Geology, Geochronology and Geochemistry of Weilasituo Sn-Polymetallic Deposit in Inner Mongolia, China

Late Mesozoic intermediate–felsic volcanics and hypabyssal intrusions are common across the western slope of the Great Xing’an Range (GXAR). Spatiotemporally, these hypabyssal intrusions are closely associated with epithermal Pb–Zn polymetallic deposits. However, few studies have investigated the petrogenesis, contributions and constraints of these Pb–Zn polymetallic mineralization-related intrusions. Therefore, we examine the representative Erdaohezi deposit and show that these mineralization-related hypabyssal intrusions are composed of quartz porphyry and andesite porphyry with concordant zircon U–Pb ages of 160.3 ± 1.4 Ma and 133.9 ± 0.9 Ma, respectively. These intrusions are peraluminous and high-K calc-alkaline or shoshonitic with high Na2O + K2O contents, enrichment in large ion lithophile elements (LILEs; e.g., Rb, Th, and U), and depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Zr, and Hf), similar to continental arc intrusions. The zircon εHf(t) values range from 3.1 to 8.0, and the 176Hf/177Hf values range from 0.282780 to 0.282886, with Hf-based Mesoproterozoic TDM2 ages. No differences exist in the Pb isotope ratios among the quartz porphyry, andesite porphyry and ore body sulfide minerals. Detailed elemental and isotopic data imply that the quartz porphyry originated from a mixture of lower crust and newly underplated basaltic crust, while the andesite porphyry formed from the partial melting of Mesoproterozoic lower crust with the minor input of mantle materials. Furthermore, a magmatic–hydrothermal origin is favored for the Pb–Zn polymetallic mineralization in the Erdaohezi deposit. Integrating new and published tectonic evolution data, we suggest that the polymetallic mineralization-related magmatism in the Erdaohezi deposit occurred in a back-arc extensional environment at ~133 Ma in response to the rollback of the Paleo-Pacific Plate.

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Structural evolution in the Moine of northwest Scotland: a Caledonian linked thrust system?

Geological Magazine ◽

10.1017/s0016756800026492 ◽

1986 ◽

Vol 123 (1) ◽

pp. 1-11 ◽

Cited By ~ 21

Author(s):

Robert W. H. Butler

Keyword(s):

Cross Sections ◽

Lower Crust ◽

Structural Evolution ◽

Amphibolite Facies ◽

Metamorphic Basement ◽

Thrust System ◽

Lower Crustal

AbstractA model is proposed whereby the Caledonian metamorphic basement-cover complex of northwest Scotland (the Moine) is considered as a linked thrust system. This system lies between the Moine thrust at its base and the Naver–Sgurr Beag slide at its top. Ductile fold and thrust zones, which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies, are interpreted as decoupling from a detachment presently situated at relatively shallow depths. This model is illustrated by two preliminary balanced cross-sections. These imply shortening across the northwest Scottish Caledonides in excess of 130 km and probably over 200 km. When these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth. The most likely location for the lower crustal wedge is beneath the Grampian Highlands.

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Petrogenesis and Tectonic Setting of Early Cretaceous Intrusive Rocks in the Northern Ulanhot Area, Central and Southern Great Xing’an Range, NE China

Minerals ◽

10.3390/min11121414 ◽

2021 ◽

Vol 11 (12) ◽

pp. 1414

Author(s):

Baoqiang Tai ◽

Wentian Mi ◽

Genhou Wang ◽

Yingjie Li ◽

Xu Kong

Keyword(s):

