Granite Complex Research Articles

Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: A hot rift to drift transition?

Rift-related thinning of continental basement along distal margins is likely achieved through the combined activity of ductile shear zones and brittle faults. While extensional detachments responsible for the latest stages of exhumation are being increasingly recognized, rift-related shear zones have never been sampled in ODP sites and have only rarely been identified in fossil distal margins preserved in orogenic belts. Here we report evidence of the Jurassic multi-stage crustal thinning preserved in the Santa Lucia nappe (Alpine Corsica), where amphibolite facies shearing persisted into the rift to drift transition. In this nappe, Lower Permian meta-gabbros to meta-gabbro-norites of the Mafic Complex are separated from Lower Permian granitoids of the Diorite–Granite Complex by a 100–250m wide shear zone. Fine-grained syn-kinematic andesine+Mg-hornblende assemblages in meta-tonalites of the Diorite–Granite Complex indicate shearing at T=710±40°C at P<0.5GPa, followed by deformation at greenschist facies conditions. 40Ar/39Ar step-heating analyses on amphiboles reveal that shearing at amphibolite facies conditions possibly began at the Triassic–Jurassic boundary and persisted until t<188Ma, with the Mafic Complex cooling rapidly at the footwall of the Diorite–Granite Complex at ca. 165.4±1.7Ma.Final exhumation to the basin floor was accommodated by low-angle detachment faulting, responsible for the 1–10m thick damage zone locally capping the Mafic Complex. The top basement surface is onlapped at a low angle by undeformed Mesozoic sandstone, locally containing clasts of footwall rocks. Existing constraints from the neighboring Corsica ophiolites suggest an age of ca. 165–160Ma for these final stages of exhumation of the Santa Lucia basement.These results imply that middle to lower crustal rocks can be cooled and exhumed rapidly in the last stages of rifting, when significant crustal thinning is accommodated in less than 5Myr through the consecutive activity of extensional shear zones and detachment faults. High thermal gradients may delay the switch from ductile shear zone- to detachment-dominated crustal thinning, thus preventing the exhumation of middle and lower crustal rocks until the final stages of rifting.

The Interplay of Evolved Seawater and Magmatic-Hydrothermal Fluids in the 3.24 Ga Panorama Volcanic-Hosted Massive Sulfide Hydrothermal System, North Pilbara Craton, Western Australia

The ~3240 Ma Panorama volcanic-hosted massive sulfide (VHMS) district is unusual for its high degree of exposure and low degree of postdepositional modification. In addition to typical seafloor VHMS deposits, this district contains greisen- and vein-hosted Mo-Cu-Zn-Sn mineral occurrences that are contemporaneous with VHMS orebodies and are hosted by the Strelley granite complex, which also drove VHMS circulation. Hence the Panorama district is a natural laboratory to investigate the role of magmatic-hydrothermal fluids in VHMS hydrothermal systems. Regional and proximal high-temperature alteration zones in volcanic rocks underlying the VHMS deposits are dominated by chlorite-quartz ± albite assemblages, with lesser low-temperature sericite-quartz ± K-feldspar assemblages. These assemblages are typical of VHMS hydrothermal systems. In contrast, the alteration assemblages associated with granite-hosted greisens and veins include quartz-topaz-muscovite-fluorite and quartz-muscovite (sericite)-chlorite-ankerite. These vein systems generally do not extend into the overlying volcanic pile. Fluid inclusion and stable isotope studies suggest that the greisens were produced by high-temperature (~590°C), high-salinity (38–56 wt % NaCl equiv) fluids with high densities (>1.3 g/cm3) and high δ 18O (9.3 ± 0.6 ‰ ). These fluids are compatible with the measured characteristics of magmatic fluids evolved from the Strelley granite complex. In contrast, fluids in the volcanic pile (including the VHMS ore-forming fluids) were of lower temperature (90°–270°C), lower salinity (5.0–11.2 wt % NaCl equiv), with lower densities (0.88–1.01 g/cm3) and lower δ 18O (−0.8 ± 2.6 ‰ ). These fluids are compatible with evolved Paleoarchean seawater. Fluids that formed the quartz-chalcopyrite-sphalerite-cassiterite veins, which are present within the granite complex near the contact with the volcanic pile, were intermediate in temperature and isotopic composition between the greisen and volcanic pile fluids (T = 240°–315°C; δ 18O = 4.3 ± 1.5 ‰ ) and are interpreted to indicate mixing between the two end-member fluids. Evidence of mixing between evolved seawater and magmatic-hydrothermal fluid within the granite complex, together with the lack of evidence for a magmatic component in fluids from the volcanic pile, suggest partitioning of magmatic-hydrothermal from evolved seawater hydrothermal systems in the Panorama VHMS system. This separation is interpreted to result from either the swamping of a relatively small magmatic-hydro-thermal system by evolved seawater or density contrasts precluding movement of magmatic-hydrothermal fluids into the volcanic pile. Variability in the salinity of fluids in the volcanic pile, combined with evidence for mixing of low- and high-salinity fluids in the massive sulfide lens, is interpreted to indicate that phase separation occurred within the Panorama hydrothermal system. Although we consider this phase separation to have most likely occurred at depth within the system, as has been documented in modern VHMS systems, the data do not allow the location of the inferred phase separation to be determined.

