Journal of Geology最新文献

{"title":"New Zircon U-Pb Age and Its Restriction on the Warming Time of the Interglacial Paleoclimate during the Cryogenian in the Yangtze Block","authors":"Minglong Li, Jingchun Tian, Lin Chen, Hui Xu, Deshun Zheng, Weiqing Liu","doi":"10.1086/705327","DOIUrl":"https://doi.org/10.1086/705327","url":null,"abstract":"The evolution of the paleoclimate and the duration of the two glacial periods during the Cryogenian in the Yangtze Block are of great geological significance for understanding “Snowball Earth.” We studied the evolution of the paleoclimate during the Cryogenian, using chemical weathering, and determined the warming time of the interglacial paleoclimate, using zircon U-Pb dating in the Yangtze region. A new laser ablation ICP-MS zircon U-Pb age of 659.9±3.2 Ma is reported from the middle Datangpo Formation. The chemical index of alteration (CIA) for the Cryogenian fine clastic rock samples from a drill core shows that the climate in the provenance area during the Gucheng and Nantuo glacial periods was dry and cold and that the chemical weathering was weak; the average CIA values were 63.3 and 64.2, respectively. In the early stage of the Datangpo period, the climate in the provenance area was still dry and cold, with a CIA average of 60.7, compatible with that in the two glacial periods; in the middle and late stages, the climate was warm and humid, chemical weathering was enhanced, and the average CIA rose to 77.4, compatible with the 76.9 of the Liantuo period before the Cryogenian. The evolution of other chemical weathering (paleoclimate) proxies, such as the chemical index of weathering, the plagioclase index of alteration, and Rb/Sr, is compatible with that of the CIA. The warming time of the interglacial paleoclimate during the Cryogenian in the Yangtze Block was ca. 660 Ma, roughly consistent with the ending time of the Sturtian in Australia and Mongolia, indicating that the lower part of the Datangpo Formation may still represent Sturtian glacial deposits, thus further suggesting the global consistency of the end of the Sturtian.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-10-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705327","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45311921","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A Mathematical Model Based on Bayesian Theory and Gaussian Copula for the Discrimination of Gabbroic Rocks from Three Tectonic Settings","authors":"Shuai Han, Mingchao Li, Qi Zhang, Heng Li","doi":"10.1086/705413","DOIUrl":"https://doi.org/10.1086/705413","url":null,"abstract":"Discriminating among tectonic settings by the chemical composition of igneous rocks is a feasible method in geochemistry. In this study, the feasibility of using gabbroic rocks to discriminate among tectonic settings is analyzed, and a mathematical model based on Gaussian copula and Bayesian theory is set up to discriminate among three tectonic settings: island arc, ocean island, and mid-oceanic ridge. The derivation of the model includes three steps: (1) determine the probability density functions (PDFs) of the elements in different tectonic settings, (2) determine the joint PDFs of the geochemical components of the rocks from different tectonic settings using copula functions, and (3) determine the tectonic settings of rocks using Bayesian theory. The optimal parameters of the mathematical model are calculated using a genetic algorithm, and finally the definitive form of the model is determined with nine basic elements: TiO2, Al2O3, FeOT, CaO, MnO, K2O, Na2O, Ni, and Sr. An experiment shows that the success rates of the mathematical model on the three tectonic settings are 84.03%, 95.48%, and 91.84%, respectively. The average percent success rate is 92.13%, which is significantly higher than using discrimination diagrams and the naive Bayes algorithm. Such an ideal result indicates that using gabbroic rocks to determine the types of tectonic settings is feasible. Moreover, this study can provide support for the application of machine learning and mathematical methods in the field of geochemistry.