ISSN:
0016-7835
Keywords:
Key words U
;
Pb zircon dating
;
SHRIMP
;
Gneiss zircons
;
KTB
;
Germany
;
Northern Gondwana
;
Cambrian/Ordovician palaeogeography
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract In order to investigate the provenance, deposition age and metamorphism of metasedimentary gneisses from the pilot and the main holes of the German Continental Deep Drilling Program (KTB), zircons from volcano-metasedimentary (v-unit) and monotonous greywacke (g-unit) gneisses were dated by ion microprobe (SHRIMP) and conventional U–Pb zircon methods. The zircons were found to consist of anhedral detrital or subhedral cores surrounded by one or two metamorphic rims. Analysis of growth zones in 37 zircons indicated ages for the detrital cores between 2.25 and 2.05 Ga in a v-unit gneiss (sample K52), and 1.8–2.35 Ga in two g-unit gneisses (K747 and K798). Neo-Proterozoic (650–545 Ma) subhedral igneous cores were identified in all investigated KTB gneiss samples, and an additional two zircon growth episodes in the source area of the sediments at 712±15 and 525±11 Ma were also recognised. Zircon overgrowths dated at 487±13 Ma are interpreted to have formed during an early metamorphic episode following incorporation of the zircons into their host metasediments. A second metamorphic overprint indicated by a date of 381+22/–11 Ma was determined for zircon overgrowth in the g-unit gneisses. The dates determined for zircon growth phases suggest that the KTB metasediments were derived from northern Gondwana. Hypothetical provenance areas and a new model of terrane positions that is consistent with the geochronological, geodynamic and palaeo-geographic preconditions at the northern border of Gondwana in Late Precambrian to Early Ordovician times, are proposed.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/PL00014657
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