Granitic Bodies Research Articles

PETROLOGY OF THE CRUSTAL PLUTONIC ROCKS OF NAWEOBA BLOCK, ZHOB OPHIOLITE, BALOCHISTAN, PAKISTAN

Zhob Ophiolite complex is composed of three detached blocks named Omzha, Ali Khanzai and Naweoba blocks. The crustal plutonic section of the Naweoba block is mapped and divided into gabbro and granite. Based on petrographical studies, the gabbros fell in the domain of gabbro, gabbronorite, and hornblende gabbro while granitic rocks fell in the vicinity of quartz-rich granitic rocks, granodiorite, plagiogranite and tonalite. Gabbroic rocks cover the maximum area of the crustal plutonic section and are usually medium-grained while at many places the grain size is quite large to be seen with naked eyes. Minerlogically gabbroic rocks consist of orthopyroxene, clinopyroxene, amphibole and plagioclase. These rocks maybe the fragments of main crustal plutonic section of the Zhob ophiolite. The granitic rocks having mafic minerals dominted in the eastern portion, while the felsic minerals dominted ones are in the west. The eastern side of the granitic body is compact and massive compared to western portion which is quite altered and shattered. Granitic rocks are composed of plagioclase, alkali feldspar and quartz where rutile and Cr-spinel exist in trace amounts. The gabbros of Naweoba block may have formed in a magma chamber as a result of fractional crystallization. While the granites maybe a late magmatic differentiate from the same magma chamber. The close correlation of gabbroic and granitic rocks of Naweoba block with Muslim Bagh, Khanozai and Bela ophiolites suggests their formation in supra subduction zone setting.

Paleoproterozoic arc magmatism in the southern Amazonian Craton, Brazil: Constraints from geology, geochemistry, and geochronology of granitic rocks

This work presents new geological, geochemical, and geochronological data of Paleoproterozoic granites located in the south-central sector of the Amazonian Craton. This region falls within the Peixoto de Azevedo Domain in the southern Tapajós-Parima Tectonic Province. The results here presented are of the utmost importance in advancing geological knowledge of the Alta Floresta Gold Province. The study area encompasses granitic rocks of the Matupá Intrusive Suite, and of the newly individualized Teles Pires Intrusive Suite. The latter includes two granitic bodies that have been previously ascribed to the Matupá Intrusive Suite. The Matupá Intrusive Suite was divided into two facies: Facies 1, composed of syenogranites and monzogranites, and Facies 2, composed of granodiorites. Granodiorites of Facies 2 have chemical characteristics similar to that of magnesian and metaluminous granites, while syenogranites and monzogranites of Facies 1 are ferrous and slightly peraluminous. Petrographic and geochemical features demonstrate the calc-alkaline nature of the Matupá Intrusive Suite which is typical for I-type granites from of volcanic arc settings. Magmatic zircon crystals yielded U–Pb LA-ICP-MS ages at 1864 ± 19 Ma and 1859 ± 8 Ma for syenogranite and monzogranite of Facies 1, respectively, and an age of 1881 ± 5 Ma for granodiorite of Facies 2. These are the crystallization ages for intrusive bodies of the Matupá Intrusive Suite. The Teles Pires Intrusive Suite varies from syenogranite to monzogranite whose calc-alkaline signature can be linked to the setting of a mature magmatic arc. U–Pb zircon in situ dating with LA-ICP-MS yielded upper intercept ages at 1790 ± 6 Ma and 1793 ± 7 Ma which are interpreted as its magmatic crystallization. Besides these crystallization ages, inherited zircon crystals also provided an older intercept age of 1870 ± 14 Ma, which is correlatable with the Matupá Intrusive Suite. Crustal assimilation and/or contamination are suggested for both the Matupá and Teles Pires intrusive suites due to the presence of inherited zircon populations of ca. 1960 Ma and 1900 Ma, and ca 1896 Ma and 1880 Ma, respectively. These ages are coherent with the Creporizão (Creporizão orogenic) Intrusive Suite, Tropas Intrusive Suite (Tropas orogenic), and Parauari Intrusive Suite (Parauari orogenic), which are all part of the Tapajós Domain. The two intrusive suites outcropping in the study area are therefore attributed to the Matupá Intrusive Suite, Peixoto de Azevedo Domain, and to the Teles Pires Intrusive Suite, Juruena Domain, which in turn is part of the Rondônia-Juruena Tectonic Province.

