The Temaguessine Fe-cordierite orbicular granite (Central Hoggar,Algeria): U–Pb SHRIMP age,petrology, origin and geodynamical consequences for the late Pan-African magmatism of the Tuareg shield

The Temaguessine high-level subcircular pluton is intrusive into the LATEA metacraton (Central Hoggar) Eburnian (2 Ga) basement and in the Pan-African (615 Ma) granitic batholiths along a major NW–SE oriented major shear zone. It is dated here (SHRIMP U–Pb on zircon) at 582 ± 5 Ma. Composed of amphibole–biotite granite and biotite syenogranite, it comprises abundant enclaves: mafic magmatic enclaves, country-rock xenoliths and remarkable Fe-cordierite (#Fe = 0.87) orbicules. The orbicules have a core rich in cordierite (40%) and a leucocratic quartz–feldspar rim. They are interpreted as resulting from the incongruent melting of the meta-wacke xenoliths collapsed into the magma: the breakdown of the biotite + quartz assemblage produced the cordierite and a quartz–feldspar minimum melt that is expelled, forming the leucocratic rim. The orbicule generation occurred at T < 850° and P < 0.3 GPa. The Fe-rich character of the cordierite resulted from the Fe-rich protolith (wacke with 4% Fe₂O₃ for 72% SiO₂). Strongly negative ε_Nd (−9.6 to −11.2), Nd T_DM model ages between 1.64 and 1.92 Ga, inherited zircons between 1.76 and 2.04 Ga and low to moderately high I_Sr (0.704–0.710) indicate a Rb-depleted lower continental crust source for the Temaguessine pluton; regional considerations impose however also the participation of asthenospheric material. The Temaguessine pluton, together with other high-level subcircular pluton, is considered as marking the end of the Pan-African magma generation in the LATEA metacraton, resulting from the linear delamination along mega-shear zones, allowing asthenospheric uprise and melting of the lower continental crust. This implies that the younger Taourirt granitic province (535–520 Ma) should be considered as a Cambrian intraplate anorogenic event and not as a very late Pan-African event.     

The “Taourirt” magmatic province, a marker of the closing stage of the Pan-African orogeny in the Tuareg Shield: review of available data and Sr–Nd isotope evidence

The Tuareg Shield, located between the Archaean to Palaeoproterozoic Saharan metacraton and the West African craton, is composed of 23 recognized terranes that welded together during the Neoproterozoic Pan-African orogeny (750–520 Ma). Final convergence occurred mainly during the 620–580 Ma period with the emplacement of high-K calc-alkaline batholiths, but continued until 520 Ma with the emplacement of alkali-calcic and alkaline high-level complexes. The last plutons emplaced in central Hoggar at 539–523 Ma are known as the “Taourirt” province. This expression is redefined and three geographical groups are identified: the Silet-, Laouni- and Tamanrasset-Taourirts. The Silet-Taourirts are cross-cutting Pan-African island arc assemblages while the two others intrude the Archaean–Palaeoproterozoic LATEA metacraton. The Taourirts are high-level subcircular often nested alkali-calcic, sometimes alkaline, complexes. They are aligned along mega-shear zones often delimiting terranes. Mainly granitic, they comprise highly differentiated varieties such as alaskite (Silet-Taourirts) and topaz–albite leucogranite (Tamanrasset-Taourirts). Different subgroups were defined on the basis of REE patterns and major and other trace elements. The Taourirt province displays a wide transition from dominant alkali-calcic to minor alkaline granite varieties. Sr isotopes indicate that these complexes were affected by fluid circulation during the Ordovician along shear zones probably contemporaneous to the beginning of the Tassilis sandstone deposition. Nd isotope systematic indicates a major interaction with the upper crust during the emplacement of highly differentiated melts, particularly in samples showing seagull wing-shaped REE patterns. On the other hand, all Taourirt plutons are strongly contaminated by the lower crust: _Nd vary from −2 to −8 and T_DM from 1200 to 1700 Ma. This implies the presence of an old crust at depth, also below the Silet-Taourirts, which are emplaced within Pan-African island arc assemblages. A model is proposed for the genesis of the Taourirt province where reworking of the mega-shear zones, which dissected the LATEA metacraton, provoked a linear delamination of the lithospheric mantle, asthenosphere uprise and partial melting of the lower crust (or strong interaction with), giving rise to a mixed source.     

