Detrital Garnets Research Articles

Redistribution of fine gold from hydrothermal sources to sedimentary sinks, Rakaia River, Canterbury, New Zealand

ABSTRACT The Rakaia River in the South Island of New Zealand is a unique catchment in which to trace active detrital gold transport, deformation and concentration from near-coeval orogenic sources. The catchment hosts widespread small late Cenozoic hydrothermal alteration zones, some of which are gold-bearing, in actively rising and eroding mountain headwaters. Detrital gold liberated from the sources is distinctively porous with abundant micron-scale silicate inclusions, especially albite. Fine gold (∼200–100 µm) has been transported >100 km downstream, but coarse gold remains highly diluted in proximal fluvial sediments. Fine gold is also diluted by coarse fluvial sediments that form the upper kilometre of the Canterbury sedimentary basin southeast of the mountain front. Minor re-concentration of fine gold occurs in a bedrock gorge at the mountain front, and on steep ocean beaches beyond the river mouth. Despite the long-distance transport, the fine gold particles have been only superficially modified by surface smearing and minor flattening. The gold may have travelled in suspension in the river, buoyed by electrostatically adhering fine micas, in contrast to detrital garnets that were rounded in bed-load. In contrast, detrital gold on the western side of the mountains has been flattened to thin flakes during bed-load transport.

Source‐To‐Sink Analysis in the Mesozoic SW Junggar Basin, Central Asia: Evidence From Detrital Garnet and Tourmaline Geochemistry

AbstractSource‐to‐sink systems respond to and therefore potentially record topographic and tectonic changes. North of the Tian Shan Belt, the Sikeshu subbasin of the SW Junggar Basin transitioned from active extension in the Triassic to post‐extensional subsidence in the Jurassic‐Cretaceous. Sediment in the Sikeshu subbasin has been shown to be derived from the Tianshan. However, the details of the source‐to‐sink system remain unclear and discrepancies exist between proxy records. The heavy minerals in the Middle Triassic in the Sikeshu subbasin are dominated by garnets. To investigate the garnet sources and decipher the Mesozoic source‐to‐sink evolution, we conducted petrological and sedimentary analysis and detrital garnet and tourmaline geochemistry. We found that the geochemistry of garnets in the Middle Triassic sandstone is most consistent with that of the skarns in the Yili Block (YB) in Tianshan, while the geochemistry of 55%–84% of garnets in other Mesozoic sandstones is consistent with that of garnets in amphibolites in the YB. The geochemistry of the tourmalines since the Upper Triassic is consistent with that of the meta‐sedimentary rocks in the YB and Central Tianshan Block. The dominance of garnets sourced from skarns in the Middle Triassic probably indicates a near‐source point provenance and the broader range of garnet compositions from the Upper Triassic–Lower Cretaceous suggests multiple sources. We infer that the point source changed to multiple sources, which is consistent with the zircon spectra changing from unimodal to multimodal. This change reflects the expansion of the drainage that accompanies a change from active rifting to a post‐rift stage.

An Ediacaran orogeny in subglacial East Antarctica is uncovered by detrital garnet geochronology

Detrital minerals provide valuable insights into the tectonic history of continents. Uranium-lead dating of detrital zircon is widely used to characterize the magmatic history of continents but is generally insensitive to metamorphism accompanying the production and reworking of crust during orogenesis. Garnet is the most important mineral for recording prograde and peak orogenic metamorphism and can occur as a common detrital phase. Here, we demonstrate laser-ablation lutetium-hafnium (Lu-Hf) geochronology of detrital garnet as a provenance tool for reconstructing orogenic histories at (super)continental scales. Detrital garnet (n = 557) from modern sands and Permo-Carboniferous glacial strata in South Australia faithfully record local garnet-grade metamorphic events but also include a major population at ca. 590 million-years with no known source in South Australia. We trace the ca. 590 million-year-old detrital garnets to a largely ice-covered orogenic province in East Antarctica, uncovering the inception of convergent margin tectonism along the palaeo-Pacific margin of Gondwana.

