Ching-Ying Lan

Thermochronological evidence for the movement of the Ailao Shan-Red River shear zone: A perspective from Vietnam

In order to explore the timing of strike-slip movement along the Ailao Shan–Red River shear zone, an 40Ar/39Ar thermochronological study of the Day Nui Con Voi metamorphic massif in northern Vietnam was undertaken. The massif, exposed in the southeastern segment of the Ailao Shan–Red River shear zone, reveals a rapid cooling in the early Miocene (25–21 Ma) after a very slow cooling ca. 34–25 Ma. The slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late Paleogene (40–30 Ma) extension in eastern Tibet and western Yunnan, China. The rapid cooling, consistent with evidence of a wider rapid cooling span from 27 to 17 Ma compiled from the entire Ailao Shan–Red River shear zone, constrains the duration of left-lateral shearing, which eventually offset Indochina from South China by about 600 km. The time constraints we have established reinforce the argument that the onset of the Ailao Shan–Red River shear zone postdates the opening of the South China Sea that began ca. 30 Ma. This result highlights the necessity for reevaluating the Cenozoic tectonic models of Southeast Asia.

Onset timing of left-lateral movement along the Ailao Shan–Red River Shear Zone: 40Ar/39Ar dating constraint from the Nam Dinh Area, northeastern Vietnam

Left-lateral motion along the Ailao Shan‐Red River (ASRR) Shear Zone has been widely advocated to be the result of the collision between the Indian and Eurasian plates and to account for sea-floor spreading in the South China Sea. Our new 40 Ar/ 39 Ar data on the south-easternmost outcrop of the Day Nui Con Voi metamorphic massif, northern Vietnam, suggest that the exhumation of metamorphic massif by shearing along the ASRR zone began 027 Ma and lasted until 022 Ma. A perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of 0 6c m y ˇ1 . Such a good correlation also indicates that the onset of the leftlateral movement of the shear zone may have occurred later than 027.5 Ma. This conclusion is consistent with our previous interpretation that collision-induced southeastward extrusion of Indochina along the ASRR Shear Zone postdates the opening of the South China Sea, and that extrusion tectonics in SE China may not be responsible for the opening of the South China Sea. # 2000 Elsevier Science Ltd. All rights reserved.

Geochemical and Sr-Nd isotopic characteristics of granitic rocks from northern Vietnam

Abstract Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has significantly higher initial Nd isotopic ratios [eNd(T)=+0.7 to +1.5] and older Nd isotopic model ages (TDM∼1.7 Ga) than the latter [eNd(T)=−4.7 to −9.7; TDM∼1.3–1.5 Ga] which were emplaced south of the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan–Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate eNd(T) values (+0.6 to −2.8) and younger TDM ages (0.6–1.1 Ga). These magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan–Red River shear zone related to the India–Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks.

Nd isotopic study of Precambrian basement of South Korea: Evidence for Early Archean crust?

We carried out a Sm-Nd isotopic reconnaissance study of South Korea. Eight river sediments gave crustal residence ages (T DM ) between 1.7 and 2.7 Ga, implying that, on average, the crust of South Korea was formed in Early to Middle Proterozoic time. Twenty-six Precambrian basement rocks yielded T DM between 1.4 and 3.8 Ga. More than half of them exceed 2.5 Ga, indicating that South Korea has a long history of crustal evolution. Four of the samples have T DM from 3.2 to 3.8 Ga suggesting the possibility that there may be ancient continental crust in Korea having ages comparable to those of the oldest cratons in the world. However, these samples also have preferentially higher Sm/Nd, so their old ages could be artifacts, thus requiring confirmation. The large variation in T DM of South Korea is reminiscent of that for the South China block. However, the existence of Early Archean ages better matches that of the North China block, because the basement of south China was thought to be mostly Proterozoic in age. The recent discovery of rare Archean basement rocks in the South China block may favor models that attempt to correlate South Korea with south China.

Subduction-related granitic rocks of Taiwan

Abstract Late Cretaceous granitic rocks constitute an essential part of the pre-Tertiary Tananao metamorphic basement complex of Taiwan. They are dominantly of granodiorite to quartz monzonite composition. Most granitic rocks are peraluminous (A/CNK > 1.0 and normative corundum > 1%) and display moderately fractionated LREE and relatively unfractionated HREE patterns with negative Eu anomalies. On a primitive mantle-normalized trace-element diagram, they show a significant Nb depletion which is typical of the calc-alkaline magmatism from the subduction-zone environment. They fall within the volcanic arc field on the discrimination diagram of Pearce, Harris and Tindle (1984). The lack of systematic inter-element relationships suggests that the role of fractional crystallization is not significant and that these granitic rocks were derived from heterogeneous protoliths. Geochemical data suggest Taiwan granitic rocks are contaminated I-type and I-type granites related to the subduction of the Paleo-Pacific plate beneath the eastern margin of the Eurasia plate during late Mesozoic time.

