We present new laser ablation (LA)‐multicollector‐inductively coupled plasma mass spectrometry detrital zircon U‐Pb age and Hf isotope data of the Triassic sedimentary rocks from the eastern and ...western sides of the Ailaoshan suture in SW China, which separate the Indochina and South China blocks. Detrital zircon grains of the Lower‐Middle Triassic sequences from western South China show single age population of 280–237 Ma (peak at 254–251 Ma) with mainly positive εHf(t) values (+0.15 to +9.11, ~60%). The Middle Triassic samples from eastern Indochina also contain single age population of 273–236 Ma (peak at 246–242 Ma), but with mainly negative εHf(t) values (−18.95 to −1.83, ~88%). The unimodal detrital zircon age pattern and Hf isotope features suggest that the Permian‐Triassic magmatic arcs had supplied detritus for the Lower‐Middle Triassic sequences on both sides of the Ailaoshan suture. In contrast, the Upper Triassic sequences on the either side of the suture display similar polymodal age populations at 290–230, 500–400, 850–700, 1000–900, 2000–1800, and 2600–2400 Ma. This indicates a dramatic change in detrital provenance across the Middle‐Late Triassic boundary. We interpret this change to be resulted from the closure of the Paleotethyan Ailaoshan Ocean in the early Late Triassic. The ocean closure and consequent Indochina‐South China continent‐continent collision may have connected the drainage systems of the two blocks, leading to the marked similarities in their post‐Middle Triassic sedimentary provenance.
Key Points
Lower‐Middle Triassic sedimentary rocks received detritus from Permian‐Triassic magmatic arcs
Upper Triassic sedimentary rocks received detritus from both the Indochina‐South China interior and the Permian‐Triassic magmatic arcs
Final closure of the Ailaoshan Ocean was likely completed by the Late Triassic
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Interactions between ocean plate subduction and the nearby mantle plume represent a possible major drive for mantle heterogeneity. We first report Late Permian (~261 Ma) Nb‐enriched basalts (NEBs) in ...the Ailaoshan region. The Ailaoshan NEBs are characterized by high Nb (10.1–11.5 ppm) and Nb/La (1.2–1.3), positive Nb‐Ta anomalies and εNd(t) (+5.65 to +6.21), and low and mantle‐like zircon δ18O (5.28–5.75‰). Our results demonstrate that the source region contains an enriched mantle component, which may have derived from the nearby Emeishan plume. The Ailaoshan NEBs were possibly generated by mixing of subduction‐derived, mid‐ocean ridge basalt (MORB)‐like components and mantle plume‐derived, enriched components. We suggest that the Late Permian rollback of the Ailaoshan oceanic slab may have entrained the Emeishan plume material into the subduction zone, which caused compositional heterogeneity in the mantle wedge and formed the Ailaoshan NEBs.
Plain Language Summary
The plume‐subduction interaction is a special and very important event in the modern history of our planet Earth. Such interaction could be one of the major mechanisms for present mantle heterogeneity. As for the behavior of mantle plumes near ancient subduction zones is still a mystery. The Paleo‐Tethyan Ailaoshan Ocean may have experienced late Paleozoic subduction beneath the South China Block, which may be influenced by the Emeishan mantle plume to the east. However, volcanic arc rocks outcropped in the subduction zone tended to be metasomatized by the subduction components, which made it difficult to trace their source compositions. We have recently identified the Late Permian Nb‐enriched basalts in the Ailaoshan region, which have little subduction component inheritance. The bulk‐rock geochemical, Sr‐Nd and zircon O isotopic data indicate that the Emeishan plume‐derived enriched component existed in their source region. Considering the Permian regional extension in South China Block, we suggest that the Ailaoshan oceanic slab rollback‐induced counterflow drove westward flow of Emeishan plume material resulted in the entrainment of enriched component around the subduction zone. This study implies that the ancient plume‐subduction interaction may have played an important role in the mantle heterogeneity.
