Post-collisional Period Research Articles

Discovery of Late Cretaceous basalts in the Asuo area, Central Tibet: Implications for orogenic root removal beneath the Lhasa-Qiangtang orogenic belt

Post-collisional mafic rocks provide valuable insights into the mantle properties and evolution of orogenic belts. However, their origins remain unknown. In this study, comprehensive age, elemental, and zircon Lu–Hf isotopic data are presented for mafic dikes from the Asuo area of the Central Tibetan Plateau. The Asuo mafic dikes exhibit typical arc-related characteristics such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements. This sodium-rich calc-alkaline mafic rocks has low K2O contents (0.98–1.26 wt%) and high Na2O/K2O ratios (2.55–2.97). The presence of xenolith zircons and varying zircon Hf isotopic composition [εHf(t): 1.33–8.32] indicate that the mafic magma assimilated juvenile crust. Zircon U-Pb dating results suggest that the Asuo mafic rocks were emplaced contemporaneously with the adakite-like rocks during ∼ 95–75 Ma. However, no genetic link between Asuo mafic rocks and the adakite-like rocks in the Lhasa–Qiangtang Orogenic belt was observed. The primary contribution of mantle-derived mafic magmas to the formation of coeval intermediate–felsic magmas is the provision of additional heat. The mass exchange between them may be limited. We suggest a geodynamic scenario in which the orogenic root may have been removed during the post-collisional period via repeated and localized lithospheric dripping. In this model, the formation of mafic and adakite-like intrusions occurred in two stages, that is, (1) partial melting of metasomatized lithospheric mantle generated mafic melts, and (2) underplating of these mafic melts beneath the thickened juvenile lower crust, which resulted in partial melting of the juvenile mafic lower crust and generation of adakite-like melts. Our results provide new insights into the magmatism in the terminal stage of an orogenic system.

Geochemical variations of anatectic melts in response to changes of P–T–H2O conditions: Implication for the relationship between dehydration and hydration melting in the Himalayan orogen

The anatectic mechanism is the most fundamental criterion in determining the petrogenetic relationship between migmatite, granulite and granite at convergent plate margins. Although two main anatectic mechanisms, namely water-absent (dehydration) and water-fluxed (hydration) melting, have been identified in many collisional orogens, their spatiotemporal relationship and genetic connection remain obscure. To address this issue, we conduct an integrated study of forward phase equilibrium and trace element modelling for partial melting of metapelite in the Himalayan orogen. The results are used to decipher changes of phase relation and melt composition in response to changes of temperature, pressure and water content. Pressure and water content mainly affect the solidus and evolving residual mineral assemblages, whereas temperature controls the anatectic reaction. Hydration melting at lower pressure and temperature would produce anatectic melts with relatively high SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios, but low Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios. In contrast, dehydration melting at higher pressure and temperature would generate anatectic melts with relatively high Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios, but low SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios. A comparison of geochemical compositions between the modelled melts and the two groups of well-characterized leucogranites in the Himalayan orogen indicates that the coeval dehydration and hydration melting would likely occur at the deeper and shallower crustal levels, respectively. In combination with the secular evolution of metamorphic facies series in the Himalayan orogen, we propose a spatiotemporally coupled dehydration-hydration melting mechanism. The early metamorphism at lower geothermal gradients would result in metamorphic dehydration in the lower crust and hydration in the upper crust during the syn-collisional period in the Early Cenozoic. As a result, the later metamorphism at higher geothermal gradients would lead to dehydration melting in the lower crust and hydration melting in the upper crust during the post-collisional period in the Late Cenozoic. This study highlights the applicability of phase equilibrium and trace element modelling to reveal the geochemical variations of anatectic melts in response to changes of P–T–H2O conditions. This provides new insights into the relationship between dehydration and hydration melting in collisional orogens.

The polyphase evolution of the mafic rocks of the Juiz de Fora Complex: The record of two supercontinent cycles

