Tonalitic Gneisses Research Articles

Magmatic and metamorphic history of Paleoarchean tonalite–trondhjemite–granodiorite (TTG) suite from the Singhbhum craton, eastern India

Texturally controlled dating of zircon from Paleoarchean tonalite–trondhjemite–granodiorites of the Older Metamorphic Tonalitic Gneisses and the Singhbhum Granite batholith (Phases I, II, and III) from the Singhbhum craton in eastern India reveals a polycyclic evolution of the Archean crust. The granitoid suites were emplaced in two pulses at 3.45–3.44Ga and 3.35–3.32Ga. Tonalites and trondhjemites of the Older Metamorphic Tonalitic Gneisses were emplaced at ca. 3.45–3.44Ga together with Phase III of the Singhbhum Granite pluton while granites belonging to the Older Metamorphic Tonalitic Gneisses were emplaced at ca. 3.35–3.32 together with Phase I and Phase II of the Singhbhum Granite pluton. Both crustal units underwent an early phase of relatively high-grade metamorphism at 3.30–3.28Ga followed by extensive fluid-induced alteration during low-grade metamorphism at 3.19–3.12Ga, and 3.02–2.96Ga. The two units have also been marginally affected at ca. 2.52Ga and 1.06Ga by major metamorphic events in the North Singhbhum Mobile Belt and the Singhbhum shear zone at the northern margin of the craton. The zircon grains in granites have inherited cores with ages of ca. 3.61Ga and 3.46–3.41Ga and with well-developed oscillatory growth zonation which suggests the granitic magmas were derived by partial melting of an igneous precursor or sedimentary rocks derived from an igneous source. The emplacement of the expansive granitoids belonging to the Older Metamorphic Tonalitic Gneisses and the Singhbhum Granite was synchronous with the amphibolite-facies metamorphism (ca. 3.32Ga) of older meta-igneous and metasedimentary rocks belonging to the Older Metamorphic Group. Major felsic crust formation in the craton occurred in a narrow time interval between 3.46 and 3.32Ma with minor contributions of material as old as 3.6Ga. The complex polycyclic evolution of the Paleoarchean crust in the Singhbhum craton can account for the wide range of often disparate ages obtained using whole rock isochron dating techniques with some of the isochron dates being geologically meaningful while others representing mixing lines or disturbance of the isotopic systems during metamorphism.

Middle Neoarchean magmatism in western Shandong, North China Craton: SHRIMP zircon dating and LA-ICP-MS Hf isotope analysis

Western Shandong Province is a typical area of the North China Craton (NCC) where Neoarchean plutonic and supracrustal rocks are widely distributed. Early studies documented that ∼2.7Ga and ∼2.5Ga magmato-tectono-thermal events are well developed in the area. Here we report SHRIMP U–Pb ages and Hf-in-zircon isotopic data from ten samples of different magmatic rock types including hornblendite, gneissic tonalite, gneissic trondhjemite and gneissic granite. Magmatic zircon grains have 207Pb/206Pb ages ranging from 2667 to 2598Ma. Some rocks contain ∼2.5Ga metamorphic rims and ∼2.7Ga zircon cores. The magmatic zircon grains have ɛHf(t) values and Hf crustal model ages of −1.1 to +11.3 and 3.02–2.4Ga, respectively. Combined with an earlier study, our main conclusions are that the middle Neoarchean rocks mainly occur together with early Neoarchean rocks in the northeastern portion of the central belt. Juvenile additions to continental crust and crustal recycling played important roles in the middle Neoarchean of western Shandong Province, and the entire Neoarchean tectonic evolution can be divided into middle to early Neoarchean (2.75–2.6Ga) and late Neoarchean (2.6–2.5Ga) events.

A comment on ultrahigh-temperature metamorphism from an unusual corundum + orthopyroxene intergrowth bearing Al–Mg granulite from the Southern Marginal Zone, Limpopo Complex, South Africa, by Belyanin et al.

