GSA Bulletin最新文献

{"title":"Apatite records metamorphic and hydrothermal fluid evolution at the large Shuangqishan orogenic gold deposit, SE China","authors":"Yingiun Ma, Shaoyong Jiang, H. Frimmel","doi":"10.1130/b36642.1","DOIUrl":"https://doi.org/10.1130/b36642.1","url":null,"abstract":"Orogenic gold deposits are currently the world’s major source of Au, but uncertainty exists in the timing of mineralization relative to the ages of host rocks, metamorphism, and magmatism. Consequently, the origin and detailed evolution of ore-forming fluids have long been elusive. Apatite occurs widespread in various types of mineral deposits and can provide valuable information on ore genesis. Here, we present textural, high-precision in situ U-Pb geochronological, trace elements, and Sr isotope data for metamorphic and hydrothermal apatite in the Shuangqishan gold deposit in southeastern China. These data, together with U-Pb dates of metavolcanic host rocks, granitic and mafic intrusive rocks, allow us to precisely constrain the timing of mineralization and reconstruct the history of the ore-forming fluids there. The apatite crystals within metamorphic and auriferous quartz veins can be grouped into three generations according to cathodoluminescence imaging and trace element concentrations: Ap1 precipitated in the pre-mineralization metamorphic stage, whereas Ap2 and Ap3 formed in the auriferous hydrothermal stage. Metamorphic Ap1 displays a negative Eu anomaly, indicating a predominance of Eu2+ and crystallization under relatively reduced conditions. On the other hand, syn-gold hydrothermal Ap2 and Ap3 have marked positive Eu anomalies. Radiometric ages for metamorphic Ap1 are at ca. 461 Ma, whereas hydrothermal Ap2 and xenotime from auriferous quartz gave U-Pb ages of 425 ± 15 Ma and 416 ± 15 Ma, interpreted as the time of gold mineralization. Emplacement ages determined by U-Pb zircon data are 439 ± 2 Ma for granitic intrusion and 427 ± 2 Ma for a mafic dike in the mining district. Mineralization, therefore, postdates the metamorphism of local host rocks and granitic magmatism by ∼35 ± 24 m.y. and ∼14 ± 17 m.y., respectively. The synchronism of gold genesis and mafic magmatism provides additional evidence for a mantle derivation of the ore-forming fluids. The highly radiogenic 87Sr/86Sr ratios obtained for Ap1 (0.7178−0.7302) are consistent with its precipitation from a metamorphic fluid. Higher Sr concentrations (Ap2 = 1228−2884 ppm, Ap3 = 2325−3169 ppm) and positive Eu anomalies (Eu/Eu*: Ap2 = 1.03−2.84, Ap3 = 2.00−4.30) but lower and variable 87Sr/86Sr ratios in hydrothermal Ap2 (0.7100−0.7165) and Ap3 (0.7086−0.7116) are mainly ascribed to extensive fluid-wall rock exchange of Sr during mineralization. It is therefore suggested that fluid-rock reaction played a vital role in the observed positive Eu anomalies of hydrothermal apatite. Our study highlights the usefulness of apatite as a novel and robust geochronological, geochemical, and isotopic indicator of complex mineralization processes in hydrothermal gold deposits.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"9 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131822280","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":"Southward continuation and slab rollback of the Neotethyan arc−back-arc system: Insights from Eocene mafic intrusions from North Sumatra, SE Asia","authors":"Xiaoqing Yu, Yuzhi Zhang, Xin Qian, A. Ghani, Thomas C. Sheldrick, Chang Xu, Yuejun Wang","doi":"10.1130/b36651.1","DOIUrl":"https://doi.org/10.1130/b36651.1","url":null,"abstract":"The Neotethyan subduction and collision with the Asian, Indian, and Australian plates is evidenced by a giant Andean-type arc system and a remarkable Neotethyan igneous belt along the southwestern Eurasian margin. However, the southward extent, and cause, of Neotethyan subduction is poorly understood. Here, a zircon U-Pb dating and whole-rock geochemical study of two groups of Eocene (50−48 Ma) mafic intrusions from Tangse, North Sumatra, helps to unravel the Neotethyan subduction story. Group 1 consists of diabase samples, which have mid-oceanic-ridge− and arc-like geochemical affinities, with low chondrite-normalized La/Yb(cn) (∼0.6) ratios and a high measured εNd(t) value of +8.7. Such geochemical signatures indicate that the magmatic rocks were derived from a refractory depleted mantle source that was metasomatized by slab melts. Group 2 consists of gabbro-diorites, which are enriched in the large-ion lithophile elements and depleted in some high field strength elements. These samples have high La/Yb(cn) (2.9−6.6), low Nb/La(cn) (0.2−0.3), εNd(t) values of +1.1 to +5.0, and εHf(t) values of +8.8 to +15.3. Group 2 samples were produced by the melting of a depleted mantle wedge metasomatized by slab fluids. The magmatism provides evidence for the southern continuation of the Neotethyan igneous belt and tectonic system, whereby slab rollback triggered melting and upwelling in an arc−back-arc geodynamic system from South Tibet to SE Asia.