Greenschist-facies sub-ophiolitic metamorphic rocks of Andaman Islands,Burma–Java subduction complex

Sub-ophiolitic greenschist facies metamorphic rocks occur at the sole of ophiolite slices and as blocks in the mélange zone beneath the Andaman ophiolite. These are represented by metabasics as actinolite schist to actinolite–chlorite schist and metasediments as garnetiferous quartzo-feldspathic mica–chlorite schist and piemontite quartzite to piemontite bearing quartz–muscovite–chlorite schist to muscovite–quartz-chlorite schist. Actinolite occurs along the schistosity and also as porphyroblasts. Syn to post-tectonic garnet shows no compositional zoning and represent almandine–spessartine solid solution (Alm_44–47, Sps_23–27, Gros_13–17, Pyr_9–10). The metabasics are enriched in LILE and depleted in Zr and Y compared to N-MORB.The lithological features suggest that residual heat was the main heat source for greenschist-facies metamorphism. Top part of the subducting slab and overlying trench sediments were metamorphosed and dislocated by the close spaced thrusts in an accretionary prism setting. The field association indicates that metamorphism and the uplift of metamorphic rocks along with ophiolite slices were bracketed in between Cretaceous and Oligocene period. These processes were later than the Pre-Cretaceous emplacement of the ophiolites of Sumatra and Java.     

The ‘subduction initiation rule’: a key for linking ophiolites,intra-oceanic forearcs,and subduction initiation

We establish the ‘subduction initiation rule’ (SIR) which predicts that most ophiolites form during subduction initiation (SI) and that the diagnostic magmatic chemostratigraphic progression for SIR ophiolites is from less to more HFSE-depleted and LILE-enriched compositions. This chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during SI. The magmatic chemostratigraphic progression for the Izu–Bonin–Mariana (IBM) forearc and most Tethyan ophiolites is specifically from MORB-like to arc-like (volcanic arc basalts or VAB ± boninites or BON) because SI progressed until establishment of a mature subduction zone. MORB-like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of SI. The contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early SI, but continued melting results in a depleted, harzburgitic residue that is progressively metasomatized by fluids from the sinking slab; subsequent partial melting of this residue yields ‘typical’ SSZ-like lavas in the latter stages of SI. If SI is arrested early, e.g., as a result of collision, ‘MORB-only’ ophiolites might be expected. Consequently, MORB- and SSZ-only ophiolites may represent end-members of the SI ophiolite spectrum. The chemostratigraphic similarity of the Mariana forearc with that of ophiolites that follow the SIR intimates that a model linking such ophiolites, oceanic forearcs, and SI is globally applicable.     

The Loch Maree Group: Palaeoproterozoic subduction–accretion complex in the Lewisian of NW Scotland

The Lewisian complex of northwest (NW) Scotland has long been correlated with intercontinental Palaeoproterozoic belts of the North Atlantic region but uncertainty about the age and origin of the supracrustal rocks of the Loch Maree Group (LMG) and the apparent lack of subduction-related intrusive rocks have precluded interpretations of a similar tectonic setting for the Lewisian. We present integrated field, geochemical and geochronological data that resolve both issues and are consistent with an intercontinental setting. The LMG is made up of two components, one oceanic (plateau basalts or primitive arcs, plus associated abyssal sediments, ferruginous hydrothermal deposits, and platform carbonates) and the other continental (deltaic flysch, greywacke shale). The metasediments have geochemical characteristics that imply a source outside the Archaean gneisses of the Lewisian, an interpretation that agrees with the detrital zircon populations (from the Flowerdale schists) that have a significant 2.2–2.0-Ga component. The Ard gneiss, formerly regarded by some as a tectonic sliver of basement, is a strongly foliated granodiorite that occurs in sheets intrusive into the LMG, and has given a UPb crystallisation age of 1903±3 Ma, consistent with its syntectonic relationship with the major D1/D2 phase of Proterozoic deformation. The gneiss has a rather primitive geochemistry, which implies that it was not generated by melting of the local metasediments but was derived by partial melting of a more mafic source. The most likely model is that the LMG evolved as an accretionary complex, modern parallels of which can be found in the Shimanto belt in Japan, Rhodope in north Greece and Colombia and the Caribbean. The various elements of the complex became tectonically intermixed and subject to extreme deformation during accretion to the overriding Lewisian continent. Eventual relaxation and exhumation of the accretionary complex may have resulted in the generation of the Ard gneiss (possibly by melting of the underplated oceanic plateau) followed by collision with the continental crust of the lower plate. The younger D3 phase of the Palaeoproterozoic deformation sequence was coincident with the emplacement of the Tollie pegmatites at 1.7 Ga, c 200 m. years after the main collisional event, and may be related to a younger accretionary event (Labradorian?).     

