Petrological, Geochemical and Isotopic Constraints ... - Oxford Journals

JOURNAL OF PETROLOGY

VOLUME 43

NUMBER 8

PAGES 1529–1549

2002

Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany ¨ NSLI1∗, ROLF L. ROMER3 AND SAKAE SANO1,2, ROLAND OBERHA ROLAND VINX4 ¨ T POTSDAM, PF 601553, D-14415 POTSDAM, GERMANY ¨ R GEOWISSENSCHAFTEN, UNIVERSITA INSTITUT FU

1 2

EARTH SCIENCE LABORATORY, FACULTY OF EDUCATION, EHIME UNIVERSITY, 3 BUNKYO-CHO,

790-8577 MATSUYAMA, JAPAN 3

GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, D-14473 POTSDAM, GERMANY ¨ T HAMBURG, GRINDELALLEE 48, D-2000 HAMBURG, MINERALOGISCH–PETROGRAPHISCHES INSTITUT, UNIVERSITA

4

GERMANY

RECEIVED DECEMBER 8, 2000; REVISED TYPESCRIPT ACCEPTED FEBRUARY 11, 2002

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 islandarc 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

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

∗Corresponding author. Fax: +49 331 977 5060. E-mail: [email protected]

 Oxford University Press 2002

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.

Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination KEY WORDS:

INTRODUCTION


VOLUME 43

(Ocker, Brocken and Ramberg granites) as well as a gabbronorite complex (Harzburg gabbro; Lossen, 1889; Erdmannsdo¨rfer & Schro¨der, 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. 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 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

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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 (Mu¨ller, 1978; Stu¨tze, 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 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

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HARZBURG MAFIC–ULTRAMAFIC INTRUSION

Fig. 1. Geological map of the Harz Mountains, central Germany [simplified and modified from Mohr (1978)]. Β, occurrences of high-pressure/ low-temperature metamorphic rocks (Ahrendt et al., 1996) and minerals (Ganssloser et al., 1996).

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)

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.

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.

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

Mixing phenomena (hybridization)

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Fig. 2. Lithological map of the Harzburg mafic–ultramafic intrusion [modified from Sohn (1956)]. Β, position of Flora I and Flora II–III drill cores.

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 anorthiterich rims. Furthermore, as described below, two types of clinopyroxenes showing an obvious compositional gap exist in small domains.

MINERAL CHEMISTRY 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.

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Fig. 3. (a)–(d)

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 fayaliterich 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 (1 wt % (Fig. 6a). Small spinel crystals are

Plagioclase shows wide compositional ranges (Fig. 5). Norite and gabbronorite (mg-number >60) include the most An [= 100Ca/(K + Na + Ca)] rich plagioclase (An>85). Intercumulus plagioclase in dunite and harzburgite is low in An content (An66–83) compared with that in norite and harzburgite (An>85). 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 Ζ 60) show two peaks in the An content at 55 and 73%. Furthermore, gabbronorites (mg-number Ζ 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

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Fig. 3. Photomicrographs of rocks from the Harzburg intrusion. Scale bars represent 1 mm. (a) Sample 378.7. Cumulate texture. Dunite with intercumulus phlogopite. It should be noted that the rectangular spinel crystals occur only in the intercumulus phases. (b) Sample H-16. Harzburgite with intercumulus orthopyroxene and pargasite. (c) Sample H-97. Norite. (d) Sample 380.8. Pargasite and phlogopite inclusions in spinel. (e) Sample H-214. Gabbronorite showing typical texture. (f ) Sample 101004. Fine-grained biotite–quartz–plagioclase inclusion in gabbronorite. (g) Sample HA974. Diorite showing hybrid texture. (h) Sample H-73. Irregular zoning of plagioclase.

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 TiO2rich 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.

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Fig. 4. Mineral assemblage as a function of whole-rock mg-number. It should be noted that amphibole and mica occur as intercumulus phases in the most high-mg-number cumulate. Clinopyroxene does not form a cumulus phase. Ilmenite is the dominant opaque mineral in gabbronorite. The essential crystallization order is olivine → spinel → plagioclase, orthopyroxene → clinopyroxene. Shaded zone indicates range of compositional variation of cumulates.