The Juiz de Fora Complex (JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, is a basement unit displaying a variety of rock types that is tectonically imbricated with the supracrustal rocks of the Raposos Group in the Upper Thrust System of the Occidental Terrane, Ribeira Belt. Here we report new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data on mafic garnet-granulites from the JFC with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. This work presents the most comprehensive metamorphic study on the Juiz de Fora Complex rocks to date. Metamorphic P-T estimates for the metamorphic peak conditions of 800–900 °C and 8–9 kbar in basic garnet-bearing granulites were obtained for the JFC, indicating that metamorphism took place at deeper crustal levels. Geochemical, zircon U–Pb geochronology, whole-rock Sm–Nd, Rb–Sr isotopes, and Lu–Hf isotopic data of mafic granulites yielded U–Pb ages of ca. 2201 Ma, and moderately juvenile εNd(t) = −0.5 and were obtained from the JFC marking the timing of volcanic arc magmatism during the Rhyacian period. This arc-related stage was followed by the post-collisional period during the Early Paleoproterozoic with ca. 1.94 Ga E-MORB signature granulites that shows evidence of magma sourced from Neoarchean (∼2.8 Ga) crust with addition of juvenile components. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which placed the Neoproterozoic metasediments and the JFC rocks at the same level. The integrated information sheds light on two collisional episodes where the oldest is represented by arc-related granulites during the Rhyacian Orogeny and the youngest is the higher-pressure Neoproterozoic overprint during the Brasiliano Orogeny.

Long-term magmatism in the Ribeira Orogen, Western Gondwana: SHRIMP U-Pb and O isotope data for two Ediacaran-Cambrian magmatic events in the Rio de Janeiro area

In the southeastern Brazilian margin, a Neoproterozoic to the Early Paleozoic long-term magmatic interval is recorded within the Oriental Terrane, part of the Ribeira Orogen, characterizing it as a hot orogenic belt. Metaigneous and igneous rock units of ca. 570–540 Ma and 510–480 Ma are coeval with two main collisional orogenies related to the Western Gondwana assembly: the Ribeira Orogeny (590–565 Ma) and the Búzios Orogeny (540–490 Ma). The Oriental Terrane, interpreted as a continental microplate amalgamated during the convergence between the São Francisco and Congo cratons, comprises Cryogenian to Ediacaran metaigneous rocks and Cambro-Ordovician plutons intrusive in metasedimentary sequences. Zircons from an orthogneiss and a porphyritic leucogneiss sample gave SHRIMP U-Pb crystallization ages of 558 ± 2 Ma and 560 ± 2 Ma, respectively, with high δ18O values from 7 ‰ to 10 ‰. Zircons from a tonalite pluton and tonalitic dikes, gave SHRIMP U-Pb crystallization ages of 483 ± 2 Ma and 492 ± 2 Ma, with lower δ18O values, between 6 ‰ to 8 ‰. These two magmatic pulses, integrated with a compiled database from other plutons in this terrane, corroborate with the hypothesis that they represent crustal scale magmatic provinces here interpreted as pre- and post-collisional in relation with the Búzios Orogeny. Their distribution within the tectonic map of the Oriental terrane reflect a paleogeography linked to their tectonic origin. The 570–540 Ma plutons are aligned ENE -WSW, parallel and close to the suture zone with the Cabo Frio Tectonic Domain, representing a crustal melt related to a NW-subduction. A subsequent magmatic hiatus of 30 ma is interpreted as the collision of the Cabo Frio Tectonic Domain. The post collisional period is marked the intrusion of non– linear 510–480 Ma pluton, widespread in the Oriental terrane.

Generation of aluminous A-type granite by partial melting of felsic restite: Evidence from Mesozoic granitoids in the southern margin of the North China Craton

Aluminous A-type granites generally show high SiO 2 contents of >70 wt%, and their petrogenesis is still hotly debated with respect to source nature, magmatic processes and tectonic setting . This paper presents a comparison of petrography , geochronology and geochemistry between a Mesozoic syenogranite and its adjacently contemporaneous granitoids in the Xiong'ershan area, the hanging wall of the Paleozoic continental collision zone between the North China Craton and the North Qinling microcontinent. Syenogranites at Taishanmiao are metaluminous to weakly peraluminous, and belong to high-K calc-alkaline to shoshonitic series. They contain embayed bipyramidal quartz crystals , quartz-feldspar intergrowths and high Fe biotite species as well as accessory fluorite , magnetite and ilmenite . They exhibit significantly negative Ba, Sr and Eu anomalies with most δEu values of <0.5, and relatively high contents of Nb, Ta and Zr. They are weakly magnesian to ferroan in lithochemistry and mostly fall into the field of aluminous A-type granite in terms of their high Zr contents and 10,000 × Ga/Al ratios. Furthermore, they have low Y/Nb ratios of <1.20, falling into the A1-type field of the Nb-Y-Ce ternary diagram. The geochronological results indicate that the Taishanmiao syenogranites were emplaced into the late Paleoproterozoic Xiong'er Group volcanics at 119 ± 1 Ma, consistent with emplacement ages of the late stage non-adakitic granitoids at Funiushan in the same area. Whole-rock Nd and zircon Hf O isotope compositions are similar between the two types of granitoids, indicating that they share a common magma source that is composed of the ancient crustal basement in the southern margin of the North China Craton and the juvenile crust of the North Qinling microcontinent. According to their petrographic and geochemical features, the syenogranites are categorized into aluminous A-type whereas the non-adakitic granitoids belong to I-type. Therefore, they were produced by partial melting of felsic restites after extraction of hydrous granitic melts to form the Funiushan I-type granites. As such, the composition of these granitoids is primarily dictated by both source composition and magmatic processes at the ancient convergent plate margin. Although the granitic magmatism would take place in the post-collisional period, the aluminous A-type granite was formed in the terminal stage of post-collisional magmatism. In addition, the post-collisional granites record reactivation of the hanging wall in the continental collision zone. • The Taishanmiao syenogranites belong to aluminous A1-type granite. • The syenogranites were derived from partial melting of the felsic restite. • The felsic restite was produced by extraction of hydrous granitic melts. • The syenogranites are petrogenetically associated with calc-alkaline granitoids. • Granite composition is also dictated by crustal anataxis and melt extraction.

