SHORT EPISODES OF CRUST GENERATION DURING PROTRACTED ACCRETIONARY PROCESSES: EVIDENCE FROM CENTRAL ASIAN OROGENIC BELT, NW CHINA

continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (~540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ~400 Ma and 280 Ma for the Altai segment, and ~430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (~20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting.Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (~540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ~400 Ma and 280 Ma for the Altai segment, and ~430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (~20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting

Short episodes of crust generation during protracted accretionary processes

This study was supported by funding from the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant nos. XDB03010600 and XDB18020204), the National Natural Science Foundation of China (grant nos. 41202041, 41630208 and 41673033), the Key Program of the Chinese Academy of Sciences (QYZDJ-SSW-DQC026), the DREAM Program of China (No. 2016YFC0600407), talent project of Guangdong Province (2014TX01Z079), and GIG–CAS 135 project 135TP201601. PAC and CJH acknowledge support from the Natural Environment Research Council (grant NE/J021822/1). The senior author thanks the grant from the NSC, Taiwan, which supported his one-year academic visit at the NTU. This is contribution no. IS-2352 from GIG–CAS.Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (∼540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ∼400 Ma and 280 Ma for the Altai segment, and ∼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (∼20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting. The Junggar segment is characterized by high and uniform Nd–Hf isotope ratios (εNd(t) = +5  to  +8; zircon εHf(t) = +10  to  +16) and it appears to have formed in an intra-oceanic arc system. In the Altai and Chinese Tianshan segments, the Nd–Hf isotope ratios (εNd(t) = −7  to  +8; zircon εHf(t) = −16  to  +16) are lower, although they increase with decreasing age of the rock units. The introduction of a juvenile component into the Chinese Tianshan and Altai granitic rocks appears to have occurred in continental arc settings and it reflects a progressive reduction in the contributions from old continental lower crust and lithospheric mantle. Within the long-lived convergent margin setting (over ∼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of ∼20–40 Ma. These intervals were associated with higher Nb/La ratios, coupled with lower La/Yb ratios, in both the mafic and granitic rocks, and these episodes of increased magmatism from intraplate-like sources are therefore thought to have been in response to lithospheric extension. The trace element and Nd–Hf isotope data, in combination with estimates of granitic magma volumes, highlight that crust generation rates are strongly non-uniform within long-lived accretionary orogens. The estimated crust generation rates range from ∼0.1 to ∼40 km3/km/Ma for the Paleozoic record of the CAOB, and only comparatively short (20–40 Ma) periods of elevated magmatic activity had rates similar to those for modern intra-oceanic and continental arcs.PostprintPeer reviewe

2.8 Ga Subduction-related magmatism in the Youanmi Terrane and a revised geodynamic model for the Yilgarn Craton

New studies in the central and southern parts of the Murchison Domain in the Youanmi Terrane, western Yilgarn Craton, reveal previously unidentified igneous rock types in addition to boninitic occurrences similar to those found in the northern part of the domain. New results also allow for a re-assessment of a volcanic suite previously described as examples of a “Karasjok type” of komatiite. The rocks are most plausibly examples of arc picrites, specifically of the type found in ophiolite settings. In addition to boninitic rocks and picrites, this study identified remobilized olivine-cumulate rich lavas that resemble examples found in the Troodos and Othris ophiolites. Examples of REE enriched high magnesium andesites exhibit pronounced high field strength element depletions on normalized plots combined with mantle-like Nb/Zr ratios. The rocks chemically resemble Phanerozoic examples of enriched boninites and their signatures are distinct from those typically attributed to crustal contamination of mantle plume magmas. Intrusions of the ∼2792 Ma Warriedar Suite have compositions analogous to those of post-Archean Alaskan Intrusive complexes and were likely derived from subduction-modified sub-cratonic mantle. The new observations can be accounted for in a geodynamic scenario involving subduction along the western Yilgarn margin at ∼2800 Ma. Similarities with the contemporaneous Superior Province raise the possibility that the nuclei of the two cratons were closely associated at this time. © 2019 Elsevier B.V.Australian Research Council-LP13010072

Crustal maturation through chemical weathering and crustal recycling revealed by Hf-O-B isotopes

Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U-Pb and Hf-O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (similar to 490-270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with epsilon(Hf)(t) values from +10.6 to +14.4. Zircon delta O-18 values and whole-rock delta B-11 values, however, are highly variable. The Silurian - Carboniferous (pre-300 Ma) rocks have distinctly lower zircon delta O-18 values ranging from 5.16 to 6.72 parts per thousand with an average of 5.78 parts per thousand, and high delta B-11 values (-7.5-+12.2 parts per thousand), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon delta O-18 values (8.24 to 10.29 parts per thousand) and lower delta B-11 values (-9.0 to -12.2 parts per thousand), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf-O-B isotope mixing calculations indicate a contribution of more than similar to 50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time. (C) 2019 Elsevier B.V. All rights reserved

