Processes and conditions during contact anatexis, melt escape and restite formation: The Huntly Gabbro complex, NE Scotland

The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic, mid-crustal, layered, mafic intrusions, emplaced into Proterozoic metaclastic rocks of NE Scotland soon after the thermal peak of static, high-T, low-P regional metamorphism. This gabbro and its associated contact metamorphic rocks illustrate a variety of processes operating during contact anatexis and subsequent melt segregation and extraction. These processes may closely mirror those occurring at much larger scales in the deep crust during high-grade regional metamorphism and the generation of granitic magmas. The emplacement of the Huntly mafic magma resulted in high-grade contact metamorphism and, locally, anatexis of metapelites, leading to the formation of migmatites. The migmatites and country-rock schists were studied to establish the physical conditions of metamorphism and anatexis, the nature of the melting reactions, the compositions of the melts produced, and the extent to which melting was a closed- or open-system process. The country-rock schists immediately to the south of the Huntly Complex contain mineral assemblages characteristic of the regional andalusite zone. Thermobarometry of an andalusite schist yields regional metamorphic conditions of 537 ± 42°C and 0·27 ± 0·12 GPa, consistent with previously published P-T estimates. The contact metamorphic rocks include sillimanite hornfelses, metatexites and diatexites. The metatexiles consist of cordierite-K-feldspar hornfels melanosomes and K-feldsper-rich garnetiferous leucosomes. The diatexites consist of schollen of fine-giained granoblastic hornfels and metatexite suspended in igneous-textured matrix rocks composed of abundant sub / euhedral garnet, cordierite, plagioclase and, locally, orthopyroxene, with minor interstitial biotite, K-feldspar and quartz. The hornfels melanosomes and schollen retained their structural integriy during partial melting, but the matrix rocks did not. In the highest-grade diatexites, the assemblage Grt + Opx + Crd + Hc + Pl characterizes both the hornfels schollen and the sub / euhedral minerals of the matrix rocks. Application of phase equilibria to Opx-bearing rocks yields estimated peak-metamorphic conditions of 900 ± 50°C, 0·45 ± 0·1 GPa and aH2O <0·3. The pressure estimate implies an emplacement depth of ∼16 ± 3 km. The prograde P-T path of contact-metamorphic rocks had a low, positive dP/dT slope, indicating that the gabbro intrusion increased the lithostatic load on the country rocks by overplating. Pseudomorph textures involving Al-silicates provide strong evidence that the diatexites evolved from andalusite schists via a sillimanite hornfels stage. Mineralogical changes reflect a sequence of dehydration reactions, followed by fluid-absent partial-melting reactions involving biotite breakdown, It was the fluid-absent reactions that generated the sub / euthedral minerals in the diatexites, as peritectic phases. In many of the highest-grade diatexites, quartz-free anhydrous solid assemblages were produced via reactions such as Bt + Sil + Grt = Crd + Hc + Kfs + melt and Bt + Grt + Crd = Opx + Hc + Kfs + melt. Whole-rock major-element geochemical studies indicate that the Opx-Crd hornfelses and diatexite matrices are depleted in Si and K relative to their schist protoliths. Mass-balance calculations indicate that (1) the Opx-Crd hornfels xenoliths represent solid Ca-, Mg-, Fe-, Al-, Na-rich residues left after extraction of ∼60% melt; (2) the Opx-bearing diatexite matrix rocks are also residual, and represent restite-enriched crystal-liquid mushes left after extraction of ∼55% melt; (3) the Opx-free diatexite matrix rocks probably represent restite-enriched mushes that retained a higher proportion of residual melt; (4) the anatectic melts were of H2O-undersaturated, peraluminous, low-Ca, potassic granite composition. Melt compositions and proportions were confirmed experimentally by partially melting local metapelite samples at 900°C and 0·5 GPa, under fluid-absent conditions. The similarity between the compositions of calculated and experimental melts implies that the melts under-went little or no fractional crystallization before their expulsion. At many localities, a large proportion of the melt escaped from the sites of its generation. Segregation of melt from restite was probably achieved through gravity-driven processes. The fugitive granitic melts did not mix with the gabbroic magma to any great extent, although contamination of mafic magma occurred locally, leading to the generation of biotite-bearing gabbros. The fugitive melts probably contributed to the contemporaneous 'Grampian' suite of S-type granites. The mid-Ordovician middle crust of NE Scotland was thus a site of crustal differentiation. The results demonstrate that crustal fusion and magma production can occur without significant chemical interaction between the mantle-derived heat source and the crustal melts, and that melt extraction can occur in the absence of regional tectonic deformation.