Studies on the Southern Marginal Zone (SMZ) of the Limpopo Belt have generally concluded that this terrain was affected by a single granulite-facies metamorphic event at 2.67–2.66 Ga (Kroner et al. 2000; Kreissig et al. 2000; Zeh et al. 2005; Elington and Armstrong 2004; Stevens and Van Reenen 1992a, b; Barton and van Reenen 1992; Barton et al. 1992; Van den Berg and Huizenga 2001; Rigby et al. 2008), with peak metamorphic conditions of 7.5–9.5 kbar and 800–850 C. In their recent paper, Belyanin et al. (2012) propose an ultrahigh-temperature (UHT) metamorphic event in the SMZ, with peak metamorphic conditions in excess of 1,000 C at approximately 12 kbar. Similar conclusions were proposed by earlier work on the same rock (Belyanin et al. 2010). The proposed UHT conditions are based on Al-rich orthopyroxene and ternary feldspar thermometry using reintegrated feldspar compositions. This evidence is derived from a single outcrop containing unusual aluminous layers within metasedimentary granulites, although Tsunogae et al. (2004) used Al-in-orthopyroxene and ternary feldspar thermometry to argue for a peak metamorphic temperature of [950 C from more representative metapelitic granulites of the Bandelierkop formation. Both these studies have proposed that UHT metamorphism affected the SMZ generally and thus that the relevant PT path of all rocks traversed UHT conditions. This interpretation is problematic because it is difficult to reconcile with the partial melting history of the rocks and with the peak metamorphic assemblage in most rocks. In essence, both the metasedimentary granulites of the Bandelierkop formation and the trondhjemitic to tonalitic Bavianskloof gray gneisses contain a significant proportion of biotite, which defines a syn-peak metamorphic fabric (Fig. 1). Within the metasediments of the Bandelierkop formation, compositional banding at the outcrop scale (Fig. 1) exists that is considered to represent primary differences in composition between different sedimentary units (van Reenen 1983). Anatexis of these rocks has occurred via three reactions (Stevens and van Reenen 1992a): Ms ? Qtz ? Pl = Sil ? Melt; Bt ? Sil ? Qtz ? Pl = Grt ? Melt; and Bt ? Qtz ? Pl = Opx ? Crd ? Melt. The first two reactions exhausted muscovite and sillimanite, respectively, while the third reaction had just begun to occur in the chemically most favorable rocks at the conditions of peak metamorphism. Pseudosection modeling of metapelitic granulites from the Bandelierkop quarry, which have undergone anatexis and that represent restitic, melt-depleted compositions, indicates that biotite in such rocks would not have survived temperatures significantly above 900 C and that the mineral compositions in such rocks are consistent with peak metamorphic temperatures of 820–860 C at 8–10 kbar (Fig. 2). At the peak metamorphic conditions proposed by Belyanin et al. (2012), these restitic granulites would have become Communicated by T. L. Grove.

Multiple intrusions and remelting-remobilization events in a magmatic arc: The St. Peter Suite, South Australia

Granitoids with juvenile signatures are common in arc environments and contribute to growth of the continental crust. Thermo-mechanical models of arcs suggest that intermittent intrusion of magma batches leads to magma hybridization, remelting, and remobilization of earlier intrusive rocks driven by fluctuations in temperature and water fluxing. While there are numerous examples in the literature of multiple intrusions and magma hybridization, field examples of remelting and remobilization of earlier intrusive rocks within an arc are rare. Here, we investigate the evolution of magmatic rocks of the Paleoproterozoic St. Peter Suite, emplaced along the SW margin of the Gawler craton, South Australia, a typical calc-alkaline arc suite. Magmatic rocks recording multiple intrusions and multiple magma interactions have undergone in situ remelting and remobilization forming migmatites. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon dating yielded crystallization ages of 1647 ± 12 Ma for a tonalitic gneiss representing the oldest intrusive suite, and 1604 ± 12 Ma for a leucogranite representing the youngest intrusive suite. Both these suites developed synmagmatic foliation and magmatic banding defined by broadly parallel dikes, elongated enclaves, and schlieren. The rock suites record two deformation events associated with anatexis. The first event, D 1 , was responsible for a dominant approximately E-W–striking foliation (S 1 ) parallel to magmatic foliation, and associated with a dominantly sinistral shearing that affected the older suite. This deformation was associated with the first anatectic event, as demonstrated by the association between leucosomes and structures. The second deformation event, D 2 , affected both suites and was characterized by isolated F 2 folds and shear planes filled with leucosomes subparallel to axial planar foliation. Leucosomes interconnected and gave rise to magma extraction channels tens of meters long. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb titanite dating of a late leucosome sample, collected from within the older magmatic suite, yielded an age of 1605 ± 7 Ma, coinciding with the crystallization age of the younger suite rather than postdating it, as expected. We interpret these results to indicate that crystallization of the younger suite and the second anatectic event occurred in the time encompassed by the error of these young ages. Leucosomes from both anatectic phases lack anhydrous peritectic phases and are interpreted to represent low-temperature anatexis resulting from water fluxing. Combined, these results suggest that a protracted and complex intrusive history can be made significantly more complex by anatexis, giving rise to evolved magmas after older ones, erasing earlier intrusive relationships, and establishing new ones. Rocks of the St. Peter Suite record many of the key processes expected in arcs, including the prediction that early intrusive arc rocks remelt to form younger and more fractionated magmas.

