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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]

[Display omitted] • Juvenile crust in Singhbhum Craton formed at ∼3.51, ∼3.45, 3.38–3.25, 3.15–3.05 and ∼2.80–2.75 Ga. • Reworking of older crust in the craton occurred at ∼3.46, ∼3.3, ∼3.1 and ∼2.8 Ga. • Maximum crustal thickness was attained at ∼3.3 Ga followed by a possible reduction over ∼3.1–2.8 Ga. • Paleo-Meso–Archean continental growth occurred through episodic intraplate juvenile magmatism and resultant crustal reworking. The composition of continental crust and its temporal variation, the mechanism of crust-mantle interaction, and the degree of mantle depletion during the early Archean have been widely debated. To address these issues related to the Singhbhum Craton, we present whole-rock sediment geochemistry, detrital zircon U-Pb date, trace element and Hf isotope data from four Archean clastic sedimentary successions. The rocks have high whole-rock K 2 O/Na 2 O, relatively high La and Th compared to Ni and Sc, distinct negative Eu anomaly and flat HREE patterns, and high detrital zircon U/Yb ratios. These features suggest attainment of early crustal stability with the sediments derived from a strongly weathered basement dominated by granitoids formed through shallow intracrustal melting. Detrital zircon U-Pb and Hf isotope data indicate episodic crust formation at ∼3.51, ∼3.46, 3.38–3.25, 3.15–3.05 and ∼2.80–2.75 Ga. Each major crust formation event is marked by addition of depleted mantle-derived juvenile crust and its quick reworking. Besides, episodic tapping of older crust, as indicated by a wide range (both positive and negative) of zircon εHf t values, are evident at ∼ 3.46, ∼3.3, ∼3.1 and ∼2.8 Ga. Pressure-sensitive zircon trace element ratios suggest that the crust attained the maximum thickness at ∼3.3 Ga followed by a possible reduction in thickness over ∼3.1–2.8 Ga. In the absence of strong evidence of subduction, the rock association and their temporal distribution (coeval granitoid and mafic-ultramafic magmatism) suggest that the Paleoarchean-Mesoarchean crustal growth in the Singhbhum Craton was a result of episodic, depleted mantle-derived, intraplate magmatic injection and attendant crustal reworking. [ABSTRACT FROM AUTHOR]

Abstract: Rutile and zircon in eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD-MH) were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron microprobe (EMP). Both single spot and profile analyses of large matrix rutiles (>60μm) generally demonstrate core–rim variability of Pb, Sr, Zr and U. Pb and Sr show enrichment at the rims, while Zr and U decrease in concentration from the cores to the rims. Correspondingly, temperatures estimated by Zr-in-rutile geothermometers are higher within the cores (mainly in the range of 680–740°C) than at the rims by 10–30°C. We note that estimated temperatures within the cores of matrix rutiles correlate positively with grain diameter. This suggests that the Zr content in rutiles is not only dependent on temperature and pressure, but is also affected by fluid activity and Zr diffusion. Estimated temperatures within the cores of matrix rutiles merely represent lower limits of the peak ultra-high pressure (UHP) metamorphic temperatures. Combined with Th/U and Ce/Lu ratios, rare earth element (REE) patterns and 206Pb/238U ages, Ti-in-zircon geothermometers can be used to distinguish the thermal evolution of eclogites during the UHP metamorphism. Zircons of 745±8.0Ma (2σ, n =3) show evidence of the temperatures at which they crystallized from the original magma. Zircons of 240±7.0Ma (2σ, n =11) with flat HREE patterns suggest a definite temperature range (689±18°C (2σ, n =11)) for the prograde metamorphism. Although 226Ma is generally considered the time of the peak UHP metamorphism for the Dabie–Sulu orogen, except for a few grains with a maximum temperature of 744°C, most of the 226±2.0Ma (2σ, n =12) zircons indicate lower temperatures (average=698±12°C (2σ, n =12)). These data imply that the 226Ma zircons were probably not products of the peak UHP metamorphism, but might instead be associated with the pressure decrease in the post-peak metamorphism. The youngest zircons (<220Ma) provide evidence for temperatures (682±14°C (2σ, n =11)) associated with the retrograde metamorphism. Geothermometers that incorporate both rutile and zircon indicate that there may have been experienced a locally short-lived heating event for eclogites from the CCSD-MH. [Copyright &y& Elsevier]

The processes responsible for the origin and evolution of Early Archean continental crust are not properly understood. Questions regarding the pattern of crustal growth (episodic vs. continuous) and the role of crust-mantle interaction in the temporal change of crustal composition and thickness are still being widely debated. In order to address the above issues, we present whole-rock geochemistry and detrital zircon U-Pb-Hf isotopes, and trace-element data from Mesoarchean clastic rocks of two selected greenstone belts of western Dharwar Craton. The mineralogically mature character of the sampled sandstones along with their high K 2 O/Na 2 O, Rb/Sr and CIA values, relatively enriched concentrations of La and Th over Cr and Sc, variably negative Eu-anomaly, and high U/Yb values of detrital zircons are indicative of their derivation from strongly weathered stable continental provenance dominated by low-HREE TTGs and K-rich granites of shallow crustal origin. Detrital zircon U-Pb ages indicate the maximum age of deposition for these sandstones is ∼3.1 Ga. The detrital zircon U-Pb and Lu-Hf data suggest two major crust formation events at 3.54–3.33 Ga and 3.27–3.12, mainly via juvenile crustal addition and subsequent rapid reworking. Besides, ∼3.6 Ga detrital zircons with negative εHft values suggest that the antiquity of the western Dharwar Craton can be traced back at least up to 3.8 Ga. Reworking of the older continental crust occurred during ∼ 3.25 Ga and ∼ 3.15–3.07 Ga, the last one represented by emplacement of K-rich granites. Zircon trace element ratios indicate a maximum increase in crustal thickness at ∼3.2 Ga followed by a crustal thinning at around ∼3.15 Ga. The detrital record in combination with geological setting suggests an episodic crustal growth and reworking in the western Dharwar Craton during Paleoarchean to early Mesoarchean period which was possibly related to intraplate processes. • Juvenile crust in Western Dharwar Craton formed at 3.54–3.33 Ga. • The juvenile crust reworked during 3.27–3.12 Ga. • Reworking of older crust occurred at ∼3.25 Ga, ∼3.15–3.07 Ga and ∼ 3.02 Ga. • Maximum crustal thickness attained at ∼3.2 Ga followed by thinning up to ∼3.1 Ga. • Link between crustal thickening and partial convective overturn is suggested. [ABSTRACT FROM AUTHOR]