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705413","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46769410","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Possibility of Mixed Origin of Rare Earth Elements in Sedimentary Marine Apatites: A Case Study from Phosphorites in the Cretaceous (Campanian-Maastrichtian) Duwi Formation, Abu-Tartur Plateau, Egypt","authors":"G. El-Habaak, M. Askalany, M. Abdel-Hakeem","doi":"10.1086/705414","DOIUrl":"https://doi.org/10.1086/705414","url":null,"abstract":"It is well known that the rare earth elements (REEs) incorporated in the crystal lattice of sedimentary apatite are commonly derived from ambient seawater. This study documents, for the first time, the possibility of mixed origin of apatite REEs present in the Egyptian Western Desert phosphorites, known as the Abu-Tartur phosphorites, one of the most important accumulations of sedimentary phosphorites in the Middle East and North Africa. Shale-like patterns of REEs, negative Ce anomalies, and a (La/Sm)N−(La/Yb)N binary diagram of the studied phosphorites indicate that the incorporation of REEs into apatite crystal lattices has occurred from the ambient seawater by substitution during late-stage diagenesis. The second origin is attributed to REE-bearing supergene fluids, which resulted in the occurrence of sedimentary britholite as green rims and patches containing about 14.93 wt% total REEs in places where the black phosphorites are gradually oxidized into the brown variety. For instance, the intensive chemical weathering induces the crystal lattice of carbonate fluorapatite to preferentially release Ca2+ and CO32− ion species to solution, resulting in the formation of a carbonate-depleted layer in which REEs, particularly heavy REEs, are incorporated into the preferential Ca2+ sites inside the altered apatite lattice, leading to britholite formation.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705414","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43528885","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Petrology and Geochemistry of Mafic Intrusive Rocks from the Sapi-Shergol Ophiolitic Mélange, Indus Suture Zone, Western Ladakh: Constraints on Petrogenesis and Tectonic Setting","authors":"I. Bhat, T. Ahmad, D. S. Rao","doi":"10.1086/704254","DOIUrl":"https://doi.org/10.1086/704254","url":null,"abstract":"This article reports the existence of subduction-related tholeiitic, normal mid-ocean ridge basalt (N-MORB)-type mafic intrusives emplaced within the Sapi-Shergol ophiolitic mélange of the Indus Suture Zone, western Ladakh. The Shergol mafic intrusives show Fe enrichment with basalt to basaltic-andesite composition. Based on their mineralogy and textures, these mafic rocks can be identified as fine- to medium-grained gabbros that have undergone greenschist-grade metamorphism. These rocks have N-MORB-type geochemical characteristics, exhibiting nearly flat to depleted light rare earth element patterns ((La/Sm)N=0.66–1.05). Petrogenetic modeling suggests <20% partial melting of a depleted MORB-type mantle source, within the spinel peridotite stability. The presence of slightly negative anomalies of high field strength elements like Nb, Zr, and Ti in multielement patterns reflect the influence of subduction zone magmatism. The presence of low-Ti clinopyroxene (En38–50Fe11–25Wo31–43; enstatite-ferrosilite-wollastonite), Ca-rich plagioclase (An2–36; andesine), and pargasitic amphibole also reflects their subduction-related depleted-mantle origin. The MORB–island arc tholeiite signature displayed by the Shergol gabbros intrusive in Shergol peridotites reflect their generation in a mantle wedge associated with the Early Cretaceous intraoceanic subduction within the Neo-Tethys Ocean. They are similar to Spongtang ophiolite gabbros intrusive in the Spongtang ophiolite mantle peridotites from south Ladakh, ophiolitic mélange gabbros from southern Tibet, and Muslim Bagh ophiolite gabbros from Pakistan. Based on this study, we offer a geodynamic model suggesting that the Sapi-Shergol ophiolitic slice was intruded by mafic intrusive rocks that represent the relict of the intraoceanic substratum of the Cretaceous Dras arc complex.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/704254","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44549140","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Paleoproterozoic Mafic-Ultramafic Magmatism in the Northern Borborema Province, Northeast Brazil: Tectonic Setting and Potential for Deposits","authors":"Alanielson C. D. Ferreira, C. F. Filho, E. Dantas, Valmir da Silva Souza","doi":"10.1086/704256","DOIUrl":"https://doi.org/10.1086/704256","url":null,"abstract":"Mafic-ultramafic intrusions form a 32-km-long NE-SW-trending array within Paleoproterozoic arc-related calc-alkaline plutonic rocks of the Rio Piranhas terrane in the Borborema Province, Northeast