Alkali Granite Research Articles

Chemical and Nd isotope constraints on granitoid sources involved in the Caledonian Orogeny in Scotland

Major- and trace-element data and Nd isotope compositions for granitoid samples from the Grampian Highlands in Scotland show a systematic evolution in the composition of their sources in the course of the Caledonian Orogeny. Granitoids of 511–451 Ma, related to the collision of the Midland Valley island arc with the Grampian terrane, show S-type affinity and fractionated REE patterns with minor Eu anomalies and low initial ϵ Nd values of −14.1 to −11.2 suggesting melting of predominantly Dalradian metasediments. Subsequently formed granitoids of 425–406 Ma derived from an assumed Andean plate margin comprise a wide spectrum of rock types including I-type granite–granodiorite, and S-type granitoids, monzonites and alkali granites. The trace-element patterns of these rocks and the range of initial ϵ Nd values of −2.1 to −6.9 are consistent with melting of variably rejuvenated crust as found in continental margin settings. We conclude that the Grampian Highlands were affected by two major crust-modifying events during the Caledonian Orogeny: predominantly recycling of older crust during docking of the Midland Valley arc and addition of juvenile, mantle-derived material to the crust during the convergence of Avalonia with Laurentia.

Late Riphean alkali granites of the Zabhan microcontinent: Evidence for the timing of Rodinia breakup and formation of microcontinents in the Central Asian Fold belt

Late Riphean alkali granites of the Zabhan microcontinent: Evidence for the timing of Rodinia breakup and formation of microcontinents in the Central Asian Fold belt

Termination of the Permo-Triassic Songrim (Indosinian) orogeny in the Ogcheon belt, South Korea: Occurrence of ca. 220 Ma post-orogenic alkali granites and their tectonic implications

The ‘Indosinian Orogeny’ is defined in a broad sense as encompassing all the Late Paleozoic to Early Mesozoic tectonic events associated with the closure of the eastern parts of the Paleo-Tethys Ocean. It includes the Permo-Triassic continent–continent collision between the North China and South China blocks that generated the Qinling–Dabie–Sulu orogen containing the largest ultrahigh-pressure (UHP) terrane in the world. The Korean Peninsula, situated at the easternmost end of the Tethyan tectonic regime, was also tectonically influenced by the Indosinian Orogeny, often known as the ‘Songrim Orogeny’ in Korea. The Songrim Orogeny, characterized by intense regional medium- to high-pressure metamorphism, extends from the Imjingang belt through the Gyeonggi massif to the Ogcheon belt. It was initiated as early as the Early Permian Period (ca. 290 Ma) and peaked in the Triassic Period (ca. 255 to 225 Ma). This paper presents new geochemical and SHRIMP U–Pb zircon ages for two alkali granites that postdated the Songrim Orogeny in the Ogcheon belt. Both granites are contemporaneous (219.3 ± 3.3 Ma and 219.6 ± 1.9 Ma) and are geochemically post-orogenic A-type granites intruded in an extensional setting subsequent to a major collision. The data provide a critical time constraint on the termination of the Indosinian Orogeny in the Korean Peninsula. Despite the absence of an UHP terrane in the Korean Peninsula, similarities in the duration, metamorphic characteristics, and post-orogenic magmatism between the Qinling–Dabie–Sulu and Songrim orogens, as parts of the broad Indosinian orogenic regime, are clearly alike, suggesting a genetic linkage in their time–space characteristics.

Petrogenesis of an Alkali Syenite-Granite-Rhyolite Suite in the Yanshan Fold and Thrust Belt, Eastern North China Craton: Geochronological, Geochemical and Nd-Sr-Hf Isotopic Evidence for Lithospheric Thinning

