New GSA Bulletin Articles Published Online Ahead of Print

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 and SE Asia; the architecture and permeability of the Sibillini Mountains, central Appenines; the Arctic Atlantic Ocean; and the northern U.S. Cordillera. You can find these articles at .

Formation of Eocene−Miocene felsic magmatic rocks along N-S−trending Yardoi-Kongbugang mountain ranges in the eastern Himalaya: New insights into surface uplift and the initiation of E-W extension in southern Tibet
Jing-Jing Fan; Xiu-Zheng Zhang; Lin Ma; Qiang Wang; Zi-Qi Jiang ...
Large-scale N-S shortening induced by India-Asia convergence caused the formation of numerous E-W−trending mountain ranges in Tibet. However, the mechanism(s) of formation of N-S−trending mountain ranges remains elusive. We report on a felsic magmatic belt located along the N-S−trending Yardoi-Kongbugang mountain ranges on the flank of the Cona rift in the eastern Tethyan Himalaya. Zircon and monazite geochronology and whole-rock geochemistry revealed three epochs of middle- to lower-crustal anatexis beneath the Cona rift at ca. 47−42 Ma, 35−34 Ma, and 24−14 Ma. The mid-Eocene and early Oligocene granitoids show adakitic signatures indicating continuous crustal thickening, while the formation of Miocene leucogranites and N-S−trending dacitic dikes was related to ductile crustal extension. Silicic melts were exposed along the whole rift since the early Oligocene, suggesting that the early Oligocene could be regarded as a transitional epoch from tectonic compression to orogen-parallel extension. Widespread mid-Eocene and Miocene magmatism in the Himalaya, together with coeval metamorphic anatexis, represents two phases of crustal weakening. The weakened crustal zones under continued India-Asia convergence may have favored uplift and subsequent lateral flow of the weak zones, which initiated E-W extension. Finally, significant upwelling of the weak zones evolved into magma extrusion and formed the N-S−trending mountain ranges. This study provides new insights into the mechanisms of surface uplift and E-W extension and challenges the common view of initiation of E-W extension in southern Tibet not earlier than the early Miocene.

Geochemical signature of a deeply subducted slab in the source of post-collisional magmatism: A case study from the Western Tianshan NW China
Ning-Bo Li; He-Cai Niu; Qiang Shan; Wu-Bin Yang
The contributions of deeply subducted slabs to post-collisional magmatism are poorly constrained. Here, we present a study of the post-collisional, Early Permian (ca. 279 Ma), Hamisite basalts from the Western Tianshan NW China to trace the geochemical fingerprints of deeply subducted slab. The Hamisite basalts have arc-like trace element patterns but high Nb/Ta ratios (18.4−23.0). They have slightly enriched Sr−Nd isotopic compositions ( 87Sr/86Sr = 0.7048−0.7052; εNd[t] = 2.02−3.85), which indicates that the basalts were derived from an enriched mantle source. Extremely low δ7Li values (−5.76‰ to 0.20‰) imply the involvement of a deeply subducted slab (i.e., eclogites) in the source. Correlations among Nb/Ta and (Dy/Yb)N ratios, and rare earth element contents, indicate that the deeply subducted slab contained residual rutile and garnet but no epidote. Given that melting of eclogites requires an anomalously hot mantle source, we propose that mantle upwelling during post-collisional extension triggered melting of the deeply subducted slab. Our results indicate that melting of a deeply subducted slab can generate a metasomatized mantle source with high Nb/Ta ratios, which could be a hidden, high-Nb/Ta reservoir on Earth. Low δ7Li values and high Nb/Ta ratios could be a fingerprint of recycling of a deeply subducted slab.

