Granitic Crust Research Articles

The formation of lithium-rich pegmatites through multi-stage melting

Abstract Lithium-cesium-tantalum–type pegmatites (the primary source of lithium) crystallize from highly evolved, volatile felsic melts that incorporated crustal material in their source. Pegmatites are classically thought to form either from extreme fractionation of a parental granite body or via low-degree partial melting of a metamorphic rock (anatectic origin). However, the processes that lead to the formation of economic lithium pegmatite deposits remain enigmatic, because precipitation of lithium ore minerals requires melt lithium concentrations in excess of 5000 ppm—~500 times upper crustal abundances. We use petrological modeling to quantify lithium enrichment in an anatectic-origin scenario and show that it is primarily driven by the relative stability of residual biotite and muscovite at medium to high pressures (~8 kbar), and biotite and cordierite at low pressures (~3 kbar). We show anatexis of an average lithium-enriched metasedimentary source cannot sufficiently elevate the lithium content of the ensuing melt to form economic deposits; however, if this first-generation melt—now crystallized as granitic crust—is re-melted, the second-generation melt will be sufficiently concentrated in lithium to crystallize lithium ore minerals. We propose a petrogenetic model for anatectic-origin lithium pegmatites, in which a region experiences at least two stages of partial melting, ultimately generating lithium-rich melts without invoking extensive fractional crystallization. This mechanism can both account for the occurrence of unzoned lithium pegmatites and explain why economic pegmatites in many terranes are younger than their inferred source granites.

Roles and perspectives of A- and I-type magmas in rare earth element and gold mineralization

A- and I-type magmas are commonly associated with rare earth element (REE) and gold (Au) mineralization. However, the behavior and solubility of the REEs and gold within the A- and I-type magmas and their synchronous magmatism remain unclear. This study focused on the synchronous Early Cretaceous A- and I-type magmatism that formed alkaline and granodioritic complexes within the Luxi and Jiaobei terranes of the central-eastern North China Block, terranes that contain the third largest known REE deposit in China and the third largest gold province in the world. Our integrated whole-rock geochemical and Nd isotopic data and zircon Hf-O isotopic data provide new insights into the petrogenesis of these contemporaneous A- and I-type magmas and their role in the generation of the REE and gold mineralization in this area. The alkaline magmas that formed the intrusions in the Luxi terrane had A- and I-type affinities, were oxidized, were fluorine-, sulfur-, and copper-rich, were H2O-poor, and were generated by low-pressure partial melting of a source region containing enriched lithospheric mantle material and minor amounts of Paleoproterozoic A-type granitic crust. In comparison, the granodioritic magmas within the Jiaobei terrane had I-type affinities, were relatively reduced, were sulfur-rich, contained low concentrations of Cu, were hydrous, and were formed by high-pressure partial melting of a source region containing enriched mantle material and large volumes of Archean to Paleoproterozoic crustal material. The A- and I-type magmas within the Luxi terrane also have higher high field strength element (HFSE; e.g., Th, Nb, Ta) and total light rare earth element (ΣLREE) concentrations as well as higher ΣLREE/total heavy rare earth element (ΣHREE) ratios than the I-type magmas enriched in large ion lithophile elements (LILEs; e.g., Ba, Sr) in the Jiaobei terrane. These characteristics suggest that the diversity of REE and gold mineralization was the result of interaction between the lithospheric mantle and hybrid sediment-derived melts and aqueous fluids under rutile-bearing eclogite-facies (>2.0 GPa) conditions within the Luxi terrane and with aqueous fluids under amphibolite- to eclogite-facies (>1.5 GPa) conditions within the Jiaobei terrane. Our study strongly suggests that the interaction between the lithospheric mantle and sediment-derived melts, the presence of coexisting REE- and fluorine-rich magmas, and low-pressure partial melting were all crucial factors in the formation of A-type intrusion-related REE deposits, whereas high-pressure partial melting and contemporaneous sulfur and chalcophile element enrichment were critical processes in the formation of I-type intrusions prospective for gold mineralization.

