Textural, geochronological and chemical constraints from polygenetic titanite and monogenetic apatite from a mid-crustal shear zone: An integrated EPMA, SIMS, and TIMS study

摘要

Titanites extracted from a 520 +/- 9 Ma syn-orogenic syenite at Orjimbingwe (Damara orogen, Namibia) show shape, colour and U-Pb systematics that vary with position in the syenite relative to a major shear zone. Most titanite fractions from syenite cropping out outside the shear zone are characterized by a medium brown anhedral core overgrown by a pale brown subhedral rim. EMPA and SIMS analyses show that the cores are enriched in Al, Fe and sometimes Na, Mg and Mn but also in REE, Nb and Zr relative to the rim. TIMS U-Pb ages from these titanites gave relatively imprecise Pb-207/Pb-206 ages between 680 +/- 21 Ma and 695 +/- 20 Ma as well as 570 +/- 8 Ma to 604 +/- 9 Ma mainly due to their low Pb-206/Pb-204 ratios. Combined Th-U-Pb relationships suggest that these titanites contain a component similar to ca. 715-750 Ma-old titanite populations observed so far only in Neoproterozoic syenites from the northern part of the orogen. Titanites from syenites within the shear zone are dark brown, mostly euhedral to subhedral and display only occasionally a pale brown rim. These titanites show decreasing concentrations of Na, Mg, Al and Fe and increasing concentrations of Mn from core to rim, interpreted to reflect growth zonation. Abundances of MREE, HREE, Y, Nb and Zr are higher in the rim than in the core. Pb-207/Pb-206 ages obtained on these titanites range from 470 +/- 2 Ma to 538 +/- 3 Ma for most samples. Only the samples with the highest Pb-206/Pb-204 ratios (between 350 and 230) have U-Pb ages similar to the previously published U-Pb zircon age of 520 +/- 9 Ma from the same complex. Other titanite fractions with lower Pb-206/Pb-204 ratios have slightly older ages than ca. 520 Ma, indicating some inheritance. Inherited titanite survived multiple metamorphic episodes in the Damara orogen between ca. 750 Ma and 470 Ma and the igneous event that lead to the intrusion of the Otjimbingwe syenite at 520 Ma; these data imply a high closure temperature of U-Pb in titanite of 650 -> 700 degrees C consistent with previous studies. Coeval apatite has no apparent inheritance, low Pb-206/Pb-204 ratios between 55 and 170 and U-Pb ages similar to, or only slightly younger than the U-Pb zircon or titanite age obtained on the titanite fractions with the highest Pb-206/Pb-204 ratios. Assuming a closure temperature for U-Pb of >= 900 degrees C for zircon, similar to 700 degrees C for titanite and similar to 450 degrees C for apatite, initial fast cooling of the complex can be inferred. Our data demonstrate that titanite can contain multiple age domains, preserving a chronological and chemical record of previous igneous events. (c) 2007 Elsevier B.V. All rights reserved.