Post-collisional Intrusions Research Articles

Ultramafic–mafic mélange, island arc and post-collisional intrusions in the Mayile Mountain, West Junggar, China: Implications for Paleozoic intra-oceanic subduction–accretion process

The West Junggar, northwestern China, is located in the southwest of the Central Asian Orogenic Belt (CAOB) and consists of Paleozoic ophiolitic mélanges, arcs and accretionary complexes. The Mayile Mountain ultramafic–mafic mélange in the southern West Junggar is the northeastern part of the Mayile ophiolitic mélange. The Mayile Mountain mélange is represented by a ca. 30 km long, 5 km wide, north–northeast-striking serpentinite-matrix mélange, which encloses serpentinized harzburgite, dunite, chromitite, wehrlite, clinopyroxenite, gabbro, rodingite, and chert blocks. It is overlain by the Middle Silurian greenschist facies and Early–Middle Devonian unmetamorphic volcano-sedimentary strata. Chemically, chromian spinel from peridotite and clinopyroxenite is similar to those of island arc lava and depleted peridotite from supra-subduction zone (SSZ). Clinopyroxene from the ultramafic–mafic rocks shows the affinities of island arc tholeiite and boninite. The ultramafic–mafic rocks also show the subduction-related geochemical signatures, such as flat REE patterns with slight LREE depletion ((La/Yb) N = 0.66–0.89), LILE enrichment and HFSE depletion. The ultramafic–mafic mélange is crosscut by intrusions. Zircon U–Pb dating reveals four phases of magmatism: Late Cambrian (501–493 Ma), Early Ordovician (488–485 Ma), Early Silurian (444–439 Ma), and Late Carboniferous–Early Permian (318–296 Ma). The Late Cambrian gabbro, diorite and granite, Early Ordovician diorite and Early Silurian granodiorite intrude the mélange in places and predate the overlying Middle Silurian and Early–Middle Devonian successions. They are I-type calc-alkaline and show strong island arc affinities, with LILE and LREE enrichment ((La/Yb) N = 3.77–21.45) and HFSE depletion. On the contrary, the Late Carboniferous–Early Permian I-type diorite, granodiorite and granite and A-type alkali-feldspar granite crosscut both the mélange and overlying strata. They are coeval with the widespread post-collisional granitoids in the West Junggar. Our data for the mélange and intrusions suggest that this mélange might be a highly mixed SSZ-type ophiolitic mélange generated in the earliest stage of south-dipping subduction–accretion during the Early–Middle Cambrian, and then an island arc was successively built in the south in the Late Cambrian (501–493 Ma), in the center in the Early Ordovician (488–485 Ma) and in the north in the Early Silurian (444–439 Ma), as a result of northward arc migration. In the Middle Silurian, the mélange and island arc intrusions might be exposed and covered by deep or hemi-deep sea volcano-sedimentary strata in a back-arc setting, and they might have been accreted southward to another arc and continental margin before the Early–Middle Devonian. After the ocean was ultimately closed at the end of the Early Carboniferous, the mélange and island arc as well as other tectonic units in the West Junggar were stitched together by the Late Carboniferous–Permian post-collisional granitoids.

The Koshrabad granite massif in Uzbekistan: petrogenesis, metallogeny, and geodynamic setting

The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.

Hercynian post-collisional magmatism in the context of Paleozoic magmatic evolution of the Tien Shan orogenic belt

The Hercynian Tien Shan (Tianshan) orogen formed during Late Palaeozoic collision between the Karakum–Tarim and the Kazakhstan paleo-continents. In order to constrain timing of Hercynian post-collisional magmatism, 27 intrusions were sampled for U–Pb zircon dating along a ca. 2000 km – long profile in Uzbekistan and Kyrgyzstan. The samples were dated utilizing sensitive high resolution ion microprobe (SHRIMP-II). The obtained ages, together with previously published age data, allowed the timing of Hercynian post-collisional magmatism to be constrained and interpreted in the context of the Paleozoic magmatic evolution of the region. Apart from Hercynian post-collisional magmatism, two older magmatic episodes have been recognized, and the following sequence of events has been established: (1) approximately 10 Ma after cessation of continuous Caledonian magmatism a number of Late Silurian–Early Devonian intrusions were emplaced in the Middle and Northern Tien Shan terranes between 420 and 390 Ma. The intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of Caledonian Paleo-Kazakhstan continent; (2) the next relatively short Late Carboniferous episode of subduction under Paleo-Kazakhstan was registered in the Kurama range of the Middle Tien Shan. Calc-alkaline volcanics and granitoids with ages 315–300 Ma have distinct metallogenic affinities typical for subduction-related rocks and are not found anywhere outside the Middle Tien Shan terrane west of the Talas–Farghona fault; (3) the Early Permian Hercynian post-collisional magmatism culminated after the closure of the Paleo-Turkestan ocean and affected the whole region across terrane boundaries. The post-collisional intrusions formed within a relatively short time span between 295 and 280 Ma. The model for Hercynian post-collisional evolution suggests that after collision the Tien Shan was affected by trans-crustal strike-slip motions which provided suitable conduits for ascending asthenospheric material and heat influx in the crust. This produced both granitoid magmas and hydrothermal fluid flow. As a result post-collisional intrusions and orogenic Au deposits, known in the region, formed coevally and were tectonically controlled; (4) between 240 and 220 Ma a Triassic thermal event affected the region resulting in resetting and growth of new zircon grains which is detected on a regional scale. Probably the influx of heat into the crust during the Triassic was tectonically focused and varied significantly in different terranes. In the region under investigation the Triassic thermal event was not accompanied by any significant magmatic activity. Thus, after cessation of Hercynian post-collisional magmatism ca. 280 Ma ago there was a long magmatically quiet period in the Tien Shan.