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aïr to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies.We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen.An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P–T–t paths have been established from>15 kbar––790 °C (eclogite) to 4 kbar––500 °C (greenschist retrogression) through 12 kbar––830 °C (garnet amphibolite) and 8 kbar––700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm–Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean–Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at ≈850 Ma to the west of LATEA, the Serouenout terrane in the 700–620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin.During the main Pan-African phase (625–580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.70839 ± 0.00016; 6WR, MSWD=0.87) and the Tisselliline granite (552 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.7074 ± 0.0001; 5WR, MSWD=1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: _Nd=−14 to −21 at 624 Ma and T_DM=1650–2320 Ma for Ounane and _Nd=−13 to −15 at 555 Ma and T_DM=1550–1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.The LATEA cratonic microcontinent remained however sufficiently rigid to preserve Archaean and Palaeoproterozoic lithologies as well as Middle Neoproterozoic oceanic thrust sheets. This corresponds to the notion of metacraton [J. African Earth Sci. 34 (2002) 119], i.e. a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological, isotopic and sometimes petrological characteristics.     

The Anfeg post-collisional Pan-African high-K calc-alkaline batholith (Central Hoggar, Algeria), result of the LATEA microcontinent metacratonization

The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km²) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (≈2 Ga) high-grade basement belonging to the LATEA metacraton at approximately 608 Ma (recalculated from the U–Pb dating of [Tectonics 5 (1986) 955]) and cooled at approximately 4 kbar, with a temperature of about 750 °C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks.The Anfeg batholith defines a Nd–Sr isotopic initial ratios trend (_Nd/(⁸⁷Sr/⁸⁶Sr)_i from −2.8/0.7068 to −11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield (_Nd/⁸⁷Sr/⁸⁶Sr)_i of +6.2/0.7028 for the depleted mantle, −22/0.708 for the old lower crust.The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature. Some mantle melts with only slight crustal contamination (Laouni troctolitic layered intrusions) are even present. This suggests that the southern boundary of LATEA microcontinent is not far south of the Tuareg shield.     

Geochronological problems related to polymetamorphism in the Limpopo Complex, South Africa

The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D₃/M₃ event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D₂/M₂ event, characterized by SW-plunging sheath folds. These major D₂/M₂ folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D₃/M₃ shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.     

Evolution of the Yunkai Terrane, South China: Evidence from SHRIMP zircon U–Pb dating, geochemistry and Nd isotope

The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430 Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8 Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains 242 Ma metamorphic zircons, as well as 440 Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have t_DM model ages of 2.17–1.91 Ga and ε_Nd (440 Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record t_DM model ages of 2.13–1.42 Ga and ε_Nd (440 Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella, Dronning Maud Land, East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Geochemical and mantle-like isotopic (Nd, Sr) composition of the Baklan Granite from the Muratdağı Region (Banaz, Uşak), western Turkey: Implications for input of juvenile magmas in the source domains of western Anatolia Eocene–Miocene granites

The (late syn)- post-collisional magmatic activities of western and northwestern Anatolia are characterized by intrusion of a great number of granitoids. Amongst them, Baklan Granite, located in the southern part of the Muratdağı Region from the Menderes Massif (Banaz, Uşak), has peculiar chemical and isotopic characteristics. The Baklan rocks are made up by K-feldspar, plagioclase, quartz, biotite and hornblende, with accessory apatite, titanite and magnetite, and include mafic microgranular enclaves (MME). Chemically, the Baklan intrusion is of sub-alkaline character, belongs to the high-K, calc-alkaline series and displays features of I-type affinity. It is typically metaluminous to mildly peraluminous, and classified predominantly as granodiorite in composition. The spider and REE patterns show that the rocks are fractionated and have small negative Eu anomalies (Eu/Eu^* = 0.62–0.86), with the depletion of Nb, Ti, P and, to a lesser extent, Ba and Sr. The pluton was dated by the K–Ar method on the whole-rock, yielded ages between 17.8 ± 0.7 and 19.4 ± 0.9 Ma (Early Miocene). The intrusion possesses primitive low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70331–0.70452) and negative ε_Nd(t) values (−5.0 to −5.6). The chemical contrast between evolved Baklan rocks (SiO₂, 62–71 wt.%; Cr, 7–27 ppm; Ni, 5–11 ppm; Mg#, 45–51) and more primitive clinopyroxene-bearing monzonitic enclaves (SiO₂, 54–59 wt.%; Cr, 20–310 ppm; Ni, 10–70 ppm; Mg#, 50–61) signifies that there is no co-genetic link between host granite and enclaves. The chemical and isotopic characteristics of the Baklan intrusion argue for an important role of a juvenile component, such as underplated mantle-derived basalt, in the generation of the granitoids. Crustal contamination has not contributed significantly to their origin. However, with respect to those of the Baklan intrusion, the generation of the (late syn)- post-collisional intrusions with higher Nd(t) values from the western Anatolia require a much higher amount of juvenil component in their source domains.     