Petrogenesis of Triassic granite from the Jintan pluton in Central Jiangxi Province, South China: implication for uranium enrichment

Numerous Triassic biotite granites and two-mica granites crop out in the interior of South China, and some of them possess high U contents, which have been regarded as the sources for later hydrothermal mineralization. Their petrogenesis is therefore crucial for constraining the possible origins of the U enrichment. Here we report new LA-ICPMS zircon U-Pb ages, mineral geochemistry of biotite and muscovite, whole rock geochemical results and Sr-Nd and zircon Hf isotope data from the Jintan pluton in Central Jiangxi Province, South China. LA-ICPMS zircon U-Pb dating indicates that both biotite granite (BG) and two-mica granite (TMG) in the Jintan pluton crystallized at similar to 220 Ma. The TMG have higher U contents (7.85 to 48.90 ppm, average 18.44 ppm) than theBG (4.99 to 17.72 ppm, average 8.64 ppm). Both BG and TMG show negative whole-rock epsilon(Nd)(t)and zircon epsilon(Hf)(t)values and contain some inherited zircons. The TMG are strongly peraluminous (A/CNK = 1.13-133), contain abundant primary muscovite, and display S-type affinity on plots of Yvs Rb and Th vs Rb, suggesting that they are S-type granites. The BG also display S-type granite affinities on plots of Yvs Rb and Th vs Rb. The suites display similar Sr-Nd isotope compositions (BG initial Sr-87/(86) Sr values = 0.711389 to 0.714225 and E-Nd(t) = -9.91 to -9.16, TMG initial Sr-87/(86) Sr values = 0.711832 and epsilon(Nd)(t) = -10.02) and are spatially associated, suggesting that the BG should also be classified as S-type granites. The TMG have higher zircon epsilon Hf(t)values (-6.4 to -1.1) than the BG (-8.7 to -3.7), indicating the TMG and BG might be derived from similar sediments but possibly with some distinct characteristics in their sources. The BG exhibit linear covariations in chemical compositions with relatively high total REE and light REE contents and MgO contents, while the TMG displays broader compositional variations but with relatively low total REE, light REE and MgO contents. Biotite geochemistry indicates the TMG formed in a more reduced magmatic system than the BG. The temperatures estimated by zircon saturation thermometry indicate the BG had distinctly higher magmatic temperatures than the TMG. The TMG display relatively high Al2O3/TiO2 ratios and low CaO/Na2O ratios than the BG but have higher Sr/Y and La/Yb ratios. The geochemical and petrological data suggest the BG were derived from clay-poor psammite sources at deeper levels with higher temperatures and higher oxygen fugacity, and underwent an extensive fractional crystallization, while the TMG was derived from clay-rich pelitic sources at higher levels and lower temperatures and oxygen fugacity with only limited fractional crystallization. We conclude that the combination of U-rich sources, physical-chemical conditions such as low partial melting temperature or low degrees of partial melting, a reduced environment and low REE and LREE contents of magmas controlled the U enrichment in TMG. (C) 2018 Elsevier B.V. All rights reserved

Magnesium and Calcium Isotopic Geochemistry of Silica‐Undersaturated Alkaline Basalts: Applications for Tracing Recycled Carbon

Abstract Carbon plays important roles in the evolution of the atmosphere and biosphere, and geochemical differentiation in Earth's interior. Most subducted C is recycled to the deep mantle and returns to the surface by degassing from erupted basalts and associated fluids. The Mg–Ca isotopic systems have been widely used in tracing recycled C. However, the idea that these geochemical proxies truly reflect the deep C cycle has been challenged. Here we present whole‐rock geochemical and Mg–Ca isotopic compositions of Miocene silica‐undersaturated alkaline basalts of the Western Qinling orogen, China. These alkaline basalts are associated with carbonatites and are characterized by low SiO2 (39.0–43.2 wt.%) and Al2O3 (6.98–9.15 wt.%) contents, high CaO/Al2O3 ratios (1.4–1.8), and positive Nb–Ta and negative Pb–Zr–Hf–Ti anomalies, suggesting they were derived by partial melting of carbonatite‐metasomatized asthenospheric mantle. The studied samples have δ26Mg values of −0.24‰ to −0.44‰, ranging from mantle‐like values (δ26Mg = −0.25‰ ± 0.07‰) to lower values. This implies that carbonatite metasomatism does not always produce low‐δ26Mg anomalies. The samples have δ44/40Ca values (0.59‰–0.77‰; relative to the standard SRM915a) that are lower than Bulk Silicate Earth (0.94‰ ± 0.05‰), which are attributed to the involvement of low δ44/40Ca recycled carbonate in the mantle source. We suggest that the shift in mantle δ26Mg values during carbonatite metasomatism is controlled by the type and amount of carbonatite involved, while Ca isotope variations depend largely on the δ44/40Ca values of subducted carbonates. Mg or Ca isotopes alone, however, may not be sufficient to track the deep carbon cycle

