Marco Filippi, Fabrice Jouffray, J. Lardeaux, Massimo Tiepolo, M. Spalla
The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite−pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610−660 °C and 1.9−2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.
在阿尔金特拉-梅尔坎图尔山丘（西阿尔卑斯山外结晶山丘）东部新发现的 Bois de Sélasse 辉绿岩对于更好地解释欧洲瓦里斯坎带南部的构造演化至关重要。该辉绿岩的全岩成分与玄武岩原岩一致，具有正常的洋中脊亲缘关系，火成岩锆石岩心的铀-铅年代测定显示其喷发年龄为 524 ± 5 Ma。该岩体可能是在冈瓦纳大陆活动边缘北部的大洋岩石圈中或在冈瓦纳大陆下俯冲过程中的弧后盆地中形成的。保存完好的前闪长岩-辉长岩-夕长岩矿物提供了沿着标准的大洋俯冲地温（≤10 °C/km）进行顺级变质作用的证据。通过热力学建模，并结合钛-锆石和锆-金红石热量测定法，将尖晶石面的峰值条件限制在610-660 °C和1.9-2.3 GPa。通过对变质锆石边缘的 U-Pb 测定，确定埃克洛辉石面变质作用的最小年龄为 339 ± 6 Ma。Bois de Sélasse斜长岩的原岩确实比瓦里斯卡大洋更古老，但它同样受到了瓦里斯卡俯冲的影响。我们讨论了这一新发现对欧洲瓦里坎带的影响。
{"title":"On the occurrence of a Variscan eclogite in the Argentera-Mercantour Massif, Western Alps: Implications for the evolution of the southern Variscan belt","authors":"Marco Filippi, Fabrice Jouffray, J. Lardeaux, Massimo Tiepolo, M. Spalla","doi":"10.1130/b37010.1","DOIUrl":"https://doi.org/10.1130/b37010.1","url":null,"abstract":"The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite−pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610−660 °C and 1.9−2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"1 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140091031","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}
Keith D. Gray, K. Johnson, David A. Foster, V. Isakson
Changes in lithospheric composition and strength associated with rifting of the Rodinian supercontinent (ca. 780−485 Ma) served to localize Mesozoic tectonic activity in the McCall region of west-central Idaho, USA. Although discrimination diagrams support slab failure melting, ca. 150−121 Ma calcic magmatism in the Hazard Creek complex and chemically similar intrusive rocks of northeastern Oregon, USA (e.g., Cornucopia stock), is best explained by progressive loading of oceanic crust. Westward-directed thrusting across the Laurentian continental margin and adjacent island-arc terranes (Olds Ferry/Wallowa; Blue Mountains province) led to partial melting of metabasaltic source rocks at pressures and depths sufficient for garnet stability (>10 kbar, ∼35 km). Early pluton emplacement during right-oblique arc−continent collision overlapped in time with subduction zone reorganization, which involved a change in dip direction (westward to eastward) under the accreting Blue Mountains block. Ensuing magmatic activity predated and was concurrent with ca. 116 Ma thrust reactivation of the Wallowa/Olds Ferry boundary, which temporally overlapped with contractional deformation along the terrane−continent accretionary boundary. Later synorogenic magmatism was focused along the accretionary boundary, where rising anatectic melts were caught between the eastward-impinging/northward-migrating (obliquely colliding) terrane block and continental margin backstop (Rodinian rift architecture). Middle Cretaceous dextral transpression viewed in this manner precludes the need for an outboard “hit-and-run” collider (Insular superterrane) and thus, a post-accretion shear zone in western Idaho.
