Granitic Xenoliths Research Articles

U-Pb monazite and zircon ages of a granite xenolith and its host alkaline basalt from the Byala Reka Dome

U-Pb monazite and zircon ages of a granite xenolith and its host alkaline basalt from the Byala Reka Dome

Xenoliths of the bode dike system: evidence for early Devonian arc-type magmatism and late Carboniferous–Permian crust reworking beneath the eastern Harz Mountains (Germany)

Xenoliths recovered from the post-Variscan Bode dike system of the eastern Harz Mountains provide evidence for the existence of an Early Devonian magmatic arc system hidden beneath very low-grade metasedimentary rocks of the Rhenohercynian Zone, but also for Late Carboniferous–Early Permian crust reworking. This interpretation is based on petrographic observations and whole-rock geochemical analyses of granite xenoliths, in addition to results of zircon U–Pb dating and Hf isotope analyses. Zircon grains recovered from variably deformed granite xenoliths yield ages between 419 and 393 Ma, interpreted to reflect the timing of granite intrusion. Rare zircon xenocrysts of Archean (ca. 2.92–2.65 Ga) and Proterozoic age (ca. 1.5 to 0.56 Ga), all with subchondritic εHf420 Ma values (− 0.8 to − 5.5) indicate reworking of older crust. Compilation of age-Hf isotope data further suggests that the pre-Variscan granitoids beneath the Harz Mountains belong to the same magmatic arc system exposed widespread in the adjacent Mid-German Crystalline Zone, and interpreted to result from NW-ward subduction of the Rheic Ocean beneath Avalonia-Baltica. Zircon in xenoliths with granophyric texture yields ages at 400 Ma and 295–310 Ma, indicating re-melting of Devonian granitoid basement during post-Variscan rift-related magmatism, immediately prior to Bode dike intrusion.Graphical abstractLeft: Late Devonian intrusion of magmatic arc granites during NW-ward subduction of the Rheic ocean beneath Avalonia-Baltica. Right: Formation of the Bode dike system and Harz granites during Late Carboniferous-Permian extension.

Contamination and dissolution-crystallisation textures of granite relics in xenolith-rich composite Coral Island Dike of the Paraná-Etendeka Magmatic Province

The Coral Island Xenolithic Dike (CIXD) is a composite intrusion with mafic borders and apophyses (Pulse 1) and a felsic centre with a high amount of granitic xenoliths and xenocrysts (Pulse 2). The xenoliths, most abundant in the dike core, are granitic and vary from millimetres up to 1.5 m in size. The xenolithic composite dike is low-TiO2 Gramado-type. The xenoliths are concentrated in the dacitic core, partially molten and accompanied by relict feldspar and partly resorbed quartz xenocrysts. Field, petrographic and geochemical data attest to contamination by assimilation processes in the central part. Flowage differentiation (Bagnold Effect) has concentrated the large xenoliths in the dacitic core. Evidence of xenolith partial melting comprises spherulitic K-feldspar/albite intergrowths and polycrystalline quartz aggregates. Coronas around the pyroxenes in the dacite and the absence of mesostasis in the dike magmas attest to supercooling of the system at shallow-level conditions. The ascent of the central part of the dike (dacite + xenoliths + relict xenocrysts) was only possible due to the proximity of the magma reservoir to the emplacement site and chamber recharging with hot basic magma input (Pulse 1) that prevents the system from cooling and crystallising.

A nearly isobaric P-T-deformation evolution path of the Cerro Negro Paleoproterozoic mylonitic igneous suites: The exhumation of the Tandilia Belt of the Rio de la Plata Craton

