Nd isotopic evidence for the position of southernmost Indian terranes within East Gondwana

Sm-Nd model ages of orthopyroxene-bearing massif charnockites from the Cardamom Hills Massif and adjoining supracrustal rocks from the Kerala Khondalite Belt in southernmost India are used to infer some of the relationships within these rocks and between them and neighboring areas. Most of these rocks have model ages of 2.1–2.8 Ga with most charnockites in the range 2.2–2.6 Ga. Thus, 3.0–3.4 Ga Archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks. The similarity of Sm-Nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks. A cordierite gneiss, representative of a relatively minor lithology in the supracrustal belt, has a model age of 1.3 Ga. The protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks. The source material of the gneiss protolith may have been located in the Wanni and Vijayan Complexes of Sri Lanka. The overlap of the model ages of rocks in this area and those in the Highland Complex of Sri Lanka supports the notion that these two sets of rocks were joined to each other in Gondwana. They belong to a belt that ran from Antarctica through Sri Lanka and India into Madagascar. This belt was involved in Pan-African tectono-metamorphism, as reflected in the 550 Ma age of the last, granulite-forming, event throughout the belt.     

U-Pb Age and Hf Isotope Study of Detrital Zircons from the Wanzi Supracrustals： Constraints on the Tectonic Setting and Evolution of the Fuping Complex, Trans-North China Orogen

Located in the middle segment of the Trans-North China Orogen, the Fuping Complex is considered as a critical area in understanding the evolution history of the North China Craton （NCC）. The complex is composed of various high-grade and multiply deformed rocks, including gray gneiss, basic granulite, amphibolite, fine-grained gneiss and marble, metamorphosed to upper amphibolite or granulite facies. It can be divided into four rock units： the Fuping TTG gneisses, Longquanguan augen gneisses, Wanzi supracrustals, and Nanying granitic gneisses. U-Pb age and Hf isotope compositions of about 200 detrital zircons from the Wanzi supracrustals of the Fuping Complex have been analyzed. The data on metamorphic zircon rims give ages of 1.82-1.84 Ga, corresponding to the final amalgamation event of the NCC, whereas the data for igneous zircon cores yield two age populations at -2.10 and -2.51 Ga, with some inherited ages scattering between 2.5 and 2.9 Ga. These results suggest that the Wanzi supracrustals were derived from the Fuping TTG gneisses （-2.5 Ga） and the Nanying granitic gneisses （2.0-2.1 Ga） and deposited between 2.10 and 1.84 Ga. All zircons with -2.51 Ga age have positive initial εHf values from ＋1.4 to ＋10.9, suggesting an important crustal growth event at -2.5 Ga through the addition of juvenile materials from the mantle. The Hf isotope data for the detrital zircons further imply that the 2.8 Ga rocks are important components in the lower crust, which is consistent with a suggestion from Nd isotope data for the Eastern Block. The zircons of 2.10 Ga population have initial εHf values of-4.9 to ＋6.1, interpreted as mixing of crustal re-melt with minor juvenile material contribution at 2.1 Ga. These results are distinct from that for the Western Block, supporting that the Fuping Complex was emplaced in a tectonic active environment at the western margin of the Eastern Block.     

Analysis of airborne magnetic and gravity anomalies of peninsular shield, India integrated with seismic and magnetotelluric results and gravity anomalies of Madagascar, Sri Lanka and East Antarctica

Modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with Moyar Bhavani Shear Zone (MBSZ) and Palghat Cauvery Shear Zone (PCSZ) are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks. This along with thick crust (44–45 km) under the Southern Granulite Terrain (SGT) indicates collision of Dharwar craton towards north and SGT towards south with N–S directed compression during 2.6–2.5 Ga. This collision may be related to contemporary collision northwards between Eastern Madagascar–Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Arcuate shaped N and S-verging thrusts, MBSZ-Mettur Shear and PCSZ-Gangavalli Shear, respectively across Cauvery Shear zone system (CSZ) in SGT also suggest that the WDC, EDC and SGT might have collided almost simultaneously during 2.6–2.5 Ga due to NW–SE directed compressional forces with CSZ as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity. Gravity anomalies of schist belts of WDC suggest marginal and intra arc basin setting.The gravity highs of EGFB along east coast of India and regional gravity low over East Antarctica are attributed to thrusted high-density lower crustal/upper mantle rocks at a depth of 5–6 km along W-verging thrust, which is supported by high seismic velocity and crustal thickening, respectively. It may represent a collision zone at about 1.0 Ga between India and East Antarctica. Paired gravity anomalies in the central part of Sri Lanka related to high density intrusives under western margin of Highland Complex and crustal thickening (40 km) along eastern margin of Highland Complex with several arc type magmatic rocks of about 1.0 Ga in Vijayan Complex towards the east may represent collision between them with W-verging thrust as in case of EGFB. The gravity high of Sri Lanka in the central part falls in line with that of EGFB, in case it is fitted in Gulf of Mannar and may represent the extension of this orogeny in Sri Lanka.     

Neoarchaean magmatism and metamorphism of the western granulites in the central domain of the Mozambique belt, Tanzania: U–Pb shrimp geochronology and PT estimates

U–Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet–biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U–Pb and Pb–Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50–100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet–biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800–850 °C at 12–14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east.     

Strain localization and large-scale block rotation in the lower continental crust, Kataragama area, Sri Lanka

In Sri Lanka a suite of rocks is regionally exposed which once formed a part of the middle to lower crust of a collisional orogen with about doubled crustal thickness. A subhorizontal to gently west-dipping major thrust zone (Highland Basal Thrust) crosses Sri Lanka from NNE to SSW. It separates a granulite facies upper unit, the Highland Complex from an amphibolite facies lower unit, the Vijayan Complex with a displacement of more than 300 km. The basal Highland Complex was migmatized during thrusting. Rheological contrasts between dry granulites and migmatites were high in the presence of partial melts. Granulite facies fragments within the migmatites vary from the decimetre to kilometre scale. Granulites in the area of Kataragama, formerly interpreted as the Kataragama Klippe, are now identified as a large-scale raft within the migmatites, which is completely separated from the overriding Highland Complex. During thrusting strain was concentrated in the migmatized basal parts of the Highland Complex. After crystallization of the migmatites thrusting continued, and strain was still localized within the migmatites. Therefore, granulite facies parageneses, structures and textures are preserved in the granulite rafts. The deviation of the granulite facies structures in the Kataragama raft from the general trend in the Highland Complex is interpreted to indicate counterclockwise rotation of the whole raft by about 90° around a subvertical rotation axis.     

General geology and petrology of some Precambrian crystalline rocks from the Vijayan Complex of Sri Lanka

Detailed field studies of the Precambrian Vijayan Complex terrain, Sri Lanka, reveal the occurrence of granites, gneisses and migmatites in association with calc-silicate gneisses, quartzites and dolerites. Microcline-rich granites and gneisses show both sharp and gradational contacts with the adjacent migmatites. Petrological observations favor a magmatic origin for the granites and gneisses. These bodies seem to have intruded into pre-existing metasedimentary rocks which were subsequently subjected to retrograde metamorphism under amphibolite facies conditions.     

Trace element geochemistry and tectonic characterization of the granulite facies rocks from Southwest Obudu Plateau,southeastern Nigeria

The trace elements characteristics of the migmatitic gneisses (biotite-garnetand hornblende-biotite), granulite facies rocks (charnockitic gneisses) and meta-peridotite in the area of Southwest Obudu Plateau indicate that the area exhibits a high degree of geochemical variability. Compatible trace elements (Ni and Cr) are comparatively high in the granulite facies rocks and meta-peridotite. Ni ranges from 28×10-6 to 266×10-6 whilst Cr ranges from 62×10-6 to 481×10-6 for the granulite facies rocks (charnocki...     

