Ages and Hf isotopes of igneous zircons from Neoarchean TTG gneisses in the Eastern Block, North China Craton: Tectonic implications

2020 ◽  
Author(s):  
Guochun Zhao
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Plate Tectonics ◽  
Basic Research ◽  
Large Igneous Province ◽  
Main Body ◽  
Juvenile Crust ◽  
Block Based ◽  
Two Phases ◽  
T Values

<p class="BODYTEXT"><span lang="EN-GB">Available zircon ages indicate that the plutonic protoliths of Neoarchean TTG (tonalitic-trondhjemitic-granodioritic) gneisses in the Eastern Block were emplaced at two phases, with the earlier one at 2.75-2.65 Ga and the younger one at 2.55-2.50 Ga. Although the 2.75-2.65 Ga rock associations are only exposed in the Luxi and Qixia areas, the ~2.7 Ga igneous event must have occurred across the whole Eastern Block and was a major crustal accretionary or mantle-extraction event that formed a thick mafic crust beneath the whole Eastern Block based on the following lines of evidence: </span></p> <p class="BODYTEXT"><span lang="EN-GB">(1) The 2.75-2.65 Ga TTG rocks in the Luxi granite-greenstone terrane have positive εHf(t) values (+2.7 to +10.0), with most zircon Hf model ages close to the rock-forming ages, which provides robust evidence that the ~2.7 Ga event that formed the 2.75-2.65 rock associations was a crustal accretion (mantle extraction) event, not a crust-reworking event.</span></p> <p class="BODYTEXT"><span lang="EN-GB">(2) The 2.55-2.50 Ga TTG rocks in the Eastern Block possess mildly positive to slightly negative εHf(t) values, with most zircon Hf model ages pointing to 2.8-2.6 Ga, similar to rock-forming ages of the 2.75-2.65 Ga TTG gneisses in the Luxi granite-greenstone terrane, suggesting that the 2.55-2.50 Ga rocks in the Eastern Block were mainly derived from the partial melting of an early Neoarchean (2.75-2.65 Ga) juvenile crust that formed at ~2.7 Ga. As the 2.55-2.50 Ga TTG gneisses are ubiquitous over the whole Eastern Block, the 2.7 Ga event must have occurred over the whole Eastern Block, forming an early Neoarchean juvenile crust that experienced partial melting or reworking to form the 2.55-2.50 Ga TTG rocks. </span></p> <p class="BODYTEXT"><span lang="EN-GB">(3) TTG rocks are generally considered to have been derived from the partial melting of a thickened mafic crust (eclogite or rutitle/garnet-bearing amphibolite). This means that an early Neoarchean (2.75-2.65 Ga) juvenile crust formed by the ~2.7 Ga event should be a mafic-dominant crust, which is either a lower continental crust or an oceanic crust. In this case, the ~2.7 Ga event in the Eastern Block may have represented a Large Igneous Province event that formed the main body of the Eastern Block. This study was financially supported by the sub-project of a NSFC Major Project, entitled “Continental Crust Growth-Stabilization and Initiation of the Early Plate Tectonics” (Project Code: 41890831) and HKU Seed Fund for Basic Research (201811159089).</span></p>

Origin of syn-collisional granitoids in the Gangdese orogen: Reworking of the juvenile arc crust and the ancient continental crust

2021 ◽  
Author(s):  
Yu-Wei Tang ◽  
Long Chen ◽  
Zi-Fu Zhao ◽  
Yong-Fei Zheng
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Continental Collision ◽  
Late Eocene ◽  
Igneous Rocks ◽  
Early Eocene ◽  
Southern Tibet ◽  
Crustal Rocks ◽  
Mafic Magmas ◽  
T Values

