Syenitic Rocks Research Articles

METASOMATIC TRANSFORMATIONS IN TWO METAMORPHIC COMPLEXES DOMINATED BY HETEROGENEOUS Ca-PYROXENE - HORNBLENDE GRANITE AND SYENITE, OTTER LAKE AREA, QUEBEC, CANADA

The Bellmount Complex (28 × 5 km) and the Polish Hills Complex (6 × 3 km), found in a high-grade metamorphic terrane in the western Grenville Province of the Canadian Precambrian Shield, consist mainly of granitic–syenitic rocks with highly variable microstructure and mineral proportions. In a 40-meter section, the mean mineral content is Qtz 21 Kfs 55 Pl 14 Cpx 10 Ttn . The atom ratio Mg/(Mg + Fe 2+ ) in Ca-pyroxene and hornblende is equal to 0.6 to 0.7, Na/(K + Na) in discrete crystals of K-feldspar is equal to 0.06, and Na/(Na + Ca) in plagioclase is 0.93. Complex exsolution-induced microstructures formed during cooling to ca. 400°C. The granite and syenite are interlayered with amphibolite, K-feldspar – biotite (veined) gneiss and other rock components. There are signs of metasomatism on the 1 to 10 meter scale within the two complexes. K-feldspar amphibolite formed from Ca-pyroxene granulite with an addition of K, Rb, Sr, and Ba, and removal of Ca and Na, leaving concentrations of Si, Ti, Al, Fe, Mg, P, Sc, V, Zr, Nb, Cs and Hf unchanged. Also, bodies of Ca-pyroxene skarn and Ca-pyroxene – scapolite skarn, possibly formed from dolomite marble with an addition of all major elements except Mg and Ca, and the addition of several trace elements, including V, Zn, Rb, Y, Ba and rare-earths. Furthermore, a mixture of K-feldspar amphibolite and hornblende granite has possibly formed by an alteration of common amphibolite to K-feldspar amphibolite (with an addition of K and Rb), followed by the local transformation of K-feldspar amphibolite to hornblende granite (with an addition of Si and removal of Fe, Mg, Mn and Ca), leaving concentrations of Al, Na, K, Rb, Sr, Y and Ba unchanged. The introduction of elements at numerous places in the two complexes was possibly facilitated by deep fractures of a northeast-trending rift structure, marked by the presence of several bodies of ca. 1.15 Ga nepheline syenite.

Textural, chemical, and isotopic effects of late-magmatic carbonatitic fluids in the carbonatite–syenite Tamazeght complex, High Atlas Mountains, Morocco

Carbonatites of the Eocene Tamazeght complex, High Atlas Mountains, Morocco, consist of calciocarbonatites (alvikite and sovite dykes) and magnesiocarbonatites (diatreme breccias and dykes rocks). These are associated with ultramafic, shonkinitic, gabbroic to monzonitic and various foid syenitic silicate units. Stable and radiogenic isotope compositions for carbonatites and silicate rocks indicate that they share a common source in the mantle, although for some carbonatitic samples contamination with sedimentary rocks seems important. The observed isotopic heterogeneity is mainly attributed to source characteristics, fractional crystallization (accompanied by various degrees of assimilation), and late- to post-magmatic fluid–rock interaction. During the late fluid–rock interaction, Sr, Mn, and possibly also Fe were mobilized and redistributed to form secondary carbonate minerals in carbonatites. These fluids also penetrated into the adjacent syenitic rocks, causing enrichment in the same elements.

