Pb SHRIMP Zircon Research Articles

U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone, Kyrgyzstan, and Ar–Ar age of deformations along the fault

A 2000km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U–Pb zircon SHRIMP ages 728±11Ma and 778±11Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and their calculated tHfc ages varied from 2.42 to 2.71Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical 206Pb/238U ages of 279±5Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and calculated tHfc ages from 2.42 to 2.71Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400°C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227Ma to 199Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic.

Petrology, geochemistry, and geochronology of the Chah-Bazargan gabbroic intrusions in the south Sanandaj–Sirjan zone, Neyriz, Iran

The Chah-Bazargan gabbroic intrusions are located in the south of Sanandaj–Sirjan zone. Precise U–Pb zircon SHRIMP ages of the intrusions show magmatic ages of 170.5 ± 1.9 Ma. These intrusions consist primarily of gabbros, interspersed with lenticular bodies of anorthosite, troctolite, clinopyroxenite, and wehrlite. The lenticular bodies show gradational or sharp boundaries with the gabbros. In the gradational boundaries, gabbros are mineralogically transformed into anorthosites, wehrlites, and/or clinopyroxenites. On the other hand, where the boundaries are sharp, the mineral assemblages change abruptly. There is no obvious deformation in the intrusions. Hence, the changes in mineral compositions are interpreted as the result of crystallization processes, such as fractionation in the magma chamber. Rock types with sharp boundaries show abrupt chemical changes, but the changes exhibit the same patterns of increasing and decreasing elements, especially of rare earth elements, as the gradational boundaries. Therefore, it is possible that all parts of the intrusions were formed from the same parental magma. Parts showing signs of nonequilibrium crystallization, such as cumulate features and sub-solidification, underwent fracturing and were interspersed throughout the magma chamber by late injection pulses or mechanical movements under mush conditions. The geological and age data show that the intrusions were formed from an Al-, Sr-, Fe-enriched and K-, Nb-depleted tholeiitic magma. The magma resulted from the partial melting of a metasomatized spinel demonstrated by negative Nb, P, Hf, and Ti, and positive Ba, Sr, and U anomalies typical of subduction-related magmas.

Geochemistry and U–Pb SHRIMP zircon chronology of granitoids and microgranular enclaves from Jhirgadandi Pluton of Mahakoshal Belt, Central India Tectonic Zone, India

The northern part of Central India Tectonic Zone (CITZ) is delineated by an arc-shaped supracrustal belt commonly referred to as Mahakoshal Belt, which is considered as a product of intense rifting of sialic crust that occurred at ca 2400–2600Ma. Several granitoid plutons intrude the Parsoi Formation of Mahakoshal Belt. Among these, an elliptical small stock-like granitoid body trending E–W is exposed in and around Jhirgadandi region of Mahakoshal Belt, referred herein as Jhirgadandi Pluton. It is composed of minor amount of mafic rocks (diorite) and predominant granitoids. Country-rock pelitic xenoliths and microgranular enclaves (ME) are commonly hosted in granitoids but are absent in diorite. The ME exhibit typical magmatic texture with a Bt(±Cpx±Hbl)-Pl-Kf-Qtz-Mag-Ap assemblage, similar to that in host granitoids but with contrasting mineral proportions. Whole-rock molar Al2O3/(CaO+Na2O+K2O) (A/CNK) ratios of diorite (0.63–0.72), ME (0.69–1.21) and granitoids (0.83–1.05) suggest their nature largely metaluminous (I-type) to rarely peraluminous (S-type) granitoids. On most binary plots involving silica, two distinct compositional paths can be recognized; one formed by an array of differentiating diorite and ME, and another by fractionating granitoids gradually depleting in compatible elements. It is most likely that ME were generated by progressive and concurrent mixing of coeval pristine mafic (diorite) and granitoid magmas and fractionation processes. However, coherent and identical trace elements (except for Sr, Th, Y and Ni) and REE patterns for ME-granitoid pairs most likely suggest partial to near-complete chemical equilibration through varying degrees of diffusion process across the ME – partly crystalline host granitoid boundary. High-precision U–Pb SHRIMP zircon 206Pb/238U ages for ME (1758±19Ma) and host granitoid (1753±9.1Ma) from Jhirgadandi Pluton further support the notion that they were coeval. The obtained age (∼1750Ma) of Jhirgadandi Pluton also points to the existence and role of Super-Columbian continental component in the evolution of Mahakoshal Belt of the CITZ.

