Sr and Pb isotopes exhibit global trends with the concentrations of major elements (SiO 2 , TiO 2 , FeO, Al 2 O 3 and K 2 O) and major elements ratios (CaO/Al 2 O 3 and K 2 O/TiO 2 ) in the shield-stage lavas from 18 oceanic hotspots (including Hawaii, Iceland, Galapagos, Cook-Australs, St. Helena, Cape Verde, Cameroon, Canary, Madeira, Comoros, Azores, Samoa, Society, Marquesas, Mascarene, Kerguelen, Pitcairn, and Selvagen). Based on the relationships between major elements and isotopes in ocean island basalts (OIBs), we find that the lavas derived from the mantle end members, HIMU (or high ‘μ’ =  238 U/ 204 Pb), EM1 (enriched mantle 1), EM2 (enriched mantle 2), and DMM (depleted MORB [mid-ocean ridge basalt] mantle) exhibit distinct major element characteristics: When compared to oceanic hotspots globally, the hotspots with a HIMU (radiogenic Pb-isotopes and low 87 Sr/ 86 Sr) component, such as St. Helena and Cook-Australs, exhibit high CaO/Al 2 O 3 , FeO T , and TiO 2 and low SiO 2 and Al 2 O 3 . EM1 (enriched mantle 1; intermediate 87 Sr/ 86 Sr and low 206 Pb/ 204 Pb; sampled by hotspots like Pitcairn and Kerguelen) and EM2 (enriched mantle 2; high 87 Sr/ 86 Sr and intermediate 206 Pb/ 204 Pb; sampled by hotspots like Samoa and Societies) exhibit higher K 2 O concentrations and K 2 O/TiO 2 weight ratios than HIMU lavas. EM1 lavas exhibit the lowest CaO/Al 2 O 3 in the OIB dataset, and this sets EM1 apart from EM2. A plot of CaO/Al 2 O 3 vs K 2 O/TiO 2 perfectly resolves the four mantle end member lavas. Melting processes (pressure, temperature and degree of melting) fail to provide an explanation for the full spectrum of major element concentrations in OIBs. Such processes also fail to explain the correlations between major elements and radiogenic isotopes. Instead, a long, time integrated history of various parent–daughter elements appears to be coupled to major element and/or volatile heterogeneity in the mantle source. End member lava compositions are compared with experimental partial melt compositions to place constraints on the lithological characteristics of the mantle end members.

Compositions of HIMU, EM1, and EM2 from Global Trends between Radiogenic Isotopes and Major Elements in Ocean Island Basalts

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2017GL076872

Opening of the South China Sea and Upwelling of the Hainan Plume

\n Cenozoic basalts with ages ranging from 28.5 to < 0.1 Ma are widely distributed in the Indochina block, the South China Sea basin and the Leiqiong area in South China including the Leizhou Peninsula and the northern Hainan Island, which form the southeastern Asian basalt province (SABP). These Cenozoic basalts share common petrological and geochemical characteristics. However, the origin of the Cenozoic intraplate volcanism in the SABP is still a controversial issue. In this work, we apply a novel technique of multiscale global tomography to study the whole-mantle 3-D P-wave velocity (Vp) structure beneath the SABP. Our results show that low-Vp anomalies prevail in the whole mantle beneath the SABP. Although the strongest low-Vp zones exist beneath Hainan, significant low-Vp anomalies are also visible in the mantle beneath other parts of the SABP. These low-Vp anomalies appear somehow independent, rather than deriving from a single plume. We deem that a cluster of plumes rather than a single plume existed in the Cenozoic and may still exist now in the mantle beneath the SABP, though the Hainan plume may be the strongest one. A geochemical study suggested that the Hainan plume upwelling might be slowing down and close to exhausting its source zone. This geochemical inference is supported by our tomographic images showing that the low-Vp zones under Hainan are weak and intermittent in the lower mantle (∼700–2889 km depths). The low-Vp zones in the mantle beneath other SABP Cenozoic volcanoes are also weak, suggesting that those mantle plumes, if any, are also dying or already dead. As compared with a strong single plume, each member in a plume cluster should be small and weak, and so hard to exist long. The SABP is surrounded by subduction zones. The hot mantle upwelling beneath the SABP might be caused by collapsing of subducted slabs down to the lowermost mantle.

Deep mantle structure and origin of Cenozoic intraplate volcanoes in Indochina, Hainan and South China Sea

Structural and kinematic analysis of Cenozoic rift basins in South China Sea: A synthesis

The Cenozoic Sulu Sea arc‐basin system is situated in the tectonic junction between the South China Sea (SCS), northern Borneo, Palawan Continental Terrane, Philippine Mobile Belt, and Celebes Sea. We compare new/published geochronological and geochemical data from across the circum‐Sulu Sea region, and summarize seven major magmatic phases from the Middle Eocene to Pleistocene. The Middle Eocene (42.65 Ma) Sabah ophiolite and Eocene‐Oligocene (34‐33 Ma) Central Palawan ophiolite have MORB‐IAT‐transitional features, representing an intraoceanic subduction setting in the Paleogene northern Borneo and central‐southern Palawan. After the SCS opening (∼32 Ma) and ridge jump (∼25 Ma), late‐stage Proto‐SCS subduction (24‐21 Ma) may have formed the Panay arc andesite and the BABB magmatism in SW Zamboanga peninsula. Starting of final convergence between the Palawan Continental Terrane and northern Borneo‐SW Philippines (∼21 Ma) likely caused regional uplift/thrusting, forming the Top Crocker Unconformity and triggering the NW‐dipping Celebes Sea subduction. The subduction may have formed arc magmatism (21‐18 Ma) in the Cagayan ridge and its continuation in Panay and NE Sabah, and opened the NW Sulu Sea back‐arc basin through continental crust attenuation. Subduction rollback likely occurred in 17‐14 Ma and 13‐9 Ma, shifting arc magmatism southeast to the Sulu ridge and opening the SE Sulu Sea back‐arc basin. NW‐dipping Celebes Sea subduction largely ceased after ∼9 Ma, followed by extension‐related uplift/exhumation and 4‐0.2 Ma intraplate volcanism in northern Borneo. SE‐dipping Sulu Sea subduction likely occurred along the Negros‐Sulu trenches, and produced arc volcanism from ∼4 Ma.

