Mean Crustal Residence Ages Research Articles

Petrogenesis of synorogenic diorite–granodiorite–granite complexes in the Damara Belt, Namibia: Constraints from U–Pb zircon ages and Sr–Nd–Pb isotopes

The synorogenic Palmental complex (central Damara Belt, Nambia) consists of ca. 545Ma old quartz diorites and rare granodiorites and ca. 520Ma-old leucogranites, representing one of the earliest and most primitive phase of crustal plutonism predating the main high-T regional metamorphism. Most quartz diorites and one granodiorite evolved through multistage, polybaric evolutionary processes involving fractionation from a lithospheric mantle-derived melt, followed by fractional crystallization of mainly hornblende, plagioclase and apatite which is shown by decreasing MgO, FeO, CaO, TiO2 and P2O5 with increasing SiO2. Assimilation of felsic basement gneisses was also important during formation of these granitoids. Although their chemical characteristics (high LILE, low HFSE) resemble those of quartz diorites and granodiorites with calc-alkaline affinity, they differ in their enriched Sr (initial 87Sr/86Sr: 0.7061–0.7098) and Nd (initial εNd: −2.7 to –9.9) isotopic composition. Neodymium depleted mantle mean crustal residence ages range from 1.3 to 1.9Ga for the quartz diorites including the granodiorite. These model ages correlate with major and trace element abundances, further substantiating that AFC processes modified the initial isotopic systematics. Lead (206Pb/204Pb: 17.43–17.68, 207Pb/204Pb: 15.61–15.66, 208Pb/204Pb: 38.19–38.49) isotopic composition are unradiogenic relative to other Damaran intrusive rocks but plot above the Stacey and Kramers (1975) reference line, indicating that the source underwent an ancient (pre Pan-African) increase in U/Pb and Th/U, followed by more recent U-depletion. Some variation in 206Pb/204Pb at high 207Pb/204Pb further indicates involvement of ancient crustal material, most likely through AFC processes. A cross-cutting leucogranite dyke has also evolved isotopic compositions (initial 87Sr/86Sr: 0.7326; initial εNd: −15.6; 206Pb/204Pb: 17.42, 207Pb/204Pb: 15.62, 208Pb/204Pb: 38.16) but in view of the apparent younger age of the granite, this rock type cannot be considered as a fractionation product of the quartz diorites and therefore represents likely a melting product of the underlying felsic basement. We also report data from two quartz diorite samples collected in the north and east of the Palmental area. They plot off the trends defined by the other quartz diorites. Palaeoproterozoic Nd model ages of 2.1Ga and distinct Pb isotopic data indicate a Palaeoproterozoic mean crustal residence age for the source material. We conclude that these samples belong to the nearby Goas and Okongava quartz diorites. For these rocks, based on a previous model and a comparison with results from amphibolite-dehydration melting experiments, a high-K meta-tholeiite melted at 1000–1050°C is a likely source. Isotopic data obtained on samples of the Palmental complex make a Pan African subduction zone setting unlikely for these rocks. It is concluded that the most primitive Palmental quartz diorites recording moderately evolved Nd and Sr isotope data (εNd(t): −2.7, 87Sr/86Sr: 0.706) reflect an isotope composition typical of aged subcrustal lithospheric mantle.

Zircon ages and geochemistry of late Neoarchean syenogranites in the North China Craton: A review