The Yanshan Fold and Thrust Belt in eastern China has been intruded by a series of alkalic igneous rocks, ranging in composition from granite and rhyolite to syenite and trachyte. Laser ablation inductively coupled plasma mass spectrometry U^Pb analyses of zircon from three alkaline suites yield Early Cretaceous ages of 130^117Ma. Three groups of rocks have been identified based on their mineralogical, geochemical and Sr^Nd^Hf isotope characteristics. The alkali granites and rhyolites are ferroan and have low Al2O3, MgO, CaO, Sr, Ba and Eu concentrations and high SiO2, total Fe2O3, K2O, Nb, Ga, Ta, Th and heavy rare earth element abundances and Ga/Al ratios. Geochemical data and Sr-, Ndand zircon Hf-isotopic compositions [(Sr/Sr)i1⁄4 0 7050^0 7164, eNd(t)1⁄4 8 4 to 13 6 and eHf(t)1⁄4 5 7 to 16 8] indicate that they were probably generated by shallow dehydration melting of biotiteor hornblende-bearing granitoid crustal source rocks and then mixed with contemporaneous magma from a mantle and/or lower crustal source. Ferroan syenites have distinct geochemical features from those of the alkaline granites and rhyolites, suggesting that they were produced by clinopyroxene and plagioclase fractionation of melt derived from an enriched mantle source, mixed with lower and upper crustal-derived magmas. The magnesian syenites and trachytes have Sr-, Ndand zircon Hf-isotopic compositions that are distinct from those of the ferroan syenites. They were mainly derived from partial melting of lower crustal materials, mixed with enriched mantle-derived alkali basaltic magma.The emplacement of an alkali syenite^granite^rhyolite suite, coeval with the formation of metamorphic core complexes and pull-apart basins in eastern China, indicates they formed in an extensional setting, possibly as a result of lithospheric thinning.

Two types of magma sources of rare-metal alkali granites

The geological setting, age, and magma sources of rare-metal ores at the Khalzanburgetdei deposit in western Mongolia and the Khan Bogd occurrence in southern Mongolia are considered. The Khalzanburgetdei deposit, 392 Ma in age, is situated in the Lake Zone of the Early Caledonides of the Central Asian Foldbelt at a branch of a triple junction of grabens. The Khan Bogd occurrence, about 290 Ma in age, is located in the South Mongolian Hercynian Zone and related to a continental rift conjugated in space and time with Hercynian island-arc systems. To estimate features of rare-metal magma sources, the Nd and O isotopic compositions were used, as well as ratios of incompatible elements (Nb-U, Nb-Zr, La-Yb, Th-Ta, and Ce-Pb). Since the granitic magma was commonly saturated with these elements, their ratios may be used for estimation of magma source compositions, trends of magma fractionation, and accumulation of rare-metal minerals. A mixture of an OIB source and crustal material (Caledonian ophiolitic and island-arc complexes) served as a source of rare-metal granitic rocks of the Khalzanburgetdei deposit, while the Khan Bogd occurrence was related to a source of subduction-related basic rocks, probably, with participation of the depleted and enriched mantle and continental crust.

High radiogenic heat production in the Kerala Khondalite Block, Southern Granulite Province, India

In situ radioelemental (K, U and Th) analysis and heat production estimates have been made at 59 sites in the Kerala Khondalite Block (KKB) of the Southern Granulite Province (SGP) of India. Together with the in situ analyses on granulites and gneisses previously reported from 28 sites, and heat production estimated from the published geochemical analyses on granites and syenites of the KKB, the new data set allows good characterization of heat production for the major granulite facies rocks and granitoids of the KKB. Garnet biotite gneisses are characterized by high levels of Th and U, with mean values of 60 and 3 ppm, respectively. Khondalites, leptynites and charnockites have slightly lower levels of Th (23, 20 and 22 ppm, respectively) and U (2.9, 2.4 and 0.9 ppm, respectively). The mean K, U, Th abundances for the granites, leucogranites and granitic gneisses ranges from 3.9 to 4.3%, 2.6 to 4.3 ppm, 22 to 50 ppm respectively, and for the syenites 4.8%, 2 ppm and 5.7 ppm. Mean radiogenic heat production values for garnet–biotite gneiss, khondalite, leptynite and charnockite are 5.5, 2.7, 2.4 and 2.2 μW m−3, respectively. For the granites, leucogranites, granitic gneisses and syenites it is 2.6, 3.4, 4.6 and 1.4 μW m−3, respectively. Heat production of granulite facies rocks, which are the most abundant rocks in KKB, correlate well with Th, but less with U, suggesting that variation is caused by Th and U bearing accessory minerals such as monazite and zircon. The high heat production of the KKB granulites are in contrast to the low heat production of the Late Archaean granulites of the Northern Block (NB) of the SGP which are highly depleted in radioelements and also the granulites of Madurai Block (MB) that have higher radioelemental abundances than in the granulites of the NB. The high heat production of the KKB granulites could be due to the nature of protoliths and/or metasomatism associated with Neoproteroic- to- Pan African alkaline magmatic activity represented by alkali granite and syenite–carbonatite emplacements and emplacement of pegmatites.

