high-K Calc-alkaline Suite Research Articles

Petrogenesis of Mediterranean lamproites and associated rocks: The role of overprinted metasomatic events in the post-collisional lithospheric upper mantle

Abstract High-MgO lamproite and lamproite-like (i.e. lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine–Himalayan collisional margin, which followed the closure of the Tethys Ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g. plugs, dykes and laccoliths) and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (northern Italy), through the Late Miocene in Corsica (southern France) and in Murcia-Almeria (southeastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (central Italy), in the Balkan peninsula (Serbia and Macedonia) and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al 2 O 3 , CaO and Na 2 O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent, and it is rarely found only in the groundmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207 Pb over 206 Pb and high time-integrated 232 Th/ 238 U. Their composition requires an originally depleted lithospheric mantle source metasomatized by at least two different agents: (1) a high Th/La and Sm/La (i.e. SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; (2) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterized by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterize areas that were affected by multiple Wilson cycles, as observed in the Alpine–Himalayan Realm.

Neoproterozoic magmatic evolution of the southern Ouaddaï Massif (Chad)

This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the poorly known southern Ouaddaï massif, located at the southern edge of the so-called Saharan metacraton. This area is made of greenschist to amphibolite facies metasediments intruded by large pre- to syn-tectonic batholiths of leucogranites and an association of monzonite, granodiorite and biotite granite forming a late tectonic high-K calc-alkaline suite. U-Pb zircon dating yields ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite (containing numerous inherited Archean and Paleoproterozoic zircon cores) and a muscovite-leucogranite, respectively. Geochemical fingerprints are very similar to some evolved Himalayan leucogranites suggesting their parental magmas were formed after muscovite and biotite dehydration melting of metasedimentary rocks. A biotite-granite sample belonging to the late tectonic high-K to shoshonitic suite contains zircon rims that yield an age of 540 ± 5 Ma with concordant inherited cores crystallized around 1050 Ma. Given the high-Mg# (59) andesitic composition of the intermediate pyroxene-monzonite, the very similar trace-element signature between the different rock types and the unradiogenic isotopic signature for Nd, the late-kinematic high-K to shoshonitic rocks formed after melting of the enriched mantle and further differentiation in the crust. These data indicate that the southern Ouaddaï was part of the Pan-African belt. It is proposed that it represents a continental back-arc basin characterized by a high-geothermal gradient during Early Ediacaran leading to anatexis of middle to lower crustal levels. After tectonic inversion during the main Pan-African phase, late kinematic high-K to shoshonitic plutons emplaced during the final post-collisional stage.

Geology, petrology and U-Pb geochronology of Serra da Rajada Granitic Pluton: implications about ediacaran magmatic evolution in NE portion of the Rio Piranhas-Seridó Domain (NE of Borborema Province)

A atividade plutônica ediacarana, relacionada à orogênese Brasiliana/Pan-Africana, constitui uma importante feição geológica na Província Borborema, representada por inúmeros batólitos, stocks e diques. O Plúton Granítico Serra da Rajada (PGSR), situado na porção central do Domínio Rio Piranhas-Seridó, representa um exemplo desses corpos, objeto de estudos cartográ- fico, petrográfico, litoquímico e geocronológico. Suas rochas são aqui descritas como monzogranitos constituídos por K-feldspato, plagioclásio (oligoclásico-An23-24%), quartzo e biotita, tendo minerais acessórios opacos, titanita, allanita, apatita e zircão. Clorita, mica branca e carbonato são minerais de alteração. Dados litoquímicos evidenciam rochas bastante evoluídas (SiO2 , 69% a 75%), ricas em álcalis (Na2 O + K2 O ≥ 8,0%), empobrecidas em MgO (≤ 0,45%), CaO (≤ 1,42%) e TiO2 (≤ 0,36%) e teores moderados de Fe2 O3 t (2,16 a 3,53%). Apresentam natureza transicional entre metaluminosas e peraluminosas (predomínio das últimas) e possuem afinidade por rochas cálcio-alcalinas de alto K. Diagramas de Harker mostram correlações negativas em Fe2 O3 t, MgO e CaO, indicando fracionamento de máficos e plagioclásio. O espectro de ETR mostra enriquecimento dos ETR leves com relação aos ETR pesados (LaN/YbN = 23,70 a 10,13) e anomalia negativa no Eu (Eu/Eu* = 0,70 a 0,23), sugerindo fracionamento ou acumulação na fonte de feldspatos (plagioclásio). A integração dos dados permite correlacionar as rochas do PGSR àquelas descritas na literatura como Suíte Cálcio-Alcalina de Alto K Equigranular. Estudos geocronológicos/ isotópicos U-Pb e Sm-Nd indicam, respectivamente, que a biotita monzogranito possui idade de cristalização de 557±13 Ma, com idade modelo TDM de 2,36 Ga, tendo valor de εNd para a idade de cristalização de -20,10, permitindo inferir fonte crustal para o magma, gerada no paleoproterozoico

