Ferroan Granites Research Articles

Geology, mineralogy and petrological affinities of the Neoarchean granitoids from the central portion of the Canaã dos Carajás domain, Amazonian craton, Brazil

The central portion of the Canaã dos Carajás domain, located in the northern part of the Carajás province was originally marked by the occurrence of undifferentiated rocks belonging to the Xingu complex and Plaque suite, as well as greenstone belts, mafic rocks of the Pium diopside-norite and high K calc-alkaline leucogranites (Boa Sorte and Cruzadão granites). A semi-detailed geological mapping (1:100.000) was carried out in the Vila União area and allowed the identification of several deformed granite bodies intrusive in the Mesoarchean units. They correspond to the most expressive unit of the studied area and are composed predominantly of monzogranites with chemical and mineralogical affinity with the Neoarchean A-type granites of the Planalto and Vila Jussara suites. The variable felsic and ferromagnesian minerals contents, as well as the different proportions between them, allowed to distinguish four major groups of granitoids: (i) biotite-hornblende monzogranites (BtHblMzG); (ii) biotite granites and leucogranites (BtLG); (iii) biotite-hornblende tonalites (BtHblTn); and (iv) quartz diorites (QD). The tectonic foliation of these rocks follows the regional E-W trend and exhibits high angles (70–85°). Well-developed core and mantle structures in quartz and feldspar crystals, as well as the presence of serrated and irregular contacts between these crystals suggest that dynamic recrystallization occurred at relatively high temperatures (>500 °C). These rocks exhibit a wide variation in silica content (57.20–75.91 wt%), are metaluminous to slightly peraluminous and show affinity with A-type (high HFSE) and ferroan granites. Based on magnetic petrology studies it was possible to distinguish two groups of rocks: (1) granites containing only ilmenite with low magnetic susceptibility values (MS; <0.570 × 10−3 SI), and (2) granites with magnetite as the main iron and titanium oxide mineral and higher MS values (>1.437 × 10−3 SI). Textural and compositional evidences indicate that magnetite and ilmenite are early-crystallized phases and titanite has magmatic origin. Amphiboles are calcic and classified as hastingsite, whereas biotites are magnesian to ferroan. The relatively high Fe/(Fe + Mg) ratios found in the amphiboles of the BtHblMzG and BtHblTn varieties indicate that these granitoids were formed under low to moderate fO2 conditions, whereas in the BtLG the lower values of this ratio suggest more oxidizing conditions. Geothermometers point to crystallization temperatures between 830 and 930 °C in the different facies. The high total Al content in the amphibole crystals suggests crystallization at pressures between 400 and 800 MPa, indicating that these granitoids were emplaced at different crustal depths.

Geochronology and geochemistry of the late Neoproterozoic A-type granitic clasts in the southwestern Tarim Craton: petrogenesis and tectonic implications

ABSTRACTDue to sparse data for deciphering the late Neoproterozoic tectonic history, there is still considerable debate on whether long-lasting superplume-related or long-duration subduction-related dynamics dominated the Tarim Craton. In this contribution, our field investigations detail the late Neoproterozoic siliciclastic successions, and we report the first granitic conglomerates with zircon U–Pb ages of 753.9 ± 3.7 Ma in the SW Tarim Craton. Importantly, detrital zircons from the thick Cryogenian sedimentary basin also contain a major zircon population at ca. 750 Ma. Together with seismic data, this suggests a large ca. 750 Ma magmatic event in the SW Tarim Craton. Geochemically, the granitic clasts exhibit A-type granite features with high SiO2, high alkali but extremely low K, high FeOT/MgO and Ga/Al, and high high-field strength elements (HFSEs) (i.e. Nb, Hf, and Ta) with significant depletion in Rb, K, Sr, P, Eu, and Ti, and significant negative Eu anomalies (Eu* = 0.13–0.36), showing ferroan granite affinities. Including the detrital zircons, the ca. 750 Ma zircons have a large range of negative εHf(t) values (−19.46 to −1.16). Elemental and zircon Hf isotope data suggest that the granites were derived from Palaeoproterozoic reworked continental crust and are probably related to crustal thinning and extension. By comparison with previous studies, we conclude that Rodinia breakup was diachronous in the outer parts of the supercontinent.

Geochemistry, U-Pb geochronology and Lu-Hf isotope systematics of a suite of ferroan (A-type) granitoids from the CGGC: Evidence for Mesoproterozoic crustal extension in the east Indian shield

Ferroan (A-type) granitoid batholiths typically intrude during the periods of crustal-scale extension either at the terminal phase of an orogenic cycle or in an unrelated ‘anorogenic’ event. Elevated magmatic temperatures coupled with the anhydrous nature of the melts warrants the involvement of the lower crust along with the mantle as material and/or heat source. Granitoid batholiths from the north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) show extreme iron enrichment with high alkali and HFSE contents associated with elevated magmatic temperatures (∼900 °C), typical of ferroan (A-type) granites. Whole-rock trace element compositions indicate that the granitoids were dominantly produced by low-pressure anhydrous melting of middle- to lower-crust. U-Pb zircon geochronology constrains the intrusion age of the granitoids to ca. 1450 Ma. Lu-Hf isotopic compositions of the dated zircon indicate a limited contribution of contemporaneous juvenile mantle material; instead point to a dominant involvement of Palaeoproterozoic crustal source. Based on these findings, we suggest that the ferroan (A-type) granitoids occurring in the north-eastern CGGC are the results of melting of Palaeoproterozoic crust, triggered by mantle upwelling and increased heat transfer resulting from crustal extension associated to the fragmentation of the Indian landmass during the breakdown of the Columbia supercontinent.

