Bedrock Sources Research Articles

Controls on the stable Mo isotopic composition of inner-continental precipitation

The molybdenum (Mo) isotope budget of the surface environment has been well characterized in recent decades, facilitating accurate mass balance modeling of the ancient redox-driven Mo cycle. One yet unresolved component is the range of possible processes and sources that result in isotopically heavy river waters relative to continental sources. Following a recent hypothesis that isotopically heavy δ98Mo of precipitation may control the final δ98Mo of river water relative to its continental source bedrock, we investigated the δ98Mo composition of 19 snow samples from three locations in Central Europe: the Swiss Alps (“Alpine” samples) from the Hochalpine Forschungsstation Jungfraujoch (HFSJ) in both summer and winter, the Swiss Jura Mountains in winter and the French Vosges Mountains in winter. Stream waters from two snowmelt-fed streams were additionally collected from the Alpine site. Snow sample δ98Mo compositions were highly variable, ranging from −0.03 to +1.93 ‰, with no clear mixing trends indicating complex sources, source pathways, or post-depositional processing. Only the winter snow samples from the high-altitude HFSJ site had δ98Mo values consistently heavier than typical continental crust. The δ98Mo results were coupled with radiogenic Sr isotopic, major ion, and trace element compositions as tracers for three major sources of airborne ion inputs: sea salt, mineral dust, and anthropogenic aerosols. We found that the most likely source of Mo to precipitation in the lower elevation Vosges and Jura Mountains samples was isotopically light soil mineral dust, which reflected the underlying bedrock sources, with a likely additional anthropogenic component. The isotopically heavy winter snow samples at the elevated HFSJ site were attributed to a higher input of carbonate mineral dust from long-distance transport of carbonate-rich Saharan dust, which was overwritten in the summer by an influx of low elevation sources transported upwards during higher vertical thermal mixing. Finally, we concluded that precipitation has a negligible direct effect on the overall Mo content and isotopic composition of inner continental European streams and rivers relative to other sources, such as bedrock weathering. Future time-series studies may augment these conclusions to show possible heterogeneity of precipitation as well as the addition of sample sites closer to marine sources, where a larger flux of isotopically heavy marine aerosols to streams might be expected.

Quartz luminescence sensitivity enhanced by residence time in the critical zone

The emerging use of quartz luminescence properties to characterize Earth-surface processes shows promise, with optically stimulated luminescence (OSL) sensitivity proposed as a valuable tool for provenance or sediment history tracing. However, the geologic processes that lead to quartz sensitization remain unclear. Here we study the impact of source rock and surface processes on the luminescence properties of quartz sand from bedrock and modern and Late Pleistocene alluvium generated from a mountainous catchment in northern Utah, USA. Continuous wave and linear modulated OSL are used to characterize the luminescence sensitivity and intensity of the fast-decay component. We compare the OSL sensitivity with sand-grain provenance and with proxies for surface processes such as topographic metrics, cosmogenic 10Be-derived erosion rates, chemical weathering indices, and magnetic susceptibility. Late Pleistocene sediment has low OSL sensitivity and a weak fast-decay component, similar to bedrock samples from the source area. In contrast, modern alluvium is dominated by the fast-decay component and has higher and more variable OSL sensitivity, with no clear relationship to upstream bedrock source. There is, however, an inverse relationship between OSL sensitivity and catchment-averaged erosion rates and a positive relationship with chemical weathering indices and magnetic susceptibility. These metrics suggest that the modern alluvium has experienced increased residence time in the shallow critical zone compared to the Late Pleistocene sediments. We suggest that changes in hillslope processes between the effectively wetter, cooler Pleistocene and the dryer, warmer conditions of the Holocene enhanced the luminescence properties. The results suggest that climatic controls on rates and processes of chemical and mechanical weathering and sediment transport and residence within the critical zone are encoded in the luminescence properties of quartz sand.