Petrography and Physicomechanical Properties of Rocks from the Ambela Granitic Complex, NW Pakistan

Petrography and physicomechanical properties of alkali granites, alkali quartz syenite, and nepheline syenite from Ambela, NW Pakistan, have been investigated. Whereas the alkali quartz syenite and most of the alkali granites are megaporphyritic, the nepheline syenite and some of the alkali granites are microporphyritic. Their phenocryst shape and size and abundance of groundmass are also different. The values of unconfined compressive strength (UCS) are the lowest and highest for megaporphyritic alkali granite and alkali quartz syenite, respectively. However, all the four rock types are moderately strong. Correspondingly, their specific gravity and water absorption values are within the permissible range for use as construction material. The UCS for the alkali quartz syenite is the highest, most probably because (i) it has roughly equal amounts of phenocryst and groundmass, (ii) it displays maximum size contrast between phenocryst and groundmass, (iii) its phenocrysts are highly irregular, and (iv) it contains substantial amounts of quartz.

Anatectic reworking and differentiation of continental crust along the active margin of Gondwana: a zircon Hf–O perspective from West Antarctica

Abstract The Fosdick migmatite–granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian–Carboniferous and another in the Cretaceous. The Hf–O isotope composition of zircons from Devonian–Carboniferous granites is explained by mixing of material from two crustal sources represented by the high-grade metamorphosed equivalents of a Lower Palaeozoic turbidite sequence and a Devonian calc-alkaline plutonic suite, consistent with an interpretation that the Devonian–Carboniferous granites record crustal reworking without input from a more juvenile source. The Hf–O isotope composition of zircons from Cretaceous granites reflects those same two sources, together with a contribution from a more juvenile source that is most evident in the detachment-hosted, youngest granites. The relatively non-radiogenic εHf isotope characteristics of zircons from the Fosdick complex granites are similar those from the Permo-Triassic granites from the Antarctic Peninsula. However, the Fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of Gondwana, including the Tasmanides of Australia and the Western Province of New Zealand, where the wider range of more radiogenic εHf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis. These new results highlight prominent arc-parallel and arc-normal variations in the mechanisms and timing of crustal reworking v. crustal growth along the former active margin of Gondwana. Supplementary material: Figs S1 and S2 are available at www.geolsoc.org.uk/SUP18625

Morphology of zircon crystals from Ukan-Kayo granitic complex, Okayama Prefecture.

Morphology of zircon crystals from Ukan-Kayo granitic complex, Okayama Prefecture.

Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): evidence for Early Jurassic rifting prior to accretion with North America

The circular Bokan Mountain complex (BMC) on southern Prince of Wales Island, southernmost Alaska, is a Jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the Alexander terrane. The BMC hosts significant rare metal (rare earth elements, Y, U, Th, Zr, and Nb) mineralization related to the last stage of BMC emplacement. U–Pb (zircon) and 40Ar/39Ar (amphibole and whole-rock) geochronology indicates the following sequence of intrusive activity: (i) a Paleozoic basement composed mainly of 469 ± 4 Ma granitic rocks; (ii) intrusion of the BMC at 177 ± 1 Ma followed by rapid cooling through ca. 550 °C at 176 ± 1 Ma that was synchronous with mineralization associated with vertical, WNW-trending pegmatites, felsic dikes, and aegirine–fluorite veins and late-stage, sinistral shear deformation; and (iii) intrusion of crosscutting lamprophyre dikes at &gt;150 Ma and again at ca. 105 Ma. The peralkaline nature of the BMC and the WNW trend of associated dikes suggest intrusion during NE–SW rifting that was followed by NE–SW shortening during the waning stages of BMC emplacement. The 177 Ma BMC was synchronous with other magmatic centres in the Alexander terrane, such as (1) the Dora Bay peralkaline stock and (2) the bimodal Moffatt volcanic suite located ∼30 km north and ∼100 km SE of the BMC, respectively. This regional magmatism is interpreted to represent a regional extensional event that precedes deposition of the Late Jurassic – Cretaceous Gravina sequence that oversteps the Wrangellia and Alexander exotic accreted terranes and the Taku and Yukon–Tanana pericratonic terranes of the Canadian–Alaskan Cordillera.