Zircon U–Pb geochronology and geochemistry of Late Cretaceous–early Eocene granodiorites in the southern Gangdese batholith of Tibet: petrogenesis and implications for geodynamics and Cu ± Au ± Mo mineralization

Cu ± Au ± Mo mineralization is found in multiple intrusive suites in the Gangdese belt of southern Tibet (GBST). However, the petrogenesis of these ore-bearing intrusive rocks remains controversial. Here, we report on mineralization-related Late Cretaceous-early Eocene intrusive rocks in the Chikang–Jirong area, southern Gangdese. Zircon U–Pb analyses indicate that the mainly granodioritic Chikang and Jirong plutons were generated in the Late Cretaceous (ca. 92 Ma) and early Eocene (ca. 53 Ma), respectively. They are high-K calc-alkaline suites with high SiO2 (64.8–68.3 wt.%) and Al2O3 (15.1–15.7 wt.%) contents. Chikang granodiorites are characterized by high Sr (835–957 ppm), Sr/Y (118–140), Mg# (58–60), Cr (21.8–36.6 ppm), and Ni (14.3–22.9 ppm), and low Y (6.0–8.1 ppm), Yb (0.54–0.68 ppm) values with negligible Eu anomalies, which are similar to those of typical slab-derived adakites. The Jirong granodiorites have high SiO2 (64.8–65.3 wt.%) and Na2O + K2O (7.19–7.59 wt.%), and low CaO (2.45–3.69 wt.%) contents, Mg# (47–53) and Sr/Y (14–16) values, along with negative Eu and Ba anomalies. Both Chikang and Jirong granodiorites have similar εHf(t) (7.6–13.1) values. The Chikang granodiorites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust, and the Jirong granodiorites were possibly generated by partial melting of Gangdese juvenile basaltic crust. In combination with the two peak ages (100–80 and 65–41 Ma) of Gangdese magmatism, we suggest that upwelling asthenosphere, triggered by the rollback and subsequent break-off of subducted Neo-Tethyan oceanic lithosphere, provided the heat for partial melting of subducted slab and arc juvenile crust. Taking into account the contemporaneous occurrence of Gangdese magmatism and Cu ± Au ± Mo mineralization, we conclude that the Late Cretaceous–early Eocene magmatic rocks in the GBST may have a significant potential for Cu ± Au ± Mo mineralization.

Petrogenesis of high-K metagranites in the Kerala Khondalite Belt, southern India: a possible magmatic-arc link between India, Sri Lanka, and Madagascar

The Proterozoic Kerala Khondalite Belt (KKB), southern India preserves a distinct high-grade terrain that is interpreted to have been situated adjacent to Madagascar and Sri Lanka during Gondwana assembly. As such, it has become a major focus for testing models of supercontinent amalgamation and dispersal. The lithounits of KKB have remarkable petrological similarities to the Highland Complex (HC) of Sri Lanka and south-central Madagascar. However, there is no well-constrained petrogenetic model for the KKB that fits explicitly within a supercontinent reconstruction model. We present here results from our on-going studies on the origin and evolution of K-rich (potassic, where K2O/Na2O>1) gneisses of KKB in relation to Proterozoic supercontinent events. Our results show, in a major departure from earlier metasedimentary origin, that potassic gneisses are metamorphosed granitoids. The metagranitoid samples display high K2O contents and low Al2O3/(FeO+MgO+TiO2) values. They are moderate to strongly peraluminous (ASI values ranging from 1.05 to 1.47) rocks showing mineralogical, petrological, and geochemical characteristics distinctive of the high-K calc-alkaline suites. Typical of igneous suites, the high-K metagranites show minor variation in chemical compositions with most oxides showing negative correlation with SiO2. Geochemistry illustrates distinctive features of arc-related magmas with LILE (K, Rb, and Th) and LREE enriched patterns and considerable depletion of HSFE (Nb, Zr, and Ti). The high-K metagranites are further characterized by strong negative anomalies of Eu (Eu/Eu*=0.10–0.44) and Sr, suggesting melting in plagioclase stability field and retention of plagioclase in the residual phase. Petrogenetic discrimination for granitoids, using major and trace elements demonstrates that the high-K metagranites of the KKB formed by partial melting of igneous source in lower- to middle-crust levels. Overall the geochemical features are supportive of origin in relation to a convergent margin setting, possibly in a continental magmatic arc system, which can be connected to the amalgamation and dispersal of continental fragments in a supercontinent event. This study, therefore, provides a lead towards more robust comparisons between the Proterozoic supercontinent events and processes.

