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Journal of Petrology | Volume 43 | Number 8 | Pages 1529-1549 | 2002
© Oxford University Press 2002
Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany
1INSTITUT FÜR GEOWISSENSCHAFTEN, UNIVERSITÄT POTSDAM, PF 601553, D-14415 POTSDAM, GERMANY
2EARTH SCIENCE LABORATORY, FACULTY OF EDUCATION, EHIME UNIVERSITY, 3 BUNKYO-CHO, 790-8577 MATSUYAMA, JAPAN
3GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, D-14473 POTSDAM, GERMANY
4MINERALOGISCH–PETROGRAPHISCHES INSTITUT, UNIVERSITÄT HAMBURG, GRINDELALLEE 48, D-2000 HAMBURG, GERMANY
We present mineralogical, petrological and geochemical data to constrain the origin of the Harzburg mafic–ultramafic intrusion. The intrusion is composed mainly of mafic rocks ranging from gabbronorite to quartz diorite. Ultramafic rocks are very rare in surface outcrops. Dunite is observed only in deeper sections of the Flora I drill core. Microgranitic (fine-grained quartz-feldspathic) veins found in the mafic and ultramafic rocks result from contamination of the ultramafic magmas by crustal melts. In ultramafic and mafic compositions cumulate textures are widespread and filter pressing phenomena are obvious. The order of crystallization is olivine 
pargasite, phlogopite, spinel plagioclase, orthopyroxene plagioclase, clinopyroxene. Hydrous minerals such as phlogopite and pargasite are essential constituents of the ultramafic cumulates. The most primitive olivine composition is Fo89·5 with 
0·4 wt % NiO, which indicates that the olivine may have been in equilibrium with primitive mantle melts. Coexisting melt compositions estimated from this olivine have mg-number = 71. The chemical variety of the rocks constituting the intrusion and the mg-number of the most primitive melt allow an estimation of the approximate composition of the mantle-derived primary magma. The geochemical characteristics of the estimated magma are similar to those of an island-arc tholeiite, characterized by low TiO2 and alkalis and high Al2O3. Geochemical and Pb, Sr and Nd isotope data demonstrate that even the most primitive rocks have assimilated crustal material. The decoupling of Sr from Nd in some samples demonstrates the influence of a fluid that transported radiogenic Sr. Lead of crustal origin from two isotopically distinct reservoirs dominates the Pb of all samples. The ultramafic rocks and the cumulates best reflect the initial isotopic and geochemical signature of the parent magma. Magma that crystallized in the upper part of the chamber was more strongly affected by assimilated material. Petrographic, geochemical and isotope evidence demonstrates that during a late stage of crystallization, hybrid rocks formed through the mechanical mixing of early cumulates and melts with strong crustal contamination from the upper levels of the magma chamber.
KEY WORDS: Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination