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Knowledge of pressure-temperature-time (P-T-t) evolution of Archean high-grade (deep crustal) metamorphic rocks is important for deciphering the nature of Archean tectonic processes. However, exposures of such rocks are limited in the present rock record. Here, we study a suite of high-grade, mafic rocks that are present along a crustal-scale shear zone (called the Mercara Shear Zone) between two Archean terrains of India, the Coorg block and the Dharwar Craton. Given that the Mercara Shear Zone is dated to be Mesoarchean, these shear zone rocks are well suited to elucidate Archean orogenic processes. Petrological investigation shows that these mafic rocks are characterized by a granulitic assemblage of orthopyroxene, clinopyroxene, plagioclase, quartz, amphibole ± garnet, and with accessory phases such as apatite, ilmenite, magnetite and rutile in some cases. We distinguish the investigated rocks into low-Mg and high-Mg varieties based on their whole rock composition as well as their mode of occurrence in the field and mineral chemistry. This difference in the bulk composition led to different reaction histories – for example, the low-Mg mafic granulites underwent partial melting while high-Mg granulites were less fertile. Combining these observations with the results of geothermobarometry, phase equilibria modelling, geochronology (U-Pb in zircon and Lu-Hf garnet geochronology) and diffusion modeling, we have reconstructed a multi-stage P-T-t history for these rocks. The first phase (Stage 1) is represented by granulite-grade metamorphism at ~750 – 900 °C and 8 – 13 kbar during ~3100 Ma (with uncertainties permitting a timing as recent as ~2700 Ma), after which they resided at T < 500 °C, likely at lower crustal levels (Stage 2). Subsequently, these rocks were reheated to a T of 700 – 750 °C at 7 – 10 kbar at ~2400 Ma (Stage 3) and then again cooled down to ~500 – 600 °C at 6 – 8 kbar (Stage 4). Application of diffusion chronometry shows that: (1) the cooling rates of these granulites at high temperatures (> 600 °C) varied in the range of 25 – 50 °C/Ma, and (2) the rocks resided for a long duration (~500 million years) at the Stage 2 metamorphic conditions i.e., at T < 500 °C. We infer that such a protracted, high-T metamorphic history involving at least two heating pulses, and the relatively slow cooling rates on the order of 10’s °C/Ma are consistent with the operation of peel-back styled orogenesis (an embryonic form of plate tectonics) on an early hotter Earth (Mesoarchean to Palaeoproterozoic). Moreover, the controls of bulk rock compositions on reaction histories provide a mechanism for intracrustal differentiation and generating Mg-rich, refractory material that may have eventually formed the lower continental crust over a protracted and pulsed thermal evolution spanning several hundred million years.