베트남 북부 네안 희토류 산출지의 지구화학탐사

베트남 북부 네안(Nghe An)성 뀌 차우(Quy Chau)에서 남동쪽으로 약 14 km 떨어진 쵸 빈(Chau Binh) 일대를 대상으로 1차 토양지구화학탐사를 수행하였다. 시료채취 간격은 가로 300 m 간격으로 14 line, 세로 500 m 간격으로 15 line이었으며 토양시료 194개를 채취하였다. 그리고, 1차 토양지구화학탐사 결과를 바탕으로 총희토류(TREO) 함량이 높은 격자점을 대상으로 2차 정밀 피트탐사를 수행하여 총 7개 피트에서 토양 56개를 채취하였다. 조사지역의 지질은 반 찌응(Ban Chieng) 흑운모 화강암 복합체와 다이 록(Dai Loc) 편마암질 화강암 복합체가 편암, 편마암, 석회암으로 구성된 부캉(Bukang)층을 관입하고 있다. 본 역의 주요 광화작용은 주석, 루비, 그리고 화강암 복합체의 주변부 변질대(albitization)에 분포하고 있는 것으로 사료되는 함희토류 모나자이트(약 300 g/t)와 제노타임(약 10 g/t)의 산출이 특징적이다. 중사시료에서 확인한 모나자이트와 제노타임의 근원암을 규명하기 위해 토양지구화학탐사를 수행하였다. 1차 토양지구화학탐사에서 채취한 토양시료에 대하여 희토류원소(La, Ce, Nd, Y 등) 및 미량원소(Th, Sr, Ba, Nb 등)의 화학성분을 ICP-MS를 이용하여 분석한 결과, 배경토양의 총희토류 함량이 지각 평균의 약 2배를 초과하고 있으며, 중희토류(약 2배)와 경희토류(약 1.84배)가 모두 부화되어 있다. 2차 정밀 피트탐사에서 채취한 토양시료에 대하여 희토류원소(La, Ce, Nd, Y 등)의 화학성분을 ICP-MS를 이용하여 분석한 결과, 격자점 4-7번 피트에서 광체로 추정되는 노두를 인지하였고 본 광체는 N40W 방향으로 발달되어 있다. 1차 토양지구화학탐사 및 2차 정밀 피트탐사를 종합적으로 고찰한 결과, 네안성 뀌차우 일대에서는 토양에서 이상대를 보이며 광체의 연장방향으로 추정되는 희토류 부존 유망 지역을 잠정적으로 선정하였으며, 향후 광체로 추정되는 구간에 대하여 지구물리탐사 및 시험시추를 수행할 예정이다. The phase I soil geochemical exploration was carried out targeting around Chau Binh area far from about 14 km with southeastern direction from Quy Chau within Nghe An province. The interval of sampling are horizontal 300 m with 14 line and longitudinal 500 m with 15 line, resulting in 194 soil samples. Based on the result of the phase I soil geochemical exploration, the phase II detailed pitting survey was carried out targeting the grid point with high TREO content, resulting in 56 soil samples within 7 pits. The geology of survey area are consisted of Ban Chieng biotite granite complex and Dai Loc gneissic granite complex intruding Bu Khang formation comprising of schist, gneiss and limestone. Main mineralization in the study area have the characteristics of occurrence with tin, ruby and REE-bearing monazite(about 300 g/t) and xenotime(about 10 g/t) to be thought as occurring at the alteration zone of granite complex. In order to elucidate the source rock of monazite and xenotime confirmed from heavy sand, soil geochemical exploration was carried out. As a analysis result with ICP-MS on the soil samples from the phase I soil geochemical exploration, total REE oxide content of background amount to about 2 times of crustal abundance, enriching the heavy rare earth(about 2 times) and light rare earth(about 1.84 times). As a analysis result with ICP-MS on the soil samples from the phase II soil detailed pit survey, we identified outcrop considering as economic ore body at the grid point 4-7 pit with N40W attitude. As a synthetic consideration on the phase I soil geochemical exploration and phase II detailed pit survey, we tentatively designated areas considering as the extension of economic ore body with REE anomaly. In the near future, we have the plan to carry out the geophysical exploration and test drilling targeting the interval anticipated to the economic ore body.

