Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics studied this month include the nature and dynamics of China and Tibet; the Dead Sea transform; and volcanism in an extensional basin along the Laurentian Iapetus margin. You can find these articles at https://bulletin.geoscienceworld.org/content/early/recent.
Short-lived intra-oceanic arc-trench system in the North Qaidam belt (NW China) reveals complex evolution of the Proto-Tethyan Ocean
Changlei Fu; Zhen Yan; Jonathan C. Aitchison; Wenjiao Xiao; Solomon Buckman ...
Abstract: Recognition of any intra-oceanic arc-trench system (IOAS) could provide invaluable information on the tectonic framework and geodynamic evolution of the vanished ocean basin. The Tanjianshan Complex and mafic-ultramafic rocks along the North Qaidam ultra-high pressure metamorphic belt in NW China record the subduction process of the Proto-Tethyan Ocean. Four lithotectonic units, including island arc, ophiolite, forearc basin, and accretionary complex, are recognized based on detailed field investigation. They rest on the northern margin of the Qaidam block and occur as allochthons in fault contact with underlying high-grade metamorphic rocks. The ophiolite unit mainly consists of ultramafic rocks, 527−506 Ma gabbro, 515−506 Ma plagiogranite, dolerite, and massive lava. High-Cr spinels in serpentinite, dolerite with forearc basalt affinity, and boninitic lava collectively indicate a forearc setting. The accretionary complex, exposed to the south of the ophiolite complex and island arc, is highly disrupted and contains repeated slices of basalt, 495−486 Ma tuff, chert, limestone, and mélange. Tuffs with positive zircon εHf(t) values indicate derivation from a nearby juvenile island arc. These lithotectonic units, as well as the back-arc basin, are interpreted to constitute a Cambrian IOAS that formed during the northward subduction of the Proto-Tethyan Ocean. Combined with regional geology, we propose a new geodynamic model involving short-lived Mariana-type subduction and prolonged Andean-type subduction to account for the complex evolution of the Proto-Tethyan Ocean. The reconstruction of a relatively complete IOAS from the North Qaidam belt not only reveals a systematic evolution of intra-oceanic subduction but also advances our understanding of the subduction and accretion history of the Proto-Tethyan Ocean.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36127.1/609274/Short-lived-intra-oceanic-arc-trench-system-in-the
Multiple sources and magmatic evolution of the Late Triassic Daocheng batholith in the Yidun Terrane: Implications for evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau
Pengsheng Dong; Guochen Dong; M. Santosh; Xuanxue Mo; Peng Wang ...
Abstract: Granitoids with diverse composition and tectonic settings provide important tools for exploring crustal evolution and regional geodynamic history. Here we present an integrated study using petrological, mineralogical, zircon U-Pb geochronological, whole-rock geochemical, and isotopic data on the Late Triassic Daocheng batholith in the Yidun Terrane with a view to understanding the petrogenesis of a compositionally diverse batholith and its implications for the evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau. The different lithological units of the batholith, including granodiorite, monzogranite, and quartz diorite, with abundant mafic microgranular enclaves in the granodiorite (MME I) and monzogranite (MME II), show identical crystallization ages of 218−215 Ma. The mineral assemblage and chemical composition of the granodiorite are identical to those of tonalitic-granodioritic melts generated under water-unsaturated conditions. The insignificant Eu anomalies and low magmatic temperatures indicate hydrous melting in the source. The relatively narrow range of whole-rock chemical and Sr-Nd isotopes, as well as the zircon trace element and Hf isotopic compositions of the granodiorite, suggest a homogeneous crustal source for the magma. Our modeling suggests that the rock was produced by 20−50% of lower crustal melting. The Daocheng monzogranites display more evolved compositions and larger variations in Sr-Nd-Hf isotopes than the granodiorite, which are attributed to assimilation and the fractional crystallization process. This is evidenced by the presence of metasedimentary enclave and inherited zircon grains with Neoproterozoic and Paleozoic ages, a non-cotectic trend in composition, and the trend shown by the modeling of initial 87Sr/86Sr ratios and Sr. The quartz diorites and MMEs showing composition similar to that of andesitic primary magma have high zircon εHf(t) values and are characterized by enrichment in LILEs and depletion of HFSEs. They were derived from the partial melting of lithospheric mantle that had been metasomatized by slab melts and fluids. The MMEs in both rocks display typical igneous texture and higher rare earth element (REE) and incompatible element concentrations than their host granites. The presence of fine-grained margins, acicular apatite, and plagioclase megacrysts suggests a magma mingling process. The overgrowth of amphibole around the pyroxene, quartz ocelli rimmed by biotite, and oscillatory zones of plagioclase are all indicative of chemical diffusion. Their enriched Sr-Nd isotopes imply isotopic equilibrium with the host granites. Based on a comparison with the coeval subduction-related magmatism, we propose that subduction and subsequent rollback of the Paleo-Tethys (Garzê-Litang Ocean) oceanic slab was the possible mechanism that triggered the diverse Triassic magmatism within the eastern Tibetan Plateau.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35970.1/609047/Multiple-sources-and-magmatic-evolution-of-the
Basin response to the Jurassic geodynamic turnover from flat subduction to normal subduction in South China
Wei Zhang; Feng-Qi Zhang; Yildirim Dilek; Kong-Yang Zhu; Hong-Xiang Wu ...
