Yang Qi , Shujuan Jiao , Lin Chen , Jiahui Liu , Yu Liu , Jinghui Guo
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引用次数: 0
摘要
揭示超高温变质作用(UHT)的热演化历史有助于揭示造山地壳的成因和演化。然而,由于可确定日期的附属矿物(如锆石和独居石)的复杂行为,一般很难确定变质作用的持续时间,尤其是加热阶段。华北克拉通孔雀石带记录了古近纪超高温变质事件,根据加权平均年龄推算,该变质事件的时间约为1920Ma。然而,锆石在顺行期和逆行期都会生长。因此,约 1920 Ma 的年龄可能是一个过于简单的解释,可能存在复杂的热演化过程。在这项研究中,结合锆石U-Pb年代测定法和锆英钛测温法,东部箜达岩带超高温变质作用的持续时间被推定为60-70 Myr,其中有两个短的减压-加热期(均持续了20 Myr),中间有一个冷却期(持续了30 Myr)。这一发现进一步扩展了我们的知识,即在长寿命的超高温变质作用中,存在一个加热-冷却-加热的循环过程,而不是一个持续延长的冷却过程。我们的研究结果表明,东部箜达岩带的超高温变质作用需要两个阶段的岩石圈延伸,这两个阶段可能分别与浅板块断裂和碰撞后岩石圈脱层有关。这进一步表明,古近纪造山作用虽然以流变性弱的板块俯冲为主,但与现代喜马拉雅造山作用的形成相当。
Heating–cooling–heating cycles within ca. 70 Myr recorded in UHT granulites in the Khondalite Belt, North China Craton
Revealing the thermal evolution history of ultrahigh-temperature metamorphism (UHT) could help shed light on the genesis and evolution of the orogenic crust. However, it is generally difficult to constrain the duration of metamorphism, especially the heating stage due to the complex behavior of the datable accessory minerals (e.g., zircon and monazite). The Khondalite Belt of the North China Craton records Paleoproterozoic UHT metamorphic event which was previously constrained to be ca. 1920 Ma by using the weighted mean age of zircon U–Pb dating results, however, zircon could grow during both prograde and retrograde periods. Thus, the age of ca. 1920 Ma may be an oversimplified explanation and there could be a complex thermal evolution. In this study, combined with zircon U–Pb dating and Ti-in-zircon thermometry, the duration of the UHT metamorphism in the eastern Khondalite Belt was constrained to be 60–70 Myr with two short periods of decompression-heating (both lasting for ∼20 Myr) intervened by a period of cooling process (lasting for ∼30 Myr). This finding further expands our knowledge that there was a heating–cooling–heating cycle rather than a continuously prolonged cooling process in a long-lived UHT metamorphism. Our results show that the UHT metamorphism in the eastern Khondalite Belt requires two stages of lithosphere extension, which were possibly related to shallow slab breakoff and post-collisional lithospheric delamination, respectively. It further indicates that Paleoproterozoic orogenesis, although dominated by subduction of rheologically weak slab, is comparable to the formation of modern Himalaya orogens.
期刊介绍:
Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as:
(1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology;
(2) Geochronology and isotope and elemental geochemistry;
(3) Precambrian mineral deposits;
(4) Geophysical aspects of the early Earth and Precambrian terrains;
(5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes.
In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes.
Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.