Trace element geochemistry and U-Pb dating of zircon inclusions in sapphire from Southern Vietnam

Indicator of basalt-related sapphire formation

Doan Thi Anh Vu a,b, Alongkot Fanka a,c, Chakkaphan Sutthirat a,d,*
a Department of Geology, Faculty of Science, Chulalongkorn University, Thailand
b Faculty of Geology, University of Science, VNU-HCM, Viet Nam
c Applied Mineral and Petrology Special Task Force for Activating Research (AMP STAR), Department of Geology, Faculty of Science, Chulalongkorn University, Thailand
d Gem and Jewelry Institute of Thailand (Public Organization), Thailand
Keywords: Cenozoic basalt, Sapphire, Southern Vietnam, U-Pb, Zircon
Trace element geochemistry of zircon inclusion in sapphire from Southern Vietnam shows a large variation in Hf (1.1 to 2.7 wt%), Y (779 – 4220 ppm), Th (329 – 5199 ppm), U (224 – 3587 ppm), and ∑REE (706 – 2710 ppm) abundances with clearly positive Ce and mildly negative Eu anomalies observed in chondrite-normalized REE patterns. These characteristics suggest that the majority of the host sapphires were crystallized from evolved felsic melt at the lower crustal levels. The U-Pb zircon dating yields the ages ranging between 35.5 ± 1.6 Ma and 14.7 ± 0.29 Ma for Krong Nang sapphire, 5.9 ± 0.13 Ma for Binh Thuan sapphire, 5.6 ± 0.07 Ma for Dak Nong sapphire, and 5.5 ± 0.08 Ma for Di Linh sapphire. Except the Krong Nang sapphire, these ages are coeval with the alkali basalt eruptions in Southern Vietnam. This implies the crystallization of sapphire during early stages of the basaltic magmatism. Sapphire and associated minerals were extensively transported from different depths to the surface during the later stages of basalt magmatism (≤5 Ma).

1. Introduction

Zircon is a common mineral inclusion in most basalt-related sap- phires, e.g., Vietnam, Australia, Thailand, Cambodia, Laos, and China (Coenraads et al., 1990; Izokh et al., 2010; Khamloet et al., 2014; Sutherland et al., 2002; 2015b; Sutthirat et al., 2020; Vu, 2017; 2020) in which these basaltic terranes and associated sapphires appear to have similarly evolved during the Cenozoic tectonics. Zircons confined to alteration domains within sapphire are normally assumed to be coge- netic with the host sapphire (Coenraads et al., 1990; Elmaleh et al., 2019; Khamloet et al., 2014; Sutherland et al., 2002; 2015b; Vu et al., 2020). Hence, trace element geochemistry combined with U-Pb dating of zircon inclusion should provide useful information for fingerprinting their host rocks. Based on these analytical results, formation of zircon inclusion and its host sapphire from alkali basalt in Lava Plains area of northeast Australia was proposed as a process of the hybrid interplay between a previously metasomatized mantle and infiltration of felsic and mafic melts at lower crustal levels (Sutherland et al., 2015). How- ever, those from Ban Huai Sai, Laos were seemingly related to highly evolved alkaline melts (Sutherland et al., 2002). Recently, the formation of zircon and sapphire from Bo Phloi, Kanchanaburi in western Thailand was also reported as a complex interplay between lower crustal and upper mantle materials (Sutthirat et al., 2020).
In Southern Vietnam, sapphires have been mined in basalt-related alluvial deposits (Fig. 1, right). Zircon inclusions were identified from these gem-quality sapphires (Izokh et al., 2010; Long et al., 2004; Smith et al., 1995; Vu et al., 2020). However, a few chemical analyses of zircon inclusions from Dak Nong were published only by Izokh et al. (2010). Their Hf, Th and U contents were higher than those of the placer zircons from the same deposit, although, ∑REE contents fell within the same range. As a result, Izokh et al. (2010) proposed that crystallization of zircon inclusion and host sapphire originated in an iron-rich syenitic melt. Nevertheless, complexity of multistage magmatism was also dis- cussed in relation with other assemblages.
This study was therefore designed to collect and analyze a lot more zircon inclusions in sapphires from various basaltic terranes, i.e., Dak Nong, Di Linh, Binh Thuan, and Krong Nang, in Southern Vietnam. The wide ranges of trace analyses lead to characterization of the original formation whereas the U-Pb ages provide detail of time span to prove the multistage magmatism model. Consequently, precise genesis model of sapphire in Southern Vietnam is reconstructed and present in the discussion part. 