Metallogeny and geodynamics of the Aktiuz–Boordu Mining District, Northern Tien Shan, Kyrgyzstan

The Aktiuz–Boordu Mining District is located in the Northern Tien Shan in the eastern part of Kyrgyzstan. The region is characterized by nappe-folding structures and comprises strongly deformed Precambrian and Lower Paleozoic sedimentary, volcano-sedimentary, and metamorphic sequences. Metamorphic rocks are represented by crystalline schists, para- and orthogneisses, marble, migmatite, amphibolite and eclogite lenses. These rocks are thought to be the oldest in the Northern Tien Shan. The 2780-Ma Aktiuz complex has a total thickness of 2800–2900 m. Available U–Pb zircon age data for the Kemin Group migmatites yield ages of 2200 ± 50 Ma. Based upon their geological setting, multistage metamorphism and isotopic ages of retrograde metamorphism (1.1–1.9 Ga), these rocks reach a few kilometers in thickness and are subdivided into Archaean and Paleoproterozoic. The Archaean and Paleoproterozoic basement metamorphic rocks contain mineralization of various ages and types, including porphyry Cu, Au-sulphide, Au–Bi, barite, epithermal base metal and Au–Ag, REE and rare-metals. Two ore fields have been identified within the Aktiuz–Boordu Mining District, they are: (1) Taldybulak–Boordu, with Au, base metal, and porphyry Cu systems; (2) Aktiuz, with REE, rare- and base-metal deposits. Within the Paleozoic Taldybulak–Boordu volcanic structure, deposits and occurrences of Au are present at Taldybulak Levoberezhny, Chimbulak Zapadny, Karamoko and Kuranjailyau; of Pb at Boordu, Taldybulak Stary, Chimbulak Vostochny and Chimbulak Zapadny; of Mo at Karabulak, and of Cu at Berkut-Kashka. Almost all are found along the periphery of deeply eroded volcanoes. Rock types in the Taldybulak–Boordu ore field mainly comprise chlorite–amphibole and amphibole schists, amphibolites, and migmatites of the Paleoproterozoic Kuperlisay suite. The younger Paleoproterozoic Kokbulak and Kapchigay suites are represented by mica schists and granite gneisses, which are separated from the Kuperlisay suite by a fault. The basement rocks are cut by a complex network of Paleozoic sub-1 volcanic intrusions, accompanied by eruptive rocks and breccia pipes. These represent the root system of Middle and Late Paleozoic volcanoes. At the Taldybulak Levoberezhny Au deposit, the principal ore-controlling structures are inter- and intraformational shear, thrust–fault zones and tectonic mélange injected by intrusions that underwent metasomatic alteration. Gold-sulphide disseminated mineralization is associated with the upper part of Taldybulak shear zone and is associated with monzodiorites and quartz–tourmaline veinlets. The Aktiuz ore field is located in an uplifted block of the basement rocks, including gneisses and crystalline schists (the Archean Aktiuz complex and Paleoproterozoic Kemin Group). They are intruded by Neoproterozoic mafic and ultramafics rocks, Late Ordovician–Silurian collision granites and granodiorites, Devonian monzodiorites, alaskites, alkalic and subalkalic two-feldspar granites, and Permian–Triassic syenites, subalkalic leucogranites, granophyres and granite-porphyries. The Kuperlisay leucogranite intrusion is an elongated body with several ridge-like projections of granophyres. For the Kuperlisay leucogranite a K–Ar age of 260 Ma was obtained, whereas the K–Ar age of granophyre is 225–230 Ma. Analyses of the Aktiuz granite samples taken from the Kutessay open pit yielded an age of 416 Ma (U–Pb zircon SHRIMP method). The Aktiuz ore field contains deposits of Th at Kuperlisay and Kutessay II, and of REE and base metals at Aktiuz and at Kalesay. All are hosted by leucocratic alkali granites, granophyres, granite-porphyries and albitites. Significant reserves and resources of REE and base metals ores for the Aktiuz and Kutessay II deposits make them economically important.