Abstract: Growing evidence supports that the early Mesozoic development of South China was deeply shaped by flat subduction of the paleo-Pacific plate. However, the Jurassic tectonic setting and processes remain controversial. Here, we present new detrital zircon U-Pb ages and sedimentary data from the Jurassic basin in northwestern Zhejiang to constrain the Jurassic tectonic evolution. The continuous Jurassic succession archives an abrupt sedimentary change from a high-energy coastal environment to a proximal and fast-filling terrestrial environment. This lithostratigraphic change was also accompanied by the shift of detrital provenance. Age spectra of the detrital zircons from the bottom of the Jurassic strata show strong 1.0−0.7 Ga and 500−400 Ma populations, which are inferred to be mainly derived from the Yangtze block. In contrast, samples from the overlying Lower−Middle Jurassic were dominated by age groups of 2.0−1.7 Ga and 300−170 Ma, which were probably sourced from the Cathaysia block. The switch of the sedimentary and provenance characteristics reveals that an earliest Jurassic broad sag basin in the inland shifted to an Early−Middle Jurassic retro-arc foreland basin along the coastal region. Abundant Jurassic-aged zircons are compatible with the re-initiation of “normal subduction” in the Early Jurassic. The re-initiation of “normal subduction” resulted in the generation of an accretionary orogeny, continental arcs, and a retroarc foreland basin along the eastern South China margin in contrast to the extensional regime in the inland. The basin response and distinct tectonic regimes of the inland and continental margin in the Early−Middle Jurassic support a geodynamic turnover from flat to normal subduction.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36059.1/608457/Basin-response-to-the-Jurassic-geodynamic-turnover
The Mesozoic magmatic, metamorphic, and tectonic evolution of the eastern Gangdese magmatic arc, southern Tibet
Ze-Ming Zhang; Hui-Xia Ding; Xin Dong; Zuo-Lin Tian; Richard M. Palin ...
Abstract: Magmatic arcs are natural laboratories for studying the growth of continental crusts. The Gangdese arc, southern Tibet, is an archetypal continental magmatic arc that formed due to Mesozoic subduction of the Neo-Tethyan oceanic lithosphere; however, its formation and evolution remain controversial. In this contribution, we combine newly reported and previously published geochemical and geochronological data for Mesozoic magmatic rocks in the eastern Gangdese arc to reveal its magmatic and metamorphic histories and review its growth, thickening, and fractionation and mineralization processes. Our results show that: (1) the Gangdese arc consists of multiple Mesozoic arc-type magmatic rocks and records voluminous juvenile crustal growth. (2) The Mesozoic magmatic rocks experienced Late Cretaceous granulite-facies metamorphism and partial melting, thus producing hydrous and metallogenic element-rich migmatites that form a major component of the lower arc crust and are a potential source for the Miocene ore-hosting porphyries. (3) The Gangdese arc witnessed crustal thickening and reworking during the Middle to Late Jurassic and Late Cretaceous. (4) Crystallization-fractionation of mantle-derived magmas and partial melting of thickened juvenile lower crust induced intracrustal chemical differentiation during subduction. We suggest that the Gangdese arc underwent the following main tectonic, magmatic, and metamorphic evolution processes: normal subduction and associated mantle-derived magmatism during the Late Triassic to Jurassic; shallow subduction during the Early Cretaceous and an associated magmatic lull; and mid-oceanic ridge subduction, high-temperature metamorphism and an associated magmatic flare-up during the early Late Cretaceous, and flat subduction, high-temperature and high-pressure metamorphism, partial melting, and associated crust-derived magmatism during the late Late Cretaceous. Key issues for further research include the temporal and spatial distributions of Mesozoic magmatic rocks, the evolution of the components and compositions of arc crust over time, and the metallogenic processes that occur in such environments during subduction.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36134.1/608458/The-Mesozoic-magmatic-metamorphic-and-tectonic
Evaluation of the exploration prospect and risk of marine gas shale, southern China: A case study of Wufeng-Longmaxi shales in the Jiaoshiba area and Niutitang shales in the Cen’gong area
Qiyang Gou; Shang Xu; Fang Hao; Yangbo Lu; Zhiguo Shu ...
