Chi-Yu Lee

Late Cenozoic basaltic volcanism around the Taiwan Strait, SE China: Product of lithosphere-asthenosphere interaction during continental extension

Abstract Late Cenozoic intraplate basaltic volcanism in SE China and Taiwan occurred as a result of lithospheric extension related to the opening of the South China Sea. It was gradually terminated by the compression which propagated westwards from the arc-continent collision in Taiwan since ∼ 12 Ma ago. The basalts show a spatial variation in chemical composition. Their alkalinity, degree of silica undersaturation and abundance of incompatible elements increase whilst Pb isotopic ratios decrease ( 206 Pb 204 Pb from 19.0 to 18.2) progressively away from a NE-trending extension axis in the western Taiwan Strait. Abundant tholeiites were emplaced near the axial zone whereas alkali basalts, basanites and nephelinites took place further away from this axis. Overall, the basalts have Sr-Nd-Pb isotopic systematics similar to those of seamount basalts in the South China Sea which are characterized by a Dupal-type Pb isotopic anomaly. Based on geochemical constraints from the basalts, depth estimates for mantle xenoliths and regional geophysical data, a passive extension model is proposed for the geodynamic evolution of this region. It suggests greatest lithosphere thinning during the Miocene beneath the axial zone. The lithospheric mantle was thermo-mechanically eroded by convective upwelling of the asthenosphere, a process that raised the lithospheric geotherm and resulted in a plum-pudding-type convecting mantle. The spatial chemical and isotopic variation in the basalts can be explained by different degrees of decompression melting of this convecting mantle, corresponding to a change in lithosphere thickness, compounded by various contributions from the continental lithospheric mantle (CLM) -derived plum component. This model, which involves continental extension followed by magma generation via lithosphere-asthenosphere interaction, should be applicable to other areas of Cenozoic rift magmatism around the South China Sea and in the eastern Eurasian continental margin.

Geochemical and Sm-Nd isotopic study of amphibolites in the Cathaysia Block, southeastern China: evidence for an extremely depleted mantle in the Paleoproterozoic

Abstract Geochemical and Sm–Nd isotopic results are reported for late Paleoproterozoic mafic amphibolites from SW Zhejiang and NW Fujian, parts of the Cathaysia Block of SE China. Two suites of contemporaneous amphibolites are distinct in their geochemical characteristics. Group 1 samples, from NW Fujian, have chemical compositions of transitional and alkali basalts, show LREE-enriched patterns and plot mainly in the field of within-plate basalt on a number of trace element discrimination diagrams. Group 2 rocks, from SW Zhejiang, have tholeiitic compositions and are characterized by flat to LREE-depleted patterns and fall into the MORB field. All the amphibolite samples have high eNd(T) values of +5.6 to +8.5 (T=1766±19 Ma). A positive correlation between eNd(T) and Nb/Th suggests possible mixing of a mantle-derived magma and a crustal component, with the least-contaminated samples having very high eNd(T) values (+8∼+8.5) and Nb/Th ratios of ∼13. The geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto-oceanic basin (e.g. the Gulf of Tadjoura). The exceptionally high eNd(T) values of up to +8.5 for some of the amphibolites suggest the presence of a time-integrated extremely depleted mantle source beneath Cathaysia during the Paleoproterozic.

Geochemical and Sr-Nd Isotopic Characteristics of Late Paleogene Ultrapotassic Magmatism in Southeastern Tibet

Geochemical and Sr-Nd isotopic data are reported for late Paleogene potassic lamprophyres from western Yunnan, southeastern margin of the Tibetan Plateau. These lamprophyres are mostly ultrapotassic in composition with K2O/Na2O = 2.1 to 5.2, except for a few samples with shoshonitic affinity showing slightly lower K2O/Na2O = 1.6 to 1.7. They are characterized by high initial 87Sr/86Sr ratios of 0.70624 to 0.70924; negative εNd(T) values of -1.7 to -4.6; enrichment in large-ion lithophile elements, light rare-earth elements, and Pb; and depletion in high-field-strength elements, resembling those of high K/Ti and low-Ti potassic magmas formed in subduction-related settings. These lamprophyres were generated by partial melting of a metasomatized, phlogopite-bearing spinel harzburgite lithospheric mantle source, followed by crystal fractionation and varying degrees of crustal assimilation. Relatively constant incompatible trace element ratios, such as Rb/ Sr (˜0.2), Rb/Ba (˜0.1), La/Sm (˜5), Th/K (˜0.0003), and Nb/La (˜0.2), and limited Sr and Nd isotopic compositions in the ultrapotassic rocks possibly reflect an evenly distributed metasomatized mantle source. With a general similarity in geochemistry, the potassic and ultrapotassic magmas from southeastern (40-30 Ma) and northern (<15 Ma) parts of the Tibetan Plateau display obvious differences in Th/U, Rb/Sr, and Sr-Nd isotopes. These differences in geochemistry and Sr-Nd isotopes suggest contrasting subcontinental lithosphere mantle bulk compositions beneath the southeastern and northern parts of the Tibetan Plateau, caused by metasomatism involving subducted sediments from distinct crustal provenances.