Provenance of low-grossular–high-pyrope detrital garnets from beach sands of East Coast of India between Gosthani and Vamsadhara rivers

Major element compositional spectrum, formation in diverse magmatic and metamorphic rocks, and relative stability during sediment transport, burial, and diagenesis make garnet an important indicator of sedimentary provenances. However, a fundamental question yet to be answered is whether the low-grossular–high-pyrope detrital garnets reflect source characteristics or record sedimentary processes. To address this problem, we have analysed garnets from the beach sands of the East Coast of India and in the source lithologies of the catchment area within the Eastern Ghats Mobile Belt. Investigated garnets show a broad compositional spectrum; most garnets are dominated by almandine49–86% with variable contents of pyrope9–47%, minor grossular0.8–15%, and low spessartine0.5–4%. Using multiple proxies, including a state-of-the-art machine-learning-based garnet discrimination scheme, we found that 96 % of analysed beach sand garnets are of metamorphic origin and 4 % igneous. Amongst the metamorphic garnets, 85 % were derived from granulite facies, 5 % from eclogite/ultrahigh-pressure facies, 5 % from amphibolite facies, and 1 % from blueschist/greenschist facies rocks. Concerning host-rock bulk composition, 93 % of garnets belong to intermediate–felsic/meta-sedimentary class and 7 % belong to the mafic category. The chemistries of the beach sand detrital garnets of the East Coast of India are readily coupled with the compositions of the garnets from source rock lithologies of the Eastern Ghats Mobile Belt. Very low-grossular garnets are derived from meta-pelitic rocks (khondalites) and mafic granulites, whereas slightly grossular-rich garnets are derived from charnockites. The present study indicates that the low-grossular–high-pyrope detrital garnets are vital signposts of sedimentary provenances. We advocate that the high P-T granulite facies mobile belts are distinctive sources for the low-grossular–high-pyrope detrital garnets.

Changes in detrital garnet grain morphology and microtextures during fluvial transport in the Western Carpathians revealed by scanning electron microscopy and 3D model analysis: Implication for paleoenvironmental reconstruction

Siliciclastic grain surfaces preserve an information about mechanical transport effects and/or various chemical processes that the detritus has undergone. Weathering, transport, and diagenesis leave the clear traces that may be used to interpret the paleo-environmental conditions. We aimed to detail surface analysis of detrital garnet by the proven scanning electron microscopy (SEM), X-ray micro-tomography (XRT) and subsequent morphometric analysis of derived 3D surface model. We tracked garnet morphological and surface microtextural changes during fluvial transport in an active Beňatinská voda and Sobranecký potok streams in the Western Carpathians from the place of garnet removal from the host igneous rocks to the last appearance of its fragments in the fluvial sediments. Analysed garnet grains show a range of almost complete solid solution between almandine and grossular (Alm70-73Grs15-19Prp5-6Sps2-3Adr0-3). Precise SEM analysis showed that conchoidal fractures, straight and arcuate steps, crescentic percussion marks and V-shaped cracks are main mechanical fluvial microtextures, whilst 3D surface models derived from XRT provided the information that no relationship between mechanical surface-microtextural development and mineral inclusion distribution exists. Thus, the presence of inclusions and their proximity to the grain surface have no influence on garnet destruction during its fluvial transport. Morphometric variables such as surface roughness and curvatures derived from 3D garnet model can be used to distinguish between mechanical impacts and corrosion-induced depressions. The results related to detritus behaviour in the modern streams are applicable to the ancient fluvial sediment research and can help to correctly reconstruct the paleoenvironmental and paleogeographic conditions in various areas.