Abstract: The Wufeng-Longmaxi shales and the Niutitang shales are the most important organic-rich marine shales in southern China. To fully understand the significant difference in drilling results between the two sets of shales, the accumulation conditions of shale gas were systematically compared. The Niutitang shales have a superior material base of hydrocarbon generation for higher total organic carbon than the Wufeng-Longmaxi shales. Due to the influence of hydrothermal activities and carbonization of organic matter, however, the porosity, pore volume, pore size, and pore connectivity of Niutitang shales is obviously lower than that of Wufeng-Longmaxi shales. The natural fractures of Wufeng-Longmaxi shales are dominated by horizontal bedding fractures, and most of them are filled by calcite. By contrast, the high dip-angle fractures are more developed in the Niutitang shales. Especially, these fractures remain open in stages during the process of serious uplift and denudation movements. Thus, the seal conditions of the Niutitang shales are poor, which is further not conducive to the enrichment of shale gas. Our work also suggests that the exploration and development of highly over matured marine shales in southern China should follow the principle of “high to find low, and strong to find weak.”
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36063.1/608459/Evaluation-of-the-exploration-prospect-and-risk-of
Identification of ca. 520 Ma mid-ocean-ridge−type ophiolite suite in the inner Cathaysia block, South China: Evidence from shearing-type oceanic plagiogranite
Longming Li; Shoufa Lin; Guangfu Xing; Fan Xiao; Wenjiao Xiao
Abstract: An ophiolite suite, predominantly composed of residual mantle peridotites, mid-ocean-ridge basalt (MORB)−like ultramafic rocks, and oceanic plagiogranites, has been identified in the Zhenghe-Dapu fault zone, Cathaysia block, South China. The peridotites experienced strong serpentinization and are characterized by low 187Os/188Os ratios of 0.11621−0.12008 and very low 187Re/188Os values of 0.031−0.129, similar to those from highly refractory mantle residues. The meta-ultramafic rocks, mainly amphibolites, can be classified into two groups. Group I is characterized by high Ni and Cr and low K2O contents. Their spoon-like rare earth element (REE) patterns, along with lower concentrations of highly incompatible elements, indicate that the protolith was of cumulate origin. Group II displays depleted REE patterns and low Nb/Yb, Th/Yb, and Ti/V ratios, which are geochemically similar to normal (N) MORB. Both groups exhibit positive εNd(t) values (1.2−4.1) and relatively high (87Sr/86Sr)i ratios (0.7046−0.7096), suggesting their origin from partial melting of depleted mantle sources in a mid-ocean-ridge setting that experienced a greater extent of fluid-rock alteration. The meta-plagiogranites intercalated with the mylonitic amphibolites are characterized by low K2O (0.09−0.21 wt%) and total REE contents along with low K2O/Na2O and Rb/Sr ratios, which are consistent with those of typical oceanic plagiogranite. They exhibit strongly positive zircon εHf(t) values (+9.5 to +15.1) and positive whole-rock εNd(t) values (+2.8 to +3.6). Their extremely low MgO (0.6−1.65 wt%), Cr (0.22−6.26 ppm), and Ni (0.77−4.74 ppm) compositions and low Mg# (22.4−31.9) preclude their origination from mantle-derived primary magma but favor oceanic crust. Low zircon δ18O values (4.02‰−5.4‰) and decoupled Sr-Nd isotope features imply the involvement of high-temperature seawater alteration in their source region. The enriched light rare earth element (LREE) patterns with strongly positive Eu anomalies, similar to the East Karmøy−type plagiogranite in western Norway, imply that the plagiogranites were derived from anatexis of amphibolite in an active shear zone near the mid-ocean ridge. The magmatic and metamorphic zircons from the meta-plagiogranites yield nearly identical secondary ion mass spectrometry (SIMS) U-Pb ages ranging from 523 to 521 Ma and from 522 to 518 Ma, respectively. The simultaneous magmatism and metamorphism also signify an active high-temperature shear zone, where the plagiogranites were formed slightly later than the oceanic crust. The age of ca. 520 Ma represents the formation age of the oceanic crust rather than its emplacement age. The identification of the ca. 520 Ma ophiolite suite along the Zhenghe-Dapu fault zone indicates that the Cathaysia block consisted of at least two different terranes rather than a single tectonic unit in the Cambrian, and the final amalgamation of the eastern and western Cathaysia block may have occurred far later than ca. 520 Ma, most likely during the late early Paleozoic.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36088.1/608460/Identification-of-ca-520-Ma-mid-ocean-ridge-type