Petrogenesis of two granites from the Nilgiri and Madurai blocks, southwestern India: Implications for charnockite–calc-alkaline granite and charnockite–alkali (A-type) granite link in high-grade terrains

Igneous charnockites share some of the characteristics of calc-alkaline and alkali (A-type) granites, and together constitute important rock types in many high-grade terrains. The Neoproterozoic Kalpatta and Munnar biotite-hornblende granites intrude the upper-amphibolite to granulite-facies terrain of southwestern India. Geochemical characteristics of the Kalpatta granite are similar to high-K calc-alkaline magnesian granitoids, whereas those of the Munnar granite are similar to alkali ferroan granitoids. Within the constraints imposed by the high temperature, K-rich nature of the magmas, comparison with experimental studies on various granitoid source compositions, and trace- and rare-earth-element modeling, the distinctive features of both granites reflect a source rock of charnockitic nature. In this context, the northern Kerala (NKM) and Cardamom hill (CM) charnockite massifs, occurring near the granites, were considered as probable source rock compositions. Both the NKM and CM charnockites consists of an intermediate (low SiO 2) type and silicic (high SiO 2) type, with the intermediate type showing similarities to high-Ba–Sr granitoids with low K 2O/Na 2O ratios and the silicic type showing similarities to high-Ba–Sr granitoids with high K 2O/Na 2O ratios. The proposed petrogenetic model involves partial melting of the intermediate NKM charnockite forming the Kalpatta calc-alkaline granite. In contrast, the Munnar alkali (A-type) granite was the product of melting, followed by fractional crystallization of the intermediate CM charnockite. It is suggested that granitoids that formed by melting of intermediate type charnockites show high-K calc-alkaline magnesian geochemical characteristics, whereas those formed by intermediate type charnockite melting-fractional crystallization show alkali ferroan geochemical characteristics.

Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S)

Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47°S–71°30′W), and occur together with scarce trachytic lava flows. Whole-rock K–Ar ages between 3.98 and 3.08 Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic–felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A 1-type granitoid affinities and are characterized by high silica and alkali contents (60–68 wt.% SiO 2; 8.7–10.8 wt.% Na 2O + K 2O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts (( 87Sr/ 86Sr) o = 0.70488–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512645). Nevertheless, some of them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province (( 87Sr/ 86Sr) o = 0.70556–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A 1-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-arc tectonic settings. In Central Patagonia, these ∼ 3–4 Ma old alkaline intrusions occur aligned along a ∼ N160–170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area, related to major regional tectonics occurring in possible relation with the collision of one segment of the Chile Spreading Ridge with the trench.

Origin of I- and A-type granitoids from the Eastern Desert of Egypt: Implications for crustal growth in the northern Arabian–Nubian Shield

I- and A-type granitoid rocks, emplaced during pre- and post-collision stages, respectively, of the Neoproterozoic Pan-African Orogeny, are widely distributed in the Eastern Desert of Egypt, constituting ∼60% of the basement outcrop. Petrological and geochemical data are presented for a selection of the two groups, the I-type, El Bula tonalite–granodiorite suite, and the A-type, Lômân alkali granites, with the aim of discussing their origin and geotectonic implications. The El Bula (EB) rocks have geochemical characteristics of medium-K calc-alkaline, metaluminous to mildly peraluminous, granitoids formed in an island-arc environment. The Lômân (LM) granites display midalkaline, metaluminous, post-orogenic, A-type characteristics. With respect to the EB granitoids, the LM granites contain lower Al 2O 3, Fe 2O 3, MgO, MnO, CaO, TiO 2, Sr, Ba, and V, but higher Na 2O, K 2O, Nb, Zr, Th, and Rb. The I-type granitoids were presumably formed by high degrees of partial melting (∼40%) of a mafic (amphibolitic), lower crustal source, whereas the A-type granites are derived from a tonalitic, middle crustal source, followed by some crystal fractionation. Such high degrees of partial melting attest to the large areal distribution of these rocks and require broad thermal anomalies, likely related to significant geodynamic features of the Arabian–Nubian Shield (ANS) evolution. We propose petrogenetic models involving an upwelling of hot asthenospheric mantle, due to oblique convergence during the pre-collision stage, and following a lithospheric delamination during the post-collision stage. Such asthenosphere uprise could account for the high crustal growth rate of the ANS.

