Geochemistry,zircon UPb ages and Hf isotopes of granites and pegmatites from Gubrunde region in the Eastern Nigeria Terrane: Insight into magmatism and tectonic evolution of the late Pan-African orogeny

This study reports a new dataset of whole-rock geochemistry, biotite chemistry, in situ zircon UPb geochronology and Hf isotope for a suite of granite and associated pegmatite samples from the Gubrunde region in the Eastern Nigeria Terrane (ENT), Nigeria. The Gubrunde granitic rocks are weakly ferroan, peraluminous and calc-alkalic to alkali-calcic in composition, and show I-type affinity. The zircon UPb geochronology gives an age of ~580 Ma for the rocks, although the presence of inherited zircons with early Pan-African ages of 696 ± 12, 647 ± 7 and 624–613 Ma are evident indicative of a complex history of their source rocks. The Gubrunde granite and the pegmatite yielded similar average Hf crustal model age T_DM2 of 1.9 ± 0.1 Ga and εHf(t) values ?6.2 ± 1.2, suggesting that they may have sourced from reworked old crustal rocks with minor contributions from the mantle. The granite and the pegmatite were likely to connect by fractional crystallization under low to moderate pressure (~2.2 to 3.0 kbar) and temperature (~717 °C), and low oxygen fugacity (<ΔNNO ?1.14). The ca. 580 Ma magmatism may have been triggered by delamination of the lithospheric mantle as a consequence of crustal thinning during waning stage of the Pan-African orogeny.     

安徽省东至县查册桥金矿地质特征及硫铅同位素示踪

东至县查册桥金矿位于扬子陆块北缘,矿床规模达中型。为了解成矿物质来源,对查册桥金矿矿石进行了硫、铅同位素分析。结果显示:δ(34S)变化范围为1.8×10~(-3)～18.1×10~(-3),平均值为11.09×10~(-3),较集中分布在10×10~(-3)～15×10~(-3)之间,分布宽泛;矿石硫化物矿物206 Pb/204 Pb值介于17.954～19.748之间,207 Pb/204 Pb值介于15.608～15.786之间,208 Pb/204 Pb值介于38.107～39.411之间,它们的μ值9.43～9.77;ω值32.63～38.83;w(Th)/w(U)值介于3.31～3.93,Δβ值18.45～30.06;Δγ值22.96～57.97。硫、铅同位素特征表明查册桥金矿床成矿物质主要来源于上地壳,其形成与陆内造山作用和岩浆作用密切相关。     

Identification of an underfilled foreland basin system in the Upper Devonian of the Central Pyrenees: implications for the Hercynian orogeny

A foreland basin succession has been identified in the Frasnian of the Central Pyrenees. This succession comprises a carbonate-dominated transgressive system which recorded the cratonward migration of the foreland basin subsidence, and siliciclastic depocenters which recorded the progression of the thrust-fold deformation. The foreland basin system has always been maintained in deep-marine environments, i.e., at an underfilled depositional state. It was associated with a thrust wedge which descended toward a deep-marine hinterland, i.e., with a type of orogenic wedge usually related to subduction zones. The Frasnian foreland basin system differs from the one known in the Carboniferous which evolved to overfilled depositional state and was associated with a thrust wedge rising toward a mountainous hinterland. Consequently, the Hercynian orogeny in the Pyrenees seems to result first, from a Frasnian thrusting controlled by a subduction zone located north of the Pyrenees, and second, from a Carboniferous thrusting controlled by the surrection of a frontal thrust belt in the Pyrenees. The association of underfilled foreland basin systems and hinterland-dipping thrust wedges, as exemplified in the Frasnian of the Pyrenees, can be interpreted as illustrative of the initial stages of thrust-wedge growth in deep-marine settings.     

Pampean Orogen: An Intraplate Component of Early Paleozoic Deformation?

