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Due to the inherently fluid‐mobile nature of W, the 182W record of the early Earth may have been obscured by fluid‐induced mobilization of W. To investigate W mobilization in Archean greenstone sequences, we analyzed 182W isotope systematics and major and trace element concentrations in samples from the 3.53 Ga old Onverwacht Group of the Kaapvaal Craton (South Africa) and the >3.51 Ga old Badampahar Group of the Singhbhum Craton (India). Our results for mafic and ultramafic metavolcanic rocks show W/Th ratios significantly higher than primary magmatic values, which suggests fluid‐induced W enrichment. Samples least affected by secondary W enrichment (W/Th < 0.26) show no resolvable W isotope anomalies from modern mantle values in both cratons. Samples from the Kaapvaal Craton with elevated W/Th exhibit deficits in 182W as low as −8.1 ± 4.3 ppm compared to the modern mantle. Covariations of μ182W with W/Th, and Ce/Pb suggest that negative isotope signatures were introduced during secondary fluid‐mediated processes. The enrichment of W is most evident in altered ultramafic rocks comprising serpentine, resulting in additional covariations between MgO, LOI, and W/Th. The W isotope composition of serpentinized komatiites reflects the composition of younger intruding granitoids. We therefore infer the latter as a possible source of W‐rich fluids. The Badampahar Group samples exhibit little W isotope variability. A well‐resolved 182W deficit of −6.2 ± 2.9 ppm was determined in a single komatiite sample, which indicates an unknown fluid source, currently not represented in any other unit of the Singhbhum Craton. Plain Language Summary: The tungsten (W) isotope composition of ancient rocks can be used to trace processes that occurred during Earth's early evolution. However, interactions between rocks and fluids may alter the W concentration and therefore influence the interpretation of W isotope data. To identify the source of such fluids and the processes by which they affect the W isotope composition of rocks, we analyzed ancient rock samples from South Africa and India. The isotope composition of rocks with a low W concentration reflects that of the modern Earth. Therefore, they do not trace the processes that occurred during Earth's early evolution. Samples from South Africa with untypically high W concentrations show a different isotopic composition. The variation in the W isotope signature correlates with other chemical indices that are susceptible to modification by fluid‐related processes. This shows that the W within the rocks is derived from an external fluid source and not from their original magmatic source. Samples with the highest W enrichment have a similar isotope composition as spatially associated intrusive rocks. By inference, the latter likely represent the source of W‐rich fluids. The samples from India show similar enrichment in W, indicating similar fluid‐related processes and W sources at both localities. Key Points: The magmatic sources of metavolcanic rocks from the Onverwacht Group and the Badampahar Group do not exhibit W isotope anomaliesNegative W isotope signatures in the Onverwacht Group are likely derived from fluids sourced from younger intrusive granitoidsFelsic intrusive rocks are a major source of W‐rich fluids in Paleoarchean greenstone units [ABSTRACT FROM AUTHOR]

Numerous Cretaceous metamorphic core complexes (MCCs) extend from Transbaikalia in Russia to northern Mongolia within the Central Asian Orogenic Belt. We investigated the Buteel and Zagan MCCs in detail. Shear sense indicators in mylonitized rocks show footwall-to-the-NW tectonic transport. Single zircon dating of footwall rocks in the Buteel MCC establishes the emplacement of granitoid orthogneiss precursors at 240-211 Ma, a felsic metavolcanic rock at 265.0 [+ or -] 1.2 Ma, a syenite at 265.5 [+ or -] 1.2 Ma and a metarhyolite of the pre-granitoid basement at 553.6 [+ or -] 2.9 Ma. A peralkaline granite intruding orthogneisses of the Zagan MCC has a new U-Pb zircon age of 151.6 [+ or -] 0.7 Ma. [sup.40]Ar/[sup.39]Ar ages of 133.5 [+ or -] 1.8 Ma of hornblende from amphibolite and 122.6 [+ or -] 1.8 Ma of biotite from mylonitized gabbro-dolerite of the Buteel MCC are interpreted as cooling ages representing the time of deformation in the footwall. Geological data suggest that the MCCs in Transbaikalia and northern Mongolia formed as a result of extension in a crust that had previously been thickened by abundant calc-alkaline magmatism in an Andean-type setting on the border of the closing Mongol-Okhotsk ocean, by widespread collisional to post-collisional thrusting, and by extensive alkaline-peralkaline magmatism.