Triassic paragonite- and garnet-bearing epidote-amphibolite from the Hida Mountains, Japan

Paragonite- and garnet-bearing high-grade epidote-amphibolite (PGEA) in the Ise area of the Hida Mountains, Japan is characterized by the high-pressure (HP) epidote-amphibolite facies parageneses (M₁), garnet + hornblende + clinozoisite + paragonite + quartz + rutile. Paragonite and garnet of the peak M₁ stage are locally replaced by retrograde albite (+ oligoclase) and chlorite (M₂), respectively. Phase equilibria constrain peak metamorphic conditions of P = 1.1–1.4 GPa and T = 530–570 °C, and a decompressional P–T path for this rock. Mineral parageneses of prograde epidote-amphibolite facies are comparable to some HP rocks from the Hongan region of western Dabie, but differ from other HP mafic schists with cooling ages of c. 330 Ma in the Hida Mountains. New paragonite K–Ar dating for the PGEA yields a Triassic cooling event at 210 Ma that is coeval with regional cooling and exhumation of the Sulu–Dabie–Qinling (SDQ) belt. Both petrological and geochronological data of the Triassic HP epidote-amphibolite in Hida Mountains support our earlier hypothesis that the SDQ belt extends across the Korean Peninsula to SW Japan.     

Petrology and CHIME geochronology of Pan-African high K and Sr/Y granitoids in the Nkambe area, Cameroon

The Central African Belt in the Nkambe area, northwestern Cameroon represents a collisional zone between the Saharan metacraton and the Congo craton during the Pan-African orogeny, and exposes a variety of granitoids including foliated and massive biotite monzogranites in syn- and post-kinematic settings. Foliated and massive biotite monzogranites have almost identical high-K calc-alkaline compositions, with 73–67 wt.% SiO₂, 17–13 wt.% Al₂O₃, 2.1–0.9 wt.% CaO, 4.4–2.7 wt.% Na₂O and 6.3–4.4 wt.% K₂O. High concentrations of Rb (264–96 ppm), Sr (976–117 ppm), Ba (3680–490 ppm) and Zr (494–99 ppm), with low concentrations of Y (mostly< 20 ppm with a range 54–6) and Nb (up to 24 ppm) suggest that the monzogranites intruded in collisional and post-collisional settings. The Sr/Y ratio ranges from 25 to 89. K, Rb and Ba resided in a single major phase such as K-feldspar in the source. Garnet was present in the source and remained as restite at the site of magma generation. This high K₂O and Sr/Y granitic magma was generated by partial melting of a granitic protolith under high-pressure and H₂O undersaturated conditions where garnet coexists with K-feldspar, albitic plagioclase. CHIME (chemical Th–U-total Pb isochron method) dating of zircon yields ages of 569 ± 12–558 ± 24 Ma for the foliated biotite monzogranite and 533 ± 12–524 ± 28 Ma for the massive biotite monzogranite indicating that the collision forming the Central African Belt continued in to Ediacaran (ca 560 Ma).     

An early Paleoproterozoic high-K intrusive complex in southwestern Tarim Block, NW China: Age, geochemistry, and tectonic implications