The provenance of a turbidite system within a tectonically active wrench basin: Insights from heavy mineral characteristics of Miocene sandstones in the Tabernas Basin, south‐east Spain

AbstractThis paper provides insight into the provenance of the Late Miocene turbidite succession of the Tabernas Basin. Although this area has been extensively studied, only limited attention has been paid to sediment provenance. Through heavy mineral analysis, it has been possible to identify provenance‐related signatures from the adjacent Sierra de los Filabres and Sierra Alhamilla uplifts. Stable mineral ratio data confirm that the Sierra de los Filabres provided sediment with generally higher chloritoid:tourmaline and higher Type Bii garnet abundances than those derived from the Sierra Alhamilla. By comparison, modern sediments derived from the Sierra Alhamilla have garnet compositions with larger proportions of Types A and C, suggesting that the basinal sediments were not sourced from the incipient Sierra Alhamilla Uplift. Heavy mineral analysis confirms that the Sierra de los Filabres was the primary source for the Tabernas succession, with minor variations indicating that the erosive part of the system migrated across the uplift. Input was predominantly from the Nevado–Filábride Complex, with minor amounts from the small remnant of the Alpujarride Complex attached to the southern margin of the Sierra de los Filabres. Evidence strongly suggests a single sediment routing system but identifies some subtle provenance variations. In particular, there was a shift in detrital garnet composition between the Sartenella Formation and the Verdelecho Formation, Solitary Channel and El Gordo Megabed, which is attributed here to a shift in catchment within the Sierra de los Filabres. This shift appears to have occurred during the deposition of the Sartenella Formation, since the garnet compositions of the Verdelecho Formation and Solitary Channel are similar to each other and differ from the preceding part of the Sartenella Formation. The Solitary Channel displays marked heterogeneities in provenance character, manifested by changes in chloritoid abundance, consistent with previous studies that suggest the depositional architecture in the channel was influenced by high‐frequency changes in sediment flux and sea level.

Remobilization of HFSE, Y, and REE during Diagenetic Alteration of Heavy Minerals in Sandstones from the Chvalčov Site, Flysch Belt of the Outer Western Carpathians, Czech Republic

An in situ electron microprobe study of detrital minerals yielded important insights into the diagenetic history of the Cretaceous-to-Paleogene flysch sandstones from the Chvalčov site, Rača Unit, Flysch Belt of the Outer Western Carpathians. Detrital titanite and a Fe-Ti mineral (probably ilmenite) were almost completely altered to TiO2 minerals, which also newly crystallized in intergranular spaces of sandstone. Brookite, anatase, and, exceptionally, rutile were identified by Raman spectroscopy. Authigenic TiO2 phases show complex composition with occasionally elevated contents of Fe, Nb, Zr, V, Sc, Cr, Al, Y, and/or P, which were likely sourced from altered neighboring heavy minerals. In addition, rare authigenic LREE- and Y-enriched apatite rims were observed on detrital apatite. The remobilization of REE, Y, and HFSE was likely mediated by acidic early diagenetic fluids enriched in fluoride and sulfate anions. The superimposed formation of calcite cement was associated with the dissolution of detrital garnet, feldspars, and quartz. The compositions of detrital apatite and garnet (Alm60-82Prp4-30Sps0-24Grs0-19) are comparable with those from adjacent parts of the Flysch Belt. Detrital rutile is enriched in Nb, V, Cr, and Zr. Our study illustrates the intensity of diagenetic alteration of detrital minerals in flysch sandstones as well as the usefulness of in-situ electron-microprobe investigations for the recognition of processes influencing heavy minerals in diagenetically altered sediments.