The Keimoes Suite redefined: The geochronological and geochemical characteristics of the ferroan granites of the eastern Namaqua Sector, Mesoproterozoic Namaqua-Natal Metamorphic Province, southern Africa

Voluminous granite gneisses and granites straddle the boundary between the Kakamas and Areachap Terranes in the eastern Namaqua Sector (NS) of the Mesoproterozoic Namaqua-Natal Metamorphic Province (NNMP). These rocks have been previously poorly defined and loosely grouped into the Keimoes Suite, but a recent U-Pb age study has suggested the suite be subdivided into syn-tectonic and post-tectonic groups relative to the main phase of the Namaqua Orogeny. This study adds new whole rock geochemical, isotopic and age data for these granites that confirms the subdivision is appropriate. The older group of syn-tectonic granite gneisses, dated between 1175 and 1146 Ma, have penetrative foliations and are largely derived from fractionated, leucogranitic metaluminous to peraluminous magmas with low maficity, low Ti, Mn and Ca. They were derived from mildly depleted sources (εNd(t): −1.47 to 1.78), with Meso-to Paleoproterozoic Nd model ages (1.57–1.91 Ga), and high initial Sr ratios (0.71970–0.75567) suggesting mixing between younger depleted and older, arc-like sources imparting an arc-like signature to the magmas. High initial Sr ratios appear to be an intrinsic character of these granites reflecting those of granites in the region and the highly radiogenic nature of the NS.The weakly to unfoliated late-to post-tectonic megacrystic granodiorites and monzogranites, including charnockites, intruded between 1110 and 1078 Ma and constitute the Keimoes Suite proper. They have I-type characteristics, being strongly metaluminous and locally hornblende- and orthopyroxene-bearing with moderate SiO2 and with arc-type affinities (LILE enrichment relative to the HFSE, Ta-Nb, Ti and P anomalies). However, the granitoids also have high Fe/Mg ratios, along with high HFSE, LILE and REE contents more indicative of A-type granites. They show an increasing maficity, metaluminous character, and general decreasing degree of fractionation with decreasing age. They are similar to the syn-tectonic granites in having εNd(t) values close to zero (−2.95 to 2.83) and Meso-to Paleoproterozoic model ages (TDM: 1.38–1.99 Ga) but lower initial Sr ratios (<0.723 in general) suggesting derivation from relatively depleted sources with a variable enriched and/or crustal component.The timing of emplacement of the post-tectonic granites places peak D2 deformation in the eastern NS predominantly at ∼1.13–1.10 Ga, with peak D2 varying from ∼1.18–1.13 Ga. There was more voluminous granitic magmatism in the Areachap Terrane to the east during the late-to post-tectonic magmatic episode, whereas the earlier, ∼1.18–1.14 Ga syn-tectonic magmatic episode is more concentrated to the west in the Kakamas Terrane. The broad, protracted period of magmatism in the eastern NS attests to a long-lived duration of high-heat flow in this portion of the southern African crust at this time. Nd model ages of Meso-to Paleoproterozoic age reflect those in other granites throughout the NS suggesting extensive reworking of Paleoproterozoic crust during the 1.2–1.0 Ga Namaquan Orogeny.

A geochemical and geochronological study of the Early Cretaceous, extension-related Honggong ferroan (A-type) granite in southwestern Zhejiang Province, southeast China

AbstractThe Honggong pluton is the largest ferroan alkalic (A-type) granite intrusion emplaced along the Jiangshan–Shaoxing fault zone in southwestern Zhejiang Province, and has important implications for understanding the Late Mesozoic tectonic evolution of SE China. U–Pb ages of 138.7 ± 0.8, 134.2 ± 1.1, 128.5 ± 1.5 and 126.1 ± 0.9 Ma were obtained from zircon by laser ablation–inductively coupled plasma–mass spectrometry, indicating that the Honggong pluton formed in the Early Cretaceous. The Honggong pluton has a clear ferroan alkalic (A-type) granite geochemical signature with, for example, high total alkali contents and FeOt/(FeOt+ MgO) values. The Sr–Nd–Hf isotopic compositions suggest that there was juvenile material in the magma source. Geochemical evidence indicates that the pluton was derived through extensive fractionation of melts that contained both asthenospheric mantle and Mesoproterozoic crustal components. These rare granites in southern China were emplaced during five episodes at 235–225, 190, 165–155, 100–90 and 140–120 Ma. The age of the Honggong pluton suggests that localized extension in southwestern Zhejiang Province began as early as ~138 Ma and continued to 126 Ma. This Early Cretaceous extensional event was triggered by localized rollback of the subducting Pacific Plate.