Brazil. The mafic-ultramafic intrusions consist of wehrlite, clinopyroxenite, and troctolite. They outcrop as small elongated bodies hosted by orthogneiss and migmatites associated with minor amphibolite and supracrustal rocks. Primary magmatic features predominate in the core of the intrusions, whereas metamorphic minerals and textures characterize their outer zones. The composition of cumulus olivine from different intrusions ranges from Fo80.6 to Fo67.6 (forsterite), indicating moderately primitive parental magmas. The compositional range of cumulus plagioclase coexisting with cumulus olivine in troctolites is characterized by high anorthite content (An79.9–An89.0). This feature, together with interstitial magmatic amphibole in troctolites, is characteristic of gabbroic intrusions in subduction-related magmatic arcs. U-Pb zircon ages indicate that mafic-ultramafic intrusions crystallized at ca. 2190 ± 5 Ma. U-Pb zircon ages of host orthogneiss indicate that they crystallized from calc-alkaline felsic magmas at ca. 2220–2223 Ma, thus providing only ∼30 My for the reported events of felsic and mafic plutonism. U-Pb zircon ages of host rocks include older ages (2.47 and 2.69 Ga) indicating that older crustal components are associated with the 2.23 Ga calc-alkaline magmatism. The presence of older crustal components is consistent with arc-related features, indicated by geochemical data and variable εNd(t) values in the mafic-ultramafic intrusions. The tholeiitic and calc-alkaline magmatism is correlated with the ca. 2.15–2.25 Ga magmatic arc reported in previous studies of the Rio Piranhas terrane. The mafic-ultramafic magmatism is interpreted as having originated in a magmatic arc, possibly resulting from partial melting of a mantle wedge above the subduction zone in late stages of the orogeny and suggests a new window of opportunity for exploration of Ni-Cu–platinum group element deposits.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/704256","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42478423","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Arc-Related Pyroxenites Derived from a Long-Lived Neoarchean Subduction System at the Southwestern Margin of the Cuddapah Basin: Geodynamic Implications for the Evolution of the Eastern Dharwar Craton, Southern India","authors":"Abhinay Sharma, R. Giri, N. V. C. Rao, W. Rahaman, D. Pandit, S. Sahoo","doi":"10.1086/704361","DOIUrl":"https://doi.org/10.1086/704361","url":null,"abstract":"Mineralogy and bulk-rock geochemistry (including Sr and Nd isotopes) of two ultramafic (pyroxenite) intrusions at Gurramkonda, situated adjacent to the southwestern margin of the Paleo-Mesoproterozoic Cuddapah Basin within the Eastern Dharwar Craton, southern India, are presented. The pyroxenites have a characteristic cumulate texture, with a predominance of pyroxene (augite) and amphibole (hornblende and actinolite), whereas chromite is present mostly as inclusions in pyroxene. Mineral chemistry of these ultramafics is akin to that of arc cumulate rocks, particularly those associated with the Alaskan-type complexes, formed in suprasubduction zones. High Mg#’s (72–74), coupled with high bulk-rock Ni (830–1130 ppm) and Cr (1110–1,360 ppm), strikingly correspond to those of continental arc cumulates. The enrichment of large ion lithophile elements over high field strength elements, conspicuous negative Nb-Ta, Zr-Hf, and Ti anomalies, and chondrite-normalized light rare earth element patterns highlight their arc affinity. Fluid-related modification of the mantle source is apparent from the various trace-element ratios. Negative ɛNd2.15Ga values (−0.93 to −2.90) also support contribution of an enriched-mantle reservoir in their genesis. Partial melting of this fertile mantle source has been attributed to the Paleoproterozic lithospheric extension with heat contributions from a mantle plume or convective asthenosphere. Ultramafic intrusions of arc-related geochemical affinity thus constitute rare, but genetically important, occurrences within the Eastern Dharwar Craton. We propose that, together with the recently studied calc-alkaline lamprophyres occurring immediately toward the western margin of the Cuddapah Basin, the Gurramkonda pyroxenites are manifestations of a long-lived and major subduction system of Neoarchean age in the Eastern Dharwar Craton, southern India.