Key Points
Ailaoshan Nb‐enriched basalts (~261 Ma) were formed coeval with Emeishan plume activity
The basaltic magma may have generated by mixing between depleted MORB‐type and enriched Emeishan plume mantle components
Ailaoshan oceanic slab rollback may have driven the entrainment of Emeishan plume material into the mantle wedge above the subducted slab
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Rationale
In situ trace element analysis of melt inclusions by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) provides important geochemistry information. However, the ...precision and accuracy of this technique are affected by many factors, such as matrix effect, laser conditions, and calibration method. In addition, many previous LA‐ICP‐MS studies ablated entire melt inclusions along with their host minerals and obtained trace element composition by deconvoluting the mixed ablation signal, which may induce much uncertainty.
Methods
A 193 nm ArF laser ablation system combined with inductively coupled plasma sector field mass spectrometry (ICP‐SF‐MS) was used to investigate matrix effect, laser conditions, choice of external calibration standards, and data reduction strategy for in situ analysis of 36 major and trace elements in six common silicate reference glasses. The validity of the protocol presented here was demonstrated by measuring trace elements in olivine‐hosted melt inclusions. Instead of ablating entire melt inclusions along with their host minerals, melt inclusions were polished to the surface to avoid laser ablating the mineral host.
Results
The calibration lines calculated from the calibration standards should cross the coordinate origin, especially for low‐concentration elements (<10 ppm). As the laser crater size increased from 17 to 33 μm, the precision was improved from <20% to <8% (2RSD), and accuracy was improved from ±20% to better than ±10%. Most measured trace elements in Dali melt inclusions are consistent with those in their host rocks. For mobile elements (Ba, Sr, Pb), melt inclusions display much smaller variations than their host rocks.
Conclusions
A simple but accurate approach for in situ analysis of trace elements in melt inclusions by LA‐ICP‐SF‐MS has been established, which should greatly facilitate the wider application of in situ trace element geochemistry to melt inclusion studies.
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Silicic magmas are the most viscous of all magmas, however some granitic plutons display remarkably homogeneous compositions, which contradicts the hypothesis that mechanical mixing is the main ...homogenizing process in the magma chamber. Thus much remains controversial about the mechanisms responsible for the homogeneities of silicic plutons. Here, we present textural observations, elemental mapping, and in situ elemental and Nd‐O isotopic data of apatites from the compositionally homogeneous Late Permian Yuanyang A‐type granitic pluton (SW Yunnan, South China). Apatite grains display oscillatory chemical zonation and resorption‐precipitation texture, suggesting incremental growth dominated by co‐genetic magma batches injection. The intra‐/inter‐grain core to rim elemental and Nd‐O isotopic variations imply crystal transfer and crystallization from different melt domains within the crystal mush. We propose that rejuvenation events associated with hotter cogenetic intermediate magma batch injection has induced crystal mush reactivation and convective stirring in silicic magma chambers, thereby homogenizing the entire reservoir.
Plain Language Summary
Magma mixing between mafic and viscous felsic magmas is usually incomplete, forming heterogeneous structures and geochemical compositions in felsic magma bodies. However, some silicic magma bodies have homogeneous compositions and structures, such as the Late Permian Yuanyang A‐type granites (YAGs) in southwestern South China, and the actual homogenizing process in silicic magma chamber is still unclear. Accessory apatite is ubiquitous in felsic plutons, whose growth zoning could reflect magma chamber evolution. Apatite grains collected from the YAGs show complex resorption‐precipitation texture. Their core‐rim oscillatory elemental and Nd‐O isotopic variations suggest compositional perturbations of ambient magma, probably caused by the injection/mixing of multiple magma pulses. Accordingly, we propose that hot magma recharge could reactivate the crystal‐mush, causing magma convection and stirring that gradually homogenized the silicic magma chamber.