Characterisation and geodynamic setting of the 1 Ga granitoids of the Karagwe-Ankole belt (KAB), Rwanda

The Karagwe-Ankole belt (KAB) is a poorly understood deformation belt in the Great Lakes area of Central Africa, with several hypotheses put forward to explain the emplacement of the various generations of granites. The youngest generation of granites (G4; 1000 Ma) are spatially associated with Sn-W-Ta ore deposits, and are considered as the parental granites. The paragenesis of these mineral resources have been extensively scrutinised, but the geodynamic context and characterisation of these parental granites has been neglected. Tectonic models for the formation of this plutonic suite range from an intra-cratonic to full-scale continent–continent collision setting.In this study, the G4 intrusive suite of the KAB granites, are characterised based on whole-rock geochemical analysis and U-Pb zircon dating.Two main populations of U-Pb zircon ages are identified in the G4 granites at 1370–1420 Ma and 950–1020 Ma. The older population is attributed to inheritance of the bimodal magmatic Kibaran event, while the younger population is ascribed to the new crystallisation. G4 granites contain xenocrystic zircon, indicating inheritance of older granites or basement rocks. G4 granites are, however, much more differentiated compared to older Kibaran granites, containing higher K and Rb and lower Ca, Sr and Ba concentrations, which implicates recycling of older crustal material. The variable geochemistry, as well as the different proportions of xenocrystic zircons in the different granite bodies, suggest melting was highly localised and derived from a highly heterogeneous source region, for which an intracontinental setting is favoured. The Th/U ratios of the parental granites are lower than 0.1, which would suggest a metamorphic origin of the granites. Together with the low luminescence signal of these zircons an anatectic origin of the melt is considered, implying that the parental granites could be considered more like migmatites rather than actual granites formed by fractionation out of large magma chambers.

Granitoids of the Kalba batholith, Eastern Kazakhstan: U–Pb zircon age, petrogenesis and tectonic implications

This is a synthesis of published and new data on the Kalba batholith in Eastern Kazakhstan, the large granitic body in the western part of the Central Asian Orogenic Belt. The batholith consists of granodiorite-granite and leucogranite rocks discriminated on the basis of major- and trace-element chemistry and isotope systematics. The granodiorite-granite rocks, which form the bulk of the batholith, are compositionally variable and can be classified as mixed S-I-type granites. The leucogranites occurring as a few large intrusions in the northwestern part of the batholith have more stable compositions, with high contents of REE, HFSE, F, Li, and B, typical of A-granites. Judging by the isotope systematics of the Kalba granites, compared with that of their potential parent rocks from the Kalba-Narym zone and its surroundings, the two groups originated by different mechanisms in two magmatic events. The granodiorite-granites were produced by large-scale melting of crustal material, including the metabasaltic basement and overlying metavolcanic and metasedimentary rocks. The origin of leucogranites was associated with low-degree partial melting of the deepest Kalba-Narym sediments under the effect of fluoride fluids. The batholith formation spanned about 21 myr: granodiorites and granites formed in the 297–286 Ma interval and leucogranites between 288 and 276 Ma. The ages of the two events bracket the intraplate postorogenic stage of the CAOB history that was coeval to the formation of the Tarim large igneous province.

Engineering characterisation of weathered rock at Sri Jaya, Pahang, Malaysia

Erosion and weathering are major processes that may reduce the strength of rock slope material or may cause significant change in the physical state of the slope. These can lead to landslide or slope failure, bringing significant risk in terms of properties damage and/or human safety. In Sri Jaya, Pahang, a large body of granite is exposed as cut rock slope, which shows different zones of weathering grade. Field and laboratory tests were carried out to determine the mechanical properties and the physical properties of weathered rocks of the slope, which involved a number of testing for the Schmidt rebound hammer test, point load test, moisture content, water absorption and specific gravity. From the tests, the study has determined the engineering properties of weathered rock of the slope where weathering grade and engineering properties of rock were correlated. A strong correlation was found between the weathering grade and engineering properties where they showed significant correlations for this site. It is proven that the engineering properties could be used as reliable parameters in determining the weathering grade of material in the field.