Structural, petrological and U–Pb SHRIMP geochronological study of the western Beaverlodge domain: Implications for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogen in the SW Rae craton, Canadian Shield

The Beaverlodge domain is a polydeformed Precambrian terrane in the southwestern Rae craton whose rocks variably record the effects of Archean tectonism and four major Paleoproterozoic tectonic events: the 2.5–2.3Ga Arrowsmith orogeny, the 1.99–1.93Ga Taltson orogeny, the ca. 1.90Ga Snowbird orogeny and the ca. 1.87–1.80Ga Hudsonian orogeny. Structural–metamorphic analysis and in situ U–Pb SHRIMP zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding: a lower level comprising mainly high-grade Archean (ca. 3.0 and 2.7–2.6Ga) metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca. 2.33 to <2.17Ga supracrustal rocks of the Murmac Bay Group at more variable metamorphic grade. Whereas the older package underwent thermal and/or metamorphic events in the Archean (ca. 2.57Ga) and early Paleoproterozoic (ca. 2.34Ga; Arrowsmith orogeny), rocks at higher structural levels bear no record of these events, recording only younger (1.94–1.90Ga) metamorphism interpreted to reflect two further stages of orogenesis. The first stage, correlated with Taltson orogeny, involved tight to isoclinal folding (D1/2) that led to development of the composite ESE-trending (S0/S1/S2) transposition foliation followed by more open folding (D3). The absence of Taltson-age plutonic rocks, except for low-volume crustal melts, suggests that this cycle was driven by crustal thickening, consistent with a clockwise P–T–t path derived for one sample. The high degree of strain along the contact between Archean rocks and the Murmac Bay Group, along with local structural discordance, indicates some degree of translation of cover rocks over basement during this stage. Pelitic rocks at higher structural levels (low- to medium-grade zone) underwent prograde metamorphism to middle amphibolite facies whereas rocks at deeper structural levels (high-grade zone) attained upper amphibolite facies conditions. Monazite data bracket prograde metamorphism (and related garnet growth) between 1939±6Ma and 1934±5Ma. The second stage, dated most precisely at 1906±7Ma, involved refolding of D1 to D3 structures about NE-trending axes (F4) and coeval dextral shearing and is correlated with Snowbird orogenesis. Rocks of the low- to medium-grade zone were held at roughly the same crustal level whereas rocks in the high-grade zone were uplifted via dextral–oblique displacement along domain-bounding shear zones, instigating decompression (garnet to cordierite) reactions. This study demonstrates that after <2.17Ga deposition of the upper Murmac Bay Group, rocks along the entire west-southwestern Rae margin, from the Northwest Territories/northern Alberta to the Snowbird tectonic zone, were affected by Taltson (1940–1930Ma) orogenesis. The Taltson tectono-metamorphic cycle then appears to have transitioned, as rocks underwent further shortening and uplift in a dextral transpressive regime, into Snowbird tectonism at ca. 1910–1900Ma. Snowbird orogenesis is therefore best viewed as part of a semi-continuous 1940–1900Ma cycle of tectonic burial and exhumation, with its inception at ca. 1910Ma triggered by a change in boundary conditions, as yet not fully understood, but consistent with arrival of the Hearne craton proposed by others. While the Beaverlodge domain has many features in common with the Tantato domain, future work needs to explore why rocks in the Tantato domain lack evidence for Arrowsmith orogeny and failed to record the cycle of 1.94–1.93Ga tectonic burial.

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

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

The metacarbonate rocks of Itatuba (Paraíba): A record of sedimentary recycling in a Paleoproterozoic collision zone of the Borborema province, NE Brazil

The Alto Moxotó Terrane (AMT) is a Paleoproterozoic domain structured within the Neoproterozoic collage that forms the Borborema Province in Northeastern Brazil. In the Itatuba region (state of Paraíba) the dominant rocks consist of banded granodioritic–monzogranitic gneisses, augen syenogranitic gneisses and migmatites with limited occurrences of metasedimentary rocks. A metaplutonic–metacarbonate complex appears as concordant intrusive bodies whose emplacement was controlled by contractional shear zones. This complex includes metamafic–ultramafic rocks, leucotonalitic orthogneisses and metacarbonate rocks cut by alkaline orthogneisses emplaced along late transcurrent shear zones. The metamafic–ultramafic complex presents patterns of a tholeiitic magmatic series and exhibits symplectite textures, suggesting that they have reached a high-pressure (eclogite?) metamorphic peak. Uncommon associated rocks are represented by massive and brecciated metacarbonates, which contain abundant inclusions of the country rocks. The structure of this brecciated facies varies from plastic clastomylonitic to a true melting state, confirming the existence of intrusive metacarbonates. Field and petrographic evidence, such as the occurrence of rare rounded pyroxenite inclusions and the association with lamprophyre, suggests the involvement of the lower crust, or even the mantle in their formation. However, the geochemical (rare earth elements (REE) and incompatible elements) and isotopic (C–O) patterns preclude a mantle source for these rocks, confirming that they are remobilized sedimentary carbonates. U–Pb SHRIMP and TIMS zircon studies have been performed on the country and intrusive orthogneisses. In the host orthogneiss, zircon cores yield an age of 2,308±22.5Ma, whereas zircon rims exhibit a cluster with a mean age of 2012±17Ma. For the leucotonalitic orthogneisses that are frequently associated with the rocks of the mafic–ultramafic complex, zircon cores exhibit a spread of ages that may be attributed to the formation of the mafic to felsic complex, and the majority of these ages are centred approximately 2,180Ma. The rims exhibit two clusters of 2,086±19Ma and 1,953±25Ma. The TDM Nd model age of this rock is 2,067Ma; consequently, it is possible that this age interval represents another rock-forming episode associated with the probable eclogite episode. It is therefore likely that the metamafic–ultramafics, leucotonalitic orthogneisses and the metacarbonate rocks represent an Orosirian collisional complex of the AMT. Sm–Nd and U–Pb data from the late alkaline Neoproterozoic transcurrent granites (549, 587, and 621Ma) show reworking of the Paleoproterozoic crust.