A tale of an orbicule in the eastern Gangdese belt of southern Tibet: Petrographic, geochemical, and submagmatic structural perspectives on its formation
Xuxuan Ma; Zhiqin Xu; Xiaohong Mao; Tarryn Cawood; Haoyu Yan ...
Owing to their intriguing appearance and unusual occurrence, as well as their significance in addressing magmatic processes, orbicules have attracted attention from geologists all over the world. We discovered an orbicule-bearing boulder in the Linzhi region, near the Eastern Himalayan Syntaxis, which is the first report of orbicules in the Gangdese magmatic belt, southern Tibet. The orbicules are composed of dioritic shells around monzodioritic cores, in a dioritic matrix. They typically have a single shell, composed of elongate and radially oriented hornblendes, with interstitial plagioclase. Zircon laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating yields an indistinguishable age of ca. 28.8 Ma for both the orbicule core and matrix, indicating approximately coeval crystallization. Zircon Lu-Hf and whole-rock Sr-Nd isotopes reveal that the original magmas of both the orbicule matrix and orbicule core were relatively juvenile (ƐHf(t) = +2.2 to +6.0 for matrix and +1.2 to +3.9 for core; ƐNd(t) = −1.68 to −1.46 for matrix and −3.65 to −2.52 for core), similar to reported Oligocene−Miocene post-collisional adakites in the Gangdese belt. Zircon and hornblende geochemistry reveals that both the orbicule matrix and cores were generated from highly oxidized, high oxygen fugacity magmas. Hornblende composition further indicates that the original magmas of the matrix and the orbicule cores were water-rich, with H2O contents greater than 4 wt%. In combination with the whole-rock geochemistry, these results suggest that the orbicules were formed in a post-collisional setting, from magma generated by partial melting of the juvenile thickened lower crust of the Lhasa terrane triggered by convective removal of the Lhasa lithospheric root. The preferred orientation of euhedral igneous minerals in the matrix and the alignment and compaction of orbicules define a magmatic to submagmatic foliation. This, together with the subsolidus deformation microstructures exhibited by minerals within the orbicules, indicates that some flow or movement of orbicules took place under magmatic to submagmatic conditions, as the subsolidus plastic orbicules were jostled and transported within a hypersolidus liquid magma.

Syn-collisional extension and Ni-Cu sulfide-bearing mafic magma emplacement along the Irtysh Shear Zone in the Central Asian Orogenic Belt
Bo Wei; Christina Yan Wang; Pengfei Li
The Central Asian Orogenic Belt was characterized by a long-lived accretionary history from Neoproterozoic to Paleozoic, followed by a collisional phase of orogeny in the latest Paleozoic with the closure of the Paleo-Asian Ocean. However, Ni-Cu sulfide mineralization associated with mantle-derived mafic magmas in the Central Asian Orogenic Belt was relatively short-lived. Thus, the coherence between the protracted accretionary and collisional history of the Central Asian Orogenic Belt and the transient Ni-Cu sulfide-bearing mafic magmatism is difficult to understand. We investigated both Ni-Cu sulfide-bearing and sulfide-barren mafic intrusions along a major suture zone of the Irtysh Shear Zone within the Central Asian Orogenic Belt to understand the origin of short-lived Ni-Cu sulfide mineralization in the context of the collision of two arc systems of the Chinese Altai and the East Junggar regions. Zircon U-Pb dating results for the Ni-Cu sulfide-bearing mafic intrusions show that they were only emplaced briefly from 290 Ma to 280 Ma after subduction terminated (ca. 312 Ma), substantially earlier than Ni-Cu sulfide-barren mafic intrusions (ca. 274−271 Ma). Zircons from all Ni-Cu sulfide-bearing/barren mafic intrusions yield arithmetic mean δ 18O values ranging from 6.0 to 7.4‰, higher than the typical mantle value (∼5.3‰), which indicates their derivation from a metasomatized mantle with inherited subduction signatures. The δ11B values of the rocks from these intrusions range from −17.2‰ to −2.3‰, significantly lower than those of typical volcanic arcs (up to +18‰), which suggests that the metasomatic components in the mantle source were dominated by sediment melts rather than fluids. The pressure calculated using amphibole thermobarometry shows that the parental magmas of the Ni-Cu sulfide-bearing mafic intrusions were emplaced shallower than those of Ni-Cu sulfide-barren mafic intrusions. This is supported by the fact that the country rocks of the Ni-Cu sulfide-bearing mafic intrusions are shallow crustal lithologies of sedimentary rocks, whereas the country rocks of the barren intrusions are relatively deeper crustal lithologies of high-grade metamorphic rocks and granites. Olivine from the Ni-Cu sulfide-barren intrusions has forsterite (Fo) contents ranging from 51.1 mol% to 76.2 mol% and Ni from 196 ppm to 1312 ppm. These values are much lower than those of the Ni-Cu sulfide-bearing intrusions (Fo = 72.6−79.4 mol%; Ni = 1022−1925 ppm), which indicates that the parental magmas of the barren intrusions may have experienced extensive olivine crystallization. Notably, the emplacement of the Ni-Cu sulfide-bearing mafic intrusions (290−280 Ma) coincidently overlapped with a transient period of orogen-parallel extension (ca. 295−280 Ma) in response to the collision of the Chinese Altai and East Junggar. Syn-collisional extension may lead to the rapid ascent and emplacement of mantle-derived mafic magmas at a shallower crustal level, preventing significant loss of Ni from olivine crystallization at depths, and thus providing metal-rich mafic magmas for potential Ni-Cu sulfide mineralization. Our results highlight that syn-collisional extension is an essential geodynamic mechanism that controls the emplacement of Ni-Cu sulfide-bearing mafic intrusions in orogenic belts.