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"19 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126704472","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":"Ultrahigh-pressure mantle metasomatism in continental collision zones recorded by post-collisional mafic rocks","authors":"Mingdi Gao, Haijin Xu, S. Foley, Junfeng Zhang, Yu Wang","doi":"10.1130/b36610.1","DOIUrl":"https://doi.org/10.1130/b36610.1","url":null,"abstract":"Post-collisional mafic magmas are melts of mixed sources formed by the interaction between crust and mantle. We report the first identification of metasomatic sources formed at >200 km in continental collision zones that led to the Triassic post-collisional Shidao mafic magmas in the Sulu ultrahigh-pressure terrane, eastern China. The Shidao magmas are ultrapotassic with low Rb/K2O and Rb/Sr ratios, highlighting a major contribution of K-richterite to the melts, and with high Dy/Yb ratios indicating residual garnet in the source. Experiments have demonstrated that K-richterite coexists with garnet only at pressures higher than 6−7 GPa, which indicates that the Yangtze continental crust was subducted to >200 km, where it interacted with the overlying mantle to produce K-richterite + garnet-bearing assemblages. This ultra-deep source of the Shidao magmas spotlights an alternative mechanism for the genesis of post-collisional magmas, which are generally of shallow origin.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"57 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128631693","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":"Episodic magmatism in the Lianhuashan tectonic belt: Implications for late Mesozoic crustal reworking in SE South China","authors":"Yujia Xin, Jianhua Li, Yueqiao Zhang, S. Dong","doi":"10.1130/b36504.1","DOIUrl":"https://doi.org/10.1130/b36504.1","url":null,"abstract":"The Lianhuashan tectonic belt preserves abundant evidence of episodic magmatism and deformation that is crucial for understanding the late Mesozoic crustal reworking in SE South China. Here, we report zircon U-Pb ages, geochemical, and Sr−Nd−Hf isotopic data for representative granitoids and volcanic rocks to clarify the episodic magmatism in the Lianhuashan tectonic belt and its association with regional tectonic evolution. Our results define three episodes of magmatism. The Late Jurassic (160−150 Ma) magmatic rocks show high CaO/Na2O and molar CaO/(MgO + FeOt) (CMF) ratios and enriched Sr−Nd−Hf isotopes that are consistent with high-temperature melting of psammitic sources. The Early Cretaceous (144−131 Ma) magmatic rocks display variable CaO/Na2O ratios and zircon εHf(t) values (−12.0 to −2.5), which implies binary mixing of melts from pelitic and basaltic sources. The Late Cretaceous (ca. 94 Ma) magmatic rocks show high A/CNK and low CaO/Na2O and CMF ratios, which are characteristics of pelite-derived melts. They have zircon Hf isotopes (−4.4 to −1.9) that are slightly higher than those of the Early Cretaceous rocks, which is consistent with partial melting of diverse sources with more involvement of juvenile crust. These magmatic records, coupled with regional structural observations, suggest that the Lianhuashan tectonic belt underwent alternating crustal shortening and extension during the Late Jurassic to Cretaceous time. The Late Jurassic magmatism possibly marks the initiation of crustal extension following Middle Jurassic retro-arc crustal shortening related to the advancing Paleo-Pacific subduction. In the Cretaceous, the two magmatic flare-ups were separated by a magmatic lull associated with crustal shortening. The magmatic flare-ups occurred coevally with two stages of regional extension, as evidenced by widespread generations of basins and extensional domes. The time consistency implies that the magmatism was associated with or facilitated crustal extension that occurred at a regional scale and may have resulted from slab rollback during subduction retreat. We suggest that the magmatism and extension provide essential evidence of the crustal reworking in SE South China governed by the Paleo-Pacific subduction. Changes in the slab dynamics (steepening and shallowing) may control the flare-up, lull, and resumption of magmatism, as well as alternating episodes of crustal extension and shortening.