High-precision U–Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach

A novel approach of thermally annealing and sequentially partially dissolving single zircon grains prior to high-precision Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) is presented. This technique is applied to complex zircon from the Precambrian Lewisian Gneiss Complex of Scotland. Up to six partial dissolutions were conducted at incrementally higher temperatures and analysed at each successive step. ID-TIMS analyses reveal the portions of zircon affected by the lowest temperature partial dissolution step have suffered Pb-loss. Successively higher temperature partial dissolution steps yield a series of analyses from the younger domains, followed by mixing trajectories with older components, presumably from the inner domains. Specifically, for a partially retrogressed granulite tonalite gneiss from the central block (Assynt), high-grade metamorphic zircon ages of c. 2500Ma and c. 2700Ma are resolved with a protolith age of c. 2860Ma also recognised. This unequivocally demonstrates two separate episodes of high-grade metamorphism affected rocks from this region. The c. 2700Ma age provides a minimum age constraint on the highest pressure event known from Archean crustal rocks. Using this technique of pseudo-spatial resolution coupled with high-precision analysis it is possible to recognise discrete Pb-loss and multiple stages of zircon growth or isotopic resetting within single grains to within 0.1–0.2% error (2σ) on individual 207Pb/206Pb ages. This method has relevance to U–Pb zircon geochronology where conventional micro-beam techniques are unable to resolve between separate ages within single grains.

Spatio-temporal Variation in Radon Concentration in Groundwater with Respect to Rock Types: A Case Study from Chitradurga District, Karnataka

Abstract: An attempt was made in the present study to delineate how the radon concentrations vary with respect to different geological formations and to evaluate annual effective dose exposure due to ingestion of radon. A total of 60 groundwater samples were collected from layered sequential aquifers in Chitradurga district having major rock types such as Bababudan Group, Charnockite, Chitradurga Group, Closepet granite, migmatites and granodiorite – tonalitic gneisses and Sargur Schist complex during pre-monsoon and post-season of the year 2011. Radon measurement was made using Durridge RAD-7 radon-in-air monitor, connected to RAD H2O accessory with closed loop aeration concept. In the present study, the radon activity ranged from 0 to 186.6 Bq/L and 0 to 150.6 Bq/L during pre- and post-monsoon seasons of the year 2011, with 56.67 % (17 samples) of samples during both the seasons exceeding the EPA’s MCL value of 11.1 Bq/L. The annual mean radon activity in the groundwater was higher in the area having Chitradurga rock group formations (78.1 Bq/L) followed by Sargur-Satyamangalam schist complex group (56.8 bq/L), migmatites and granodiorite – tonalitic Gneisses group (56.3 Bq/L), Closepet granite (42.7 Bq/L), Charnonkite (29.1 Bq/L) and Bababudan Group (22.2 Bq/L). It is inferred that radon concentration found to depend on the tectonic structure, geology of the area and on the presence of uranium minerals in these rocks. The annual effective dose resulting from radon in groundwater in the Chitradurga district were significantly lower than UNSCEAR and WHO recommended limit of 1 mSv/y.

STRUCTURAL AND SUBSURFACE RELATIONSHIPS BETWEEN THE FORT RIXON-SHANGANI GREENSTONE BELT AND THE NALATALE PLUTON, ZIMBABWE CRATON, AS DERIVED FROM GRAVITY AND AEROMAGNETIC DATA

