GSA Bulletin最新文献

{"title":"Cenozoic intraplate volcanism in central Asia: Mantle upwelling induced by India-Eurasia collision","authors":"Jian Wang, Yuping Su, Jianping Zheng, H. Dai, G. Tang, Qiang Ma, J. Zhao, E. Belousova, W. Griffin, S. O’Reilly","doi":"10.1130/b36545.1","DOIUrl":"https://doi.org/10.1130/b36545.1","url":null,"abstract":"Most of Earth’s volcanism occurs at tectonic plate boundaries associated with subduction or rifting processes. The mantle plume hypothesis is an important supplement to plate tectonics for explaining some high-volume intraplate volcanic fields. However, many intraplate magmatic provinces occur as low-volume, monogenetic basaltic-suite fields that are neither associated with plate-boundary processes nor attributable to mantle plumes, and the origin of such magmatism has long been debated. Identification of their source characteristics and possible mechanisms that trigger mantle melting will provide essential insights into Earth’s mantle heterogeneity and also develop our knowledge of tectonic plate movement through time. Here, we report new geochronology, mineral chemistry (especially olivine), and whole-rock chemical and Sr-Nd-Pb-Hf isotopic compositions on Cenozoic intracontinental alkaline basalts from the northwestern Tarim craton (central Asia), aiming to better assess the origin of Earth’s low-volume effusive intraplate volcanic fields. The basalts (ca. 42 Ma) have olivine (e.g., mean Ni abundances of ∼2250 ppm, mean Mn/Zn ratios of 13.7) and whole-rock chemistry consistent with their derivation from a mixed peridotite-pyroxenite source. Moderately depleted Sr-Nd-Pb-Hf isotopes (87Sr/86Sr = 0.7039−0.7053; εNd = +4.0 to +5.5; 206Pb/204Pb = 18.247−18.535; εHf = +8.1 to +8.7) require a young (ca. 500 Ma) oceanic crust recycled into the source, possibly related to subduction events during the assembly of Pangea. Estimated thermal-chemical conditions indicate that the original melting occurred in a relatively dry (H2O = 1.4 ± 0.9 wt%) and reduced (logfO2 ΔFMQ = −0.97 ± 0.21, where FMQ is fayalite-magnetite-quartz) asthenosphere under a mantle potential temperature of ∼1420 °C and a pressure of ∼3.7 GPa (corresponding to a depth of ∼120 km). Combining these data with regional tectonic history and geophysical data (high-resolution P-wave tomography), we propose that the long-lasting India-Eurasia collision triggered asthenospheric upwelling, focusing melts along translithospheric zones of weakness; this model provides a robust explanation for the observed Cenozoic intracontinental volcanism in central Asia. The integrated geochemical and geophysical evidence reveals that plate subduction−induced mantle upwelling represents a likely mechanism for the generation of many regions of plume-absent intraplate magmatism within continents.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"207 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-11-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132047774","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Rifted margin with localized detachment and polyphase magmatism: A new model of the northern South China Sea","authors":"Shihao Hao, L. Mei, X. Pang, L. Gernigon, D. Paton, Jinyun Zheng, Qing Ye, Zhichao Zhou, Ye Zhong","doi":"10.1130/b36264.1","DOIUrl":"https://doi.org/10.1130/b36264.1","url":null,"abstract":"Continental passive margins are commonly classified into volcanic (or magma-rich) and non-volcanic (or magma-poor) types based on the magmatic budget. It is difficult, however, to use either of these models to define the northern margin of the South China Sea (SCS). Based on 2-D/3-D time/depth-migrated seismic data and borehole data from the Pearl River Mouth Basin in the northern SCS, we examine the detachment fault systems and syn-rift magmatism in the Baiyun and Liwan Sags. The detachment fault systems and related tilted blocks of the northern SCS established in the Baiyun and Liwan Sags in previous studies may be mainly constrained to the Baiyun Sag. The Liwan Sag, however, acts as a “sag-type” basin accompanied by magmatic domes. Radiometric age of igneous samples from wells, coupled with multi-horizon onlapping terminations within syn-rift layers, reveal four-phase rift-related magmatism in the northern SCS. The crustal thinning in the Baiyun Sag is controlled by a detachment fault, but in the Liwan Sag, it is mainly controlled by a preferential thinning of the lower crust. High thermal conditions and intense syn-rift magmatism weakened the lower crust, which contributes to the depth-dependent thinning in the Liwan Sag.