U-Pb speleothem geochronology reveals a major 6 Ma uplift phase along the western margin of Dead Sea Transform
O. Chaldekas; A. Vaks; I. Haviv; A. Gerdes; R. Albert
Abstract: The timing of vertical motions adjacent to the Dead Sea Transform plate boundary is not yet firmly established. We utilize laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb geochronology of carbonate cave deposits (speleothems) to constrain paleo-groundwater levels along the western margin of the Dead Sea Transform and provide a proxy for the timing of large-scale incision and tectonic uplift. Phreatic speleothems can form in caves that are located slightly below the groundwater level. Tectonic uplift and/or base level subsidence can trigger incision of canyons and induce a drop in the groundwater table. This can cause dewatering of the caves, cessation of the deposition of phreatic speleothems, and initiation of growth of vadose speleothems. The transition between deposition of phreatic and vadose speleothems can therefore reflect tectonic or erosive events. We obtained 102 U-Pb ages from 32 speleothems collected from three cave complexes across a 150-km-long, north-to-south transect. These ages indicate that phreatic deposition began between 14.68 ± 1.33 and 11.34 ± 1.62and ended by 6.21 ± 0.59 Ma. Later, vadose speleothems grew intermittently until the Quaternary. These results suggest an abrupt drop in the water table starting at ca. 6 Ma with no re-submergence of the caves. We interpret this to indicate river incision of ∼150−200 m that was driven by uplift and folding of the western margin of the Dead Sea Transform and by inland morpho-tectonic, base-level subsidence in the Dead Sea area. The observed timing corresponds with a change in the Euler pole of the plates motion along the Dead Sea Transform. The growth period of phreatic speleothems suggests groundwater level stability and limited vertical tectonic motions between 14 Ma and 6 Ma.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36051.1/608120/U-Pb-speleothem-geochronology-reveals-a-major-6-Ma
Concurrent MORB-type and ultrapotassic volcanism in an extensional basin along the Laurentian Iapetus margin: Tectonomagmatic response to Ordovician arc-continent collision and subduction polarity flip
Deta Gasser; Tor Grenne; Fernando Corfu; Reidulv Bøe; Torkil S. Røhr ...
Abstract: Arc-continent collision, followed by subduction polarity flip, occurs during closure of oceanic basins and contributes to the growth of continental crust. Such a setting may lead to a highly unusual association of ultrapotassic and mid-ocean ridge basalt (MORB)-type volcanic rocks as documented here from an Ordovician succession of the Scandinavian Caledonides. Interbedded with deep-marine turbidites, pillow basalts evolve from depleted-MORB (εNdt 9.4) to enriched-MORB (εNdt 4.8) stratigraphically upward, reflecting increasingly deeper melting of asthenospheric mantle. Intercalated intermediate to felsic lava and pyroclastic units, dated at ca. 474−469 Ma, are extremely enriched in incompatible trace elements (e.g., Th) and have low εNdt (−8.0 to −6.6) and high Sri (0.7089−0.7175). These are interpreted as ultrapotassic magmas derived from lithospheric mantle domains metasomatized by late Paleoproterozoic to Neoproterozoic crust-derived material (isotopic model ages 1.7−1.3 Ga). Detrital zircon spectra reveal a composite source for the interbedded turbidites, including Archean, Paleo-, to Neoproterozoic, and Cambro-Ordovician elements; clasts of Hølonda Porphyrite provide a link to the Hølonda terrane of Laurentian affinity. The entire volcano-sedimentary succession is interpreted to have formed in a rift basin that opened along the Laurentian margin as a result of slab rollback subsequent to arc-continent collision, ophiolite obduction and subduction polarity flip. The association of MORBs and ultrapotassic rocks is apparently a unique feature along the Caledonian-Appalachian orogen. Near-analogous modern settings include northern Taiwan and the Tyrrhenian region of the Mediterranean, but other examples of strictly concurrent MORB and ultrapotassic volcanism remain to be documented.
View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36113.1/608121/Concurrent-MORB-type-and-ultrapotassic-volcanism
GSA BULLETIN articles published ahead of print are online at https://bulletin.geoscienceworld.org/content/early/recent. Representatives of the media may obtain complimentary copies of articles by contacting Kea Giles. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to The Geological Society of America Bulletin in articles published. Non-media requests for articles may be directed to GSA Sales and Service, firstname.lastname@example.org.
# # #