Field Evidence of Magma Mixing from Microgranular Enclaves Hosted in Palaeoproterozoic Malanjkhand Granitoids, Central India

The Malanjkhand granitoids (MG) pluton (about 1500 sq km) occurs in the Balaghat district of Madhya Pradesh. The MG (~2400 Ma) represent an episode of Palaeoproterozoic felsic magmatism in Central India and hosts potential Cu (±Mo±Au) deposits. The enclaves hosted in MG can be broadly classified into two categories: microgranular enclaves (dark-coloured, fine-grained magmatic) and xenoliths of country rocks. The microgranular enclaves (ME) may be rounded, ellipsoidal, discoid, elongated, lenticular or tabular, and their size commonly reaches up to 2 metres across. The ME have sharp and in places, diffuse contacts with their host granitoids. The shape and size of ME indicate contemporaneous flow and mingling of partly crystalline felsic-mafic magmas. Some ME exhibit dark crenulated margins giving them a pillow-like form that has been attributed to undercooling of a ME magma as globules intruded into a granitoid magma. The presence of corroded felsic and mafic minerals (xenocrysts) in ME is interpreted as the result of mechanical transfer during the mafic-felsic magma interaction and mixing event. Mafic minerals (biotite) rim the quartz xenocrysts giving rise to ocellar texture, which exhibit signatures of resorption under hybrid (enclave) magma conditions. All these features suggest an origin for the calc-alkaline intermediate granitoid magma in Malanjkhand involving a magma mixing process.     

Petrography and major elements geochemistry of microgranular enclaves and neoproterozoic granitoids of south Khasi,Meghalaya: Evidence of magma mixing and alkali diffusion

Neoproterozoic (690±19 Ma) felsic magmatism in the south Khasi region of Precambrian northeast Indian shield, referred to as south Khasi granitoids (SKG), contains country-rock xenoliths and microgranular enclaves (ME). The mineral assemblages (pl-hbl-bt-kf-qtz-mag) of the ME and SKG are the same but differ in proportions and grain size. Modal composition of ME corresponds to quartz monzodiorite whereas SKG are quartz monzodiorite, quartz monzonite and monzogranite. The presence of acicular apatite, fine grains of mafic-felsic minerals, resorbed maficfelsic xenocrysts and ocellar quartz in ME strongly suggest magma-mixed and undercooled origin for ME. Molar Al₂O₃/CaO+Na₂O+K₂O (A/CNK) ratio of ME (0.68–0.94) and SKG (0.81–1.00) suggests their metaluminous (I-type) character. Linear to sub-linear variations of major elements (MgO, Fe₂O₃ ^t, P₂O₅, TiO₂, MnO and CaO against SiO₂) of ME and SKG and two-component mixing model constrain the origin of ME by mixing of mafic and felsic magmas in various proportions, which later mingled and undercooled as hybrid globules into cooler felsic (SKG) magma. However, rapid diffusion of mobile elements from felsic to mafic melt during mixing and mingling events has elevated the alkali contents of some ME.     

Magma mixing/mingling in the Eocene Horoz (Nigde) granitoids, Central southern Turkey: evidence from mafic microgranular enclaves

Mafic microgranular enclaves (MMEs) are widespread in the Horoz pluton with granodiorite and granite units. Rounded to elliptical MMEs have variable size (from a few centimetres up to metres) and are generally fine-grained with typical magmatic textures. The plagioclase compositions of the MMEs range from An₁₈?CAn₆₄ in the cores to An₁₇?CAn₂₉ in the rims, while that of the host rocks varies from An₁₇ to An₅₅ in the cores to An₀₇ to An₃₃ in the rims. The biotite is mostly eastonitic, and the calcic-amphibole is magnesio-hornblende and edenite. Oxygen fugacity estimates from both groups?? biotites suggest that the Horoz magma possibly crystallised at fO₂ conditions above the nickel?Cnickel oxide (NNO) buffer. The significance of magma mixing in their genesis is highlighted by various petrographic and mineralogical characteristics such as resorption surfaces in plagioclases and amphibole; quartz ocelli rimmed by biotite and amphibole; sieve and boxy cellular textures, and sharp zoning discontinuities in plagioclase. The importance of magma mixing is also evident in the amphiboles of the host rocks, which are slightly richer in Si, Fe³⁺ and Mg in comparison with the amphiboles of MMEs. However, the compositional similarity of the plagioclase and biotite phenocrysts from MMEs and their host rocks suggests that the MMEs were predominantly equilibrated with their hosts. Evidence from petrography and mineral chemistry suggests that the adakitic Horoz MMEs could be developed from a mantle-derived, water-rich magma (>3 mass%) affected by a mixing of felsic melt at P >2.3?kbar, T >730°C.     