Phenocryst-poor rhyolites of bimodal, tholeiitic provinces: the Rattlesnake Tuff and implications for mush extraction models

We consider the origin of rhyolites associated with tholeiitic basalt in bimodal provinces, as exemplified by the Rattlesnake Tuff of the High Lava Plains of eastern Oregon, in comparison to rhyolites associated with calcalkaline suites in light of recent models of extraction of rhyolite from crystal mush (Hildreth, J Volcanol Geotherm Res, 136:169–198, 2004; Bachmann and Bergantz, J Petrol, 45:1565–1582, 2004). The High Lava Plains encompass a strongly bimodal, tholeiite-rhyolite suite, spatially and compositionally related to the Snake River Plain and Yellowstone Plateau. In our assessment we draw the distinction between fractionation dominated processes to make rhyolites from rhyolites and processes required to make the parental rhyolite melt. New isotopic data and compositional zoning profiles in phenocrysts confirm that crystal fractionation dominated the generation of progressively more evolved, discrete rhyolites in the zoned Rattlesnake Tuff and are consistent with an origin of the least evolved high-silica rhyolites by partial melting of a mafic crust. While the most evolved rhyolites are compositionally virtually indistinguishable from those of calcalkaline suites, the parental rhyolites from bimodal suites are more Fe-rich than their calcalkaline counterparts. Oxygen isotope thermometry yields pre-eruptive temperatures of 860°C, in keeping with 800–880°C zircon saturation temperatures. High magmatic temperatures are common among rhyolites of bimodal suites, distinguishing them from cooler rhyolites of calcalkaline suites. Extraction of interstitial melt from a granodioritic mush cannot produce compositions of the Rattlesnake Tuff on the basis of major and trace element arguments (especially Fe, Ba, Sr, and Eu) and on the basis of temperature considerations. Chemically viable parental crystal mushes are syenite and alkali (A-type) granites for the production of all more evolved Rattlesnake Tuff rhyolites; ferro-dacitic mush is required for production of the least-evolved, parental Rattlesnake Tuff rhyolite. Paucity of such ferro-dacitic compositions in tholeiitic bimodal suites, especially compared to the abundance of dacitic (granodioritic) compositions in calcalkaline suites, argues against the mush extraction model for the parental rhyolite. Furthermore, rhyolites of bimodal suites lack associated voluminous eruptions of crystal-rich ignimbrite that might represent a parental mush, as exemplified by the “monotonous intermediate” Fish Canyon Tuff in calcalkaline suites. We conclude that extensive fractionation is common among rhyolites and may obscure their ancestry. Fe-rich parental rhyolites common in bimodal tholeiitic suites, as represented by Rattlesnake Tuff, may often be the result of partial melting of mafic to intermediate crust, in contrast to calcalkaline high-silica rhyolites that are related to voluminous suites of intermediate intrusive rocks where the pre-plutonic mush-extraction model works better.

Petrochemistry of the Siah-Kuh granitoid stock southwest of Kerman, Iran: Implications for initiation of Neotethys subduction

The Upper Triassic Siah-Kuh granitoid stock is part of a series of Triassic-Cretaceous intrusions in the Phanerozoic Sanandaj-Sirjan zone in the Zagros orogenic belt, south-central Iran. The Siah-Kuh stock intrudes Lower Paleozoic amphibolites of the Abshur metamorphic complex and Upper Devonian-Lower Carboniferous greenschists of the Sargaz metamorphic complex. The Siah-Kuh granitoid stock consists mainly of coarse to medium grained leucogranodiorite, leucomonzogranite and alkali granite with subordinate syenite. It also contains a number of mafic enclaves of different sizes and is intruded by alkali granite and granophyre dikes along with monzonitic and a few dioritic dikes. The Siah-Kuh granitoid rocks are characterized by enrichment in large ion lithophile elements (LILE) such as Rb, Ba, K, Ce and depletion in high field strength elements (HFSE) such as Y, Nb and Zr. The chondrite normalized REE patterns are characterized by moderate LREE enrichment [(La/Yb) N = 1.31–4.82] and unfractionated HREE [(Gd/Yb) N = 0.50 to 1.33]. Granodiorites with the least fractionated HREE and a general absence of Eu anomalies suggest the involvement of plagioclase and absence of garnet during the melting processes. Alkali granites with the highest (La/Yb) N values and pronounced negative Eu anomalies are more differentiated products. Nd isotope analyses of the Siah-Kuh granitoids yield an “errorchron” age of 199 ± 30 Ma (MSWD = 1.30), consistent with the field relationships and give Nd model ages ( T DM) from 0.81 to 1.64 Ga and εNd( T) values from +1.81 to +2.45 at 200 Ma. These geochemical data suggest that the Siah-Kuh granitoid stock has characterisitics of metaluminous to slightly peraluminous, calc-alkaline, I-type granite formed in a volcanic arc setting. The onset of subduction of the Neotethys oceanic crust beneath the Central Iranian microcontinent in Triassic time could have accounted for the continental arc volcanism. Dehydration of subducted oceanic crust and partial melting of mantle wedge caused partial melting of subcontinental lithosphere, which resulted in the formation of metasomatised and enriched mafic arc magmas, at variable water fugacity and led to the formation of the Siah-kuh granitoid rocks.