At the end of the Neoarchean continental evolution, voluminous syenogranites were emplaced in the North China Craton, together with other magmatic rocks (trondhjemite–tonalite–granodiorite (TTG), monzogranite, diorite, gabbro). Syenogranites are widely distributed in Anshan-Benxi, Qinhuangdao and western Shandong, and also occur in southern Jilin, northern Liaoning, northwestern Hebei and central Henan. Based on geological relationships, degree of metamorphism, deformation and magmatic zircon ages, two phases of syenogranite magmatism are recognized. Rocks produced during the first phase show a gneissic texture and were formed between 2.53 and 2.52Ga and locally comprise abundant TTG. Rocks of the second phase cut late Neoarchean TTG and supracrustal rocks, display a massive structure, and mainly formed between 2.52 and 2.50Ga. All syenogranites share the same features in major element compositions, being high in SiO2 and low in CaO, total FeO, MgO, TiO2 and P2O5. However, they are different in trace and REE compositions and can be subdivided into three types. (1) Type 1 shows a large variation in total REE contents, low (La/Yb)n ratios, strong negative Eu*/Eu anomalies and Ba depletion; (2) Type 2 is similar to Type 1 but has higher (La/Yb)n ratios. (3) Type 3 shows a large variation in total REE and (La/Yb)n ratios and significantly do not show strongly negative Eu*/Eu anomalies and Ba depletion. Whole-rock Sm–Nd isotopic compositions show large variations in ɛNd(t) values and tDM(Nd) modal ages, ranging from −9.49 to −4.72 and 3.70 to 3.25Ga (Type 1), 0.55–1.03 and 2.77–2.71Ga (Type 2) and −2.35 to 1.23 and 2.93–2.66Ga (Type 3), respectively. Hf isotopic compositions of zircons from three samples have ɛHf(t) values and tDM1(Hf) ages of 0.7–7.2 and 2.84–2.56Ga (Type 1), 2.6–7.4 and 2.74–2.56Ga (Type 2) and 2.1–6.3 and 2.76–2.60Ga (Type 3). It is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages, and most of them were emplaced during the second phase (2.52–2.50Ga) in an extensional tectonic regime. The formation of these voluminous syenogranites marks a tectono-magmatic event resulting in stabilization of the North China Craton at the end of the Neoarchean.

Geochemistry and Nd - isotopic composition of high silica rhyolites in the neoarchaean Musoma-Mara greenstone belt, northern Tanzania: evidence for the presence of older continental crust

The high silica rhyolites of the Kibasuka Hill, although volumetrically minor, constitute a part of the greenstone sequence in the northern Musoma-Mara Greenstone Belt (MMGB). They are characterized by high silica contents (63.38 – 81.65 wt %), very low CaO (! 0.05 wt %),variable to unusually high K2O (2.24 – 13.43 wt %) most likely attributed to strong sericite alteration, and their major element composition are similar to those of melts derived from partial melting of metapelites. The samples have very low concentration of the transition elements, Sr (3.13 – 48.4 ppm) and very large negative Europium anomalies (Eu/Eu* = 0.27 –0.48). Their Nd isotopic composition reveals that the T-depleted mantle (DM) model ages of the samples are highly variable and range from 2867 Ma to 4015 Ma. These geochemical features are consistent with generation of the high silica rhyolites by partial melting of a heterogeneous source composed of juvenile Archaean rocks of the MMGB and older sedimentary rocks. The later imparts to the rocks very old mean crustal residence ages. Partial melting of this suite took place at low-pressure conditions within the continental crust where plagioclase was a stable phase. Such old TDM model ages of up to 4015 Ma recorded by the high silica rhyolites suggest the presence of very old continental crust in the MMGB which has so far not been identified anywhere else in the Tanzania Craton.

Record of 1.82 Ga Andean-type continental arc magmatism in NE Rajasthan, India: Insights from zircon and Sm–Nd ages, combined with Nd–Sr isotope geochemistry