Basaltic thermal and material inputs in rhyolite petrogenesis: the Chhapariyali rhyolite dyke, Saurashtra, Deccan Traps

The Saurashtra peninsula in the northwestern Deccan Traps continental flood basalt province, India, contains notable concentrations of rhyolitic rocks. The Chhapariyali rhyolite dyke, part of the compositionally diverse Southeastern Saurashtra dyke swarm, intrudes basaltic lava flows. It shows vitrophyric portions, basaltic magma enclaves, gabbroic mineral assemblages (plagioclase + clinopyroxene ± Fe-Ti oxide aggregates), spectacular quench textures, and intense spherulitisation. Thermobarometric calculations on equilibrium mineral–whole-rock (or glass) pairs indicate clinopyroxene crystallisation at 1120–890 °C overlapping with plagioclase crystallisation at 948–932 °C, and a pressure range of 3.6–0.1 kbar, indicating crystallisation during magma storage or ascent in the upper crust. Petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data suggest advanced fractional crystallisation of a mafic magma with considerable assimilation of ancient granitic crust, or anatexis of such crust, due to heating by a basaltic magma chamber. In either scenario, abundant gabbroic cumulates were left by the crystallising basaltic magmas. A new basalt magma recharging the chamber entrained the gabbroic crystal mush and formed enclaves by mingling with the rhyolite magma stored above. Continued basaltic recharge pushed the enclave-bearing, gabbroic cargo-laden rhyolite magma out of the chamber and into the basaltic lava flows via a dyke. Strong supercooling within cold basaltic rock, possibly aided by meteoric water ingress, led to the development of spectacular quench textures in the crystallising rhyolite, whereas extensive low-temperature alteration produced intense spherulitisation. This case study of the Chhapariyali dyke underscores significant thermal and material inputs from basalt in rhyolite petrogenesis.

Ultrahigh-Pressure Mineral Inclusions in a Crustal Granite: Evidence for a Novel Transcrustal Transport Mechanism

Spherical crystals in minerals from prismatine-bearing rock from Waldheim, including ultrahigh-pressure (UHP) minerals such as stishovite and coesite, were previously described in uncommon crustal environments. To determine if this was an outlier phenomenon, we searched for equivalent inclusions in other rocks, which we indeed discovered in a Variscan tin-bearing granite sensu stricto from the Erzgebirge/Germany. The identification of more examples of this phenomenon implies a novel, very rapid transcrustal transport mechanism, which, however, is not unique. We demonstrate the unusual occurrence of UHP minerals (moissanite, diamond, lonsdaleite, stishovite, coesite, kumdykolite, and cristobalite-II) in topaz the investigated granitic samples, which reflects the direct interaction of mantle and crust via supercritical fluids or extremely volatile-rich melts. Mostly, the UHP minerals we recognized occur as tiny inclusions in moissanite. The trapping by this mineral prevents a fast reaction in an exogenous environment.

Depth wide distribution and metabolic potential of chemolithoautotrophic microorganisms reactivated from deep continental granitic crust underneath the Deccan Traps at Koyna, India.

Characterization of inorganic carbon (C) utilizing microorganisms from deep crystalline rocks is of major scientific interest owing to their crucial role in global carbon and other elemental cycles. In this study we investigate the microbial populations from the deep [up to 2,908 meters below surface (mbs)] granitic rocks within the Koyna seismogenic zone, reactivated (enriched) under anaerobic, high temperature (50°C), chemolithoautotrophic conditions. Subsurface rock samples from six different depths (1,679-2,908 mbs) are incubated (180 days) with CO2 (+H2) or HCO3 - as the sole C source. Estimation of total protein, ATP, utilization of NO3 - and SO4 2- and 16S rRNA gene qPCR suggests considerable microbial growth within the chemolithotrophic conditions. We note a better response of rock hosted community towards CO2 (+H2) over HCO3 -. 16S rRNA gene amplicon sequencing shows a depth-wide distribution of diverse chemolithotrophic (and a few fermentative) Bacteria and Archaea. Comamonas, Burkholderia-Caballeronia-Paraburkholderia, Ralstonia, Klebsiella, unclassified Burkholderiaceae and Enterobacteriaceae are reactivated as dominant organisms from the enrichments of the deeper rocks (2335-2,908 mbs) with both CO2 and HCO3 -. For the rock samples from shallower depths, organisms of varied taxa are enriched under CO2 (+H2) and HCO3 -. Pseudomonas, Rhodanobacter, Methyloversatilis, and Thaumarchaeota are major CO2 (+H2) utilizers, while Nocardioides, Sphingomonas, Aeromonas, respond towards HCO3 -. H2 oxidizing Cupriavidus, Hydrogenophilus, Hydrogenophaga, CO2 fixing Cyanobacteria Rhodobacter, Clostridium, Desulfovibrio and methanogenic archaea are also enriched. Enriched chemolithoautotrophic members show good correlation with CO2, CH4 and H2 concentrations of the native rock environments, while the organisms from upper horizons correlate more to NO3 -, SO4 2- , Fe and TIC levels of the rocks. Co-occurrence networks suggest close interaction between chemolithoautotrophic and chemoorganotrophic/fermentative organisms. Carbon fixing 3-HP and DC/HB cycles, hydrogen, sulfur oxidation, CH4 and acetate metabolisms are predicted in the enriched communities. Our study elucidates the presence of live, C and H2 utilizing Bacteria and Archaea in deep subsurface granitic rocks, which are enriched successfully. Significant impact of depth and geochemical controls on relative distribution of various chemolithotrophic species enriched and their C and H2 metabolism are highlighted. These endolithic microorganisms show great potential for answering the fundamental questions of deep life and their exploitation in CO2 capture and conversion to useful products.