The morphology and chemistry of K-feldspar megacrysts from Ikizdere Pluton: evidence for acid and basic magma interactions in granitoid rocks, NE-Turkey

Ikizdere Pluton consists of granite, granodiorite, tonalite, monzonite, quartz monzonite containing pinkish colored K-feldspar megacrysts (KFMs). The crystal sizes of the KFMs range from 1 to 4 cm. The lath-shaped megacrysts are uniformly (i.e., randomly) distributed in the host plutonic rocks and have mafic and felsic inclusions whose crystal sizes are smaller than 1 mm. The crystal inclusions are biotite, slightly annitic in composition with X_Mg[=Fe_tot/(Fe_tot+Mg)]=0.50-0.58, amphibole (magnesio-hornblende, X_Mg[=Mg/(Mg+Fe_tot)]=0.70-0.79), iron-titanium oxide (low titanium magnetit and ilmenite), plagioclase (Ab₇₅₋₂₅An₆₅₋₃₅) and as minor quartz. The compositions of the KFMs range from Or₉₅Ab₅An₀ to Or₈₂Ab₁₇An₁. BaO contents of the megacrysts increase from core to rim. The mafic and felsic inclusions are compositionally similar those of the host rocks.The chemical and textural features of K-feldspar are typical for megacrysts that grew as phenocrysts in dynamic granitoidic magma systems. The overgrowth of KFMs and mafic magma injections (magma mixing) may be related to temperature, pressure and compositional fluctuations in the magma chamber. Remnant of earlier formed K-feldspar crystals remain in the felsic magma system, while the mafic injection can decompose some earlier precipitated KFMs. The remnant of K-feldspar remaining after mafic injection are overgrown by rapid diffusion of Ba, K and Na elements in liquid phase, during the later stages of crystallization of the host magma.     

Mineralogy and geochemistry of biotites from Proterozoic granitoids of western Arunachal Himalaya: Evidence of bimodal granitogeny and tectonic affinity

Felsic magmatic rocks in Kameng corridor of western Arunachal Himalaya are represented by extensively exposed Palaeoproterozoic porphyritic muscovite-biotite granite (GGn) of the Bomdila Group and small stock-like Mesoproterozoic hornblende-biotite granite (HBG) of the Salari Group. Mineralogy and chemical composition of biotites from GGn and HBG have been utilized to understand the nature and tectonic environment of their parental felsic melts. Biotites in GGn (FeO^t/MgO=3.1–4.6) are Fe-biotites and have shown affinity with primary biotites co-precipitating with muscovite in a peraluminous (S-type) felsic melt of syn-collisional tectonic environment. Biotites in HBG (FeO^t/MgO=1.3–2.2) are transitional between Fe and Mg biotites evolved from Fayalite-Magnetite-Quartz (FMQ) to Nickel-Nickel Oxide (NNO) buffers and are related to primary biotites co-existing with amphibole and other ferromagnesian minerals in a calc-alkaline metaluminous (I-type) felsic melt mostly formed in a subduction setting. Both GGn and HBG biotites exhibit Mg⇌Fe substitution, which is more pronounced in HBG biotites. GGn biotites exhibit 2Al⇌3Fe²⁺ substitution as expected in peraluminous melt, whereas 3Mg⇌2Al substitution normally expected to operate in metaluminous melt is less pronounced in HBG biotites. GGn biotites are markedly enriched in siderophyllite, and depleted in phlogopite components as compared to HBG biotites, which point to diverse genetic conditions. HBG biotites indicate oxidizing environment of the felsic melt unlike the reducing nature of the porphyritic granite (GGn).     

