Zircon Evaporation Ages Research Articles

Zircon evaporation ages and geochemistry of metamorphosed volcanic rocks from the Vinjamuru domain, Krishna Province: evidence for 1.78 Ga convergent tectonics along the southeastern margin of the Eastern Dharwar Craton

Palaeoproterozoic intermediate to potassic felsic volcanism in volcano‐sedimentary sequences could either have occurred in continental rift or at convergent magmatic arc tectonic settings. The Vinjamuru domain of the Krishna Province in Andhra Pradesh, SE India, contains such felsic and intermediate metavolcanic rocks, whose geochemistry constrains their probable tectonic setting and which were dated by the zircon Pb evaporation method in order to constrain their time of formation. These rocks consist of interlayered quartz–garnet–biotite schist, quartz–hematite–baryte–sericite schist as well as cherty quartzite, and represent a calc‐alkaline volcanic sequence of andesitic to rhyolitic rocks that underwent amphibolite‐facies metamorphism at ~1.61 Ga.Zircons from four felsic metavolcanic rock samples yielded youngest mean 207Pb/206Pb ages between 1771 and 1791 Ma, whereas the youngest zircon age for a meta‐andesite is 1868 Ma. A ~2.43 Ga zircon xenocryst reflects incorporation of Neoarchaean basement gneisses. Their calc‐alkaline trends, higher LILE, enriched chondrite‐normalized LREE pattern and negative Nb and Ti anomalies on primitive mantle‐normalized diagrams, suggest formation in a continental magmatic arc tectonic setting. Whereas the intermediate rocks may have been derived from mantle‐source parental arc magmas by fractionation and crustal contamination, the rhyolitic rocks had crustal parental magmas. The Vinjamuru Palaeoproterozoic volcanic eruption implies an event of convergent tectonism at the southeastern margin of the Eastern Dharwar Craton at ~1.78 Ga forming one of the major crustal domains of the Krishna Province. Copyright © 2012 John Wiley & Sons, Ltd.

Single zircon evaporation ages: Evidence for the Mesoproterozoic crust in the southeastern Nigerian basement complex

The Ukwortung area of the Obudu Plateau exposes high-grade metamorphic rocks of upper amphibolite to granulite facies. Zircon grains from three locations of Southwest Obudu Plateau were dated by the Pb-Pb dating method. This single zircon radiometric dating method confirms the occurrence of Mesoproterozoic crust in the southeastern Nigeria basement complex. The pyroxene gneiss (granulite facies) sample from Biereberi near Ukpe yielded a mean age of 2061.4±0.4 Ma, suggesting the presence of Paleoproterozoic crust. The combined 207Pb/206Pb ratios from the garnet-biotite gneiss in the area gave a mean age of 1794.4±0.4 Ma. This further confirms the presence of Paleoproterozoic crustal components within the Obudu Plateau. The Mesoproterozoic (1548.8±0.5 Ma) and Neoproterozoic (619.8±0.9 Ma) ages were obtained from two populations of zircon grains from the leucogranite in Okordem, Southwest Obudu. These “signatures” collaborate the occurrence of Mesoproterozoic ages in the Afar region and Cameroon basements along with the southeastern Nigerian basement complex, constituting the major portion of the West Central African mobile belt. The zircon age of 1794.4±0.4 Ma may probably be the maximum depositional age of the meta-sediment (garnet-biotite gneiss) and the age of 1548.8±0.5 Ma may be interpreted as the time of emplacement of the granitoid. The Neoproterozoic age (619.8±0.9 Ma) probably records the metamorphic event that was prevalent during the Pan-African period and thus affected the area.

New Constraints from Pb-Evaporation Zircon Ages of the Méiganga Amphibole-Biotite Gneiss, Central Cameroon, on Proterozoic Crustal Evolution

