| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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
Received December 8, 2000; Revised typescript accepted February 11, 2002
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
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
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The Harz Mountains of central Germany represent a portion of uplifted, mainly Palaeozoic, crust of the Rhenohercynian belt of the European Variscides (Fig. 1). The Ordovician to Permian rock sequence is surrounded by Permian to Mesozoic sediments and was uplifted during the Cretaceous. The Harz Mountains contain a Proterozoic polymetamorphic terrane (Ecker gneiss) and several post-orogenic granite intrusions (Ocker, Brocken and Ramberg granites) as well as a gabbronorite complex (Harzburg gabbro; Lossen, 1889; Erdmannsdörfer & Schröder, 1927). The emplacement of the Harzburg gabbronorite and the Brocken and Ocker granites occurred contemporaneously at 293–297 Ma (Baumann et al., 1991). The crustal evolution and geodynamic setting of this Variscan basement has been the focus of numerous studies (Sohn, 1956; Anderson, 1975; Vinx, 1982; Hentschke, 1985; Wachendorf et al., 1995; Ganssloser et al., 1996; Franz et al., 1997; Gabriel et al., 1997). The Harzburg gabbronorite complex provides one of the clues to the evolution and origin of the late Variscan magmas of the Harz Mountains. So far, geochemical investigations (Vinx, 1982; Hentschke, 1985) have mainly concentrated on the mafic to intermediate portions of the intrusion, as fresh ultramafic rocks are not available at the surface. Vinx (1982) also studied the ultramafic rocks; however, these rocks were strongly serpentinized. A limited number of new samples from commercial exploration drilling (Flora I, II, III) have been made available to us. The cores from the drill holes revealed unweathered mafic and ultramafic material with a relatively low degree of serpentinization.
|
, occurrences of high-pressure/low-temperature metamorphic rocks (In this paper, we reconstruct the possible primary magma composition of the Harzburg mafic–ultramafic intrusion and discuss the processes that led to the evolution of the magma and its contamination. The study focuses on the petrology and isotope geochemistry of the drill core samples. The Harzburg intrusion includes the type locality of harzburgite. It should be noted that harzburgites from the type locality exhibit a poikilitic texture, which is composed of euhedral olivine crystals with surrounding large oikocrysts of clinopyroxene, plagioclase and/or orthopyroxene, hornblende and phlogopite.
|
REGIONAL GEOLOGY |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The Harzburg intrusion (Fig. 2) consists of two oval bodies defining a SSW–NNE trend at the surface level (Sohn, 1956). The two separate bodies are detached outcrops of the same intrusion with a thin bridge of hornfels roof rocks within a graben structure. The intrusion is composed of gabbronorite with locally occurring dunite, harzburgite, norite, diorite and quartz diorite. The body has been previously considered to represent a layered intrusion (Vinx, 1982). Along the eastern side, the gabbronorite massif is in contact with the Ecker gneiss. Its western part intruded Upper Devonian to Upper Carboniferous pelites and siliceous slates and produced hornfelses. The internal distribution of ultramafic and mafic rocks reveals gross concentric structures. These show advanced fractionation in the centre of the northern body (Fig. 2) and more ‘primitive’ cumulates at the southern rim of the northern body. The most evolved rock types are quartz diorites and they occur in the NW and exhibit signs of contamination, e.g. xenoliths of sedimentary rocks. The close relations to the contemporaneous granitic intrusions surrounding the gabbronorite massif were pointed out earlier (Müller, 1978; Stütze, 1980). The Harz Mountains host several mafic complexes (Fig. 1) including pillow lavas and tuffs. These rocks, however, are of Devonian age and not related to the Carboniferous Harzburg intrusion.
