Timing the tectonic mingling of ultramafic rocks and metasediments in the southern section of the coastal accretionary complex of central Chile

Uranium-lead ages are reported for zircons from ultramafic bodies and metamorphic host rocks of the Western Series that outcrop at La Cabaña, in the southern section of the coastal accretionary complex of central Chile. Metasedimentary mica schists hosting the ultramafic bodies contain a main detrital zircon population of Devonian age (365–380 Ma) clustering around ~368 Ma, differing significantly from neighbouring areas where Devonian zircons are scarce. Zircons from the metasomatic reaction zones (albitites and chloritites), formed during the emplacement and alteration of the ultramafic bodies, are mainly Ordovician (~478 Ma) and lack Devonian zircons, resembling a typical detrital zircon pattern from other locations in the Western Series. Zircons from the chloritite reaction zone of the Lavanderos serpentinite, the easternmost ultramafic body in La Cabaña, are in textural equilibrium with metamorphic ilmenite. Some of these zircons yield an average age of 283.4 ± 7 Ma (<i>n</i> = 6) which is identical, within error, to a previously reported K-Ar fuchsite cooling age of 282 ± 6 Ma from the reaction zone. Most zircons extracted from chromitite boulders have euhedral oscillatory-zoned growth patterns with a similar range of ages than those reported for the Western Series (324–1090 Ma; <i>n</i> = 12), except for two zircons with cloudy appearance and high U/Th ratios which yielded an average age of 285.5 ± 7 Ma. The presence of Early Permian zircons (~280–290 Ma) in all studied rocks suggests remobilization of Zr, possibly triggered by metasomatic fluids released during the disequilibrium reaction associated with the tectonic emplacement of the ultramafic rocks into the metasedimentary rock. Simultaneously with the formation of metasomatic zircons, Palaeozoic and Mesoproterozoic zircons from the metasedimentary rocks were mechanically incorporated into the ultramafic rocks, thus providing a record of the timing of crustal emplacement of the ultramafic rocks into the accretionary complex.

本文报道了智利中部海岸增生杂岩（coastal accretionary complex）南部露头区拉卡巴尼亚（La Cabaña）产出的西部岩系（Western Series）超镁铁质岩体（ultramafic body）及其变质围岩（metamorphic host rock）中的锆石（zircon）铀-铅年龄数据。赋存于超镁铁质岩体中的副变质岩（metasedimentary rock）云母片岩，其碎屑锆石（detrital zircon）主体为泥盆纪（Devonian，365~380 Ma），年龄峰值集中在约368 Ma，这与周边缺乏泥盆纪锆石的区域存在显著差异。超镁铁质岩体侵位（emplacement）及蚀变过程中形成的交代反应带（metasomatic reaction zone，包括钠长石岩（albitite）与绿泥岩（chloritite））中的锆石，主体为奥陶纪（Ordovician，约478 Ma），且不含泥盆纪锆石，其碎屑锆石特征与西部岩系其他区域的典型模式相似。拉卡巴尼亚最东侧的超镁铁质岩体——拉瓦德罗斯蛇纹岩（Lavanderos serpentinite）的绿泥岩反应带中的锆石，与变质钛铁矿（ilmenite）达到结构平衡（textural equilibrium）。其中部分锆石的平均年龄为283.4±7 Ma（n=6），在误差范围内与此前报道的该反应带中铬云母（fuchsite）K-Ar冷却年龄（282±6 Ma）完全一致。从铬铁矿岩（chromitite）砾石中提取的绝大多数锆石均具有自形振荡环带生长结构，年龄范围与西部岩系已报道数据一致（324~1090 Ma；n=12），仅两颗外观浑浊、U/Th比值较高的锆石例外，其平均年龄为285.5±7 Ma。所有研究岩石中均存在早二叠世（Early Permian）锆石（约280~290 Ma），这表明锆石发生了Zr的重新活化（remobilization），其驱动因素可能是超镁铁质岩体侵入副变质岩系时的不平衡交代反应所释放的交代流体（metasomatic fluid）。与交代锆石形成的同时，副变质岩系中的古生代（Palaeozoic）及中元古代（Mesoproterozoic）锆石被机械包裹进入超镁铁质岩体中，由此记录了超镁铁质岩体侵入海岸增生杂岩的地壳就位时间。