Architecture and permeability structure of the Sibillini Mts. Thrust and influence upon recent, extension-related seismicity in the central Apennines (Italy) through fault-valve behavior
Manuel Curzi; Angelo Cipriani; Luca Aldega; Andrea Billi; Eugenio Carminati ...
The central Apennines are a fold-thrust belt currently affected by post-orogenic extensional seismicity. To constrain the influence that the inherited thrust-related structures exert on the present seismic behavior of the belt, we provide the high-resolution structural and hydraulic characterization of one of the most external exposed thrust fault systems of the central Apennines, the Sibillini Mts. Thrust Front (STF). We integrate structural mapping, multiscale structural analysis, and in situ air permeability on the brittle structural facies of the thrust zone. We also performed K-Ar dating of selected fault rocks to better constrain structural inheritance. The STF is defined by a complex, ∼300-m-thick deformation zone involving Meso-Cenozoic marl and limestone that results from the accommodation of both seismic and aseismic slip during shortening. Permeability measurements indicate that the low permeability (10 −2 ÷ 10−3 D) of the marly rich host rock diminishes within the thrust zone, where the principal slip surfaces and associated S-C structures represent efficient hydraulic barriers (permeability down to ∼3 × 10−10 D) to sub-vertical fluid flow. Field data and K-Ar dating indicate that the STF began its evolution ca. 7 Ma (early Messinian). We suggest that the studied thrust zone may represent a barrier for the upward migration of deep fluids at the hypocentral depth of present-day extensional earthquakes. We also speculate on the influence that similar deformation zones may have at depth on the overall regional seismotectonic pattern by causing transient fluid overpressures and, possibly, triggering cyclic extensional earthquakes on normal faults prone to slip while crosscutting the earlier thrust zones (as per a classic fault valve behavior). This mechanism may have controlled the origin of the 2016−2017 central Apennines devastating earthquakes.

Paleomagnetism and geochronology of Early Cretaceous volcanic rocks in the eastern segment of the Lhasa terrane, Tibetan Plateau, and their tectonic implications
Yong Cao; Zhiming Sun; Haibing Li; Zhenyu Yang; Junling Pei ...
Paleogeographic reconstructions of the Lhasa terrane for the Cretaceous provide important constraints on the evolution of the Neo-Tethys Ocean and crustal shortening within Asia. However, the Cretaceous paleogeography of the Lhasa terrane remains contentious. A direct way to study this issue is to conduct paleomagnetic investigations of the Cretaceous rocks of the Lhasa terrane; however, most previous Cretaceous paleomagnetic investigations of the Lhasa terrane were conducted in the middle and western segments of this terrane. Different vertical-axis rotations affected different parts of the Lhasa terrane following the India-Asia collision, and therefore paleomagnetic data from the western and middle segments of the Lhasa terrane cannot necessarily be used to directly constrain the paleolatitudes of its eastern segment. This study presents paleomagnetic data from the Early Cretaceous volcanic rocks with well-constrained ages from the Luolong area in the eastern segment of the Lhasa terrane. Isotope geochronology reveals that these rocks formed at 127−124 Ma. The mean high-temperature direction obtained from 17 paleomagnetic sites is declination/inclination (Ds/Is) = 21.8°/19.0° with κs = 65.8 and α95 = 4.4° (κ—best estimate of the precision parameter; α95—radius of the 95% probability ellipse around the mean direction; s—stratigraphic coordinates). Petrographic investigations, a positive fold test, a reversal test, and a paleosecular variation test indicate the primary origin of this characteristic remanence. A paleomagnetic pole of 60.9°N, 227.2°E with dp/dm = 2.4°/4.6° (dp/dm—semi-axes of the 95% probability ellipse around the mean pole) yields a paleolatitude of 9.2 ± 2.4°N for the eastern segment of the Lhasa terrane. Combined with reliable results from previous paleomagnetic studies, we draw the following conclusions. (1) During the Early Cretaceous, the Lhasa terrane was oriented WNW-ESE as a whole, and the eastern-middle segments may have been oriented nearly E-W. (2) Asia has accommodated 2050 ± 230 km of N-S crustal shortening along 96°E longitude since the Early Cretaceous. (3) The minimum N-S width of the Neo-Tethys Ocean at ca. 125 Ma was 4185 ± 300 km.