The origin of magmas that formed the Bushveld Complex remains highly debated in spite of many decades of intense research. Previous geochemical–petrological studies have shown a strong mantle derivation resulting ultimately in highly economic ore bodies of platinum group elements and chromium. However, geochemistry also points to the contribution of a significant crustal component, which may have been derived singly or in combination from a number of different sources. These include subcontinental lithospheric mantle that was enriched prior to Bushveld magma formation, possibly by subduction, assimilation of lower and upper crust during magma ascent, and contamination during magma chamber accretion within sedimentary rocks of the enclosing Transvaal Supergroup. In this study, the contributions of these various reservoirs will be evaluated by employing Hf isotopic data of well-characterized zircon grains in mafic, felsic and metasedimentary rocks, together with Zr–Hf bulk-rock compositions. The results reveal that magmatic zircon grains in mafic cumulate rocks from the floor to the roof of the c. 9 km thick Rustenburg Layered Suite (RLS) show essentially the same variations in εHf2·055 Ga from −7·5 to −10·2 as those of metamorphic zircon grains and overgrowths in the immediate surrounding quartzite and metapelitic rocks, as well as in granitic melt batches, granophyres, and the upper Rooiberg volcanics. The same values are also obtained by estimating the average Hf isotopic compositions of detrital zircon grains in many quartzite and metapelitic rocks from the surrounding Magaliesberg (εHf2·055 Ga = −6·2 to −10·8, six samples, maximum deposition age at 2080 Ma) and Houtenbeck formations (εHf2·055 Ga = −7·1 to −8·9, three samples, maximum deposition age at 2070 Ma), and by a six-point isochron of a garnet-schist from the Silverton Formation (εHft = −6·6 ± 0·7; age = 2059·4 ± 2·7 Ma). Zircon morphologies, zoning patterns, Hf isotopic data and petrological constraints furthermore reveal that metamorphic zircon was precipitated from aqueous fluids and/or felsic melts at temperatures between 550 and 900 °C, and that the Hf isotopic composition became homogenized during fluid transport in the contact aureole. However, results of numerical modelling indicate that fluid infiltration had only a minor effect on the Zr–Hf budget and Hf isotopic composition of the RLS, and that these parameters were mainly controlled by the mixing of melts derived from three major sources: (1) the asthenospheric mantle (>20 %); (2) enriched subcontinental lithospheric mantle (1200 °C) mantle-derived magmas with relatively low Zr–Hf concentrations having a similar εHf2·055 Ga of −8·5 ± 1·9 to that of upper crust rocks surrounding the RLS; (2) significant assimilation of volcanic and metasedimentary rocks with high Zr–Hf concentration; (3) mingling, mixing and/or diffusive exchange of Zr and Hf between crust and mantle-derived melts and aqueous fluids prior to late-magmatic crystallization of zircon at temperatures between 700 and 900 °C. This study shows that the combination of Zr–Hf bulk-rock data with Hf isotopic data of well-characterized zircon grains provides a powerful tool to quantify various mantle and crustal reservoirs of mafic layered intrusions, and allows new insights into magma chamber and related contact metamorphic processes.

The Adamello intrusive suite is a composite batholith in Northern Italy, with an estimated 2000 km3 volume, assembled incrementally over a time span of 10 to 12 million years. The history of crystallization has been studied in detail through laser ablation ICP-MS and SIMS U–Pb geochronology of zircon, which records prolonged crystallization of each of the different intrusive units at mid-crustal levels between 43·47 and 33·16 Ma. The magmas were episodically extracted from this storage area and ascended to the final intrusion level at ∼6 km paleo-depth. Each batch of melt cooled very rapidly down to the ambient temperature of 250°C, evidenced by distinct cooling paths recorded by amphibole, biotite and K-feldspar 40Ar/39Ar dates. The magma source area was moving from SW to NE with time, causing increasing thermal maturity in the mid-crustal reservoir. The resulting temporal trend of higher degrees of crustal assimiliation in the course of the evolution of the magmatic system can be traced through Hf and O isotopes in zircon. Rough estimates of magma emplacement rates (‘magma flux’) yield very low values in the range of 10-4 km3/yr, typical of mid-to-upper crustal plutons and increase with time. Although we cannot discern a decrease of magma flux from our own data, we anticipate that a dramatic decrease of magma flux between 33 and 31 Ma along the northern contact lead to cessation of magma emplacement. ISSN:0022-3530 ISSN:1460-2415

Kimberlite-borne mantle eclogites represent an important diamond source rock. Although the origin and stability of diamond, as opposed to its low-pressure polymorph graphite, have been studied for decades, their relationship in rare natural samples where both polymorphs coexist remains poorly constrained. To shed new light on this issue, seven graphite–diamond-bearing eclogites from the kimberlite pipe Udachnaya, Siberian craton were comprehensively investigated with respect to their petrography, mineral chemical composition and omphacite 87Sr/86Sr, acquired in situ by laser ablation multicollector inductively coupled plasma mass spectrometry. The calculated P–T conditions for basaltic group eclogites (Eu/Eu* < 1) correspond to a pressure range of 4·8–6·5 GPa and temperatures of 1060–1130 °C, whereas gabbroic eclogites with positive Eu- and Sr-anomalies have a smaller pressure variation (4·8–5·8 GPa), but a larger range in temperature (990–1260 °C). Reconstructed bulk compositions for gabbroic eclogites indicate an oceanic crustal origin for their protoliths, with accumulation of plagioclase and olivine ± clinopyroxene (gabbronorite or olivine gabbro). The protoliths of basaltic eclogites probably formed from the complementary residual melt. The presence of coesite and low Mg# in basaltic eclogites suggest that their light rare earth element depletion was the result of