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/704361","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47263829","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Upper Visean Magdalen Islands Basalts of Eastern Quebec, Canada: A Complex Assemblage of Contrasting Mafic Rock Types Erupted in Peak Stages of Transtensional Basin Development above a Mantle Plume","authors":"P. Jutras, J. Dostal","doi":"10.1086/704382","DOIUrl":"https://doi.org/10.1086/704382","url":null,"abstract":"A thick succession of mafic upper Asbian volcanic rocks is exposed as cap rocks in the Magdalen Islands of eastern Quebec. This succession records peak stages of a Late Devonian to Early Carboniferous magmatic system that regionally developed from transtensional tectonics above a mantle plume. In the upper part of the succession, tholeiitic basalts with enriched mid-oceanic ridge basalt (E-MORB) affinities are tightly interbedded with highly alkaline basalts with ocean island affinities, whereas the lower part is composed of moderately alkaline basalts and pyroclastic deposits that are interpreted as the products of mixing between these two compositionally distinct melts. Based on trace-element contents and geological constraints, the tholeiitic basalts were produced by decompressional melting in depleted uppermost asthenospheric mantle material enriched over the average (E-MORB-type source) at the top of the plume, whereas the highly alkaline basalts were produced subsequently when decompressional melting reached down into the enriched mantle (ocean island basalt–type source). These two primary melts may have been thermally stratified below the lithosphere before being tapped by transtensional structures to feed a thick magmatic underplating that regionally developed at the base of the crust, where magma mixing may have occurred. Interbedding of the two contrasting melts in the upper part of the succession implies that, although the two primary mantle sources were still being tapped into, magma mixing was by then no longer occurring. Above a small gap, the uppermost flow is a fluorine-rich tholeiitic basalt with arc affinities that suggest partial melting of the regional subcontinental lithospheric mantle source, which was previously enriched by nearly continuous subduction below eastern Canada in early to middle Paleozoic times. The latter setting seemingly corresponds to the local waning of the magmatic system in uppermost Asbian times, possibly because of the westward migration of the overridden plume toward New Brunswick, where alkaline volcanism from a sublithospheric source resumed in Brigantian times.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/704382","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43665154","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"An Evaluation of Boulder Deposits along a Granite Coast Affected by the 2004 Indian Ocean Tsunami Using Revised Hydrodynamic Equations: Batu Ferringhi, Penang, Malaysia","authors":"S. Haslett, Bernardine R. Wong","doi":"10.1086/704255","DOIUrl":"https://doi.org/10.1086/704255","url":null,"abstract":"A field survey of wave-transported granite boulder deposits occurring at Batu Ferringhi on the north coast of the island of Penang, Malaysia, was undertaken to evaluate whether the boulders had more likely been transported by storm waves or by the 2004 Indian Ocean tsunami. Field measurements of boulder dimensions, imbrication, and depositional position in relation to the tidal frame (using a local intertidal biozonation) were collected from seven boulder groups. Revised hydrodynamic equations for wave height and flow velocity for subaerial and submerged boulders were employed, as well as formulas for joint-bounded pretransport settings. Under subaerial pretransport settings, five of the boulder groups could have been transported under local storm wave conditions. A joint-bounded setting would have precluded the transport of three of the boulder groups by the 2004 tsunami event; otherwise all seven groups could have been transported by this tsunami event under subaerial or submerged pretransport settings. This event is known to have fatally affected this coastline, and, although it is very likely that there have been other historic tsunami events within the region, based on evaluation of the boulder deposits presented here, there is no evidence to suggest that any wave event of a magnitude higher than the 2004 tsunami, be it tsunami or storm wave, has been experienced along this coastline.