A ocorrência alcalina de Cananéia, litoral sul do estado de São Paulo: petrologia e geoquímica

The Cananeia massif (Ar-Ar, 83.6±0.9 Ma) in SE Brazil is represented by two small bodies - Morro de Sao Joao, 1.8 km 2 , and Morrete, 0.4 km 2 . The intrusion is covered by Quaternary sediments, has as country rocks the Acungui Group and it is tectonically associated with the Ponta Grossa Arch (Guapiara Lineament). Syenitic rocks clustered into two groups, alkali feldspar syenites and quartz-alkali feldspar syenites, are dominant and microsyenites, subordinate. Alkali feldspar (mesoperthite), quartz, clinopyroxene, amphibole and biotite are common minerals, whereas plagioclase and olivine occur rarely. Accessory phases include opaques, apatite, titanite and zircon. The Cananeia rocks are silica-saturated to oversaturated in composition, have potassic affinity and are highly evolved (mg#<0.33, low Cr and Ni contents). The normalized distribution for the incompatible elements indicate pronounced Sr, P and Ti negative anomalies and evident Zr and Nd positive anomalies. The earth rare elements behaviour displays high concentration, LREE to HREE fractionation and strong Eu negative spike. 87 Sr/ 86 Sr (R o ) initial ratios vary from 0.7065 to 0.70700 and from 0.7054 to 0.7078 for the less and more evolved rocks types, respectively. The positive correlation between R o and SiO 2 is a clear evidence that these syenites were subjected to assimilation-crustal contamination processes. Sr-Nd isotopic signature similar to other syenitic occurrences of southern Brazil allow to suggest for these rocks an origin from a basanitic parental magma, the mantelic source, subcontinental lithospheric and heterogeneous in composition, being submitted to metassomatic enrichment processes in Proterozoic times.

A ocorrência alcalina de Cananéia, litoral sul do estado de São Paulo: petrologia e geoquímica

The Cananéia massif (Ar-Ar, 83.6±0.9 Ma) in SE Brazil is represented by two small bodies - Morro de São João, 1.8 km², and Morrete, 0.4 km². The intrusion is covered by Quaternary sediments, has as country rocks the Açungui Group and it is tectonically associated with the Ponta Grossa Arch (Guapiara Lineament). Syenitic rocks clustered into two groups, alkali feldspar syenites and quartz-alkali feldspar syenites, are dominant and microsyenites, subordinate. Alkali feldspar (mesoperthite), quartz, clinopyroxene, amphibole and biotite are common minerals, whereas plagioclase and olivine occur rarely. Accessory phases include opaques, apatite, titanite and zircon. The Cananéia rocks are silica-saturated to oversaturated in composition, have potassic affinity and are highly evolved (mg#<0.33, low Cr and Ni contents). The normalized distribution for the incompatible elements indicate pronounced Sr, P and Ti negative anomalies and evident Zr and Nd positive anomalies. The earth rare elements behaviour displays high concentration, LREE to HREE fractionation and strong Eu negative spike. 87Sr/86Sr (Ro) initial ratios vary from 0.7065 to 0.70700 and from 0.7054 to 0.7078 for the less and more evolved rocks types, respectively. The positive correlation between Ro and SiO2 is a clear evidence that these syenites were subjected to assimilation-crustal contamination processes. Sr-Nd isotopic signature similar to other syenitic occurrences of southern Brazil allow to suggest for these rocks an origin from a basanitic parental magma, the mantelic source, subcontinental lithospheric and heterogeneous in composition, being submitted to metassomatic enrichment processes in Proterozoic times.

An investigation of the significance of local site effects in Plovdiv, Bulgaria

Plovdiv is the second largest city in the Republic of Bulgaria. A large part of the city is located on Holocene alluvial sediments and the oldest neighborhoods are situated on syenitic rock outcrops. We believe that local site effects may be an important contributor to the destruction caused by earthquakes. The primary objective of this study was to estimate quantitatively the local site effects in the central area of Plovdiv in terms of fundamental site frequency and amplification factor. Another important objective was to see how these correlate with the geological structures underlying the city. Measurements of the seismic noise at more than two hundred regularly placed points were made in the central area of the city. The H/V spectra were then calculated and analyzed to determine the spatial distribution of the fundamental site frequency and the amplification factor. The results exhibit very good correlation with the local geology. They were also compared with an intensity map from the strong 1928 Plovdiv earthquake. The comparison clearly demonstrates that the local site effects were the main factor in the destruction of buildings—the zones where the most damage was observed are also the zones where we have low fundamental site frequencies and high amplifications. Similarly the areas with high fundamental site frequencies and low amplification factors cover the neighborhoods where less damage has been observed. This study may form a basis for a more comprehensive and systematic microzonation study in Plovdiv.

