Palaeozoic Granites Research Articles

Structural and geochemical studies of the Palaeozoic granites from Dalhousie region of Himachal Himalaya: Implications on petrogenesis and deformation of strongly peraluminous S‐type granites

The Palaeozoic granites forming the Dalhousie pluton occur as an elongated body intruded into the core of an antiform in Salkhala metasedimentary rocks and are referred to herein as Dalhousie granites (DG). The studied mesoscopic structures, for example, well‐developed and randomly distributed megacrysts of K‐feldspar at the core part of massif, show undeformed porphyritic nature, whereas augen along with the mylonitic foliation and parallelly aligned feldspar megacrysts are dipping towards north‐north‐east (NNE) in the marginal part. The presence of aplite veins is evidence of final stage crystallization, where these rocks rapidly crystallized from siliceous residual fluids/solutions that escaped along fractures in the granites. Moreover, the granites show intrusive as well as thrusted contact with the Chamba metamorphics. The microscopic study from the marginal parts shows the dynamic recrystallization of quartz along with the fractures filled with secondary material oriented opposite to the top‐to‐the‐SW. Whereas euhedral quartz grains with undeformed grain boundaries, compositional zoning in plagioclase, and magmatic (perthite) texture of K‐feldspar are present in the core of Dalhousie pluton. In addition, a new whole‐rock geochemical dataset of DG is presented and investigated to elucidate the petrogenesis and tectonic environment. The geochemical data shows that DG (dominantly monzogranites) are formed from a pelitic source‐derived, (molar Al2O3/CaO + Na2O + K2O > 1.1) strongly peraluminous (S‐type) calc‐alkaline magma. This magma was generated by muscovite vapour absent dehydration melting. Harker bivariate plots indicate that fractional crystallization was a dominant process during the evolution of these granites. The tectonic discrimination diagrams suggest that DG were generated in syn‐collisional setting. Exceptionally high Pb in DG occurred due to primary melt generation at low‐temperature during partial melting (low‐degree melting) of the pelitic source rock. Trace and rare earth elements characteristics, such as positive anomaly of Zr show a notable amount of zircon as one of the accumulating phases. A pronounced negative Eu anomaly (EuN/Eu* = 0.04–0.66), along with high silica content and highly varied trace element ratios in these rocks, show that these are fairly evolved granites.

Multiple Palaeozoic–Mesozoic orogenic events in the SE Yangtze Block, China: evidence from the petrogenesis and deformation of gneissic granites from the Nanwenhe metamorphic dome and regional correlation analysis

ABSTRACT The Nanwenhe metamorphic dome contains a rare Silurian granitic pluton that developed in the SE Yangtze Block during early Palaeozoic orogenesis, and this pluton comprises the Nanwenhe gneissic granites. The petrogenetic and structural deformation histories of the gneissic granites recorded the multi-stage interaction of the SE Yangtze Block with other blocks during the Palaeozoic and Mesozoic, including the Cathaysia, Indochina, and Paleo-Pacific blocks. Zircon U–Pb and Hf isotopes and geochemical data indicate that the Early Palaeozoic granites (433–420 Ma) are S-type granitic rocks, and they were derived from the partial melting of ancient continental crust with no contribution from mantle material. And the large variations and positive values of εHf(t) (−12.25 to +10.69) of the gneissic granites are ascribed to a heterogeneous source and disequilibrium melting. Here we compare early Palaeozoic granitoids in the SE Yangtze Block (foreland belt) with those in the orogenic core, and suggest that the SE Yangtze Block underwent limited syn-collisional crustal thickening and metamorphism, and following which post-collisional granitoids were formed through partial melting of continental crust. Structural analysis of the Nanwenhe gneissic granites allows two stages of deformation to be identified: northward detachment (D1) and northwestward thrusting (D2). 40Ar–39Ar isotope dating of muscovites from the gneissic granites yields a well-defined plateau age of 229.61 Ma, which is interpreted as the timing of Late Triassic structural overprinting associated with the formation of the Nanwenhe metamorphic dome. Integration of our new results with previous research findings from the dome and South China allows us to conclude that the detachment deformation (D1) represents Late Triassic post-collisional extension, and D2 represented Jurassic intracontinental orogeny associated with subduction of the Paleo-Pacific Block.