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"33 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123656622","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Quantitative and geomorphologic parameterization of megaclasts within mass-transport complexes, offshore Taranaki Basin, New Zealand","authors":"Wei Li, Yan Li, Kamaldeen O. Leif Omosanya, Tiago M. Alves, Song Jing, Xiujuan Wang, Nan Wu, Wenhuan Zhan","doi":"10.1130/b36446.1","DOIUrl":"https://doi.org/10.1130/b36446.1","url":null,"abstract":"Mass-transport complexes (MTCs) in sedimentary basins reflect the gravitational transport of sediments from the shelf edge to the abyssal plain. Megaclasts, large sedimentary blocks of hundreds of meters long within MTCs, can record kinematic and sedimentary information deemed essential to understanding source-to-sink systems. Yet, deformation structures in such megaclasts remain poorly understood. This study uses high-quality, three-dimensional (3-D) seismic reflection data from the deep-water Taranaki Basin offshore New Zealand to analyze the morphological character of 123 megaclasts and propose a new classification scheme based on their morphometric properties. The megaclasts are up to 400 m tall, 1900 m long, and 1200 m wide. In the study area, they are high- to moderate-amplitude features owing to their different lithologies and continuous-to-contorted seismic facies. The megaclasts can be classified as undeformed, rotated, deformed, and highly deformed based on their internal deformational styles. Two different kinds of morphological depressions observed on their basal shear zones further indicate whether the megaclasts are transported or formed in situ. Our study demonstrates that quantitative parameterization of the megaclasts provides important information about their deformational processes and a more complete understanding of megaclast emplacement along continental margins.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"34 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124813653","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Formation of authigenic carbonates contributed by sulfate- and metal-driven anaerobic oxidation of methane in the northern Okinawa Trough, East China Sea","authors":"Lei Liu, H. Guan, Lanfang Xu, Zhilei Sun, N. Wu","doi":"10.1130/b36400.1","DOIUrl":"https://doi.org/10.1130/b36400.1","url":null,"abstract":"Authigenic carbonates of the northern Okinawa Trough have been shown to be contributed by both sulfate- and Fe-driven anaerobic oxidation of methane (SD-AOM and Fe-AOM, respectively); however, the microorganisms involved remain poorly constrained. To better understand the types and roles of microorganisms in specific biological methane oxidation processes, authigenic carbonates collected from the northern Okinawa Trough at water depths between ∼540 m and 700 m were studied using petrographic, mineralogical, elemental, and lipid biomarker analyses. The carbonate minerals were primarily aragonite, high-magnesium calcite, low-magnesium calcite, or dolomite. The extremely low δ13C values of bulk carbonates (−59.2‰), molecular fossils of anaerobic methane oxidizing archaea (ANME; −131‰), and sulfate-reducing bacteria (SRB; −77‰) indicated that the authigenic carbonates formed as products of AOM. Biogenic methane was confirmed to be the main carbon source for all carbonates, whereas bio-degraded oil and hydrothermal input complemented carbon sources, as revealed by the occurrence of unresolved complex mixture, strongly enriched As and Sb, and their correlations with Fe/Al. Combined with enriched δ18O values (as high as +5‰), the methane fluids were suggested to derive from the dissociation of gas hydrates. ANME-2 and a mixture of ANME-1 and ANME-2 were identified for the two calcitic carbonates, respectively, as revealed by their specific biomarker patterns. The relatively higher contents of 13C-depeleted isoprenoids and SRB−related fatty acids, strong Mo enrichment, and biological debris in both calcite- and aragonite-dominated carbonates reflect precipitation driven by SD-AOM in a shallow sulfate-methane transition. For other seep carbonates, low amounts of 13C-depeleted isoprenoids and the near absence of crocetane suggest dominance of the ANME-1 assemblage. The extremely low amounts and non-13C-depleted iso-/anteiso-C15:0 fatty acids (−27‰) in the dolomite-containing carbonates indicate the decoupling of ANME and SRB. These carbonates were most likely formed below or at the bottom of the sulfate-methane transition and included contributions from Fe-AOM, as indicated by the low Mo content, occurrence of dolomite and siderite, and decreased δ56Fe values with increasing Fe/Al ratios.