Detrital zircon geochronology of the Lützow-Holm Complex,East Antarctica: Implications for Antarctica–Sri Lanka correlation

The Lützow-Holm Complex (LHC) of East Antarctica has been regarded as a collage of Neoarchean (ca. 2.5 Ga), Paleoproterozoic (ca. 1.8 Ga), and Neoproterozoic (ca. 1.0 Ga) magmatic arcs which were amalgamated through the latest Neoproterozoic collisional events during the assembly of Gondwana supercontinent. Here, we report new geochronological data on detrital zircons in metasediments associated with the magmatic rocks from the LHC, and compare the age spectra with those in the adjacent terranes for evaluating the tectonic correlation of East Antarctica and Sri Lanka. Cores of detrital zircon grains with high Th/U ratio in eight metasediment samples can be subdivided into two dominant groups: (1) late Meso- to Neoproterozoic (1.1–0.63 Ga) zircons from the northeastern part of the LHC in Prince Olav Coast and northern Sôya Coast areas, and (2) dominantly Neoarchean to Paleoproterozoic (2.8–2.4 Ga) zircons from the southwestern part of the LHC in southern Lützow-Holm Bay area. The ca. 1.0 Ga and ca. 2.5 Ga magmatic suites in the LHC could be proximal provenances of the detrital zircons in the northeastern and southwestern LHC, respectively. Subordinate middle to late Mesoproterozoic (1.3–1.2 Ga) detrital zircons obtained from Akarui Point and Langhovde could have been derived from adjacent Gondwana fragments (e.g., Rayner Complex, Eastern Ghats Belt). Meso- to Neoproterozoic domains such as Vijayan and Wanni Complexes of Sri Lanka, the southern Madurai Block of southern India, and the central-western Madagascar could be alternative distal sources of the late Meso- to Neoproterozoic zircons. Paleo- to Mesoarchean domains in India, Africa, and Antarctica might also be distal sources for the minor ∼2.8 Ga detrital zircons from Skallevikshalsen. The detrital zircons from the Highland Complex of Sri Lanka show similar Neoarchean to Paleoproterozoic (ca. 2.5 Ga) and Neoproterozoic (ca. 1.0 Ga) ages, which are comparable with those of the LHC, suggesting that the two complexes might have formed under similar tectonic regimes. We consider that the Highland Complex and metasedimentary unit of the LHC formed a unified latest Neoproterozoic suture zone with a large block of northern LH–Vijayan Complex caught up as remnant of the ca. 1.0 Ga magmatic arc.     

Development of the archean crust in the medina mountain area,wind river range,wyoming (U.S.A.)

Evidence for an extensive Archean crustal history in the Wind River Range is preserved in the Medina Mountain area in the west-central part of the range. The oldest rocks in the area are metasedimentary, mafic, and ultramafic blocks in a migmatite host. The supracrustal rocks of the Medina Mountain area (MMS) are folded into the migmatites, and include semi-pelitic and pelitic gneisses, and mafic rocks of probable volcanic origin. Mafic dikes intrude the older migmatites but not the MMS, suggesting that the MMS are distinctly younger than the supracrustal rocks in the migmatites. The migmatites and the MMS were engulfed by the late Archean granite of the Bridger, Louis Lake, and Bears Ears batholiths, which constitutes the dominant rock of the Wind River Range.Isotopic data available for the area include Nd crustal residence ages from the MMS which indicate that continental crust existed in the area at or before 3.4 Ga, but the age of the older supracrustal sequence is not yet known. The upper age of the MMS is limited by a 2.7 Ga RbSr age of the Bridger batholith, which was emplaced during the waning stages of the last regional metamorphism. The post-tectonic Louis Lake and Bears Ears batholiths have ages of 2.6 and 2.5 Ga, respectively (Stuckless et al., 1985).At least three metamorphic events are recorded in the area: (1) an early regional granulite event (M₁) that affected only the older inclusions within the migmatites, (2) a second regional amphibolite event (M₂) that locally reached granulite facies conditions, and (3) a restricted, contact granulite facies event (M₃) caused by the intrusion of charnockitic melts associated with the late Archean plutons. Results from cation exchange geobarometers and geothermometers yield unreasonablu low pressures and temperatures, suggesting resetting during the long late Archean thermal evenn     

The bimodal rift-related Juscelândia volcanosedimentary sequence in central Brazil: Mesoproterozoic extension and Neoproterozoic metamorphism

