Sun-Lin Chung

The Amount of Recycled Crust in Sources of Mantle-Derived Melts

One proposed strategy for controlling the transmission of insect-borne pathogens uses a drive mechanism to ensure the rapid spread of transgenes conferring disease refractoriness throughout wild populations. Here, we report the creation of maternal-effect selfish genetic elements in Drosophila that drive population replacement and are resistant to recombination-mediated dissociation of drive and disease refractoriness functions. These selfish elements use microRNA-mediated silencing of a maternally expressed gene essential for embryogenesis, which is coupled with early zygotic expression of a rescuing transgene.The phosphoinositide phosphatase PTEN is mutated in many human cancers. Although the role of PTEN has been studied extensively, the relative contributions of its numerous potential downstream effectors to deregulated growth and tumorigenesis remain uncertain. We provide genetic evidence in Drosophila melanogaster for the paramount importance of the protein kinase Akt [also called protein kinase B (PKB)] in mediating the effects of increased phosphatidylinositol 3,4,5-trisphosphate (PIP3) concentrations that are caused by the loss of PTEN function. A mutation in the pleckstrin homology (PH) domain of Akt that reduces its affinity for PIP3 sufficed to rescue the lethality of flies devoid of PTEN activity. Thus, Akt appears to be the only critical target activated by increased PIP3 concentrations in Drosophila.Using genomic and mass spectrometry-based proteomic methods, we evaluated gene expression, identified key activities, and examined partitioning of metabolic functions in a natural acid mine drainage (AMD) microbial biofilm community. We detected 2033 proteins from the five most abundant species in the biofilm, including 48% of the predicted proteins from the dominant biofilm organism, Leptospirillum group II. Proteins involved in protein refolding and response to oxidative stress appeared to be highly expressed, which suggests that damage to biomolecules is a key challenge for survival. We validated and estimated the relative abundance and cellular localization of 357 unique and 215 conserved novel proteins and determined that one abundant novel protein is a cytochrome central to iron oxidation and AMD formation.

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.

Petrologic and geochemical constraints on the petrogenesis of Permian-Triassic Emeishan flood basalts in southwestern China

Abstract The Emeishan flood basalt is a large igneous province erupted during the Permian–Triassic period in southwestern China. Based on petrographic, major and trace element, and Sr–Nd isotope data, the Emeishan basalts can be classified into two major magma types. These are: (1) a low-Ti (LT) type that exhibits low Ti/Y ( 500). The HT lavas can be further divided into three subtypes. HT1 lavas exhibit significantly high TiO2 (3.65–4.7%), Fe2O3* (12.7–16.4%), Nb/La (0.75–1.1), coupled with higher eNd(t) (1.1–4.8) and lower SiO2 (45–51%); HT2 lavas are compositionally similar to the HT1 lavas but show conspicuous depletion in U and Th. The HT3 type has higher Mg# (0.51–0.61) than the HT1 and HT2 lavas. It differs from the LT type in having higher TiO2 (∼3%) at comparable Mg#. Elemental and isotopic data suggest that the chemical variations of the LT and HT lavas cannot be explained by crystallization from a common parental magma. Instead, they may originate from different mantle sources under various melting conditions and underwent distinct differentiation and contamination processes. REE inversion calculations indicate that the HT magmas were generated by low degrees of partial melting (1.5%) of a mantle source that has eNd(t) of ∼+5 and 87Sr/86Sr(t) of ∼0.704 within the garnet stability field. These magmas were then subjected to shallow level gabbroic fractionation, which led to larger chemical variations. In contrast, parental magmas of the LT type were generated by higher degree of partial melting (16%) of a distinct mantle source (eNd(t)≈+2, 87Sr/86Sr(t)≈0.705) around the spinel–garnet transition zone. The chemical evolution of the LT lavas is controlled by an olivine (ol)+clinopyroxene (cpx) fractionation. The Emeishan flood basalts may result from a starting mantle plume. The petrogenesis of both the LT and HT magmas was further complicated by contamination of upper crust and lithospheric mantle. While the HT1 lavas have experienced an AFC style of contamination in the upper crust, the HT2 lavas that mark with U–Th depletions may result from additional interaction with melts derived from a gabbroic layer near the crust–mantle boundary. In contrast, a temperature-controlled style of contamination was associated with the LT lavas. Our data show that both temporal and spatial geochemical variations exist in the Emeishan flood basalt province. The occurrence of thick LT lavas in the western part of the province may record the main episode of the flood basalt emplacement. In contrast, the less abundant overlying HT basalts may imply a waning activity of the plume. In fact, the HT basalts are the dominant magma type in the periphery of the province. The lower degrees of mantle melting of the HT lavas may be a result of relatively thicker lithosphere and lower geotherm.

