Multiple magma mingling, enclave typology and textural evolution in the Ross of Mull Granite, Scotland
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Abstract:
The Ross of Mull Igneous Complex consists of a suite of syn-post tectonic lamprophyric-dioritic-microdioritic-monzogranitic calc-alkaline bodies emplaced hito Moinian metasediments (P=2-3 kbar) throughout the closing stages of the Caledonian orogeny (414±3 Ma; Halliday et al., 1979). The pluton occupies an area approximately 140 km[sup]2, of which only half is exposed on the mainland - the rest is submerged. Microdioritic and dioritic bodies are confined to the core of the pluton and occupy topographic lows, while granites occupy the highs. The distribution of these plutonic rocks is interpreted as a compositionally zoned (reversed) magma chamber. Prior to and throughout the main phase of monzogranite emplacement, a series of basaltic (alkalic) magma pulses intruded the monzogranitic magma chamber, inducing mechanical mixing, homogenisation and the production of a hybrid porphyritic monzogranite. Binary mixing equations allow the proportions of granitic magma involved in the mixing event to be estimated, which vary between 66-80%. Continued injection of basaltic magma into the evolving, crystal-ladden, porphyritic monzogranitic magma chamber resulted in the fragmentation of the basaltic magma and the formation (preservation) of megacrystic, microdioritic enclaves. On the basis of alkali feldspar crystal growth rates in granitic magmas, as well as thermodynamical considerations (e.g. Furman & Spera, 1985), the time elapsed between the formation of the porphyritic monzogranite and the injection of additional basaltic magma pulses was approximately 15000 years. Based on detailed field mapping and petrographic analysis, microdioritic enclaves can be subdivided into four texturally distinct populations, depending on their megacrystic mineralogy. The mineralogy and textures of the enclaves reflect and record the point at which the basaltic magma intruded the crystallising porphyritic monzogranitic magma chamber. Generally, highly megacrystic microdiorites are interpreted as having been intruded relatively early in the crystallisation history of the porphyritic monzogranite. Microdioritic enclaves with fewer megacrysts are likely to have been emplaced late in the crystallisation of the granite, when the rheological differences between the two magmas would have inhibited mingling.
In exceptional circumstances, microdioritic bodies and enclaves become veined by thin (c. 5 mm wide) leucocratic (monzonitic) veins composed of plagioclase + alkali feldspar ± quartz. Typically, these veins occupy 5-30% volume of the microdiorite. Field and mineralogical evidence cannot equivocally explain the formation of the monzonitic veins. Partial melting experiments on megacryst-free microdioritic enclaves at crustal pressures and temperatures (i.e. 750-950 °C, 50 MPa), have therefore been carried out in order to shed light on the origin of the veining phenomena. The composition of the melt generated during these experiments requires high (950 °C) temperatures and is less sodic but richer in quartz than that of the leucocratic veins. Integrated field, mineral chemistry and geochemical data suggests that mechanical mixing of basaltic and porphyritic monzogranite magma (at depth or in a conduit) produced a heterogeneous mixture which was injected into a porphyritic monzogranitic magma chamber. The higher liquidus of the basaltic magma coupled with the input of additional heat from new basaltic magma pulses induced fluid-present partial melting of the more fusible components in the mixture (i.e. the granitic end-member). Where the mixture was almost crystalline prior to incorporation into the porphyritic monzogranite, re-heating of the mixture caused recrystallisation of the microdioritic matrix, partial melting of the granitic material and thermal expansion leading to the formation of a feldspar-rich, pseudo-polygonal monzonitic vein network (e.g. pink veined microdiorites). However, in the case where the mixture was still ,largely molten prior to incorporation into the porphyritic monzogranite, fluid-present partial melting of the granitic material in the mixture caused the formation of feldspar + quartz-rich leucocratic veins without recrystallisation of the microdiorite matrix (e.g. white veined microdiorites). As melting of the granitic magma ensued, monzonitic melt exfiltrated through the partially molten microdiorite matrix via porous flow and deformation enhanced melt segregation mechanisms.