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/704255","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41558958","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Paleoenvironmental Archives in Rock Rinds and Sand/Silt Coatings","authors":"W. Mahaney","doi":"10.1086/703537","DOIUrl":"https://doi.org/10.1086/703537","url":null,"abstract":"Organo-mineral chemical coatings in rock rinds and sands and silts, underappreciated research until recently, offer a wealth of paleoenvironmental records derived from chemical and biomineral weathering over varying time frames. Rock rinds as well as sands and silts in paleosols carry various detailed records ranging from thin weathered films in young (Holocene) samples to thicker, often overprinted records in recycled sands/rinds of Pleistocene and older age, some as old as Miocene and Late Oligocene. Whereas rinds on Earth are confined to the Late Paleogene/Neogene, similar examples on Mars extend into the Noachian (∼4 Ga). New research outlined here shows that coatings tend to produce altered microsurfaces conducive to microbe growth with fossil forms found in a variety of different microenvironments, including extreme polar and tropical locales. Organo-chemical coatings on sands in paleosols reveal a wealth of biomarkers important in paleoenvironmental reconstruction from pole to equator. Rock rinds, revealed here, are shown to carry weathering zones in microcosm that often bear a close similarity to juxtaposed paleosols, the latter carrying mature A/B/C/Cu horizonation, indicating that weathering forces affecting a substrate are fine-tuned to similar kinetic forces acting on clasts in surface deposits. Aeolian and cosmic airburst inputs to rock rinds and chemical reconstruction of Si/Al wreckages leading to clay mineral reconstruction are also found in some rind and paleosol records. Analyses of such coatings using the light microscope, scanning electron microscope/energy dispersive spectrometer, focused ion beam, and secondary ion mass spectrometer offer the prospect of revealing still more secondary organo-mineral compounds, whose relation to primary mineral weathering is of importance to understanding pedogenic and diagenetic processes. The sum total of environmental change over time, written or embedded in rinds and sand/silt coatings, when subjected to high-resolution microscopy offers a novel mineral archive at the nanometer level and above.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/703537","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43163124","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Abyssal and Suprasubduction Peridotites in the Lajishan Ophiolite Belt: Implication for Initial Subduction of the Proto-Tethyan Ocean","authors":"Changlei Fu, Zhen Yan, J. Aitchison, Xianqing Guo, Wenjing Xia","doi":"10.1086/703488","DOIUrl":"https://doi.org/10.1086/703488","url":null,"abstract":"Several mantle peridotite massifs crop out along the northern margin of the South Qilian belt, but their origin remains controversial. The Xigou peridotite massif is the largest one within the Lajishan ophiolite belt and mainly consists of lherzolite, harzburgite, and serpentinized dunite, in association with anorthosite, amphibolite, dolerite dikes, and pillow basalt. Lherzolites show typical features of relatively fertile abyssal peridotites, with iron-rich olivine, Al-spinel, and aluminous pyroxene. Harzburgites and serpentinized dunites are characterized by U-shaped rare earth element patterns and low Pd-group platinum-group elements resembling the geochemical features of suprasubduction peridotites. Spinel Cr#’s ( Cr / ( Cr + Al ) ) range from 0.38 in harzburgite to 0.75 in dunite. The progressive increase in spinel Cr#’s is related to changing melt compositions from mid-ocean ridge basalt (MORB) like to boninitic during subduction initiation. The Xigou suprasubduction peridotites, MORB-like mafic rocks, and boninites probably constitute an incomplete forearc crust-mantle section. The abyssal lherzolite preserved in the forearc region might represent older oceanic lithosphere or younger fertile mantle residue along a trench–fracture zone intersection with a transform fault. Therefore, we suggest that the intraoceanic subduction of the Proto-Tethyan Ocean may have begun no later than 535 Ma, before the development of the Lajishan intraoceanic arc-trench system.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2019-06-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/703488","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46845543","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}