Systematic geochronologic, geochemical, and Nd isotopic analyses were carried out for an early Paleoproterozoic high-K intrusive complex exposed in southwestern Tarim, NW China. The results provide a better understanding of the Paleoproterozoic tectonic evolution of the Tarim Block. Zircon U–Pb age dating indicates two Paleoproterozoic magmatic episodes occurring at ca. 2.41 Ga and ca. 2.34 Ga respectively, which were followed by a ca. 1.9 Ga metamorphic event. The 2.41 Ga granodiorite–adamellite suite shares characteristics of late to post-orogenic metaluminous A-type granites in its high alkalinity (Na₂O + K₂O = 7.6–9.3%), total REE (410–788 ppm), Zr (370–660 ppm), and Y (21.7–58.4 ppm) contents. εNd(t) values for the suite range from − 3.22 to − 4.71 and accordingly the Nd modal ages (T_2DM) vary between 3.05 Ga and 3.17 Ga. Based on geochemical data, the 2.34 Ga suite can be subdivided into two sub-suites, namely A-type and S-type. However, both types have comparable Nd isotope compositions (εNd(t) ≈ − 0.41 to − 2.08) and similar narrow T_2DM ranges (2.76–2.91 Ga).Geochemical and Nd isotopic data for the high-K intrusive complex, in conjunction with the regional geological setting, suggest that both the 2.41 Ga suite and the 2.34 Ga A-type sub-suite might have been produced by partial melting of the Archean mafic crust in a continental rift environment. The S-type sub-suite is thought to have formed by partial melting of felsic pelites and/or metagreywackes recycled from Archean crust (TTG?). Gabbro enclaves with positive εNd(t) value (2.15) have been found to be intermingling within the 2.34 Ga suite; ca. 2.34–2.36 Ga gabbroic dykes and adamellites have previously been documented in eastern Tarim. These observations indicate that the high-K intrusions may reflect the emergence of depleted mantle upwelling beneath the Tarim Block at that time. We suggest a three-stages model for the Precambrian crustal evolution in the Tarim Block: (1) the formation of proto-crust (TTG) by ca. 2.5 Ga, (2) episodes of felsic magmatism possibly occurring in continental rift environments at ca. 2.41 Ga and ca. 2.34–2.36 Ga, and (3) ca. 1.9 Ga metamorphism that may represent the solidification of the Precambrian basement of the Tarim Block.     

Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian mobile belt: geological and isotopic evidence

The available geological, geochronological and isotopic data on the felsic magmatic and related rocks from South Siberia, Transbaikalia and Mongolia are summarized to improve our understanding of the mechanisms and processes of the Phanerozoic crustal growth in the Central Asian mobile belt (CAMB). The following isotope provinces have been recognised: ‘Precambrian’ (T_DM=3.3–2.9 and 2.5–0.9 Ga) at the microcontinental blocks, ‘Caledonian’ (T_DM=1.1–0.55 Ga), ‘Hercynian’ (T_DM=0.8–0.5 Ma) and ‘Indosinian’ (T_DM=0.3 Ga) that coincide with coeval tectonic zones and formed at 570–475, 420–320 and 310–220 Ma. Continental crust of the microcontinents is underlain by, or intermixed with, ‘juvenile’ crust as evidenced by its isotopic heterogeneity. The continental crust of the Caledonian, Hercynian and Indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments. The crustal growth in the CAMB had episodic character and important crust-forming events took place in the Phanerozoic. Formation of the CAMB was connected with break up of the Rodinia supercontinent in consequence of creation of the South-Pacific hot superplume. Intraplate magmatism preceding and accompanying permanently other magmatic activity in the CAMB was caused by influence of the long-term South-Pacific plume or the Asian plume damping since the Devonian.     

Two magma series and associated ore deposit types in the Permian Emeishan large igneous province, SW China

The Late Middle Permian ( 260 Ma) Emeishan large igneous province in SW China contains two magmatic series, one comprising high-Ti basalts and Fe-rich gabbroic and syenitic intrusions, the other low-Ti basalts and mafic–ultramafic intrusions. The Fe-rich gabbros are spatially and temporally associated with syenites. Each series is associated with a distinctive type of mineralization, the first with giant Fe–Ti–V oxide ore deposits such as Panzhihua and Baima, the second with Ni–Cu–(PGE) sulfide deposits such as Jinbaoshan, Limahe and Zhubu. New SHRIMP zircon U–Pb isotopic data yielded 263 ± 3 Ma for the Limahe intrusion, 261 ± 2 Ma for the Zhubu intrusion and 262 ± 2 Ma for a syenitic intrusion. These new age dates, together with previously reported SHRIMP zircon U–Pb ages, suggest that all these intrusions are contemporaneous with the Emeishan flood basalts and formed during a major igneous event at ca. 260 Ma.The oxide-bearing intrusions have higher Al₂O₃, FeO (as total iron) and total alkalis (Na₂O + K₂O) but lower MgO than the sulfide-bearing intrusions. All intrusions are variably enriched in LREE relative to HREE. The oxide-bearing intrusions display positive Nb- and Ti-anomalies and in certain cases negative Zr–Hf anomalies, whereas the sulfide-bearing intrusions have obvious negative Nb- and Ti-anomalies, a feature of crustal contamination. Individual intrusions have relatively small ranges of Nd(t) values. All the intrusions, however, have Nd(t) values ranging from − 3.9 to + 4.6, and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7039 to 0.7105. The syenites have very low MgO (< 2 wt.%) but highly variable Fe₂O₃ (2.5 to 13 wt.%) with initial ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7039 to 0.7089. Magmas from both series could have derived by melting of a heterogeneous mantle plume: the high-Ti series from a Fe-rich, more fertile source and the low-Ti series from a Fe-poor, more refractory source. In addition, the low-Ti series underwent significant crustal contamination. The two magma series evolved along different paths that led to distinct mineralization styles.     