The geochemistry of a dying continental arc: the Incapillo Caldera and Dome Complex of the southernmost Central Andean Volcanic Zone (~28°S)

The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central Volcanic Zone (~28°S), where the steeply dipping (~30°) segment of the subducting Nazca plate transitions into the Chilean “flatslab” to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3–1 Ma effusive phase characterized by ~40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive “caldera-forming” event at 0.51 Ma that produced the 14 km³ Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO₂ = 67–72 wt%; La/Yb = 37–56; Ba/La = 16–28; La/Ta = 30–50; ⁸⁷Sr/⁸⁶Sr = 0.70638–0.70669; ε _Nd = −4.2 to −4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor. The strong HREE depletion (Sm/Yb = 6–8) that distinguishes Incapillo magmas from most of the large ignimbrites of the Altiplano–Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (>40 km) in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated “adakite-like” dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone.     

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell pluton

We describe field occurrences of sapphirine-bearing granulites, charnockites and migmatites in the Gruf complex, Central Alps and present a new geological map and a structural analysis of the entire Gruf complex for the first time. We have carried out an accurate analysis of the relationships between granulite facies metamorphism, migmatisation and deformation within the complex, in relation to the intrusion of the Bergell pluton. Granulites and charnockites display fabrics different from those defined by the regional foliation and lineation, which are, typically for migmatites, disordered on the mesoscale. On a regional scale, strike variations are also related to the structural complexity of migmatites within which no major antiform could be identified. Irregular interfingering of sub-parallel leucosome veins and back-veining along the contact between the Gruf migmatites and the Bergell tonalite are evidence for contemporaneous emplacement and crystallisation at about 740 °C and 6.5–7.5 kbar in Oligocene times (ca 30 Ma). Metamorphic conditions in the charnockites and granulites (>920 °C for 8.5–9.5 kbar) largely exceed these regional metamorphic conditions and are dated at 282–260 Ma. We propose that the ascending Bergell pluton entrained the polymetamorphic, granulitic lower crust enclosed within the peripheral migmatitic Gruf complex.     

The Mammoth Peak sheeted complex,Tuolumne batholith,Sierra Nevada,California: a record of initial growth or late thermal contraction in a magma chamber?

The Mammoth Peak sheeted intrusive complex formed in the interior of a ~7–10 km deep magma chamber, specifically in the Half Dome granodiorite of the Tuolumne batholith, central Sierra Nevada, CA (USA). The sheets consist of fractionated melts with accumulated hornblende, biotite, magnetite, titanite, apatite, and zircon. The accumulation, especially of titanite, had a profound effect on minor and trace elements (Nb, Ta, Ti, REE, U, Th, P, Zr, Hf, etc.), increasing their contents up to five to six times. Our thermal–mechanical modeling using the finite element method shows that cooling-generated tensile stresses resulted in the inward propagation of two perpendicular sets of dilational cracks in the host granodiorite. We interpret the sheeted complex to have formed by a crack-seal mechanism in a high strength, crystal-rich mush, whereby outward younging pulses of fractionated magma were injected into these syn-magmatic cracks at the margin of an active magma chamber. Thermal–mechanical instabilities developed after the assembly of the sheeted complex, which was then overprinted by late ~NW–SE magmatic foliation. This case example provides a cautionary note regarding the interpretation that sheeted zones in large granitoid plutons imply a diking mechanism of growth because the sheeted/dike complexes in plutons (1) may display inverse growth directions from the growth of the overall intrusive sequence; (2) need not record initial chamber construction and instead may reflect late pulsing of magma within an already constructed magma chamber; (3) have an overprinting magmatic fabric indicating the continued presence of melt after construction of sheeted complexes and thus a prolonged thermal history as compared to dikes; and (4) because the scale of the observed sheeted complexes may be small (<1%) in comparison to large homogenous parts of plutons, in which there is no evidence for sheeting or diking. Thus, where extensive dike complexes in plutons are absent, such as in much of the Tuolumne batholith, the application of an incremental diking model to explain chamber construction is at best speculative.     