赣杭构造带灵山花岗岩体黑云母的矿物化学特征及其对岩石成因的指示意义

灵山花岗岩体在平面上为一环状分布的侵入体,中心为角闪石黑云母花岗岩,外围为黑云母花岗岩。在角闪石黑云母花岗岩中分布有大量的暗色镁铁质微粒包体。黑云母是大多数中酸性火成岩中比较重要的一种镁铁质矿物,它能很好地反映寄主岩浆的属性和成岩时的物理、化学条件,因此,本文对这两种花岗岩及镁铁质微粒包体中的黑云母开展了系统的岩相学观察和电子探针化学组成研究,探讨灵山岩体的物质来源、成岩条件和岩浆的混合作用过程。研究结果表明两种花岗岩体的黑云母具有不同化学成分,而暗色镁铁质微粒包体中黑云母的化学成分则变化较大。三种黑云母均在低氧逸度条件下晶出。两种花岗岩中的黑云母均富Fe贫Mg,属于铁质黑云母,含铁系数[(Fe~(3+)+Fe~(2+))/(Fe~(3+)+Fe~(2+)+Mg~(2+))]分别为0.65~0.70,0.72~0.78,FeOT/MgO均接近7.04。MF值[2×Mg/(Fe~(2+)+Mg+Mn)]分别为0.64~0.76和0.48~0.60,指示两种花岗岩的物质来源都是以壳源为主。镁铁质微粒包体中黑云母的MF值变化范围比较大,为0.63~1.06,为铁质黑云母到镁质黑云母,暗示包体岩浆经历过不同程度的岩浆混合作用。镁铁质微粒包体中部分黑云母与角闪石黑云母花岗岩中黑云母的结晶条件相似,而部分则有明显差异,推测是由于基性的镁铁质包体岩浆注入到酸性的花岗岩浆是一个连续多阶段的过程。     

The origin of chromitites and related PGE mineralization in the Bushveld Complex: new mineralogical and petrological constraints

This article reports a study of chromitites from the LG-1 to the UG-2/3 from the Bushveld Complex. Chromite from massive chromitite follows two compositional trends on the basis of cation ratios: trend A—decreasing Mg/(Mg + Fe) with increasing Cr/(Cr + Al); trend B—decreasing Mg/(Mg + Fe) with decreasing Cr/(Cr + Al). The chromitites are divided into five stages on the basis of which trend they follow and the data of Eales et al. (Chemical Geology 88:261–278, 1990) on the behaviour of the Mg/Fe ratio of the pyroxene and whole rock Sr isotope composition of the environment in which they occur. Following Eales et al. (Chemical Geology 88:261–278, 1990), the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aAl₂O₃ and, in the case of stage 5, the appearance of cumulus plagioclase buffering the aAl₂O₃. The similarity of PGE profiles across the MG-3 and MG-4 chromitites that are separated laterally by up to 300 km and the variation in V in the UG-2 argue that the chromitites have largely developed in situ. Modelling using the programme MELTS shows that increase in pressure, mixing of primitive and fractionated magma, felsic contamination of primitive magma or addition of H₂O do not promote crystallization of spinel before orthopyroxene (in general they hinder it) and that the Cr₂O₃ content of the magma was of the order of 0.25 wt.%. Less than 20% of the chromite in the magma is removed before orthopyroxene joins chromite, which implies a >13-km thickness of magma for the Critical Zone. It is suggested that the large excess of magma has escaped up marginal structures such as the Platreef. The PGE profile of chromitites depends on whether sulphide accumulated or not along with chromite. Modelling shows that contamination of Critical Zone magma with a felsic melt will induce sulphide immiscibility, although not chromite precipitation. The LG-1 to LG-4 chromitites developed without sulphide, whilst those from the LG-5 upwards had associated liquid sulphide. Much of the sulphide originally in the LG-5 and above has been destroyed as a result of reaction with the chromite.     

