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The South Beishan Orogenic Belt (SBOB) in the southwestern Central Asian Orogenic Belt (CAOB) is thought to be an eastern extension of the Eastern Tianshan Orogen. Up to now, it has been suggested to consist of a well-preserved Paleozoic magmatic and sedimentary sequence with a possible former Neoproterozoic microcontinent. We provide new geochemical and geochronological data for gneissic granites of the SBOB in order to constrain its tectonic evolution and setting. The gneissic granitoids belong to the high-K, calc-alkaline series and are characterized by an enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE), a depletion of Nb, Ta, Ti, Sr, and Ba and a positive Pb anomaly. The petrography and geochemical signatures reveal a possible I-type granite affinity and are in accordance with typical Andean Arc granites. Zircon grains, yielding 206 Pb/ 238 U magmatic crystallization ages of 933 ± 2 Ma and 900–890 Ma for the Shibanshan and Huaniushan arc granites, respectively, manifest the former presence of an early Neoproterozoic Precambrian microcontinent in the SBOB. Zircon ɛ Hf ( t ) values range from −16.1 to 10.2, indicating that juvenile material and reworked ancient crust were involved in the source of the gneissic granites. Furthermore, in conjunction with studies of the adjacent regions, the results of the present study suggest that the former Precambrian microcontinent in the SBOB neither belonged to the nearby Dunhuang Block nor the Tarim Craton, but has a common affinity with the Central Tianshan Arc Terrane. It is also suggested that the 900–890 Ma plutons originated from the mixing of juvenile material with older crust in an Andean-type active continental arc setting during the assembly of Rodinia. Thus the SBOB plays a key role in understanding the tectonic evolution of the CAOB. [ABSTRACT FROM AUTHOR]

Abstract: Hydrothermally altered rocks are products of fluid–rock interactions, and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks. Thus, quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure. In order to decipher the fluid activity in the Sulu ultrahigh-pressure (UHP) terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. In one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies overprinting of the eclogite lens decreases from the rim to the core of the lens, with fresh eclogite preserved in the core. The foliated biotite gneiss contains felsic veins and residual phengites. Zircon rims from the gneiss are characterized by melt-related signatures with steep HREE patterns, high Hf contents and negative Eu anomalies, and a pool of weighted average 206Pb/238U analyses reveal an age of 219±3Ma (2σ), which is younger than the UHP metamorphic age (236±2Ma, 2σ) recorded by zircons from the eclogite lens. This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting due to phengite dehydration during the “hot” exhumation stage. The formation age of the quartz vein (219±2Ma, 2σ) defined by zircon rims agrees well with the partial melting time (219±3Ma, 2σ) of the host gneiss. The initial 176Hf/177Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217±5Ma (2σ), overlap with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced a significant fluid flow during the exhumation stage, and thus contributed significantly to the extensive retrogression of eclogites in the Sulu UHP terrane. [Copyright &y& Elsevier]

Abstract: In situ U–Pb dating and trace element analysis of zircons, combined with a textural relationship investigation in thin section, is a powerful tool to constrain the ultra high-pressure stage of high-grade metamorphism. Two types of zircon grains have been identified in thin sections of a retrograde eclogite from the main hole of the Chinese Continental Scientific Drill project in the Sulu UHP terrane. Type 1 zircon grains occur as inclusions in fresh garnet and omphacite, and Type 2 zircon grains were found in symplectite around omphacite. The fresh rims of Type 1 zircons and mantles of a few Type 2 zircons exhibit remarkably lower REE, Y, Nb and Ta contents than the inherited zircon cores, suggesting coeval growth with garnet, rutile and apatite during UHP metamorphism. These may have formed in the UHP metamorphism and survived retrograde metamorphism. The weighted average 206Pb/238U age of these zircon domains (230±4Ma, 2σ) agrees well with the published age of coesite-bearing zircon separates (230±1Ma, 2σ), suggesting that the peak UHP metamorphism in the Sulu terrane may have occurred at ~230Ma. Zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism. Together, they provide a younger weighted average 206Pb/238U age of 209±4Ma (2σ). These retrograde zircon domains have similar REE compositions to the ~230Ma UHP zircon domains. These observations imply that the ~209Ma zircon domains could have formed by fluid activity-associated alterations in the amphibolite-facies metamorphism, which could have resulted in the complete loss of Pb but not REEs in these domains. [Copyright &y& Elsevier]