Two high-grade mylonites from Cerro Negro hill were studied in detail to shed light on the igneous-metamorphic evolution of the northwestern extreme of the Azul Megashear Zone (AMSZ) of the Tandilia Belt, Rio de la Plata craton. Bulk-rock compositions of a garnet-bearing hornblende gneissic mylonite (ribbon mylonite) and a xenolith of garnet-bearing granite mylonite (striped gneiss) included in the former were modeled with the GeoPs software (Xiang and Connolly, 2021) to construct P-T pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H by Gibbs energy minimization. Pseudosections were contoured for isopleths parameters and mineral isomodes of interest. Garnet, with a homogeneous composition of pyr5(gro + andr)18.5spes6.5alm70, present in the younger granite crystal mush (<8 vol% of melt for 0.6 wt% of H2O) formed at 820 °C and 7.2 kbar. These conditions were encompassed by biotite with 0.21–0.23 Ti pfu and amphibole with Mg# of 0.30–0.33. A nearly isobaric cooling is outlined during a high-grade deformational event (D1) until core with the composition of pyr5(gro + andr)10spes5alm80, in slightly zoned garnet of the granite xenolith (modeled with 0.3 wt% of H2O), yields conditions of 540–580 °C and 7.2–8.2 kbars. Rimwards, a subsequent cooling with exhumation was recorded in garnet composition of pyr3(gro + andr)17spes6alm72 to reach conditions of ∼420–440 °C and ∼6.6 kbar. The integrated P-T path holds conditions of middle crust realms (ca. 24–25 km) from the initiation of deformation (D1) coevally to, or shortly, after the younger granite crystallization with a waning apparent geothermal gradient to 20 C°Km−1. Consistent mechanisms of deformation in minerals (d) generated a subvertical E-W mylonitic foliation (D1) under granulite to amphibolite metamorphic facies conditions until greenschist metamorphic facies conditions (D2). Fragile events (D3 and D4) occurred at higher crustal levels concomitant with water influx and sporadic retrogression. Hitherto, the studied rocks experienced a somewhat different evolution path when compared to adjacent Paleoproterozoic areas around the AMSZ or from other places of the Tandilia Belt, which show gently partial exhumation P-T paths after their solidus.

NOVOUKRAINSKYI MASSIF: SOURCE OF ORIGINAL MAGMAS AND TIME OF FORMATION

NOVOUKRAINSKYI MASSIF: SOURCE OF ORIGINAL MAGMAS AND TIME OF FORMATION

Geochronology and Geochemistry of the Granite Xenolith within the Lamprophyre at the Zhenyuan Gold Deposit (Yunnan Province, SW China)

AbstractThe xenoliths in host rocks of hydrothermal gold deposits can generally provide much geochemical information of the deep rocks, which may have an implication for the sources of ore‐forming materials. Geochronology and geochemical characteristics are reported for a granite xenolith, which is enclosed by the lamprophyre in the Zhenyuan gold deposit (Yunnan Province, SW China). This granite xenolith mainly consists of K‐feldspar, quartz, and plagioclase with trace amounts of magnetite, titanite, apatite, zircon, and sulfides. Zircons from the granite xenolith yield a weighted average U‐Pb age of 281.1 ± 1.3 Ma (MSWD = 2.1), which could represent the crystallization age of this granite xenolith. The Ti‐in‐zircon geothermometer and the Mn concentration in apatite calculate that this granite xenolith formed at 685 ± 43°C with the logfO2 values ranging from NNO + 3.7 to NNO + 6.4. The zircons in the granite xenolith have a restricted range of positive ∊Hf(t) values ranging from +9.4 to +10.8, and the corresponding Hf TDM2 model ages range from 588 to 678 Ma. The zircon U‐Pb age and the Hf isotopic compositions indicate this granite xenolith within the lamprophyre is mainly derived from partial melting of juvenile crustal rocks before the completed closure of the Ailaoshan Ocean. The pyrites in the granite xenolith have higher Bi concentrations, and lower As, Sb, and Tl concentrations than the gold‐bearing pyrites of the ores in the Zhenyuan gold deposit. The low Au concentrations of the pyrites together with the ore‐forming age of the Zhenyuan gold deposit (Oligocene) indicate the granite xenolith may have limited contribution to the gold mineralization of the Zhenyuan gold deposit.

Tibetan Plateau growth linked to crustal thermal transitions since the Miocene

Abstract The topographic transition of central–northern Tibet since the early Miocene has created a consistently high and flat plateau similar to that of today. However, to date, the associated deep crust and/or /mantle events are poorly understood, mainly due to an early Miocene metamorphic–magmatic lull within the Qiangtang Block. To address this issue, we undertook a study of crustal xenoliths and zircon xenocrysts in 6.0–2.3 Ma lavas in the Qiangtang Block. The occurrence of 22.6–12.9 Ma high-temperature–low-pressure granulite xenoliths implies that the middle crust of the block has been very hot since that time. Zircon xenocrysts and granitic xenoliths from 6.0–2.3 Ma lavas were studied and shown to have high δ18O values, which supports Miocene crustal melting and the formation of unexposed, coeval felsic plutons. Combined with paleoelevation data from the Tibetan Plateau, our results suggest that the early Miocene cold–hot thermal transition of the middle–lower crust was near-synchronous with topographic evolution from high-relief mountains to a flat plateau, which supports crustal flow as the main topographic smoothing mechanism for central–northern Tibet.

