Mafic Granites Research Articles

Geologic-tectonic setting of the gold-endowed Kilo Terrane in the eastern Central Kibalian Superterrane, northeastern Democratic Republic of Congo

Solid geological mapping, granite geochemistry and robust U-Pb SHRIMP geochronology confirm our subdivision of the eastern Central Kibalian Superterrane (ECKS) into two terranes: the ca 2570–2552 Ma Western Ituri Terrane and the ca 2680–2570 Ma Kilo Terrane. The ECKS is part of the Central, Northern and Southern Kibalian Superterranes, first-order interpretative crustal blocks, which constitutes the Kibalian Greenstone Region in the Bomu-Kibalian craton. The Kilo Terrane comprises: 1) the lower Kibalian lithostratigraphic units of deep-sea facies mafic–ultramafic volcanic rocks, interlayers of pelitic-sedimentary rocks and mafic granite and diorite-tonalite complexes in the Galaya-Kilo Sub-Terrane; and 2) the upper Kibalian lithostratigraphic units of intermediate-felsic volcanic-sedimentary, mafic–ultramafic intrusive rocks and widespread quartz-dioritic, tonalitic to granodioritic granites in the Balazana-Bunia Sub-Terrane.Mafic granite and diorite-to-tonalite magmatism in the Kilo Terrane is demonstrated by: 1) the ca 2680–2640 Ma syn-volcanic mafic-dioritic granite magmatism, including the ca 2650–2640 Ma albitic granites extensive in the Nizi-Tsi Belt; 2) the ca 2640–2600 Ma syn-volcanic hornblende tonalite-to-granodiorite magmatism in the Kilo and Moto-Zani Terranes; and 3) a ca 2600–2570 Ma syenite, quartz-monzonite and felsic granite intrusions along extensional zones and reactivated major terrane boundaries. The coincidence between the ca 2603 Ma tectonic uplifting and ca 2605–2590 Ma hydrothermal activity in the Kilo Terrane is associated with the gold mineralization.The 2711–2790 Ma xenocryst ages point to potential existence of the > 2790 Ma basement rocks extending from underneath the Western Ituri Terrane into underneath the Bunia West Domain. It’s absence in the gold-hosting domains of the Kilo Terrane does not exclude the existence of > 2.8 Ga basement, but instead it invokes on the existence of an unconformity separating the basement rocks from rootless, ca 2680–2570 Ma complexly folded and faulted juvenile granite-greenstone domains, sutured together by first- to second-order fault/shear zones. Protracted periods of Archean to post-Archean tectonic uplifting and inversion were responsible for preservations and erosion of upper crustal-level granite greenstone belts, including orogenic gold depositsThe ca 2680–2570 Ma juvenile crustal addition in the Kilo Terrane occurred some 30 m.y. after the ca 2850–2600 Ma juvenile crustal addition events in well-researched Archean cratons, worldwide.

Geology and geochronology of the Archean plutonic rocks in the northeast Democratic Republic of Congo

Here we report the first regional-scale study of Archean plutonic rocks from 50,000 km2 of the northeast Democratic Republic of Congo (DRC). We include 50 new U-Pb zircon Sensitive High-Resolution Ion MicroProbe (SHRIMP) ages supported by petrographic and whole rock geochemical data from a further ~400 samples, and 39 additional U-Pb zircon ages reported by Kabete et al. (submitted) and Bird (2016).Felsic-intermediate magmatism across the northeast DRC occurred from ~ 3200 to 2530 Ma. Plutonism before ~ 2670 Ma is restricted to the West Nile Gneiss in eastern and northern-most DRC, whereas plutonism across the remainder of the northeast DRC initiated between 2670 and 2640 Ma. Two Neoproterozoic granite plutons ~ 1000–950 Ma are also present within the West Nile Gneiss. Magmatism between ~ 2670 and 2530 Ma marks the principal period of crustal growth in the northeast DRC. Felsic-intermediate plutonism between ~ 2670 and 2600 Ma included two suites of magnesian, calc-alkaline rocks, (1) a sodic-calcic suite with high Sr/Y ratios (>40) comparable with tonalite-trondhjemite-granodiorite (TTG) suites in other Archean terranes, and (2) a calc-alkaline to high-K suite of granitoids with low Sr/Y ratios (<40) and lower Na and Al contents, comparable with “low-Ca” granites in the Yilgarn Craton. In contrast, most granitoids emplaced between ~ 2600 and 2530 Ma have ferroan, high-K calc-alkaline compositions, and are enriched in high field strength (HFSE) and rare Earth elements (REE). Although the HFSE-elevated granitoids are comparable with Closepet-type, and “mafic granites” in other Neoarchean terranes, their volume within the northeast DRC appears unprecedented.Before ~ 2600 Ma partial melting of hydrous metabasalts at depths > 40 km is inferred to have produced the high Sr/Y magnesian granitoids, whereas the low Sr/Y magnesian granitoids were derived from melting-assimilation-storage-hybridization (MASH) processes in the overlying low-mid crust during the same period. Trans-lithospheric extension after ~ 2600 Ma, enabled asthenospheric mantle to interact with the base of the crust, resulting in high-temperature melting that produced the HFSE-elevated granitoids. Inherited Meso- and Paleoarchean zircons are almost completely restricted to the area within 30–40 km of the West Nile Gneiss, implying most of the northeast DRC is underlain by juvenile Neoarchean crust.

