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首页> 外文期刊>Contributions to Mineralogy and Petrology >Constraints on the thermal evolution of continental lithosphere from U-Pb accessory mineral thermochronometry of lower crustal xenoliths, southern Africa
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Constraints on the thermal evolution of continental lithosphere from U-Pb accessory mineral thermochronometry of lower crustal xenoliths, southern Africa

机译：非洲南部下地壳异岩的U-Pb辅助矿物热年代计对大陆岩石圈热演化的限制

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摘要

U-Pb isotopic thermochronometry of rutile, apatite and titanite from kimberlite-borne lower crustal granulite xenoliths has been used to constrain the thermal evolution of Archean cratonic and Proterozoic off-craton continental lithosphere beneath southern Africa. The relatively low closure temperature of the U-Pb rutile thermochronometer (approx 400-450 deg C) allows its use as a particularly sensitive recorder of the establishment of "cratonic" lithospheric geotherms, as well as subsequent thermal perturbations to the lithosphere. Contrasting lower crustal thermal histories are revealed between intracratonic and craton margin regions. Discordant Proterozoic (1.8 to 1.0 Ga) rutile ages in Archean (2.9 to 2.7 Ga) granulites from within the craton are indicative of isotopic resetting by marginal orogenic thermal perturbations influencing the deep crust of the cratonic nucleus. In Proterozoic (1.1 to 1.0 Ga) granulite xenoliths from the craton-bounding orogenic belts, rutiles define discordia arrays with Neoproterozoic (0.8 to 0.6 Ga) upper intercepts and lower intercepts equivalent to Mesozoic exhumation upon kimberlite entrainment. In combination with coexisting titanite and apatite dates, these results are interpreted as a record of post-orogenic cooling at an integrated rate of approximately 1 deg C/Ma, and subsequent variable Pb loss in the apatite and rutile systems during a Mesozoic thermal perturbation to the deep lithosphere. Closure of the rutile thermochronometer signals temperatures of <= 450 deg C in the lower crust during attainment of cratonic lithospheric conductive geotherms, and such closure in the examined portions of the "off-craton" Proterozoic domains of southern Africa indicates that their lithospheric thermal profiles were essentially cratonic from the Neoproterozoic through to the Late Jurassic. These results suggest similar lithospheric thickness and potential for diamond stability beneath both Proterozoic and Archean domains of southern Africa. Subsequent partial resetting of U-Pb rutile and apatite systematics in the cratonic margin lower crust records a transient Mesozoic thermal modification of the lithosphere, and modeling of the diffusive Pb loss from lower crustal rutile constrains the temperature and duration of Mesozoic heating to <= 550 deg C for >= 50 ka. This result indicates that the thermal perturbation is not simply a kimberlite-related magmatic phenomenon, but is rather a more protracted manifestation of lithospheric heating, likely related to mantle upwelling and rifting of Gondwana during the Late Jurassic to Cretaceous. The manifestation of this thermal pulse in the lower crust is spatially and temporally correlated with anomalously elevated and/or kinked Cretaceous mantle paleogeotherms, and evidence for metasomatic modification in cratonic mantle peridotite suites. It is argued that most of the geographic differences in lithospheric thermal structure inferred from mantle xenolith thermobarometry are likewise due to the heterogeneous propagation of this broad upper mantle thermal anomaly. The differential manifestation of heating between cratonic margin and cratonic interior indicates the importance of advective heat transport along pre-existing lithosphere-scale discontinuities. Within this model, kimberlite magmatism was a similarly complex, space- and time-dependent response to Late Mesozoic lithospheric thermal perturbation.
机译：来自金伯利岩传播的下地壳麻粒岩异质岩的金红石，磷灰石和钛铁矿的U-Pb同位素测年法已被用来限制南部非洲下古宙克拉通和元古代离克拉通大陆岩石圈的热演化。 U-Pb金红石型温度计时器的相对较低的闭合温度（大约400-450摄氏度）使其可用作建立“克拉通”岩石圈地热以及随后对岩石圈的热扰动特别敏感的记录器。在克拉通边缘和克拉通边缘区域之间发现了相反的下地壳热历史。克拉通内部太古代（2.9至2.7 Ga）颗粒中元古代（1.8至1.0 Ga）不协调的金红石年龄表明，通过边缘造山热扰动影响了克拉通核的深壳，同位素复位。在克拉通边界造山带的元古代（1.1至1.0 Ga）粒状异岩中，金红石定义了带有新元古代（0.8至0.6 Ga）上截距和下截距的discordia阵列，相当于金伯利岩夹带时的中生代碎屑。与钛铁矿和磷灰石共存相结合，这些结果被解释为以约1℃/ Ma的综合速率进行造山后冷却的记录，以及随后的中生代热扰动下磷灰石和金红石系统中可变的Pb损失。深层岩石圈。金红石型温度计时器的关闭表明在达到克拉通岩性圈层地热传导过程中下地壳中的温度<= 450摄氏度，而在非洲南部“克拉通”元古代的被测区中，这种封闭表明其岩石圈的热剖面从新元古代到侏罗纪晚期基本上都是克拉通的。这些结果表明，在南部非洲的元古代和太古代地区下，岩石圈厚度相似，金刚石具有稳定的潜力。随后在克拉通边缘下地壳中将U-Pb金红石和磷灰石系统进行了部分复位，记录了岩石圈的瞬时中生代热变质，而对下地壳金红石的扩散性Pb损失的建模将中生代加热的温度和持续时间限制为<= 550 > = 50 ka这一结果表明，热扰动不仅是与金伯利岩有关的岩浆现象，而且是岩石圈加热的更持久的表现，可能与侏罗纪至白垩纪晚期冈瓦纳地幔上涌和裂谷有关。在下地壳中这种热脉冲的表现在空间和时间上与异常升高和/或扭结的白垩纪地幔古地热有关，并为克拉通地幔橄榄岩组中的交代修饰提供了证据。有人认为，由地幔异种岩热气压计推断出的岩石圈热结构的大多数地理差异，同样是由于该上地幔热异常的不均匀传播所致。在克拉通边缘与克拉通内部之间的加热差异表明，对流传热沿已存在的岩石圈尺度不连续性的重要性。在该模型中，金伯利岩岩浆作用是对晚期中生代岩石圈热扰动的类似复杂，时空依赖的反应。

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