Basaltic dykes and their xenoliths from the Gerroa–Kiama region, southern Sydney Basin, New South Wales: evidence for multiple intrusive episodes

The orientation, petrography and geochemistry of numerous basaltic dykes between Kiama and Gerroa were investigated in this study. Geochemical analysis shows that the dykes range from sub-alkaline to alkaline in composition. The dykes fit into three distinct geochemical groups, with most being transitional from alkali basalts to foidites, followed by alkali basalts and lastly andesites/basaltic andesites. We suggest that three intrusive episodes occurred based on petrographic analysis and geochemical classification. The dykes contain phenocrysts of olivine, plagioclase and clinopyroxene in a fine-grained groundmass. Multiple microtextures were found, including porphyritic, glomeroporphyritic, amygdaloidal, aphanitic and intergranular. Most dykes strike northwest–southeast, two strike east–west, and four strike northeast–southwest. One dyke to the south of Boat Harbour Reserve contains numerous xenoliths, including gabbroic, granitic and charnockitic types. The gabbroic xenoliths were likely sourced from the mid-crust, whereas the granitic xenoliths were likely derived from the upper crust. Joints in the study area strike northwest–southeast, north–south, east–west and northeast–southwest. The dominant joint set strikes northwest–southeast, and secondary joint sets strike northeast–southwest. The least pervasive joint sets strike east–west and approximately north–south. The dykes intruded along long-lived inherent structural weaknesses in the host rocks, as their orientations match the orientations of dominant joint sets. Geochemically, the dykes are distinct in having an abundance of foidites, which are rare within the Sydney Basin. They are clearly different from the nearby upper Permian K-rich Gerringong Volcanics, but many of the alkali basaltic dykes are like those of the Sydney Basin, and the foidites are similar in composition to the lower Eocene Kulnura basalt on the NSW Central Coast. The orientation, geographic distribution and geochemistry of the dykes strongly suggest that the basaltic magmas have been injected through the same upper-crustal weaknesses over a protracted period. KEY POINTS Dykes occurring between Kiama and Gerroa fall under three separate geochemical groups: alkali basalts to foidites, alkali basalts and andesites/basaltic andesites. Three intrusive episodes occurred based on petrographic analysis and geochemical classification. The dykes intruded along long-lived inherent structural weaknesses in the host rocks.

PALEOZOIC XENOLITHS IN EOCENE PLUTONS: THE EVIDENCE FOR THE DESTRUCTION OF PRE-JURASSIC CRYSTALLINE BASEMENT BENEATH ADJARA–TRIALETI BELT, LESSER CAUCASUS

In Georgia the Paleogene Adjara–Trialeti riftogenic belt (length 350 km, width 50–2 km) is dominantly composed of trachytic and trachytic–andesitic pyroclastic deposits, though plutonic rocks also play an important role in the structure. In this article, we report new data on the (LA-ICP-MS) U–Pb zircon geochronology and petrochemistry of the plutons, their xenoliths and restite from this belt. The results indicate that the magmatism in the basin began in the Early Eocene (~50 Ma) associated with the formation of pyroclastic rocks. The mafic intrusions (~46–44 Ma) led to the assimilation and contamination of sialic crust and formation of monzo-syenite melts emplaced at ~43–42 Ma. The Eocene monzo-syenite plutons contain xenoliths of Paleozoic granites (312±7 to 474±5 Ma) and tholeiitic basalts that contain inherited zircon grains ranging in age from Neo-Proterozoic (747±33 Ma, 632±29 Ma) to Cambrian (515±9 Ma). Paleozoic granite xenoliths show complete mineralogical and age similarity to the Adjara–Trialeti belt adjacent pre-Jurassic massif granites. Inherited zircon grains most likely are captured by magmas during ascent that cuts through the Gondwana-derived old continental crust. Obtained results and regional geological analysis demonstrate that the riftogenic basin of the Adjara–Trialeti belt developed on the pre-Jurassic crystalline basement, from Late Cretaceous to Eocene into a back-arc extensional regime to post-collision geodynamic setting.