Chemical mineralogy of the Monchique alkaline complex,southern Portugal

Some 500 microprobe analyses from 43 rock samples, covering the entire range of major and minor intrusive rock-types, define the following ranges of mineral composition: plagioclases (An_70-30), alkali feldspars (Or_98-20 and Ab_90–99), olivines (Fo_85-82), clinopyroxenes (aluminous titanaugite through salite and aegirine-augite to acmite), amphiboles (subsilicic kaersutite through pargasite to hastingsite, edenite or katophorite), biotites (titanbiotite to titaniferous manganiferous lepidomelane). Varied discontinuous reaction relationships are evident petrographically between these minerals (e.g. amphibole overgrowths on pyroxenes or biotites), but most appear to reflect reequilibration during slow magmatic cooling, or perhaps local disequilibrium effects, and there is little evidence for significant subsolidus, hydrothermal or deuteric modification of the primary mineralogy.Although these mineral ranges are reasonably typical of differentiated alkaline gabbroic-syenitic intrusions, Monchique also shows many unusual features: e.g. a restricted stability range for olivine, the absence of amphiboles from all rocks with 54<%SiO₂<58, a lack of alkali amphiboles corresponding to the acmite-rich pyroxenes, the presence of acmite-poor pyroxenes and aluminous biotites in peralkaline rocks, and irregular Ti variation in biotites. Mineral/ host-rock relationships also show peculiarities: e.g. Mg/ (Mg+Fe) ratios of mafic minerals and An contents of plagioclases increase as host-rock fractionation index (FI) increases from gabbroic (FI 30–40%) to alkali feldspar-bearing essexitic (FI c. 50) rocks. Thereafter, Mg ratios decrease only slightly, such that many malignites and miaskitic syenites (FI 70–90) carry pyroxenes and biotites which are no more, and sometimes less evolved than those in the gabbroic rocks (FI 30–40).Such features confirm earlier suggestions from whole-rock geochemistry, that a major evolutionary process in the complex was a relatively discontinuous segregation of already somewhat evolved essexitic magma into more basic and more felsic magmas, rather than an incremental gabbrosyenite crystal fractionation. They also confirm that the complex represents neither a single in situ differentiating magma pulse, nor a series of simply related pulses, but an irregular and largely irresolvable juxtaposition of magma-batches of widely varying compositions and evolutionary histories.     

Geochemistry and petrogenesis of radioactive Palaeoproterozoic granitoids of Kinwat crystalline inlier,Nanded and Yeotmal districts,Maharashtra

Kinwat crystalline inlier exposes Palaeoproterozoic granitoids belonging to the northern extensions of younger phase of Peninsular gneissic complex (PGC) within Deccan Trap country in Eastern Dharwar Craton (EDC) and bounded in south by a major NW-SE trending lineament (Kaddam fault). Geochemically, the Kinwat granitoids are similar to high-K, calc-alkaline to shoshonite magnesian granitoids and subdivided into two major groups, i.e. felsic group (pink and grey granites) and intermediate to felsic group (hybrid granitoids). The felsic group (∼67–74% SiO₂) shares many features with Neoarchaean to Palaeoproterozoic high potassic granites of PGC such as higher LILE and LREE content and marked depletion in Eu, P and HFSE, especially Nb, Ti, relative to LILE and LREE. The hybrid granitoids (∼58–67% SiO₂) have comparatively higher Ca, Mg and Na contents and slightly lower REE content than the granitoids of felsic group. Both, felsic and hybrid granitoids are metaluminous to weakly peraluminous and belong to highly fractionated I-type suite as evidenced by negative correlation of SiO₂ with MgO, FeO^t, CaO, Na₂O, Al₂O₃, whereas K₂O, Rb and Ba show sympathetic relationship with SiO₂. Moderate to strong fractionated REE patterns (Ce/Yb_N: ∼54–387) and strong negative Eu anomalies (Eu/Eu*: 0.13–0.41) are quite apparent in these granitoids. The geochemical characteristics together with mineralogical features such as presence of biotite±hornblende as the dominant ferromagnesian mineral phases point towards intracrustal magma source, i.e. derivation of magma by partial melting of probably tonalitic igneous protolith at moderate crustal levels for felsic granites, whereas hybrid granitoids appear to be products of juvenile mantle-crust interaction, in an active continental margin setting.     

