Yuejun Wang

Petrogenesis and tectonic implications of Late-Triassic high ɛNd(t)-ɛHf(t) granites in the Ailaoshan tectonic zone (SW China)

High ɛNd(t)-ɛHf(t) granites are robust evidence for crustal growth. In this paper we report results of petrologic, geochronological and geochemical investigations on the Huashiban granites from the Ailaoshan tectonic zone in western Yunnan (SW China). Zircon grains separated from the two samples (10HH-119A and 10HH-120A) yield the weighted mean 206Pb/238U ages of 229.9 ± 2.0 Ma and 229.3 ± 2.3 Ma, respectively, interpreted as the crystallization ages of the granites. Based on our results, in combination with the existing U-Pb geochronological data for the Ailaoshan metamorphic rocks, we propose that the Ailaoshan Group might be a rock complex composed of the Mesoproterozoic, Neoproterozoic, Hercynian, Indosinian and Himalayan components, rather than a part of the crystalline basement of the Yangtze block. The zircon grains show highly depleted Lu-Hf isotope compositions, with positive ɛHf(t) values ranging from 8.4 to 13.1. The Huashiban granites have high SiO2 (72.66 wt%–73.70 wt%), low Mg# (0.28–0.34) with A/CNK=1.01–1.05, and can be classified as peralumious high-K calc-alkaline I-type granites. A synthesis of these data indicates that the Ailaoshan tectonic zone had evolved into a post-collisional setting by the Late-Triassic (229 Ma). Genesis of the Huashiban high ɛNd(t)-ɛHf(t) granites involved into two processes: (1) underplating of the sub-arc mantle into the lower crust, and (2) remelting of the juvenile crustal materials in response to the upwelling of the asthenospheric mantle in the post-collisional setting.

Petrogenesis for the Chiang Dao Permian high-iron basalt and its implication on the Paleotethyan Ocean in NW Thailand

The Changning-Menglian suture in SW Yunnan has been accepted as the Paleotethyan main ocean. However, it has been a matter of debate as to its southerly extension in NW Thailand (the Chiang Mai-Chiang Rai vs. Nan-Uttaradit zone). Our field investigation identified the high-iron basaltic rocks in the Chiang Dao Permian standard profile in NW Thailand. The high-iron rocks provide crucial records for understanding the controversy on the location of Paleotethyan main ocean in NW Thailand. The Early Permian high-iron samples show extremely high FeOt (20.96 wt.%–25.56 wt.%) and TiO2 (6.07 wt.%–6.34 wt.%) and low SiO2 (38.54 wt.%–43.46 wt.%) and MgO (1.61 wt.%–2.40 wt.%) contents. Such characteristics are similar to those of the Fenner differentiation trend rarely observed in the natural system, distinct from those of the “normal” Bowen trend. Their chondritenormalized REE and primitive mantle-normalized patterns are generally similar to those of typical OIB. The initial 87Sr/86Sr ratios and εNd(t) values range from 0.704 677 to 0.705 103 and 3.16 to 3.48, respectively, falling near the field of typical OIB (oceanic-island basalt). These data synthetically suggest that the Chiang Dao high-iron rocks are the products of high-degree partial melting of peridotite with Fe-rich eclogitic blobs/streaks in response to a seamount setting. In comparison with the Permian tectonic setting in SW Yunnan and NW Thailand, it is inferred that the Paleotethyan Ocean was located between the Shan-Thai terrane of Sibumasu and Sukhothai arc along the Inthanon zone of the Chiang Mai-Chiang Rai rather than Nan-Uttaradit zones.

Evolving Yangtze River reconstructed by detrital zircon U‐Pb dating and petrographic analysis of Miocene marginal Sea sedimentary rocks of the Western Foothills and Hengchun Peninsula, Taiwan

The timing of the establishment of the Yangtze River, whether prior to the early Miocene (~24 Ma) or more recently (~2 Ma), has been a point of much debate. Here we applied detrital zircon U-Pb dating to Miocene sedimentary rocks from Taiwan and to estuary sands from modern rivers in SE China to trace sediment provenance and to further constrain the evolution of the Yangtze River. Detrital zircon U-Pb ages from Miocene sandstones of the Western Foothills show similar age spectra to Miocene and modern sediments in the Yangtze River drainage and some similarity to the Minjiang River sediments. However, they differ significantly from ages in some sandstones from the Hengchun Peninsula accretionary prism and from the estuary sands of the Jiulongjiang River. This information, together with petrographic and sedimentary facies analysis, argues that the Jiulongjiang and Minjiang Rivers were major sources to some Hengchun Peninsula turbidites (~12 Ma), while synchronous sedimentation in the Western Foothills was supplied from the Yangtze, Minjiang (or similar river), and possibly even the Yellow River. These sediments were transported southward/eastward via rivers or channels to the marginal sedimentary basins now inverted in the Western Foothills in Taiwan. The Yangtze River must have been established prior to the middle Miocene.