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 mgnumber 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 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 (ICPMS) (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

1535

Opx-cumulate

Ol-norite

Pl-‘harzburgite’

‘Harzburgite’

Ol-gabbronorite 0·807

Gabbronorite

Gabbronorite

Gabbronorite

H-8

H-11

H-13

H-16

H-17

H-55

H-66

H-69

Troctolite

Norite

Ol-norite

‘Harzburgite’

Ferrogabbro

Norite

Ol-gabbronorite 0·854

Diorite

Diorite

Dunite

Dunite

Dunite

Dunite

H-91

H-97

H-112

H-114

1536

H-116

H-151

H-214

H-315

H-316

Flora-380.0

Flora-380.8

Flora-381.3

Flora-382.0

89·5 (0·3)

89·5 (0·2)

89·5 (0·2)

89·6 (0·4)

0·29 (0·05)

0·31 (0·04)

0·30 (0·04)

0·29 (0·04)

0·02 (0·03)

0·17 (0·06)

72·3 (5·4)

69·4 (2·1)

80·5 (0·5)

52·1 (3·1)

61·9 (4·5)

83·1 (1·2)

77·9 (9·4)

76·2 (2·9)

18·4 (5·3)

85·1 (4·5)

85·7 (3·6)

71·1 (14·4)

64·9 (8·5)

58·5 (7·5)

72·2 (2·8)

73·6 (6·1)

79·6 (2·0)

85·1 (0·7)

84·4 (2·4)

76·1 (6·2)

62·4 (8·0)

0·381 (0·007)

0·428 (0·007)

0·855 (0·011)

0·748 (0·019)

0·366 (0·004)

0·867 (0·004)

0·874 (0·004)

0·883 (0·006)

0·879 (0·005)

0·819 (0·015)

0·462 (0·008)

0·479 (0·007)

0·475 (0·005)

0·816 (0·009)

0·871 (0·005)

0·888 (0·004)

0·865 (0·006)

0·860 (0·007)

0·332 (0·006)

Al2O3 (SD)

0·41 (0·04)

0·45 (0·08)

1·95 (0·21)

0·92 (0·28)

0·39 (0·04)

2·46 (0·34)

2·55 (0·26)

2·39 (0·23)

1·65 (0·12)

1·42 (0·41)

0·44 (0·11)

0·63 (0·13)

0·36 (0·06)

1·63 (0·29)

2·10 (0·56)

3·00 (0·29)

2·38 (0·64)

2·02 (0·23)

0·51 (0·03)

Opx

mg-no. (SD)

0·915 (0·018)

0·918 (0·011)

0·911 (0·013)

0·495 (0·019)

0·554 (0·010)

0·885 (0·015)

0·819 (0·012)

0·479

0·917 (0·004)

0·926 (0·002)

0·848 (0·065)

0·611 (0·007)

0·621 (0·009)

0·604 (0·010)

0·874 (0·009)

0·903 (0·010)

0·924 (0·010)

0·897 (0·009)

0·900 (0·009)

0·832 (0·005)

Cpx

TiO2 (SD)

0·51 (0·19)

0·61 (0·19)

0·66 (0·18)

0·17 (0·09)

0·19 (0·11)

0·31 (0·12)

0·40 (0·19)

0·11

0·41 (0·05)

0·46 (0·03)

0·42 (0·22)

0·15 (0·11)

0·10 (0·07)

0·17 (0·09)

0·72 (0·15)

0·40 (0·16)

0·44 (0·11)

0·64 (0·15)

0·50 (0·10)

0·15 (0·10)

Cpx

Al2O3 (SD)

3·41 (1·12)

3·97 (0·67)

4·03 (0·35)

0·72 (0·24)

0·72 (0·23)

2·65 (0·55)

1·21 (0·41)

