Volcanic Basement Research Articles

Integration of new zircon U–Pb ages with biostratigraphy to establish a high-precision age model of the Miocene Nakayama Formation on Sado Island in Central Japan

The most common age constraint for the diatomaceous sediments is biostratigraphy of siliceous microfossils. Although biostratigraphy is a powerful tool to establish stratigraphy and correlate with sedimentary sequences in other sites, biostratigraphy generally includes uncertainties difficult to evaluate. In this study, we measured zircon U–Pb ages of eight tuff beds intercalated with diatomaceous mudstone of the Nakayama Formation on Sado Island in Central Japan and integrated the U–Pb ages with diatom and radiolarian biostratigraphy, whose ages and errors were re-evaluated by this study, to establish an age model precisely representing the sedimentary age. Two tuff beds in the upper and middle part of the formation offered zircon U–Pb ages of 6.7 ± 0.2 Ma and 10.87 ± 0.07 Ma, which are consistent with biostratigraphy, and provided a good example of effective integration of zircon U–Pb ages with the biostratigraphy. On the other hand, zircon U–Pb ages of the other six tuff beds in the lower part are around 12 Ma and not distinguishable from each other. In addition, older zircon grains in the 6 tuff beds are assembled in the interval from 30 to 20 Ma, which is consistent with the age of the volcanic basement rocks forming most part of Sado Island. Similarities in chemical compositions of glass shards and age distributions of zircon grains indicate that the volcaniclastic components in the tuff beds should originate from single or associated magmatic activities.

On Volganism and Tectonics in the Evolution of the Guyots of the Magellan Seamounts (Pacific Ocean)

This report is devoted to the analysis of original geological materials on the Magellan Seamounts in the Pacific Ocean, obtained by the authors in numerous voyages of R/V ʺGelendzhikʺ. This chain of guyouts does not have a common volcanic basement and apparently was formed in the second half of the Early Cretaceous on the oldest (middle-last Jurassic) fragment of the Pacific plate. The main points of view on the genesis of the Magellan Mountains are as follows: either they originated at the crossing of the transform fractures, or the Pacific plate moved them from the southern hemisphere to their present-day position. Because of their high degree of study, the Magellan Seamounts are one of the key sites for understanding the mechanism of the origin of linear chains in the ocean. A comprehensive analysis of new geological on the Magellan Seamounts has established the important role of magmatism and tectonics in the formation of the modern morphological shapes, sedimentation and influence on the paleooceangraphy. The periodic renewal of these processes from the Early Cretaceous to Late Cenozoic can be traced in the cyclicity of sedimentation, the permanent growth of ore crusts and the formation of secondary volcanic domes and cones.

Reinterpretation of Marine Magnetic Anomalies in the Bay of Bengal: Implications for Plate Tectonics During the Mid‐Cretaceous

AbstractThe breakup of India from Antarctica in the Early Cretaceous and subsequent seafloor spreading in the NW‐SE direction initiated the formation of the Bay of Bengal and its conjugate Enderby Basin. Two hypotheses are proposed to explain the evolutionary tectonics between India and Antarctica. One involves a single breakup, while the second invokes another breakup episode. Further, a major reorganization of the plate boundaries in the mid‐Cretaceous caused the spreading center to align E‐W by Late Cretaceous. The tectonics involved in this spreading direction change is unknown and difficult to understand as it occurred in the mid‐Cretaceous, coincident with the Cretaceous Normal Superchron. In the present study, we have generated a revised magnetic isochron map of the Bay of Bengal and used the modulus of its analytic signal to understand the evolutionary tectonics between the two plates, India and Antarctica. Mesozoic magnetic anomalies M12n to M0, and mid‐Cretaceous incursion Q2 (108 Ma) are identified in the western basin of the Bay of Bengal, while Q2, Q1 (92 Ma) and Late Cretaceous isochron A34 are inferred in its eastern basin. Gravity grid reconstructions demonstrate asymmetrical crustal accretion in the conjugate corridors. Fan‐shaped spreading and a southward ridge jump between 108 and 92 Ma in the eastern basin, aided the ∼40° clockwise rotation of the spreading center. The presence of volcanic basement beneath the additional crust suggests on‐axis mode of formation for the Ninetyeast Ridge by 100 Ma. The present study favors the two‐phase breakup model for the evolution of these plates.