• Granitoids of Singhbhum Craton. • Two generation of TTGs at 3.42 and 3.32 Ga formed by melting of juvenile mafic crust. • Later K-rich, silicic granites at 3.28 and 3.25 Ga formed by melting of tonalites. • Recurring crustal melting in a gradually thickening oceanic plateau. • Final cratonisation of the Singhbhum Craton at 3.25 Ga. A significant part of many Archean cratons formed during Paleoarchean. Yet, the mechanism and tectonic setting of formation of Paleoarchean continental crust remain highly debated. In this contribution, we present field, petrographic, geochemical, zircon U-Pb age and Hf isotope data on Paleoarchean granitoids from west-central part of the Singhbhum Craton (Champua-Hat Gamharia corridor), India. The whole process starting from nucleation of a juvenile continent to its evolution and final stabilization is documented. The core of the craton nucleated with formation of 3.45–3.40 Ga TTGs showing juvenile character (zircon ɛHf t = +0.6 to +7.1). These rocks show slightly depleted HREE and Y, negligible Eu-anomaly (Eu/Eu* = 0.90 to 1.00) and moderate Sr/Y (25–64), consistent with derivation from a low-K mafic crust at a pressure near the lower end of the garnet stability field, causing subordinate garnet retention in the residue and negligible role of plagioclase. A second generation of TTG formation took place at 3.32 Ga in the area by deeper melting of a juvenile mafic crust (zircon ɛHf t = +1.3 to +5.7) as suggested by strongly depleted HREE and Y, and high Sr/Y (52–155) implying significant amount of garnet retention in the residue. Subsequently, the area witnessed intracrustal melting at 3.28 and 3.25 Ga which tapped moderately old to juvenile (zircon ɛHf t = −1.9 to +4.5), mostly TTG sources at variable depths generating potassic, LILE-enriched, high-silica granites. Intrusion of these potassic granites marks the final cratonization of the Singhbhum Craton. The sequence of events is interpreted in terms of repeated crustal melting and granitoid generation in a gradually thickening oceanic plateau with a progressive change in granitoid source from mafic to felsic in composition. A synthesis of rock assemblage, regional geological setting and structural pattern also supports intraplate nature of the magmatism in Singhbhum Craton, which might have been a significant mechanism of crustal growth worldwide during Paleoarchean. Further, a comparison of juvenile crustal growth and crustal reworking events of the Singhbhum and other Indian cratons show that these cratons record distinct evolutionary histories and were probably nucleated at different sites. [ABSTRACT FROM AUTHOR]

Archean cratons display spatial and temporal changes in the composition and thickness of the crust, which has important implications for intracrustal differentiation, attainment of long-term stability, and operation or absence of plate tectonics. The geodynamic reason(s) for these changes, though, is a controversial subject. To address this issue, the present contribution investigates the spatial and temporal changes in granitoid compositions of the well-exposed Singhbhum Craton. It reports new zircon U-Pb and Lu-Hf, and whole-rock geochemical data on Paleoarchean granitoids from some of the less-studied parts of the Singhbhum Craton. The new information is then collated with previously reported data from the central part (cratonic core) of the Singhbhum Craton to illustrate the spatial and temporal changes in granitoid chemistry, using selected major oxide and pressure-sensitive elemental ratios. Temporal changes in the characteristics of the granitoids suggest that the crust building in Singhbhum Craton possibly started in a relatively thin oceanic plateau with the emplacement of low-pressure TTGs and diorites during ∼3.53–3.47 Ga. The ∼3.47–3.42 Ga period marks a gradual increase in crustal thickness, inferred from progressively increasing Al 2 O 3 and pressure-sensitive trace element ratios of the TTGs. Subsequently, as a result of protracted mantle upwelling and consequent juvenile TTG addition, the composition of the bulk continental crust differentiated into a more evolved felsic composition by ∼3.35 Ga, when the first potassic granite was emplaced. Spatial distribution of the granitoids suggests partial convective overturn (contemporaneous sinking of greenstones and diapiric rise of shallow- to mid-crustal granitoid domes) played a role in bringing the older as well as juvenile TTGs to a melting depth, generating ∼3.35–3.25 Ga spatially restricted high-silica, high-potassic, low-pressure K-rich granite and transitional granitoid domes. TTG magmatism also continued contemporaneously during this period. During ∼3.33–3.32 Ga, continental crust in Singhbhum Craton reached the maximum thickness, inferred from the formation of high-pressure (or low-HREE) TTGs. Afterward, possibly delamination of dense lower crustal residue caused the crust to become thinner, as recorded by progressively low-pressure K-rich granite and transitional granitoid emplacement until ∼3.28 Ga. Near the end of Paleoarchean (∼3.26–3.25 Ga), the crust became thicker again, further yielding high-pressure TTGs and K-rich granites. We suggest an oceanic plateau-like setting (characterized by relatively inefficient heat extraction compared to a modern-day subduction zone) where plume magmatism and delamination of anatectic residues of crustal melting caused the time-transgressive thickening and thinning of the continental crust, respectively. This process resulted in change in melting depth and, in turn, in the granitoid chemistry. Therefore, the Paleoarchean crustal architecture and the bulk crustal composition of the Singhbhum Craton controlled the temporal change in granitoid chemistry in an ongoing tectonic regime (partial convective overturn-dominated setting) without any distinct tectonic shift. [ABSTRACT FROM AUTHOR]