Differentiation of the late-Archaean sanukitoid series and some implications for crustal growth: Insights from geochemical modelling on the Bulai pluton, Central Limpopo Belt, South Africa

The late-Archaean (∼2.59Ga) Bulai pluton from North-Eastern South Africa mostly consists in granodioritic rocks and subordinate amounts of mafic lithologies, mainly monzodiorites, both belonging to the sanukitoid series. Many lines of evidence point to a genetic link between these two components, such as similar mineralogical assemblages, parallel trace-element patterns and isotopic homogeneity. The nature of this genetic link has been investigated using semi-quantitative geochemical modelling.Several models reproduce the geochemistry of the Bulai granodiorites, including magma mixing, crystallization from a primary magma and partial melting of mafic rocks. Pure mixing between two magmas that are highly contrasted in terms of geochemistry and physical properties is of doubtful reliability in generating massive volumes of homogeneous hybrid magma. Thus, the models that best fit the data involve either equilibrium or fractional crystallization from a mafic mantle-derived magma, coupled with increasing amounts of mixing with felsic melt in course of differentiation. Alternatively, melts computed by dehydration melting of monzodiorites (incongruent melting of biotite) are also similar in composition to the most differentiated Bulai granodiorites (SiO2>65wt.%). However, formation of the least differentiated granodiorites (SiO2<65wt.%) by such a process requires unrealistic melting conditions. Rather, they would result from entrainment of the (mafic) residual solid after melting.The widespread differentiated rocks of the sanukitoid suite were probably derived from differentiation of mafic mantle-derived melts (sanukitoids s.s.), either as residual liquids (crystallization and mixing) or partial melts. Thus, the presence of pre-existing differentiated crust is not a requirement to the generation of these granitoids that therefore can be considered as a juvenile addition to the crust. In addition, the composition of the sanukitoid series matches the one of some post-Archaean, high-K calc-alkaline suites of late-orogenic affinity, as well as the average composition of the continental crust. As a result, growth and differentiation of the crust since the end of the Archaean were likely accommodated by similar processes that gave rise to the sanukitoid series and post-Archaean high-K calc-alkaline granitoids.

Oligocene magmatism in the eastern margin of the east Himalayan syntaxis and its implication for the India–Asia post-collisional process

Post-collisional high Sr/Y-type magmatism in the southern Lhasa terrane extends ~1500km along the strike of the orogen. The origin and magma generation mechanism for these rocks remain controversial. We report a detailed study of such high Sr/Y-type magmatism in the east Himalayan syntaxis, SE Tibet, and explain its significance for understanding post-collisional geodynamical processes. The two-mica granites from the Beibeng, Damu, Bolonggong and Bomi plutons emplaced along the eastern margin of the east Himalayan syntaxis represent a high-K calc-alkaline suite (SiO2=67–75wt.%, Al2O3=13.3–15.8wt.%, K2O/Na2O=0.7–1.9). An outstanding geochemical feature is that they have high Sr/Y ratios (19–88, mostly >30), coupled with low Y and HREE, indicating that they were derived from partial melting of thickened lower crustal materials. Sr–Nd isotopic data show that the potential magma sources are the intermediate intrusive rocks related to the Jurassic–Eocene Gangdese magmatism and basement metasedimentary rocks. The contribution of the latter source increases from south to north. The two-mica granites from the Beibeng, Damu, Bolonggong and Bomi intrusions have magma crystallization ages of 29.9–27.5Ma, 27.6Ma, 27.1–26.5Ma and 23.7Ma, respectively. Our data show decreasing magma crystallization ages both from south to north along the eastern margin of the east Himalayan syntaxis and from east to west along the Gangdese belt. We suggest that such variations resulted from the diachronous breakoff of the subducted Indian continental slab. Beneath the east Himalayan syntaxis, the breakoff of the subducted Indian continental slab was initiated in the south and propagated towards north probably due to the thermal impact of a widening window opened by slab breakoff. Along the Gangdese belt, the breakoff of the subducted Indian continental slab commenced in east, and then propagated westwards.