Microtextural and mineral chemical analyses of andesite–dacite from Barren and Narcondam islands: Evidences for magma mixing and petrological implications

Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An₅₅) are frequently overgrown by thin calcic (An₇₂) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.     

Mineral chemistry of banded migmatites from Hafafit and Feiran areas, Egypt

The migmatitic rocks exposed in Hafafit and Feiran areas exhibit some migmatitic structures as the banded, agmatic, boudinage and schlieren structures. The dominant type of these structures is the stromatic migmatites. Electron microprobe analyses of plagioclases, biotites and amphiboles from Hafafit and Feiran areas, in the Eastern Desert and Sinai, Egypt, are carried out and the metamorphic conditions are discussed. The present study revealed marked differences in the composition of plagioclases, biotites and amphiboles from Hafafit and Feiran localities. The obtained data indicated that plagioclases of the Feiran migmatites are of andesine and oligoclase composition, and display anorthite content from An₂₀ to An₃₈; whereas the Hafafit migmatites show a wider range of plagioclases from An₁₀ to An₆₀, and therefore plagioclases have labradorite, andesine and oligoclase composition. This may be due to the slow rate of the crystallisation processes. The analyses indicated that biotites of the studied areas are of metamorphic origin showing significant variation in Fe–Mg. It is worth mentioning that biotites from Hafafit migmatites have Mg–biotite composition while that of Feiram migmatites have Fe–biotite composition. High Mg and low Fe contents in biotite suggest higher crystallisation temperature. The composition of amphiboles in Hafafit migmatites is ferro-tschermakitic hornblende, while amphiboles from Feiram migmatites are magnesio-hornblende. High Ti content in the hornblende of Feiran migmatites suggests that they were formed at slightly higher temperatures and lower pressure than the Hafafit migmatites (i.e. Feiram migmatites and Hafafit migmatites were formed at granulite and amphibolite facies, respectively). Discrimination diagrams show that the muscovite is of secondary origin. Moreover, the present study confirmed that these migmatites are mainly formed by metamorphic differentiation via partial melting.     

Mineralogy and geochemistry of granitoids from Kinnaur region,Himachal Higher Himalaya,India: Implication on the nature of felsic magmatism in the collision tectonics

Felsic magmatism in the southern part of Himachal Higher Himalaya is constituted by Neoproterozoic granite gneiss (GGn), Early Palaeozoic granitoids (EPG) and Tertiary tourmaline-bearing leucogranite (TLg). Magnetic susceptibility values (<3 ×10^?3 SI), molar Al₂ O ₃/(CaO + Na₂ O + K ₂O) (≥1.1), mineral assemblage (bt–ms–pl–kf–qtz ± tur ± ap), and the presence of normative corundum relate these granitoids to peraluminous S-type, ilmenite series (reduced type) granites formed in a syncollisional tectonic setting. Plagioclase from GGn (An₁₀–An₃₁) and EPG (An₁₅–An₃₃) represents oligoclase to andesine and TLg (An₂–An₁₅) represents albite to oligoclase, whereas compositional ranges of K-feldspar are more-or-less similar (Or₈₈ to Or₉₅ in GGn, Or₈₆ to Or₉₇ in EPG and Or₈₇ to Or₉₄ in TLg). Biotites in GGn (Mg/Mg + Fe^t= 0.34–0.45), EPG (Mg/Mg + Fe^t= 0.27–0.47), and TLg (Mg/Mg + Fe^t= 0.25–0.30) are ferribiotites enriched in siderophyllite, which stabilised between FMQ and HM buffers and are characterised by dominant 3Fe\(\rightleftharpoons \)2Al, 3Mg\(\rightleftharpoons \)2Al substitutions typical of peraluminous (S-type), reducing felsic melts. Muscovite in GGn (Mg/Mg + Fe^t=0.58–0.66), EPG (Mg/Mg + Fe^t=0.31?0.59), and TLg (Mg/Mg + Fe^t=0.29–0.42) represent celadonite and paragonite solid solutions, and the tourmaline from EPG and TLg belongs to the schorl-elbaite series, which are characteristics of peraluminous, Li-poor, biotite-tourmaline granites. Geochemical features reveal that the GGn and EPG precursor melts were most likely derived from melting of biotite-rich metapelite and metagraywacke sources, whereas TLg melt appears to have formed from biotite-muscovite rich metapelite and metagraywacke sources. Major and trace elements modelling suggest that the GGn, EPG and TLg parental melts have experienced low degrees (～13, ～17 and ～13%, respectively) of kf–pl–bt fractionation, respectively, subsequent to partial melting. The GGn and EPG melts are the results of a pre-Himalayan, syn-collisional Pan-African felsic magmatic event, whereas the TLg is a magmatic product of Himalayan collision tectonics.     

