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

• Published in: Journal of petrology 2003-06, Vol.44 (6), p.995-1029
• Main Authors: DROOP, G. T. R., CLEMENS, J. D., DALRYMPLE, D. J.
• Format: Article
• Language: English
• Subjects: diatexite; fluid-absent melting; melt segregation; migmatite; peraluminous granite
• ISSN: 0022-3530; 1460-2415; 1460-2415
• DOI: 10.1093/petrology/44.6.995

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 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 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 d P/d T slope, indicating that the gabbro intrusion increased the lithostatic load on the country roc