Key Points
The apatites from the compositionally homogeneous Yuanyang A‐type granites are texturally and geochemically heterogeneous
The identified four apatite zoning domains documented magma replenishment, mixing and crystal mush reactivation
Magma convection and stirring induced by magma batches injection are key to homogenizing the silicic magma chamber
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The geometric transformation of a descending plate, such as from steep to flat subduction in response to a change from normal to overthickened oceanic crust during subduction, is a common and ...important geological process at modern or fossil convergent margins. However, the links between this process and the metamorphic evolution of the exhumation of oceanic (ultra)high‐pressure eclogites are poorly understood. Here we report detailed petrological, mineralogical, phase equilibria, and secondary ion mass spectrometry zircon and rutile U‐Pb age data for the Dong Co eclogites at the western segment of the Bangong‐Nujiang suture zone, central Tibet. Our data reveal that the Dong Co eclogites experienced peak eclogite‐facies metamorphism (T = 610–630°C, P = 2.4–2.6 GPa) and underwent multiple stages of retrograde metamorphism. P‐T pseudosections and compositional isopleths of garnet define a complex clockwise P‐T‐t path (including two stages of decompression‐dominated P‐T path and one of isobaric heating), suggesting varying exhumation velocities. Combining previous studies with our new results, we suggest that the transformation from rapid to slow exhumation is dominated by the transition from steep to flat subduction. The flat‐slab segment, caused by subduction of buoyant oceanic plateau, led to an extremely slow exhumation and a strong overprinting of HP granulite facies at a depth of ~50 km at ~177 Ma. The slab rollback that followed in response to a substantial density increase of the eclogitized oceanic plateau resulted in another rapid exhumation process at ~168 Ma and triggered the formation of abundant near‐simultaneous or later magmatic rocks.
Key Points
HP‐UHP eclogites from western segment of the Bangong‐Nujiang suture zone, central Tibet
Multistage retrograde metamorphism, phase equilibria, and SIMS zircon and rutile U‐Pb ages
Granulite facies overprinting during the exhumation of eclogites linked to flat‐slab subduction
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The Cenozoic Sulu Sea arc‐basin system is situated in the tectonic junction between the South China Sea (SCS), northern Borneo, Palawan Continental Terrane, Philippine Mobile Belt, and Celebes Sea. ...We compare new/published geochronological and geochemical data from across the circum‐Sulu Sea region, and summarize seven major magmatic phases from the Middle Eocene to Pleistocene. The Middle Eocene (42.65 Ma) Sabah ophiolite and Eocene‐Oligocene (34‐33 Ma) Central Palawan ophiolite have MORB‐IAT‐transitional features, representing an intraoceanic subduction setting in the Paleogene northern Borneo and central‐southern Palawan. After the SCS opening (∼32 Ma) and ridge jump (∼25 Ma), late‐stage Proto‐SCS subduction (24‐21 Ma) may have formed the Panay arc andesite and the BABB magmatism in SW Zamboanga peninsula. Starting of final convergence between the Palawan Continental Terrane and northern Borneo‐SW Philippines (∼21 Ma) likely caused regional uplift/thrusting, forming the Top Crocker Unconformity and triggering the NW‐dipping Celebes Sea subduction. The subduction may have formed arc magmatism (21‐18 Ma) in the Cagayan ridge and its continuation in Panay and NE Sabah, and opened the NW Sulu Sea back‐arc basin through continental crust attenuation. Subduction rollback likely occurred in 17‐14 Ma and 13‐9 Ma, shifting arc magmatism southeast to the Sulu ridge and opening the SE Sulu Sea back‐arc basin. NW‐dipping Celebes Sea subduction largely ceased after ∼9 Ma, followed by extension‐related uplift/exhumation and 4‐0.2 Ma intraplate volcanism in northern Borneo. SE‐dipping Sulu Sea subduction likely occurred along the Negros‐Sulu trenches, and produced arc volcanism from ∼4 Ma.