Late Permian appinite–granite complex from northwestern Liaoning, North China Craton: Petrogenesis and tectonic implications

The appinite–granite suite commonly occurs within post-subduction regimes during the final stage of an orogeny and thus holds the key to monitoring critical geodynamic and crustal evolutionary processes. The present zircon U–Pb dating and geochemical study documents the Late Permian appinite–granite complex from northwestern Liaoning, North China Craton. The hornblende-rich appinitic rocks have an SiO2 range from 41.8 to 53.4%, moderate to high alkali contents, enrichment in large ion lithophile elements and depletion in high field strength elements, with initial 87Sr/86Sr ratios of 0.7066 to 0.7128, εNd(t) of −8.3 to −13.3 and zircon εHf(t) from −10.1 to −18.7. These features suggest that their generation most likely involves metasomatism of mantle peridotites by sediment-derived liquids related to subduction and a later partial melting. The associated monzogranites range in SiO2 from 72.6 to 75.0% and exhibit a high-K calc-alkaline character, with high Sr–Ba abundances and elevated Sr/Y and La/Yb ratios. Such potassic adakitic signatures, plus their evolved isotopic compositions (87Sr/86Sri=0.7063 to 0.7066, εNd(t)=−15.6 to −16.3, zircon εHf(t)=−16.5 to −18.8), are consistent with partial melting of mixed protoliths including newly underplated enriched mafic lower crust and minor old lower crustal materials. Such a mafic and felsic magma association not only attests to a heterogeneously enriched sub-continental lithospheric mantle beneath the northern North China Craton, but also indicates reworking within a post-orogenic transcurrent regime of lithospheric delamination. The temporal coincidence of the present appinite–granite complex with regional transcurrent fault activity leads to the further suggestion that such mafic–felsic magmatic suites could provide spatial markers for monitoring important post-orogenic structural and geodynamic processes.

Petrogenesis of Cretaceous mafic intrusive rocks, Fosdick Mountains, West Antarctica: Melting of the sub-continental arc mantle along the Gondwana margin

A diorite pluton and widely distributed mafic dykes occur in the Fosdick migmatite–granite complex, which is interpreted to represent middle-to-lower crustal rocks of the paleo-Pacific active continental margin of Gondwana. The mafic dykes exhibit a variety of relationships with host rocks in the field ranging from undeformed dykes with sharp contacts with host gneisses to dismembered dykes with commingled textures and numerous back-veins of leucosome intruded from host migmatitic gneisses suggestive of significant interaction with crustal rocks. New U–Pb ages for magmatic zircon in these rocks yield Cretaceous crystallization ages ranging from ca 113Ma to ca 98Ma for the mafic dykes and ca 100Ma for the diorite pluton. These mafic intrusive rocks, which contain abundant hydrous minerals, are medium- to high-K-series calc-alkaline rocks with basic–intermediate compositions (47–59wt.% SiO2 for the mafic dykes and 52–56wt.% SiO2 for the diorite pluton). They have trace element patterns characterized by LILE enrichments and negative Nb anomalies indicating an origin from a hydrous mantle source metasomatized by slab-derived components. The samples without evidence of interaction with crustal rocks (11 of 14 samples), which are likely to better reflect the mantle source composition, have positive εSr(100Ma) values (+8.1 to +14.5) and negative to slightly positive εNd(100Ma) values (−1.6 to +2.5) consistent with derivation from an enriched mantle source. These eleven samples may be divided into two groups either characterized by higher LILE/HFSE ratios, less radiogenic εSr(100Ma) values and more radiogenic εNd(100Ma) values, or characterized by relatively lower LILE/HFSE ratios, more radiogenic εSr(100Ma) values and less radiogenic εNd(100Ma) values suggesting differences in the mantle source. The results of this study are consistent with the melting of a variably metasomatized sub-arc mantle source during a transition from a wrench to a transtensional tectonic setting, but are inconsistent with a mantle plume origin.

