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1.
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.  相似文献   

2.
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.  相似文献   

3.
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 XMg[=Fetot/(Fetot+Mg)]=0.50-0.58, amphibole (magnesio-hornblende, XMg[=Mg/(Mg+Fetot)]=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.  相似文献   

4.
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.  相似文献   

5.
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 (FeOt/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 (FeOt/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).  相似文献   

6.
叶茂  赵赫  赵沔  舒珣  张若曦  杨水源 《岩石学报》2017,33(3):896-906
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。  相似文献   

7.
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.  相似文献   

8.
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 (An70-30), alkali feldspars (Or98-20 and Ab90–99), olivines (Fo85-82), 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.  相似文献   

9.
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, FeOt, CaO, Na2O, Al2O3, whereas K2O, Rb and Ba show sympathetic relationship with SiO2. Moderate to strong fractionated REE patterns (Ce/YbN: ∼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.  相似文献   

10.
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.  相似文献   

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