Partial melting of oceanic sediments in subduction zones and its contribution to the petrogenesis of peraluminous granites in the Chinese Altai

2018 ◽  
Vol 156 (4) ◽  
pp. 585-604 ◽  
Author(s):  
QUN LUO ◽  
CHEN ZHANG ◽  
SHU JIANG ◽  
LUOFU LIU ◽  
DONGDONG LIU ◽  
...  
Keyword(s):  
Partial Melting ◽  
Subduction Zones ◽  
Peraluminous Granite ◽  
Isotopic System ◽  
Central Asian ◽  
Oceanic Sediments ◽  
Peraluminous Granites ◽  
Isotopic Analyses ◽  
Geodynamic Processes

AbstractLate Carboniferous magmatism in the Chinese Altai provides an important view of geodynamic processes active during crustal growth in the Central Asian Orogenic Belt (CAOB). In this study, five representative peraluminous granite plutons from the Chinese Altai were selected for systematic geochronological, geochemical and Sr–Nd–Hf isotopic analyses (Table 1). These granites were emplaced between 449 and 327 Ma in an active subduction zone, and have moderate to high SiO2 (66.54–76.13 wt%), moderate Na2O+K2O (6.27–7.66 wt%), and high Al2O3 contents (12.43–16.18 wt%). All granite samples in this study showed significant decoupling of the Nd and Hf isotope systems. Results show negative εNd(t) values (−3.3 to −0.9), and predominantly positive εHf(t) values (+0.24 to +8.01, n=57) except for a few negative εHf(t) values (−7.44 to −0.03, n=9), high Mg# values (28.69–53.33), high Nd/Hf ratios (4.26–43.57), and enrichment of large-ion lithophile elements (LILEs; e.g. Pb, Th, and U), suggesting that the granites were derived from the partial melting of oceanic sediments and the associated mantle wedge, with fractionation of plagioclase, K-feldspar and biotite. In situ zircon Hf isotopic analyses yield negative εHf(t) values from −30.6 to −13.7 for the zircon xenocrysts. The U–Pb ages and Hf isotopic ratios of these zircon xenocrysts were probably inherited from oceanic sediments. Zircon saturation temperatures suggest that these peraluminous granites were emplaced at 537–765°C. We propose that: (1) the Nd isotopic system more faithfully reflects the source of peraluminous magmas in the Chinese Altai than the Hf isotopic system, and (2) the oceanic sediment recycling was an important process during continental growth in the CAOB.

scholarly journals Geochemical characteristics and conditions of formation of the Chah-Bazargan peraluminous granitic patches, ShahrBabak, Iran

2017 ◽  
Vol 68 (5) ◽  
pp. 445-463
Author(s):  
Abdolnaser Fazlnia
Keyword(s):  
Trace Elements ◽  
Partial Melting ◽  
Adhesion Force ◽  
High Viscosity ◽  
Metamorphic Complex ◽  
Peraluminous Granites ◽  
Sanandaj Sirjan Zone ◽  
In Situ Crystallization ◽  
Zagros Orogen

Abstract Xenoliths of garnet-biotite-kyanite schist from the Qori metamorphic complex (southern part of the Sanandaj-Sirjan zone, northeast Neyriz, Zagros orogen in Iran) in the 173.0±1.6 Ma Chah-Bazargan leuco-quartz diorite intrusion were studied. This intrusion caused these schist xenoliths to be metamorphosed to the pyroxene hornfels facies (approximately 4.5±1.0 kbar and 760±35 °C), converting them to diatexite migmatite as a result of partial melting of the xenoliths. These melts are granites in composition. Melt volumes of 20 to 30 vol. % were calculated for small patches of the peraluminous granites. It is possible that anatectic melting affected only the leucosome, such that melting was more than 20 to 30 vol. %. It is possible that a large amount of melt was not extracted due to balanced in situ crystallization, the adhesion force between melt and crystal (restite), and high viscosity of the leucosome. The Chah-Bazargan peraluminous granites are depleted in trace elements such as REEs, HFSE (Ti, Zr, Ta, Nb, Th, U, Hf, Y), Ba, Pb, and Sr. These elements are largely insensitive to source enrichment, but sensitive to the amounts of main and accessory minerals. These elements were hosted by minerals such as garnet, biotite, muscovite, K-feldspar, plagioclase, ilmenite, apatite, monazite, and zircon in the source (diatexitic migmatitic xenoliths).