A variety of models has been proposed for the origin of late Devonian low-grade greywacke, juxtaposed with the high-grade Variscan basement of the Black Forest. Here we interpret detrital zircon ages, and geochemical and Sm-Nd isotopic data of greywacke samples as evidence for their sources and tectonic setting of deposition. Detrital zircon ages of 371-384 Ma reveal igneous activity shortly before the deposition of the greywacke sequence at c. 365 Ma. [C.sub.Nd] values of c. +3 to -10 in the greywacke samples indicate mixing of juvenile and older crustal material. Samples with low MgO concentrations and [C.sub.Nd] values of -10 to -4 are interpreted as mostly derived from older granite gneiss basement. Involvement of a juvenile mafic source is documented by samples with up to 7 wt% MgO and [C.sub.Nd] values up to +3. Assessment of the composition of the juvenile component shows high concentrations of light REE. and low Sm/Nd and Nb/La ratios, all consistent with a subduction-related origin, We conclude that the greywacke sequence was derived from a late Devonian Andean-type continental margin, implying a cryptic suture in the internal high-grade Variscan basement of the southern Black Forest. Keywords: Variscan foldbelt, Black Forest, greywacke, provenance, neodymium.

The Mozambique belt of southern Tanzania is underlain by locally restricted 1100-950 Ma (late Kibaran) granitoid gneisses that were derived from remelting of Archaean continental crust, as suggested by Nd isotopic systematics. These rocks were deformed and metamorphosed during an intense Neoproterozoic (Pan-African) event at around 630 Ma together with tectonically interlayered and widespread 800-650 Ma granitoid gneisses and minor clastic metasediments. The 800-650 Ma granitoids were derived predominantly from Neoproterozoic juvenile melts. There is no evidence for pre-800 Ma deformation. The 630 Ma event led to extensive migmatization in all gneisses and caused local melting. Similarities in age and tectonometamorphic evolution between these rocks and similar gneisses in northern Mozambique and southern Malawi suggest a similar geological evolution. However, distinctly different ages for the peak of metamorphism in Tanzania, Malawi and Mozambique indicate diachronous high-grade events that may be associated with terrane accretion and continental collision during orogenesis and favour the view that East Gondwana was not a coherent block during formation of the Gondwana supercontinent. Keywords: Tanzania, Gondwana, Pan-African orogeny, absolute age.

The beginning of the Grampian episode of tectonometamorphism in Scotland is dated by collision and obduction of the Ballantrae Ophiolite Complex at 478 +/- 8 Ma. For the first time we have used zircon and garnet to radiometrically date the peak of the Caledonian Buchan and Barrovian tectonometamorphism in their type areas as contemporaneous at 467 +/- 2.5 Ma. Detrital garnet from the neighboring Southern Uplands flysch terrane has the same radiometric age and occurs in 465 +/- 2.5 Ma molasse in the Midland Valley. We show that the entire Grampian episode lasted 15 +/- 2.5 m.y. and that it took fewer than 7.6 m.y. to expose high-grade rocks following the peak of metamorphism. Those who study Precambrian orogeny should note the brevity of this Scottish tectonometamorphic episode and the speed of exhumation. Keywords: fast, Barrovian, tectonometamorphism, Scotland.

Low-volume melts from volatile-rich mantle domains, such as those represented by the Tertiary melilitites from the Bad Urach - Hegau volcanic district of southern Germany, are ideally suited for a study of the fluids involved in intraplate volcanism. The [[Delta].sup.18]O values of unaltered primitive melilitites (4.9[per thousand] to 6.1[per thousand]) as well as those of hornblende (4.7[per thousand] to 5.5[per thousand]) and phlogopite (5.4[per thousand] to 6.7[per thousand]) megacrysts from chemically evolved melilitite tuffs are consistent with an origin from typical mantle. The [Delta]D values of -91[per thousand] to -98[per thousand] of the unaltered primitive melilitites support this inference, as they are similar to those of other European Tertiary intraplate basalts. In contrast, the [Delta]D values for the hornblende and phlogopite megacrysts of -24[per thousand] to -42[per thousand] indicate involvement of recycled seawater probably during melt fractionation in the lithospheric mantle. The combined H, O, and Sr isotope evidence is consistent with a two-stage melt-evolution process in which melting was initiated in a normal European-type mantle, followed in some cases by magma ponding and fractionation in the lithospheric mantle that had previously been overprinted by recycled seawater.

Rb-Sr dating of phlogopites isolated from Sturgeon Lake kimberlite, Saskatchewan, Canada points to a crytallization age of 98 Ma in accordance with the geological age. The Ar(super 40)/Ar(super 39) heating analysis of a phlogopite sample shows high amounts of excess argon indicating to an age of nearly 410 Ma. The results of oxygen and carbon isotopic analyses are discussed.