When zircon drowns: Elusive geochronological record of water-fluxed orthogneiss melting in the Velay dome (Massif Central, France)

Zircon U-Pb geochronology is routinely performed to unravel the timing and duration of melting events within the continental crust. A comprehensive understanding of the zircon behavior during anatexis is therefore paramount to accurately assess the impact of crustal melting on orogenic processes. We report on an anatectic system from the southern part of the Velay dome (Variscan French Massif Central) encompassing late Carboniferous migmatites developed at the expense of peraluminous orthogneisses and associated with (leuco)granite bodies intrusive at higher structural levels. Our zircon U-Pb geochronological survey (494 analyzed grains) evidences a very scant record of the Variscan anatectic event. Indeed, oscillatory-zoned melt-precipitated zircon grains extracted from metatexites, leucosomes and (leuco)granites almost systematically yielded the late Neoproterozoic crystallization age of the felsic igneous protolith. Field and petrographic observations, whole-rock geochemical signatures, phase equilibrium and thermodynamically-constrained trace element modelling collectively indicate that partial melting of the orthogneisses took place at T ~ 700 °C and was fluxed by ingress of external water. The lack of new zircon growth is ascribed to a range of processes ultimately related to the low melting temperature including limited zircon solubility in the melt phase, occlusion of zircon in non-reacting source biotite and sluggish zircon dissolution kinetics. Peraluminous orthogneisses, yet highly fusible due to their “eutectic” subsolidus mineral assemblage, exhibit a very limited zircon record of low temperature water-fluxed melting. Thus, zircon has little potential to provide reliable chronological constraints on water-fluxed melting episodes affecting such lithologies.

Contrasting high-Mg, high-K rocks in Central Iberia: the appinite—vaugnerite conundrum and their (non-existent) relation with arc magmatism

The central zone of the Iberian Massif contains abundant granitic batholiths, mostly consisting of peraluminous granodiorites to granites, with minor volumes of high-Mg and high-K ultramafic, mafic, and intermediate rocks (UMI rocks), belonging to two associations: vaugnerites and appinites. The UMI rocks crop out as small stocks within anatectic domains or granite bodies, as enclaves (scattered, clusters or swarms), even as syn-plutonic dikes. SHRIMP zircon U–Pb ages range from ~ 291 Ma to ~ 328 Ma, being broadly coeval with the granites and slightly younger than the anatexites that host them. The vaugnerites are alkaline, with Na2O + K2O concentrations similar to shoshonites, whereas the appinites are comparable to high-K calc-alkaline series, but in no way can they be considered arc-rocks indicative of subduction. Compared with arc-related shoshonites and high-K calc-alkaline rocks, the Iberian UMI rocks, especially the vaugnerites, are much more enriched in mica-compatible elements such as Li, Rb, Cs, Be, Sn, Nb, etc. as well as in Th and U. They also have extraordinarily elevated production of radiogenic heat, the appinites about 1.5 µWm−3, and the vaugnerites about 3.9 µWm−3. These values contrast with the average heat production of the upper mantle (0.02 µWm−3) and the lower continental crust (0.6 µWm−3) and are even higher than the average continental crust (1.4 µWm−3) indicating extensive crustal contamination. The negative whole-rock eNdt, usually between − 2 and – 6, the elevated initial 87Sr/86Sr, usually > 0.706, and the old Nd model ages (1 Ga to 1.5 Ga), i.e., much higher than the zircon crystallization ages, reinforces this idea. Despite both rock associations being affected by crustal contamination, the processes that operated were different. Whereas appinites may have resulted from bulk contamination with anatexites plus crystal fractionation, the vaugnerites require contamination with supercritical fluids resulting from biotite breakdown. Accordingly, we propose that the production of appinites or vaugnerites depends on the fertility of the metamorphic rocks they traversed en route from the mantle to the middle crust: if the host rock was fertile and partially molten, the intrusion of mafic magma increased the melt fraction so permitting bulk contamination to produce hybrid appinite magmas; if the rock was infertile, the fluids released from micas breakdown during the metamorphism could migrate as such to the mafic magmas, contaminating them to yield vaugnerite magmas.