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129334553","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":"Petrogenesis and tectonic setting of Neoarchean tonalitic-trondhjemitic-granodioritic gneisses in the Xiwulanbulang area of the Yinshan block, North China craton","authors":"X. Wang, Jian Zhang, Qian Liu, H. Zhou, C. Yin, Shuhui Zhang, Ying Chen, Changquan Cheng, Minjie Guo","doi":"10.1130/b36641.1","DOIUrl":"https://doi.org/10.1130/b36641.1","url":null,"abstract":"As one of the major Archean microcontinental blocks in the North China craton, the Yinshan block consists of the Guyang granite-greenstone belt and the Wuchuan high-grade complex, both of which preserve large volumes of Neoarchean tonalitic-trondhjemitic-granodioritic (TTG) gneisses. The petrogenesis and tectonic setting of these TTG gneisses are of great significance to understanding the Neoarchean crustal growth and evolution of the Yinshan block. Although extensive investigations have been carried out on the TTG gneisses in the Yinshan block, controversy remains regarding their petrogenesis and tectonic setting. In this study, we present new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for the Neoarchean TTG gneisses in the Xiwulanbulang area from the Wuchuan high-grade complex. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating results from six TTG gneissic samples show that their igneous TTG plutons were emplaced in the period 2.61−2.48 Ga. The results from zircon Lu-Hf isotopes show that εHf(t) values (−3.2 to +7.5) and depleted mantle model ages (3270−2651 Ma) are like those of the oldest trondhjemite (ca. 2.7 Ga) exposed in the Xiwulanbulang area, indicating that it mainly resulted from partial melting of 2.7 Ga juvenile mafic crust. Geochemically, the Xiwulanbulang TTG gneisses contain high Sr (280−800 ppm) contents, low Y (1.05−9.54 ppm) and Yb (0.05−0.90 ppm) contents, and relatively high LaN/YbN (12.8−105) and Sr/Y (39.91−1392.6 ppm) ratios, with relative enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), showing the characteristics of adakitic rocks. In addition, their lower contents of MgO, Ni, Cr, and Mg# indicate that the Xiwulanbulang TTG rocks belong to adakite sourced from thickened continental lower crust. Also, the TTG gneisses show similar initial 87Sr/86Sr ratios (0.701659−0.702635) and εNd(t) values (−4.37 to + 3.55), with 206Pb/204Pb(t) = 13.910485−15.241164, 207Pb/204Pb(t) = 14.806723−15.259938, and 208Pb/204Pb(t) = 33.758621−35.592727, suggesting a depleted mantle source modified with some enriched components. Collectively, the Xiwulanbulang TTG rocks were most likely derived from the partial melting of thickened mafic crust between 2.61 and 2.48 Ga. Combining previous studies with data presented in this paper, we propose that the Yinshan block underwent two major igneous events during Neoarchean time, with the earlier one occurring ca. 2.7 Ga, forming a thickened mafic crust and minor TTG rocks, and the second igneous event at 2.61−2.48 Ga, resulting in the partial melting of the ca. 2.7 Ga continental lower (mafic) crust to form the TTG plutons. The geochemical data for the Xiwulanbulang TTG gneisses are inconsistent with a model for the partial melting of subducted slabs, but combined with other geologic considerations, a model in which the partial melting of a thickened continental lower (mafic) crust was heated by a mantle plume is","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"178 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122598705","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":"Orbital and sub-orbital pacing of mudstones in the Dongying Depression, eastern China: Implications for middle Eocene East Asian climate evolution","authors":"Xiangxin Kong, Zaixing Jiang, Yuan