The amphibole biotite gneiss (ABGn) in the Meiganga area forms part of a meta volcano-sedimentary sequence of the Adamawa Yade domain (AYD), Central African Fold Belt (CAFB). This sequence shows affinity with immature sediments (greywackes, arkoses) with intercalation of mafic lavas or tuffs. New 207Pb/206Pb zircon evaporation ages for two ABGn samples range from 1887 - 2339 Ma and from 675 - 889 Ma, respectively. These ages and evidence from internal zircon structures indicate that igneous rocks of Archean to Paleoproterozoic and of early Neoproterozoic age contributed to the detritus of the sedimentary sequence. The deposition of detritus took place prior to 614 - 619 Ma which represent the syntectonic emplacement of the Meiganga metadiorite. Leucogranites north to the Meiganga area were generated by melting of crust identical to that which provided the source of the ABGn. The metasedimentary sequence investigated in this study is similar to that of the southern part of the AYD and in the Borborema Province, NE Brazil. The tectonic and geochronologic characteristics of the AYD in the Meiganga area support the idea that during the Proterozoic, Central Africa and NE Brazil were part of the same continental landmass.

Granitóides proterozóicos como marcadores da evolução geotectônica da região nordeste do Pará, Brasil

Proterozoic granitoids as indicators of the tectonic evolution of northeastern Para, Bra- zil. Granitoids cropping out inside Phanerozoic sedimentary cover in northeast Para, Brazil (Gurupi region), records the geotectonic evolution of this region. 207 Pb/ 206 Pb single zircon evaporation ages were obtained for different granitoid bodies of the region, besides Sm-Nd isotopic studies. Almost all the plutons are genetically related with regional geological processes operative during this crustal segment formation, which includes the Sao Luis Craton. Such processes are associated with amalgamation of island arcs and Archean nuclei during the Pal- aeoproterozoic (2.15 to 2,07 Ga). The reworking of the southwestern margin of the Sao Luis Craton at the end of the Neoproterozoic formed the Gurupi Belt and is recorded by the occurrence of a granitic pluton of 549 ± 4 Ma.

Granitóides proterozóicos como marcadores da evolução geotectônica da região nordeste do Pará, Brasil

Granitoids cropping out inside Phanerozoic sedimentary cover in northeast Pará, Brazil (Gurupi region), records the geotectonic evolution of this region. 207Pb/206Pb single zircon evaporation ages were obtained for different granitoid bodies of the region, besides Sm-Nd isotopic studies. Almost all the plutons are genetically related with regional geological processes operative during this crustal segment formation, which includes the São Luís Craton. Such processes are associated with amalgamation of island arcs and Archean nuclei during the Palaeoproterozoic (2.15 to 2,07 Ga). The reworking of the southwestern margin of the São Luís Craton at the end of the Neoproterozoic formed the Gurupi Belt and is recorded by the occurrence of a granitic pluton of 549 ± 4 Ma.

Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area, central Cameroon

The Méiganga area is situated in the Adamawa–Yadé domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites. The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation. Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma 207Pb/206Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the 207Pb/206Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (207Pb/206Pb zircon evaporation age).

Sedimentary provenance and palaeoenvironment of the Baixo Araguaia Supergroup: constraints on the palaeogeographical evolution of the Araguaia Belt and assembly of West Gondwana

Abstract Provenance studies on metasedimentary rocks of the Baixo Araguaia Supergroup of the Brasiliano Araguaia Belt, central Brazil, yield 207 Pb/ 206 Pb zircon evaporation ages for detrital zircons from quartzites concentrated around 1000–1200 Ma and 2800–2900 Ma; Sm–Nd T DM model ages of schists and phyllites scatter around 1600–1700 Ma. Facies analysis of low-grade metasedimentary rocks from drill cores suggests a sedimentary environment of basin floor and lower- to upper-slope turbidites. Nearby sources are indicated by the textural and mineralogical immaturity; together with structural geological data indicating tectonic transport of the supracrustal pile towards the NW, this suggests probable provenance from the southeastern portion of the Araguaia Belt and not from the Amazonian Craton as usually believed. The Goiás Massif, Goiás Magmatic Arc, São Francisco Craton and Paranapanema block are considered to be the best candidates. They may have formed a larger continental mass during West Gondwana amalgamation, prior to their collision with the Amazonian Craton to form the Araguaia Belt. Final timing of this collision is constrained by c . 550 Ma syntectonic granites. Similar ages for high-grade gneisses in the Rokelide Belt suggest coeval collision and coetaneous metamorphism of the Araguaia and Rokelide belts, but more geological and geophysical data are required for a decisive correlation between these belts.

