Ultrahigh-temperature granites and a curious thermal eye in the post-collisional South Bohemian batholith of the Variscan orogenic belt (Europe)
Abstract
Comprehensive zircon thermometry that takes into account zircon saturation tempera-tures, Ti-in-zircon measurements, and zircon morphologies and microstructures can provide key information on the thermal evolution of a granite batholith. The Variscan South Bohe-mian batholith (Germany, Austria, and Czech Republic) comprises a series of granitoid units that intruded between ca. 330 and ca. 300 Ma.
We categorize the granitic rocks according to their emplacement temperature into very low temperature (T) (VLT; 750 °C), low T (LT; 750-800 °C), medium T (MT; 800-850 °C), high T (HT; 850-900 °C), and ultrahigh T (UHT; 900 °C). The first stage of batholith formation (ca. 330-325 Ma) is characterized by LT to MT melting of mainly metasedimentary sources driven by their isothermal exhuma-tion.
In turn, ca. 322 Ma HT and UHT granites in the southern half of the batholith reveal an ephemeral thermal anomaly in the subbatholithic crust, which is presumably linked to a hidden mafic intrusion. The HT and UHT granites are weakly peraluminous, high-K, I-type rocks.
Although sharing some features with A-type granites such as high Zr and rare earth element contents, they differ from classical A-type granites in being magnesian, not enriched in Ga over Al, and having high Ba and Sr contents. A ring structure of ca. 317 Ma MT and/ or LT plutons is observed around the HT and/or UHT granite complex and interpreted as an aftermath of the hotspot event.
This study is an example of how deep-crustal hotspots, presumably caused by mantle magmatism, can significantly enhance the effects of decom-pressional crustal melting in a post-collisional setting.