Tracing uplift and erosion in orogenic settings using quartz luminescence sensitivity: Insights from the Northern Andes uplift

The optically stimulated luminescence (OSL) sensitivity of the fast component in quartz has been increasingly used in provenance analysis of Quaternary sediments. Quartz OSL natural sensitization is thought to be mainly controlled by the formation conditions of the source bedrock and surface processes occurring mainly in sediment source areas. Thus, quartz OSL sensitivity can be linked to distinct sediment source regions, based on their tectonic setting and erosion conditions. In this way, changes in quartz OSL sensitivity within siliciclastic successions would track variations in sediment provenance. So far, few works evaluated how the OSL sensitivity of quartz sand grains varies in sedimentary successions that experienced long-term cycles of deep burial, exhumation, and erosion. Therefore, this work aims to characterize the sensitivity of the fast OSL component of quartz sand grains retrieved from the Cenozoic sedimentary rocks of the Northern Andes basins and assess its spatiotemporal changes. We found that quartz grains with the lowest OSL sensitivity are sourced by crystalline and volcanic rocks related to the Andean Continental Arc emplaced in the Colombian Central Cordillera, reflecting the onset of denudation in orogenic sources during the Paleocene. Subsequently, increasing trends in OSL sensitivity are related to the sedimentary recycling during the Andean orogeny, reaching maximum values as a result of the progressive unroofing of Cretaceous rocks in the Colombian Eastern Cordillera, originally sourced from the low-relief Amazon Craton. Changes in quartz OSL sensitivity measured in the Cenozoic sedimentary basin-fill sequences of the Northern Andes vary according to the shifts in sediment provenance related to the orogenic construction and sediment recycling of the Andean range.

Detrital zircons and the magmatic history of Viti Levu, Fiji

We integrate the existing detrital zircon data from multiple modern river sediment samples on Viti Levu, Fiji, with the most current available geological and topographic mapping of the respective river drainage basins to compare detrital populations with potential bedrock sources. The temporal and spatial variations in zircon geochemistry supplement what is known from igneous rocks and confirm the petrological differences between plutonic and volcanic rocks from the Eocene to early Oligocene (Yavuna age, >30 Ma), middle Oligocene to middle Miocene (Wainimala age, 30–12.5 Ma), late Miocene (Colo age, 12.5–6.5 Ma) and latest Miocene (Namosi age, 6.5–5 Ma). The >30 Ma Yavuna-age zircons are restricted to areas that drain the previously mapped Yavuna Group. The 30–12.5 Ma zircons are found across central Viti Levu from west to east, and the 30–15 Ma zircons have distinctively low U/Yb and high Dy/Yb ratios. They are the best radiometric evidence of widespread early to middle Miocene arc magmatism in Fiji that was relatively U-poor. Peak deconvolution of the Colo age zircons from individual basins suggests the following ages for undated or poorly dated plutons from central Viti Levu. The large Mavuvu pluton is probably composed of multiple intrusions in the 12–10 Ma range, the Waiqa pluton is probably ca 10 Ma, and the Noikoro pluton is probably ca 9 Ma. There are zircons from unknown plutonic or volcanic sources between 8 and 7 Ma in western Viti Levu that have distinct Eu/Eu* ratios. We attribute the highest U/Yb ratios in some Colo age zircons to crustal anatexis. Namosi-age zircons are abundant in the Medrausucu Group and can be found scattered across Viti Levu.

Revisiting Ediacaran to early Cambrian depositional history of western North China: did it remain passive until the mid-Paleozoic?

Whether the North China Block (NCB) remained in extension from its cratonization to the mid-Paleozoic is questionable. Here, we conduct a synthesis of zircon U–Pb data of Statherian–Ordovician sandstones to provide a historical review of provenance changes in the western NCB through time. In contrast to typical NCB basement sources characterized by c. 2.7–1.8 Ga ages with spectral peaks at c. 2.5 and 1.9 Ga during much of c. 1.8–0.45 Ga, Ediacaran to Cambrian Stage 3 succussions contain abundant zircons with Meso- to Neoproterozoic ages. The exotic provenance, further verified by southeastward palaeo-flow, implies sources from the western Bainaimiao arc terrane (BAT), where basement rocks with suitable ages exist. Hence, the BAT should have evolved at the NCB margin before c. 0.56 Ga, but after rifting of the NCB (until c. 773 Ma). This event led to a craton-wide stratigraphic break during the Mesoproterozoic–Ediacaran. The presence of 521–515 Ma detrital zircons in Cambrian Stage 3 strata indicates subduction onset of the Palaeo-Asian Ocean before c. 515 Ma, coincident with the Precambrian–Cambrian boundary paraconformity. A sequence of depositional shifts triggered by the tectonic activities of the BAT unveils a complicated plate reorganization history of the northern NCB, contesting the view that the NCB remained passive from the Statherian. Supplementary material : Figures S1, S2 and Tables S1 and S2 are available at https://doi.org/10.6084/m9.figshare.c.7040743

