give TDepleted Mantle model source ages of 260–462 Ma and TCrustal ages of 391–754 Ma. The source magmas reflect variably depleted lithospheric mantle with little supracrustal input. Zircon U-Pb (0.88–1.56 Ma) and (U-Th)/He (0.86–1.02 Ma) ages are older than host basalt ages (~0.7 Ma), which suggests limited residence before transport. Zircon genesis suggests Zr-saturated, Al-undersaturated, carbonatitic-influenced, low-degree partial melting (<1%) of peridotitic mantle at ~60 km beneath the Indochina terrane. View Full-Text
Keywords: zircon; xenocryst; alkali basalt; Ratanakiri Volcanic Province; trace elements; O-isotopes and Hf-isotopes; U-Pb; (U-Th)/He zircon; xenocryst; alkali basalt; Ratanakiri Volcanic Province; trace elements; O-isotopes and Hf-isotopes; U-Pb; (U-Th)/He
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Piilonen, P.C.; Sutherland, F.L.; Danišík, M.; Poirier, G.; Valley, J.W.; Rowe, R. Zircon Xenocrysts from Cenozoic Alkaline Basalts of the Ratanakiri Volcanic Province (Cambodia), Southeast Asia—Trace Element Geochemistry, O-Hf Isotopic Composition, U-Pb and (U-Th)/He Geochronology—Revelations into the Underlying Lithospheric Mantle. Minerals 2018, 8, 556.

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