Plume-lithosphere interaction in generation of the Emeishan flood basalts at the Permian-Triassic boundary

The Emeishan flood volcanism that erupted at Permian-Triassic boundary time produced a large igneous province of at least 2.5 X 10 5 km 2 in the western margin of the Yangtze craton, southwestern China. The volcanic successions, suggested to have resulted from a starting mantle plume, comprise thick piles of basaltic flows and subordinate picrites and pyroclastics. The picrites, which have high magnesian contents (MgO ≊ 20–16 wt%), variable degrees of light rare earth element enrichment [(Ce/Yb) N ≊ 4–25] and heterogeneous isotope ratios [ϵ Nd ≊ (T) +4 to −4], are proposed to have been generated by mixing between the dominant plume-derived magmas and small amounts of lamproitic liquids from the continental lithospheric mantle.

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.

Zircon U-Pb and Hf isotope constraints on the Mesozoic tectonics and crustal evolution of southern Tibet

The first in situ Hf and U-Pb isotope analyses of zircon separates from Mesozoic granites in southern Tibet identify a significant, previously unknown stage of magmatism. Igneous zircons (n = 34) from a granite within the Gangdese batholith show a weighted mean 206 Pb/ 238 U age of 188.1 ± 1.4 Ma and e Hf (T) (the parts in 10 4 deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values between +10.4 and +16.8, suggesting predominantly Early Jurassic intrusive activity with a juvenile mantle contribution. Of 40 inherited zircons from two Cretaceous S-type granites in the northern magmatic belt, 23 delineate a slightly older 206 Pb/ 238 U age cluster between 188 and 210 Ma. These zircons have e Hf (T) values from −3.9 to −13.7, yielding crustal Hf model ages from ca. 1.4 to 2.1 Ga, suggesting a major episode of crustal growth in Proterozoic time and remelting of this crust in Early Jurassic time. Combining these with literature data, we interpret the Jurassic Gangdese magmatism as an early product of the Neo-Tethyan subduction that played a long-lasting role in the tectonic evolution of southern Tibet prior to the India-Asia collision.

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.

Geologic, geochemical, and geophysical consequences of plume involvement in the Emeishan flood-basalt province

Prevolcanic kilometer-scale lithospheric doming in the Emeishan large igneous province, southwest China, allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography, geology, geochemistry, and geophysics of the region. Systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments, the extent of erosion, thickness, and chemistry of volcanic rocks, and the crust-mantle structure. These features, which are best explained by a mantle plume, may be used to track older plume sites in the geologic record.

Geochemical and Sr-Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin

The Okinawa Trough is an incipient intracontinental back arc basin that has developed behind the Ryukyu arc-trench system. To explore its magmatic evolution and the nature of the mantle source, we present major and trace element and Sr-Nd isotopic data for mafic volcanic rocks from the Middle and Southern Okinawa Trough and the Central Ryukyu Arc. Two episodes of activity formed the latter: older (∼21–13 Ma) and younger (∼6–4 Ma). Although Quaternary basalts from the Middle Okinawa Trough (MOT) have major element compositions comparable to basalts from intra-oceanic back arc basins, they are characterized by relative enrichments of large ion lithophile elements and Pb and by depletions of high field strength elements analogous to those observed in Ryukyu Arc volcanics. Two components have been involved in the generation of MOT basalt, which we identify as an E-MORB type (or oceanic island basalt type) upwelling asthenospheric mantle and a “subduction component.” Quaternary basalts from the Southern Okinawa Trough (SOT) have uniform Nd but heterogeneous Sr isotopic ratios and incompatible trace element compositions. This may be ascribed to more complicated tectonic and magmatic processes in the SOT compared with the MOT, such as oblique subduction of the Philippine Sea Plate and interaction with postcollisional extension in the northern Taiwan orogenic belt. Integrating geological information available from nearby regions, we emphasize that the SOT is an “atypical” back arc basin because its development essentially occurred synchronous with or even prior to development of the arc-trench system.

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.