Stability assessment of the restored Benggang units in a weathered granite crust region of South China

Benggang, a unique and severe form of gully erosion, is widely distributed in weatherd granite crust regions of South China and causes great harms to ecosystem and human. In the recent decades, large investments had been to restore the collapsing Benggang units. In the comprehensive benefit assessment protocol for soil and water conservation effectiveness of the Benggang restoration, stability assessment of the restored Benggang units lacks. This study developed a protocol to assess the stability of the restored Benggang units in a weathered granite crust region via diagnosing sedimentary layers in a soil profile (a meter away from its debris dam). Characteristic sedimentary layers formed by collapse, or strong or weak surface erosion were defined by biplotting ratios of coarse particles (> 1 mm) (PR) and quartz proportion (QR) in each sedimentary layer to those in the collapsing wall (original soil). Among thirty restored Benggang units investigated after 2–10 years of restoration in Anxi and Changting counties in Fujian province, 27 units were still collapsing or collapsed, consisting of 12 very active ones, while only 3 units were fully stabilized. Rainfall intensity threshold causing collapse (RITC) of the units was identified via matching local rainfall events with the characteristic sedimentary layers of collapsion in the dam-front soil profile, significantly higher RITCs in the stable or collapsing-to-stable units than the continuously or late collapsing units (p < 0.05). Based on the estimated RITCs, no significant differences in stability of the restored units were observed between Changting strategy (ecological restoration only with vegetation) and Anxi strategy (restoration with cashcrop plantation). The findings of this study suggested that the characteristic sedimentary layers in the dam-front soil profile determined by the PR-QR biplotwas feasible for stability assessment of the restored Benggang units and provided a quantitative RITC estimation.

Thorium zoning in monazite: A case study from the Ivrea–Verbano zone, NW Italy

AbstractMetamorphism and partial melting of the lower crust is commonly assumed to cause depletion in heat producing elements (HPEs; K, U, Th). In the deep crust, volumetrically subordinate metasedimentary layers, which are source to crustal granites (sensu lato), host the majority of Th ± U, primarily within the REE + Th + U + Y phosphate mineral monazite. We examine the spatial and temporal distribution of Th within monazite grains in metasedimentary rocks from the lower crustal section of the Ivrea–Verbano Zone (Italy), using textural, compositional and geochronological data. We link this to outcrop and regional scale trends described by in‐field gamma‐ray spectrometry data (in‐field GRS) for the purpose of understanding how Th distribution is controlled by progressive metamorphism and partial melting. In‐field GRS data shows that the whole rock budget of Th does not change between granulite facies rocks and their unmelted equivalents but is significantly lower in rocks that have undergone more significant melt loss at ultra‐high temperature (UHT) conditions. Concurrently, the bulk Th budget of monazite increases with metamorphic grade to granulite facies conditions and is greatly reduced in UHT samples. Monazite geochronology returns dates mostly in the range 240–320 Ma with two main peaks at circa 290 and 270 Ma. Textural and chemical constraints indicate that these dates record the timing of pre‐peak to peak metamorphic conditions. Amphibolite facies monazite compositional zones are absent from granulite facies monazite, in contrast to examples from lower‐pressure terranes. This is consistent with the expanded stability of allanite relative to monazite with increasing pressure having an important role in determining the internal structure, composition and extent of inheritance of monazite in going from amphibolite facies to granulite facies rocks. We propose high‐pressure granulites should preserve less monazite inherited from amphibolite facies conditions than low‐pressure granulites. Monazite is preserved at all metamorphic grades and presents a mineralogical mechanism for retaining Th in residual deep crust during partial melting and after melt loss.

