Petrogenesis of the Anorthosite Dyke Swarm of Tromso, North Norway: Experimental Evidence for Hydrous Anatexis of an Alkaline Mafic Complex

The 456 ± 4 Ma Skattøra migmatite complex in thenorth Norwegian Caledonides consists of migmatitic nepheline-normativemetagabbros and amphibolites that are net-veined by numerousnepheline-normative anorthositic and leucodioritic dykes. Plagioclase(An_20–50) is the dominant mineral (85–100%) in thedykes and the leucosome, but amphibole is generally presentin amounts up to 15%. The following observations strongly suggestformation of the anorthositic magma by anatexis of the surroundinggabbro in the presence of an H₂O-bearing fluid phase: (1) themigmatites have plagioclase-rich (anorthositic) leucosomes andamphibole-rich restites; (2) crystallization of amphibole inthe anorthositic and leucodioritic dykes suggests high H₂O activity;(3) the presence of coarse-grained to pegmatitic dykes and miaroliticcavities indicates a fluid-rich magma; (4) hydration zones thatsurround many anorthosite dykes suggest that the magma probablyexpelled H₂O-rich fluids during crystallization. Water-saturatedmelting experiments at 0·5–1·5 GPa and temperaturesfrom 800 to 1000°C have been performed on a nepheline-normativegabbro to test the proposed petrogenesis of the Skattøraanorthosites. The glasses produced close to the solidus aretonalitic in composition, but they become richer in plagioclaseat higher temperatures. At and below 1·0 GPa, the residuesare composed of amphibole. Experiments above 1·0 GPaproduced residual garnet and/or zoisite in addition to amphibole,suggesting that the anorthositic dykes in the Skattøramigmatite complex formed below 1·25 GPa. The experimentsshow that the high Na₂O content of the anorthosite dykes canonly be produced if Na is added to the charges. The glass thatbest fits the composition of the Skattøra dykes was producedat 1·0 GPa and 900°C with 2 wt % Na(OH) added. KEY WORDS: anorthosite; dyke swarm; anatexis; experimental petrology     

The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1{middle dot}0 to 3{middle dot}2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes

Fluid-absent melting experiments on a zoisite- and phengite-bearingeclogite (omphacite, garnet, quartz, kyanite, zoisite, phengiteand rutile) were performed to constrain the melting relationsof these hydrous phases in natural assemblages, as well as themelt and mineral compositions produced by their breakdown. From1·0 to 3·2 GPa the solidus slopes positively from1·5 GPa at 850°C to 2·7 GPa at 1025°C,but bends back at higher pressures to 975°C at 3·2GPa. The melt fraction is always low and the melt compositionsalways felsic and become increasingly so with increasing pressure.The normative Ab–An–Or compositions of the initialmelts vary from tonalites at 1·0 GPa to tonalite–trondhjemitesat 1·5 GPa, adamellites at 2·1 and 2·7GPa, and to true granites at 3·2 GPa. At pressures <     

Vapour-Absent Melting from 10 to 20 kbar of Crustal Rocks that Contain Multiple Hydrous Phases: Implications for Anatexis in the Deep to Very Deep Continental Crust and Active Continental Margins

Dehydration-melting experiments from 10 to 20 kbar were performedon a metavolcanoclastic rock containing (in vol. %) biotite(16), amphibole (15) and epidote (13) in addition to plagioclaseand quartz. At 10 and 12.5 kbar traces of biotite and epidoteremain at 850C, amphibole becomes more abundant, and the meltfraction is 5–10 vol. %. These relationships reflect thatthe thermal stability of biotite is lowered in the presenceof epidote through the dehydration-melting reaction biotite+epidote+quartz=amphibole+garnet+alkalifeldspar+melt. Amphibole dehydration-melting produces an additional25 vol. % melt between 875 and 925C. At 15 kbar and 875C themelt fraction is 22 vol. %, amphibole is present in trace amounts,and biotite constitutes 8 vol. %. These relationships suggestthat the curves marking biotite- and amphibole-out intersectclose to 15 kbar, and that the fertility of the rock increasesfrom 10 to 15 kbar at 850C. At 20 kbar the melt fraction isonly 5 vol. % at 850C, amphibole is transformed to omphaciteand biotite constitutes 5% of the mode. This result shows thatthe fertility decreases from 15 to 20 kbar at 850C, mainlybecause much Na is locked up in omphacite. Along active continentalmargins, intrusion of hot mantle-derived magmas is common, andmelting of metavolcanoclastic rocks may be an important granitoid-formingprocess. Intersection of the amphibole- and biotite-out reactionsbetween 12.5 and 15 kbar suggests that fusion of biotite- andhornblende-bearing rocks can produce magmas ranging in compositionfrom granitic (biotite dehydration-melting) to granodioritic(amphibole dehydration-melting) in either order depending onpressure. KEY WORDS: amphibole; biotite; dehydration-melting; epidote; metavolcanoclastic rock ^*Corresponding author.     

Fluid-Absent Melting Behavior of an F-Rich Tonalitic Gneiss at Mid-Crustal Pressures: Implications for the Generation of Anorogenic Granites

Fluid-absent melting experiments on a biotite (20 wt.%) andhornblende (2 wt.%) bearing tonalitic gneiss were conductedat 6 kbar (900–975C), 10 kbar (875–1075C), and14 kbar (950–975C) to study melt productivity from weaklyperaluminous quartzofeldspathic metamorphic rocks. At 6 kbar,biotite dehydration–melting is completed at 975C viaincongruent melting reactions that produce orthopyroxene, twooxides, and {small tilde}25 wt.% granitic melt. At 6 kbar, hornblendedisappears at 900C, probably in reaction with biotite. At 10kbar, biotite dehydration–melting produces <10 wt.%melt up to 950C via incongruent melting reactions that produceorthopyroxene, garnet, and granitic melt. Hornblende disappearsin the satne temperature interval either by resorption or byreaction with biotite. Widespread biotite dehydration–meltingoccurs between 950 and 975C and produces orthopyroxene, twooxides, and {small tilde}20 wt.% fluorine-rich (up to 0•31wt.%) granitic melt. At 14 kbar only a trace of melt is presentat 950C, and the amounts of hornblende and biotite are virtuallythe same as in the starting material. At 975C, hornblende isgone and {small tilde}10 wt.% granitic melt is produced by meltingof both biotite and hornblende. Our results show that hornblende-bearing assemblages cannotgo through dehydration–melting on their own (althoughthey can in combination with biotite) if the Ca content in thesource rock is too low to stabilize clinopyroxene. In such rocks,hornblende will either resorb or melt by reaction with biotite.Under fluid-absent conditions, intrusion of hot, mantle-derivedmagmas into the lower crust is necessary to initiate widespreaddehydration–melting in rocks with compositions similarto those discussed here. We argue that the high thermal stabilityof biotite in our starting material is caused mainly by theincorporation of fluorine. The relatively high F content inbiotite in the starting material (0•47 wt.%) suggests thatthe rock has experienced dehydroxylation in its past. F enrichmentby a previous fluid-absent partial melting event is excludedbecause of the lack of phases such as orthopyroxene and garnetwhich would have been produced. Our experiments show that thedehydration–melting of such F-enriched biotite producesF-rich granitic liquids, with compositions within the rangeof A-types granites, and leaves behind a granulitic residuedominated by orthopyroxene, quartz, and plagioclase. This studytherefore supports the notion that A-type granites can be generatedby H₂O-undersaturated melting of rocks of tonalitic composition(Creaser et al., 1991), but does not require that these sourcerocks should be residual after a previous melting event.