Uranium-Lead Geochronology of Two-Feldspar Granites of the Inhul Megablock (Ukrainian Shield) by Monazite

In the area of the Novooleksandrivka village, the valley of the Bokovenka river crosses a powerful strip of metamorphic rocks of the Inhulo-Inhulets series with numerous small granitoid bodies of the Kirovohrad complex. In the right bank of the river valley north of Novooleksandrivka there are almost continuous rock outcrops of porphyry-like, mostly coarse-grained garnet-biotite granites, which are cut by veins of aplite-pegmatoid and pegmatoid granites. Uranium-lead isotope systems of accessory monazites from porphyry-like granite, layered body of uneven-grained granite and from veined body of aplite-pegmatoid granite have been studied. The age of the first two, more coarse-grained granite varieties, is 2043.2 ± 2.6 and 2041 ± 2.3 million years, respectively. Significantly younger are the monocytes from the vein of aplite-pegmatoid granite - 2030 ± 0.3 million years. In granites in this sequence, in addition to structural and textural characteristics (in general, decrease in grain size), there is a decrease in the amount of SiO2 (from 73.14 to 70.93%) connected with a significant increase in K2O (from 3.96 to 7.58%), (their inverse correlation coefficient is 0.98), a significant decrease in the CaO content from 2.04 to 0.97%, and a slight decrease in the MgO content from 1.14% to 0.82%. These changes are probably caused by the crystallizational differentiation of the original granite melt.

Whiteschist genesis through metasomatism and metamorphism in the Monte Rosa nappe (Western Alps)

Whiteschists from the Monte Rosa Nappe were examined in the field as well as with petrographic, geochemical, and isotopic methods to constrain the controversial origin of these rocks in their Alpine metamorphic context. Whiteschists occur as ellipsoidal-shaped, decametric-sized bodies, within a Permian metagranite, and consist mainly of chloritoid, talc, phengite, and quartz. The transition from whiteschist to metagranite is marked by multiple sharp mineralogical boundaries defining concentric zones unrelated to Alpine deformation. The development of reaction zones, as well as the geometry of the whiteschist suggest a pervasive fluid infiltration, facilitated and canalized by reaction fingering. Whole-rock compositions of whiteschists and metagranites indicate an enrichment in MgO and H2O and depletion of Na2O, CaO, Ba, Sr, Pb, and Zn in the whiteschist relative to the metagranite. Trace- and rare-earth elements, together with all other major elements, notably K2O and SiO2, were within uncertainty not mobile. Such a K and Si saturated, Na undersaturated fluid is not compatible with previous interpretations of fluids derived from ultramafic rocks, evaporites, or Mg-enriched seawater due to mantle interactions. Together with the large variations in δD and δ18O values, this indicates large fluid fluxes during metasomatism. Calculated δD and δ18O values of fluids in equilibrium with the whiteschist support a magmatic–hydrothermal fluid source, as does the chemical alteration pattern. Bulk rock 87Sr/86Sr ratios in whiteschists confirm a pre-Alpine age of fluid infiltration. The 87Sr/86Sr ratios in whiteschists were subsequently partially homogenized in a closed system during Alpine metamorphism. In conclusion, the granite was locally affected by late magmatic–hydrothermal alteration, which resulted in sericite–chlorite alteration zones in the granite. The entire nappe underwent high-pressure metamorphism during the Alpine orogeny and the mineralogy of the whiteschist was produced during dehydration of the metasomatic assemblage under otherwise closed-system metamorphism. While each whiteschist locality needs to be studied in detail, this in-depth study suggests that many whiteschists found in granitic bodies in the Alps might be of similar origin.

Metallogenic characteristics and genesis of granite type uranium ore bodies in South China

South China is the key producing area of granite-type uranium deposits in China. After decades of exploration, many important progress has been made in the study of metallogenic regularity of granite type uranium deposits in this area. On the basis of previous studies, this paper attempts to sort out the geological conditions and characteristics of diagenesis and mineralization of granite type uranium deposits in South China, and discuss their metallogenic models, so as to better summarize the metallogenic regularity and serve the prospecting and prediction.