Late Neoproterozoic (c. 555 Ma) high-pressure-ultrahigh-temperature (HP-UHT) metamorphism has been documented for MgAl-rich migmatitic granulites from the Palni Hills in the Southern Granulite Terrane (South India). Conspicuous reaction textures indicate a clockwise P-T evolution, which is constrained through P-T pseudosection modelling and thermobarometry. The transformation of sillimanite to kyanite, which coexisted with orthopyroxene and/or garnet, records an early stage of loading. During subsequent heating to UHT conditions at deep-crustal levels (c. 1000 degrees C, 13 center dot 0 kbar) kyanite was transformed to sillimanite, and distinct peak-temperature assemblages (orthopyroxene + sillimanite + mesoperthite + rutile +/- garnet +/- quartz +/- sapphirine, garnet + biotite + sillimanite + spinel + corundum + rutile + plagioclase and garnet + orthopyroxene + rutile + plagioclase +/- quartz) formed in specific bulk compositions through biotite-dehydration-melting reactions. A sequence of corona and sapphirine-bearing symplectite textures records subsequent isothermal decompression of the order of c. 6 kbar at persistent extreme temperatures (1010-920 degrees C). UHT decompression is consistent with the uniformly high Al contents of porphyroblastic, coronitic and symplectitic orthopyroxene (up to 10 center dot 4 wt % Al(2)O(3)). Regrowth of garnet and biotite documents post-decompressional cooling to subsolidus conditions of < 800 degrees C at mid-crustal levels (c. 6 kbar). HP-UHT metamorphism and the clockwise P-T path of the Palni Hills granulites is attributed to a single late Neoproterozoic tectono-metamorphic event, which has been consistently dated at c. 555 Ma through laser ablation inductively coupled plasma mass spectrometry U-Pb analyses of zircon and in situ electron microprobe U-Th-total Pb analyses of monazite. The MgAl-rich granulites occur as enclaves in enderbitic orthogneiss. The intrusion of the orthogneiss in the late Archean (2534 +/- 28 Ma) marks the beginning of voluminous granitoid emplacement in the Southern Granulite Terrane between 2530 and 2440 Ma, which presumably caused a first high-grade metamorphic event in the early Paleoproterozoic (2469 +/- 13 Ma), recorded by zircon cores in the MgAl-rich granulites. The clockwise P-T-t evolution indicates that HP-UHT metamorphism in the central part of the Southern Granulite Terrane is related to collisional tectonics during the final assembly of Gondwana in the late Neoproterozoic. Extreme heating is ascribed to upwelling of the asthenosphere during delamination of the thickened lithospheric mantle. Fast uplift of the rocks followed by mid-crustal isobaric cooling reflects extension of the hot overthickened crust and its subsequent cooling to a normal geotherm.

SUMMARY OF THEP^TPATH Petrological investigations of the three samples investi-gated provide evidence that the Mahalapye Complexunderwent a clockwise P^T evolution with a progradedecompression-heating from c.6508C/7kbar to8008C/5·5^6kbar (metamorphic peak), followed bydecompression-cooling to 6508C/3kbar (Fig. 5). Thisevolution is best constrained by mineral assemblages andreaction textures observed in the metapelitic samplesMa3a and Ma3c, but additionally is confirmed by themineral assemblages and compositions of sample Ma1h.The P^T path indicates that the rocks of the MahalapyeComplex underwent heating to high temperatures of8008C at moderate pressures of up to only 5·5kbar,equivalent to a crustal depth of 19km (assuming anaverage lithostatic pressure gradient of 270bar/km).Furthermore, it indicates that the rocks underwent apost-peak metamorphic uplift from 19 km to 11kmdepth (from 5·5 to 3kbar), whilst undergoing coolingfrom 8008Cto6508C. The lower end of the retrogradeP^T path seems to continue straight into the andalusitestability field, in agreement with the finding of retrogradeandalusite in metapelitic rocks by Hisada et al.(2005).