The emergence of an ‘evolutionary geomorphology’?

Earth surface processes and landforms are modified through the actions of many microorganisms, plants and animals. As organism-driven landform modifications are sometimes to the advantage of the organism, some of these landform features have become adaptive functional components of ecosystems, concurrently affecting and responding to ecological and evolutionary processes. These recent eco-evolutionary insights, focused on feedback among geomorphologic, ecological and evolutionary processes, are currently leading to the emergence of what has been called an ??evolutionary geomorphology??, with explicit consideration of feedbacks among the evolution of organisms, ecosystem structure and function and landform organization at the Earth surface. Here we provide an overview in the form of a commentary of this emerging sub-discipline in geosciences and ask whether the use of the term ??evolutionary geomorphology?? is appropriate or rather misleading.     

The early Cambrian Chahmir shale-hosted Zn–Pb deposit,Central Iran: an example of vent-proximal SEDEX mineralization

The Chahmir zinc–lead deposit (1.5 Mt @ 6 % Zn + 2 % Pb) in Central Iran is one among several sedimentary-exhalative Zn–Pb deposits in the Early Cambrian Zarigan–Chahmir basin (e.g., Koushk, Darreh-Dehu, and Zarigan). The deposit is hosted by carbonaceous, fine-grained black siltstones, and shales interlayered with volcaniclastic sandstone beds. It corresponds to the upper part of the Early Cambrian volcano-sedimentary sequence (ECVSS), which was deposited on the Posht-e-Badam Block during back-arc rifting of the continental margin of Central Iran. Based on crosscutting relationships, mineralogy, and texture of sulfide mineralization, four different facies can be distinguished: stockwork (feeder zone), massive ore, bedded ore, and distal facies (exhalites with barite). Silicification, carbonatization, sericitization, and chloritization are the main wall-rock alteration styles; alteration intensity increases toward the proximal feeder zone. Fluid inclusion microthermometry was carried out on quartz associated with sulfides of the massive ore. Homogenization temperatures are in the range of 170–226 °C, and salinity is around 9 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore facies suggests anoxic to locally suboxic event for the host basin. δ³⁴S_(V-CDT) values of pyrite, sphalerite, and galena range from +10.9 to +29.8?‰. The highest δ³⁴S values correspond to the bedded ore (+28.6 to +29.8?‰), and the lowest to the massive ore (+10.9 to +14.7?‰) and the feeder zone (+11.3 and +12.1?‰). The overall range of δ³⁴S is consistent with a sedimentary environment where sulfide sulfur was derived from two sources. One of them was corresponding to early ore-stage sulfides in bedded ore and distal facies, consistent with bacterial reduction from coeval seawater sulfate in a closed or semiclosed basin. However, the δ³⁴S values of late ore-stage sulfides, observed mainly in massive ore, interpreted as a hydrothermal sulfur component, leached from the lower part of the ECVSS. Sulfur isotopes, along with the sedimentological, textural, mineralogical, fluid inclusion, and geochemical characteristics of the Chahmir deposit are in agreement with a vent-proximal (Selwyn type) SEDEX ore deposit model.     