Geology, Geochronology, and Geodynamics of the Khan Bogd alkali granite pluton in southern Mongolia

The Khan Bogd alkali granite pluton, one of the world’s largest, is situated in the southern Gobi Desert, being localized in the core of the Late Paleozoic Syncline, where island-arc calc-alkaline differentiated volcanics (of variable alkalinity) give way to the rift-related bimodal basalt-comendite-alkali granite association. The tectonic setting of the Khan Bogd pluton is controlled by intersection of the near-latitudinal Gobi-Tien Shan Rift Zone with an oblique transverse fault, which, as the rift zone, controls bimodal magmatism. The pluton consists of the eastern and the western ring bodies and comes into sharp intrusive contact with rocks of the island-arc complex and tectonic contact with rocks of the bimodal complex. The inner ring structure is particularly typical of the western body and accentuated by ring dikes and roof pendants of the country island-arc complex. According to preliminary gravity measurements, the pluton is a flattened intrusive body (laccolith) with its base subsiding in stepwise manner northwestward. Reliable geochronologic data have been obtained for both plutonic and country rocks: the U-Pb zircon age of alkali granite belonging to the main intrusive phase is 290 ± 1 Ma, the 40Ar/39Ar ages of amphibole and polylithionite are 283 ± 4 and 285 ± 7 Ma, and the Rb-Sr isochron yields 287 ± 3 Ma; i.e., all these estimates are close to 290 Ma. Furthermore, the U-Pb zircon age of red normal biotite granite (290 ± 1 Ma) and the Rb-Sr age of the bimodal complex in the southern framework of the pluton are the same. The older igneous rocks of the island-arc complex in the framework and roof pendants of the pluton are dated at 330 Ma. The geodynamic model of the Khan Bogd pluton formation suggests collision of the Hercynian continent with a hot spot in the paleoocean; two variants of this model are proposed. According to the first variant, the mantle plume, after collision with the margin of the North Asian paleocontinent, reworked the subducted lithosphere and formed a structure similar to an asthenospheric window, which served as a source of rift-related magmatism and the Khan Bogd pluton proper. In compliance with the second variant, the emergence of hot mantle plume resulted in flattening of the subducted plate; cessation of the island-arc magmatism; and probably in origin of a local convective system in the asthenosphere of the mantle wedge, which gave rise to the formation of a magma source. The huge body of the Khan Bogd alkali granite pluton and related volcanic rocks, as well as its ring structure, resulted from the caldera mechanism of the emplacement and evolution of magmatic melts.

Weathering damage evaluation of rock properties in the Bunhwangsa temple stone pagoda, Gyeongju, Republic of Korea

The stone pagoda of the Bunhwangsa temple in Republic of Korea was made of piling small brick-shaped stones. The majority of stone bricks are andesitic rocks with variable geneses. Rock properties of the pagoda roof suffer partial significant deterioration, such as multiple peel-offs, exfoliation, onion-peel-like decomposition, cracks forming round lines and falling-off stone pieces. The stylobates and tabernacles at the four corners are composed of granitic rocks, which are heavily contaminated by lichens and mosses. Some of these contamination marks show dark black or yellowish brown colors by inorganic secondary hydrates. The four tabernacles and northern face of the pagoda body have been exposed to relatively high humidity, which causes light gray efflorescence as stalactites between the northern and western sides of the body. The efflorescences are composed of calcite, gypsum and clay minerals. The stone lion statues at the southeast and northeast corners are made of alkali granite, while the others are lithic tuff. Total rock properties of the pagoda consist of 9,708 stone bricks. Among them, 11.0% are fractured, 6.7% are fallen off, and 7.0% show considerable surface efflorescence, which shows that the pagoda has been highly deteriorated by physical, chemical and biological weathering. The authors strongly suggest long-term monitoring and comprehensive conservation researches.