Late Palaeozoic magmatism in the eastern Tseel Terrane of SW Mongolia evidenced by chronological and geochemical data

The eastern Tseel Terrane of SW Mongolia, which is an important part of the Central Asian Orogenic Belt (CAOB), contains a segment of imbricated sheets of Late Palaeozoic granites as well as a bimodal volcanic suite. Late Devonian (380–382 Ma) granites that are enriched in Th, U, and K and depleted in Nb, Ta, P, Ba, Sr, and Ti indicate arc magmatism and are low‐K tholeiitic to medium‐K calc‐alkaline series. Zircon εHf(t) values are of +8.30 to +14.1, with crustal model ages of 0.83–0.48 Ga. The bimodal volcanic suite is composed of N‐MORB basalt and 302 ± 2 Ma rhyolite having A2‐type granite affinities, εHf(t) values ranging from +9.9 to +11.9, and crustal model ages from 0.69 to 0.59 Ga. The rhyolite mighthave originated from partial melting of juvenile lower crust, rather than fractional crystallization of coeval basaltic magma. Late Carboniferous (299–303 Ma) granites are medium‐K to high‐K calc‐alkaline series, cross‐cutting carbonaceous siltstone, emplaced into the deformed volcanic rocks and are characterized by enrichment in K, Th, and U accompanied by depletion in Ba, Nb, Ta, Sr, P, and Ti in a primitive mantle‐normalized diagram with εHf(t) values ranging from +6.05 to +10.6 and crustal model ages from 0.70 to 0.65 Ga. Thus, LA‐ICP‐MS zircon ages in combination with geochemical data document a phase of arc‐related magmatism in the Tseel Terrane at 380–382 Ma. These rocks were subsequently deformed under low‐grade metamorphic conditions and intruded by undeformed granites at c. 299–303 Ma. The coeval 302 Ma rhyolite in bimodal volcanic suite shows typical A2‐type granitic geochemical affinities and probably documents a back‐arc basin extensional environment, which was probably related to the roll‐back of the Palaeo‐Asian Oceanic Plate during the northward subduction under the Central Mongolia microcontinent.

Petrogenesis of the Carboniferous Ghaleh-Dezh metagranite, Sanandaj–Sirjan zone, Iran: constraints from new zircon U–Pb and40Ar/39Ar ages and Sr–Nd isotopes

AbstractThe Ghaleh-Dezh metagranites in the northern Sanandaj–Sirjan Zone (SaSZ) in western Iran are found in a 0.5 km long by 0.3 km wide unit emplaced within the older Precambrian basement. New zircon U–Pb ages confirm that crystallization and emplacement of the protolith of the metagranites occurred at 312 ± 10 Ma and 298 ± 17 Ma in the Upper Carboniferous (Pennsylvanian) – Early Permian, which is consistent with the ages of recently discovered Palaeozoic granites in the northern SaSZ. The studied metagranitic body has been metamorphosed at lower greenschist facies and deformed in ductile–brittle regime due to subsequent reheating events during the Mesozoic. The rocks are metaluminous to slightly peraluminous granites with an A2-type affinity. Initial87Sr/86Sr ratios andεNd(t) contents vary from 0.7037 to 0.7130 and −0.70 to 0.34, respectively.143Nd/144Nd(i)values for the granitic rocks are fairly uniform at ∼0.5123. The geochemical and isotopic evidence indicates that these rocks were generated from a mantle magma with crustal contamination and fractional crystallization. Rb–Sr isochron and40Ar/39Ar K-feldspar ages are 274 Ma and 60–70 Ma, respectively. The older event,c.270 Ma, was likely related to the opening of Neotethys, whereas the younger ages likely relate to collisional events in the region during the closure of Neotethys.