|
|
PETROGRAPHY OF THE MAFIC AND ULTRAMAFIC MEMBERS |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The Harzburg mafic–ultramafic intrusion is composed of dunite, harzburgite, norite, gabbronorite, diorite and quartz diorite. Gabbronorite is the most common rock type in the body. Dunite is not found in surface outcrops, but observed in the core from the Flora I drill site (Fig. 2). Ultramafic rocks such as dunite and harzburgite show cumulate textures (Fig. 3). They never show porphyroclastic textures, which are characteristic mantle tectonite features. Dunite displays adcumulate textures with minor intercumulus pargasitic hornblende, phlogopite, plagioclase or clinopyroxene (Fig. 3a). Chromian spinel occurs only in the intercumulus phases as small, rectangular crystals. Harzburgites show typical orthocumulate textures with cumulus olivine and intercumulus clinopyroxene, hornblende, phlogopite, plagioclase and orthopyroxene (Fig. 3b). Norite also shows cumulate textures (Fig. 3c). In the ultramafic rocks, olivine appears as the first cumulus phase. Spinel occurs as euhedral crystals included in intercumulus minerals, but is never found included in olivine (Fig. 3a). Pargasite and phlogopite occur also as inclusions in spinel (Fig. 3d) underlining their magmatic origin. Although orthopyroxene occurs as an intercumulus phase in ultramafic rocks, it appears as a cumulus phase in norite together with plagioclase. On the basis of microscopic observations, the order of crystallization is olivine
pargasite, phlogopite, spinel orthopyroxene, plagioclase clinopyroxene, plagioclase (Fig. 4). Gabbronorite forms the dominant member of the intrusion. It does not show cumulate textures, but is characterized by holocrystalline textures (Fig. 3e). Sometimes, fine-grained biotite–feldspar rock fragments of meta-sedimentary origin are observed in the gabbronorite (Fig. 3f). Contamination by sedimentary material is confirmed by the occurrence of blue corundum in xenoliths (Harries, 1999). The fragments are usually assimilated within the host gabbronorite. Olivine and spinel are rare, but ilmenite, biotite and quartz are widespread in the gabbronorite. Apatite and zircon occur commonly as accessory phases (Fig. 4). Diorite and quartz diorite are exposed mainly along the northwestern rim of the intrusion (Fig. 2). They show heterogeneous hypidiomorphic granular textures (Fig. 3g). The constituent minerals are relatively large crystals of plagioclase and orthopyroxene, and interstitial smaller hornblende, biotite and quartz. Fine-grained quartz–feldspar rocks are found as small pods or veinlets in ultramafic rocks from the drill core.
|
|
Drill core samples (Flora I and II–III)
The locations of drill holes Flora I and II–III are shown in Fig. 2. The maximum depth sampled is 390 m at site Flora I. Most of the drill core is composed of ultramafic rocks such as dunite and harzburgite (below 100 m), although gabbronorite is widely distributed in the surface outcrops. Although dunite is absent in surface outcrops, it occurs in the lowermost section of the Flora I drill core (below 350 m). Leucocratic rocks such as gabbronorite, diorite and quartz diorite are found only in the first 100 m of the drill hole. At deeper levels, fine-grained quartz–feldspar rocks appear as small pods or veinlets in the ultramafic host rock. Feldspar is strongly saussuritized. Most ultramafic rocks are strongly serpentinized, except at the deepest position. Ultramafic rocks from levels below 300 m have preserved their original mineral assemblages and textures relatively well. Plagioclase, pargasite and phlogopite are heterogeneously distributed as intercumulus phases between cumulus olivine and spinel in the dunite. In the shallower levels, most of the cumulus olivine is altered, although cumulus spinel and intercumulus phlogopite and/or pargasite survived serpentinization.
Mixing phenomena (hybridization)
Along the northwestern margin of the intrusion, a wide variety of features related to hybridization processes are observed. As reported by Vinx (1982), intrusive relationships among mafic to intermediate magmas indicate mixing phenomena. The lithology of the intermediate rocks is heterogeneous at the outcrop scale. Fine-grained biotite-rich patches are often found in the heterogeneous dioritic to quartz dioritic rock. Xenoliths of quartzo-feldspathic gneiss and hornfels are also recognized in the intermediate rocks. Many gabbronorites and diorites show peculiar textures that originate from mixing. Such textures include fine-grained biotite–feldspar–quartz-rich patches (Fig. 3f) with reaction rims of orthopyroxene + ilmenite + quartz. Irregular zoning in plagioclase is often found in gabbronorite (Fig. 3h). The plagioclase zoning shows albite-rich cores with anorthite-rich rims. Furthermore, as described below, two types of clinopyroxenes showing an obvious compositional gap exist in small domains.
|
MINERAL CHEMISTRY |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Mineral compositions (Tables 1 and 2) were determined by JEOL 8600 electron microprobe at Ehime University. Beam conditions for microprobe analyses were 15 keV accelerating voltage and 15 nA beam current. The complete dataset is available for downloading from the Journal of Petrology Web site at http://www.petrology.oupjournals.org.
|
|
Olivine
Olivine (Fig. 5) in dunite and harzburgite has the highest Fo [= 100Mg/(Mg + Fe)] content of 89·5 with 0·3–0·4 wt % NiO, similar to olivine from mantle peridotites (Fig. 6b). The Fo content of olivine in norites is slightly lower than that in dunite and harzburgite. The gabbronorite with a high mg-number (>60) includes olivine that ranges in composition from Fo78·4 to Fo89·2. With decreasing Fo contents, NiO contents also decrease to 0·1 to 0·2 wt %. The gabbronorite with a significantly lower whole-rock mg-number (44) contains fayalite-rich olivine of Fo22 (Fig. 5). Thus a compositional gap between Fo76 and Fo24 is observed in these rocks. The most differentiated gabbronorite with low mg-number (<40) does not contain olivine.