Paleoproterozoic S- and A-type granites in southwestern Zhejiang: Magmatism, metamorphism and implications for the crustal evolution of the Cathaysia basement

Paleoproterozoic granites and metamorphic rocks of the Badu complex, the oldest rocks found in Cathaysia Block, provide a window for investigating the crustal evolution of the Cathaysia basement. SHRIMP and LA-ICP-MS zircon U–Pb ages of five representative granite samples from Jinluohou (Lizhuang) and Jingju granitic complexes indicate that the magmatism took place during Paleoproterozoic, and overprinted by early Mesozoic thermal event. The Jinluohou garnet-bearing biotite granite (1878±28Ma) has high A/CNK (1.28–1.42), low contents of P2O5 and low FeOT/(FeOT+MgO). It contains a large amount of biotites and garnets with minor inherited zircons but no primary muscovites, showing the S-type granite affinity. It was formed at low water fugacity and high Zr saturation temperature (792–809°C). The Jingju medium-coarse grained K-feldspar granite (1861±35Ma), Jingju medium-fine grained K-feldspar granite (1849±30Ma) and Jinluohou gneissic granodiorite (1877±10Ma) can reach strongly peraluminous (A/CNK=0.97–1.26), are enriched in alkalis and HFSE (Zr, Nb, Ce, Y) and have high Ga/Al and FeOT/(FeOT+MgO) ratios. Their high Zr saturation temperatures (840–854°C, most of them are higher than 830°C) together with high Rb/Nb and Y/Nb ratios suggest that they belong to the A2 subgroup of A-type granites. The ages of inherited zircon cores from Jinluohou garnet-bearing biotite granite (2088–2929Ma) reveal the existence of the Archean basement, and the Hf and Nd isotopes and chemical compositions suggest that these S- and A-type granites were originated from the Paleoproterozoic and Archean crustal sources with different proportions of input from juvenile crust materials. The Paleoproterozoic S- and A-type granites intruded simultaneously, and formed under intraplate extensional geodynamic setting, which is an important event of episodic crustal growth and reworking. The age of Jingju porphyritic quartz monzonite (226.2±1.4Ma) and the lower intercept age of Jinluohou gneissic granodiorite, Jingju K-feldspar granite (224–231Ma) may indicate another important thermal overprinting event in this region.

Geologic structure of Western Primorye: Structuring dynamics

The first data are reported on the structuring dynamics of Western Primorye. The analysis of the structural parageneses of the least studied macro- and mesolevels of the arrangement of the geologic space allowed us to distinguish three sequentially superposed structural patterns (parageneses), each of which was formed by its own stress field during separate geodynamic periods: the Late Proterozoic-Early Paleozoic, the Middle-Late Paleozoic, and the Mesozoic-Cenozoic. During the first two episodes of geodynamic activation (Late Proterozoic-Early Paleozoic), the main compression axis was oriented longitudinally. The first episode was characterized by the formation of a thrust-fold structural pattern. The second episode resulted in the superposition of the meridional riftogenic structures and, respectively, in the pull-apart normal-fault segmentation of the crystalline basement into mainly longitudinal blocks, which partially coincide with the previously distinguished terranes. During the latitudinal compression and oblate deformation that predominanted during the Middle-Late Paleozoic geodynamic period, the down-thrown and uplifted blocks were transformed into depressions and arches, respectively. The depressions were filled with Paleozoic sediments, while the primary rocks in the arches experienced granitization with the formation of two granitic complexes that joined terranes. The granites are characterized by shadow and shear-type fold and pseudofolded forms. The Mesozoic-Cenozoic period governed by NNW compression is divided into the Triassic-Jurassic, Early Cretaceous, and Tertiary tectonic episodes. During the Triassic-Jurassic period, the ENE-trending narrow arches and depressions were formed by warping. The arches accumulated granitized rocks, while the depressions were compensated for by the Early-Late Triassic sedimentary-volcanogenic sediments. All these processes led to the formation of structural complexes that joined and overlapped older complexes of meridional blocks (terranes) across the strike. Two subsequent Mesozoic-Cenozoic episodes manifested themselves in the discrete formation of coal-bearing depressions due to the NNW compression and ENE extension of the warping products.

Syndeformational granite crystallisation along the Mount Magnet Greenstone Belt, Yilgarn Craton: evidence of large-scale magma-driven strain localisation during Neoarchean time

The Neoarchean is characterised by a global-scale period of crust formation, extensive mafic to granitic magmatism, crustal reworking and intracrustal partial melting. Since a dramatic strength drop is associated with the presence of melt in crystallising or melting rocks, Neoarchean continental deformation is thought to have been largely accommodated by shearing of high-grade gneisses and by syntectonic granitic magmatism. However, direct field examples of such strain localisation in hot and partially molten granites are relatively scarce. The Archean Yilgarn Craton, which is made up of volumetrically dominant granites and granitic gneiss, represents a natural laboratory to study the effect of melting on continental deformation. This paper describes meso- and microstructural evidence of large-scale, synmagmatic deformation recorded during crystallisation of a Neoarchean granitic complex. Two main kilometre-scale synmagmatic shear zones have been identified, and they are inferred to have been active during granite crystallisation. Structural evidence suggests that the dome-shaped structure that characterises the granitic complex represents a primary feature developed during pluton crystallisation. In the Mount Magnet greenstone belt, early structures predate granite emplacement, while the kilometre-scale, Boogardie synform developed during pluton crystallisation in response to granite doming. The studied complex represents a natural example of strain localisation within a low-viscosity corridor that may have accommodated large amounts of strain during the Neoarchean, as predicted by numerical experiments and tectonic models.

