Kuo Lung Wang

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.

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

Three Fe-Ti oxide ore-bearing gabbro-granitoid complexes in the panxi region of the Permian Emeishan large igneous province, SW China

The Permian (∼260 Ma) Emeishan large igneous province of SW China contains three nearly identical gabbro-granitoid complexes that host giant Fe-Ti oxide deposits. The Fe-Ti oxide deposits are within the lower portions of evolved layered gabbroic intrusions and are spatially and temporally associated with A-type granitic plutons. The 264 ± 3 Ma Taihe layered gabbroic intrusion hosts a large magmatic Fe-Ti oxide deposit and is coeval with the Taihe peralkaline, A-type granitic pluton, which is dated at 261 ± 2 Ma. Within the A-type granitic pluton are microgranular enclaves, which have compositions intermediate between the gabbro and host granite. Primitive mantle-normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks. The chondrite-normalized REE patterns show Eu-anomalies changing from positive (Eu/Eu* = 1.5 to 5.9) in the gabbroic intrusion to negative in the enclaves (Eu/Eu* = 0.4 to 0.6) and granites (Eu/Eu* = 0.2 to 0.5). Whole rock εNd(T) values of the gabbroic intrusion (εNd(T) = +2.5 to +3.3) are similar to those of the enclaves (εNd(T) = +1.0 to 2.0) and granite (εNd(T) = +1.5 to +1.9) whereas the zircon εHf(T) values of the gabbro (εHf(T) = +8.1 ± 0.8) are indistinguishable from those of the granites (εHf(T) = +9.2 ± 1.0), suggesting that the parent magmas for all rock types originated from the same mantle source. Geochemical modeling indicates that the gabbros and granites can be generated by fractional crystallization of a common parental magma similar to high-Ti Emeishan flood basalt. The compositional jump from the gabbro to the enclaves is attributed to the crystallization of Fe-Ti oxide minerals. The results of this study and other studies suggest that the magmatic conditions (for example, pressure, composition, fO2), which led to the formation of at least three Fe-Ti oxide bearing gabbro-granitoid complexes, were relatively common during the development of the Emeishan large igneous province.

Proterozoic mantle lithosphere beneath the extended margin of the South China block: In situ Re-Os evidence

The Os isotope compositions of sulfides in mantle xenoliths from the Penghu Islands, Taiwan Strait, reveal the presence of Proterozoic subcontinental lithospheric mantle beneath the highly extended southeast margin of the South China block. Both T RD (Re depleted) model ages for individual sulfides and model ages estimated from the initial 187Os/188Os ratios of Re-Os mixing lines require that some volumes of the subcontinental lithospheric mantle formed prior to 2.3–1.9 Ga. Later events in the subcontinental lithospheric mantle may be recorded by T RD model ages of 1.5–1.2 Ga and ca. 0.9 Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those known in the crust of the mainland South China block. The sulfide Os isotope data show that Proterozoic lithosphere beneath the South China block has survived the extensive Mesozoic Yanshanian magmatism on the continental margin and has not been delaminated even during the severe lithospheric extension that led to the subsidence of the Taiwan Strait.

Hf isotope and REE compositions of zircon from jadeitite (Tone, Japan and north of the Motagua fault, Guatemala): implications on jadeitite genesis and possible protoliths

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solution-precipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf( t ) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of whole-sale metasomatic replacement at ~80 Ma from an igneous protolith of juvenile origin with an age of 136 ± 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 ± 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.