The timing and tectonic context of Pan-African gem bearing pegmatites in Malawi: Evidence from Rb–Sr and U–Pb geochronology

The Pan-African belts of Malawi contain a largely unexplored endowment of gem bearing pegmatites. We present U–Pb in zircon (LA-ICPMS) and Rb–Sr mineral isochron geochronological and isotope data from pegmatites across Malawi. The pegmatites contain tourmaline, beryl, aquamarine, zircon, amethyst and sunstone as gemstone species. Two zircon bearing pegmatites in southern Malawi intruded early in the Pan-African orogenic cycle at 719 ± 5 Ma and 729 ± 4 Ma and are associated with the emplacement of alkaline rocks that formed during an intra-continental rifting episode in the eastern part of former Rodinia. One further zircon pegmatite containing inherited zircon of a similar age (746 ± 44 Ma) was emplaced at 598 ± 15 Ma, after the assembly of Western and Eastern Gondwana and the formation of the East African Orogen (EAO). The majority of the analysed pegmatites, however, are significantly younger. The ∼550 Ma pegmatites were emplaced during the Kuunga Orogeny, correlating with the collision of northern and southern Gondwana cratonic entities. During a prolonged post-collisional period, possibly related to crustal collapse and extension, further gem-mineralised pegmatites formed at ∼520 ± 6 Ma and ∼500–485 Ma. The youngest pegmatite intruded in the southern Malawian Ntcheu area in the Middle Ordovician at ∼460 Ma. A large spread in 87Sr/86Sr initial isotopic ratios between 0.70556 and 0.79018 suggests a variety of magma sources for the Kuunga-related pegmatites with a variably strong crustal affinity.

Petrogenesis of the late Cretaceous Budongla Mg-rich monzodiorite pluton in the central Lhasa subterrane, Tibet, China: Whole-rock geochemistry, zircon U-Pb dating, and zircon Lu-Hf isotopes

Mg-rich monzodiorite are found in the Budongla gold ore district, Zhongba County, Xizang (Tibet) Autonomous Region, P.R. China. Studying the petrogenesis of this intermediate pluton can provide effective information to explore the geological evolution of the Lhasa terrane. One monzodiorite sample yielded a weighted mean 206Pb/238U age of 92.7 ± 1.1 Ma (mean square weighted deviation=0.33) using LA-ICP-MS zircon U-Pb dating, which represents the late phase of Late Cretaceous magmatism. The rock-forming minerals in the Budongla Mg-rich monzodiorite mainly include K-feldspar, plagioclase, quartz, biotite augite, and amphibole, and its accessory minerals mainly include magnetite, titanite, zircon, and apatite. The rocks are rich in K2O+Na2O and K2O with medium contents of SiO2, CaO, and A12O3,, suggesting these rocks belong to the high-K calc-alkaline series. These rocks have high MgO, Fe2O3, and FeO, with high Mg# values and low DI, which implies they are Mg-rich intermediate intrusive rocks. The rocks are enriched in LREEs and LILEs and depleted in HREEs and HFSEs. They have negative Eu anomalies, no obvious negative Ce anomalies, and slightly negative εHf(t). We infer that the Budongla pluton is a high-K calc-alkaline metaluminous Mg-rich monzodiorite and intruded during the post-collisional period of the Lhasa and Qiangtang terranes.

Shoshonitic magmatism in the Southwestern Borborema Province (NE Brazil): The plutons of Campos Sales-Assaré and Padre Marcos

The extensive magmatic record and the large network of shear zones are the most striking features in the Borborema Province (BP) northeast of Brazil. In this context, high-potassium magmatism (Itaporanga Suite) is one of the suites with the most significant expression and volume, usually related to a post-collisional period during the transition from the convergent to the transcurrent tectonic regime. Hence, the Campos Sales-Assaré Batholith (CSAB) represents an extensive sigmoidal magmatic body, comprising the extreme west portion of the Transversal Zone, within the BP, cropping out between the Phanerozoic Araripe and Parnaíba basins. Three different trends are recognized in the AFM diagram, named A, B, and C, suggesting that the batholite comprises more than one magmatic series or different plutons. The field aspects and petrography suggest that the main active petrogenetic process was the magmatism mixing between mafic and felsic magmas. The intrinsic parameters obtained for the magmatic body also corroborate the mixing environment, suggesting that the system may have been reheated. Moreover, differences in mineral chemistry and geochemical data indicate that the batholith may be composed of two distinct plutons: Campos Sales-Assaré Pluton (CSAP) and Padre Marcos Pluton (PMP), separated by the Fronteiras shear zone. The estimated oxygen fugacity in amphiboles from one sample of trend B is higher than in amphiboles of trend A. Furthermore, the classification of magmatic bodies as potassic shoshonitic indicates a metasomatized subcontinental lithospheric mantle as a source for the basic rocks and an amphibolite lower crust source for the leucogranites shoshonitic-like. The difference between trends A and B in the multi-elementary diagram suggests a more enriched source for the latter. According to the data, these plutons can be associated with post-collisional within the Brasiliano Orogeny.

Terrane geodynamics: Evolution on the subduction conveyor from pre-collision to post-collision and implications on Tethyan orogeny

Terranes are passengers within drifting oceanic plate, and ride with the plate to subduction plate boundaries. Although oceanic lithosphere readily subducts into the mantle at the plate boundary, terranes may resist sinking or not, depending on a variety of controlling factors that are not very well understood. Further, the tectonic development of terranes prior to their arrival at a subduction zone is not well documented. We performed numerical experiments to explore these unknowns of terrane geodynamics during the whole pre- to post-collisional period. Our analyses reveal that terranes can undergo considerable extension prior to their arrival to subduction plate boundaries owing to the pull force exerted by sinking oceanic slabs. Increasing terrane crustal thickness, decreasing terrane width or imposed convergence velocity, and an intra-oceanic subduction setting aggrandizes the pre-collisional terrane extension. The numerical models identify increasing terrane crustal thickness and width, and decreasing imposed convergence velocity as factors that promote terrane accretion. Additionally, having a continental overriding plate at the subduction boundary increases the propensity for terrane accretion. Some intra-oceanic subduction experiments demonstrate ablative subduction and subduction polarity reversal events in connection with terrane collisions/subduction. We compare our models to the evolution of a controversial Tethyan terrane, the Nilufer oceanic plateau. Our models suggest that the Nilufer terrane may have undergone pre-collisional extension owing to the pull of the sinking Tethys Ocean plate. Further, uninterrupted subduction of the Tethys Ocean, despite the accretion of the Nilufer terrane, is illustrated by our results which have implications on other regions along the Tethyan orogenic belt.

Intrusion of the Ediacaran multi-pulse Quatro Ilhas Granitoids under inclined transpression, Dom Feliciano Belt, Southern Brazil

The Quatro Ilhas Granitoids are syntectonic to a major NE-SW dextral strike-slip shear zone at the northeastern part of the Dom Feliciano Belt in Southern Brazil. They comprise high-K calc-alkaline, coarse-grained porphyritic granodiorite to monzogranite emplaced at 625 to 615 Ma. Their foliation is marked by the alignment of biotite and euhedral to stretched K-feldspar phenocrysts and dips gently to steeply to the SE. Mineral and stretching lineations plunge at a low angle to both NE and SW. Several fine-grained dioritic to granitic synplutonic sheets crosscut the porphyritic granites parallel or oblique to the foliation. This assemblage is affected by regional tectonics, resulting in centimetre-to metre-scale, NW-verging asymmetrical folds, whose axis is parallel to the observed lineations. Progressive verticalization of foliation due to folding leads to local steeply-dipping, dominantly dextral shear zones. The observed structures and coeval strike-slip and contractional structures point to oblique/inclined transpressive deformation of the monzogranite. The fine-grained granites are geochemically grouped into low- and high-K types. The first group is relatively depleted in K2O and enriched in Ba and Sr, whereas the second resembles A-type granitoids. The low-K intrusions are interpreted to be crustally-derived, and the high-K type results from differentiation of tholeiitic magmas with crustal contamination. The presence of both gently-dipping and steeply-dipping structures active during the emplacement of these granitoids is further evidence of the transpressive character of the regional tectonic event. It is interpreted as a marker of the transition between the main collision and overall oblique transpression (650–635 Ma) and strict strike-slip tectonics (≤611 Ma) during the post-collisional period of the Dom Feliciano Belt construction.