Tectono-magmatic evolution of the Greenland−Iceland−Norway ridge complex and the Jan Mayen hotspot in the Arctic Atlantic Ocean: Constraints from in situ trace elements and Sr isotopes of minerals
Shuang-Shuang Chen; Rui Gao; Jian-Ping Zheng; Jia-Qi Liu
The subarctic North Atlantic Ocean has experienced a complex and gradual magmatic evolution, including continental breakup, mature seafloor spreading, and episodic ridge jumps. Here, we present new in situ major-element, trace-element, and Sr isotopic compositions for minerals (olivine, clinopyroxene, and plagioclase) from the Jan Mayen microcontinent (Integrated Ocean Drilling Program [IODP] Sites 907 and 985), Reykjanes Ridge (Site 983), Greenland Basin (Site 913), and Knipovich Ridge (Site 911) volcanic samples in the Arctic Atlantic Ocean. The Jan Mayen hotspot−type and Iceland plume−type volcanic rocks have a common magma source. The former were likely derived from the latter by further fractional crystallization, resulting in their distinct geochemical features. The in situ incompatible element and isotopic compositions show that the normal mid-ocean-ridge basalt spreading ridge feature for the older (>14 Ma) system and the enriched plume feature for the younger (1.521 Ma and 1.049 Ma) system likely indicate plume-ridge interaction and the evolution from mid-ocean-ridge−type to plume-type magmatism beneath the Norwegian−Greenland Sea. The disequilibrium of Sr isotopes in plagioclase among the groundmass, phenocryst cores, and rims is likely due to heterogeneous compositions beneath the North Atlantic Ocean basin, with the contamination of thick continental crust and the influence of H2 O-enriched melts originating from the deep Iceland plume.

Astronomical and tectonic influences on climate and deposition revealed through radioisotopic geochronology and Bayesian age-depth modeling of the early Eocene Green River Formation, Wyoming, USA
Benjamin T. Bruck; Brad S. Singer; Mark D. Schmitz; Alan R. Carroll; Stephen Meyers ...
The Wilkins Peak Member (WPM) of the Green River Formation in Wyoming, USA, comprises alternating lacustrine and alluvial strata that preserve a record of terrestrial climate during the early Eocene climatic optimum. We use a Bayesian framework to develop age-depth models for three sites, based on new 40Ar/39Ar sanidine and 206Pb/ 238U zircon ages from seven tuffs. The new models provide two- to ten-fold increases in temporal resolution compared to previous radioisotopic age models, confirming eccentricity-scale pacing of WPM facies, and permitting their direct comparison to astronomical solutions. Starting at ca. 51 Ma, the median ages for basin-wide flooding surfaces atop six successive alluvial marker beds coincide with short eccentricity maxima in the astronomical solutions. These eccentricity maxima have been associated with hyperthermal events recorded in marine strata during the early Eocene. WPM strata older than ca. 51 Ma do not exhibit a clear relationship to the eccentricity solutions, but accumulated 31%−35% more rapidly, suggesting that the influence of astronomical forcing on sedimentation was modulated by basin tectonics. Additional high-precision radioisotopic ages are needed to reduce the uncertainty of the Bayesian model, but this approach shows promise for unambiguous evaluation of the phase relationship between alluvial marker beds and theoretical eccentricity solutions.

Large-scale basement mobilization endows the giant Carlin-type gold mineralization in the Youjiang Basin, South China: Insights from mercury isotopes
Wei Gao; Ruizhong Hu; Xueyun Wang; Runsheng Yin; Xianwu Bi ...
The metal source and genesis of hydrothermal mercury-rich gold metallogenic systems occurring far away from active continental margins remain puzzling. The Youjiang Basin of South China, where exists numerous Carlin-type gold deposits and some synmineralization hidden intrusions, is a natural laboratory to address this issue due to it was up to 1000 km inward from the active continental margins of South China when mineralization. Here, we use mass-independent fractionation of mercury isotope ratios (reported as ∆ 199Hg), which is predominantly generated during Hg photochemical reactions on Earth’s surface and has superiority of isotopic inheritance during hydrothermal processes, to address the metal source of the Youjiang Carlin-type gold deposits. Ore-associated sulfides from seven representative deposits display negative to near-zero ∆199Hg values (−0.29‰ to 0.04‰), which fall in between that of the regional Precambrian basement rocks (−0.21‰ to 0.06‰) and deep magmatic-hydrothermal systems (∼0‰), suggesting a binary mixing of Hg from these two sources. An isotope mixing model and mass balance calculations demonstrate that ∼1000 km3 of the basement rocks, which contributed to 86% of Hg budget, were leached and remobilized by magmatic-hydrothermal fluids and deep-circulating crustal fluids to endow the gold reserves of these deposits. Given that traditional S, Pb, C, and O isotopic data yielded indirect and ambiguous constraints on metal source due to their complex evolution processes and isotope fractionation during the fluids ascended. Our results, therefore, highlight the great advantage of using Hg isotope as a new tracer to understand metal sources of hydrothermal deposits.