An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes

Modern oceanic crust is constantly produced at oceanic ridges and recycled back into the mantle at subduction zones via plate tectonics. An outstanding question in geology is whether the Earth started in a non-plate tectonic regime, and if it did, when the transition to the modern regime occurred. This is a complicated question to address because Archaean rocks lack modern equivalents to anchor interpretations. Here, we present a silicon isotopic study of 4.0–2.8-Gyr-old tonalite–trondhjemite–granodiorites, as well as Palaeozoic granites and modern adakites. We show that Archaean granitoids have heavier silicon isotopic compositions than granites and adakites, regardless of melting pressure. This is best explained if Archaean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater. Before the appearance of silica-forming organisms 0.5–0.6 billion years ago, the oceans were close to silicon saturation, which led to extensive precipitation of cherts on the seafloor. This is in contrast to modern oceans, where silica biomineralization maintains dissolved silicon at low concentration. The unique heavy silicon isotope signature of cherts has been transferred to Archaean granitoids during an oceanic subduction process, which was probably responsible for the formation of felsic rocks on Archaean emerged lands. Archaean granitic rocks formed by melting of silica-enriched subducted basaltic crust through interaction with seawater, according to heavy silicon isotopes measured in Archaean samples.

Does faecal matter reflect location? An initial assessment of isotopic variability between consumed prey remains and faecal matter for wild jaguars

ABSTRACTFaecal isotopic analysis may complement other non-invasive wildlife survey tools for monitoring landscape use by carnivores, such as motion-detecting cameras and non-invasive genetic sampling. We analysed carbon, nitrogen, and strontium isotopes in faecal matter produced by jaguars (Panthera onca) as well as bones from consumed prey at the Mountain Pine Ridge Forest Reserve (MPR) in Belize, Central America. The MPR is ideally suited for a spatial isotope study as vegetation and geology both vary considerably. The isotopic composition of faecal matter should reflect the habitat and geology where consumed prey lived. We used bone from consumed prey recovered from jaguar scats as a proxy for diet. Faecal matter and bone showed comparable spatial isotopic trends, suggesting that the isotopic composition of jaguar faeces can be used to detect foraging in different habitats (pine forest versus broadleaf forest) or on different geologies (Mesozoic carbonates; Palaeozoic granite, contact metamorphics, and metasediments). This result is reassuring as bones are not always present in carnivore scats. Studying landscape use by cryptic and wide-ranging carnivore species like jaguars remains challenging. Isotopic analysis of faecal matter complements the existing array of non-invasive spatial monitoring tools.

Geochronology and geochemistry of the Carboniferous Ulann Tolgoi granite complex from northern Inner Mongolia, China: Petrogenesis and tectonic implications for the Uliastai continental margin

This study presents zircon U–Pb ages and geochemical and Sr–Nd isotopic data for acid granites from the Uliastai continental margin, which is located in the Ulaan Tolgoi area of the south‐eastern Central Asian Orogenic Belt (CAOB). Although most samples have similar calc‐alkaline (K2O/Na2O = 0.55–1.70, average of 1.35) and peraluminous (A/CNK = 1.02–1.20 and A/NK = 1.09–1.87) characteristics, the granodiorites, monzogranites, and alkali feldspar granites exhibit distinct light rare earth element (LREE)/heavy REE (HREE) anomalies, including negative Nb, P, Ti, Sr, and Ba anomalies and negative εNd(t) values. These characteristics indicate that this complex massif comprises lower Early Carboniferous subduction‐related calc‐alkaline granodiorites (348 ± 1 Ma); an upper Early Carboniferous syn‐collisional segment characterized by S‐type, high‐K, calc‐alkaline, and peraluminous monzonitic granites (334 ± 1 Ma); and Late Carboniferous post‐orogenic extensional A2‐type peraluminous alkali feldspar granites (317 ± 1 Ma). Previous studies performed on coeval volcanic rocks and Early Palaeozoic granites in the same area have demonstrated that the Early Carboniferous granodiorites have the same geochemical characteristics as volcanic arc granites (VAGs), thus indicating that the northward subduction of the Hegenshan Ocean may have continued through the Early Carboniferous. The monzogranite and alkali feldspar granites, in conjunction with the Late Carboniferous lagged arc volcanic rocks observed in this area, suggest a syn‐collisional to post‐collisional setting. The new data presented in this study provide constraints on the timing of the transition from island‐arc magmatism to post‐collisional extension‐related magmatism in this region during the later stage of the Early Carboniferous.