Geochemistry and petrogenesis of post-collisional ultrapotassic syenites and granites from southernmost Brazil: the Piquiri Syenite Massif

The Piquiri Syenite Massif, southernmost Brazil, is part of the post-collisional magmatism related to the Neoproterozoic Brasiliano-Pan-African Orogenic Cycle. The massif is about 12 km in diameter and is composed of syenites, granites, monzonitic rocks and lamprophyres. Diopside-phlogopite, diopside-biotite-augite-calcic-amphibole, are the main ferro-magnesian paragenesis in the syenitic rocks. Syenitic and granitic rocks are co-magmatic and related to an ultrapotassic, silica-saturated magmatism. Their trace element patterns indicate a probable mantle source modified by previous, subduction-related metasomatism. The ultrapotassic granites of this massif were produced by fractional crystallization of syenitic magmas, and may be considered as a particular group of hypersolvus and subsolvus A-type granites. Based upon textural, structural and geochemical data most of the syenitic rocks, particularly the fine-grained types, are considered as crystallized liquids, in spite of the abundance of cumulatic layers, schlieren, and compositional banding. Most of the studied samples are metaluminous, with K2O/Na2O ratios higher than 2. The ultrapotassic syenitic and lamprophyric rocks in the Piquiri massif are interpreted to have been produced from enriched mantle sources, OIB-type, like most of the post-collisional shoshonitic, sodic alkaline and high-K tholeiitic magmatism in southernmost Brazil. The source of the ultrapotassic and lamprophyric magmas is probably the same veined mantle, with abundant phlogopite + apatite + amphibole that reflects a previous subduction-related metasomatism.

Postorogenic carbonatites at Eden Lake, Trans-Hudson Orogen (northern Manitoba, Canada): Geological setting, mineralogy and geochemistry

The Eden Lake pluton in the Trans-Hudson Orogen is the first known occurrence of carbonatites in Manitoba. The pluton is largely made up of modally and geochemically diverse syenitic rocks derived from postorogenic magma(s) of shoshonitic affinity. Their diversity can be accounted for by a combination of crystal fractionation and fluid release in the final evolutionary stage (crystallization of quartz alkali-feldspar syenite). At Eden Lake, carbonatites, represented predominantly by coarse-grained massive to foliated sövite, occur as branching veins and lenticular bodies up to 4 m in thickness showing crosscutting relations with respect to all of the syenitic units. The host rocks are intensely fenitized at the contact, and there is also abundant mineralogical and textural evidence for assimilation of silicate material by carbonatitic magma through wallrock reaction and xenolith fragmentation and digestion. The bulk of the carbonatites are composed of (in order of crystallization): Sr–REE-rich fluorapatite, aegirine–augite, and coarse calcite crystals surrounded by fine-grained calcite (on average, ∼ 90 vol.% of the rock). Noteworthy accessory constituents are celestine, bastnäsite-(Ce) (both as primary inclusions in calcite), Nb–Zr–rich titanite, low-Hf zircon, allanite-(Ce) and andradite. The calcite is chemically uniform (Sr-rich, Mg–Mn–Fe-poor and low in 13C), but shows clear evidence of ductile deformation and syndeformational cataclasis. Geochemically, the carbonatites are enriched in Sr, Ba, light rare-earth elements, Th and U, but depleted in high-field-strength elements (particularly, Ti, Nb and Ta). The stable-isotope composition of coarse- and fine-grained calcite from the carbonatites and interstitial calcite from syenites is remarkably uniform: ca. − 8.16 ± 0.27‰ δ 13C (PDB) and + 8.04 ± 0.19‰ δ 18O (SMOW). The available textural and geochemical evidence indicates that the Eden Lake carbonatites are not consanguineous with the associated syenites and may have been derived from a Nb–Ti-retentive and 13C-depleted source such as the subducted crustal material underlying the Eden Lake deformation corridor.