U^Pb and Lu^Hf isotope analyses, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry on zircon grains from 37 granitoid samples indicate that the Kalahari Craton consists of at least five distinct terranescBarberton South (BS), Barberton North (BN), Murchison^Northern Kaapvaal (MNK), Limpopo Central Zone (LCZ), and Francistowncwhich underwent different crustal evolutions, and were successively accreted at c. 3� 23 Ga, 2� 9 Ga and 2� 65^2� 7 Ga. The investigated granitoids were emplaced over a period of c. 1.5 billion years, and are exposed along a c. 1000 km long traverse from the Barberton Mountain Land/ Swaziland to the Francistown arc complex, Botswana. The presented datasets reveal that most granitoids of the BS (3� 45^3� 10 Ga), MNK (2� 93^2� 67 Ga), Francistown (2� 70^2� 65 Ga) and LCZ terranes (3� 2^2� 03 Ga) show near-chondritic to subchondritic eHft (BS ¼ ^1� 7t oþ 0� 5; MNK ¼ ^3� 4t oþ 0� 7; Francistown ¼ ^0� 5t oþ 1� 1; LCZ ¼ ^12� 4t o ^1 � 8), indicating that crustal recyclingcperhaps by mixing of an older crust with a depleted mantle reservoircplayed an important role during their formation and growth. Higher, superchondritic eHft values, as indicative for an important depleted mantle influence, were obtained only from some granitoids of the BN terrane (eHf3� 23Ga ¼þ 2� 5 � 0� 8), the Gaborone Granite Suite (eHf2� 80Ga ¼þ 2� 0 � 1� 6), and from a few detrital zircons from the Mahalapye complex of the Limpopo Belt. In addition, the datasets show that the internal Hf isotope variation of magmatic zircon domains from most granitoids is commonly less than � 1� 5 e-units, and only in rare cases up to � 3� 1 e-units. The rare significant eHft variations may be ascribed to incomplete mixing of different sources during magma crystallization. It is also shown that the combined approach of cathodoluminescence imaging with U^Pb and Lu^Hf isotope analysis provides a powerful tool to distinguish zircon domains formed and/or altered at different times.

A combined set of U^Pb and Lu^Hf in situ laser ablation ICP-(MC)-MS zircon analyses were obtained from orthogneisses and granitoids in the Central Zone of the Limpopo Belt, which comprises the Beit Bridge and Mahalapye complexes. The results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events, even if the distinct zircon domains underwent multiple Pb loss and the texture^age relationships, as obtained by cathodoluminescence images and U^Pb analyses, are ambiguous. Furthermore, the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination. The combined U^Pb and Lu^Hf data reveal that the Sand River gneiss suite of the Beit Bridge Complex was emplaced at 3283 8Ma and formed from melting of an older Archaean crust, which was derived from a depleted mantle source at around 3 65 Ga.The hafnium model age (TDM) is significantly older than those obtained from zircons from numerous Neoarchaean granitoids of the Beit Bridge Complex, comprising the Singelele gneiss (2647 12Ma), the Bulai granite (2612 7Ma), the Regina gneiss (2649 9Ma) and two samples of the Zanzibar gneiss (2613 6Ma). These granitoids show initial eHf(t) values between þ 0 5 and 7 1, which correspond to initialTDM between 3 46 and 3 01Ga. These variable TDMinitial and eHf(t)initial values are interpreted to be the result of different mixtures of reworked 3 65 Ga Palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the Neoarchaean at 2 65 Ga.This conclusion is supported by results obtained from the Mahalapye Complex, which was affected by migmatization and granite intrusions during the Palaeoproterozoic at 2 02^2 06 Ga. The Mokgware granite (2019 9Ma) contains zircon xenocrysts with Pb^Pb ages of 2 52^2 65 Ga and 2 93 Ga and hafnium model ages of 3 0^3 4 Ga, indicating that this granite is derived from remelting of Archaean crust. In contrast, uniformTDM Hf initial ages of 2 61^2 67 Ga obtained from a diorite gneiss (2061 6Ma) of the Mahalapye Complex indicate that its protolith may have been formed from remelting of a Neoarchaean juvenile crust. Variable eHf(t)initial values from 3 7 to þ6 3 of zircon cores (2711 11Ma) in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the Neoarchaean.