Genesis of Manganese Ore Occurrences of the Olkhon Terrane the Western Baikal Region

Geological and mineralogical data are reported on the manganese occurrences of the Olkhon terrane (Western Baikal region), which are localized in metadolerites of the Ustkrestovsky Complex, high-temperature mafic hornfels, granites, calcitic marbles and calciphyres, and occasionally are developed as separate veins in gneiss granites or small lenses in quartzites. Most of them are made up of high-temperature mineral assemblages (Opx + Cpx + Pl + Ilm ± Grt ± Bt ± Amp), the main manganese carriers in which are ferrorhodonite (33–36 wt % MnO), orthopyroxene (6–12 wt % MnO), and ilmenite (3–16 wt % MnO). Obtained data are in conflict with traditional concepts that these rocks are gondites (manganese-rich metamorphosed sediments) or that manganese flux in carbonate sediments was related to the volcanic activity that occurred simultaneously with sedimentation at about 500 Ma. The diversity of manganese occurrences was produced by metasomatic processes that occurred almost simultaneously with regional metamorphism and emplacement of subalkaline mafic bodies during collisional tectonogenesis (about 470 Ma).

Phase equilibrium modelling of granite magma petrogenesis: B. An evaluation of the magma compositions that result from fractional crystallization

Several fractional crystallization processes (flow segregation, gravitational settling, filter-pressing), as well as batch crystallization, have been investigated in this study using thermodynamic modelling (pseudosections) to test whether they are able to reproduce the compositional trends shown by S-type granites. Three starting compositions comprising a pure melt phase and variable amounts of entrained minerals (0, 20 and 40wt.% of the total magma) have been used to study a wide range of likely S-type magma compositions. The evolution of these magmas was investigated from the segregation from their sources at 0.8GPa until emplacement at 0.3GPa in an adiabatic path, followed by isobaric cooling until the solidus was crossed, in a closed-system scenario. The modelled magmas and the fractionated mineral assemblages are compared to the S-type granites of the Peninsula pluton, Cape Granite Suite, South Africa, which have a composition very similar to most of the S-type granites. The adiabatic ascent of the magmas digests partially the entrained mineral assemblage of the magmas, but unless this entrained assemblage represents less than 1wt.% of the original magma, part of the mineral fraction survives the ascent up to the chosen pressure of emplacement. At the level of emplacement, batch crystallization produces magmas that only plot within the composition of the granites of the Peninsula pluton if the bulk composition of the original magmas already matched that of the granites. Flow segregation of crystals during the ascent and gravitational settling fractional crystallization produce bodies that are generally more mafic than the most mafic granites of the pluton and the residual melts have an almost haplogranitic composition, producing a bimodal compositional distribution not observed in the granites. Consequently, these two processes are ruled out. Filter-pressing fractional crystallization produces bodies in an onion-layer structure that become more felsic with increasing crystallization, culminating in a haplogranitic melt, and is able to reproduce the compositional trends of the granites, but only if the original magmas already had the composition of the granites. Filter-pressing fractionation produces a mineral assemblage that is 1.5 times more mafic than the magma fraction from which it is derived. However, the mineral assemblages produced by crystallization of an originally pure melt phase are still too felsic to account for the bulk of the granites of the Peninsula pluton. For filter-pressing to produce the most mafic granites of the pluton, the original magmas must already contain an entrained mafic mineral assemblage and have the same composition of the granites, otherwise the modelled trends do not match the maficity (FeO+MgO) or the slope against maficity of the granites. Crystallization of the magma in filter-pressing releases a free water phase, whose amount depends on the amount of water of the original magma, and whose behaviour may be controlled by a water-saturation front. In summary, the main control in the composition of S-type granites is the amount and nature of the entrained mineral assemblage, and filter-pressing fractional crystallization can only modify slightly the compositions of the granitic bodies derived from these magmas.