We document a Palaeoproterozoic Andean-type continental arc granitoid emplacement event in the northwestern segment of the Aravalli mountain range, NW India. Our study is focused on the Jasrapur pluton which is dominated by monzogranites with a subordinate amount of granodiorite and quartz monzodiorite. These rocks carry typical continental arc geochemical signatures, such as high-K calc-alkaline, mildly peraluminous, I-type and magnesian features, as well as high Rb and low Nb, Y and Rb/Zr ratios, decoupling between LILE and HFSE, and negative Nb–Ta and Ti anomalies. Conventional geothermobarometric estimates suggest high temperature (~ 800 °C) and pressure (≥ 600 MPa) conditions for the crystallisation of the Jasrapur granitoids. Whole-rock Sm–Nd isotope dating yielded an isochron age of 1800 ± 59 Ma, in good agreement with a zircon Pb–Pb evaporation age of 1821.7 ± 0.4 Ma, which we interpret to reflect the age of emplacement. A Ba-enrichment trend (400 to 1600 ppm), together with mantle-like initial Sr ratios (Sr i = 0.7060 ± 0.0007), suggest that the primary Jasrapur magma was generated from partial melting of a depleted mantle source enriched by slab-induced LILE fluids. Nevertheless, negative values of initial Nd (ε Nd(t) = − 5.01 ± 0.13) and a 2.6 Ga mean crustal residence age suggest the involvement of an Archaean component in the petrogenesis of Jasrapur magma. The eventual Jasrapur magma was generated from a zone of melting, assimilation, storage, and homogenisation, i.e. a MASHed zone at the base of an Archaean continental crust. Sr–Nd isotope mixing modelling, assessment of major element data based on various melts produced by experimental dehydration melting as well as comparison of the trace element data with those from the Central and Southern volcanic zones (CVZ and SVZ) of the Andes suggest that the Jasrapur magma underwent crustal contamination while traversing through a relatively thick continental crust. Appraisal of the REE patterns of the granitoids and tectonically similar and contiguous amphibolites signify that the granitoid magma was produced from a garnet-free but plagioclase-rich MASHed zone formed at a relatively shallow depth. Isotopic comparisons with other correlated events documented farther south in the central Rajasthan indicate a 1850–1822 Ma event of continental arc magmatism in the northwestern peninsular Indian shield which also corresponds in age to the amalgamation of the Palaeoproterozoic supercontinent Columbia.

Discovery of Archaean crust within the Akitkan orogenic belt of the Siberian craton: New insight into its architecture and history

The Akitkan orogenic belt separates the Aldan and Anabar superterranes of the Siberian craton and was previously suggested to consist exclusively of Palaeoproterozoic juvenile crust. Foliated granitoids in the Baikal basement salient within the Akitkan belt were studied geochronologically and geochemically. Thermal ionisation mass spectrometry (TIMS) analysis of six zircons of a tonalite sample yielded a discordia line with an upper concordia intercept age of 2884 ± 12 Ma which is interpreted as the time of tonalite emplacement. Geochemical data of the foliated granitoids confirm their affinity to the tonalite–trondhjemite–granodiorite (TTG) series. They are characterized by high Al 2O 3 contents (14.4–17.2 wt.%), high Sr/Y ratios (18–99) and show significant rare-earth element (REE) fractionation with La n /Yb n = 11–89. The ɛNd (T) values vary from −1.6 to +0.8 and Nd mean crustal residence ages range from 3.23 to 3.07 Ga. Geochemical and isotopic data confirm their genesis through interaction of mantle and crustal material under high pressure near the base of thickened crust either after accretion of ancient continental crust and an oceanic plateau, or after underplating of oceanic plateau-like basaltic magmas. The Akitkan orogenic belt is a composite crustal unit of Archaean and Palaeoproterozoic age. We propose two possible scenarios of its evolution. In the first scenario Palaeoproterozoic complexes developed along the active margin of the Archaean Anabar superterrane at 2.1–2.0 Ga. Alternatively Archaean and Palaeoproterozoic components of the Akitkan belt could have a distal origin and were accreted to the Anabar superterrane. These accretional processes were followed by a major collision of the Aldan and Anabar superterranes at ∼1.9 Ga finalised by post-collisional granitoid magmatism at 1.87–1.84 Ga.

Crustal age domains in the Kibaran belt of SW-Uganda: Combined zircon geochronology and Sm–Nd isotopic investigation

Combination of geochemistry, Nd mean crustal residence ages and single zircon ages reveal distinct provinces not recognized previously within the Kibaran belt of SW-Uganda. Geochemical analyses (HFSE and REE) of four granitoids, namely the Ntungamo, Chitwe, Rwentobo and Kamwezi from the Karagwe Tinfield have been interpreted to be mainly composed of melt components from subduction modified parts of the continental lithospheric mantle. In addition we can demonstrate, from these distinctive batholiths crystallization ages that lead to the result of three age groups, one at around 1566 Ma, the second at 1444 Ma to and the third clusters between 1329 Ma and 1363 Ma. The ε Nd( t) values from the now precisely dated intrusions of the Kibaran belt indicate subsequently two distinctive groups. The older granites have ε Nd( t) values of −20.1 to −23.1 and Nd mean crustal residence ages, based on a depleted mantle model, from 3.52 Ga to 3.84 Ga while the younger granite batholiths show lower ε Nd( t) values of −10.1 to −14.0 and Nd mean crustal residence ages from 2.86 Ga to 3.06 Ga, respectively. The distinct geochemical, geochronological and isotopic characteristics and the unrelated geological evolution reconstructed from these data strongly implies that the two now adjacent Precambrian complexes were geologically unrelated crustal packages prior to high grade metamorphism and, as such, represent “suspect terranes” which were most probably brought together during a middle Proterozoic collision event.