Platy pyroxene: New insights into spinifex texture

New evidence has emerged for a different type of platy spinifex texture that has not previously been documented in the existing literature, in this case from 2.8 Ga high-Mg basalts in the Murchison Domain of the Yilgarn Craton, where petrographic and geochemical evidence shows that the dominant platy mineral is pyroxene, rather than olivine. In our samples, two scales of plates are evident. Larger plates have lengths and widths that are approximately equal and range from ~1000 to 15 000 mm, with thicknesses typically ≲120 mm. These plates have ≲25 mm thick augite rims, and cores that are now a mixture of low-temperature hydrous alteration minerals. They occur in sets of similarly oriented crystals, and typically intersect other sets of crystals at oblique angles. A second population of smaller augite-only plates occur within the interstices of the larger plates; they have lengths and widths that range from 200 to 1500 mm, and thicknesses that are typically ≲50 mm. Pyroxene dendrites are also a typical component of this texture and represent a third scale of crystal growth, which probably crystallized shortly before the remaining liquid quenched to glass. All scales of pyroxene contained within this texture exhibit skeletal features and are considered to have crystallized rapidly. We discuss possible conditions that led to the crystallization of platy habits instead of the typical acicular ones. The exposed volcanic sequence in our study area is volcanologically similar to other Archean komatiites, such as those from the 2.7 Ga Abitibi greenstone belt, for example, and has probably experienced a similar cooling history; however, apart from having similar textures, we cannot demonstrate a komatiitic association. Liquid compositions, estimated from chilled flow margins, are distinctly lower in MgO (14.4-15.8wt %) and higher in SiO2 (50.9-52.1wt %) than those for most platy olivine spinifex-textured komatiites; from these compositions, we calculate dry liquidus temperatures of 1312-1342°C and mantle potential temperatures of 1440-1480°C. On the basis of these temperatures we question whether a mantle plume is a necessary element of their petrogenesis. 'Platy olivine spinifex' is an igneous texture that characterizes komatiites and its observation in outcrops or drill core (typically prior to, or in lieu of chemical analysis) leads geologists to classify a rock as a komatiite. Field descriptions may therefore drive assumptions and interpretations surrounding the prevailing tectonic or geodynamic setting at the time of emplacement. We emphasize the importance of careful discrimination between a variety of spinifex textures within a local volcanological framework and caution against the habit of making direct interpretations of rock type based on the existence of spinifex textures alone. © The Author 2017.British Geological Survey, Australian Research Council, University of Sydney, University of Western Australia, Western Universit

Crust-mantle mixing and crustal reworking of southern Tibet during Indian continental subduction: Evidence from Miocene high-silica potassic rocks in Central Lhasa block

Collisional zones are commonly considered as important regions for crustal reworking, but the reworking mechanism remains debated. The well-known Himalayan-southern Tibetan orogen, built by India-Asia collision and convergence, has the thickest continental crust on Earth and is therefore an ideal region for studying crustal reworking during collisional orogenesis. Here we revisit the Miocene high-silica potassic rocks (trachytes) in the Konglong area of the central Lhasa block, southern Tibet. Integrated studies of geochronology, mineral compositions, bulk-rock major- and trace-element geochemistry, and Sr-Nd-Pb-Hf-O isotopes unequivocally indicate that the Konglong trachytes formed by mixing between enriched mantle-derived ultrapotassic and thickened ancient crust-derived magmas. Combined with post-collisional magma mixing recently identified in the southern Lhasa block, we suggest that magma underplating and subsequent mantle-crust interaction (i.e., the matter and energy transfer from the mantle to the crust) has been a common and important crustal reworking process in southern Tibet during Indian continental subduction. This process may be related to Indian plate flat subduction and subsequent foundering during the post-collisional stage. In combination with the nature of Cenozoic magmatism in the Himalaya block, we suggest that in addition to partial melting of the subducted continental crust, magma underplating and subsequent crust-mantle mixing beneath the obducted continent has also played an important role in crustal reworking of the collisional zone. (C) 2019 Elsevier B.V. All rights reserved