U–Pb age and Hf-isotope geochemistry of zircon from felsic volcanic rocks of the Paleoproterozoic Aillik Group, Makkovik Province, Labrador

U–Pb zircon SHRIMP geochronology and Lu–Hf zircon LA–MC–ICPMS isotope geochemistry are used to constrain the ages of Paleoproterozoic felsic magmatism and their crustal sources in the Aillik Group, Makkovik Province of Labrador. This information is important for understanding the extent of crustal reworking vs. juvenile crustal growth in subduction-related magmatism during the Paleoproterozoic and improves framework knowledge for uranium exploration in the Central Mineral Belt of Labrador. U–Pb SHRIMP zircon geochronology from four felsic tuff samples collected from two different areas of the Aillik Group yield magmatic 207Pb/206Pb ages of 1852±7Ma (2σ), 1854±7Ma (2σ), 1861±7Ma (2σ), and 1862±7Ma (2σ). The ages fall within the range of previous U–Pb zircon dates reported for felsic volcanic rocks of the Aillik Group. Altogether these dates indicate that felsic volcanism spanned at least 30m.y. and was perhaps most intense during the last 10m.y. of that interval. A foliated syn-deformational monzogranite in the Aillik Group (Cross Lake granite) yields a 207Pb/206Pb zircon date of 1805±6Ma (2σ), which indicates that development of a pervasive planar fabric associated with northwestward thrusting of the Aillik Group continued after this time.Initial εHfst values for magmatic zircons from the four felsic tuffs (−4.8 to −1.9, −1.7 to +2.2, −11.8 to +3.8, −9.0 to −4.6) and monzogranite (−4.8 to −1.5) exhibit significant variability, which indicates heterogeneous sources. Most of the zircon grains have negative initial εHfst values and calculated crust formation Hf-isotope TDM model ages point to substantial contributions from melts of ancient crust, ranging in age from ca. 2200 to 2900m.y. and clustering at ca. 2500m.y. Inherited zircon grains in the felsic tuffs and monzogranite, mainly have U–Pb ages of ca. 1880–1920Ma, though others are older, up to ca. 2743Ma. Hf-isotope TDM model ages for the inherited zircons are similar to those of the magmatic zircons. The Hf-isotope results indicate that subduction-related felsic magmatism in the Makkovik Orogen involved significant crustal reworking (∼80%) during formation of the Aillik Group.

Fractionation and incipient self-granulitization during deep-crust emplacement of Lower Ordovician Valle Fértil batholith at the Gondwana active margin of South America

Large granite batholiths were emplaced at the Gondwana active margin during Lower Ordovician in South America. These have contributed to crustal growth by net addition of silicic rocks to the continental crust. New U–Pb SHRIMP zircon age determinations, together with thermobarometric and geochemical data, yield that batholith magma intrusion is the responsible of heating and self-granulitization of early gabbro pulses. Partially molten granulitic gabbros, which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith-forming Qtz-diorites, contain Opx-bearing, trondhjemitic leucosomes surrounding Hbl+Opx+Pl mafic mesosomes forming typical agmatitic structures. Hornblende–Plagioclase equilibria, applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes, yield temperatures in the range 850–910°C for core-to-core pairs and in the range 1000–1075°C for rim-to rim pairs, at pressures of about 0.7GPa. SHRIMP zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20Ma from 465 to 485Ma, with most ages clustered at about 470Ma. The age of metagabbros is 473±7Ma for older zircons and 454±4 for younger zircons. These ages are almost coincident within error with the age of host migmatites (477±5Ma) and those of batholith intrusion of 476±9Ma and 475±3Ma for Qtz-diorites and 475±5Ma for granites. Zircon overgrowths of these intrusive rocks yield sages clustered around 450Ma, revealing a protracted thermal history, more complex than previously believed. The geochemical study reveals that Qtz-diorites, tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma. A weak adakitic tendency, with Sr/Y>15 in several samples, implies the presence of Grt in the source or magma chamber at a minimum pressure of 1.0GPa, higher than the depth of emplacement at 0.7GPa. The high temperature of magma emplacement, which induced the incipient self-granulitization of early magmatic pulses, together with the cotectic-like fractionation linking coeval Qtz-diorites, tonalites and granodiorites, is compatible with fractionation at the lower crust of a deep-generated, infracrustal, (sublithospheric?) intermediate magma.