岩石圈成分和强度的变化与罗迪尼亚超大陆的断裂（约 780-485 Ma）有关，这种变化使美国爱达荷州中西部麦考尔地区的中生代构造活动局部化。尽管判别图支持板块崩塌熔融，但美国俄勒冈州东北部哈扎德溪复合岩和化学性质类似的侵入岩（如 Cornucopia 储量）中约 150-121 Ma 的钙质岩浆活动最好解释为大洋地壳的渐进加载。劳伦大陆边缘和邻近的岛弧地层（奥尔德斯费里/瓦洛瓦；蓝山省）的西向推移导致新元古代源岩在足够的压力和深度（>10千巴，∼35千米）下部分熔融，从而使石榴石变得稳定。在右斜弧-大陆碰撞过程中的早期柱状构造与俯冲带重组在时间上重叠，俯冲带重组涉及蓝山地块增生下倾角方向的改变（由西向东）。随后的岩浆活动早于瓦洛瓦/奥尔德斯渡口边界约 116 Ma 的推力重新激活，并与之同时发生，在时间上与沿陆块-大陆增生边界的收缩变形重叠。后来的同生岩浆活动主要集中在增生边界沿线，在那里，上升的北极岩浆被夹在向东挤压/向北迁移（斜向碰撞）的陆相块体和大陆边缘挡板（罗迪尼断裂构造）之间。从这个角度来看，中白垩世的右旋转位不需要外侧的 "撞击 "碰撞器（Insular superterrane），因此在爱达荷州西部也不需要后发剪切带。
{"title":"Jura-Cretaceous synorogenic magmatism and relations to supercontinental rifting in the northwestern U.S. Cordillera","authors":"Keith D. Gray, K. Johnson, David A. Foster, V. Isakson","doi":"10.1130/b37203.1","DOIUrl":"https://doi.org/10.1130/b37203.1","url":null,"abstract":"Changes in lithospheric composition and strength associated with rifting of the Rodinian supercontinent (ca. 780−485 Ma) served to localize Mesozoic tectonic activity in the McCall region of west-central Idaho, USA. Although discrimination diagrams support slab failure melting, ca. 150−121 Ma calcic magmatism in the Hazard Creek complex and chemically similar intrusive rocks of northeastern Oregon, USA (e.g., Cornucopia stock), is best explained by progressive loading of oceanic crust. Westward-directed thrusting across the Laurentian continental margin and adjacent island-arc terranes (Olds Ferry/Wallowa; Blue Mountains province) led to partial melting of metabasaltic source rocks at pressures and depths sufficient for garnet stability (>10 kbar, ∼35 km). Early pluton emplacement during right-oblique arc−continent collision overlapped in time with subduction zone reorganization, which involved a change in dip direction (westward to eastward) under the accreting Blue Mountains block. Ensuing magmatic activity predated and was concurrent with ca. 116 Ma thrust reactivation of the Wallowa/Olds Ferry boundary, which temporally overlapped with contractional deformation along the terrane−continent accretionary boundary. Later synorogenic magmatism was focused along the accretionary boundary, where rising anatectic melts were caught between the eastward-impinging/northward-migrating (obliquely colliding) terrane block and continental margin backstop (Rodinian rift architecture). Middle Cretaceous dextral transpression viewed in this manner precludes the need for an outboard “hit-and-run” collider (Insular superterrane) and thus, a post-accretion shear zone in western Idaho.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"9 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139957676","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}
Luther Strayer, R. Catchings, Joanne Chan, Mark Goldman, Adrian McEvilly, John Suppe
We developed Vp, Vs, Vp/Vs ratio, and Poisson’s ratio models of the uppermost crust (<4 km depth) from the eastern San Francisco (SF) Bay (California, USA) to near the Calaveras fault along a 15-km-long, linear profile. Upper crustal velocities are highly variable beneath, west, and well east of the Hayward fault. We observe eight notable features, from west to east: (1) Near San Francisco Bay, there is an ∼2-km-wide structure with high Vp/Vs ratios (up to 5) and Poisson’s ratios (up to 0.48) extending from the surface to the base of our model, which we suggest the structure is a near-vertical fault that lies along a straight-line projection between the Silver Creek fault to the south and the Point Richmond fault to the north. The structure may be part of an ∼90-km-long fault along the eastern SF Bay. (2) The western East Bay Plain, the lower lying area between the bay and the hills, includes up to 800 m of low-velocity sediments (Vp ∼1600−3000 m/s, Vs ∼500 m/s to ∼1000 m/s), underlain by higher velocity basement rocks (Vp ∼3000−5800 m/s; Vs ∼1000−1500 m/s). (3) Between ∼1 km and 3 km east of the Bay shoreline, sediments thin in a series of steps (likely faults) toward the Hayward fault. (4) Between ∼3 km west and ∼1 km east of the Hayward fault (at the East Chabot fault) at depths greater than 1 km, basement Vp (up to 6000 m/s) and Vs (up to 2800 m/s) are high, and Vp/Vs ratios (<2) and Poisson’s ratios (<0.3) are low, suggesting crystalline rocks. Furthermore, a near-vertical zone of low Vp/Vs ratios and Poisson’s ratios is between near-surface traces of the Hayward and East Chabot faults, likely corresponding to the San Leandro Gabbro of Ponce et al. (2003). (5) Eastward of the East Chabot fault in the upper 1.5 km, basement Vp (∼3000 m/s to ∼4200 m/s) and Vs (∼1200−2000 m/s) are lower than those west of the fault. (6) In the eastern Hayward/Oakland Hills, there are zones of laterally varying, high- and low-velocity (Vp ∼2500−3000 m/s) Jurassic−Cretaceous and Tertiary sediments in the shallow subsurface that likely extend much deeper than imaged. (7) Seismic energy that propagates westward from sources east of the Hayward fault (HF) appear weaker than energy that propagates eastward from sources west of the HF, suggesting that the HF acts as a partial barrier to shallow seismic energy propagation into the more populated eastern SF Bay area. (8) Unlike many fault zones, it appears that the active trace of the Hayward fault (in our study area) is not cored by a prominent, low-velocity zone relative to rocks to the east and west of the active trace. However, the active trace does mark a prominent change from relatively higher velocities to the west and lower velocities to the east.