Cai","doi":"10.1130/b36606.1","DOIUrl":"https://doi.org/10.1130/b36606.1","url":null,"abstract":"The middle Eocene was a key phase in East Asian and global climate evolution; however, the understanding of the astronomical driving mechanisms of key climate changes during this phase in eastern China is lacking. A 390-m-thick, continuous lacustrine mudstone succession in the Dongying Depression (eastern China) provides an ideal middle Eocene sedimentary record. This study established an astronomical time scale (ca. 38.97−43.55 Ma) for the Dongying mudstones by double tuning of gamma-ray logs and calcite series. Based on the tuned pollen, geochemical element and nitrogen isotope data, paleo-humidity and paleo-salinity evolution profiles were constructed, showing that Dongying mudstones experienced a phased decrease in salinity and an increase in humidity. The rapid wetting accompanied by abnormal high salinity during ca. 40.0−40.6 Ma indicates that Dongying mudstones responded to the middle Eocene Climatic Optimum (MECO) event. Multi-parameter analysis showed that the rhythmic changes of Dongying mudstones at different scales were driven by multi-scaled astronomical cycles. This study focused on the intensity changes of recorded obliquity and semi-precession signals. The signals of semi-precession weakened in the dry pre-MECO period, while those of obliquity weakened in the wetter MECO period. Considering the limitation of latitude on the influence of the two cycles on the surface sunshine, we speculate that the signals of obliquity and semi-precession may reflect the intensities of high-latitude dry airflow and low-latitude wet airflow, respectively, liking to the current East Asian monsoon (EAM) system. This study expends understanding of the role of astronomical forcing on the onset and strengthening of the EAM.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"23 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127410013","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":"Two stages of porphyry Cu mineralization at Jiru in the Tibetan collisional orogen: Insights from zircon, apatite, and magmatic sulfides","authors":"Xiang Sun, Jun Deng, Yongjun Lu, Xiaobo Si, P. Hollings, M. Santosh, Qiangliang Li, Xu Zheng","doi":"10.1130/b36741.1","DOIUrl":"https://doi.org/10.1130/b36741.1","url":null,"abstract":"Continental collision−related porphyry copper (Cu) deposits provide significant global copper resources, but their genesis remains controversial because it is not clear whether remelting of remnant arc-derived lower-crustal Cu-rich cumulates is critical to their formation. We investigated zircon and apatite compositions of the Jiru porphyry Cu deposit in the Gangdese belt of southern Tibet, which is characterized by weaker early Eocene mineralization and more pronounced Miocene mineralization. Our data demonstrate that apatite hosted in zircon can record the volatile compositions of magma before fluid exsolution. Zircon-hosted apatites from the early Eocene granite that have low XF/XCl ratios (≤3) and are interpreted to have crystallized from volatile-undersaturated magma have Cl contents of 0.96−2.14 wt% (1.86 ± 0.38 wt%; n = 15) and SO3 contents of 0.01−0.14 wt% (0.08 ± 0.04 wt%; n = 15). In contrast, zircon-hosted apatites from the Miocene porphyry that have low XF/XCl ratios (≤10) and are interpreted to have crystallized from volatile-undersaturated magma have Cl contents of 0.40−0.56 wt% (0.47 ± 0.06 wt%; n = 11) and SO3 contents of 0.20−0.92 wt% (0.59 ± 0.29 wt%; n = 11). In addition, the early Eocene magma was relatively reduced (ΔFMQ = 0.86 ± 0.55, where FMQ is fayalite-magnetite-quartz) compared with the oxidized Miocene magma (ΔFMQ = 2.04 ± 0.43). The contrasting magmatic oxidation states and apatite/melt S contents were likely critical in controlling the different scales of Cu mineralization during the early Eocene and Miocene, as more highly oxidized magma can dissolve much larger amounts of sulfur and metals (e.g., Cu). Melt Cl content may not play a critical role in magma fertility, since the Jiru early Eocene granite had higher melt Cl contents than the Miocene porphyry. Magmatic sulfides (pyrrhotite, chalcopyrite, and pyrite) were not recognized in the Miocene zircons but did occur in the early Eocene zircons, and these sulfides were demonstrated to have crystallized before fluid exsolution in the shallow magma reservoir. The Jiru early Eocene magmatic sulfide saturation might not have enhanced the amount of copper available to hydrothermal fluids during the early Eocene. Late-stage sulfide saturation in the previous arc magmas might reduce the potential for collision-related porphyry Cu mineralization that is associated with remelting of the previous arc lower crust.