Neoproterozoic tectonic evolution of the Jebel Saghro and Bou Azzer—El Graara inliers, eastern and central Anti-Atlas, Morocco

New mapping, geochemistry, and 17 U–Pb SHRIMP zircon ages from rocks of the Sirwa, Bou Azzer–El Graara, and Jebel Saghro inliers constrain the Neoproterozoic evolution of the eastern Anti-Atlas during Pan-African orogenesis. In the Sirwa inlier, Tonian quartzite from the pre Pan-African passive margin deposits of the Mimount Formation contains detrital zircon derived entirely from the West African Craton (WAC), with most grains yielding Eburnean Paleoproterozoic ages of about 2050Ma. Cryogenian Pan-African orogenic activity (PA1) from about 760 to 660Ma included northward-dipping subduction to produce a volcanic arc, followed by ophiolite obduction onto the WAC. In the Bou Azzer–El Graara inlier, calc-alkaline granodiorite and quartz diorite, dated at 650–646Ma, are syn- to post-tectonic with respect to the second period of Pan-African orogenesis (PA2), arc-continent accretion, and related greenschist facies metamorphism. Slab break-off and lithospheric delimination may have provided the source for the supra-subduction calc-alkaline plutons. At about 646Ma, quartz diorite intruded the Tiddiline formation placing an upper limit on molassic deposition. Widespread Ediacaran high-K calc-alkaline to shoshonitic plutonism and volcanism during the final stage of Pan-African orogenesis (PA3) occurred in a setting related to either modification of the margin of the WAC or formation of a continental volcanic arc above a short-lived southward-dipping subduction zone. In the Saghro inlier, eight plutonic rocks yield ages ranging from about 588 to 556Ma. Sampled plutonic rocks previously considered to be Cryogenian yielded Ediacaran ages. Peraluminous rhyolitic volcanic rocks in the lower part of the Ouarzazate Supergroup, including ash-flow tuffs of the Oued Dar’a caldera, yield ages between about 574 and 571Ma. The Oued Dar’a caldera developed in a pull-apart graben produced by a left-step in a northeast-trending, left-lateral strike-slip fault zone, and much of the lower Ouarzazate Supergroup volcanic rocks in the area are probably related to caldera out-flow facies and collapse. Late stage PA3 intrusive rocks include the Bouskour–Sidi Flah and Timijt rhyolitic dike swarms at about 563Ma, the voluminous pink Isk-n-Alla granite (559±5Ma), and volumetrically minor gabbro of Tagmout (556±5Ma). Rhyolite flows from the upper part of the Ouarzazate Supergroup, above a regional angular unconformity, yielded ages of 558±4 and 556±4Ma. The youngest ages place an upper limit on block faulting and weak folding during latest Pan-African tectonic activity (PA3), coincident with the departure of the Cadomian crustal fragment from the northern margin of the WAC.

The Mesoproterozoic Karagwe-Ankole Belt (formerly the NE Kibara Belt): The result of prolonged extensional intracratonic basin development punctuated by two short-lived far-field compressional events