3.7 billion year old detrital sediments in Greenland are consistent with active plate tectonics in the Eoarchean

AbstractPlate tectonic processes modulate element cycling, crust generation, and differentiation, yet at what point in Earth’s history these processes emerged remains debated. Here we present evidence that parts of the >3.7 Ga Isua Supracrustal Belt formed within a fore-arc setting, consistent with the operation of plate tectonics in the Eoarchean. We show that the oldest known sequence of detrital meta-sedimentary rocks were deposited conformably above chemical sediments on a volcanic basement. Mineral and trace elemental compositions show that turbiditic and pelagic detrital sediments were derived from terrains, comprising both basalts and differentiated tonalitic igneous rocks. The boninitic volcanic basement would have formed in a tensile environment before the adjacent terrains which sourced the clastic sediments. This suggests formation within a fore-arc during the initial few million years of subduction. This environment may have facilitated the local proliferation of life suggested by the frequent occurrence of layers rich in biogenic graphite.

The Early Cenozoic Volcanism of the Northern Okhotsk Region System of Grabens and Faults

Abstract —We studied geochemical compositions of the early Paleocene basaltic and andesite dikes associated to linear zones of the Lankovo–Omolon shearing system (Northern Okhotsk region) and basalts of the Evdyreveem volcanic field associated to the Okhotsk–Penzhinsk fault system, and compared them to other synchronous manifestations of basic volcanism: andesibasalts and andesites of the studied earlier Garmanda field, as well as with the Late Cretaceous basalts of the Mygdykit Formation of the Northern Okhotsk region, roofing the Okhotsk–Chukotka volcanic belt. The isotopic composition of Sr and Nd in dikes, the distribution of major and trace elements with the ratios of noncoherent elements indicate the formation of volcanic bodies in the environment of continental margin rifting, which is confirmed by the combination of depleted, intraplate and above subduction geochemical features of their composition. Such behavior of the elements indicates multi-stage processes of the earlier Mesozoic supra-subduction fluid metasomatosis. Melting of an ancient buried Cretaceous slab may explain the appearance of such “above subduction” marks as the Nb–Ta negative anomalies in the studied basaltoids. Andesite dikes are characterized by higher isotope ratios of Nd and lower Sr, with lower absolute concentrations of trace elements and more pronounced anomalies on spider plots.

Characteristic and tectonic significance of zircon SHRIMP U-Pb geochronology and Hf-O isotopes from ~2.9Ga metamorphic basic volcanics in Qixia area, North China Craton

Characteristic and tectonic significance of zircon SHRIMP U-Pb geochronology and Hf-O isotopes from ~2.9Ga metamorphic basic volcanics in Qixia area, North China Craton

Coal facies of the Middle Permian Baruunnaran deposit, South Mongolia

The Baruunnaran coal deposit is located in the northeastern part of the South Gobi Basin, southern Mongolia, and hosted in the middle Permian Tavantolgoi Formation. In this paper we present indices of coal facies determined from 34 coal samples obtained from three seams in the lower and upper part of the formation (III, IXG and X), by studying their organic petrography and the geochemistry of the coal ash. The results of coal petrography revealed that seams III, IXG and X are composed of 46.9-80.9 vol.% vitrinite, 11.6-47.5 vol.% inertinite and 1.2-18.2 vol.% liptinite. In samples from seams III and X the average content of mineral matter is low at 11 and 13.4 vol.%, respectively, and 6.3 vol.% in seam IXG, and consists of clay, silica, pyrite, carbonate, and other minerals. The inorganic content mostly occurs as fillings of cell cavities, cracks, and fissures of vitrinite and inertinite macerals. The vitrinite random reflectance values range from 0.81-1.07%. The Gelification Index and Tissue Preservation Index suggest the peats accumulated in wet forest swamp environments with high tree density. The majority of the seams accumulated in mildly oxic to anoxic conditions with good tissue preservation. The peat mire water ranged from weakly to strongly acidic. Further, it was determined by Al2O3/TiO2 ratios that the clastic sediments were probably sourced from volcanic basement characterized by intermediate felsic composition.