The geology, structure and subsurface mass distribution in the Fort Rixon-Shangani (FRSh) granite-greenstone terrain, Zimbabwe craton, has been investigated using regional aeromagnetic and gravity data, as well as density-constrained gravity modelling along four traverses/profiles. Processed aeromagnetic data reveal sets of north-northwest-trending dykes and faults, west-northwest-trending faults and a major north-northeast-trending shear zone bounding the FRSh greenstone belt in the southeast. The distribution of dykes is more widespread and continuous than previously recognised, while concealed mafic and ultramafic horizons or extensions are revealed. Cross-cutting relationships and magnetic anomaly displacements provide relative age constraints for the various dykes and associated faults in the area, with the north-northwest-trending dyke set being the youngest and the north-northeast-trending set the oldest. There has been repeated movement along the faults over time. The regional Bouguer gravity anomaly map shows that the Fort Rixon (FR) greenstone belt is marked by a 30 mGal positive anomaly, relative to a background of −120 mGal over the surrounding tonalitic gneisses. Both the Bouguer anomaly and first vertical gravity gradient maps suggest a smaller body than the current known outcrop, and this is confirmed by gravity modelling which shows a reduced extent of the greenstone belt lithologies with depth. The maximum anomaly appears to be significantly offset to the southwest, suggesting continuation of greenstone belt lithologies beyond surface exposures in to the southwest. The porphyritic Nalatale granite intrudes and bisects the FRSh greenstone belt, and is associated with a gravity low (~7 mGal amplitude), which appreciably reduces the FR greenstone belt positive anomaly in the northeast. The regional setting suggests a western granite-greenstone contact that may be explained by doming, an eastern contact that is marked by the Irisvale-Lancaster strike-slip shear zone, and a northern contact with an intrusive and post-tectonic granite. Interpretation of the gravity data by 2½D modelling along three profiles shows that the FR greenstone belt has a boat or v shape and a depth extent of 3.0 to 4.0 km. The thinnest part of the greenstone belt occurs in the northeast near the Nalatale granite, suggesting that the pluton underlies much of the greenstone belt in the area. The Nalatale pluton is on average 2.5 km thick under the greenstone belt but includes a root up to 4.5 km thick, and a steep contact with the tonalitic gneisses to the east. It is probably a wedge-shaped pluton that was emplaced along steeply oriented fractures in the brittle crust. The models show that the Nalatale pluton has a shallow outward dip with steep contacts at depth. These results, together with the surface and subsurface disposition of the various geological units, suggest that the greenstone belt underwent deformation by granite intrusion through diapiric-type geotectonic processes.

Geochemistry and P–T–t evolution of the Abu-Barqa Metamorphic Suite, SW Jordan, and implications for the tectonics of the northern Arabian–Nubian Shield

The Abu Barqa Metamorphic Suite (ABMS) represents the oldest part of the Arabian–Nubian Shield in southern Jordan. It comprises tonalitic gneiss, metasediments including schist and paragneiss, and granitic gneiss, intruded by later granitic bodies. Geochemically, the majority of the schist samples have shale and Fe-shale protoliths, while the paragneisses represent metagreywacke. Tectonic discrimination diagrams indicate that the protolith of the ABMS was deposited at an active continental margin/island arc setting.U–Pb zircon (SIMS) ages from metamorphic and igneous rocks of the ABMS indicate that it evolved between ∼800 and ∼610Ma. A tonalitic gneiss has a crystallization age of 787±3Ma. Detrital zircon from the metasediment has a range of concordant ages from 680 to 860Ma. The entire metamorphic complex was intruded by calc-alkaline granitoids (∼615–610Ma) and quartz diorite dated (∼600Ma).Field and petrographic investigations of ABMS metasediment elucidate the development of three metamorphic zones, from north to south: (1) andalusite–staurolite (andalusite+staurolite+biotite+muscovite+plagioclase±garnet+quartz±chlorite+fibrolitic sillimanite+accessories), (2) garnetsillimanite (sillimanite+garnet+biotite+plagioclase+quartz+K-feldspar+cordierite (pinitized)±rutile±ilmenite), and (3) cordierite–sillimanite (sillimanite+biotite+plagioclase+cordierite+hercynite+quartz+accessories).The maximum metamorphic conditions (M1) were attained in the garnet–sillimanite zone (5–6kbar, ∼700°C), while peak conditions of ∼3.2kbar and 540°C were obtained for the andalusite–staurolite zone (using both forward-pseudosection modeling and inverse-modeling with multi-equilibrium approach). Compositional isopleth calculations of small relics of garnet yield about 3.5kbar and 600°C for the sillimanite–cordierite zone. These values are in agreement with results obtained by TWQ inverse modeling. The M1 event most probably occurred around ∼625 and was followed by a decompressional thermal phase (M2) contemporaneous with post-tectonic granitoid emplacement at ∼615–610Ma; and lastly went through a retrograde cooling phase (M3) accompanying uplift of the whole complex to the surface at ∼605Ma.