Mineralogy, whole-rock and Sr-Nd isotope geochemistry of mafic microgranular enclaves in Cretaceous Dagbasi granitoids, Eastern Pontides, NE Turkey: Evidence of magma mixing, mingling and chemical equilibration

Rocks of the Late Cretaceous Dagbasi Pluton (88-83 Ma), located in the eastern Pontides, include mafic microgranular enclaves (MMEs) ranging from a few centimetres to metres in size, and from ellipsoidal to ovoid in shape. The MMEs are composed of gabbroic diorite, diorite and tonalite, whereas the felsic host rocks comprise mainly tonalite, granodiorite and monzogranite based on both mineralogical and chemical compositions. MMEs are characterized by a fine-grained, equigranular and hypidiomorphic texture. The common texture of felsic host rocks is equigranular and also reveals some special types of microscopic textures, e.g., oscillatory-zoned plagioclase, poikilitic K-feldspar, small lath-shaped plagioclase in large plagioclase, blade-shaped biotite, acicular apatite, spike zones in plagioclase and spongy-cellular plagioclase textures and rounded plagioclase megacrysts in MMEs. Compositions of plagioclases (An₃₃-An₆₀), hornblendes (Mg#=0.77-1.0) and biotites (Mg#=0.61-0.63) of MMEs are slightly distinct or similar to those of host rocks (An_12-57; hbl Mg#=0.63-1.0; Bi Mg#=0.50-0.69), which suggest partial to complete equilibration during mafic-felsic magma interactions.The felsic host rocks have SiO₂ between 60 and 76 wt% and display low to slightly medium-K tholeiitic to calc-alkaline and peraluminous to slightly metaluminous characteristics. Chondrite-normalized rare-earth element (REE) patterns are fractionated (La_cn/Lu_cn=1.5-7.3) with pronounced negative Eu anomalies (Eu/Eu*=0.46-1.1). Initial ε_Nd(i) values vary between −3.1 and 1.6, initial ⁸⁷Sr/⁸⁶Sr values between 0.7056 and 0.7067.Compared with the host rocks, the MMEs are characterized by relatively high Mg-number of 22-52, low contents of SiO₂ (53-63 wt%), low ASI (0.7-1.1) and low to medium-K tholeiitic to calc-alkaline, metaluminous to peraluminous composition. Chondrite-normalized REE patterns are relatively flat [(La/Yb)_cn=1.4-3.9; (Tb/Yb)_cn=0.9-1.5] and show small negative Eu anomalies (Eu/Eu*=0.63-1.01). Isotope signatures of these rocks (⁸⁷Sr/⁸⁶Sr_(i)=0.7054-0.7055; ε_Nd(i)=-1.0 to 1.9) are largely similar to the host rocks. Gabbroic diorite enclaves have relatively low contents of SiO₂, ASI; high Mg#, CaO, Al₂O₃, TiO₂, P₂O₅, Sr and Nb concentrations compared to dioritic and tonalitic enclaves.The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, interacted with a crustal melt that originated from dehydration melting of the mafic lower crust at deep crustal levels. The existence of compositional and textural disequilibrium and the nature of chemical and isotopic variation in these rock types indicate that magma mixing/mingling between an evolved mafic and a granitic magma was involved in their genesis. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched lithospheric mantle source. Al-in-amphibole estimates the pluton emplacement at ca. 0.3-3.8 kbar, and therefore, magma mixing and mingling must have occurred at 3.8 kbar or below this level.     

