Development of a synorogenic composite sill at deep structural levels of a continental arc (Odenwald, Germany). Part 1: Sederholm-type emplacement portrayed by contact melt in shrinkage cracks

Abstract

We present geochronological, thermobarometric and computertomographic data from a composite sill exposed in the Weschnitz pluton of the Variscan Odenwald. The wall rock consists of quartzmonzodiorite, which intruded at deep structural levels (P = ca. 0.50 GPa, ca. 18 km depth) in a continental arc setting. Zircons and titanites of the quartzmonzodiorite yielded similar UPb ages at 344.3 ± 0.6 and 343.2 ± 2.1 Ma, respectively, reflecting fast initial cooling (≥76 °C/m.y.) until the solidus was attained at ca. 680 °C. Under these conditions, the quartzmonzodiorite was cut by a spessartite sill, which yielded a UPb titanite age at 342.0 ± 1.0 Ma. The short period between wall rock and sill emplacement (≤3.9 m.y.) suggests that both are genetically related.The quartzmonzodiorite situated adjacent to the spessartite sill is enriched in quartz and plagioclase and depleted in hornblende. As geothermobarometric data of these felsic parts yielded T = ca. 710 °C and P = 0.42 GPa (ca. 15 km depth), they are interpreted as contact melt, which invaded into shrinkage cracks of the cooling sill resulting in thin felsic veins. The migration of contact melt from the wall rock into the shrinkage cracks was suction-induced because of volume gain by contact melting in the wall rock and volume loss by cooling of the sill.The formation of such Sederholm-type composite veins seems to be very fast. Titanites of the felsic veins yielded a UPb age at 341.8 ± 1.5 Ma, which is the same like that of the sill. Moreover, results of thermal modelling indicate a maximum period of 4 months between sill emplacement and the time when both the quartzmonzodiorite and the sill had attained the ambient temperature. The modelling further explains, why Sederholm-type veins, described from several places around the world, are restricted to deep crustal levels.