Ediacaran post-collisional K-rich granitic magmatism within the Major Gercino Shear Zone, Southern Brazil: An example of prolonged magmatism and differentiation under active transcurrent tectonism

The post-collisional period of the Brasiliano/Pan-African Orogenic Cycle along the Dom Feliciano Belt in southern Brazil (650–580 Ma) is marked by intense magmatism, comprised of substantial volumes of coeval granitic and mafic rocks emplaced along transcurrent shear zones that constitute the Southern Brazilian Shear Belt. Some representatives of this magmatism are the Estaleiro Granodiorite (EG) and associated fine-grained granites that compose the Estaleiro Granitic Complex (EGC), together with minor volumes of mafic material as enclaves and tabular bodies. The EGC is a syntectonic rock association that registers the main transcurrent phase of the Major Gercino Shear Zone (MGSZ). The Estaleiro Granodiorite and fine-grained granites share the same MGSZ dextral transcurrent kinematics and evolution from magmatic to solid-state conditions under deformational temperatures consistent with upper-greenschist to mid-amphibolite facies. New zircon UPb LA-MC-ICP-MS crystallisation ages, 612 ± 3.5 and 611 ± 5.3 Ma, for the Estaleiro Granodiorite undeformed and deformed portions constrain the timing of transcurrent tectonic activity of the Major Gercino Shear Zone. Geochemical features as SiO2 contents above 63%, the alkaline and potassic predominant character [K2O > (Na2O – 2)], enrichment in Na2O + K2O (6.9–9.2 wt%), high K2O/Na2O ratios (0.8–1.4), high Al2O3 (14.1–16.4 wt%), low TiO2 (0.1–0.8 wt%) and high Fe2O3/FeO (~1.11), Ba (488–1701 ppm),Sr (260–1006 ppm), strong enrichment in LILE relative to HFSE, REE with moderate fractionation [(La/Yb)N = 10.6–51.1] and absence of strong Eu negative anomalies [(Eu/Eu*)N = 0.54–1.23] of the Estaleiro Granitic Complex granitoids are compatible with rocks of shoshonitic affinity. New 87Sr/86Sri ratios and ƐNdt values obtained for fine-grained granites point to increasing crustal contamination along the differentiation of the complex. Whole-rock chemical and SrNd isotope data point to mantle sources modified by previous, subduction-related metasomatism. The primary magmas have evolved through AFC processes, more prominent in the most differentiated and/or deformed rocks. We suggest that the transcurrent tectonism of the Major Gercino Shear Zone played an important role in the activation of mantle sources capable of originating shoshonitic magmas and facilitated their interaction with crustal melts to generate the Estaleiro Granitic Complex granitoids.

Ediacaran Na-alkaline Acampamento Velho volcanism in the Ramada Plateau, southernmost Brazil: Sr–Nd–Pb isotopic data and petrogenetic evolution

In the Ramada Plateau, there is an expressive succession of volcanic rocks from the Acampamento Velho Formation related to the post-collisional period of the Pan-African/Brasiliano orogeny. This unit is predominantly composed of felsic pyroclastic rocks (ignimbrites) with subordinate rhyolitic and basaltic flows, as well as subvolcanic basalts, trachytes, and rhyolites generated between approximately 580 and 550 Ma. These rocks have a sodic alkaline affinity, which is typical for a post-collisional environment. Sr-Nd-Pb isotopic data suggest the rocks of the Acampamento Velho Formation in the Ramada Plateau were produced by the fusion of an asthenospheric mantle, modified by previous subduction with an EMI-type mantle signature. Petrogenetic modelling using isotopes, major and trace elements allowed us to conclude that fractional crystallization processes were more effective in differentiating the basic-intermediated terms involving the fractionation of approximately 60% of melt. For the intermediate and acid compositions, a greater discrepancy between the modelling values for the major/trace elements and isotopes was observed despite the occurrence of fractional crystallization associated with crustal assimilation, especially in trace elements and isotopes diagrams. Field and petrographic evidence, as well as parabolic geochemical trends allowed us to conclude that the intermediate and acid terms of the Acampamento Velho Formation magmatism evolved largely by magma mixing processes