O maciço alcalino de Tunas, PR: novos dados geocronológicos

The NW-SE elongated Tunas intrusion, occupying an area of 22 km², crops out about 80 km from the city of Curitiba, and it is emplaced into Precambrian metavolcanic-sedimentary rocks of the Acungui Group. Five sub-circular volcanic structures are recognized. Petrographically, the massif mainly consists of syenites and alkali syenites with subordinate alkali gabbros, essexites, syenogabbros and syenodiorites; small late syenitic dykes and magmatic breccias are also found. Previous K-Ar data give a Late Cretaceous mean age of 82.2 Ma for Tunas rocks. In this article we present new U-Pb age determinations on zircon crystals from syenitic rocks (ID-TIMS, 82.7 ± 0.7 Ma; SHRIMP, 84.7 ± 1.2 Ma) which confirm that age, placing the massif into the Late Cretaceous group of alkaline intrusions tectonically associated with the Sao Jeronimo-Curiuva Lineament, a NW-SE-trending branch of the Ponta Grossa Arch in southern Brazil.

The Serra da Graciosa A-type granites and syenites, southern Brazil. Part 1: regional setting and geological characterization

The Serra da Graciosa region includes important occurrences of granites and syenites of the Graciosa A-type Province (formerly Serra do Mar Suite), southern Brazil. Using fieldwork, petrography, and remote sensing imagery, we characterize the geology of the plutons in the region. Five individual plutons were recognized. Two correspond to the previously defined Marumbi and Anhangava Plutons. We divide the former "Graciosa Pluton" into three new plutons: Capivari, Órgãos, and Farinha Seca. The plutons are elliptical with northeast-southwest orientation. Two petrographic associations can be recognized: an alkaline association that includes peralkaline and metaluminous hypersolvus alkalifeldspar granites and syenites (Anhangava, Farinha Seca, Órgãos), and an aluminous association composed of metaluminous and weakly peraluminous subsolvus granites (Capivari, Órgãos, Anhangava, Marumbi). Occurrences of each association are limited to one individual pluton or to portions of a pluton, and the age relationships are not well established. Monzodioritic rocks are found marginal to the Órgãos and Farinha Seca Plutons, and interaction with silicic magmas locally produced hybrid quartz syenites (Farinha Seca Pluton). Geothermobarometry indicates emplacement at shallow crustal levels (P = 2 ± 0.6 kbar), and crystallization temperatures within the interval 900-700ºC for the granitic and syenitic rocks, and 1000-750ºC for the monzodioritic rocks.

Isotope dilution ICP-MS analysis of trace elements in a Syenite rock reference material

Isotope dilution inductively coupled plasma magnetic sector field mass spectrometry (ID-ICP-MS) was applied to the trace analysis of selected elements of Syenite rock-type reference material SARM 2 as part of the continuing work programme on the re-evaluation of certain South African Reference Materials. The results for Ba, Cd, Cu, Mo, Ni, Pb, Sr and Zn are compared with the initial certification data for SARM 2, as well as published data for these elements since 1978. Complete uncertainty statements were calculated according to the ISO GUM and EURACHEM/CITAC guidelines for quantification of uncertainty of measurement.

Paleoproterozoic potassic–ultrapotassic magmatism: Morro do Afonso Syenite Pluton, Bahia, Brazil

