Suite Of Gneisses Research Articles

Zircon U-Pb-Hf constraints on the geological evolution of the Bihar Mica Belt, Central Indian Tectonic Zone, and its link to Columbia assembly

In this study, U-Pb-Hf isotope composition and trace element chemistry of zircon from basement gneisses and quartzites of the Bihar Mica Belt (BMB), a belt of supracrustal rocks within the Central Indian Tectonic Zone, is used to constrain crustal growth and reworking. The protoliths of BMB gneisses were emplaced at 1651 ± 5 Ma. The magmas were derived from juvenile and 1747 ± 7 Ma I-type long-lived felsic precursors that survive as xenocrystic cores. Zircon grains from quartzites display two distinct morphologies. Grains with little rounding of edges have internal structures comprising 1750 ± 5 Ma inherited cores mantled by 1657 ± 4 Ma igneous zones that are identical to those from the granite gneisses. The εHf (t) and trace element composition of both the xenocrystic cores and the igneous mantles in these detrital grains are identical to those of zircon in the gneisses. They constitute molasse-like sediments derived from the nearby basement. More rounded grains furnish age clusters between 1857 and 2848 Ma, suggestive of long-distance transport and derivation from multiple sources. In-situ formed metamorphic rims around zircons in gneisses and quartzites furnish 1476–1466 Ma ages, dating the earliest metamorphic overprint. The age of sedimentation is bracketed between 1657 Ma and 1466 Ma. The 1651 ± 5 Ma gneisses and its 1747 ± 7 Ma felsic precursor were I-type, and emplaced in an arc setting. They constitute part of the expansive suite of Paleoproterozoic gneisses of the Chhota Nagpur Gneiss Complex emplaced during protracted period of arc-magmatism between 1.75 Ga and 1.65 Ga. Strongly positive εHf(t) of 2.0–2.85 Ga detrital zircons conforms to the global trend of positive εHf(t) excursions and high dT/dP gradients, and may correspond to passive margin juvenile magmatism associated with the break-up of Archean supercratons prior to Columbia assembly. The 1.86 Ga and 1.75 Ga detrital and xenocrystic zircons from the BMB have enriched εHf(t), in accordance with negative excursions seen in global zircon between 2.0 Ga and 1.70 Ga, reflecting progressive assembly of Columbia through collisional orogenesis. An excursion to strongly positive εHf(t), coinciding with low dT/dP gradients at ca. 1.65 Ga, reflects wide-spread formation of arcs linked to accretionary growth of Columbia.

Long-lived (>100 m.y.) postcollisional exhumation and cooling in the Paleoproterozoic Trans−North China orogen: Evidence from phase equilibria modeling and monazite petrochronology of granulite-facies metapelites in the Fuping Complex

Long-lived collisional orogens that formed over tens to hundreds of millions of years are common in the geologic record. The Trans−North China orogen marks the collision between the Eastern and Western blocks of the North China craton, and it preserves metamorphic rocks with ages between 1.98 Ga and 1.80 Ga. These units allow detailed assessment of the time scale and duration of crustal thickening, exhumation, and cooling associated with a major Proterozoic orogeny. In this study, we present integrated petrography, mineral chemistry, phase equilibria modeling, and texturally controlled in situ mass spectrometry of monazite U-Th-Pb and trace-element analyses performed on a suite of orthopyroxene-bearing pelitic granulites and garnet-biotite gneisses from the Fuping Complex within the Trans−North China orogen. These rocks record clockwise pressure-temperature (P-T) paths involving granulite-facies peak conditions of 9.9−11.0 kbar and 850−880 °C for pelitic granulites, and 10.9−11.6 kbar and 860−880 °C for garnet-biotite gneisses, followed by postpeak decompression to ∼8−9 kbar and later cooling, with final solidification of melt at <840 °C. Four monazite populations were identified in these samples. Group I grains are irregular and elongated and occur in contact with or embay garnet. They have high rare earth element (REE) and Y contents and metamorphic ages of 1.90−1.86 Ga, which correspond to the breakdown of garnet during postpeak decompression. Most monazite grains crystallized from melt are represented by groups II + III + IV and are associated with orthopyroxene, biotite, plagioclase, and quartz in the matrix. They have crystallization ages between 1.86 Ga and 1.76 Ga and relatively low REE and Y concentrations. These data imply a long-lived (>100 m.y.) postcollisional exhumation and cooling involving decompression from 10−12 kbar to ∼9 kbar during 1.90−1.86 Ga, followed by retrograde cooling from 1.86 to 1.76 Ga under prolonged residence in the middle to lower crust. Initial collision and peak metamorphism occurred before 1.90 Ga, ultimately leading to the final cratonization of the North China craton and its incorporation into the Columbia supercontinent.