Whither the megathrust? Localization of large-scale subduction slip along the contact of a mélange

Long-lived subduction complexes, such as the Franciscan Complex of California, include tectonic contacts that represent exhumed megathrust horizons that collectively accommodated thousands of kilometres of slip. The chaotic nature of mélanges in subduction complexes has spawned proposals that these mélanges form as a result of megathrust displacement. Detailed field and petrographic relationships, however, show that most Franciscan mélanges with exotic blocks formed by submarine landsliding. Field relationships at El Cerrito Quarry in the eastern San Francisco Bay area suggest that subduction slip may have been accommodated between the blueschist facies metagreywacke of the Angel Island nappe above and the prehnite-pumpellyite facies metagreywacke of the Alcatraz nappe below. Although a 100–200 m-thick mélange zone separates the nappes, this mélange is a variably deformed, prehnite-pumpellyite facies sedimentary breccia and conglomerate deposited on the underlying coherent sandstone, so the mélange is part of the lower nappe. A 20–30 m-thick fault zone between the top of the mélange, and the base of the Angel Island nappe displays an inverted metamorphic gradient with jadeite-glaucophane-lawsonite above lawsonite-albite assemblages. This zone has a strong seaward (SW)-vergent shear fabric and hosts ultracataclasite and pseudotachylite. These relationships suggest that significant subduction megathrust displacement at depths of 15–30 km was accommodated within the 20–30 m-thick fault zone. Field studies elsewhere in the Franciscan Complex suggest similar localization of megathrust slip, with some examples lacking mélanges. The narrow megathrust zone at El Cerrito Quarry, its uniform sense-of-shear, and the localization of slip along the contact of, rather than within a mélange, contrast sharply with the predictions of numerical models for subduction channels.     

Late Holocene marine terraces of the Cartagena region,southern Caribbean: The product of neotectonism or a former high stand in sea-level?

The detailed stratigraphic survey and paleontological study (mollusks, corals, foraminifera and ostracods) of four low-level, ～3 m, marine terrace sections: Punta Canoas, Manzanillo del Mar, Playa de Oro, and Tierra Bomba Island, from the Cartagena region, southern Caribbean, supplemented with 22 radiocarbon dates, reveals that the northern terraces were deposited as parasequences in a clastic depositional system compared to the Tierra Bomba Island succession that was deposited in a carbonate depositional system between ～3600 and ～1700 cal yrs BP. Drier conditions and the southern location of the ITCZ at about 3 ka triggered stronger easterly Trades and more dynamic southwestward sediment drift fed by the Magdalena River mouth, thus promoting the formation of sand spits that ultimately isolated the Cienaga de Tesca coastal lagoon from the Caribbean Sea. Our estimates support the hypothesis that the present position of the terraces is the product of neotectonism rather than a higher 3 ka, sea-level. Upheaval of the terraces varies between ～3.8 mmyr^?1 at Punta Canoas and ～2.2 mmyr^?1 at Tierra Bomba to ～1.5 mmyr^?1 at Manzanillo del Mar and Playa de Oro terraces. Our study corroborates previous contentions on the role of mud diapirism and the dynamics of the Dique Fault as late Holocene upheaval mechanisms.     

Kinematic analysis of a transtensional fault system: The Atuel depocenter of the Neuquén basin,southern Central Andes,Argentina

The Atuel depocenter of the Neuquén basin originated as an Upper Triassic to Lower Jurassic rift system, later inverted during the Andean contractional deformation. In order to study the extensional architecture and the kinematic evolution of this depocenter, we collected a large amount of field and sub-surface data, consisting of slip data from outcrop-scale normal faults, thickness and facies distribution within the synrift deposits, and structural data from angular and progressive unconformities. The Atuel depocenter has a NNW trend, showing a bimodal distribution of NNW and WNW major faults (first and second order faults). On the other hand, from kinematic indicators measured on outcrop-scale faults (third and fourth order faults), we found a mean NE internal extension direction, which is oblique to the general trend of the sub-basin. Taking these particular characteristics into account, we interpreted the Atuel depocenter as an oblique rift system. We evaluated two mechanisms in order to explain the development of this transtensional system: 1) reactivation of upper-crustal NNW-oriented Paleozoic shear zones, and 2) oblique stretching of a previous NNW-oriented lithospheric weakness zone.     