The Morro do Afonso Syenitic Pluton (MASP) is a small, bimodal suite of lamprophyric and syenitic rocks, located in the Serrinha Nucleus, São Francisco Craton, northeastern Brazil. The unit consists predominantly of alkali feldspar syenite with numerous mafic xenoliths and dykes that are predominantly lamprophyric. The syenite shows geochemical and isotopic features typical of the Roman potassic magmas, i.e. low TiO 2 (up to1.2%), low K 2O/Al 2O 3 (0.33–0.49), strong enrichment in LILE, and high LILE/HFSE ratios. Fractionation, mainly of clinopyroxene and amphibole, explains the transition from lamprophyre to syenite. Magma mixing is required to explain the most evolved compositions. MASP syenites have Paleoproterozoic crystallization ages (2.11 Ga). The Nd isotope composition of mafic and felsic phases are similar ( ε N d mafic = −2.36 to −2.61 and ε N d felsic = −2.47) and they have Archaean model ages ( T DM = 2.56–2.58 Ga) that match the age of an inherited zircon grain (ca. 2.6 Ga). MASP is similar to many other post-orogenic syenitic plutons distributed over a wide area of São Francisco Craton (SFC), which are closely comparable in age and also typically associated with ultrapotassic lamprophyre. The present results suggest that a major proportion of the potassic–ultrapotassic syenitic rocks of the SFC were emplaced in a narrow time interval from 2111 to 2080 Ma, limiting the time of metamorphism peak of the Transamazonian Orogeny to at least 2.1 Ga. These plutons show some evidence of having interactions with older felsic crustal rocks and are interpreted to be derived from partial melting of lithospheric mantle that was enriched by subduction.

Petrogenesis of mica-amphibole-bearing lamprophyres associated with the Paleoproterozoic Morro do Afonso syenite intrusion, eastern Brazil

Mica-amphibole-lamprophyres, identified as vogesites, are associated with the Paleoproterozoic Morro do Afonso syenite intrusion in northeastern Brazil. The lamprophyres occur mainly as dykes that crosscut the syenitic rocks and occasionally as enclaves. Lamprophyric rocks are formed by the early magmatic paragenesis amphibole-clinopyroxene-apatite-phlogopite-ilmenite; feldspars are found in the groundmass. Near liquidus amphibole is edenite, close to the boundary with pargasite, which is enriched in alkalis relative to the other amphiboles (Mg-hornblende and actinolite). Clinopyroxene is diopside, and inclusions of phlogopite are analyzed in both clinopyroxene and amphibole phenocrysts. The chemical evolution of the mafic minerals is consistent with increasing oxygen fugacity during late magmatic stages. Whole-rock geochemical data suggest a metaluminous, ultrapotassic parental liquid, with silica saturation close to the limit of undersaturation. Trace element concentrations, such as enrichment in large ion lithophile and strong depletion of some high field-strength elements, indicate a mantle source that was partially modified by a subduction event. In this metasomatic mantle, it is important to emphasize the strong enrichment of light rare-earth elements, which is higher than those typically associated with basaltic rocks from active continental margins, and corresponding concentrations similar to those determined in lamproitic rocks. Major element modeling shows that fractional crystallization and magma flow segregation are the main petrogenetic processes involved in the magmatic evolution of lamprophyre magma, and it is possible to generate syenite magma by these mechanisms.

Alkaline Ultrapotassic A-Type Granites Derived from Ultrapotassic Syenite Magmas Generated from Metasomatized Mantle

Ultrapotassic Neoproterozoic granites crop out in southern and northeastern Brazil, associated with syenitic suites. The granites occur as small circular bodies and dikes hosted by the syenitic rocks. The southern intrusions reflect post-collisional magmatism, whereas the northeastern granites were emplaced in mobile belts produced during intraplate continental collision. Systematic mineralogical differences of the granites reflect higher K2O/Na2O ratios in the northeastern assemblages. Bulk-rock compositions of the ultrapotassic granites show very high alkalis (mainly K2O) and LIL elements, with significant depletions of Nb, Y, Zr, Hf, Ga, and REE relative to anorogenic granites. These are typical characteristics of ultrapotassic, silica-saturated alkaline rocks from postcollisional settings. Trace-and rare-earth—element patterns suggest derivation by fractional crystallization from syenitic parental magmas. Geochemical and mineralogical characteristics of these granites are completely different from within-plate alkaline granites. Most compositional data are comparable to those of shoshonitic granitoids, suggesting a probable mantle source modified by subducted-slab dehydration. In spite of this, the geochemical composition of the analyzed ultrapotassic granites is also quite different from shoshonitic and calc-alkaline granitoids in collisional settings, defining a silica-saturated ultrapotassic rock series.

