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

• Published in: Minerals (Basel) Vol. 8; no. 12; p. 556
• Main Authors: Paula C. Piilonen, F. Lin Sutherland, Martin Danišík, Glenn Poirier, John W. Valley, Ralph Rowe
• Format: Journal Article
• Language: English
• Published: MDPI AG 01-12-2018
• Subjects: (U-Th)/He; alkali basalt; O-isotopes and Hf-isotopes; Ratanakiri Volcanic Province; trace elements; U-Pb; xenocryst; zircon
• ISSN: 2075-163X
• DOI: 10.3390/min8120556

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.