0·90

3·15 (0·81)

4·21 (0·02)

1·84 (0·81)

0·76 (0·19)

0·58 (0·16)

0·74 (0·12)

2·44 (0·33)

3·41 (0·22)

4·33 (0·61)

2·93 (0·31)

2·73 (0·12)

0·59 (0·17)

Cpx

Na2O (SD)

0·61 (0·11)

0·65 (0·11)

0·63 (0·11)

0·18 (0·03)

0·15 (0·02)

0·31 (0·08)

0·19 (0·03)

0·16

0·53 (0·13)

0·64 (0·05)

0·31 (0·16)

0·16 (0·03)

0·15 (0·03)

0·15 (0·02)

0·40 (0·08)

0·59 (0·08)

0·56 (0·12)

0·36 (0·07)

0·35 (0·03)

0·16 (0·03)

Cpx

Cr2O3 (SD)

1·14 (0·35)

1·25 (0·32)

1·38 (0·35)

0·02 (0·01)

0·04 (0·03)

0·75 (0·12)

0·20 (0·07)

0·00

0·94 (0·18)

1·01 (0·01)

0·04 (0·04)

0·07 (0·06)

0·05 (0·04)

0·05 (0·04)

0·26 (0·06)

1·05 (0·11)

1·00 (0·07)

0·80 (0·18)

0·59 (0·08)

0·09 (0·06)

Cpx

mg-no. (SD)

0·540 (0·033)

0·508 (0·040)

0·539 (0·027)

0·562 (0·014)

0·463 (0·058)

0·424 (0·015)

0·492 (0·031)

0·518 (0·025)

0·583 (0·033)

0·442 (0·015)

Spinel

cr-no. (SD)

0·470 (0·014)

0·505 (0·030)

0·464 (0·029)

0·450 (0·015)

0·487 (0·052)

0·525 (0·004)

0·479 (0·022)

0·420 (0·025)

0·435 (0·034)

0·418 (0·009)

Spinel

TiO2 (SD)

0·74 (0·19)

1·36 (0·66)

1·11 (0·24)

0·64 (0·23)

1·19 (0·73)

0·43 (0·03)

0·58 (0·35)

0·49 (0·19)

0·42 (0·10)

1·59 (0·71)

Spinel

The complete dataset is available at the Journal of Petrology Web site. mg-number = Mg/(Mg + Fe2+) using cation fractions. XFo calculated as 100Mg/( Mg + Fe2+) and XAn as 100Ca/(Ca + Na) using cation fractions. cr-number = Cr/(Cr + Al + Fe3+) using cation fractions. WR, whole rock; Opx, orthopyroxene; Cpx, clinopyroxene. Concentrations are in wt %, with standard deviation given in parentheses.

0·889

0·889

0·889

20·5 (1·3)

85·6 (0·1)

0·28 (0·04)

0·20 (0·04)

0·25 (0·05)

0·27 (0·07)

0·10 (0·04)

0·17 (0·03)

0·23 (0·04)

0·21 (0·04)

0·22 (0·04)

mg-no. (SD)

NUMBER 8

0·887

0·439

0·410

0·749

85·9 (0·4)

86·7 (0·4)

88·1 (0·3)

87·0 (0·3)

78·0 (0·6)

78·9 (0·3)

86·0 (0·5)

88·2 (0·4)

85·9 (0·3)

XAn (SD) Plagioclase Opx

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0·286

0·853

0·869

0·863

Gabbronorite

H-73

0·811

0·481

0·457

0·473

0·855

0·865

0·853

Diorite

0·852

Gabbro

H-4

NiO (SD) Olivine

WR

Olivine

mg-no. XFo (SD)

H-1

Sample no. Rock type

Table 1: Summary of compositional data for anhydrous minerals, Harzburg intrusion, Germany JOURNAL OF PETROLOGY AUGUST 2002

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Dunite

Dunite

Flora-381.3

Flora-382.0

88·9

88·9

88·9

88·7

43·9

41·0

85·4

74·9

28·6

85·3

86·9

86·3

81·1

48·1

45·7

47·3

80·7

85·5

0·920 (0·012)