Microbial diversity and function in crystalline basement beneath the Deccan Traps explored in a 3 km borehole at Koyna, western India.

Deep terrestrial subsurface represents a huge repository of global prokaryotic biomass. Given its vastness and importance, microbial life within the deep subsurface continental crust remains under-represented in global studies. We characterize the microbial communities of deep, extreme and oligotrophic realm hosted by crystalline Archaean granitic rocks underneath the Deccan Traps, through sampling via 3000 m deep scientific borehole at Koyna, India through metagenomics, amplicon sequencing and cultivation-based analyses. Gene sequences 16S rRNA (7.37× 106 ) show considerable bacterial diversity and the existence of a core microbiome (5724 operational taxonomic units conserved out of a total 118,064 OTUs) across the depths. Relative abundance of different taxa of core microbiome varies with depth in response to prevailing lithology and geochemistry. Co-occurrence network analysis and cultivation attempt to elucidate close interactions among autotrophic and organotrophic bacteria. Shotgun metagenomics reveals a major role of autotrophic carbon fixation via the Wood-Ljungdahl pathway and genes responsible for energy and carbon metabolism. Deeper analysis suggests the existence of an 'acetate switch', coordinating biosynthesis and cellular homeostasis. We conclude that the microbial life in the nutrient- and energy-limited deep granitic crust is constrained by the depth and managed by a few core members via a close interplay between autotrophy and organotrophy.

Magma Chamber Formation by Dike Accretion and Crustal Melting: 2D Thermo‐Compositional Model With Emphasis on Eruptions and Implication for Zircon Records

AbstractWe present a 2D thermo‐elastic model of a magma body formation in a granitic crust by injection of rhyolitic or basaltic dikes and sills. Elastic analytical solutions enable the computation of rock displacement in response to magma intrusion and evacuation during volcanic eruptions. Phase diagrams for magma and rocks govern melting/crystallization behavior and temperature evolution in 16 million computational cells. Calculated temperature histories are used to predict zircon crystallization/dissolution, their ages, and isotopic ratios within individual batches of magma and rocks. Incremental dike injection naturally generates magma batches of melt that form kilometer‐wide clusters appearing in different parts of the domain with diverse shapes, horizontal and vertical interconnectivity. The volume of eruptive melt strongly depends on magma influx rates Q, the width of the injection region W, and eruptions. For example, rhyolitic dike injection with Q = 0.125 m3/s with W = 5 km during 100 ka generates ∼50 km3 of eruptive melt while no significant melt forms if W = 10 km. Injection of basaltic dikes into the granitic crust generates comparable amounts of rhyolitic melt from dike‐residual and country rocks. High injection intensities produce magma reservoirs capable of large eruptions, while repetitive eruptions lead to shrinkage of magma bodies. High heat input causes host rock zircons to lose a significant portion of their old cores. Magmatic zircons in the periphery form quickly due to rapid cooling of intruded dikes while in the central part crystals can grow during several hundreds of ka. The model predicts highly heterogeneous O and Hf isotopic ratios recorded in zircons.

Geological knowledge advances on the Alto Rio Negro region, northwestern Amazonian Craton, Brazil: a review

This paper presents a literature review of the geology of the Alto Rio Negro region, located in northwestern Brazil. Firstly, we summarize the Amazonian Craton tectonic models and how the study area fits in these different tectonic models. The aim is to organize the literature data and better guide the consultation for new studies. The first geological reports about the Alto Rio Negro are from the Mid-19th century, when the British naturalist Alfred Russel Wallace travels to this country and, since then, long periods of scientific production absence occur. However, after the RADAMBRASIL Project in the 1970s, there was a significant research advance. Recent publications reveal a complex tectonic evolution, with three events that mark periods of generation and reworking of the Proterozoic basement. Two Paleoproterozoic magmatic arc systems generated the geological basement. The Calymmian collision of these terrains produced a large volume of crustal granites, in addition to pre-, and post-tectonic granites. A crustal reworking period (1.4-1.1 Ga) reactivated the basement structures and, possibly, generated the Ectasian carbonatitic intrusions. There are many difficulties to access this region, so the literature data are of great value and provide a background for new studies. The lack of more detailed geological maps covering the Alto Rio Negro region is the main obstacle. Still, it offers a fertile research field for geologists interested in contributing to diverse research fields of Geology focusing on the Amazonian Craton.