Granitoid gneisses of the Morokweng impact structure: Implications for Neoarchaean evolution of the western Kaapvaal craton

One of the most enigmatic aspects of the geology of the Kaapvaal craton of southern Africa concerns the age and nature of the rocks involved in the formation of its western parts, which are almost completely obscured beneath extensive Neoarchaean to Cenozoic supracrustal sequences. A 369-m-deep drillhole near the centre of the ~70 km wide Morokweng impact structure intersects a suite of Neoarchaean, calc-alkaline, granitic-granodioritic, trondhjemitic and monzonitic gneisses. The gneisses are LREE-enriched, but most display primitive mantle-like to slightly depleted HREE concentrations suggestive of a mantle source with garnet retention in the restite. The monzonitic gneiss is distinctly more enriched in trace and REE, and particularly LREE, which is attributed to differentiation processes in parent magmas to the granitoids. Microbeam zircon U Pb geochronology indicates an emplacement age of 2922 ± 5 Ma for the oldest granite, with the remaining granitic, granodioritic and monzonitic rocks being emplaced coevally at 2906 ± 6 Ma. Rare xenocrystic cores preserve an inherited >3.3 Ga crustal component. These crystallization ages coincide with the proposed culmination of westward-directed subduction beneath a significant continental fragment – the Kimberley Block - that led to its collision with the proto-Kaapvaal craton (Witwatersrand Block) between 2.93 and 2.88 Ga, and support a magmatic arc setting. Based on the age data, the granitoid gneisses intersected in the M4 drillhole predate ca. 2.88 Ga unfoliated granitoids found in outcrops and other drillcores in the vicinity that have been interpreted as post-orogenic intrusions but which nonetheless show similar geochemical characteristics to the M4 core granitoids. The distinct age of the M4 granitoid gneisses relative to other granitoid rocks in the Morokweng region may reflect the greater exhumation that occurred in the central parts of the 146 Ma Morokweng impact structure relative to its margins. • Granitoid gneisses exposed by the 146 Ma Morokweng impact belong to a 2.90–2.92 Ga suite of calc-alkaline TTG gneisses • Monzonitic gneiss is strongly enriched in REE, suggesting derivation from a residual magma after fractional crystallization • The granitoid gneisses are interpreted as mid-crustal intrusions formed in a magmatic arc setting • Their intrusion marks the climax of collision and amalgamation of the two major crustal blocks constituting the Kaapvaal craton • Collision affected the tectonic style and sedimentation in the Witwatersrand basin, assisting formation of its gold deposits

The geology of the Clifden district, Connemara Co. Galway, Ireland and present understanding of Connemara geology

The Clifden district, illustrated with an accompanying new detailed 1:12,500 geological map and fold trace map, includes most of the stratigraphy of the Connemara Dalradian and is used to summarise present understanding of the older geological history of Connemara in the light of new structural work and recent age determinations. The Grampian metamorphism peaked with sillimanite formation during and after the late D2 syntectonic intrusions of metagabbros at 475–470Ma and the formation of the major D2 Derryclare fold which was then repeatedly folded by numerous major D3 folds synchronous with the injection in the south of the 467Ma Quartz diorite gneiss suite, maintaining the high-grade metamorphism. Late D3 saw the general initiation of cooling and uplift, the latter pronounced in eastern Connemara above and around the subterranean gathering of the Oughterard Granite magma, plus movement on the late D3 Renvyle-Bofin slide, generating an area of lower pressure metamorphism with cordierite and andalusite not found in the Clifden area.

The geology of the Clifden district, Connemara Co. Galway, Ireland and present understanding of Connemara geology

The Clifden district, illustrated with an accompanying new detailed 1:12,500 geological map and fold trace map, includes most of the stratigraphy of the Connemara Dalradian and is used to summarise present understanding of the older geological history of Connemara in the light of new structural work and recent age determinations. The Grampian metamorphism peaked with sillimanite formation during and after the late D2 syntectonic intrusions of metagabbros at 475–470Ma and the formation of the major D2 Derryclare fold which was then repeatedly folded by numerous major D3 folds synchronous with the injection in the south of the 467Ma Quartz diorite gneiss suite, maintaining the high-grade metamorphism. Late D3 saw the general initiation of cooling and uplift, the latter pronounced in eastern Connemara above and around the subterranean gathering of the Oughterard Granite magma, plus movement on the late D3 Renvyle-Bofin slide, generating an area of lower pressure metamorphism with cordierite and andalusite not found in the Clifden area.