Plain Language SummaryThe Sulu Sea and the adjacent sea basins (e.g., Celebes and SCS) are situated at the junction between the Eurasian, Philippine Sea, and Indo‐Australian plates. The opening and closure (when Australia‐Eurasia eventually collide) of these basins represent the final tectonic episode of the Neo‐Tethys, an ocean that separates the northern and southern continents. When and how these sea basins were created are long disputed. Here we present a regional tectonic reconstruction model by compiling new/published radiometric age and chemical data of major igneous suites from across the circum‐Sulu Sea region. Our model suggests that the Sulu Sea was formed by the NW‐dipping Celebes Sea subduction at ∼21 Ma, in response to the collision between the South China‐derived Palawan Continental Terrane and northern Borneo‐SW Philippines. The Sulu Sea basin may have continued to expand till ∼9 Ma, when NW‐dipping subduction of the Celebes Sea stopped. Afterward, to the west of the Sulu Sea, within‐plate extension in northern Borneo occurred and continues to the present‐day; whereas to the east, subduction of the Sulu Sea may have occurred along the Negros‐Sulu trenches, and produced arc volcanism from ∼4 Ma.
Key PointsAge and geochemical data compilation for the circum‐Sulu Sea region summarized 7 Cenozoic (Mid Eocene to Pleistocene) magmatic phasesAfter Palawan Continental Terrane accretion, Sulu Sea back‐arc basin was opened by Celebes Sea subduction (∼21 Ma) and rollback (17‐9 Ma)Northern Borneo underwent intraplate extension and magmatism after ∼9 Ma, and the Sulu Sea subducted along Sulu‐Negros trench from ∼4 Ma
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The Palaeozoic magmatic history of Central Iran is related to the subduction of the Proto‐Tethys Ocean and the amalgamation of Gondwana continental fragments. This study presents whole‐rock ...geochemistry, Sr–Nd isotope and zircon U–Pb–Hf ages of shoshonitic plutonic rocks (e.g., monzonite–syenite) from the north Posht‐e‐Badam (NPB) district with a view to constrain their petrogenetic and tectono‐magmatic evolution during the Palaeozoic. The geochemical data show moderate enrichment in large‐ion lithophile elements and Ti, large positive Pb anomalies, and depletion or moderate enrichment in high‐field‐strength elements (e.g., Nb and Ta), typical of continental arcs. Zircon LA–ICP–MS U–Pb ages are in the range of 444–428 Ma. The Sr–Nd–Hf isotope data, for example, (87Sr/86Sr)i = 0.7073 to 0.7089, εNd(t) = −1.4 to −0.2, and εHf(t) = −3.6 to +7.7, indicate derivation of the magma from a mantle source. The numerical modelling shows a contribution of less than 10% crustal assimilation and ~3% sediment into the melt‐derived mantle. We propose that metasomatic enrichment occurred during the subduction of Proto‐Tethyan Ocean between the Iranian microcontinents, leading to the formation of a shoshonitic melt. We infer that the recorded ages coincide with a continental rift episode followed by a rear arc tectonic region that spread along the northern active margin of Gondwana. Further, the change in magma affinity from calc‐alkaline (~474–512 Ma) to shoshonitic (444–428 Ma) in Central Iran is probably related to the gradual steepening of the slab dip angle and trench‐ward migration of the front arc into the region above metasomatically enriched rear arc mantle.
Schematic model for tectono‐magmatic evolution of the mantle beneath Central Iran. A contribution of 10% crustal components and 3% sediment into the melt‐derived mantle for 447–426 Ma age of the shoshonitic rocks. Change in affinity from calc‐alkaline to shoshonitic is related to the gradual steepening of the slab dip angle and trench‐ward migration of the front arc into rear arc mantle.