The Nadun Cu–Au mineralization,central Tibet: Root of a high sulfidation epithermal deposit

A new high sulfidation epithermal Cu–Au occurrence (Nadun) has been discovered adjacent to the Cretaceous Duolong porphyry Cu–Au deposit within the Bangong–Nujiang metallogenic belt, central Tibet. The Nadun Cu–Au mineralization is hosted in a tectonic–hydrothermal breccia with advanced argillic alteration, which occurs above sandstone, associated with quartz–pyrite veins. The granodiorite porphyry with strong argillic alteration yields a zircon U–Pb age of 119.1 ± 1.3 Ma, whereas the weakly argillic granodiorite porphyry intruded into the breccia has a younger age of 116.1 ± 1.3 Ma. This indicates that Cu–Au epithermal mineralization likely occurred between ~ 116 Ma and ~ 119 Ma, consistent with the duration of magmatic–hydrothermal activity at Duolong (~ 115–118 Ma), and providing evidence that Nadun and Duolong were formed during the same event. Moreover, the Nadun and Duolong porphyries have similar Hf isotopic compositions (εHf(t) values ranging from − 8.8 to 8.1; mean = 5.0 ± 1.1, n = 32), likely indicating that the deposits are comagmatic. In addition, boiling assemblages in vapor-rich inclusions coexisting with brines occur in early stage quartz–pyrite veins, and likely record phase separation at a temperature of > 550–300 °C and pressure of 700–110 bars. Most liquid-rich fluid inclusions formed at the breccia stage show similar salinity (1.7–19.3 wt.% NaCl equiv) to vapor-rich inclusions from the underlying quartz–pyrite veins, likely indicating vapor contraction during cooling at elevated presssure. This suggests that quartz–pyrite veins may act as conduits for ore-forming fluid traveling from the porphyry to the epithermal hydrothermal system. O and H isotopic compositions (δ¹⁸O_fluid = 0.42–9.71‰ and δD = − 102 to − 66‰) suggest that ore-forming fluids are dominantly from a magmatic source with a minor addition of meteoric water at a later stage. The S and Fe isotope compositions of sulfides (δ³⁴S = − 5.9 to 0.5‰ and δ⁵⁷Fe = − 2.15 to 0.17‰) decrease from the quartz–pyrite vein to breccia ore, indicating that ore-forming fluids gradually become SO₄²-enriched and relatively oxidized. This body of evidence suggests that the Nadun Cu–Au mineralization may represent the root of a high sulfidation epithermal deposit.     

The Rwenzori Mountains,a landslide-prone region?

With its exceptionally steep topography, wet climate, and active faulting, landslides can be expected to occur in the Rwenzori Mountains. Whether or not this region is prone to landsliding and more generally whether global landslide inventories and hazard assessments are accurate in data-poor regions such as the East African highlands are thus far unclear. In order to address these questions, a first landslide inventory based on archive information is built for the Rwenzori Mountains. In total, 48 landslide and flash flood events, or combinations of these, are found. They caused 56 fatalities and considerable damage to road infrastructure, buildings, and cropland, and rendered over 14,000 persons homeless. These numbers indicate that the Rwenzori Mountains are landslide-prone and that the impact of these events is significant. Although not based on field investigations but on archive data from media reports and laymen accounts, our approach provides a useful complement to global inventories overlooking this region and increases our understanding of the phenomenon in the Rwenzori Mountains. Considering the severe impacts of landslides, the population growth and related anthropogenic interventions, and the likelihood of more intense rainfall conditions, there is an urgent need to invest in research on disaster risk reduction strategies in this region and other similar highland areas of Africa.     