Introduction to Journal of South American Earth Sciences special issue on “Magmatism of southernmost South America”

During the Neoproterozoic West-Gondwana assembly, the Araçuaí belt together with its African counterpart, the West Congo belt, formed a confined orogenic system between the São Francisco and Congo cratons. As a result, a widespread magmatism took place from ca. 630 to 480 Ma at the Araçuaí belt, finishing with a ca. 530 to 480Ma post-collisional period. The Alto Chapéu pluton is one of the numerous intrusions emplaced during this post-collisional event. In order to investigate the crystallization conditions involved in the evolution of this pluton, we conducted a study based on zircon chemistry, concerning the granitic composition of this magmatism at the early stages of the Araçuaí orogenic collapse. Coupled zircon saturation and Ti-in-zircon temperatures along with REE-in-zircon oxygen fugacity estimatives indicate high temperatures and relative reducing conditions intervening magmatic differentiation given by autocrystic zircon domains (maximum 814 ± 14 °C to average 749 ± 24 °C and ΔQFM -3.78 ± 1.02). Even more reducing and hotter isolated magma pulses at Alto Chapéu Pluton are indicated by antecrystic zircon domains (isolated peaks of 864 ± 28 °C and 920 ± 30 °C and ΔQFM reaching −5.11 ± 0.84 at antecrystic rim). Inherited grains interpreted to derive from host rock point to slightly lower temperatures, and also reducing conditions (maximum 756 ± 16 °C to averages of 679 ± 22 °C and ΔQFM -3.88 ± 1.09). Older inherited zircons are problematic to constraint and because of this zircon trace element chemistry can shed light to characteristics of lost melts. The chemistry of inherited zircon points out to the presence of former continental-like crust at least 70 Ma prior to the Araçuaí main granite production events. The wide time-lapse involving zircon inheritance and crystallization seems to be related to post-collisional nature in recording a variety of geochemical sources signature.

Constraining crystallization conditions during the Cambro-Ordovician post-collisional magmatism at Araçuaí belt (SE Brazil): Zircon as key petrologic witness

During the Neoproterozoic West-Gondwana assembly, the Araçuaí belt together with its African counterpart, the West Congo belt, formed a confined orogenic system between the São Francisco and Congo cratons. As a result, a widespread magmatism took place from ca. 630 to 480 Ma at the Araçuaí belt, finishing with a ca. 530 to 480Ma post-collisional period. The Alto Chapéu pluton is one of the numerous intrusions emplaced during this post-collisional event. In order to investigate the crystallization conditions involved in the evolution of this pluton, we conducted a study based on zircon chemistry, concerning the granitic composition of this magmatism at the early stages of the Araçuaí orogenic collapse. Coupled zircon saturation and Ti-in-zircon temperatures along with REE-in-zircon oxygen fugacity estimatives indicate high temperatures and relative reducing conditions intervening magmatic differentiation given by autocrystic zircon domains (maximum 814 ± 14 °C to average 749 ± 24 °C and ΔQFM -3.78 ± 1.02). Even more reducing and hotter isolated magma pulses at Alto Chapéu Pluton are indicated by antecrystic zircon domains (isolated peaks of 864 ± 28 °C and 920 ± 30 °C and ΔQFM reaching −5.11 ± 0.84 at antecrystic rim). Inherited grains interpreted to derive from host rock point to slightly lower temperatures, and also reducing conditions (maximum 756 ± 16 °C to averages of 679 ± 22 °C and ΔQFM -3.88 ± 1.09). Older inherited zircons are problematic to constraint and because of this zircon trace element chemistry can shed light to characteristics of lost melts. The chemistry of inherited zircon points out to the presence of former continental-like crust at least 70 Ma prior to the Araçuaí main granite production events. The wide time-lapse involving zircon inheritance and crystallization seems to be related to post-collisional nature in recording a variety of geochemical sources signature.

Post-collisional mafic magmatism: Record of lithospheric mantle evolution in continental orogenic belt

In order to better understand the role of post-collisional mafic magmatism at convergent plate boundaries in revealing the earth’s evolution, this paper has systematically summarized the research history of post-collisional mafic magmatism, different types of collision and their influence on the nature of orogenic mantle, the concept and implication of post-collisional magmatism, and the relationship between post-collisional mafic magmatism and orogenic mantle evolution and mineralization. Post-collisional mafic igneous rocks are not only the direct records for studying the nature and evolution of orogenic mantle, but also the important carriers for regional mineralization. However, the type and quantity of the crustal materials involved in modifying the overlying lithospheric mantle during collisional orogeny, the process and mechanism of such modification, and the major control factors and mechanism of mafic magmatism-related mineralization during the post-collisional period are the main contents and direction of future researches in this field. Therefore, the study of post-collisional mafic magmatism is of significant implications for developing the theory of plate tectonics.

