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

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/Sm N 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 (∆T = 10-50 • C). The δ 18 O values (4.88 to 5.01 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, mantlederived 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 (T = 10-50 C). The  18 O 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 176 Hf/ 177 Hf 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.

Gem-Quality Zircon Megacrysts from Placer Deposits in the Central Highlands, Vietnam—Potential Source and Links to Cenozoic Alkali Basalts

“…Although zircon megacrysts may be potentially derived from carbonatitic, metasomatized mantle material and do not primarily relate to alkali basalts, the zircon ages obtained were consistent with bi-episodal eruptive events at Buon Ma Thuot, Dak Lak (5.8-1.67 Ma), Pleiku (4.3-0.8 Ma), and the younger basalt eruptions exposed at Xuan Loc (0.83-0.44 Ma) ( Figure 1B) [4]. Zircon megacrysts from North-East Cambodia not only occur geographically close to the megacrysts found in the present study, but also share a similar U-Pb age connected to the younger period of volcanic activity in the region (0.98 ± 0.04 Ma, Cong et al [10]; 0.88 ± 0.22 Ma to 1.56 ± 0.21 Ma, Piilonen et al [22]). Obtained zircon U-Pb ages were just slightly older than the eruption events that were dated based on the K-Ar ages for the whole rock [4].…”

“…Trace-element chemical signatures of the zircon megacrysts of south-central Vietnam were found to be very comparable to those of zircon from Ratanakiri, Northeast Cambodia (Figure 7 in Cong et al [10]; Figure 7 in this study) and other places such as New Zealand [11], East Australia [18], eastern and northeast China [5,8]. Results of a detailed study by Cong et al [10] and Piilonen et al [22] using δ 18 O and 176 Hf/ 177 Hf isotopes clearly demonstrated that zircon from Ratanakiri, Cambodia, had a mantle origin and may be derived from metasomatized, partially melted lithospheric mantle material (such as peridotite, harzburgite), with strong carbonatitic geochemical fingerprint. In fact, experimental studies by Foley et al [51], have demonstrated that mantle metasomatism, as induced by the presence of CO 2 and H 2 O, lowers the solidus temperature at upper mantle conditions and promotes partial melting of the peridotites that results in melts with carbonatitic compositions at low degree, and carbonated silicate melts with higher degrees of melting.…”

“…Obtained zircon U-Pb ages were just slightly older than the eruption events that were dated based on the K-Ar ages for the whole rock [4]. This might be due to the difference in closure temperatures for U-Pb ages of zircon and K-Ar ages from the basalt, that indicated rapid extraction of zircon from its formation source, and the comparably fast uplift by the alkali-basalt eruption events [17,22]. Note, however, that the rim zones of resorption and re-growth (visualized by PL images in Figure 5) gave the very same ages as the zircon core regions within statistical errors.…”

“…According to Cong et al [10], zircon megacrysts from Cenozoic basalts in northeastern Cambodia crystallized in the mantle during metasomatic events caused by phosphate-rich fluids and/or silicate melts enriched in zirconium. Piilonen et al [22] recently reported findings of megacrystic zircons as single crystals enclosed in the alkali basalt from Ratanakiri, Cambodia, and suggested a single stage growth in a carbonate-influenced environment.…”

Editorial for Special Issue “Mineralogy and Geochemistry of Gems”

“…Zircon is another important mineral geologically [27,28], which can be of gem-quality. Studies on gem-quality zircons from two occurrences are also included in this Special Issue [29,30]. Data by Piilonen et al [29] on zircon xenocrysts from alkali basalts in Ratanakiri Province (Cambodia) suggest that their genesis involved zirconium-saturated, aluminium-undersaturated, carbonatitic-influenced, low-degree partial melting (<1%) of peridotitic mantle at ca.…”