Formation mechanism of collapsing gully in southern China and the relationship with granite residual soil: A geotechnical perspective

Recently, there has been an increase in collapsing gullies in the south of China as one of the most destructive types of soil erosion. Most collapsing gullies are formed on a well-developed granite crust; thus, granite residual soil plays a critical role. However, the extent to which the geotechnical features of residual soil, especially soil disintegration, affects collapsing gully formation is poorly understood. This study performed laboratory disintegration tests on granite residual soil taken from the red soil, sandy soil, and detritus layers of a collapsing gully. The disintegration behaviour was quantified by defining the disintegration ratio, Rd, and three equivalent disintegration rates, vI, vII, and vIII, corresponding to Rd = 10%, 30%, and 50%, respectively. The results revealed that the red soil layer (depth < 1.3 m) and the soil at the shallower depth of the sandy soil layer (depth < 3.0 m) showed similar disintegration behaviours, which were complete (Rd = 100%) and rapid, with vI values in the range of 66.7–266.7%/min. The soil disintegration in the sandy soil layer was characterised by an incremental increase in Rd to 100% within 100 min. The residual soil at the bottom of the sandy soil layer and the top of the detritus layer (depths of 4.0–8.0 m) disintegrated consistently at the first, after which the disintegration rate gradually decreased with vIII lower than 1%/min. The detritus layer soil at a depth greater than 10.0 m showed incomplete disintegration, and the ultimate Rd was approximately 60%. The formation mechanism for the soil disintegration and gully collapse was also proposed. The weakening of cementation triggered the breakup of soil aggregates and led to soil disintegration and the occurrence of a gully collapse. This study provided new insights on gully erosion.

Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhood

Prior studies have hypothesized that some polluted white dwarfs record continent-like granitic crust—which is abundant on Earth and perhaps uniquely indicative of plate tectonics. But these inferences derive from only a few elements, none of which define rock type. We thus present the first estimates of rock types on exoplanets that once orbited polluted white dwarfs—stars whose atmospheric compositions record the infall of formerly orbiting planetary objects—examining cases where Mg, Si, Ca and Fe are measured with precision. We find no evidence for continental crust, or other crust types, even after correcting for core formation. However, the silicate mantles of such exoplanets are discernable: one case is Earth like, but most are exotic in composition and mineralogy. Because these exoplanets exceed the compositional spread of >4,000 nearby main sequence stars, their unique silicate compositions are unlikely to reflect variations in parent star compositions. Instead, polluted white dwarfs reveal greater planetary variety in our solar neighborhood than currently appreciated, with consequently unique planetary accretion and differentiation paths that have no direct counterparts in our Solar System. These require new rock classification schemes, for quartz + orthopyroxene and periclase + olivine assemblages, which are proposed here.

A Pilbara perspective on the generation of Archaean continental crust

Models for the generation and evolution of continental crust are informed by a detailed understanding of regional geology and geochemistry. This contribution therefore evaluates the increasing amounts of high quality geochemical data, on well-characterized samples, now available from the magmatic rocks of the Pilbara Craton, Western Australia. The East Pilbara Terrane is characterized by vertical dome and basin tectonics, and the younger (Mesoarchean) rocks to the west are associated with lateral tectonics. The craton stabilized at ~2.95 Ga (Hickman, 2012), and it was overlain by voluminous, dominantly mafic lavas of the Fortescue Group in an intra-cratonic setting at ~2.7 Ga. The magmatic rocks of the Pilbara Craton are bimodal in SiO2, with the volcanic rocks typically having <60% SiO2, and the plutonic rocks >60% SiO2, and such differences are in part linked to different depths of magma emplacement.The volcanic rocks are divided into two groups, with Dy/La > and < 0.6. The geochemistry of the former group is consistent with broadly closed system differentiation, whereas the low Dy/La rocks largely reflect open system magmatic processes, and contributions from high silica magmas. The high Dy/La rocks have restricted Th/Nb ratios similar to those of primitive mantle, whereas the low Dy/La rocks range to higher Th/Nb values. In most cases these higher Th/Nb values reflect open system magma differentiation, but in some of the younger volcanic suites (the Bookingarra Group and Cooneeina Basalt) these values appear to be associated with subduction. There is no evidence for garnet or amphibole in the generation of the volcanic rocks, however the composition of the plutonic rocks reflects the involvement of both garnet and plagioclase, during melt generation and differentiation.The high Dy/La rocks have Sm/Nd ratios similar to CHUR, but their Lu/Hf ratios are significantly sub-chondritic. Many of the Pilbara rocks from 3.5–3.0 Ga were generated from mantle sources with broadly chondritic Hf isotope ratios, and so in the preferred model the mean composition of the high Dy/La group was generated by ~20% melting of chondritic mantle. Apart from the high Th/Nb magmas at 3.05–2.97 Ga (Smithies et al., 2005), there are surprisingly few changes in the minor and trace element features of the magmas associated with the generation of the craton (3.5–3.0 Ga), and after it stabilized at ~2.95 Ga. The 3.05–2.97 Ga plutonic rocks in the granitic complexes also have elevated Th/Nb, suggesting that such ratios were inherited from their mafic source rocks. This observation implies that the mafic sources and the granitic rocks were of similar age, and that there was rapid conversion of young mantle-derived basaltic crust into felsic continental crust, with little contribution from older basement material. The volcanic rocks are characterized by relatively low water contents (~<2%) on the basis of their low maximum Al2O3 values, a mean Fe3+/ΣFe of 0.19 ± 0.01, and models in which the high Dy/La rocks were derived from primitive mantle. Such low water contents relative to modern subduction-related magmas are a feature of the magmatic rocks of the Pilbara Craton, and the development of ‘granitic crust’ appears to be linked to the development of higher water contents associated with subduction, locally commencing at ca. 3.1 Ga, and to globally dominant plate tectonics.