Granitoids at convergent plate boundaries can be produced either by partial melting of crustal rocks (either continental or oceanic) or by fractional crystallization of mantle-derived mafic magmas. Whereas granitoid formation through partial melting of the continental crust results in reworking of the pre-existing continental crust, granitoid formation through either partial melting of the oceanic crust or fractional crystallization of the mafic magmas leads to growth of the continental crust. This category is primarily based on the radiogenic Nd isotope compositions of crustal rocks; positive εNd(t) values indicate juvenile crust whereas negative εNd(t) values indicate ancient crust. Positive εNd(t) values are common for syn-collisional granitoids in southern Tibet, which leads to the hypothesis that continental collision zones are important sites for the net growth of continental crust. This hypothesis is examined through an integrated study of in situ zircon U-Pb ages and Hf isotopes, whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral O isotopes for felsic igneous rocks of Eocene ages from the Gangdese orogen in southern Tibet. The results show that these rocks can be divided into two groups according to their emplacement ages and geochemical features. The first group is less granitic with lower SiO2 contents of 59.82−64.41 wt%, and it was emplaced at 50−48 Ma in the early Eocene. The second group is more granitic with higher SiO2 contents of 63.93−68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The early Eocene granitoids exhibit relatively depleted whole-rock Sr-Nd-Hf isotope compositions with low (87Sr/86Sr)i ratios of 0.7044−0.7048, positive εNd(t) values of 0.6−3.9, εHf(t) values of 6.5−10.5, zircon εHf(t) values of 1.6−12.1, and zircon δ18O values of 5.28−6.26‰. These isotopic characteristics are quite similar to those of Late Cretaceous mafic arc igneous rocks in the Gangdese orogen, which indicates their derivation from partial melting of the juvenile mafic arc crust. In comparison, the late Eocene granitoids have relatively lower MgO, Fe2O3, Al2O3, and heavy rare earth element (HREE) contents but higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb)N ratios. They also exhibit more enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7070−0.7085, negative εNd(t) values of −5.2 to −3.9 and neutral εHf(t) values of 0.9−2.3, and relatively lower zircon εHf(t) values of −2.8−8.0 and slightly higher zircon δ18O values of 6.25−6.68‰. An integrated interpretation of these geochemical features is that both the juvenile arc crust and the ancient continental crust partially melted to produce the late Eocene granitoids. In this regard, the compositional evolution of syn-collisional granitoids from the early to late Eocene indicates a temporal change of their magma sources from the complete juvenile arc crust to a mixture of the juvenile and ancient crust. In either case, the syn-collisional granitoids in the Gangdese orogen are the reworking products of the pre-existing continental crust. Therefore, they do not contribute to crustal growth in the continental collision zone.

Zircon U–Pb ages, geochemistry and Sr–Nd isotopes of the Golshekanan granitoid, Urumieh–Dokhtar magmatic arc, Iran: evidence for partial melting of juvenile crust

2020 ◽  
pp. 1-16
Author(s):  
Fatemeh Sarjoughian ◽  
Bahareh Zahedi ◽  
Hossein Azizi ◽  
Wenli Ling ◽  
David R. Lentz ◽  
...  
Keyword(s):  
Partial Melting ◽  
Active Continental Margin ◽  
Late Eocene ◽  
The Body ◽  
Central Iran ◽  
Primitive Mantle ◽  
Magmatic Arc ◽  
Juvenile Crust ◽  
Continental Lithospheric Mantle ◽  
T Values

Abstract The Golshekanan granitoid body is situated in the central part of the Urumieh–Dokhtar magmatic arc (UDMA) in central Iran, and includes granite and granodiorite with minor monzonite and diorite. Zircon U–Pb dating yields a late Eocene (Priabonian) crystallization age of 37.6 ± 0.2 Ma. The body is calc-alkaline and metaluminous to weakly peraluminous (A/CNK ≤ 1.10) with SiO2 ranging from 61.1 to 71.5 wt% and MgO from 0.8 to 3.3 wt%, with Na2O + K2O of 4.0–8.5 wt%. Primitive mantle-normalized trace-element patterns display enrichments in the large-ion lithophile elements (LILE), such as Rb, Cs, Ba and K, and depletion from the high-field-strength elements (HFSEs), such as Nb, Ti, Ta and P. The rocks are enriched in LREEs relative to HREEs (average (La/Yb)CN = 4.3) and exhibit weak negative Eu anomalies (average Eu/Eu* = 0.75), revealing typical active continental margin arc affinity. The low initial 87Sr/86Sr ratios (0.70440–0.70504) and notable positive ϵNd(t) values (+4.0 to +5.2) indicate an origin by partial melting of juvenile rocks in the lower crust, possibly with some involvement of sub-continental lithospheric mantle beneath Central Iran. These processes probably occurred due to the Neo-Tethys oceanic slab retreat and (or) rollback during late Eocene time.

scholarly journals The largest plagiogranite on Earth formed by re-melting of juvenile proto-continental crust

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Hamed Gamal El Dien ◽  
Zheng-Xiang Li ◽  
Mohamed Abu Anbar ◽  
Luc S. Doucet ◽  
J. Brendan Murphy ◽  
...  
Keyword(s):  
Continental Crust ◽  
Plate Tectonics ◽  
High Sodium ◽  
Melt Extraction ◽  
The Earth ◽  
Nubian Shield ◽  
Low Potassium ◽  
Juvenile Crust ◽  
Geochemical Analyses ◽  
Arabian Nubian Shield