|
|
Spinel
Ferric and ferrous iron contents in spinel were calculated by fitting to stoichiometry. Spinel occurs only as a cumulus phase. As shown in Fig. 6a, the mg-number [= 100Mg/(Mg + Fe2+)] ranges from 0·4 to 0·6 with cr-number [= 100Cr/(Cr + Al)] of 0·4–0·55. These compositional ranges are different from those of abyssal peridotites and normal layered intrusions (Dick & Bullen, 1984). The mg-number of spinel from the Harzburg cumulates is obviously lower than the range from abyssal peridotites. The range of cr-number of spinels from layered intrusions is much higher than that from the Harzburg cumulates. The TiO2 content in Harzburg spinels is up to 2·5 wt %, but usually 1 wt % (Fig. 6a). Small spinel crystals are dark brown, whereas larger ones are light brown. Dark brown spinel with a high TiO2 content is also characterized by high Fe3+ contents, which may reflect more oxidizing conditions than in spinel with low TiO2 content.
Plagioclase
Plagioclase shows wide compositional ranges (Fig. 5). Norite and gabbronorite (mg-number > 60) include the most An [= 100Ca/(K + Na + Ca)] rich plagioclase (An85). Intercumulus plagioclase in dunite and harzburgite is low in An content (An66–83) compared with that in norite and harzburgite (An85). Some of the plagioclase in the dunite shows very low An content (An66–72) for plagioclase in ultramafic rocks. In gabbronorite, the An content shows a bimodal distribution as a function of the whole-rock mg-number. Gabbronorites (40 < mg-number 
60) show two peaks in the An content at 55 and 73%. Furthermore, gabbronorites (mg-number 40) show two distinct compositional ranges of An52–64 and An68–84. In one sample (H-73), the core of a crystal is formed by plagioclase with An49–56 surrounded by plagioclase with An81–84, with an abrupt compositional gap (Fig. 3h).
Orthopyroxene and clinopyroxene
The mg-number of orthopyroxene shows a wide range from 0·90 to 0·32 with a compositional gap between 0·50 and 0·72. The Al2O3 content continuously decreases from 3·5 to 0·5 wt % with decreasing mg-number (Fig. 6c). Orthopyroxene in iron-rich gabbronorite includes small clinopyroxene exsolution lamellae. This is common for orthopyroxenes throughout the Harzburg rock sequence. Clinopyroxene in cumulus rocks is characterized by higher Al2O3, TiO2 and Na2O contents than in gabbronorites, as shown in Fig. 6d. With decreasing mg-number from 0·95 to 0·80, concentrations of these elements decrease drastically. The maximum Al2O3 content in the most Mg-rich clinopyroxene is 5 wt %. The TiO2 and Na2O contents at the highest mg-number are 1·0 and 0·8 wt %, respectively.
Amphibole
For the classification and calculation of site occupation of amphiboles the program PROBE-AMPH (Tindle & Webb, 1994) was used. Amphibole shows wide compositional variations (Fig. 7a). The compositions of intercumulus amphibole in ultramafic rocks and magnesian gabbronorites range from pargasite to pargasitic hornblende. Additionally, actinolites to actinolitic hornblende occur in iron-rich gabbronorites, where they were apparently formed by later processes. As shown in Fig. 7, TiO2, Na2O and K2O contents are high in intercumulus amphiboles in dunite and harzburgite. The dunite and harzburgite contain TiO2-rich amphiboles, titanian-pargasite, whereas magnesian gabbronorite hosts edenitic–pargasitic hornblende. Norite includes magnesio-hornblende. These dark yellowish brown amphiboles occur as interstitial phases among the cumulus crystals. However, amphiboles in more iron-rich gabbronorites are colourless actinolite or actinolitic hornblende, which may have formed during a secondary alteration stage.
|
Biotite–phlogopite
Biotite–phlogopite is a ubiquitous phase throughout the rocks of the Harzburg intrusion. Phlogopite is found as an intercumulus phase among the cumulus crystals in ultramafic rocks. Norite, however, does not usually include phlogopite. With falling mg-number, TiO2 and K2O contents increase, but Na2O and Cr2O3 contents decrease (Fig. 7b). Biotite in gabbronorites is associated with ilmenite.
