g-Hong Chen

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.

A double island arc between Taiwan and Luzon: consequence of ridge subduction

Analysis of geomorphological, geochronological, geochemical and geophysical features in the segment of the Taiwan-Luzon Arc between Taiwan and Luzon (the Bashi Segment) allows the recognition of a double arc structure. The two volcanic chains are separated by 50 km just north of Luzon (18°N), and converge near 20°N. Islets in the western chain are older and largely composed of volcanic rocks of Miocene to Pliocene age. They all show low relief, lateritic platforms and wave-cut terraces, and are covered by massive recrystallized limestone. In contrast, all active volcanoes in this segment of the Taiwan-Luzon Arc belong to the eastern chain, where most islets are Quaternary in age. The volcanoes have well-developed cone shapes, and well-preserved deposits of near-vent facies. Magmas of the eastern chain have higher KSi, (La)n,(La/Yb)n, and lowereNd than their counterparts at the same latitude in the western chain. Therefore, the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain. Moreover, the available seismological data seem to suggest an abrupt increase of the dip angle from 30° at 18°N to 80° at 20°N. Thus, the double arc structure is located in the region where the Benioff zone suddenly changes. In analogy with the Lesser Antilles Arc, we propose a geodynamic model in which the double arc in the Bashi Strait is the tectonic manifestation of the subduction of the aseismic Scarborough Seamount Chain, the extinct mid-ocean ridge of the South China Sea. Before that ridge reached the Manila Trench, the western chain was the volcanic front. When the ridge reached the subduction zone at 5–4 Ma, its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity. Furthermore, this ridge-arc collision was probably also responsible for regional uplift causing extensive sub-aerial weathering and erosion as well as massive reef formation in the western chain. When subduction started again, the dip angle became shallower in response to the extra buoyancy of the downgoing ridge. If the depth of magma generation remained constant, the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain. Moreover, we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest, thus imprinting a geochemically more enriched signature on the magmas of the eastern chain.

Miocene basalts in northwestern Taiwan: Evidence for EM-type mantle sources in the continental lithosphere

Abstract Cenozoic extension around the Taiwan Strait resulted in intraplate basalt volcanism in the Fujian-Taiwan region of the southeastern China continental margin. In northwestern Taiwan, the basalt volcanism took place in two distinct periods: the early Miocene (23–20 Ma), with the eruption of alkali basalt only, and the late Miocene (13–9 Ma), marked by the emplacement of various basalt types. The early Miocene basalts have uniform SrNdPb isotope compositions comparable with those of the other Fujian-Taiwan basalts, which are believed to have originated from a “plum-pudding” type convecting mantle. These basalts display EM2-type lead isotope signature like that observed in seamount basalts from the South China Sea. By contrast, the late Miocene basalts show distinctive isotope characteristics indicating additional involvement of an EM1-type mantle source that has never been identified before for any volcanics in southern China. We propose that the “enriched mantle” components (EMI and EM2) reside in different levels of the continental lithospheric mantle. Reactivation of the unique EM1 source may be ascribed to the arc-continent collision in Taiwan during the last 12 million years which terminated the intraplate volcanism around this region.

Post-collisional magmatism around northern Taiwan and its relation with opening of the Okinawa Trough

Being part of an active mountain belt formed by oblique collision of the Luzon arc with Asia, northern Taiwan and the offshore islets are marked by a series of latest Pliocene‐Quaternary volcanoes whose eruptions have been conventionally ascribed to westward propagation of the Ryukyu volcanic arc. On the basis of new geochemical data, along with supporting geologic and geophysical evidence, we propose instead that this young volcanism resulted from post-collisional lithospheric extension in the northern Taiwan mountain belt and the mantle source regions involved in the melt generation have been significantly modified by the nearby Ryukyu subduction-related processes. Magmas thus produced through the northeast to the southwest in the Northern Taiwan Volcanic Zone (NTVZ) vary from low-K to calc-alkaline and then shoshonitic compositions. Such a spatial geochemical variation, characterized by southwestward increase in enrichments of potassium and incompatible trace elements, runs subparallel to the southwestern part of the present-day Ryukyu Trench. The geochemical variation that can be explained by southwestward decrease in degrees of partial melting of the mantle sources is reconciled with a southwestward-weakening extensional regime observed in the NTVZ. The post-collisional extension in northern Taiwan, furthermore, might have played a role in reactivation of the opening in the middle Okinawa Trough, and gave way to its rapid southwestward propagation with associated development of the Ryukyu subduction zone west of124oE. This nascent subduction, in turn, resulted in abundant submarine volcanoes which delineate an embryo volcanic front along the southern margin of the trough. Therefore, the southwestern Okinawa Trough is not a ‘fore-arc’ basin as had been previously alleged, but represents an ‘atypical’ back-arc basin which developed broadly synchronously or prior to its arc‐trench system in this particular collision=extension=subduction tectonic environment.