The influence of bedrock faulting and fracturing on sediment availability and Quaternary slope systems, Talla, Southern Uplands, Scotland, UK

In bedrock-dominated upland terrains, local heterogeneity in the erodibility of rock masses is a critical but under-explored factor constraining sediment erosion, mobilisation and transport. Here we examine how fault-related fracturing controls variations in the erodibility and grain-size of bedrock source material at the hillslope-scale. We then assess how this influences the evolution of slope sediment systems using a case-study from the Southern Uplands, Scotland, UK. Faults are associated with fracture densities that are an order of magnitude greater than background joint- and bedding-related fractures in weakly metamorphosed sedimentary rocks. Thus, fault zones are enhanced source areas yielding more abundant, smaller clasts. They are associated with enhanced erosion, gullying and debris flows, and the development of blanket colluvium on steep open hillsides. The orientation at which faults intersect the hillslope constrains the evolution of the sediment system. Faults with trends closely aligned to the direction of slope are associated with higher erosion via confined-channel debris flow activity in strongly coupled gullies. Faults that are oblique to slope direction disrupt and segment gully systems developed on minor transfer faults. Overall, faults that are oblique to slope direction are associated with lower erosion and give rise to decoupling within debris flow systems. Inclusion of geological weighting parameters in the formulation of a sediment connectivity index to characterise the effect of faulting on the erodibility and mobility of source material improves correspondence of the model with observations and provides a simple approach that could be adapted for other sources of geological heterogeneity.

The Kuching Formation: A deep marine equivalent of the Sadong Formation, and its implications for the Early Mesozoic tectonic evolution of western and southern Borneo

The pre-Cretaceous history of Borneo remains relatively poorly studied. Limited exposures of Palaeozoic and lower Mesozoic rocks are located in NW Kalimantan and in West Sarawak, an area interpreted as the West Borneo basement. Lower Mesozoic sedimentary rocks in West Sarawak were analysed to study their depositional environments and implications for the tectonic evolution. Upper Triassic turbidites in West Sarawak, exposed in the northern part of Kuching city, informally named the Kuching Formation, are the deep marine equivalent to the more widespread, shallow marine Sadong Formation. The Kuching Formation comprises thinly-bedded stacked turbidites, consisting of incomplete Bouma sequences, with multiple, erosive channel sandstone bodies deposited under upper flow regime waning flows. Thin debrites with abundant coaly-material are interbedded with the channel sandstones. The Kuching and Sadong formations both contain volcaniclastic detritus that was derived from the westward-subducting Palaeo-Pacific plate, forming a Triassic Andean-type arc which extended from West Borneo in the south to southern China, Taiwan and Japan in the north. Palaeoproterozoic to Archean detrital zircons in the Kuching and Sadong formations reveal a Cathaysian basement source, providing insights into the nature of the West Borneo basement. Quartz-mica schists (Kerait Schist, Tuang Formation) in fault-contact with the two sedimentary successions may have formed during accretion.

Crustal Imaging across the Princess Elizabeth Land, East Antarctica from 2D Gravity and Magnetic Inversions

