过碱性岩
岩石成因
地球化学
地质学
分步结晶(地质学)
玄武岩
等时线
部分熔融
放射性核素
深熔
岩石学
地幔(地质学)
火山
作者
Fang Shao,Yaoling Niu,Marcel Regelous,Di‐Cheng Zhu
出处
期刊:Lithos
[Elsevier]
日期:2015-02-01
卷期号:216-217: 196-210
被引量:36
标识
DOI:10.1016/j.lithos.2014.12.015
摘要
We report petrological and geochemical data on coeval trachybasalts, syenites with enclaves, trachytes, peralkaline rhyolites and peraluminous rhyolites from the Glass House Mountains–Maleny–Noosa area, southeast Queensland, Australia. This rock association and the unique characteristics of the peralkaline rhyolites offer convincing lines of evidence that the petrogenesis of the peralkaline rhyolites is a straightforward consequence of protracted fractional crystallization from basaltic melts of alkali-rich composition. Compared to the common peraluminous rhyolites elsewhere, the peralkaline rhyolites here are characterized by elevated abundances of most incompatible elements, especially the very high Nb (vs. Th) and Ta (vs. U), the very low Ba, Sr and Eu and the extremely high 87Sr/86Sr ratio. The high Nb and Ta are inherited from the parental alkali basaltic melts. The low Ba, Sr and Eu result from removal of plagioclase during the protracted fractional crystallization. These rocks altogether define a Rb–Sr isochron of ~ 28 Ma, which is similar to Ar–Ar age data on these rocks in the literature. The extremely high 87Sr/86Sr ratio of the peralkaline rhyolites (up to 1.88) is actually characteristic of peralkaline rhyolites because of extreme Sr (also Eu and Ba) depletion and thus the very high Rb/Sr ratio. That is, the Sr in these rocks is essentially radiogenic 87Sr accumulated from the 87Rb decay since the volcanism. We suggest that the petrogenesis of the peralkaline rhyolites from the Glass House Mountain area may be of general significance globally. The coeval peraluminous rhyolites apparently result from crustal anatexis in response to the basaltic magma underplating. The small “Daly Gap” exhibited in this rock association is anticipated during the protracted fractional crystallization from basaltic parent to the more evolved felsic varieties.
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