Early Cretaceous ◽

Tectonic Setting ◽

Granite Porphyry ◽

Granitic Rocks ◽

Magma Source ◽

Isotopic Data ◽

Quartz Porphyry ◽

Intrusive Rocks ◽

Quartz Monzonite ◽

Great Xing’An Range

Abundant Early Cretaceous magmatism is conserved in the central and southern Great Xing’an Range (GXR) and has significant geodynamic implications for the study of the Late Mesozoic tectonic framework of northeast China. In this study, we provide new high-precision U–Pb zircon geochronology, whole-rock geochemistry, and zircon Hf isotopic data for representative intrusive rocks from the northern part of the Ulanhot area to illustrate the petrogenesis types and magma source of these rocks and evaluate the tectonic setting of the central-southern GXR. Laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U–Pb dating showed that magmatism in the Ulanhot area (monzonite porphyry: 128.07 ± 0.62 Ma, quartz monzonite porphyry: 127.47 ± 0.36, quartz porphyry: 124.85 ± 0.34, and granite porphyry: 124.15 ± 0.31 Ma) occurred during the Early Cretaceous. Geochemically, monzonite porphyry belongs to the metaluminous and alkaline series rocks and is characterized by high Al2O3 (average 17.74 wt.%) and TiO2 (average 0.88 wt.%) and low Ni (average 4.63 ppm), Cr (average 6.69 ppm), Mg# (average 31.11), Y (average 15.16 ppm), and Yb (average 1.62 ppm) content with enrichment in Ba, K, Pb, Sr, Zr, and Hf and depletion in Ti, Nb, and Ta. The granitic rocks (e.g., quartz monzonite porphyry, quartz porphyry, and granite porphyry) pertain to the category of high-K calc-alkaline rocks and are characterized by high SiO2 content (>66 wt.%) and low MgO (average 0.69 wt.%), Mg# (average 31.49 ppm), Ni (average 2.78 ppm), and Cr (average 8.10 ppm) content, showing an affinity to I-type granite accompanied by Nb, Ta, P, and Ti depletion and negative Eu anomalies (δEu = 0.57–0.96; average 0.82). The Hf isotopic data suggest that these rocks were the product of the partial melting of juvenile crustal rocks. Notably, fractionation crystallization plays a crucial role in the process of magma emplacement. Combining our study with published ones, we proposed that the Early Cretaceous intrusive rocks in the Ulanhot area were formed in an extensional tectonic background and compactly related to the subduction of the Paleo-Pacific Ocean plate.

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Hafnium isotopic record of crustal maturation during Middle Triassic magmatism in the Southern Alps (Italy)

10.5194/egusphere-egu2020-14303 ◽

2020 ◽

Author(s):

Julian-Christopher Storck ◽

Jörn-Frederik Wotzlaw ◽

Ozge Karakas ◽

Peter Brack ◽

Axel Gerdes ◽

...

Keyword(s):

Isotopic Composition ◽

Lower Crust ◽

Middle Triassic ◽

Southern Alps ◽

Crustal Material ◽

Crustal Melting ◽

Magmatic System ◽

Magma Flux ◽

Lower Crustal ◽

Over Time

Tracing the origin and evolution of magmas on their pathway through the lithosphere is key to understanding the magmatic processes that eventually produce eruptions. For ancient magmatic provinces, isotope-geochemical tracers are powerful tools to probe the source regions and magma-crust interaction during ascent and storage.We present new hafnium isotopic compositions of ID-TIMS dated zircons to trace the evolution of the Middle Triassic magmatic province in the Southern Alps (northern Italy) at high temporal resolution [1]. Systematic changes in hafnium isotopic composition with time reveal a coherent temporal evolution from depleted mantle signatures towards crust-dominated signatures within less than four million years. This trend can be ascribed to progressive influence of a crustal source, incorporated into the reservoir from which these zircons crystallized. Towards the end of the magmatic episode, the &#949;Hf compositions abruptly revert within one-million-years back towards more juvenile compositions mainly recorded by the mafic to intermediate intrusive pulses (e.g. Monzoni and Predazzo), the effusive climax of basaltic lavas and the post-intrusive ash beds (e.g. Punta Grohmann) in the Dolomite region. We interpret the variation of Hf-isotopic signatures over time as a protracted contamination signal induced by interaction of the mantle-derived magmas with the lower crust.The dataset obtained in this study is further implemented into a two-component mixing model employing a range of potential crust and mantle endmember Hf isotope signatures and Hf concentrations which is directly translated into crustal melt/total melt (=sum of crustal and mantle-derived melt) ratios over time. Based on these observations we explored the thermal evolution and crustal melting as a function of time, lithology, water content and magma flux for a lower crustal magmatic system by numerical modelling. Dykes and sills of basaltic composition are incrementally emplaced at the mantle-crust boundary, which leads to changes in crustal over mantle melt ratios over time. Initial intrusions of basaltic dykes into the relatively cold lower crust cause only limited crustal melting and assimilation but ensuing magma injections into progressively hotter crust results in more extensive partial melting and assimilation of crustal material. Subsequent intrusions into the magmatic lower-crustal roots cannibalize previous intrusions with progressively less isotopic contrast due to dilution with mantle-derived magmas. This is potentially accompanied by an increase in magma flux, e.g. by delamination of dense lower crustal cumulates into the subcontinental lithospheric mantle.The observed trends in hafnium isotopic composition therefore do not necessarily require tectonic re-organizations or changes in mantle sources. Instead these trends may trace variations in mantle-crust interaction during thermally induced chemical maturation of the lower crustal magmatic roots progressively replacing ancient pelitic to mafic lower crustal lithologies by juvenile cumulates.&#160;[1] Storck, J.-C., Wotzlaw, J.-F., Karakas, O., Brack, P., Gerdes, A., Ulmer, P. Hafnium isotopic record of mantle-crust interaction in an evolving continental magmatic system, Earth and Planetary Science Letters, (in press).