Detrital Garnet Geochronology by In Situ U‐Pb and Lu‐Hf Analysis: A Case Study From the European Alps

AbstractDetrital geochronology employing the widely‐used zircon U‐Pb proxy is biased toward igneous events and metamorphic anatexis; additionally, zircon is highly refractory and frequently polycyclic. Garnet, a rock‐forming and thus commonly occurring mineral, is predominantly metamorphic and much less refractory. Here, we report in situ U‐Pb and Lu‐Hf ages from detrital garnet hosted in ancient and modern sediments of the European Alps. Both geochronometers are biased toward the most recent garnet‐crystallizing metamorphic event in the source area, with fewer inherited ages. This likely reflects efficient removal of inherited garnet during diagenesis and metamorphism, and is in contrast to detrital zircon, apatite, and rutile U‐Pb data, which largely record pre‐Alpine ages. Neither the U‐Pb nor Lu‐Hf system in garnet exhibits a relationship between age recovery and composition. However, the Lu‐Hf system in garnet yields significantly better age recovery than the U‐Pb system. Estimated initial 238U/206Pbc values at the time of crystallization are near unity, suggesting that garnet does not significantly partition U from Pb during crystallization, at least for the generally almandine‐rich garnets analyzed in this study. Hence, Lu‐Hf geochronology of detrital garnet offers an effective method to detect and date the most recent phase of mid‐grade metamorphism in sub‐anatectic source areas, in which detrital zircon U‐Pb analysis may be of less utility.

Detrital garnet petrology challenges Paleoproterozoic ultrahigh-pressure metamorphism in western Greenland

Abstract. Modern-style plate tectonics is characterised by the global operation of cold and deep subduction involving blueschist facies and ultrahigh-pressure metamorphism. This has been a common process since the Neoproterozoic, but a couple of studies indicate similar processes were active in the Paleoproterozoic, at least on the local scale. Particularly conspicuous are extreme ultrahigh-pressure conditions of ∼ 7 GPa at thermal gradients < 150 ∘C GPa−1 proposed for metamorphic rocks of the Nordre Strømfjord shear zone in the western part of the Paleoproterozoic Nagssugtoqidian Orogen of Greenland. By acquiring a large dataset of heavy minerals (n = 52 130) and garnet major-element composition integrated with mineral inclusion analysis (n=2669) from modern sands representing fresh and naturally mixed erosional material from the metamorphic rocks, we here intensely screened the area for potential occurrences of ultrahigh-pressure rocks and put constraints on the metamorphic evolution. Apart from the absence of any indications pointing to ultrahigh-pressure and low-temperature–high-pressure metamorphism, the results are well in accordance with a common Paleoproterozoic subduction–collision metamorphic evolution along a Barrovian-type intermediate temperature and pressure gradient with a pressure peak at the amphibolite–granulite–eclogite-facies transition and a temperature peak at medium- to high-pressure granulite-facies conditions. In addition, we discuss that all “evidence” for ultrahigh-pressure metamorphism proposed in the literature for rocks of this area is equivocal. Accordingly, the Nordre Strømfjord shear zone is not an example of modern-style plate tectonics in the Paleoproterozoic or of very low thermal gradients and extreme pressure conditions in general.

Mid-cretaceous rapid denudation of Eastern Tibetan plateau: Insights from detrital records at the Southwestern corner of Sichuan basin

Reconstruction of the Cretaceous tectonic evolution of the eastern margin of the Tibetan Plateau is of great significance to understanding the formation and early evolution of the Tibetan Plateau. The thick late Mesozoic sedimentary sequence in the Western Sichuan Basin may record the evolution of the basin itself and the tectonic uplift history of the eastern margin of the Tibetan Plateau during the Cretaceous period. Here we provide new multi-proxy provenance data from conglomerate clast populations, sandstone petrography, heavy mineral assemblages, U-Pb dating of detrital zircon, paleocurrent data, and detrital garnet geochemistry from the Cretaceous clastic units in the southwest corner of Sichuan Basin. Our analysis reveals two distinct changes in sediment provenance recorded in the Cretaceous strata at the southwest corner of the Sichuan Basin. The first significant change of detrital provenance was identified in the mid-Cretaceous Jiaguan Formation. Metamorphic rocks and volcanic rocks clast, as well as feldspar and mica, increased significantly, a heavy mineral assemblage dominated by hematite–limonite and ilmenite with high ATi and RuZi and low ZTR values, predominantly type Bi garnet and mostly 164 Ma, 207 Ma, 440 Ma, 780 Ma–824 Ma, and 1840 Ma detrital zircon ages, interpreted to be sourced from the Songpan-Ganzi fold belt and Longmenshan orogenic belt. The second shift, in the Guankou Formation, is marked by a low compositional maturity, a distinct increase in fossiliferous carbonate clasts and high GZi index with type Bii garnet, and Triassic zircon ages, indicating the exhumation of Longmenshan orogenic belt during the late Cretaceous. These data collectively indicate that the significant surface uplift and rapid denudation of the eastern margin of the Tibetan Plateau were probably initiated in the mid-Cretaceous (∼120 Ma). In conclusion, a detailed hands-on provenance analysis of the clastic sedimentary sequences in the southwest corner of the Sichuan Basin enabled us to determine catchment areas and shifts hitherto unstudied, thus contributing to the exhumation history of the eastern margin of the Tibetan Plateau.