U‐Pb age and Hf isotope compositions of zircons from the north‐western region of the Ukrainian shield: mantle melting in response to post‐collision extension

Terra Nova, 24, 373–379, 2012AbstractThe North‐Western region of the Ukrainian shield represents Palaeoproterozoic continental crust formed essentially during two orogenic events at ca. 2.1–2.05 and 2.0–1.95 Ga. At 1.8–1.74 this area was intruded by huge Korosten anorthosite–mangerite–charnockite granite complex. At least three generations of mafic dykes can be distinguished in the region. We report new U‐Pb data obtained for a suite of Ni‐enriched dolerite dykes and layered gabbroic intrusions. Our results indicate that the Tomashgorod dyke was formed at 1787.4 ± 6.4 Ma, the Prutivka intrusion at 1777.0 ± 4.7 Ma and the Kamenka massif at ca. 1.79 Ga. This time coincides with the collision of the Fennoscandian and Sarmatian segments of the East European craton and it is contemporaneous with the initial stages of evolution of the Korosten complex. Hf isotopic compositions of zircons indicate that these rocks originated due to melting of moderately depleted mantle material.

The role of fractional crystallization in the genesis of early syn-D3, tin-mineralized Variscan two-mica granites from the Carrazeda de Ansiães area, northern Portugal

A multiphase granitic complex (G1–G6) in the Carrazeda de Ansiães area, Central Iberian Zone, was emplaced during syn-kinematic stages of the Variscan orogeny, between 329.9±0.8Ma and 318.9±1.9Ma, as determined by ID-TIMS U–Pb on zircon and monazite. The complex intruded the metasedimentary Douro Group in a NW trending antiform formed during the D1 and D3 deformation phases, but was itself affected by ductile and brittle deformation. The granites have equal or higher amounts of muscovite than biotite, and contain surmicaceous enclaves and more rarely xenoliths, “schlieren” and microgranular enclaves. They are S-type, mainly alkaline-calcic and peraluminous with ASI ranging between 1.21 and 1.45 and normative corundum of 2.63–4.77%. They are enriched in LREE relatively to HREE, with negative Eu anomalies, (87Sr/86Sr)i=0.7097–0.7159, εNd=−6.2 to −9.1, and δ18O=10.96–11.81‰. Geochemical and isotopic characteristics show that granites G1, G2, G4 and G5 correspond to distinct pulses of magma formed by partial melting of heterogeneous metasedimentary material. Geochemical fractionation trends recorded by whole rocks and minerals permit to distinguish and model two fractional crystallization series G2–G3 and G5–G6, the latter also affected by crustal assimilation during fractionation. Granites G1, G2, G3 and G6 have ≥ 20ppm Sn and G5 16ppm Sn; they are also associated with mineralized Sn- and W-bearing quartz veins. Fractional crystallization was the responsible process for the increase in Sn content in granites from the series and their micas.

Geochemistry of phosphorite deposits around Hirapur-Mardeora area in Chhatarpur and Sagar Districts, Madhya Pradesh, India

The Hirapur-Mardeora phosphorite deposits are found to occur in Bijawar group of Gangau iron formations over the basement of Bundelkhand granite complex in the Archean period. There are four distinct units of phosphorites which include shale-phosphorite, secondary-phosphorite, ironstone-phosphorite and quartz-breccia phosphorite. Chemical analysis of 24 representative samples of these different units was carried out for qualitative and quantitative determination of their major and trace element concentration, geochemical behaviour and inter-element relationship. The study reveals that these elements were precipitated by direct, inorganic, syngenetic, and authigenic processes in the primary phosphorites and by epigenetic, leaching, remobilization and reprecipitation in the cavities, voids, fractures and fissure fillings in the secondary phosphorites during diagenesis. The phosphogenic processes might have taken place during the slight reduction of fairly oxidising conditions under tropical to arid climate in shallow water marine environment of the basin. Key words: Hirapur-Mardeora, phosphorite, geochemistry, Chhatarpur and Sagar districts, diagenesis.