Evaluation of structural and geological factors in orogenic gold type mineralisation in the Kervian area, north-west Iran, using airborne geophysical data

The Piranshahr-Sardasht-Saqqez Zone (PSSZ) in the north-west of the Sanandaj-Sirjan metamorphic zone (SSZ) hosts some major Iranian gold deposits. In the south-east of PSSZ, there is a north-east trending orogenic gold belt which contains three gold deposits/occurrences (Qolqoleh, Kervian and Ghabaghloujeh). In this research, studies are focused on processing and analysing airborne magnetic and radiometric data in order to find applicable indicators for prospecting gold in this area.Former studies on the gold deposits/occurrences in the study area suggest three essential factors in local orogenic gold mineralisation: (1) intersecting deep bending structures/shear zones, (2) Fe-rich mafic meta-volcanic lithologies (primary source and host rocks) and (3) altered mylonitic granites (secondary host rock). Geological structures and lithological contacts can be mapped based on locating edges in the magnetic field at different depths.In this study, we extracted the structure from aeromagnetic data by reduction to the pole, upward continuation and applying a tilt derivative filter to the horizontal derivative of the upward continued data. Upward continuation was to several levels from 500 to 4000 m. Afterwards, a 3D architecture was built based on extracted subsurface lineaments in different levels. This 3D model can assist in the visualisation of the underground shape of structures that may influence gold mineralisation.Moreover, mafic meta-volcanic rocks in the study area, which contain magnetic minerals such as magnetite, titanomagnetite and ilmenite, can be mapped using aeromagnetic data. Mylonitic granites, which are the other host rock in the deposits, were mapped using airborne radiometric data.

강우 침투 특성 분석을 위한 전기비저항 모니터링 탐사

강우에 수반된 급경사지의 붕괴는 지반에 수분이 침투하여 평형상태에서 중력방향으로 지반이 이동하는 현상으로 지형과 지질 등에 의한 내적 요소와 강우, 지진 등에 의한 외적요인에 따라 붕괴의 정도가 달라진다. 암종에 따른 수분의 침투특성을 파악하기 위해 백악기말 고철질화강암이 분포하는 부산 금정산 지역과 퇴적암이 분포하는 지역 중 단양지역(사암), 진주지역(셰일), 포항지역(이암-미고결퇴적암) 등 4개 지역을 시범지역으로 선정하여 전기비저항 모니터링탐사를 실시하였다. 탐사기간은 우기가 시작되는 6월부터 하절기를 지나 건기에 해당하는 11월까지 수행하였다. 모니터링 결과 금정 지역과 진주 지역은 표준 강우량 산정 시 우기 동안의 선행강우량의 영향은 6월에서 10월까지의 전체 강우를 대상으로 산정할 필요성이 있고 반감기의 산정 시 이에 대한 고려가 필요할 것으로 판단된다. 또한 저비저항 이상대의 수직적인 분포는 수분의 침투가 지표에서 하부 지반으로 균일하게 발생하는 것이 아니라 풍화대의 균열이나 토층의 이완된 간극을 따라 발생하고 있다. 포항지역 미고결퇴적암은 수분의 침투 형태가 화강암 및 쇄설퇴적암과는 상이하며, 강우 이후 하부지반으로 침투되는 속도가 빠르고 하부의 암반대수층으로 함양되는 기간이 짧아 현재 사용되는 표준강우량 산정 시 적용하는 선행강우량의 영향기간과 유사한 시간을 보인다. 지반의 함수 정도에 의해 단양의 쇄설성 퇴적암 분포지의 하부 지반에서 관찰되는 저비저항 이상대의 유지 기간은 포항지역과 유사하며, 집중호우 시측방으로 수분이 확산되는 정도는 진주의 쇄설성 퇴적암과 같은 형태를 보인다. 이상의 분석결과에서 단양 지역은 선행 강우량이 미치는 기간이 3주 정도로 추정된다. During rainfall period, to identify the characteristics of the infiltration of moisture, electrical resistivity monitering survey was carried out to weathered zone. Four regions of geophysical exploration areas with different rock types, four regions were selected. An area consists of mafic granite and three areas are composed of sedimentary rocks (Sandstone, Shale, Unconsolidated Mudstone). Survey was conducted from June (rainy season) to November (dry season), and during the period the change in resistivity was observed. According to the result of monitoring exploration on Geumjeong and Jinju areas, for the estimation of the standard rainfall, it is necessary to estimate the effects of the antecedent rainfall during the rainy season based on the overall rainfall from June till October and also necessary to consider this for the estimation of the half period. Also, the vertical distribution of the low resistivity anomaly zone does not show that the infiltration of moisture does not occur uniformly from the surface of the ground to the lower ground but shows that it occurs along the relaxed gap of the crack or soil stratum of the weathering zone. In Pohang area, the type of moisture infiltration is different from that of the granite or sedimentary rock. Since, after the rainfall, the rate of infiltration to the lower ground is high and the period of cultivation to the lower bedrock aquifer is short, it has similar effect to that of the antecedent rainfall applied for the estimation of the standard rainfall being presently used. In Danyang, due to the degree of water content of the ground, the duration period of the low resistivity anomaly zone observed in the lower ground of the place where clastic sedimentary rock is distributed is similar to that in Pohang area. The degree of lateral water diffusion at the time of localized heavy rain is the same as that of the sedimentary rock in Jinju. According to the above analysis results, in Danyang area, the period when the antecedent rainfall has its influence is estimated as three weeks or so.