A geochemical perspective on the genesis of Cenozoic basic volcanism in northeastern Turkey: an overview of metasomatism and heterogeneity of the sub-continental lithospheric mantle in a post-collisional setting

A geochemical perspective on the genesis of Cenozoic basic volcanism in northeastern Turkey: an overview of metasomatism and heterogeneity of the sub-continental lithospheric mantle in a post-collisional setting

Geochronology, Eruption Sequence and Geochemistry of Mid–Late Jurassic Volcanics South of Manzhouli: Petrogenesis and Implications for Mesozoic Tectonic Regime Transformation

AbstractTo the south of Manzhouli, Hulunbuir, Inner Mongolia, experienced a tectonic regime transformation from compression to extension in the mid‐Mesozoic. Based on systematic research of the volcanics, petrology, volcanic facies, chronology and geochemistry of rocks in the Buridun area, two stages of volcanics are identified. The first stage named the trachyte series was formed in the late Middle Jurassic (167–163 Ma), its eruption rhythm is pyroxene trachyandesite–trachyandesite–trachyte, and its origin rock is basic volcanics from thickened lower crust, with a tectonic setting in the collision orogeny after the closure of the Mongolia Okhotsk Ocean (MOO). The second stage is a bimodal volcanic rock, formed in the early Late Jurassic (163–160 Ma). The eruption rhythm of basic volcanics in this stage is basaltic andesite–basalt–olivine basalt, which comes from the metasomatized lithospheric mantle, the acidic volcanics of which being characterized by the eruption rhythm of sedimentary‐explosive‐overflow facies, which came from the partial melting of newly formed lower crust, and this shows the characteristics of A‐type granite; the tectonic setting is extension of the lithosphere after collision and closure of the MOO. The changes in the formation age and tectonic setting of the two stages of volcanics demonstrate that the transition time from the compressive system to the extensional system south of Manzhouli is about 163 Ma.

Petrogenesis of the Granitic Dykes in the Yangshan Gold Belt: Insights from Zircon U-Pb Chronology, Petrography, and In-Situ Hf Isotope Analysis

The Yangshan gold belt is renowned for its igneous rock formations, particularly dykes that form in tectonically weak zones. Some of these rock formations exhibit a close spatial relationship with gold mineralization, and a tiny portion of the granitic dykes serve as gold ore bodies by themselves. In order to investigate the nature of granitic dykes and their association with gold mineralization, we conducted a comprehensive study consisting of zircon U-Pb chronology, petrography, and in situ Hf isotope analysis of 25 granitic dyke samples collected from east to west across the belt. According to LA-ICP-MS zircon U-Pb dating results, the granitic dykes inherited zircon ages that are concentrated between 745.0 and 802.0 Ma, and magmatic intrusion ages that mainly fall between 201.0 and 213 Ma. Moreover, the granitic dykes display a calc-alkaline to high-K calc-alkaline peraluminous series, which is relatively enriched in light over heavy REE, with moderate Eu anomalies. These dykes are rich in large-ion lithophile elements and poor in high-field-strength elements. The zircon Lu-Hf isotope data range from εHf(t) values of −1.5 to 0.1, mantle model (TDM1) ages range from 859 to 937 Ma, and crustal model (TDM2) ages range from 1111 to 1218 Ma. The granitic dykes found in the Yangshan gold belt were formed between 200 and 213 Ma ago, during a period of intracontinental extension following the late collision between the Yangtze plate and Qinling microplates. These dykes originated from the volcanic basement of the Mesoproterozoic Bikou Group, which was formed by the melting of the upper crust under the crustal thickening caused by the subduction and collision of the Qinling microplate. Subsequently, the dykes were transported along a tectonically weak zone, assimilating surrounding rocks and undergoing a transformation from “I”-type to “S”-type granite before finally evolving into granite with specific “A”-type characteristics. Our study provides new insights into the petrogenesis of granitic dykes in the Yangshan gold belt, as well as the relationship between gold mineralization and magmatic activity, which has significant implications for mineral exploration and the geological understanding of gold mineralization in this region.