Palaeozoic arc magmatism in the Central Asian Orogenic Belt of Kazakhstan: SHRIMP zircon ages and whole-rock Nd isotopic systematics

Early Palaeozoic tonalite to granodiorite intrusions in northern Kazakhstan are associated with lode gold mineralization and have SHRIMP zircon ages of 457.3 ± 6.6 Ma (Aksu), 452.9 ± 5.6 Ma and 447.4 ± 5.4 Ma (both Zholymbet). The Stepnyak intrusion contains large xenoliths with an age of 480.6 ± 5.0 Ma. One early Palaeozoic zircon from a porphyritic diorite at Stepnyak has a core with a near-concordant 207Pb/ 206Pb age of 3888 ± 1.5 Ma, whereas other xenocrystic grains are between 983 and 2698 Ma old. The early Archaean age is probably inherited from unexposed basement of the Kokchetav Massif and represents the oldest crustal material so far known from the Asian continent. It appears that the Aksu, Zholimbet and Stepnyak granitoids were emplaced in the late Ordovician in an Andean- or Japan-type continental arc environment on the margin of the Kokchetav Massif. Late Palaeozoic granitoids in central Kazakhstan have Devonian zircon ages of 407.1 ± 3.9, 381.1 ± 3.1 and 369.2 ± 4.9 Ma, whereas the youngest sample from the Topar Massif has a late Carboniferous emplacement age of 314.1 ± 5.1 Ma. Initial ε Nd values range from +5 to −1 corresponding to Nd-model ages of 1.1–0.6 Ga. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly juvenile crustal protoliths. Our data also confirm that Devonian and Carboniferous arc magmatism in central Kazakhstan assimilated older and already accreted island arc systems, yet is derived from primitive sources, probably in a subduction setting. The range of ε Nd values with corresponding Nd mean crustal residence ages in the range of 0.6–1.1 Ga for most samples indicates variable recycling of late Proterozoic largely juvenile continental crust in the formation of the Palaeozoic magmatic arcs of central Kazakhstan. The orogenic evolution terminated with Permian anorogenic, rift-related alkaline granite magmatism ( ε Nd = +2 to +8) which reflects melting of juvenile, mantle-derived lower crust.

Archaean crustal accretion at the northern border of the Congo Craton (South Cameroon). The charnockite-TTG link

Abstract Magmatic charnockitic rocks and TTG (tonalite-trondhjemite-granodiorite) plutons intruded successively the Archaean greenstone belts of the Ntem Complex, at the northwestern margin of the Congo Craton. Geochemical data, zircon Pb-Pb ages, and Sr-Nd isotope studies, constrain the magmatic features, the genetical timing, and the geodynamic settings of these different suites. Charnockites and TTGs are characterized by high Al2O3 contents, high Na/K ratio, low Th content, LREE enrichment, HREE depletion, negative anomalies in Nb, Ta, and Ti, and positive anomalies in Sr. The Pb-Pb zircon ages indicate that charnockites were emplaced at ca. 2900 Ma and TTGs, which cross-cut the charnockites, at ca. 2830 Ma. TDM Nd mean crustal residence ages of both suites range between 3.10 and 2.93 Ga. The charnockites show slightly positive initial εNd2.9Ga (+0.3 to +1.3), whereas the TTGs have slightly negative values (+0.1 to −1.5). The charnockitic and TTG magmas may have resulted from different partial melting processes of the primitive Archaean basaltic crust and contaminated mantle, possibly in a hot slab subduction convergent regime. They contribute to a two-stage crustal growth of the Archaean craton.