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

Shoshonitic intrusive suite in SE Guangxi: Petrology and geochemistry

A NE-direction Mesozoic shoshonitic intrusive suite in SE Guangxi has been identified in terms of geological, petrological and geochemical investigations. The shoshonitic intrusives are characterized by enrichment of LILE, HFSE and LREE and no obvious Nb-Ta depletion, similar to those potassic rocks formed in within-plate and rifting environments. Unlike most shoshonitic rocks forming in arc settings, the SE Guangxi shoshonitic intrusives were likely generated during regional lithosphere extension induced by upwelling of asthenosphere mantle.

Cretaceous fractionated I-type granitoids and metaluminous A-type granites in SE China: the Late Yanshanian post-orogenic magmatism

The Late Yanshanian Orogeny (130-90 Ma) encompasses an important Mesozoic magmatic event in the crustal evolution of SE China. Products of post-orogenic magmatism, widely distributed in the eastern part of Zhejiang and Fujian provinces known as the Southeast Coast Magmatic Belt (SCMB), are dominated by large amounts of slightly Nb and Ta depleted, high-K calc-alkaline granites (I-type) and small amounts of strongly Ba, Sr, Eu, Ti and P depleted, metaluminous granites (A-type). 40 Ar/ 39 Ar dating from amphiboles suggests that emplacement of A-type granites mostly postdates (94-90 Ma) the intrusion of voluminous I-type granitoids (110-99 Ma). Using the Al-in-amphibole geobarometer, I-type suites were estimated to have been emplaced at shallow depths (5-7 km). Along with the fact that A-type granites are phyric or miarolitic in texture, it can be concluded that all these post-orogenic suites in the SCMB belong to shallow intrusives. They have also undergone a rapid cooling (higher than 100°C/Ma at T > 300 °C) as indicated by the thermochronology of hornblende, biotite and K-feldspar; therefore, generation of A-type granites from I-type magmas through fractional crystallisation would be a difficult process. Alternatively, their geochemical characteristics are attributed to partial melting in the residual lower crust under an elevated geothermal environment. On the other hand, I-type magmas are considered to be middle-crust-derived melts largely modified with mantle-derived melts that had been depleted with Nb and Ta by earlier tectonic processes. Such a tectonic environment is explained by the underplating of basaltic magmas, most probably due to lithospheric delamination taking place at c. 110 Ma, which marks the beginning of the postorogenic episode in this area. Numerical modelling for a heat source provided by the underplating of basaltic magma supports such a proposition.

The igneous provinciality in Taiwan: consequence of continental rifting superimposed by Luzon and Ryukyu subduction systems

Abstract Geochemical characteristics of late Cenozoic volcanics in Taiwan demonstrate that these rocks were produced in diverse tectonic environments. In Taiwan, three igneous provinces, namely, Western, Eastern and Northern, respectively (in order of the initiation of volcanic activities), can be distinguished. The Western Province comprises intraplate basalts, erupted in the rifted continental margin of southeastern China; whereas the Eastern and Northern Provinces are composed of arc volcanics genetically related to the Luzon and the Ryukyu subduction zones, respectively. These volcanic rocks were sequentially emplaced on the island of Taiwan as a result of the collision between the Luzon arc and the Asian continent since about 12 Ma. This collision, likewise, induced westward migration of the Ryukyu Trench system and subsequent opening of the Okinawa Trough. As a consequence, the collision-derived compression diminished in northern Taiwan. A post-collisional extension regime thus formed has generated some recent volcanic rocks in this region.

Metamorphosed mafic rocks with N-type MORB geochemical features in Hainan Island—Remnants of the Paleo-Tethys Oceanic Crust?

A number of metamorphosed mafic rocks occurred within the Paleozoic strata in the Chenxing and Bangxi regions at the northern side of the Changjiang-Qionghai Fault in Central Hainan Island. These metamorphosed mafic rocks are tholeiites in chemistry. They are characterized by extreme depletion of Th, Nb, Ta and LREEs, resembling the depleted N-type mid-ocean ridge basalts (MORB). Field relations suggest that the protolith of the metamorphosed mafic rocks were likely formed in Paleozoic. These metamorphosed mafic rocks with N-type MORB geochemical features were probably the remnants of the Paleo-Tethys oceanic crust.