0·920 (0·004)

0·921 (0·001)

0·391 (0·024)

0·460 (0·010)

0·820 (0·001)

0·772 (0·016)

0·343

0·893 (0·005)

0·919 (0·012)

0·835 (0·030)

0·499 (0·012)

0·485 (0·024)

0·485 (0·009)

0·862 (0·009)

0·897 (0·003)

0·893

0·908 (0·004)

0·890 (0·016)

0·307 (0·041)

5·50 (0·70)

3·58 (0·75)

3·46 (0·30)

2·22 (0·05)

5·20 (0·44)

4·35 (0·24)

3·99 (0·42)

4·46 (1·49)

4·25

4·07 (0·97)

0·44 (0·38)

2·81 (1·06)

5·27 (0·30)

5·46 (0·27)

4·66 (0·53)

2·77 (0·92)

4·35 (0·16)

1·57

2·15 (0·21)

2·72 (1·13)

4·82 (0·42)

1·16 (0·44)

1·19 (0·05)

1·07 (0·02)

0·29 (0·04)

0·12 (0·03)

0·25 (0·00)

0·16 (0·03)

0·29

1·10 (0·17)

1·44 (0·47)

1·03 (0·35)

0·14 (0·02)

0·16 (0·03)

0·16 (0·03)

1·05 (0·23)

1·16 (0·08)

0·60

1·48 (0·68)

1·31 (0·07)

0·13 (0·03)

0·20 (0·03)

Mica

Na2O (SD) Mica

Cr2O3 (SD)

1·12

0·07

8·09 (0·83) 1·88 (0·21)

8·49 (0·14) 1·56 (0·22)

7·98 (0·07) 0·70 (0·07)

9·45 (0·41) 0·07 (0·04)

9·54 (0·21) 0·06 (0·05)

8·82 (0·02) 1·92 (0·08)

8·93 (0·72) 0·28 (0·06)

8·45

8·19 (0·24) 1·68 (0·16)

7·45 (0·46) 0·38 (0·44)

7·64 (2·64) 0·02 (0·02)

10·23 (0·38) 0·09 (0·04)

10·05 (0·23) 0·13 (0·03)

9·69 (1·13) 0·08 (0·02)

8·61 (0·70) 0·09 (0·04)

8·38 (0·29) 1·14 (0·09)

4·51

6·27 (1·64) 0·19 (0·04)

7·93 (0·28) 1·29 (0·39)

10·16 (0·15) 0·08 (0·08)

9·26 (0·54) 0·15 (0·11)

Mica

K2O (SD)

0·918 (0·030)

0·887 (0·011)

0·914 (0·017)

0·971 (0·027)

0·540 (0·099)

0·924 (0·025)

0·459 (0·091)

0·878 (0·007)

0·998 (0·003)

0·978

0·939 (0·008)

0·871 (0·036)

0·754

0·772 (0·055)

0·722 (0·027)

0·851 (0·024)

0·880 (0·026)

0·997 (0·005)

1·000 (0·000)

0·983 (0·021)

0·403 (0·079)

0·840 (0·084)

Amphibole

mg-no. (SD)

2·44 (0·87)

4·13 (0·15)

2·06 (0·73)

0·86 (0·41)

0·14 (0·06)

0·78 (0·24)

0·24 (0·24)

2·81 (0·27)

0·20 (0·16)

0·56

2·71 (0·18)

2·62 (0·52)

0·24

0·49 (0·13)

0·55 (0·09)

3·22 (0·71)

3·67 (0·66)

0·81 (0·64)

1·77 (0·20)

0·93 (0·35)

0·15 (0·17)

0·26 (0·16)

Amphibole

TiO2 (SD)

3·01 (0·43)

2·84 (0·10)

3·31 (0·51)

2·96 (0·15)

0·31 (0·19)

0·49 (0·10)

0·28 (0·25)