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 Miocene Galápagos ash layer record of Integrated Ocean Drilling Program Site Legs 334 and 344: Ocean-island explosive volcanism during plume-ridge interaction

Drilling at Integrated Ocean Drilling Program Site U1381 on the Cocos Ridge offshore Costa Rica recovered 67 primary Miocene (ca. 8 Ma to ca. 16.5 Ma) marine fallout ash layers. Geochemical, volcanological, and geological criteria link these ashes to Plinian eruptions that carried ash to at least 50–450 km from the Galapagos hotspot. These ash layers are the first documentation of highly explosive Miocene Galapagos hotspot volcanism. This volcanism is bimodal with two-thirds of the tephra layers generated by basaltic magmas (glass compositions <57 wt% SiO2) and one-third by rhyolitic magmas. The temporal distribution of the tephra layers, inferred from sediment accumulation rates calibrated by 40Ar/39Ar and biostratigraphic ages, reveals a distinct increase in eruption frequency and hence increased volcanic activity of the Galapagos hotspot after 14 Ma which we interpret in the context of dynamic interaction between the Galapagos plume and spreading ridge.

Mesozoic age of the uril formation of the Amur Group, Lesser Khingan terrane of the Central Asian foldbelt: results of U-Pb and Lu-Hf isotopic studies of detrital zircons

U-Pb (LA-ICPMS) geochronological studies established the minimum age of detrital zircons from metasedimentary rocks of the Uril Formation of the Amur Group of ∼240 Ma, which approximately corresponds to the lower age boundary of formation of their protoliths. The upper boundary of accumulation of sedimentary rocks of this formation is governed by the age of superimposed structural-metamorphic transformations (220–210 Ma). It follows that the age of protoliths of metasedimentary rocks of the Uril Formation is Triassic in contrast to the previously suggested Early Precambrian age. At the same time, previous estimations of the Nd model age of metasedimentary rocks of the Tulovchikha Formation of the Amur Group and intruding gabbroic rocks are 1.7 and 0.5 Ga, respectively. In other words, the age of this formation is 1.7–0.5 Ga. All of this indicates a combination of sedimentary and volcanic rocks of different ages in the section of the Amur Group. Judging from the Lu-Hf isotopic-geochemical studies of zircons, the major sources of protoliths for metasedimentary rocks of the Uril Formation are Neoproterozoic igneous rocks and also Early and Late Paleozoic and Early Mesozoic igneous rocks, the formation of which was related to the reworking of the Neo- and Mesoproterozoic continental crust.

Multiple mantle sources of the Early Permian Panjal Traps, Kashmir, India

The Early Permian Panjal Traps of northern India are the volcanic remnants of continental rifting that led to the formation of the Neotethys Ocean and the ribbon-like continent Cimmeria. The Traps are one of at least five major mafic eruptions of flood basalts during the Late Palaeozoic however their origin and petrogenesis are poorly constrained. Basalts from the Kashmir Valley were collected and analyzed for chemical and isotopic (Sr, Nd) compositions in order to characterize their mantle source and evaluate the petrogenetic processes related to opening of the Neotethys Ocean. Samples collected from the eastern side (Guryal Ravine, Pahalgam, PJ3) of the Kashmir Valley are chemically similar to mildly alkaline to tholeiitic, within-plate flood basalts. The TiO2 contents (TiO2 = 0.8 to 3.1 wt.%), La/YbN values (La/YbN = 1.8 to 6.1) and εNd(t) values (εNd(t) = −5.3 to +1.3) along with partial melt modeling indicates that the basalts were likely derived from a spinel peridotite source. In contrast, samples collected from the western side (PJ4) of the Kashmir Valley (Buta Pathri) are more primitive in composition and show evidence for clinopyroxene fractionation. The basalts from the western side of the Kashmir Valley have higher Mg# (Mg# = 60 to 78) values and εNd(t) values (εNd(t) = +0.3 to +4.3) suggesting they were derived by slightly higher amounts of partial melting and from a more depleted spinel peridotite source. The changing bulk composition of the basalts from ‘enriched OIB-like’ on the eastern side to ‘depleted MORB-like’ compositions on the western side is likely due to the changing nature of the Panjal rift from a nascent continental setting to one transitioning to a mature ocean basin. In comparison to Pangaean and post-Pangaean flood basalt provinces, the Panjal Traps are more chemically similar to the flood basalts from the post-Pangaean provinces that are associated with plate separation.