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The isotopic composition of strontium of the Early Jurassic North Mountain Basalts, Nova Scotia

Canadian Journal of Earth Sciences ◽

10.1139/e88-094 ◽

1988 ◽

Vol 25 (6) ◽

pp. 942-944 ◽

Cited By ~ 9

Author(s):

Lois M. Jones ◽

David J. Mossman

Keyword(s):

Isotopic Composition ◽

Lower Crust ◽

High Ratio ◽

Early Jurassic ◽

Magma Source ◽

Crustal Material ◽

Initial Ratio ◽

Flow Units ◽

Lower Unit ◽

Heterogeneous Mantle

The initial isotopic composition of strontium of the Early Jurassic North Mountain Basalts was determined for two of three flow units: the lower unit and upper unit. Each unit was sampled along the outcropping basalts over a distance of 170 km. The initial 87Sr/86Sr ratios are remarkably constant for each unit. For the lower unit, the initial ratios range from 0.70591 to 0.70609; for the upper unit, the range is 0.70675–0.70687. The relatively high ratio could represent tapping of a magma source in an isotopically heterogeneous mantle. Alternatively, the magma could have resided for a time in the lower crust and assimilated crustal material, thereby increasing the radiogenic 87Sr content. Mixing had to have been very efficient, as indicated by similar initial ratios over considerable distances. With the extrusion of the upper unit, an isotopically different part of the mantle was tapped, or, more likely, the magma was retained within the lower crust long enough to assimilate additional crustal material and mix sufficiently well to yield the consistently higher initial ratio observed throughout the upper unit.

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Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

10.26686/wgtn.16958932 ◽

2021 ◽

Author(s):

◽

Richard J Wysoczanski

Keyword(s):

Upper Mantle ◽

Lower Crust ◽

Mineral Assemblage ◽

Geophysical Methods ◽

Crustal Material ◽

West Antarctica ◽

Rock Composition ◽

Temperature Oxidation ◽

Crustal Xenoliths ◽

Lower Crustal

Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.

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Lithospheric Xenoliths from the Marie Byrd Land Volcanic Province, West Antarctica

10.26686/wgtn.16958932.v1 ◽

2021 ◽

Author(s):

◽

Richard J Wysoczanski

Keyword(s):