A Late Variscan tin province associated to the ilmenite-series granites of the Sardinian Batholith (Italy): The Sn and Mo mineralisation around the Monte Linas ferroan granite

The characterisation of the late Variscan intrusion of Monte Linas (southern Sardinia) allowed to firstly study some Sn-bearing ores of this area and their relationships with the surrounding intrusives. The Monte Linas pluton was emplaced at ca. 290Ma at a shallow crustal level between allochthonous units and the foreland of the Sardinian Variscan belt. The pluton emplaced in a post-collisional regime into a previously exhumed low-grade basement, forming a coarse-grained monzogranite that is capped by an almost continuous sub-horizontal sheet of fine-grained rocks. The rocks within the pluton are ferroan F-bearing granites, belonging to the ilmenite-series. They have alumina saturation index values indicative of sub-aluminous to slightly peraluminous granitoids, as also indicated by the chemical composition of biotite within the pluton. The new data of this study indicate that the Monte Linas pluton formed from water-undersaturated magmas under low-fO2 conditions at temperatures >850°C. The later stages of magmatism were characterised by boiling and fluid expulsion at confining pressures of <1kbar, producing fayalite-bearing pegmatites, miarolitic facies and greisening.A wide variety of mineral deposits are associated with the Monte Linas pluton, including different types of Sn-bearing and Mo-bearing ores. Sn-bearing ores are represented by 1) Sn–As and Sn–Pb–Zn–Cu veins, and 2) “wrigglite” skarn Fe-Zn-Sn ores. Field and analytical studies, including fluid inclusion analyses, EMPA and SEM-EDS, allow to refer these deposits to the evolution of initial highly saline, hypothermal magmatic fluids, as confirmed by parageneses and fluid inclusion analyses of cassiterite in veins, which provide evidence of polyphase processes that started at temperatures close to 400°C. The hydrothermal systems were initially characterised by low-fO2 and high-chlorine solutions that mobilised Sn and underwent rapid changes in physicochemical conditions that led to the deposition of cassiterite. In the “wrigglite” F-rich genetic environment, a role of fluoride as complexing agent for Sn can be inferred. The Monte Linas pluton is also characterised by Mo deposits, not typical in ilmenite series granites; they occur both as greisens and veins. Mo was mobilized as a result of rapid increases in fO2 within the magmatic system, and precipitated in a fS2-rich environment nearby the contact between the intrusion and the surrounding country rocks.The variety of Sn mineralising events around the Monte Linas pluton confirms the role of physicochemical characters of magmas and of magmatic processes in the genesis of tin deposits. The occurrence in southern Sardinia of a wide suite of ilmenite-series ferroan granites emplaced in similar geological contexts allows to consider the idea of a low-grade tin province and opens the way to further explorations.

Geochemistry and geochronology of the rapakivi granites and associated rocks in the midwest portion of the Serra da Providência composite batholith, SW of Amazonian craton, Rondônia, Brazil

The Serra da Providência batholith includes the type area of the homonymous suite, the oldest rapakivi magmatic assemblage in the SW of the Amazonian craton (1.60–1.53 Ga). In the midwest portion of this massif, besides wiborgites/pyterlites and granophyric syenogranites, a leucosyenogranite facies and porphyritic rhyolites constitute new rock varieties recently described in that area. UPb LA-MC-ICP-MS zircon ages of 1574 ± 9 Ma and 1604 ± 3 Ma, respectively, were obtained for these new varieties and confirm their link with the Serra da Providência magmatism, whereas the subvolcanic rocks are older than the main rock varieties and were formed in a precursor event. These granitic facies are metaluminous to peraluminous, alkali-calcic, A2-type, ferroan granites. Their FeOt/(FeOt + MgO) ratios vary from 0.83 to 0.98 and suggest that these rocks were crystallized from oxidized-to reduced-A-type magmas, where the leucosyenogranites and granophyric sienogranites tend to be formed under more reduced conditions. They show fractionated REE patterns with very pronounced to weak negative Eu anomalies. The presence of granophyric textures and miarolitic cavities in equigranular syenogranitic facies suggests that these rocks were formed at shallow crustal depths, lower than 3 km. Three samples of leucosyenogranite have silica contents higher than 75% and low K/Rb ratios (<150), similarly to the tin specialized granites described in the Amazonian craton. Two distinctive groups of mafic rocks were recognized in the study area: porphyritic and equigranular gabbronorites. They correspond to tholeiitic basalts, with #Mg varying from 37 to 41 in porphyritic gabbronorites and 51 to 65 in equigranular gabbronorites. The low to moderate #Mg suggests that these rocks were crystallized from more evolved basaltic magmas. The porphyritic gabbronorites are enriched in TiO2, FeOt, K2O, P2O5 and REE compared to the equigranular gabbronorites that are enriched in MgO and CaO. The porphyritic gabbronorites have significant negative Eu anomalies a feature not observed in the equigranular gabbronorites. Porphyritic gabbronorites geochemical characteristics are similar to calc-alkaline basalts, whereas equigranular gabbronorites are similar to continental basalts. Petrographic, geochemical, and geological data of the felsic facies and the presence of associated mafic rocks corroborate the bimodal and post-collisional character of this magmatism. The occurrence of porphyritic rhyolites associated with shallow level plutonic granites in the Serra da Providência batholith reinforces the similarities between the Rondonian granites and the classical Fennoscandian rapakivi granites.