The Princess Elizabeth Land landscape in East Antarctica was shaped by a complex process, involving the supercontinent’s breakup and convergence cycle. However, the lack of geological knowledge about the subglacial bedrock has made it challenging to understand this process. Our study aimed to investigate the structural characteristics of the subglacial bedrock in the Mount Brown region, utilizing airborne geophysical data collected from the China Antarctic Scientific Expedition in 2015–2017. We reconstructed bedrock density contrast and magnetic susceptibility models by leveraging Tikhonov regularized gravity and magnetic inversions. The deep bedrock in the inland direction exhibited different physical properties, indicating the presence of distinct basement sources. The east–west discontinuity of bedrock changed in the inland areas, suggesting the possibility of large fault structures or amalgamation belts. We also identified several normal faults in the western sedimentary basin, intersected by the southwest section of these survey lines. Furthermore, lithologic separators and sinistral strike-slip faults may exist in the northeast section, demarcating the boundary between Princess Elizabeth Land and Knox Valley. Our study provides new insights into the subglacial geological structure in this region, highlighting the violent impact of the I-A-A-S (Indo-Australo-Antarctic Suture) on the subglacial basement composition. Additionally, by identifying and describing different bedrock types, our study redefines the potential contribution of this region to the paleocontinent splicing process and East Antarctic basement remodeling.

Detrital gold morphology and recycling around the early Miocene St Bathans paleovalley, Central Otago, New Zealand

ABSTRACT The St Bathans paleovalley developed as a major southwest-draining fluvial system during early Miocene rejuvenation of the Central Otago landscape. Rounded quartz pebbles and detrital gold were recycled from Eocene quartz pebble conglomerates into the headwaters of this Miocene paleodrainage system on the northern margin of the Otago Schist belt. Gold particle morphology is mostly irregular and rough, locally with relict gold crystal shapes, plus a minor component of well-defined flakes. Incremental rounding and surface deformation at the 10 µm scale has occurred through several recycling events in Central Otago, hindering linkage of Miocene and younger placer gold to ultimate basement sources. More intense deformation of the gold, forming thin flakes, occurred in the lower reaches of the Miocene paleodrainage system as gold was transported >100 km to eastern Southland. Hence, the St Bathans paleovalley has been an important source of gold for historic mining in Central Otago and Southland.

Spatial trace and rare earth elemental compositions of surficial sediments on the high Mountains, Central Asia, and implications for desert sand and Asian dust

As one of the major mountain systems in Central Asia, the Tianshan Mountains provide vast amounts of sediment to adjacent areas. The elemental compositions of surficial sediments on the piedmont of the Tianshan Mountains were investigated to track surface processes. However, spatial comparisons of the elemental compositions of the sediments in the Tianshan Mountains and adjacent areas remain limited. To address this issue, this study investigated the trace element compositions (including rare earth elements (REEs)) of surficial sediments in a vast region ranging from the Pamirs, Tarim Basin, and Tianshan Mountains to the Junggar Basin. According to statistical analyses of the selected source tracers of trace element and REE ratios, this large region can be geochemically divided into two regions. One region comprises the Pamirs, Tarim Basin, and South and Central Tianshan Mountains (PTSC), where the sediments are characterized by contrary source tracer ratios compared with the other region comprising the Yili Basin, North Tianshan, and Junggar Basin (YNTJ). We suggest that the source bedrock is the primary factor controlling the trace element and REE compositions in these two regions. The trace element and REE compositions in the PTSC and YNTJ regions represent signatures of the two endmembers of Asian dust. The trace compositions of loess deposits on the Chinese Loess Plateau (CLP) and Asian dust archived in the North Pacific Ocean (NPO) mostly resemble those of sediments in the PTSC region, suggesting that Asian dust is mainly derived from sediments eroded from the North Tibetan Plateau. Our study enriches our understanding of the source materials of Asian dust archived in the CLP and NPO.

GEMAS: Chemical weathering of silicate parent materials revealed by agricultural soil of Europe