The Barro Alto Complex and Juscelândia volcanosedimentary sequence are exposed in the central part of the Neoproterozoic Brasília belt of central Brazil. The former is a large (approximately 150 km long), boomerang-shaped, mafic-ultramafic, layered complex formed by two different intrusions metamorphosed under granulite facies. These rocks are tectonically overlain by rocks of the Juscelândia volcanosedimentary sequence, represented mainly by biotite-gneiss and amphibolite, or amphibolite facies metamorphic equivalents of rhyolite and basalt, respectively. New SIMS U–Pb zircon data and Sm–Nd isochron data presented herein help clarify the igneous and metamorphic evolution of the Juscelândia volcanosedimentary sequence, as well as its relationship with the Barro Alto Complex. Zircon grains from two biotite gneisses were analyzed by SIMS (SHRIMP) and indicate Mesoproterozoic dates, approximately 1.28 Ga, interpreted as the time of bimodal volcanism in a tectonic setting transitional between a continental rift and an ocean basin. Metamorphism is constrained by Sm–Nd garnet-whole-rock isochrons for garnet amphibolite and pelitic schists of the Juscelândia sequence, as well as for clinopyroxene-garnet amphibolite and garnet granulite of the Barro Alto Complex, which give ages between 0.74 and 0.76 Ga, in agreement with SIMS dates for metamorphic zircon rims. These new data are significant, because they establish that a single metamorphic event affected both the Barro Alto Complex and the Juscelândia sequence. Based on these new data, we present a modified tectonic model for the Brasília belt.     

Xes-Xen thermochronology of the Rayner metamorphic complex,Enderby Land (East Antarctica,Molodezhnaya Station Area)

The Xe_s-Xe_n dating of zircons from rocks of the Rayner Complex of the Enderby Land at the Molodezhnaya Station area (coast of the Alasheyev Bight) yielded age estimates of 550 ± 50 and 1040 ± 30 Ma. The metamorphic rocks of the Rayner Complex record two main events: first, the crystallization of the magmatic protoliths of charnockitic and enderbitic gneisses and, second, superimposed structural and metamorphic alterations under conditions transitional from the amphibolite to granulite facies (metamorphism manifested regionally in the rocks of the Rayner Complex). The most reliable Xe_s-Xe_n age estimates for magmatic zircons from the charnockitic and enderbitic gneisses correspond to the Grenville stage of the development of the Rayner Complex (～1.0 Ga). The Xe isotopic systematics of metamorphic zircons reflect a pan-African stage in the evolution of the Rayner Complex (600–550 Ma). Pan-African events are reflected in the U-Xe isotope system in two cases: if metamorphic zircons crystallized at the same time (which probably resulted in the formation of a plateau in the Xe_s-Xe_n age spectrum) and if the initial isotopic systems were disturbed (which resulted in a decrease in apparent age toward low-temperature gas fractions). It is important that secondary alterations and a decrease in apparent ages to 600–550 Ma affected only those components (i.e., caused xenon release only from those traps) that were unstable under the maximum metamorphic temperatures and yielded T _cl values lower than 750°C (conditions transitional from the amphibolite to granulite facies). At a higher xenon retention, “primary” isotopic systems are preserved. Consequently, the age of metamorphism transitional between the amphibolite and granulite facies can be estimated at 600–550 Ma on the basis of Xe_s-Xe_n dating. In general, the results of our study indicate that the age of regional metamorphism of the Rayner complex at the Molodezhnaya area is approximately 600–550 Ma rather than ～1.0 Ga, as was previously supposed.     

Charnockite genesis across the Archaean–Proterozoic terrane boundary in the South Indian Granulite Terrain: Constraints from major–trace element geochemistry and Sr–Nd isotopic systematics

