“…These characteristics suggest that continental crustal contamination has been minimal during the genesis of intraplate volcanism in the wider South China Sea region, as previously suggested for the more restricted SCS basin 14 , 27 , 29 , 30 , Hainan Island 9 , 10 and the Indochina block 23 , 25 . Some authors have however argued for a significant role of crustal contamination for Vietnamese basalts 20 , 24 . Figure 8 Mg# vs. 87 Sr/ 86 Sr ( a ), 143 Nd/ 144 Nd ( b ), and 176 Hf/ 177 Hf ( c ) isotopic ratios for late Cenozoic volcanic rocks from Thailand.…”
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.
“…These characteristics suggest that continental crustal contamination has been minimal during the genesis of intraplate volcanism in the wider South China Sea region, as previously suggested for the more restricted SCS basin 14 , 27 , 29 , 30 , Hainan Island 9 , 10 and the Indochina block 23 , 25 . Some authors have however argued for a significant role of crustal contamination for Vietnamese basalts 20 , 24 . Figure 8 Mg# vs. 87 Sr/ 86 Sr ( a ), 143 Nd/ 144 Nd ( b ), and 176 Hf/ 177 Hf ( c ) isotopic ratios for late Cenozoic volcanic rocks from Thailand.…”
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.
“…The considerable variation of negative ε Nd ( t ) values suggests effective contamination of the mafic rocks by crustal materials during magma emplacement. The effect of crustal contamination and assimilation fractionation crystallization (AFC) can be evaluated by correlations between major elements (such as MgO, SiO 2 and FeO T ) with trace element ratios (Nb/La, Nb/Th, Th/Ta and Zr/Nb) and isotope compositions 18 – 20 . Generally, La/Sm ratios decrease and ε Nd ( t ) values increase with increasing Nb/La ratios (Fig.…”
Early Cretaceous mafic rocks are first reported in the northern Guangxi region from the western Qin-Hang belt in the interior South China Block. A systematic investigation of zircon U–Pb dating, whole-rock geochemistry, Sm–Nd isotopes and zircon Hf–O isotopes for these mafic rocks reveals their petrogenesis and the mantle composition as well as a new window to reconstruct lithospheric evolution in interior South China Block during Late Mesozoic. Zircon U–Pb dating yielded ages of 131 ± 2 Ma to 136 ± 2 Ma for diabase and gabbro from Baotan area, indicating the first data for Early Cretaceous mafic magmatism in the western Qing-Hang belt. These mafic rocks show calc-alkaline compositions, arc-like trace element distribution patterns, low zircon εHf(t) of − 9.45 to − 6.17 and high δ18O values of + 5.72 to + 8.09‰, as well as low whole-rock εNd(t) values of − 14.27 to − 9.53. These data suggest that the studied mafic rocks are derived from an ancient lithospheric mantle source that was metasomatized during Neoproterozoic subduction. Thus, the occurrence of these mafic rocks indicates a reactivation of Neoproterozoic subducted materials during an extension setting at Late Mesozoic in the western Qin-Hang belt, an old suture zone that amalgamates the Yangtze and Cathaysia blocks.
“…Additionally, the upper‐middle crust of CYP is also shortened in S‐N direction with the obstruction of ICB, in which a small part of displacement of individual branch faults of XJFZ appears to be absorbed by thrust‐faulting (JSF and QJF) and folding (BB’ in Figures and a). For the most, the bending of RRF (60 km) suggests that the total left‐shear (60 km~) across XJFZ remains constant, which may pass though RRF into ICB, where the left‐lateral (~12.5 km, 5 Ma~) Dien Bien Phu Fault would be the counterpart of the XJFZ, though left‐slip displacement on the individual branch faults of XJFZ becomes less southwards (Koszowska, Wolska, Zuchiewicz, Cuong, & Pécskay, ; Lai, Chen, & Lâm, ; Wang et al, ; Figure c). Therefore, most of the displacement of XJFZ is absorbed by the bending of RRF and the NW‐SE trending faults (Wuliangshan Fault, Amojiang Fault, etc.)…”
The Central Yunnan Plateau (CYP) is located in the southeastern end of the Sichuan‐Yunnan rhombic block. CYP is surrounded by the South China Block to the east and the Indo‐China Block to the southwest. Stratigraphy and topography research shows the relative surface uplift of the CYP since the Pliocene. However, the widely accepted model of lower crustal flow is very difficult to fully explain the surface uplift of CYP, with faults and fold structures in the brittle shallow crust. The lower crust thickening led by the lower crustal flow only contributes 34–54% (600–950 m) of the total surface uplift. The wide‐angle seismic profiles and topography analysis show that the CYP surface uplift is predominately a consequence of isostatic compensation due to surface erosion, upper‐middle crustal shortening, and lower crustal thickening. The average isostatic adjustment caused by surface erosion of CYP is about 500 m, which is about 29% of the total surface uplift (1750 m). The GPS station velocities, focal mechanisms, and seismic anisotropy show the strong decoupling relationship between upper‐middle crust and lower crust. In detail, there is a movement with southeast‐directed and clockwise rotation around the Eastern Himalayan syntaxis in the upper‐middle crust, although south‐directed flow in the lower crust. The southeast motion of the upper‐middle crust could be resolved into two portions. The eastward component of upper‐middle crust motion is obstructed by the South China Block in the east. The result is the W‐E direction shortening of upper‐middle crust, shown by the S‐N trending CYP anticlinorium. The W‐E direction shortening of upper‐middle crust contributes 17–37% (300–650 m) of the total surface uplift. In contrast, the southward component of upper‐middle crust motion, shown by the sinistral strike‐slip displacement of the Xiaojiang fault zone, could pass through the Red River fault zone into the Indo‐China Block, where the left‐lateral Dien Bien Phu Fault would be the counterpart. The contribution of upper‐middle crustal shortening in a S‐N direction is very limited. The lower crustal flow could have been playing a prominent role in the growth of the whole southeast margin of the Tibetan Plateau, and the contribution from upper‐middle crustal shortening cannot be ignored in some local areas.
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