The Mesoproterozoic Kibara Belt (also Kibaran Belt or Kibarides in some references) of Central Africa was often portrayed as a continuous, c. 1500km long orogenic belt, trending NE to NNE from Katanga, Democratic Republic of Congo (DRC) in the south, up into SW Uganda in the north. Recently however, the Karagwe-Ankole Belt (KAB; formerly the NE Kibara Belt) has been redefined as the part north of a NW oriented Palaeoproterozoic basement high of the Ubende-Rusizi Belts, while the Kibara Belt (KIB) is now limited to the part south of this rise.We present a lithostratigraphy for the KAB that takes into account two rheologically contrasting structural domains (Western and Eastern Domain); each of them being characterised by independent sedimentary sub basin(s) and depositional conditions: the ED with Archaean basement versus the WD with Palaeoproterozoic basement. We document new volcanic and detrital U–Pb SHRIMP zircon data which provide new constraints on the timing of deposition and on the detrital provenance of the sedimentary sequences in the KAB. We discuss the evolution of the KAB in a wider regional context, comparing it to other Mesoproterozoic units and with reference to the general geodynamic history of this part of the African continent in Proterozoic times.The lithostratigraphic successions of the KAB are only valid respectively in the ED (Kagera Supergroup) or in the WD (Akanyaru Supergroup), with no correlations between them. Deposition of the Kagera Supergroup in the ED is bracketed between 1.78Ga and 1.37Ga and the deposits have to be considered an Eburnean-age “molasse”. Detrital components comprise material only of Archaean and Palaeoproterozoic age, consistent with derivation from nearby source regions. In the WD, deposition of the two lowermost groups of the Akanyaru Supergroup is bracketed between 1.42Ga and 1.37Ga. The large contribution of detrital Palaeoproterozoic components in the WD strengthens the view that this domain is underlain by Palaeoproterozoic basement and supports the concept that part of the Akanyaru Supergroup sediments consists of reworked Eburnean-aged molasse. In the WD of the Kivu-Maniema area (DRC), later sedimentation periods are documented at respectively 1222Ma and 710Ma. The KAB documents a long-lived period of intracratonic intermittent depositional activity (with periods of interruption of deposition, erosion and magmatism) showing a recurrent subsidence trend controlled by structural activity moving with time from E to W.On a regional scale, we postulate that since 1.8Ga, following the amalgamation of Archaean and Palaeoproterozoic landmasses into a single coherent ‘proto-Congo Craton’, various long-lived shallow-water intracratonic basins (aulacogenes) developed. These basins underwent a comparable Mesoproterozoic geodynamic evolution, as shown not only in the sequences of the KAB and of the relatively close Kibara (KIB), Bangweulu Block and Northern Irumide Belts, but even in more distant sequences located in SW Angola and E Brazil.The long-lived aulacogene history of the KAB within the proto-Congo Craton is interrupted only twice by short-lived compressional deformation reflecting far-field effects of global orogenic events, external to the proto-Congo Craton. The first event at 1.0Ga is related to Rodinia amalgamation. The second event at 550Ma results from Gondwana amalgamation and develops a N–S Pan African overprint in the KAB which has previously been underestimated or even overlooked. Three mineralisation provinces occurring in the KAB, respectively the Bushveld-type, the tin-coltan-wolfram and the gold province, can be ascribed successively to the 1375Ma Kibaran magmatic event, the 1.0Ga Rodinia and the 550Ma Gondwana amalgamation events.Our results give additional weight to the recent redefinition of the KAB and the KIB, forming two distinct Belts respectively north and south of the Palaeoproterozoic basement high of the Ubende-Rusizi Belts, the more that within this basement rise local Mesoproterozoic strike-slip basins, with their own unique lithostratigraphic and geodynamic characteristics (e.g. Itiaso Group) are documented, which differ from those of the KAB or the KIB.

Late Permian–Early Triassic igneous activity in the Attic Cycladic Belt (Attica): New geochronological data and geodynamic implications

Igneous rocks of the Attic Cycladic Belt (ACB) formed prior to the Alpine orogeny have received relatively little attention, especially regarding their crystallisation age. New U–Pb SHRIMP zircon data from mylonitised metagranitoid rocks within metapelites and metabasites of the Pentelikon mountain, lower tectonic unit (LTU) of Attica, in combination with internal zircon characteristics reveal the following: the metagranitoid precursor was very likely an I-type pluton crystallising in two successive stages, at 255±3Ma and 246±2Ma (weighted mean 206Pb/238U ages; error at 95% c.l.). These age results are respectively marginally older than and within uncertainty of the 240±4Ma protolith age reported for metagranitoids of the same unit in the area of Lavrion, Attica. They are in line with the protolith ages of several meta-igneous rocks within the Cycladic Blueschist Unit of Aegean islands. One core zircon domain and one single zircon grain yielded 206Pb/238U ages of 311±3Ma and 318±4Ma (1σ errors), providing evidence for inheritance from a previous Hercynian event. The new data, whilst interpreted to favour a rift-related geotectonic setting of formation (poly-episodic magmatism), do not provide additional strong arguments for a rifting versus a subduction scenario. Assuming that the dated rocks formed indeed in a rift setting, prominent at that time in the broad European area, they probably represent a higher level of the crust, compared to Permo-Triassic metagabbros of the Rhodope or the European Alps, which formed at deeper crustal levels (underplated gabbros). The new data, in combination with earlier petrological data on the type of the metamorphic path, favour the view that the LTU of Attica belongs geotectonically to the Cycladic Blueschist Unit of the ACB. This inference is thus filling an important missing link in the geotectonic configuration of the geology of Attica.

Fast sediment underplating and essentially coeval juvenile magmatism in the Ordovician margin of Gondwana, Western Sierras Pampeanas, Argentina

Metasedimentary high-pressure upper amphibolite facies gneisses (1.2±0.1GPa and 780±45°C) at Las Chacras, Sierra de Valle Fértil, are tectonically juxtaposed to the westernmost parts (outboard) of the Famatinian (Early Ordovician) magmatic arc, which underwent syn-plutonic middle crust high-grade metamorphism at lower pressure. U–Pb SHRIMP zircon data suggest that the gneisses contain Famatinian igneous detritus, so that their sedimentary protoliths were probably deposited in a forearc basin and then rapidly underthrust and accreted to the lower crust of the arc, essentially coevally with arc magmatism at 468±4Ma. Chemically and isotopically juvenile garnetiferous amphibolites within the gneisses are recognised as representing the most primitive magmas so far observed in this belt, which has often been considered to be a continental arc derived from isotopically mature sources. This is consistent with the idea that at least part of the dominant Famatinian magmatism originated in depleted mantle but was heavily contaminated by crustal components.