{"title":"Upper crustal seismic velocity structure of the Hayward fault zone, San Francisco Bay, California, USA: Results from the 2016 East Bay Seismic Experiment (EBSI-16)","authors":"Luther Strayer, R. Catchings, Joanne Chan, Mark Goldman, Adrian McEvilly, John Suppe","doi":"10.1130/b36919.1","DOIUrl":"https://doi.org/10.1130/b36919.1","url":null,"abstract":"We developed Vp, Vs, Vp/Vs ratio, and Poisson’s ratio models of the uppermost crust (<4 km depth) from the eastern San Francisco (SF) Bay (California, USA) to near the Calaveras fault along a 15-km-long, linear profile. Upper crustal velocities are highly variable beneath, west, and well east of the Hayward fault. We observe eight notable features, from west to east: (1) Near San Francisco Bay, there is an ∼2-km-wide structure with high Vp/Vs ratios (up to 5) and Poisson’s ratios (up to 0.48) extending from the surface to the base of our model, which we suggest the structure is a near-vertical fault that lies along a straight-line projection between the Silver Creek fault to the south and the Point Richmond fault to the north. The structure may be part of an ∼90-km-long fault along the eastern SF Bay. (2) The western East Bay Plain, the lower lying area between the bay and the hills, includes up to 800 m of low-velocity sediments (Vp ∼1600−3000 m/s, Vs ∼500 m/s to ∼1000 m/s), underlain by higher velocity basement rocks (Vp ∼3000−5800 m/s; Vs ∼1000−1500 m/s). (3) Between ∼1 km and 3 km east of the Bay shoreline, sediments thin in a series of steps (likely faults) toward the Hayward fault. (4) Between ∼3 km west and ∼1 km east of the Hayward fault (at the East Chabot fault) at depths greater than 1 km, basement Vp (up to 6000 m/s) and Vs (up to 2800 m/s) are high, and Vp/Vs ratios (<2) and Poisson’s ratios (<0.3) are low, suggesting crystalline rocks. Furthermore, a near-vertical zone of low Vp/Vs ratios and Poisson’s ratios is between near-surface traces of the Hayward and East Chabot faults, likely corresponding to the San Leandro Gabbro of Ponce et al. (2003). (5) Eastward of the East Chabot fault in the upper 1.5 km, basement Vp (∼3000 m/s to ∼4200 m/s) and Vs (∼1200−2000 m/s) are lower than those west of the fault. (6) In the eastern Hayward/Oakland Hills, there are zones of laterally varying, high- and low-velocity (Vp ∼2500−3000 m/s) Jurassic−Cretaceous and Tertiary sediments in the shallow subsurface that likely extend much deeper than imaged. (7) Seismic energy that propagates westward from sources east of the Hayward fault (HF) appear weaker than energy that propagates eastward from sources west of the HF, suggesting that the HF acts as a partial barrier to shallow seismic energy propagation into the more populated eastern SF Bay area. (8) Unlike many fault zones, it appears that the active trace of the Hayward fault (in our study area) is not cored by a prominent, low-velocity zone relative to rocks to the east and west of the active trace. However, the active trace does mark a prominent change from relatively higher velocities to the west and lower velocities to the east.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"73 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139526626","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}
Patricia Ascanio-Pellon, D. Stockli, Daniel Ruiz-Arriaga, L. Stockli
The tectonic affinity and origins of the Jurassic, siliciclastic San Cayetano and Constancia formations exposed in western and central Cuba in the Caribbean region remain debated. The scarcity of modern geochronologic studies on these Cuban units hampers both sedimentary provenance and palinspastic reconstructions, resulting in Caribbean models that tend to oversimplify the formation of Cuba and correlate the Jurassic strata to various regions such as North America, South America, the Yucatán margin, or the conceptual Caribeana domain. Using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS), we conducted a detailed detrital zircon (DZ) U-Pb provenance study of these Cuban siliciclastic strata that provides critical insights into understanding the formation of Cuba during the Early Jurassic rifting stages of Pangea. Results from 19 San Cayetano Formation samples show a dominant Oaxaquia (ca. 1 Ga) and Chiapas batholith (ca. 250 