Deformation mechanisms, mineral chemistry and zircon U-Pb geochronological constraints in the south Patos shear zone: implications for the crustal evolution of the Borborema Province, NE Brazil

The southern boundary of the Patos Shear Zone (PSZ) is characterized by mylonites resulting from the deformation of Paleoproterozoic basement gneisses, Neoproterozoic metavolcanic and metasedimentary rocks, granitic and granodioritic intrusions. Among the latter, the Santa Terezinha and Catingueira plutons show stretched shapes in agreement with the the shear zone’s regional trend. Recrystallized quartz grains in these plutons accommodate deformation by dislocation creep. The host banded gneisses show similar deformation structures to the plutons. Quartz crystallographic fabrics mainly record the activity of basal and prism slip planes with local contribution of rhomb planes, whilst feldspar clasts show evidence of intracrystalline deformation in the larger porphyroclasts. These characteristics suggest that shearing occurred at lower-amphibolite facies conditions. U-Pb zircon data for the Santa Terezinha pluton yields an age of 625 ± 7 Ma, which, coupled with the pluton elongated shape and microstructure, suggests that its emplacement was pre-kinematic in relation to the Patos shear zone. Post-emplacement elongation of the granite body occurred along the strike-slip trend. Such structural and geochronological constraints highlight the close association between magma emplacement and shear zone deformation during distinct stages of the Brasiliano-Pan-African orogeny in the Borborema Province.

The Effect of Addition of ZnO to Granite Body on Sintering and Mechanical Properties

In granite body, 1-3-5 wt% ZnO was replaced instead of feldspar and fired in an industrial continuous production kiln. Physical properties of the tiles were determined including dried strength, fired strength, water absorption, fired loss, and colorimeter values. X-Ray Diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-Ray (EDX) measurements for distinctive microstructural changes and phases formed were done. The fired strength values of the standard and 3% ZnO added body are 399.94kg/cm2, 510.50 kg / cm2, respectively. Water absorption and firing shrinkage values are close to each other. In the analysis of the sample with 3% added ZnO with XRD, it was observed that the solubility of quartz was increased, mullite formation was prevented; besides 27.4 wt.% spinel phase and 13.8 wt.% albite formation was observed.

MAGMATISMO PÓS-TECTÔNICO INVESTIGADO POR MEIO DOS MÉTODOS GEOCRONOLÓGICOS U-Pb E Lu-Hf, COMPLEXO PEDRA BRANCA, RIO DE JANEIRO – RJ

O Complexo Pedra Branca está localizado geograficamente na zona oeste da cidade do Rio de Janeiro e está inserido tectonicamente no Domínio Costeiro, pertencente ao Terreno Oriental da Faixa Ribeira Central. Os litotipos que formam o Complexo foram gerados em eventos relacionados a Orogênese Brasiliana (~600 Ma), abrigando assim rochas pré- a- pós-colisionais. O estudo de corpos graníticos pós-tectônicos na Faixa Ribeira revela que o magmatismo ocorreu em dois pulsos distintos, com intervalo de aproximadamente 30Ma entre eles. O presente estudo analisou 9 amostras do Complexo Pedra Branca, por meio dos métodos geocronológicos U-Pb e Lu-Hf, referentes às litologias granito Pedra Branca, possível representante do pulso mais antigo e granito Favela, possível representante do pulso mais jovem, a fim de melhorar a compreensão do magmatismo pós-tectônico na região. Os resultados, em sua maioria, corroboraram com os anteriormente descritos na literatura, porém indicaram que o magmatismo no complexo, ocorreu na forma de pulsos progressivos, diminuindo o tempo de intervalo esperado para o mesmo (~20Ma). O indicador petrogenético εHf indicou participação de fontes crustais e mantélicas, dando caráter bimodal ao magmatismo.

Timing and origin of the Shesuo skarn Cu-polymetallic deposit in the northern Lhasa subterrane, central Tibet: Implications for Early Cretaceous Cu(Mo) metallogenesis in the Bangong-Nujiang metallogenic belt