U–Pb zircon geochronology of the eastern part of the Southern Ethiopian Shield

The Southern Ethiopian Shield (SES) in the central East African Orogen lies at the junction of Neoproterozoic (880–550Ma) largely greenschist-facies juvenile crust of the Arabian-Nubian Shield in the north and more metamorphosed and remobilized older crust of the Mozambique Belt to the south. The SES exposes a polycyclic sialic gneissic basement (represented by the Alghe Terrane in this study) with interleaved ophiolitic–volcano–sedimentary (Kenticha, Megado, and Bulbul) terranes. U–Pb zircon SHRIMP and LA-ICP-MS dating of 8 igneous and metamorphic intermediate and felsic bodies in the eastern SES yield Neoproterozoic crystallization ages: 847±11, 855±14, 732±5, 665±8, 657±6, 560±8, and 548±8Ma. From these and previously published ages, we infer 4 principal magmatic episodes: 840–890Ma (Late–Tonian–Early Cryogenian), 790–700Ma (Cryogenian), ∼660Ma (Moyale Event), and Pan-African (630–500Ma; Ediacaro-Cambrian). Neoproterozoic zircon xenocrysts (941, 884, 880, 863, 762, 716 and 712Ma) confirm the dominance of Neoproterozoic crust in the study area. One sample of undeformed granite from the Alghae Terrane has abundant Archean zircons and may be a ∼550Ma melt of ∼2.5Ga crust, demonstrating for the first time that Archean crust or sediments with abundant Archean zircons exists in the SES. In spite of ∼300 million years of Neoproterozoic igneous activity, we see no evidence of systematic compositional evolution in SES igneous rocks from early low-K suites to late high-K suites. Ediacaran deformation and magmatism of the SES reflects Late Tonian and Cryogenian formation of mostly juvenile crust that was subsequently deformed and chemically modified as a result of collision between large fragments of East and West Gondwana. Terminal collision began at ∼630Ma and caused crustal thickening, melting, uplift, erosion, orogenic collapse, and tectonic escape over a broad region of the East African Orogen, including up to ∼25km of erosion of SES crust. Plate convergence was likely continuous from ∼630Ma, forming major N-S structures. Deformation stopped at ∼550Ma and was followed by exhumation between ∼ 530 and 500Ma.

Composition, age, and origin of the ~620 Ma Humr Akarim and Humrat Mukbid A-type granites: no evidence for pre-Neoproterozoic basement in the Eastern Desert, Egypt

The Humr Akarim and Humrat Mukbid plutons, in the central Eastern Desert of Egypt, are late Neoproterozoic post-collisional alkaline A-type granites. Humr Akarim and Humrat Mukbid plutonic rocks consist of subsolvus alkali granites and a subordinate roof facies of albite granite, which hosts greisen and Sn–Mo-mineralized quartz veins; textural and field evidence strongly suggest the presence of late magmatic F-rich fluids. The granites are Si-alkali rich, Mg–Ca–Ti poor with high Rb/Sr (20–123), and low K/Rb (27–65). They are enriched in high field strength elements (e.g., Nb, Ta, Zr, Y, U, Th) and heavy rare earth elements (La n /Yb n = 0.27–0.95) and exhibit significant tetrad effects in REE patterns. These geochemical attributes indicate that granite trace element distribution was controlled by crystal fractionation as well as interaction with fluorine-rich magmatic fluids. U–Pb SHRIMP zircon dating indicates an age of ~630–620 Ma but with abundant evidence that zircons were affected by late corrosive fluids (e.g., discordance, high common Pb). eNd at 620 Ma ranges from +3.4 to +6.8 (mean = +5.0) for Humr Akarim granitic rocks and from +4.8 to +7.5 (mean = +5.8) for Humrat Mukbid granitic rocks. Some slightly older zircons (~740 Ma, 703 Ma) may have been inherited from older granites in the region. Our U–Pb zircon data and Nd isotope results indicate a juvenile magma source of Neoproterozoic age like that responsible for forming most other ANS crust and refute previous conclusions that pre-Neoproterozoic continental crust was involved in the generation of the studied granites.

Early Carboniferous sub- to mid-alkaline magmatism in the Eastern Sierras Pampeanas, NW Argentina: A record of crustal growth by the incorporation of mantle-derived material in an extensional setting

A recently discovered granitic intrusion at Cerro La Gloria in western Sierra de Famatina (NW Argentina) is representative of sub- to mid-alkaline Carboniferous magmatism in the region. The main rock type consists of microcline, quartz and plagioclase, with amphibole, magnetite, ilmenite, biotite, epidote, zircon, allanite and sphene as accessory minerals. We report a U–Pb zircon SHRIMP age for the pluton of 349±3Ma (MSWD=1.1), i.e., Tournaisian. Whole-rock chemical composition and Nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust (εNdt between −0.58 and +0.46 and TDM values of about 1.1Ga). The pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back-arc basalts. Coeval, Early Carboniferous A-type granites occur farther east in the Sierras Pampeanas, probably generated during lithospheric stretching. Overall, the Early Carboniferous granitic rocks show a west-to-east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west. Based on the observed patterns of geochronology, geochemistry and field relationships we suggest that A-type magma genesis in the Eastern Sierras Pampeanas was linked to an Andean-type margin where the lithospheric mantle played a role in its generation.

