Multiple magma mingling, enclave typology and textural evolution in the Ross of Mull Granite, Scotland

Pugliese, Stefano (1999) Multiple magma mingling, enclave typology and textural evolution in the Ross of Mull Granite, Scotland. (PhD thesis), Kingston University, .


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.

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