Alojamento do granito Lavras e a mineralização aurífera durante evolução de centro vulcano-plutônico pós-colisional, oeste do Escudo Sul-riograndense: dados geofísicos e estruturais

&lt;p&gt;A construção do granito Lavras é analisada mediante integração de dados geológicos, geofísicos e estruturais originais e compilados, em consonância com a evolução do centro vulcano-plutônico. Esse centro engloba o complexo intrusivo Lavras do Sul e a sequência traquiandesítica da Formação Hilário (604 - 590 Ma), ambos formados em posição de antepaís durante o período pós-colisional da Orogênese Dom Feliciano, no oeste do Escudo Sul-riograndense. A análise de estruturas rúpteis e lineamentos magnéticos indica que o vulcanismo teve início próximo ao colapso dessa orogenia, condicionado por sistemas transtensivos dextrais NW-SE a WNW-ESE que invertem para sinistrais com o relaxamento tectônico. A formação do complexo intrusivo, principiando com a intrusão subvulcânica do monzonito Tapera no norte, acompanhou a inversão no regime de &lt;italic&gt;stress&lt;/italic&gt; regional ao longo de zona de falha N70-75°W que o seciona. Ao final, ocorreu o posicionamento do granito Lavras no sul, o qual possui dimensões modestas (325 km³) e forma tabular (comprimento - L: espessura - E ≈ 3:1) afinando para sul, como deduzido dos dados gravimétricos. Dois domínios composicional-estruturais, equivalentes aos granitos magnesianos centrais (granodiorito e monzogranito) e os ferrosos da borda (sienogranito e feldspato alcalino granito), são definidos pela trama ASM (anisotropia de suscetibilidade magnética). Tais dados, aliados aos de estruturas rúpteis, apontam um &lt;italic&gt;pluton&lt;/italic&gt;multicíclico construído em dois eventos de ressurgência, envolvendo: (1) o lacólito central decorrente do alojamento do granodiorito sob a soleira de monzogranito; e (2) as intrusões anulares de granitos ferrosos, induzidas pela expansão do reservatório epizonal em razão da recarga com magmas máfico-ultramáficos lamprofíricos. O controle estrutural, a distribuição espacial e a associação com diques lamprofíricos corroboram o vínculo da mineralização aurífera com o último episódio de ressurgência em um centro vulcano-plutônico maduro.&lt;/p&gt;

Petrology of ferroan alkali-calcic granites: Synorogenic high-temperature melting of undepleted felsic lower crust (Damara orogen, Namibia)

The 556±4Ma-old Bloedkoppie granite (Central Damara orogen, Namibia) is a metaluminous to slightly peraluminous, alkali-calcic to calc-alkalic and ferroan granite. Its composition implies high-temperature, reduced, and anhydrous conditions during granite formation. The granite is fractionated, but heterogeneous radiogenic isotope data (87Sr/86Sr(init.): 0.712 to 0.727; εNd(init.): −7.2 to −13.1; 206Pb/204Pb: 17.30–17.72; 207Pb/204Pb: 15.54–15.67; 208Pb/204Pb: 37.80–38.23) indicate also that combined assimilation–fractional crystallization processes played an important role in the generation of the granite. Major and trace element compositions and isotope data of the least evolved samples and U–Pb data from zircon cores demonstrate that the source rocks are dominated by ca. 1.95Ga old felsic orthogneisses from the underlying basement. Zircon saturation temperatures and normative Qz–Ab–Or compositions indicate minimum melting P–T conditions of ca. 860°C at >5kbar and <5wt.% H2O. The most likely petrogenetic model involves high temperature partial melting of a Paleoproterozoic felsic source in the lower crust ca. 10–20Ma before the first peak of regional high-temperature metamorphism. Underplating of the lower crust by magmas derived from the lithospheric mantle may have provided the heat for melting of the undepleted basement to produce reduced and anhydrous melts.