The chemical composition of soil represents to a large extent the primary mineralogy and geochemistry of the source bedrock, the effects of pre- and post-depositional weathering and element mobility, either by leaching or mineral sorting with the addition of secondary products such as clays. Agricultural soil in Europe (0–20 cm, 33 countries, 5.6 million km2) was sampled during the continental-scale soil mapping project (GEochemical Mapping of Agricultural and grazing land Soil - GEMAS). Total element concentrations were determined by wavelength dispersive X-ray fluorescence spectrometry. Bulk content of major elements (SiO2, TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, P2O5) in soil has been used to calculate weathering indices such as chemical index of alteration (CIA), reductive and oxidative mafic indices of alteration (MIA), the change in mass balance (τ calculation relative to immobile Nb) for soil derived from silicate bedrocks (granite, gneiss and schist) at the European continental-scale, which are useful tools for evaluating chemical weathering trends.The greatest variation of CIA values is exhibited by soil derived from granite, followed by soil derived from schist. The CIA results show clearly that Ca and Na are removed from agricultural soil (0–20 cm) during soil development. Geographical distribution of weathering indices shows that weak chemical weathering of agricultural soil samples, sourced from gneissic and granitic bedrock, occurs in the Fennoscandian Shield; intermediate chemical weathering in granite, schist and gneiss occurring in the Iberian Peninsula and France, and intense chemical weathering in schist sourced agricultural soil occurring in areas with intense rainfall and moderate temperature (Wales, England, Ireland and western Iberia). In addition, values of CIA and MIA, higher than those for standard rock compositions, suggest more active enrichment/depletion processes during soil formation and subsequent soil weathering/maturation.The range of the elemental mass transfer coefficient τ is wide for agricultural soil derived from gneiss and granite, and narrow for agricultural soil sourced from schist parent rocks. The weathering behaviour of chemical elements, assessed by using the τ mass transport model, suggests an order of susceptibility of K ≈ Na > Al > Fe > Ca > Mg > Ti.Weathering indices and gain-loss mass transfer coefficients, tested on agricultural soil samples, provide an insight to the weathering processes affecting the silicate parent rocks and their impact on soil development at the European scale. This can have further implications for evaluation of soil nutritional conditions and mitigating of soil erosion and deprivation.

The Use of Pyrite Chemistry to Vector toward Gold Mineralization, an Example from the Drogo Prospect, Nunavut, Canada

Abstract Anomalous high grade Au mineralization has been found in float material at the Drogo Prospect, Nunavut, Canada. However, traditional exploration methods have thus far been unable to find the bedrock source of the mineralization. Recent studies have shown that pyrite trace element chemistry, as determined using LA-ICP-MS, varies between Au-forming and barren fluids. To test whether this also occurred at the Drogo Prospect, we analyzed pyrite in rock samples, including Au-rich samples, moderate Au grade samples, and barren samples. However, little variation in pyrite trace element chemistry was observed, suggesting that pyrite trace element chemistry may not be an effective exploration tool at the Drogo Prospect. To confirm these observations, cluster analysis was used to determine if natural clustering of the data could highlight which samples were more likely to be Au rich; again, no statistical differences could be found between the samples. In contrast to our working hypothesis, this suggests that pyrite trace element chemistry is not an effective exploration tool to find the bedrock source of the Drogo boulder trend.

Variscan zircons everywhere? Multi-stage sedimentary recycling in Central Europe

Abstract The North German Basin is part of a Central European-wide sedimentary recycling system that has existed since at least the Neoproterozoic. Understanding the evolution of such a system is crucial for further studies, as the North German Basin inherits vast natural gas resources and may act as an intermediate sink for younger strata. This study presents new detrital zircon morphology, trace element and U–Pb age data obtained from Upper Rotliegend II strata (Upper Permian). Detrital zircon dating revealed Cambrian, Carboniferous and Permian main age clusters. There are also several minor Paleo-, Meso- and Neoproterozoic age clusters. Zircon grain morphologies show completely unrounded to completely rounded grains throughout each age range. The heterogeneity of the data is key to deciphering the sedimentary history of the Central German Basin, as the basin fill is most likely a mixture of (repeatedly) recycled material and also directly derived from bedrock sources. These results are supported by trace element data, which show a wide range of values indicating different magma sources. This study further explores the dispersal patterns of detrital zircon over time and demonstrates their complexity.