Evolution of the Archean continental crust in the nucleus of the Yangtze block: Evidence from geochemistry of 3.0 Ga TTG gneisses in the Kongling high-grade metamorphic terrane, South China

Archean Tonalite-Trondhjemite-Granodiorite (TTG) rocks are scattered within the Kongling high-grade metamorphic terrane (KHMT) in the northern South China block. A comprehensive geochronological and geochemical study is carried out on the Taoyuan granitic gneisses, a newly recognized TTG suite in the northwestern KHMT. This suite has long been regarded as a Mesoproterozoic magmatic pluton, but U-Pb zircon ages of 2994 ± 22 Ma and 2970 ± 15 Ma are obtained by LA-ICP-MS method in this study. The Taoyuan gneiss suite is trondhjemitic in composition, and has high SiO2 (67.80–74.93 wt.%), Na2O (5.11–5.81 wt.%) contents with Na2O/K2O ratios greater than unity, and low Ni (2.56–7.61 ppm), Cr (1.26–7.67 ppm), Yb (0.32–0.82 ppm) and Y (4.48–11.5 ppm) contents. Plots show large variation in La/Yb and Sr/Y ratios and pronounced depletion in Nb, Ta and Ti in the primitive mantle-normalized spiderdiagram. The gneiss suite also displays two-stage Nd model ages close to its crystallization age with corresponding εNd(t) values of −2.5 to +3.5. It is thus suggested that the Taoyuan gneisses, in fact, is part of the Archean Kongling basement complex.Geochemical evidence implies that the TTG rocks may be derived from partial melting of subducted oceanic crust from a garnetiferous amphibolite source with residual assemblage of garnet + amphibole + plagioclase. Our study further indicates that the nucleus of the Yangtze block might experience a juvenile continental crustal growth during Mesoarchean. We also suggest that the Yangtze block may have its own crustal evolutionary history independent from the North China craton and the Tarim block before Paleoproterozoic.

Petrology and U–Pb geochronology of zircon in a suite of charnockitic gneisses from parts of the Chotanagpur Granite Gneiss Complex (CGGC): evidence for the reworking of a Mesoproterozoic basement during the formation of the Rodinia supercontinent

Abstract Understanding the evolution of the Chotanagpur Granite Gneiss Complex (CGGC) of the East Indian Shield is crucial to decipher the role of the Indian Shield in the formation of the Rodinia supercontinent. The area around Deoghar–Dumka exposes a suite of granulite-facies orthogneisses (variably retrogressed to amphibole–biotite gneiss) that enclose remnants of Palaeoproterozoic metasedimentary and meta-igneous rocks. Results from mineral chemistry, laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) U–Pb dating of zircon and limited bulk-rock compositions of the studied rocks suggest that the magmatic protoliths of the felsic orthogneisses had A-type chemistry, and that these were emplaced at approximately 1450 Ma presumably in a continental rift setting. Intense deformation and metamorphism of the felsic rock culminated at approximately 9 kbar and 850°C along an apparent geothermal gradient of 26°C km −1 . These peak metamorphic conditions were successively followed by initially a steeply decompressive and then a weakly decompressive retrograde pressure–temperature path. The shape of the retrograde pressure–temperature path and the estimated geothermal gradient at the metamorphic peak are interpreted to be the products of continent–continent collision; U–Pb dates of metamorphic zircon overgrowths suggest an age of approximately 943 Ma for the collisional event. This study demonstrates that ‘Grenville-age’ orogenesis thoroughly reworked the approximately 1450 myr-old basement of the CGGC during the formation of the Rodinia supercontinent.

Two orogenic events separated by 2.6 Ga mafic dykes in the Central Zone, Limpopo Belt, southern Africa

The Limpopo Belt of southern Africa is a typical early Precambrian orogen that experienced two high-grade metamorphic events which are a key for understanding its tectonic evolution. There has been a long-standing debate on whether the Neoarchean (c. 2.65Ga) or the Palaeoproterozoic (c. 2.0Ga) tectono-thermal event records continental collision. The clear field relationship between deformed mafic dykes and the surrounding rocks is a powerful tool to help reconstructing the deformation history. Mafic dykes intruded the 3.3–3.1Ga Sand River Gneiss Suite in the Central Zone of the Limpopo Belt near Musina, South Africa, and were classified in the past into older Causeway dykes and younger Stockford dykes according to assumed field relationships, deformational patterns, and doubtful Rb-Sr dating. Our detailed field observations suggest that both dyke types belong to the same generation and cut the foliation and anatectic melt veins in the Sand River Gneiss Suite. They also experienced strong post-emplacement deformation and high-grade metamorphism, so they separate two high-grade metamorphic events. SHRIMP zircon dating yielded a likely emplacement age of ∼2.6Ga for two Causeway dyke samples as well as metamorphic ages of 2002±4Ma and 2002±8Ma. The zircons of two Stockford dyke samples yielded igneous emplacement ages of 2607±5 and 2604±6Ma, respectively, and metamorphic ages of 2014±13Ma and 2005±11Ma. Combining these data with similarities in the dyke geochemistry, we propose that the Causeway and Stockford dykes formed at the same time, but had different orientations. Our preferred tectonic model for the evolution of the Central Zone of the Limpopo Belt involves continental collision between the Zimbabwe and Kaapvaal Cratons at∼2.65Ga, followed by intracontinental transpressive orogeny and high-grade metamorphism at ∼2.0Ga. We therefore propose that the widespread ∼2.6Ga mafic dykes in the Central Zone formed in an extensional environment during post-collisional orogenic collapse. Both the Neoarchaean and Palaeoproterozoic orogenic events resulted in high-grade metamorphism and are separated in time by emplacement of the ∼2.6Ga mafic dykes.