Mesoproterozoic bimodal magmatism along the Protogine Zone, S Sweden: three magmatic pulses at 1.56, 1.22 and 1.205 Ga, and regional implications

The Protogine Zone is a major tectonic zone situated at the eastern boundary of the Sveconorwegian orogenic belt in southern Sweden. This structure hosts Mesoproterozoic dolerite dykes and plutonic rocks including ultramafic cumulate, syenite, monzonorite and granite. Pb–Pb zircon evaporation ages are reported for seven samples of rocks along the Protogine Zone, including the Vaggeryd syenite pluton. Two samples of the main syenite facies of the Vaggeryd pluton yield ages of 1220±3 and 1219±3 Ma. Three samples of anorthosite gabbro, aplite and pegmatite, all of which form dykes in the Vaggeryd pluton, yield equivalent ages of 1220±3, 1221±3 and1218±3 Ma. They demonstrate that the different facies recorded in the pluton belong to a single magmatic pulse at 1220 Ma, significantly older than previous estimate of 1204±4 Ma (Jarl 2002). Samples of the Gumlösa-Glimåkra granite in NE Scania and the Åker metabasite, located immediately west of the Vaggeryd syenite, were dated at 1204±3 and 1567±3 Ma, respectively, hence reproducing earlier reported ages for these intrusions determined by conventional TIMS and SIMS U–Pb techniques (1204−15 +16 Ma, Johansson 1990 and 1562±6 Ma, Söderlund et al. 2004a). The 1575–1562 Ma suite of mafic intrusions along the Protogine Zone shows temporal correspondence with rapakivi magmatism in SW Finland and suggests that the Protogine Zone first may have originated as a zone of crustal weakness, which later was reactivated (e.g. at 1224–1215 and ca. 1204 Ma) in an extensional, possibly back-arc, tectonic setting prior to the 1.13–0.95 Ga Grenvillian-Sveconorwegian orogeny.

Reavaliação e novos dados geocronológicos (Ar/Ar, Rb/Sr e Sm/Nd) do Batólito Pelotas no Rio Grande do Sul: implicações petrogenéticas e idade de reativação das zonas de cisalhamento

SHRIMP and conventional U-Pb and Pb/Pb evaporation dating of zircon from the Pelotas batholith indicates one group of ages between 625 and 633 Ma for the Pinheiro Machado suite, and another group between 575 and 599 Ma for the Viamão, Encruzilhada do Sul and Dom Feliciano suites. Sr87/Sr86 isotopic ratios of 0.7060 - 0.7016, µNd values of -5 and -10, and Nd T DM model ages between 2200 and 1600 Ma suggest that the suites were mainly derived by melting of Paleoproterozoic crust formed during the Tranzamazonic Cycle. Basic magmatism and heterogeneous magma mixing in the Pinheiro Machado, Viamão, Encruzilhada do Sul and Dom Feliciano suite and the Piquiri alkaline magmatism, with Pb/Pb zircon evaporation ages between 610 and 615 Ma, suggest that a mantle component also contributed to the formation of the Pelotas batholith. 39Ar/40Ar determinations in micas from mylonitic rocks of high and low-dip shear zones showed ages between 540 and 530 Ma, which show that important tectonic events occurred after the emplacement of the batholith. This event developed in a tranpressive regime, was responsible for the reactivation of older shear zones, and was probably contemporaneous with the development of the Camaquã basin.

Ca. 750–1100 Ma magmatic events and Grenville-age deformation in Sri Lanka: relevance for Rodinia supercontinent formation and dispersal, and Gondwana amalgamation

Large volumes of ∼880–1100 Ma calc-alkaline granitoid rocks in the Wanni and Vijayan crustal provinces of Sri Lanka make it likely that these domains were produced in active margin settings, probably Grenville-age magmatic arcs. We report new single zircon evaporation ages and Nd isotopic systematics for dioritic to granodioritic gneisses of the Kadugannawa Complex of central Sri Lanka which record a period of magmatic arc activity between 1006 and 881 Ma and show this complex to be part of the Wanni domain. Both provinces were probably generated in arc-related settings at the outer margin of Rodinia, but this cannot be ultimately proven on the basis of the currently available data. In spite of a strong Pan-African tectono-thermal overprint at ∼600–550 Ma that we relate to involvement of Sri Lanka in the amalgamation of the Gondwana Supercontinent, rare ∼1000–900 Ma structures are preserved in the gneisses of the Kadugannawa Complex that may constitute remnants of the accretion history of Rodinia. We further suggest that the basement rocks of Sri Lanka do not correlate with crustal domains in southern India or in East Africa, since these were not part of Rodinia.