Spinel–quartz high temperature paragenesis in Al–Fe granulites from the Ihouhaouene area (NW Hoggar, Algeria)

The spinel–quartz-bearing Al–Fe granulites from Ihouhaouene (In Ouzzal, West Hoggar) have a migmatitic appearance with quartzo-feldspathic layers intercalated with restitic layers. These granulites are characterized by a hercynitic spinel–quartz assemblage typical of high grade terranes. The stability of the spinel–quartz assemblage is attributed to an elevation of temperature (from 800 to >1100 °C) at high pressures (10–11 kbar), followed by an isothermal decompression from 9 to 5 kbar, an evolution typical of the In Ouzzal clockwise P–T path. The Al–Fe granulites’ history can be subdivided into different successive crystallisation stages. During the first stage, the spinel–quartz assemblage formed, probably following a prograde event that also produced partial melting. During a second stage, the primary spinel–garnet–sillimanite–quartz paragenesis broke-down to give rise to the secondary assemblage. The metamorphic evolution and phase relations during this stage are shown in P–T–X pseudosections calculated for the simple FMASH system. These pseudosections show that the orthopyroxene–cordierite–spinel symplectite appeared during a high temperature decompression, as a product of destabilisation of garnet in sillimanite-free microdomains with high X_Mg values. At the same time, the spinel–quartz association broke-down into cordierite in Fe-rich microdomains. Average pressure and temperature estimates for the orthopyroxene–spinel–garnet–cordierite–quartz association are close to the thermal peak of metamorphism (1000 ± 116 °C at 6.3 ± 0.5 kbar). With decreasing temperatures garnet–sillimanite corona developed from the breakdown of the primary spinel–quartz assemblage in the Fe-rich microdomains, whereas cordierite–spinel formed at the expense of primary sillimanite and garnet in the Mg-rich microdomains.     

Geology and paleomagnetism of El Potrero pluton, Baja California: Understanding criteria for timing of deformation and evidence of pluton tilt during batholith growth

The 102 Ma El Potrero pluton, in the western foothills of Sierra San Pedro Mártir, in north-central Baja California, was emplaced during a long period of contractional deformation bracketed between 132 and 85 Ma that affected this segment of the Peninsular Ranges Batholith. The pluton records regional and emplacement related deformation manifested by: (1) a solid-state fabric developed on its eastern contact, which is produced by eastward lateral pluton expansion; (2) cleavage triple point zones in the host-rock NW and SE of the pluton; (3) subhorizontal ductile shear zones indicative of top-to-the-east transport; (4) magmatic and tectonic foliations parallel to regional structural trends and regional shear zones; (5) variable axial ratios of microgranitoid enclaves close to pluton–wall rock contacts; (6) evidence of brittle-emplacement mechanisms in the western border of the pluton, which contrast with features indicating mainly ductile mechanisms toward the east; and, (7) markedly discordant paleomagnetic directions that suggest emplacement in an active tectonic setting. The overall mean for 9 accepted paleomagnetic sites is Dec = 34.6°, I = 25.7° (k = 88.3, α₉₅ = 5.5°), and is deviated  35° with respect to the reference cratonic direction. This magnetization is interpreted to indicate a combination of tilt due to initial drag during vertical diapiric ascent (or westward lateral-oblique expansion) of the adjacent San Pedro Mártir pluton and later rotation ( 15°) by Rosarito Fault activity in the southwest; this rotation may have occurred as eastward contraction acted to fill the space emptied by the ascending San Pedro Mártir pluton. The Rosarito fault may have tilted several plutons in the area (Sierra San Pedro Mártir, El Potrero, San José, and Encinosa). Magnetic susceptibility fabrics for 13 sites reflect mostly emplacement-related stress and regional stress. Paleomagnetic data and structural observations lead us to interpret the El Potrero pluton as a syntectonic pluton, emplaced within a regional shear zone delimited by the Main Mártir Thrust and the younger Rosarito Fault.     