Hadean origins of Paleoarchean continental crust in the central Wyoming Province

The scarce remnants of Earth’s earliest history make it challenging to describe the crust-forming processes that operated during that time, and all evidence that survived subsequent tectonism and recycling deserves to be studied closely. We present geologic, petrologic, geochemical, and isotopic descriptions of Paleoarchean gneisses in the central Wyoming Province. We identify two groups of gneisses: a bimodal suite of amphibolite and tonalite-trondhjemite-granodiorite (TTG) layered gneisses (3385−3450 Ma), and a suite of massive trondhjemite and granite gneisses (3300−3330 Ma). Here, 3.82 Ga inherited zircon components are present in several samples. Negative bulk rock initial e Nd values also indicate that older crust was involved. Oxygen isotopic compositions of zircon mainly fall within the range of mantle zircon δ 18 O values, but several analyses extend up to ∼6.5‰. Initial Hf isotopic compositions of 3.82 Ga zircon are negative and require derivation from Hadean crustal sources. Our data reveal the record of a 3.82 Ga differentiation event during which Hadean crust was partially melted, and zircon crystallized from those melts. Hadean crust must have persisted in the central Wyoming Province until 3.45−3.37 Ga, when mafic crust partially melted to form the TTG layered gneisses, which also incorporated the 3.82 Ga zircons. Subsequent intracrustal recycling at 3.33−3.30 Ga produced calc-alkalic granites. The central Wyoming Province provides a significant addition to the sparse record of Hadean crust being magmatically reworked to form the abundant quartzofeldspathic gneisses common in Archean terrains worldwide, which effectively transformed Earth’s evolving continental crust from mafic to felsic in composition.

Regional tectonics, geology, magma chamber processes and mineralisation of the Jinchuan nickel-copper-PGE deposit, Gansu Province, China: A review

The Jinchuan Ni-Cu-PGE deposit (>500 Mt @ 1.2% Ni, 0.7% Cu, ∼0.4 g/t PGE), one of the largest magmatic sulphide deposits in the world, is located within the westernmost terrane of the North China Craton. It is hosted within the 6.5 km long, Neoproterozoic (∼0.83 Ga) Jinchuan ultramafic intrusion, emplaced as a sill-like body into a Palaeoproterozoic suite of gneisses, migmatites, marbles and amphibolites, below an active intracratonic rift. The parental magma was high-Mg basalt, generated through melting of sub-crustal lithospheric mantle by a mantle plume during the initiation of Rodinia supercontinent breakup. The lower Palaeozoic collision of the exotic Qilian Block with the breakup-related southern margin of the craton accreted a subduction complex, and emplaced voluminous granitic intrusions and foreland basin sequences within the craton, to as far north as Jinchuan. During the Cainozoic, allochthonous lower Palaeozoic rocks were thrust up to 300 km to the northeast over cratonic basement, to within 25 km of the Jinchuan deposit.The Jinchuan ultramafic intrusion was injected into three interconnected sub-chambers, each containing a separate orebody. It essentially comprises an olivine-orthopyroxene-chromite cumulate, with interstitial orthopyroxene, clinopyroxene, plagioclase and phlogopite, and is predominantly composed of lherzolite (∼80%), with an outer rim of olivine pyroxenite and cores of mineralised dunite. Mineralisation occurs as disseminated and net-textured sulphides, predominantly within the dunite, with lesser, PGE rich lenses, late massive sulphide accumulations, small copper rich pods and limited mineralised diopside skarn in wall rock marbles. The principal ore minerals are pyrrhotite (the dominant sulphide), pentlandite, chalcopyrite, cubanite, mackinawite and pyrite, with a variety of platinum group minerals and minor gold. The deposit underwent significant post-magmatic tremolite-actinolite, chlorite, serpentine and magnetite alteration. The volume of the Jinchuan intrusion accounts for <3% of the total parental magma required to generate the contained olivine and sulphide. It is postulated that mafic melt, intruded into the lower crust, hydraulically supported by density contrast buoyancy from below the Moho, ponded in a large staging chamber, where crystallisation and settling formed a lower sulphide rich mush. This mush was subsequently injected into nearby shallow dipping faults to form the Jinchuan intrusion.