Noble gases in South Indian carbonatites: Trapped and in situ components

We have studied noble gases in carbonates and apatites from three carbonatites of South India, namely Hogenakal (2400 Ma), Sevattur (770 Ma) and Khambamettuu (523 Ma) by vacuum crushing. Apatite has also been analysed by pyrolysis. Vacuum crushing mostly releases the trapped gas components. The ratios 21Ne/ 20Ne, 22Ne/ 20Ne and 40Ar/ 36Ar increase with progressive crushing due to preservation of different composition gases in smaller inclusions released in later steps. This heterogeneity of isotopic composition of fluid inclusions is a consequence of the involvement of magmas carrying different noble gas signatures. The inclusions with lower ratios suggest the presence of a subducted atmospheric component, while the higher 21Ne/ 20Ne, 22Ne/ 20Ne and 40Ar/ 36Ar can be attributed to the presence of an enriched lithospheric mantle component. In addition, very minor trapped gases from less degassed, deeper mantle may also be present but overprinted by lithospheric and/or nucleogenic components. We propose that these carbonatites were generated only in an advanced stage of magmatism when this lithospheric component overwhelmed any contribution from the deeper mantle source. The lithospheric mantle underwent enrichment during an ancient subduction process through mantle metasomatism manifested in nucleogenic/radiogenic isotopic ratios of 21Ne/ 20Ne, 22Ne/ 20Ne and 40Ar/ 36Ar. The apatites analysed by pyrolysis clearly show nucleogenic 21Ne from 18O(α,n) reaction. We have demonstrated the potential of using U,Th– 21Ne systematics as a thermo-chronometer in conjunction with the established U,Th– 4He and U– 136Xe clocks. While for Hogenakal, the U,Th– 21Ne age of 845 ± 127 Ma is in agreement with the age of emplacement of other adjacent younger carbonatites, syenites and alkali granites, for the Sevattur apatite (738 ± 111 Ma) it indicates the crystallisation age.

Geochemistry of boron in the Ilímaussaq alkaline complex, South Greenland

The distribution of boron has been studied in rocks and minerals of the Ilímaussaq complex, South Greenland, using optical emission spectrometry. In the silica-undersaturated rocks of intrusive phases 1 and 3, average B contents increased from 5.6 ppm in augite syenite to a maximum of 8.9 ppm in sodalite-rich agpaitic nepheline syenite (naujaite roof cumulate) and then decreased to 5.4 ppm in the final lujavrites. Boron only behaved as an incompatible element during certain stages of the fractionation history. Starting at the naujaite stage, sodalite crystals (60–45 ppm B) were fractionated by flotation and were also trapped among the heavy cumulus phases of the bottom cumulates. This prevented the significant build-up of B in late derivatives as seen in other nepheline syenites. Nevertheless, in late pegmatites and veins associated with the agpaitic rocks, B was locally concentrated in certain Be minerals and metamict/reworked minerals. In the silica-oversaturated rocks of intrusive phase 2, average B contents increased from 8.6 ppm in quartz syenite to 13 ppm in alkali granite.

PETROCHEMISTRY AND RADIOACTIVITY OF GABAL ABU AQARIB ALKALI FELDSPAR GRANITE, CENTRAL EASTERN DESERT, EGYPT

The present work deals with geology, geochemistry and radiometry of Gabal Abu Aqarib Alkali Granite, Central Eastern Desert, Egypt. Gabal Abu Aqarib about 136 km2, and represents a part of the late Precambrian Arabian Nubian Shield. It is of alkali feldspar granite composition and may be considered epeiorogenic uplifts granitoids. Geochemically, the granite rocks of G. Abu Aqarib were originated frommetaluminous calc-alkaline magmas that developed in volcanic arc post orogenic tectonic setting. The studied granite rocks of G. Abu Aqarib area were generated at depth of about 9-20 km equivalent to 4-6 kb and temperature ranging from 760oC to 840oC with multi-processes of both assimilation and fractional crystallization involving plagioclase, hornblende and Fe-Ti oxides from a partial melted lithospheric magma. The U and Th contents in the studied area are associated with hematitization and silicification. Also, the radioactivity are controlled by the presence of higher content of zircon as accessory minerals and iron oxides while iron oxide and hydroxides are known to absorb U from circulating fluids.