New evidence for two sharp replacement fronts during albitization of granitoids from northern Aravalli orogen, northwest India

We present new evidence of infiltration metasomatism in granitoids that were albitized in a process that produced two sharp replacement fronts, both of which are clearly visible in the field. The two fronts advanced through the original granite simultaneously, but at different rates. Here we focus mainly on the Ajitgarh intrusive in the northern Aravalli orogen of northwest India. This intrusion shows geographically well-defined metasomatic zones on the outcrop scale as well as a large volume of original ferroan granite, both of which were poorly preserved in most of the previously studied Khetri granites. Stage I metasomatism transformed the grey original granite to pink microcline–albite granite, and stage II converted the microcline–albite granite to white albite granite. Both these reaction fronts are sharp and are easily recognized in the field by their different colours. The mineralogical and chemical changes during the first stage are expressed by transformation of original oligoclase to albite, biotite (annite-rich) and hastingsite (amphibole) to hastingsite with low XFe values, dehydration, gain in Na, and losses in Fe and Rb. The second stage of metasomatism caused almost complete conversion of microcline to albite and complete or nearly complete disappearance of amphibole. Chemically, these changes are manifested by substantial gain in Na and extreme losses in K, Rb, Ba, Ca, Sr, Fe, and Mg. Depending on the modal abundances of amphibole, stage II albitized rocks are depleted in light rare earth elements or heavy rare earth elements or both, signifying that rare earth elements are principally hosted by mafic phases. The disparity in whole-rock δ18O values during both stages of albitization is related to the variations in modal amounts of Si-bearing phases. The replacement microstructures are in accord with the fluid-mediated phase transformations by a coupled dissolution–precipitation mechanism. The albitizing event took place at low temperatures of 350–400 °C and the fluid was metamorphic in nature.

Metasomatism of ferroan granites in the northern Aravalli orogen, NW India: geochemical and isotopic constraints, and its metallogenic significance

The late Palaeoproterozoic (1.72–1.70 Ga) ferroan granites of the Khetri complex, northern Aravalli orogen, NW India, were extensively metasomatised ~900 Ma after their emplacement, at around 850–830 Ma by low-temperature (ca. 400 °C) meteoric fluids that attained metamorphic character after exchanging oxygen with the surrounding metamorphic rocks. Albitisation is the dominant metasomatic process that was accompanied by Mg and Ca metasomatism. A two-stage metasomatic model is applicable to all the altered ferroan intrusives. The stage I is represented by a metasomatic reaction interface that developed as a result of transformation of the original microcline–oligoclase (An12–14) granite to microcline–albite (An1–3) granite, and this stage is rarely preserved. In contrast, the stage II metasomatic reaction front, where the microcline-bearing albite granite has been transformed to microcline-free albite granite, is readily recognisable in the field and present in most of the intrusives. Some of them lack an obvious reaction interface due to the presence of stage II albite granites only. When studied in isolation, these intrusives were incorrectly classified and their tectonic setting was misinterpreted. Furthermore, our results show that the mafic mineralogy of metasomatised granites has a significant impact on the characterisation of such rocks in the magmatic classification and discrimination diagrams. Nevertheless, the stage I metasomatised granites can be appropriately characterised in these diagrams, whereas the characterisation of the stage II granites will lead to erroneous interpretations. The close spatial association of these high heat producing ferroan granites with iron oxide–copper–gold (IOCG), U and REE mineralisation in the region indicates a genetic link between the metasomatism and the mineralisation. World-class IOCG, U and REE deposits are associated with metasomatised ferroan granites, suggesting that such a relationship may act as a critical first-order exploration target for undiscovered mineral deposits.

The Aguilar pluton (23°12′ S–65°40′ W; NW Argentina): Petrological implications on the origin of the Late Jurassic intraplate magmatism in the Central Andes

The Late Jurassic Aguilar pluton is located in NW Argentina, about 300–400 Km east of the Tarapacá basin, representing the backarc basin linked to the Jurassic volcanic arc. This small-size and compositionally heterogeneous pluton intruded the metasedimentary rocks of the Ordovician Santa Victoria Group, along the Cobres-Salinas Grandes lineament. A revision of published geochemical data in the light of new field and petrological results, allows us to propose a model concerning the petrogenesis and emplacement mechanisms of Aguilar pluton and to discuss its geodynamic setting. The pluton is mainly composed of metaluminous and nearly peraluminous granitoids, showing the geochemical characteristics of ferroan granites. The volumetrically subordinate mafic rocks are both ne- and hy-normative, and their primary magmas were generated by partial melting of a pristine Proterozoic mantle. Aguilar rocks display a rather limited range in (87Sr/86Sr)i, compared to the entire rift-related plutonic suite, i.e., 0.703198–0.704601, and εNdt from −1.06 to 3.82, calculated at 149 Ma. Fractional crystallization of mantle-derived magmas and crustal contamination processes explain the evolution to produce strongly silica-oversaturated magmas, which emplaced in the continental crust. The petrological data indicate that magma emplacement and cooling occurred at rather shallow depth. Multiple injections of magma batches into the magma chamber caused mingling and mixing processes early in the crystallization history. The Aguilar pluton is one of the several igneous complexes whose formation was associated with the extensional tectonics active during Late Jurassic-Early Cretaceous in NW Argentina. Based on the geological position and the igneous rocks affinity, we exclude that the Late Jurassic magmatism was generated in an orogenic setting and envisage that it was linked to the early extensional phase that preceded the Cretaceous continental rifting, related to the break–up of the South America-Africa continents.