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"225 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134344299","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Cambrian−Ordovician magmatic flare-up in NE Gondwana: A silicic large igneous province?","authors":"Wei Dan, J. Murphy, G. Tang, Xiu-Zheng Zhang, W. M. White, Qiang Wang","doi":"10.1130/b36331.1","DOIUrl":"https://doi.org/10.1130/b36331.1","url":null,"abstract":"The origin of the Cambrian−Ordovician tectono-magmatic events affecting NE Gondwana and the adjacent peri-Gondwanan terranes (e.g., Himalaya, Lhasa, Southern Qiangtang, Baoshan, Tengchong, Sibumasu, Helmand, and Karakorum) is controversial. Although its volume is poorly constrained, we propose that an extensive belt of granitic rocks that formed in various pulses between ca. 510 Ma and 460 Ma may represent the remains of a potential silicic large igneous province (LIP), which is referred to here as the Pinghe silicic LIP, with an areal extent of ∼2.5 Mkm2. The putative Pinghe silicic LIP is composed predominantly of S-type granites with subordinate A-type granites and minor intraplate mafic rocks. The recognition of this belt of granitic rocks aids in the refinement of tectonic reconstructions of Gondwana and of models for the rifting of terranes from its northern margin. The Pinghe silicic LIP broadly coincides with the adjacent 511 Ma Kalkarindji LIP in northern Australia, and the plume or mantle upwelling that triggered the Kalkarindji LIP may have been responsible for driving crustal melting that generated the granitic rocks, in a manner analogous to the Karoo−Chon Aike association.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"11 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115391997","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lithologic controls on microfracturing from legacy underground nuclear explosions","authors":"E. Swanson, Jennifer E. Wilson, A. Sussman","doi":"10.1130/b36466.1","DOIUrl":"https://doi.org/10.1130/b36466.1","url":null,"abstract":"Detection and verification of underground nuclear explosions (UNEs) can be improved with a better understanding of the nature and extent of explosion-induced damage in rock and the effect of this damage on radionuclide migration. Much of the previous work in this area has focused on centimeter- to meter-scale manifestations of damage, but to predict the effect of damage on permeability for radionuclide migration, observations at smaller scales are needed to determine deformation mechanisms. Based on studies of tectonic deformation in tuff, we expected that the heterogeneous tuff layers would manifest explosion-induced damage differently, with welded tuffs showing more fractures and nonwelded tuffs showing more deformation bands. In comparing post-UNE samples with lithologically matched pre-UNE equivalents, we observed damage in multiple lithologies of tuff through quantitative microfracture densities. We find that the texture (e.g., from deposition, welding, alteration, etc.) affects fracture densities, with stronger units fracturing more than weaker units. While we see no evidence of expected deformation bands in the nonwelded tuffs, we do observe, as expected, much larger microfracture densities at close range (<50 m) to the explosive source. We also observe a subtle increase in microfracture densities in post-UNE samples, relative to pre-UNE equivalents, in all lithologies and depths. The fractures that are interpreted to be UNE-induced are primarily transgranular and grain-boundary microfractures, with intragranular microfracture densities being largely similar to those of pre-UNE samples. This work has implications for models of explosion-induced damage and how that damage may affect flow pathways in the subsurface.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"142 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133478793","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Cretaceous−Cenozoic cooling history of the Qiangtang terrane and implications for Central Tibet formation","authors":"Wenjun Bi, Yalin Li, P. Kamp, Ganqing Xu, Jiawei Zhang, Zhongpeng Han, L. Du, Chengshan Wang, Haiyang He, Tiankun Xu, Zining Ma","doi":"10.1130/b36313.1","DOIUrl":"https://doi.org/10.1130/b36313.1","url":null,"abstract":"Reconstructing the growth process of the Qiangtang terrane in space and time is crucial for understanding the geological evolution of Central Tibet. However, its growth process and dynamic mechanism remain unclear. Here, we present new fission track data obtained along a N-S transect near the Puruo Kangri Mountain in the central zone of the Northern Qiangtang terrane. The completely reset apatite fission track ages of detrital samples range from 65.1 to 89.6 Ma, which show a northward younging trend. The thermal history modeling results indicate that this region underwent northward-propagating exhumation during the Late Cretaceous (ca. 92−65 Ma). Our data, combined with previously reported low-temperature thermochronology data for the Qiangtang terrane, suggest that the Qiangtang terrane experienced three main stages of cooling: ca. 120−65, ca. 55−35, and <25−0 Ma. The first stage (ca. 120−65 Ma) displays an outward-propagating