First evidence for Archean continental crust in northern Vietnam and its implications for crustal and tectonic evolution in Southeast Asia

Southeast Asia, or, in a more strict sense, the Indochinese continent, was previously considered to be composed entirely of Proterozoic to Phanerozoic rocks, and reliable evidence for Archean crust was lacking. Gneisses from the Cavinh Complex, south of the Red River shear zone, northern Vietnam, however, show Archean Nd model ages of 3.4–3.1 Ga. Zircon separates from the rocks yielded U-Pb dates of 2.8–2.5 Ga, the first convincing evidence for the presence of a Late Archean complex in Southeast Asia. Given that the Red River shear zone was propagating in the South China block with a left-lateral offset of ∼600 km, the Cavinh Complex can be correlated with the Late Archean Kangding Complex in the western margin of the Yangtze craton, southwestern China. The Cavinh Complex therefore represents one of the oldest crustal nuclei of the South China block.

High-Mg potassic rocks from Taiwan: implications for the genesis of orogenic potassic lavas

Taiwan is an active mountain belt formed by oblique collision between the Luzon are and the Asian continent. Regardless of the ongoing collision in central and southern Taiwan, a post-collisional extension regime has developed since the Plio-Pleistocene in the northern part of this orogen, and led to generation of the Northern Taiwan Volcanic Zone. Emplaced at similar to0.2 Ma in the southwest of the Volcanic Zone, lavas from the Tsaolingshan volcano are highly magnesian (MgO approximate to 15 wt.%) and potassic (K2O approximate to 5 wt.%, K2O/Na2O approximate to 1.6-3.0). Whereas these basic rocks (SiO2 approximate to 48 wt.%) have relatively low Al2O3 approximate to 12 wt.%, total Fe2O3 approximate to 7.5 wt.% and CaO approximate to 7.2 wt.%, they are extremely enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, Ba, Th and U). The Rb and Cs abundances, > 1000 and 120 ppm, respectively, are among the highest known from terrestrial rocks. In addition, these rocks are enriched in light rare earth elements (LREE), depleted in high Field strength elements (HFSE), and display a positive Pb spike in the primitive mantle-normalized variation diagram. Their REE distribution patterns mark with slight Eu negative anomalies (Eu/Eu * approximate to 0.90 - 0.84), and Sr and Nd isotope ratios are uniform (Sr-87/Sr-86 approximate to 0.70540-0.70551; Nd-143/(144) Nd approximate to 0.51268-0.51259). Olivine, the major phenocryst phase, shows high Fo contents (90.4 +/- 1.8; 1 sigma deviation), which are in agreement with the whole rock Mg-values (83 - 80). Spinel inclusions in olivine are characterized by high Cr/Cr+Al ratios (0.94-0.82) and have compositions similar to those from boninites that originate from highly refractory peridotites. Such petrochemical characteristics are comparable to the Group I ultrapotassic rocks defined by Foley et al. [Earth-Sci. Rev. 24 (1987) 81], such as orogenic lamproites from central Italy, Span and Tibet, We therefore suggest that the Tsaolingshan lavas resulted from a phlogopite-bearing harzburgitic source in the lithospheric mantle that underwent a recent metasomatism by the nearby Ryukyu subduction zone processes. The lavas exhibit unique incompatible trace element ratios, with Rb/Cs approximate to 8, Ba/Rb approximate to 1, Ce/Pb approximate to 2, Th/U approximate to 1 and Nb/ U approximate to 0.8, which are significantly lower than the continental crust values and those of most mantle-derived magmas. Nonmagmatic enrichment in the mantle source is therefore required. Based on published experimental data, two subduction-related metasomatic components, i.e., slab-released hydrous fluid and subducted sediment, are proposed, and the former is considered to be more pervasive for causing the extraordinary trace element ratios observed. Our observations lend support to the notion that dehydration from subducting slabs at convergent margins, as a continuing process through geologic time, can account for the fractionation of these elemental pairs between the Earths crust and mantle

Late Triassic–Late Cretaceous accretion/subduction in the Taiwan region along the eastern margin of South China – evidence from zircon SHRIMP dating