Geochemistry and mineral phases of REE in Jammi carbonatite veins and fenites, southern end of the Sokli complex, NE Finland

The Jammi area consists of Archaean mafic volcanic rock and tonalitic gneiss, which were intruded by the Devonian Sokli carbonatite, causing large-scale and intense fenitization of the country rocks. The rocks are albite-fenites, crosscutting late carbonatite veins, with the mineral assemblages of calcite-dolomite-aegirine-albite-apatite-phlogopite. Geochemical analyses indicate that the total content of rare earth elements (REE) in the carbonatite veins is 0.11–1.83%, including 0.11–1.81% LREE and 0.002–0.041% HREE. Seven rare earth minerals have been analysed in detail from the area, including Sr-apatite, monazite, ancylite (Ce), bastnäsite (Ce), strontianite, baryte and brabanite. The Jammi carbonatite and fenite samples display similar REE patterns, with highest enrichment of LREE, and steep slopes towards the HREEs. The REE distribution in the whole-rock data is controlled by carbonates and apatite. REE-rich accessory phases such as monazite and ancylite (Ce) influence the REE pattern only when present in large amounts. Mineralogical and chemical evidence demonstrates that hydrothermal processes were responsible for the REE mineralization. During late-stage processes, apatite and carbonate minerals were replaced by various assemblages of REE–Sr–Ba minerals. Apatite could be a potential source for REE as a by-product of phosphate production in the Sokli area.

OLDEST U-PB CRYSTALLISATION AGE FOR THE WEST AFRICAN CRATON FROM THE OUDALAN-GOROUOL BELT OF BURKINA FASO

U-Pb geochronological data from the northern and western extensions of the Oudalan-Gorouol volcano-sedimentary belt (OGB) on 3 samples of granodiorite gneiss (sample BF\_1144), migmatitic gneiss (sample BF\_1299), and granite clast from a metaconglomerate (sample BF_1147) gave the oldest U-Pb crystallisation ages for outcropping Palaeoproterozoic rocks yet identified in the West African Craton (WAC). The U-Pb concordant crystallisation ages of 2253 ± 9 Ma and 2253 ± 15 Ma for samples BF\_1144 and BF\_1299, respectively, likely represent the age of crystallisation of parent granodiorite gneiss and may constrain the age of a gneissic protolith on which the OGB basin was deposited, or an early group of granitoids that was emplaced in the hinterland during formation of the OGB basin. A concordant age of 2255 ± 26 Ma for sample BF_1147 is interpreted as the age of crystallisation of a granite pluton that was eroded from a site nearby before becoming part of the Dembam Member, and constrains the maximum age of the Birimian Supergroup in the OGB. The older Pb-Pb ages on zircons from the granodiorite gneiss, migmatitic gneiss and granite clast are in the range ~2.34 to ~2.27 Ga, and are interpreted as inherited from pre-existing zircon cores. They are similar to Pb-Pb ages reported as inherited zircon ages from the Dabakala tonalitic gneiss (in Cote d’Ivoire).

Geology of the Monapo Klippe, NE Mozambique and its significance for assembly of central Gondwana

The Monapo Klippe in north-east Mozambique is an ovoid-shaped outcrop measuring approximately 35×40km and is clearly visible on satellite and geophysical images. Based on recent field mapping, geochemical studies and new geochronological data, we present a revision of the lithostratigraphy of the klippe and offer a model for its origin and emplacement in the framework of regional tectonics. There are three main groups of rocks within the klippe: (1) the Metachéria Metamorphic Complex; (2) the Mazerapane Intrusive Suite; and (3) the Ramiane Intrusive Suite. The Metachéria Metamorphic Complex consists of a mélange of granulite gneiss, including mafic, felsic, pelitic and carbonate rocks, characterised by a strong penetrative shear fabric. The largely undeformed Mazerapane and Ramiane Suites have intruded into the Metachéria Metamorphic Complex. The Mazerapane Suite consists of foid-bearing ultramafic and mafic gneisses and intrudes into the western half of the complex, whereas the Ramiane Suite is dominated by alkaline granitic rocks, contains no foid-bearing units and intrudes into the eastern half of the complex. In addition to these three main units, there are a number of minor but structurally important units, the main ones of which include amphibolite-facies tonalitic gneisses and the Evate calcite carbonatite. Underlying all of these units is a narrow, high strain mylonite zone. Undeformed pegmatite bodies and dykes cross-cut all rock types of the Monapo Klippe including the marginal mylonite. Near identical dates for the intrusion of the Ramiane Suite at 637±5Ma and metamorphism of the Metachéria Complex at 634±8Ma indicates a major episode of granulite-facies metamorphism and crust generation at this time. The ∼635Ma age for the granulite-facies metamorphism is comparable to granulite-facies events identified in other parts of the East African Orogen in Tanzania, Madagascar and other parts of northern Mozambique to the north of the Lúrio Belt. The absence of granulite-facies rocks in the underlying Nampula Block is consistent with structural arguments that the Monapo Klippe is the remnant of an allochthonous thrust sheet. In this context, the Monapo Complex is very similar to other granulite-facies “klippe” in East Africa, Antarctica and Sri Lanka, lending support to the idea of a Pan-African mega-nappe formerly existing across greater East Gondwanaland.