Syn‐ and post‐rift lower crustal flow under the Sunda Shelf, southern Vietnam: A role for climatically modulated erosion

AbstractTectonic subsidence on rifted, passive continental margins are largely controlled by patterns of extension and the nature of strain partitioning in the lithosphere. The Sunda Shelf, adjacent to the SW South China Sea, is characterized by deep basins linked to regional Cenozoic extension associated with propagating seafloor spreading caused by slab pull from the south. Analysis of seismic reflection profiles and drilled sections crossing the Nam Con Son and Cuu Long basins highlight Oligocene extension, with most of the thinning concentrated in the ductile mid‐lower crust. Upper crustal extension was modest and ductile flow is inferred to be directed northwestwards, towards the oceanic crust. Basin inversion occurred in the Mid Miocene, associated with the collision of the Dangerous Grounds Block and Borneo. Subsequent accelerated tectonic subsidence exceeded predictions from uniform extension models assumed to relate to extensional collapse after inversion. We correlate this to a period of faster erosion onshore driven by strong monsoon rains in Indochina and Peninsular Thailand at that time. Erosion of the onshore basement, inducing rock uplift and coupled with loading of the basins offshore, drives ductile mid‐lower crustal flow, likely to the northeast under Indochina, and/or to the west where Plio‐Pleistocene subsidence of the shelf is very slow. Significant sediment delivery from the Mekong River into the Cuu Long Basin began in the Late Miocene and migrated seawards as the basin filled. Mass balancing suggests that the basins of this part of the Sunda Shelf are filled through erosion of bedrock sources around the Gulf of Thailand. There is no need for sediment delivery from a major river draining the Tibetan Plateau to account for the deposited volumes.

EVALUATION OF DEPTH TO BASEMENT, HEAT FLOW, AND LEAD-ZINC MINERALIZATION FROM ANALYSIS OF AEROMAGNETIC DATA IN SOME PARTS OF SOUTHERN BENUE TROUGH

The techniques of aeromagnetic data analysis were employed to elucidate depth to magnetic basement sources, lead-zinc (pb-zn) mineralisation and heat flow in parts of Southern Benue Trough. The study area covers an area extent of about 18,150 sq. Km, latitude 05 0 30’N - 06 0 30’N and longitude 007 0 30’E - 009 0 00’E. The digital aeromagnetic data consisting of Total Magnetic Intensity with geospatial attributes covering six (6) sheets 302 (Nkalagu), 303 (Abakaliki), 304 (Dangha), 313 (Afikpo), 314 (Ugep) and 315 (Ikom) were used in this study. Several magnetic digital filtering methods such as reduce to pole, low/high pass, etc were applied on the data before analysis for lineaments, mineralisation, and depth to magnetic sources. Zones of broad magnetic anomalies were found in the north eastern and south eastern part of the study area like Otam-Izekwe, Enyigba, Uburu, Abakaliki, Ndieze Izzi, Isiagu, Obubra, Oturekpe, Mkpani, Idomi, Enona, Ugep, etc. Depth estimates to the magnetic basement sources from spectral analysis range from 0.65km to 1.155 km, while depth estimate from source parameter imaging (SPI) stretches from 0.679km to 7.723km. The north western part of the study area is the deepest. Pb-zn mineralization within the study area is found to be structurally controlled trending in NE-SW and NW – SE directions. The heat flow values range from 0.65 – 1.95Wm -2 , pb-zn mineralization is within 0.9 – 1.4Wm -2 and Curie point depth range from 0.75km to 2.1km. Keywords: Depth to basement, Heat flow, Lead-Zinc, Mineralization, Aeromagnetic data DOI: 10.7176/JEES/13-2-06 Publication date: March 31 st 2023

