 |
| Titel |
Detrital zircons - the unique source of information on tectonics, paleogeography and denudation processes of East Antarctica (subglacial challenge) |
| VerfasserIn |
Boris Belyatsky, German Leitchenkov, Nickolay Rodionov, Anton Antonov, Sergey Sergeev, Helen Savva |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042164
|
|
|
|
|
|
| Zusammenfassung |
| Vast (about 7 billions km2) almost wholly (98%) covered with ice continental mass of East
Antarctica is the central fragment of ancient supercontinents of Rodinia and Gondwana.
Any information on its geologic structure is of the greatest importance for solving
the problems of formation and amalgamation of lithosphere of ancient continents,
processes of intraplate activity, denudation and peneplanation of the earth crust
and for geodynamic reconstruction. Geologic structure of central part of the East
Antarctica is still absolutely unknown due to the thick (up to 4000 m) ice cover,
which is the obstacle even for modern drilling technology to sample directly the
underlying rocks. The main goal of the study has been to make an attempt in fill up the
hiatus in geologic knowledge on the origin of subglacial continental crust of the
Antarctica.
We studied detrital zircons from sedimentary and metasedimentary rocks outcropped in
Prince-Charles Mts (PCM, East Antarctica). Rock specimens were sampled from the
Permian-Triassic sedimentary succession outcropped along the Beaver Lake coast
(sandstones and siltstones) and from moraine of the Fisher Massive (metasandstone) and
Meridith Massive (sandstone). A lump of zircons which are characterized by different grain
morphology from well-rounded to poorly-rounded has been extracted from rock specimens
for isotopic studies and dating. The age determinations of 302 zircons from 6 specimens were
conducted using secondary ion-microprobe SHRIMP-II and laser-ablation ICP-MS. The age
of zircons ranges from 500 to 3200 Ma. Isotopic analysis and probability distribution
diagrams for zircon populations show heterogeneity of provenance. Zircons of 500 Ma old
are proposed to come from the eastern flank of Lambert Glacier and/or from central
Antarctica; 900–1100 Ma old - from Proterozoic Mobile Belt (central-northern
PCM); while 2400–3200 Ma old - from granite-greenstone terrain (southern PCM).
Additionally, 21 trace elements and isotopic composition of hafnium in detrital zircons
from two moraine samples was studied using SIMS CAMECA-4f and laser device
coupled with HR-MC-ICPMS. Distribution of trace elements allows us to suggest
that 70–85% zircons have been crystallized from magma of intermediate-silicic
composition with temperature of crystallization of 700-800Ë C (65% SiO2) and
about 10–15% - from low-temperature water-saturated granitoid melts. Only 5–7%
zircons are formed as a result of metamorphic processes. Four zircon grains from
metasandstone (Fisher Massive) have specific distribution of trace elements which are
typical to high-temperature (up to 950Ë C) magmas of alkaline or mafic affinity. Hf
isotope signatures show according to two-step evolution model that zircons from
metasandstone of Fisher Massive were formed in crust protoliths of 1400-2200
m.y. old, whereas zircons from sandstone of Meridith Massive – in protolith of
1400-1600 m.y. and 3000-3400 m.y. old. Generally, Hf isotope composition of
studied zircons corresponds to composition of chondrite unfractionated reservoir
(CHUR).
The first opportunity to obtain direct information about the bedrock geology of the central
East Antarctic arose when the 3650 m deep borehole at the Vostok station, located in the
southern part of Lake Vostok (largest subglacial freshwater lake in Antarctica) recovered the
basal layer of the ice sheet. We studied a small (4.7 mm long) clast of siltstone extracted from
the 3607 m depth ice core of the Vostok Station Borehole. This clast was entrapped from
bottom sediments in the shallow area of the lake and incorporated into the accreted ice.
Siltstone consists of poorly-rounded quartz and a minor amount of accessories
including zircon and monazite. We infer that the bedrock upstream (northwest) of
Lake Vostok from where the siltstone clast was scraped off bedrock by ice and
transported to the lake is of sedimentary nature. 23 zircon and 5 monazite grains in the
siltstone clast have yielded two age clusters ranging between 0.8 - 1.2 Ga and 1.6 -1.8
Ga. These ages testify that their provenance presumably the area of Gamburtsev
Subglacial Mountains; central part of East Antarctica is dominated by 1.6-1.8 Ga
and 0.8-1.2 Ga assemblages. Younger rocks are not excluded because one zircon
grain has an age of 600 Ma although this determination is so far statistically very
poor.
The study has been partly supported by RFBR grant 07-05-00736. |
|
|
|
|
|
|