Paleoarchean partial convective overturn in the Singhbhum Craton, Eastern India

Analyses of fold superposition structures and kinematics of shear zones along a 120km long E–W corridor across the Singhbhum Craton (Eastern India) are combined with U–Pb–Th chemical ages of texturally constrained monazites in supracrustals, gneisses and granitoids to reconstruct the Paleoarchean tectonic history of the western part of the Craton. Metamorphic sub-greenschist facies shallow marine to platformal sediments of the Iron Ore Group (IOG) and the accordantly deformed Jagnnathpur meta-lavas were accreted onto a basement composed of >3.4Ga amphibolite facies para-amphibolites/quartzites/muscovite schists (Older Metamorphic Group, OMG) and anatectic Older Metamorphic Tonalite Gneisses (OMTG). Crustal shortening was followed by emplacement-ascent of the Singhbhum granitoid pluton, SGP (3.2–3.3Ga) synchronous with sinking of supracrustal screens along N/NNE-trending steep-dipping sinistral–normal ductile shear zones. Deformation microstructures indicate the bulk of the pluton responded to exhumation-strain by low-T (≤500°C) ductile flow of quartz and K-feldspar, with plagioclase being largely unaffected. But granitoids along the western margin of pluton emplaced concurrent with westward-retreating exhumation of the thickened crust was deformed at T≥650°C. Since all granitoids in the pluton share the same N/NNE-trending tectonic fabric, microstructural features imply phased emplacement of the pluton during exhumation strain. The transition from crustal shortening to exhumation interrupted by phased granitoid emplacement between 3.2 and 3.4Ga is explained by a two-stage partial convective overturn model involving structures typical of supracrustal-down, granitoid-up sense of movement. The tectonic restoration in the western part of the Singhbhum Craton is new and complements growing evidence that accretion and convective overturn may have operated during the Paleoarchean.

Geochemistry, petrography, and zircon U–Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon–Tanana terrane

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

The latest Neoarchean–Paleoproterozoic evolution of the Dunhuang block, eastern Tarim craton, northwestern China: Evidence from zircon U–Pb dating and Hf isotopic analyses

The Dunhuang block, in the easternmost segment of the Tarim craton, exposes Precambrian mafic granulite and felsic gneiss. To better understand the poorly known formation and evolution of the Tarim craton, we carried out detailed field-based petrological and geochronological investigations on the major lithologies of the Dunhuang block. U–Pb dating and Hf isotopic analyses on the mafic granulite and felsic gneiss (TTG gneiss) of the Dunhuang block reveal two distinct age populations: the Latest Neoarchaean (∼2.5Ga) and the Late Palaeoproterozoic (1.82–1.85Ga). The zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of ∼2.5Ga, supporting the existence of Archean rocks in the Dunhuang block. The short time interval between Latest Neoarchean magmatism and high-grade metamorphism suggests that they were related to the same Latest Neoarchean tectonothermal event. The ∼2.5Ga zircons show Hf-depleted mantle model ages between 2.55Ga and 2.8Ga with a peak at ∼2.7Ga, suggesting that ∼2.7Ga juvenile crust of the Dunhuang block was reworked during a 200–300m.y. period at the end of the Neoarchean. The age of ∼1.85Ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high-pressure granulite facies metamorphism. Combined with the Hf isotopic data, these data suggest that the Dunhuang block experienced ∼2.7Ga crustal growth, a ∼2.5Ga magmatic–metamorphic event and a 1.82–1.85Ga (HP) granulite facies metamorphic event. This sequence of events is very similar to that of the North China Craton, implying that the evolutionary history of the Dunhuang block was consistent with that of the North China Craton from the Neoarchean to Paleoproterozoic.

EXTREME FRACTIONATION AND DEFORMATION OF THE LEUCOGRANITE - PEGMATITE SUITE AT RED CROSS LAKE, MANITOBA, CANADA. I. GEOLOGICAL SETTING