The Lisov Granulite Massif differs from neighbouring granulite bodies in the Moldanubian Zone of southern Bohemia (Czech Republic) in including a higher proportion of intermediate–mafic and orthopyroxene-bearing rocks, associated with spinel peridotites but lacking eclogites. In addition to dominantly felsic garnet granulites, other major rock types include quartz dioritic two-pyroxene granulites, tonalitic granulites and charnockites. Minor bodies of high-pressure layered gabbroic garnet granulites and spinel peridotites represent tectonically incorporated foreign elements. The protoliths of the mafic–intermediate granulites (quartz-dioritic and tonalitic) crystallized 360–370 Ma ago, as indicated by laser ablation inductively coupled plasma mass spectrometry U–Pb ages of abundant zircons with well-preserved magmatic zoning. Strongly metamorphically recrystallized zircons give ages of 330–340 Ma, similar to those of other Moldanubian granulites. For the overwhelming majority of the Lisov granulites peak metamorphic conditions probably did not exceed 800–900 C at 4–5 kbar; the equilibration temperature of the pyroxene granulites was 670– 770 C. This is in sharp contrast to conditions of adjacent contemporaneous Moldanubian granulites, which are characterized by a distinct HP–HT signature. The mafic–intermediate Lisov granulites are thought to have originated during Visean metamorphic overprinting of metaluminous, medium-K calc-alkaline plutonic rocks that formed the mid-crustal root of a Late Devonian magmatic arc. The protolith resembled contemporaneous calc-alkaline intrusions in the European Variscan Belt.

The Kyffhauser Crystalline Complex, Central Germany, forms part of the Mid-German Crystalline Rise, which is assumed to represent the Variscan collision zone between the East Avalonian terrane and the Armorican terrane assemblage. High-precision U–Pb zircon and monazite dating indicates that sedimentary rocks of the Kyffhauser Crystalline Complex are younger than c. 470 Ma and were intruded by gabbros and diorites between 345 ± 4 and 340 ± 1 Ma. These intrusions had magmatic temperatures between 850 and 900°C, and caused a contact metamorphic overprint of the sediments at P–T conditions of 690–750°C and 5–7 kbar, corresponding to an intrusion depth of 19–25 km. At 337 ± 1 Ma the magmatic–metamorphic suite was intruded by granites, syenites and diorites at a shallow crustal level of some 7–11 km. This is inferred from a diorite, and conforms to P–T paths obtained from the metasediments, indicating a nearly isothermal decompression from 5–7 to 2–4 kbar at 690–750°C. Subsequently, the metamorphic–magmatic sequence underwent accelerated cooling to below 400°C, as constrained by garnet geospeedometry and a previously published K–Ar muscovite age of 333 ± 7 Ma. With respect to P–T–D–t data from surrounding units, rapid exhumation of the KCC can be interpreted to result from NW-directed crustal shortening during the Visean.

This study presents new geochemical (major and trace element, Nd–Sr isotope) and U–Pb zircon, monazite, titanite and rutile data for various rock types (eclogite, high-pressure granulite, amphibolite, orthogneiss, leucosome) of the high-grade metamorphic Marianske Lazně Complex in the western Bohemian Massif. Concordant U–Pb zircon analyses from the mafic to intermediate samples disclose the Marianske Lazně Complex as a mixture of c. 540 Ma oceanic rocks juxtaposed at depth with lower-crustal rocks of the structurally overlying Tepla–Barrandian Unit. This interpretation refutes earlier models in which the Marianske Lazně Complex was interpreted as a dismembered Cambro-Ordovician ophiolite complex affected by Variscan subduction. Metamorphic zircon in mafic rocks of the Marianske Lazně Complex, as well as monazite from orthogneiss, and titanite in leucosome yield ages around 380 Ma. On the basis of detailed petrography and cathodoluminescence imaging we conclude that the c. 380 Ma age reflects the timing of Variscan exhumation and associated decompression melting under upper amphibolite-facies to granulite-facies conditions. Thereby, the data derived from metamorphic zircon of eclogites and high-pressure granulite, unexpectedly, do not date the timing of eclogitization, which could have happened just before Variscan exhumation, or even shortly after Late Cadomian protolith formation.