Upper Mantle ◽

Lower Crust ◽

Mineral Assemblage ◽

Geophysical Methods ◽

Crustal Material ◽

West Antarctica ◽

Rock Composition ◽

Temperature Oxidation ◽

Crustal Xenoliths ◽

Lower Crustal

Studies of the Earths lithosphere, and particularly the lower crust, have in the past relied on geophysical methods, and on geochemical studies of granulite terrains exposed at the surface. Geophysical studies can not evaluate the compositions to any large extent. Granulite terrains typically represent ancient rather than present day sections, have invariably suffered retrograde metamorphism, and have been affected by fluids during uplift. More recently, studies of lithospheric xenoliths (fragments of the lithosphere brought to the surface by entraining (typically alkaline) melts) have been used to study the composition of, and processes influencing, the lithosphere. Xenoliths have the advantage of representing relatively unaltered and young fragments of the lithosphere, and together with other studies have added much to our understanding of the Earths composition and processes. The study of the lithosphere in Marie Byrd Land (MBL), West Antarctica, is complicated by the difficult access and harsh climate of the region. Geophysical studies are limited, and deep crustal exposures are entirely absent. In an attempt to study the composition and structure of the MBL lithosphere, xenoliths were collected from various volcanic edifices in MBL, including the volcanoes of the Executive Committee Range (ECR), and the USAS Escarpment in central MBL, and Mount Murphy on the Walgreen coast. The xenolith suite consists of peridotites, pyroxenites and granulites, spanning a vertical section from upper mantle to lower crust, that are in pristine condition, due to the arid Antarctic conditions. The peridotite suite from MBL consists of spinel Iherzolites from Mounts Hampton and Cumming in the ECR, the USAS Escarpment, and Mount Murphy. Cr-diopside rich peridotites also occur at Mounts Hampton and Murphy, indicating a more chemically diverse upper mantle in these regions (e.g. Mg# 75-92 in Cr-diopside rich peridotites compared to Mg# 87-92 in spinel Iherzolites). REE contents of the peridotites vary from LREE-depleted (up to 0.293 (La/Yb)n in USAS Escarpment peridotites) to LREE-enriched (up to 10.015 (La/Yb)n in Mount Hampton peridotites), further indicating the extreme heterogeneity of the MBL upper mantle. Lower crustal xenoliths from Mounts Sidley and Hampton in the ECR, and from Mount Murphy have meta-igneous textures ranging from pyroxenite to gabbro. They consist of varying amounts of olivine, clinopyroxene, orthopyroxene, plagioclase and spinels; garnet is entirely absent. Orthopyroxene is absent in Mount Sidley xenoliths, whereas olivine is rare in Mount Hampton xenoliths. Mineral P-T equilibria indicate crystallisation of Mounts Sidley and Murphy pyroxenites at lower levels (7-11 kb and 6.5-12 kb respectively) than the granulites (3-5.5 kb and 3-9 kb), with Mount Hampton pyroxenites (6-7.5 kb) and granulites (5.5-8.5 kb) crystallising at similar crustal levels. High temperatures of equilibration (> 1000 [degrees] C) are consistent with a rift-like geotherm in the MBL lithosphere. Whole rock composition of the lower crustal xenoliths is controlled by the mineral assemblage, reflecting their origin as mafic cumulate rocks. Elements that partition readily into the xenolith mineral assemblage are present in higher abundances (e.g. up to 1700 ppm Sr in plagioclase rich xenoliths, and 3745 ppm Cr in clinopyroxene rich pyroxenites) than elements that do not (e.g. Rb < 6 ppm in all lower crustal xenoliths). 87Sr/86Sr (0.702861 [plus or minus] 7 to 0.704576 [plus or minus] 15) and 143Nd/144Nd (0.512771 [plus or minus] 6 to 0.512870 [plus or minus] 5) ratios indicate that the melts were primitive magmas, that did not assimilate any isotopically evolved crustal material prior to or during crystallisation. The single-pyroxene mineral assemblage of Mount Sidley (and possibly Mount Murphy) xenoliths crystallised from an alkaline melt, whereas the two-pyroxene assemblage of Mount Hampton xenoliths crystallised from a sub-alkaline melt. Xenoliths from Mount Sidley reveal petrographic and geochemical evidence for melt-fluid interaction at lower crustal depths. This interaction is inferred to be associated with late Cenozoic plume-related volcanism. It is manifested by high-temperature oxidation of olivine, replacement of clinopyroxene by kaersutite, traces of alkaline mafic glass, and the growth of apatite, Fe-Ti oxides and plagioclase. The xenolith suite has been enriched in elements that readily partition into these mineral phases (e.g. Ti, K, P, Sr, Ba), as well as in mobile elements (e.g. LILEs and LREEs). Pb isotopic ratios (e.g. 206Pb/204Pb from 18.005 - 19.589) and REEs define mixing lines between unradiogenic lower crust (206Pb/204Pb = 18.005) and small volume melts (206Pb/204Pb > 19.53) approaching HIMU composition, sourced from the inferred mantle plume. The composition of the infiltrating melts has also evolved, by percolative fractional crystallisation in the lower crust. The chemical heterogeneity detected in the MBL lower crust indicates a lower crustal discontinuity in the ECR, between Mount Sidley and Mount Hampton, here termed the ECR lower crustal discontinuity. Granulites from Mount Sidley are similar in composition to granulites from the Transantarctic Mountains (TM) in the McMurdo Sound region, Mount Ruapehu and Fiordland (New Zealand). Granulites from Mount Hampton are similar in composition to granulites from Mount Murphy, and the Ross Embayment (RE). These groups have been termed the TM Group and the RE Group respectively. The compositional similarity of granulites in each group may indicate the derivation of the lower crust in these regions from similar melts, and possibly indicate their juxtaposition as parts of the Gondwana supercontinent. The mafic cumulate character of the xenolith suite is inferred to represent original oceanic crust, and a model for the growth of the crust since its formation in latest pre-Cambrian - early Cambrian is presented here.