2·58 (0·43)

0·99 (0·17)

1·89

2·50 (0·07)

2·11 (0·16)

0·37

0·47 (0·09)

0·48 (0·07)

2·35 (0·09)

2·56 (0·09)

2·27 (0·25)

2·80 (0·04)

1·45 (0·37)

0·22 (0·21)

0·21 (0·14)

Amphibole

Na2O (SD)

0·67 (0·42)

1·00 (0·03)

0·55 (0·44)

0·28 (0·15)

0·19 (0·13)

0·34 (0·08)

0·08 (0·14)

0·86 (0·45)

0·22 (0·07)

0·60

0·45 (0·01)

0·69 (0·24)

0·19

0·20 (0·05)

0·23 (0·04)

0·95 (0·18)

0·90 (0·12)

0·39 (0·05)

0·22 (0·02)

0·37 (0·09)

0·11 (0·18)

0·08 (0·07)

Amphibole

K2O (SD)

1·625 (0·292)

1·831 (0·030)

1·747 (0·057)

1·633 (0·102)

0·434 (0·274)

0·576 (0·082)

0·325 (0·327)

1·741 (0·046)

0·695 (0·173)

1·405

1·721 (0·045)

1·589 (0·098)

0·450

0·577 (0·058)

0·607 (0·072)

1·720 (0·082)

1·829 (0·035)

1·535 (0·134)

1·706 (0·082)

1·105 (0·258)

0·286 (0·327)

0·287 (0·156)

Amphibole

Al(IV) (SD)

0·791 (0·131)

0·811 (0·029)

0·863 (0·096)

0·710 (0·090)

0·047 (0·011)

0·061 (0·014)

0·033 (0·021)

0·757 (0·031)

0·149 (0·071)

0·477

0·617 (0·024)

0·532 (0·079)

0·035

0·036 (0·010)

0·041 (0·007)

0·695 (0·046)

0·757 (0·041)

0·554 (0·062)

0·581 (0·013)

0·313 (0·095)

0·036 (0·022)

0·017 (0·012)

Amphibole

A-site (SD)

pargasitic hornblende

pargasite

pargasite–pargasitic hornblende


actinolitic hornblende

tremolitic hornblende

hornblende

cummingtonite/actinolitic


hornblende

tremolitic hornblende/magnesio-

magnesio-hornblende


hornblende

pargasitic hornblende/magnesio-

actinolite




pargasite

hornblende

edenitic hornblende/pargasitic

magnesio-hastingsitic hornblende

magnesio-hornblende

hornblende

cummingtonite/ferro-actinolitic

actinolite

Nomenclature

The complete dataset is available at the Journal of Petrology Web site. mg-number = Mg/(Mg + Fe2+) using cation fractions. Concentrations are in wt %, with standard deviation given in parentheses. Nomenclature of amphiboles was determined by PROBE-AMPH (Tindle & Webb, 1994).

Dunite

Dunite

Flora-380.0

Diorite

H-316

Flora-380.8

Ol-gabbronorite

Diorite

H-214

Norite

H-151

H-315

‘Harzburgite’

Ferrogabbro

H-114

Ol-norite

H-112

H-116

Troctolite

Norite

H-91

Gabbronorite

H-73

H-97

Gabbronorite

Gabbronorite

H-66

H-69

Ol-gabbronorite

Gabbronorite

H-17

H-55

‘Harzburgite’

H-16

86·5

85·3

Ol-norite

Pl-‘harzburgite’

H-11

Opx-cumulate

H-8

H-13

85·2

Gabbro

Diorite

H-1

0·665 (0·011)

Mica

Whole Mica

rock

TiO2 (SD)

mg-no. mg-no. (SD)

H-4

Rock type

Sample no.