Selective entrainment of peritectic garnet into S-type granitic magmas: Evidence from Archaean mid-crustal anatectites

Entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites, as well as the chemical connection between granite magmas and their sources. This concept has become linked to models for magma migration out of the source, as restite entrainment is considered to take place when diatexitic sources mobilise en masse. This is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water-undersaturated magmas that ascended through narrow conduits. We investigate pelitic migmatites from the Mkhondo Valley Metamorphic Suite (MVMS) in Swaziland, where a mid-crustal heating event produced metatexitic migmatites with minimal post-anatectic recrystallisation. In these rocks all the garnet is peritectic, having arisen through biotite fluid-absent melting, which produced garnet poikiloblasts characterised by inclusions of melt, quartz and biotite. Leucosomes that represent sites of melt transfer carry similar, smaller (typically < 1 mm), entrained garnet poikiloblasts, which were capable of amalgamating to form larger composite grains. In anatectic structures where melt was present for longer, entrained garnet was extensively recrystallised, via a dissolution-precipitation process, to adopt a more magmatic character. The peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and HREE-rich accessory phases, while the recrystallised garnet in the larger melt-filled structures became progressively better equilibrated with these minerals. Thus, peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk-rock composition, and, concurrent rapid magma segregation prevented the development of diatexitic source conditions. The segregated magma consisted of melt, the peritectic assemblage (principally garnet) and the accessory minerals monazite and zircon. These rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction. Importantly, under the circumstances which produced the MVMS anatectites, peritectic garnet is entrained as < 1 mm poikiloblasts, demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic.

The late- to postorogenic transition in the Apiaí domain, SE Brazil: Constraints from the petrogenesis of the Neoproterozoic Agudos Grandes Granite Batholith

A remarkable change in the typology of granite magmatism occurred during the late evolution of the Agudos Grandes Batholith (Apiaí domain, SE Brazil), marked by the appearance of A-type granites some 10–15 Myr after the last manifestations of the compressional period. The elemental and Sr–Nd isotope geochemistry of the approximately 600 Ma, “late-orogenic”, moderately peraluminous Piedade, Serra dos Lopes, and Pilar do Sul granites shows that the middle crust was heated to approximately 820–850 °C during the latest stages of the compressional period and generated large amounts of melt through biotite dehydration–melting of gneiss protoliths. These crustal magmas either formed independent intrusions or mixed in varied proportions with raising high-K calc-alkaline magmas, as suggested by continuous trends toward more mafic granites (65–70 wt% SiO 2), which have the geochemical signature of contaminated calc-alkaline magmas. The postorogenic granites occur as two subgroups with different age (∼585 and ∼565 Ma), both with the chemical fingerprints typical of A-type granites (low mg#, Ba, and Sr; high Zr, Hf, Nb, and Y), interpreted as a result of crustal melting at lower P, fO 2, and a(H 2O) and higher T (up to 870 °C, indicated by zircon saturation). Significant trace-element and radiogenic isotope contrasts (e.g., εNd T = −16 vs. −11) show that the two postorogenic subgroups derived from different sources. A Sr–Nd isotope signature nearly identical to the “end-member” late-orogenic crustal melts suggests that the older postorogenic granites derived from a similar (but probably less fertile) source. The less negative εNd T of the young subgroup is unmatched by known crustal sources in the Apiaí domain and may have resulted from either an infracrustal component or mobilization of an unexposed section with less crustal residence time.