Trans-alkaline magmatism in the Serrinha–Pedro Velho Complex, Borborema Province, NE Brazil and its correlations with the magmatism in eastern Nigeria

The Solânea Complex (572 ± 8 Ma) comprises various intrusions that total 200 km 2. It cuts Palaeoproterozoic migmatized orthogneisses of the Serrinha–Pedro Velho Complex, which is part of the São José do Campestre Massif, North Tectonic Domain of the Borborema Province, NE Brazil. It is a post-collisional intrusion constituted of coarse-grained biotite–amphibole monzogranites, fine-grained leucogranites and diorites. Major, trace and rare earth elements data, used to constrain the origin and nature of these rocks, indicate that the monzogranites display chemical features of weakly peraluminous ferro-potassic trans-alkaline granitoids, similar to coeval occurrences described in eastern Nigerian. Sm–Nd data together with the whole rock geochemical data suggest that: (I) the monzogranites were generated by partial melting of tonalitic gneiss from the lower crust, leaving behind a granulitic residue; (II) the diorites originated by partial melting of an enriched lithospheric mantle; (III) the evolution of the Solânea Complex involved magma mixing between the monzogranites and diorites. K-feldspar and amphibole fractionation appear to have occurred during the evolution of the monzogranites. Leucogranites constituting an elliptical shaped intrusion (80 km 2), the Dona Inês Pluton, are intruded to the north of the Solânea Complex. They also occur as small intrusions cross-cutting the border of the Solânea Complex. Geochemical and isotopic data suggest that the leucogranites were generated by melting of crustal Palaeoproterozoic rocks. Rb–Sr data gave an age of 544 ± 16 Ma. Trans-alkaline magmatism is associated with ductile–brittle deformation in the Serrinha–Pedro Velho Complex of the São José do Campestre Massif. This event follows the peak of an extensional-related high-temperature metamorphism (upper amphibolite to granulite facies), with estimated age of 578 to 574 Ma. If a pre-Gondwana reconstruction is considered, this event is seen to extend to the African side of Gondwana in eastern Nigeria.

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 (SiO 2, 62–71 wt.%; Cr, 7–27 ppm; Ni, 5–11 ppm; Mg#, 45–51) and more primitive clinopyroxene-bearing monzonitic enclaves (SiO 2, 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.

Age of Variscan magmatism from the Balkan sector of the orogen, central Bulgaria

New high-resolution, secondary ion mass spectrometry U–Th–Pb data obtained from zircon and monazite from representative Variscan intrusions in the Balkan sector of the Variscan orogen (Central Bulgaria) constrain the ages of magmatism to the interval ∼315–285 Ma. The samples investigated are granitoids from two contrasting intrusive suites. Calc-alkaline granitoids of the first suite include the San Nikola granite (311.9±4.1 Ma) and the Petrohan granodiorite (304.6±4.0 Ma) from the Balkan Terrane, and the Smilovene (304.1±5.5 Ma) and Hisara (303.5±3.3 Ma) granites from the Sredna Gora Terrane. Zircons from these intrusions predominantly lack inherited cores. The second suite consists of two-mica, leucocratic granites and includes the Koprivshtitsa (312.0±5.4 Ma) and Strelcha (289.5±7.8 Ma) plutons. Zircons from intrusions of the second suite are 80–90% xenocrystic. Apparent ages of inherited cores are the magmatic age and slightly older (∼300–400 Ma), to Ordovician (∼450 Ma) ages, and to Neoproterozoic (∼600–900 Ma) ages. Gabbroic magmas related to the first suite are found only in the more northerly Balkan Terrane, while the leucogranites occur only in the southerly Sredna Gora Terrane. Although coeval, the two suites of intrusions are not genetically related, and furthermore were likely derived from different source regions via different melt generating mechanisms. We propose a tectonic and petrogenetic model, whereby following juxtaposition of the Balkan and Sredna Gora Terranes during Variscan collision, the two suites of magmas were generated by continued subduction under eastern Europe due to the influx of mafic magmas into the base of the crust, and by influx of water-rich fluids into high-grade metapsammitic rocks of the Sredna Gora metamorphic series. The new ages are somewhat younger than ages for post-collisional intrusions in the central European Variscan Massifs (i.e., Bohemian, Black Forest, and Vosges), where ages of intrusions are predominantly ∼340–320 Ma, although some younger intrusions may also occur. Within the intra-Alpine massifs, both older (∼340–320 Ma) and younger (∼310–290 Ma) granitoids are present. Farther west, intrusions in the Iberian Massif appear to be predominantly younger ∼325–290 Ma, similar to the intrusions of the Balkans. This may indicate that collision of adjacent crustal blocks culminated at earlier times in the central and more northerly portions of the orogen, and moved both east, west, and south as the evolution of the orogen progressed.