Geodynamic significance of the A-type granites in the Sawuer region in west Junggar, Xinjiang: Rock geochemistry and SHRIMP zircon age evidence

Sawuer region is located in west Junggar, Jimunai County of Altay district and Hefeng County of Tacheng district, Xinjiang. The region is along the north margin of Kazakstan-Junggar plate. The intrusions (mainly acid) are widespread. Qiaqihai and Kuoyitasi intrusions are important in the region, with the characteristics of A-type granite. Further investigations indicate that they belong to A2-type granite that formed in extension tectonic setting of post collision. The REE chondrite-normalized patterns of the intrusions show LREE enrichment and the δ Eu values are lower. The Nd, Sr, Pb isotope compositions of the intrusions indicate a mantle source, while the low δ 18 O values resulted from the isotope exchange between intrusion and meteoric water. According to SHRIMP U-Pb age analysis results, the crystallization age of Qiaqihai intrusion is 290.7 ± 9.3 Ma (1 σ ), and that of Kuoyitasi intrusion is 297.9 ± 4.6 Ma (1 σ ), corresponding to the beginning of early Permian. The A2-type granites indicate that the region was in the extension period of the post-collisional stage at the beginning of early Permian in the Sawuer region. The A-type granites in the Sawuer region in west Junggar discovered from this work is analogous to the A-type granites found in east Junggar of the Ulungur alkali granites belt. The confirmation of post-collisional A-type granites of early Permian in the Sawuer region provides new evidence for the regional vertical continental crust growth in early Permian. The former proposed Ulungur alkali granites belt can extend from east Ulungur through west Ulungur and to Zhaisang in Kazakstan westwards.

New U–Pb SHRIMP zircon age for the Schurwedraai alkali granite: Implications for pre-impact development of the Vredefort Dome and extent of Bushveld magmatism, South Africa

The Schurwedraai alkali granite is one of a number of prominent ultramafic-mafic and felsic intrusions in the Neoarchaean to Palaeoproterozoic sub-vertical supracrustal collar rocks of the Vredefort Dome, South Africa. The alkali granite intruded the Neoarchaean Witwatersrand Supergroup and has a peralkaline to peraluminous composition. A new zircon SHRIMP crystallization age of 2052 ± 14 Ma for the Schurwedraai alkali granite places it statistically before the Vredefort impact event at 2023 ± 4 Ma and within the accepted emplacement interval of 2050–2060 Ma of the Bushveld magmatic event. The presence of the alkali granite and associated small ultramafic-mafic intrusions in the Vredefort collar rocks extends the southern extremity of Bushveld-related intrusions to some 120 km south of Johannesburg and about 150 km south of the current outcrop area of the Bushveld Complex. The combined effect of these ultramafic-mafic and felsic bodies may have contributed to a pronouncedly steep pre-impact geothermal gradient in the Vredefort area, and to the amphibolite-grade metamorphism observed in the supracrustal collar rocks of the Vredefort Dome.

Geology and U–Pb geochronology of the Kipawa Syenite Complex — a thrust related alkaline pluton — and adjacent rocks in the Grenville Province of western Quebec

The Kipawa Syenite Complex, a thin, folded sheet of amphibole syenite, quartz syenite and minor nepheline syenite, lies along a west-vergent thrust separating a lower slice comprising the Kikwissi granodiorite and biotite tonalite dated at 2717 +15–11 Ma, and unconformably overlying metasedimentary rocks from an overlying slice containing the Red Pine Chute orthogneiss, an alkali granite gneiss, and the Mattawa Quartzite. The syenite complex, dated at 1033 ± 3 Ma, lies within the lower slice but has metasomatically altered the overlying slice. Texturally guided U–Pb spot analyses on partially metasomatised zircons from the alkali granite gneiss yield a cluster of 207Pb/206Pb ages at 1389 ± 8 Ma, interpreted as the time of igneous crystallization and four ages overlapping the time of syenite emplacement, interpreted as in situ, metasomatic growth. The highest structural slice comprises garnet amphibolite separated from lower slices by the Allochthon Boundary Thrust. Metamorphic grade increases upward from greenschist grade in the biotite tonalite to amphibolite grade (690 °C, 9 kbar (1 kbar = 100 MPa)) at the lower boundary of the alkali granite. Emplacement of the Kipawa Syenite Complex took place after assembly of the thrust stack had begun and after emplacement of the allochthon or hot slab responsible for the inverted metamorphic gradient. Origin of the syenite is tentatively ascribed to anatexis of material metasomatized by flow of alkaline solutions along a major shear surface. Crystallization of new zircon in the margins of the syenite shows that metasomatism continued from ca. 1035 to 990 Ma, redistributing alkalies, fluorine, rare-earth elements and zirconium.