Geochemistry and Nd-isotopic composition of potassic magmatism in the Neoarchaean Musoma-Mara Greenstone Belt, northern Tanzania

The Neoarchaean Musoma-Mara Greenstone Belt (MMGB) of northern Tanzania is underlain in part by the ∼2649 Ma post-orogenic potassic-rich granites, which are the most abundant intrusive rocks in the belt. The rocks are composed of plagioclase + K-feldspar + quartz + biotite ± sphene ± zircon ± hornblende ± chlorite. They are characterized by high contents of SiO 2 (68.90–77.76 wt%), K 2O (3.71–5.44 wt%) and low Na 2O (3.27–5.70 wt%) leading to low ratios of Na 2O/K 2O (0.63–1.02). The rocks are depleted in CaO (0.22–2.41 wt%) as well as in Sr (15–412 ppm), Cr (≤16 ppm) and Ni (≤5 ppm); and their major element composition are similar to those of experimental melts derived from partial melting of tonalite. On chondrite-normalized REE patterns, these rocks show fractionated patterns (La/Yb CN = 1.22–41.32) that are characterised by moderate to strongly negative Eu anomalies (Eu/Eu* = 0.04–0.86). On primitive mantle-normalized spidergrams, these rocks are generally enriched in Th, U, K and Pb and depleted in Ba, Sr, Nb, Ta and Ti relative to adjacent elements. The K-rich granites have ɛNd o (at 2.649 Ga) values of +0.55 to +1.70 that compare well with those of associated volcanic rocks and TTG (ɛNd = +0.44 to +2.66) which predate the emplacement of the K-rich granitoids. Their mean crustal residence ages are 170 to 450 Ma older than their emplacement ages. The overall geochemical features of this suite of rocks, together with evidence from experimental results, are consistent with their generation by partial melting of relatively juvenile igneous rocks within the continental crust at pressures corresponding to depths <15 km where plagioclase was a stable phase. The transition from earlier TTG magmatism to potassic magmatism in the MMGB is interpreted as marking a transition from growth of the Neoarchaean continental crust through the addition of juvenile mantle-derived material to intra-crustal recycling of pre-existing material.

Geochemistry, zircon ages and whole-rock Nd isotopic systematics for Palaeoproterozoic A-type granitoids in the northern part of the Delhi belt, Rajasthan, NW India: implications for late Palaeoproterozoic crustal evolution of the Aravalli craton

Determination of zircon ages as well as geochemical and Sm–Nd isotope systematics of granitoids in the Khetri Copper Belt of the Aravalli mountains, NW India, constrain the late Palaeoproterozoic crustal evolution of the Aravalli craton. The plutons are typical A-type within-plate granites, derived from melts generated in an extensional tectonic environment. They display REE and multi-element patterns characterized by steep LREE-enriched and almost flat HREE profiles and distinct negative anomalies for Sr, P and Ti. Initial εNd values range from −1.3 to −6.2 and correspond to crustal sources with mean crustal residence ages of 2.5 to 2.1 Ga. A lower mafic crustal anatectic origin is envisaged for these granitoids, and the heterogeneous εNd(t) values are inferred to have been acquired from the magma source region. Zircon Pb–Pb evaporation and U–Pb ages indicate widespread rift-related A-type magmatism at 1711–1660 Ma in the northern Delhi belt and also suggest a discrete older magmatic event at around 1800 Ma. The emplacement ages of the compositionally distinct A-type granitoid plutons, and virtually coeval granulite metamorphism and exhumation in another segment of the Aravalli mountains, further signify that part of the Aravalli crust evolved during a widespread extensional event in late Palaeoproterozoic time.

Neoproterozoic to Paleozoic Geology of the Altai Orogen, NW China: New Zircon Age Data and Tectonic Evolution

Abstract We present a synthesis and a new account of the geological and tectonic history of the terranes of the Chinese Paleozoic Altai orogen together with new, single zircon ages for granitic and rhyodacitic rocks. A central terrane consists of Neoproterozoic to Silurian, amphibolite facies, metasedimentary rocks, and abundant Devonian‐Carboniferous granites. The presence of Precambrian basement is indicated by Sinian fossils, our xenocryst ages, and published Nd mean crustal residence ages of granites. Felsic arc‐type lavas on the southern margin of the terrane have a mean 207Pb/206Pb zircon age of 505 Ma, reflecting the time of arc volcanism, and the presence of xenocysts with ages between 614 and 921 Ma suggests derivation by intracrustal melting. Accordingly, we suggest that a Cambro‐Ordovician continental magmatic arc was built on the southern margin of the central terrane by northward subduction. A low‐grade Ordovician Andean‐type arc with a continental basement is situated above a normal fault on...