Low- and high-temperature granites

ABSTRACTI-type granites can be assigned to low- and high-temperature groups. The distinction between those groups is formally based on the presence or absence of inherited zircon in relatively mafic rocks of a suite containing less than about 68% SiO2, and shown in many cases by distinctive patterns of compositional variation. Granites of the low-temperature group formed at relatively low magmatic temperatures by the partial melting dominantly of the haplogranite components Qz, Ab and Or in H2O-bearing crustal source rocks. More mafic granites of this type have that character because they contain restite minerals, often including inherited zircon, which were entrained in a more felsic melt. In common with other elements, Zr contents correlate linearly with SiO2, except sometimes in very felsic rocks, and Zr generally decreases as the rocks become more felsic. All S-type granites are apparently low-temperature in origin. After most or all of the restite has been removed from the magma, these granites may evolve further by fractional crystallisation. High-temperature granites formed from a magma that was completely or largely molten, in which zircon crystals were not initially present because the melt was not saturated in that mineral. High-temperature suites commonly evolved compositionally through fractional crystallisation and they may extend to much more mafic compositions through the production of cumulate rocks. However, it is probable that, in some cases, the compositional differences within high-temperature suites arose from varying degrees of partial melting of similar source rocks. Volcanic equivalents of both groups exist and show analogous differences. There are petrographic differences between the two groups and significant mineralisation is much more likely to be associated with the high-temperature granites. The different features of the two groups relate to distinctive source rock compositions. Low-temperature granites were derived from source rocks in which the haplogranite components were present throughout partial melting, whereas the source materials of the high-temperature granites were deficient in one of those components, which therefore, became depleted during the melting, causing the temperatures of melting to rise.

Common Crustal Source for Contrasting Peraluminous Facies in the Early Paleozoic Capillitas Batholith, NW Argentina

The Ordovician Capillitas batholith, part of the northern Pampean ranges, NW Argentina, exhibits two peraluminous granitic facies in its eastern portion: (a) coarse- to medium-grained, porphyritic mafic facies with biotite, cordierite and aluminosilicates, carrying sillimanite-, cordierite-, and andalusite-bearing migmatitic enclaves and schlieren and (b) enclave free, mica poor, coarse-grained, porphyritic felsic facies, with andalusite and sillimanite. Banded hornfels aureoles contain cordierite poikiloblasts, biotite, plagioclase and quartz. The low-P mineral assemblage in these granites, enclaves and restites, suggests partial fusion of a supracrustal protolith. The two facies plot as two separate groups in geochemical variation diagrams, suggesting that they evolved from different magma batches derived from the same source, rather than from in situ fractional crystallization. The composition of felsic facies granites corresponds to pelite and metagraywacke-derived melts, whereas cordierite-bearing mafic granites follow a trend indicating mixing of pelite-derived melts and corresponding restites. The mafic-facies granites approach more the continental crust composition than the felsic-facies ones, which display more pronounced Ba and Sr negative anomalies. The average (La/Yb) N ∼11 rules out a high pressure garnet-rich source and the low normalized Sr contents, in both granitic facies, suggest a recycled metasedimentary protolith. Absence of mafic intrusives that could have assimilated pelitic schists, allow us to infer that melting took place at rather low T and under high water activity. Heat to trigger partial fusion could have been radiogenically generated and stored in the upper crust during deformation and thickening of the continental crust, with further release during decompression. The Capillitas batholith, emplaced close to an I, S-type granite boundary line in this region appears to be an Argentinean analogue of the Cooma Series supersuite in the Lachlan Fold Belt, emplaced close to the eastern Australian I, S-type granite boundary line.

Geochemistry and petrogenesis of S-type granites in the East Greenland Caledonides

In the Kong Oscar Fjord region, East Greenland Caledonides, the formation of peraluminous leucogranite by low-temperature (<800°C) anatexis of metasedimentary rocks, and collection of the resulting melts into granite sheets and plutons can be studied in detail. Field observations suggest that a significant proportion of the biotite in the granites represents remnants of biotite from the metasedimentary source. This complicates geochemical modelling of granite formation, because chemical analyses do not necessarily represent melt compositions. Rb, Sr, Ba, and Eu concentrations in the granites are variable and suggest variations in the proportions of feldspar and mica retained in the residue. Despite uncertainties in modelling, Rb–Sr relationships suggest that granite formation was aided by the presence of externally derived H 2O-rich fluids, which could have been introduced along fluid pathways such as shear zones. The distribution of granite plutons indicates that they did not move over great distances, consistent with relatively wet magmas. Some biotite-rich granitoid rocks were formed by partial melting of the same metasediments, but at higher temperatures, during an earlier metamorphic event, while other mafic granites may have a totally different petrogenesis.

Tectonic implications of Palaeoproterozoic post-collisional, high-K felsic igneous rocks from the Kimberley region of northwestern Australia

Palaeoproterozoic high-K I-type granites, high-level porphyry intrusions, and felsic volcanic rocks of the Whitewater Volcanics dominate the Hooper and Lamboo Complexes in the Kimberley region of northwestern Australia. The granites, porphyries and volcanic rocks are gradational into each other in the field, and they have the same mineralogy, similar major and trace element abundances, and indistinguishable SHRIMP U–Pb zircon ages of 1865–1850 Ma. There is evidence of widespread mingling between the granites and coeval gabbros. Magma mixing may be important in the formation of some of the mafic granites, but most of the rocks probably formed from felsic parent magmas that underwent variable degrees of fractional crystallization. The felsic igneous rocks may have formed by partial melting of intermediate to felsic, calc-alkaline rocks along the southern and eastern margins of the Kimberley Craton, following accretion of various earlier Palaeoproterozoic terranes to the craton. Therefore, models for Palaeoproterozoic high-K granites in northern Australia that invoke intracratonic rifting of a stable Archaean craton may need to be revised.

Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites

Granites of the Lachlan Fold Belt resulted from partial melting of the crust. In most cases, fusion involved mainly quartz and feldspar, producing felsic melts. Varying degrees of separation of those melts from the unmelted source rock (restite) were responsible for much of the compositional variation seen in the granites of the belt. Less commonly, melting occurred at higher temperatures forming more mafic melts, such as for the I-type Boggy Plain Supersuite and the S-type Koetong Suite. Hence, the felsic haplogranites of the Lachlan belt dominantly formed initially as primary melts that separated from restite and less often by the fractionation of more mafic melts. Source rocks of the I- and S-type granites were undersaturated or oversaturated in Al, respectively, and the more mafic granites share that characteristic with their source. As the magmas of the Boggy Plain Supersuite evolved progressively by fractional crystallization, the rocks trended towards saturation in Al, to eventually form a mode close to Al saturation. Other felsic I-type magmas, formed directly by partial melting, were generally more oversaturated in Al, as were the corresponding S-type melts derived from peraluminous source rocks. In an unfractionated state, there are some overlaps in the degree of Al saturation in these magmas produced by partial melting. However, when extended fractional crystallization of these felsic partial melts took place, an almost complete separation in Al-saturation developed between I-type and more peraluminous S-type melts. Because apatite is soluble in peraluminous melts, P became progressively more abundant in the S-type melts as they fractionated. This led to contrasts in the abundances of P and of elements such as Y, the rare earth elements, and Th, between the strongly fractionated I- and S-type granites. Hence, such granites can easily be distinguished from each other.

High‐ and Low‐Temperature I‐type Granites

Abstract: I– and S‐type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S‐type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I‐type analogues.Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S‐types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S‐type granites are likely to be closely related in origin, with the former comprising both restite‐rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation.We propose a subdivision of I‐type granites into two groups, formed at high and low temperatures. The high‐temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low‐temperature I–type granites, the compositions extend to lower SiO2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO2 in the more mafic rocks. While the high‐temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low‐temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low‐temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite.High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H2O.

Geochemistry and Nd isotope systematics of Archaean granites of the Eastern Goldfields, Yilgarn Craton, Australia: implications for crustal growth processes

The Archaean granites and granitic gneisses of the Eastern Goldfields in the Yilgarn Craton, Western Australia, can be divided into two major groups (high-Ca and low-Ca), and three minor groups (high-HFSE (high field strength elements), mafic, and syenitic). The high-Ca group (68–77% SiO 2) with high Al 2O 3, Na 2O and Sr, and low Y, shares many features with typical Archaean tonalite-trondhjemite suites, but has higher K 2O, Rb, and Th contents. The low-Ca group (70–76% SiO 2) differs from the high-Ca group in having lower Al 2O 3, CaO, and Na 2O, but higher K 2O, Rb, Th, Zr, Y, La and Ce contents. Granites of the high-HFSE, mafic and syenitic groups form a minor component (10–20%) of the Eastern Goldfields granites. The siliceous (74–77% SiO 2) high-HFSE granites are restricted to a narrow NNW-trending zone and are characterised by high TiO 2, total FeO, MgO, Y, Zr and Ce, but only moderate Rb, Th and Pb contents. The A-type syenites (50–68% SiO 2) are distinguished by their high total alkalies and mainly occur along tectonic lineaments. The mafic group (55–70% SiO 2) is lithologically diverse and exhibits a wide range of K 2O, Rb, Th, La and Ce contents. Granite emplacement occurred between 2.69 and 2.60 Ga, contemporaneous with, or post-dating, greenstone formation. ϵ Nd values for the high-Ca, mafic and syenitic groups are similar (mostly 0.0 to +2.5), whereas those for the low-Ca group show a pronounced polarity, from −4.5 in the west to +2.0 in the east. Nd depleted-mantle model ages for the low-Ca granites range from 3.2 Ga in the west to 2.75 Ga in the east. Model ages for the high-Ca group (2.75–2.9 Ga) overlap with ages for inherited zircons from these granites. The high-Ca granites appear to have been derived at high pressures by partial melting of a mafic to intermediate source (subducted oceanic crust, crustal underplate or thickened crust), or remelting of an older tonalitic source derived by such a process. If the high-Ca granites were largely crust derived, then their extensive source rocks must have formed less than 250 Ma prior to granite formation. The low-Ca granites appear to be the products of crustal reworking, probably from a tonalitic to granodioritic protolith that was progressively younger towards the east, perhaps in the order of several hundred million years younger, or involved an additional younger component. The localisation of the crustally-derived high-HFSE granites suggests either geochemical zonation of the crust or specific tectonic processes, such as rifting. The mafic granites, and possibly the syenites, imply a significant mantle contribution and new crustal growth, although the extent is equivocal. The crust of the Eastern Goldfields appears to have been generated by both lateral and vertical accretion over a significant period from, or before, 3.0 Ga until the time of granite emplacement.

