 |
| Titel |
The age, nature and likely genesis of the Cambrian Khantaishir arc, Lake Zone, Mongolia |
| VerfasserIn |
Vojtěch Janoušek, Yingde Jiang, Karel Schulmann, David Burianek, Pavel Hanžl, Ondrej Lexa, Turbat Ganchuluun, Altanbaatar Battushig |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091797
|
| Publikation (Nr.) |
 EGU/EGU2014-6108.pdf |
|
|
|
|
|
| Zusammenfassung |
| Recent discovery of the huge Cambrian arc in the Khantaishir Mountain Range (SE
Mongolian Altai) suggests that the principal Neoproterozoic and Devonian–Carboniferous
episodes of crustal growth in the Central Asian Orogenic Belt (CAOB) (Sengör et
al. 1993) have to be revised. This probably the largest arc system known in the
Mongolian tract of the CAOB is seemingly intrusive into the Neoproterozoic accretionary
wedge (the Lake Zone) in the N and underthrust southwards below the Palaeozoic
volcanosedimentary prism (Gobi Altai Zone). The arc shows a section from deep, ultramafic
cumulates to shallower crustal levels of the magmatic system and thus provides an
excellent opportunity to study this important period of crustal growth in the Mongolian
CAOB.
The magmatic rocks are intermediate to ultrabasic (SiO2 = 39.2–61.8 wt. %), rather
primitive (mg# = 45–60) Amp–Bt tonalites to coarse-grained Amp gabbros and
hornblendites. They are Na-rich (Na2O/K2O = 1.3–9.7 by wt.), exclusively metaluminous
and mostly subalkaline, except for the ultrabasic types that enter the alkaline domain due
to accumulation of Amp crystals. The P–T conditions calculated using the Amp
thermobarometer of Ridolfi et al. (2010) show that the gabbro crystallized at 930–950 °C
and 0.36–0.43 GPa. The (normal-) calc-alkaline chemistry and characteristic trace-element
enrichment in hydrous-fluid mobile large-ion lithophile elements (LILE: Rb, Ba, Th, U, K
and Pb) over high-field strength elements (HFSE: Nb and Ta) confirm an origin within an
igneous arc.
The newly obtained LA ICP-MS zircon ages for three tonalites–diorites range between
516 ± 2 Ma and 494 ± 3 Ma. While zircons in two of them give high initial ÉHf
values (+8 to +14), implying a derivation by (near) closed-system fractionation from
little modified, depleted-mantle derived magmas, the third contains significantly
different component (ÉHf = +3 to +6). The latter component may have come from a
distinct, less depleted/metasomatized mantle domain or, more likely, originated by
remelting of a juvenile metabasic crust, presumably by the advected heat from the
voluminous mantle-derived melts. The single gabbro yielded two zircon populations, 521
± 6 Ma (avg. 238U–206Pb age, 3 grains) and 538 ± 3 Ma (8 grains). The zircons
contain both components distinguished in the more felsic samples (ÉHf = +4 to +13),
compatible with the field evidence for the presence of several generations of the
basic magmas, as well as magma mixing/mingling between contrasting magma
batches.
Taken together, our data provide an evidence for a Cambrian (c.520–495 Ma)
magmatic-arc activity in the Khantaishir Range. The lack of significant zircon inheritance and
rather primitive ÉHfvalues (all > +3) imply that the arc was not funded on mature continental
crust. On the contrary, a key role was played by depleted-mantle derived magmas, perhaps
with some recycling of a juvenile metabasic crust.
Financial support: Grant Agency of the CR (P210/12/2205), Ministry of Education of the
CR (LK11202) and Hong Kong Research Council (HKU705311P and HKU704712P).
Ridolfi et al. 2010. Contrib. Mineral. Petrol. 160, 45–66.
Sengör et al. 1993. Nature 364, 299–307. |
|
|
|
|
|
|