cooling pattern from the Central Qiangtang terrane, which was related to the crustal shortening and thickening driven by the Lhasa-Qiangtang collision. The low exhumation rate, flat lavas, and paleoaltimetry studies imply that the central and southern zones of the Northern Qiangtang terrane and Central Qiangtang terrane may form plateau landscape by 65−55 Ma. The second stage (ca. 55−35 Ma) of cooling is mainly documented in the Southern Qiangtang terrane, and the northern zone of the Northern Qiangtang terrane. This cooling event was caused by the crustal deformation and shortening driven by intracontinental subduction related to ongoing convergence of the Indian and Asian plates. Subsequently, both the transition to low erosion rates (<0.05 mm/yr) and paleoaltimetry data indicate that the Qiangtang terrane became a primitive plateau by ca. 35 Ma. The final stage (<25−0 Ma) of cooling was linked to the E-W extension in the Qiangtang terrane.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130712684","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Growth of the southern Tian Shan-Pamir and its impact on central Asian climate","authors":"F. Richter, Jozi Pearson, Marius Vilkas, R. Heermance, C. Garzione, M. Cecil, G. Jepson, Annelisa Moe, Jianhong Xu, Langtao Liu, Jie Chen","doi":"10.1130/b36471.1","DOIUrl":"https://doi.org/10.1130/b36471.1","url":null,"abstract":"Uplift and amalgamation of the high-elevation (>3000 m) Tian Shan and Pamir ranges in Central Asia restricts westerly atmospheric flow and thereby limits moisture delivery to the leeward Taklimakan Desert in the Tarim Basin (<1500 m), the second largest modern sand dune desert on Earth. Although some research suggests that the hyper-arid conditions observed today in the Tarim Basin developed by ca. 25 Ma, stratigraphic evidence suggests the first erg system did not appear until 12.2 Ma. To address this controversy and to understand the tectonic influences on climate in Central Asia, we studied a continuous, 3800-m-thick stratigraphic section deposited from 15.1 to 0.9 Ma now exposed within the western Kepintagh fold-and-thrust belt in the southern Tian Shan foreland. We present new detrital zircon data (n = 839), new carbonate oxygen (δ18Oc) and carbon (δ13Cc) stable isotope compositions (n = 368), structural modeling, and stratigraphic observations, and combine these data with recently published magnetostratigraphy and regional studies to reconstruct the history of deposition, deformation, and climate change in the northwestern Tarim Basin. We find that basins along the southern (this study) and northern (i.e., Ili Basin) margins of the Tian Shan were likely receiving similar westerly precipitation by 15 Ma (δ18Oc = ∼−8‰) and had similar lacustrine-playa environments at ca. 13.5 Ma, despite differences in sedimentary provenance. At ca. 12 Ma, an erg desert formed adjacent to the southern Tian Shan in the northwestern Tarim Basin, coincident with a mid- to late Miocene phase of deformation and exhumation within both the Pamir and southern Tian Shan. Desertification at ca. 12 Ma was marked by a negative δ18Oc excursion from −7.8 ± 0.4‰ to −8.7 ± 0.7‰ in the southern Tian Shan foreland (this study), coeval with a negative δ18Oc excursion (∼−11 to −13‰) in the Tajik Basin, west of the Pamir. These data suggest that only after ca. 12 Ma did the Pamir-Tian Shan create a high-elevation barrier that effectively blocked westerly moisture, forming a rain shadow in the northwestern Tarim Basin. After 7 Ma, the southern Tian Shan foreland migrated southward as this region experienced widespread deformation. In our study area, rapid shortening and deformation above two frontal foreland faults initiated between 6.0 and 3.5 Ma resulted in positive δ13Cc excursions to values close to 0‰, which is interpreted to reflect exhumation in the Tian Shan and recycling of Paleozoic carbonates. Shortening led to isolation of the study site as a piggy-back basin by 3.5 Ma, when the sediment provenance was limited to the exhumed Paleozoic basement rocks of the Kepintagh fold belt. The abrupt sedimentologic and isotopic changes observed in the southern Tian Shan foreland appear to be decoupled from late Cenozoic global climate change and can be explained entirely by local tectonics. This study highlights how tectonics may overprint the more regional and global climate signa","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"44 2","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132433718","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geological architecture and history of the Antigua volcano and carbonate platform: Was there an Oligo−Miocene lull in Lesser Antilles arc magmatism?","authors":"Leny Montheil, M. Philippon, J. Cornée, M. BouDagher-Fadel, D. V. van Hinsbergen, P. Camps, M. Maffione, F. Audemard, Brechtje