After accretion of the Pampean continental fragment to the western Rio de la Plata craton margin (530 Ma), subsequent deformation, crustal anatexis and plutonism may have been intraplate responses to Brasiliano-PanAfrican collisional tectonism on the eastern margin during the amalgamation of Gondwana. Investigations of intraplate orogens such as the Tien Shan and the Ancestral Rocky Mountains, as well as of analogue and numerical models, permit discrimination of two early Paleozoic tectonomagmatic phases in the Sierras Pampeanas. The first involved marginal trough subduction and calc-alkaline magmatism, culminating in accretion of the Pampean terrane to the western craton edge; the second was characterized by crustal anatexis and peraluminous plutonism, penetrative deformation and high-angle reverse faulting resulting from continental collision on the eastern craton margin.Field observations from modern (Tien Shan) and ancient (Ancestral Rocky Mountains) intraplate chains, deep seismic and borehole data, radiometric and fission-track data constitute control for analogue and numerical models of intraplate deformation resulting from continental collision. Near-simultaneous continent-wide deformation, regularly spaced ranges/buckles, reverse-fault initiation at fold hinges of buckles, and doubling of crustal thickness are replicated in structural arrays formed in four-layer analogue models of lithospheric buckling. These data have significant implications for the ductile deformation, crustral thickening and post-subduction plutonism that spanned central South America in Late Cambrian time.     

Evidence for Holocene megafloods down the Tsangpo River gorge, southeastern Tibet

Lacustrine and alluvial terraces and sediments record the extent of at least two Holocene glacially dammed lakes immediately upstream of the Tsangpo River gorge at the eastern syntaxis of the Himalaya. The larger lake covered 2835 km², with a maximum depth of 680 m and contained an estimated 832 km³ of water; the smaller lake contained an estimated 80 km³ of water. Radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 ± 40 and 9870 ± 50 ¹⁴C yr B.P. for the higher set of lake terraces, and 1220 ± 40 and 1660 ± 40 ¹⁴C yr B.P. for sediments from the lower terraces. Catastrophic failure of the glacial dams that impounded the lakes would have released outburst floods down the gorge of the Tsangpo River with estimated peak discharges of up to 1 to 5 × 10⁶ m³ s⁻¹. The erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post-glacial megafloods down the Tsangpo River were likely among the most erosive events in recent Earth history.     

Tectonic and climatic control of the changes in the sedimentary record of the Karnali River section (Siwaliks of western Nepal)

Abstract A multidisciplinary study was conducted on the section of the Siwalik Group sediments, approximately 5000 m thick, exposed along the Karnali River. Analysis of facies, clay mineralogy and neodymium isotope compositions revealed significant changes in the sedimentary record, allowing discussion of their tectonic or climatic origin. Two major changes within the sedimentary fill were detected: the change from a meandering to a braided river system at ca 9.5 Ma and the change from a deep sandy braided to a shallow sandy braided river system at ca 6.5 Ma. The 9.5‐Ma change in fluvial style is contemporaneous with an abrupt increase of ?_Nd(0) values following a ?_Nd(0) minimum. This evolution indicates a change in source material and erosion of Lesser Himalayan rocks within the Karnali catchment basin between 13 and 10 Ma. The tectonic activity along the Ramgarh thrust caused this local exhumation. By changing the proximity and morphology of relief, the forward propagation of the basal detachment to the main boundary thrust was responsible for the high gradient and sediment load required for the development of the braided river system. The change from a deep sandy braided to a shallow sandy braided river system at approximately 6.5 Ma was contemporaneous with a change in clay mineralogy towards smectite‐/kaolinite‐dominant assemblages. As no source rock change and no burial effect are detected at that time, the change in clay mineralogy is interpreted as resulting from differences in environmental conditions. The facies analysis shows abruptly and frequently increasing discharges by 6.5 Ma, and could be linked to an increase in seasonality, induced by intensification of the monsoon climate. The major fluvial changes deciphered along the Karnali section have been recognized from central to western Nepal, although they are diachronous. The change in clay mineralogy towards smectite‐/kaolinite‐rich assemblages and the slight decrease of ?_Nd(0) have also been detected in the Bengal Fan sedimentary record, showing the extent and importance of the two major events recorded along the Karnali section.     