Sediment provenances of a Mesozoic intracontinental basin enclosed by multiple orogenic belts, Junggar Basin, NW China: insights from detrital ilmenite, Cr-spinel geochemistry, and zircon U–Pb geochronology

ABSTRACT The Junggar Basin was an intracontinental basin enclosed by multiple orogenies in the Mesozoic. Unravelling the provenances development in the western Junggar Basin enclosed by the West Junggar Orogenic Belt (WJOB) and Tianshan Orogenic Belt (TSOB) would elucidate the exhumation history of these orogenic belts during the post-orogenic period. The TSOB includes Central Tianshan which includes a continental block with the Precambrian to early Palaeozoic metamorphic basements, while the WJOB includes Palaeozoic accretionary complexes. Given their contrasting geological histories, we use detrital garnets, metamorphic ilmenites, epidotes, and Precambrian zircons to distinguish the TSOB and detrital Cr-spinel to distinguish the WJOB. Combined with sedimentary analysis, palaeocurrents, heavy mineral data, seismic profiles, and zircon U-Pb geochronology, here we reconstruct a four-phase source-to-sink model. During the syn-rift phase, the southern basin was stably fed by the TSOB in the Triassic, evidenced by high Garnet-zircon index and stable single-peak Carboniferous detrital zircon age spectra. During the post-rift sag basin phase, the source-to-sink system experienced a significant change, because detrital zircon age spectra change into multiple peaks. During the transpressional basin phase, the WJOB replaced the TSOB as the dominant provenance in the late Middle Jurassic and the TSOB became the dominant provenance again in the Late Jurassic. Syndepositional volcanic materials provided a large amount of sediments. During the sag basin phase, lake transgression happened and the TSOB provenance could not reach the northern basin. We concluded that the WJOB experienced transpression and obvious denudation in the late Middle Jurassic, and the TSOB uplifted under transpression in the Late Jurassic. The transpression controlled the change of major provenances. The aridification in the Late Jurassic was recorded by the decrease in the Zircon-Tourmaline-Rutile index and the increase in epidotes and magnetites. The humidification in the Early Cretaceous was responded to by lake transgression and sediment recycling.

Two glaciers and one sedimentary sink: the competing role of the Aare and the Valais glaciers in filling an overdeepened trough inferred from provenance analysis