New geochronological data from the Paleozoic and Mesozoic nappe structures, igneous rocks, and molybdenite in the North Wuyi area, Southeast China

The Wuyishan metallogenic belt, a part of the circum-Pacific tectonomagmatic belt, is considered one of the 19 key metallogenic belts in China because of its abundant mineral resources and vast prospecting potential. Specifically, the North Wuyi area is an important Cu–Pb–Zn polymetal ore-concentrated district in East China. We report new geochronological dates in this area, which suggest that the formation of mineralization-related magmatic rocks have mainly occurred during the Paleozoic and Mesozoic in four stages: the Caledonian (530Ma to 430Ma), Early–Middle Jurassic (183Ma to 160Ma), Early Cretaceous (140Ma to 110Ma), and Late Cretaceous (63Ma to 85Ma). The mineralization events are either coeval or slightly postdate the related magmatic activities. Nappe structures are widespread in the North Wuyi area and play an important role in the formation and preservation of ore deposits, as determined from field observations and structural analyses of mining tunnels. The regional distribution and structural styles of the thrust faults in the North Wuyi area and new 40Ar–39Ar dating results (415Ma to 390Ma) confirm that intensive tectonic activity during the Early Paleozoic, when the South China Block was about to separate from Gondwana, has wiped out evidence of Caledonian mineralization. All nappe structures with regional scales (several thousand meters) were formed during the Early Cretaceous period, with an 40Ar–39Ar age of 120Ma to 129Ma. The thrust direction of the nappe structures is mainly southeast during the Early Cretaceous in the main body and north piedmont of the Wuyi Mountain. These early thrust faults have not only broken or reformed the existing ore deposits, but have also provided hydrothermal ore-forming channels and spaces for metallogenism that are coeval or posterior to the thrust. The subsequent nappe structures thrust mainly from southeast to northwest and overprint the previous southeastward thrust faults. Using 40Ar–39Ar dating of the structural belt, U–Pb SHRIMP zircon dating of the wall rocks, and Re–Os dating of the ore formation, we analyze the genetic connection between the structures and ore bodies in different ore fields. Our results serve as useful information on prospecting for the latent ore body in the North Wuyi area, especially the ore bodies destroyed by faulting activities.

Paleoproterozoic eclogites of MORB-type chemistry and three Proterozoic orogenic cycles in the Ubendian Belt (Tanzania): Evidence from monazite and zircon geochronology, and geochemistry

Eclogites and metapelites from the Ubende Terrane of the Proterozoic Ubendian Belt were studied for the purpose of establishing their metamorphic history. Geochemical, petrological and geochronological data of these rocks indicate that oceanic lithosphere was subducted in the Paleoproterozoic and experienced repeated regional metamorphic cycles following their emplacement into continental lithosphere in the Proterozoic. Eclogite facies metamorphic conditions are recorded by well-preserved porphyroclasts of omphacite (Jd 17), matrix plagioclase (Ab 72) and garnet core, which give a minimum peak pressure of 15 kbar at 700 °C reflecting a geothermal gradient lower than 13 °C/km. These eclogites have chondrite normalized REE patterns that resemble those of N-MORB and E-MORB. U–Pb SHRIMP dating of zircon in the eclogites reveal metamorphic dates of 1886 ± 16 and 1866 ± 14 Ma. These data indicate that eclogites of the Ubende Terrane in the Ubendian Belt may represent former oceanic crust that was metamorphosed during Paleoproterozoic subduction. The subduction was followed by a regional metamorphic event dated at 1831 ± 11 Ma (monazite in metapelite) and 1817 ± 26 Ma (zircon in metapelite). Mylonitic textures are common in all lithologic units of the Ubende Terrane and reflect regional Mesoproterozoic and Neoproterozoic metamorphic overprints. The Mesoproterozoic event is dated at 1091 ± 9 Ma (U–Pb SHRIMP date on zircon rims in a metapelite), while Neoproterozoic dates of 596 ± 41 Ma (U–Pb SHRIMP zircon age of an eclogite) and 601 ± 7 Ma (U–Th total Pb age of a monazite rim from a metapelite) reflect later metamorphic events. The Neoproterozoic mylonitization of eclogites occurred under high-pressure amphibolite-facies conditions at 680–750 °C/10–11 kbar.