Potassium-rich magmatism in the Western Outer Carpathians: Magmagenesis in the transitional zone between the European Plate and Carpathian–Pannonian region

Miocene igneous rocks of the Western Outer Carpathians in Poland and Moravia, Czech Republic, were emplaced in a tectonic setting transitional between the European foreland and the Carpathian–Pannonian arc system. They form medium- to high-K calc-alkaline suites which are compositionally different to each other and to other calc-alkaline sequences in the region. Geographically and compositionally the Moravia rocks can be seen as transitional between the rift-related alkaline magmatism of the foreland and the dominantly calc-alkaline magmatism of the Carpathian–Pannonian region. Certain Moravia rocks are unusually enriched in incompatible trace elements, e.g. Nb≤122ppm and Th≤31ppm. The primary magmas of both suites were formed in metasomatised lithosphere mantle of the European plate but the enrichment mechanism differed: in Poland, it was by ancient subduction-related events whereas in Moravia metasomatism was mainly by melts and/or fluids from deeper mantle sources. Partial melting was promoted by impingement of rising asthenosphere on the base of the lithosphere, possibly related to reactivation of trans-lithospheric, NW–SE-trending fault zones.

Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating.

In situ LA-MC-ICP-MS U-Pb zircon geochronology combined with cathodoluminescence imaging were carried out to determine protolith and metamorphic ages of orthogneisses from the Western Tatra Mountains (Central Western Carpathians). The metamorphic complex is subdivided into two units (the Lower Unit and the Upper Unit). Orthogneisses of the Lower Unit are mostly banded, fine- to medium-grained rocks while in the Upper Unit varieties with augen structures predominate. Orthogneisses show a dynamically recrystallised mineral assemblage of Qz + Pl + Bt ± Grt with accessory zircon and apatite. They are peraluminous (ASI = 1.20–1.27) and interpreted to belong to a high-K calc-alkaline suite of a VAG–type tectonic setting. LA-MC-ICP-MS U-Pb zircon data from samples from both units, from crystals with oscillatory zoning and Th/U > 0.1, yield similar concordia ages of ca. 534 Ma. This is interpreted to reflect the magmatic crystallization age of igneous precursors. These oldest meta-magmatics so far dated in the Western Tatra Mountains could be linked to the fragmentation of the northern margin of Gondwana. In zircons from a gneiss from the Upper Unit, cores with well-developed oscillatory zoning are surrounded by weakly luminescent, low contrast rims (Th/U < 0.1). These yield a concordia age of ca. 387 Ma corresponding to a subsequent, Eo-Variscan, high-grade metamorphic event, connected with the formation of crustal-scale nappe structures and collision-related magmatism.

Petrogenesis of Carboniferous adakites and Nb-enriched arc basalts in the Alataw area, northern Tianshan Range (western China): Implications for Phanerozoic crustal growth in the Central Asia orogenic belt