The Layos Granite, Hercynian Complex of Toledo (Spain): an example of parautochthonous restite-rich granite in a granulitic area

ABSTRACTThe Layos Granite forms elongated massifs within the Toledo Complex of central Spain. It is late-tectonic with respect to the F2 regional phase and simultaneous with the metamorphic peak of the region, which reached a maximum temperature of 800–850°C and pressures of 400–600 MPa. Field studies indicate that this intrusion belongs to the “regional migmatite terrane granite” type. This granite is typically interlayered with sill-like veins and elongated bodies of cordierite/garnet-bearing leucogranites. Enclaves are widespread and comprise restitic types (quartz lumps, biotite, cordierite and sillimanite-rich enclaves) and refractory metamorphic country-rocks including orthogneisses, amphibolites, quartzites, conglomerates and calc-silicate rocks.These granites vary from quartz-rich tonalites to melamonzogranites and define a S-type trend on a QAP plot. Cordierite and biotite are the mafic phases of the rocks. The particularly high percentage of cordierite (10%–30%) varies inversely with the silica content. Sillimanite is a common accessory mineral, always included in cordierite, suggesting a restitic origin. The mineral chemistry of the Layos Granite is similar to that of the leucogranites and country-rock peraluminous granulites (kinzigites), indicating a close approach to equilibrium. The uniform composition of plagioclase (An25), the high albitic content of the K-feldspar, the continuous variation in the Fe/Mg ratios of the mafic minerals, and the high Ti content of the biotites (2.5–6.5%) suggest a genetic relationship.Geochemically, the Layos Granite is strongly peraluminous. Normative corundum lies between 4% and 10% and varies inversely with increase in SiO2. The CaO content is typically low (&lt;1.25%) and shows little variation; similarly the LILE show a limited range. On many variation diagrams, linear trends from peraluminous granulites to the Layos Granite and associated leucogranite can be observed. The chemical characteristics argue against an igneous fractionation or fusion mechanism for the diversification of the Layos Granite. A restite unmixing model between a granulitic pole (represented by the granulites of the Toledo Complex) and a minimum melt (leucogranites) could explain the main chemical variation of the Layos Granite. Melting of a pelitic protolith under anhydrous conditions (biotite dehydration melting) could lead to minimum-temperature melt compositions and a strongly peraluminous residuum.For the most mafic granites (61–63% SiO2), it is estimated that the trapped restite component must have been around 65%. This high proportion of restite is close to the estimated rheological critical melt fraction, but field evidence suggests that this critical value has been exceeded. This high restite component implies high viscosity of the melt which, together with the anhydrous assemblage of the Layos Granite and the associated leucogranites, indicates H2O-undersaturated melting conditions. Under such conditions, the high viscosity magma (crystal-liquid mush) had a restricted movement capacity, leading to the development of parautochthonous plutonic bodies.

I- and S-type granites in the Lachlan Fold Belt

ABSTRACTGranites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth's surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe3+/Fe2+, and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks.S-type granites have higher δ18O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks.Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.

Petrogenesis of the magmatic complex at Mount Ascutney, Vermont, USA

The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had 87Sr/86Sr=0.7039, 143Nd/144Nd=0.512678 (ɛNd=+3.8) and δ18O=6.1‰, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in δ18O, to about 7.8‰, is the result of fractional crystallization. Mafic rocks show a large range in initial 143Nd/144Nd ratio, from about 0.51267 to 0.51236 (ɛNd= +3.7 to −2.5), which is correlated with elevated 87Sr/86Sr ratios and δ18O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement.

