Explore the vast range of titles available.

The Singhbhum craton is one of five Archean cratons constituting the Indian subcontinent. It consists of four major lithotectonic units with broadly defined ages from Eoarchean to Neoarchean: the Older Metamorphic Group (3.7-3.2 Ga), Older Metamorphic Tonalite Gneisses (3.8-3.1 Ga), Singhbhum Granite (3.5-3.0 Ga), and Iron Ore Group (3.51-2.55 Ga). In this study, 270 zircons were separated from modern sediment of the Baitarani River, which is wholly contained within the craton. Zircons were analyzed with laser ablation ICP-MS for their U-Pb systematics, >50% were less than 5% discordant. Three primary age groupings account for ∼98% of analyses: 3.62-3.55 Ga (5%), 3.50-3.22 Ga (87%), and 3.10-3.06 Ga (6%). The preponderance of 3.50-3.22 Ga zircons is consistent with the local basement that includes a 3.47 Ga tonalite gneiss enclave within a 3.35-3.30 Ga outcrop of the Singhbhum Granite near Keonjhar. Lu-Hf systematics of zircons yielded 67% with positive initial εHf, scattered above and below the mantle growth curve and 33% with negative initial εHf, indicating contributions from both depleted mantle and older crustal sources. Single-stage model ages range from 4.29 to 3.10 Ga. Of note is a single zircon with a 207Pb/206Pb age of 4015±9 Ma (1.3% discordant), which is the first Hadean zircon documented from any of the Indian cratons. This grain yielded an initial εHf of -5.30, which indicates an episode of Hadean felsic crust formation in the Singhbhum craton comparable to that proposed for the Jack Hills of the Yilgarn craton (Australia).

High-silica rhyolites and granites (>75 wt% SiO2, anhydrous basis) are common features of the crust as part of both the volcanic and the plutonic records. While low crystallization pressure (<250 MPa) is typically inferred, it has been suggested that they form via polybaric evolution, with initial crystallization at relatively high pressures (>500 MPa). We use glass compositions derived from the EarthChem portal, selected natural examples from the literature, and rhyolite-MELTS calculations to show that the phase relations in the quartz-albite-orthoclase ternary dictate the silica content of silicic melts. In particular, we show that silica content of melts increases with decreasing pressure as a result of the displacement of the quartz-sanidine cotectic toward the Qz apex with decreasing pressure. It follows from our analysis that (1) the crust is expected to be stratified in terms of the silica content of residing melts; (2) high-silica glasses form at low pressure, requiring shallow-level crystallization, and preclude a polybaric evolution for many systems (e.g., Bishop Tuff); (3) the existence of high-silica pumice requires fractionation (or melting) at low pressure, showing that high-silica rhyolites are intrinsic to the shallow crust; and (4) low-pressure cumulates (or melting residues) must exist in the shallow crust, weighing in favor of the cumulatic nature of many granitoids found in plutons.

We report a newly determined age (1647±18 Ma) for a rare magmatic zircon extracted from a very fine-grained felsic ash bed in the type area of the Porcellanite Formation, Vindhyan Supergroup, India, and chemical compositions of two samples from this ash bed. The new results, in the context of published literature (1) firmly establish the age of the Porcellanite Formation between ∼1640 and 1630 Ma and (2) suggest that Plinian eruptions took place from isolated vents along a 300-km crustal fracture.

Multiple lines of evidence point to one or more moderately nearby supernovae, with the strongest signal at ∼2.6 Ma. We build on previous work to argue for the likelihood of cosmic ray ionization of the atmosphere and electron cascades leading to more frequent lightning and therefore an increase in nitrate deposition and wildfires. The potential exists for a large increase in the prehuman nitrate flux onto the surface, which has previously been argued to lead to CO2 drawdown and cooling of the climate. Evidence for increased wildfires exists in an increase in soot and carbon deposits over the relevant period. The wildfires would have contributed to the transition from forest to savanna in northeast Africa, long argued to have been a factor in the evolution of hominin bipedalism.

We describe a previously unrecognized major dextral strike-slip fault system in the South Carpathians, hereafter referred to as the Transcarpathian fault system. The master fault has been active since the mid-Cretaceous and has a total offset of ∼150 km, of which only <35 km are post-Oligocene. The fault acted as a subduction-transform edge propagator (STEP) fault during the mid-Cretaceous subduction of the Ceahlau-Severin ocean system and separated an area to the north where the subduction system was accretionary (the East Carpathians) from an area to the south (the western half of the South Carpathians) where the subduction system was erosive. In the South Carpathians, the oceanic basin closed during the mid-Cretaceous after commencement of higher convergence rates and subduction erosion of the trench, leading to tectonic underplating and continental collision between the Dacia and Moesian microplates. The results bolster the idea that STEP-type strike-slip faults are critical in the development of highly curved orogens and that accretionary versus erosional trench segments lead to very different structural configurations along the same subduction/collisional system.