Three types of subvertical leucogranites and pegmatites populate the Red Cross Lake pegmatite field, located in the Red Cross Lake greenstone belt of the Sachigo Subprovince, in the northwestern part of the Archean Superior Province of the Canadian Shield. The greenstone belt is situated in the northern margin of the Oxford–Stull domain of the North Caribou terrane, in tectonic contact with the Hudson Bay terrane. The tholeiitic greenstone belt, flanked by plutonic masses of tonalitic gneiss to the north and tonalite–granodiorite gneiss to the south, consists of a bimodal assemblage of metabasalt + subvolcanic metagabbro and felsic lapilli tuff of andesitic to rhyolitic compositions, mafic mylonite with a mylonitized granitoid, and volcaniclastic metagreywacke. Regional context and petrochemical features of the south-flanking tonalitic-granodioritic masses and of the greenstone-belt lithologies indicate a largely juvenile, deep-oceanic origin. To the north, the isotopic signatures of tonalitic-granodioritic masses indicate an older continental association. The leucogranites and pegmatites and related intrusions were subvertically emplaced into, or alongside, the predominantly mylonitized metavolcanic rocks of the WNW–ESE-trending North Kenyon Shear Zone. Some of the leucogranites are geochemically rather primitive, but they grade into more evolved pegmatite bodies and ultimately into extremely fractionated lepidolite-subtype pegmatites. Investigation of the origin of the rare-element enrichment in these pegmatites, within the limits of our reconnaissance sampling, did not show above-average enrichment of rare lithophile elements in any of the main regional map units.

Delineation of the ca. 2.7 Ga TTG gneisses in the Zanhuang Complex, North China Craton and its geological implications

Through detailed studies we have delineated a suite of banded TTG gneisses from the Zanhuang Complex. The protolith of the gneisses, predominantly tonalite, has undergone intensive metamorphism, deformation and anatexis and in a banded structure is intimately associated with melanocratic dioritic gneiss and leucocratic trondhjemitic veins. SHRIMP Zircon U–Pb data show that the tonalite was formed ca. 2692±12Ma ago. The tonalitic gneiss has the features of high SiO2 (67.76–73.31%), high Al2O3 (14.38–15.83%), rich in Na2O (4.48–5.07%) and poor in K2O (0.77–1.93%). The gneiss is strongly fractioned in REE ((La/Yb)N=12.02–24.65) and shows a weak positive Eu anomaly (Eu/Eu*=1.05–1.64). It has high contents of Ba (199–588ppm) and Sr (200–408ppm), low contents of Yb (0.32–1.00ppm) and Y (3.41–10.3ppm) with high Sr/Y ratios (21.77–96.77) and depletion in HFSE Nb, Ta and Ti. These characteristics are similar to those of the high-Si adakitic rocks. The melanocratic dioritic gneiss has low SiO2 (59.81%), high MgO (6.34%), high Al2O3 (14.02%) contents, rich in Na2O (3.7%) and poor in K2O (1.79%), with high Mg index (Mg#=67). REE and trace elements are on the whole similar to that of the tonalitic gneiss, but compatible element abundances V (116ppm), Cr (249ppm), Co (37ppm) and Ni (179ppm) are higher. The leucocratic felsic bands (approximating trondhjemite in composition) have major oxides similar to that of the TTG gneisses but the REE and compatible elements are extremely low, which are indicative of the products of anatexis. The tonalitic gneiss has positive εNd(t) (2.37–3.29) and low initial Sr (0.69719–0.70068) values with depleted mantle Nd model age of ca. 2.8Ga, suggesting its generation from partial melting of mantle-derived juvenile crust. The dioritic gneiss was also derived from subduction environment, but has undergone significant metasomatism of mantle wedge. The delineation of the ca. 2.7Ga TTG gneisses in the Zanhuang Complex further proves that the North China Craton experienced large-scale continental crustal accretion in early Neoarchean, and gives new constraints on the subdivision of the early blocks and greenstone belts of the craton.

Late Proterozoic juvenile arc metatonalite and adakitic intrusions in the Sør Rondane Mountains, eastern Dronning Maud Land, Antarctica

This study is a detailed investigation of the petrology and geochronology of the Late Proterozoic metatonalite in the Sør Rondane Mountains, eastern Dronning Maud Land, Antarctica. Metatonalite is dominant over roughly 100×20km2 area in the southwestern end of the mountain range and is classified into five lithologies: gneissose Bt–Hbl metatonalite, weak gneissose Hbl–Bt metatonalite, Hbl metagabbro, Hbl–Bt tonalitic gneiss, and Bt metatonalite. The gneissose Bt–Hbl metatonalite is the main lithotype widely distributed over this area, which is geochemically categorized as low-K tholeiitic granitoid. Petrological studies suggest that the tholeiitic magma was derived from low-K basalt melting at the crustal depth, and the most plausible tectonic setting is a juvenile oceanic arc. The other four metaplutonic rocks are scattered as stocks or small intrusions in this area. They are geochemically regarded as calc-alkaline adakites related to oceanic slab melting. U–Pb SHRIMP zircon ages of the tholeiitic metatonalite are concentrated at 998–995Ma, whereas the calc-alkaline adakitic rocks are younger and divided between ages 945–920Ma and 772Ma. We believe that the tholeiitic metatonalite was formed first as a juvenile arc component between 998 and 995Ma, followed by adakitic magmatism and oceanic slab melting at 945–920Ma and 772Ma. Magmatism during this stage is not recorded in the western to central Dronning Maud Land. Moreover, the exposure of an adakitic equivalent continues at the farther eastern side of the Sør Rondane Mountains. This suggests that the tectonic framework of the Sør Rondane Mountains, eastern Dronning Maud Land, is different from the western to central Dronning Maud Land.