Petrochemistry and tectonic setting of the Middle Triassic arc-like volcanic rocks in the Sayabouli area, NW Laos

The volcanic rocks from the Sayabouli area in northwestern Laos have been poorly studied. These volcanic rocks are traditionally mapped as the Permian–Early Triassic sequences on the geological map. One basaltic-andesite from the Sayabouli area yields a zircon U-Pb age of 237.7±1.7 Ma, suggesting a Middle Triassic origin. All basalt and basaltic-andesite samples from the Sayabouli area show depletions in HFSEs (e.g., Nb, Ta, Ti) and have high LILE/HFSE ratios, and exhibit the geochemical affinity to the continental arc volcanic rocks and are geochemically similar to the continental arc volcanic rocks from the Phetchabun belt in northeastern Thailand, suggesting a Late Permian–Middle Triassic continental margin in the Sayabouli area of northwestern Laos and Phetchabun area of northeastern Thailand. Our data indicate that the Phetchabun arc volcanic belt through the western Loei sub-belt can be linked to the Sayabouli area in northwestern Laos.

Detrital zircon U-Pb geochronology and Lu-Hf isotopic compositions of the Wuliangshan metasediment rocks in SW Yunnan (China) and its provenance implications

The Wuliangshan Group occurs to the east of the Lancang giant igneous zone in SW Yunnan, and is mainly composed of low-grade metamorphosed sedimentary rocks. The group has been considered as the syn-orogenic product of the Baoshan with Simao-Indochina blocks. However, its depositional time and provenance remain to be poorly constrained. This paper presents zircon U-Pb dating and Lu-Hf-isotopic data for five representative sandstone samples from the Wuliangshan Group. The detrital grains yield a major age-peak at ∼259 Ma, and four subordinary age-peaks at ∼1 859, ∼941, ∼788, and ∼447 Ma, respectively. Our results suggest that the Wuliangshan metasedimentary sequence was deposited after Middle Triassic rather than previously-thought Cambrian. The detrital zircon age spectrum, along with in-situ Lu-Hf isotopic data suggest that the Wuliangshan Group might be a syncollisional sedimentary product related to the collision of Baoshan with Simao-Indochina blocks. It is inferred that the provenance of the Wuliangshan Group is mainly from the Simao/Yangtze blocks to the east rather than the Baoshan Block or Lancang igneous zone to the west.

Zircon U-Pb geochronology of the Cenozoic granitic mylonite along the Ailaoshan-Red river shear zone: New constraints on the timing of the sinistral shearing

The Ailaoshan-Red River (ASRR) shear zone in SW China represents an important discontinuity believed to have accommodated eastward extrusion of the Tibetan Plateau in response to the collision of the Indian and Eurasian plates. The onset timing and duration of the ASRR sinistral strike-slip shearing have been hotly disputed. In this paper we present new zircon LA-ICP-MS U-Pb geochronological data from six syntectonic granitic mylonite and leucosomes samples from the ASRR shear zone. Our data reveal a metamorphic age of ∼40 Ma, most likely suggesting the maximum age of the shearing initiation. Rocks showing syn-kinematic signatures yield crystallization ages of 38–22 Ma, with inherited components ranging from 716 to 108 Ma. These results, together with existing geological and geochronological data, indicate that the sinistral shearing along the ASRR zone probably began at 40 Ma, mainly activated at 29–22 Ma and lasted at least to ∼22 Ma. Our data suggest a continuous extrusion between the Indochina and South China blocks during ∼35–17 Ma. The ASRR sinistral shearing has accommodated large scale eastward displacement of the southeastern Tibetan syntaxis, and is likely responsible for the opening of the South China Sea.

Episodic slab rollback and back-arc extension in the Yunnan-Burma region: Insights from Cretaceous Nb-enriched and oceanic-island basalt–like mafic rocks

One highly debated topic related to the nature of the Cretaceous mantle and geodynamic processes in SE Asia is whether they were dominated by the Tethyan or Pacific Ocean subductions. The main reason for this puzzle is the lack of geochemical studies on mantle-derived mafic rocks in SE Asia. In this paper, we report Early Cretaceous Nb-enriched diabases (ca. 139 Ma) and Late Cretaceous Nb-enriched (ca. 68 Ma) and oceanic-island basalt (OIB)−like (ca. 73 Ma) amphibolites from the Ailaoshan tectonic zone in the Yunnan-Burma region. The ca. 139 Ma diabases and ca. 68 Ma amphibolites display similar geochemical features with typical Nb-enriched basalts, e.g., high Na 2 O, TiO 2 , and Nb (>7 ppm) contents, and high (Nb/Th) PM , (Nb/La) PM , and Nb/U ratios. Unlike adakite-associated Nb-enriched basalts, our Nb-enriched diabases and ampibolites were derived through mixing between OIB-like and arc-like mantle source components in the spinel stability field ( 2 and Nb (20.0−26.0 ppm) contents, and OIB-like rare earth element patterns and trace-element spidergrams. The OIB-like rocks were generated by a low degree of partial melting of garnet-facies peridotite (>80 km). In combination with previous Cretaceous to early Eocene age data from granitoids, three significant magmatic flare-ups are identified at 140−115 Ma, 75−64 Ma, and 55−50 Ma, with two magmatic gaps at 115−75 Ma and 64−55 Ma. Given the distances of the studied area from the Pacific and Neotethyan trenches (2400−5000 km and 320 km, respectively), these Cretaceous igneous rocks more likely formed in a Neotethyan subduction-related tectonic setting, rather than a Pacific Ocean−related setting. We propose a tectonic model involving episodic flat slab subduction, slab rollback, and intracontinental back-arc extension for Neotethyan subduction in the Yunnan-Burma region.