Neoarchaean high-grade metamorphism in the Central Zone of the Limpopo Belt (South Africa): Combined petrological and geochronological evidence from the Bulai pluton

The Bulai pluton represents a calc-alkaline magmatic complex of variable deformed charnockites, enderbites and granites, and contains xenoliths of highly deformed metamorphic country rocks. Petrological investigations show that these xenoliths underwent a high-grade metamorphic overprint at peak P–T conditions of 830–860 °C/8–9 kbar followed by a pressure–temperature decrease to 750 °C/5–6 kbar. This P–T path is inferred from the application of P–T pseudosections to six rock samples of distinct bulk composition: three metapelitic garnet–biotite–sillimanite–cordierite–plagioclase–(K-feldspar)–quartz gneisses, two charnoenderbitic garnet–orthopyroxene–biotite–K-feldspar–plagioclase–quartz gneisses and an enderbitic orthopyroxene–biotite–plagioclase–quartz gneiss. The petrological data show that the metapelitic and charnoenderbitic gneisses underwent uplift, cooling and deformation before they were intruded by the Bulai Granite. This relationship is supported by geochronological results obtained by in situ LA-ICP-MS age dating. U–Pb analyses of monazite enclosed in garnet of a charnoenderbite gneiss provide evidence for a high-grade structural-metamorphic–magmatic event at 2644 ± 8 Ma. This age is significantly older than an U–Pb zircon crystallisation age of 2612 ± 7 Ma previously obtained from the surrounding, late-tectonic Bulai Granite. The new dataset indicates that parts of the Limpopo's Central Zone were affected by a Neoarchaean high-grade metamorphic overprint, which was caused by magmatic heat transfer into the lower crust in a ‘dynamic regional contact metamorphic milieu’, which perhaps took place in a magmatic arc setting.     

Geologic evolution of the Serrinha nucleus granite–greenstone terrane (NE Bahia, Brazil) constrained by U–Pb single zircon geochronology

U–Pb single zircon crystallization ages were determined using TIMS and sensitive high resolution ion microprobe (SHRIMP) on samples of granitoid rocks exposed in the Serrinha nucleus granite–greenstone terrane, in NE Brazil. Our data show that the granitoid plutons can be divided into three distinct groups. Group 1 consists of Mesoarchaean (3.2–2.9 Ga) gneisses and N-S elongated TTG (Tonalite-Trondhjemite-Granodiorite) plutons with gneissic borders. Group 2 is represented by ca. 2.15 Ga pretectonic calc-alkaline plutons that are less deformed than group 1. Group 3 is ca. 2.11–2.07 Ga, late to post-tectonic plutons (shoshonite, syenite, K-rich granite and lamprophyre). Groups 2 and 3 are associated with the Transamazonian orogeny. Xenocryst ages of 3.6 Ga, the oldest zircon yet recorded within the São Francisco craton, are found in the group 3 Euclides shoshonite within the Uauá complex and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement.Group 1 gneiss-migmatitic rocks (ca. 3200 Ma) of the Uauá complex constitute the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100 Ma) to the south. Subsequent TTG melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162 ± 26 Ma), Araci (3072 ± 2 Ma), Requeijão (2989 ± 11 Ma) and others, which together form a major part of the Archaean nucleus. Some of these plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt. The older gneissic rocks occur as enclaves within younger Archaean plutons. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9 Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton.Group 2 granitoid plutons were emplaced at 2.16–2.13 Ga in a continental arc environment floored by Mesoarchaean crust. These plutons were subsequently deformed and intruded by late to post-tectonic group 3 alkaline plutons. This period of Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon from the basement.     

