Two types of ultrapotassic plutonic rocks in the Bohemian Massif - Coeval intrusions at different crustal levels

摘要

We present U-Pb zircon age determinations of two Variscan ultrapotassic plutonic rocks from the Moldanubian Zone (Bohemian Massif). Equant, multifaceted zircons without inherited cores from a two-pyroxene-biotite quartz monzonite of the Jihlava Pluton yielded a precise age of 335.12 ±0.57 Ma, interpreted as dating magma crystallization. The majority of both tabular and prismatic grains from the amphibole-biotite melagranite ("durbachite") from the Treble Pluton plot along a discordia intersecting the concordia at 334.8 ± 3.2 Ma; prismatic zircon grains commonly contain inherited cores and yield an upper intercept age of 2.2 Ga, indicating early Proterozoic inheritance. We therefore suggest that both types of the ultrapotassic plutonic rocks from the Bohemian Massif crystallized at ca 335 Ma, and the previously published ages higher than ca 340 Ma for "durbachites" were biased by a small amount of unresolved inheritance. The ultrapotassic magma emplacement in the middle crust was related to rapid exhumation of a deep crustal segment, considered as isothermal decompression between high-pressure (～340 Ma) and medium-pressure (～333 Ma) stages recorded in granulites. Mineral assemblages as well as external and internal zircon morphology suggest that the Jihlava intrusion was deep and dry, whereas the Treble intrusion was shallow and wet. Low ε_(Hf) values of zircons (- 4.4 to - 7.5) in both rock types suggest a similar source with a predominant crustal component. However, inherited grains in the Treble melagranite indicate its contamination with crustal material during emplacement, and thus possibly a slower rate of exhumation and/or of magma ascent through the crust