Isotope constraints on the origin of Pan-African granitoid rocks in the Kaoko belt, NW Namibia

The geodynamic evolution of the Kaoko belt, northwest Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, is considered to be the result of collision between the Congo (Africa) and Rio de la Plata (South America) cratons. In the coastal area of this belt, Pan-African dioritic to granitic melts intruded at ~650 Ma and between ~580 and ~550 Ma into sedimentary sequences of the Kuiseb Formation. Field observations suggest that the later intrusive episode coincided with the peak of high-grade metamorphism in this region, which is characterised by granulite facies conditions and anatexis. Franz et al. (1999) state that both intrusive episodes can be connected with high-grade metamorphic events. The δ18O values of the Pan-African granitoids range from +8.5 to +10.3‰ and tend to be slightly higher than those of the Palaeoproterozoic and late Archaean basement gneisses (+7.6 to +9.9‰ and +7.4 to +9.0‰, respectively). The Pan-African granitoids have low to moderate initial 87Sr/86Sr ratios between ~0.7036 and ~0.7121. Sm-Nd isotopic data of the Pan-African granitoids have moderate initial ϵNd values ranging from −1.2 to −6.8. They are distinct from Palaeoproterozoic as well as late Archaean orthogneisses from the adjacent Congo Craton, which reveal much lower ϵNdvalues between −18 and −28 during the time of emplacement of the Pan-African granitoids. Thus, the derivation of these granitoid melts exclusively from much older crust can be precluded. Although the Pan-African granitoids and the Pan-African high-grade metasedimentary rocks have similar initial ϵNd values of −1.2 to −6.8 and also similar mean crustal residence ages between 1.2 and 1.6 Ga, the derivation of the granitoids from these metasedimentary rocks alone seems also improbable since the granitoids reveal significantly lower initial 87Sr/86Sr ratios than the metasediments. It is here suggested that the Nd, Sr and O isotopic features of the Pan-African granitoids reflect mixing between upper crustal material (metasediments) and remelted basement gneisses from lower crustal levels.

Single zircon ages and whole-rock Nd isotopic systematics of early Palaeozoic granitoid gneisses from the Czech and Polish Sudetes (Jizerské hory, Krkonoše Mountains and Orlice-Sněžník Complex)

Granitoid orthogneisses make up the predominant rock type in the West Sudetes from Jizerske hory in the NW to the Orlicke hory and Sněžnik Mountains in the SE. These generally strongly foliated gneisses are calc-alkaline in composition and display trace element characteristics suggesting generation in a volcanic arc setting. Single zircon ages reflecting the time of emplacement of the gneiss protoliths define a relatively narrow Cambro-Ordovician range between 502 and 515 Ma. This is similar to previously reported zircon ages from the Czech and Polish West Sudetes and documents an important and regionally extensive post-Cadomian magmatic event that we relate to continental arc magmatism on the margin of Avalonia that developed during closure of the Tornquist Ocean. An age of 492 Ma for a microgranite dyke cutting deformed and metamorphosed orthogneisses in the Orlicke hory shows the main deformation to be early Paleozoic. Zircon xenocryst minimum ages range between 546 and 2070 Ma and show maxima in the Cadomian/Pan-African (550–850 Ma) and Grenvillian (1000–1300 Ma) time brackets. The Grenvillian event is also evident from Nd mean crustal residence ages that vary between 1.34 and 1.87 Ga. From these data we suggest that the pre-Variscan granitoid gneisses of the Czech West Sudetes were largely generated by melting of a predominantly Grenville-age basement that was part of the northern margin of Gondwana and may have been related to Grenville-age basement now identified in northern South America.