Geochemistry of phosphate bearing sedimentary rocks in parts of Sonrai block, Lalitpur District, Uttar Pradesh, India

Major, trace and rare earth elements of phosphatic rocks around Sonrai block of Paleo-Mesoproterozoic age having phosphatic breccia, quartzite, shale, sandstone, limestone and ironstone, have been determined to evaluate their correlation, relationship with the phosphorus content, the nature of possible substitution of various elements and regional distribution pattern over the area. The study indicates that the number of elements is substituted in the apatite structures; few of them are associated with phosphate and carbonate minerals. The variable concentration of major, trace and rare earth elements in the phosphatic rocks has been influenced by various physico-chemical processes involved during weathering and leaching of the source rocks. The distribution of the major, trace and rare earth elements is controlled by the environmental variations in the sediment water interface. The majority of trace elements were mainly influenced by the principle adsorbents like the phosphate minerals in addition to clay, iron oxides and silicate minerals. The PAAS normalized REE patterns of Sonrai block of phosphorites are characterized by negative Ce anomalies and Positive Eu anomalies. It is inferred from the distribution and interrelationship of major, trace and rare earth elements that the deposition of phosphate minerals might have occurred in highly oxidizing to slightly reducing conditions in supratidal to intertidal continental margins and shallow marine environment. The deposition was controlled by marine upwelling leading to excess charge of phosphate in certain zones of phosphogenic basins, lithologic facies variations in restricted circulations of basinal waters and electrochemical factors such as negative Eh, pH and other factors, which influenced the deposition of phosphates. The replacement, precipitation in voids and fissures and diagenesis were also important mechanisms of phosphate generation in Sonrai basin. The main source for various elements may be the minerals of cratonic mass of Bundelkhand Granitic Complex, a basement of Bijawar Basin, which also provided land derived phosphorus through weathering of the terrestrial cover.

ANALYSIS OF MAGNETIC ANOMALY OVER THE RIRUWAI YOUNGER GRANITE RING COMPLEX: A GEODYNAMIC IMPLICATION

The Riruwai Complex constitutes one of the Younger Granite Complexes in Nigeria within the Niger- Nigeria anorogenic belt province, located about 140 Km south of Kano. It provides one of the finest examples of the complete cycle of anorogenicmagmaticactivities in Nigerian Province, and consists of alkaline volcanic and plutonic rocks ranging from p yroxene bearing porphyries, rhyolites and comendite s, to riebeckite granites. The aeromagnetic intensity field anomaly of the Riruwai Younger Granite ring complex has been studied and analyzed to establish the tectonic nature of the complex and its influenc e on the geodynamics. The application of second derivati ve both in the x- and y-directions; (d 2 U / dx 2 ) and (d 2 U / dy 2 ) filters respectively, and Laplacian and polymonia l regression analysis revealed a general NE-SW regional anomaly pattern within the Complex and along which the Riruwai Younger Granite is emplaced. Very low magnetic anomaly is associated with the bi otite granites and the riebeckite bearing granites of the Complex. The very low anomaly is deemed to be attributed to highly evolved, late formed low density residual fluid rich magmatic material that were emp laced as granites as a separate magmatic phase, aft er an initial eruption of not so evolved volcanic phase. The general tectonic stress field traversing the Co mplex coincides with the NE-SW structural fabric within t he Basement Complex of the region.

Zircon U–Pb ages of the metamorphic supracrustal rocks of the Xinghuadukou Group and granitic complexes in the Argun massif of the northern Great Hinggan Range, NE China, and their tectonic implications