Timing and geochemistry of potassic magmatism in the eastern part of the Svecofennian domain, NW Ladoga Lake Region, Russian Karelia

Abstract The Puutsaari intrusion is a potassium-rich magmatic complex in the eastern part of the Svecofennian domain close to the Archaean border. The intrusion is generally undeformed in contrast to 1880–1875 Ma-old country rock tonalitic migmatites and diatectites. The main rock types are: (1) mafic rocks of a gabbro–norite–diorite–quartz monzodiorite series; (2) quartz diorite–tonalite–granodiorite; and (3) coarse-grained microcline granite. The three rock-types intruded coevally forming a peculiar three-component mingling system. The mafic rocks, enriched in K, P, Ba, Sr and LREE, have marked shoshonitic affinities (K2O=1.97–5.40, K2O/Na2O=0.6–2.37). On a regional scale they demonstrate transitional geochemistry between less enriched syn-orogenic 1880 Ma-old gabbro–tonalite complexes and strongly enriched 1800 Ma post-collisional shoshonitic intrusions. The microcline granite as well as the tonalite–granodiorite rocks are geochemically similar to crustal anatectic granitoids of the NW Ladoga Lake area. The three rock groups do not form a single trend on Harker-type diagrams and are unlikely to be related by fractional crystallisation or mixing. Zircons from the Puutsaari microcline granite and from the mafic rock series have been dated by ion-microprobe (NORDSIM) at 1868.2±5.9 and 1869±7.7 Ma, respectively. Most zircons recovered from a granite sample had zoned or homogeneous cores and unzoned fractured rims. No statistically significant variation of zircon core and rim ages from the granite was established in the course of this study. Zircons from the mafic rock are unzoned. It is suggested that the mafic rocks at Puutsaari were derived from an enriched mantle shortly after the main Svecofennian collisional event and the roughly 1.88 Ga regional metamorphic culmination. The emplacement of the mafic melt caused anatectic melting of various crustal protoliths and produced coeval granitic and tonalitic compositions.

Tectonic setting of post-collisional magmatism in the Palaeoproterozoic Svecofennian Orogen, SW Finland

Five bimodal post-collisional intrusions in southwestern Finland have been investigated. Geochemically, the mafic rocks are shoshonitic monzodiorites, which are highly enriched in Fe, P, Ti, F, LREE and in incompatible trace elements. The felsic rocks are garnet bearing peraluminous, S-type anatectic granites. New data on the mafic and the felsic intrusions yielded the same U–Pb zircon age of 1815 Ma. Therefore, the mafic and felsic intrusions are coeval but not cogenetic. Narrow contact metamorphic aureoles around the mafic intrusions contain garnet–orthopyroxene bearing assemblages, and thermobarometry indicates an intrusion depth of at least 15 km. Hence, there was little or no unroofing after peak regional metamorphism at 4–6 kbar. The geochemical characteristics of the mafic rocks suggest that they were derived from subcontinental lithospheric mantle that was previously enriched by fluids released during Svecofennian subduction. It is suggested here that hot upwelling asthenosphere convectively removed subcontinental lithospheric mantle and triggered partial melting of the enriched parts of the mantle. Uprising mafic melts increased the already high temperatures at mid-crustal levels and caused granulite facies metamorphism, crustal anatexis and production of granitic melts.