P–T–t evolution of spinel–cordierite–garnet gneisses from the Sauwald Zone (Southern Bohemian Massif, Upper Austria): is there evidence for two independent late-Variscan low-P/high-T events in the Moldanubian Unit?

The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An_35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An_10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An_0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.     

湘南宝山矿床花岗闪长斑岩中黑云母的矿物学特征及其指示意义

湖南宝山矿床处于坪宝矿带的北端,是湘南地区最大的铜多金属矿床。为了进一步探讨矿区内花岗质岩石的形成条件及成矿潜力,文章在详细的野外地质和岩相学观察的基础上,对与成矿密切相关的花岗闪长斑岩中的黑云母进行了详细的矿物化学分析。电子探针分析结果表明:宝山花岗闪长斑岩中的黑云母为铁质黑云母和镁质黑云母,其中,Ti介于0.18~0.30,且Mg/(Mg+Fe2+)值介于0.42~0.58,属于典型的岩浆成因黑云母;黑云母的氧化系数为0.16~0.26,w(Mg O)为8.17%~11.72%,平均9.3%,MF值范围为0.38~0.50,指示其岩体属于壳幔混源型的I型花岗岩;岩体中以黑云母的全铝含量计算的结晶压力为97~174 MPa,相应的结晶深度为3.67~6.57 km,平均深度为5.12 km。其log f(O2)变化范围为-14.5~-12.8,表明黑云母是在较高氧逸度条件下结晶形成的,有利于铜矿的形成。     

Experimental evidence for a relationship between liquid immiscibility and ore-formation in felsic magmas

Geological studies demonstrate that liquid immiscibility in felsic magma closely associates with the ore forming process. In order to obtain experimental evidence demonstrating the relationship between the ore forming process and liquid immiscibility in felsic magma, we carried out a series of experiments at high temperature and atmospheric pressure. The experimental results show that the granite ∼ KBF₄∼Na₂MoO₄ system is a homogeneous melt at high temperature. With decrease in temperature, however, the melt decomposes into two immiscible melts: silicate melt and ore-forming melt. The ore-forming melt exists as globules in the silicate phase. Molybdenm, Ca, Na, Mg, P, Mn, F, B, and OH are concentrated in these globules. The ore forming melt is characterized with very low SiO₂ and Al₂O₃ concentrations but the concentration of MoO₃ and CaO is very high. In contrast, the silicate melts are significantly enriched in SiO₂ and Al₂O₃, and depleted in MoO₃ and CaO. In the silicate melt the concentrations of network modifying elements (e.g. Mo, Ca, Na, P, Mg) and volatiles (F, OH) are very low. The differences between the two immiscible melts exist not only in chemical composition but also in structure. The ore-forming melt structurally consists of [MoO₄], [MoOF₄], [B(OH)₄], and OH, while the silicate melt is [Si0₄]. Because of the difference in composition and structure the two immiscible melts possess different physical properties. Compared to silicate melt, the ore-forming melt has a lower density and viscosity, which permits the globules to behave as bubbles in granite magma and to move and concentrate in the upper part of magma chamber. This process is probably responsible for the concentration of ore-forming elements in the upper part of granite bodies and their immediate aureoles. The present experimental results suggest that liquation in felsic magma can be the first step in the ore-forming process during granitoid evolution.     

Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa

Lateral variations in the amount of trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex are constrained by new Sr-isotopic, whole-rock and mineralogical data from three profiles that are separated laterally by ca.100 km and represent thicknesses of 551–1,127 m of Main Zone gabbronorites below the Pyroxenite Marker. An analysis of the An-contents (100×Ca/(Ca+Na)) of plagioclase cores within the Thornhill (north), Roossenekal and Stoffberg (south) profiles show similar systematic, up-section variations from An₆₆ to An₅₉. In contrast, both the An-contents of bulk plagioclase separates and the Mg-numbers (100×Mg/(Mg+Fe_T)) of orthopyroxenes show pronounced lateral variations from Thornhill (An_67-61; Mg#_67-61), through Roossenekal (An_64-58; Mg#_64-55) to Stoffberg (An_59-55; Mg#_59-50). These mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south. Modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0% at Thornhill, through 10–30% at Roossenekal to 30–45% at Stoffberg. A two- to eightfold southward increase in whole-rock concentrations of P, Ti, Y, Zr and Ba is consistent with the trapped liquid model. However, the 14-fold increase in Rb from Thornhill to Stoffberg is too great to be accounted for by trapped liquid alone, but can be explained by local assimilation of partial melts of the country rock. Constant initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of Main Zone plagioclase separates (0.7081–0.7085) in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0.6924–0.7096. The southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at Stoffberg. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorised users.     

Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Querigut Complex, French Pyrenees

The Quérigut mafic–felsic rock association comprisestwo main magma series. The first is felsic comprising a granodiorite–tonalite,a monzogranite and a biotite granite. The second is intermediateto ultramafic, forming small diorite and gabbro intrusions associatedwith hornblendites and olivine hornblendites. A U–Pb zirconage of 307 ± 2 Ma was obtained from the granodiorite–tonalites.Contact metamorphic minerals in the thermal aureole providea maximum emplacement pressure of between 260 and 270 MPa. Petrographiccharacteristics of the mafic and ultramafic rocks suggest crystallizationat <300 MPa, demonstrating that mantle-derived magmas ascendedto shallow levels in the Pyrenean crust during Variscan times.The ultramafic rocks are the most isotopically primitive components,with textural and geochemical features of cumulates from hydrousbasaltic magmas. None of the mafic to ultramafic rocks havedepleted mantle isotope signatures, indicating crustal contaminationor derivation from enriched mantle. Origins for the dioritesinclude accumulation from granodiorite–tonalite magma,derivatives from mafic magmas, or hybrids. The granitic rockswere formed from broadly Proterozoic meta-igneous crustal protoliths.The isotopic signatures, mineralogy and geochemistry of thegranodiorite–tonalites and monzogranites suggest crystallizationfrom different magmas with similar time-integrated Rb/Sr andSm/Nd isotope ratios, or that the granodiorite–tonalitesare cumulates from a granodioritic to monzogranitic parent.The biotite granite differs from the other felsic rocks, representinga separate magma batch. Ages for Quérigut and other Pyreneangranitoids show that post-collisional wrenching in this partof the Variscides was under way by 310 Ma. KEY WORDS: Variscan orogeny; Pyrenees; Quérigut complex; epizonal magmatism; post-thickening; mafic–felsic association     

Experimentally derived thermochemical data for pargasite and reinvestigation of its stability with quartz in the system Na2O–CaO–>MgO–Al2O3–SiO2–H2O

 The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C in the system Na₂O– CaO–MgO–Al₂O₃–SiO₂–H₂O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the following three equilibria: (a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An₅₀)+1.5 enstatite+H₂O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An₅₀)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An₅₀). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature, and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic amphiboles, which favor a single-coupled Na^A-Al^T1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is estimated as for ΔG ⁰ _f ,Pg=−12022.11±5.2 kJ mole^-1, and S ⁰ _Pg=591.7 ±7.9 JK^-1 mole^-1. Received: 31 July 1995/Accepted: 3 June 1996     

Evidence for hybridisation in the Tynong Province granitoids,Lachlan Fold Belt,eastern Australia

The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ?_Hf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ?_Hf in two of the three analysed samples (8.8 and 10.1 ?_Hf units) exceeds that expected from a single homogeneous population (～4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ?_Nd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for ⁸⁷Sr/⁸⁶Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.