Geochronological, geochemical, and Sr–Nd–Hf isotopic studies of the Aketas adakitic granites in Eastern Junggar: Petrogenesis and tectonic implications

The Aketas adakitic granites are located in the Dulate Arc, in the northern part of the eastern Junggar region, Xinjiang (NW China). In this study, we determined the zircon U–Pb isotopic compositions, bulk‐rock Sr–Nd–Hf isotopic compositions, and major and trace elements of Aketas granite samples. These samples show a narrow range in SiO2 content (65.05–67.19 wt.%) as well as high Al2O3 (14.95–18.09 wt.%; A/CNK = 0.96–1.08), Na2O + K2O (7.94–8.42 wt.%), and CaO (2.05–3.48 wt.%) content. Similar to modern adakites, the samples have high Na2O (4.55–5.61 wt.%) and Sr (554.00–915.00 ppm) content, high Sr/Y ratios (85.10–127.38), and low Y (6.51–7.12 ppm) and Yb (0.42–0.79 ppm) content. The low initial 87Sr/86Sr ratios (0.70298–0.70350) and positive εNd(t) (+5.1 to +7.0) and εHf(t) values (+10.4 to +14.9) suggest that the Aketas adakitic granites were derived from the partial melting of lower crustal material in a subduction setting. This is further evidenced by high Mg# values (42.00–65.67), high SiO2 and K2O content, and low Cr (<17.30 ppm), Co (<9.62 ppm), and Ni (<12.60 ppm) content. Bidirectional subduction occurred between eastern Junggar and the Altai Orogen during the Late Carboniferous, and in combination with other exposed Palaeozoic granites in eastern Junggar, the Aketas adakitic granites may represent the latest record of this subduction event. The emplacement of the Aketas adakitic granites implies that eastern Junggar was involved in southward subduction at 308 ± 2.3 Ma; therefore, the final collision between the Siberian and Kazakhstan blocks likely occurred between 310.3 and 307 Ma. Copyright © 2017 John Wiley & Sons, Ltd.

Polygonal cracking in granite and considerations for a morphological classification (La Pedriza de Manzanares, Spain)

The area known as Pedriza de Manzanares forms part of the Upper Manzanares River Basin and of the recently (June 2013) listed Sierra de Guardarrama National Park, located in the Spanish province of Madrid. The region is home to Late Palaeozoic granites characteristic of the Spanish Central Systam, which were intruded during the Variscan Orogeny and uplifted to their present position during the Alpine Orogeny. Previous studies of polygonal cracking in granite suggest several possible control mechanisms. Some authors contend that weathering patterns are governed by internal geodynamic factors, such as the final stages of magmatic consolidation or differential movements in fault planes as a result of their structural position. Others claim that climate-related external factors, specifically insolation rates and thermal differences, can explain the predominance of a given orientation in such patterns. The present paper is a study of the effect of case hardening on polygonal cracking, and the role of variables such as temperature and rock surface hardness in granite. weathering. It also aims to establish a general systematic classification for polygonal cracking, focusing particularly on the cracking associated with the sub-vertical planes of fracture at Pedriza de Manzanares. The morphometric parameters associated with cracks and fractures, including strike and dip, are analysed, along with the height of occurrence and shape of polygonal cracks.