First evidence of early Birimian (2.21 Ga) volcanic activity in Upper Guinea: the volcanics and associated rocks of the Niani suite

During the 1:200,000-scale mapping of northeastern Guinea, a suite of volcanic rocks comprising porphyritic lavas, pyroclastics and pyroclastic breccias, was identified in the Niani area in the northeast of the Siguiri Basin. The texture of the rocks, the abundance of pyroclastic deposits and the diversity of lapilli they contain all point to explosive eruptive mechanisms. A Pb–Pb zircon evaporation age of 2212±6 Ma obtained from a sample of rhyodacite has yielded the first evidence of early Birimian volcanic activity in Guinea. Most of the volcanic rocks of the Niani volcanic suite may be defined as intermediate to acidic medium-K calc-alkaline igneous rocks. Some of them show HREE fractionation comparable to the one of high-Al/low-Yb TTGs and recent adakites. It seems probable that these lavas were emplaced in relation with the subduction of a young and hot oceanic plate.

Timing of accretion and collisional deformation in the Central Asian Orogenic Belt: implications of granite geochronology in the Bayankhongor Ophiolite Zone

Growing evidence suggests that the mechanism of Palaeozoic continental growth in Central Asia was by subduction–accretion with punctuated collisions that produced ophiolitic sutures between accreted blocks. The Bayankhongor ophiolite is the largest ophiolite in Mongolia and possibly all of Central Asia, and is interpreted to mark the collisional suture between the Baidrag and Hangai continental blocks. New 207Pb/ 206Pb zircon evaporation ages for granite plutons and dykes that intrude the ophiolite and its neighbouring lithotectonic units suggest that the ophiolite was obducted at c . 540 Ma at the beginning of a collisional event that lasted until c . 450 Ma. The new data, combined with that of previous studies, indicate regional correlation of isotopic ages north-westward from Bayankhongor to southern Tuva. These data record oceanic crust formation at c . 570 Ma, followed by approximately 30 million years of subduction–accretion that culminated in obduction of ophiolites, collision related metamorphism, and magmatism in the period c. 540–450 Ma. Correlation of isotopic-age data for the ophiolites of western Mongolia and southern Tuva suggests that the ophiolites define a major collisional suture in the Central Asian Orogenic Belt (CAOB) that defines the southern and western margins of the Hangai continental block.

U–Pb geochronological constraints on the timing of Brioverian sedimentation and regional deformation in the St. Brieuc region of the Neoproterozoic Cadomian orogen, northern France

During the Neoproterozoic Cadomian orogeny in northern France, supracrustal rocks of the Brioverian Supergroup were deposited in marginal and back arc basins, and were subsequently variably deformed and metamorphosed. New U–Pb analyses of single, and small multigrain fractions of zircon from selected plutons from the Baie de St. Brieuc region provide robust geochronological constraints on the timing of these events. The Jospinet granodiorite forms part of the local basement directly overlain by Brioverian metasediments and basic volcanics, and yields a U–Pb zircon date of 625.9+3.6/−1.9 (2 σ) Ma. The pre-tectonic Port Moguer tonalite, which has been strongly sheared along with its amphibolite facies country rocks, has a crystallization age of 600.4±0.9 Ma. Emplacement ages of 576.3+1.5/−1.2 Ma for the syn-tectonic Fort La Latte quartz diorite and 574.6+1.8/−1.5 Ma for the late-tectonic St. Quay quartz diorite place limits on termination of deposition and timing of subsequent regional deformation of the Brioverian sequence in the Baie de St. Brieuc region. The new dates constrain the age of Brioverian sedimentation to the interval 626–575 Ma, a range consistent with a previously published Pb–Pb zircon evaporation age of ca. 588±22 Ma for Brioverian volcanic rocks (Lanvollen Formation). Deformation within this sector of the Cadomian belt is believed to have occurred shortly before 575 Ma, revising previously published estimates for the age of this major tectonothermal Cadomian event by 10–20 My.