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Secondary ion mass spectrometry (SIMS) measurement of oxygen isotopes in apatite has been employed more and more in petrogenetic, metallogenic, and climate change studies. Well‐characterised ...reference materials are needed due to the matrix effect, but they are yet to be well established. In this study, we conducted in‐situ oxygen isotopic and chemical analyses on six commonly used apatite reference materials (ie, Emerald, Kovdor, McClure, Mud Tank, Otter Lake, and Slyudyanka) and two in‐house apatite references (Qinghu and GEMS 203) to assess their oxygen isotope homogeneity and applicability for microbeam analyses. Our results show that all these apatite references are in general chemically homogeneous. In terms of oxygen isotopes, GEMS 203 (δ18O = 9.85 ± 0.40‰ 2SD, corrected by Durango 3), Kovdor (δ18O = 6.55 ± 0.38‰, 2SD), and McClure (δ18O = 5.94 ± 0.42‰, 2SD) are fairly homogeneous, whereas Emerald (δ18O = 10.37 ± 0.45‰, 2SD), Mud Tank (δ18O = 6.35 ± 0.46‰, 2SD), Otter Lake (δ18O = 9.71 ± 0.47‰, 2SD), Qinghu (δ18O = 5.44 ± 0.49‰, 2SD), and Slyudyanka (δ18O = 17.49 ± 0.43‰, 2SD) are less homogenous. This indicates that the former group represents better reference materials for in‐situ oxygen isotopic analyses, whilst the latter group can be used as secondary reference material for analytical quality control.
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A new natural zircon reference material SA01 is introduced for U‐Pb geochronology as well as O and Hf isotope geochemistry by microbeam techniques. The zircon megacryst is homogeneous with respect to ...U‐Pb, O and Hf isotopes based on a large number of measurements by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ion mass spectrometry (SIMS). Chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA‐ID‐TIMS) U‐Pb isotopic analyses produced a mean 206Pb/238U age of 535.08 ± 0.32 Ma (2s, n = 10). Results of SIMS and LA‐ICP‐MS analyses on individual shards are consistent with the TIMS ages within uncertainty. The δ18O value determined by laser fluorination is 6.16 ± 0.26‰ (2s, n = 14), and the mean 176Hf/177Hf ratio determined by solution MC‐ICP‐MS is 0.282293 ± 0.000007 (2s, n = 30), which are in good agreement with the statistical mean of microbeam analyses. The megacryst is characterised by significant localised variations in Th/U ratio (0.328–4.269) and Li isotopic ratio (−5.5 to +7.9‰); the latter makes it unsuitable as a lithium isotope reference material.
Keypoints
The homogeneity of the SA01 zircon megacryst was assessed by microbeam techniques for U‐Pb geochronology, and Li, O and Hf isotope geochemistry.
SA01 is homogeneous in terms of U‐Pb, O and Hf isotopes.
SA01 is unsuitable as a lithium isotope reference material.
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•A Matlab program named “IsotopeMaker” was created for convenient isotopic data reductions.•IsotopeMaker can process Sr, Nd and Hf isotope data in LA-MC-ICP-MS mode.•IsotopeMaker provides a fast and ...convenient mass bias correction for double spike data.
Mass fractionation during isotope measurement by mass spectrometer hinders the acquirement of true isotopic composition and tedious and complex mathematical calculations are required to correct for mass bias. This study provides a Matlab program named “IsotopeMaker” for convenient isotopic data reductions. IsotopeMaker has two main aspects of application. For in situ LA-MC-ICP-MS isotopic analyses, IsotopeMaker can process Sr, Nd and Hf isotope data. For double spike data, IsotopeMaker provides a fast and convenient mass bias correction for all elements that have at least four isotopes. The visualization windows of IsotopeMaker make it easy to operate. Isotopic Sr, Nd and Hf data of a modern coral, a reference zircon and an in-house apatite and Mo isotopic data measured using the double spike method have been used to test the program capability.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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