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"69 13","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131875012","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":"Fossil divergent double-subduction zone in the Great Xing’an Range, NE China: Evidence from a deep seismic reflection profile","authors":"Xiaomiao Tan, R. Gao, Jian‐Bo Zhou, S. Wilde, H. Hou, Haiyan Wang, Rui Qi, Wenhui Li","doi":"10.1130/b36738.1","DOIUrl":"https://doi.org/10.1130/b36738.1","url":null,"abstract":"Divergent double-subduction is an important component of plate tectonics and continental growth at modern active convergent margins. However, such a process is rarely identified by deep seismic reflection (DSR) profiles, which casts doubt on its past significance. We report here a 416 km DSR profile of large dynamite shots across the Great Xing’an Range (NE China), which shows multiple arc-shaped reflections in the upper-middle crust of the Xing’an terrane, and densely layered reflection structures suggesting tectonic nappes in the lower crust. This new evidence, together with recent geological studies, indicates that the Xing’an terrane may not be a micro-continental block as previously thought, but a Paleozoic accretionary complex. In addition, there is a strong west-dipping reflection at the boundary between the Xing’an and Erguna terranes in the western DSR profile extending from the lower crust to the lithospheric mantle, suggesting that this fossil subduction zone is most likely the Toudaoqiao-Xinlin suture, which separates the Xing’an and Erguna terranes. In addition, an east-dipping strong reflection was also found in the eastern profile between the Xing’an and Songliao terranes, corresponding to the Hegenshan-Nenjiang suture at the surface. Given the newly identified crust-mantle structure, the opposite subduction zone, and other geological evidence, we propose a new model that the Xing’an terrane was developed by divergent double-subduction of the Paleo-Xing’an Ocean (part of the Paleo-Asian Ocean in the area) from ca. 500 Ma to ca. 300 Ma. Our study provides the first DSR evidence for a divergent double-subduction in the Paleozoic, which emphasizes its importance in past accretionary orogens.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"2 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115225691","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":"Diachronous subduction, closure of the Proto-Tethys Ocean, and collisional accretion of microcontinents: Insights from the early Paleozoic intermediate-mafic rocks in the Amdo microcontinent of the Tibetan Plateau","authors":"Yin‐biao Peng, Sheng‐yao Yu, San-zhong Li, Yong‐jiang Liu, Pei Lv, Chuanzhi Li, L. Dai, Yunshuai Li, Xiangyu Gao, Xingzhou Jiang, Wentao Ji","doi":"10.1130/b36689.1","DOIUrl":"https://doi.org/10.1130/b36689.1","url":null,"abstract":"The Proto-Tethys Ocean played an important role in the formation and evolution of the East Asian continental blocks, and the final closure of the Proto-Tethys Ocean may have led to the first assembly of nearly all East Asian blocks at the northern margin of Gondwana. However, controversy remains about the reconstruction of East Asian blocks at the northern margin of Gondwana. The Tibetan Plateau is located in the northern margin of Gondwana and recorded the entire tectonic evolution from continental rift and drift to subduction and collision during the early Paleozoic. Therefore, it is an important region in which to address tectonic evolution processes of the Proto-Tethys Ocean and Gondwana. Here, we present petrology, zircon U-Pb, Sr-Nd-Hf isotope, and whole-rock geochemical data from newly recognized intermediate-mafic magmatic rocks from the Amdo microcontinent of central Tibet, with a view to gain insights into the nature and geotectonic evolution of the northern margin of Gondwana. Zircon grains from meta-andesites and meta-basites in Amdo yielded magmatic crystallization ages of 490 Ma and 455−450 Ma, respectively. The meta-andesites are characterized by enrichment in large ion lithophile