Convergent margin processes during Archean–Proterozoic transition in southern India: Geochemistry and zircon U–Pb geochronology of gold-bearing amphibolites, associated metagabbros, and TTG gneisses from Nilambur

The northern domain of the Southern Granulite Terrane (SGT) in India comprises a collage of Mesoarchean to Neoarchean crustal blocks which preserve important imprints of crustal growth during the early history of the Earth. Here we investigate the zircon U–Pb geochronology and geochemistry of a suite of amphibolites, metagabbros, TTG gneisses and charnockites from the Nilambur region, which forms part of the Wynad Gold Belt in the SGT. Magmatic zircons from three amphibolites yield weighted mean 207Pb/206Pb ages of 2661±59Ma, 2499±19Ma, 2570±19Ma and 2542±49Ma, closely followed by metamorphism at ca. 2.45Ga. Zircons from the TTG gneisses show protolith emplacement ages of 2619±21Ma and the overgrowth rims define an age of 2524±6Ma. Zircons from the metagabbro show spot ages between 2576Ma and 2717Ma and a weighted mean 206Pb/207Pb age of 2644±30Ma, with metamorphism at 2503±28Ma. The U–Pb data suggest prominent magmatic and metamorphic events during the Archean–Proterozoic transition.Geochemical features of the Nilambur rock suite suggest that the amphibolites, metagabbros, and TTG gneisses could be related to a common basaltic protolith. Their data are consistent with formation in primitive arc magmatic settings with MORB-like components in the source followed by magmatic differentiation. The amphibolites and metagabbros show negative Nb–Ta, Zr–Hf and Ti anomalies, typical of subduction-related intraoceanic tholeiitic arc basalt. Their low Th/Ce ratios (<0.1) preclude any significant contribution from subducted sediments. The rocks are characterized by marked enrichment in LILE and LREE, and show relative depletion of HFSE. Most of the samples show a flat or slightly LREE enriched patterns, with P, Ti, Th and Nb depletion. The results are consistent with magma derivation from MORB-like mantle wedge, without metasomatism by LILE–LREE-rich fluids derived through the dehydration of the subducted slab. The data obtained in this study, and those from recent studies suggest that the crustal blocks adjacent to the southern margin of the Dharwar Carton in Peninsular India preserve important evidence for active convergent margin tectonics during Archean–Proterozoic transition associated with the generation and emplacement of subduction-related arc magmas and continental growth.

Integrated water resources assessment and management in a small watershed – a geomorphic approach

Depleting water resources and increasing human demands on the available fresh water resources suggest the need and scope for integrated water resources assessment and management. Narava basin, a small watershed spread over 105 km2 with a khondalitic suite of rocks and gneisses of Archaean age in the north coastal region of Andhra Pradesh, India was studied to propose effective water resources assessment and management strategies. The watershed, with an undulating topography and gentle to steep slopes, is drained by a dendritic to sub-dendritic drainage system. The watershed receives an annual average rainfall of 980 mm. With increasing agricultural activity, monsoon failures, and indiscriminate developmental activities, groundwater during the last two decades has become the main trusted source for irrigation and drinking. A hydrogeomorphological approach integrating remote sensing applications, geoelectrical investigations and hydrological studies was proposed to delineate and study the groundwater potential and recharge dynamics in the Narava watershed. Linking the remote sensing data, morphometric characteristics, topographic analysis, land use/cover assessment and groundwater conditions, hydrogeomorphic maps were prepared. Synergizing the lineament densities with layer parameters, aquifer yields, groundwater occurrence and dynamics, groundwater potential zones were demarcated and classified. Merging the characteristics of groundwater potential, hydrogeomorphology and water balance, suitable water harvesting structures and management strategies are suggested.

Protoliths of enigmatic Archaean gneisses established from zircon inclusion studies: Case study of the Caozhuang quartzite, E. Hebei, China