Ma) signature, while six Constancia samples display variable ca. 1 Ga and ca. 250 Ma grains. The Lower Cretaceous Polier Formation and the Paleocene Moncada Formation also display the same ca. 1 Ga and ca. 250 Ma signatures. After comparing these Cuban data with data from other regional DZ studies, we propose that that the San Cayetano and Constancia formations are correlative to the Todos Santos Formation located in the southwestern Yucatán region of Mexico. These Cuban units were predominantly deposited adjacent to the Chiapas batholith during the Early Jurassic in northwest−southeast-trending basins created by Pangean rifting. They were eventually sheared during eastward migration of the Caribbean plate and transported northward until they collided with the North American continent in the Paleogene. This DZ study provides new constraints on the tectonic provenance of western and central Cuba and improves plate tectonic reconstructions of the Caribbean.
加勒比地区古巴西部和中部出露的侏罗纪硅质碎屑岩圣卡耶塔诺地层和康斯坦西亚地层的构造亲缘关系和起源仍存在争议。对古巴这些地层单元的现代地质年代研究稀少，妨碍了沉积产地和古地层重构，导致加勒比模型倾向于过度简化古巴地层的形成，并将侏罗纪地层与北美、南美、尤卡坦边缘或加勒比概念域等不同地区相关联。利用激光烧蚀-电感耦合等离子体质谱法（LA-ICP-MS），我们对这些古巴硅质碎屑岩地层进行了详细的碎屑锆石（DZ）U-Pb 出处研究，为了解古巴在泛大陆早侏罗世断裂阶段的形成提供了重要的见解。来自 19 个 San Cayetano Formation 样品的研究结果显示了主要的 Oaxaquia（约 1 Ga）和 Chiapas 浴岩（约 250 Ma）特征，而 6 个 Constancia 样品则显示了可变的约 1 Ga 和约 250 Ma 岩粒。1 Ga 和约 250 Ma 的颗粒。下白垩统波利埃地层和古新统蒙卡达地层也显示出相同的约 1 Ga 和约 250 Ma 的特征。1 Ga 和大约 250 Ma 的特征。在将古巴的这些数据与其他地区的 DZ 研究数据进行比较之后，我们认为 San Cayetano 和 Constancia 地层与位于墨西哥尤卡坦西南部地区的 Todos Santos 地层相关。侏罗纪早期，这些古巴单元主要沉积在恰帕斯熔岩附近的西北-东南走向盆地中，这些盆地是由潘加山脉断裂形成的。它们最终在加勒比板块东移过程中被剪切并向北迁移，直至在古近纪与北美大陆相撞。这项 DZ 研究为古巴西部和中部的构造成因提供了新的制约因素，并改进了加勒比地区的板块构造重建。
{"title":"Insights into Caribbean tectonics from a detrital zircon U-Pb provenance study of siliciclastic strata in western and central Cuba","authors":"Patricia Ascanio-Pellon, D. Stockli, Daniel Ruiz-Arriaga, L. Stockli","doi":"10.1130/b37114.1","DOIUrl":"https://doi.org/10.1130/b37114.1","url":null,"abstract":"The tectonic affinity and origins of the Jurassic, siliciclastic San Cayetano and Constancia formations exposed in western and central Cuba in the Caribbean region remain debated. The scarcity of modern geochronologic studies on these Cuban units hampers both sedimentary provenance and palinspastic reconstructions, resulting in Caribbean models that tend to oversimplify the formation of Cuba and correlate the Jurassic strata to various regions such as North America, South America, the Yucatán margin, or the conceptual Caribeana domain. Using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS), we conducted a detailed detrital zircon (DZ) U-Pb provenance study of these Cuban siliciclastic strata that provides critical insights into understanding the formation of Cuba during the Early Jurassic rifting stages of Pangea. Results from 19 San Cayetano Formation samples show a dominant Oaxaquia (ca. 1 Ga) and Chiapas batholith (ca. 250 Ma) signature, while six Constancia samples display variable ca. 1 Ga and ca. 250 Ma grains. The Lower Cretaceous Polier Formation and the Paleocene Moncada Formation also display the same ca. 1 Ga and ca. 250 Ma signatures. After comparing these Cuban data with data from other regional DZ studies, we propose that that the San Cayetano and Constancia formations are correlative to the Todos Santos Formation located in the southwestern Yucatán region of Mexico. These Cuban units were predominantly deposited adjacent to the Chiapas batholith during the Early Jurassic in northwest−southeast-trending basins created by Pangean rifting. They were eventually sheared during eastward migration of the Caribbean plate and transported northward until they collided with the North American continent in the Paleogene. This DZ study provides new constraints on the tectonic provenance of western and central Cuba and improves plate tectonic reconstructions of the Caribbean.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"29 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139527058","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}
Chao Wang, Lin Ding, Fulong Cai, Deng Zeng, Jinxiang Li, Liyun Zhang, Yahui Yue