Carboniferous volcanic rocks in the Alataw area, Northern Tianshan Range (Xinjiang), consist of early Carboniferous (ca. 320 Ma) adakites and Nb-enriched arc basalts and basaltic andesites (NEBs), and late Carboniferous (ca. 306–310 Ma) mainly high-K calc-alkaline andesites, dacites and rhyolites. The adakites are calc-alkaline, and characterized by high Na 2O/K 2O (1.52–3.32) ratios, negligible to positive Eu anomalies, strong depletion of heavy rare earth elements (e.g., Yb = 0.74–1.47 ppm) and Y (6.7–14.9 ppm), positive Sr and Ba but negative Nb and Ti anomalies, and relatively constant ε Nd( T) values (+ 3.4–+ 6.6) and ( 87Sr/ 86Sr) i ratios (0.7035–0.7042). Some andesitic and dacitic adakite samples exhibit high MgO contents similar to magnesian andesites. The NEBs are sodium-rich (Na 2O/K 2O = 2.03–8.06), and differ from the vast majority of arc basalts in their higher Nb, Zr, TiO 2 and P 2O 5 contents and Nb/Th, Nb/La and Nb/U ratios, and minor negative to positive anomalies in Ba, Nb, Sr, Zr and Ti. They have the highest ε Nd( T) values (+ 6.4–+ 11.6) but varying ( 87Sr/ 86Sr) i ratios (0.7007–0.7063). The high-K calc-alkaline suite is similar to typical ‘normal’ arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative Eu, Nb, Sr and Ti anomalies. They have ε Nd( T) values (+ 1.2–+ 6.4) and ( 87Sr/ 86Sr) i ratios (0.7018–0.7059). Geochemically, they are similar to coeval I-type granitoids in the Alataw area. Given the presence of early Carboniferous ophiolites in the Northern Tianshan Range, and the isotopically inappropriate compositions of Proterozoic metamorphic basement in the Alataw area, we argue that the Alataw adakites were most probably related to the melting of young subducted crust of the Northern Tianshan Ocean. The NEBs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab-derived fluids. The later high-K calc alkaline suite was generated by AFC processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids. The larger range of isotopic compositions exhibited by both the NEB and high-K suite, relative to the adakites, suggests that the mantle wedge was heterogeneous prior to slab- or fluid-mediated metasomatism. Continental crustal growth of the Central Asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 Ma and possibly afterwards. The results of this study suggest that other Carboniferous Nb-enriched basalts in the Tianshan Range were generated by subduction processes rather than by intraplate tectonics as previously proposed.

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 K 2O/Na 2O ratios. They show fairly fractionated REE patterns (La N/Yb N 6–22) with no Eu anomalies. The BMGs are characterized by higher K 2O/Na 2O 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 SiO 2 with K 2O/Na 2O > 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.

Do coeval mafic and felsic magmas in post-collisional to within-plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review

The end of major orogenic episodes is marked by uplift and erosion, transcurrent to extensional tectonic regimes induced partly by gravitational collapse of the thickened crust, partly by delamination of the lithosphere, and emplacement of voluminous igneous formations. This period starts soon after the completion of the continent–continent collisional event. Contrasting igneous suites, regarding their nature, evolution and original sources, are emplaced. Two distinctive and successive igneous associations can be evidenced: (i) The post-collisional association is the more complex. Peraluminous silicic rocks, bearing Al–Fe–Mg silicates, such as garnet, cordierite, and sillimanite, are coeval with metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic to ultrapotassic. (ii) The postorogenic association yields less potassic and more sodic compositions. The igneous suites, comprising mafic and felsic rocks, range from alkali-calcic metaluminous to alkaline and peralkaline. They evolve progressively into more markedly alkaline within-plate suites. The post-collisional association identifies two contrasting sources. Peraluminous granitoids and related volcanic rocks contain frequently mafic enclaves corresponding to blobs of undercooled shoshonitic to ultrapotassic basic to intermediate magmas. The peraluminous suite originates by dehydration incongruent melting of muscovite±biotite in the continental crust. Medium-K to high-K calc-alkaline suites originate in an amphibole–spinel peridotite metasomatised lithospheric source. Shoshonitic to ultrapotassic metaluminous suites are issued from partial melting involving phlogopite of a garnet-bearing depleted lithospheric upper mantle, metasomatised by subducted material. The onset of the successive short-lived magmatic episodes is induced by lithosphere stacking and slab breakoff. Mantle-derived magmas emplaced within lower crust provide enough heat to enhance crustal anatexis and, then, they are injected into and mix with the crustal-derived liquids. During postorogenic and within-plate episodes, commingling of mafic and felsic magmas is documented by synplutonic dykes and sills, and by mafic enclaves. Radiogenic isotope systematics suggest that coexisting mafic and felsic magmas come from the same mantle source yielding depleted but LILE-enriched compositions, with subsequent wall-rock crustal contamination. The onset of the magmatic episode is promoted by lithosphere delamination. The role played by continental crust, though still disputed, becomes increasingly minor with time to null. The transition from post-collisional to within-plate geodynamic settings documents the waning role played by crustal anatexis in magma generation, with regard to the increasing role of enriched OIB mantle sources.

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 km 2) 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 ( ε 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 ( ε 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.