Late Miocene or older canyon incision in the northern U.S. Cordillera shown by erosion rates, incision models, and basalt flow ages
Nate Mitchell; Brian Yanites; Alison Duvall; Eugene Humphreys; Jonathan Perry-Houts ...
Deep canyons along the Salmon, Snake, and Clearwater rivers in central Idaho, USA suggest long-lasting transient incision, but the timing and drivers of this incision are not well understood. The perturbation of the Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho all occurred within or near to this region, but the relationship among these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17 new) of fluvial sediment across the region to quantify erosion rates, calibrate stream power models, and estimate incision timing. We estimate that transient incision along the Salmon River began prior to ca. 10 Ma. However, canyon age decreases to ca. 5 Ma or earlier farther to the north. For a group of tributaries underlain by basalt, we use the age of the basalt to estimate that local transient incision began between ca. 11.5 and 5 Ma. Based on these timing constraints, the canyons along the Salmon and Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon incision was triggered by events related to the Yellowstone hotspot (e.g., basalt lava damming, subsidence of the Columbia Basin, reactivation of faults, and/or lower crustal flow). Furthermore, our models suggest basalt may be more erodible than the other rock types we study. We show that lithology has a significant influence on fluvial erosion and assumptions regarding river incision model parameters significantly influence results. Finally, this study highlights how geodynamic processes can exert a significant influence on landscape evolution.

High-magnesium igneous associations record final-stage geodynamic process of the southeastern segment of the Central Asian Orogenic Belt
Yan Jing; Wen-chun Ge; Hao Yang; Yu Dong; Zheng Ji ...
Much controversy exists regarding the evolution and termination of accretionary orogens where typical continent-continent collision features are lacking. The Central Asian Orogenic Belt provides a key opportunity to explore the genetic relationships between the formation of high-Mg igneous associations and the evolution of accretionary orogens. In this study, two-stage Triassic magmatic events were identified in the southeastern segment of the Central Asian Orogenic Belt (including ca. 247 Ma andesites and ca. 222 Ma diorites), through precise secondary ion mass spectrometry (SIMS) and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon dating. The Olenekian andesites show typical sanukitoid affinities with high TiO2, MgO, Ni, and Cr contents, and enrichment in large ion lithophile elements (LILEs; K, Ba, and Sr). They yield depleted Hf isotopic compositions and high δ18O values, which along with Hf-O isotope modeling results indicate that they originated from the partial melting of mantle wedge peridotites with input of 20% terrigenous sediments. The Norian diorites have moderate SiO 2, relatively high MgO, Cr, and Ni contents, Mg# values of 63−65, high Sr, low Y and Yb contents, relevant high La/Yb ratios, and negligible Eu anomalies, which makes them comparable to high-Mg andesites with adakitic affinity. Their depleted isotopic compositions are similar to coeval A-type granites that resulted from lithospheric delamination after the final collision. In addition, with high K 2O/Na2O ratios and incompatible element ratios similar to those of the continental crust (e.g., Nb/U, Ce/Pb, and Nd/Sm), they conform to an origin in which partial melting of delaminated mafic lower crust subsequently interacted with mantle materials. These two-stage abnormal Triassic high-Mg rocks archive a hot slab window triggered by the break-off of the Paleo-Asian oceanic slab and lower crustal delamination related to collapse of the southeastern segment of the Central Asian Orogenic Belt. These findings provide beneficial information for the reconstruction of the East Asian blocks in Pangea.