THE U-Pb AGE AND Lu-Hf ISOTOPE SYSTEMATICS OF ZIRCON FROM THE HULIAIPOLE MET AVOLCANICS, THE AZOV DOMAIN OF THE UKRAINIAN SHIELD: EVIDENCE FOR THE PALEOARCHEAN-HADEAN CRUST

The Azov Domain occurs as a part of a larger Mesoarchean (3.2-3.0 Ga) craton, fragments of which are preserved in the eastern part of the Ukrainian Shield and as a block of the Kursk Magnetic Anomaly (KMA). In the Neoarchean-Palaeoproterozoic time, it was fragmented into several tectonic blocks: Vovcha, Remivka, Huliaipole, Bilotserkivka, and Saltych. The northern part of the Huliaipole Block is composed of tonalite-trondhjemite-granodiorite (TTG) rock association, that hosts the Kosivtsevo greenstone structure. It is composed of metamorphosed rocks of the jaspilite-komatiite-tholeiite association (the Kosivtsevo unit), which corresponds to the Sura Suite of the Konka Series of the Middle Dnieper Domain. The Neoarchean-Paleoproterozoic formations are represented by volcano-sedimentary rocks of the Huliaipole Suite and granitoids of the Dobropillya and Anadol complexes. Granitoids of the Dobropillya complex host numerous pyroxenite, gneiss, and plagioclase granite xenoliths. The U-Pb zircon age of granitoids of the Dobropillya Complex is 2040 Ma and inherited zircon has an age up to 3400 Ma. Small intrusions of two-feldspar granites of the Anadol Complex are widespread in the Ternuvate structure. Their U-Pb monazite age is 2190 Ma. In the central part of the Huliaipole Block, the NW-striking Huliaipole syncline (3.5 × 9 km) occurs. This structure is composed of volcano-sedimentary rocks of the Huliaipole Suite, which unconformably overlie Archean TTG. Felsic and intermediate metavolcanics are confined mainly to ferruginous quartzites of the middle Subsuites. To a limited extent, meta-andesites and felsic metavolcanics are also found in the lower and upper Huliaipole Subsuites. Zircons from meta-andesites and felsic metavolcanics of the Huliaipole Suite are very heterogeneous, indicating their crustal derivation. The U-Pb age of zircon populations from metadacite of the Huliaipole Suite was determined using the LA-ICP-MS method at 3085-2850 and 3700-3360 Ma. In addition, the age of the two crystals exceeded 3800 Ma. According to geological and geochronological data, the Huliaipole Block, 30 × 50 km in size, is composed of rocks and relicts of the Hadean, Archean, and Palaeoproterozoic eons. The oldest nucleoid of the Azov Domain.

The Cuitzeo granitic xenolith: evidence of an Early Miocene magma plumbing system in central Mexico

Relevant data on the structure and composition of the crystalline basement in Central México can be found by means of plutonic, metamorphic, and sedimentary xenoliths transported by Neogene and Quaternary volcanic eruptions within the Trans-Mexican Volcanic Belt (TMVB). We present detailed major oxide and trace elements concentrations, isotopic analysis and thermobarometric estimations for a granitic xenolith found in an Early Miocene ignimbrite in Cuitzeo Lake, Michoacán. The xenolith is a calc-alkaline quartz-plagioclase-K-feldspar-biotite-amphibole granite-monzogranite with 73.7 wt.% SiO2. Trace element and isotopic signatures are compatible with a volcanic arc signature. According to amphibole-plagioclase pairs and Ti-in-zircon thermobarometry, the studied xenolith suggests that the granitic system crystallized between 655–737 °C and 1.3–1.9 kbar. U-Pb isotopic analyses of zircon grains from this rock have provided a concordia age of 20.76 ± 0.11 Ma. The presence of granitic xenoliths in Quaternary eruptions produced by the TMVB are not rare. However, this is the first zircon age of a Miocene granitic rock showing evidence of the shallow plutonic counterpart of the magma plumbing system of the Early Miocene (~23 to ~16 Ma) Mil Cumbres - Angangueo voluminous, andesitic-dacitic-rhyolitic episode. Implications for faulting, erosion, and Miocene-Pliocene ignimbrite emplacement in Cuitzeo region are also discussed.