Compositional heterogeneity of minerals
From the above description of mineral composition and comparison with bulk-rock mg-number, it is evident that the noritic and gabbronoritic rocks from surface outcrops show enormous compositional heterogeneities that are not always reflected by hybrid textures. In such rocks the bulk mg-number and the mineral mg-number strongly correlate. The few pristine samples made available from the drill cores allow us to identify the most primitive rock types and their primary phases. In these rocks, cumulus minerals that crystallized from a primitive melt show only restricted compositional variations (Table 3), demonstrating that at least local equilibria were achieved at an early stage of magma evolution.
|
|
GEOCHEMISTRY |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Analytical procedure
Six mafic and ultramafic cumulates, two fine-grained quartz–feldspar rocks in the ultramafic sequence (felsic pod) from the drill core, and two hybrid rocks from surface outcrops were analysed for major and trace element abundances and Sr, Nd and Pb isotope compositions. Major and trace element data and Sr, Nd and Pb isotope data were analysed at GeoForschungsZentrum (GFZ) in Potsdam. Major and trace elements were analysed by X-ray fluorescence (XRF). Additional concentration data of some trace elements, including rare earth elements (REE), were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) (Table 4). Sr, Nd and Pb isotopic compositions were obtained on a Finnigan MAT262 multicollector mass spectrometer. Sr, Nd and Pb were separated and purified by ion-exchange chromatography using standard procedures. Analytical details are given in the footnote to Table 5.
|
|
Whole-rock major element composition
Results are shown in Table 4. Whole-rock chemical variation against the mg-number is shown in Fig. 8. Most of the data plotted are from R. Vinx (unpublished data, 1988). Many oxides display good correlations with mg-number. Cumulates range from mg-number = 75 to 90. SiO2, Al2O3 and CaO show scattered convex patterns. The variation within the cumulates is probably controlled by the olivine–plagioclase distribution. Data for the Harzburg intrusion mainly follow a tholeiitic fractionation trend, although chemical diversity, probably as a result of melt–wall-rock interaction, is recognized. Here, the term hybrid is used for rocks showing mixing phenomena such as xenoliths in various stages of assimilation, or inverse plagioclase zoning. Compositions of many metasedimentary rocks plot in the same area as the gabbronorites.
|
, metasedimentary rocks (quartz–feldspathic gneiss and/or hornfels); open crosses, leucocratic rocks (quartz–feldspar rock, diorite and quartz diorite); •, all other data from R. Vinx (unpublished data, 1988). Vertical lines in the plots represent estimated mg-number for a primary melt equilibrated with the most Fo-rich olivine (Fo89·5).
Whole-rock trace element composition
Primitive mantle-normalized trace element abundances are illustrated in Fig. 9. The normalization values are from Sun & McDonough (1989). Figure 9a shows trace element patterns for dunite, harzburgite and magnesian gabbro. These rocks have positive Cs and Pb anomalies and weak positive Rb, Th and U, or negative Ba anomalies. The pod-forming quartz–feldspar rocks within ultramafic samples show enriched patterns for highly incompatible elements. These rocks have positive Cs and Pb, and negative Ba, Sr, P, Eu and Ti anomalies (Fig. 9a). The normalized patterns of the hybrid rocks show positive Cs and Pb, and negative Ba, P and Ti anomalies (Fig. 9a). Their compositional variation is intermediate between ultramafic and quartz–feldspar rocks. Chondrite-normalized REE abundance of mafic and ultramafic cumulates, hybrid rocks and felsic pods in ultramafic rock are also shown (Fig. 9b). Normalization values are from Anders & Grevesse (1989). Dunites display the lowest REE abundance (x 1 chondrite at La to Lu) with flat patterns, whereas harzburgites show almost parallel patterns with a much higher REE abundance (x 5 to x 7 chondrite at La) as compared with those of dunites. REE patterns of the felsic pods from the ultramafic sequence are characterized by light REE (LREE) enrichment (x 150 chondrite at La) and a negative Eu anomaly. Hybrid rocks show intermediate patterns between the ultramafic cumulates and the felsic pods in the ultramafic sequence (Fig. 9).
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Estimation of primary melt composition
In general, cumulus minerals can be thought as of having equilibrated with the surrounding melt in a magma system. The most primitive mineral is olivine with Fo89·5 and high NiO contents of
0·4 wt %. (Fig. 6b). Such olivine compositions can be in equilibrium with the mantle or a primitive basaltic melt. On the basis of the primary cumulus olivine compositions, we estimated the coexisting melt compositions. The estimated melt composition has mg-number = 71, if the partition coefficient between olivine and melt for Fe–Mg exchange is 0·3 (Roeder & Emslie, 1970). As there is no evidence for cumulus texture in the many gabbronorite samples, and chemical compositions of the rocks constituting the Harzburg intrusion exhibit a good and single compositional trend, especially for higher mg-number (> 50), we infer a primary melt composition from the chemical trend. If there is a melt composition coexisting with the primary olivine on the chemical trend represented by the whole-rock variation shown in Fig. 8, the composition can be represented by the intersection between the whole-rock chemical trend and mg-number = 71. On the basis of this approach, the best estimate of the primary melt composition is shown in Table 6. Whole-rock compositions of continental flood basalts, island-arc basalts and mid-ocean ridge basalt are also shown for comparison in Table 6. The estimated melt composition is characterized by low TiO2 and P2O5 and relatively higher Al2O3 and SiO2 as compared with continental flood basalts and mid-ocean ridge basalts. The estimated melt composition with its low TiO2 is similar to that of island-arc tholeiites.