Typologie du zircon dans les granitoïdes de la boutonnière précambrienne de Sidi Flah-Bouskour, Saghro, Anti Atlas, Maroc

The Precambrian inlier at Sidi Flah-Bouskour is cut by several successive intrusions of Neoproterozoic granitoids, whose zircon crystal morphology allows precise definition of granite types (Pupin, 1976, 1980, 1988) and, by inference, an assessment of tectonic settings. The first intrusion consists of gabbro, quartz-diorite and amphibole-granodiorite, belonging to the Pan-African B1 phase related to a pre-collision domain. It shows a calc-alkaline granodioritic composition. The second intrusion includes the Bouskour biotite-granite, which belongs to the high-K calc-alkaline suite emplaced at the time of the second Pan-African B2 phase in the context of syn-collision. These units are intruded by a Late Proterozoic pink granite followed by injection of submeridian sigmoidal cracks of rhyolitic dykes that represent the last Precambrian magmatic event in this area.

Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

The Atesina Volcanic District, the Monte Luco volcanics, and the Cima d'Asta, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in the central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280–260 Ma). The magmatism originated during a period of post-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continuum spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. They constitute a metaluminous to weakly peraluminous series showing mineralogical, petrographic and chemical characteristics distinctive of the high-K calc-alkaline suites. In the MORB-normalized trace element diagrams, the most primitive volcanic and plutonic rocks (basaltic andesites and gabbros with Mg No.=66 to 70; Ni=25 to 83 ppm; Cr=248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb–Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial 87 Sr / 86 Sr ratios from 0.7057 to 0.7114; ϵ Nd values from −2.7 to −7.4; ∂ 18 O values between 7.6 and 9.5‰) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magmas and crustal materials. Only the scanty andalusite–cordierite and orthopyroxene–cordierite bearing peraluminous granites in the Cima d'Asta and Bressanone-Chiusa intrusive complexes can be interpreted as purely crustal melts (initial 87 Sr / 86 Sr =0.7143–0.7167; initial ϵ Nd values between −7.9 and −9.6, close to average composition of the granulitic metasedimentary crust from the Ivrea Zone in the western Southern Alps). Although the Permian magmatism shows geochemical characteristics similar to those of arc-related suites, palaeogeographic restorations, and geological and tectonic evidence, seem not to support any spatial and/or temporal connection with subduction processes. The magmatism is post-collisional and post-orogenic, and originated in a regime of lithospheric extension and attenuation affecting the whole domain of the European Hercynian belt. A change in the convergence direction between Gondwana and Laurasia, combined with the effects of gravitational collapse of the Hercynian chain, could have been the driving mechanism for lithosphere extension and thinning, as well as for upwelling of hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like signature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s), with felsic crustal melts.

Geochemical Signatures of Main Neoproterozoic Late-Tectonic Granitoids from the Proterozoic Sergipano Fold Belt, Brazil: Significance for the Brasiliano Orogeny

The Proterozoic Sergipano fold belt is intruded by various Neoproterozoic late-tectonic granitic plutons of contrasting composition, ranging from (1) metaluminous normal-K calc-alkaline and high-K calc-alkaline suites to (2) peralkaline shoshonitic/ultrapotassic suites and (3) a peraiuminous crustal leucocratic complex. Whole-rock trace-element data, however, show an overlapping LILE enrichment, except for the ultrapotassic suite. HFSE distributions range from 24 to 350 ppm, with the highest values belonging to the ultrapotassic suite and the lowest to the suite derived from the crust. High LILE/HFSE ratios of these rocks suggest their subduction-related character, which may be inherited from orogenic cycles older than the Brasiliano. High LILE and medium to high Cr and Ni contents in some facies from these granites suggest a lower-mafic-crust to metasomatized subcontinental-lithospheric-mantle component. The observed LILE content, degree of alkalinity, and REE polarity toward the northern limit of the Sergipano fold belt suggest an increasing crustal thickness in the same direction. Negative ϵNd for all granites studied suggests a common, recycled origin for them, implying the involvement of old lithospheric mantle in the case of the ultrapotassic suite. Mantle separation ages support this evidence, as the majority of them (even that for a paleosome migmatite) lie between Transamazonian and Cariris Velhos ages; the most juvenile components show values closer to 1.0 Ga. The identified geochemical signature of these rocks suggests that they have undergone a complex, multiple-stage evolution during a collisional event that involved the partial melting of compositionally distinct source rocks of different ages, suggesting that the Sergipano fold belt crustal section is complexely structured into vertical age domains. This study records as well that the Brasiliano orogeny did not accrete depleted mantle material to the continental crust of the Sergipano fold belt.