A diverse suite of Archaean gneisses at Huangbaiyu village in the North China Craton, includes rare fuchsite-bearing (Cr-muscovite) siliceous rocks – known as the Caozhuang quartzite. The Caozhuang quartzite is strongly deformed and locally mylonitic, with silica penetration and pegmatite veining common. It contains abundant 3880–3600 Ma and some Palaeoarchaean zircons. Because of its siliceous nature, the presence of fuchsite and its complex zircon age distribution, it has until now been accepted as a (mature) quartzite. However, the Caozhuang quartzite sample studied here is feldspathic. The shape and cathodoluminescence petrography of the Caozhuang quartzite zircons show they resemble those found in immature detrital sedimentary rocks of local provenance or in Eoarchaean polyphase orthogneisses, and not those in mature quartzites. The Caozhuang quartzite intra-zircon mineral inclusions are dominated by quartz, with lesser biotite, apatite (7%) and alkali-feldspar, and most inclusions are morphologically simple. A Neoarchaean orthogneiss from near Huangbaiyu displays morphologically simple inclusions with much more apatite (73%), as is typical for fresh calc-alkaline granitoids elsewhere. Zircons were also examined from a mature conglomerate quartzite clast and an immature feldspathic sandstone of the overlying weakly metamorphosed Mesoproterozoic Changcheng System. These zircons have oscillatory zoning, showing they were sourced from igneous rocks. The quartzite clast zircons contain only rare apatite inclusions (<1%), with domains with apatite habit now occupied by intergrowths of muscovite + quartz ± Fe-oxides ± baddeleyite. We interpret that these were once voids after apatite inclusions that had dissolved during Mesoproterozoic weathering, which were then filled with clays ± silica and then weakly metamorphosed. Zircons in the immature feldspathic sandstone show a greater amount of preserved apatite (11%), but with petrographic evidence of replacement of other apatites by quartz and mica. From the zircon morphology and inclusions studies, our studied Caozhuang quartzite is most likely <3500 Ma immature detrital sedimentary rock of local provenance. This strengthens the case for Eoarchaean rocks with a substantial 3880–3800 Ma component occur close to Huangbaiyu.

A comparison of Nd model ages and U–Pb zircon ages of Grenville granitoids: constraints on the evolution of the Laurentian margin from 1.5 to 1.0 Ga

Terra Nova, 24, 7–15, 2012AbstractConcordance and discordance between Nd model ages and U–Pb ages is still widely misunderstood in the geological community. In the SW Grenville Province, plutonic orthogneisses often yield TDM ages significantly older than U–Pb crystallization ages, but this can be understood in a model where a series of crustal Nd isotope evolution lines departs at intervals from the depleted mantle (DM) evolution line. Seven samples with concordant U–Pb and Nd model ages have initial ε Nd on the DM line, attributed to melting of juvenile crust generated in an Elzevirian back‐arc rift zone. In contrast, plutons with discordant U–Pb and Nd model ages lie near crustal evolution lines defined by suites of basement gneisses on the flanks of the rift zone. This study shows that Nd data from large suites of geographically constrained, undated grey gneiss provide a crustal evolution framework for orogen‐scale interpretation of more scattered but precisely dated plutons.

COMMENTS ON THE PAPER "PETROLOGY AND GEOCHEMISTRY OF THE GRANITOID ROCKS OF THE JOHANNESBURG DOME, CENTRAL KAAPVAAL CRATON, SOUTH AFRICA" BY D.M. VAN TONDER AND H. MOURI (SAJG, 113, 257-286)

The writer wishes to comment on some aspects of the geology, petrology and geochemistry of the paper by Van Tonder and Mouri (2010) that are deemed to be problematic. In their paper the authors have chosen to subdivide the Johannesburg Dome (JD) granitoids into three main suites, namely: 1. a Tonalite Gneiss suite (TG) around the southern boundary of the dome; 2. a Granodiorite to Adamellite Gneiss suite (GAG) across the northern part of the dome; and 3. a Granodiorite/Adamellite to Granodiorite suite (GG) occurring between the TG and GAG suites. This subdivision differs from earlier explanations of the geology of the granitic rocks on the dome (Anhaeusser, 1971; 1973). Under normal circumstances, when presenting new interpretations of previously studied geological environments, it is customary to provide new data and argument in support of any suggested changes to the geological status quo. In their study, the authors state that previous geochemical work recorded on the JD is very limited, with only one dataset of major, some trace element, and some rare earth element analyses of isolated areas. The authors then state that in their work an attempt is made, using a complete set of petrographic, mineral chemistry, major-, trace- and REE data, to identify the subcomponents of the Archaean granitoid rocks of the JD, to classify them within the current framework of understanding of the TTG suites and to propose processes involved in the petrogenesis of these granitoids. The writer takes issue with the claim by the authors that they have provided a more complete dataset than the information made available earlier by Anhaeusser (1971; 1973 1999). It is accepted that a copious supply of major-, trace- and REE data was not supplied previously (due to analytical cost considerations), but what was provided were very selective analyses of the best preserved, …

REPLY ON COMMENTS ON THE PAPER "PETROLOGY AND GEOCHEMISTRY OF THE GRANITOID ROCKS OF THE JOHANNESBURG DOME, CENTRAL KAAPVAAL CRATON, SOUTH AFRICA" BY D.M. VAN TONDER AND H. MOURI (SAJG, 113, 257-286)