|
Mixing phenomena
As shown in Fig. 5, we can distinguish two types of clinopyroxenes in one specimen (H-73). In this specimen, clinopyroxene showing low mg-number (70) is always located in the centre of large plagioclase crystals as small fragments, and never shows euhedral shape (Fig. 3h). The larger clinopyroxenes forming individual crystals have higher mg-number (86–90). Furthermore, plagioclase also shows two different compositions, such as low An (50–58) in the core and high An (76–86) in the rim (Fig. 5). These chemical variations of the minerals point to mixing of a fractionated magma having low mg-number clinopyroxene and low-An plagioclase with a more primitive magma. The dunitic cumulates with the most primitive composition contain hydrous minerals such as amphibole and phlogopite included in cumulus and intercumulus phases, indicating a primary, hydrous mantle magma. Fractionation processes from such magma lead to highly evolved, hydrated felsic compositions. Mixing phenomena in such a magma chamber involve fluid phases and may be related to changes in O2 fugacity. The occurrence of felsic veins containing An-rich plagioclase and phlogopite within the dunite documents this mixing phenomenon. Cumulates and fractionated portions of a mantle-derived magma occur together and demonstrate hybridization as a result, perhaps, of the overturn of a magma chamber.
Petrographically visible effects of magma evolution and mixing provide evidence for a primitive ultramafic melt, the effects of fractional crystallization, mixing with a crustal magma and fluid interaction at different stages of the magmatic evolution. Hence, assimilation–fractional crystallization calculations for the magmatic rocks of the Harzburg intrusion remain somewhat poorly constrained, as fluid composition may have been highly variable over time. Furthermore, the rocks used for this investigation are all ultramafic, for example, eight dunites and one harzburgite with the exception of one gabbro and two felsic veins. These rocks do not properly describe a magmatic rock suite. Nevertheless, a mixture of our harzburgite (Flora III 87.9) with a granitic assimilant (Brocken granite, PKG-17; Baumann et al., 1991) requires a ratio of the rate of assimilation to the rate of crystallization of 0·3 to fit our dataset.
Magma evolution constrained by Nd–Sr–Pb isotopes
Three isotopically distinctive reservoirs necessary to explain the isotopic variability of the sample suite match against two petrologically distinct reservoirs (mantle melt and crustal melt) and a fluid of unknown provenance (hydrous magmatic phases). Because of contrasting solubilities of Sr, Pb and Nd in fluids (e.g. Michard, 1989), a fluid represents an ideal tool to uncouple the isotope systematics of Nd from those of Sr and Pb. Such a fluid phase may have been involved in the formation of plagioclase-bearing rocks, which all fall to the right of the 
Nd(T)–87Sr/86Sr(T) mixing hyperbola shown for a two-component crust–mantle system (Fig. 10a). Plagioclase-bearing rocks received variable additions of more radiogenic Sr and Pb from the fluid, but essentially no additions of Nd, which results in subhorizontal trends in the Nd(T)–87Sr/86Sr(T) and Nd(T)–206Pb/204Pb diagrams (Fig. 10a and e). The Pb, Sr and Nd isotope composition of the fluid probably was spatially and temporally highly variable, and therefore is not well known. Its Pb isotope composition was rather radiogenic, with 206Pb/204Pb values as high as 18·77 (Fig. 10). Such values are distinctly more radiogenic than the Pb isotope compositions of hydrothermal ore deposits in the Harz (Fig. 10), which are thought to reflect the isotopic character of the Rhenohercynian crust (Bielicki & Tischendorf, 1991). It is well known that fluids leaching the rock preferentially mobilize radiogenic over non-radiogenic lead (e.g. Johansson, 1983; Romer & Wright, 1993). This is commonly explained by in situ grown radiogenic Pb and unradiogenic Pb occurring in different environments in the crystal. In situ grown radiogenic Pb has been displaced within the crystal structure by a series of alpha recoils, whereas unradiogenic Pb is still hosted in the crystal on sites that are suitable for Pb. Thus, the anomalously radiogenic Pb isotope signature of the fluid-derived component may indicate that the fluid preferentially mobilized radiogenic Pb. Strontium in the fluid was radiogenic and had an 87Sr/86Sr of at least 0·708, that is, the composition of the affected sample (Table 5), whereas the isotopic composition of Nd in the fluid is not constrained. Although the Pb and Sr isotope signatures do not necessarily represent the isotope signature of the fluid source, the radiogenic compositions of Pb and Sr demonstrate that they were derived from wall rocks and not from the mantle melt. As the emplacement of a hot mafic magma into the crust will cause dehydration reactions, it appears possible that not only the Pb and Sr but also the fluid was derived from the crust.