Knowledge of the evolution of the Bangong-Nujiang Tethyan Ocean is crucial for reconstructing the paleography of the Tethyan Realm, given its significance as a key component of the eastern Tethys. Nonetheless, there has been uncertainty regarding both the timing and the processes involved in the closure of this ocean. This study focused on a 110−106 Ma igneous complex comprising basalts−basaltic andesites, trachyandesites, and granodiorites from the Sumxi area in the western part of the Qiangtang terrane of west-central Tibet. The basalts−basaltic andesites have SiO2 contents of 52.5−58.7 wt% and MgO contents of 2.89−4.63 wt%, and exhibit some arc-like geochemical signatures. However, these rocks also have elevated Nb contents (>10 ppm) and Nb/La ratios (>0.5), as well as enriched Sr-Nd isotopic composition [εNd(t) = −7.40 to −6.00], implying that they are products of a mantle source metasomatized by adakitic melts. The trachyandesites are characterized by intermediate compositions (SiO2 = 63.6−65.2 wt%), high Mg number (40−60), and more enriched εNd(t) values (−8.37 to −7.49). Comparing their geochemical composition to that of mélange rocks, it is postulated that these trachyandesites were formed through the partial melting of a mantle source including mélange matrix rocks within a subduction zone. The granodiorites exhibit adakitic geochemical features (Sr = 830.14−1032.70 ppm, Y = 14.86−15.37 ppm, Sr/Y = 54−68), indicating that they originated from the partial melting of a thickened lower crust in a continental arc setting. Our results, in combination with a synthesis of tectonomagmatism along the Bangong-Nujiang suture zone, provide convincing evidence for subduction of an oceanic plateau and subsequent slab roll-back. The Sumxi igneous complex, with its clear arc affinity, suggests that the Bangong-Nujiang Tethyan Ocean, or at least its western part, remained open until the late Early Cretaceous (ca. 106 Ma).
{"title":"Late Mesozoic subduction-accretion in the southern Qiangtang: Insights from the Sumxi igneous complex of west-central Tibet","authors":"Chao Wang, Lin Ding, Fulong Cai, Deng Zeng, Jinxiang Li, Liyun Zhang, Yahui Yue","doi":"10.1130/b36945.1","DOIUrl":"https://doi.org/10.1130/b36945.1","url":null,"abstract":"Knowledge of the evolution of the Bangong-Nujiang Tethyan Ocean is crucial for reconstructing the paleography of the Tethyan Realm, given its significance as a key component of the eastern Tethys. Nonetheless, there has been uncertainty regarding both the timing and the processes involved in the closure of this ocean. This study focused on a 110−106 Ma igneous complex comprising basalts−basaltic andesites, trachyandesites, and granodiorites from the Sumxi area in the western part of the Qiangtang terrane of west-central Tibet. The basalts−basaltic andesites have SiO2 contents of 52.5−58.7 wt% and MgO contents of 2.89−4.63 wt%, and exhibit some arc-like geochemical signatures. However, these rocks also have elevated Nb contents (>10 ppm) and Nb/La ratios (>0.5), as well as enriched Sr-Nd isotopic composition [εNd(t) = −7.40 to −6.00], implying that they are products of a mantle source metasomatized by adakitic melts. The trachyandesites are characterized by intermediate compositions (SiO2 = 63.6−65.2 wt%), high Mg number (40−60), and more enriched εNd(t) values (−8.37 to −7.49). Comparing their geochemical composition to that of mélange rocks, it is postulated that these trachyandesites were formed through the partial melting of a mantle source including mélange matrix rocks within a subduction zone. The granodiorites exhibit adakitic geochemical features (Sr = 830.14−1032.70 ppm, Y = 14.86−15.37 ppm, Sr/Y = 54−68), indicating that they originated from the partial melting of a thickened lower crust in a continental arc setting. Our results, in combination with a synthesis of tectonomagmatism along the Bangong-Nujiang suture zone, provide convincing evidence for subduction of an oceanic plateau and subsequent slab roll-back. The Sumxi igneous complex, with its clear arc affinity, suggests that the Bangong-Nujiang Tethyan Ocean, or at least its western part, remained open until the late Early Cretaceous (ca. 106 Ma).","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"7 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139532663","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}