1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield

At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in southern Finland and Russian Karelia in a 600-km-long belt from the Åland Islands to the NW Lake Ladoga region. The rocks range from ultramafic, calc-alkaline, apatite-rich potassium lamprophyres to peraluminous HiBaSr granites, and form a shoshonitic series with K 2O+Na 2O>5%, K 2O/Na 2O>0.5, Al 2O 3>9% over a wide spectrum of SiO 2 (32–78%). Although strongly enriched in all rocks, the LILE Ba and Sr and the LREE generally define a decreasing trend with increasing SiO 2. Depletion is noted for HFSE Ti, Nb and Ta. Available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded. Initial magmas (Mg#>65) in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post-collisional uplift some 30 Ma after the regional Svecofennian metamorphic peak. However, prior to the melting episode, the lithospheric mantle was affected by carbonatite metasomatism; more extensively in the east than in the west. The melts generated in the more carbonate-rich mantle are extremely enriched in P 2O 5∼4%, F∼12,000 ppm, LILE: Ba∼9000 ppm, Sr∼7000 ppm, LREE: La∼600 ppm and Ce∼1000 ppm. The parental magma underwent 55–60% fractionation of biotite+clinopyroxene+apatite+magnetite+sphene whereupon intermediate varieties were produced. After further fractionation, 60–80%, of K-feldspar+amphibole+plagioclase±(minor magnetite, sphene and apatite), leucosyenites and quartz-monzonites were formed. In the west, where the source was less affected by carbonatite metasomatism, calc-alkaline lamprophyres (vogesites, minettes and spessartites) and equivalent plutonic rocks (monzonites) were formed. Removal of about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene produced peraluminous HiBaSr granites. The impact of crustal assimilation is considered to be low. At about 1.8 Ga, the post-collisional shoshonitic magmatism brought juvenile material, particularly enriched in alkalis, LILE, LREE and F, into the crust. Although areally restricted, the regional distribution of the post-collisional intrusions may indicate that larger volumes of 1.8 Ga juvenile material resides in unexposed parts of the crust.

LAJINHA, UMA INTRUSÃO TARDI-OROGÊNICA E PÓSCOLISIONAL NO EXTREMO OESTE DO COMPLEXO PARAÍBA DO SUL, MINAS GERAIS, ESPÍRITO SANTO, BRASIL

This is an interpretation of geological, petrological and geochemical data of the Lajinha intrusion,which belongs to the Alto Capim Subsuite of the Espírito Santo Magmatic Suite. The Lajinhaintrusion is a plutonic body of medium extensions (48km2) with diapiric structure and magmaticzoning. Two magmatic groups build up this body, one of granitic affinity and the other group ofdioritic affinity. Between this two groups do not exist “commingling” but a chemical interaction (e.g.: “mixing” in the contact region). The acid rocks, that crop out in a ringlike form, can be observedin two distinct litological groups. The main group is coarse-grained, has a great number of big potassicfeldspar-phenocrysts (up to 15cm), small part of matrix and it do not have a preferencial orientation.In some parts exist a flux orientation of the phenocrystals and at the contact with the host rockapears weak intrusive foliation. The other subordinate group is formed by a middle to coarse grainedmatrix, with isotropic apearence and with a very low content of K-feldspar phenocrystals. Theseparation of the two groups may be a result of “filter-pressing” and/or “gravity separation”. Thegranites are constituted principally of quartz, K-feldspar, plagioclase, biotite, accessory and opaqueminerals. Their chemical composition ranges from alkaligranite to granodiorite. The basic rockgroup,which crops out in an oval formed elonged central nuclei, has a middle to coarse granulometry,homogeneous and isotropic apearence over all of the visited outcrops. This rock is built up byamphibole, biotite, plagioclase, small percentage of pyroxene, accessory and opaque minerals. Thechemical composition vary between syeno-diorite, diorite and gabbrodiorite. The contact betweenthe granitc rockgroup and the basic rockgroup is very well defined and shows only metasomatismeffects and not “commingling”. The host of the intrusion are para-rocks like migmatites, bandedgneisses, quartzites, and ortho-rocks like an homogeneous gneisse (Galileia Intrusive Suite; PocraneComplex) and amphibolitic. They are very strong foliated. The contact between the host and theintrusive rocks is normally formed by gradational evolution (transformation) and in some parts ofthe north western contact by faults. From the host towards the intrusion growth up the number ofblasts and the distribuion of fusion effects. Pegmatitic veins cut the mainbody in the same directionas distension fractures (NE-SW). In the northeastern part this fractures are invaded by a fine grainedalkaline basalt of probably cretaceous age. This veins cut the host and the granites of the intrusion,too. The observations make possible the idea of an intrusive process under late-orogeneous to postcollisionalregime. The differentiation trends of the two rock groups are calco-alkaline (trondhjemiticto monzonitic), with peraluminous to metaluminous and alkaline to subalkaline tendency. Thisintrusion make part of the symetric body distribution within the Espírito Santo Magmatic Arc,beginning with late to post-collisional intrusion in the west and east and early to syn-collisionalintrusions in the central region, the later influenced by compressional and transcorrent movements.