Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland

Abstract

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 closedor 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 metatexites consist of cordierite±K-feldspar hornfels melanosomes and K-feldspar-rich garnetiferous leucosomes. The diatexites consist of schollen of fine-grained 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 integrity during partial melting, but the matrix rocks did not. In the highestgrade 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 aH2O5 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/euhedral minerals in the diatexites, as peritectic phases. In many of the highestgrade diatexites, quartz-free anhydrous solid assemblages were produced via reactions such as Bt ‡ Sil ‡ Grt ˆ Crd ‡ Hc ‡ Kfs ‡ melt and Bt‡ Grt ‡ Crd ˆ 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, JOURNAL OF PETROLOGY VOLUME 44 NUMBER 6 PAGES 995±1029 2003