The authors of the paper would like to acknowledge the substantial pioneering work done on the Johannesburg Dome (JD) by C.R. Anhaeusser as was referenced in the said paper. Although the subdivision of the JD granitoids proposed in the paper differs from previous work by Anhaeusser (1973), it is based on new and existing geochronological data, field relationships and geochemistry. The proposed suites are broadly similar to those proposed in the new book “The Geology of South Africa” by Robb et al. (2008) of which Anhaeusser was a co-editor: 1. a Granodiorite- to Adamellite Gneiss suite (GAG) with local development of tonalite and trondhjemite gneiss, i.e. probably the ~3.34 Ga rocks of the same composition of Poujol and Anhaeusser (2001) (Lanseria Gneiss, Robb et al, 2008) occurring over much of the northern …

PETROLOGY AND GEOCHEMISTRY OF THE GRANITOID ROCKS OF THE JOHANNESBURG DOME, CENTRAL KAAPVAAL CRATON, SOUTH AFRICA

The Johannesburg Dome (JD) in the central Kaapvaal Craton (KC) is dominated by granitoid rocks of the tonalite-trondhjemite-granodiorite (TTG) series. Based on modal analysis as well as a major and trace element investigation the JD granitoids could be subdivided into three main suites, i.e. a Tonalitic gneiss suite (TG) around the southern boundary, a Granodiorite to Adamellite Gneiss suite (GAG) across the northern part, and a Granodiorite/adamellite to Granodiorite suite (GG) occurring between the TG and GAG suites. These rocks are dominantly I-type and peraluminous with the tonalites (TG and partly the GAG suites) falling in the metaluminous field. TTGs of the JD are high-K calc-alkaline to calc-alkaline and are dominantly high silica rocks (~70 weight %), aluminous (Al2O3 >15wt%) with low Yb ( 30), high Na2O/K2O (>1), and have Na2O contents of between 3wt% and 5wt%, comparable to that of the average TTG. The JD tonalities (TG suite) have higher Al2O3, Sr, Na2O/K2O, Mg#, Ni, Cr and LILE contents compared to the more calc-alkaline granitoids (GG suite and trondhjemites of the GAG suite), which are typically richer in HREE (lower REE fractionation), Y and show a negative Sr and Eu anomaly. Other characteristic features of the JD TTG’s include HFSE depletion and distinct enrichment of fluid sensitive elements such as Pb. The strongly fractionated REE pattern, high (La/Yb)N ratio and depletion in HREE (Yb) of the JD TTGs are characteristics shared with modern adakites. The TG suite most probably formed through melting of a subducted oceanic slab with the melt interacting with mantle peridotite during its accent through a thin mantle wedge. The remaining JD granitoids (GAG and GG) most probably formed through the remelting of a TTG protolith, which has a subducted slab and mantle wedge signature (similar to the TG suite).

Geochemistry of TTG and TTG-like gneisses from Lushan-Taihua complex in the southern North China Craton: Implications for late Archean crustal accretion

The Late Archean Taihua complex, mainly composed of amphibolite and TTG and TTG-like gneisses, is volumetrically most important metamorphic rock suites scattered along the southern margin of the North China Craton (NCC). Zircon SHRIMP U–Pb dating shows two episodes (2765 ± 13 and 2723 ± 9 Ma) of Archean magmatism in the Lushan area with distinct geochemical features. The early (2765 ± 13 Ma) suite (TTG-like gneisses) has low-SiO 2 (52.5–66.1 wt%), high-Mg # (0.47–0.68), low HREE (Yb N = 3.0–5.4) and Y (8.07–13.9 ppm) with low to moderate (La/Yb) N (6.7–37.1) and Sr/Y (25.9–119.3). The younger (2723 ± 9 Ma) suite (TTG gneisses) has high-SiO 2 (63.5–74.3 wt%), low-Mg # (0.13–0.52), very low REE (Yb N < 1.8) and Y (<4 ppm) with a wide range of (La/Yb) N (5.2–86.6), Sr/Y (71.4–949) and showing Eu/Eu* > 1 (1.20–2.43). Both suites show pronounced negative Nb–Ta and Ti anomalies on the primitive mantle-normalized spidergram. The TTG-like gneiss suite has similar bulk-rock Nd and zircon Hf model ages (∼3.0 Ga) with ɛ Nd( t) > 0 (0.26–1.46), and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet + clinopyroxene + rutile ± amphibole, which favors subducted slab model for the late Archean TTG. The TTG gneiss suite has abundant relic zircons (2.95–2.80 Ga) with ɛ Nd( t) < 0 (−1.31 to −0.23), which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet-amphibolite residue (garnet + amphibole ± rutile). Significant high-pressure fractional crystallization (garnet ± amphibole) and accumulation (plagioclase) are also required in the petrogenesis. The Lushan amphibolite with nearly flat primitive mantle-normalized trace-element pattern is interpreted to represent early ocean crust metamorphism. These observations suggest a possible model of late Archean crustal accretion from ocean crust to continental terrain in the southern North China Craton.