Early Oligocene gravity-flow deposits at the base of the Meiji Reef system in the South China Sea, and their palaeoclimate significance

In this paper, we describe gravity flow facies that form the base of the Meiji Reef system in the South China Sea, and discuss their palaeoclimate significance. These Early Oligocene (26.1–23.5 Myr) deposits occur in Well NK1 and comprise a 110.6 m thick succession developed on top of volcanic basement. The deposits can be divided into two intervals, both of which are dominated by debris flow facies. The lower 17.6 m thick interval has a gravel content of 50–65% with parent rocks including dacite, basalt, tuff, and coral reef fragments. From base to top, these units coarsen upward and show increasing gravel contents. Gravel clasts are well-rounded but poorly sorted (diameter 10–350 mm), which, together with their complex composition, indicate transportation by the gravity flows from the near-shore volcanic rock basement to the slope. The upper 93 m thick interval comprises abundant bioclastic materials, including benthic foraminifera, crustal red algae, and red algae detritus, with a few volcanic gravel clasts. The content of volcanic rock gravels decreases towards the top, which may be attributed to the reduction or disappearance of volcanic rocks in the provenance area due to the establishment of coral reef facies that cap the succession and dominate the overlying interval from a core depth of 885.6 m to the seafloor. Based on U-Pb dating, the cap-shaped red algae laminae at the facies transition were formed at 24.6 ± 1.1 Ma, dating the earliest initiation of reef building to end-Oligocene times. The emergence of gravity flow sediments during the early Oligocene may be attributed to the onset of the Antarctic glaciation and concurrent regional tectonic activities that led to a shallowing of the sea. The origin of debris with high roundness characteristics is believed to be associated with transportation along the slope triggered by tectonic and climatic events, resulting in the formation of debris flow deposits. Studying the gravity flow facies at the Meiji Reef system is crucial for determining the onset of reef-building and helps establish the relationship between gravity flow deposition, paleoclimate, and regional tectonic evolution in the South China Sea.

Sverdrup Basin Composite Tectono-Sedimentary Element, Canadian Arctic Islands

Abstract The Sverdrup Basin Composite Tectono-Sedimentary Element (CTSE) covers 210 000 km 2 in the Canadian Arctic Archipelago. The CTSE was initiated in the Early Carboniferous by rifting of highly deformed Early Paleozoic strata and contains a maximum of 15 km of Carboniferous–Eocene strata. Eight phases of basin development have been recognized, with each being characterized by a specific combination of tectonic and depositional regimes. The phases are separated by intervals of uplift and tectonic reorganization, and each resultant first-order sequence is regarded as a separate TSE. Carbonate sedimentation was dominant in the Late Carboniferous and Early Permian, with clastic sedimentation becoming more common in the Middle Permian. Source areas lay to the east, south and north. In the Triassic, clastic sedimentation rates increased; and by the end of the Triassic, the central basin was filled. A shallow seaway was present throughout the Jurassic. In the Early Cretaceous, subsidence rates and clastic supply increased significantly, and basic volcanism occurred over most of the CTSE. Subsidence rate, sediment supply and volcanism decreased in the Late Cretaceous. The basin was progressively deformed in the Paleogene (Eurekan Orogeny), with local foreland deposits reaching 3 km. Seventeen oil and gas fields have been discovered on salt-cored, Eurekan anticlines. A combination of stratigraphic and structural traps, involving Triassic–Jurassic strata, has the greatest potential for future hydrocarbon discoveries.