Caledonide development offshore–onshore Svalbard based on ocean bottom seismometer, conventional seismic, and potential field data

The western Barents Sea and the Svalbard archipelago share a common history of Caledonian basement formation and subsequent sedimentary deposition. Rock formations from the period are accessible to field study on Svalbard, but studies of the near offshore areas rely on seismic data and shallowdrilling. Offshore mapping is reliable down to the Permian sequence, but multichannel reflection seismic data do not give a coherent picture of older stratigraphy. A survey of 10 Ocean Bottom Seismometer profiles was collected around Svalbard in 1998. Results show a highly variable thickness of pre-Permian sedimentary strata, and a heterogeneous crystalline crust tied to candidates for continental sutures or major thrust zones. The data shown in this paper establish that the observed gravity in some parts of the platform can be directly related to velocity variations in the crystalline crust, but not necessarily to basement or Moho depth. The results from three new models are incorporated with a previously published profile, to produce depth-to-basement and -Moho maps south of Svalbard. There is a 14 km deep basement located approximately below the gently structured Upper Paleozoic Sørkapp Basin, bordered by a 7 km deep basement high to the west, and 7–9 km depths to the north. Continental Moho-depth range from 28 to 35 km, the thickest crust is found near the island of Hopen, and in a NNW trending narrow crustal root located between 19°E and 20°E, the latter is interpreted as a relic of westward dipping Caledonian continental collision or major thrusting. There is also a basement high on this trend. Across this zone, there is an eastward increase in the V_P, V_P/V_S ratio, and density, indicating a change towards a more mafic average crustal composition. The northward basement/Moho trend projects onto the Billefjorden Fault Zone (BFZ) on Spitsbergen. The eastern side of the BFZ correlates closely with coincident linear positive gravity and magnetic anomalies on western Ny Friesland, apparently originating from an antiform with high-grade metamorphic Caledonian terrane. A double linear magnetic anomaly appears on the BFZ trend south of Spitsbergen, sub-parallel to and located 10–50 km west of the crustal root. Based on this correlation, it is proposed that the suture or major thrust zone seen south of Svalbard correlates to the BFZ. The preservation of the relationship between the crustal suture, the crustal root, and upper mantle reflectivity, challenges the large-offset, post-collision sinistral transcurrent movement on the BFZ and other trends proposed in the literature. In particular, neither the wide-angle seismic data, nor conventional deep seismic reflection data south of Svalbard show clear signs of major lateral offsets, as seen in similar data around the British Isles.     

云南兰坪白秧坪铜钴多金属矿集区矿石中石英的40Ar-39Ar年龄

云南兰坪白秧坪铜钴多金属矿集区矿石中石英的⁴⁰Ar-³⁹Ar快中子活化年龄谱呈马鞍形,坪年龄为56.53±0.43Ma,最小视年龄为55.52±1.78Ma,等时线年龄为55.90±0.29Ma,三者在误差范围内一致(1σ)。⁴⁰Ar/³⁶Ar初始值为294.7±1.14,与尼尔值(295.5±5)十分接近,坪年龄和等时线年龄均可作为石英的形成时代。因此,55.90~56.53Ma(喜马拉雅早期)代表了矿床的成矿年龄。     

The devastating Muzaffarabad earthquake of 8 October 2005: New insights into Himalayan seismicity and tectonics