The Bangong-Nujiang metallogenic belt is a newly discovered copper-polymetallic belt in the Tethyan domain. To date, the Shesuo skarn Cu-polymetallic deposit is the only productive Early Cretaceous Cu deposit in the middle segment of this belt. We conducted zircon U-Pb and molybdenite Re-Os dating of this deposit, and the results show that the Shesuo skarn Cu-polymetallic deposit formed at 117–112 Ma. Within the mining area, granodiorites and monzogranites are the main metallogenic granitic bodies. The petrogeochemical features imply that the granodiorites and monzogranites were derived from the partial melting of juvenile lower crust resulting from mantle-derived magmatic underplating, with the additional involvement of ancient crust in the magma chamber, whereas the monzogranites experienced significant fractional crystallization. The δ34S values of sulfide ores suggest that they were mainly sourced from the magma and slightly contaminated by surrounding strata. The Pb isotope compositions of sulfides and Re contents of molybdenite indicate that the metals were derived from the juvenile lower crust, with a small fraction from ancient crustal materials. Our study suggests that the generation of the Shesuo deposit was most likely triggered by slab breakoff during the southward subduction of the Bangong-Nujiang Ocean lithosphere. The Shesuo and Xiongmei deposits have low zircon Ce4+/Ce3+ and EuN/Eu* ratios and low whole-rock average V/Sc, Sr/Y and (EuN/Eu*)/YbN ratios. We propose that these values are due to the input of reduced components, leading to the generation of metallogenic magmas with relatively low magmatic oxidation states and water contents and rendering the magmas infertile for later porphyry-skarn Cu formation in the Xiongmei area.

The Yichun Ta-Sn-Li Deposit, South China: Evidence for Extreme Chemical Fractionation in F-Li-P-Rich Magma

Abstract The Yichun Ta-Sn-Li deposit occurs within the 9.5-km2 Yashan Igneous Complex, which is made up of felsic, peraluminous intrusions including two-mica, Li-mica, and topaz-lepidolite granites. The topaz-lepidolite granite forms a thin sheet confined to the upper part of the Li-mica granite and is separated from the host metasediments by a marginal pegmatite. The topaz-lepidolite granite is characterized by the presence of snowball textures in which quartz, K-feldspar, and topaz phenocrysts contain albite laths and rare lepidolite and columbite arranged along growth zones. These textures indicate simultaneous crystallization of these phases and attest to the subsolvus character of the topaz-lepidolite granite. Minor and accessory phases in the topaz-lepidolite granite include amblygonite, cassiterite, columbite-tantalite, microlite, wodginite, zircon, monazite, and pollucite. The topaz-lepidolite granite is characterized by low SiO2 (68.6–69.7 wt %), TiO2 (0.01 wt %), Fe2O3(total) (0.15–0.33 wt %), MgO (0.01–0.05 wt %), and CaO (0.15–0.19 wt %), whereas Al2O3 (17.97–18.26 wt %), Li2O (0.90–2.09 wt %), P2O5 (0.43–0.54 wt %), and F (1.09–2.30 wt %) are all very high. Na2O ≫ K2O due to both high Na2O (5.18–6.30 wt %) and low K2O (2.63–3.05 wt %). The topaz-lepidolite granite contains extremely high concentrations of Rb (>3,300 ppm), Cs (>340 ppm), Sn (>140 ppm), and Ta (>120 ppm), with very low Zr/Hf and Nb/Ta. Gaps of >3.5 wt % SiO2 and >2.5 wt % Al2O3 together with large increases in F, Li2O, and P2O5 between the Li-mica and topaz-lepidolite granites are not compatible with an evolution by Rayleigh fractionation processes. It is proposed instead that early crystallization of the Li-mica granite led to the formation of an Si-poor, Al-, Na-, and flux-rich boundary layer whose low viscosity and density allowed effective separation from the remaining melt and crystals. This melt concentrated incompatible elements including tin and tantalum and accumulated in the upper part of the magma chamber where it crystallized the sheet-like bodies of marginal pegmatite and topaz-lepidolite granite. The uppermost part of the topaz-lepidolite granite consists of aplite and overlying quartz lepidolite rock, which is composed mainly of quartz, lepidolite, albite, K-feldspar, topaz, amblygonite, cassiterite, and columbite. Compared to the topaz-lepidolite granite the aplite and quartz-lepidolite rock shows a number of complementary element enrichments and depletions. The quartz-lepidolite rock is enriched in Fe, Mn, K, Rb, Cs, Li, F, Ba, Sr, Ti, Zr, Nb, W, Th, and Sm. The formation of the quartz-lepidolite rock is interpreted to result from separation of an F- and Li-rich alkaline phase, which accumulated beneath the already crystallized marginal pegmatite. Complementary to this was crystallization of the strongly peraluminous sodic aplite with the highest tantalum contents measured in this study.