Origin of heavy rare earth elements in highly fractionated peraluminous granites

Rare earth elements (REEs + Y) play an important role in modern industry. Heavy REEs (HREEs) are particularly critical because of their relative scarcity in nature. Global HREE resources are predominantly present in granitoid weathering crusts in southern China. Although it is well known that REEs are generally enriched in alkaline rocks, in contrast, the parental granitoids in most Chinese HREE deposits are peraluminous. Here, we examined different REE mineral compositions, distribution patterns, and Nd isotope ratios of the granitoid complex in Zudong, South China, whose weathering crusts form the largest HREE deposits globally. The complex is composed of granodiorite, biotite-muscovite, and muscovite alkali-feldspar granites, whose REE patterns change from HREE depletion to enrichment relative to light REEs (LREEs) with increasing negative Eu anomalies. They are not fractional crystallization products from the same parental melts owing to the different zircon U-Pb ages (∼191, ∼169, and ∼154 Ma for granodiorite, biotite-muscovite, and muscovite granites, respectively). However, magmatic zircons from the three types of granitoids show consistent initial Hf isotope compositions [εHf(t) = 3.4 to 8.6], indicating they evolved from the same sources. The granodiorite contains primary LREE minerals with low εNd(t) values (−13.8 to −10.1), decoupled from high zircon Hf isotopes. This indicates a garnet-containing basement source resulted in Lu (HREE) remaining preferentially in the garnet phase, with Hf entering the melt. This contradicts the high HREE concentration recorded in the biotite-muscovite and muscovite granites, which contain abundant HREE minerals associated with muscovite, fluorite, and recrystallized quartz. The whole-rocks and their secondary REE minerals show variable and higher initial Nd isotope ratios [εNd(t) = −3.7 to 2.9 and −11.4 to −7.1 for biotite-muscovite and muscovite granites, respectively] than the early granodiorites and their primary REE minerals, indicating minimal 143Nd was derived from radioactive decay of 147Sm in the basement and wall rocks. This constrained the contribution of external REE-, volatile-rich liquids, which drove the segregation of highly evolved silicate melts from the magma chamber. This increased volatile saturation and fluid exsolution, which may have mobilized the HREEs and metasomatized the granites during their emplacement. Therefore, we infer that external fluid metasomatism appears to be crucial for the HREE enrichment in highly fractionated peraluminous granites.

Provenance analysis and thermochronology of the Chivillas Formation, Mexico: a record of basin formation and inversion in the southern Sierra Madre Oriental during the Early Cretaceous–Palaeogene

ABSTRACT Sandstone petrography, zircon and apatite U-Pb geochronology, and apatite geochemistry of Early Cretaceous siliciclastic turbidites interbedded with basalt (Chivillas Formation) in the southern Sierra Madre Oriental, Mexico, indicate local provenance from westerly basement sources. Three main detrital populations were observed in both the zircon and apatite geochronology: Meso–Neoproterozoic, Carboniferous–Permian, and Early Cretaceous. Apatite Sr/Y vs light rare earth element discrimination diagrams indicate that most Precambrian grains have affinity with high-grade metamorphic rocks and most Phanerozoic grains with igneous rocks. These results are consistent with derivation from exposed crystalline sources within the Acatlán-Oaxacan block including the Grenvillian Oaxacan Complex, the East Mexico Arc, and the Early Cretaceous continental arc. The compositional disparities observed in individual samples reflect differences in source of individual turbiditic flows comprising the Chivillas Formation. We surmise that various rivers from the northern and southern Acatlán-Oaxacan block drained into the basin during the Early Cretaceous. Furthermore, detrital apatite fission-track analyses of Chivillas sandstones yielded mostly 42–40 Ma ages (Eocene) that post-date the shortening of the Mexican orogen in the southern Sierra Madre Oriental. Thermal models calculated based on track lengths indicate rapid exhumation of Chivillas rocks at 46–36 Ma, which is the time of the development of the Tehuacán Valley and reactivation of the Oaxaca Fault System as a normal fault.

Provenance, tectonic setting and palaeoclimate of Proterozoic Jiran Sandstone, Southeastern Rajasthan, India: A petrographic approach

Proterozoic Jiran Sandstone rests unconformably on Binota Shale and Khori-Malan Conglomerate. The Jiran Sandstone is comprised mainly of fine to medium-grained, varicolored, thickly bedded sandstones, showing diverse primary sedimentary structures such as ripple marks, planar, and trough cross-bedding. Petrographically, Jiran Sandstone is of mainly quartzarenite which is composed of varieties of quartz with ultra-scarcity of feldspar, lithic fragments, micas, and heavy minerals. Quartz is more abundant mineral shown by X-ray Diffraction Analysis. The provenance, tectonic setting, and paleoclimatic condition of the Jiran sandstone were evaluated using integrated petrographic studies. Analysis pursuant, monocrystalline and polycrystalline quartz grains and heavy minerals are driven primarily from metamorphic and plutonic Precambrian basement source rocks of a craton interior setting with a minor quartzose recycled sedimentary source material. Intensive chemical weathering in warm and humid paleoclimate is indicated by lack of feldspar and rock fragments.