A window on mantle-derived magmas within the Central Andes: eruption style transitions at Cerro Overo maar and La Albóndiga lava dome, northern Chile

Cerro Overo maar and La Albondiga lava dome are two independent monogenetic volcanoes located in the Central Volcanic Zone of the Andes in northern Chile, close to the active Lascar and Chiliques volcanoes. Cerro Overo maar was formed <77 ka ago by explosive-effusive eruptions, including phreatomagmatic activity, while La Albondiga lava dome (Pleistocene) is the result of magmatic explosive-effusive activity alone. Cerro Overo and La Albondiga are characterized by deposits composed of pyroclastic rocks and dense coherent lava blocks. At Cerro Overo, these deposits are located around the crater rim, while at La Albondiga, they form coherent in situ lava dome features. Cerro Overo also displays thin ash- and lapilli-dominated deposit layers presumed to be pyroclastic surge deposits, which include juvenile pyroclasts exhibiting cauliflower textures, numerous exotic accidental lithics, and ballistically transported lapilli, bombs, blocks, and bomb sags. These fragments include recycled, juvenile pyroclasts, as well as material from older volcanic and intrusive rocks from the stratigraphic units immediately below. These small-volume volcanoes represent two of the less silicic volcanoes (~54 wt.% SiO2) in the northern Chilean Pleistocene to Holocene volcanic provinces. They are characterized by a fine grain size (mainly fine lapilli), olivine phenocrysts with skeletal textures, ignimbritic, dioritic, and granitic xenoliths, and quartz xenocrysts, with high concentrations of incompatible trace elements and light rare earth elements. The general magmatic evolution of the Cerro Overo and La Albondiga systems has been controlled by fast ascent (e.g., skeletal olivine phenocrysts) of mantle-derived magma associated with mixing, fractional crystallization, and a low degree of crustal assimilation during turbulent ascent processes. Such eruptions provide evidence that various factors play an essential role in the transition from explosive-effusive magmatic (Cerro Overo and La Albondiga) to phreatomagmatic (Cerro Overo) volcanic eruption styles.

Age of the basement beneath the Mesozoic Lusitanian Basin revealed by granitic xenoliths from the Papôa volcanic breccia (West Iberia)

The dyke of the Papôa volcanic breccia cross-cutting the Lower Jurassic sequence of the Lusitanian Basin (West Iberia) contains granitic xenoliths. In this study, for the first time, U-Th-Pb zircon analysis of two xenoliths yielded 298±4Ma for biotite granite and of 292±2Ma for two-mica granite, indicating that the pre-Mesozoic basement of the Lusitanian Basin includes Permian intrusions. These ages are close within the margin of error of the age of the Late Carboniferous granites of the Berlengas isle that with the Late Devonian high-grade metamorphic rocks of the Farilhões isles, located northwest of the study area, which form the pre-Mesozoic basement of the Lusitanian Basin. These new geochronological findings enable it to be established that Permo-Carboniferous magmatism lasted at least 13Ma, in this region of the Appalachian-Variscan belt. Furthermore, a comparison with available data from Paleozoic tectonic units of the Appalachian-Variscan belt located both in the Iberian Massif and outside it enables the suggestion to be made that the Lusitanian Basin (Peniche) most probably rests on the South Portuguese Zone, which may also be correlated with the Rhenohercynian Zone present in southwest England, and the Meguma terrane of Nova Scotia.

Composition of spherules and lower mantle minerals, isotopic and geochemical characteristics of zircon from volcaniclastic facies of the Mriya lamproite pipe