Morphology, Structure, and Composition of Zircon from Metasandstones of the Alkesvozhskaya Sequence (Polar Urals)

A description of the features of morphology, internal structure, and analysis results of Th/U and Zr/Hf ratios in zircons from sandstones of the gold-bearing Alkesvozhskaya sequence, located at the basement of the Paleozoic section in the Subpolar Urals, is presented. It has been established that most of the studied zircons are primary igneous grains of the first weathering cycle. The detrital zircon source was probably in formations close in age, underlying or located nearby felsic and basic volcanics of igneous complexes of different stages of formation of Timanides-protouralides, associated granitoids, and intrusive mafic formations. Small part of well-rounded zircon Pre-Riphean grains can be redeposited from the Riphean metasedimentary formations, which, in turn, inherited it from the rocks of the ancient basement of the East European Platform.

Timing of Tectonic and Magmatic Events in the Philippine Sea Plate Since 50 Ma From High‐Resolution Magnetostratigraphy of IODP Site U1438

The Philippine Sea Plate in the West Pacific is a unique natural laboratory to study subduction dynamics and the evolution of upper plate magmatism following subduction initiation. To investigate these processes, International Ocean Discovery Program Expedition 351 recovered at Site U1438, located in a rear-arc position, a complete sedimentary sequence from recent age to the early Eocene and the top of the underlying volcanic basement. The recovered cores offered the opportunity to study for the first time and in unprecedented detail the styles, products, and timing of the volcanic events that marked the emplacement, growth, and demise of the Kyushu-Palau volcanic arc following the inception of the Izu-Bonin-Mariana subduction. Here, we report a magnetostratigraphy for Site U1438 based on ∼60,000 remanence directions isolated from 1,063 archive half core sections and 429 discrete specimens. We identified 142 magnetic reversals and correlated 115 of them with the geomagnetic polarity timescale. When combined with additional biostratigraphic and geochronological constraints, our magnetostratigraphy allowed construction of a high-resolution age model for Site U1438 and the determination of changes in sedimentation rates. We show that following subduction initiation at 52–50 Ma and the emplacement of basalts in the rear-arc at 48.7 Ma, a diffuse volcanism in the rear-arc (48.4–45.6 Ma) preceded the true emplacement of the Kyushu-Palau arc at 40.2 Ma, which then grew through four compositionally distinct eruptive phases until 28.8 Ma. Subsequent rollback of the Pacific slab triggered rifting of the arc (28.8–24.3 Ma) and ultimately back-arc spreading in the Shikoku and Parece Vela basins.

Lithostratigraphy of the Bumbeni Complex and its associated subdivisions, South Africa

Abstract The Bumbeni Complex represents an Early Cretaceous volcanic complex that developed at the southern termination of the Lebombo mountain range in northern KwaZulu-Natal, South Africa. The volcano-sedimentary complex intrudes and disconformably overlies the Lebombo Group of the Karoo Large Igneous Province. The volcano-stratigraphy is defined by an assemblage of volcanic and volcaniclastic rocks as well as late stage quartz-syenite intrusions. Based on current geological and geophysical studies the complex is considered to represent the remnants of a collapsed cauldron structure. The stratigraphic succession is intricate and laterally discontinuous, comprising mafic and felsic lava flows, welded and non-welded tuffs, ignimbrites, lahar deposits, rhyolite domes and dykes and coarse-grained syenitic rocks. Rhyolite dome formation and collapse, and associated plinian eruptions and column collapse, were largely responsible for the variety of volcaniclastic rocks, including classic air-fall tuff deposits. The complex forms part of a northeast trending volcanic basement lineament known as the Bumbeni Ridge defined by aeromagnetic data.