Two-Stage, Extreme Albitization of A-type Granites from Rajasthan, NW India

Albitization is a common process during which hydrothermal fluids convert plagioclase and/or K-feldspar into nearly pure albite; however, its specific mechanism in granitoids is not well understood. The c. 1700 Ma A-type metaluminous ferroan granites in the Khetri complex of Rajasthan, NW India, have been albitized to a large extent by two metasomatic fronts, an initial transformation of oligoclase to nearly pure albite and a subsequent replacement of microcline by albite, with sharp contacts between the microcline-bearing and microcline-free zones. Albitization has bleached the original pinkish grey granite and turned it white. The mineralogical changes include transformation of oligoclase (similar to An(12)) and microcline (similar to Or(95)) to almost pure albite (similar to An(0 center dot 5-2)), amphibole from potassian ferropargasite (X-Fe 0 center dot 84-0 center dot 86) to potassic hastingsite (X-Fe 0 center dot 88-0 center dot 97) and actinolite (X-Fe 0 center dot 32-0 center dot 67), and biotite from annite (X-Fe 0 center dot 71-0 center dot 74) to annite (X-Fe 0 center dot 90-0 center dot 91). Whole-rock isocon diagrams show that, during albitization, the granites experienced major hydration, slight gain in Si and major gain in Na, whereas K, Mg, Fe and Ca were lost along with Rb, Ba, Sr, Zn, light rare earth elements and U. Whole-rock Sm-Nd isotope data plot on an apparent isochron of 1419 +/- 98 Ma and reveal significant disturbance and at least partial resetting of the intrusion age. Severe scatter in the whole-rock Rb-Sr isochron plot reflects the extreme Rb loss in the completely albitized samples, effectively freezing Sr-87/Sr-86 ratios in the albite granites at very high values (0 center dot 725-0 center dot 735). This indicates either infiltration of highly radiogenic Sr from the country rock or, more likely, radiogenic ingrowth during a considerable time lag (estimated to be at least 300 Myr) between original intrusion and albitization. The albitization took place at similar to 350-400 degrees C. It was caused by the infiltration of an ascending hydrothermal fluid that had acquired high Na/K and Na/Ca ratios during migration through metamorphic rocks at even lower temperatures in the periphery of the plutons. Oxygen isotope ratios increase from delta O-18 = 7 parts per thousand in the original granite to values of 9-10 parts per thousand in completely albitized samples, suggesting that the fluid had equilibrated with surrounding metamorphosed crust. A metasomatic model, using chromatographic theory of fluid infiltration, explains the process for generating the observed zonation in terms of a leading metasomatic front where oligoclase of the original granite is converted to albite, and a second, trailing front where microcline is also converted to albite. The temperature gradients driving the fluid infiltration may have been produced by the high heat production of the granites themselves. The confinement of the albitized granites along the NE-SW-trending Khetri lineament and the pervasive nature of the albitization suggest that the albitizing fluids possibly originated during reactivation of the lineament. More generally, steady-state temperature gradients induced by the high internal heat production of A-type granites may provide the driving force for similar metasomatic and ore-forming processes in other highly enriched granitoid bodies.

U[sbnd]Mo mineralization potential in Pan-African granites, southwestern Cameroon: Economic geology of the Ekomédion prospect

Uranium and molybdenum mineralization at the Ekomédion prospect of southwestern Cameroon occurs within pegmatite pods in Pan-African two-mica granite. The two-mica granite and the associated pegmatite experienced brittle deformation. Mineralization occurs as veinlets following early fractures and as irregular bodies intimately associated with pegmatite pods. Syn-tectonic alteration comprises albitization, silicification, ferruginization, seriticitization and chloritization. Mica in the ore zone is less magnesian and F-rich relative to mica in the host two-mica granite. Uraninite crystals are anhedral to subhedral and molybdenite is commonly encapsulated by phengitic muscovite. The two-mica granite is a strongly fractionated high-K, low P ferroan granite. The pegmatite pods hosting UMo mineralization are suggested to have been derived from the fractionation of magma parental to the two-mica granite as observed from Rare Earth Element (REE) patterns and the trend shown by Th/U ratios of the host granite and the pegmatite. This implies that uranium and molybdenum fractionated and concentrated preferentially into the late stage pegmatitic fluid that formed the pods and subsequently into the hydrothermal fluid phase. This therefore points to the two-mica granite as well as the metamorphic wall rocks as potential sources for UMo mineralization in southwestern Cameroon.

Geochemistry and zircon geochronology of the Permian A-type Hasanrobat granite, Sanandaj–Sirjan belt: A new record of the Gondwana break-up in Iran

