Mafic dykes of the East Antarctic shield : experimental, geochemical and petrological studies focusing on the proterozoic evolution of the crust and mantle

摘要

The Vestfold Hills block of the East Antarctic Shield is an Archeanudgranulite facies complex crossed by hundreds of mafic dykes. The dykesudcompose five chemically distinct suites emplaced in three separate magmaticudepisodes during the Early-Middle Proterozoic. Sampling of Chilled marginsudcoupled with detailed documentation of cross-cutting relationships form theudbasis for studying the chemical and temporal evolution of each dyke suite udas well as providing insight into the chemical evolution of theudsubcontinental mantle during the Proterozoic. Furthermore, the emplacementudof the swarms is bracketed by tectonothermal metamorphic events atudca.2500 Ma and ca.1100 Ma. Previous studies of the metamorphic assemblagesudwhich developed during these events have provided constraints on theudP,T, time path of the Shield. A principal purpose of the present study hasudbeen to determine emplacement depths of the dyke suites in order to furtherudrestrict the possible uplift path of the shield during the -1400 Maudinterval separating, the final two major deformations. High pressure experimental techniques were used to determine the loadudpressure at the emplacement depth of two samples representing dyke suitesudintruded at ca.2400 Ma and ca.1360 Ma. This was accomplished byudexperimentally reproducing the chilled margin phenocryst assemblage and theudphase compositions after first rigorously evaluating crystal/liquid KD's toudensure the studied material represented liquid compositions: The resultsudof the experimental studies were also applied to compositionally similaruddykes from the Napier Complex of Enderby Land. The studies indicate theudNapier Complex and the Vestfold Hills are characterized by -2000 Ma ofudcrustal stability following the peak metamorphic event at ca .3100 Ma. Bothudterrains followed identical isobaric cooling paths, to ca.2500 Ma, Afterudwhich the Napier Complex remained at deep crustal levels ( -28 km) untiludca.1000 Ma, while the Vestfold Hills was exhumed at a rate of -1cm/1000 auduntil ca. 1100 Ma. This long period of crustal stability experienced byudboth terrains was terminated by a Himalayan-style tectonic event whichudresulted in isothermal decompression at a minimum net rate of 11cm/1000 audin the Napier Complex, while crustal loading, depressed the Vestfold Hillsudfrom depths of -16 km to -22 km resulting in the formation of garnet in theudmafic dykes. In the absence of more recent deformational events, it isudassumed that these terrains experienced slow, erosion controlled upliftudsince ca.1000 Ma. The oldest undeformed mafic dyke swarm in the Vestfold Hills wasudemplaced at ca.2400 Ma and is composed of two contemporaneous suites, audSi02 -rich, high-Mg tholeiite suite and a Fe-rich tholeiite suite, whichudcompare favourably to basaltic komatiite -Fe tholeiite associations ofudArchean greenstone belts. Normalized plots of incompatible trace elements,udphase compositions, and major element ratios divide the high-Mg suite intoudthree distinct subgroups. The trace element characteristics and isotopicudstudies prohibit these subgroups from being related by crystaludfractionation and previous isotopic studies preclude crustal contaminationudas a source of chemical variations. Major and trace element evaluationudalso indicates that two of the chemically distinct subgroups were derivedudfrom a primitive liquid extracted from "chondritic" mantle sources leavingudan olivine + orthopyroxene residue. Superimposed upon the chondriticudcharacteristics is evidence for selective trace element contaminationudthrough a "wall-rock" reaction-type process, possibly with a plagioclasebearingudlherzolite. Comparison of an estimated parental liquid compositionudwith experimental melting studies indicates magma extraction took place atudpressures of -10 kbar (35 km), consistent with the geochemical signatureudindicating partial melting with a plagioclase-bearing mantle. The moreuddifferentiated Fe-rich tholeiites are depleted in LIL elements compared toudthe high-Mg tholeiites which excludes any simple relationship between theudsuites by a fractional crystallization process. Three petrographically and chemically distinct groups can beudidentified within the Fe-rich suite. Two groups can be related in bothudmajor and trace element variations by crystal fractionation, but fieldudrelationships preclude this simple interpretation. All the groups have REEudpatterns indicating derivation from an unfractionated mantle, whichudprecludes a relationship with the high-Mg suite by polybaric meltudextraction of a single source. The two closely related groups displayudenrichment in P and Zr in excess of that due to crystal fractionation.udThis may represent a relatively deep level wall-rock reaction process asudthe third group displays elemental enrichments identical to that of theudhigh-14g tholeiites (formed at -10 kbar).udThe youngest and volumetrically largest dyke swarm was emplaced inudthe Vestfold Hills at ca.1360 Ma and is dominated by Fe-rich tholeiitesudwith subsidiary, though widespread alkaline dykes. The major and traceudelement diversity of the Fe-rich suite can be accounted for by a crystaludfractionation dominated process. However, the geochemical trends areuddefined by clusters of analyses which in themselves define differentiationudtrends that cannot be related to fractionation of the phenocryst phases. Furthermore, the ordering of the groups does not relate to a simple,udmonotonic emplacement sequence. The nature of the small-scale geochemicaludtrends and the complex emplacement style may be the result of "open system"udmagma chamber processes. Rare earth element profiles, as well as theudincompatible element plots indicate the parental liquids to this suite wereudderived from a primitive source which had not experienced previous meltingudevents.