Geochronology and geochemistry of Ordovician felsic volcanism in the Southern Armorican Massif (Variscan belt, France): Implications for the breakup of Gondwana

Low-grade, porphyritic felsic metavolcanics make up a significant volume of the South Armorican domain (Variscan belt, France). Their age and geochemistry are the first focus of this paper. These rocks have long been considered to be Silurian in age because they are either thrust over Silurian sediments (the so-called “Porphyroid Nappe”) or inserted within the Silurian sediments, acting as a decollement level (Para-autochthon), and because of a previous zircon age. Five new U–Pb SHRIMP zircon dates for the felsic volcanics from the Porphyroid Nappe and its Para-autochthon define Ordovician ages between 473±5 and 494±4Ma. These Ordovician metavolcanics are mainly of calc-alkaline affinity, probably in response to the reworking of the Ediacaran sediments derived from the Cadomian belt (Brioverian), variably mixed with a Paleoproterozoic (Icartian) component in metatuffs.A second focus of this paper is the integration of this volcanism at the scale of the Variscan belt. Because of its Early Ordovician age and calk-alkaline chemistry, the felsic volcanism from the Porphyroid Nappe is similar to the well-known Ollo de Sapo Formation in the Iberian Massif. Hence, the volcanism allows a better understanding of (i) the Early Ordovician rifting along the northern Gondwana margin, and (ii) the large-scale structure of the Variscan belt from the Iberian Massif to the French Massif Central (i.e. along the Ibero-Armorican Arc).

Age and composition of granulite xenoliths from Paso de Indios, Chubut province, Argentina

Granulite xenoliths enclosed in Paleogene alkali basalts from the locality of Paso de Indios in The Argentinean Patagonia, have been studied for petrology , geochemistry and U–Pb SHRIMP zircon geochronology . These are lower crust xenoliths composed of pyroxene and plagioclase dominantly. Symplectitic overgrowths with olivine and new pyroxene-plagioclase formation around the large pyroxene crystals are common. The study of phase equilibrium yields a decompression path from pressure 0.9 GPa at temperatures of about 1000 °C. The major element composition does not correspond with any known basaltic or andesitic magmas . They have SiO 2 < 50 wt% and MgO = 6 wt% and lower, very close in composition to the bulk lower continental crust composition. They are extremely depleted in incompatible elements with values as low as Rb = 15.3 to 4.1 ppm; Y = 6.2 to 2.5 ppm; Th = 0.92 to 0.13 ppm; U = 0.51 to 0.22 ppm; ΣREE = 53 ppm. This depletion is attributed to a residual origin of the granulites. In situ age determination with SHRIMP yields a Concordia age of 175.9 ± 4.9 Ma with MSWD = 1.4. This age represent the time of complex lithosphere evolution in relation with the formation of this residual crust. ► Patagonia middle Jurassic granulite xenoliths show crustal growth processes. ► Granulite development may be related to Aluk plate subduction and Gondwana break up. ► The granulites are extremely depleted in incompatible elements. ► The depletion is attributed to a residual origin of the granulites.

Petrology and SHRIMP U–Pb zircon geochronology of Cordilleran granitoids of the Bariloche area, Argentina

A petrological and geochronological study of Cordilleran granitoid intrusions in the Bariloche area (Argentina) point to a complex time-compositional evolution of magmatic processes in relation with oblique subduction of the Phoenix plate below the South America active margin during Jurassic times. The observed geochemical variations in both major and trace elements, together with the textural and mineralogical relations, point to a roughly defined, overall process of magmatic “filtering” linking all the intrusive batholithic rocks of the Bariloche area. These data suggest that the composition of the parental magma that underwent fractionation may be an intermediate magma with SiO2 = 58–60 wt%, MgO = 2.5 wt%, FeO = 6.5 wt%, CaO = 6.1. These are coincident with the typical compositions of evolved andesites. Magnetite, amphibole and plagioclase are the main phases involved in the fractionation process. According to Hbl thermobarometry, fractionation may have taken place, at least in part, at shallow pressures (P = 0.5–1.5 kbar), possibly at the level of emplacement. The coupled observations of the two pressure dependent ratios, namely Sr/Y and La/Yb are pointing to a low-pressure, low-temperature final fractionation dominated by only Pl. The geochronologic study by U–Pb SHRIMP zircon determinations of 14 samples from granites, tonalites and diorites yield a broad range of about 20 Ma, between 150 and 170 Ma at the Medium Jurassic. The batholith was accomplished by a protracted magmatic activity that lasted for about 20 Ma. This time is much longer than the time elapsed from intrusion to complete crystallization of shallow magma chambers. It is concluded that amalgamation of discrete magma pulses is the dominant process that built-up the batholith. The observed structures suggest that the fractures conditioning the emplacement of the magma batches were arranged en échelon and show a right-stepping. The resulting geometry is compatible with the activity of a large-scale, sinistral, N–S trending, strike-slip fracture zone permitting the emplacement of each magma pulse. This major, strike-slip fault system should be deeply entrenched in the crust to allow intruding magmas generated and fractionated at depth. Because batholith generation is a direct consequence of subduction, structural relations and ages can be used to constraint the plate motion relations during Jurassic in this region of the South America active margin.