Cenozoic evolution of the Sulu Sea arc-basin system: an overview

Intraplate volcanism initiated shortly after the cessation of Cenozoic seafloor spreading in the South China Sea (SCS) region, but the full extent of its influence on the Indochina block has not been well constrained. Here we present major and trace element data and Sr-Nd-Pb-Hf isotope ratios of late Cenozoic basaltic lavas from the Khorat plateau and some volcanic centers in the Paleozoic Sukhothai arc terrane in Thailand. These volcanic rocks are mainly trachybasalts and basaltic trachyandesites. Trace element patterns and Sr-Nd-Pb-Hf isotopic compositions show that these alkaline volcanic lavas exhibit oceanic island basalt (OIB)-like characteristics with enrichments in both large-ion lithophile elements (LILE) and high field strength elements (HFSEs). Their mantle source is a mixture between a depleted Indian MORB-type mantle and an enriched mantle type 2 (EMII). We suggest that the post-spreading intraplate volcanism in the SCS region was induced by a Hainan mantle plume which spread westwards to the Paleozoic Sukhothai arc terrane.

/pdf/hainan-mantle-plume-produced-late-cenozoic-basaltic-rocks-in-4vn4em19sk.pdf

Hainan mantle plume produced late Cenozoic basaltic rocks in Thailand, Southeast Asia.

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.

https://www.mdpi.com/2075-163X/9/7/446/pdf

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

\n Since the early Cenozoic, the West Philippine Basin (WPB) and the whole Philippine Sea Plate (PSP) has undergone a complex geological evolution. In this study, we presented K-Ar ages, in situ trace element, and major element compositions of minerals of basalts collected from the Benham Rise and the Central Basin Fault (CBF) in the WPB, to constrain their magmatic process and regional geological evolution. Olivine phenocrysts and microlites in the alkali basalts (20.9 Ma) from the Benham Rise have forsterite (Fo) contents of 56.90%–76.10% and 53.13%-66.41%, respectively. The clinopyroxenes in the tholeiites (29.1 Ma) from the CBF is predominantly diopside and augite, and it is depleted in light rare earth elements (LREEs) (LaN/YbN=0.13–3.40) and large-ion lithophile elements (LILEs). The plagioclases in the basalts from both of the Benham Rise and the CBF are predominantly labradorite and andesine, with a minor amount of bytownite, and it is enriched in LREEs, Ba, Sr, and Pb and exhibits strong positive Eu anomalies. However, there exist obvious differences in plagioclase compositions between these two tectonic sites. The source lithology of the Benham Rise basaltic rocks could be garnet pyroxenite, and yet that of the CBF could be spinel-lherzolite. The calculated mantle potential temperature beneath the Benham Rise is 1439°C–1473°C, which is significantly higher than that beneath the CBF (1345°C–1381°C), suggesting there existed thermal anomaly beneath the Rise during basaltic magmatism. This study also calculated the temperature and pressure of the clinopyroxenes and plagioclases, which have been used to indicate magmatic processes. Finally, we suggest that the Benham Rise basaltic rocks may be related to a mantle plume (e.g., the Oki-Daito mantle plume), and the CBF was once located in a back-arc spreading center behind an active subduction zone. The extinction of the Oki-Daito mantle plume activity might be at about 20.9 Ma, and cessation of the back-arc spreading of WPB was at about 29.1 Ma or younger.

/pdf/in-situ-geochemical-compositions-of-the-minerals-in-basaltic-4d0oogxdnd.pdf

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

The Chemical Composition of Global Subducting Sediments and Its Geological Significance

ABSTRACT Shortly after the cessation of the Cenozoic seafloor spreading in the South China Sea, a period of basaltic magmatism occurred in the Indochina Peninsula. The various types of xenoliths/xenocrysts present in these igneous rocks can provide information about the lithospheric mantle under the Indochina Peninsula. In this study, we present new ages and in situ major and trace element compositions for the xenoliths and olivine xenocrysts from the Wichian Buri and Mae Tha basalt flows in Thailand. The new ages obtained in this study for the Wichian Buri and Mae Tha basalt flows are 9.24 and 2.35 Ma, respectively. Under Thailand, the lithology of the sub-continental mantle lithosphere (SCLM) could be spinel-lherzolite, with a mantle temperature range of 936°C–1096°C. The mantle source of the Cenozoic basalts from Thailand and its adjacent regions could be affect by recycled oceanic crust and carbonates. The origin of spongy textures in clinopyroxene should be resulted from partial melting induced by decompression. SCLM under Thailand were transformed by peridotite-melt interaction, and basaltic melt trapped the mantle xenocryst and xenolith quickly ascended to the surface.

Long Yuan

Papers

Hainan mantle plume produced late Cenozoic basaltic rocks in Thailand, Southeast Asia.

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

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

The Chemical Composition of Global Subducting Sediments and Its Geological Significance

The nature of the sub-continental lithospheric mantle beneath Thailand: evidence from xenocrysts in Cenozoic basalts