Mineralogy of an Appinitic Hornblende Gabbro and Its Significance for the Evolution of Rising Calc-Alkaline Magmas

The magmatic and sub-solidus evolution of calcic amphiboles and Fe–Ti oxides was investigated in the Neoproterozoic Frog Lake pluton, Nova Scotia, Canada, in order to understand the relationship between the history of hydrous magma and the resulting mineralogy. The pluton occurs as sheet-like bodies of hornblende gabbro and hornblendite, with lesser tonalite dykes and granite bodies, interlayed with screens of medium-grade metamorphic country rock. Small, diffuse clots of felsic minerals are present in the gabbro. The subsolidus growth of actinolite occurs in early clinopyroxenes and amphiboles. Ilmenite is the dominant Fe–Ti oxide, as interstitial magmatic crystals. The increase of Mn towards the margin of the ilmenite crystals indicates a gradual increase in oxygen fugacity with time, leading to the precipitation of titanite and ferrohypersthene. The replacement of titanite by ilmenite and ilmenite lamellae in the amphiboles suggests subsequent reducing conditions during the sub-solidus crystallisation. The gabbros in the coeval, but apparently shallower, Jeffers Brook granodiorite laccolith have dominant magnetite and Mg-rich subsolidus amphiboles, which are indicative of high oxygen fugacity. The differences between the two plutons suggest that there was a greater flux of hydrothermal water through the sheet-like architecture of the Frog Lake pluton.

Deep geothermal energy in northern England: Insights from 3D finite difference temperature modelling

Many of the most widely used deep geothermal resource maps for the UK are produced by contouring around sparsely distributed and often unreliable data points. We thus present a MATLAB-based 3D finite difference temperature modelling methodology, which provides a means for producing more resolute and geologically realistic versions of these maps. Our case study area in northern England represents an area where both sedimentary basins and radiothermal granite bodies comprise potential geothermal resources. We divide our 3D model into geological units, which are then assigned separate thermal properties. Assuming conductive heat transfer and steady-state and fixed boundary conditions, we calculate 3D regional subsurface temperature. Due to our averaging technique for thermal properties, the resolution of our geological model is scarcely compromised with respect to similar finite element methods. One predicted ‘hot spot’ at 1 km depth in the central part of our case study area corresponds with the granitic North Pennine Batholith. Other shallow hot spots correspond with thermally insulating sedimentary rock units and geological structures that incorporate these units. Predictive heat flow density maps highlight areas with accelerated surface heat flow associated with shallow conductive basement rock and heat producing granite bodies. Our predicted subsurface temperatures show broad similarities with measured equilibrium borehole temperatures. Inaccuracies may relate to convective heat transfer involving fault systems, or input variables relating to the geological model. Our predictive subsurface temperature and heat flow density maps are more resolute and geologically realistic relative to pre-existing contoured maps. The method presented here represents a useful tool for understanding controls on subsurface temperature distribution and geothermal potential.

Zircon U-Pb and Lu-Hf isotopes of Huai'an complex granites, North China Craton: Implications for crustal growth, reworking and tectonic evolution