|
In the two-component Nd(T)–87Sr/86Sr mixing system (Fig. 10a), dunite and the felsic pods represent the two end-members. The dunite displays the least radiogenic Pb and Sr isotope composition and the most radiogenic Nd, and therefore might most closely approach the isotopic signature of the mantle source. However, the dunites also have the lowest contents of REE, Sr and Pb of the entire sample suite, and therefore represent those samples that will most easily acquire a different isotopic composition through contamination with crustal material. As a result of the large contrast in contents of Pb between dunite and crustal rocks, the large variation in Pb isotope composition among different reservoirs, and the ready mobilization of Pb, the Pb isotope signature of dunite will possibly be more strongly affected than the isotope signature of Sr and Nd. Thus, if Pb shows no crustal contamination, the isotope signature of Sr and Nd is likely not to have been affected by crustal material. The Pb isotopes of the dunite samples are much higher in 207Pb/204Pb and 208Pb/204Pb than a typical mantle reservoir (Zartman & Doe, 1981). They fall close to typical upper-crustal Pb evolution curves (Zartman & Doe, 1981). Thus, although dunite shows the least radiogenic Pb among the rocks of the Harzburg intrusion, its Pb signature is entirely dominated by a crustal signature, which implies that its 87Sr/86Sr(T) represent maximum values and its Nd(T) represent minimum values for the mantle melt. The predominance of a crustal signature in the dunite sample, furthermore, implies that the crustal Pb acquired by the rocks of the Harzburg intrusion originates from at least two isotopically distinct crustal reservoirs.
The felsic pods best represent one crustal end-member. Their initial Sr isotope composition falls within the range of Sr isotope compositions known from the Harz (Fig. 10c). As a result of the wide range of lithologies of contrasting age, it is not possible to define one crustal end-member. Instead, there may be a range of end-members. This becomes especially clear in the Nd(T)–206Pb/204Pb and 87Sr/86Sr–206Pb/204Pb diagrams, where the samples do not fall on a single mixing trend, but rather on a suite of mixing trends (see Fig. 10).
Rocks that fall on the Nd(T)–87Sr/86Sr mixing trend include the dunite and hybrid gabbro samples and samples from the felsic pods within the dunites. Samples that fall off this trend are characterized by rock–fluid interaction and include the dunite, gabbro, harzburgite and plagioclase-bearing dunite samples. These rocks display a small variation in Nd(T), which implies only minor contributions of crustally derived Nd. The Pb–Nd–Sr systematics of these rocks is compatible with a fractionation-controlled genetic relationship in the presence of an externally derived fluid phase. Petrological data, especially the bimodal distribution of compositional ranges of plagioclase and pyroxene, demonstrate that the hybrid gabbro samples represent a mixture of cumulates with more felsic material similar to the one in the felsic pods. The felsic pods represent either crustal material remobilized by the heat of the Harzburg mafic–ultramafic intrusion or, more likely, represent the upper part of the magma chamber that experienced extensive contamination by crustal material through assimilation processes. In such a scenario, the hybrid gabbro may have originally evolved in the same magma chamber as the dunites, harzburgites and gabbros. The hybrid gabbro received its hybrid character during a late stage of fractionation, when the highly contaminated upper parts of the magma chamber mixed with less contaminated parts beneath.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Although the Harzburg intrusion shows signs of strong assimilation of surrounding crustal rocks, the ultramafic and some of the mafic rocks such as dunite, harzburgite, norite and gabbronorite preserve essential magma features. The primary magma composition estimated from the constituent minerals and the whole-rock chemical compositions is similar to that of an island-arc tholeiite rather than a magma of continental affinity. Crystallization occurred in the primary magma during its ascent to crustal levels, where it strongly assimilated crustal rocks. As a result, the intrusion shows an unusually wide compositional range from dunite to quartz diorite, characterized by a wide range of Sr, Nd and Pb isotope compositions.
|
ACKNOWLEDGEMENTS |
|---|
We are indebted to Dr. G. von Bronsart, who gave us the permission to work on samples from the Flora core, formerly drilled by his company Bronsart–Hamburger Metallkontor GmbH. We also wish to thank the Niedersächsisches Landesamt and the Bundesanstalt für Geowissenschaften und Rohstoffe, both in Hanover, for their co-operation. Drs. K. P. Burgäth and M. Mohr were very helpful at the core storage facility in Grubenhagen. Dr. P. Dulski provided ICP-MS analyses, Dr. R. Naumann helped with XRF analyses and Mrs. C. Schulz with isotope analyses. We appreciated constructive comments made by C. W. Devey, K. Mezger, J. Geldmacher and an anonymous reviewer. This work was supported by Overseas Research Program and Grant-in-Aid for Scientific Research (No. 11640454) from the Japanese Ministry of Education (Monbu-sho) for S. Sano.
|
FOOTNOTES |
|---|
*Corresponding author. Fax: +49 331 977 5060.