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

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

Fission-track dating of volcanics in the northern part of the Taiwan-Luzon Arc: eruption ages and evidence for crustal contamination

Igneous rocks from the Coastal Range and offshore islets of eastern Taiwan, Camiguin island and northern Luzon of the Philippines, which collectively constitute the northern part of the Taiwan-Luzon Arc, have been dated by the fission-track technique. The pooled ages range widely from 49.9 Ma to 0.02 Ma. Some of the samples produce bimodal or even multiple age distributions in composite probability density plots, indicating the presence of inherited “ages” in the dated zircons. A χ2 test shows that most of the χ2 ages are in good agreement with the youngest peak date. Except for one intrusive basement sample and five volcanic samples affected by hydrothermal alteration, most of the χ2 ages can be interpreted as the ages of volcanic eruptions. The inherited ages of the zircon populations present in many samples record thermal activities which occurred prior to the eruption of their host magmas. This suggests that upper level crustal contamination has taken place, thereby introducing some xenocrystic zircons into the magmas erupted. The fission-track ages are always younger than or occasionally equal to the ages obtained by other methods for the samples from the same localities. Of six samples dated by the ArAr stepwise heating method four samples have plateau age close to the FTD ages. Some samples do not contain inherited, partially annealed FTD ages and the corresponding KAr ages are consistent with the FTD ages. A detailed study of RbSr isotopic systematics of phenocryst minerals by Lan C. Y., Shen J. J. S. and Lee T. (1986). A RbSr isotopic study of andesites from Lutao, Lanhsu, and Hsiao-Lanhsu, eruption ages and isotopic heterogeneity. Bull. Inst. Earth Sci.6, 211–226 showed isotopic disequilibrium suggesting retention of unequilibrated older volcanic products. Many of the KAr dates, which are commonly older than the eruption ages determined by the FTD technique, may reflect inherited argon. Based on results from a combination of dating methods, volcanism in the northern part of the Taiwan-Luzon Arc can be deduced to have been active since 26 Ma. The igneous activity in the Taiwan Segment (northern part of North Luzon Arc as exposed in the Coastal Range) ceased at 2.2 Ma, because of the arc-continent collision. Further southward, away from the present location of the collision zone, the volcanoes are still active.

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.

Geochemical constraints on the petrogenesis of high-Mg basaltic andesites from the Northern Taiwan Volcanic Zone

Abstract The Northern Taiwan Volcanic Zone (NTVZ) is a Late Pliocene–Quaternary volcanic field that occurred as a result of extensional collapse of the northern Taiwan mountain belt. We report here mineral compositions, major and trace element and Sr/Nd isotope data of high-Mg basaltic andesites from the Mienhuayu, a volcanic islet formed at ∼2.6 Ma in the central part of the NTVZ. The rocks are hypocrystalline, showing porphyritic texture with Mg-rich olivine (Fo≈81–80), bronzite (En≈82–79) and plagioclase (An≈66–58) as major phenocryst phases. They have uniform whole-rock compositions, marked by high magnesium (MgO≈5.9–8.1 wt.%, Mg value≈0.6) relative to accompanying silica contents (SiO2≈52.8–54.5 wt.%). The high-Mg basaltic andesites contain the highest TiO2(∼1.5 wt.%) and lowest K2O (∼0.4 wt.%) among the NTVZ volcanic rocks. In the incompatible element variation diagram, these Mienhuayu magmas exhibit mild enrichments in large ion lithophile (LILE) and light rare earth elements (LREE), coupled with an apparent Pb-positive spike. They do not display depletions in high field strength elements (HFSE), a feature observed universally in the other NTVZ volcanics. The high-Mg basaltic andesites have rather unradiogenic Nd (eNd≈+5.1–7.2) but apparently elevated Sr (87Sr/86Sr≈0.70435–0.70543; leached values) isotope ratios. Their overall geochemical and isotopic characteristics are similar to mid-Miocene (∼13 Ma) high-Mg andesites from the Iriomote-jima, southern Ryukyus, Japan. Despite these magmas have lower LILE and LREE enrichments and Pb positive spike, their “intraplate-type” incompatible element variation patterns are comparable to those of extension-induced Miocene intraplate basalts emplaced in the Taiwan–Fujian region. Therefore, we interpret the Mienhuayu magmas as silica-saturated melts derived from decompression melting of the ascended asthenosphere that had been subtly affected by the adjacent Ryukyu subduction zone processes. This interpretation is consistent with the notion that in the northern Taiwan mountain belt post-orogenic lithospheric extension started in Plio–Pleistocene time.

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.