Effects of crustal eclogitization on plate subduction/collision dynamics: Implications for India-Asia collision

Abstract2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continental subduction. Crustal eclogitization directly increases the average slab density and accordingly the slab pull force, which makes the slab subduct deeply and steeply. Numerical results demonstrate that the duration time from initial continental collision to slab breakoff largely depends on the slab pull force. Specifically, eclogitization of subducted crust can greatly decrease the duration time, but increase the breakoff depth. The detachment of oceanic slab from the pro-continental lithosphere is accompanied with obvious exhumation of the subducted continental crust and a sharp uplift of the collision zone in response to the disappearance of downward drag force and the induced asthenospheric upwelling, especially under the condition of no or incomplete crustal eclogitization. During continental subduction, the slab dip angle is strongly correlated with eclogitization of subducted continental lower crust, which regulates the slab buoyancy nature. Our model results can provide several important implications for the Himalayan-Tibetan collision zone. For example, it is possible that the lateral variations in the degree of eclogitization of the subducted Indian crust might to some extent contribute to the lateral variations of subduction angle along the Himalayan orogenic belt. Moreover, the accumulation of highly radiogenic sediments and upper continental crustal materials at the active margin in combination with the strong shear heating due to continuous continental subduction together cause rising of isotherms in the accretionary wedge, which facilitate the development of crustal partial melting and metamorphism.

Carboniferous Arc Setting in Central Hainan: Geochronological and Geochemical Evidences on the Andesitic and Dacitic Rocks

Volcanic rocks in the Bangxi-Chenxing tectonic zone provide important carries for better understanding the Late Paleozoic tectonic evolution in Hainan and its temporal-spatial pattern of the eastern Paleotethyan evolution. This paper presents a set of new geochronological and geochemical data on the andesitic and dacitic rocks along the Bangxi-Chenxing tectonic zone in central Hainan. The representative andesitic and dacitic samples yield similar zircon U-Pb ages of 353±3 and 351±7 Ma, respectively, being of Early Carboniferous origin. These volcanic rocks are characterized by low TiO2 and high Al2O3 contents and are enriched in LILEs and LREEs but depletion in HFSEs, along with negative εNd(t) values of -1.4– -4.7 and high 87Sr/86Sr(i) ratios of 0.707 2–0.710 1. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a metasomatized wedge modified by the slab-derived component in a continental arc setting. In combination with the available data, it is proposed for the development of a Carboniferous continental arc in response to the eastern Paleotethyan evolution. The Bangxi-Chenxing tectonic zone might westerly link with the Jinshajiang-Ailaoshan-Song Ma suture zone, constituting an assemblage boundary between the South China and Indochina blocks.

Petrogenesis of the early Silurian Dashuang high-Mg basalt–andesite–dacite in eastern South China: origin from a palaeosubduction-modified mantle

The early Paleozoic Kwangsian event, the first compressive orogeny after the Rodinia assemblage in the South China Block (SCB), is characterized by the angular unconformity between the Devonian and pre-Devonian sequences and widespread granitoids and small-volume mafic rocks. However, the mechanism triggering this event is highly debated and the associated mantle source is poorly known. Our recent investigations identified the occurrence of early Silurian high-Mg basalt–andesite–dacite in the Dashuang area along the northwestern Yunkai Domain in the SCB. This study presents a set of new zircon U–Pb geochronological, whole-rock geochemical and Sr–Nd isotopic data for these volcanic rocks. They yield weighted mean 206Pb/238U ages of c. 430–443 Ma, similar to the age-peaks of early–middle Paleozoic magmatism and metamorphism in the SCB. These volcanic rocks have high MgO, Cr and Ni contents and Mg-numbers. They also exhibit arc-like geochemical affinities with crustal-like Sr–Nd isotopic compositions, similar to the Neoproterozoic high-Mg basalt–andesite in the eastern SCB. Such signatures, combined with other geological observations, suggest that the Dashuang high-Mg rocks were derived from an ancient (probably early Neoproterozoic) mantle wedge source. The early Paleozoic intra-continental reactivation in response to late-orogenic extension triggered the melting of this palaeosubduction-modified mantle, which led to the formation of these volcanic rocks.