Table 2: Summary of compositional data for hydrous minerals, Harzburg intrusion, Germany

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Fig. 5. Frequency of Fo (%) in olivine, An (%) in plagioclase and mg-number in clinopyroxene. It should be noted that obvious bimodal compositional ranges in An (%) are recognized in gabbronorite (40 < mg-number < 60), gabbronorite (mg-number 70) always occurs as small fragments located in the core of large plagioclase crystals (see Fig. 3h). The surrounding plagioclase has much lower An content (>48–58) in the core and higher An content (>80–84) in the rim.

by ion-exchange chromatography using standard procedures. Analytical details are given in the footnote to Table 5.

metasedimentary rocks plot in the same area as the gabbronorites.

Whole-rock trace element composition 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 mgnumber. 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

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

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SANO et al.


Fig. 6. Anhydrous mineral compositions. (a1) mg-number vs cr-number in spinels. Field for spinel composition from oceanic peridotites is from Dick & Bullen (1984) and Dick (1989). Field for spinel compositions for layered intrusions is from McLaren & De Villiers (1982), Wilson (1982), Engelbrecht (1985), Nicholson & Mathez (1991), Scoon & Teigler (1994) and Roach et al. (1998). Spinel field from Alaskan arc cumulates is from Himmelberg et al. (1986). (a2) cr-number vs TiO2 in spinel. (b) mg-number vs NiO in olivine. Field of olivine composition in mantle peridotites is from Sato (1977). (c) mg-number vs Al2O3 in orthopyroxene. (d) mg-number vs TiO2, Al2O3, Na2O and Cr2O3 in clinopyroxene.

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 (× 1 chondrite at La to Lu) with flat patterns, whereas harzburgites show almost parallel patterns with a much higher REE abundance (× 5 to × 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 (× 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).

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Fig. 7. Hydrous mineral compositions. (a) Amphibole compositions. Nomenclature from Leake (1978). Intercumulus amphiboles in dunite and harzburgite are Ti-rich pargasite–Ti-rich pargasitic hornblende compositions. (b) Biotite–phlogopite compositions. K2O and TiO2 concentrations increase with decreasing mg-number.

DISCUSSION 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 wholerock 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

1540

41·486

1541

101·635

1·011

0·002

0·007

0·905

99·622

1·002

0·207

0·004

1·779

0·001

0·006

0·896

Total

Si

Fe2+

Mn

Mg

Ca

Ni

XMg

0·064

1·791

0·005

0·220

0·372

0·036

0·294

CaO

49·431

0·257

10·661

NiO

0·171

48·430

MnO

MgO

40·652

10·039

SiO2

FeO

Average

Comment:

Max.

Flora-380.0 (n=27)

Sample:

0·890

0·004

0·000

1·757

0·001

0·187

0·994

98·255

0·212

0·013

47·537

0·071

9·064

40·201

Min.

0·004

0·001

0·000

0·007

0·001

0·008

0·004

0·580

0·040

0·013

0·341

0·048

0·370

0·257

SD

0·900

0·006

0·001

1·787

0·003

0·198

1·003

100·055

0·306

0·033

48·954

0·161

9·643

40·959

Average

0·930

0·007

0·002

1·846

0·005

0·213

1·009

101·003

0·354

0·059

51·571

0·236

10·382

41·665

Max.

Flora-380.8 (n=23)

0·893

0·004

0·000

1·762

0·001

0·138

0·998

98·206

0·226

0·010

47·919

0·059

6·712

40·554

Min.

0·012

0·001

0·000

0·025

0·001

0·025

0·003

0·648

0·038

0·014

0·965

0·039

1·176

0·280

SD

0·895

0·006

0·001

1·784

0·004

0·210

0·998

99·637

0·307

0·028

48·505

0·172

10·181

40·444

Average

0·898

0·007

0·002

1·794

0·005

0·218

1·003

100·523

0·379

0·061

49·031

0·230

10·579

40·776

Max.

Flora-381.3 (n=22)

0·891

0·005

0·000

1·776

0·002

0·202

0·991

98·453

0·240

0·010

48·000

0·105

9·810

39·893

Min.