Over one billion years of Archean crust evolution revealed by zircon U-Pb and Hf isotopes from the Saglek-Hebron complex

The Saglek-Hebron Complex (SHC) located in Northern Labrador, Canada, includes some of the oldest crustal rocks on Earth. This granite-greenstone terrain is dominated by trondhjemite, Mg-rich tonalite, granite, with subordinate granodiorite, and recorded over one billion years of early magmatic history between ∼3900 Ma and ∼2700 Ma. New in-situ U-Pb analyses on zircons from eighteen rock samples, suggest that six distinct magmatic events formed the felsic crust of the SHC, during the Archean Eon. Age deconvolution analysis constrains these magmatic episodes at ca. 3857 Ma, 3744 Ma, 3575 Ma, 3330 Ma, 3224 Ma and 2750 Ma, respectively defined as the Iqaluk, the Uivak I, the Uivak II, the Iluilik, the Lister and the late granite events. The 3330 Ma Iluilik represents a newly defined magmatic event, distinct from the Lister event previously recognized at ∼3200 Ma. Uranium-Pb geochronology, combined with in-situ Hf isotopic data on zircons and geochemistry of the host rocks, reveal a complex and protracted crustal history of the SHC marked by the reworking of mafic and felsic crust over one billion years. Zircons from the oldest Iqaluk and Uivak I samples yield slightly suprachondritic initial εHf values (up to ∼+4), consistent with the involvement of a long-term depleted source in the Eoarchean. The 3330 Ma Iluilik granodiorite exhibits a low initial εHf value of −6, clearly diverging from the near-chondritic Hf isotopic composition of the 3224 Ma Lister tonalites. Considering a mafic crustal precursor (with a 176Lu/177Hf ratio of ∼0.02) for trondhjemite-tonalite-granodiorite (TTG) samples, the initial Hf isotopic compositions of the SHC Eoarchean to Paleoarchean zircons would be consistent with the reworking of Hadean mafic crust. The zircon εHf vs. time trend for granite samples corresponds to a 176Lu/177Hf ratio of ∼0.01, suggesting that the granitic crust was mainly produced by crustal reworking of the oldest TTG. Similarly to the felsic crust of the Itsaq Gneiss Complex of SW Greenland, the SHC granitoids appear to record a shift to more juvenile compositions at the end of the Paleoarchean. However, contrary to what is observed in SW Greenland, almost no input of juvenile material has been recorded by the SHC Neoarchean felsic crust.