Distribution patterns of natural radioactivity and delineation of anomalous radioactive zones using in situ radiation observations in Southern Tamil Nadu, India

In situ radiation measurements in the beach sectors and adjacent hinterlands and along rivers in the interiors of southern peninsular India were carried out using a portable radiation survey meter. A very high intrinsic anomalous radioactivity >26 μGy/h has been observed in the hinterlands within a fresh quarry and weathered boulders in the syenite rock body around Puttetti in the western Kanyakumari district of southern Tamil Nadu. Over the weathered hillocks in the hinterlands adjacent to the coast around Inayam, Kurumpanai and Midalam, the in situ radiation measurements have also exhibited high radioactivity ranging from 4 to 22 μGy/h which is significantly higher than the radiation exposure rates (RER) observed along the beach sectors at various locations from Chavara to Tuticorin (1–14 μGy/h). The observed radiation levels are presumably the highest concentration in southern India and it is the first time that such a high intrinsic radiogenic source in the hinterlands is reported in southwest coast of India. It is also observed based on the laboratory analysis of samples and in situ radiation data that the rivers/channels in this region contain insignificant level of radioactivity concentration and hence they do not contribute much to the placer deposits on the beaches. The placer deposits associated with significant RER (both in situ observations as well as laboratory estimates from samples) in the beach sectors from Kadiapattanam to Inayam are inferred to be derived through the country rocks/weathered hillocks in the immediate hinterlands.

Epilithic Lichen Communities in High Arctic Greenland: Physical, Environmental, and Geological Aspects of Their Ecology in Inglefield Land (78°–79°N)

The present investigation of High Arctic epilithic lichens and their substrate is based on field observations in Inglefield Land, North-West Greenland, mainly in 1999, with subsidiary observations from 1995. Eighteen rock samples, all glacial erratics, were specifically selected on the basis of macroscopic mineralization features such as iron and copper staining, vein and breccia structures, and ore minerals. The samples are representative of the crystalline shield, and their lithologies can be matched with exposures in Inglefield Land. Seven lichen communities are recognized, viz. Pleopsidium chlorophanum c., Xanthoria elegans var. splendens c., Dimelaena oreina–Physcia caesia–Xanthoria elegans c., Xanthoria elegans–Umbilicaria virginis c., Orphniospora moriopsis c., Porpidia flavicunda c., and Tremolecia atrata c. The studied material on the 18 samples reveals no conspicuous correlation between metal concentrations in the rock samples and the lichen communities and, broadly speaking, it can be stated that the lichens reflect more the properties of the rock surface, such as, for example, nitrogen- and iron-bearing weathering crusts, than the mineralogical composition of the rocks. However, there is a very close affinity between the Orphniospora moriopsis community and one variety of syenitic rocks with elevated magnetite and phosphorus.

Natural radionuclides in the rocks of the Valle del Cervo Pluton in Piedmont

Monitoring of the gamma radiation in Valle del Cervo Pluton was performed by determining U and Th contents in the main rock types cropping out over the entire area and pertaining to the granitic complex, syenitic complex and monzonitic complex. In particular, syenitic rocks were largely used as building and ornamental materials (e.g. Sienite della Balma). All the samples are fresh and do not present joints or fractures filled with U minerals. In the crushed samples the activity of uranium varies from 346 to 764 Bq/kg. Concentration of thorium varies from 202 to 478 Bq/kg. For all the analysed rocks uranium activity is higher than thorium one. The lowest value of radioactive concentration is referred to rocks of the granitic complex. The most active rocks are syenites. The data confirm the high activities of Valle del Cervo rock types, strongly connected with high K content of the source magma (geochemical signature); on the contrary, the activity seems to be not related to the location of the samples.