Abstract. The extent and distribution of glaciers on the Swiss Plateau during the Last Glacial Maximum (LGM) can be determined from the geological record. However, similar reconstructions for the glaciations that preceded the LGM are far more difficult to be made due to the destruction of suitable sedimentary records through recurring glaciations or due to the inaccessibility of preserved records. Here, we explored Quaternary sediments that were deposited during the Marine Isotope Stage (MIS) 8 glaciation at least around 250 ka, and which were recovered in a drilling that was sunk into an overdeepened bedrock trough west of Bern (Switzerland). We analyzed the sediment bulk chemical composition of the deposits to investigate the supply of the material to the area by either the Aare Glacier, the Saane Glacier, or the Valais Glacier, and we complement this investigation with the results of heavy mineral analyses and geochemical information from detrital garnet. The potential confluence of the Valais and the Aare glaciers in the Bern area makes this location ideal for such an analysis. We determined the sediment bulk chemical signal of the various lithological units in the central Swiss Alps where the glaciers originated, which we used as endmembers for our provenance analysis. We then combined the results of this fingerprinting with the existing information on the sedimentary succession and its deposition history. This sedimentary suite is composed of two sequences, Sequence A (lower) and Sequence B (upper), both of which comprise a basal till that is overlain by lacustrine sediments. The till at the base of Sequence A was formed by the Aare Glacier. The overlying lacustrine deposits of an ice-contact lake were mainly supplied by the Aare Glacier. The basal till in Sequence B was also formed by the Aare Glacier. For the lacustrine deposits in Sequence B, the heavy mineral and garnet geochemical data indicate that the sediment was supplied by the Aare and the Saane glaciers. We use these findings for a paleogeographic reconstruction. During the time when Sequence A and the basal till in Sequence B were deposited, the Aare Glacier dominated the area. This strongly contrasts with the situation during the LGM, when the Aare Glacier was deflected by the Valais Glacier towards the northeast. The Valais Glacier was probably less extensive during MIS 8, but it was potentially present in the area, and it could have been essential for damming a lake in which the material supplied by the Aare and the Saane glaciers accumulated. In conclusion, combining provenance with sedimentological data, we could document how sediment was supplied to the investigated overdeepened basin during the MIS 8 glacial period and how glaciers were arranged in a way that was markedly different from the LGM.

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism, thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (∼45%) and low-Cr megacrystic (∼39%) chemistries, with a significant proportion of eclogitic garnets (∼11%), and only subordinate contribution from harzburgitic garnets (∼5%) with variable Cr2O3 contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ∼1150–1250 °C) indicate their derivation from at least ∼220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ∼45 mW/m2 or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ∼50–65 kbar diamond window of the Olenek province by the same time.

Garnet major-element composition as an indicator of host-rock type: a machine learning approach using the random forest classifier

The major-element chemical composition of garnet provides valuable petrogenetic information, particularly in metamorphic rocks. When facing detrital garnet, information about the bulk-rock composition and mineral paragenesis of the initial garnet-bearing host-rock is absent. This prevents the application of chemical thermo-barometric techniques and calls for quantitative empirical approaches. Here we present a garnet host-rock discrimination scheme that is based on a random forest machine-learning algorithm trained on a large dataset of 13,615 chemical analyses of garnet that covers a wide variety of garnet-bearing lithologies. Considering the out-of-bag error, the scheme correctly predicts the original garnet host-rock in (i) > 95% concerning the setting, that is either mantle, metamorphic, igneous, or metasomatic; (ii) > 84% concerning the metamorphic facies, that is either blueschist/greenschist, amphibolite, granulite, or eclogite/ultrahigh-pressure; and (iii) > 93% concerning the host-rock bulk composition, that is either intermediate–felsic/metasedimentary, mafic, ultramafic, alkaline, or calc–silicate. The wide coverage of potential host rocks, the detailed prediction classes, the high discrimination rates, and the successfully tested real-case applications demonstrate that the introduced scheme overcomes many issues related to previous schemes. This highlights the potential of transferring the applied discrimination strategy to the broad range of detrital minerals beyond garnet. For easy and quick usage, a freely accessible web app is provided that guides the user in five steps from garnet composition to prediction results including data visualization.