The Blaník Gneiss in the southern Bohemian Massif (Czech Republic): a rare rock composition among the early palaeozoic granites of Variscan Central Europe

Metamorphosed and deformed tourmaline-bearing leucogranites with a Cambro-Ordovician formation age are widespread in the Monotonous Group of the Variscan southern Bohemian Massif, Czech Republic. The rocks, known locally as Blanik gneiss, are strongly peraluminous and classify as phosphorus-rich low-T, S-type granite. The magma formed from a metapelitic source, most likely through muscovite dehydration melting. With respect to its low-T origin and the abundance of tourmaline, the Blanik gneiss is exotic within the spectrum of Early Palaeozoic granites of the Variscan fold belt of Central Europe. Coeval granitic gneisses in the neighbouring Gfohl unit of the Bohemian Massif can be classified as higher T S-type granites and were probably generated through biotite dehydration melting. The geochemical differences between the Early Palaeozoic granitic magmatism in the Gfohl unit and the Monotonous Group support models claiming that these two geological units belonged to independent peri-Gondwana terranes before the Variscan collision. It is suggested here, that the Gfohl unit and the Monotonous Group represent zones of higher and lower heat flow within the Early Palaeozoic northern Gondwana margin, respectively. The geochemical data presented in this study could be helpful for terrane correlations and palaeogeographic reconstructions.

Age, composition, and tectonic significance of Palaeozoic granites in the Altyn orogenic belt, China

ABSTRACTWe examined the petrography, litho-geochemistry, and geochronology of granite plutons in the Altyn orogenic belt, northern margin of the Qinghai–Tibet Plateau, to investigate the geodynamic history of the belt. The granites are peraluminous, with variable chondrite-normalized rare earth element patterns but generally similar trace element compositions; all are depleted in Ba, Nb, Sr, P, and Ti, and enriched in Rb, Th, Ta, Zr, and Hf. Their U–Pb zircon ages record multiple magmatic episodes between 500 and 404 Ma (north Altyn block (NAB)), 522 and 432 Ma (central Altyn block (CAB)), and 483 and 226 Ma (south Altyn block (SAB)). We propose a geodynamic model for the Altyn orogenic belt. Prior to 467 Ma, the north Altyn Ocean subducted southward below the NAB. The north Altyn region saw a change from subduction to collision between the Dunhuang block and the NAB during 467–450 Ma. The collision lasted until 430 Ma and was followed by post-collisional extension until 400 Ma. Farther south, the central Altyn Ocean subducted northwards beneath the CAB, resulting in 520–500 Ma granitic magmatism and leading to collision of the south Altyn micro-block (SAMB) with the CAB. Deep subduction of SAMB crust below the CAB resulted in granite intrusions during 490–455 Ma. The SAMB–CAB collision gave way to post-collisional extension, slab break-off, and delamination during 450–430 Ma. Bidirectional subduction of the south Altyn Ocean after ~485 Ma, dominated by southward subduction, continued until 456 Ma. Closure of the south Altyn Ocean led to continent–continent collision between the SAMB and the Qaidam block at 450–430 Ma, followed after ~410 Ma by repeated break-offs of slab segments until 337 Ma. Subsequent intrusions formed at 264–226 Ma in a setting of either post-collisional extension or sinistral strike-slip.

Timing, petrogenesis, and setting of granites from the southern Beishan late Palaeozoic granitic belt, Northwest China and implications for their tectonic evolution