Major, trace element compositions and Sr–Nd isotopic characteristics of charnockitic gneisses from the Southern Granulite Terrain (SGT), South India are presented. The study region encompasses the central segment of the Cauvery Shear Zone system (CSZ) and regions within the Madurai Block (MB) immediately south of it (designated here as the CSZ/MB and MB domains). Differences in the compositions and source characteristics between charnockitic rocks of the CSZ vis-à-vis those of the CSZ/MB and MB regions are highlighted. Foremost, the charnockites and enderbites of the CSZ show highly fractionated REE patterns with positive Eu-anomalies, depleted HREE, Y and near chondritic εNd₀ and initial-⁸⁷Sr/⁸⁶Sr at ca. 2.5 Ga, consistent with hydrous partial melting of amphibolitic crust with residual garnet and hornblende for the parental melts. By contrast, modeled at ca. 1.8 Ga and 0.8 Ga, the CSZ/MB and MB charnockitic rocks, which show a wider range of Ti and P, relatively lower degree of HREE depletion, commonly negative Eu-anomalies and undepleted Y, present clear evidence for involvement of Archaean crustal components in sources of their magmatic protoliths. There is also evidence for significant intracrustal melting processes within a thickened crust at elevated temperatures between 800 and 1000 °C. Implications to the controversial Archaean–Neoproterozoic terrane boundary problem of the SGT are discussed.     

Archean geodynamics in the Central Zone, North China Craton: constraints from geochemistry of two contrasting series of granitoids in the Fuping and Wutai complexes

The Archean to Paleoproterozoic Central Zone of the North China Craton is situated between the Eastern and Western Archean continental blocks and contains two contrasting series of Neoarchean granitoids: the 2523–2486 Ma tonalite−trondhjemite–granodiorite (TTG) gneisses in the Fuping Complex, and the 2555–2525 Ma calc-alkaline granitoids (tonalite, granodiorite, granite and monzogranite) in the Wutai Complex. The Fuping TTG gneisses most likely formed from partial melting of 2.7 Ga basalts at >50 km, with an involvement of 3.0 Ga crustal material. The Wutai granitoids have higher K₂O, LILE and Rb/Sr, but lower Sr/Y and La_N/Yb_N than the Fuping TTG gneisses, are characterized by Nd T_DM from 2.5 to 2.8 Ga and _Nd(t) from 0.49 to 3.34, and are derived from partial melting of a juvenile source at <37 km.The geochemistry of these two contrasting series of Neoarchean granitoids provides further evidence that the Wutai Complex originated and evolved separately from the Fuping Complex. The Wutai Complex most likely formed as an oceanic island arc with volcanism and synvolcanic granitoid intrusions at 2555–2525 Ma. The Wutai Complex was subsequently accreted onto the Eastern Archean Continental Block, and was probably responsible for crustal thickening and TTG magmatism at 2523–2486 Ma in the Fuping Complex (as part of the Taihangshan–Hengshan block), at the western margin of the Eastern Archean Continental Block.     

Zircon ages,geochemistry, and Nd isotopic systematics of pre-Variscan orthogneisses from the Erzgebirge,Saxony (Germany), and geodynamic interpretation

High grade granitoid orthogneisses occur in several metamorphic units of the Erzgebirge in the Saxothuringian Zone of the Variscan Belt. The determination of protolith ages and the geochemical characterization of these rocks permit a reconstruction of the Neoproterozoic to early Palaeozoic magmatic and geodynamic history of the Erzgebirge. Single zircon Pb-Pb evaporation and SHRIMP ages combined with major and trace element data and Sm-Nd isotope systematics indicate at least two discrete magmatic events concealed in the so-called red gneisses, one at ~550 Ma in rocks of the medium pressure—medium temperature (MP-MT) unit and the other at ~500–480 Ma in rocks of the high pressure units. The transition zones comprise both Neoproterozoic granitoids and early Palaeozoic metarhyolites. The granitoid gneisses represent Neoproterozoic calc-alkaline granitoids with REE patterns similar to those produced in Andean-type continental margins. The early Palaeozoic muscovite gneisses are geochemically distinct from the older granitoids and may be derived from melts generated in a back-arc setting. Initial _Nd values in all samples overlap and range from –4.1 to –9.2, corresponding to crustal sources with average residence times of 1.5 to 1.9 Ga. Zircon xenocryst ages as old as 2992 Ma provide evidence for Grenvillian, Svecofennian-Birimian-Aazonian and older age components and suggest an association of the Erzgebirge with Avalonia.B. Mingram and A. Kröner have shared senior authorship     