M. Person, J.C. McIntosh, J.-H. Kim, C. Noyes, L. Bailey, S. Lingrey, R. Krantz, D. Lucero, P. Reiners, G. Ferguson
Conceptual models of sedimentary basin groundwater flow systems typically assume that the crystalline basement acts as an impermeable boundary and can be neglected. In this study, we use hydrologic models constrained by isotopic and geochemical datasets to argue that the La Sal Mountains, Utah, USA, act as a hydrologic window into the Paradox Basin’s lower aquifer system and underlying crystalline basement. We conducted a sensitivity study in which we varied crystalline basement/laccolith permeability as well as fault zone connectivity along a cross-sectional transect from the La Sal Mountains to Lisbon Valley. When the crystalline basement/laccolith units are set at relatively permeable levels (10−14 m2), simulated tracers that include total dissolved solids, oxygen isotopic composition of pore fluids (δ18O), and groundwater residence times are in closest agreement with field measurements. Model results indicate that pore fluids in the basal aquifer system underlying the Paradox Formation confining unit are a mixture of relatively young meteoric fluids and older Paradox Formation brines. The presence of faults did not significantly modify fluid exchange between the upper and lower aquifer systems. This was due, in part, to underpressuring within the Paradox Formation. Our study concludes that the Paradox Basin represents a regional recharge area for the Colorado Plateau, with groundwater discharge occurring along the Colorado River within the Grand Canyon some 375 km away to the southwest. This is only possible with a permeable crystalline basement. Our findings help explain the genesis of Mississippi Valley-type ore deposits of the US Midcontinent, where the presence of a permeable basement may be useful in addressing issues related to solute mass and energy balance.
{"title":"Hydrologic windows into the crystalline basement and their controls on groundwater flow patterns across the Paradox Basin, western USA","authors":"M. Person, J.C. McIntosh, J.-H. Kim, C. Noyes, L. Bailey, S. Lingrey, R. Krantz, D. Lucero, P. Reiners, G. Ferguson","doi":"10.1130/b37063.1","DOIUrl":"https://doi.org/10.1130/b37063.1","url":null,"abstract":"Conceptual models of sedimentary basin groundwater flow systems typically assume that the crystalline basement acts as an impermeable boundary and can be neglected. In this study, we use hydrologic models constrained by isotopic and geochemical datasets to argue that the La Sal Mountains, Utah, USA, act as a hydrologic window into the Paradox Basin’s lower aquifer system and underlying crystalline basement. We conducted a sensitivity study in which we varied crystalline basement/laccolith permeability as well as fault zone connectivity along a cross-sectional transect from the La Sal Mountains to Lisbon Valley. When the crystalline basement/laccolith units are set at relatively permeable levels (10−14 m2), simulated tracers that include total dissolved solids, oxygen isotopic composition of pore fluids (δ18O), and groundwater residence times are in closest agreement with field measurements. Model results indicate that pore fluids in the basal aquifer system underlying the Paradox Formation confining unit are a mixture of relatively young meteoric fluids and older Paradox Formation brines. The presence of faults did not significantly modify fluid exchange between the upper and lower aquifer systems. This was due, in part, to underpressuring within the Paradox Formation. Our study concludes that the Paradox Basin represents a regional recharge area for the Colorado Plateau, with groundwater discharge occurring along the Colorado River within the Grand Canyon some 375 km away to the southwest. This is only possible with a permeable crystalline basement. Our findings help explain the genesis of Mississippi Valley-type ore deposits of the US Midcontinent, where the presence of a permeable basement may be useful in addressing issues related to solute mass and energy balance.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"43 9","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139532336","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}
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