Origin of infracrustal (I-type) granite magmas

ABSTRACTI-type granites are produced by partial melting of older igneous rocks that are metaluminous and hence have not undergone any significant amount of chemical weathering. In the Lachlan Fold Belt of southeastern Australia and the Caledonian Fold Belt of Britain and Ireland there was a major magmatic event close to 400 Ma ago involving a massive introduction of heat into the crust. In both areas, that Caledonian-age event produced large volumes of I-type granite and related volcanic rocks. Granites of these two areas are not identical in character but they do show many similarities and are markedly different from many of the granites found in Mesozoic and younger fold belts. These younger, dominantly tonalitic, granites have compositions similar to those of the more felsic volcanic rocks forming at the present time above subduction zones. The Palaeozoic granites show little evidence of such a direct relationship to subduction. Within both the Caledonian and Lachlan belts there are some granites with a composition close to the younger tonalites. A particularly interesting case is that of the Tuross Head Tonalite of the Lachlan Fold Belt, which can be shown to have formed from slightly older source rocks by a process that we refer to as remagmatisation which has caused no significant change in composition. Since remagmatisation has reproduced the former source composition in the younger rocks, the wrong inference would result from the use of that composition to deduce the tectonic conditions at the time of formation of the tonalite. Granites, particularly the more mafic ones, will generally have compositions reflecting the compositions of their source rocks, and attempts to use granite compositions to reconstruct the tectonic environment at the time of formation of the granite may be looking instead at an older event. This is probably also the case for some andesites formed at continental margins.Several arguments can be presented in favour of a general model for the production of I-type granite sources by underplating the crust, so that the source rocks are infracrustal. Such sources may contain a component of subducted sediments with the consequence that some of the compositional characteristics of sedimentary rocks may be present in I-type source rocks and in the granites derived from them. The small bodies of mafic granite and gabbro associated with island arc volcanism have an origin that can be related to the partial melting of subducted oceanic crust or of mantle material overlying such slabs and can be referred to as M-type. These rocks have compositions indistinguishable from those of the related volcanic rocks, except for a small component of cumulative material. The tonalitic I-type granites characteristic of the Cordillera are probably derived from such M-type rocks of basaltic to andesitic composition, which had been underplated beneath the crust. Some of the more mafic tonalites of the Caledonian-age fold belts may also have had a similar origin. More commonly, however, the plutonic rocks of the older belts are granodioritic and these probably represent the products of partial melting of older tonalitic I-type source rocks in the deep crust, these having compositions and origins analogous to the tonalites of the Cordillera. In this way, multiple episodes of partial melting, accompanied by fractionation of the magmas, can produce quite felsic rocks from original source rocks in the mantle or mantle wedge. These are essential processes in the evolution of the crust, since the first stages in this process produce new crust and the later magmatic events redistribute this material vertically without the addition of significant amounts of new crust.

Tin in volcanogenic massive sulfide deposits; an example from the Geco Mine, Manitouwadge District, Ontario, Canada

Tin and tin-bearing minerals are prominent in the metamorphosed (amphibolite-granulite facies) Geco Cu-Zn volcanogenic massive sulfide deposit as well as in some of the smaller massive sulfides in the Manitouwadge district. Cassiterite and minor Sn-bearing andradite, epidote, rutile, and phlogopite occur within the massive sulfide ores. Nigerite-24R of composition (Sn, Ti) x (Zn, Mg, Fe, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 2, and mixed polytypes of hoegbomite with the general formula (Ti, Sn) x (Fe, Mg, Zn, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 1, occur in the footwall rocks which comprise the feeder zone. Hoegbomite is the primary tin-bearing mineral in the districtwide cordierite-gedrite gneisses which represent footwall conformable alteration zones.A relatively high f (sub O 2 ) during metamorphism and also during the formation of the ores accounts for the presence of cassiterite rather than stannite or the simple tin sulfides in the ores. Nigerite and hoegbomite occur only in Ca-Na-deficient Mg-Fe-rich aluminous rocks and are associated with Zn staurolite, corundum, cordierite, sillimanite, (Zn, Fe) spinels, and Fe-Ti oxides.Bulk-rock tin assays systematically increase as the Geco orebody is approached: districtwide cordierite-gedrite gneisses (10-30 ppm), feeder zone (50-330 ppm), and orebody (100-3,000 ppm). Hanging-wall felsic gneisses and nearby mafic gneisses and granites contain less than 3 ppm tin. Small nearby pegmatites contain 2 to 10 ppm tin.From textural evidence, phase equilibria calculations, and the lithologic distribution of tin, a model for the Geco deposit is developed in which the districtwide cordierite-gedrite gneisses represent one segment of the fossil hydrothermal system and in which Cu-Zn and Sn mineralizations are closely associated. Nigerite, hoegbomite, and spinel from Geco occupy distinct and characteristic (Zn, Fe, Mg, Mn) solid solution fields. Hence, these species, which are known to occur as detrital heavy minerals may be useful pathfinders in geochemical exploration for eroded volcanogenic massive sulfide deposits.