华北克拉通南缘太华杂岩组成及演化

太华杂岩位于华北克拉通南部,其组成复杂,记录了几乎所有早前寒武纪各阶段重要的地质事件;此外,由于其所处特殊地理位置,研究太华杂岩对于华北克拉通早前寒武纪地壳形成和演化、构造单元划分和基底拼合等都具有举足轻重的科学价值。本文综合已有的岩石学、变质作用、地球化学以及同位素年代学等诸多研究工作,得到以下阶段性结论和认识:1)将鲁山地区太华划分为以深成侵入岩为主的片麻岩系和以变质沉积-火山岩为主的表壳岩系;前者形成于中太古代晚期-新太古代早期,后者形成于古元古代晚期。而小秦岭地区太华杂岩中变质深成侵入岩形成时间跨度较大,为中太古代晚期-古元古代早期;而其上覆的火山-沉积岩可与鲁山太华杂岩的表壳岩类比,形成时间亦为古元古代晚期。2)中太古代-新太古代(2.91～2.50Ga)为华北克拉通南部大陆最主要的地壳形成时期。提出太华杂岩在太古宙经历了两期明显的地壳生长时间,一期发生在2.85～2.70Ga,以鲁山太华片麻岩系中的深成侵入岩和斜长角闪岩为代表;另一期发生在～2.50Ga,以小秦岭华山和崤山地区太华杂岩中各类花岗质岩石为代表。3)太华杂岩在所谓的全球陆壳生长"沉寂期(2.45～2.20Ga)"岩浆活动异常发育,推测这一时期的岩石形成于古元古代俯冲-汇聚环境,可能是与华北克拉通南部太古宙陆块和其他陆块汇聚-碰撞相关。4)太华杂岩在古元古代晚期普遍遭受了强烈的变质和变形,其变质程度主体为高角闪岩相,局部可达麻粒岩相,且记录了包含近等温降压退变质片段的顺时针变质作用P-T轨迹,经历了一个漫长的变质演化过程(1.97～1.80Ga),变质作用的时限跨度可达150Myr。5)提出华北克拉通南部曾经为一个统一基底,称之为"南部太古宙地块",此地块形成时间为新太古代末期(～2.5Ga)。该古老陆块经历了如下5个构造-演化阶段:(1)冥古宙-始太古代初始陆核形成;(2)中太古代-新太古代陆壳快速生长;(3)古元古代早期(～2.3Ga)岩浆活动异常活跃;(4)古元古代(2.30～1.97Ga)陆内拉伸-破裂;和(5)古元古代末期(1.97～1.80Ga)陆块最终拼合。     

The geochemistry of beryllium and fluorine in the gem fields of Sri Lanka

A study of the abundance of Be in the gem sediments of Sri Lanka shows that Be is found in the range of 1–13 ppm. Be shows an irregular distribution among sediments. It occurs in the silicate form and due to the proximity to the beryllium bearing rocks, namely granites and pegmatites of the Highland and Southwest Groups of Sri Lanka, very little decomposition of the Be-bearing minerals had taken place. This is further aided by the high resistance to weathering of the beryllium minerals, particularly beryl and chrysoberyl.The beryllian granites and pegmatites of the Precambrian of Sri Lanka are presumed to have been formed due to the magmatic activity associated and related to charnockitic rocks abundant in the main gem bearing areas of Sri Lanka.Fluorine is found in the range of 400–2,000 ppm and the F/Be ratios for all the areas studied show a range of 54–441. The analysis of the averages of these ratios do not show any particular anomaly in any of the areas studied. The narrow ranges of the F/Be ratios indicate the similar conditions under which weathering and geochemical transportation had taken place in the gem fields of Sri Lanka.     

Sm–Nd isotope geochemistry of metamorphic volcano-sedimentary successions in the São Gabriel Block,southernmost Brazil: evidence for the existence of juvenile Neoproterozoic oceanic crust to the east of the Rio de la Plata craton