The emplacement age of gabbroic rocks and associated granitoids of the Liganga-Msanyo complex, south eastern Tanzania.

The Msanyo Gabbro and the associated Mdando granitoids, which intrude Paleoproterozoic high-grade metamorphic rocks of the Ubendian Belt in southern Tanzania, yield imprecise Sm-Nd whole rock isochron ages of 1608±134 Ma (e (Nd) = 2.1) and 1642±100 Ma (e (Nd) = 1.6), respectively. These ages are indistinguishable at the 95% confidence level, indicating that the two rock suites were intruded at the same time. A more precise estimate of the time of emplacement is provided by a 5-point mineral isochron age of 1505±42 obtained from one of the gabbro samples. Calculated initial e (Nd) values and mean crustal residence ages are similar for both rock suites and range from –0.7 to 1.5 and 1.7 to 1.9 Ga. Respectively. Initial Sr isotope ratios lie between 0.7032 and 0.7039 and these values are similar to those obtained in the mantle and the lower continental crust. The rather evolved Nd isotope systematics, combined with the relatively non-radiogenic Sr isotopic signature, suggest that both suites cystallised from mantle-derived magma that had assimilated a significant amount of lower continental crustal material. Subsequent to their emplacement, the rocks experienced localised shearing and sericitisation during the Ukingan tectonic event. A Rb-Sr plagioclase-whole rock age dates this event at 1137±50 Ma. Tanzania Journal of Science Volume 27A (Special Issue) 2001, pp. 1-14

UPb SHRIMP and SmNd age constraints on the timing and sources of the Pan-African Cape Granite Suite, South Africa

Geochemical and isotopic UPb SHRIMP and SmNd studies in three granitic plutons from the Pan-African Neoproterozoic/Cambrian Saldania Mobile Belt, southwestern South Africa supports differences in the sources and timing of emplacement of the main plutons related to the Cape Granite Suite as established by previous researchers. The Phase I, early syn-tectonic Darling Batholith Granitoid reflects an overall peraluminous chemical signature compatible with derivation mainly from melts extracted from paraderived crustal sources (Sa1 association). The Phase II, late to post-tectonic Robertson Pluton, reflects affinities to the Australian l-type granites (1a association). Despite the good structural constraints on the syn- and post-tectonic origin, the UPb ages point to a broadly synchronous crystallisation episode at 547±6 Ma and 536±5 Ma, respectively. In addition to UPb, Nd isotopic studies were also carried out for both plutons, as well as for the Riviera Granite, another phase II (1a association) pluton. The initial ε Nd (550 Ma) based on a depleted mantle model range from −3.5 (Darling), to −3.1 (Robertson) and to −2.6 (Riviera). The Nd mean crustal residence ages are 1559 Ma for Darling, 1626 Ma for Robertson and 1243 Ma for Riviera. Despite the small databank, a dominant Mesoproterozoic (∼ 1600 Ma) crust may be seen as the best candidate to explain the model T DM ages obtained. All the data largely overlap with others recently obtained for other plutons within the Cape Granite Suite and cast doubts on the current correlation between Saldania and the southeastern Brazilian, Dom Feliciano Belts.

Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

Single zircon U–Pb dating combined with 207Pb/206Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial eNd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K2O/Na2O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc.

Geochemistry, PbPb single zircon ages and NdSr isotope composition of Precambrian rocks from southern and eastern Ethiopia: implications for crustal evolution in East Africa