We report U-Pb zircon ages and in situ δ18O values for crystals of zircon, rutile, and corundum erupted in the course of a March 2009 phreatomagmatic explosion of Koryaksky volcano, Eastern Volcanic Front, Kamchatka, Russia. Zircon crystals display a wide spectrum of crystallization ages ranging from Cenozoic to Archean, including single grains and grain-age clusters, ca. 1.7, 11, 37–56, 89–99, 152, 1461, 1946, 2584–2734, and 3314 Ma. The older end of this spectrum represents the oldest known zircon ages from Kamchatka. Oxygen isotopic values span from normal to low δ18O in zircon (3.3‰ to 5.5‰), corundum (4.3‰ to 5.5‰), and rutile (2‰ to −2.8‰). As Koryaksky volcano is built over Cretaceous-Eocene and younger crust, the presence of older zircon crystals requires that these accessory minerals with their diverse ages and δ18O values were derived from heterogeneous detrital components in sediment that contaminated the magma shortly before or during the phreatomagmatic eruption. We also present a compilation of published U-Pb zircon ages in Kamchatka for detrital and metamorphic grains for comparison purposes. We propose that sediment was delivered from the metamorphic Ganal and Sredinny Massifs along the Petropavlovsk fracture zone via a paleodrainage system whose remnants are now covered by the modern edifices of the Eastern Volcanic Front volcanoes. The termination of this paleodrainage system connecting to the modern north-flowing Kamchatka River postdates 1.7 Ma, on the basis of the age of the youngest detrital zircon present.

Dehydration (fluid-absent) melting of garnetiferous amphibolite is now widely believed to have given rise to the tonalite-trondhjemite-granodiorite (TTG) suite of plutonic rocks that is the major component of all large Precambrian Shield terranes. The presence of garnet as a residual mineral of melting is implicated by geochemistry and for efficient gravitational segregation and disposal of the ultramafic residuum. The great volume of the ancient TTG rocks suggests that early large-scale continental accretion involved a conveyor belt process that delivered hydrated oceanic basalts to depths in the earth where garnet amphibolite is stable at the melting temperature. This process became active at some time during secular cooling of the earth. Experimental definition of the maximum (hence, oldest) geothermal gradients that could have sustained dehydration melting of garnet amphibolite has been hampered by the difficulty of achieving chemical equilibrium in unfluxed (fluid-absent) silicate systems at the low temperatures (∼800°C) where melting first occurs. An alternative approach, termed “geo-experimental,” attempts to define the PT stability field of garnet amphibolite on the basis of quantitative geothermometry-geobarometry of actual mafic rocks bearing the assemblage aluminous hornblende-almandine-rich garnet-plagioclase-quartz. The results show that garnet is stable to much lower pressures than previously thought. The garnet-in line has a positive dP/dT slope in both subsolidus and melt-present fields. A subduction temperature gradient as high as 36°C/km could have accompanied melting of garnet amphibolite and generation of TTG magmas in the earlier Archean. The near parallelism of the garnet-in line and geothermal curves could have acted like an on switch during cooling of the earth, resulting in a surge of felsic continent, with accumulation of nearly the present volume before the end of the Archean.

A sample from the top of the Banxi Group (early Neoproterozoic) in Hunan Province (South China) was dated by the SHRIMP zircon U-Pb method. The resulting age of 725±10 Ma probably solves the debate between the synglacial and the preglacial correlation of the Liantuo Formation and indicates that the Liantuo Formation most likely correlates with the preglacial Banxi Group. Since the Banxi (Danzhou) Group is upwardly transitional to the glacial Jiangkou Group, the age becomes a new maximum age constraint on the onset of the Jiangkou glaciation, which therefore most likely correlates with the Sturtian glaciation.

This study uses detrital zircon U-Pb geochronology from shallow-water carbonates of the Bighorn Dolomite in Wyoming, USA, to provide robust evidence for long-distance eolian sediment transport during the Ordovician. The Bighorn Dolomite was deposited in a shallow-water carbonate platform that developed approximately 10° south of the Ordovician paleoequator on the western edge of Laurentia. The ages and textures of detrital zircons from the Bighorn indicate that the grains were transported by winds through saltation and suspension from the paleo east where rocks of the Paleoproterozoic Trans-Hudson orogenic belt were exposed in present-day Manitoba and Saskatchewan. Our interpretation of long-distance eolian transport is consistent with the paleogeography of Laurentia and expected prevailing wind directions and draws on modern analogs where Saharan sediment is transported by trade winds for distances of more than 3000 km.

In the absence of vegetation, wind systems would have been very efficient in reworking terrestrial sediments. As a consequence, the Precambrian sedimentary succession should theoretically be replete with aeolian deposits; however, Precambrian aeolianites are rare. Aeolian reworking of Precambrian fluvial sediments has been described by a number of authors. Earlier researchers have reported aeolian reworking of fluvial quartz-rich sandstone unit of the Beasley River Quartzite Member of the Lower Wyloo Group (LWG). The Nummana Member is the topmost lithostratigraphic unit of the LWG. Earlier researchers interpreted the Nummana Member as a shallow marine deposit. In this note, published petrographical data from the Nummana Member are reviewed, and new field, as well as petrographical, evidence is reported. Earlier researchers did not adequately explain the origin of rounded to well-rounded grains in the Nummana sandstone. Rounded to well-rounded quartz grains, in combination with pinstripe lamination, adhesion features, and dunes with high-angle (up to 31°) foresets, confirm the aeolian origin of the Nummana Member. The occurrence of subaerial basalts with continental tholeiite affinity (the Cheela Springs Basalt) on top of the Nummana Member is consistent with its nonmarine interpretation. The nearshore to terrestrial LWG succession, including the Nummana aeolianite, indicates regression and consequent emergence of the depositional surface and high continental freeboard during the early Paleoproterozoic in Western Australia.