İZMİR-ANKARA-ERZİNCAN SUTUR ZONUNDAKİ (SİVAS KUZEYDOĞUSU) EOSEN YAŞLI KARATAŞ VOLKANİTLERİNİN JEOKİMYASAL ÖZELLİKLERİ

Karatas volcanics, is the product of Eocene volcanics crop out in the form of two belts along the northern and southern boundaries of the Izmir-Ankara-Erzincan Suture Zone. According to geochemical data, these volcanics have alkaline basic-intermediate character and consist of basaltic trachyandesite, trachyandesite and trachyte. This volcanic activity has been controlled by fractional crystallization and crustal contamination from basaltic trachyandesite to trachyte. Orientation of the samples towards amphibole area on the Rb/Sr - Ba/Rb ratio diagrams, dispersion of the Zr/Ba ratios (0.08 - 0.33) in the lithospheric mantle range, increase in the Ba/Rb ratio, decreases in the MgO, Ni and Cr contents point out that this volcanism originated from enriched lithospheric mantle rather than asthenospheric mantle. Geochemical data show that this enriched lithospheric mantle material is upper continental crustal material, main part of enrichment resulted by the subduction related fluids and also the contribution of the sedimentary materials. This situation may be explained that; the melts, derived from N-MORB or OIB bearing material ascended into the continental crust in a pre-collisional period and were reactivated by extensional tectonic and/or delamination processes during the post-collisional period, possibly caused the partial melting within the upper continental crust and produced the Karatas volcanics.

Lower Oligocene Calc-Alkaline Spessartitic Lamprophyres from Central Iran (East of Anarak Area); an Evidence from the Eastern Branch of Neotethys Subduction-Related Mantle Enrichment

The Lower Oligocene Kal-e-kafi (East of Anarak, Central Iran) lamprophyres occur as stocks and dikes, which cross-cut the Eocene volcanic and Cretaceous sedimentary rocks. The predominant minerals of these lamprophyres are hornblende (magnesiohastingsite) and clinopyroxene (diopside) phenocrysts set in a fine- to medium-grained matrix of the same minerals plus plagioclase (labradorite to bytownite), sanidine, apatite, and magnetite. Secondary minerals are chlorite, magnetite, calcite, and epidote. Petrography, mineral chemistry, and whole rock compositions classify these rocks as calc-alkaline lamprophyre, in general, and spessartite in particular. These samples have intermediate compositions (SiO2 ~ 58 wt %). The chondrite-normalized REE patterns and primitive mantle-normalized multi-element spider diagram of Kal-e-kafi lamprophyres are remarkably parallel and suggest that these dikes and stocks were derived from the same parental magma and underwent similar melt extraction. These rocks are enriched in alkalies, large-ion lithophile elements (e.g., Rb, Ba, K), and light rare-earth elements (e.g., La, Ce), and exhibit moderate to high fractionation in LREE patterns, with an average La/Lu ratio of 112. The large amount of hydrous fluids coming from the subducted slab rather than sediments caused to the enrichment and metasomatism of subcontinental lithospheric mantle source. Crustal contamination and assimilation of host rocks also played role in the genesis of these lamprophyres. Geochemical characteristics of the studied rocks suggest that parental magma have been derived from partial melting of a metasomatized amphibole-bearing spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism to explain enrichment in volatiles of the mantle, and the lamprophyric magmatism of the Kal-e-kafi area in Lower Oligocene times. Several tectonomagmatic discrimination diagrams indicate that the Kal-e-kafi lamprophyres occurred during postcollisional period of lithospheric extension.