The article presents the results of studying the rocks of the pyroclastic facies of the Mriya lamproite pipe, located on the Priazovsky block of the Ukrainian shield. In them the rock's mineral composition includes a complex of exotic mineral particles formed under extreme reduction mantle conditions: silicate spherules, particles of native metals and intermetallic alloys, oxygen-free minerals such as diamond, qusongite (WC), and osbornite (TiN). The aim of the research is to establish the genesis of volcaniclastic rocks and to develop ideas of the highly deoxidized mantle mineral association (HRMMA), as well as to conduct an isotopic and geochemical study of zircon. As a result, groups of minerals from different sources are identified in the heavy fraction: HRMMA can be attributed to the juvenile magmatic component of volcaniclastic rocks; a group of minerals and xenoliths that can be interpreted as xenogenic random material associated with mantle nodules destruction (hornblendite, olivinite and dunite xenoliths), intrusive lamproites (tremolite-hornblende) and crystalline basement rocks (zircon, hornblende, epidote, and granitic xenoliths). The studied volcaniclastic rocks can be defined as intrusive pyroclastic facies (tuffisites) formed after the lamproites intrusion. Obviously, the HRMMA components formed under extreme reducing conditions at high temperatures, which are characteristic of the transition core-mantle zone. Thus, we believe that the formation of primary metal-silicate HRMMA melts is associated with the transition zone D".

Niobium Mineralogy of Pliocene A1-Type Granite of the Carpathian Back-Arc Basin, Central Europe

A1-type granite xenoliths occur in alkali basalts erupted during Pliocene–Pleistocene continental rifting of Carpathian back-arc basin (Central Europe). The Pliocene (5.2 Ma) peraluminous calc-alkalic granite contains unusually high concentrations of critical metals bound in Nb, Ta, REE, U, Th-oxides typical for silica-undersaturated alkalic granites, and syenites: columbite-Mn, fergusonite-Y, oxycalciopyrochlore, Nb-rutile, and Ca-niobate (fersmite or viggezite). In contrast, it does not contain allanite and monazite—the main REE-carriers in calc-alkalic granites. The crystallization of REE-bearing Nb-oxides instead of OH-silicates and phosphates was probably caused by strong water deficiency and low phosphorus content in the parental magma. Increased Nb and Ta concentrations have been inherited from the mafic parental magma derived from the metasomatized mantle. The strong Al- and Ca-enrichment probably reflects the specific composition of the mantle wedge modified by fluids, alkalic, and carbonatitic melts liberated from the subducted slab of oceanic crust prior to the Pliocene-Pleistocene rifting.

Carboniferous 40Ar/39Ar Age of Rare-Metal Enriched Rhyolites and Ignimbrites in the Sakmara Allochthon (Southern Urals)

The Sakmara Allochthon (Southern Urals) comprises a widely distributed folded stratum of rhyolites and ignimbrites, which are characterized by Nb–Zr–REE geochemical specification. This stratum overlies unconformably folded Paleozoic complexes, including Devonian ones. Rhyolites contain potassium feldspar and quartz phenocrysts, as well as granite xenoliths. According to geochemical features, volcaniс rocks were formed under the intraplate setting; in terms of the chemical composition, they belong to the A-type granites. The 40Ar/39Ar age of the felsic rock groundmass of rhyolites is 303 ± 2 Ma. The feldspar phenocrysts yielded ages of 306 ± 3 and 337 ± 3 Ma. The first estimate, as well as the matrix age, is associated with the formation of the volcanic complex, while the second value is probably associated with the age of xenogenic material. The complex of rare-metal enriched rhyolites characterizes the stage of the terminal Late Carboniferous rift magmatism in the rear part of the active continental margin. This complex formed prior to collision and thrusting of ophiolitic allochthons.

CARBONIFEROUS 40Ar/39Ar AGE OF THE RARE METAL-ENRICHED RHYOLITES AND IGNIMBRITES IN THE SAKMARA ALLOCHTHON OF THE SOUTHERN URALS, THEIR GEOCHEMICAL FEATURES AND GEODYNAMIC SETTING

In the structure of the Sakmara allochthon of the Southern Uralssequence with rhyolites and ignimbrites locally occures. They have Nb-Zr-REE geochemical specialization. This sequence unconformably overlays folded Paleozoic complexes, including the Devonian ones. Rhyolite contains K-feldspar and quartz phenocrysts, K-feldspar glomeroporphyrites and granite xenolith. Geochemical features of the rhyolites show intraplate-originated affinities and A-type granite composition.40Ar/39Ar age of the felsitic matrix of the rhyolites of 303±2 Ma defines the age of the volcanic complex origin. For feldspar phenocrysts the age of 306±3 Ma and 337±3 Ma is obtained. The first value coincides to the matrix age and connected with formation of the volcanic complex. The second value belongs, apparently, to xenogenic material. Obtained age values reflect the evolution of Carboniferous active continental margin magmatism, widespread in different structural zones of the Urals. Rare-metal rhyolites characterize the final late Carboniferous intraplate (rift-related) back-arc magmatism at the active continental margin. Volcanism preceded to a collision-related ophiolitic thrust nappes emplacement.