2. Geologic setting

Three main tectonic terranes, Kontum massif, Srepok orogenic belt, and Da Lat zone, in Southern Vietnam (Fig. 1, left) have been distinguished based on geological structures and rock formations (Tri and Khuc, 2011). The Kontum massif appears to be the oldest basement remaining in the northern part. The Srepok terrane in the western region belongs to the Indosinian orogeny whereas a large-scale structure of active continental margin is defined as the Da Lat zone located in the eastern region. All of them appear to have separated from Gondwana and further evolved since Devonian (S¸ engo¨r et al., 1988; Hutchison, 1989; Metcalfe, 1988; 1996; 2009; 2011). The main geological structures and rock formations can be simplified and shown in Fig. 1 (right). Precambrian remnants remain as the oldest basements in the extensive Kontum massif in the north. According to lithological and chronological data, they can be subdivided into the Kan Nack Formation, the Dak Min Formation, and the Chu Lai-Sa Huynh Complex (Luong and Bao, 1988). The Kan Nack Formation is generally characterized by granulite facies, particularly two pyroxenes-plagioclase gneiss, and quartz-biotite- sillimanite-garnet-cordierite schist, which range widely in age from Archean to Proterozoic. The Dak Min Formation contains a variety of amphibolite facies, such as biotite-plagioclase gneiss, quartz-biotite- garnet schist, and quartz-biotite-sillimanite schist, originated in Paleoproterozoic era. Regarding the Neoproterozoic Chu Lai-Sa Huynh Complex, the youngest Precambrian (Neoproterozoic) remnant is indicated by igneous-metamorphic suites including granite gneiss, gneissic granite, and migmatite. Most of the Precambrian basements seem to have been influenced by several magmatic processes during Carboniferous to Triassic periods. These igneous suites contain the Dak Lin andesite (Carboniferous-Permian), Permian-Triassic diorite, granodio- rite, and granite of the Ben Giang Complex, and Triassic granite and granosyenite of the Van Canh Complex. They appear to be confined in the northern parts of both Srepok belt and Da Lat zone. In addition, Triassic volcanogenic sedimentary sequences of the Mang Giang For- mation particularly expose surrounding those older igneous suites and Precambrian remnants. Furthermore, thick sequences of Jurassic sedi- mentary rocks including the Ban Don Group (Early Jurassic) and the Deo Bao Loc Formation (Late Jurassic) distribute over the regional area (Fig. 1, right). They are similarly composed of sandstone, siltstone, and shale (Luong and Bao, 1988). Subsequent Cretaceous magmatism caused by the Paleo-Pacific plate subduction (Hutchison, 1989; Hutchison, 2014; Metcalfe, 2011), yielded a variety of intrusive rocks varying in composition from diorite to granodiorite, granite, and granosyenite which are locally named as the Deo Ca, the Dinh Quan, and the Ankroet plutons. They situate in the central and southern parts, especially in the Da Lat zone. In addition, the Cretaceous Nha Trang rhyolite and andesite are frequently observed around those plutons (Hoa et al., 2005). Quaternary fluvial sediments have extensively deposited throughout the regional area as well as basaltic volcanisms have erupted since Neogene. Regarding the Cenozoic basalts (Neogene-Quaternary), they are partly associated with sapphire deposits in Southern Vietnam (Garnier et al., 2005; Hoang and Flower, 1998). They crop out over an area of approximately 23,000 km2 (Hoang and Flower, 1998). These basalts have different ages such as Da Lat (13.3 – 7.9 Ma), Phuoc Long (<8 – 5 Ma), Buon Ma Thuot (<6.3–1.63 Ma), Pleiku (<6.3–2.1 Ma), Xuan Loc (0.83 – 0.44 Ma), and Ile des Cendres (<2 Ma) (Hoang et al., 1996), and Dak Nong (7.1–1.1 Ma) (Garnier et al., 2005) (Fig. 2). These data reflect two main eruptive episodes. The shield-building phases mostly include quartz-olivine tholeiite with less alkali basalt whereas the later phases are made up of small-volume flows of alkali basalt, basanite and rare nephelinite. These younger basalts are usually associated with sapphire occurrences (Garnier et al., 2005; Izokh et al., 2010; Long et al., 2004; Smith et al., 1995; Vu, 2017; 2020). 


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