Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Abstract

Abstract. Dehydration-melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high-grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of uHzO. The element-partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aHZO is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T s 850°C is relatively small (10-20%), and probably insufficient to mobilize a partially melted rock body.Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse-grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration-melting reactions: Bt + Sil + Qtz + PI = Grt ? Crd + Kfs + L (metapelite), Bt + Qtz + P1 = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + PI + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.

Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada

“…The qualitative relationships between anatexis, subsolidus dehydration and v a n m e a,,, may be i-iustrated with isobaric T-u,,, diagrams (Greenwood, 1975;Powell, 1983;Grant, 1985;Waters, 1988). Corresponding P-T and T-a,,, diagrams for selected reactions involving hornblende (Hbd), quartz (Qtz), orthopyroxene (Opx), clinopyroxene (Cpx), plagioclase (PI), silicate liquid (L) and vapour (V) in the system Ca0-Mg0-A1,03-Si02-H20 (CMASH) are illustrated schematically in Fig.…”

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

“…The MRPSZ is localized in mid-crustal rocks of the Namaqua Metamorphic Complex that experienced high-temperature (T )-low-pressure (P ) amphiboliteto granulite-facies peak metamorphic conditions during the Mesoproterozoic at ca. 1200-1050 Ma (Jackson, 1976;Blignaut, 1977;Waters and Whales, 1984;Waters, 1986Waters, , 1988Robb et al, 1999;Clifford et al, 2004;Cornell et al, 2009;Diener et al, 2013;Bial et al, 2015). The development of the MPRSZ post-dates peak metamorphism, with deformation having occurred under retrograde amphibolite-to greenschist-facies conditions, at ca.…”

Formation and evolution of granite magmas from migmatites :

“…Fine-grained interstitial quartz, plagioclase and biotite located in the interstices of a crystal framework comprising large, euhedral plagioclase and K-feldspar is evident in figure 17e. Crystal faces where framework-forming plagioclase is in contact with interstitial quartz or K-feldspar is evidence for crystallization from an anatectic melt (Platten 1983;Waters 1988;Harte & Pattison 1991). The K-feldspar is, in some cases, clear and homogeneous or cryptoperthitic, but in other examples the K-feldspar is cloudy due to alteration and to perthite exsolution.…”

“…Higher temperature anatexis involving the incongraent melting of biotite, or hornblende, without excess water, is generally considered to be the principal means of generating large volumes of waterundersaturated felsic melts and granulite facies mineral assemblages (Brown & Fyfe 1970;Fyfe 1973;Powell 1983;Ellis & Thompson 1986;Clemens & Vielzeuf 1987;Waters 1988;Clemens 1990;Vieizeuf & Montei 1994;Le Breton & Thompson 1988;Stevens & Clemens 1993;Patino Douce & Beard 1995Stevens et al, 1997).…”