Petrogenesis of granitoids and associated xenoliths in the early Paleozoic Baoxu and Enping plutons, South China: Implications for the evolution of the Wuyi-Yunkai intracontinental orogen

The early Paleozoic Wuyi-Yunkai orogen was associated with extensive felsic magmatic activities and the orogenic core was mainly distributed in the Yunkai and Wugong domains located in the western Cathaysia block and in the Wuyi domain located in the central part of the Cathaysia block. In order to investigate the evolution of the Wuyi-Yunkai orogen, elemental and Sr-Nd isotopic analyses were performed for granites from the Baoxu pluton in the Yunkai domain and from the Enping pluton in the central part of the Cathaysia block. The Baoxu pluton consists of biotite granite with abundant xenoliths of gneissic granite, granodiorite and diorite, and the Enping pluton is mainly composed of massive granodiorite. Biotite granites (441 ± 5 Ma) and gneissic granite xenolith (443 ± 4 Ma) of the Baoxu pluton are all weakly peraluminous (A/CNK = 1.05–1.10). They show high Sr/Y and La/Yb ratios and have negative bulk-rock εNd(t) values (−7.0 to −4.4), which are similar to coeval gneissic S-type granites in the Yunkai domain and were probably derived from dehydration melting of a sedimentary source with garnet residue in the source. Granodiorites (429 ± 3 Ma) from Enping and granodiorite xenolith (442 ± 4 Ma) from Baoxu are metaluminous and have REE patterns with enriched light REE and flat middle to heavy REE, possibly generated by the dehydration melting of an igneous basement at middle to lower crustal level. Diorite xenolith from Baoxu is ultrapotassic (K2O = 4.9 wt%), has high contents of MgO (7.0 wt%), Cr (379 ppm) and Ni (171 ppm) and shows pronounced negative Nb, Ta and Ti anomalies. This xenolith also has negative εNd(t) value (−3.6) and low Rb/Ba and high Ba/Sr ratios, and is thus interpreted to be derived from an enriched lithospheric mantle with the breakdown of phlogopite. Early Paleozoic I- and S-type granites in the Wuyi-Yunkai orogen mostly have negative εNd(t) values and do not have juvenile components, consistent with genesis by an intracontinental orogenic event. These early Paleozoic granites occur near the ancient suture zone between the Yangtze and Cathaysia blocks and have high La/Yb and Sr/Y ratios, likely due to the existence of residual garnet in the source, suggesting the thickened crust at ca. 440 Ma. The 450–440 Ma gneissic S-type granites near the suture zone are earlier than those in the central part of the Cathaysia block (∼430 Ma). The crustal thickening along the ancient suture zone at 440 Ma propagated into the central part of the Cathaysia block as evidenced by the 430 Ma granites. Early Paleozoic I-type granites near the suture zone clearly show involvement of significant mantle-derived materials, in contrast to granites in the central part of the Cathaysia block. The ancient suture zone may have acted as channels for the emplacement of mafic magmas during the collapse of an intracontinental orogen.

Elemental and Sr-Nd-Pb isotope geochemistry of the Florianópolis Dyke Swarm (Paraná Magmatic Province): crustal contamination and mantle source constraints