The genesis of ca. 1.78 Ga granitoids in the Xiong’er large igneous province: Implications for continental crust generation
Jian-Feng Ma; Chuan-Hao Qu; Yan-Yan Zhou; Tai-Ping Zhao
The genesis of intermediate-felsic rocks and the implications for crustal growth are highly controversial. These issues are particularly relevant to large igneous provinces, which are one of the most effective methods of crustal generation. Mafic magmas, even those with continental marks, have been considered as the contribution from mantle to crust, whereas the intermediate-felsic series in large igneous provinces has uncertain implications for crustal growth due to uncertainty about their modes of generation. In this paper, we focus on the intermediate-felsic granitoids, i.e., Gushicun diorites and granites, in the Xiong’er large igneous province (ca. 1780‒1750 Ma) through detailed study of their petrology, chronology, and geochemistry. Our new data show that both diorites and granites have the same emplacement age of ca. 1780 Ma and were synchronized with the Xiong’er large igneous province. The granitoids are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Li, Ba, and Rb) but depletion in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). They have almost the same ΣREE (rare earth element) contents and TTi-Zircon (∼850 °C), whole-rock Nd (εNd(t) from −8.24 to −5.76), zircon Hf (εHf(t) isotope ratios of −14 to −8 isotope ratios), and O-isotope compositions (average δ18O = 5.3‰). The fine-grained Gushicun diorites have low Sr/Nd (<10) and Eu/Sm (<0.3) but high Si/Al (>3.2) ratios and are similar to the basaltic andesites in the Xiong’er large igneous province, which suggests that the diorites are crystallization melts that differentiated along the liquid line of descent. The Rayleigh fractionation model based on the REE contents shows that the Gushicun diorites were formed by 40%‒50% fractional crystallization of Xiong’er basaltic melts. The characteristics of high K2O (3.8‒4.7 wt%) and Yb contents (4.0‒6.5 ppm), low Sr/Y (1.4‒3.3), and low metaluminous values (A/CNK = 0.8‒1.1) of the Gushicun granites with mantle-like O-isotope values rule out the melting genesis of low K2O (<0.5 wt%) amphibolite and trondhjemite−tonalite−granodiorites (TTGs) of the surrounding Taihua Complex. The Gushicun granites show a geochemical affinity with andesites in the Xiong’er large igneous province. The alphaMELTs simulation based on the major elements argues against the partial melting of amphibolite and TTGs but supports the melting genesis of Xiong’er intermediate igneous rocks with low water contents (<1 wt%). These findings shed light on the formation of the felsic series in large igneous provinces, which remelted the previous mafic-intermediate rocks via underplating basaltic magmas. Despite having enriched Nd- and Hf-isotope features, the Gushicun diorites represent juvenile crust, while the granites were formed by the reworking of juvenile crust instead of ancient crust. This paper also suggests that radiogenic isotopic ratios cannot be reliably used to assess types of continental crust.

Evolutionary, paleoecological, and biostratigraphic implications of the Ediacaran-Cambrian interval in West Gondwana
Lucas V. Warren; Lucas Inglez; Shuhai Xiao; Luis A. Buatois; M. Gabriela Mángano ...
The Ediacaran-Cambrian transition interval is described for the west part of the Gondwana Supercontinent. This key interval in Earth’s history is recorded in the upper and lower part of the Tagatiya Guazú and Cerro Curuzu formations, Itapucumi Group, Paraguay, encompassing a sedimentary succession deposited in a tidally influenced mixed carbonate-siliciclastic ramp. The remarkable presence of cosmopolitan Ediacaran shelly fossils and treptichnids, which are recorded in carbonate and siliciclastic deposits, respectively, suggests their differential preservation according to lithology. Their distribution is conditioned by substrate changes that are related to cyclic sedimentation. The associated positive steady trend of the δ13C values in the carbonate facies indicates that the Tagatiya Guazú succession is correlated to the late Ediacaran positive carbon isotope plateau. Sensitive high-resolution ion microprobe U-Pb ages of volcanic zircons from an ash bed ∼30 m above the fossil-bearing interval in the Cerro Curuzu Formation indicate an Early Cambrian (Fortunian) depositional age of 535.7 ± 5.2 Ma. As in other coeval sedimentary successions worldwide, the co-occurrence of typical Ediacaran skeletal taxa and relatively complex trace fossils in the studied strata highlights the global nature of key evolutionary innovations.