Crustal origin of Early Proterozoic syenites in the Congo Craton (Ntem Complex), South Cameroon

Two petrographically different syenite suites can be distinguished in the mostly Archean Ntem Complex of Cameroon: a suite of two-pyroxene syenites and a suite of clinopyroxene syenites. The syenites occur as discrete intrusions surrounded by Archean gneisses. Based on structural relationships the syenites are of late to post-orogenic origin. The Pb–Pb zircon evaporation ages indicate that the syenites intruded at ca. 2.3 Ga and thus postdate significantly the Archean episode of crust formation and differentiation that generated the surrounding gneiss terrane. The isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin. The syenite suites do not show simple chemical trends, have highly variable trace element contents and display only a small variation in their major element contents, particularly in their SiO 2 contents. The isotope characteristics, including 143Nd/ 144Nd, 87Sr/ 86Sr and common Pb, point to a heterogeneous source or multiple sources for the syenites. The Sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87Sr/ 86Sr ratio of 0.690 to high values of 0.713. In contrast the initial 143Nd/ 144Nd ratios are more homogenous with εNd (2320) from −5.8 to −7.5. The Pb isotopes from leached K-feldspars indicate that the sources of the syenites had a history of U depletion and Th/U enrichment. The syenites show the same isotope characteristics as the surrounding gneiss terrane, thus the deep continental crust of the Ntem Complex itself is the most likely source for the syenite melts. This conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source. In addition, inherited zircons from the syenites yield Pb—Pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses. In contrast to many other syenites worldwide, the syenites in the Ntem Complex do not contain juvenile mantle material, but are the sole products of crustal reworking. Thus, the syenites did not contribute to the crustal growth in the Ntem Complex. However, this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust. The cause for the generation of these large syenite bodies in the Ntem Complex is most likely an episode of deep-seated thermal activity that was so far not known from this part of the Congo Craton.

THE SOURCE OF THE ESPINHAÇO DIAMONDS: EVIDENCES FROM SHRIMP U-PB ZIRCON AGES OF THE SOPA CONGLOMERATE AND PB-PB ZIRCON EVAPORATION AGES OF METAVOLCANIC ROCKS

We present here the results of the SHRIMP U/Pb zircon ages obtained from the diamond-bearing Sopa Conglomerate and also Pb-Pb zircon evaporation age of the associated metavolcanic rocks, outcropping in the Diamantina region of the Espinhaco Range. The Sopa Conglomerate, a clast-supported rock that appear in the lower part of the Espinhaco Supergroup, was deposited mainly in alluvial fan, braided river and fan delta systems. The apparent 238 U- 206 Pb and 207 Pb- 2()6 Pb zircon ages varied from 1442 Ma to 3683 Ma and from 1726 Ma to 3599 Ma peaks, respectively. Different generations of zircon populations showed by the radiometric ages surely demonstrate the age of cratonic rocks, however it is still unknown if one of them could be correlated with a primary source rock of the diamonds. The series of K- and Fe-rich metavolcanics is coeval with the Sopa Conglomerate. The results of a single date of these rocks yielded ages of 1710±12 Ma to the crystallization of the volcanic protolith. Detrital zircons of the Sopa Conglomerate matrix are older than the primary zircons of the metavolcanic rocks outcropping in the lower Espinhaco Supergroup. Therefore, geochronological data do not provide additional evidence for the idea that diamonds are originated from these metavolcanic rocks, as supposed by some authors after geologic regional studies.

THE SOURCE OF THE ESPINHAÇO DIAMONDS: EVIDENCES FROM SHRIMP U-PB ZIRCON AGES OF THE SOPA CONGLOMERATE AND PB-PB ZIRCON EVAPORATION AGES OF METAVOLCANIC ROCKS