Petrography and geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: Implications for their sources and geological setting

The Nagoundéré Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as the Adamawa-Yade batholith. The granites were emplaced into a ca. 2.1 Ga remobilised basement composed of metasedimentary and meta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The granitoids comprise three groups: the hornblende–biotite granitoids (HBGs), the biotite ± muscovite granitoids (BMGs), and the biotite granitoids (BGs). New Th–U–Pb monazite data on the BMGs and BGs confirm their late Neoproterozoic emplacement age (ca. 615 ± 27 Ma for the BMGs and ca. 575 Ma for the BGs) during the time interval of the regional tectono-metamorphic event in North Cameroon. The BMGs also show the presence of ca. 926 Ma inheritances, suggesting an early Neoproterozoic component in their protolith.The HBGs are characterized by high Ba–Sr, and low K₂O/Na₂O ratios. They show fairly fractionated REE patterns (La_N/Yb_N 6–22) with no Eu anomalies. The BMGs are characterized by higher K₂O/Na₂O and Rb/Sr ratios. They are more REE-fractionated (La_N/Yb_N = 17–168) with strong negative Eu anomalies (Eu/Eu^* = 0.2–0.5). The BGs are characterized by high SiO₂ with K₂O/Na₂O > 1. They show moderated fractionated REE patterns (La_N/Yb_N = 11–37) with strong Eu negative anomalies (Eu/Eu^* = 0.2–0.8) and flat HREE features (Gd_N/Yb_N = 1.5–2.2). In Primitive Mantle-normalized multi-element diagrams, the patterns of all rocks show enrichment in LILE relative to HFSE and display negative Nb–Ta and Ti anomalies. All the granitoids belong to high-K calc-alkaline suites and have an I-type signature.Major and trace element data of the HBGs are consistent with differentiation of a mafic magma from an enriched subcontinental lithospheric mantle, with possible crustal assimilation. In contrast, the high Th content, the LREE-enrichment, and the presence of inherited monazite suggest that the BGs and BMGs were derived from melting of the middle continental crust. Structural and petrochemical data indicate that these granitoids were emplaced in both syn- to post-collision tectonic settings.     

Paleoproterozoic subduction-related metasomatic signatures in the lithospheric mantle beneath NE Brazil: inferences from trace element and Sr–Nd–Pb isotopic compositions of Neoproterozoic high-K igneous rocks

Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/Yb_CN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (_Nd from −9.3–−20.1) isotopic compositions. T_DM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb=16–17.3, ²⁰⁷Pb/²⁰⁴Pb=15.1–15.6, ²⁰⁸Pb/²⁰⁴Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust.     

Tectonomagmatism in continental arcs: evidence from the Sark arc complex

The island of Sark (Channel Islands, UK) exposes syntectonic plutons and country rock gneisses within a Precambrian (Cadomian) continental arc. This Sark arc complex records sequential pulses of magmatism over a period of 7 Ma (ca. 616–609 Ma). The earliest intrusion (ca. 616 Ma) was a composite sill that shows an ultramafic base overlain by a magma-mingled net vein complex subsequently deformed at near-solidus temperatures into the amphibolitic and tonalitic Tintageu banded gneisses. The deformation was synchronous with D₂ deformation of the paragneissic envelope, with both intrusion and country rock showing flat, top-to-the-south LS fabrics. Later plutonism injected three homogeneous quartz diorite–granodiorite sheets: the Creux–Moulin pluton (150–250 m; ca. 614 Ma), the Little Sark pluton (>700 m; 611 Ma), and the Northern pluton (>500 m; 609 Ma). Similar but thinner sheets in the south (Derrible–Hogsback–Dixcart) and west (Port es Saies–Brecqhou) are interpreted as offshoots from the Creux–Moulin pluton and Little Sark pluton, respectively. All these plutons show the same LS fabric seen in the older gneisses, with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling. The cooling rate increased rapidly with decreasing crystallisation age: >9 Ma for the oldest intrusion to cool to lower amphibolite conditions, 7–8 Ma for the Creux Moulin pluton, 5–6 Ma for the Little Sark pluton, and <3 Ma for the Northern pluton. This cooling pattern is interpreted as recording extensional exhumation during D₂. The initiation of the D₂ event is suggested to have been a response to the intrusion of the Tintageu magma which promoted a rapid increase in strain rate (>10⁻¹⁴ s⁻¹) that focussed extensional deformation into the Sark area. The increased rates of extension allowed ingress of the subsequent quartz diorite–granodiorite sheets, although strain rate slowly declined as the whole complex cooled during exhumation. The regional architecture of syntectonic Cadomian arc complexes includes flat-lying “Sark-type” and steep “Guernsey-type” domains produced synchronously in shear zone networks induced by oblique subduction: a pattern seen in other continental arcs such as that running from Alaska to California.