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首页> 外文期刊>International Geology Review >Deciphering the geochronology of a large granitoid pluton (Karkonosze Granite, SW Poland): an assessment of U-Pb zircon SIMS and Rb-Sr whole-rock dates relative to U-Pb zircon CA-ID-TIMS
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Deciphering the geochronology of a large granitoid pluton (Karkonosze Granite, SW Poland): an assessment of U-Pb zircon SIMS and Rb-Sr whole-rock dates relative to U-Pb zircon CA-ID-TIMS

机译：解读大型花岗岩类岩体的地质年代学（波兰西南部Karkonosze花岗岩）：相对于U-Pb锆石CA-ID-TIMS的U-Pb锆石SIMS和Rb-Sr全岩石日期的评估

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摘要

Granitoid plutons are often difficult to radiometrically date precisely due to the possible effects of protracted and complex magmatic evolution, crustal inheritance, and/or partial re-setting of radiogenic clocks. However, apart from natural/geological issues, methodological and analytical problems may also contribute to blurring geochronological data. This may be exemplified by the Variscan Karkonosze Pluton (SW Poland). High-precision chemical abrasion (CA) ID-TIMS zircon data indicate that the two main rock types, porphyritic and equigranular, of this igneous body were both emplaced at ca. 312 Ma, while field evidence points to a younger age for the latter. This is in contrast to the earlier reported SIMS (SHRIMP) zircon dates that scattered mainly between ca. 322 and 302 Ma. In an attempt to overcome this dispersion, at least in part caused by radiogenic lead loss, the CA technique was used before SHRIMP analysis. The ~(206)Pb/~(238)U age obtained in this way from a sample of porphyritic granite is 322 ± 3 Ma, ～16 Ma older than the untreated zircons; another porphyritic sample yielded a mean age of 319 ± 3 Ma, and the mean age was 318 ± 4 Ma for an equigranular granite sample - all three somewhat older than the age obtained by ID-TIMS. Older SIMS dates of ca. 318-322 Ma might indicate either faint inheritance or that zircon domains crystallized during earlier stages of Karkonosze igneous evolution. The ID-TIMS results have been used to re-assess the whole-rock Rb-Sr data. Excluding a porphyritic granite with excess radiogenic ~(87)Sr, it appears that isotopic homogeneity was achieved for most samples during the 312 Ma event, as shown by a pooled 21 -point isochron with an age of 311 ±3 Ma and an initial ~(86)Sr/~(86)Sr of 0.7067 ± 4. Local crustal contamination by stopping of metapelitic material might account for the more radiogenic Sr isotope signature observed in biotite-rich schlieren. A critical re-evaluation of all available SHRIMP data using the ID-TIMS age of 312 Ma as a benchmark suggests that the observed scatter may be partly attributed to analytical and methodological problems, in particular failing to distinguish subtly discordant spots from truly concordant ones, which is a serious limitation of the microbeam analytical approach. Other likely pitfalls contributing to geochronological scatter are identified in the published Re-Os ages on molybdenite and the ~(40)Ar/ Ar data on micas. A scenario postulating a 15-20 milliion year evolution of the Karkonosze Pluton cannot be established on the basis of available geochronological data, which rather supports a brief igneous event, although a more protracted pre-emplacement evolution is possible. A short timescale for crystallization of large igneous bodies, as suggested by the ID-TIMS data from the Karkonosze Granite, is in line with models of transport of granitic magmas through dikes to form large plutons.
机译：由于长期和复杂的岩浆演化，地壳遗传和/或放射源钟的部分重设可能产生的影响，通常很难精确地对花岗岩类小行星进行放射线测年。但是，除了自然/地质问题外，方法和分析问题也可能导致地球年代数据模糊。 Variscan Karkonosze Pluton（波兰SW）可以举例说明。高精度化学磨蚀（CA）ID-TIMS锆石数据表明，该火成岩体的两种主要岩石类型（斑状和等粒）都位于约。 312 Ma，而现场证据表明后者年龄较小。这与早期报道的SIMS（SHRIMP）锆石日期主要散布在大约两个月之间相反。 322和302 Ma。为了克服这种分散，至少部分是由放射源性铅损失引起的，在SHRIMP分析之前使用了CA技术。用这种方法从斑岩花岗岩样品中获得的〜（206）Pb /〜（238）U年龄为322±3 Ma，比未处理的锆石大了约16 Ma。另一个斑状样品的平均年龄为319±3 Ma，等粒花岗岩样品的平均年龄为318±4 Ma，这三个年龄均比ID-TIMS获得的年龄稍大。 SIMS的较早日期。 318-322 Ma可能表明微弱的遗传或在Karkonosze火成岩演化的早期阶段结晶了锆石域。 ID-TIMS结果已用于重新评估整个岩石的Rb-Sr数据。不包括具有过量放射源〜（87）Sr的斑岩质花岗岩，看来在312 Ma事件期间大多数样品都实现了同位素同质性，如年龄为311±3 Ma的集合21点等时线和初始〜 （86）Sr /〜（86）Sr为0.7067±4。通过变质材料停止而造成的局部地壳污染可能解释了在富含黑云母的schlieren中观察到的更具放射性的Sr同位素特征。对所有可用的SHRIMP数据进行严格的重新评估，使用312 Ma的ID-TIMS年龄作为基准，这表明观察到的散布可能部分归因于分析和方法问题，特别是未能区分出明显不一致的斑点和真正一致的斑点，这是微束分析方法的严重局限。在已发表的辉钼矿的Re-Os年龄和云母的〜（40）Ar / Ar数据中，还发现了其他可能导致年代学分散的陷阱。无法根据现有的年代学数据建立假设Karkonosze Pluton演化为15-20百万年的情景，这虽然支持更短暂的火成岩演化，但它支持短暂的火成事件。 Karkonosze花岗岩的ID-TIMS数据表明，短时间的大火成岩体结晶时间与花岗岩岩浆通过堤坝形成大Plut的运输模型相吻合。

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