Late Palaeozoic granites are widely distributed in the southeastern Beishan area, which is located in the central part of the southern Central Asian Orogenic Belt (CAOB). U–Pb zircon dates of five late Palaeozoic granitic plutons from the southeastern Beishan area yield Permian ages: 285 ± 2 Ma (Shuwojing and Western Shuwojing plutons), 269 ± 3 Ma (Jianquanzi and Jiuquandihongshan plutons), and 260 ± 1 Ma (Jiujing pluton). The early Permian Shuwojing pluton, an unfractionated calc-alkaline biotite monzogranite with slightly positive εNd(t) (+0.7 and +0.6) and low initial 87Sr/86Sr (0.70722 and 0.70712), and the early Permian Western Shuwojing pluton, a high-K calc-alkaline biotite granite with slightly negative εNd(t) (−0.2 and −0.1) and low initial 87Sr/86Sr (0.70390 and 0.70919), are likely derived from a mixture of depleted (juvenile) mantle and upper continental crustal (or sedimentary) material. The mid-Permian Jianquanzi and Jiuquandihongshan monzogranites have highly fractionated potassium-rich calc-alkaline characteristics with negative εNd(t) (−4.3) and very high initial 87Sr/86Sr (0.71949), reflecting a continental crustal component. The compositionally diverse Jiujing pluton and the granodiorite and high-Sr monzogranite phases display adakite-like compositions with relatively low εNd(t) (−0.1 and −2.2) and high initial 87Sr/86Sr (0.70822 and 0.70913). The Jiujing low-Sr monzogranite has higher initial 87Sr/86Sr (0.73464) and lower εNd(t) (−2.8), indicating a significant continental crustal component in its genesis. This work, combined with the regional geology and previous studies, suggest that the early to middle Permian southern Beishan plutons formed in a post-collisional environment, probably an intracontinental rift environment linked to asthenospheric upwelling in response to the break-off of a subducted slab. In the late Permian, the southern Beishan area was in a compressive tectonic regime and thickening of the continental crust resulted in the formation of the Jiujing adakite-like granite.

Early Palaeozoic Granites in the Northern Tian Shan, Kyrgyzstan: Their Zircon Ages and Evolution Processes

Early Palaeozoic Granites in the Northern Tian Shan, Kyrgyzstan: Their Zircon Ages and Evolution Processes

Marine bioerosion in rocks of the prehistoric tholos of La Pastora (Valencina de la Concepción, Seville, Spain): archaeological and palaeoenvironmental implications

In this study we examined the materials used in the building of the tholos of La Pastora located in the archaeological site of Valencina de la Concepción (Seville, Spain), which represents the largest settlement of the Guadalquivir Valley during the Third millennium BC. Three different types of rock were found: Palaeozoic sandstone-shale, which forms the walls of both the corridor and the chamber; Palaeozoic granite, which makes up part of the roof slabs; and Neogene calcareous sandstone, which forms the remainder of the roof and floor slabs, as well as the jambs, lintels and some blocks of the walls of the chamber. Intense marine bioerosion as a result of the activity of bivalves, annelids and sponges was observed in some elements of the calcareous sandstone. In fact, the remains of some of the tracemakers were unusually well-preserved, as was the case for the bivalve Petricola lithophaga, whose shells remain in many of their burrows. We infer that these bioerosion structures were formed in a shallow coastal environment (intertidal), and that bioerosion was possible until the few moments before its extraction as construction material. Thereby, the environment in which rock extraction took place was likely a coastal wave-cut platform, in which calcareous sandstone levels outcropped and stood out between clay layers. The radiocarbon dates obtained from these bivalve shells provide a time range between 4780 and 4400 cal yr BP (2830–2450 yr BC) as the highest probability for rock extraction and the subsequent building of the tholos. Thus, the date represents a “Terminus Post Quem” for the construction of the monument of La Pastora.

Investigation of the inner structure of La Crosa de Sant Dalmai maar (Catalan Volcanic Zone, Spain)

La Crosa de Sant Dalmai volcano is the largest volcanic edifice of the Catalan Volcanic Zone (NE Spain). It is a very well preserved maar-type structure, 1.5km in diameter, excavated at the contact between a hard substrate and a soft substrate formed by Palaeozoic granites and Pliocene and Quaternary gravels, respectively. In order to infer the uppermost inner structure of La Crosa de Sant Dalmai maar and to characterise its main geological and tectonic constraints, we have performed a multiparametric geophysical study including gravimetry, magnetometry, self-potential, and electrical resistivity tomography. The results obtained together with a field geology revision and additional geological data from two drill cores, provide a detailed picture of the post-eruptive maar infill sequence as well as of the uppermost part of the maar-diatreme structure. This information helps in understanding the origin and subsequent evolution of the volcano, which included an alternation of phreatomagmatic and Strombolian phases. Geophysical data show that the last Strombolian phase, which culminated with the formation of a scoria cone inside the maar, was associated with a NW–SE trending regional fault. The little erosion and degradation of the original tephra ring suggest a much younger age of La Crosa de Sant Dalmai maar than was previously stated.

The role of glacio-isostasy in the formation of post-glacial river terraces in relation to the MIS 2 ice limit: evidence from northern England

In recent years it has been demonstrated that the formation of long-timescale river terrace sequences, which are generally found in areas beyond the extent of most if not all of the Middle and Late Pleistocene ice sheets, has invariably been a response to uplift during the Late Cenozoic and especially the Quaternary. Climatic fluctuation at a Milankovitch timescale has driven the alternations of aggradation and incision recorded in such terraces. It has been widely observed, however, that fluvial terraces also occur in areas glaciated during the Last Glacial Maximum (LGM), which coincides with marine oxygen isotope stage (MIS) 2. This paper, in seeking to compare records from inside and outside of the LGM ice limit, concentrates on a single English river system, that of the Humber. The Humber estuary is shared by the largely Pennine-derived drainage of the Yorkshire Ouse, to the north and entirely within the MIS 2 glacial limit, and, to the south, the Trent, which is almost wholly outside the LGM limit. Thus the Trent has a terrace sequence extending back to the Middle Pleistocene, whereas in the component rivers of the Ouse system, records begin with the melting of the last glacial ice. Importantly, there is considerable difference in the disposition of the post-LGM fluvial deposits in these two subsystems. In the Ouse system there are modest terrace staircases, commencing with full glacial deposits that stand up to 30 m above the modern floodplain. In the Trent, in contrast, last glacial gravels form the foundation of the modern floodplain, with Holocene sediments emplaced directly above them. Thus there is little or no post-LGM incision in the Trent, whereas in the Ouse several incision events are recorded, continuing into the later stages of the Holocene. Wider comparison reveals that the Ouse system is an exemplar for other sequences within the MIS 2 limit, whereas systems beyond this glaciation typically have last glacial sediments beneath their modern floodplains and show little evidence of Holocene incision. The various possible explanations of these differences are discussed, with emphasis placed on glacio-isostatic uplift of areas glaciated during MIS 2 as the main reason for the significant post-glacial incision that typifies valleys in such regions. A new approach to modelling glacio-isostatic adjustment is outlined, from which it is concluded that lower-crustal flow plays a significant role in this process in regions of relatively hot and dynamic crust, like northern England, in addition to the mantle flow that is considered in conventional analyses of glacio-isostasy. Lower-crustal flow has a significant effect due to the combination of the small spatial scale of the glaciated region of northern England and the high mobility of the lower-crustal layer beneath it, due to the heating effect of the widespread Palaeozoic granite in the area.

Quaternary uplift of northern England

Upland flats, attributable to erosion, have long been recognised in the landscape of the Lake District region of NW England, at altitudes of up to ~ 800 m O.D. Extrapolation using uplift rates derived from dated Pleistocene sites (karstic caves and other features) in the adjacent Pennine uplands suggests that if this succession of flats formed close to sea-level they date from the Middle Pliocene onwards, indicating a subsequent time-averaged uplift rate of almost 0.3 mm a − 1. Numerical modelling indicates that erosion of surrounding areas at a typical rate of 0.2 mm a − 1 since 3.1 Ma could have caused this uplift, as well as constraining the local effective viscosity of the lower crust as ~ 4 × 10 18 Pa s and the typical local Moho temperature as ~ 650 °C. It is thus feasible that most of the topography of northern England has developed since the Middle Pliocene, as a consequence of coupling between erosion and the resulting induced flow in the lower continental crust. The much faster vertical crustal motions indicated in this part of northern England, compared with SE England, are thus mainly a consequence of much greater mobility of the lower crust in the north, due to its younger thermal age and the heating effect of radioactive Palaeozoic granites. Uplift of this magnitude, which has previously gone unrecognised, may have affected post-Pliocene global climate.

