In Situ LA-ICP-MS Analysis of Minerals Hosted by Late Cenozoic Basaltic Rocks from Thailand

Abstract

Shortly after the cessation of seafloor spreading, intraplate magmatism affected large areas in the South China Sea (SCS) region. The origin and geodynamic setting of the post-spreading volcanism is still in debate, for many previous studies have focused on petrogenesis and mantle source of the late Cenozoic basalts from the SCS region. In this study, we obtained in situ major element compositions (by using Electron microprobe analysis—EMPA) and trace element compositions (by using laser ablation inductively coupled plasma mass spectrometry— LA-ICP-MS) for minerals (clinopyroxenes (Cpx), plagioclases (Pl), and olivines (Ol)) hosted by late Cenozoic basaltic rocks from Thailand. The results showed that the olivines had forsterite contents between 60.12% and 84.74%. Clinopyroxene were diopside and augite, and they were enriched in light rare earth elements (LREEs) (LaN/YbN = 1.93–4.27) and depleted in large-ion lithophile elements (LILEs). Mineral compositions (mainly based on clinopyroxene) confirmed that these late Cenozoic basaltic rocks were of an intraplate affinity and were similar to contemporaneous basaltic fields in the SCS region (Southern Vietnam, Northern Hainan, and SCS seamounts). Plagioclases were predominantly labradorite, with a few andesine and bytownite, and they were enriched in LREEs and Ba, Sr, and Pb, and most of them exhibited strong positive Eu anomalies. The source lithology of Thailand basaltic rocks could be garnet pyroxenite. The mantle potential temperature beneath Thailand is in the range of 1448–1467 °C, which can be comparable to those beneath Southern Vietnam and Northern Hainan, indicating the Thailand basaltic rocks could be produced by the Hainan mantle plume. In addition, the crystallization temperature of clinopyroxenes (1145–1214 °C) and plagioclase (1067–1133 °C) and their composition characteristics indicate that the magmatic processes have a conspicuous characteristic of fast rate of magma upwelling. Thus, we proposed that the deep geodynamic setting of Thailand late Cenozoic basaltic rocks is similar to those of the whole SCS region, and Hainan mantle plume plays a significant role in the petrogenesis of these basaltic rocks.

In Situ Geochemical Compositions of the Minerals in Basaltic Rocks from the West Philippine Basin: Constraints on Source Lithology and Magmatic Processes

“…While the plagioclase phenocrysts from sample 94DS0 exhibit negative Eu anomalies; Th, U, Pb, and Hf enrichments; and Ba, Nb, Sr, and Ti depletions ( Figure 9). The plagioclase phenocrysts from sample 94DS0 are similar to the MATA-1 plagioclase xenocrysts from the Late Cenozoic basalts from Thailand [36], indicating that the plagioclase phenocrysts may be xenocrysts, rather than magmatic in origin, which were trapped by sample 94DS0 during its ascent to the surface. In addition, the trace element and rare earth element contents of the plagioclase in the basalts from site 100DS are significantly higher than those from site 94DS, and the trace element and rare earth element characteristics of the plagioclase from site 100DS are similar to those from Thailand [36].…”

“…The plagioclase phenocrysts from sample 94DS0 are similar to the MATA-1 plagioclase xenocrysts from the Late Cenozoic basalts from Thailand [36], indicating that the plagioclase phenocrysts may be xenocrysts, rather than magmatic in origin, which were trapped by sample 94DS0 during its ascent to the surface. In addition, the trace element and rare earth element contents of the plagioclase in the basalts from site 100DS are significantly higher than those from site 94DS, and the trace element and rare earth element characteristics of the plagioclase from site 100DS are similar to those from Thailand [36]. The estimated primary compositions of the basalts from the CBF and sites DSDP447 and ODP1201 plot within the experimental field are defined by the partial melting of peridotite, while the estimated primary compositions of the Benham Rise plot within the silica-deficient eclogite field ( Figure 10) [38].…”

“…The ΣREE values of the clinopyroxene from sample 94DS0 range from 15.98 to 17.39 ppm, with an average of 17.32 ppm, which are significantly lower than those of the clinopyroxene from the Late Cenozoic basalts from the South China Sea (average of 62 ppm) and Thailand (average of 84 ppm) [35,36]. Compared to the clinopyroxene from the Late Cenozoic basalts from the South China Sea and Thailand [35,36], the clinopyroxene from sample 94DS0 exhibits LREE depletion ( 40 Ar rad (grammol/gram) 40 5 Lithosphere average of 0.78) but no obvious Ce anomalies, indicating the fractionation of plagioclase during the magmatic processes.…”

“…This indicates that the plagioclase from samples 100DS7 and 100DS18 crystallized under similar physical conditions. The crystallization temperatures and pressures of the plagioclase from the Benham Rise can be comparable to those from Thailand (1145°C-1214°C and 0.4-0.9 GPa) and the South China Sea basin (927°C-1179°C) [36,71]. In addition, the temperatures of the plagioclase from the Benham Rise basalts are similar to those from the CBF, while the pressures of the Table 3: Crystallization temperatures and pressures of the clinopyroxenes of basaltic rocks from CFB.…”

“…clinopyroxene from the Late Cenozoic basalts from the South China Sea (average of 62 ppm) and Thailand (average of 84 ppm) [35,36]. Compared to the clinopyroxene from the Late Cenozoic basalts from the South China Sea and Thailand, the clinopyroxene from sample 94DS1 exhibits LREE depletion, with relatively low ratios of LREE/HREE (1.…”