The Florianópolis Dyke Swarm is located in Santa Catarina Island, comprising also the adjacent continental area, and belongs to the Paraná Magmatic Province (PMP). The dyke outcrops in the island are 0.1–70m thick and most of them are coast-parallel (NE-SW trending), with subordinate NW-SE trending. The vast majority of the dykes has SiO2 varying from 50 to 55wt% and relatively high-Ti (TiO2>3wt%) contents and these rocks were divided using the criteria commonly used to distinguish the different magma-types identified in the volcanic rocks from the PMP. The Urubici dykes (Sr>550μg/g) are the most abundant and some of them experienced crustal contamination reaching to 10%, as evidenced by low P2O5/K2O (0.30–0.21), high (Rb/Ba)PM (1.0–2.2), and radiogenic Sr and Pb isotope compositions (87Sr/86Sri up to 0.70716 (back to 125Ma) and 206Pb/204Pbm up to 19.093). The Pitanga (Sr<550μg/g) and the basaltic trachyandesite dykes are less abundant and almost all of them were also substantially affected by at least 15% of crustal assimilation, evidenced by high (Rb/Ba)PM (up to 2.6) and Sr (87Sr/86Sri=0.70737–0.71758) and Pb (206Pb/204Pbm=18.446–19.441) isotope ratios, as well as low P2O5/K2O values (0.30–0.18). The low-Ti (TiO2<2wt%) dykes are scarce and show a large compositional variability (SiO2: 50.4–64.5wt%), with similar geochemical characteristics of the low-Ti volcanic rocks (Gramado-Palmas) from southern PMP, although the most primitive dykes show hybrid characteristics of Ribeira and Esmeralda magmas. The presence of granitic xenoliths with border reactions and dykes with diffuse contacts indicate that crustal contamination probably occurred by assimilation from re-melted the host rocks. Considering only the high-Ti Urubici dykes that were not affected by crustal contamination, the Sr, Nd and Pb isotope mixing modelling indicates the participation of a heterogeneous metasomatized (refertilized) subcontinental lithospheric mantle (SCLM). This mantle source was originated by partial melting of a depleted sublithospheric mantle (DMM – Depleted Mantle MORB), which was hybridized by addition of pyroxenite (<5%) and carbonatite (up to 2%) melts. The isotope mixing modelling also points to a significant participation (up to 50%) of Archean SCLM, not evidenced in the mantle sources of the northern PMP high-Ti Pitanga flows (dominated by Neoproterozoic SCLM).

Geochemistry, mineralogy, and zircon U–Pb–Hf isotopes in peraluminous A-type granite xenoliths in Pliocene–Pleistocene basalts of northern Pannonian Basin (Slovakia)

Anorogenic granite xenoliths occur in alkali basalts coeval with the Pliocene–Pleistocene continental rifting of the Pannonian Basin. Observed granite varieties include peraluminous, calcic to peralkalic, magnesian to ferroan types. Quartz and feldspars are dominant rock-forming minerals, accompanied by minor early ilmenite and late magnetite–ulvospinel. Zircon and Nb–U–REE minerals (oxycalciopyrochlore, fergusonite, columbite) are locally abundant accessory phases in calc-alkalic types. Absence of OH-bearing Fe, Mg-silicates and presence of single homogeneous feldspars (plagioclase in calcic types, anorthoclase in calc-alkalic types, ferrian Na-sanidine to anorthoclase in alkalic types) indicate water-deficient, hypersolvus crystallization conditions. Variable volumes of interstitial glass, absence of exsolutions, and lacking deuteric hydrothermal alteration and/or metamorphic/metasomatic overprint are diagnostic of rapid quenching from hypersolidus temperatures. U–Pb zircon ages determined in calcic and calc-alkalic granite xenoliths correspond to a time interval between 5.7 and 5.2 Ma. Positive eHf values (14.2 ± 3.9) in zircons from a 5.2-Ma-old calc-alkalic granite xenolith indicate mantle-derived magmas largely unaffected by the assimilation of crustal material. This is in accordance with abundances of diagnostic trace elements (Rb, Y, Nb, Ta), indicating A1-type, OIB-like source magmas. Increased accumulations of Nb–U–REE minerals in these granites indicate higher degree of the magmatic differentiation reflected in Rb-enrichment, contrasting with Ba-enrichment in barren xenoliths. Incipient charnockitization, i.e. orthopyroxene and ilmenite crystallization from interstitial silicate melt, was observed in many granite xenoliths. Thermodynamic modeling using pseudosections showed that the orthopyroxene growth may have been triggered by water exsolution from the melt during ascent of xenoliths in basaltic magma. Euhedral-to-skeletal orthopyroxene growth probably reflects contrasting ascent rates of basaltic magma with xenoliths, intermitted by the stagnation in various crustal levels at a <3 kbar pressure. The Tertiary suite of intra-plate, mantle-derived A1-type granites and syenites is geochemically distinct from pre-Tertiary, post-orogenic A2-type granites of the Carpatho–Pannonian region, which exhibit geochemical features diagnostic of crustal melting along continental margins.