Mesozoic felsic arc magmatism and continental olivine tholeiites in Zhejiang Province and their relationship with the tectonic activity in southeastern China

The Mesozoic geology of Zhejiang Province in southeastern China is characterized by two distinct geodynamic settings separated by a major tectonic event. During the Late Jurassic-Early Cretaceous, a continental magmatic arc developed with a westward subduction of the Kula or Izanagi plate under the South China Block. During the development of this magmatic arc, microsyenite-porphyries and quartz-bearing porphyries were emplaced in western Zhejiang about 124 Ma ago. These porphyritic rocks intruded felsic tuffs that rest unconformably on Carboniferous and Permian folded strata. Geochemically, they are enriched in LIL and light rare earth elements (LREE) which are features of acid shoshonitic suites. Isotopically, they are characterized by negative ϵ Nd( T = 124) (−5.3 to −4.7) values and high initial 87 Sr 86 Sr ratios (0.7079–0.7084). Later, in eastern Zhejiang, quartz-phyric ignimbrites and subordinate acid andesites were emplaced and intruded by nearly contemporaneous microsyenitic and rhyolitic domes during the Cretaceous, about 97 Ma ago. All these rocks belong to the high-K calc-alkaline suites. They show geochemical features of volcanic arc granites emplaced in an active continental margin and are characterized by negative ϵ Nd( T = 97) (−8.8 to −7.1) and high initial Sr isotopic ratios (0.7081–0.7089). The negative ϵ Nd( T = 97) values of all the Cretaceous calc-alkaline and shoshonitic rocks of intermediate to acid composition suggest that these rocks were probably derived from a mantle wedge source, contaminated by variable amounts of old crustal components, likely represented by the Proterozoic basement rocks of the South China Block. The contemporaneous igneous rocks in Vietnam, as well as in the Fujian and Guangdong Provinces (China) represent the lateral counterparts of the Zhejiang calc-alkaline plutonic and volcanic rocks. In the late Early Cretaceous, an important tectonic event occurred which provoked the ending of the Inagazi or Kula plate subduction and affected the continental magmatic arc. This tectonic event is likely related to the collision of the China-Indochina margin with the West Philippine Block. After this tectonic event, in the beginning of the Late Cretaceous, continental stretching and crustal attenuation affected the South China Block leading to the development of NE-SW-faulted basins in which continental olivine-tholeiites and minor alkaline rhyolites were erupted, both characterized by low ϵ Nd( T = 97) ratios (ca. −4). Meanwhile, continental conglomerates and sandstones were also deposited. This rifting process began likely before the India—Asia indentation and lasted at least until the Late Tertiary.

Geochemistry of the Karamea Batholith, New Zealand and comparisons with the Lachlan Fold Belt granites of SE Australia

The Karamea Batholith in the Buller terrane of the South Island New Zealand forms part of an extensive Middle-Late Devonian belt of magmatic activity along, or close to, the Paleo-Pacific margin of Gondwana. The belt includes the I- and S-type granites of the Lachlan Fold Belt in SE Australia and coeval rocks in Antarctica. The northern half of the Karamea Batholith comprises five main intrusive phases: Zetland Diorite, Whale Creek Granite, Karamea Granite, O'Sullivans Granite and Dunphy Granite. To the east of the Karamea Batholith in the Takaka terrane, ultramafic-mafic Devonian igneous rocks are represented by the Riwaka Complex. The rocks forming the Karamea Batholith are a high-K calc-alkaline suite ranging in composition from metaluminous (ASI for Zetland Diorite = 0.8) to strongly peraluminous (ASI for Dunphy Granite = 1.2–1.3). Initial 87 Sr 86 Sr ratios exhibit a large range from 0.705 in the Zetland Diorite to 0.719 in the Dunphy Granite. The corresponding values for ϵ Nd are −0.3 and −9.2. There is a strong inverse correlation between ϵ Nd and initial 87 Sr 86 Sr , which suggests that the Karamea rocks were generated by a simple mixing process. The mafic end-member (with ϵ Nd = 0), which is itself probably derived from a mixed lithospheric source, is taken to be the Zetland Diorite/Riwaka Complex, and the crustal end-member is represented by Ordovician Greenland Group greywackes that form the country rocks to the batholith. Mixing is also supported by recent U-Pb zircon studies. The inherited zircon population in the granites matches the detrital zircon population in the Greenland Group greywackes. The Whale Creek Granite, Karamea Granite and O'Sullivans Granite can be modelled by 20–30% crustal material, whereas the Dunphy Granite appears to represent 65–85% crustal material. In terms of the I-S classification scheme developed for the Lachlan Fold Belt granites in SE Australia, both types are present in the Karamea Batholith. However, in New Zealand there appears to be a continuum from one extreme to the other, which is consistent with the mixing model presented here.