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Melt Percolation, Melt-Rock Reaction and Oxygen Fugacity in Supra-Subduction Zone Mantle and Lower Crust from the Leka Ophiolite Complex, Norway

Journal of Petrology ◽

10.1093/petrology/egab078 ◽

2021 ◽

Author(s):

Brian O’Driscoll ◽

Julien Leuthold ◽

Davide Lenaz ◽

Henrik Skogby ◽

James M D Day ◽

...

Keyword(s):

Single Crystal ◽

Subduction Zone ◽

Oxygen Fugacity ◽

Lower Crust ◽

Oceanic Lithosphere ◽

Ophiolite Complex ◽

Global Comparison ◽

Oceanic Spreading ◽

Melt Percolation ◽

Lower Crustal

Abstract Samples of peridotites and pyroxenites from the mantle and lower crustal sections of the Leka Ophiolite Complex (LOC; Norway) are examined to investigate the effects of melt-rock reaction and oxygen fugacity variations in the sub-arc oceanic lithosphere. The LOC is considered to represent supra-subduction zone (SSZ) oceanic lithosphere, but also preserves evidence of pre-SSZ magmatic processes. Here we combine field and microstructural observations with mineral chemical and structural analyses of different minerals from the major lithologies of the LOC. Wehrlite and websterite bodies in both the mantle and lower crust contain clinopyroxene likely formed at a pre-SSZ stage, characterised by high Al, high Cr, low Mg crystal cores. These clinopyroxenes also exhibit low Al, low Cr, high Mg outer rims and intracrystalline dissolution surfaces, indicative of reactive melt percolation during intrusion and disruption of these lithologies by later, SSZ-related, dunite-forming magmas. Chromian-spinel compositional variations correlate with lithology; dunite-chromitite Cr-spinels are characterised by relatively uniform and high TiO2 and Al2O3, indicating formation by melt-rock reaction associated with SSZ processes. Harzburgite Cr-spinel compositions are more variable but preserve a relatively high Al2O3, low TiO2 endmember that may reflect crystallisation in a pre-SSZ oceanic spreading centre setting. An important finding of this study is that the LOC potentially preserves the petrological signature of a transition between oceanic spreading centre processes and subsequent supra-subduction zone magmatism. Single crystal Cr-spinel Fe3+/ΣFe ratios calculated on the basis of stoichiometry (from electron microprobe [EPMA] and crystal structural [X-ray diffraction; XRD] measurements) correlate variably with those calculated by point-source (single crystal) Mössbauer spectroscopy. Average sample EPMA Fe3+/ΣFe ratios overestimate or underestimate the Mössbauer-derived values for harzburgites, and always overestimate the Mössbauer Fe3+/ΣFe ratios for dunites and chromitites. The highest Fe3+/ΣFe ratios, irrespective of method of measurement, are therefore generally associated with dunites and chromitites, and yield calculated log(fO2)FMQ values of up to ~+1.8. While this lends support to the formation of the dunites and chromitites during SSZ-related melt percolation in the lower part of the LOC, it also suggests that these melts were not highly oxidised, compared to typical arc basalts (fO2FMQ of >+2). This may in turn reflect the early (forearc) stage of subduction zone activity preserved by the LOC and implies that some of the arc tholeiitic and boninitic lava compositions preserved in the upper portion of the ophiolite are not genetically related to the mantle and lower crustal rocks, against which they exhibit tectonic contacts. Our new data also have implications for the use of ophiolite chromitites as recorders of mantle oxidation state through time; a global comparison suggests that the Fe3+/ΣFe signatures of ophiolite chromitites are likely to have more to do with local environmental petrogenetic conditions in sub-arc systems than large length-scale mantle chemical evolution.