E-mail: roob{at}geo.uni-potsdam.de
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGYPETROGRAPHY OF THE MAFIC...MINERAL CHEMISTRYGEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Ahrendt, H., Franzke, H. J., Marheine, D., Schwab, M. & Wemmer, K. (1996). Zum Alter der Metamorphose in der Wippraer Zone/Harz: Ergebnisse von K/Ar-Alterssdatierungen an schwachmetamorphen Sedimenten. Zeitschrift der Deutschen Geologischen Gesellschaft 147(1), 11–28.
Anders, E. & Grevesse, N. (1989). Abundances of the elements: meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197–214.[Web of Science]
Anderson, T. A. (1975). Carboniferous subduction complex in the Harz Mountains, Germany. Geological Society of America Bulletin 86, 77–82.
Basaltic Volcanism Study Project (1981). Basaltic Volcanism on the Terrestrial Planets. New York: Pergamon Press, 1286 pp.
Baumann, A., Grauert, B., Mecklenburg, S. & Vinx, R. (1991). Isotopic age determinations of crystalline rocks of the Upper Harz Mountains, Germany. Geologische Rundschau 80, 669–690.
Bielicki, K.-H. & Tischendorf, G. (1991). Lead isotope and Pb–Pb model age determinations of ores from Central Europe and their metallogenetic interpretation. Contributions to Mineralogy and Petrology 106, 440–461.[Web of Science]
Cox, K. G. (1983). The Karoo province of southern Africa: origin of trace element enrichment patterns. In: Hawkesworth, C. J. & Norry, M. J. (eds) Continental Basalts and Mantle Xenoliths. Nantwich: Shiva, pp. 139–157.
Dick, H. J. B. (1989). Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 71–105.
Dick, H. J. B. & Bullen, T. (1984). Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas. Contributions to Mineralogy and Petrology 86, 54–76.[Web of Science]
Engelbrecht, J. P. (1985). The chromites of the Bushveld Complex in the Nietverdiend area. Economic Geology 80, 896–910.
Erdmannsdörfer, O. H. & Schröder, H. (1927). Erläuterungen zur geologischen Karte von Preußen, Blatt Bad Harzburg, 3. Berlin: Preußisch geologische Landesanstalt.
Franz, L., Schuster, A. & Strauss, K. (1997). Basement evolution in the Rhenohercynian segment: discontinuous exhumation history of the Eckergneis Complex (Harz Mountains, Germany). Chemie der Erde 57, 105–135.[Web of Science]
Gabriel, G., Jahr, T., Jentzsch, G. & Melzer, J. (1997). Deep structure and evolution of the Harz mountains: results of three-dimensional gravity and finite-element modelling. Tectonophysics 270, 279–299.[Web of Science]
Ganssloser, M., Theye, T. &, Wachendorf, H. (1996). Detrital glaucophane in greywackes of the Rhenohercynian Harz mountains and the geodynamic implications. Geologische Rundschau 85, 755–760.
Harries, L. (1999). Saphirblauer Korund aus dem Harz. Lapis 11, 30.
Hentschke, U. (1985). The late fractionation stage of the Harzburg gabbro intrusion, Harz mountains, North Germany. Neues Jahrbuch für Mineralogie, Abhandlungen 152, 177–185.[Web of Science]
Himmelberg, G. R., Loney, R. A. & Craig, J. T. (1986). Petrogenesis of the ultramafic complex at the Blashke Islands, southeastern Alaska. US Geological Survey Bulletin, 1662, 14 pp.
Hofmann, A. W. (1988). Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth and Planetary Science Letters 90, 297–314.[Web of Science]
Johansson, Å. (1983). Lead isotope composition of Caledonian sulphide-bearing veins in Sweden. Economic Geology 78, 1674–1688.
Leake, B. E. (1978). Nomenclature of amphiboles. Canadian Mineralogist 16, 501–520.
Lévèque, J. & Haack, U. (1993). Pb isotopes of hydrothermal ores in the Harz. Monograph Series on Mineral Deposits 30, 197–210.