The dynamics of big mantle wedge,magma factory,and metamorphic–metasomatic factory in subduction zones

The East Asian continental margin is underlain by stagnant slabs resulting from subduction of the Pacific plate from the east and the Philippine Sea plate from the south. We classify the upper mantle in this region into three major domains: (a) metasomatic–metamorphic factory (MMF), subduction zone magma factory (SZMF), and the ‘big mantle wedge’ (BMW). Whereas the convection pattern is anticlockwise in the MMF domain, it is predominantly clockwise in the SZMF and BMW, along a cross section from the south. Here we define the MMF as a small wedge corner which is driven by the subducting Pacific plate and dominated by H₂O-rich fluids derived by dehydration reactions, and enriched in large ion lithophile elements (LILE) which cause the metasomatism. The SZMF is a zone intermediate between MMF and BMW domains and constitutes the main region of continental crust production by partial melting through wedge counter-corner flow. Large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of SZMF. More fertile and high temperature peridotites are supplied from the entrance to this domain. The domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab. The BMW occupies the major portion of upper mantle in the western Pacific and convects largely with a clockwise sense removing the eastern trench oceanward. Sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading. We envisage that the heat source in BMW could be the accumulated TTG (tonalite–trondhjemite–granodiorite) crust on the bottom of the mantle transition zone. The ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra-oceanic arcs on the subducting Philippine Sea plate from the south, in addition to the sediment trapped subduction by the Pacific plate and Philippine Sea plate. The dynamics of MMF, SZMF and BMW domains are controlled by the angle of subduction; a wide zone of MMF in SW Japan is caused by shallow angle subduction of the Philippine Sea plate and the markedly small MMF domain in the Mariana trench is due to the high angle subduction of Pacific plate. The domains in NE Japan and Kyushu region are intermediate between these two. During the Tertiary, a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of Tethyan Asia, which may be related to the collision of India with Asia and the indentation. The volume of upper mantle under Asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of Asia, leading to the formation of a series of marginal basins. The western Pacific domain in general is characterized by double-sided subduction; from the east by the oldest Pacific plate and from the south by the oldest Indo-Australian plate. The old plates are hence hydrated extensively even in their central domains and therefore of low temperature. The cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle. Thus, a fluid dominated upper mantle in the western Pacific drives a number of microplates and promote the plate boundary processes.     

Alkali amphiboles from Late Archean gneiss of the Wuhe Group. Anhui, Central China——not an indication of high P/T metamorphism

在中国中部晚太古代五河群的碱性片麻岩中,曾报导有蓝色角闪石和硬玉的存在,并且认为它们是与安徽张八岭群同期的高压变质矿物。本文作者经过对岩石和矿物成分的详细研究,证实五河群中的碱性角闪石+锥辉石+斜长石+黑云母+石英±钠长石±赤铁矿等矿物组合是典型的角闪岩相变质矿物组合,这些碱性角闪石的核部是岩浆岩成因的镁钠铁闪石,其边部是角闪岩相变质成因的镁钠闪石,这些碱性闪石的Al_2O_3含量特别低,而FeO和Fe_2O_3含量很高,它们与张八岭群的青铝闪石在化学成分上差别很大。五河群片麻岩中的钠质辉石是很纯的锥辉石而没有硬玉组分。从含大量微斜长石的矿物组合、蓝色角闪石的矿物成分、低硬玉质的钠辉石和估算的温度和压力条件来看,我们认为五河群的片麻岩并不是高压蓝闪片岩相的变质产物。此外,以前关于五河群和中国中部其他变质岩中(如张八岭群)有关硬玉的报导中所发表的电子探针和化学分析资料与辉石的化学分子式不符,我们的岩石学研究也没有证实硬玉的存在。     

The freshwater lens of Benjamín Aceval,Chaco, Paraguay: a terrestrial analogue of an oceanic island lens

The occurrence of a freshwater lens in the Paraguayan Chaco, 900 km away from the ocean, is reported. It is located underneath sandstone hills, surrounded by lowlands with predominantly saline groundwater. Its geometry was delineated using geoelectrical and electromagnetic investigations. The unusual height of the fresh groundwater level can be attributed to the presence of a confining layer at depth. The lens receives its recharge exclusively from rainfall during the hot and humid summer months. It predominantly contains water predating the atmospheric atomic bomb tests, some of it probably up to a thousand or more years old. The water balance shows that extraction currently does not exceed recharge in normal years. However, the available volume of groundwater leaves little room for a further increase of extraction in the future. Recharge is augmented by return flow from thousands of latrines and cess pits, and this has lead to widespread contamination of the groundwater by faecal bacteria.