Molybdenum mineralization genetically linked with magmatism at the Shipingchuan deposit, SE China
Yan-Jun Li; Jun-Hao Wei; Meng-Ting Chen; Zheng-Hui Chen; Yann Lahaye ...
Most molybdenum (Mo) deposits are considered to be associated with magmatic systems; however, their genetic link is not clearly resolved when using, for example, bulk sulfur (S) and lead (Pb) isotopes of sulfides dominated by pyrite separates. Here, we present microtextures and in situ trace-element results of pyrite, in situ S isotopic compositions of molybdenite and pyrite, and Pb isotopes of pyrite and K-feldspar determined by laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and multicollector (MC) LA-ICP-MS. Combined with geochronologic data for both the magmatism and Mo mineralization, these in situ methods allowed the genetic link between Mo mineralization and magmatism to be unraveled at Shipingchuan, SE China. The Shipingchuan deposit is characterized by molybdenite-quartz veins hosted by faults or fractures around a biotite K-feldspar granite. Zircon LA-ICP-MS U-Pb and molybdenite Re-Os dating results confirm that the biotite K-feldspar granite and mineralization were coeval and formed during the end of the Early Cretaceous (ca. 105 Ma), whereas postmineralization monzogranite porphyry dikes formed at 93.7 Ma. Pyrite shows a close mutual relationship with molybdenite and is characterized by Co-Ni-As-Te-Cu-Pb-Ag-Bi−rich cores (PyI) revealed by LA-ICP-MS data. Co/Ni and calculated (Se/S)fluid ratios for both cores (PyI) and rims (PyII) indicate a magmatic-hydrothermal origin. The in situ S isotopic compositions (δ34S) of molybdenite are 2.1‰ to 6.8‰, which are identical to those of molybdenite separates. PyI has δ 34S values of −6.9‰ to 0.8‰, whereas PyII exhibits slightly more negative values of −7.5‰ to −0.2‰. These analyses indicate reliable results for in situ S isotopes in molybdenite, which could be more appropriate to constrain the origin of sulfur in hydrothermal fluids than results from pyrite analysis. The in situ S isotopic compositions of sulfides confirm a magmatic source for sulfur. The in situ Pb isotopic compositions of pyrite are consistent with those of K-feldspar from the biotite K-feldspar granite, indicating a common source. The heterogeneous As/Ni, As/Sb, and As/Bi values, as well as the variations of δ34S values of both PyI and PyII, indicate progressive oxidization of the hydrothermal fluids. These results illustrate that the metal-rich fluids were released from the granite and migrated along faults and fractures around the granite in an extensional setting. The precipitation of molybdenite-quartz veins at Shipingchuan was triggered by changes in temperatures and redox conditions, which were caused by mixing of magmatic fluids and meteoric water. Results in this study provide an example of tracing the link between Mo mineralization and magmatism using in situ S isotopic compositions of molybdenite and Pb isotopes of pyrite and K-feldspar.

Apatite records metamorphic and hydrothermal fluid evolution at the large Shuangqishan orogenic gold deposit, SE China
Ying Ma; Shao-Yong Jiang; Hartwig E. Frimmel
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.

Southward continuation and slab rollback of the Neotethyan arc−back-arc system: Insights from Eocene mafic intrusions from North Sumatra, SE Asia
Xiaoqing Yu; Yuzhi Zhang; Xin Qian; Azman A. Ghani; Thomas C. Sheldrick ...
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.

Ultrahigh-pressure mantle metasomatism in continental collision zones recorded by post-collisional mafic rocks
Mingdi Gao; Haijin Xu; Stephen F. Foley; Junfeng Zhang; Yu Wang
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.

Petrogenesis and tectonic setting of Neoarchean tonalitic-trondhjemitic-granodioritic gneisses in the Xiwulanbulang area of the Yinshan block, North China craton
Xiao Wang; Jian Zhang; Qian Liu; Hai Zhou; Changqing Yin ...
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/Yb N (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 initial87Sr/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 208 Pb/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 favored for the formation of these Neoarchean TTG rocks in the Yinshan block.

Episodic magmatism in the Lianhuashan tectonic belt: Implications for late Mesozoic crustal reworking in SE South China
Yujia Xin; Jianhua Li; Yueqiao Zhang; Shuwen Dong
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.

Orbital and sub-orbital pacing of mudstones in the Dongying Depression, eastern China: Implications for middle Eocene East Asian climate evolution
Xiangxin Kong; Zaixing Jiang; Yuan Cai
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.

Two stages of porphyry Cu mineralization at Jiru in the Tibetan collisional orogen: Insights from zircon, apatite, and magmatic sulfides
Xiang Sun; Jun Deng; Yongjun Lu; Xiaobo Si; Pete Hollings ...
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 SO 3 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 SO 3 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.

Fossil divergent double-subduction zone in the Great Xing’an Range, NE China: Evidence from a deep seismic reflection profile
Xiao-Miao Tan; Rui Gao; Jian-Bo Zhou; Simon A. Wilde; He-Sheng Hou ...
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.

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
Yinbiao Peng; Shengyao Yu; Sanzhong Li; Yongjiang Liu; Pei Lv ...
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.

Large detrital zircon data set investigation and provenance mapping: Local versus regional and continental sediment sources before, during, and after Ancestral Rocky Mountain deformation
Tyson M. Smith; Joel E. Saylor; Tom J. Lapen; Ryan J. Leary; Kurt E. Sundell
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. 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.