The recent earthquake of 8 October 2005 in the Muzaffarabad region in western Himalaya destroyed several parts of Pakistan and the north Indian state of Jammu and Kashmir. The earthquake of magnitude 7.6 claimed more than 80,000 lives, clearly exposing the poor standards of building construction — a major challenge facing the highly populated, earthquake prone, third world nations today. In this paper, we examine variations in the stress field, seismicity patterns, seismic source character, tectonic setting, plate motion velocities, GPS results, and the geodynamic factors relating to the geometry of the underlying subsurface structure and its role in generation of very large earthquakes. Focal mechanism solutions of the Muzaffarabad earthquake and its aftershocks are found to have steep dip angles comparable to the Indian intra-plate shield earthquakes rather than the typical Himalayan earthquakes that are characterized by shallow angle northward dips. A low p-value of 0.9 is obtained for this earthquake from the decay pattern of 110 aftershocks, which is comparable to that of the 1993 Latur earthquake in the Indian shield — the deadliest Stable Continental Region (SCR) earthquake till date. Inversion of focal mechanisms of the Harvard CMT catalogue indicates distinct stress patterns in the Muzaffarabad region, seemingly governed by an overturned Himalayan thrust belt configuration that envelops this region, adjoined by the Pamir and Hindukush regions. Recent developments in application of seismological tools like the receiver function technique have enabled accurate mapping of the dipping trends of the Moho and Lithosphere–Asthenosphere Boundary (LAB) of Indian lithosphere beneath southern Tibet. These have significantly improved our understanding of the collision process, the mechanism of Himalayan orogeny and uplift of the Tibetan plateau, besides providing vital constraints on the seismic hazard threat posed by the Himalaya. New ideas have also emerged through GPS, macroseismic investigations, paleoseismology and numerical modeling approaches. While many researchers suggest that the Himalayan front is already overdue for several 8.0 magnitude earthquakes, some opine that most of the front may not really be capable of sustaining the stress accumulation required for generation of great earthquakes. We propose that the occurrence of great earthquakes like those of 1897 in Shillong and 1950 in Assam have a strong correlation with their proximity to multiple plate junctions conducive for enormous stress build up, like the eastern Himalayan syntaxis comprising the junction of the India, Eurasia plates, and the Burma, Sunda micro-plates.     

Trace element fractionation during high-grade metamorphism and crustal melting—constraints from ion microprobe data of metapelitic, migmatitic and igneous garnets and implications for Sm–Nd garnet chronology

Rare earth element (REE) and other trace element (Y, Sr, Ti, Cr, V, Na) abundances in garnet from a garnet-bearing metapelite, a pelitic migmatite, a syn-tectonic granite and a post-tectonic leucogranite were measured by secondary ion mass spectrometry (SIMS) in order to identify the effective variables on the trace element distribution between garnet and the host rock. Garnet from the garnet-bearing metapelite, the pelitic migmatite and the syn-tectonic granite is zoned with respect to REE. The cores are enriched by a factor of 2–3 relative to the rims. For the garnets from the garnet-bearing metapelite equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for Ti, V, Cr and REE from core to rim. These features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open-system behaviour in these rocks at a time when the garnet, originally nucleated in the metamorphic environment was incorporated into the melt. The garnet from the syn-tectonic granite shows stepwise decreasing concentrations in REE from core to rim: a REE-rich core can be distinguished from a broad REE-depleted rim. Notably, from core to rim an inflection of the Yb / Er and Yb / Dy ratios is visible. Whereas the decrease of HREE abundance in the core region of the garnet from the syn-tectonic granite may arise from equilibrium partitioning during garnet growth, the inflection can be interpreted as a result of partial melting. Garnet cores with high Yb / Er and Yb / Dy >  1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower Yb / Er and Yb / Dy <  1 crystallized in the presence of a melt. Garnet from the leucogranite has lower REE abundances and is considered to be of igneous origin. In contrast to garnet from the other samples, its core has low trace element abundances, whereas its rim is significantly enriched in REE but depleted in Ti. These features suggest that only the outermost rim was in equilibrium with the melt. For this garnet, liquid diffusion controlled partitioning is more likely to explain the extreme trace element variation. An evaluation of Sm and Nd concentrations in garnet and a comparison of Sm–Nd and U–Pb garnet ages and U–Pb monazite ages form the terrane indicate that the observed LREE systematics in the different garnet species are a primary feature and are not homogenized by volume diffusion during high grade amphibolite facies conditions.