Brons, Koen J.R. Van der Looij, P. Münch","doi":"10.1130/b36465.1","DOIUrl":"https://doi.org/10.1130/b36465.1","url":null,"abstract":"Since the acceptance of plate tectonics, the presence of calc-alkaline magmatic rocks has been recognized as evidence of subduction. But under specific geodynamic circumstances, subduction may occur without generating magmas. Here, we investigate the Cenozoic northern Lesser Antilles arc where, from sparsely exposed magmatic records, Eocene−Oligocene and Pliocene magmatic flare-ups and a Miocene lull were postulated. Nevertheless, most of the arc is submarine, so it is challenging to discern lulls and flare-ups from sampling bias. We review the magmatic evidence exposed onshore in the Lesser Antilles and investigate in detail the island of Antigua, which exposes an Eocene to Miocene volcanic sequence and platform carbonate series that coincide with the postulated lull. By combining lithostratigraphic analysis, structural mapping, 40Ar/39Ar geochronology, and biostratigraphy, we refine the magmatic history of the island and date the arrest of extensive arc magmatism at 35 Ma, with minor activity until 27 Ma. No magmatic products are interleaved with the platform sequence until the latest Oligocene, which confirms a lull in northern Lesser Antilles arc magmatism that may have lasted ca. 20 Ma. Flare-up of magmatic activity contributed to crustal thickening and land emersion, whereas magmatic lulls and related thermal cooling induced subsidence/submersion. Thus, we propose that the paleo-(bio)-geographical evolution of the eastern Caribbean region has been partly controlled by magmatic activity.\u0000 Fault kinematic analysis, along with anisotropy of magnetic susceptibility, suggest that, at the island scale, magmatic arrest is not associated with a change in stress field during the Oligocene. We speculate that slab flattening triggered by progressive curvature played a role in the temporal shutdown of the northern Lesser Antilles arc.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"32 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116638396","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"New insights into deep carbon recycling and formation of nepheline-bearing alkaline rocks from Sr-Nd-Mg isotope compositions","authors":"Weiliang Kong, Zhaochong Zhang, Dongyang Zhang, Changhong Wang, M. Santosh, Bingxiang Liu, Bowen Wei","doi":"10.1130/b36555.1","DOIUrl":"https://doi.org/10.1130/b36555.1","url":null,"abstract":"Magnesium isotopes are widely used to trace recycled carbonates in the mantle source. We recently recognized extremely light Mg isotope values (δ26Mg = −0.50 to −0.62‰) in nepheline syenites in the Tarim large igneous province (TLIP), NW China. To evaluate the significance of the light Mg isotopes, we conducted petrological, mineral chemical, zircon U-Pb ages, and geochemical and isotopes (Sr-Nd-Mg) analyses on the nepheline syenite to understand its petrogenesis. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating yields an age of 272.5 ± 1.4 Ma for the nepheline syenite. Petrographic and geochemical studies show that the nepheline syenite and nephelinite in the TLIP display similar mineral assemblages, clinopyroxene Sr isotope compositions and bulk-rock Sr and Nd isotope compositions (87Sr/86Sr(i) = 0.70364−0.70399, εNd(t) = +3.51 to +4.49 versus 87Sr/86Sr(i) = 0.70348−0.70371, εNd(t) = +3.28 to +3.88 for nepheline syenite and nephelinite, respectively), indicating they are possibly co-magmatic. Rhyolite-MELTS modeling shows that the nepheline syenite formed from nephelinite by fractional crystallization of spinel, olivine, clinopyroxene, apatite, and biotite. In combination with information from previous studies, we correlated the extremely light magnesium isotopes of nepheline syenite to “genetic genes” of nephelinite (δ26Mg = −0.35 to −0.55‰) which were produced by the reaction between peridotite and carbonated silicate melt derived from the carbonated eclogite. We invoke a three-stage model for the genesis of the nepheline syenite in the TLIP. Initially, the subduction of oceanic crust delivered the sedimentary carbonate rocks into the deep mantle and formed carbonated eclogite. The carbonated silicate melt derived by the melting of the carbonated eclogite reacted with ambient peridotite to form primary nephelinitic magma. Finally, fractional crystallization of nephelinitic melt during ascent produced the nepheline syenite. Our study provides insights into the implication of light magnesium isotopes for deep carbon recycling in the origin of alkaline rocks.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"93 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2022-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"117303121","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}