Petrology and geochemistry of the Palaeo‐Mesoarchean Banded Iron formations (BIFs) from the central Bundelkhand greenstone belts, Bundelkhand Craton, India: Source characteristics and depositional environment

Palaeo‐Mesoarchean Banded Iron formations (BIFs) of central Bundelkhand greenstone belts occur from Babina (in the west) to Mahoba (in the east) via Mauranipur along with the E–W trending of the Bundelkhand Tectonic Zone (BTZ). In the present study, we report petrological, geochemical, and mineralogical data of BIFs from the Babina, Mauranipur, and Mahoba greenstone belts of central Bundelkhand Greenstone Complex to address their source characteristics and deposition environment. The lithological association of BIFs in the Babina and Mauranipur greenstone belts consists of pillowed basic–ultrabasic volcanics and metasediments, whereas in the Mahoba greenstone belt, BIFs and quartzite occur within basic volcanics. These lithological associations collectively suggest that the entire succession was deposited in a marine environment. Based on geological setting, they are classified as Algoma‐type. The main mineral assemblages of the Babina and Mauranipur BIFs are composed of magnetite, quartz, ±garnet, hornblende, chlorite, and apatite, whereas the Mahoba BIFs along with magnetite and quartz, poised of two distinct mineral assemblages; the first one is Opx–Cpx–Grt and the other one is Grt–Hbl. The preliminary P–T study of these BIFs implies that they had gone through amphibolite‐ to granulite‐facies metamorphism. The geochemical characteristics show that BIFs from the Babina and Mauranipur greenstone belts are dominated by higher quartz and magnetite (total content = 90.63–99.95 wt.%), whereas the Mahoba greenstone belt is characterized by lesser quartz and magnetite components (total content = 86.68–94.58 wt.%). The PAAS (Post Archean Average of Australian Sediments)‐normalized REE patterns display significant positive Eu anomaly (Eu/Eu* up to 2.50) with well‐represented negative Ce anomalies (Ce/Ce* = 0.27–1.20). The REE and trace element ratios of BIFs from the Babina and Mauranipur greenstone belts suggest that they were formed by high‐T hydrothermal fluid activity without or very less continental input. Moreover, the Mahoba greenstone belt BIFs are formed by low‐T hydrothermal fluid activity with substantial continental input. Their chemical variation suggests the shallowing of the proto‐basin from the Babina in the west to Mahoba in the east. Tectonic discrimination plots imply the BIFs of the Babina, Mauranipur, and Mahoba greenstone belts of the central Bundelkhand Greenstone Complex are formed in a back‐arc tectonic setting.

Formation and development of coral reefs in the South China Sea

Coral reefs are unique geologic bodies that are formed by biological activity in tropical and subtropical oceans and coastal areas. When and how these reefs came into being and reached their present scales in the ocean remain controversial. Based on strontium, carbon and oxygen isotopes and mineralogical, micropaleontological and elemental analyses of the coral Meiji Reef in the Nansha Islands, South China Sea, we reconstructed the formation and evolutionary history of the island. The results show that the coral reef developed on top of a volcanic basement under the combined influence of diverse factors, especially regional tectonic movement and long-term climatic change. The evolution of the reef was divided into eight stages based on these factors. Extensive exposure was frequently found on the island, and an almost 1000-m thick coral reef eroded simultaneously. The entire development process of coral reefs on the island may be explained by a phased erosion deposition model, which was also found to be suitable for studying the development of coral reefs in the Xisha Islands. We suggest that many coral reefs in the tropics may not flourish continuously over long periods of time, and sedimentary hiatuses normally occur in the layers of coral reefs.