The Sanandaj–Sirjan metamorphic–plutonic Belt (SSB) in west central Iran is a polyphase metamorphic terrain composed of dominantly greenschist-grade metasedimentary and metavolcanic rocks, and felsic to mafic plutons, of Neoproterozoic–Phanerozoic ages. The Hasanrobat granite in central SSB occurs as a single pluton, ~20km2 surface area, with relatively consistent mineralogy and chemistry. Quartz, alkali feldspars (microcline and perthite), sodic plagioclases and biotite are the main constituents, commonly associated with minor amphibole. Accessory phases include zircon, allanite, apatite, and magnetite.The country rocks are Upper Carboniferous–Lower Permian sandstones and dolomitic limestones. Scattered patches of skarn-type assemblages dominated by tremolite and talc occur in the dolomitic limestones, and sandstones are recrystallized to a coarse-grained quartzite at contact with the granite. The granite is metaluminous to slightly peraluminous, and is distinguished by high FeOt/MgO ratios, typical of ferroan (A-type) granites. The A-type affinity is also reflected by high Na2O+K2O, high Ga/Al ratios, high contents of large ion lithophile elements (LILE), high field strength elements (HFSE) and rare earth elements (REE), as well as low contents of Sr, and distinct negative Eu anomalies. The biotites are aluminous, Fe-rich, and plot near the siderophyllite corner in the quadrilateral biotite diagram. They are further distinguished by high fluorine contents (0.61 to 1.33wt.%). Amphibole is ferrohastingsite in composition. Ion microprobe analyses of zircon grains separated from a representative granite sample yielded concordant U–Pb ages with weighted mean 206Pb/238U age of 288.3±3.6Ma.The granite and the country rocks are cut by a set of mafic dykes with asthenosphere-like geochemical signatures. Such association suggests anorogenic intraplate magmatism in Lower Permian in the region. This provides further evidence for, and significantly constrains timing of, a major extension in Upper Paleozoic in Iran, previously inferred from the rock record. The extension led to the Gondwana break-up and the opening of Neotethys Ocean between Sanandaj–Sirjan and Zagros in Permian.

Geologia, Petrografi a e Geoquímica do Batólito Seringa, Província Carajás, SSE do Pará

The Seringa Granite, with 2250 km² of outcropping area represents the biggest granite batholith of the Carajas Province. It is intrusive in Archean units of the Rio Maria Granite-Greenstone Terrane, located in the southeastern of the Amazonian Craton. The Seringa Granite is formed by two great petrographic groups: A) monzogranite rocks, represented by biotite-amphibole coarse-grained monzogranite, amphibolebiotite coarse-grained monzogranite; B) syenogranite rocks, represented by, porphyry amphibole-biotite syenogranite, heterogranular leuco-syenogranite, leuco-microsyenogranite, and heterogranular amphibolebiotite syenogranite. Biotite and amphibole are the varietal minerals and zircon, apatite, opaque, and allanite the accessories minerals. The Seringa Granite is subalkaline, metaluminous to peraluminous, display K2O/ Na2O ratios between 1 and 2 and FeOt/(FeOt +MgO) between 0.86 and 0.97. The patterns of REE show increase in negative europium anomalies from the less evolved facies to the more evolved facies. In these sense, it is enriched in light REE parallel to the impoverishment of heavy REE. It shows geochemical affinities with within-plate ferroan granites, of the A2-subtype and oxidized A-type granites. The fi eld relations and the petrographic and geochemical features of the Seringa Granite are not consistent with the evolution of its facies from a single magma pulse by fractional crystallization. The Seringa Granite show petrographic, geochemical and magnetic susceptibility properties that suggest its inclusion in the Serra dos Carajas Suite.

Three-step continental-crust growth from subduction accretion and underplating, through intermediary differentiation, to granitoid production

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb dating, laser-ablation multi-collector ICPMS Hf isotope and electron microprobe element analyses of inherited/antecrystal and magmatic zircons from five granitoid intrusions of Linxi area, in the southern segment of the Great Xing’an Range of China were integrated to solve continental crustal growth mechanisms. These intrusions were divided into two suites. Suites 1 and 2 are mainly granodiorite and syenogranite and correspond to magnesian and ferroan granites, respectively. SHRIMP dating establishes an Early Cretaceous (135–125 Ma) age for most Linxi granitoids and a time of ∼146 Ma when their source rocks were generated or re-melted. However, some granitoids were generated in Early Triassic (241 Ma) and Late Jurassic (146 Ma), after their source rock experienced precursory melting episodes at 263 Ma and 165 Ma, respectively. All zircon 206Pb/238U ages (<300 Ma, n = 100), and high positive zircon eHf(t) values (n = 175) suggest juvenile source materials with an absence of Precambrian basement. Hf–Nd isotopic decoupling of Linxi granitoids suggests a source component of pelagic sediments, i.e. Paleozoic subduction accretion complexes. Zircon eHf(t) values (t = 263–165 Ma) form a trend sub-parallel to the depleted mantle Hf isotope evolution curve, whilst those with t = 146–125 Ma fall markedly below the latter. The first trend indicates a provenance from essentially subducted oceanic slabs. However, the abrupt eHf(t) decrease, together with extensive Early Cretaceous magmatism, is interpreted as reflecting mantle upwelling and resultant underplating, and exhumation of subducted oceanic slabs. Suite 1 granitoids derive mainly from subducted oceanic slabs or Paleozoic subduction accretion complex, whereas Suite 2 from underplated mafic rock and, subordinately, Paleozoic subduction accretion complex. Compositions of Suites 1 and 2 depend on the hydrous, oxidized or relatively anhydrous, reduced nature of source rocks. Among each of these five intrusions, magmatic zircons have systematically lower 176Hf/177Hf than inherited/antecrystal zircons. Hf isotopic and substituting element profiles through inherited/antecrystal zircons (t = 263 to ∼146 Ma) indicate repeated low melt-fraction melting in the source region. In contrast, profiles through inherited/antecrystal and magmatic zircons (t = 146–125 Ma) reveal melting region expansion with a widening range of source compositions and increasing melt fractions. These results lead to the conclusion that continental growth in this region involved a three-step process. This included subduction accretion and repeated underplating, intermediary differentiation of juvenile rocks, and granitoid production from these differentiated rocks.