We present here the results of the SHRIMP U/Pb zircon ages obtained from the diamond-bearing Sopa Conglomerate and also Pb-Pb zircon evaporation age of the associated metavolcanic rocks, outcropping in the Diamantina region of the Espinhaço Range. The Sopa Conglomerate, a clast-supported rock that appear in the lower part of the Espinhaço Supergroup, was deposited mainly in alluvial fan, braided river and fan delta systems. The apparent 238 U- 206 Pb and 207 Pb- 206 Pb zircon ages varied from 1442 Ma to 3683 Ma and from 1726 Ma to 3599 Ma peaks, respectively. Different generations of zircon populations showed by the radiometric ages surely demonstrate the age of cratonic rocks, however it is still unknown if one of them could be correlated with a primary source rock of the diamonds. The series of K- and Fe-rich metavolcanics is coeval with the Sopa Conglomerate. The results of a single date of these rocks yielded ages of 1710±12 Ma to the crystallization of the volcanic protolith. Detrital zircons of the Sopa Conglomerate matrix are older than the primary zircons of the metavolcanic rocks outcropping in the lower Espinhaço Supergroup. Therefore, geochronological data do not provide additional evidence for the idea that diamonds are originated from these metavolcanic rocks, as supposed by some authors after geologic regional studies.

Neoarchæan crustal evolution in the Congo Craton: evidence from K rich granitoids of the Ntem Complex, southern Cameroon

The Neoarchæan (2.6 Gal K rich granitoids of the Ebolowa area (Ntem Complex, Congo Craton) in southern Cameroon form small massifs trending north-northwest-south-southeast to north-south and contain xenoliths of the surrounding tonalite-trondhjemite-granodiorite (TTG) series and of the greenstone belt country rocks. The granitoids range in modal composition from granodiorite to granite and leucogranite. The rocks display high SiO 2 (69–76.5%), Na 2O+K 2O (5.5–9.1 %) and K 2 O Na 2 O (0.5–1.9). They are metaluminous to slightly peraluminous (0.9 <A/CNK < 1.1) and define a classic calc-alkaline suite. Their trace element distribution diagrams are characterised by pronounced enrichment in LILE and LREE (93 < La N < 705), and negative Nb, P and Ti anomalies. The intrusion temperatures, based on the major elements and zircon saturation, indicate a range from 700°C to 800°C. The Sr and Nd isotopes, combined with the trace element signatures and intrusion temperatures, are consistent with a derivation of the granitoids by partial melting of the surrounding or underlying Archæan gneisses during granulite-facies metamorphism. The Sr and Nd isotopes of the high K granitoids do not show any evidence for a new mantle contribution in their petrogenesis. PbPb zircon evaporation ages of 2666±2 and 2687±3 Ma indicate that this final magmatic episode post-dates the main crust formation by more than 200 Ma. The formation of these late granitoids in the Ebolowa area is attributed to a large-scale tectonothermal event around 2.6 Ga that led to the formation of similar granitoids throughout the extensive Ntem Complex. The intrusion of these granitoids is probably the terminal tectono thermal episode following the Archæan accretion and differentiation of the northwest Congo Craton.

Late Archaean (2550–2520 Ma) juvenile magmatism in the Eastern Dharwar craton, southern India: constraints from geochronology, Nd–Sr isotopes and whole rock geochemistry

The results of field, geochronologic, geochemical and isotopic studies are presented for the granitoids that occur east of the Closepet batholith up to the Kolar schist belt (KSB). Field data, such as common foliation, strong shear deformation occasionally leading to mylonitization, together with petrographic data, including reduction in grain size with corroded borders, show characteristics of the syn-kinematic emplacement of the granitoids. Single zircon evaporation ages define a minimum age of 3127 Ma for the tonalitic–trondhjemitic–granodioritic (TTG) basement and 2552–2534 Ma plateau ages for the emplacement of the granitoids, which slightly predate (20–30 Ma) the emplacement of the 2518 Ma Closepet batholith.Major and trace element data, together with isotopic data, suggest at least four magmatic suites from Closepet batholith to the east, which have independent magmatic evolution histories. The observed data are compatible with magma mixing for the Closepet batholith, melting of TTG and assimilation–fractional crystallization processes for Bangalore granites, either melting of heterogeneous source or different degree of melting of the same source for the granitoids of Hoskote–Kolar and fractional crystallization for the western margin of the KSB. Isotopic (Nd–Sr) and geochemical data (LREE and LIL elements) suggest highly enriched mantle and ancient TTG crust for the Closepet batholith, enriched mantle and TTG crust for the Bangalore granites, c.a. chondritic mantle source for the granitoids of Hoskote–Kolar and the quartz monzonites of the western margin of the KSB and slightly depleted mantle for granodiorites of the eastern margin of the KSB.We interpret all these geochronologic, geochemical and isotopic characteristics of granitoids from the Closepet batholith to the east up to the KSB in terms of a plume model. The centre of the plume would be an enriched ‘hot spot’ in the mantle that lies below the present exposure level of the Closepet batholith. Melting of such an enriched mantle hot spot produces high temperature magmas (Closepet) that penetrate overlying ancient crust, where they strongly interact and induce partial melting of the surrounding crust. These magmas cool very slowly, as the hot spot maintains high temperatures for a long time; thus they appear younger (2518 Ma). On the contrary, to the east the plume induces melting of c.a. chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas, which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older (2552–2534 Ma). This plume model can also account for late Archaean geodynamic evolution, including juvenile magmatism, heat source for reworking, inverse diapirism and granulite metamorphism in the Dharwar craton.