A 40Ar / 39Ar and U / Pb isotopic study of the Ilímaussaq complex, South Greenland: Implications for the 40K decay constant and for the duration of magmatic activity in a peralkaline complex

Magmatism in the Gardar Province, South Greenland, is related to two main rifting events at 1280 Ma and 1180 to 1140 Ma. Little is known about the duration of the magmatic activity in a specific complex. The Ilímaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types, which intruded and fractionated in three successive magmatic events. As the intrusion contains some of the most evolved, incompatible element-rich rocks on Earth, it was chosen for a detailed geochronological study to quantify the duration of melt production, intrusion, fractionation and cooling. Amphiboles, which are abundant in all rock types, including pegmatites and late magmatic veins, were dated using the 40Ar / 39Ar technique. Since the solidus temperature of the most evolved melts is below the closure temperature of amphibole, and no later heating event occurred, the 40Ar / 39Ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation. The 40Ar / 39Ar plateau ages range between 1142.6 ± 2.2 Ma and 1152.3 ± 3.7 Ma using the Steiger and Jäger [Steiger, R.H. and Jäger, E., (1977). Subcommision on Geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362.] 40K decay constant. These ages are younger than an U–Pb age of 1160 ± 5 Ma on baddeleyite from the first magma batch. Our results indicate that the 40K decay constant of Steiger and Jäger may be too high; a lambda similar to that proposed by Kwon et al. [Kwon, J., Min, K., Bickel, P. and Renne, P.R., (2002). Statistical methods for jointly estimating decay constant of 40K and age of a dating standard. Math. Geol. 34, 457–474.] is required to make the 40Ar / 39Ar match the U–Pb age.

Characterization of the Mefjell plutonic complex from the Sør Rondane Mountains, East Antarctica: Implications for the petrogenesis of Pan‐African plutonic rocks of East Gondwanaland

Abstract The Mefjell plutonic complex consists of 500–550‐Ma Pan‐African plutonic rocks, which intrude into the Precambrian crystalline basement in the Sør Rondane Mountains, East Antarctica, and forms part of the Sør Rondane Suture Zone. The complex comprises syenitic and granitic (mostly monzogranitic) rocks, and is characterized by the presence of iron‐rich hydrous mafic minerals and primary ilmenite, both of which imply its formation at high temperature and under low oxygen fugacity conditions. The syenitic rocks are metaluminous, and are high in alkalis, K2O/Na2O, Al2O3, FeOt/(FeOt + MgO) (0.88–0.98), K/Rb (800–1000), Ga (18–28 p.p.m.), Zr (up to 2100 p.p.m.) and Ba. They also have a low Mg♯ (Mg/[Mg + Fe2+]), Rb, Sr, Nb, Y and F, low to moderate light rare earth element (LREE)/heavy rare earth element (HREE) ratios and positive Eu anomalies in their rare earth element (REE) patterns. The granitic rocks are metaluminous to peraluminous, and have a high Rb content, high Sr/Ba and LREE/HREE ratios, low K/Rb and negative Eu anomalies. Most of the syenitic and granitic rocks have Y/Nb ratios greater than 1.2, and are depleted in Nb, Ti and Sr on the primitive mantle‐normalized spider diagrams, indicating a crustal origin with subduction zone signatures. We interpret both the syenitic and granitic rocks to be derived from an iron‐rich lower crustal source by dehydration melting induced by the heat of mantle‐derived basaltic intrusion, after which they then underwent limited fractional crystallization. The Mefjell plutonic complex has a high Zr content and tectonic discrimination diagram signatures indicative of normal A‐type granitic rocks. Both rock suites may have been generated under the same postorogenic tectonic setting. The Mefjell syenitic rocks are chemically comparable to charnockites in the Gjelsvikjella and western Mühlig‐Hofmannfjella areas of East Antarctica, whereas the granitic rocks are comparable to aluminous A‐type granitic rocks in South India, which were emplaced during formation and evolution of the Gondwanaland supercontinent.