Juvenile Neoproterozoic dioritic, tonalitic, trondhjemitic and granodioritic gneisses in the São Gabriel block, southern Brazil, have been identified by geochronologic studies. Age proposals for associated (ultra-)mafic metavolcanic and metasedimentary rocks, however, range from Archean to Neoproterozoic. Whole rock Sm–Nd analyses presented here support a Neoproterozoic age for these rocks. TDM model ages of the (ultra-)mafic metavolcanic rocks range between 0.65 and 1.35 Ga with ɛNd(t) positive values between 3.16 and 6.87; TDM model ages of metasedimentary and metavolcanoclastic rocks vary between 0.77 and 1.19 Ga with ɛNd(t) values between 1.2 and 6.23; tonalitic calc-alkaline gneisses show ɛNd(t) values of 4.34 and 6.3 and TDM model ages of 0.89 and 0.72 Ga, respectively. A late-kinematic granite (Santa Zélia granite) display slightly negative ɛNd(t) values (−1.6) and a higher TDM model age of about 1.4 Ga. These data support the existence of Meso/Neoproterozoic juvenile oceanic crust and island arc rocks during the Brasiliano orogenic events. The main source rocks of the metasedimentary units are previously formed juvenile rocks. The data also indicate minor assimilation of older crustal material and/or contamination of the melts by radiogenic Nd released from older rocks on the subducting slab. Existence of widespread old sialic crust in the subduction zone environment, however, can be ruled out indicating important orogenic accretion between 0.9 and 0.7 Ga. A geotectonic model for the São Gabriel block and the eastern margin of the Rio de la Plata craton comprises eastward subduction and following accretion of an intra-oceanic island arc between 0.9 and 0.8 Ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the Rio de la Plata craton between 0.8 and 0.7 Ga. We postulate that the juvenile rocks of São Gabriel block represent relics of a Neoproterozoic ocean between the Rio de la Plata craton and a continental block (Encantadas block) possibly derived from the Kalahari craton. Subduction and arc accretion began roughly coeval with the initial stages of the break-up of Rodinia (0.9 Ga) and indicate a peripheric Rio de la Plata craton in relation to the Rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the Neoproterozoic Brasiliano orogenic events.     

U–Pb zircon (TIMS and SIMS) and Sm–Nd whole-rock geochronology of the Gour Oumelalen granulitic basement, Hoggar massif, Tuareg shield, Algeria

Two major granulitic units are recognized in the Gour Oumelalen area. One of the units is composed partially of Archean gneisses (Red Gneiss complex) with U–Pb zircon SIMS and TIMS ages of approximately 2.7 Ga. Although they were formed from 3.0- to 3.2-Ga-old precursors, as indicated by Nd model ages, we find no evidence of any older history (≈3.5 Ga) as suggested by previous Pb–Pb ages. The other formation (Gour Oumelalen supergroup) is a metasedimentary sequence at least partly of Paleoproterozoic age, as indicated by zircon dates of a metavolcanic rock at approximately 2.2 Ga. A later magmatic event is recorded at approximately 1.9 Ga in both units and related to coeval granulite-facies metamorphism that affected both units. Nd model ages at approximately 2.0 Ga suggest an accretion of juvenile crust formation at that time. The existence of T_DM Nd model ages intermediate between 2.5 and 2.9 Ga could result from the mixing of 3.2 and 2.0-Ga-old material or may reflect separate events.     

990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico: roots of an orogen

Inliers of 1.0–1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012±12 Ma anorthosite–mangerite–charnockite–granite (AMCG) suite; (2) El Catrı́n unit: ≥1350 Ma orthogneiss migmatized at 1106±6 Ma; and (3) El Marquez unit: ≥1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U–Pb isotopic analyses of zircon: (a) the 1106±6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004–978±3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 °C at 977±12 Ma is documented by ⁴⁰Ar/³⁹Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential–longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of 25–30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia.     

U/Pb and Sm/Nd geochronologic studies of the eastern Borborema Province, Northeastern Brazil: initial conclusions

The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the São Luis-West Africa craton and the São Francisco-Congo-Kasai cratons. New Sm/Nd and U/Pb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma.The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandão massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic São Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province.Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (T_DM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandão massif to the east yields both Transamazonian and Archean U/Pb zircon and Nd (T_DM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaço Group.Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeú-Paraíba (SPP), Riacho Pontal (SRP), and Piancó-Alto Brígida (SPAB) fold belts in the central domain formed ˜ 1.0 ± 0.1 Ga, based on U/Pb ages of zircons. Nd model ages (T_DM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3–1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments.Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the São Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandão massifs and the older crustal blocks of the central domain were originally part of the São Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age.