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The Role of Lower Crustal Rheology in Lithospheric Delamination During Orogeny

Frontiers in Earth Science ◽

10.3389/feart.2021.755519 ◽

2021 ◽

Vol 9 ◽

Author(s):

Lin Chen

Keyword(s):

Lower Crust ◽

Numerical Models ◽

The Tibetan Plateau ◽

Mantle Lithosphere ◽

Laboratory Studies ◽

Surface Uplift ◽

Continental Lower Crust ◽

Lower Crustal ◽

Flow Laws

The continental lower crust is an important composition- and strength-jump layer in the lithosphere. Laboratory studies show its strength varies greatly due to a wide variety of composition. How the lower crust rheology influences the collisional orogeny remains poorly understood. Here I investigate the role of the lower crust rheology in the evolution of an orogen subject to horizontal shortening using 2D numerical models. A range of lower crustal flow laws from laboratory studies are tested to examine their effects on the styles of the accommodation of convergence. Three distinct styles are observed: 1) downwelling and subsequent delamination of orogen lithosphere mantle as a coherent slab; 2) localized thickening of orogen lithosphere; and 3) underthrusting of peripheral strong lithospheres below the orogen. Delamination occurs only if the orogen lower crust rheology is represented by the weak end-member of flow laws. The delamination is followed by partial melting of the lower crust and punctuated surface uplift confined to the orogen central region. For a moderately or extremely strong orogen lower crust, topography highs only develop on both sides of the orogen. In the Tibetan plateau, the crust has been doubly thickened but the underlying mantle lithosphere is highly heterogeneous. I suggest that the subvertical high-velocity mantle structures, as observed in southern and western Tibet, may exemplify localized delamination of the mantle lithosphere due to rheological weakening of the Tibetan lower crust.

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Thermodynamic constraints on assimilation of silicic crust by primitive magmas

10.5194/egusphere-egu21-1251 ◽

2021 ◽

Author(s):

Jussi S Heinonen ◽

Frank J Spera ◽

Wendy A Bohrson

Keyword(s):

Phase Equilibria ◽

Lower Crust ◽

Primitive Mantle ◽

Upper Crust ◽

Basaltic Composition ◽

Primitive Magma ◽

Partial Melts ◽

Thermodynamic Limits ◽

Melt Composition ◽

Lower Crustal

Some studies on basaltic and more primitive rocks suggest that their parental magmas have assimilated more than 50 wt.% (relative to the initial uncontaminated magma) of crustal silicate wallrock. But what are the thermodynamic limits for assimilation by primitive magmas? This question has been considered for over a century, first by N.L. Bowen and many others since then. Here we pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems &#8212; the Magma Chamber Simulator (MCS; https://mcs.geol.ucsb.edu).In the models, komatiitic, picritic, and basaltic magmas of various ages and from different tectonic settings assimilate progressive partial melts of average lower, middle, and upper crust. In order to pursue the maximum limits of assimilation constrained by phase equilibria and energetics, the mass of wallrock in the simulations was set at twice that of the initially pristine primitive magmas. In addition, the initial temperature of wallrock was set close to its solidus at a given pressure. Such conditions would approximate a rift setting with tabular chambers and high magma input causing concomitant crustal heating and steep geotherms.Our results indicate that it is difficult for any primitive magma to assimilate more than 20&#8722;30 wt.% of upper crust before evolving to intermediate/felsic compositions. However, if assimilant is lower crust, typical komatiitic magmas can assimilate more than their own weight (range of 59&#8722;102 wt.%) and retain a basaltic composition. Even picritic magmas, more relevant to modern intraplate settings, have a thermodynamic potential to assimilate 28&#8722;49 wt.% of lower crust before evolving into intermediate/felsic compositions.These findings have important implications for petrogenesis of magmas. The parental melt composition and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition. The fact that primitive mantle melts have potential to partially melt and assimilate significant fractions of (lower) crust may have fundamental importance for how trans-Moho magmatic systems evolve and how crustal growth is accomplished. Examples include generation of siliceous high-magnesium basalts in the Precambrian and anorogenic anorthosite-mangerite-charnockite-granite complexes with geochemical evidence of considerable geochemical overprint from (lower) crustal sources.

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