Mineralogical and Isotope-Geochemical Characteristics of the Zharnichikha River Basanites (Southern Transbaikalia)

Abstract —Cenozoic alkali basaltoids are developed in southern Transbaikalia (Dauria–Khentei Ridge). They are represented by thin flows and sheets. Unlike the volcanic fields of the South Baikal Volcanic Region (SBVR), the investigated fields are not associated with rifting. Volcanics from the flow of the Zharnichikha River were studied. Compositionally, they belong to basanites. For the first time, the mineral compositions of the volcanics have been determined. In their geochemical parameters, the basanites correspond to basic volcanics of oceanic islands. The P–T parameters of the equilibrium crystallization of olivine and clinopyroxene phenocrysts have been determined. For the calculated parental melt, the temperature of formation was 1536 °C, and the pressure was 3.28 GPa. The obtained original data on Sr, Nd, and Pb isotope composition have shown that the isotope composition of the magmatic melt formed from a PREMA source with a minor contribution of the latter. The mantle source for the studied basanites is assumed to be olivine pyroxenites (Ol + Cpx + Grt). The generation of basaltic melts occurred under the influence of a mantle plume, which caused Cenozoic activation.

A New Insight into the Biostratigraphy of Usfan and Haddat Ash Sham Formations, West Central Saudi Arabia

A clastic-dominated sedimentary succession is exposed in different localities at the central part of western Saudi Arabia, unconformably overlying the weathered Precambrian Arabian Shield rocks. The age and chronological relationships of the different formational names applied to that succession is still a matter of debate among geologists. The succession is assigned to Cretaceous-Eocene or Oligo-Miocene according to the ages of the overlying Tertiary basic volcanics (harrat).

Lithologic, geomorphic, and climatic controls on sand generation from volcanic rocks in the Sierra Nevada de Santa Marta massif (NE Colombia)

Tectonics, climate, and lithology are the three main factors controlling the generation and transfer of sediments as well as their textural and compositional signatures. In this paper, we investigate the generation of sands from volcanic rocks in nine drainages with mixed lithological assemblage in a high-relief – subtropical climate setting (Sierra Nevada de Santa Marta, NE Colombia). The integration of geomorphological analysis with sand petrography, and bulk-rock and clay geochemistry allows us to constrain the dynamics regulating the generation of sediments derived from felsic and mafic-intermediate volcanic basement rocks in relation to plutonic and metamorphic lithologies. The petrographic composition of modern fluvial sands from the area is extremely variable (litho-feldspatho-quartzose, feldspatho-quartzose, and quartzose-feldspatho-lithic), but it reveals a strong dilution of (especially mafic) volcanic compositional signals, and a consequent overrepresentation of plutonic and metamorphic lithologies. Traditional chemical weathering indices fail to represent the extent of chemical alteration, but instead seem to favour preservation of provenance signals. Chemical weathering primarily affects feldspars and the ferromagnesian groundmass of mafic volcanic rock fragments. Enrichment of ferromagnesian elements in the clay fraction supports the notion that mafic volcanic rock fragments are unstable relative to plutonic and metamorphic rocks. Drainage features (particularly river network and relief) control the abundance of volcanic lithic fragments and the rates and extent of chemical weathering. A linear log-ratio model permits us to quantify sediment generation from volcanic rocks in this mixed-lithology setting. The results indicate that even in the case where volcanic grains are practically absent in the sand fraction, there could still be up to 20% of volcanic rocks in the drainage basin, and in the case that 50% of the sand grains are volcanic, around 75% of drained lithologies are expected to be volcanic. Our results demonstrate that the contribution of volcanic detritus to modern sands systematically underrepresents the areal proportion of volcanics rocks in mixed source areas. Post-depositional diagenetic modifications are likely to alter and/or dissolve volcanic lithic fragments, making accurate reconstruction of sediment provenance even more difficult. Sediment-generation models based on a range of mixed lithologies in various tectonic and climatic settings will be needed to improve estimates of the abundance of volcanic rocks in drainage basins of ancient sediment routing systems.