AbstractThe growth of continental crust through melt extraction from the mantle is a critical component of the chemical evolution of the Earth and the development of plate tectonics. However, the mechanisms involved remain debated. Here, we conduct petrological and geochemical analyses on a large (up to 5000 km2) granitoid body in the Arabian-Nubian shield near El-Shadli, Egypt. We identify these rocks as the largest known plagiogranitic complex on Earth, which shares characteristics such as low potassium, high sodium and flat rare earth element chondrite-normalized patterns with spatially associated gabbroic rocks. The hafnium isotopic compositions of zircon indicate a juvenile source for the magma. However, low zircon δ18O values suggest interaction with hydrothermal fluids. We propose that the El-Shadli plagiogranites were produced by extensive partial melting of juvenile, previously accreted oceanic crust and that this previously overlooked mechanism for the formation of plagiogranite is also responsible for the transformation of juvenile crust into a chemically stratified continental crust.

INFLUENCE OF PULSED MAGMATIC ACTIVITY, LATENT HEAT, AND PARTIAL MELTING ON THE STRENGTH OF THE CONTINENTAL CRUST

2020 ◽  
Author(s):  
Aleksi Rantanen ◽  
◽  
David Whipp ◽  
Jussi S. Heinonen ◽  
Lars Kaislaniemi ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Latent Heat ◽  
Magmatic Activity

Mesozoic adakitic rocks from the Xuzhou-Suzhou area, eastern China: Evidence for partial melting of delaminated lower continental crust

2006 ◽  
Vol 27 (4) ◽  
pp. 454-464 ◽  
Author(s):  
Wen-Liang Xu ◽  
Qing-Hai Wang ◽  
Dong-Yan Wang ◽  
Jing-Hui Guo ◽  
Fu-Ping Pei
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Eastern China ◽  
Lower Continental Crust ◽  
Adakitic Rocks

Structure and spatial-temporal relationship of potassic magmatism linked to a continental-scale strike-slip shear zone and post-collisional Cenozoic extension

2021 ◽  
Author(s):  
junyu Li ◽  
shunyun Cao ◽  
Xuemei Cheng ◽  
Haobo Wang ◽  
Wenxuan Li
Keyword(s):  
Partial Melting ◽  
Shear Zone ◽  
Continental Crust ◽  
Red River ◽  
Compositional Variation ◽  
Continental Scale ◽  
Magmatic Rocks ◽  
Western Yunnan ◽  
Potassic Rocks ◽  
Potassic Magmatism

<p>Adakite‐like potassic rocks are widespread in post-collisional settings and provide potential insights into deep crustal or crust-mantle interaction processes including asthenosphere upwelling, partial melting, lower crustal flow, thickening and collapse of the overthickened orogen. However, petrogenesis and compositional variation of these adakite‐like potassic rocks and their implications are still controversial. Potassic magmatic rocks are abundant developed in the Jinshajiang–Ailaoshan tectono-magmatic belt that stretches from eastern Tibet over western Yunnan to Vietnam. Integrated studies of structure, geochronology, mineral compositions and geochemistry indicate adakite-like potassic rocks with different deformation are exposed along the Ailaoshan-Red River shear zone. The potassic felsic rocks formed by mixing and partial melting between enriched mantle-derived ultrapotassic and thickened ancient crust-derived magmas. The mixing of the mafic and felsic melts and their extended fractional crystallization of plagioclase, K-feldspar, hornblende and biotite gave rise to the potassic magmatic rocks. Zircon geochronology provide chronological markers for emplacement at 35–37 Ma of these adakite-like potassic rocks along the shear zone. Temperature and pressure calculated by amphibole-plagioclase thermobarometry range from 3.5 to 5.9 kbar and 650 to 750 ℃, respectively, and average emplacement depths of ca. 18 km for granodiorite within this suite. In combination with the results of the Cenozoic potassic magmatism in the Jinshajiang–Ailaoshan tectono-magmatic belt, we suggest that in addition to partial melting of the thickened ancient continental crust, magma underplating and subsequent crust-mantle mixing beneath the ancient continental crust have also played an important role in crustal reworking and strongly affected the rheological properties and density of rocks. The exhumation underlines the role of lateral motion of the Ailaoshan-Red River shear zone initiation by potassic magma-assisted rheological weakening and exhumation at high ambient temperatures within the shear zone.</p>