Pure sediment-derived granites in a subduction zone

2021 ◽  
Author(s):  
Jian Xu ◽  
Xiao-Ping Xia ◽  
Qiang Wang ◽  
Christopher J. Spencer ◽  
Chun-Kit Lai ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Subduction Zones ◽  
Geochemical Modeling ◽  
Peraluminous Granite ◽  
Elemental Ratios ◽  
Isotopic Signatures ◽  
Yunnan Region ◽  
Isotopic Compositions ◽  
Peraluminous Granites ◽  
Regional Tectonic

The Earth is unique in the Solar System due to significant volumes of granite in the lithosphere. However, the origins of granites are still highly debated, especially sediment-derived granites, which are often treated as a geochemical end-member of the continental crust. In the Yunnan region of South China, we identify the occurrence of pure sediment-derived granite in a subduction system. The suite of strongly peraluminous granite reported herein is interpreted to represent pure metasedimentary melts based on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopic compositions. These Late Permian−Early Triassic (ca. 254−248 Ma) granites are characterized by radiogenically enriched Sr, Nd, and Hf isotopic signatures. They show δ11B and δ18O signatures akin to those of continental shales. Geochemical modeling indicates no contributions from the mantle that can be detected. Considering the regional tectonic evolution, these granites are suggested to be formed in a subduction zone by decompression melting of rapidly exhumed back-arc sediments. We posit that decompression melting was triggered by widespread extension and thinning of the crust prompted by rollback of the subducting oceanic crust. These granites thus provide evidence that granite formation in subduction zones does not necessarily contribute to crustal growth. These subduction-related pure sediment-derived granites have different elemental ratios and contents (e.g., Al2O3/TiO2 and Yb) from the Himalayan leucogranites. Considering their source compositions (e.g., pelitic rocks), which are similar to those of the Himalayan leucogranites, these differences are likely due to their higher formation temperature and lower pressure despite a great similarity in isotopic compositions. Identification of pure sediment-derived, strongly peraluminous granites (SPGs) in subduction systems provides an important geodynamic mechanism for crustal anatexis, which can both geochemically and tectonically complement their collisional counterparts identified in the Himalayas.

The Origins of Strongly Peraluminous Granitoid Rocks

2019 ◽  
Vol 57 (4) ◽  
pp. 529-550 ◽  
Author(s):  
D. Barrie Clarke
Keyword(s):  
Partial Melting ◽  
Bulk Composition ◽  
Aqueous Fluid ◽  
Granitic Rocks ◽  
Peraluminous Granite ◽  
Granite Magma ◽  
Metasedimentary Rocks ◽  
South Mountain Batholith ◽  
Peraluminous Granites ◽  
Granitoid Rocks

Abstract Strongly peraluminous granites (SPAGs), with 1.20 < A/CNK < 1.30, are relatively rare rocks. They contain significant modal abundances of AFM minerals such as Bt-Ms-Crd-Grt-And-Toz-Tur-Spl-Crn of potentially magmatic, peritectic, restitic, and xenocrystic origin. Determining the origin of a SPAG depends to a large extent on establishing the correct origin of these AFM minerals. Strongly peraluminous granitic rocks can form in eight distinctly different ways: (1) as the melt fraction resulting from dehydration partial melting of peraluminous metasedimentary rocks; (2) as the bulk composition of diatexitic migmatite resulting from extensive partial melting of peraluminous metasedimentary rock; (3) as a diatexite modified by incomplete restite unmixing; (4) by bulk contamination of a less strongly peraluminous granite magma with highly peraluminous metasedimentary rocks; (5) by selective acquisition or concentration of AFM minerals by a less strongly peraluminous granite magma; (6) by fractional crystallization of quartz and feldspar from a less strongly peraluminous granite magma; (7) by removal of alkalies (Ca, Na, K) by release of a suprasolidus aqueous fluid from a less strongly peraluminous granite magma; and (8) by subsolidus hydrothermal alteration of a less strongly peraluminous granite rock. Contamination by pelitic material is the most effective process for creating SPAG plutons. A detailed case study of the South Mountain Batholith shows that its early SPAGs contain high modal abundances of Bt-Crd-Grt, largely of external origin, whereas its later SPAGs contain high modal abundances of Ms-And-Toz, largely the products of fluido-magmatic processes.