Timing and Evolution of Cretaceous Island Arc Magmatism in Central Cuba: Implications for the History of Arc Systems in the Northwestern Caribbean

AbstractSHRIMP and conventional zircon dating place temporal constraints on the evolution of the Cretaceous Volcanic Arc system in central Cuba. The arc has a consistent stratigraphy across strike, with the oldest and deepest rocks in the south (in tectonic contact with the ∼5–10-km-wide Mabujina Amphibolite Complex [MAC]) and younger rocks in the north. The MAC is thought to represent the deepest exposed section of the Cretaceous Volcanic Arc and its oceanic basement in Cuba. We undertook a single zircon geochronological study of five gneisses and two amphibolites from the MAC and seven rocks from the Manicaragua Batholith, which intrudes both the MAC and the Cretaceous Volcanic Arc. A SHRIMP zircon age of Ma for a trondhjemitic orthogneiss (MAC) from the Jicaya River dates the oldest phase of granitoid magmatism in this area and the entire Caribbean (Antillean) region. A tonalitic gneiss collected near the previous sample yielded an age of Ma, and a further tonalitic gneiss had an age of Ma, with one in...

The Rayner tectonic Province of East Antarctica: Compositional features and geodynamic setting

Geochemical and new isotopic (U-Pb, Sm-Nd) data on the Mesoproterozoic metaigneous complexes of the Rayner Province in central East Antarctica (Enderby Land-Kemp Land and the northern Prince Charles Mountains) are presented. These territories are mainly composed of amphibolite-to-granulite-facies orthogneisses, many of which are Y-depleted tonalite gneisses and mafic schists. The igneous complexes of their protolith are largely products of anatexis of the lower crust; mantle-derived and upper crustal rocks are less abundant. The geochemical features of the mafic rocks indicate that they crystallized from high-temperature plume-related mantle melts and low-temperature lithospheric melts. As follows from the published and new Nd model ages, the Rayner Province formed and evolved over the Paleo-to-Mesoproterozoic in the regime of accretionary and collisional tectonics with predominance of accretion of the juvenile Paleoproterozoic crust between 1500–2400 Ma. New data show that in the northern Prince Charles Mountains, granite-gneiss protoliths were emplaced ca. 1040 and 930 Ma ago. The Rayner Province is considered to be a long-living mobile belt formed as a result of collision of Paleoproterozoic island-arc terranes and Archean blocks amalgamating into a continental massif 1050–1000 Ma ago in the course of the growth of the Rodinia supercontinent. In the northern Prince Charles Mountains, thermal processes related to magmatic underplating at the base of the crust were probably important.

Time constraints on magmatism along the Major Gercino Shear Zone, southern Brazil: Implications for West Gondwana reconstruction

The Dom Feliciano Belt, situated in southernmost Brazil and Uruguay, contains a large mass of granite–gneissic rocks (also known as Florianópolis/Pelotas Batholith) formed during the pre-, syn- and post-orogenic phases of the Brasiliano/Pan-African cycle. In the NE extreme of this granitic mass, pre-, syn- and post-tectonic granites associated with the Major Gercino Shear Zone (MGSZ) are exposed. The granitic manifestation along the MGSZ can be divided into pre-kinematic tonalitic gneisses, peraluminous high-K calc-alkaline early kinematic shoshonitic, and metaluminous post-kinematic granites. U–Pb zircon data suggest an age of 649±10Ma for the pre-tectonic gneisses, and a time span from 623±6Ma to 588±3Ma for the early to post-tectonic magmatism. Negative εHf (t) values ranging from −4.6 to −14.6 and Hf model ages ranging from 1.64 to 2.39Ga for magmatic zircons coupled with whole rock Nd model ages ranging from 1.24 to 2.05Ga and εNd (t) values ranging from −3.84 to −7.50, point to a crustal derivation for the granitic magmatism. The geochemical and isotope data support a continental magmatic arc generated from melting of dominant Paleoproterozoic crust, and a similar evolution for the granitic batholiths of the eastern Dom Feliciano Belt and western Kaoko Belt.