elements (e.g., Rb, Th, U, and K) and depletion in high field strength elements (e.g., Nb, Ta, and Ti) with positive zircon εHf(t) values (+3.4 to +6.4), which were probably derived from partial melting of a depleted mantle source enriched by minor fluids from subducted sediments in a continental arc setting. The meta-basites are tholeiitic and exhibit both mid-ocean-ridge basalt (e.g., flat rare earth element patterns) and arc-like (e.g., elevated Th/Yb ratios) geochemical affinities, in combination with negative zircon εHf(t) (−4.6 to −0.3) and whole-rock εNd(t) (−0.21 to −0.45) values, suggesting that they were probably generated by varying degrees of partial melting of a spinel peridotite source with ∼5%−10% crustal assimilation in a back-arc setting. In this regard, we favor the interpretation that the Amdo microcontinent experienced long-lasting (ca. 530−450 Ma) subduction of the Proto-Tethys Ocean slab before the subsequent collisional accretion of microcontinents. Integrating results from previous studies, we propose that the Amdo microcontinent, South Qiangtang terrane, Qilian-Qaidam terrane, and other East Asian blocks were all located at the northern margin of East Gondwana, which recorded the arc-related magmatism along the proto-Tethyan margin in the early Paleozoic. The closure of the Proto-Tethys Ocean and initial collision along the East Gondwana Proto-Tethyan margin may have been diachronous between ca. 455 and 430 Ma.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"2 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128527340","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":"Large detrital zircon data set investigation and provenance mapping: Local versus regional and continental sediment sources before, during, and after Ancestral Rocky Mountain deformation","authors":"Tyson M. Smith, J. Saylor, T. Lapen, R. Leary, K. Sundell","doi":"10.1130/b36285.1","DOIUrl":"https://doi.org/10.1130/b36285.1","url":null,"abstract":"The increasing number and size of detrital geochronology data sets offer new opportunities for increased accuracy and resolution of sediment routing models. However, the new opportunities come coupled with challenges in large data integration and visualization. We address these challenges by outlining two novel approaches that aid in analyzing and interpreting large detrital geochronology data sets: (1) combination of bottom-up and top-down detrital zircon source modeling, and (2) sediment provenance mapping. Combining source-modeling methods provides guidance in identifying empirical detrital zircon sources and determining source proportions. Provenance mapping integrates source proportions from modeling results and complimentary geologic data (e.g., paleocurrents, paleogeography, and stratal thickness maps) to extrapolate provenance information through areas with sparse or ambiguous data, thus mitigating issues of data distribution heterogeneity. Sediment provenance maps also provide a synoptic view of data that, along with detrital zircon source modeling, aids in circumventing lengthy descriptions of individual age modes for data sets containing hundreds of samples, which can obscure underlying trends in the data.\u0000 We apply this approach to late Paleozoic−early Mesozoic strata, using 329 published and new U-Pb detrital zircon samples, and document five sediment-routing episodes in the core zone of intraplate deformation in western Laurentia (i.e., the Ancestral Rocky Mountains (ARM)). The transitions between these episodes are defined by changes in sediment source distribution, which are illustrated by provenance maps that show (1) the degree and extent of ARM basin isolation from transcontinental sediment sources and (2) ARM-driven changes in transcontinental sediment routing systems. We map possible sediment pathways of distally derived sediment around the ARM core, illustrating that ARM uplifts diverted transcontinental systems around areas of intense intraplate deformation. Further, the evolution of sediment routing in western Laurentia before, during, and after ARM deformation provides an example of the interaction between transcontinental sediment routing and intraplate deformation.","PeriodicalId":242264,"journal":{"name":"GSA Bulletin","volume":"27 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122404785","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}