In-situ reaction-replacement origin of hornblendites in the Early Cretaceous Laiyuan complex, North China Craton, and implications for its tectono-magmatic evolution

2021 ◽  
Author(s):  
Jia Chang ◽  
Andreas Audétat ◽  
Jian-Wei Li
Keyword(s):  
Partial Melting ◽  
North China Craton ◽  
Lithospheric Mantle ◽  
Lower Crust ◽  
North China ◽  
Ultramafic Rocks ◽  
Magmatic Evolution ◽  
Mafic Magmas ◽  
Felsic Rocks

Abstract Two suites of amphibole-rich mafic‒ultramafic rocks associated with the voluminous intermediate to felsic rocks in the Early Cretaceous Laiyuan intrusive-volcanic complex (North China Craton) are studied here by detailed petrography, mineral- and melt inclusion chemistry, and thermobarometry to demonstrate an in-situ reaction-replacement origin of the hornblendites. Moreover, a large set of compiled and newly obtained geochronological and whole-rock elemental and Sr-Nd isotopic data are used to constrain the tectono-magmatic evolution of the Laiyuan complex. Early mafic‒ultramafic rocks occur mainly as amphibole-rich mafic‒ultramafic intrusions situated at the edge of the Laiyuan complex. These intrusions comprise complex lithologies of olivine-, pyroxene- and phlogopite-bearing hornblendites and various types of gabbroic rocks, which largely formed by in-situ crystallization of hydrous mafic magmas that experienced gravitational settling of early-crystallized olivine and clinopyroxene at low pressures of 0.10‒0.20 GPa (∼4‒8 km crustal depth); the hornblendites formed in cumulate zones by cooling-driven crystallization of 55‒75 vol% hornblende, 10‒20 vol% orthopyroxene and 3‒10 vol% phlogopite at the expense of olivine and clinopyroxene. A later suite of mafic rocks occurs as mafic lamprophyre dikes throughout the Laiyuan complex. These dikes occasionally contain some pure hornblendite xenoliths, which formed by reaction-replacement of clinopyroxene at high pressures of up to 0.97‒1.25 GPa (∼37‒47 km crustal depth). Mass balance calculations suggest that the olivine-, pyroxene- and phlogopite-bearing hornblendites in the early mafic‒ultramafic intrusions formed almost without melt extraction, whereas the pure hornblendites brought up by lamprophyre dikes required extraction of ≥ 20‒30 wt% residual andesitic to dacitic melts. The latter suggests that fractionation of amphibole in the middle to lower crust through the formation of reaction-replacement hornblendites is a viable way to produce adakite-like magmas. New age constraints suggest that the early mafic-ultramafic intrusions formed during ∼132‒138 Ma, which overlaps with the timespan of ∼126‒145 Ma recorded by the much more voluminous intermediate to felsic rocks of the Laiyuan complex. By contrast, the late mafic and intermediate lamprophyre dikes were emplaced during ∼110‒125 Ma. Therefore, the voluminous early magmatism in the Laiyuan complex was likely triggered by the retreat of the flat-subducting Paleo-Pacific slab, whereas the minor later, mafic to intermediate magmas may have formed in response to further slab sinking-induced mantle thermal perturbations. Whole-rock geochemical data suggest that the early mafic magmas formed by partial melting of subduction-related metasomatized lithospheric mantle, and that the early intermediate to felsic magmas with adakite-like signatures formed from mafic magmas through strong amphibole fractionation without plagioclase in the lower crust. The late mafic magmas seem to be derived from a slightly different metasomatized lithospheric mantle by lower degrees of partial melting.

scholarly journals The role of sulphur on the melting of Ca-poor sediment and on trace element transfer in subduction zones: an experimental investigation

2021 ◽  
Author(s):  
Anne-Aziliz Pelleter ◽  
Gaëlle Prouteau ◽  
Bruno Scaillet
Keyword(s):  
Trace Element ◽  
Partial Melting ◽  
Subduction Zones ◽  
Sediment Flux ◽  
Arc Magmas ◽  
Trace Element Contents ◽  
The Stability ◽  
Element Transfer ◽  
High Sediment

Abstract We performed phase equilibrium experiments on a natural Ca-poor pelite at 3 GPa, 750-1000 °C, under moderately oxidizing conditions, simulating the partial melting of such lithologies in subduction zones. Experiments investigated the effect of sulphur addition on phase equilibria and compositions, with S contents of up to ∼ 2.2 wt. %. Run products were characterized for their major and trace element contents, in order to shed light on the role of sulphur on the trace element patterns of melts produced by partial melting of oceanic Ca-poor sediments. Results show that sulphur addition leads to the replacement of phengite by biotite along with the progressive consumption of garnet, which is replaced by an orthopyroxene-kyanite assemblage at the highest sulphur content investigated. All Fe-Mg silicate phases produced with sulphur, including melt, have higher MgO/(MgO+FeO) ratios (relative to S-free/poor conditions), owing to Fe being primarily locked up by sulphide in the investigated redox range. Secular infiltration of the mantle wedge by such MgO and K2O-rich melts may have contributed to the Mg and K-rich character of the modern continental crust. Addition of sulphur does not affect significantly the stability of the main accessory phases controlling the behaviour of trace elements (monazite, rutile and zircon), although our results suggest that monazite solubility is sensitive to S content at the conditions investigated. The low temperature (∼ 800 °C) S-bearing and Ca-poor sediment sourced slab melts show Th and La abundances, Th/La systematics and HFSE signatures in agreement with the characteristics of sediment-rich arc magmas. Because high S contents diminish phengite and garnet stabilities, S-rich and Ca-poor sediment sourced slab melts have higher contents of Rb, B, Li (to a lesser extent), and HREE. The highest ratios of La/Yb are observed in sulphur-poor runs (with a high proportion of garnet, which retains HREE) and beyond the monazite out curve (which retains LREE). Sulphides appear to be relatively Pb-poor and impart high Pb/Ce ratio to coexisting melts, even at high S content. Overall, our results show that Phanerozoic arc magmas from high sediment flux margins owe their geochemical signature to the subduction of terrigenous, sometimes S-rich, sediments. In contrast, subduction of such lithologies during Archean appears unlikely or unrecorded.