Early calc-alkaline magmatism in the Mogollon-Data Volcanic Field, New Mexico, USA

The Mogollon-Datil Volcanic Field in southwest New Mexico was active between 40 and 20 Ma. It is situated c. 500 km from the western North American plate margin, and yet the early pre-30 Ma volcanic rocks comprise a high-K calc-alkaline suite ranging from basaltic andesites to dacites. All the samples are characterized by high and variable LILE and LREE contents, and LILE/HFSE ratios, and variable Nd and Sr isotope ratios of 0.51231–0.51250 and 0.7055–0.7065 respectively. In detail, three groups are recognized: Group 1 rocks have the lower SiO 2 , incompatible element abundances and 87 Sr/ 86 Sr ratios; Group 2 preserves the clearest evidence for open system differentiation, and Group 3 exhibits little variation in 87 Sr/ 86 Sr and the most coherent major and trace element arrays. The Group 2 rocks have been modelled in terms of simple binary mixing between selected Group 1 basaltic andesites and a higher silica crustal endmember, which appears to have been derived from the mid-lower crust in the presence of residual garnet. The Group 3 rocks, by contrast, are consistent with near closed system differentiation, modelled in three stages and involving Plag-Aug-O1-Mag and subsequently hornblende and K-feldspar. Published experimental data indicate that the differentiation of the parental basaltic magmas took place at elevated pressures of c . 4-5 kbar, and water contents, c . 2wt% H 2 O. It is argued that the parental magmas to these calc-alkaline suites were generated in a period of lithospheric extension, and that they were derived from within the continental mantle lithosphere, as inferred for mid- to Late Tertiary rocks elsewhere in the western US, with no significant contribution from contemporaneous subduction processes.

Late Proterozoic granitoid magmatism in the state of São Paulo, southeastern Brazil

The Precambrian basement in the state of São Paulo is made up of a series of metamorphic units exposed at various erosional levels and in part separated by large faults. A strong overprint of the Brasiliano cycle (700-500 Ma) is evident from isotopic data over most of the region; nevertheless, a large part of the metamorphic belts belong to older (Archaean to Middle Proterozoic) reworked sequences. About 30% of the total basement area consists of intrusive granitoids of Late Proterozoic age. These can be grouped into six major groups, on the basis of petrography, geochemistry, rock association, associated manifestations and (whenever possible) age relationships. Gneissic mangerite-charnockite-hastingsite granite associations appear as folded tabular bodies within high-grade regions. The São José do Rio Pardo suite is chemically comparable to the “aluminous A-type” granitoid series, and its emplacement possibly signals a period of extensional tectonics prior to the onset of the Brasiliano cycle (∼1000 Ma?). A series of elongated batholiths (∼650 Ma) with predominant porphyritic hornblende-biotite granitoids is present mainly in high-grade areas. The compositions are generally granitic to granodioritic, but intermediate (e.g. monzodioritic) rocks are locally common. These high-K calc-alkaline suites are in some aspects comparable to the Cordilleran I-type granitoids, and their origin might be related to a subductional environment. Migmatitic granitoids constitute a peculiar association, occurring as a series of small bodies within high-grade allochthonous domains. These polyphase, heterogeneous, often nebulitic granitoids are source-related products of crustal anatexis. Syn- to late-orogenic Brasiliano granitoids in the low- to medium-grade domains are dominantly porphyritic calc-alkaline biotite granites and granodiorites, reminiscent of Caledonian I-types. In contrast, a peraluminous association (porphyritic biotite granitoid to equigranular two-mica granite) predominates in high-grade domains, which possibly reflects a different tectonic evolution for the two regions. Late- to post-orogenic Brasiliano granitoids intruded after the major orogenic development of the region (∼610-590 Ma). I-type calc-alkaline and aluminous A-type granitoids are geographically associated in the Itu belt, and appear to show some temporal and geochemical (?) overlapping. Wiborgitic facies and some Sn-W mineralization are locally present in the A-type suites. A post-orogenic extensional period is marked in the south of the area by the emplacement of shallow A-type granitoids in the Serra do Mar belt (< 590 Ma). This period was diachronous throughout the region, as shown by the rather young (590-540 Ma) deformation ages registered in the Ubatuba Domain.