Reconstruction of the exhumation history of the eastern Nepal Himalaya based on provenance changes

The Siwalik Group extending east-west collinear to the Himalayan southern front records Himalayan denudation history. In this study, we present sandstone petrography, heavy mineral assemblages, Nd isotopes and chemical composition of detrital garnet from the Siwalik succession along the Muksar Khola section. This study reveals the Tibetan Tethys Himalaya, Higher Himalayan Crystalline and Lesser Himalaya Sequence as the source of sediments and records a provenance change from the middle Miocene to Pliocene/Pleistocene. This was observed as a shift from shallow to deeper parts of the Higher Himalayan Crystalline as a source area around 10.6 Ma. The Lesser Himalaya Sequence supplied a significant amount of sediments after 7.5 Ma, but the deeper part was only exposed after the late Pliocene to Pleistocene. Around 4.0 Ma an increase in the sediments from the Higher Himalayan Crystalline was observed. This provenance study reveals two stages of the exhumation of the eastern Nepal Himalaya. The first occurred between 11.0 and 5.5 Ma and the second after 4.0 Ma, due to the activation of an out-of-sequence thrust known as the Sun Koshi Thrust, and formation of the Lesser Himalayan duplex respectively.

Provenance of Neogene sandstones in western Taiwan traced with garnet geochemistry and zircon geochronology

AbstractIn this study, we use provenance analysis to shed light on the Neogene evolution of river drainage and sediment dispersal in eastern Asia as a consequence of the Himalayan orogeny and topographic rise of the Tibetan Plateau. As an attempt to envisage how sediment‐routing systems operated in the past, and to identify the ultimate source of siliciclastic detritus originally generated in mainland China and now incorporated in, and recycled from, the western Taiwan accretionary prism, we have used Raman spectroscopy of detrital garnet in modern river sands and upper Neogene sandstones, complemented by detrital‐zircon geochronology and by petrographic and heavy‐mineral analysis. Garnet grains are relatively abundant and largely represented by almandine with various percentages of pyrope molecules in Yangtze sand as in western Taiwan sandstones and modern river sand derived from them. Instead, they are rare and largely represented by grossular or andradite in the sand of the Pearl River and of coastal rivers of SE China facing the Taiwan Strait. This finding supports the result of detrital geochronology, which documents a sharp difference between U‐Pb age spectra of Pearl River and coastal SE China zircons (characterised by a Paleozoic‐Mesozoic triple peak) and Yangtze and western Taiwan zircons (characterised by prominent Neoproterozoic, Paleoproterozoic, and latest Archean clusters). The Pearl River and minor rivers facing the Taiwan Strait are thus ruled out as ultimate sources of upper Neogene sandstones exposed today along the western front of the Taiwan fold‐thrust belt. In the envisaged scenario, sand mostly supplied by the paleo‐Yangtze River was entrained for ~ca.1,000 km southward by longshore currents and deposited on the Chinese passive margin before being accreted along the front of the Taiwan orogen since ~6 Ma. Littoral sand drift represents a major factor of long‐distance sediment transport in modern shelves and needs to be taken into full account in source‐to‐sink studies and paleogeographic reconstructions.

Garnet sand reveals rock recycling processes in the youngest exhumed high- and ultrahigh-pressure terrane on Earth

Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene-Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO2 inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO2 (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth's history.

Miocene provenance change in Himalayan foreland basin and Bengal Fan sediments, with special reference to detrital garnet chemistry

AbstractBengal Fan Miocene sediments were collected during International Ocean Discovery Program Expedition 354 and investigated using petrographic and detrital garnet chemistry analyses. The Miocene Siwalik Group, which is composed of sediments deposited in the Himalayan foreland basin, was also analyzed for comparison with the Bengal Fan data for the provenance change during the Miocene. Our petrographic analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik Group consist predominantly of Higher Himalayan Crystalline (HHC)‐derived detritus such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy; most of the garnet is rich in almandine and poor in spessartine and pyrope. However, pyrope‐rich garnet, which is considered to originate from the HHC core (granulite facies), was found in the lower to upper Miocene deposits. The deposition of HHC‐derived detrital garnet began before the Middle Miocene (15 Ma) and before the Late Miocene (10–9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast, indicated that pyrope‐rich garnet appeared in the Early Miocene (17.3 Ma) and Late Miocene (8.5–6.5 Ma). We conclude that the Bengal Fan sediments record the erosion of the HHC zone since the Early Miocene that appears in the Siwalik sediments. Furthermore, we found that the HHC‐derived inputs decreased from the late Middle Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya foreland basin and the Bengal Fan. The disappearance of the HHC‐derived detritus is probably the result of dilution by Lesser Himalayan detritus, which suggests that the Lesser Himalayan zone, which is composed of metamorphosed and unmetamorphosed sedimentary rocks, was uplifted.