GEOLOGIA, PETROGRAFIA E GEOQUÍMICA DO GRANITO ANOROGÊNICO BANNACH, TERRENO GRANITO-GREENSTONE DE RIO MARIA, PARÁ

The Bannach Granite is intrusive in to Archean rocks of the Rio Maria Granite-Greenstone Terrane, located at the southeastern border of the Amazonian craton. Eigth varieties of monzogranites have been recognized in this Paleoproterozoic batholith: amphibole + biotite ± clinopyroxene coarse-grained monzogranites: clinopyroxene-biotite-amphibole monzogranite, biotite-amphibole monzogranite, and amphibole-biotite monzogranite; Porphyritic biotite monzogranite; leucomonzogranites (coarse-grained, early and late medium-even-grained monzogranite and fine, even-grained monzogranite). The massif is zoned with the less evolved facies situated along the border and the more evolved facies in the center. The different facies are subalkaline, metaluminous to peraluminous, display K 2 O/Na 2 O ratios between 1 and 2 and FeOt/(FeOt + MgO) between 0.86 and 0.97. They show weak or moderate fractionation of heavy REE and negative europium anomalies that increase from the less evolved to the more evolved facies. The different facies display geochemical affinities with within-plate granites and A-type granites of the A2-subtype and are classified as ferroan granites. The Bannach granite evolved through fractional crystallization of titanomagnetite + ilmenite + zircon + apatite + hornblende ± clinopyroxene and feldspars minerals. The diferentiation trend was in the sense: ↔biotite-amphibole monzogranite↔amphibole-biotite monzogranite↔Worphyritic biotite monzogranite↔leucomonzogranites. The late medium-even-grained leucomonzogranites facies is interpreted as a separate intrusion derived from strongly evolved liquids. The compositional gap between the clinopyroxene-biotite-amphibole monzogranite and the biotite-amphibole monzogranite facies suggests that the biotite-amphibole monzogranite liquid was not derived from the clinopyroxene-biotite-amphibole monzogranite by a simple fractional crystallization process. The clinopyroxene-biotite-amphibole monzogranite had a particular magmatic evolution, involving the participation of cumulatic processes. The Bannach granite displays a remarkable similarity with the Jamon, Musa and Redenção granites which constitute the Jamon Suite, justifying the attribution of the former to this suite.

GEOLOGIA, PETROGRAFIA E GEOQUÍMICA DO GRANITO ANOROGÊNICO BANNACH, TERRENO GRANITO-GREENSTONE DE RIO MARIA, PARÁ

The Bannach Granite is intrusive in to Archean rocks of the Rio Maria Granite- Greenstone Terrane, located at the southeastern border of the Amazonian craton. Eigth varieties of monzogranites have been recognized in this Paleoproterozoic batholith: amphibole + biotite ± clinopyroxene coarse-grained monzogranites: clinopyroxene- biotite-amphibole monzogranite, biotite-amphibole monzogranite, and amphibole-biotite monzogranite; Porphyritic biotite monzogranite; leucomonzogranites (coarse-grained, early and late medium-even-grained monzogranite and fine, even-grained monzogranite). The massif is zoned with the less evolved facies situated along the border and the more evolved facies in the center. The different facies are subalkaline, metaluminous to peraluminous, display K 2 O/Na 2 O ratios between 1 and 2 and FeOt/(FeOt + MgO) between 0.86 and 0.97. They show weak or moderate fractionation of heavy REE and negative europium anomalies that increase from the less evolved to the more evolved facies. The different facies display geochemical affinities with within-plate granites and A-type granites of the A2-subtype and are classified as ferroan granites. The Bannach granite evolved through fractional crystallization of titanomagnetite + ilmenite + zircon + apatite + hornblende ± clinopyroxene and feldspars minerals. The diferentiation trend was in the sense: biotite-amphibole monzogranite amphibole- biotite monzogranite Worphyritic biotite monzogranite leucomonzogranites. The late medium-even-grained leucomonzogranites facies is interpreted as a separate intrusion derived from strongly evolved liquids. The compositional gap between the clinopyroxene-biotite-amphibole monzogranite and the biotite-amphibole monzogranite facies suggests that the biotite-amphibole monzogranite liquid was not derived from the clinopyroxene-biotite-amphibole monzogranite by a simple fractional crystallization process. The clinopyroxene-biotite-amphibole monzogranite had a particular magmatic evolution, involving the participation of cumulatic processes. The Bannach granite displays a remarkable similarity with the Jamon, Musa and Redencao granites which constitute the Jamon Suite, justifying the attribution of the former to this suite.