Ching-Hua Lo

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.

Diachronous uplift of the Tibetan plateau starting 40 Myr ago

The uplift of the Tibetan plateau is generally regarded as a response to the convective removal of the lower portion of the thickened Asian lithosphere. This removal is also thought to be responsible for the east–west extension that took place during the India–Asia collision. The timing of these events has been a subject of great interest for understanding mountain-building processes, collisional tectonics and the influence of these processes on climate change,. In western Tibet, potassic lavas related to east–west extension were found to have been extruded over the past 20 Myr (refs 5, 6). Here we report the widespread occurrence of magmas in eastern Tibet which show similar geochemical signatures to the potassic lavas to the west but formed 40–30 Myr ago. These magmatic activities suggest a diachronous uplift history for the Tibetan plateau, with the convective removal of the lower lithosphere inducing rapid uplift in the east beginning some 40 Myr ago and in the west about 20 Myr later. This observation is consistent with sedimentation records from the Ganges–Brahmaputra delta to the Bengal fan, and can better account for the tectonically driven models for strontium isotope evolution in the ocean and global cooling over the past 40 Myr.

Intraplate extension prior to continental extrusion along the Ailao Shan-Red River shear zone

Left-lateral movement of the Ailao Shan–Red River shear zone lends support to the hypothesis of continental extrusion resulting from the collision of India with Asia. Our new observations from northwestern Yunnan, China, and northwestern Vietnam on different sides of the shear zone demonstrate that the sinistral offset was ∼600 km according to correlations of Permian-Triassic flood basalt successions and late Paleogene highly potassic mafic magmas. We conclude that the shear was propagating on the South China continental margin and does not correspond to a suture between South China and Indochina. Furthermore, the highly potassic magmas were emplaced from ca. 40 to 30 Ma, before the shear movement, which was caused by the late Oligocene to early Miocene (ca. 27–22 Ma) extrusion activity. This suggests that a late Eocene to early Oligocene intraplate extension, possibly induced by delamination of thickened continental lithosphere, took place in northwestern Yunnan (or eastern Tibet) as a response to the India-Asia collision. This extension, and sea-floor spreading of the South China Sea that began ca. 30 Ma, could have accounted for the initiation of the Ailao Shan–Red River shear zone.

Highly fractionated I-type granites in NE China (II): isotopic geochemistry and implications for crustal growth in the Phanerozoic

NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184–137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that these rocks are highly fractionated I-type granites. In this paper, we present Sr–Nd–O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic. Despite their difference in emplacement age, Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial 87 Sr/ 86 Sr ratios (0.7045F0.0015), positive eNd(T) values (+1.3 to +2.8), and young Sm–Nd model ages (720–840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment. The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic. D 2003 Elsevier Science B.V. All rights reserved.

Age of the Emeishan flood magmatism and relations to Permian–Triassic boundary events

The Permian^Triassic (P^T) mass extinction, the greatest biological mortality event in the Earth’s history, was probably caused by dramatic and global forcing mechanisms such as the Siberian flood volcanism. Here we present the first set of high-precision 40 Ar/ 39 Ar dating results of volcanic and intrusive rocks from the Emeishan Traps, South China, which define a main stage of the flood magmatism at V251^253 Ma and a subordinate precursory activity at V255 Ma. This time span is generally coeval with, or slightly older than, the age of the P^T boundary estimated by the ash beds in the Meishan stratotype section and the main eruption of the Siberian Traps. Our data reinforces the notion that the eruption of the Emeishan Traps, rather than eruption of the Siberian Traps, accounted for the formation of the P^T boundary ash beds in South China. The Emeishan flood magmatism, which occurred in the continental margin comprising thick marine limestone formations, moreover, may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global N 13 C excursion and associated environmental crisis leading to the mass extinction at the P^T boundary. fl 2002 Elsevier Science B.V. All rights reserved.

Unusually low δ18O ultra-high-pressure metamorphic rocks from the Sulu Terrain, eastern China

Eclogite and quartzite interlayers, containing ultra-high-pressure (UHP) metamorphic mineral assemblages, occur at Qinglongshan in the Sulu terrain, eastern China. These rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks. The δ18 value of minerals in eclogite ranges from −10.4 to −9.0%o and in quartzite ranges from −10.2 to −7.3‰ Near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the UHP metamorphism, through near surface meteoric water-rock interactions. The rocks were then subducted to a great depth during continent-continent collision, forming UHP minerals with concomitant local oxygen isotope redistribution. It is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle.

Sm/Nd, Rb/Sr, and 40Ar/39Ar Isotopic Systematics of the Ultrahigh-Pressure Metamorphic Rocks in the Dabie-Sulu Belt, Central China: A Retrospective View

Because of a complicated metamorphic history, the isotopic systematics of the ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected. Depending on the degree of retrograde metamorphism and on the retentivity of isotopes, the radiogenic isotopic systematics in the UHP metamorphic rocks yielded a wide range of radiometric ages. Some of these ages are geologically meaningful, but others may not be. In some fine-grained UHP metamorphic rocks, Sm/Nd isotopic systematics appear to be in equilibrium among the UHP phases, showing the best estimate for the age of peak metamorphism at 226 ± 3 Ma. On the other hand, retrograde overprinting often makes the interpretation of isotopic data more difficult. It is common to find that the Sm/Nd and Rb/Sr isotopic systematics among the UHP phases and retrograde phases are not in equilibrium. Regression of isotopic data involving both UHP and retrograde minerals in isotopic correlation diagrams oft...

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.

Jurassic intraplate magmatism in southern Hunan-eastern Guangxi: Ar-40/Ar-39 dating, geochemistry, Sr-Nd isotopes and implications for the tectonic evolution of SE China

Abstract The Mesozoic geology of SE China is characterized by intensive and widespread magmatism. However, the tectonic regime that accounted for the Mesozoic magmatism has been an issue with little consensus. A comprehensive study of 40Ar-39Ar dating, geochemistry and Sr-Nd isotopes has been conducted on basalts from southern Hunan and syenite intrusions from eastern Guangxi. Three episodes of Jurassic magmatism, i.e. alkaline basalts of c.175 Ma in age, syenitic intrusions of c.160 Ma and high-Mg basalts of c.150 Ma, are identified. The older, c.175 Ma alkaline basalts are characterized by low Sr (ISr = 0.7035–0.7040) and high Nd (εNd(T) = 5 to 6) isotopic compositions and OIB-like trace-element patterns (e.g. Nb/La > 1). In contrast, the younger, c.150 Ma high-Mg basalts have high Sr (ISr c.0.7054) and low Nd (εNd(T) c.-2) isotopic compositions and incompatible trace-element patterns of arc affinity. The c.160 Ma syenitic intrusions display a relatively large range of Sr and Nd isotopic compositions (ISr = 0.7032–0.7082, εNd(T) = 5.5 to −4.1), with the Qinghu syenites having the lowest ISr, highest εNd(T) and OIB-type incompatible trace-element patterns analogous to the c.175 Ma alkaline basalts. Such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources, which are inferred to have resulted from the post-Indosinian orogenic lithosphere extension and thinning. The c.175 Ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the asthenosphere or an enriched lithospheric mantle source accreted by asthenosphere-derived melts during the initial extension. The c.160 Ma syenitic and c.150 Ma high-Mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning. This interpretation is at odds with the active continental margin related to the subduction of palaeo-Pacific plate, but consistent with continental rifting and extension for the Mesozoic of SE China.