Bifurcation of mantle plumes by interaction with stagnant slabs in the mantle transition zone: Evidence from late Cenozoic basalts within Southeast Asia
Fan Yang; Xiao-Long Huang; Yi-Gang Xu; Peng-Li He
Mantle plumes can encounter stagnant slabs in the mantle transition zone (MTZ) during their ascent from the lower mantle to Earth’s surface. However, the interactions between stagnant slabs and mantle plumes remain poorly understood. This study presents new data on the spatial thermal−lithological−chemical heterogeneity in the mantle over a vast area of Southeast Asia that reflects the variable influence of a late Cenozoic mantle plume. Site U1433 of International Ocean Discovery Program (IODP) Expedition 349 in the southwest sub-basin of the South China Sea (SCS) is located near the center of this seismically imaged mantle upwelling. This area also contains mid-ocean ridge basalt (MORB) derived from a peridotite-dominated and normal temperature mantle source with a depleted Nd-Hf and enriched 207Pb/206Pb isotopic composition. In contrast, MORB from Site U1431 of IODP Expedition 349 within the eastern sub-basin of the SCS and other intraplate basalts from the Hainan-Leizhou area, the eastern Indochina block, and elsewhere in the SCS are dominantly sourced from higher temperature pyroxenite-enriched mantle material that records mantle plume activity. These observations are in contrast with previous models involving a single plume rising from the lower mantle beneath the SCS that then tilted toward Hainan Island, China. Instead, it is likely that the observations from this region are indicative of plume-stagnant slab interaction. The shallow upper mantle beneath Site U1433 has not been influenced by a mantle plume as the presence of stagnant slabs within the MTZ prevented further upwelling of the plume. In contrast, a mantle plume passed through the weaker MTZ within the eastern sub-basin and circum-SCS areas, allowing plume-related magmatism to occur. The bifurcation of a deep-rooted mantle plume by stagnant slabs in the MTZ produced a cluster of small mantle upwellings that ascended to shallow depths, consistent with new multiscale global tomographic data for Southeast Asia.

Genesis and source affinities of heterogeneous ultramafic rocks in the North Kongling Complex, Yangtze craton: Architecture of a Paleoproterozoic accretionary orogen
Hongtao Peng; Hao Deng; Qingsen Han; Songbai Peng; Ali Polat ...
Ultramafic massifs are minor but meaningful components of orogens because they record the mantle properties and the processes of plate generation, subduction, accretion, and collision, thus providing key information about crust-mantle interaction processes and the evolution of orogens. Here, we present an integrated study of field observations, whole-rock and mineral geochemistry, zircon U-Pb ages, and whole-rock Sm-Nd and Re-Os isotopes for serpentinized harzburgites and olivine pyroxenites from the Paleoproterozoic Shuiyuesi mélange belt, North Kongling Complex, to constrain the tectonic evolution of the Yangtze craton, South China. The serpentinized harzburgites are characterized by enrichment of light rare earth elements (LREEs) and negative εNd(t) (−4.3 to −3.9) values, but they have high olivine Mg# (91.7−93.1) and very low Re contents (<0.09 ppb) and negative γOs(t) (−5.1 to −4.0) values. The Archean Re-Os model ages (TRD = 2.82 Ga) of the harzburgites that have the most depleted Re-Os isotope composition represent estimates for the age of the initial melt extraction. Metasomatic zircons from the serpentinized harzburgites yield a weighted average age of ca. 2.04 Ga, which is interpreted to represent the age of metasomatism during a subduction event. These geochemical and isotopic features and geochronologic results suggest that the protolith of the serpentinized harzburgites in the North Kongling Complex originated as fragments of late Mesoarchean (ca. 2.82 Ga) subcontinental lithospheric mantle and experienced melt/fluid metasomatism in a subarc mantle wedge at 2.04 Ga. The olivine pyroxenites are cumulates, and they are characterized by high MgO, Ni, and Cr contents and depleted to slightly enriched LREEs but depletion in high field strength elements (HFSEs). They have positive ε Nd(t) (+2.0 to +6.6) values and Paleoproterozoic Nd model ages (2.41−2.05 Ga), suggesting that the olivine pyroxenites were derived from a Paleoproterozoic depleted mantle source. Metamorphic zircons from the olivine pyroxenites indicate subsequent metamorphism at 2.0 Ga, in response to a collision. The olivine pyroxenites together with the metabasites in the Shuiyuesi mélange belt represent the dismembered fragments of oceanic lithosphere that formed in a suprasubduction zone during the Paleoproterozoic. Combined with the Archean and Paleoproterozoic rocks in the North Kongling Complex, it appears that the Mesoarchean subcontinental lithospheric mantle peridotites and Paleoproterozoic ophiolitic rocks were simultaneously accreted during a Paleoproterozoic orogenic event. We further propose that a subduction channel may have played an important role in the architecture of the accretionary orogen, in which both oceanic lithospheric materials and subcontinental lithospheric mantle wedge materials would have been simultaneously scraped off and accreted during slab subduction-accretion.

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For Immediate Release
29 March 2023
GSA Release No. 23-19

Justin Samuel