The topology of the vein network will have a fundamental bearing on the efficacy of chemical homogenisation within the microdiorites, as well as controlling the rate of material transport (advection) within the veins. Leucocratic veins are clearly linked in three dimensions and in order to quantify the pore structure of the
veins, a veined microdioritic enclave was collected for serial sectioning. Image analysis software and 3D modelling packages were then used to reconstruct the vein network in 3D. The results show that the vein network posseses a high effective porosity (17%), as well as a complex bifurcating and branching network. Based on the 3D topology, the specific permeability (k) of the vein network has been estimated and ranges from 8x10[sup]-7 to 1x10[sup]-12 m[sup]2. Based on these permeabilities and estimates of granitic melt viscosities (10[sup]4 to 10[sup]8 Pas), Darcian flow velocities range from 10[sup]-6 to 3 m[sup]2 yr[sup]-1. The extensive connectivity of the channel network in the veined microdiorites suggests that element mobility during active flow would have been extensive.Keywords:
Porphyritic
Magma chamber
Igneous differentiation
Orogeny
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An integrated study involving whole-rock and Sr–Nd–Hf isotope geochemistry and zircon geochronology and trace element combined with detailed field investigation was carried out for the composite Meiwu batholith in the West Qinling orogenic belt of central China to probe the origins of its compositional diversity and its emplacement history. The batholith is composed of quartz diorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves and minor tonalitic enclaves in the granodiorite. The crystallization age of the batholith is ∼240–245 Ma. Geochemical and Sr–Nd–Hf isotopic data indicate that the magmas that formed the quartz diorite and the mafic enclaves were derived by partial melting of enriched lithosphere mantle, followed by variable degrees of hybridization with crustal magmas in deep crustal hot zones. These initially heterogeneous, hybrid magmas successively intruded into the upper crust and coalesced into a large magma chamber. Zircon trace element and Hf isotopic compositions suggest that the outer fine-grained part of the quartz diorite pluton crystallized from a less differentiated magma as a result of rapid cooling and thus preserved its initial heterogeneities, whereas the inner medium-grained part of the quartz diorite pluton crystallized from a convecting, isotopically homogeneous magma that had undergone advanced magmatic differentiation. The Sr–Nd–Hf isotope compositions of the mafic magmatic enclaves are strikingly similar to those of the host granodiorite, implying their isotopic equilibration. The tonalitic enclaves have high Sr/Y ratios and most probably represent magmas derived from partial melting of thickened mafic lower crust. The Sr–Nd–Hf isotope data suggest that the granodiorite and biotite granite were dominantly derived from isotopically heterogeneous crustal sources. However, the granodiorite also has relatively high Mg#, Cr, and Ni, indicating a minor contribution from a mantle source. The granodiorite was constructed incrementally from a number of discrete melt batches that were generated by partial melting of mafic lower crust under variable water fugacity. These melt batches did not assemble into a large magma chamber and thus preserved their source chemical features. The granodiorite magma was also replenished by mafic and high Sr/Y magmas, resulting in abundant and compositionally diverse magmatic enclaves. The biotite granite formed by the successive accumulation of discrete magma pulses generated by dehydration melting of mafic lower crust under water-absent conditions. These magma pulses coalesced into a small single magma chamber where they underwent fractional crystallization. The various rock types exhibit distinct geochemical variations, indicating that the Meiwu batholith was constructed from multiple injections of magma over a protracted period. Fractional crystallization, assimilation, magma mixing and/or mingling occurred during magma ascent and at the emplacement level. Distinct magma sources played a primary role in controlling the chemical diversity of the igneous bodies at pluton and batholith scale.
Batholith
Diorite
Fractional crystallization (geology)
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Porphyritic
Magma chamber
Igneous differentiation
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Magma chamber
Fractional crystallization (geology)
Layered intrusion
Igneous differentiation
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Cretaceous granitic rocks in the Waryongsan area occur as a stock and show compositional changes with altitude. They include mafic microgranular enclaves (MME) with various sizes and types. The MMEs present clear evidence of magma mingling such as supercooling zone, mantling texture and back veining. The granitic rocks are divided into porphyritic granite, porphyritic granodiorite and fined-grained granite by their petrographic characteristics and modal compositions. The MMEs are discriminated to quartzdioritie, quartzmonzodiorite and tonalite. They have varying areal proportions in each granitic rock-type: 10∼l5% in the porphyritic granite, about 50% in the porphyritic granodiorite, and about 20% in the fined-grained granite. SiO₂ contents shows compositional change of 61.2∼72.0wt.%. Mean SiO₂ contents have 61.7wt.% in the porphyritic granodiorite, 68.6wt.% in the porphyritic granite. and 71.9wt.% in the fined-grained granite, respectively. Major oxide contents of the granitic rocks linearly vary with SiO₂ contents from the porphyiritic granodiorite to the fine-grained granite on Harker diagrams. Linear compositional variations seem to have been caused by differential degrees of mingling between mafic magma and host granite. Where larger amount of mafic magma was injected into the host granitic magma, the two magmas reached to thermal equilibrium more quickly and eventually chemical mixing occurred to produce the composition of the porphyritic granodiorite. On the other hand. less amount of injected mafic magma would have been responsible for mechanical mixing to produce the compositions of the porphyritic granite and the fined-grained granite. Therefore, it is considered that the granitic rocks in the Waryongsan area experienced magmas mingling resulting from the injection of more mafic magma into differentiating granitic magma, and that the compositional changes of the granitic rocks were ascribed to the degree of mingling between the two magmas.
