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• W1017881544 abstract "The Vestfold Hills block of the East Antarctic Shield is an Archeangranulite facies complex crossed by hundreds of mafic dykes. The dykescompose five chemically distinct suites emplaced in three separate magmaticepisodes during the Early-Middle Proterozoic. Sampling of Chilled marginscoupled with detailed documentation of cross-cutting relationships form thebasis for studying the chemical and temporal evolution of each dyke suite as well as providing insight into the chemical evolution of thesubcontinental mantle during the Proterozoic. Furthermore, the emplacementof the swarms is bracketed by tectonothermal metamorphic events atca.2500 Ma and ca.1100 Ma. Previous studies of the metamorphic assemblageswhich developed during these events have provided constraints on theP,T, time path of the Shield. A principal purpose of the present study hasbeen to determine emplacement depths of the dyke suites in order to furtherrestrict the possible uplift path of the shield during the -1400 Mainterval separating, the final two major deformations. High pressure experimental techniques were used to determine the loadpressure at the emplacement depth of two samples representing dyke suitesintruded at ca.2400 Ma and ca.1360 Ma. This was accomplished byexperimentally reproducing the chilled margin phenocryst assemblage and thephase compositions after first rigorously evaluating crystal/liquid KD's toensure the studied material represented liquid compositions: The resultsof the experimental studies were also applied to compositionally similardykes from the Napier Complex of Enderby Land. The studies indicate theNapier Complex and the Vestfold Hills are characterized by -2000 Ma ofcrustal stability following the peak metamorphic event at ca .3100 Ma. Bothterrains followed identical isobaric cooling paths, to ca.2500 Ma, Afterwhich the Napier Complex remained at deep crustal levels ( -28 km) untilca.1000 Ma, while the Vestfold Hills was exhumed at a rate of -1cm/1000 auntil ca. 1100 Ma. This long period of crustal stability experienced byboth terrains was terminated by a Himalayan-style tectonic event whichresulted in isothermal decompression at a minimum net rate of 11cm/1000 ain the Napier Complex, while crustal loading, depressed the Vestfold Hillsfrom depths of -16 km to -22 km resulting in the formation of garnet in themafic dykes. In the absence of more recent deformational events, it isassumed that these terrains experienced slow, erosion controlled upliftsince ca.1000 Ma. The oldest undeformed mafic dyke swarm in the Vestfold Hills wasemplaced at ca.2400 Ma and is composed of two contemporaneous suites, aSi02 -rich, high-Mg tholeiite suite and a Fe-rich tholeiite suite, whichcompare favourably to basaltic komatiite -Fe tholeiite associations ofArchean greenstone belts. Normalized plots of incompatible trace elements,phase compositions, and major element ratios divide the high-Mg suite intothree distinct subgroups. The trace element characteristics and isotopicstudies prohibit these subgroups from being related by crystalfractionation and previous isotopic studies preclude crustal contaminationas a source of chemical variations. Major and trace element evaluationalso indicates that two of the chemically distinct subgroups were derivedfrom a primitive liquid extracted from chondritic mantle sources leavingan olivine + orthopyroxene residue. Superimposed upon the chondriticcharacteristics is evidence for selective trace element contaminationthrough a reaction-type process, possibly with a plagioclasebearinglherzolite. Comparison of an estimated parental liquid compositionwith experimental melting studies indicates magma extraction took place atpressures of -10 kbar (35 km), consistent with the geochemical signatureindicating partial melting with a plagioclase-bearing mantle. The moredifferentiated Fe-rich tholeiites are depleted in LIL elements compared tothe high-Mg tholeiites which excludes any simple relationship between thesuites by a fractional crystallization process. Three petrographically and chemically distinct groups can beidentified within the Fe-rich suite. Two groups can be related in bothmajor and trace element variations by crystal fractionation, but fieldrelationships preclude this simple interpretation. All the groups have REEpatterns indicating derivation from an unfractionated mantle, whichprecludes a relationship with the high-Mg suite by polybaric meltextraction of a single source. The two closely related groups displayenrichment in P and Zr in excess of that due to crystal fractionation.This may represent a relatively deep level wall-rock reaction process asthe third group displays elemental enrichments identical to that of thehigh-14g tholeiites (formed at -10 kbar).The youngest and volumetrically largest dyke swarm was emplaced inthe Vestfold Hills at ca.1360 Ma and is dominated by Fe-rich tholeiiteswith subsidiary, though widespread alkaline dykes. The major and traceelement diversity of the Fe-rich suite can be accounted for by a crystalfractionation dominated process. However, the geochemical trends aredefined by clusters of analyses which in themselves define differentiationtrends that cannot be related to fractionation of the phenocryst phases. Furthermore, the ordering of the groups does not relate to a simple,monotonic emplacement sequence. The nature of the small-scale geochemicaltrends and the complex emplacement style may be the result of open systemmagma chamber processes. Rare earth element profiles, as well as theincompatible element plots indicate the parental liquids to this suite werederived from a primitive source which had not experienced previous meltingevents." @default.
• W1017881544 created "2016-06-24" @default.
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• W1017881544 date "1986-01-01" @default.
• W1017881544 modified "2023-09-26" @default.
• W1017881544 title "Mafic dykes of the East Antarctic shield : experimental, geochemical and petrological studies focusing on the proterozoic evolution of the crust and mantle" @default.
• W1017881544 hasPublicationYear "1986" @default.
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