The Huai'an Complex, located in the north-central North China Craton (NCC), consists of tonalite-trondhjemite-granodiorite (TTG) gneisses and minor intrusive bodies of granite. Its late Neoarchean to Paleoproterozoic crustal evolution is still unclear, and the study of this evolution will constrain the tectonic evolution of the NCC. The LA-ICP-MS zircon U-Pb ages of ten granites from the Huai'an Complex reveal three episodes of magmatism: 2.6–2.3 Ga, 2.0–1.7 Ga, and a few ages from ~2.3–2.0 Ga. The 2.6–2.3 Ga, 2.3–2.0 Ga and 2.0–1.7 Ga zircons have average εHf(t) values of 2.79, −4.0, and − 6.05, average TDM2 values of 2.78 Ga, 2.98 Ga, and 2.86 Ga, and average 176Lu/177Hf ratios of 0.033, 0.021, and 0.027, respectively, indicating that their magma sources were from the melting of the Neoarchean “mildly depleted” SCLM, the mafic Mesoarchean lower crust, and the mafic Meso-Neoarchean lower crust. Zircon geochemical characteristics indicate that the Huai'an Complex was a long-lived continental magmatic arc during 2.6–2.0 Ga. The juvenile crustal proportion was 100% during 3.1–2.6 Ga and decreased through 50% at 2.5 Ga to 20% at 2.45 Ga; it then increased to 45% at 2.3 Ga and finally decreased to 0% during 1.95–1.7 Ga, indicating that the crust experienced a growth stage during 3.1–2.5 Ga and reworking stage during 2.5–1.7 Ga and that the Huai'an Complex underwent compression-extension-compression processes during 2.6–1.7 Ga. The compression-dominant tectonic setting during 2.6–2.45 Ga may have been linked to the subduction and amalgamation of microblocks. The compressive setting during 2.3–1.7 Ga may have been related to amalgamation between the Eastern and Western Blocks. The modeled crustal growth curve suggests that approximately 67% of the existing continental crust of the Huai'an Complex formed before 2.6 Ga, which is similar to the NCC and several other cratons worldwide. Our results favor the two-stage cratonization model of the NCC.

SHRIMP U-Pb ages and REE patterns for zircon from an anatectic Variscan two-mica granite from the Bemposta Migmatite Complex (Central Iberian Zone)

ABSTRACT The Variscan Bemposta Migmatite Complex (BMC) in northern Portugal (Central Iberian Zone) is a NE–SW-trending high-grade metamorphic core complex comprising upper-amphibolite- to lower-granulite-facies metapelites and metagreywackes of Ediacaran-Cambrian age and subordinate Ordovician orthogneisses showing evidence of intense migmatization. The available petrological data indicate that these rocks attained peak metamorphic conditions at the end of the first Variscan contractional deformation event (D1), followed an isothermal decompression path during D2 crustal extension, and underwent subsequent retrogression during D3 shearing. The whole BMC complex hosts numerous concordant intrusive bodies (sheets several meters thick) of syn-D2 two-mica granites, genetically linked to the leucosomes, suggesting that the emplacement of these magmas was synchronous with core complex extension. U-Pb SHRIMP ages obtained from zircons from one of these syn-D2 tabular plutons (the Faia d'Água Alta granite) yielded a crystallization age of 324 ± 3 Ma, providing a good estimate for the age of the D2 anatectic event in the region.

Efficient regional scale 3D potential field geophysical modelling to redefine the geometry of granite bodies beneath prospective, geologically complex, northwest Tasmania

In this study, a regional model that defines the three-dimensional geometry of the subsurface geology beneath the complex, prospective northwestern Tasmania has been developed. This has been achieved using a series of potential field inversions constrained by surface geology, geological sections, seismic interpretations and a newly extended petrophysical dataset. Three major episodes of granitic magmatism are preserved in Tasmania: in the Neoproterozoic, Cambrian and Devonian. Granite bodies are hence considered important indicators of mineralization for explorers in an area of challenging vegetation, topography and cover sequences. Forward modelling and property-based inversions of the pre-existing geological model show that the previously interpreted subsurface geometry is not compatible with potential field data. Four sub-regions displayed a large discrepancy between calculated and observed data. This study redefines the subsurface geometries of these sub-regions through individual geometry inversions. The density and magnetic susceptibility ranges of units are further refined through property inversions. The modified geometry of the Devonian granites in the four sub-regions may be summarized as follows: 1) the Housetop Granite is relatively thin (≤5 km thickness), whereas 2) the Heemskirk and Meredith Granites are very thick and granite extends to a shallower depth between these bodies than previously interpreted. This region between plutons is thus a more prospective region than previously thought. 3) For the first time, an intrusive body underlying the eastern part of the Rocky Cape Group has been identified. Its petrophysical properties are similar to that of a granite, and its top is interpreted at a depth of >3 km. This interpreted low density (granitic) unit may be either Neoproterozoic or Devonian. 4) A new non-magnetic, low density Cambrian granite, with a minimum burial depth of 1 km, is also modelled in 3D, within the Mount Read Volcanics, in the south of the study area. Our approach, whereby sub-regions are identified for more detailed modelling, enables new constraints to be introduced in a computationally efficient way, and has general application to refining the geometry of key structures in prospective regions.