Archaean to Proterozoic Crustal Evolution in the Central Zone of the Limpopo Belt (South Africa-Botswana): Constraints from Combined U-Pb and Lu-Hf Isotope Analyses of Zircon

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 ± 8 Ma 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 (TDMHf) is significantly older than those obtained from zircons from numerous Neoarchaean granitoids of the Beit Bridge Complex, comprising the Singelele gneiss (2647 ± 12 Ma), the Bulai granite (2612 ± 7 Ma), the Regina gneiss (2649 ± 9 Ma) and two samples of the Zanzibar gneiss (2613 ± 6 Ma). These granitoids show initial εHf(t) values between + 0·5 and −7·1, which correspond to initial TDMHf between 3·46 and 3·01 Ga. These variable TDMHfinitial and εHf(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 ± 9 Ma) 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, uniform TDMHfinitial ages of 2·61–2·67 Ga obtained from a diorite gneiss (2061 ± 6 Ma) of the Mahalapye Complex indicate that its protolith may have been formed from remelting of a Neoarchaean juvenile crust. Variable εHf(t)initial values from −3·7 to +6·3 of zircon cores (2711 ± 11 Ma) in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the Neoarchaean.

Composition and single zircon U-Pb emplacement and metamorphic ages of the Aggeneys Granite Suite, Bushmanland, South Africa

Research Article| March 01, 2007 Composition and single zircon U-Pb emplacement and metamorphic ages of the Aggeneys Granite Suite, Bushmanland, South Africa Russell Bailie; Russell Bailie Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa, Present address: Paleoproterozoic Mineralisation Research Group, Dept. of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa e-mail: russellb@postgrad.uj.ac.za Search for other works by this author on: GSW Google Scholar Richard Armstrong; Richard Armstrong Research School of Earth Sciences, The Australian National University, Canberra, 0200, A.C.T., Australia e-mail: richard.armstrong@anu.edu.au Search for other works by this author on: GSW Google Scholar David Reid David Reid Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa e-mail: dlr@geology.uct.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information Russell Bailie Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa, Present address: Paleoproterozoic Mineralisation Research Group, Dept. of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa e-mail: russellb@postgrad.uj.ac.za Richard Armstrong Research School of Earth Sciences, The Australian National University, Canberra, 0200, A.C.T., Australia e-mail: richard.armstrong@anu.edu.au David Reid Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa e-mail: dlr@geology.uct.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2006 Geological Society of South Africa South African Journal of Geology (2007) 110 (1): 87–110. https://doi.org/10.2113/gssajg.110.1.87 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Russell Bailie, Richard Armstrong, David Reid; Composition and single zircon U-Pb emplacement and metamorphic ages of the Aggeneys Granite Suite, Bushmanland, South Africa. South African Journal of Geology 2007;; 110 (1): 87–110. doi: https://doi.org/10.2113/gssajg.110.1.87 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The central Bushmanland portion of the Namaqua Province of South Africa is underlain by three voluminous suites of granitic gneisses, termed the Achab, Hoogoor and Aroams Gneisses, in the Aggeneys District, the site of major world-class Broken Hill-type Pb-Zn-Cu-Ag deposits. Geochemically the granitic gneisses have similar characteristics with only slight differences in the degree of fractionation present. The gneisses exhibit a predominance of Kibaran magmatic zircon ~1.21 to ~1.17 Ga ages, which correspond closely to a similar tectono-magmatic event in the Okiep District in Namaqualand to the west and to similar aged granitic gneisses in the Natal Province of the Namaqua-Natal Belt. During this period the Aggeneys District was subjected to peak prograde M2 amphibolite facies grade metamorphism associated with peak D2 deformation. This period was characterised by continental collision and accretion throughout the belt. After a 140 m.y. period of tectonic quiescence a discrete tectono-magmatic episode is recorded in U-Pb ages of ~1.04 to ~1.01 Ga Namaquan Orogeny age from low Th/U metamorphic overgrowths also found in the Okiep District in Namaqualand to the west. Unlike Namaqualand and Natal, however, the Aggeneys District was not subjected to extensive magmatism at this time, and metamorphic conditions did not reach granulite facies grade as it did in Namaqualand. All of the granitic gneisses were intruded syn- to post-collision over a short time period. As the granitic gneisses are texturally, petrographically and geochronologically similar it is argued that they were coeval. The generally I- to S-type metaluminous to peraluminous nature of all of the granitic gneisses, along with previously published depleted mantle (TDM) Sm-Nd model ages of between ~2.42 and ~2.05 Ga, suggests derivation by partial melting of pre-existing ~2.0 Ga crust at ~1.2 Ga in a continental collision tectonic setting. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.