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Dharwar Craton
Igneous differentiation
Magma chamber
Crenulation
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Hornblende
Magma chamber
Fractional crystallization (geology)
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Silicic
Fractional crystallization (geology)
Igneous differentiation
Hornblende
Felsic
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Felsic
Porphyritic
Magma chamber
Fractional crystallization (geology)
Igneous differentiation
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Field, petrographic and geochemical evidence from the K-feldspar megacrystic Kameruka pluton, Lachlan Fold Belt, southeastern Australia, suggests that complex, multicomponent, mafic microgranular enclaves (MME) are produced by two-stage hybridisation processes. Stage 1 mixing occurs in composite dykes below the pluton, as mafic and silicic melts ascend through shared conduits. Pillows formed in these conduits are homogeneous, fine-to medium-grained stage 1 MME, which typically range from basaltic to granitic compositions that plot as a sublinear array on Harker diagrams. Stage 2 hybridisation occurs in the magma chamber when the composite dykes mix with the resident magma as synplutonic dykes. The stage 2 hybrids also form linear chemical arrays and range from basaltic to granodioritic compositions, the latter resembling the more mafic phases of the pluton. Stage 2 MME are distinguished from stage 1 types by the presence of K-feldspar xenocrysts and a more heterogeneous nature: they commonly contain stage 1 enclaves. Subsequent disaggregation and dispersal of stage 2 hybrid synplutonic dykes within the magma chamber produces a diverse array of multi-component MME. Field evidence for conduit mixing is consistent with published analogue experimental studies, which show that hybrid thermo-mechanical boundary layers (TMBL) develop between mafic and silicic liquids in conduits. A mechanical mixing model is developed, suggesting that the TMBL expands and interacts with the adjacent contrasting melts during flow, producing an increasing compositional range of hybrids with time that are mafic in the axial zone, grading to felsic in the peripheral zones in the conduit. Declining flow rates in the dyke and cooling of the TMBL zones produce a pillowing sequence progressing from mafic to felsic, which explains the general observation of more MME in more silicic hosts. The property of granitic magmas to undergo transient brittle failure in seismic regimes allows analogies with fractured solids to be drawn. The fracture network in granitic magmas consists of through-going ‘backbone’ mafic and silicic ± composite dykes, and smaller ‘dangling’ granitic dykes locally generated in the magma chamber. Stage 1 hybrids form in composite backbone dykes and stage 2 hybrids form where they intersect dangling dykes in the magma chamber. With subsequent shear stress recovery, the host magma chamber reverts to a visco-plastic material capable of flow, resulting in disaggregation and dispersal of these complex, hybrid synplutonic dykes, and a vast array of double and multicomponent enclaves potentially develop in the pluton.
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ABSTRACT The High Tatra granite intrusion is an example of a Variscan syn-tectonic, tongue-shaped intrusion. In some portions of the intrusion, structures occur which appear to be of sedimentary origin. These include structures similar to graded bedding, cross-bedding, troughs and flame structures, K-feldspar-rich cumulates and magmatic breccias. Formation of these structures might be related to changing magma properties, including crystal fraction, development of a crystal mush and a decrease in magma viscosity, stimulated by influx of mafic magma and high volatile content. The suggested processes in operation are: gravity-controlled separation, magma flow segregation, deposition on the magma-chamber floor, filter pressing and density currents stimulated by tectonic activity. %The formation of the sedimentary structures was also aided by the presence of large numbers of xenoliths that acted as a heat sink and influenced the thermal field in the intrusion, stimulating rapid cooling and crystal nucleation. Sinking xenoliths deformed the layering and, to some extent, protected the unconsolidated crystal mush from erosion by magma flowing past. %Areas with well-developed sedimentary magmatic structures can be viewed as having involved magma rich in crystals locally forming closely-packed networks from which residual melt was extracted by filter pressing, and preserved in leucocratic pods and dykes. Interleaved, non-layered granite may be interpreted to have formed from the magma with initially low crystal fractions. %It is suggested that the intrusion was formed from numerous magma injections representing different stages in the mixing and mingling of felsic and mafic sources. It solidified by gravitation-driven crystal accumulation and flow sorting on the magma chamber floor and on the surfaces of large numbers of xenoliths. Shear stress acting during intrusion might have influenced the formation of magmatic structures.
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