U–Pb detrital zircon geochronology and source provenance in the Moroccan Meseta (Variscan belt): A perspective from the Rehamna massif

Metasandstones from early Cambrian to early Carboniferous stratigraphic successions were sampled in the Rehamna massif of the Western Meseta in Morocco. The early Cambrian sample shows a single Paleoproterozoic population at ca. 2 Ga suggesting a local basement source. The Ordovician sample is largely dominated by a Cryogenian-Ediacaran population and minor Paleoproterozoic peaks. The Devonian sample reveals age populations similar to North-West African Cambrian to Devonian age spectra indicating that the southern-derived West Gondwana source essentially pertained up to the Devonian. The two early Carboniferous samples show more heterogeneous zircon age spectra with a marked Ediacaran peak accompanied by Paleoproterozoic and Mesoproterozoic sub-peaks indicating important re-organization of the drainage systems. One sample also shows presence of Upper Devonian to early Carboniferous zircon grains, which suggests local magmatic sources associated to the formation of intracontinental extensional basins. The comparison of detrital zircon spectra with paleogeographic reconstructions indicate that the early Carboniferous change in detrital zircon sources can be interpreted in the framework of the opening of the Paleotethys ocean with coeval erosion of orogenic topographies linked to the emplacement of a Mid-Variscan Allochthon, and/or collision of an Avalonian indenter to the north.

Early Devonian volcanic facies, central Lachlan Orogen, New South Wales: implications for tectonic and metallogenic models

The central Lachlan Orogen volcanic belt contains predominantly silicic and explosive eruptive products emplaced between ca 428 and 412 Ma within elongate basins over an area ∼350 km long and 20 km wide. The volcanic rocks are exposed in two parallel, south-southeast-trending belts between Cobar and Albury, NSW, and have S-, I- or A-type magmatic affinities. The southernmost sequences include subaerial rocks of the Gurragong Volcanics and Walleroobie Volcanics (herein renamed the Gurragong Group), and the newly described Culcairn Group. The Gurragong Group comprises crystal-rich Walleroobie Ignimbrite, lithic-rich Corella Ignimbrite, Mount Ariah Rhyolite coherent facies and newly recognised airfall Indarra Tuff. The Culcairn Group includes the Budginigi and Wallandoon ignimbrites. The Budginigi Ignimbrite includes crystal-rich ignimbrite, the Hadrells Hill Member hyaloclastite breccia and the Soldiers Quarry Member (coherent rhyodacite). The Wallandoon Ignimbrite is compositionally intermediate, metaluminous and amphibole-bearing. The volume estimate of southern pyroclastic units ranges from 15 to 75 km3 per succession. The northern volcanic sequences include the Canbelego-Mineral Hill Volcanic Belt, Mount Hope Group and Ural Volcanics, all deposited in a shallow- to deep-marine environment during the Cobar Basin opening. The subaerial depositional environment of the southern volcanic successions, the proximity of exposed Silurian granitoid, and contact metamorphic overprint of the Devonian volcanic successions by contemporaneous intrusions implies the volcanic pile was thicker, and synchronous uplift was occurring. Variation in the aluminium saturation index from peraluminous to metaluminous, and a variation in high-field-strength elements in the almost exclusively felsic igneous rocks may reflect variation in the source rocks and temperature of partial melting. The resulting metallogenic patterns reflect these basement differences. Metals in the south resulted in intrusion-related Sn–W; in the north, the marine environment proximity to a major transcrustal structure and variation in the fertility of the lower crustal rocks led to the development of diverse mineral systems. Key points The central Lachlan Orogen Siluro-Devonian volcanic belt is predominantly silicic, containing explosive eruptive products emplaced within a series of elongate basins between Cobar and Albury, NSW. The southernmost sequences, the Gurragong Group and the newly described Culcairn Group are subaerial; the northern sequences include the Canbelego-Mineral Hill Volcanic Belt, Mount Hope Group and Ural Volcanics and are wholly submarine. Variations in the igneous rock chemistry and regional mineralisation reflect the variation in the basement source rocks, related fertility and associated structures.