Archaeogeological implication of lithic artifacts from the Unjeonri Bronze Age site, Cheonan, Republic of Korea

The Bronze Age lithic artifacts from the Unjeonri sites in the Republic of Korea were studied on the basis of petrological, geochemical and archaeogeological interpretations. The use and rock names of the artifacts are a stone-grinding pestle (medium-grained two-mica granite), two stone-grinding plates (fine-grained two-mica granite and fine-grained quartz diorite), two stone axes (coarse-grained syenitic rock and graphitic schist), an unknown use artifact (biotite gneiss) and a stone knife (black slate). Values of magnetic susceptibility (MS) of the lithic artifacts are mainly less than 0.5 (10 −3 SI unit); however, the syenitic rock (stone axes) ranges from 2.0 to 2.7. In the rocks around the relic sites, the MS values discovered were lower than 0.5. However, the syenitic rock stone axe, the graphitic schist stone axe, and the black slate stone knife showed different values from some of the measured rocks of an outcrop. Various kinds of rocks observed around the Unjeonri sites along the stream could be used for the stone-grinding pestle, the stone-grinding plate, and one of the stone axes. Each artifact was coupled with raw material samples rocks, and analyzed geochemically. The geochemical pattern of both lithic artifacts and sampled rock were identical when normalization was used to identify the behavior, enrichment, compatibility, and incompatibility of the elements. Therefore, it is possible that the source rocks of the interpreted lithic artifacts are distributed in the vicinity of the relic sites with the exception of one a stone axe (graphitic schist), the stone knife (black slate), and a stone arrowhead (gray shale); they did not conform to typical occurrences within a 10 km radius distribution area of the site. Consequently, in the Unjeonri sites, the artifacts suggest that the two types of materials coexist as domestic and foreign lithic artifacts. Using raw materials in situ, the domestic rock types (grinding pestle, plate, and axe) are self-sufficient. The foreign rock types (knife, arrowhead, and axe) are presumed to have been introduced from a different area. The foreign types should be examined for various introduction possibilities such as tribal migration, diffusion, plundering, or trade.

A Zona Tectônica Teixeira Terra Nova - ZTTTN - fundamentos da geologia regional e isotópica

A sigmoidal-shape linear tectonic occurs from Teixeira (Paraiba, to the north-northeast)- to Terra Nova (Pernambuco, to the south-southwest) as a line diagonal to theTransversal Zone, which is the central tectonic portion of the Borborema Province, thus dividing this province into two different litho-structural and tectonic domains. To the north and northwest occur the Early Neoproterozoic Riacho Gravata (RG) and the Neoproterozoic Pianco-Alto Brigida (PAB) terranes, both of them characterized by supracrustal rocks with low-grade of regional metamorphism. To the south-southwest occur the Alto Pajeu terrane (TAP, Early Neoproterozoic, orthognaisses and mesozonal supracrustal rocks) and the domain of its basement, the Alto Moxoto terrane (TAM, Paleoproterozoic in age), with high grade metamorphic rocks. This proposedtectonic zone is mostly composed by orthogneisses (from TAP and its Paleoproterozoic basement) pierced by an almost continuous series of intrusive granitic and syenitic rocks (plutons and stocks) of the Ediacaran period (ages between 630 and 570 Ma). During the evolutionary history of this linear portion of the plateau of the Borborema province there are some (direct and indirect) records of polycyclic geologic events of the Paleoproterozoic, Early Neoproterozoic, Ediacaran, Cambrian and Cenozoic era (a sedimentary veneer, evolution of a supergenic crust and regional uplift) with interesting implications for the regional geology, geography and social-economic development. In this paper the emphasis will be given to the isotopic data (Rb-Sr, Sm-Nd, mostly) along this special and the domains that it separates, as well its role on the regional tectonic framework. For this purpose, we take into consideration an expressive amount of available geological and isotopic data, obtained along the last decade (most of them not published) from different sources. Additionally, some first interpretative hypotheses will be postulated, where this tectonic (ZTTTN) would be formed as by-product of three parallel Neoproterozoic subduction zones.