Sources and settings of Ediacaran post-collisional syenite-monzonite-diorite shoshonitic magmatism from southernmost Brazil

The Arroio do Silva Pluton comprises quartz-rich syenite-monzonite-diorite rocks, with subordinate lamprophyre as enclaves. Compositional and textural variations lead to the identification of three main varieties: (i) quartz monzonite to syenite, (ii) quartz monzonite to monzodiorite and (iii) quartz diorite. The transition between all rock types is gradational and marked mostly by the increase of mafic mineral phases, especially biotite, and by the K-feldspar content, which is characteristically euhedral in the quartz monzonite to syenite variety and becomes subhedral to anhedral towards diorite compositions. Geochemical features corroborate field relations and both point to magma mingling as the main differentiation process. Syenite is considered to be the felsic endmember, whilst the mafic one has dioritic composition. Different degrees of interaction between these magmas result in hybrid varieties represented by quartz monzonite to monzodiorite. A third magma type is observed as ultrapotassic lamprophyric enclaves, but its influence in the mixing processes seems to have been minor. Both mafic and felsic magmas have K2O/Na2O ratios between 1 and 2, compatible with their shoshonitic affinity, whilst ratios slightly above 2 characterize lamprophyric enclave magmas as ultrapotassic. The source of shoshonitic and ultrapotassic magmas, according to trace element and isotope evidence, is probably a phlogopite-bearing mantle which was affected by previous subduction of Paleoproterozoic or Neoproterozoic age. The variation of ultrapotassic to shoshonitic magmas is probably controlled by variation of phlogopite contents in the mantle source. UPb zircon crystallization age of 578 Ma (LA-MC-ICP-MS) determined for the ASP quartz monzonite to syenite variety is in agreement with the Neoproterozoic post-collisional period of the Dom Feliciano Belt. This age value is also in agreement with that of other syenitic associations from south Brazil, such as the Piquiri Syenite Massive, that records similar chemical signatures. Therefore, these syenitic associations are taken to represent previously metasomatized mantle sources in late post-collisional setting from the Brasiliano/Pan-African Cycle in southermnmost Brazil.

Sr‐Nd‐Hf‐O isotope geochemistry of the Ertaibei pluton, East Junggar, NWChina: Implications for development of a crustal‐scale granitoid pluton and crustal growth

AbstractTo better understand the compositional diversity of plutonic complexes and crustal growth of the Central Asian Orogenic Belt (CAOB), we conducted an integrated study of the Ertaibei pluton, which obtained geochronological, petrological, geochemical, and isotopic (including whole rock Sr‐Nd, in situ zircon Hf‐O) data. The pluton (ca. 300 Ma) is composed of granodiorites that contain mafic microgranular enclaves (MMEs), dolerite dikes, and granite dikes containing quartz‐tourmaline orbicules. The dolerite dikes were possibly generated by melting of an asthenospheric mantle source, with discrete assimilation of lower crustal components in the MASH (melting, assimilation, storage, and homogenization) zone. The MMEs originated from hybridization between mantle and crust‐derived magmas, which spanned a range of melting depths (∼25–30 km) in the MASH zone and were episodically tapped. Melting of the basaltic lower crust in the core of the MASH zone generated magmas to form the granodiorites. The granite dikes originated from melting of an arc‐derived volcanogenic sedimentary source with a minor underplated basaltic source in the roof of the MASH zone (∼25 km). The compositional diversity reflects both the magma sources and the degree of maturation of the MASH zone. Although having mantle‐like radiogenic isotope compositions, the Ertaibei and other postcollisional granitoids show high zircon δ18O values (mostly between +6 and +9‰), indicating a negligible contribution to the CAOB crustal growth during the postcollisional period.

Post-collisional basalts of the Acampamento Velho Formation, Camaquã Basin, São Gabriel Terrane, southernmost Brazil

ABSTRACT: The basic volcanic rocks in the Palma region, southern portion of the São Gabriel Terrane, have always been interpreted as generated during the active subduction period of the São Gabriel orogeny (Cryogenian). This terrane was built as the result of the Charrua Ocean closure between 900-680 Ma. The basalts show a subhorizontal igneous flow foliation and porphyritic texture, with plagioclase phenocrysts in a thin matrix composed of plagioclase, augite and magnetite, commonly altered to actinolite, chlorite and epidote. They have amygdales and veinlets reflecting a pervasive hydrothermal phase and are affected by thermal metamorphism related to Jaguari granite intrusion. Two samples were dated by the U-Pb zircon geochronology and yielded crystallization ages of 563±2 Ma and 573±6 Ma. The basalts have transitional composition from tholeiitic to calc-alkaline, metaluminous character, trace elements patterns rich in large-ion lithophile element (LILE) with negative anomalies of Nb, P and Ti, slight enrichment in light rare-earth elements (LREE) and horizontal pattern of heavy rare earth elements (HREE). The data allow interpreting the basalts as belonging to the Acampamento Velho Formation of the Camaquã Basin, and related to the basalts of the Ramada and Taquarembó plateaus. These associations represent the final evolution of the volcanism generated in the post-collisional period of the Dom Feliciano Belt.