K-rich plutonic rocks and lamprophyres from the Meissen Massif (northern Bohemian Massif): Geochemical evidence for variably enriched lithospheric mantle sources

摘要

K-rich plutonic rocks and lamprophyres from the Meissen Massif (northern Bohemian Massif) intruded at about 330 Ma in the Elbe Zone. The plutonic complex consists of about 50 percent gabbroic to mainly monzonitic rocks and of about' '50 percent granites. The gabbroic to monzonitic rocks are characteristic of the shoshonitic suite in their strong enrichment in LWE and LREE and a relative depletion of HFSE. The slightly silica-undersaturated gabbroic rocks (SiO_2 <50 wt. percent) with ~(87)Sr/~(86) Sr_((330 Ma)) ratios of 0.70479 to 0.70509 and #epsilon# Nd_((330 Ma)) of -0.9 to -1.7 are interpreted to be mainly composed of melt components from subduction-modified parts of the continental lithospheric mantle (CLM). Mixing and AFC modeling show that contamination of astheno-spheric basalt neither by upper crustal gneiss nor by hypothetical Sr-Ba-rich mafic lower crust can reproduce the isotope and trace element patterns of the gabbroic rocks. The monzonites (SiO_2 >53 wt. percent) crystallized mainly from a more differentiated, more enriched magma pulse. Additionally, the monzonites are affected by crustal contamination including mechanical entrainment of zircon and apatite. Similar #epsilon# Nd_((330 Ma))-values of the majority of the gabbroic to monzonitic rocks and a kersantite demonstrate that these rock types can be derived from similar sources probably enriched by subduction of oceanic crust. In contrast, associated minette dikes originated from CLM sources with evolved ~(87)Sr/~(86)Sr_((330 Ma)) ratios of 0.70686 to 0.70738 and #epsilon# Nd_((330 Ma)) of -4.1 to-4.3. The most primitive minette sample with the lowest Nb content is most enriched in Th (91 ppm) and U (23 ppm) which is typical for K-rich rocks from CLM modified by subduction of upper continental crust. Upper Devonian/Lower Carboniferous subduction and collision processes are thought to be responsible for the formation of variably enriched CLM domains from which the several K-rich Meissen magmatic rocks were extracted about 20 Ma later. Melt formation and intrusion are directly related to strike-slip along the Elbe Zone.