Late Proterozoic Rodinia breakup in Mexico - A new approach using SIMS dating of zircon coronas and baddeleyite

作为地球深部全球地球动力学过程的结果，地球历史一直被超大陆组合和分裂的循环所主导。人们对更古老的前寒武纪超级大陆知之甚少，因为相关的地质单元经常被较年轻的岩石覆盖，或者被后来的构造热事件覆盖。墨西哥位于古代大陆（如劳伦西亚和冈瓦纳）之间的缝合线上，这些大陆反复碰撞形成超级大陆，首先是罗迪尼亚（约1000 Ma），然后是盘古大陆（约300 Ma）。在墨西哥，新元古代的罗迪尼亚分裂和Iapetus洋的开放至今没有受到限制。然而，最近的研究表明，新元古代玄武岩岩浆（现在的角闪岩）可能侵入了墨西哥南部的前寒武纪基底单元，这是Iapetus洋打开期间地壳变薄的结果。到目前为止，显生宙的多次变质叠印和玄武岩的化学成分都排除了对这一火成岩事件的直接测年。以前的间接证据来自玄武岩岩浆侵入的变质沉积岩，其沉积年龄是根据600-580 Ma的化学地层数据和从斜长岩和伴生钛铁矿中获得的次生锆石的U-Pb年龄估算的，这些锆石的年龄产生于新元古代。本文的主要目的是通过高空间分辨率SIMS定年来确定这些新元古代玄武质岩浆的确切年龄和起源：(1)在由锆逸出形成的金红石和钛铁矿周围的接触变质日冕中的变质锆石；目标样品来自与玄武岩角闪岩岩脉接触的斜长岩和钛铁矿，热套印释放出Zr，导致新的锆石生长。(2)角闪岩中的原生坏长岩。U-Pb地质年代学将通过测定锆石和坏辉岩的微量元素和稳定同位素（氧）组成以及整个岩石样品中的放射性同位素（Hf, Nd）来补充，以阐明晶体及其寄主岩石的起源。该提案旨在启动海德堡大学与拟议的墨卡托研究员Bodo Weber博士（CICESE，墨西哥）之间的新合作，Bodo Weber博士是墨西哥构造演化方面的知名专家。预期结果对于重新定义新元古代的复杂构造对比以及开发复杂多变质岩的新时间化学方法具有重要意义。

Earth's history has been dominated by cycles of supercontinent assemblage and breakup as a consequence of global geodynamic processes in the deep earth. Little is known about older, Precambrian supercontinents, since the relevant geologic units often are covered by younger rocks, or overprinted by later tectonothermal events. Mexico is situated at the suture between ancient continents (like Laurentia and Gondwana) that collided repeatedly to form supercontinents, first Rodinia (ca. 1000 Ma), and then Pangea (ca. 300 Ma). In Mexico, Rodinia breakup and opening of the Iapetus Ocean during the Neoproterozoic has not been constrained so far. Recent research, however, revealed that Neoproterozoic basaltic magmas (now amphibolite) probably intruded Precambrian basement units in southern Mexico as a consequence of crustal thinning during the opening of the Iapetus Ocean. Repeated metamorphic overprints during the Phanerozoic and the chemical composition of basalts thus far have precluded direct dating of this igneous event. Previous indirect evidence is derived from metasedimentary rocks that are intruded by basaltic magmas whose sedimentary age has been estimated on the basis of chemostratigraphic data at 600-580 Ma and from U-Pb ages of secondary zircon from anorthosite and associated ilmenite ores that yielded Neoproterozoic dates. The main goal of this proposal is to determine the exact age and origins of these Neoproterozoic basaltic magmas through high-spatial resolution SIMS dating of: (1) metamorphic zircon in contact-metamorphic coronas around rutile and ilmenite formed by Zr exsolution; targeted samples are from anorthosites and ilmenitites in contact with basaltic amphibolite dikes where thermal overprinting liberated Zr leading to new zircon growth. (2) Primary baddeleyite in amphibolite. U-Pb geochronology will be complemented by determining trace element and stable isotope (oxygen) compositions of zircon and baddeleyite, as well as radiogenic isotopes (Hf, Nd) in whole rock samples to elucidate the origin of crystals and their host rocks. This proposal aims at initiating a new collaboration between Heidelberg University and the proposed Mercator Fellow Prof. Dr. Bodo Weber (CICESE, Mexico) who is a renowned expert on the tectonic evolution of Mexico. The expected results are essential to redefine the complex tectonic correlations during the Neoproterozoic as well as to develop novel chronochemistry approaches for complex polymetamorphic rocks.