New zircon ages and regional significance for the evolution of the Pan-African orogen in Madagascar

New 207 Pb/ 206 Pb single zircon evaporation ages for granulites, gneisses and granites in southern and central Madagascar record a widespread Pan-African metamorphic and magmatic event in the period c. 650–556 Ma, but also earlier ages in the range 1890–1710 Ma, inherited from protolith material and reflecting heterogeneous crustal sources. South of the Ranotsara shear zone, metasedimentary gneisses and granulites contain an early population of detrital zircons with ages in the range 1890–1740 Ma; a detrital grain with an age of 899 ± 2 Ma suggests that some sedimentary protoliths were deposited later than c. 900 Ma. Metamorphic zircons have a mean age of 564.2 ± 0.9 Ma. North of the Ranotsara shear zone, our data provide information on the age of source material of metamorphic rocks: 788.6 ± 0.7 Ma for the time of emplacement of the granitic precursor of a granulite-facies charnockite and 650.9 ± 0.9 Ma for the protolith age of an amphibolite-facies migmatitic gneiss. A structurally conformable alkali granite sheet with a crystallization age of 568.7 ±1.6 Ma contains xenocrystic zircons, one of which has an age of 1229.6 ± 1.0 Ma, inherited from the source of the anatectically derived material. The post-tectonic, alkalic Carion granite has an emplacement age of 556.0 ± 1.7 Ma and provides a minimum age for granulite- and amphibolite-facies metamorphism. Our field data indicate that extensional shear zones are common in central Madagascar, locally controlling amphibolite-facies retrogression of granulite-facies rocks and the emplacement of crustal melt granites. These events record the widespread extensional collapse of the Pan-African orogen in Madagascar.

Late Precambrian metamorphism and cooling in the Arabian–Nubian Shield: Petrology and 40Ar/ 39Ar geochronology of metamorphic rocks of the Elat area (southern Israel)

Representative samples of metamorphic rocks and one sample of undeformed granite from the Elat area of southern Israel have been analyzed by microprobe and conventional 40Ar/ 39Ar geochronological methods. The highest-grade metamorphic assemblages, formed during the collision of greater east and west Gondwanaland, give equilibrium P–T conditions of 7±1 kbar and 650–700°C. The timing of regional metamorphism in Elat has been estimated at 620±10 Ma on the basis of published single zircon evaporation ages, correlations with geochronological data from rocks that adjoined the Elat area prior to later faulting and 40Ar/ 39Ar age spectra from amphibole and phlogopite of the Shahmon metabasite complex. Samples of muscovite and biotite from the Elat schists, Taba gneiss, Elat granitic gneiss and Elat granite yield remarkably uniform ages ∼600 Ma, interpreted to record the potentially rapid cooling and unroofing of the metamorphic basement. Unroofing of the Elat metamorphic basement was synchronous with widespread igneous activity in the Sinai Peninsula and elsewhere in the Arabian–Nubian Shield, reflecting its transition from orogenic to post-orogenic tectonics. The P–T and 40Ar/ 39Ar data agree well with field, geochemical, geochronological and structural data consistent with a single metamorphic event at 620±10 Ma followed by large-scale extension and tectonic escape in the northern Arabian–Nubian Shield during the late stages of the Pan-African orogenic cycle. The lack of any evidence of a Devonian thermal event in the Elat area, previously inferred from complex 40Ar/ 39Ar age spectra from altered dikes, indicates that any such event was very localized.