To examine the tectonic history of the Taiwan segment of the eastern margin of South China, six rock samples from the Tailuko belt, the metamorphic basement of Taiwan, were selected for zircon SHRIMP dating. The aim was to identify evidence shedding light on the timing of the change from passive to active tectonics for this part of the continental margin since South China separated from the supercontinent of Rodinia. The results lead to two age groups, 190–200 and 88–90 Ma. These age groups, augmented by the previously published age data, suggest that they could have resulted from two Mesozoic accretion/subduction events. In addition, this mid‐late Mesozoic Tailuko belt might have also been reactivated and structurally complicated by the late Cenozoic collision/accretion of the Luzon arc with the Eurasian continent. Records of older tectonic events, such as those derived from the Japanese Islands, are absent in this metamorphic basement. An important finding of this study is the existence of the 191±10 Ma Talun metagranite, the oldest granitic intrusion ever reported in the Taiwan region and along the eastern coast area of South China. In spite of a large age uncertainty, the occurrence of this metagranite is not consistent with the apparent younging trend of Jurassic‐Cretaceous igneous activity toward the coastline in South China, and should be taken into consideration by future studies.

UPb and Sr isotopic studies on granitoids from Taiwan and Chinmen-Lieyü and tectonic implications

Abstract UPb zircon/monazite dating and Sr-isotope results for granitoids from Taiwan and Chinmen-Lieyu, SE China place important constraints for the tectonic evolution and the timing of felsic magmatism associated with mid-Cretaceous subduction process. The Chinmen gneiss (139.4 Ma) and granite (100.9 Ma) represent the oldest granitic units and belong to the previously recognized late Yanshanian phase of magmatism. A younger suite of granitoids occurs in the Tananao Metamorphic Complex of Taiwan and includes the Yuantoushan and Tachoshui granites (87.3 Ma) and the Kanagan gneiss (90.2 Ma). These mid-Cretaceous ages for the younger granitoid suite mitigate against the existence of the hypothetical late Paleozoic “Dongnanya” microcontinent in the study area as proposed by other investigators. UPb zircon inheritance ages, ranging between 1.67 and 2.09 Ga indicate the presence of a Precambrian basement beneath much of coastal SE China and Taiwan, however, there is no evidence of Archean crust as indicated by zircon inheritance patterns for rocks west of the present study area. On a 87 Sr 86 Sr vs. 87 Rb 86 Sr plot, the available data for the granitoids in Chinmen and coastal Fujian, SE China, however, define pseudoisochrons with apparent ages of 158–167 Ma. Although the pseudoisochron could be interpreted as a result of a two-end-member mixing process with subsequent fractional crystallization, the apparent age of the pseudoisochron, substantiated by the circumstantial evidence, is postulated as the approximate time of Sr-isotope homogenization of the crustal protoliths of the granitoids during the previous (early Yanshanian) tectonic event in this region. Such a Sr-isotope homogenization process must have occurred over a large area, possibly facilitated by a metamorphic fluid phase or by large-scale anatexis. The Cretaceous granitoids in this area were probably produced by partial melting of this crust in a more restricted scale. The Cretaceous calc-alkaline granitoids in coastal Fujian and Taiwan are of I-type or contaminated I-type. A subduction component is inferred during the magma genesis. Although the Taiwan granitoids are suggested to be subduction related, the possible tectonic environment for the genesis of coastal Fujian granitoids is yet to be solved.

Geochemical, Sr and Nd isotopic characteristics and tectonic implications for three stages of igneous rock in the Late Yanshanian (Cretaceous) orogeny, SE China

Late Yanshanian magmatism in SE China includes three stages of thermal event induced by the interaction between the continental margin of Eurasia and the paleo-Pacific plate during the Cretaceous period. Products of syn-orogenic magmatism (130–110 Ma) include high-Al gabbros (HAG), and gneissic tonalite, trondhjemite and granodiorite (TTG), which intruded into the deep basement (18–24 km). Rocks of the post- and an-orogenic magmatism are shallow-level (6–8 km) I-type granitoids (110–99 Ma), and miarolitic A-type granites plus rhyolite-dominate bimodal volcanics (94–81 Ma), respectively. Geochemically, HAG and TTG belong to the medium-K calc-alkaline affinity with high Sr/Y, whereas other granitoids are mainly high-K calc-alkaline to shoshonitic rocks with low Sr/Y. Sr and Nd isotope compositions suggest different sources of HAG and TTG from other rocks. Progressive depletions of Ba, Sr, Eu and P from I- to A-type granites reflect partial melting of felsic granulites from hydrous to dry conditions, whereas high Sr/Y in HAG and TTG are compatible with dehydration melting of amphibolites. Tectonic models which accommodate HAG and TTG may involve thickening of the lithosphere to convert the pre-existing lower-crust basic rocks into amphibolites. It was followed by basaltic underplating which is attributed to delamination of the thickened lithosphere and led to triggering of crust melting under exten