Expansion-Oriented View On Origin of Oceans

Both the concepts of plate tectonics and continental drift conceive that the planet earth’s dimension, associated with its oceans, has remained unchanged throughout the past geological periods. In contrast, Hilgenberg’s model of earth expansion endorses that initially the planet was considerably small and devoid of oceans [1]. Based on earth expansion theory the author has pointed out that since the primordial condensed or small earth was devoid of oceans, initially the ocean-forming water must have been associated with the mantle, thereby turning that geosphere considerably fluid and pre-eminently suitable for planetary expansion. Expansion of the planet appears to have been caused owing to swelling up of the semi-fluid mantle in response to an external gravitational pull caused by an extra-terrestrial planetary body, probably the Moon. The primordial earth was completely covered with a relatively thin granitic crust, which, due to swelling up of the mantle developed a number of long and sinuous expansion cracks. Through these expansion cracks widespread eruption of molten magma took place spreading on both sides of the cracks to form rudimentary oceans basins. With continued expansion, the dimension of the oceans was broadened while the expansion cracks turned in to mid-oceanic ridges. Associated with expulsion of molten lava, large quantum of volatiles, chiefly constituted of water was released from the mantle that formed the ocean water while due to desiccation of the mantle, the process of expansion was eventually stopped.

A New Magma Type in the Continental Collision Zone. The Case of Capraia Island (Tuscany, Italy)

The Tuscany Magmatic Province consists of a Miocene to Pleistocene association of a wide variety of rock types, including peraluminous crustal anatectic granites and rhyolites, calcalkaline and shoshonitic suites and ultrapotassic lamproites. In addition to the magma types already recognised, the occurrence of a new, distinct magma type at Capraia and Elba islands and in mafic enclaves in the San Vincenzo rhyolites has been suggested by recent studies. This particular type of magma, represented by intermediate to acidic calcalkaline rocks showing high Sr, Ba, and LREE, is restricted to the northwestern sector of the province and to a time interval of about 8 to 4.5 Ma. New data obtained on rocks from Capraia Island have allowed for the verification of the occurrence of this new magma type, the exploration of its origin and a discussion of its possible geodynamic significance. The high-Sr-Ba andesite-dacite rocks occurring in the Laghetto area at Capraia display a composition that is intermediate between adakitic and calcalkaline rocks. It is suggested that they represent a distinct type of magma that originated at mantle pressure by melting of the lower continental crust, followed by mixing with other Capraia magmas. The geodynamic model that best explains the composition of the studied rocks is the thickening of the continental crust during continental collision, followed by extension that favoured melting of the lower crust.

The Southeastern Saurashtra dyke swarm, Deccan Traps: Magmatic evolution of a tholeiitic basalt–basaltic andesite–andesite–rhyolite suite

The Deccan Traps continental flood basalt (CFB) province of India contains several dyke swarms, which are dominated by tholeiitic basalts and basaltic andesites. The Southeastern Saurashtra dyke swarm, containing mainly these rock types, also contains an andesite and several rhyolites. Based on petrographic, mineral chemical, and whole-rock major and trace element and Sr-Nd isotopic data, we discuss the magmatic evolution and pressure-temperature conditions of crystallisation of these dyke magmas. The tholeiitic basalts and basaltic andesites have low to moderate TiO2 contents. The andesite and the rhyolites have low CaO, MgO, Fe2O3t, TiO2 and P2O5, and high K2O, Rb, Ba and light rare earth element contents. Thermobarometric calculations for equilibrium mineral–whole-rock pairs indicate plagioclase crystallisation at 1200–1170 °C, overlapping with clinopyroxene crystallisation at 1181–1143 °C, and a pressure range of 0.1–3.6 kbar indicating crystallisation during magma ascent or storage at shallow crustal levels. Major and trace element modelling is consistent with the rhyolites being produced by advanced fractional crystallisation of basaltic parental magmas. However, a broad trend of increasingly radiogenic Sr and nonradiogenic Nd isotopic ratios, from the mafic rocks through the andesite to the rhyolites, suggests a combined assimilation and fractional crystallisation (AFC) process between basaltic magma and ancient granitic crust. The rhyolites contain the largest crustal contributions and display the most “enriched” Sr-Nd isotopic characteristics yet recorded from the Deccan Traps (εNdt = −20.6, (87Sr/86Sr)t = 0.74855). The widespread occurrence of mafic enclaves in the rhyolite dykes suggests that processes such as magma mixing may also be responsible for some of the andesitic rocks known in Saurashtra.

Cloud-fracture networks as a means of accessing superhot geothermal energy

Superhot geothermal environments (above ca. 400 °C) represent a new geothermal energy frontier. However, the networks of permeable fractures capable of storing and transmitting fluids are likely to be absent in the continental granitic crust. Here we report the first-ever experimental results for well stimulation involving the application of low-viscosity water to granite at temperatures ≥400 °C under true triaxial stress. This work demonstrates the formation of a network of permeable microfractures densely distributed throughout the entire rock body, representing a so-called cloud-fracture network. Fracturing was found to be initiated at a relatively low injection pressure between the intermediate and minimum principal stresses and propagated in accordance with the distribution of preexisting microfractures, independent of the directions of the principal stresses. This study confirms the possibility of well stimulation to create excellent fracture patterns that should allow the effective extraction of thermal energy.