Quantifying the growth of continental crust through crustal thickness and zircon Hf-O isotopic signatures: A case study from the southern Central Asian Orogenic Belt

2021 ◽  
Author(s):  
Yujian Wang ◽  
Dicheng Zhu ◽  
Chengfa Lin ◽  
Fangyang Hu ◽  
Jingao Liu
Keyword(s):  
Continental Crust ◽  
Crustal Thickness ◽  
Orogenic Belt ◽  
Central Asian Orogenic Belt ◽  
Crustal Thickening ◽  
Crustal Growth ◽  
Isotopic Signatures ◽  
Central Asian ◽  
Southern Central ◽  
Juvenile Crust

Accretionary orogens function as major sites for the generation of continental crust, but the growth model of continental crust remains poorly constrained. The Central Asian Orogenic Belt, as one of the most important Phanerozoic accretionary orogens on Earth, has been the focus of debates regarding the proportion of juvenile crust present. Using published geochemical and zircon Hf-O isotopic data sets for three belts in the Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we first explore the variations in crustal thickness and isotopic composition in response to tectono-magmatic activity over time. Steady progression to radiogenic zircon Hf isotopic signatures associated with syn-collisional crustal thickening indicates enhanced input of mantle-derived material, which greatly contributes to the growth of the continental crust. Using the surface areas and relative increases in crustal thickness as the proxies for magma volumes, in conjunction with the calculated mantle fraction of the mixing flux, we then are able to determine that a volume of ∼14−22% of juvenile crust formed in the southern Central Asian Orogenic Belt during the Phanerozoic. This study highlights the validity of using crustal thickness and zircon isotopic signatures of magmatic rocks to quantify the volume of juvenile crust in complex accretionary orogens. With reference to the crustal growth pattern in other accretionary orogens and the Nd-Hf isotopic record at the global scale, our work reconciles the rapid crustal growth in the accretionary orogens with its episodic generation pattern in the formation of global continental crust.

scholarly journals An anticlockwise metamorphic P-T path and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

2018 ◽  
Author(s):  
Carly Faber ◽  
Holger Stünitz ◽  
Deta Gasser ◽  
Petr Jeřábek ◽  
Katrin Kraus ◽  
...  
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Large Scale ◽  
Structural Data ◽  
Geothermal Gradient ◽  
Continental Collision ◽  
Crustal Thickening ◽  
Northern Norway ◽  
Nappe Complex ◽  
T Path

Abstract. This study investigates the Caledonian metamorphic and tectonic evolution in northern Norway, examining the structure and tectonostratigraphy of the Reisa Nappe Complex (RNC; from bottom to top, Vaddas, Kåfjord and Nordmannvik nappes). Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and P-T conditions of deformation and metamorphism that formed the nappes and facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P-T path attributed to the effects of early Silurian heating followed by thrusting. An early Caledonian S1 foliation in the Nordmannvik Nappe records kyanite-grade partial melting at ~ 760–790 °C and ~ 9.4–11 kbar. Leucosomes formed at 439 ± 2 Ma (U-Pb zircon) in fold axial planes in the Nordmannvik Nappe indicate that compressional deformation initiated while the rocks were still partially molten. This stage was followed by pervasive solid-state shearing as the rocks cooled and solidified, forming the S2 foliation at 680–730 °C and 9.5–10.9 kbar. Multistage titanite growth in the Nordmannvik Nappe records this extended metamorphism between 444 and 427 Ma. In the underlying Kåfjord Nappe, garnet cores record lower P-T (590–610 °C and 5.5–6.8 kbar) but a similar geothermal gradient as the S1 migmatitic event in the Nordmannvik Nappe, indicating formation at a higher relative position in the crust. S2 shearing in the Kåfjord Nappe occurred at 580–605 °C and 9.2–10.1 kbar, indicating a considerable pressure increase during nappe stacking. Gabbro intruded in the Vaddas Nappe at 439 ± 1 Ma, synchronously with migmatization in the Nordmannvik Nappe. In the Vaddas Nappe S2 shearing occurred at 630–640 ºC and 11.7–13 kbar. Titanite growth along the lower RNC boundary records S2-shearing at 432 ± 6 Ma. It emerges that early Silurian heating (~ 440 Ma), probably resulting from large-scale magma underplating, initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual nappe units. This tectonic style contrasts subduction of mechanically strong continental crust to great depths.

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