Pre-survey: Magmatic evolution of the Tiger gabbroic complex (Northern Victoria Land, Antarctica): insights into
the lower crust of a Cambrian immature Island Arc
Subduction-zone related mafic/ ultramafic complexes marking the suture between the Wilson Terrane and the Bowers Terrane in
Northern Victoria Land (Antarctica) are well-suited for evaluating the magmatic and structural evolution at the active
paleo-Pacific continent margin of Gondwana during the Cambrian. One of these intrusions is the "Tiger Gabbro Complex"
(TGC), a Cambrian arc-related, layered mafic complex which is characterized by extraordinary fresh rock sequences including
ultramafic and evolved lithologies and by the presence of spectacular high-temperature shear zones. It is our overall goal
to establish the petrogenesis of the TGC in order to evaluate the magmatic/ structural/ geological evolution of deep crustal
sequences active during the Cambrian at the margin of Gondwana. The project will focus on microanalytical and bulk rock
geochemical studies on archive samples and on new samples which will be collected during the GANOVEX X expedition. This
project will serve as pilot project for our far-reaching goal which will include a comprehensive experimental study as
well as detailed geochronological and isotope geochemical work. A very close petrographic similarity with the Chilas
Complex in Kohistan (Pakistan), a world site for deep, juvenile arc crust, opens interesting perspectives for the global
understanding of crustal accretion beneath young island arcs.
First results (whole-rock geochemistry):
Whole-rock geochemistry was performed by XRF at the BGR on sample materials free of weathering crusts or macroscopically
visible alteration. Figs. 1 a-i depict Harker-type diagrams for various compatible and incompatible elements and the molar
Mg/(Mg+Fe2+) (“Mg#”) all plotted versus MgO content. In addition to the data for the BGR samples, XRF whole-rock data for
samples of the TGC investigated by Bracciali et al. (2009) are depicted. Only a very preliminary interpretation of the data
can be given so far:
(a) The samples cover a very broad range of compositions from ultrabasic rocks with MgO higher than 20 wt.% and Mg# close to 80 to
strongly evolved “anorthositic” (s.l.) compositions with MgO lower than 1 wt.% and Mg# of c. 30 on the one hand and strongly fractionated
“ferrogabbroic” compositions with elevated Fe2O3 and TiO2 contents (up to 14 wt.% and 4 wt.%, respectively) at relatively low
MgO (c. 5 wt.%) and low Mg# (c. 45). Samples enriched in Fe and Ti may also show elevated concentrations of Zr, K, Na and
Sr.
(b) Most of the samples from Apostrophe Island define linear trends between two hypothetical end-members: one is high in Mg,
Ni, Fe, and low in Al, Na and Sr while the other is low in Mg, Ni, Fe, and relatively high in Al, Na and Sr. A relatively
constant Mg# close to 75 for all these samples may indicate that these chemical variation trends do not mimic igneous
fractionation processes s.str.. Instead, “mechanical” mixing of major cumulate phases olivine, orthopyroxene and
clinopyroxene (all with high MgO and Mg# indicating lack of chemical evolution in the magma) on the one hand with
plagioclase on the other hand may well explain these trends.
(c) Most of the samples from Spatulate Ridge show chemically evolved “gabbroic” compositions with MgO lower than 10 wt.% and Mg# varying
between 75 and 45. Together with the chemical variations in other elements (e.g. Fe, Ti, Zr, Na, K), this is interpreted to
mimic fractional crystallization processes of a chemically evolving magma. Elevated Ti and Fe abundances in part of these
samples indicate a tholeiitic fractionation trend.
The apparent differences in bulk rock compositions of samples from Apostrophe Island and from Spatulate Ridge suggest that
they represent two units of the TGC formed under contrasting magmatic conditions. In this respect, Apostrophe Island may
represent the deeper part of the igneous complex dominated by ultramafic to mafic cumulate rocks originating from a chemically
not evolving magma chamber while Spatulate Ridge represents the upper part in which fractional crystallization processes in a
chemically evolving magma dominated. Very similarily, Jagoutz et al. (2007) showed that in the Chilas Complex two principal
units, ultramafic rocks and gabbronoritic suites, exist. There, the different units are interpreted to originate from a common
parental magma, but evolved along different mineral fractionation trends. Field work during GANOVEX X is primarily aimed to
further investigate these compositional differences and to provide additional samples which then will be used to test the
hypothesis that the two units of the TGC at Apostrophe Island and Spetulate Ridge represent different levels of a large
layered complex formed under contrasting magmatic conditions.
Fig. 1 a-i: Harker-type diagrams for selected element ratio for whole-rock samples of the Tiger Gabbro sampled during
GANOVEX III at Spatulate Ridge and Apostrophe Island (NVL, Antarctica). Whole-rock data of Bracciali et al. (J Geol Soc Ln
166:711-724; 2009) are shown for comparison. Samples investigated by microprobe are marked by "X".
Scientists
Jürgen Koepke, Leibniz Universität Hannover
Friedhelm Henjes-Kunst, BGR Hannover
Solveig Estrada, BGR Hannover
Dominique Kosanke, Leibniz Universität Hannover
Research areas
Topographic map showing the outcrops of the
Tiger Gabbro complex (Ca-Ttg; marked by blue lines) close to the suture
between Wilson and Bowers terranes (indicated by dashed line in red) in Northern Victoria Land. The contact to Bowers
terrane metavolcanic and metasedimentary rocks occurring to the NW is indicated by a dotted line (in black).
Publications
---
Homepage
---
Research funding organisation
German Research Foundation
Project number: KO 1723/11
Funding period: Since 2008