Geochemical and isotope data for granitoid rocks from southern and eastern Ethiopia delineate the presumed margin of the Pan-African juvenile terrain of the Arabian-Nubian Shield against an older crustal segment of unknown origin extending from eastern Ethiopia to northern Somalia. Granitoids from southern Ethiopia have higher Na 2O and Na 20 K 2 O and lower Cr and Ni than granitoids with comparable SiO 2 values from eastern Ethiopia. In southern Ethiopia three periods of magmatism are identified on the basis on single zircon 207 Pb 206 Pb evaporation ages, namely at ∼850, ∼750-700 and ∼650-550 Ma, and these correlate well with events documented from other parts of Ethiopia and the Arabian-Nubian Shield. The initial ϵ Nd(700 Ma) and ϵ sr(700 Ma) values range from −1.2 to +3.2 and from −13.4 to + 3.7, respectively, which precludes any significant contribution from much older continental crust in the generation of these rocks. Neodymium mean crustal residence ages, based on a depleted mantle model, range from 0.96 to 1.26 Ga. These data support the interpretation that southern Ethiopia constitutes part of the Arabian-Nubian Shield. In contrast, granitoids from eastern Ethiopia show geochemical features of S-type granites. In eastern Ethiopia Pal aeo-Neoproterozoic zircon ages (781–2489 Ma) are found. Initial ϵ Nd(700 Ma) and ϵ sr(700 Ma) values range from−4.3 to −18.3 and + 33.3 to + 99.8, respectively. Neodymium mean crustal residence ages range from 1.62 to 2.88 Ga. These data, in comparison to the western and southern parts of Ethiopia, are indicative of considerable reworking of pre-Pan-African crust. Variations in age, SrNd isotope ratios and chemistry of the granitoids on a regional scale also suggest the existence of two separate basement terrains between southern and eastern Ethiopia, which may be separated by a tectonic line now concealed by Phanerozoic rocks. This tectonic line may represent a major tectonic boundary between the juvenile Arabian-Nubian Shield in the west and a pre-Pan-African gneissic terrain to the east, thus delineating the eastern margin of the Arabian-Nubian Shield.

Latest precambrian (Cadomian) zircon ages, Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge, Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the ‘Grey Gneiss’ and ‘Red Gneiss’, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a 207Pb/206Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean 207Pb/206Pb age of 555±7 Ma as well as xenocrysts ranging in age between ∼ 850 and ∼ 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform 207Pb/206Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. ɛNd(t) values for the grey gneisses are −7.5 and −6.0 respectively, (mean crustal residence ages of ∼1.7–1.8 Ga). The red gneisses have a wider range in ɛNd(t) values from −7.7 to −2.8 (TDM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism (∼ 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.

Silurian turbidites used to reconstruct a volcanic terrain and its Mesoproterozoic basement in the Irish Caledonides

The Lettergesh Formation, Co. Galway, Ireland, comprises a 1.5 km thick sequence of turbidites deposited c. 430 Ma ago during closure of the Iapetus Ocean. Petrographic and geochemical data show that the turbidites consist overwhelmingly of detritus derived from a young volcanic terrain. Palaeocurrent data indicate that the detritus was derived from the north. Modelling of the chemical compositions of the turbidites indicates provenance from a high-K calc-alkaline, subduction-related volcanic province built on continental crust. A subordinate ultrabasic component may represent detritus from either an ophiolite or tectonically emplaced serpentinite. The turbidites are characterized by low variability of initial ɛ Nd (−6.5 to −3.2) and highly variable initial 87 Sr/ 86 Sr ratios. Modelling of the isotopic data suggests a mean crustal residence age of 1440–1600 Ma for the continental foundation of the volcanic terrain. Such a model age is consistent with a mixture of crust having T DM ages of c. 1.9 Ga and c. 1.3 Ga, both of which are known from the Annagh Gneiss Complex of NW Ireland.

Pan-African reworked early/middle Proterozoic crust in NE Africa west of the Nile: Sr and Nd isotope evidence

Whole-rock Rb-Sr studies on six suites of highly metamorphosed basement rocks from southern Egypt and northern Sudan, west of the River Nile, yield Pan-African ages (562–918 Ma): these ages are interpreted as reset. Orthogneisses give Nd model ages of 1600 to 2600 Ma for their calc-alkaline plutonic precursors. A gneiss derived from sedimentary precursors gave a mean crustal residence age of 2200 Ma. A series of I-type, late-tectonic, granitoids were intruded during a period of uplift, erosion, and wrench faulting at 560–620 Ma. Geochemical characteristics and Nd model ages (1200–1700 Ma) indicate that these rocks were derived from a mixed source of juvenile Pan-African mantle material and older continental crust of mainly early/middle Proterozoic age. Negative ɛNd t , values for granitoid samples (–18.9 to –5.3) show that the Pan-African episode in NE Africa, west of the Nile, involved considerable reworking of pre-existing crust.