The formation of gneisses in the Southern East Uralian Zone—a result of Late Palaeozoic granite ascent and emplacement

Geochemical and radiogenic age data are incompatible with the idea that the gneiss-complexes of the East Uralian Zone are microcontinental blocks that collided with the Magnitogorsk island arc during the Uralian orogeny. Field and microstructural data from the Dzhabyk and Suunduk complexes presented in this paper enable us to suggest an alternative model that interprets the gneisses as deformed margins of the batholiths of the Uralian Granite Axis. The gneisses have been formed from a granitic protolith by a continuous fabric transformation on a retrograde metamorphic path. All structures in the gneisses can be assigned to the ascent and emplacement of batholiths in an active strike–slip belt.

Significance of Devonian–Carboniferous igneous activity in Tasmania as derived from U–Pb SHRIMP dating of zircon

A series of new Sensitive High-Resolution Ion MicroProbe (SHRIMP) U – Pb ages is presented for Palaeozoic (mainly Devonian and Carboniferous) granites from Tasmania. In virtually all instances the new ages are significantly older than previously determined Rb – Sr and K – Ar ages, even though the level of emplacement had been thought to be too shallow to allow loss of radiogenic daughter products. In two extreme cases, granite bodies at South West Cape and Elliott Bay that had previously yielded Carboniferous Rb – Sr and Early Devonian K – Ar ages, respectively, are now both shown to be Late Cambrian. In northeast Tasmania, granitic activity in the Blue Tier Batholith lasted for about 22 million years, with I-type magmas being followed by S-types only toward the end of that time. The exclusively I-type granites of the Scottsdale Batholith formed about 10 million years after the initiation of igneous activity in the Blue Tier Batholith, and were emplaced over a comparatively short time interval (4 – 5 million years). The new data confirm a previously held view, based on Rb – Sr analysis, that the economically important Lottah Granite crystallised roughly 9 million years later than the nearby Poimena Granite and, therefore, could not have been derived by magmatic fractionation of the latter. A regional deformation equated with the Tabberabberan Orogeny has been dated at about 390 Ma in northeastern Tasmania, based on the presence or absence of a northwest-trending foliation in the different granite bodies. The oldest granites occur in the northeast of Tasmania, with an irregular progression of ages to the west coast. A trend of this type could have arisen in an arc-free or arc-related environment. If the latter applies, either flat subduction or processes associated with the amalgamation of eastern and western basement terranes might be the controlling mechanism. Eastern Tasmania experienced a trend from mafic I-type to progressively more felsic, largely S-type igneous activity, but the trend for western Tasmania is not as obvious. The trend for eastern Tasmania is an exception to the general rule for the Lachlan Orogen, possibly signifying that the mid-crust was relatively cool when the first I-type granites were generated. Crustal thickening during the Tabberabberan Orogeny may have been a prerequisite for the generation of later, more felsic, S- and I-types.

Age of Palaeozoic granites and metamorphism in the Tuvino-Mongolian Massif of the Central Asian Mobile Belt: loss of a Precambrian microcontinent

The Tuvino-Mongolian Massif (TMM) was previously interpreted as a Precambrian block within the Central Asian Mobile Belt. According to this idea, it consists of tectonic slices composed of metamorphic rocks of pre-Mesoproterozoic basement that experienced two episodes of regional metamorphism, and Mesoproterozoic ‘cover rocks’ that were reworked together with the basement during high-grade metamorphism. Zircon U–Pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536±6 Ma, significantly later than the deposition of the cover rocks. Regional upper amphibolite-facies metamorphism, which affected all metasedimentary units of the TMM, occurred in the short time interval of 497±4 to 489±3 Ma. We propose that there is no simple basement-cover relationship in the Tuvino-Mongolian Massif. Instead, the massif consists of tectonic domains, composed of heterogeneous metasedimentary successions with distinct pre-metamorphic and pre-tectonic histories that were juxtaposed by thrusting prior to 497 Ma and then metamorphosed under upper amphibolite facies conditions. We suggest that this thrusting event was associated with early Palaeozoic collisional processes related to amalgamation of the Central Asian Mobile Belt. Our new model for the evolution of the TMM implies derivation of Neoproterozoic terrigeneous metasediments from ∼800–900 Ma granitoids formed in an Andean-type active continental margin setting.