Vliv diagenetických procesů na asociaci těžkých minerálů v pískovcích z lokality Slivotín (ždánická jednotka, flyšové pásmo Vnějších Západních Karpat, Česká republika)

An electron microprobe study of polished sections prepared from a sample of fine-grained sandstone from the locality Slivotín (Ždánice-Hustopeče Formation, Ždánice Unit, Flysch Belt of the Outer Western Carpathians, Czech Republic) allowed to yield in addition to data on chemical composition also the detailed information on in situ textural relationships of individual minerals. During our study, emphasis was given to accessory phases belonging to the translucent heavy mineral fraction. The detrital garnet (Alm36-82Grs2-45Prp2-22Sps0-15) was extensively dissolved and replaced by calcite cement from its margins and along the cracks. Detrital fluorapatite was dissolved in a similar way, however, dissolution episode was followed by growth of authigenic rims composed of carbonate-fluorapatite. Other observed heavy minerals (zircon, chrome spinel, TiO2 phase, monazite, tourmaline) probably remained unaltered by diagenetic processes. The chemical composition of chrome spinels varies mostly between magnesiochromite and chromite, whereas spinel is very rare. The chemical composition of garnets and chrome spinels is comparable with published data from Czech, Polish and Slovak parts of the Flysch Belt of the Western Carpathians, and indicates the primary source of detrital material in rocks of deeper parts of orogen, characterized especially by the presence of catazonal metamorphites and almost lacking volcanic rocks. Redeposition of heavy minerals from older sediments cannot also be ruled out. The pronounced diagenetic alteration of garnet, if not very scarce in the area of Flysch Belt, could help to explain the earlier observations of wide fluctuations of contents of garnet in heavy mineral concentrates.

Minerály těžké frakce arkózových pískovců z Tismic u Českého Brodu (perm blanické brázdy, Česká republika)

The heavy mineral concentrate originating from Permian freshwater sandstones/arkoses from the Tismice site (northern part of the Blanice Furrow, Bohemian Massif, Czech Republic) was studied in terms of mineral composition and chemical composition of selected phases. Ilmenite, to various degree altered to a TiO2 phase and/or unidentified non-stoichiometric Fe-Ti (hydro)oxides, is the predominating constituent of heavy mineral fraction. Garnet, tourmaline, apatite and baryte are subordinate components. Garnet (with commonly etched “drusy” surface) belongs exclusively to almandine (Alm45-91Prp4-27Sps1-32Grs0-17Adr0-5). Tourmaline has variable chemical composition, but oxy-dravite prevails. Accessory phases include biotite, REE-rich goyazite (Goy45-59Flo29-43Cra11-17Gor0-1), zircon, pyrite, limonite, gahnite (Ghn57-68Hrc21-32Spl7-10Mgt1-2Gal1), staurolite, xenotime and monazite. Baryte and goyazite were likely formed during diagenesis of the host sediments or during later hydrothermal activity. Detrital garnet and tourmaline were probably sourced from the granulites, mica schists and migmatites of the Malín segment of the neighbouring Kutná Hora Crystalline Complex (KHCC). Surprisingly, amphibolites or serpentinites, frequently present in areas of the KHCC more proximal to the Permian sedimentary basin, did not contribute their garnets. We suggest that these areas were not exposed to erosion during the Permian period. Spectacular etching of surface of garnets and pervasive alteration of ilmenite were associated with burial diagenesis of the host sediments.