In situ Fe isotopic analyses of fourteen reference materials using a synthetic Cr standard for mass bias and isobaric interference corrections by femtosecond LA-MC-ICP-MS

2021 ◽  
Author(s):  
Lei Xu ◽  
Wen Zhang ◽  
Tao Luo ◽  
Jin-Hui Yang ◽  
Zhaochu Hu
Keyword(s):  
Reference Materials ◽  
Isotope Ratios ◽  
Isobaric Interference ◽  
Analytical Technique ◽  
Mass Bias ◽  
Icp Ms ◽  
Isotopic Analyses

High precise and accurate measurements of Fe isotope ratios for fourteen reference materials from the USGS, MPI-DING and CGSG were successfully carried out using a developed analytical technique by fs...

From migmatites to crustal granites: Pb–Pb isotopic system of feldspars of partial melting products, Northern Ladoga region, Fennoscandinavian Shield

2016 ◽  
Vol 468 (2) ◽  
pp. 541-544
Author(s):  
Sh. K. Baltybaev ◽  
G. V. Ovchinnikova ◽  
V. A. Glebovitskii ◽  
I. M. Vasil’eva ◽  
N. G. Rizvanova
Keyword(s):  
Partial Melting ◽  
Ladoga Region ◽  
Isotopic System

scholarly journals Magmatic flare-up causes crustal thickening at the transition from subduction to continental collision

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Carlos E. Ganade ◽  
Pierre Lanari ◽  
Daniela Rubatto ◽  
Joerg Hermann ◽  
Roberto F. Weinberg ◽  
...  
Keyword(s):  
Production Rate ◽  
Continental Margin ◽  
Subduction Zones ◽  
Continental Collision ◽  
Crustal Thickening ◽  
Time Intervals ◽  
West Gondwana ◽  
Continental Arc ◽  
Order Of Magnitude

AbstractAbove subduction zones, magma production rate and crustal generation can increase by an order of magnitude during narrow time intervals known as magmatic flare-ups. However, the consequences of these events in the deep arc environment remain poorly understood. Here we use petrological and in-situ zircon dating techniques to investigate the root of a continental arc within the collisional West Gondwana Orogen that is now exposed in the Kabyé Massif, Togo. We show that gabbros intruded 670 million years ago at 20–25 km depth were transformed to eclogites by 620 million years ago at 65–70 km depth. This was coeval with extensive magmatism at 20–40 km depth, indicative of a flare-up event which peaked just prior to the subduction of the continental margin. We propose that increased H2O flux from subduction of serpentinized mantle in the hyper-extended margin of the approaching continent was responsible for the increased magma productivity and crustal thickening.

scholarly journals Supplemental Material: Himalayan Miocene adakitic rocks, a case study of the Mayum pluton: Insights into geodynamic processes within the subducted Indian continental lithosphere and Himalayan mid-Miocene tectonic regime transition

2020 ◽  
Author(s):  
Chao Lin ◽  
Jinjiang Zhang ◽  
et al.
Keyword(s):  
Isotopic Composition ◽  
Continental Lithosphere ◽  
Isotopic Data ◽  
Element Geochemistry ◽  
Adakitic Rocks ◽  
Icp Ms ◽  
Geodynamic Processes ◽  
Data Statistics

Table S1: Bulk element geochemistry and isotopic composition of the Mayum pluton; Table S2: Zircon LA-ICP-MS U-Pb data for the Mayum pluton; Table S3: Zircon in situ Lu-Hf isotopic data for the Mayum pluton; Table S4: Data statistics for Himalayan Eocene and Miocene adakitic rocks.

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