0·002

0·001

0·000

0·005

0·001

0·004

0·003

0·465

0·045

0·012

0·263

0·038

0·184

0·225

SD

0·897

0·006

0·001

1·782

0·003

0·204

1·002

99·060

0·289

0·038

48·267

0·164

9·829

40·474

Average

0·928

0·007

0·002

1·843

0·005

0·219

1·012

99·898

0·381

0·066

50·626

0·243

10·597

41·077

Max.

Flora-382.0 (n=21)

0·890

0·003

0·000

1·765

0·002

0·142

0·996

97·422

0·170

0·000

47·198

0·081

6·974

39·939

Min.

Table 3: Olivine analyses from Flora drill cores (average, minimum, maximum and standard deviation), Harzburg intrusion, Germany

0·010

0·001

0·001

0·020

0·001

0·019

0·004

0·558

0·049

0·019

0·793

0·042

0·891

0·258

SD

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Table 4: Whole-rock major and trace element abundance of Harz Gabbro intrusion Samples from drill core

Sample:

Samples from surface

Flora I

H-16

Comment: harz.

Flora II

Flora III

50.8

H-73

HA974

253.3

364.6–7 369.8

378.7–8 380.9

382.1

382.2

382.4

40.6

hybrid

hybrid

felsic

dunite

dunite

dunite

dunite

dunite

gabbro felsic

dunite

dunite

87.9 harz.

SiO2

40·04

48·83

52·55

65·72

36·96

36·91

36·92

37·17

36·73

36·26

36·24

39·05

57·30

TiO2

0·15

0·26

0·26

1·04

0·06

0·08

0·13

0·08

0·07

0·05

0·05

0·11

0·54

38·45 0·22

Al2O3

6·10

14·50

14·27

14·49

2·52

2·15

1·64

2·34

2·11

2·35

2·29

10·28

16·11

3·14

Fe2O3

3·43

1·34

1·88

1·33

4·33

4·58

4·94

4·08

4·62

4·58

4·85

4·93

0·98

4·82

FeO

6·58

5·98

5·31

5·25

5·11

5·07

4·87

5·39

5·00

4·92

4·61

3·62

1·67

6·08

MnO

0·165

0·154

0·144

0·112

MgO

31·96

17·32

12·61

3·47

0·155 39·96

0·150 41·16

3·806

0·179

9·80

35·61

0·97

5·87

4·37

2·21

0·44

1·48

0·13

8·16

0·23

0·75

1·10

0·67

K2O

0·074

0·374

0·754

2·095

0·014

0·028

0·082

0·030

0·030

0·010

0·020

0·065

0·176

0·116

P2O5

0·026

0·040

0·018

0·079

0·016

0·020

0·033

0·021

0·021

0·015

0·017

0·013

0·176

0·042

H2O+

5·81

1·45

2·38

4·18

7·84

7·41

7·78

6·90

7·26

7·96

8·49

8·47

2·96

7·53

CO2

0·20

0·17

0·10

1·00

0·31

0·29

0·33

0·26

0·41

0·36

0·34

0·27

0·32

0·25

Total

99·04

99·95

99·54

99·67

98·72

98·55

98·71

98·5

98·54

98·26

98·38

99·11

99·68

98·76

mg-no.

85·49

81·13

76·24

49·00

88·78

88·80

88·74

88·92

88·87

88·89

88·91

85·14

87·25

85·91

Sc

16

24

27

15

8

8

8

8

7

7

7

13

10

15

V

79

104

153

131

40

48

70

50

51

43

47

70

34

74

Cr

3267

615

696

233

5119

6079

5852

3897

5300

5172

5089

3501

79

4671

Ni

1328

260

655

92

2182

2124

2113

2056

2127

2217

1983

1084

48

1805

Zn

65

57

56

66

57

59

56

54

55

53

57

47

29

67

17·3

35·4

87·2

Sr Y Zr Cs Ba

28·2 4·48 12·7 0·13 16·8

142 6·91 26·0 0·98 68·7

136 10·2 19·1 1·73 98

35·7 20·8 124 5·77 238

1·08 20·2