The basement of the Argun massif in the northern Great Hinggan Range consists of the metamorphic supracrustal rocks of the Xinghuadukou Group and associated granitic complexes. The metamorphic supracrustal rocks were previously interpreted as Paleoproterozoic, while the granitic complexes were considered Mesoproterozoic in age. This paper presents new zircon LA-MC-ICP-MS U–Pb ages of biotite-plagioclase leptynite and biotite schist from the Xinghuadukou Group in the Lulin Forest area, Heilongjiang Province; zircon SHRIMP U–Pb ages of biotite-plagioclase gneiss from the Xinghuadukou Group in Lulin Forest; and quartz diorite and monzogranite from the granitic complexes in Mohe County, Heilongjiang Province. New geochronological data from the three metasedimentary rock samples of the Xinghuadukou Group can be preliminarily divided into five groups: (1) 2017–2765Ma, (2) 1736–1942Ma, (3) 1359–1610Ma, (4) 749–1239Ma, and (5) 448–716Ma. Except for the zircons of the 448–716Ma group belonging to a metamorphic origin, the other age groups had the youngest age of 749±17Ma, indicating that the Xinghuadukou Group formed during the Neoproterozoic era (at least <749Ma). These detrital zircon ages cluster at ca. 2.0–1.8Ga and ca. 1.0–0.80Ga, suggesting that the Argun massif had connections with both Columbia and Rodinia and implying that the provenance of the Xinghuadukou Group metamorphosed sediments must be characteristic of felsic–intermediate igneous rocks with ages of ca. 2.0–1.8Ga and ca. 1.0–0.80Ga. The quartz diorite and monzogranite from the granitic complexes of the basement within the Argun massif yielded weighted mean ages of 516±10Ma and 504±9Ma, respectively, indicating that these rocks emplaced in the Early Paleozoic. Considering the geochemical and chronological data together, we propose that the Xinghuadukou Group was most likely deposited in a back-arc basin environment, whereas the granitic complexes emplaced in a post-collisional setting. A wide age spectrum of detrital zircons ranging from 749±9 to 2765±11Ma with ages clustering approximately 2.0–1.8Ga and 1.0–0.80Ga suggests that the basement rocks of the Argun massif are chiefly composed of the Neoproterozoic metasedimentary sequence and Early Paleozoic granitic complexes, with a few Meso- to Paleoproterozoic and even Neoarchean metamorphic rocks.

Accessory minerals from the Primorsky rapakivi granite complex, West Baikal Region

The composition of accessory minerals from granites of the second phase, quartz-muscovite (+fluorite), and quartz-muscovite-topaz greisens from the Primorsky rapakivi granite complex, West Baikal region, were studied using backscattered scanning electron microscopy. Ilmenite from granites contains inclusions of cassiterite, titanocolumbite, fergusonite-(Y), polycrase-(Y), and betafite. Allanite-(Ce), bastnaesite-(Ce), xenotime-(Y), Y- and Zr-thorite, zircon, and cyrtolite have been identified in granites. Greisens contain cassiterite, ferrocolumbite (Ta/Nb = 0.02−0.06), pyrochlore-group minerals, ilmenorutile, rutile, wolframite, polycrase-(Y), monazite-(Ce), fluocerite-(Ce), bastnaesite-(Ce), cerphosphorhuttonite, thorite, and other minerals. The ferrocolumbite + ilmenorutile assemblage is typical of quartz-muscovite greisen, whereas the rutile + ilmenorutile + wolframite + W-columbite assemblage is contained in the quartzmuscovite-topaz greisen as a result of an increase in Eh and decrease in pH and potassium activity of solution in the back zone. The compositions of Th- and REE-bearing minerals indicate the important role of phosphate and fluorine complexes in the transport of these elements.

Guangtoushan granites and their enclaves: Implications for Triassic mantle upwelling in the northern margin of the North China Craton

Mineral electron microprobe, bulk rock major, trace elements and Nd isotopes, and zircon U–Pb and Hf isotopic data are reported for granites and dioritic enclaves from the Guangtoushan granitic complex in the northern margin of the North China Craton. The zircon U–Pb dating identifies two stages of magmatism. The early stage enclave and host granite have identical ages (250±2Ma) and similar bulk rock Nd isotopes (εNd(t)=−11.5 to −12.2) as well as similar zircon Hf isotopes (εHf(t)=−13.3 to −17.4). However, although the late stage enclave (206±2Ma), host granite (208±1Ma) and leucogranite (207±3Ma) display comparable ages, but exhibit distinct isotopic features: the granite displays moderate bulk rock Nd isotopic value (εNd(t)=−8.3) and zircon εHf(t) from −2.6 to −10.4; the enclave shows least negative bulk rock εNd(t) of −3.9 and zircon εHf(t) from 0 to −5.4; and the leucogranite possesses more pronounced negative bulk rock Nd (εNd(t)=−17.2) and zircon Hf isotopic compositions (εHf(t)=−6.2 to −19.0). An integration of field observations, geochronology, geochemistry and zircon Hf isotopic data points to a complex petrogenetic history, where the early stage enclave–granite pair is a product of the early crystallization of felsic magmas that evolved from a mafic magma, whereas the late stage enclave and granite were derived by magma mixing. The late stage leucogranite is considered as a product of remelting of Phanerozoic igneous rocks within this region. The Guangtoushan granitic complex offers robust evidence for a gradual mantle upwelling during Triassic in the northern margin of the North China Craton.