Computer Science
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.1363z&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 7, pp.1363-1382
Computer Science
9
Scientific paper
Voluminous syenites were intruded during the waning stage of the granulite facies metamorphism in the Yamato Mountains of East Antarctica. The area has been interpreted as part of a Cambrian continental collision zone with regional upper amphibolite to granulite facies metamorphism occurring during ca . 500-660 Ma period. Regardless of minor geochemical variations between different groups, all syenites are characterised by high K 2 0 + Na 2 0 (8-12%), K 2 0/Na 2 0 (~2), Sr (800-3500 ppm), Ba (2000-8500 ppm), and comparatively high TiO 2 , P 2 0 5 , Zr, and light REES relative to I-type granites. They are significantly higher in Mg number (50-75) compared with typical calc-alkaline suites, igneous charnockites, or A-type granites and define a distinctive trend on an AFM (alkali-FeO tot -MgO) diagram. Their trace element distribution diagrams are characterised by pronounced enrichment in LIL and REES, large negative Nb and Ti anomalies, and no depletion in Sr or Ba relative to the neighbouring elements. In this regard, they closely resemble the ~500 Ma post-tectonic mela-syenite to alkali basalt dikes widely occurring in East Antarctica. Such geochemical features are distinct from rift- or hotspot-related syenites, which are usually characterised by low K/Na ratios, negative Ba and Sr anomalies, and a lack of negative Nb anomalies. Initial isotopic compositions of the syenites are characterised by relatively low initial Nd values (-2.6 to -5.5) and high Sri ratios (0.7057-0.7088). Since the syenites are extremely enriched in Sr and Nd, such isotopic signatures are interpreted as reflecting the nature of the mantle source, rather than significant crystal contamination. Such isotopic signatures are also distinct from those of the rift- or hotspot-related syenites which are thought to be derived from depleted asthenospheric mantle. Considering the distinctive geochemical signatures of the Yamato syenites and their analogy to posttectonic alkaline mafic dikes in Antarctica, it is proposed that the syenites were generated by fractionation and magma mixing (with a crystal melt) of a Si-undersaturated alkali basaltic magma in a lower-crust magma chamber, followed by further crystal fractionation/cumulate-melt unmixing at middle to upper crustal levels (<5 kbars). Tectonically, it is proposed that the syenites were probably formed within the hinterland of the proposed Cambrian continental collision zone, with the parental magma being derived ultimately by partial melting of the metasomatised mantle wedge above the deepest part of the subduction zone. Similar models may also apply to the origin of some post-tectonic alkaline dikes in East Antarctica, with their sources being the continental lithospheric mantle previously modified by subduction-related processes.
Ellis David J.
Sheraton J. W.
Shiraishi Kenji
Zhao Jun-xiao
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