Electrical conductivities of two granite samples in southern Tibet and their geophysical implications

There are clear differences in the electrical conductivities of the crustal granites of the Qinghai-Tibet Plateau. Because these granites are among the major rock types on the Qinghai-Tibet Plateau, it is very important to detect the electrical conductivity of granites under high temperatures and pressures to study the electrical conductivity structure of this area. Using impedance spectroscopy at a frequency range of 10−1–106 Hz, the electrical conductivity of the muscovite-granite collected from Yadong was investigated at a confining pressure of 1.0 GPa and temperatures ranging from 577 to 996 K, while the electrical conductivity of the biotite-granite collected from Lhasa was investigated at a pressure of 1.0 GPa and temperatures ranging from 587 to 1382 K. The calculated activation enthalpies of the Yadong muscovite-granite sample is 0.92 eV in the low-temperature range (577–919 K) and 2.16 eV in the high-temperature range (919–996 K). The activation enthalpies of the Lhasa biotite-granite sample is 0.48 eV in the low-temperature range (587–990 K) and 2.06 eV in the high-temperature range (990–1382 K). The change in the activation enthalpies of the granites at different temperature ranges may be associated with the dehydration of the two samples. The electrical conductivities of the granite samples obtained in the laboratory using impedance spectroscopy correspond well with field observations conducted near the sampling points, both in terms of the actual conductivity values and the observed variations between the low-temperature and high-temperature regimes. This correlation of laboratory and field conductivities indicates that the conductivities of the crustal rocks in the two regions closely correspond to granite conductivities. We calculated the electrical conductivities of muscovite-granite and biotite-granite samples using the effective medium and HS boundary models. When applied to the crustal rocks of southern Tibet, the results of the geophysical conductivity profiles lie within the range of laboratory data. Thus, the electrical characteristics of the crustal rocks underlying the southern Qinghai-Tibet Plateau can largely be attributed to granites, with the large changes to high conductivities at increasing depths resulting from the dehydration of crustal rocks with granitic compositions.

Processes of crust formation in the early Earth imaged through Hf isotopes from the East Pilbara Terrane

The Pilbara Craton, Western Australia, is one of the best preserved Palaeo- to Mesoarchaean terrains on Earth. The East Pilbara Terrane is the archetypical granite-greenstone belt, the dome-like complexes of which were formed through three major magmatic events. These granite domes are comprised of metamorphosed granitic igneous rocks that exhibit a magmatic evolution from early tonalite-trondhjemite-granodiorite (TTG) rocks towards K-rich granites over the period 3.53–2.83Ga. Accordingly, East Pilbara has been a focus for workers seeking to constrain early Archaean geodynamic processes. One way to inform on this debate is to interrogate successive igneous supersuites using tools sensitive to magmatic source. We present new zircon Hf and whole-rock Nd isotope data from four major supersuites of the Mount Edgar Dome, of the East Pilbara Terrane. Early ca. 3.45Ga TTGs exhibit isotopic signatures that imply their partial derivation from existing crust, with addition of some juvenile material. Subsequent Palaeoarchaean magmatic events show a secular trend towards more evolved isotopic signatures, interpreted as a dominance of increasing reworking of existing crust, with only minor addition of new juvenile crust. The implication of this is that these later Palaeoarchaean supersuites were largely derived from the melting of older granitic crust, with mass balance modelling suggesting an input of ca. 20% juvenile (depleted mantle) material. The limited addition of juvenile material and increased reworking of existing crust with time, does not support a model of modern-style subduction, but is consistent with vertical tectonic processes in a volcanic plateau-type setting for the East Pilbara Terrane >3.2Ga. All Palaeoarchaean Mount Edgar samples resolve to two-stage Hf model ages of ca. 3.7Ga and Nd model ages of ca. 3.6Ga. These Eoarchaean model ages support the existence of a cryptic pre-3.5Ga protocrust, albeit of unknown extent. Analysis of late Mesoarchaean granites yields highly evolved Hf isotope signatures of K-rich monzogranites. This is consistent with a trend in the East Pilbara Terrane from sodic TTG type magmatism in the early Palaeoarchaean to K-rich magmas in the late Mesoarchaean.