Lossen, K. A. (1889). Mittheilungen des Herrn K.A. Lossen über die geologische Kartenaufnahme im Harzburger Revier (im Jahre 1888). Jahrbuch der Königlich Preußischen Geologischen Landesanstalt 1888 VIII, XXXV–XLIII.
McLaren, C. H. & De Villiers, J. P. R. (1982). The platinum-group chemistry and mineralogy of the UG-2 chromitite layer of the Bushveld Complex. Economic Geology 77, 1348–1366.
Michard, A. (1989). Rare earth element systematics in hydrothermal fluids. Geochimica et Cosmochimica Acta, 53, 745–750.[Web of Science]
Mohr, K. (1978). Geologie und Minerallagerstätten des Harz. Stuttgart: Schweizerbartsche Verlagsbuchhandlung.
Müller, G. (1978). Die magmatischen Gesteine des Harzes. Clausthaler Geologische Abhandlungen 31, 92 pp.
Nicholson, D. M. & Mathez, E. A. (1991). Petrogenesis of the Merensky Reef in the Rustenburg section of the Bushveld Complex. Contributions to Mineralogy and Petrology 107, 293–309.[Web of Science]
Perfit, M. R., Gust, D. A., Bence, A. E., Arculus, R. J. & Taylor, S. R. (1980). Chemical characteristics of island arc basalts: implications for mantle sources. Chemical Geology 30, 227–256.[Web of Science]
Roach, T. A., Roeder, P. L. & Hulbert, L. J. (1998). Composition of chromite in the upper chromitite, Muskox layered intrusion, Northwest Territories. Canadian Mineralogist 36, 117–135.[Web of Science]
Roeder, P. L. & Emslie, R. F. (1970). Olivine–liquid equilibrium. Contributions to Mineralogy and Petrology 29, 275–289.[Web of Science]
Romer, R. L. & Wright, J. E. (1993). Lead mobilization during tectonic reactivation of the western Baltic Shield. Geochimica et Cosmochimica Acta 57, 2555–2570.[Web of Science]
Romer, R. L., Förster, H.-J. & Breitkreuz, C., (2001). Intracontinental extensional magmatism with a subduction fingerprint: the late Carboniferous Halle Volcanic Complex (Germany). Contributions to Mineralogy and Petrology 141, 201–221.[Web of Science]
Sato, H. (1977). Nickel content of basaltic magmas: identification of primary magmas and a measure of the degree of olivine fractionation. Lithos 10, 113–120.[Web of Science]
Scoon, R. N. & Teigler, B. (1994). Platinum-group element mineralization in the Critical Zone of the western Bushveld Complex. I. Sulphide-poor chromitites below the UG-2. Economic Geology 89, 1094–1121.
Sohn, W. (1956). Der Harzburger Gabbro. Geologisches Jahrbuch 72, 117–172.
Stütze, B. (1980). Leukokrate Gesteine im Harzburger Gabbromassiv. Ein Beitrag zum Verständnis der genetischen Beziehungen zwischen Brockengranit (Westrand), Okerintrusion und Ganggesteinen im Harzburger Gabbromassiv. Doctoral thesis, University of Hamburg, 166 pp.
Sun, S. S. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.
Thompson, R. N., Morrison, A. P., Dickin, A. P. & Hendry, G. L. (1983). Continental flood basalts—arachnids rule OK? In: Hawkesworth, C. J. & Norry, M. J. (eds) Continental Basalts and Mantle Xenoliths. Nantwich: Shiva, pp. 158–185.
Tilton, G. R. (1973). Isotopic lead ages of chondritic meteorites. Earth and Planetary Science Letters 19, 321–329.[Web of Science]
Tindle, A. G. & Webb, P. C. (1994). PROBE-AMPH—a spreadsheet program to classify microprobe-derived amphibole analyses. Computer Geosciences 20, 1201–1228.
Todt, W., Cliff, R.-A., Hanser, A. & Hofman, A. W. (1993). Re-calibration of NBS lead standards using a 202Pb + 205Pb double spike. Terra Abstracts 5(Suppl. 1), 396.
Vinx, R. (1982). Das Harzburger Gabbromassiv, eine orogenetisch geprägte Layered Intrusion. Neues Jahrbuch für Mineralogie, Abhandlungen 144, 1–28.[Web of Science]
Wachendorf, H., Buchholz, P. & Zellmer, H. (1995). Fakten zum Harz-Paläozoikum und ihre geodynamische Interpretation. Nova Acta Leopoldina NF 71, 119–150.
Wilson, A. H. (1982). The geology of the Great ‘Dyke’, Zimbabwe: the ultramafic rocks. Journal of Petrology 23, 240–292.
Zartman, R. E. & Doe, B. R. (1981). Plumbotectonics—the model. Tectonophysics 75, 135–162.[Web of Science]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||